Effects of microinjection of vasopressin in dorsal pontine reticular structures on the gain of vestibulospinal reflexes in decerebrate cats.

1. The possibility that vasopressin (VP) acts on the dorsal pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) system to control posture as well as the vestibulospinal reflexes has been investigated by injecting small doses of VP in precollicular decerebrate cats. 2. Unilateral microinjection of VP (0.25 microliters at the concentration of 10(-11) micrograms/microliters saline) in the pRF decreased the extensor rigidity in the ipsilateral limbs, while that of the contralateral limbs either decreased or increased. The same injection also produced a moderate or a prominent increase in gain of the multiunit EMG responses of the ipsilateral triceps brachii to roll tilt of the animal (t-test, P less than 0.001 for either group of responses). In the first instance the response gain of the contralateral triceps brachii to animal tilt slightly increased, while the pattern of response remained always of the alpha-type, as shown for the ipsilateral responses (increased EMG activity during ipsilateral tilt and decreased activity during contralateral tilt). In the second instance, however, the response gain showed only slight changes, while the pattern of responses reversed from the alpha- to the beta-type. These findings occurred 5-20 min after the injection, fully developed within 30-60 min and disappeared in about 2-3 hours. 3. The structures responsible for the postural and reflex changes described above were located in the dorsal pontine tegmental region immediately ventral to the LC, and included the peri-LC alpha and the surrounding dorsal pRF. The induced effects depended upon the injected neuropeptide, since previous injection of an equal volume of saline stained by the pontamine sky blue dye into the same dorsal pontine area was ineffective. 4. We postulated that VP exerts an excitatory influence on ipsilateral dorsal pRF neurons. The increased discharge of these neurons and the related medullary inhibitory RS neurons would lead to a decreased postural activity in the ipsilateral limbs. However, since these inhibitory RS neurons fire out of phase with respect to the excitatory vestibulospinal neurons, it appears that the higher the firing rate of the RS neurons in the animal at rest, the greater the disinhibition that affects the limb extensor motoneurons during ipsilateral tilt. These motoneurons would then respond more efficiently to the same excitatory volleys elicited by given parameters of stimulation, thus leading to an increased gain of the EMG responses.(ABSTRACT TRUNCATED AT 400 WORDS)     

Tonic inhibitory influences of locus coeruleus on the response gain of limb extensors to sinusoidal labyrinth and neck stimulations

Previous experiments had shown that in decerebrate cats activation of limb extensor motoneurons during side-down roll tilt of the animal or side-up neck rotation depends on both an increased discharge of excitatory vestibulospinal (VS) neurons and a reduced discharge of inhibitory reticulospinal (RS) neurons of the medulla, thus leading to disinhibition of limb extensor motoneurons. The present experiments were performed to find out whether the locus coeruleus (LC) complex keeps under its tonic inhibitory control the medullary inhibitory RS neurons and, if so, whether this structure intervenes in the gain regulation of the vestibular and neck reflexes acting on the limb extensor musculature. In precollicular decerebrate cats with good postural rigidity of the four limbs, the amplitude of modulation and thus the response gain of the first harmonic component of multiunit EMG responses of limb extensors to sinusoidal stimulation of labyrinth and neck receptors (at 0.15 Hz, +/- 10 degrees) were quite small in forelimb muscles (triceps brachii) and almost negligible or absent in hindlimb muscles (triceps surae). Electrolytic lesion limited to the LC complex decreased the tonic contraction of limb extensors, but greatly increased in the forelimbs (and brought to the light in the hindlimbs) the response modulation of extensor muscles to the same parameters of labyrinth or neck stimulation. Correspondingly, the response gain increased, but no change in the phase angle of the responses was observed. Both changes in posture, as well as in response gain of the limb extensors to labyrinth and neck stimulation, fully developed some time after the LC lesion. This increase in response gain of the vestibular and neck reflexes acting on the limb extensor muscles did not depend on the decrease in postural activity following the LC lesion, since it was still obtained when an increased static stretch of the extensor muscle following passive flexion of the limb compensated for the reduced EMG activity. Moreover, the slope of the regression line relating the gain of the multiunit EMG response of the triceps brachii to animal tilt with the base frequency greatly increased following lesioning of the LC, thus indicating that for the same background discharge of the muscle the amplitude of modulation, and thus the response gain, increased significantly. The effects described above involved mainly, but not exclusively, the limbs ipsilateral to the side of the lesion.(ABSTRACT TRUNCATED AT 400 WORDS)     

Tonic facilitatory influences of dorsal pontine reticular structures on the response gain of limb extensors to sinusoidal labyrinth and neck stimulations

Experiments were performed to find out whether changes in resting discharge of the inhibitory reticulospinal (RS) neurons of the medulla, produced either by selective destruction or by cholinergic activation of a pontine tegmental reticular system, may modify the response gain of limb extensor muscles to given parameters of roll tilt of the animal or neck rotation. In precollicular decerebrate cats, an electrolytic lesion of the dorsal aspect of the pontine tegmentum, which slightly increased the tonic contraction of limb extensors, greatly decreased the amplitude of the multiunit EMG response of forelimb extensor muscles, i.e. of the medial head of the triceps brachii, to roll tilt of the animal and neck rotation (at 0.15 Hz, +/- 10 degrees), leading to selective stimulation of labyrinth or neck receptors. Correspondingly, the response gain of the forelimb extensors to labyrinth and neck stimulation decreased, but no change in the phase angle of the responses was observed. These findings did not depend on the increased postural activity, since they were still observed in the absence of any change in spontaneous EMG activity of the triceps brachii following the lesion. The changes in posture as well as in response gain of the forelimb extensors to labyrinth and neck stimulation produced by the pontine lesion appeared suddenly, and persisted for several hours throughout the survival period. Moreover, these changes involved mainly, but not exclusively, the limbs ipsilateral to the side of the lesion. Histological controls indicated that the structure responsible for the postural and reflex changes described above corresponded to the dorsal aspect of the pontine tegmentum located immediately ventral to the locus coeruleus (LC); this area corresponded to the peri-LC region as well as the surrounding pontine reticular formation (RF), including the dorsal aspect of the central tegmental field. This region closely corresponds to the area from which a tegmentoreticular tract, ending on the medullary inhibitory area, originates. It was previously shown that unilateral or bilateral lesion of the LC, which decreased the extensor tonus in the ipsilateral limbs, greatly enhanced the response gain of the triceps brachii to sinusoidal stimulation of labyrinth and neck receptors. These findings were attributed to suppression of an inhibitory influence that the LC exerts on the dorsal pontine reticular structures described above.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of microinjection of a cholinergic agonist into the locus coeruleus on the gain of vestibulospinal reflexes in decerebrate cats

1. Experiments were performed in precollicular decerebrate cats to determine whether activation of locus coeruleus (LC) neurons elicited by local injection of the cholinergic agonist carbachol modifies the dynamic characteristics of responses of forelimb extensors to selective stimulation of labyrinth receptors resulting from roll tilt of the animal. 2. Injection of 0.1-0.4 microliter (usually 0.25 microliter) of carbachol at a concentration of 0.02-0.1 micrograms/microliter of sterile saline into the LC of one side, which slightly increased the tonic contraction of limb extensors ipsilateral to the side of the injection, greatly decreased the amplitude of the multiunit EMG response of the ipsilateral triceps brachii to animal tilt at 0.15 Hz, +/- 10 degrees. Correspondingly, the response gain of this forelimb extensor decreased. Moreover, a significant increase in phase lag of the responses was observed. These findings did not result from the increased postural activity, since they were still observed when the limb position was adjusted so that the spontaneous EMG activity remained constant throughout the experiments. 3. The changes in posture as well as in response characteristics of the forelimb extensor to labyrinth stimulation produced by carbachol injection appeared a few min after the injection and soon reached a plateau level which persisted for several hours before returning to the control levels. 4. The effects described above involved mainly, if not exclusively, the limbs ipsilateral to the side of the injection. However, the effects of local injection into the LC of one side could be reproduced on the contralateral side following injection into the LC of that side. 5. The increase in phase lag of the multiunit EMG responses of the triceps brachii to labyrinth stimulation appeared at a threshold lower than that required to decrease the response gain of this extensor muscle. These findings suggest that different neuronal populations within the LC complex, one projecting directly to the spinal cord, the other projecting indirectly through the pontine reticular formation, are involved in the control of phase angle and gain of the vestibulospinal reflexes, respectively. However, as soon as the threshold was reached the effects described above were dose-dependent. 6. Histological controls indicated that the structure responsible for the postural and reflex changes described above corresponded to the LC. In fact, postural and reflex changes opposite in sign to those described above were obtained when the same amount of carbachol was injected into the dorsal aspect of the pontine reticular formation (pRF) located immediately ventral to the LC.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inhibition of vestibulospinal reflexes following cholinergic activation of the dorsal pontine reticular formation

1. The multiunit EMG activity of the forelimb extensor muscle triceps brachii was recorded in precollicular decerebrate cats, either at rest or during roll tilt of the animal at 0.15Hz, +/- 10 degrees leading to sinusoidal stimulation of labyrinth receptors. Both the spontaneous EMG activity as well as the labyrinthine-induced EMG responses were tested before and after pontine microinjection of a cholinergic agonist. 2. Local injection of the cholinergic agonist carbachol into the dorsal aspect of the pontine tegmentum (usually 0.25 microliter, 0.01-0.2 microgram/microliter) produced a state of postural atonia, and abolished both the spontaneous EMG activity as well as the EMG responses of the triceps brachii to sinusoidal stimulation of labyrinth receptors. This suppression was generally ipsilateral to the side of the injection and persisted throughout the episode of postural atonia, but sometimes it involved also the contralateral limbs. In these instances it could be accompanied by a spontaneous nystagmus, interspersed at regular intervals with bursts of rapid eye movements. 3. Similar effects were also obtained following injection of carbachol in the gigantocellular tegmental field (FTG) (0.25 microliter, 0.5-1.0 microgram/microliter). However, this structure was not critically responsible for the phenomena reported above, which persisted unaltered after kainic acid lesion of the FTG performed ipsilaterally to the side of the pontine injection. 4. Local infusion of the muscarinic blocker atropine sulphate reversed the effects of carbachol injection into the dorsal aspect of the pontine tegmentum, thus indicating that muscarinic receptors were involved. 5. It is postulated that the postural atonia as well as the tonic depression of vestibulospinal reflexes, which occur in the decerebrate cat after local injection of a cholinergic agonist depends, at least in part, on the activation of cholinoceptive neurons located in dorsal pontine reticular structures. These may in turn excite medullary reticulospinal neurons, which are finally responsible for the inhibition of extensor motoneurons.     

Injections of a beta-adrenergic antagonist in pontine reticular structures modify the gain of vestibulospinal reflexes in decerebrate cats

1. The norepinephrine (NE)-containing locus coeruleus (LC) neurons control posture as well as the gain of the vestibulospinal reflexes either through direct coeruleospinal (CS) projections or by inhibiting the dorsal pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) system. The question whether these inhibitory influences on the pRF are mediated through beta-adrenoceptors was investigated by injecting in precollicular decerebrate cats small doses of the non-selective beta-adrenergic antagonist propranolol in different pontine tegmental structures. 2. Injection of propranolol (usually 0.25 microliters at the concentration of 4.5 micrograms/microliters of saline) in dorsal pontine structures, which decreased the tonic contraction of limb extensors ipsilateral to the side of the injection, greatly increased the amplitude of the multiunit EMG responses of the ipsilateral triceps brachii to roll tilt of the animal at 0.15 Hz, +/- 10 degrees. Correspondingly, the response gain of the forelimb extensor to labyrinth stimulation increased. Moreover, a slight decrease in phase lead of the responses was observed. These responses were always characterized by an increased EMG activity during ipsilateral tilt and a decreased activity during contralateral tilt, as shown in the control records (alpha-responses). The same injection also produced in some instances an increase of the extensor tonus of the contralateral limbs, associated with an increased EMG activity of the contralateral triceps brachii; on the other hand, the amplitude of modulation and thus the response gain of this muscle to the same parameters of labyrinth stimulation decreased, while the response pattern reversed (beta-responses), thus being opposite to that displayed by the triceps brachii ipsilateral to the side of the injection. 3. The changes in posture and reflexes described above appeared 10-20 min after unilateral injection of propranolol in the pRF, reached in highest values in about 60-100 min and persisted for more that three hours before returning to the control level. These effects were not due to irritative phenomena following injection of the fluid, since neither changes in posture nor in the response gain of the triceps brachii to labyrinth stimulation were observed after injection of an equal volume of saline in the pRF of that side. Moreover, the magnitude of the effects increased to some extent in relation to the dose of the beta-adrenergic blocker. 4. Histological controls indicated that the structure responsible for these postural and reflex changes was located in the dorsal pontine tegmental region immediately ventral to the LC and included the peri-LC alpha and the surrounding dorsal pRF.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cholinoceptive pontine reticular structures modify the postural adjustments during the limb movements induced by cortical stimulation

1. Activation of the pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) system decreases the postural activity. This effect can be achieved either by local injection into the dorsal pontine tegmentum of cholinergic agonists which excite cholinoceptive pRF neurons, or by injection of noradrenergic agents which block the inhibitory influence exerted by the locus coeruleus (LC) neurons on the pRF. The main aim on the present study was to analyze the effects of tonic activation of these pRF neurons on the postural adjustments accompanying limb movements induced by motor cortex stimulation. In particular, electrodes were implanted chronically in the motor cortex of cats and stainless steel guide tubes of small size, later used for drug injection, were set bilaterally into sites just above the responsive regions. 2. Limb flexion elicited by stimulation of the motor cortex was accompanied by a diagonal pattern of postural adjustment, characterized by a decreased force exerted by the limb diagonally opposite to the moving one and an increased force exerted by the other two. 3. Microinjection into the pRF of both sides of 0.25 microliter of the muscarinic agonist bethanechol at the concentration of 8 or 16 micrograms/microliters in buffered artificial cerebrospinal fluid produced a short-lasting episode of postural atonia followed by a period of reduced postural activity, during which the cats were still able to stand on the measurement platform. Under this condition no changes in threshold, latency and amplitude of the flexion response were observed in the performing limb; however, the postural responses were considerably affected. In particular, when the performing limb was a forelimb, the other anterior limb showed a dissociation of the postural response in two distinct components. The first anticipatory component, which had a short latency (12-15 msec) and was considered to be centrally triggered, decreased in amplitude after injection of bethanechol and sometimes disappeared; on the other hand the second component, which had a long latency (50-60 msec) and was thus considered to be of reflex origin, increased in amplitude, due to the instability resulting from the depression of the early postural response. Similar results also affected to a lesser extent the hindlimbs. Moreover, body oscillations were observed and monitored from the force platforms following the late component of the postural responses.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inhibition of vestibulospinal reflexes during the episodes of postural atonia induced by unilateral lesion of the locus coeruleus in the decerebrate cat

1. The spontaneous EMG activity of the forelimb extensor triceps brachii of both sides as well as their responses to roll tilt of the animal at 0.15 Hz, +/- 10 degrees leading to sinusoidal stimulation of labyrinth receptors were tested in precollicular decerebrate cats, before and after unilateral electrolytic lesion of the locus coeruleus (LC). 2. Lesion of the LC of one side decreased the tonic contraction of the ipsilateral limb extensors, but greatly increased the amplitude of modulation and the response gain of the corresponding triceps brachii to animal tilt; however, no change in the phase angle of the responses was observed. A slight increase in the response gain affected also the contralateral triceps brachii. 3. The postural asymmetry described above was followed from time to time by short-lasting episodes of postural atonia, which affected not only the ipsilateral but also the contralateral limb extensors. These episodes were also associated with a suppression of the EMG responses of the triceps brachii of both sides to sinusoidal stimulation of labyrinth receptors. 4. The episodes of postural atonia which appeared after unilateral lesion of the LC were not associated with rapid eye movements; however, the slow horizontal eye movements, which may occur in normal decerebrate animals, increased in amplitude throughout these episodes. Both the postural atonia as well as the related suppression of the vestibulospinal reflexes, which lasted for 5-10 min, disappeared either spontaneously or following acoustic or somatosensory stimulations. 5. Histological controls indicated that unilateral lesions limited to the caudal part of the LC produced only a permanent decrease in postural activity of the ipsilateral limbs, associated with an increase in gain of the vestibulospinal reflex. However, in order to elicit episodes of bilateral postural atonia associated with the suppression of the vestibulospinal reflexes it was necessary to extend the lesion to more rostral aspects of the LC. 6. Since the effects described above were similar to those elicited in decerebrate cats by local injection of cholinergic agonists into the dorsal part of the pontine reticular formation, we postulated that the postural atonia as well as the related suppression of the vestibulospinal reflexes was due to transient release from LC inhibition of these dorsal pontine reticular structures, which might in turn excite the medullary reticulospinal neurons, thus leading to inhibition of the extensor motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of microinjection of cholinergic agonists into the pontine reticular formation on the gain of vestibulospinal reflexes in decerebrate cats

1. The question of which pontine neuronal groups and related receptors can mediate the cholinergic induction of the increased gain of vestibulospinal reflexes elicited by sinusoidal stimulation of labyrinth receptors was investigated by injecting in precollicular decerebrate cats either carbachol, which is a mixed muscarinic-nicotinic agonist, or bethanechol, which is a pure muscarinic agonist, via a cannula stereotaxically oriented in different pontine tegmental structures. 2. Injection of 0.1-0.2 microliter of carbachol solution (0.01-0.2 microgram/microliter of sterile saline) into the dorsal aspect of the pontine reticular formation (pRF), which slightly decreased the tonic contraction of limb extensors ipsilateral to the side of the injection, greatly increased the amplitude of the multiunit EMG response of the ipsilateral triceps brachii to roll tilt of the animal at 0.15 Hz, +/- 10 degrees, leading to selective stimulation of labyrinth receptors. Correspondingly, the response gain of the forelimb extensor to labyrinth stimulation increased. Moreover, a slight decrease in phase lead of the responses was observed. These findings were not attributable to decreased postural activity, since they were still observed when postural EMG activity was reflexly maintained by an increased static stretch of the muscle. No changes in the dynamic characteristics of the responses were observed in the contralateral triceps brachii. 3. The changes in posture as well as in response gain produced by the carbachol injection appeared suddenly, but partially declined to reach a plateau level which persisted for several hours before returning to the control level. Moreover, the magnitude of the effects increased in relation to the dose of the cholinergic agonist. 4. Histological controls indicated that the structure responsible for these postural and reflex changes was located in the dorsal aspect of the pontine tegmentum immediately ventral to the principal locus coeruleus (LC); this area corresponds to the peri-LC region and the surrounding pRF including the dorsal aspect of the central tegmental field. The effects were still obtained after chronic kainic acid lesioning of the gigantocellular area of the medulla. 5. An increase in gain of the vestibulospinal reflex which was as potent, dose-dependent, and site-specific as that previously observed with carbachol, appeared after injection of the pure muscarinic agonist bethanechol.(ABSTRACT TRUNCATED AT 400 WORDS)     

Tonic inhibition of dorsal pontine neurons during the postural atonia produced by an anticholinesterase in the decerebrate cat.

1. Experiments performed in precollicular decerebrate cats indicate that neurons located in the caudal part of the locus coeruleus and locus subcoeruleus as well as in the surrounding reticular formation were greatly depressed during the cataplectic episodes induced by i.v. injection of 0.1 mg/kg of eserine sulphate. 2. These units actually showed a slow regular firing rate when the rigidity was present. Moreover their firing rate greatly decreased during the episodes of postural atonia produced by the anticholinesterase. In some instances a complete abolition of firing occurred during these episodes. The depression of unit discharge anticipated the onset of postural atonia and lasted throughout the episodes. 3. Some of the neurons described above responded with steady changes in their discharge rate to natural stimulation of macular labyrinthine receptors during postural rigidity. However, the response of these neurons to lateral tilts was suppressed during the episodes of postural atonia induced by the anticholinesterase, This and other arguments suggested that these units were tonically inhibited during the induced cataplectic episodes. 4. The time course of the rate deceleration shown by these neurons during transition from postural rigidity to muscular atonia represents a mirror image of the rate acceleration which affects most of the pontine reticular neurons located in the gigantocellular tegmental field (FTG) during the induced cataplectic episodes. These reciprocal rate relations suggest that a functional interaction exists between the two cell groups. In particular it is postulated that the pontine FTG neurons are self-excitatory and excitatory to the locus coeruleus neurons, while the last neurons may be self-inhibitory and inhibitory to FTG neurons. These findings can be related to previous observations showing that neurons located in the region of locus coeruleus undergo a rate deceleration during desynchronized sleep which mimics the time course of firing to the pontine reticular neurons. 5. In conclusion it appears that the decerebrate rigidity is present in so far as the cholinergic reticular neurons, which trigger the bulbospinal inhibitory system, are tonically inhibited by neurons located in the monoaminergic structures of the dorsolateral pontine tegmentum. On the other hand the suppression of the decerebrate rigidity ,which occurs during the cholinergically induced cataplectic episodes results from activation of the cholinergic reticular neurons, which escape tonic inhibition from monoaminergic structures.     

Modulatory effects of the GABAergic basal ganglia neurons on the PPN and the muscle tone inhibitory system in cats

Pedunculopontine tegmental nucleus (PPN) contributes to the control muscle tone by modulating the activities of pontomedullary reticulospinal systems during wakefulness and rapid eye movement (REM) sleep. The PPN receives GABAergic projection from the substantia nigra pars reticulata (SNr), an output nucleus of the basal ganglia. Here we examined how GABAergic SNr-PPN projection controls the activity of the pontomedullary reticulospinal tract that constitutes muscle tone inhibitory system. Intracellular recording was made from 121 motoneurons in the lumbosacral segments in decerebrate cats (n=14). Short train pulses of stimuli (3 pulses with 5 ms intervals, 10-40 mA) applied to the PPN, where cholinergic neurons were densely distributed, evoked eye movements toward to the contralateral direction and bilaterally suppressed extensor muscle activities. The identical PPN stimulation induced IPSPs, which had a peak latency of 40-50 ms with a duration of 40-50 ms, in extensor and flexor motoneurons. The late-latency IPSPs were mediated by chloride ions. Microinjection of atropine sulfate (20 mM, 0.25 ml) into the pontine reticular formation (PRF) reduced the amplitude of the IPSPs. Although conditioning stimuli applied to the SNr (40-60 mA and 100 Hz) alone did not induce any postsynaptic effects on motoneurons, it reduced the amplitude of the PPN-induced IPSPs. Subsequent injection of bicuculline (5 mM, 0.25 ml) into the PPN blocked the SNr effects. Microinjections of NMDA (5 mM, 0.25 ml) and muscimol (5 mM, 0.25 ml) into the SNr reduced and increased the amplitude of the PPN-induced IPSPs, respectively. These results suggest that GABAergic basal ganglia output controls postural muscle tone by modulating the activity of cholinergic PPN neurons which activate the muscle tone inhibitory system. The SNr-PPN projection may contribute to not only control of muscle tone during movements in wakefulness but also modulation of muscular atonia of REM sleep. Dysfunction of the SNr-PPN projection may therefore be involved in sleep disturbances in basal ganglia disorders.     

Injections of beta-adrenergic substances in the locus coeruleus affect the gain of vestibulospinal reflexes in decerebrate cats

1. The tonic discharge of the noradrenergic locus coeruleus (LC) neurons is dampened by norepinephrine (NE) which acts not only on alpha2-adrenoceptors located on the somatodendritic membrane, through mechanisms of recurrent inhibition, but also on beta-receptors. Experiments were performed to find out whether inactivation of LC neurons by local injection of the beta-adrenergic agonist isoproterenol into the LC complex of one side produced changes in posture as well as in the gain of vestibulospinal reflexes acting on forelimb extensors. 2. In precollicular decerebrate cats the amplitude of modulation and thus the gain of the multiunit EMG responses of the forelimb extensor triceps brachii to animal tilt at 0.15 Hz, +/- 10 degrees, leading to sinusoidal stimulation of labyrinth receptors, were quite small. Microinjection of 0.25 microliter of a solution of the beta-adrenergic agonist isoproterenol at the concentration of 4.5-9.0 microgram/microliter of sterile saline into the LC complex of one side decreased the extensor rigidity in the ipsilateral limbs and to a lesser response gain of the ipsilateral triceps brachii to the same parameters of labyrinth stimulation greatly increased (t-test, P less than 0.001); moreover, a slight but significant increase in phase lead of the responses was observed. These findings appeared within 5-10 min after the injection of isoproterenol, fully developed within 20-30 min and persisted for about 2-3 hours after the injection. 3. The increased gain of the vestibulospinal reflexes acting on the triceps brachii did not depend on the decreased postural activity following injection of the beta-adrenergic agonist, since it was still observed if the reduced EMG activity of the extensor muscle following the injection was compensated for by an increased static stretch of the muscle. The positive correlation (t-test, P less than 0.001) between gain of the multiunit EMG response of the triceps brachii to animal tilt and base frequency observed in the control experiment disappeared and was substituted by a slight negative correlation (t-test, P less than 0.05) after injection of isoproterenol into the LC complex, probably due to a more prominent recruitment of motor units for low level of background discharge of the muscle. 4. In addition to the effects which involved the triceps brachii ipsilateral to the side of the injection, a smaller but significant increase in response gain affected the contralateral extensor muscle. This increase in gain was also associated with a slight increase in phase lead of the responses.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cholinergic mechanism controlling the response gain of forelimb extensor muscles to sinusoidal stimulation of macular labyrinth and neck receptors

The multiunit EMG activity of the triceps brachii was recorded in precollicular decerebrate cats during roll tilt of the animal or neck rotation at the frequencies of 0.026-0.15 Hz and at the peak amplitude of 10 degrees, leading to selective stimulation of labyrinth or neck receptors. The first harmonic component of the EMG responses to labyrinth stimulation was characterized by an increased activity during side-down tilt of the animal and a decreased activity during side-up tilt; however, just the opposite changes were elicited for the same directions of neck rotation. The peak of the responses was closely related to the extreme animal or neck displacement, thus being attributed to stimulation of position-sensitive macular labyrinth and receptors. Moreover, the modulation as well as the gain of the EMG responses were small in amplitude. Intravenous injections of an anticholinesterase at a dose which in some instances slightly decreased the extensor tonus as well as the background activity of the triceps brachii (eserine sulphate, 0.05-0.075 mg/kg), greatly enhanced the response gain of this extensor muscle to animal tilt or neck rotation at the parameters reported above. This finding was also observed in the absence of any decrease in spontaneous EMG activity of the extensor muscle after injection of the anticholinesterase. In no instance did the phase angle of the response change following these injections. The increased gain of the EMG response of the forelimb extensor muscle to sinusoidal stimulation of labyrinth and neck receptors was first observed 5-10 min after the injection and reached the highest value in about one hour. This effect, was not only time-dependent, but also state-dependent. In fact, the increase in response gain described above either did not occur or was negligible during the sudden recovery of the extensor rigidity which occurred either spontaneously or after somatosensory stimulations. The effects elicited by eserine sulphate were reversed within seconds by a 0.1-0.5 mg/kg dose of atropine sulphate, an anticholinergic drug. It is postulated that for the same labyrinthine or neck signal giving rise to excitatory vestibulospinal volleys acting on extensor motoneurons, the amplitude of the EMG modulation of limb extensor muscles depends on the activity of a cholinergic system.(ABSTRACT TRUNCATED AT 400 WORDS)     

Pineal gland hormone and idiopathic scoliosis: possible effect of melatonin on sleep-related postural mechanisms

Experimental and clinical evidences indicate that endocrine mechanisms, particularly involving the pineal gland, exert a role in the development of postural deficits leading to the occurrence of idiopatic scoliosis (IS). In particular, experiments performed in bipedal animals have shown that removal of the pineal gland, which secretes melatonin (M), induced a scoliosis, and that in such preparations, administration of this hormone prevented the development of this deformity (cf. 131). It appears also that adolescents with IS showed a reduced level of serum M with respect to age-related control subjects. The possible mechanisms involved in the M regulation of the tonic contraction of the axial musculature have been discussed. It is known that the pineal gland is implicated in the control of circadian rhythms, including the sleep-waking cycle, and that during this cycle there are prominent changes in postural activity, which affect not only the limbs, but also the axial musculature. These changes are characterized by a decrease followed by a suppression of postural activity, which occur particularly during transition from wakefulness to synchronized sleep and, more prominently, to rapid eye movement (REM) sleep. Episodes of postural atonia may also occur during the cataplectic episodes, which are typical of narcolepsy. Cholinergic and/or cholinoceptive neurons located in the dorsal pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) system, intervene in the suppression of posture during REM sleep, as well as during the cataplectic episodes which occur in narcolepsy. These structures are under the modulatory (inhibitory) influence of the dorsomedial and the dorsolateral pontine tegmentum, where serotoninergic raphe nuclei (RN) neurons and noradrenergic locus coeruleus (LC) neurons are located. We postulated that M may act not only on the circadian pacemaker, but also directly on the pontine tegmental structures involved in the regulation of posture during the animal states indicated above. This hypothesis is supported by the facts that: 1) the dorsal pRF may contain specific binding sites for M; 2) this structure is particularly sensitive to M in adolescents, as well as in adult subjects affected by narcoleptic disturbances leading to cataplexy; 3) M increases the release of serotonin (5-HT), a neurotransmitter which enhances the postural tone by acting on the dorsal pRF: on the other hand, deficits in M levels may lower the activity of the serotoninergic raphe system, thus leading to a decrease or suppression of postural activity similar to that occurring either during REM sleep or during the cataplectic episodes typical of narcoleptic patients; 4) IS patients may show episodes of sleep apnea, a phenomenon which has been attibuted to a reduced tonic contraction of primary and accessory respiratory muscles during REM, resulting from a reduced release of 5-HT at dorsal pontine level. It has been postulated that, if the reduced M and 5-HT levels are subliminal to produce a complete suppression of posture under the conditions reported above, the reduced postural tone, which results from this condition may lead to the development of IS, due to hypotonia which affects the axial musculature. M secretion could be regulated not only by the activity of the serotoninergic raphe neurons projecting to the pineal gland, but probably also by the activity of noradrenergic LC neurons. It is likely that the development of IS, which results from a reduced level of M and 5-HT, may occur provided that the noradrenergic LC inhibition of the pontine structures is impaired. Such impairment could depend upon genetic factors, similar to those postulated to play a role in narcolepsy. In conclusion, the possibility exists that an impaired activity of brain monoaminergic systems may lead to disfunction in the production of M, which is apparently an important factor in the etiopathogenesis of IS.     

Mapping of cholinoceptive brainstem structures responsible for the generation of paradoxical sleep in the cat

In order to determine the cholinoceptive brainstem structures critical for PS generation, we investigated the effect on PS induction of the injection of a small dose and volume (0.4 microgram/0.2 microliter) of the cholinergic agonist carbachol in the following caudal brainstem structures: 1) the caudal mesencephalic reticular formation, especially the nucleus pedunculopontinus pars compacta or X area; 2) the mediodorsal pontine tegmentum, in particular the nuclei locus coeruleus (LC), locus coeruleus alpha (LC alpha), peri-locus coeruleus alpha (peri-LC alpha) and laterodorsalis tegmenti (Ldt); 3) the pontine; and 4) bulbar gigantocellular (FTG) and magnocellular tegmental fields (FTM). We found that the only brainstem area from which a high amount of PS was induced by carbachol applications with short latencies, less than 5 minutes, is the mediodorsal pontine tegumentum, namely the LC alpha and peri-LC alpha, where ChAT-and TH- immunoreactive neurons are intermingled. Injections in an area immediately ventral to the peri-LC alpha induced physiological states resembling PS but lacking certain electrophysiological (PS-like) and behavioral components of PS (dissociated states I and II). The weak PS induction following carbachol administration in the anteromedial part of the FTG was due to the spread of the drug toward the efficient site since the latencies to PS onset were in the range of 20 to 60 minutes. No effects on PS generation were obtained after carbachol microinjections in the LC and the laterocaudal part of the FTG, while carbachol injections in the X area or in the bulbar FTG or FTM resulted in the increase of waking and the decrease of PS. In addition to these effects on PS induction, we also found that carbachol induced: 1) stereotyped PGO-like bursts when injected in the ventral part of the FTG and the rostral part of the FTM, 2) postural atonia with very short latencies, less than two minutes, when injected in the LC alpha and peri-LC alpha; and 3) hippocampal theta waves of 3-5 Hz persisting during light slow wave sleep (S1) when injected in and around the LC alpha and peri-LC alpha and in some points of the mediocaudal part of the FTG. These results support the hypothesis that PS is generated by highly localized neuronal populations and suggest that the mediodorsal pontine tegmentum (namely the nuclei LC alpha and peri-LC alpha) may represent a cholinoceptive PS generator.     

Effects of repetitive stimulation of the locus coeruleus on spinal inhibitory responses to suprasegmental stimulation in cats

Effects of repetitive stimulation of the locus coeruleus on spinal responses to activation of cortico-, reticulo-, and vestibulospinal tracts were studied in decerebellate cats anesthetized with chloralose. Descending influences of these structures were assessed from changes in amplitude of extensor and flexor monosynaptic discharges or from the magnitude of postsynaptic potentials recorded from the corresponding motoneurons. Stimulation of the motor cortex or modullary reticular formation as a rule evoked two-component inhibitory responses in extensor motoneurons and excitatory-inhibitory responses in flexor motoneurons. Stimulation of locus coeruleus effectively depressed the amplitude of the late component and, to a lesser degree, that of the early component of inhibition arising after stimulation of the cerebral cortex or reticular formation. During stimulation of the locus coeruleus no marked changes were found in inhibitory responses evoked by vestibulospinal influences in flexor motoneurons, and also in excitatory responses arising after stimulation of the above-mentioned descending pathways in both groups of motoneurons.     

Selective discharge of pontine neurons during the postural atonia produced by an anticholinesterase in the decerebrate cat.

I. Episodes of postural atonia associated with bursts of REM similar to those which occur spontaneously either in the intact preparation during desynchronized sleep, or in the chronic decorticate or decerebrate preparations, can be elicited in acute decerebrate cats following intravenous injection of small doses of an anticholinesterase. The present experiments were performed in precollicular decerebrate animals in order to identify the pontine neurons which show increases in their firing rate related in time with the appearance of the cataplectic episodes. In particular long-term recordings of single units were obtained before, during and after the episodes of postural atonia produced by i.v. injection of 0.03-0.1 mg/kg of eserine sulphate. Spontaneous discharge rates were used to measure the selectivity of each individual unit, i.e., the tendency of the unit to discharge more during the cataplectic episode than during the postural rigidity. The physiological data obtained from neurons histologically localized in different nuclear groups were then averaged. 2. Neurons localized in the pontine reticular formation as well as in the region of the locus coeruleus and the raphe system showed low rates of discharge when rigidity was present. The same units, however, showed a remarkable increase in firing rate which preceded by several tenths of seconds the onset of postural atonia and lasted throughout the cataplectic episodes. 3. The neurons of the pontine reticular formation had a selectivity which was higher than that of the neurons located in the locus coeruleus-raphe system; moreover the cells of the gigantocellular tegmental field (FTG) had the highest selectivity of all pontine reticular structures studied. 4. The relation of the discharge rate curves to the occurrence of the cataplectic episodes suggests that these neurons constitute output elements of a generator system for postural atonia. It is postulated that these pontine reticular neurons are directly involved in the activation of the bulbospinal inhibitory system, which is finally responsible for the abolition of the decerebrate rigidity. 5. During cataplectic episodes these pontine neurons showed some clustered discharges which appeared in association with bursts of eye movements. In most instances, however, there was no constant relationship of the unit activity to individual eye movements. Moreover large phasic increases in firing rate appeared also during the intervals between successive bursts of REM. 6. The striking increase in firing rate of the FTG neurons observed during the cataplectic episodes cannot be attributed to an increased excitatory input to these neurons. In fact excitatory influences following intense somatic stimulation are unlikely to occur during the cataplectic episodes; moreover the response of these neurons to intense somatosensory stimulations did not reach rates comparable with those occurring spontaneously during the induced cataplectic episodes...     

Coerulospinal enhancement of repetitive firing with correlative changes in postspike afterhyperpolarization of cat spinal motoneurons

The present study was aimed at determining if inputs from the locus coeruleus (LC) have any effect on repetitive firing of ventral horn motoneurons in cats. In hindlimb flexor and extensor motoneurons stimulated intrasomatically with current below the threshold for repetitive discharges, added LC-evoked excitatory post-synaptic potentials (EPSPs) were consistently found to produce repetitive firing, suggesting a lowering in the repetitive firing threshold attributable to excitatory LC inputs. With those extensor motoneurons showing episodic, self-sustained firing, LC-EPSPs introduced during the quiescent period were capable of starting a continuous discharge with rhythmic frequencies higher than the spontaneous activity. In some of these cells, intracellularly applied hyperpolarizing current was able to stop the spontaneous discharges. Subsequently, LC stimuli were found to reinitiate repetitive discharges, thus counteracting the ongoing suppression of the motoneurons. Quantitative analysis of the single-spike afterhyperpolarization (AHP) indicated a consistent reduction in its amplitude and time course (duration, time-to-peak, half-decay time) for flexor and extensor motoneurons in response to LC conditioning stimuli. Present results suggest an excitatory LC action on the repetitive discharges of cat motoneurons accompanied by a concurrent decrease in the amplitude and time course of the single-spike AHPs.     

Musings on the wanderer: what's new in our understanding of vago-vagal reflex? IV. Current concepts of vagal efferent projections to the gut

Vagal efferents, consisting of distinct lower motor and preganglionic parasympathetic fibers, constitute the motor limb of vagally mediated reflexes. Arising from the nucleus ambiguus, vagal lower motor neurons (LMN) mediate reflexes involving striated muscles of the orad gut. LMNs provide cholinergic innervation to motor end plates that are inhibited by myenteric nitrergic neurons. Preganglionic neurons from the dorsal motor nucleus implement parasympathetic motor and secretory functions. Cholinergic preganglionic neurons form parallel inhibitory and excitatory vagal pathways to smooth muscle viscera and stimulate postganglionic neurons via nicotinic and muscarinic receptors. In turn, the postganglionic inhibitory neurons release ATP, VIP, and NO, whereas the excitatory neurons release ACh and substance P. Vagal motor effects are dependent on the viscera's intrinsic motor activity and the interaction between the inhibitory and excitatory vagal influences. These interactions help to explain the physiology of esophageal peristalsis, gastric motility, lower esophageal sphincter, and pyloric sphincter. Vagal secretory pathways are predominantly excitatory and involve ACh and VIP as the postganglionic excitatory neurotransmitters. Vagal effects on secretory functions are exerted either directly or via release of local mediators or circulating hormones.     

A mathematical model for the mechanism of rapid eye movements induced by an anticholinesterase in the decerebrate cat.

The oculomotor pattern which appears in intact preparations during desynchronized sleep is characterized by the irregular occurrence of isolated ocular movements and bursts of rapid eye movements (REM). This complex oculomotor pattern results from the activity of two premotor structures which influence the extraocular motoneurons during this phase of sleep: one is located in the pontine reticular formation, the other in the vestibular nuclei. In the decerebrate preparation the intravenous injection of an anticholinesterase leads to the appearance of a typical pattern of oculomotor activity, which differs from that occurring during physiological sleep in so far as it consists quite exclusively of bursts of REM which appear at very regular intervals. Lesion experiments as well as unit recordings have shown that these bursts of REM depend in particular upon rhythmic discharges of the vestibular nuclear neurons. The underlying anatomical structures responsible for these bursts of REM are therefore the vestibular nuclei, the oculomotor nuclei and the oculo-orbital system. This mechanism is under the influence of cholinergic reticular neurons which generate the oculomotor rhythm. We have postulated the existence of a self-excitatory cholinergic system, located in the pontine reticular formation, whose steady discharge impinges upon an oscillatory neuronal system located in the dorso-lateral pontine tegmentum, which transforms the tonic input into a sinusoidal final output. We have assumed also that the periodic increases in the discharge frequency of this oscillatory system trigger a fast phase generator acting on the different components of the REM system, and that the behavior of each component follows a first-order differential equation. The state of excitation of the components of the system is defined as proportional to frequency of nerve impulses. Assuming ipsilateral and crossed connections, a pattern of oculomotor activity is obtained that simulates the experimental oculomotor output fairly well. The repetition of the eye jerks is described by a Fourier series. The model proposed in this paper may be taken as a first approach in describing the generation mechanism of REM, and as a theoretical guide to new experimental researches and the development of other more realistic models.