Cortical Presynaptic Control of Dorsal Horn C–Afferents in the Rat

Lamina 5 sensorimotor cortex pyramidal neurons project to the spinal cord, participating in the modulation of several modalities of information transmission. A well-studied mechanism by which the corticospinal projection modulates sensory information is primary afferent depolarization, which has been characterized in fast muscular and cutaneous, but not in slow-conducting nociceptive skin afferents. Here we investigated whether the inhibition of nociceptive sensory information, produced by activation of the sensorimotor cortex, involves a direct presynaptic modulation of C primary afferents. In anaesthetized male Wistar rats, we analyzed the effects of sensorimotor cortex activation on post tetanic potentiation (PTP) and the paired pulse ratio (PPR) of dorsal horn field potentials evoked by C–fiber stimulation in the sural (SU) and sciatic (SC) nerves. We also explored the time course of the excitability changes in nociceptive afferents produced by cortical stimulation. We observed that the development of PTP was completely blocked when C-fiber tetanic stimulation was paired with cortex stimulation. In addition, sensorimotor cortex activation by topical administration of bicuculline (BIC) produced a reduction in the amplitude of C–fiber responses, as well as an increase in the PPR. Furthermore, increases in the intraspinal excitability of slow-conducting fiber terminals, produced by sensorimotor cortex stimulation, were indicative of primary afferent depolarization. Topical administration of BIC in the spinal cord blocked the inhibition of C–fiber neuronal responses produced by cortical stimulation. Dorsal horn neurons responding to sensorimotor cortex stimulation also exhibited a peripheral receptive field and responded to stimulation of fast cutaneous myelinated fibers. Our results suggest that corticospinal inhibition of nociceptive responses is due in part to a modulation of the excitability of primary C–fibers by means of GABAergic inhibitory interneurons.     

Effects of Stimulation of the Periaqueductal Gray and <Emphasis Type="Italic">Locus Coeruleus</Emphasis> on Postsynaptic Reactions of Cat Somatosensory Cortex Neurons Activated by Nociceptors

In cats, we studied the influences of stimulation of the periaqueductal gray (PAG) and locus coeruleus (LC) on postsynaptic processes evoked in neurons of the somatosensory cortex by stimulation of nociceptive (intensive stimulation of the tooth pulp) and non-nociceptive (moderate stimulations of the infraorbital nerve and ventroposteromedial nucleus of the thalamus) afferent inputs. Twelve cells activated exclusively by nociceptors and 16 cells activated by both nociceptive and non-nociceptive influences (hereafter, nociceptive and convergent neurons, respectively) were recorded intracellularly. In neurons of both groups, responses to nociceptive stimulation (of sufficient intensity) looked like an EPSP-spike-IPSP (the latter, of significant duration, up to 200 msec) complex. Electrical stimulation of the PAG (which could itself evoke activation of the cortical neurons under study) resulted in long-term suppression of synaptic responses evoked by excitation of nociceptors (inhibition reached its maximum at a test interval of 600 to 800 msec). We observed a certain parallelism between conditioning influences of PAG activation and effects of systemic injections of morphine. Isolated stimulation of LC by a short high-frequency train of stimuli evoked primary excitatory responses (complex EPSPs) in a part of the examined cortical neurons, while in other cells high-amplitude and long-lasting IPSP (up to 120 msec) were observed. Independently of the type of the primary response to PAG stimulation, the latter resulted in long-term (several seconds) suppression of the responses evoked in cortical cells by stimulation of the nociceptive inputs. The mechanisms of modulatory influences coming from opioidergic and noradrenergic brain systems to somatosensory cortex neurons activated due to excitation of high-threshold (nociceptive) afferent inputs are discussed.Neirofiziologiya/Neurophysiology, Vol. 37, No. 1, pp. 61–73, January–February, 2005.     

Effect of vasopressin on neuronal responses of theNucl. Caudalis of the spinal trigeminal tract in rats

Effect of vasopressin on responses of individual neurons of thenucl. caudalis of the spinal trigeminal tract was studied on rats under urethan anesthesia; the responses were evoked by nociceptive (stimulation of the tooth pulp) or non-nociceptive (stimulation of Aa fibers of the infraorbital nerve) afferent activation. After injection of 10 nM vasopressin into the recording zone, responses evoked by stimulation of the tooth pulp were suppressed in all studied neurons of the high-threshold group; the same was true as to responses induced by stimulation of the tooth pulp and infraorbital nerve in most neurons of the convergent group. At the same time, vasopressin did not change the responses evoked by stimulation of Aa fibers of the infraorbital nerve in neurons of the low-threshold group. Possible involvement of vasopressin in the process of pain suppression is discussed.     

Functional organization of mechanisms of presynaptic inhibition induced by stimulation of cutaneous afferents

Changes in the N₁-component and P-phase of the dorsal surface potential (DSP) of the spinal cord evoked by test stimulation of the posterior tibial nerve after conditioning stimulation of the sural nerve were investigated in anesthetized cats. The test responses were inhibited if stimulation was applied at short intervals. They then recovered to some extent, but after 1.8–2.2 msec, a further prolonged period of inhibition began. The initial inhibition was connected with occlusion of synaptic action, and the subsequent prolonged inhibition with the development of presynaptic inhibition. The latent periods of prolonged inhibition of the N₁-component and P-phase of the DSP (2 msec) were almost exactly identical, and the curves showing the diminution of the initial occlusion of these components were very similar. The results demonstrate that presynaptic inhibition of the interneurons generating the N₁-component of the DSP and of cells of the substantia gelatinosa which participate in depolarization of the presynaptic terminals of the cutaneous afferents is due to the action of depolarizing systems with similar temporal characteristics.Dnepropetrovsk State University. Translated from Neirofiziologiya, Vol. 4, No. 5, pp. 510–515, September–October, 1972.     

Depolarization of primary afferents during fictitious scratching in thalamic cats

The dorsal cord and dorsal root potentials were recorded in immobilized thalamic cats during fictitious scratching evoked by mechanical stimulation of the ear. Depolarization of primary afferents was shown to be simulated by the central scratching generator. Antidromic spike discharges appeared at the peak of the primary afferent depolarization waves in certain afferent fibers. Similar discharges arise in the resting state in response to stimulation of limb mechanoreceptors. It is suggested that during real scratching primary afferent depolarization and antidromic spikes evoked by it may effectively modulate the level of the afferent flow to spinal neurons.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 173–176, March–April, 1978.     

Role of propriospinal neurons in inhibition of the afferent flow

The effects of stimulation of the dorsal funiculus on dorsal surface potentials (DSPs) of the spinal cord evoked by stimulation of a peripheral nerve and on antidromic action potentials (AAPs) evoked by stimulation of terminal branches of primary afferent fibers and recorded from the afferent nerve or dorsal root, were investigated in acute experiments on spinal cats and on cats anesthetized with pentobarbital and chloralose. Stimulation of the dorsal funiculus led to biphasic inhibition of the N₁-component of the DSP with maxima at the 15th–30th and 60th–80th milliseconds between the conditioning and testing stimuli. Maximal reinforcement of the AAP was found with these intervals. Bilateral division of the dorsal funiculi between the point of application of the conditioning stimuli and the point of recording the DSP abolished the first wave of inhibition of the DSP and the reinforcement of the AAP. After total transection of the cord above the site of conditioning stimulation the picture was unchanged. It is concluded that the initial changes in DSP and AAP are due to activation of the presynaptic inhibition mechanism by antidromic impulses traveling along nerve fibers running in the dorsal funiculus. Repeated inhibition of the DSP, like reinforcement of the AAP, can possibly be attributed to activation of similar inhibitory mechanisms through the propriospinal neurons of the spinal cord.Dnepropetrovsk State University. Translated from Neirofiziologiya, Vol. 5, No. 4, pp. 401–405, July–August, 1973.     

Participation of nonsegmentary neurons of the spinal cord in execution of presynaptic control

Experiments were carried out on cats six days after complete transection of the spinal cord. Cord dorsum potentials (CDP) were recorded in the vicinity of the third lumbar segment during stimulation of the isolated dorsolateral funiculus (DLF). The CDP consist of a rapid monophasic potential (which apparently reflects antidromic excitation of the cells of Clarke's column) and two subsequent slow negative waves, which are replaced by a long positive oscillation. In form, time characteristics, and behavior during thythmic stimulation, this potential differs considerably from the CDP recorded during stimulation of the afferent nerves. The presence of a positive phase of the CDP indicates that stimulation of the DLF evokes primary afferent depolarization (PAD). Stimulation of the DLF causes inhibition of the CDP evoked by stimulation of the afferent nerve. The time course of this inhibition corresponds to the time course of presynaptic inhibition. It is demonstrated that stimulation of the afferent nerve (n. femoralis) inhibits slow components of the CDP evoked by stimulation of the DLF. This inhibition reaches a maximum at the 16th millisecond; its duration exceeds 300 msec. Stronger and more prolonged inhibition of the same components is observed when both the conditioning and the testing stimuli are administered to the DLF. Since primary afferents do not take part in CDP emergence during stimulation of the DLF, it may be hypothesized that the observed inhibition develops as a result of depolarization of interneuron axon terminals.Dnepropetrovsk State University. Translated from Neirofiziologiya, Vol. 2, No. 5, pp. 520–527, September–October, 1970.     

Disturbance and restoration of functions after division of spinal afferent pathways in the cat

The possibility and degree of recovery of motor and sensory functions in cats were studied after one-stage or two-stage bilateral division of the posterior columns and spinocervical tracts at the cervical level. Blocking the afferent inflow along these systems led to severe and prolonged disturbances of sensation and motor activity and was accompanied by a sharp decrease in nociceptive sensation. Weak (6–8 V) electrical stimulation of the skin of the limbs, which evoked a primary response of maximal amplitude in intact waking animals, evoked no electrical response in the somatosensory cortex of the chordotomized animals. However, on increasing the intensity of stimulation by 2, 3, or more times, low-amplitude negative waves with a spike latency of about 15 msec, together with slow late waves, were recorded in foci of maximal activity of the cortex. Recovery of motor activity and, to some extent, of proprioception was observed 2–4 months after injury; responses to tactile stimulation were not restored. In the course of compensatory reconstruction evoked activity in the somatosensory cortex did not recover. It is concluded that the recovery of motor activity in cats after injury to the afferent systems of the spinal cord can take place despite a considerable defect of somatic sensation.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 5, No. 3, pp. 281–288, May–June, 1973.     

Synaptic activation of thoracic spinal interneurons by reticulospinal pathways

Synaptic processes of 119 thoracic spinal interneurons (T10–11) were investigated in anesthetized cats in response to stimulation of the medial and central zones of the gigantocellular nucleus in the medulla and the ventral columns of the spinal cord. Fast (90–130 m/sec) reticulospinal fibers running in the ventral column were found to produce monosynaptic or disynaptic excitation of interneurons of Rexed's layers VII–VIII, which are connected monosynaptically with group I muscle afferents, and interneurons excited both by group I muscle afferents and low-threshold cutaneous afferents. In most neurons of layer IV, connected monosynaptically with low-threshold cutaneous afferents, and in neurons of layers VII–VIII excited by afferents of the flexor reflex no marked postsynaptic processes were observed during stimulation of the reticular formation. Excitatory, inhibitory, and mixed PS Ps during activation of reticulospinal fibers were found in 14 neurons, high-threshold afferents in which evoked predominantly polysynaptic IPSPs. Seventeen neurons activated monosynaptically by reticulospinal fibers and not responding to stimulation of segmental afferents were found in the medial part of the ventral horn (layers VII–VIII).A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 566–578, November–December, 1972.     

Inhibition of jaw opening reflexes by stimulation of the central gray matter and raphe nuclei in cats

The effect of stimulation of the mesencephalic central gray matter and raphe nuclei on jaw opening reflexes evoked by excitation of high-threshold (dental pulp) and low-threshold (A-alpha) fibers of the infraorbital nerve afferents was studied in cats anesthetized with chloralose and pentobarbital. The jaw opening reflex evoked by stimulation of the dental pulp was shown to be effectively suppressed by conditioning stimulation of the central gray matter and raphe nuclei. The reflex evoked by stimulation of low-threshold infraorbital nerve afferents also was depressed (but less deeply and for a shorter period than the reflex evoked by stimulation of the dental pulp) during stimulation of the raphe nuclei and caudal zone of the central gray matter, but was unchanged after stimulation of the points located in the rostral zone of the central gray matter. Application of single stimuli or bursts of five stimuli with a frequency of 100 Hz had no effect on the reflexes studied. Short-term stimulation with a burst of 10–20 stimuli with a following frequency of 200–400 Hz led to inhibition of the reflexes, which lasted 450–1000 msec. Long-term stimulation of the central gray matter and raphe nuclei for 30 sec with a frequency of 50 Hz caused inhibition of jaw opening reflexes evoked by stimulation of both high- and low-threshold afferents for 60 min. Impulses from the central gray matter and raphe nuclei thus have a mainly inhibitory action on the jaw opening reflex evoked by stimulation of high-threshold afferents, but they act less effectively on the reflex evoked by stimulation of low-thres-hold afferents. The duration of inhibition depends essentially on the parameters of stimulation.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 374–387, May–June, 1984.     

Role of different types of fibers of the infraorbital nerve in synaptic activation of masseter and digastric motoneurons

Postsynaptic potentials of motoneurons of the masseter and digastric muscles evoked by stimulation of the infraorbital nerve with a strength of between 1 and 10 thresholds were investigated in cats anesthetized with a mixture of chloralose and pentobarbital. Depending on their ability to be activated by low-threshold afferents of this nerve, motoneurons of the masseter were divided into two groups. Stimuli with a strength of 1.2–2.5 times above threshold for the most excitable fibers of the infraorbital nerve evoked short-latency EPSPs in the motoneurons of the first group; a further increase in stimulus strength (3–9 thresholds) led to the appearance of IPSPs with latent periods of 2.8–3.5 msec. Motoneurons of the second group responded to stimulation of the infraorbital nerve with a strength of 3–9 thresholds by IPSPs whose latent periods varied from 6 to 8 msec. Stimuli below 3 thresholds in strength evoked no responses in these motoneurons. Stimulation of the infraorbital nerve with pulses of between 1 and 2 thresholds in strength evoked EPSPs in digastric motoneurons, but an increase in the strength of stimulation led to action potential generation. The presence of many excitatory and inhibitory inputs formed by afferent fibers of different types evidently provides a basis for functional diversity of jaw-opening and jaw-closing reflexes.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 6, pp. 596–603, November–December, 1980.     

Effectiveness of ascending synaptic influences from various spinal funiculi on reticulospinal neurons in cats

Activity of reticulospinal neurons evoked by stimulation of the ventral, ventrolateral, dorsolateral, and dorsal funiculi of the spinal cord was recorded extracellularly in cats anesthetized with chloralose. Responses of 57 reticulospinal neurons, of which 22 projected into the ventral funiculus, 20 into the ventrolateral, and 15 into the dorsolateral, were studied. The functional properties (conduction velocity and refractory period) and the location of the neurons of the above-mentioned groups in the medulla did not differ appreciably. The most effective synaptic activation of all neurons was observed during stimulation of the dorsal and dorsolateral funiculi. Responses to stimulation of the dorsal funiculus had the lowest threshold. These responses arose in reticulospinal neurons of the ventral and ventrolateral funiculi after the shortest latent period. The effectiveness of synaptic influences from the dorsal and dorsolateral funiculi was identical in the group of neurons of the dorsolateral funiculus. Correlation between activity evoked by stimulation of the dorsal funiculus in reticulospinal neurons and peripheral nerves indicated that the responses appeared in these cells to stimulation of muscular (groups I and II) and cutaneous (group II) afferent fibers. The results indicate that impulses from low-threshold muscular and cutaneous afferents, which effectively activate reticulospinal neurons, are transmitted along ascending pathways of the dorsal funiculi.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 3, pp. 254–263, May–June, 1979.     

Convergence of impulses on anterior hypothalamic neurons during local temperature stimulation of the skin

Unit responses in the medial preoptic region of the anterior hypothalamus and septum to local temperature stimulation of various parts of the skin were studied in chronic experiments on rabbits. The temperature of an area of skin on the back (zone I) and head (zone II) was altered by means of thermodes: heated to 38–40°C and cooled to 22–26°C. Of 111 neurons tested 21 responded to a change of skin temperature (mainly to cooling). Temperature-sensitive hypothalamic neurons were shown to react to temperature stimulation of both skin zones stimulated. The types of the responses recorded are described.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 4, pp. 365–370, July–August, 1981.     

Selective involvement of opioids in mechanisms of synapse-specific plasticity in Helix lucorum snail during sensitization acquisition

Effects of met-enkephalin (opioid peptide) and naloxone (opioid antagonist) on nociceptive sensitization were studied in L-RP11 Helix neurons. In control snails sensitizing stimulation produced reversible membrane depolarization and depression of neural responses evoked by sensory stimuli during the short-term stage of sensitization and facilitation of these responses at the long-term stage. Met-enkephalin (10 but not 0.1 microM) suppressed the neural responses evoked by nociceptive stimuli. Sensitizing stimulation during metenkephalin application prevented the facilitation of neural responses evoked by tactile stimulation of snail head, whereas facilitation of neural responses evoked by chemical stimulation of head or tactile stimulation of foot were similar to that in control sensitized snails. Sensitizing stimulation during met-enkephalin and/or naloxone application prevented the facilitation of neural responses evoked by chemical stimulation of snail head, whereas responses evoked by tactile stimulation of snail head or foot were facilitated (as in neurons of control sensitized snails). Opioids are suggested to be involved in regulation of nociceptive mechanisms and selective induction of long-term plasticity in L-RP11 neural inputs activated by tactile of chemical stimulation of snail head.     

Neuronal activity of raphe nuclei of the brain stem in the cat

Evoked potentials were recorded in the system of raphe nuclei in experiments on unanesthetized, immobilized cats. Somatic stimulation proved to be the most effective of the different stimulations used (light flash, sound click, electrical stimulation of the skin of the limbs). Sound and light stimulation did not evoke pronounced responses, or the latter (to sound) were of a very low amplitude and irregular. In the second series of experiments on cats narcotized with nembutal (30–35 mg/kg) the spontaneous activity and activity evoked by somatic stimulation of single neurons of the caudal part of the raphe nuclei were studied. The overwhelming majority of neurons were characterized by spontaneous activity which changed (inhibited or facilitated) under the effects of somatic (especially repeated) stimulation; most of them reacted to stimulation of the skin of any limb. In the case of paired stimulation of the skin of limbs on different sides at large intervals (40–60 msec), inhibition of the test discharge occurred, whereas at small intervals summation (simple addition) of the impulses occurred. In their general characteristics the neurons of the raphe nuclei apparently differ little from the neurons of the reticular formation of the brain stem.Institute of Electrophysiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 32–42, January–February, 1971.     

Postsynaptic Reactions in Somatosensory Cortex Neurons Activated by Stimulation of Nociceptors: Modulation upon Stimulation of the Central Grey, <Emphasis Type="Italic">Locus Coeruleus</Emphasis>, and <Emphasis Type="Italic">Substantia Nigra</Emphasis>

In experiments on cats, we studied the effects of electrical stimulation of the cerebral central grey (CG), locus coeruleus (LC), and substantia nigra (SN) on postsynaptic processes evoked by nociceptive volleys in somatosensory cortex neurons. Nineteen cells activated exclusively by stimulation of nociceptors (intense stimulation of the dental pulp) and 26 cells activated by both nociceptive and non-nociceptive (near-threshold) stimulations of the n. infraorbitalis and thalamic nucl. ventroposteromedialis (VPM) were intracellularly recorded (nociceptive and convergent cortical neurons, respectively). In neurons of both groups, stimulation of both nociceptive afferents and the VPM evoked complex responses having on EPSP-spike-IPSP patterns (duration of IPSPs about 200-300 msec). Electrical stimulation of the СG, which per se could activate the examined cortical neurons, induced prolonged suppression of synaptic responses evoked by stimulation of nociceptors; maximum inhibition was observed at 600- to 800-msec-long conditioning–test intervals. A certain parallelism was observed between the conditioning effects of СG stimulation and effects of systemic introduction of morphine. Isolated stimulations of the LC and SN by short high-frequency pulse series evoked primary complex EPSPs in a part of the examined cortical neurons, while high-amplitude IPSPs (up to 120 msec long) were observed in other units. Independently of the type of the primary response, conditioning stimulations of the LC and SN induced long-lasting (several seconds) suppression of synaptic responses evoked in cortical neurons by stimulation of nociceptive inputs. Mechanisms of modulating influences coming from opioidergic, noradrenergic, and dopaminergic cerebral systems to neurons of the somatosensory cortex activated upon excitation of high-threshold (nociceptive) afferent inputs are discussed.     

Secondary hair cells and afferent neurones of the squid statocyst receive both inhibitory and excitatory efferent inputs

Summary Intracellular recordings were obtained from the hair cells and afferent neurones of the angular acceleration receptor system of the statocyst of the squid,Alloteuthis subulata. Electrical stimulation of the efferent fibres in the crista nerve (minor) evoked responses in all of the secondary hair cells recorded from (n=211). 48% of the secondary air cells responded with a small depolarization, 15% with a hyperpolarization, and 37% with a depolarization followed by a hyperpolarization. The depolarizations and hyperpolarizations had mean stimulus to response delays of 6.7 ms and 24 ms, and reversal potentials of about –1 mV and –64 mV, respectively. Both types of potential increased in amplitude, up to a point, when the stimulus shock was increased and facilitation and/or summation effects could be obtained by applying multiple shocks. These data, together with the fact that both responses could be blocked by bath application of cobalt or cadmium, indicate that the secondary hair cells receive both inhibitory and excitatory efferent inputs and that these are probably mediated via chemical synapses. No efferent responses were seen in the primary hair cells but both depolarizing and hyperpolarizing efferent responses were obtained from the afferent neurones.     

Neuronal activity of the lateral vestibular nucleus of the guinea pig evoked by tilting the animal about the longitudinal axis during locomotion

In experiments on decerebrate guinea pigs, the impulse activity of neurons of the lateral vestibular nucleus evoked by tilting the animal about the longitudinal axis was investigated under conditions of spontaneous and mesencephalon stimulation-evoked locomotor activity. In most investigated neurons, locomotor activity led to changes in their responses to adequate vestibular stimulation. The dominant reaction was intensification of such responses, which was observed in almost all vestibulospinal neurons and in 2/3 of cells not having descending projections. Responses were suppressed only in 1/4 of the neurons not projecting to the spinal cord. The changes in the evoked responses had an amplitude character; the lag of the changes in the discharge frequency relative to the acceleration that caused them was constant. It is suggested that intensification of dynamic reactions of vestibular neurons during locomotion provides maintenance of the animal's equilibrium during movements in space by various gaits and along different trajectories.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 5, pp. 541–549, September–October, 1991.     

Responses of neurons of different hypothalamic structures to stimulation of tooth pulp and Aß afferents in the cat sciatic nerve

The response of neurons of different hypothalamic structures to stimulation of painful tooth pulp afferents and painless sciatic nerve Aß afferents was investigated during acute experiments on cats. It was found that 80.7%, 81.5%, and 71.4% of neurons of the posterior, tuberal, and anterior hypothalamus respectively, responded to stimulation of the tooth pulp. Shortest latency of response was recorded in the posterolateral hypothalamus. Latency of response was shorter in the lateral than in the medial structures throughout the hypothalamus. A distinct prevalence of excitatory response was found in neurons of the posterior area and an almost equal proportion of excitatory and inhibitory response in neurons of the tuberal and anterior hypothalamus. A high degree of convergence between noxious and nonnoxious somatic afferents were discovered in hypothalamic neurons: 85.8% of those studied responded to stimulation of the sciatic nerve Aß afferents. The comparable unidirectional response pattern of hypothalamic neurons to stimulation of tooth pump painful afferents and painless sciatic nerve Aß fibers point to the nonspecific nature of the response observed in the mainstream population of multisensory hypothalamic neurons. A small population of unimodal nociceptive neurons (14.2%) was found in the hypothalamus. Nociceptive responses of anterior hypothalamic neurons were distinguished by their long refractory phase, lasting 200–500 msec, and their low rate of reproduction during rhythmic stimulation of tooth pulp (1.5–2 Hz). Neuronal organization of the nociceptive hypothalamic afferent system is discussed together with the role of convergent and specific "nociceptive" neurons in the shaping of thalamic regulatory functions.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 18, No. 2, pp. 171–180, March–April, 1986.     

Contralateral input modulates the excitability of dorsal horn neurons involved in noxious signal processes. Potential role in neuronal sensitization

Wide Dynamic Range (WDR) neurons in the spinal cord receive inputs from the contralateral side that, under normal conditions, are ineffective in generating an active response. These inputs are effective when the target WDRs change their excitability conditions. To further reveal the mechanisms supporting this effectiveness shift, we investigated the weight of the excitation of the contralateral neurons on the target WDR responses. In the circuit of presynaptic (sending) and postsynaptic (receiving) neurons in crossed spinal connections the fibres that form the presynaptic neurons impinge on postsynaptic neurons can be considered the final relay of this contralateral pathway. The enhancement of the presynaptic neuron excitability may thus modify the efficacy of the contralateral input. Pairs of neurons each on a side of the spinal cord, at the L5-L6 lumbar level were simultaneously recorded in intact, anaesthetized, paralysed rats. The excitatory aminoacid NMDA and strychnine, the antagonist of the inhibitory aminoacid glycine, were iontophoretically administrated to presynaptic neurons to increase their excitability. Before and during the drug administration, spontaneous and noxious-evoked activities of the neurons were analysed. During the iontophoresis of the two substances we found that noxious stimuli applied to the receptive field of presynaptic neurons activated up to 50% of the previously unresponsive postsynaptic neurons on the opposite side. Furthermore, the neurons on both sides of the spinal cord showed significantly increased spontaneous activity and amplified responses to ipsilateral noxious stimulation. These findings indicate that the contralateral input participates in the circuit dynamics of spinal nociceptive transmission, by modulating the excitability of the postsynaptic neurons. A possible functional role of such a nociceptive transmission circuit in neuronal sensitization following unilateral nerve injury is hypothesized.