Role of the midbrain periaqueductal gray matter in conditioned reflexes

Unit activity in the midbrain periaqueductal gray matter (PGM) during an instrumental placing reflex, its extinction, differentiation, and conditioned inhibition, was studied in chronic experiments on cats. Spike responses 1–2 sec in duration in 69 (36.7%) of 182 neurons preceded by 400–800 msec the beginning of conditioned-reflex and voluntary intertrial movements. These advanced responses appeared 200 msec before the corresponding advance responses of motor cortical neurons. Fifty-eight neurons (30.9%) responded directly to acoustic stimulation with a latent period of 10–50 msec for 2–6 sec, 19 neurons (10.1%) generated double responses, linked with both the acoustic stimulus and subsequent conditioned-reflex movement, and 42 neurons (22.3%) did not respond to acoustic stimulation, although individual neurons of this group changed the level of their spontaneous activity in response to repeated conditioned stimulation, and this change was maintained for some tens of minutes. Extinction, differentiation, and conditioned inhibition all abolished conditioned-reflex movements, but each type of internal inhibition was accompanied by its own characteristic changes in the firing pattern of PGM neurons. Functional independence of neurons of the first and second groups was demonstrated during extinction and recovery of the conditioned-reflex. The results indicate the important role of PGM not only in the mechanism of the conditioned reflex, but also in the development of its internal inhibition.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 403–419, May–June, 1984.     

Neuronal organization of the periaqueductal gray matter in the cat midbrain

It is shown by the use of Golgi's method in Antonova's modification that the neuronal structure of the periaqueductal gray matter (PGM) in the frontal plane is characterized by the presence of small and medium-sized cells of "reticular type," which can be subdivided into three types: fusiform, triangular, and multipolar. On the basis of the visual distribution of these types of neurons and also of statistical analysis of 800 identified neurons, two regions can be distinguished: medial, directly surrounding the aqueduct of Sylvius, containing small neurons, among which the fusiform kind predominate significantly (P<0.001), and a lateral region with larger neurons, with significantly (P<0.001) more triangular cells. Neurons in the medial region show a characteristic and strong (P<0.001) tendency for their dendrites to be oriented toward the lumen of the aqueduct, and through them the physiologically active substances of the CSF may influence the functional activity of neurons of PGM.Central Research Institute of Reflex Therapy, Moscow City Council Main Health Board, Moscow. Translated from Neirofiziologiya, Vol. 16, No. 6, pp. 773–777, November–December, 1984.     

Corticofugal projections to the periaqueductal gray matter in the cat midbrain

Stereotaxic microinjections of horseradish peroxidase (HP) were made into different parts of the rostral and caudal periaqueductal gray (PAG) in cats to study corticofugal projections to the PAG. The method of retrograde axonal transport of HP demonstrated labeled neurons in the I and II somatosensory areas, frontal, cingular and insular cortex of the brain. It was shown that the II somatosensory cortex projects to all the areas of the rostral and caudal PAG. The frontal cortex projects to the dorsolateral quadrant of the PAG. The findings obtained enabled the detection of the morphological substrate of the corticofugal effects on one of the antinociceptive brain structures--the PAG.     

G protein activation by endomorphins in the mouse periaqueductal gray matter

The midbrain periaqueductal gray matter (PAG) is an important brain region for the coordination of mu-opioid-induced pharmacological actions. The present study was designed to determine whether newly isolated mu-opioid peptide endomorphins can activate G proteins through mu-opioid receptors in the PAG by monitoring the binding to membranes of the non-hydrolyzable analog of GTP, guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS). An autoradiographic [(35)S]GTPgammaS binding study showed that both endomorphin-1 and -2 produced similar anatomical distributions of activated G proteins in the mouse midbrain region. In the mouse PAG, endomorphin-1 and -2 at concentrations from 0.001 to 10 microM increased [(35)S]GTPgammaS binding in a concentration-dependent manner and reached a maximal stimulation of 74.6+/-3.8 and 72.3+/-4.0%, respectively, at 10 microM. In contrast, the synthetic selective mu-opioid receptor agonist [D-Ala(2),NHPhe(4), Gly-ol]enkephalin (DAMGO) had a much greater efficacy and produced a 112.6+/-5.1% increase of the maximal stimulation. The receptor specificity of endomorphin-stimulated [(35)S]GTPgammaS binding was verified by coincubating membranes with endomorphins in the presence of specific mu-, delta- or kappa-opioid receptor antagonists. Coincubation with selective mu-opioid receptor antagonists beta-funaltrexamine or D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2) (CTOP) blocked both endomorphin-1 and-2-stimulated [(35)S]GTPgammaS binding. In contrast, neither delta- nor kappa-opioid receptor antagonist had any effect on the [(35)S]GTPgammaS binding stimulated by either endomorphin-1 or -2. These findings indicate that both endomorphin-1 and -2 increase [(35)S]GTPgammaS binding by selectively stimulating mu-opioid receptors with intrinsic activity less than that of DAMGO and suggest that these new endogenous ligands might be partial agonists for mu-opioid receptors in the mouse PAG.     

c-Fos expression in the midbrain periaqueductal gray during static muscle contraction

The periaqueductal gray (PAG) of the midbrain is involved in the autonomic regulation of the cardiovascular system. The purpose of this study was to determine if static contraction of the skeletal muscle, which increases arterial blood pressure and heart rate, activates neuronal cells in the PAG by examining Fos-like immunoreactivity (FLI). Muscle contraction was induced by electrical stimulation of the L7 and S1 ventral roots of the spinal cord in anesthetized cats. An intravenous infusion of phenylephrine (PE) was used to selectively activate arterial baroreceptors. Extensive FLI was observed within the ventromedial region (VM) of the rostral PAG, the dorsolateral (DL), lateral (L), and ventrolateral (VL) regions of the middle and caudal PAG in barointact animals with muscle contractions, and in barointact animals with PE infusion. However, muscle contraction caused a lesser number of FLI in the VM region of the rostral PAG, the DL, L, and VL regions of the middle PAG and the L and VL regions of the caudal PAG after barodenervation compared with barointact animals. Additionally, the number of FLI in the DL and L regions of the middle PAG was greater in barodenervated animals with muscle contraction than in barodenervated control animals. Thus these results indicated that both muscle receptor and baroreceptor afferent inputs activate neuronal cells in regions of the PAG during muscle contraction. Furthermore, afferents from skeletal muscle activate neurons in specific regions of the PAG independent of arterial baroreceptor input. Therefore, neuronal cells in the PAG may play a role in determining the cardiovascular responses during the exercise pressor reflex.     

Activation of the midbrain periaqueductal gray induces airway smooth muscle relaxation.

In this study, we examined effects of chemical stimulation of the ventrolateral region of the midbrain periaqueductal gray (vl PAG) on airway smooth muscle tone. We observed that in anesthetized, paralyzed, and artificially ventilated ferrets, vl PAG stimulation elicited airway smooth muscle relaxation. To clarify the mechanisms underlying this observation, we examined the GABA-GABA(A) receptor signaling pathway by 1) examining the expression of GABA(A) receptors on airway-related vagal preganglionic neurons (AVPNs) located in the rostral nucleus ambiguus region (rNA), by use of receptor immunochemistry and confocal microscopy; 2) measuring GABA release within the rNA by using microdialysis; and 3) performing physiological experiments to determine the effects of selective blockade of GABA(A) receptors expressed by AVPNs in the rNA region on vl PAG-induced airway relaxation, thereby defining the role of the GABA(A) receptor subtype in this process. We observed that AVPNs located in the rNA region do express the GABA(A) receptor beta-subtype. In addition, we demonstrated that activation of vl PAG induced GABA release within the rNA region, and this release was associated with airway smooth muscle relaxation. Blockade of the GABA(A) receptor subtype expressed by AVPNs in the rNA by bicuculline diminished the inhibitory effects of vl PAG stimulation on airway smooth muscle tone. These data indicate, for the first time, that activation of vl PAG dilates the airways by a release of GABA and activation of GABA(A) receptors expressed by AVPNs.     

c-Fos expression in the midbrain periaqueductal gray after chemoreceptor and baroreceptor activation

The pattern of Fos-like immunoreactivity (FLI) in the periaqueductal gray (PAG) associated with activation of arterial chemoreceptors versus baroreceptor afferents was examined in urethane-anesthetized rats. Chemoreflex responses elicited by repeat intravenous injections of potassium cyanide (KCN; 90 microg/kg) significantly increased FLI in all columns of the PAG relative to saline-injected animals. Pressor responses elicited by intravenous phenylephrine (PE) produced a similar pattern of increased FLI throughout the PAG except in the dorsomedial and lateral columns of the caudal PAG, where FLI was minimal. Chemoreflex responses were unaltered by blockade of excitatory amino acid receptors in the dorsomedial PAG, and < 10% of the neurons of the caudal PAG that expressed FLI after KCN stimulation were retrogradely labeled from the A5 region of the caudal ventrolateral pons. These results indicate that integration of chemoreceptor inputs occurs primarily in the dorsal and lateral columns of the caudal PAG, but these neurons have little direct descending influence over lower brain stem regions integral to the central arterial chemoreflex arc.     

Effect of electroacupuncture on the activity of neurons in the central gray matter of the midbrain

The effect of electroacupuncture in locally-segment and general analgetic points on background impulse activity of central gray substance neurons and their activity caused by nociceptive stimulation of the dental pulp, infraorbital nerve and forearm skin surface was studied in acute experiments on cats. It has been established that general analgetic points are better represented in the central gray substance, as compared to locally-segment points. Different degree involvement of central gray substance in the realization of acupuncture analgetic effect in different points is postulated. The role of dorsal and ventral compartments of the central gray substance in acupunctural analgesia is discussed.     

Combined electrical stimulation of the periaqueductal gray matter and sensory thalamus

11 patients with chronic intractable pain of at least 3 years' duration underwent a morphine infusion test, the results of which suggested a syndrome of superimposed somatogenic and neurogenic pain components. They then underwent stereotactic implantation of a dual-channel brain stimulation system with two brain electrodes, one in the left periaqueductal gray matter (PAG) and the other in the sensory thalamus contralateral to the neurogenic pain. Using this system, all patients have obtained excellent simultaneous relief of both pain components (follow-up 12-36 months). The findings support a notion of two separate sensory modulating systems. They indicate that combined electrical stimulation of the PAG and sensory thalamus is a technically feasible and clinically satisfactory modality for the control of pain in humans, and they appear to indicate that better pain control is obtained by continuous, cycled stimulation of the PAG than by the conventional mode of stimulation.     

Opioid peptides within the midbrain periaqueductal gray suppress affective defense behavior in the cat

The effects of the methionine-enkephalin analog [D-Ala2-Met5]-enkephalinamide (DAME) upon the threshold for affective defense behavior were determined following microinjections placed into midbrain periaqueductal gray sites from which this response was elicited. Affective defense behavior was elicited by electrical stimulation through a cannula electrode situated in the dorsal aspect of the midbrain periaqueductal gray. Dose-response curves characterizing the effects of DAME upon affective defense behavior were determined utilizing the following doses: 0.25, 0.5 and 1.0 microgram in 0.5 microliter saline, pH = 7.4 or vehicle control (saline). Response thresholds were tested 10-30, 30-60, 60-90, 120-150, 180-210, 1440-1470 and 2880-2910 min postinjection. The results obtained indicated that injections of DAME at a dose of 1.0 microgram/0.5 microliter produced significant, long duration elevations in affective defense thresholds, lasting up to 1440-1470 min postinjection. Lower doses of DAME (0.25 and 0.5 microgram/0.5 microliter) also resulted in significant increases in affective defense thresholds, but these effects were of shorter durations (60-90 and 120-150 min) postinjection, respectively. The suppressive effects of DAME were blocked when animals were pretreated with naloxone (10 micrograms/0.5 microliter) microinjected into the same midbrain periaqueductal gray site into which 0.25 microgram DAME was injected and affective defense behavior was elicited.     

Analgesic effect of corticotropin-releasing factor administered into midbrain periaqueductal grey matter

Corticotropin-releasing factor (CRF) participates in development of stress-induced analgesia. Midbrain periaqueductal grey matter (MPAG) is one of crucial structures of the brain antinociceptive system. The aim of the study was to investigate effects of the CRF administration into the MPAG on pain sensitivity in alert rats and contribution of opioid mechanisms to these CRF-induced effects. Somatic pain sensitivity was tested by tail flick response latency following thermal stimuli. The opioid antagonist naltrexone administered systemically or centrally into the MPAG was used to study involvement ofopioid mechanisms in the CRF-induced effects. The CRF administration (0.7 microg/rat) into the MPAG caused analgesic effect. The CRF-induced analgesic effects were eliminated by systemic as well as central naltrexone pretreatment. Effect of central naltrexone on the CRF-induced analgesia manifested itself faster as compared with effect of systemic naltrexone. The data obtained suggest that one of central mechanisms of the CRF-induced analgesic effect on somatic pain sensitivity in alert rats may be mediated by the MPAG neurons and provided by involvement of opioid mechanisms.     

Comparative effects of amygdala and central gray matter of the midbrain on lateral hypothalamic unit activity

The effect of stimulation of the stria terminalis, the main afferent component of the amygdalo-hypothalmic system, and of the central gray matter and tegmentum of the midbrain on lateral hypothalamic unit activity was investigated in acute experiments on rats. Five types of unit responses were discovered: phasic excitation and inhibition, tonic activation and inhibition, and a biphasic response. In response to stimulation of the stria terminalis and lateral hypothalamus mainly inhibitory responses (62.7%) were recorded. As a result of stimulation of the central gray matter most lateral hypothalamic neurons (87%) were activated. Convergence of influences from the amygdala and tegmentum was observed on 14.4% of responding neurons. The structures had an antagonistic action on most (84.6%) of the lateral hypothalamic neurons tested.Institute of Physiology, Siberian Branch, Academy of Medical Sciences of the USSR, Novosibirsk. Translated from Neirofiziologiya, Vol. 9, No. 1, pp. 25–32, January–February, 1977.     

Suppression of postsynaptic response in trigeminal motoneurons by stimulating the midbrain central gray matter

The effects of stimulating the midbrain central gray matter (CGM) on motoneuronal response in trigeminal nerves were investigated in anesthetized cat. It was found that stimulating the CGM did not induce postsynaptic response in these motoneurones. Conditioning stimulation of the CGM brought about suppression of motoneuronal postsynaptic response to stimulation of tooth pulp and high threshold infraorbital nerve afferents without affecting motoneuronal antidromic response and jaw-opening reflex as induced by stimulating the caudal nucleus of the spinal trigeminal tract. It was thus concluded that stimulating the CGM exerts no direct effect on motoneurons but does have an influence on postsynaptic response — a result of modulation of the afferent spike flow at interneuronal level.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 543–549, July–August, 1990.     

Intranuclear bodies in neurons of the periaqueductal gray matter in the cat.

The nucleoplasm of neurons in the nucleus lateralis of the periqueductal gray matter in the cat contains fibrillar structures which have no limiting membranes. These intranuclear bodies are associated with neither the nucleolus nor the nuclear membrane and have two characteristic forms. The first, the rodlet, is a compact bundle of fibrils 2 to 8 nm in diameter. It is usually elongated in shape although it appears spherical when sectioned transversely. This rod-like structure appears to correspond to Roncoroni's rodlet or the accessory body of Cajal in light microscopy. The second and more commonly observed form is a long slender bundle of five rows of parallel fibrils. Although similar intranuclear structures have frequently been observed in the highly differentiated neurons of the sympathetic ganglia and the retina, this is the first report of their pbesence in the undifferentiated neurons of the isodendritic core of the brainstem.     

Effects of stimulating the periaqueductal gray matter on high- and low-threshold startle reflexes

The effects of stimulating the periaqueductal gray matter (PAG) on two types of startle reflex (spino-bulbo-spinal reflex produced by intensive stimulation of the peripheral nerves and low-threshold tactile spino-reticulo-spinal reflex) as well as high-threshold jaw-opening reflex arising in response to tooth pulp stimulation were investigated in cats anesthetized with chloralose. Simulating most PAG test sites led to pronounced inhibition of jaw-opening reflex, profound depression of spino-bulbo-spinal reflex, and moderate inhibition of tactile reflexes. The facilitatory effect of stimulating a number of PAG sites on the latter reflexes was demonstrated. Effects of PAG stimulation fell into two classes: brief, measurable in hundreds of msec and more prolonged, measured in minutes and seconds. Findings would indicate certain differences between the effects of PAG stimulation low-threshold (non-nociceptive) and high-threshold (nociceptive) startle reflexes, of which the possible mechanisms are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 1, pp. 71–78, January–February, 1989.     

Cytoarchitecture of the periaqueductal gray matter in the cat: a quantitative Nissl study

We studied the periaqueductal gray matter (PAG) in cresyl-violet-stained serial sections from 5 cats applying quantitative determinations. No significant variations were observed in the cytological aspects in the various sites examined (lateral, ventral and dorsal regions, and external and internal portions). The neuronal density was constant in the different regions, but showed a gradual and significant increase in the most external regions of the PAG. Our findings do not, therefore, confirm the existence in the PAG of subnuclei with a specific cytoarchitecture; this does not, however, rule out the possibility that there are specific regions for connections, histochemical properties or functions.     

Neurochemical mechanisms in inhibitory effects of stimulation of the periaqueductal gray matter on high-threshold reflex activity of the reticular formation

A study was made of microinjections of antagonists of various neuromodulators on the dynamics of inhibition of the spino-bulbo-spinal reflexes which were evoked by stimulation of the central gray matter (PAG) in rats anesthetized with chloralose. Injections were made into the reticular gigantocellular nucleus (GN), which is the basic supraspinal center of this reflex. Administering methysergide (a blocker of serotonin receptors) was accompanied by significant (two to four times) diminution of inhibition evoked by PAG stimulation with a short, high-frequency series of stimuli. Long inhibition caused by long, rhythmic stimulation of the PAG was diminished less significantly: from 6–10 to 2.5–4 min. When the opiate receptors of the GN neurons were blocked with naloxone, duration of inhibition was reduced by two to five times. The most clearly expressed diminution of both types of inhibitions was noted with injections of haloperidol, an antagonist of catecholamines. Our data indicate that evidently all of these neuromediator (neuromodulator) systems participate in inhibition of high-threshold, reflex activity of the reticular formation evoked by stimulation of the PAG, but their participation in this process is unequal.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 4, pp. 455–463, July–August, 1991.