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.     

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.     

Participation of midbrain periaqueductal gray matter in defense conditioning in rats

Responses of neurons of the periaqueductal gray matter (PAG) were studied in chronic experiments on cats during formation and extinction of a defensive conditioned reflex to sound and its differential inhibition. In response to conditioned stimulation these neurons developed phasic-tonic spike responses up to 3 sec in duration. During combination of stimuli these responses were formed long before the conditioned reflex and disappeared long after the latter was extinguished. In the case of an established conditioned reflex, the onset of spike responses occurred 100–200 msec before the appearance of motor responses. An increase in spike activity of tonic character in neurons of PAG preceded voluntary movements by 100–500 msec. The responses of these neurons to presentation of a differential stimulus consisted of groups of spikes 150–200 msec in duration. They were formed with difficulty, and their manifestation was made even more difficult by an interruption during the experiment and by preceding positive stimuli. On the basis of the results a conditioned reflex can be regarded as the result of a multilevel hierarchic process of readjustment of unit activity, which begins in the nonspecific structures of the midbrain.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 15, No. 3, pp. 278–287, May–June, 1983.     

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.     

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.     

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.     

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.     

Micturition-suppressing region in the periaqueductal gray of the mesencephalon of the cat

The periaqueductal gray (PAG) of the mesencephalon has been implicated to be involved in the control of micturition. We investigated the micturition-suppressing region in the PAG of the cat. Decerebrated 27 adult cats were used. A microelectrode was inserted stereotaxically into the PAG, and a region was searched where electrical stimulation suppressed isovolumetric bladder contractions. Simultaneous stimulation of the pontine micturition center (PMC) and micturition-suppressing region in the PAG was performed before and after an injection of bicuculline (GABA(A) blocker) into the PMC. The micturition-suppressing region was found at the dorsolateral margin of the rostral PAG. Bladder contractions were not provoked by simultaneous stimulation of the PMC and micturition-suppressing region in the PAG. However, after bicuculline injection into the PMC, partial bladder contractions were provoked by simultaneous stimulation of the PMC and the micturition-suppressing region in the PAG. These results suggest that the dorsolateral margin of the rostral PAG includes the micturition-suppressing region that seems to have neural connections with the PMC. GABA is assumed to be one of the neurotransmitters that are involved in the PMC inhibition from the micturition-suppressing region in the PAG.     

Effects of lordosis-relevant neuropeptides on midbrain periaqueductal gray neuronal activity in vitro.

Certain neuropeptides can facilitate lordosis by acting on midbrain periaqueductal gray (PAG) in estrogen-primed female rats. Here, we investigated responses of individual PAG neurons in vitro, to five neuropeptides: substance P (SP), luteinizing hormone-releasing hormone (LHRH), prolactin (PRL), oxytocin (OT), and thyrotropin-releasing hormone (TRH). Substance P, OT, and TRH excited spontaneous activity of PAG neurons through neurotransmitter-like actions in a dose-dependent manner, whereas LHRH and PRL virtually never affected PAG neurons this way. Oxytocin acted through oxytocin receptors located on the recorded PAG neurons, since excitatory actions of OT were 1) not abolished by synaptic blockade, 2) mimicked by the OT-specific agonist [Thr4, Gly7]OT but not by arginine vasopressin, and 3) blocked by the OT-specific antagonist [d(CH2)5,Tyr(Me)2,Orn8]vasotocin. Although LHRH had no neurotransmitter-like action on spontaneous activity of PAG neurons, it, as well as SP, could modulate responses of some dorsal PAG neurons to GABAA and GABAB agonists or norepinephrine. Neuromodulatory actions of LHRH and SP could help facilitate lordosis through PAG neurons.     

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.     

褪黑素对大鼠中脑导水管周围灰质内阿片肽释放的影响

本文采用推挽灌流技术、放射免疫测定法,观察褪黑素（ｍｅｌａｔｏｎｉｎ,ＭＥＬ）对大鼠中脑导水管周围灰质（ＰＡＧ）推挽灌流液中β－内啡肽（β－Ｅｐ）、亮氨酸脑啡肽（Ｌ－ＥＫ）含量的影响,以探讨ＭＥＬ镇痛效应的中枢机制。结果显示,给药组大鼠腹腔注射（ｉｐ）ＭＥＬ１１０ｍｇ／ｋｇ后３０－５０ｍｉｎ,ＰＡＧ灌流液中β－Ｅｐ含量显著增加,而Ｌ－ＥＫ含量未见显著变化;在推挽灌流同时用５０℃热水刺激甩尾法测定痛     

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.     

Modulation of neuron activity of the midbrain periaqueductal gray matter influenced by monoaminergic brainstem structures

A study was done on base activity and changes in base activity (BA) of neurons in periaqueductal gray matter (PAG) during stimulation of monoaminergic structures of the brainstem: the nucleus raphe magnus (NRM), the locus coeruleus (LC), and the substantia nigra (SN), in rats anesthetized with hexenal (200 mg/kg). Three types of PAG neurons that differed in BA structure were identified. NRM, LC and SN stimulation changed BA only in type III neurons. Stimulation of these structures evoked an increase in BA frequency in 11.0–14.5%, and inhibition in 31.0–47.5% of type III neurons. Simultaneous stimulation of two structures led to a marked drop in intensity of effects. A depressing effect on BA was always detected if stimulation of one of the structures suppressed BA. Stimulation of two structures, with one being the NRM, was most effective. The role of PAG in the organization of the brain-stem component of the antinociceptive mechanism is discussed.A. A. Bogomolets Institute of Physiology, Ukrainian Academy of Sciences, Kiev. Translated from Neirofiziologiya, Vol. 24, No. 1, pp. 52–60, January–February, 1992.     

Glutamate release in midbrain periaqueductal gray by activation of skeletal muscle receptors and arterial baroreceptors

We have previously reported that both skeletal muscle receptor and arterial baroreceptor afferent inputs activate neurons in the dorsolateral (DL) and lateral regions of the midbrain periaqueductal gray (PAG). In this study, we determined whether the excitatory amino acid glutamate (Glu) is released to mediate the increased activity in these regions. Static contraction of the triceps surae muscle for 4 min was evoked by electrical stimulation of the L7 and S1 ventral roots in cats. Activation of arterial baroreceptor was induced by intravenous injection of phenylephrine. The endogenous release of Glu from the PAG was recovered with the use of a microdialysis probe. Glu concentration was measured by the HPLC method. Muscle contraction increased mean arterial pressure (MAP) from 98 +/- 10 to 149 +/- 12 mmHg (P < 0.05) and increased Glu release in the DL and lateral regions of the middle PAG from 0.39 +/- 0.10 to 0.73 +/- 0.12 microM (87%, P < 0.05) in intact cats. After sinoaortic denervation and vagotomy were performed, contraction increased MAP from 95 +/- 12 to 158 +/- 15 mmHg, and Glu from 0.34 +/- 0.08 to 0.54 +/- 0.10 microM (59%, P < 0.05). The increases in arterial pressure and Glu were abolished by muscle paralysis. Phenylephrine increased MAP from 100 +/- 13 to 162 +/- 22 mmHg and increased Glu from 0.36 +/- 0.10 to 0.59 +/- 0.18 microM (64%, P < 0.05) in intact animals. Denervation abolished this Glu increase. Summation of the changes in Glu evoked by muscle receptor and arterial baroreceptor afferent inputs was greater than the increase in Glu produced when both reflexes were activated simultaneously in intact state (123% vs. 87%). These data demonstrate that activation of skeletal muscle receptors evokes release of Glu in the DL and lateral regions of the middle PAG, and convergence of afferent inputs from muscle receptors and arterial baroreceptors in these regions inhibits the release of Glu. These results suggest that the PAG is a neural integrating site for the interaction between the exercise pressor reflex and the arterial baroreceptor reflex.     

Neuronal activity within the ventrolateral periaqueductal gray during simulated hemorrhage in conscious rabbits

The ventrolateral (vl) periaqueductal gray (PAG) has been proposed as a site responsible for the active process triggering the onset of hypotension (i.e., phase 2) during blood loss in conscious animals (Cavun S and Millington WR. Am J Physiol Regul Integr Comp Physiol 281: R747-R752, 2001). We recorded the extracellular activity of PAG neurons in conscious rabbits to test the hypothesis that vlPAG neurons change their firing frequency before the onset of hypotension during simulated hemorrhage. Arterial and venous catheters, an intrathoracic vena caval occluder, and midbrain microelectrodes on a microdrive were implanted in 10 rabbits. During simulated hemorrhage, the occluder was inflated until arterial pressure < or = 40 mmHg. We compared changes in neuronal activity during simulated hemorrhage with those during a similar length control period for 64 vlPAG and 29 dorsolateral (dl) PAG neurons. Arterial pressure pulse modulation of neuronal activity was present in 45 and 76% of vlPAG and dlPAG neurons, respectively. When we evaluated the absolute change in activity, thus accounting for both increases and decreases, simulated hemorrhage had a significant effect on activity of vlPAG but not dlPAG neurons. The majority (56%) of vlPAG neurons did not appear to respond to simulated hemorrhage. Of the 28 responsive vlPAG neurons, 11 showed an abrupt change in firing frequency during the time interval preceding the onset of hypotension; 13 responded after the onset of hypotension; and 4 showed a consistent direction of change across the entire simulated hemorrhage. Thus 24 (38%) of the vlPAG neurons recorded responded at a time consistent with a contribution to the hypotension associated with simulated hemorrhage.     

Oxytocin in the periaqueductal gray participates in pain modulation in the rat by influencing endogenous opiate peptides

Periaqueductal gray (PAG) plays a very important role in pain modulation through endogenous opiate peptides including leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek), β-endorphin (β-Ep) and dynorphin A_1-13 (DynA_1-13). Our pervious study has demonstrated that intra-PAG injection of oxytocin (OXT) increases the pain threshold, and local administration of OXT receptor antagonist decreases the pain threshold, in which the antinociceptive role of OXT can be reversed by pre-PAG administration of OXT receptor antagonist. The experiment was designed to investigate the effect of OXT on endogenous opiate peptides in the rat PAG during the pain process. The results showed that (1) the concentrations of OXT, L-Ek, M-Ek and β-Ep, not DynA_1-13 in the PAG perfusion liquid were increased after the pain stimulation; (2) the concentrations of L-Ek, M-Ek and β-Ep, not DynA_1-13 in the PAG perfusion liquid were decreased by the OXT receptor antagonist; (3) the increased pain threshold induced by the OXT was attenuated by naloxone, an opiate receptor antagonist; and (4) the concentrations of L-Ek, M-Ek and β-Ep, not DynA_1-13 in the PAG perfusion liquid were increased by exogenous OXT administration. The data suggested that OXT in the PAG could influence the L-Ek, M-Ek and β-Ep rather than DynA_1-13 to participate in pain modulation, i.e. OXT in the PAG participate in pain modulation by influencing the L-Ek, M-Ek and β-Ep rather than DynA_1-13.