Ventrolateral Periaqueductal Gray Research Articles

Medullary catecholaminergic projections to the ventrolateral periaqueductal gray region activated by halothane anaesthesia

Under anaesthesia, blood loss and deep pain can evoke a premature, centrally-mediated sympathoinhibition leading to decompensated shock and sometimes even death. The central circuits evoking premature vasodepressor syncope are unknown, although medullary catecholaminergic pathways have been implicated. The ventrolateral periaqueductal gray region is one of only three brain regions in which catecholamine content is increased during halothane anaesthesia. The ventrolateral periaqueductal gray also contains neurons which are selectively activated by blood loss and deep pain, and recent work from our laboratory has suggested that it is a pivotal structure in central sympathoinhibitory circuits. Using retrograde tracing techniques combined with the immunohistochemical detection of: (i) the catecholamine synthetic enzyme, tyrosine hydroxylase and (ii) the protein product of the immediate-early gene c-fos as a marker of neuronal activation; the results of this study indicate that catecholaminergic projections from the A1, C1 and C2 regions of the medulla to the ventrolateral periaqueductal gray are activated by halothane anaesthesia. These data are consistent with the hypotheses that ascending catecholaminergic projections to the ventrolateral periaqueductal gray: (i) are a component of the central neural circuitry responsible for the sympathoinhibitory effects of halothane anaesthesia, and (ii) may contribute to the premature elicitation of vasodepressor syncope following blood loss and deep pain under conditions of anaesthesia.

Periaqueductal gray matter input to cardiac-related sympathetic premotor neurons

The periaqueductal gray matter (PAG) serves as the midbrain link between forebrain emotional processing systems and motor pathways used in the defense reaction. Part of this response depends upon PAG efferent pathways that modulate cardiovascular-related sympathetic outflow systems, including those that regulate the heart. While it is known that the PAG projects to vagal preganglionic neurons, including possibly cardiovagal motoneurons, no information exists on the PAG circuits that may affect sympathetically mediated cardiac functions and, thus, the purpose of this study was to use neuroanatomical methods to identify these pathways. First, viral transneuronal retrograde tracing experiments were performed in which pseudorabies virus (PRV) was injected into the stellate ganglion of rats. After 4 days survival, five PAG regions contained transynaptically infected neurons; these included the dorsomedial, lateral and ventrolateral PAG columns as well as the Edinger–Westphal and precommissural nuclei. Second, the descending efferent PAG projections were studied with the anterograde axonal marker Phaseolus vulgaris leuco-agglutinin (PHA-L) with a particular focus on determining whether the PAG projects to the intermediolateral cell column (IML). Almost no axonal labeling was found throughout the thoracic IML suggesting that the PAG modulates sympathetic functions by indirect pathways involving synaptic relays through sympathetic premotor cell groups, especially those found in the medulla oblongata. This possibility was examined by a double tracing study. PHA-L was first injected into either the lateral or ventrolateral PAG and after 6 days, PRV was injected into the ipsilateral stellate ganglion. After an additional 4 days survival, a double immunohistochemical procedure for co-visualization of PRV and PHA-L was used to identify the sympathetic premotor regions that receive an input from the PAG. The PAG innervated specific groups of sympathetic premotor neurons in the hypothalamus, pons, and medulla as well as providing reciprocal intercolumnar connections within the PAG itself (Jansen et al., Brain Res. 784 (1998) 329-336). The major route terminates in the ventral medulla, especially within the medial region which contains sympathetic premotor neurons lying within the raphe magnus and gigantocellular reticular nucleus, pars alpha. Both serotonergic and non-serotonergic sympathetic premotor neurons in these two regions receive inputs from the PAG. Weak PAG projections to sympathetic premotor neurons were found in the rostral ventrolateral medulla (including to C1 adrenergic neurons), locus coeruleus, A5 cell group, paraventricular and lateral hypothalamic nuclei. In summary, both the lateral and ventrolateral PAG columns appear to be capable of modulating cardiac sympathetic functions via a series of indirect pathways involving sympathetic premotor neurons found in selected sites in the hypothalamus, midbrain, pons, and medulla oblongata, with the major outflow terminating in bulbospinal regions of the rostral ventromedial medulla.

The role of mu and kappa opioid receptors within the periaqueductal gray in the expression of conditional hypoalgesia

The periaqueductal gray (PAG) is a midbrain structure involved in the modulation of pain and expression of classically conditioned fear responses. Non-selective opioid antagonists applied to the PAG block the expression of hypoalgesia in rats exposed to a Pavlovian signal for shock. This study was conducted to determine the anatomical and pharmacological specificity of the PAG's role in conditional hypoalgesia. Rat subjects received injections of either the mu opioid antagonist CTAP (6.6 nMol), the kappa opioid antagonist Nor-binaltorphimine (Nor-BNI, 6.6 nMol) or saline. Injections were made into either the dorsolateral (dlPAG) or ventrolateral (vlPAG) PAG prior to the presentation of an auditory stimulus that had previously been paired with foot shock while measuring nociception with the radiant heat tail flick (TF) test. Elevation in TF latency in response to the auditory stimulus was blocked only by administration of CTAP into the vlPAG. These results suggest that conditional hypoalgesia (CHA) is subserved by mu but not kappa opioid receptors located in the vlPAG but not the dlPAG.

Vascular head pain selectively activates ventrolateral periaqueductal gray in the cat

Electrical stimulation of the superior sagittal sinus is an experimental model of migraine which activates neurons within the upper cervical spinal cord. The ventrolateral periaqueductal gray has been proposed as an integrative centre for the autonomic and behavioural responses to deep pain and also receives significant inputs from the upper cervical spinal cord. The noxious-stimulation evoked expression of the immediate-early gene c-fos, was used to determine if sagittal sinus stimulation activates neurons of the ventrolateral periaqueductal gray. The superior sagittal sinus was stimulated in anesthetised cats and Fos-protein was detected in coronal brain sections using standard avidin-biotin immunohistochemistry. A pattern of Fos-positive cells restricted to the caudal ventrolateral periaqueductal gray was revealed suggesting that this region may mediate the pattern of somatic and autonomic responses characteristic of migraine.

Endogenous opioid peptides acting at mu-opioid receptors in the dorsal horn contribute to midbrain modulation of spinal nociceptive neurons.

Activation of neurons in the midbrain periaqueductal gray (PAG) inhibits spinal dorsal horn neurons and produces behavioral antinociception in animals and analgesia in humans. Although dorsal horn regions modulated by PAG activation contain all three opioid receptor classes (mu, delta, and kappa), as well as enkephalinergic interneurons and terminal fields, descending opioid-mediated inhibition of dorsal horn neurons has not been demonstrated. We examined the contribution of dorsal horn mu-opioid receptors to the PAG-elicited descending modulation of nociceptive transmission. Single-unit extracellular recordings were made from rat sacral dorsal horn neurons activated by noxious heating of the tail. Microinjections of bicuculline (BIC) in the ventrolateral PAG led to a 60-80% decrease in the neuronal responses to heat. At the same time, the responses of the same neurons to iontophoretically applied NMDA or kainic acid were not consistently inhibited. The inhibition of heat-evoked responses by PAG BIC was reversed by iontophoretic application of the selective mu-opioid receptor antagonists, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP). A similar effect was produced by naloxone; however, naloxone had an excitatory influence on dorsal horn neurons in the absence of PAG-evoked descending inhibition. This is the first demonstration that endogenous opioids acting via spinal mu-opioid receptors contribute to brain stem control of nociceptive spinal dorsal horn neurons. The inhibition appears to result in part from presynaptic inhibition of afferents to dorsal horn neurons.

Local connections between the columns of the periaqueductal gray matter: a case for intrinsic neuromodulation

Chemical stimulation of the midbrain periaqueductal gray matter (PAG) in conscious animals produces opposite responses (viz., defensive behavior and pressor responses from the lateral PAG column vs. quiescence and depressor responses from the ventrolateral PAG column), raising the possibility that PAG columns are interconnected. To test this hypothesis, two types of anatomical experiments were performed in rats. First, the anterograde axonal marker Phaseolus vulgaris leuco-agglutinin (PHA-L) was injected into individual PAG columns or adjoining regions which included the Edinger–Westphal, dorsal raphe, and precommissural nuclei. Ten days later, the intrinsic PAG projections were studied. Our results show that each PAG column projects bilaterally to all of the other PAG columns, and also provides local connections within its own column. Furthermore, the Edinger–Westphal and precommissural nuclei project to all four PAG columns, while the dorsal raphe nucleus projects only to the ventrolateral and lateral columns. In a second experiment, we found that cardiovascular-related PAG projection neurons of both the lateral and ventrolateral columns receive inputs from the reciprocal PAG column. This was demonstrated by a double tracer neuroanatomical study in which PHA-L was first iontophoretically ejected into either the lateral or ventrolateral PAG columns and then, several days later the retrograde transneuronal viral tracer, pseudorabies virus, was injected into the stellate sympathetic ganglion. Intra-PAG circuits were visualized by a dual immunohistochemical procedure. These results suggest that during the fight-or-flight response when the `fight' program is activated, the `flight' PAG network may be inhibited and the converse situation may occur during the flight response.

Antinociception following opioid stimulation of the basolateral amygdala is expressed through the periaqueductal gray and rostral ventromedial medulla

The amygdala, periaqueductal gray (PAG), and rostral ventromedial medulla (RVM) are critical for the expression of some forms of stress-related changes in pain sensitivity. In barbiturate anesthetized rats, microinjection of agonists for the μ opioid receptor into the amygdala results in inhibition of the tail flick (TF) reflex evoked by radiant heat. We tested the idea that TF inhibition following opioid stimulation of the amygdala is expressed through a serial circuit which includes the PAG and RVM. Rats were anesthetized and prepared for microinjection of DAMGO (0.5 μg/0.25 μl) into the basolateral amygdala (BLA) and lidocaine HCl (2.5%/0.4–0.5 μl) into either the ventrolateral PAG or RVM. Lidocaine did not significantly alter baseline values for TF latency or TF amplitude. When injected into the PAG prior to DAMGO application in the BLA, lidocaine significantly attenuated DAMGO-induced antinociception for the entire 40 min testing session. Similar treatment in the RVM also resulted in an attenuation of antinociception although rats showed significant recovery of TF inhibition by 40 min after lidocaine injection. Since acute injection of lidocaine into the RVM also affected baseline heart rate, separate animals were prepared with small electrolytic lesions placed in the RVM. Chronic RVM lesions also blocked TF inhibition produced by amygdala stimulation but did not affect heart rate. These results, when taken together with similar findings in awake behaving animals, suggest that a neural circuit which includes the amygdala, PAG, and RVM is responsible for the expression of several forms of hypoalgesia in the rat.

Functional Relationship between the Hypothalamic Vigilance Area and PAG Vigilance Area

Duan, Y.-F., R. W. Winters, P. M. M cCabe, E. J. Green, Y. Huang and N. Schneiderman. Functional relationship between the hypothalamic vigilance area and PAG vigilance area. Physiol Behav 62(3) 675–679, 1997.—The vigilance reaction is characterized by a large bradycardia, a pressor response, and inspiratory apnea in anesthetized rabbits and the inhibition of movement in conscious rabbits. This affective response pattern can be elicited by electrical stimulation of the dorsolateral hypothalamus (the hypothalamic vigilance area) or the ventrolateral periaqueductal gray (the periaqueductal gray vigilance area). The present study sought to advance our understanding of the functional relationship between the hypothalamic vigilance area (HVA) and the periaqueductal gray vigilance area (PVA) by measuring the effects of transverse transections of the caudal portion of the ventrolateral PAG (vlPAG) upon the cardiovascular responses elicited from the dorsolateral hypothalamus and the rostral vlPAG. Selective transverse transections of the caudal vlPAG significantly reduced the magnitudes of the bradycardia and pressor response elicited by stimulation of the PVA rostral to the transection site, but had minimal impact on the cardiovascular responses evoked by stimulation of the HVA. These findings suggest that the cardiovascular responses elicited by stimulation of the vlPAG are mediated by a neural pathway that is parallel, at least in part, to the one that subserves the response elicited from the HVA. The results also provide support for the view that the PAG is not an essential structure in the mediation of the autonomic components of affective behaviors involving behavioral inhibition.

Periaqueductal gray matter projection to vagal preganglionic neurons and the nucleus tractus solitarius

The periaqueductal gray matter (PAG) has been implicated in a variety of different functions, including autonomic regulation. Chemical stimulation of the lateral PAG produces hypertension and tachycardia while activation of the ventrolateral PAG produces the opposite effect. While these effects are the result of alterations in sympathetic activity, little is known about whether the PAG can modulate vagal functions as well. The anterograde axonal tracing method using the plant lectin Phaseolus vulgaris leucoagglutinin (PHA-L) was used to determine whether both of the lateral and ventrolateral PAG columns project to vagal preganglionic neurons and/or to the nucleus tractus solitarius (NTS). Highly restricted PHA-L injections were made in all four PAG columns throughout their rostrocaudal extent in rats. Labeled fibers were visualized by immunohistochemistry and studied in relationship with choline acetyltransferase (ChAT) immunostained parasympathetic preganglionic neurons of the dorsal motor vagal nucleus (DMV) and nucleus ambiguus (NA). The lateral PAG projects to the lateral DMV and to the caudal part of the external NA. The ventrolateral PAG innervates the same regions and also projects to the rostral part of the external NA – a site that contains cardiac parasympathetic preganglionic neurons. Both the lateral and ventrolateral PAG project to the NTS in a similar fashion innervating the medial, ventrolateral and commissural subnuclei. In summary, the lateral and ventrolateral PAG have similar patterns of innervation of the NTS and DMV, but their projection to the NA is different: the rostral external NA receives innervation only from the ventrolateral PAG and the lateral PAG innervates the caudal part.

Effect of midbrain stimulations on thermoregulatory vasomotor responses in rats.

1. Efferent projections eliciting vasodilatation when the preoptic area is warmed were investigated by monitoring tail vasomotor responses of ketamine-anaesthetized rats when brain areas were stimulated electrically (0.2 mA, 200 microseconds, 30 Hz) or with the excitatory amino acid D,L-homocysteic acid (1 mM, 0.3 microliter). 2. Both stimulations elicited vasodilatation when applied within a region extending from the most caudal part of the lateral hypothalamus to the ventrolateral periaqueductal grey matter (PAG) and the reticular formation ventrolateral to the PAG. 3. Vasodilatation elicited by preoptic warming was suppressed when either stimulation was applied within the rostral part of the ventral tegmental area (VTA). 4. Sustained vasodilatation was elicited by knife cuts caudal to the VTA, and vasodilatation elicited by preoptic warming was suppressed by cuts either rostral to the VTA or in the region including the PAG and the reticular formation ventrolateral to it. 5. These results, together with the results of earlier physiological and histological studies, suggest that warm-sensitive neurones in the preoptic area send excitatory signals to vasodilatative neurones in the caudal part of the lateral hypothalamus, ventrolateral PAG and reticular formation, and send inhibitory signals to vasoconstrictive neurones in the rostral part of the VTA.

Projections from dorsal raphe nucleus to the periaqueductal grey matter: studies in slices of rat midbrain maintained in vitro

In coronal slices of rat midbrain localised injections of FluoroGold or DiO into the dorsolateral periaqueductal grey matter (PAG) labelled cells retrogradely in the dorsal half of the dorsal raphe nucleus (DRN) and the ipsilateral ventrolateral PAG. Low intensity electrical stimulation in the dorsolateral PAG (22.9±1.9 μA) evoked antidromic responses in neurones recorded intracellularly in the dorsal subnucleus of the DRN. Antidromic responses could also be evoked in neurones in the ventral DRN and its wings but only at much higher currents (40.9±2.5 μA) which likely spread to activate axons in the ventrolateral PAG that originated from perikarya in the ventral DRN. The findings are discussed in relation to modulation of the excitability of the aversive system in the dorsolateral PAG by the DRN.

Involvement of the Cholinergic System and Periaqueductal Gray Matter in the Modulation of Tonic Immobility in the Guinea Pig

Monassi, C.R.; A. Hoffmann and L. Menescal-De-Oliveira. Involvement of the cholinergic system and periaqueductal gray matter in the modulation of tonic immobility in the guinea pig. Physiol Behav 62(1) 53–59, 1997.—Unilateral microinjection of carbachol (CCh, 1.0 μg/0.2 μl) into the ventrolateral periaqueductal gray matter (vPAG) increased the duration of tonic immobility (TI) episodes induced by postural inversion and by movement restriction maneuvers in adult male guinea pig (Cavia porcellus), while stimulation with the same drug at the same concentration into the dorsolateral and dorsomedial periaqueductal gray matter (dl/dmPAG) decreased the duration of TI. Pretreatment with atropine (7.6 μg/0.4 μl) showed that the action of CCh is mediated by muscarinic receptors in the ventrolateral PAG but not in the dorsomedial and dorsolateral regions. These data suggest that the PAG and the cholinergic system are involved in the modulation of TI episodes and that different regions of the guinea pig PAG play distinct roles in the organization of this behavior

Responses of neurones in the medullary raphe nuclei to inputs from visceral nociceptors and the ventrolateral periaqueductal grey in the rat.

The ventrolateral periaqueductal grey matter (PAG) is believed to have a role in mediating cardiovascular responses to noxious visceral stimuli. The present study was carried out as a first stage in establishing whether the ventrolateral PAG may exert these influences after a relay in the caudal medullary raphe nuclei (nucleus raphe obscurus and nucleus raphe pallidus). Single unit extracellular recordings were made from neurones in the caudal raphe nuclei and, for comparison, in the more rostral nucleus raphe magnus in Saffan-anaesthetized and paralysed rats. Neurones in the mid-line medulla were tested for their responses to electrical stimulation at chemically identified depressor sites in the ventrolateral PAG and to noxious visceral stimuli (distensions of the urinary bladder and electrical stimulation of the greater splanchnic nerve). Fifty-two per cent of caudal and 74% of rostral mid-line neurones gave short latency excitatory responses to stimulation of depressor sites in the ventrolateral PAG. Of the neurones that were also tested with noxious visceral stimuli, 5% of the caudal and 47% of the rostral neurones responded to bladder distension, while 33 and 35%, respectively, of caudal and rostral neurones responded to splanchnic nerve stimulation. These results indicate that many mid-line medullary neurones receive inputs from both the ventrolateral PAG and visceral nociceptors and may, therefore, be part of the output pathway by which the ventrolateral PAG produces integrated physiological responses to noxious visceral stimuli.

Role of the Midbrain Periaqueductal Gray in Maternal Nurturance and Aggression:c-fosand Electrolytic Lesion Studies in Lactating Rats

the upright, crouched, or kyphotic, nursing posture of lactating rats is dependent on suckling stimulation from pups. Because of the neuroanatomical connections of the periaqueductal gray (PAG) and its sensorimotor integration of the analogous lordosis posture displayed by sexually receptive female rats, the possible role of the PAG in kyphosis was investigated using c-fos immunocytochemistry and electrolytic lesions. Lactating rats interacting with and nursing a litter of suckling pups showed greater Fos-immunoreactive nuclei in the lateral and ventrolateral caudal PAG (cPAGl,vl) compared with dams receiving nonsuckling somatosensory, distal, or no stimulation from pups. In contrast, this pattern was not evident in the rostral PAG, where the highest Fos levels occurred in nonsuckled dams, or in five other brainstem sites with either no group differences (peripenduncular, dorsal raphe, and pontine nuclei) or negligible Fos (ventral tegmental area, spinal trigeminal nuclei). After bilateral electrolytic lesions of the cPAGl,vl during gestation or on day 7 postpartum, active maternal behaviors, such as retrieval and licking of pups, and total nursing time were essentially normal. Kyphotic nursing, however, was reduced by 85%, nursing in prone and supine postures increased substantially, and 24 hr litter weight gains were reduced, particularly early in lactation (by 26%). Furthermore, lesioned rats attacked a strange male twice as often as controls did, which is suggestive of reduced fearfulness. These results extend the known roles of the PAG in reproductive and defensive behaviors to the postural control of suckling-induced kyphotic nursing and the modulation of maternal aggression.

Inhibitory effects evoked from both the lateral and ventrolateral periaqueductal grey are selective for the nociceptive responses of rat dorsal horn neurones

In rats anaesthetized with alphaxalone/alphadolone a comparative study was made of the inhibitory effects on dorsal horn neurones evoked by chemical stimulation at identified pressor and depressor sites in the lateral and ventrolateral periaqueductal grey (PAG). Stimulating micropipettes were inserted stereotaxically into the lateral or ventrolateral PAG at sites where microinjection of dl-homocysteic acid (DLH) evoked increases or decreases respectively in mean arterial blood pressure. The effects of DLH microinjection at these sites were tested against the responses of dorsal horn neurones to noxious and innocuous stimuli applied to their cutaneous receptive fields. Single unit extracellular recordings were made from 15 Class 1 (low-threshold) and 37 Class 2 (wide dynamic range) dorsal horn neurones in laminae II–VI of the lower lumbar spinal cord. The responses of Class 1 neurones to innocuous prodding of their receptive fields were unaffected by neuronal activation in either the lateral or ventrolateral PAG. The nociceptive (noxious pinch/heat) responses of most Class 2 neurones were strongly inhibited by chemical stimulation in either sector of the PAG. The low threshold (prod) responses of the same neurones were generally unaffected or only weakly inhibited by identical stimulation, regardless of stimulation site. No significant differences were found between the effects of lateral vs. ventrolateral PAG stimulation on the responses of dorsal horn neurones. These results do not support the view that dorsal horn neurones may be inhibited with different selectivities by hyper- and hypotensive regions of the PAG.

Conditioned fear to context is associated with increased Fos expression in the caudal ventrolateral region of the midbrain periaqueductal gray.

Immunohistochemical detection of Fos was used to determine which regions of the periaqueductal gray are activated during conditioned fear to a context in the rat. More specifically, the aim of the study was to test the role of its lateral and ventrolateral columns in freezing behaviour during fear. Conditioned fear was evoked by re-exposing rats to the same footshock chamber in which they had received footshocks 4 h earlier. Conditioned–Re-exposed rats were compared to Not Conditioned–Re-exposed and Conditioned–Not Re-exposed rats. Freezing was observed in the Conditioned–Re-exposed group only. It was associated with an overall increase in Fos expression in the entire periaqueductal gray that was significantly greater than in the two other groups. The largest and most significant increase in Fos immunoreactivity was found in the ventrolateral column (especially in its caudal part), whereas only a moderate increase was found in the lateral column. The present results argue in favour of the ventrolateral column as the region of the periaqueductal gray that is preferentially involved in expression of conditioned fear. As previous lesion studies suggested, the ventrolateral periaqueductal gray may play a role in mediating the immobility component of freezing induced by fear. Other lines of evidence suggest that it may also play a role in mediating the quiescence immobility associated with deep pain. We propose that the ventrolateral column of the periaqueductal gray acts as an integrating centre mediating behavioural inhibition.

Actions of the ORL1Receptor Ligand Nociceptin on Membrane Properties of Rat Periaqueductal Gray NeuronsIn Vitro

The actions of the endogenous ORL1-receptor ligand nociceptin on the membrane properties and synaptic currents in rat periaqueductal gray (PAG) neurons were examined by the use of whole-cell patch-clamp recording in brain slices. Nociceptin produced an outward current in all neurons tested, with an EC50 of 39 +/- 7 nM. The outward current was unaffected by naloxone. Outward currents reversed polarity at -110 +/- 3 mV in 2.5 mM extracellular potassium, and the reversal potential increased when the extracellular potassium concentration was raised (slope = 66.3 mV/log[K+]o mM). Thus, the nociceptin-induced outward current was attributable to an increased K+ conductance. Nociceptin inhibited evoked fast GABAergic (IP-SCs) and glutamatergic (EPSCs) postsynaptic currents and increased paired-pulse facilitation in a subpopulation of PAG neurons. Nociceptin inhibited evoked IPSCs and EPSCs in approximately 50% of neurons throughout the PAG, except in the ventrolateral PAG, where nociceptin inhibited evoked IPSCs in most neurons. Nociceptin decreased the frequency of spontaneous miniature postsynaptic currents (mIPSCs and mEPSCs) in a subpopulation of PAG neurons but had no effect on their amplitude distributions. Thus, nociceptin had a presynaptic inhibitory effect on transmitter release. These findings suggest that nociceptin, via its pre- and postsynaptic actions, has the potential to modulate the analgesic, behavioral, and autonomic functions of the PAG.

Opiate withdrawal increases ProTRH gene expression in the ventrolateral column of the midbrain periaqueductal gray.

The midbrain periaqueductal gray matter (PAG) has a critical role in the modulation of behavioral and autonomic manifestations of the opiate withdrawal syndrome. We report a nearly 5-fold increase in proTRH gene expression in neurons of the ventrolateral column of the PAG following naltrexone precipitated morphine withdrawal. The accumulation of immunoreactive proTRH-derived peptides, but not the mature TRH tripeptide was concomitantly observed in these cells. These findings indicate that proTRH-derived peptides synthesized in neurons of the ventrolateral PAG may function as modifiers of opiate withdrawal responses.

Opiate withdrawal increases ProTRH gene expression in the ventrolateral column of the midbrain periaqueductal gray

The midbrain periaqueductal gray matter (PAG) has a critical role in the modulation of behavioral and autonomic manifestations of the opiate withdrawal syndrome. We report a nearly 5-fold increase in proTRH gene expression in neurons of the ventrolateral column of the PAG following naltrexone precipitated morphine withdrawal. The accumulation of immunoreactive proTRH-derived peptides, but not the mature TRH tripeptide was concomitantly observed in these cells. These findings indicate that proTRH-derived peptides synthesized in neurons of the ventrolateral PAG may function as modifiers of opiate withdrawal responses.

Spinal cholinergic and monoamine receptors mediate the antinociceptive effect of morphine microinjected in the periaqueductal gray on the rat tail, but not the feet

The antinociceptive effects of morphine (5 μg) microinjected into the ventrolateral periaqueductal gray were determined using both the tail flick and the foot withdrawal responses to noxious radiant heating in lightly anesthetized rats. Intrathecal injection of appropriate antagonists was used to determine whether the antinociceptive effects of morphine were mediated by α 2-noradrenergic , serotonergic, opioid, or cholinergic muscarinic receptors. The increase in the foot withdrawal response latency produced by microinjection of morphine in the ventrolateral periaqueductal gray was reversed by intrathecal injection of the cholinergic muscarinic receptor antagonist atropine, but was not affected by the a2-adrenoceptor antagonist yohimbine, the serotonergic receptor antagonist methysergide, or the opioid receptor antagonist naloxone. In contrast, the increase in the tail flick response latency produced by morphine was reduced by either yohimbine, methysergide or atropine. These results indicate that microinjection of morphine in the ventrolateral periaqueductal gray inhibits nociceptive responses to noxious heating of the tail by activating descending neuronal systems that are different from those that inhibit the nociceptive responses to noxious heating of the feet. More specifically, serotonergic, muscarinic cholinergic and α 2-noradrenergic receptors appear to mediate the antinociception produced by morphine using the tail flick test. In contrast, muscarinic cholinergic, but not monoamine receptors appear to mediate the antinociceptive effects of morphine using the foot withdrawal response.