Contribution of brainstem GABAergic circuitry to descending antinociceptive controls: II. Electron microscopic immunocytochemical evidence of GABAergic control over the projection from the periaqueductal gray to the nucleus raphe magnus in the rat

The 'in's and out's' of descending pain modulation from the rostral ventromedial medulla

The descending-pain modulating circuit controls the experience of pain by modulating transmission of sensory signals through the dorsal horn. This circuit's key output node, the rostral ventromedial medulla (RVM), integrates 'top-down' and 'bottom-up' inputs that regulate functionally defined RVM cell types, 'OFF-cells' and 'ON-cells', which respectively suppress or facilitate pain-related sensory processing. While recent advances have sought molecular definition of RVM cell types, conflicting behavioral findings highlight challenges involved in aligning functional and molecularly defined types. This review summarizes current understanding, derived primarily from rodent studies but with corroborating evidence from human imaging, of the role of RVM populations in pain modulation and persistent pain states and explores recent advances outlining inputs to, and outputs from, RVM pain-modulating neurons.

Cell type-specific dissection of sensory pathways involved in descending modulation

Decades of research have suggested that stimulation of supraspinal structures, such as the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM), inhibits nocifensive responses to noxious stimulation through a process known as descending modulation. Electrical stimulation and pharmacologic manipulations of the PAG and RVM identified transmitters and neuronal firing patterns that represented distinct cell types. Advances in mouse genetics, in vivo imaging, and circuit tracing methods, in addition to chemogenetic and optogenetic approaches, allowed the characterization of the cells and circuits involved in descending modulation in further detail. Recent work has revealed the importance of PAG and RVM neuronal cell types in the descending modulation of pruriceptive as well as nociceptive behaviors, underscoring their roles in coordinating complex behavioral responses to sensory input. This review summarizes how new technical advances that enable cell type-specific manipulation and recording of neuronal activity have supported, as well as expanded, long-standing views on descending modulation.

Excitatory and inhibitory neuronal signaling in inflammatory and diabetic neuropathic pain

Pain, although unpleasant, is an essential warning mechanism against injury and damage of the organism. An intricate network of specialised sensors and transmission systems contributes to reception, transmission and central sensitization of pain. Here, we briefly introduce some of the main aspects of pain signal transmission, including nociceptors and nociceptive signals, mechanisms of inflammatory and neuropathic pain, and the situation of diabetes-associated neuropathic pain. The role of glia-astrocytes, microglia, satellite glia cells-and their specific channels, transporters and signaling pathways is described. A focus is on the contribution of inhibitory synaptic signaling to nociception and a possible role of glycine receptors in glucose-mediated analgesia and treatment-induced diabetic neuropathy. Inhibitory receptors such as GABA_A- and glycine receptors are important contributors to nociceptive signaling; their contribution to altered pain sensation in diabetes may be of clinical relevance, and they could be promising therapeutic targets towards the development of novel analgesics.

Cellular and circuit diversity determines the impact of endogenous opioids in the descending pain modulatory pathway

The descending pain modulatory pathway exerts important bidirectional control of nociceptive inputs to dampen and/or facilitate the perception of pain. The ventrolateral periaqueductal gray (vlPAG) integrates inputs from many regions associated with the processing of nociceptive, cognitive, and affective components of pain perception, and is a key brain area for opioid action. Opioid receptors are expressed on a subset of vlPAG neurons, as well as on both GABAergic and glutamatergic presynaptic terminals that impinge on vlPAG neurons. Microinjection of opioids into the vlPAG produces analgesia and microinjection of the opioid receptor antagonist naloxone blocks stimulation-mediated analgesia, highlighting the role of endogenous opioid release within this region in the modulation of nociception. Endogenous opioid effects within the vlPAG are complex and likely dependent on specific neuronal circuits activated by acute and chronic pain stimuli. This review is focused on the cellular heterogeneity within vlPAG circuits and highlights gaps in our understanding of endogenous opioid regulation of the descending pain modulatory circuits.

D2 Receptors in the Periaqueductal Gray/Dorsal Raphe Modulate Peripheral Inflammatory Hyperalgesia via the Rostral Ventral Medulla

Dopamine neurons in the periaqueductal gray (PAG)/dorsal raphe are key modulators of antinociception with known supraspinal targets. However, no study has directly tested whether these neurons contribute to descending pain inhibition. We hypothesized that PAG dopamine neurons contribute to the analgesic effect of D-amphetamine via a mechanism that involves descending modulation via the rostral ventral medulla (RVM). Male C57BL/6 mice showed increased c-FOS expression in PAG dopamine neurons and a significant increase in paw withdrawal latency to thermal stimulation after receiving a systemic injection of D-amphetamine. Targeted microinfusion of D-amphetamine, L-DOPA, or the selective D2 agonist quinpirole into the PAG produced analgesia, while a D1 agonist, chloro APB, had no effect. In addition, inhibition of D2 receptors in the PAG by eticlopride prevented the systemic D-amphetamine analgesic effect. D-amphetamine and PAG D2 receptor-mediated analgesia were inhibited by intra-RVM injection of lidocaine or the GABA_A receptor agonist muscimol, indicating a PAG-RVM signaling pathway in this model of analgesia. Finally, both systemic D-amphetamine and local PAG microinjection of quinpirole, inhibited inflammatory hyperalgesia induced by carrageenan. This hyperalgesia was transiently restored by intra-PAG injection of eticlopride, as well as RVM microinjection of muscimol. We conclude that D-amphetamine analgesia is partially mediated by descending inhibition and that D2 receptors in the PAG are responsible for this effect via modulating neurons that project to the RVM. These results further our understanding of the antinociceptive effects of dopamine and elucidate a mechanism by which clinically available dopamine modulators produce analgesia.

Opioid presynaptic disinhibition of the midbrain periaqueductal grey descending analgesic pathway

BACKGROUND AND PURPOSE

The midbrain periaqueductal grey (PAG) plays a central role in modulating pain through a descending pathway that projects indirectly to the spinal cord via the rostroventral medial medulla (RVM). While opioids are potent analgesics that target the PAG, their cellular actions on descending projection neurons are unclear.

EXPERIMENTAL APPROACH

Patch clamp recordings in voltage- and current-clamp mode were made from acutely prepared PAG slices from animals that received retrograde tracer injections into the RVM.

KEY RESULTS

The μ-agonist DAMGO reduced GABAergic evoked inhibitory postsynaptic currents (IPSCs) in retro-labelled, RVM-projecting neurons to a greater extent than in unlabelled neurons. The κ-opioid agonist U69593 reduced evoked IPSCs to a similar extent in both neuronal groups, while the δ-opioid agonist deltorphin-II was without effect. DAMGO and U69593 both produced a reduction in the rate, but not amplitude of spontaneous miniature IPSCs and asynchronous evoked IPSCs in retro-labelled neurons. DAMGO and U69593 also suppressed glutamatergic EPSCs in retro-labelled and unlabelled neurons. The DAMGO inhibition of evoked EPSCs, however, was less than that for evoked IPSCs in retro-labelled, but not unlabelled neurons. In current clamp, DAMGO produced a depolarizing increase in evoked postsynaptic potentials in retro-labelled neurons, but directly inhibited unlabelled neurons.

CONCLUSION AND IMPLICATIONS

These findings suggest that μ-opioids activate the descending analgesic pathway from the midbrain PAG by a combination of presynaptic disinhibition of RVM-projecting neurons and postsynaptic inhibition of presumptive interneurons.

The Role of Glutamatergic and Dopaminergic Neurons in the Periaqueductal Gray/Dorsal Raphe: Separating Analgesia and Anxiety

The periaqueductal gray (PAG) is a significant modulator of both analgesic and fear behaviors in both humans and rodents, but the underlying circuitry responsible for these two phenotypes is incompletely understood. Importantly, it is not known if there is a way to produce analgesia without anxiety by targeting the PAG, as modulation of glutamate or GABA neurons in this area initiates both antinociceptive and anxiogenic behavior. While dopamine (DA) neurons in the ventrolateral PAG (vlPAG)/dorsal raphe display a supraspinal antinociceptive effect, their influence on anxiety and fear are unknown. Using DAT-cre and Vglut2-cre male mice, we introduced designer receptors exclusively activated by designer drugs (DREADD) to DA and glutamate neurons within the vlPAG using viral-mediated delivery and found that levels of analgesia were significant and quantitatively similar when DA and glutamate neurons were selectively stimulated. Activation of glutamatergic neurons, however, reliably produced higher indices of anxiety, with increased freezing time and more time spent in the safety of a dark enclosure. In contrast, animals in which PAG/dorsal raphe DA neurons were stimulated failed to show fear behaviors. DA-mediated antinociception was inhibitable by haloperidol and was sufficient to prevent persistent inflammatory pain induced by carrageenan. In summary, only activation of DA neurons in the PAG/dorsal raphe produced profound analgesia without signs of anxiety, indicating that PAG/dorsal raphe DA neurons are an important target involved in analgesia that may lead to new treatments for pain.

Divergent Modulation of Nociception by Glutamatergic and GABAergic Neuronal Subpopulations in the Periaqueductal Gray

The ventrolateral periaqueductal gray (vlPAG) constitutes a major descending pain modulatory system and is a crucial site for opioid-induced analgesia. A number of previous studies have demonstrated that glutamate and GABA play critical opposing roles in nociceptive processing in the vlPAG. It has been suggested that glutamatergic neurotransmission exerts antinociceptive effects, whereas GABAergic neurotransmission exert pronociceptive effects on pain transmission, through descending pathways. The inability to exclusively manipulate subpopulations of neurons in the PAG has prevented direct testing of this hypothesis. Here, we demonstrate the different contributions of genetically defined glutamatergic and GABAergic vlPAG neurons in nociceptive processing by employing cell type-specific chemogenetic approaches in mice. Global chemogenetic manipulation of vlPAG neuronal activity suggests that vlPAG neural circuits exert tonic suppression of nociception, consistent with previous pharmacological and electrophysiological studies. However, selective modulation of GABAergic or glutamatergic neurons demonstrates an inverse regulation of nociceptive behaviors by these cell populations. Selective chemogenetic activation of glutamatergic neurons, or inhibition of GABAergic neurons, in vlPAG suppresses nociception. In contrast, inhibition of glutamatergic neurons, or activation of GABAergic neurons, in vlPAG facilitates nociception. Our findings provide direct experimental support for a model in which excitatory and inhibitory neurons in the PAG bidirectionally modulate nociception.

Hyperactivity and impaired attention in Gamma aminobutyric acid transporter subtype 1 gene knockout mice

OBJECTIVES

Attention-deficit hyperactivity disorder (ADHD) is a common neurobehavioural disorder. It is conceivable that Gamma aminobutyric acid (GABA) neurotransmission is implicated in the pathophysiology of ADHD. This study investigated the effect of GABA transporter 1 (GAT-1) on the anxiety-like behaviours and cognitive function in knockout mice.

METHODS

In all, 20 adult male mice were divided into two groups: wild-type (WT) group and GAT-1-/- group. The open field test, elevated O-maze (EZM) and Morris water maze were used to evaluate behavioural traits relevant to ADHD.

RESULTS

Compared with WT mice, the GAT-1-/- mice travelled longer and displayed an enhanced kinematic velocity with the significant reduction of rest time in the open field test (p<0.05). The EZM showed that GAT-1-/- mice displayed a significant increase in total entries into the open sectors and the closed sectors compared with the WT mice. The WT mice showed shorter latencies after the training session (p<0.01), whereas the GAT-1-/- mice made no difference during probe test, the GAT-1-/- mice spent less time in the target quadrant (p<0.01).

CONCLUSION

Our results demonstrated that GAT-1-/- mice have phenotypes of hyperactivity, impaired sustained attention and learning deficiency, and the performance of GAT-1-/- mice is similar to ADHD symptoms. So, the study of the GAT-1-/- mice may provide new insights into the mechanisms and the discovery of novel therapeutics for the treatment of ADHD.

Mechanisms of electroacupuncture-induced analgesia on neuropathic pain in animal model

Neuropathic pain remains as one of the most difficult clinical pain syndromes to treat. Electroacupuncture (EA), involving endogenous opioids and neurotransmitters in the central nervous system (CNS), is reported to be clinically efficacious in various fields of pain. Although multiple experimental articles were conducted to assess the effect of EA-induced analgesia, no review has been published to assess the efficacy and clarify the mechanism of EA on neuropathic pain. To this aim, this study was firstly designed to evaluate the EA-induced analgesic effect on neuropathic pain and secondly to guide and help future efforts to advance the neuropathic pain treatment. For this purpose, articles referring to the analgesic effect of acupuncture on neuropathic pain and particularly the work performed in our own laboratory were analyzed. Based on the articles reviewed, the role of spinal opioidergic, adrenergic, serotonergic, cholinergic, and GABAergic receptors in the mechanism of EA-induced analgesia was studied. The results of this research demonstrate that μ and δ opioid receptors, α₂-adrenoreceptors, 5-HT_1A and 5-HT₃ serotonergic receptors, M₁ muscarinic receptors, and GABA_A and GABA_B GABAergic receptors are involved in the mechanisms of EA-induced analgesia on neuropathic pain.

Analgesia induced by localized injection of opiate peptides into the brain of infant rats

BACKGROUND

Stimulation of a variety of brain sites electrically or by opiates activates descending inhibitory pathways to attenuate noxious input to the spinal cord dorsal horn and produce analgesia. Analgesia induced by electrical stimulation of the periaqueductal grey (PAG) of the midbrain or medial rostral ventral medulla (RVM) matures late, towards the end or past the pre-weaning period. Descending facilitation takes precedence over inhibition. Yet opiates injected intracerebroventricularly or directly into the PAG induce analgesia relatively early in development. Our goal was to re-examine the role of opiates specific to individual receptor types in analgesia at several supraspinal sites.

METHODS

Antinociception was tested following microinjection of DAMGO (μ-opiate agonist), DPDPE (∂-opiate agonist) or U50,488 (κ-opiate agonist) into the PAG, RVM or dorsal lateral pons (DLP) in 3-, 10- and 14-day-old rats.

RESULTS

DAMGO produced analgesia at 3 days of age at each brain area; the RVM was the most effective and the dorsal PAG was the least effective site. DPDPE produced modest analgesia at 10 and 14 days of age at the ventral PAG, RVM or DLP, but not the dorsal PAG. U50,488H was ineffective at all sites and all ages.

CONCLUSIONS

Antinociception could be elicited at all three sites by DAMGO as early as 3 days of age and DPDPE at 10 and 14 days of age. The degree of analgesia increased gradually during the first 2 weeks of life, and likely reflects the maturation of connections within the brain and of descending inhibitory paths from these sites.

Capsaicin in the periaqueductal gray induces analgesia via metabotropic glutamate receptor-mediated endocannabinoid retrograde disinhibition

BACKGROUND AND PURPOSE

Capsaicin, an agonist of transient receptor potential vanilloid 1 (TRPV1) channels, is pro-nociceptive in the periphery but is anti-nociceptive when administered into the ventrolateral periaqueductal gray (vlPAG), a midbrain region for initiating descending pain inhibition. Here, we investigated how activation of TRPV1 channels in the vlPAG leads to anti-nociception.

EXPERIMENTAL APPROACH

We examined synaptic transmission and neuronal activity using whole-cell recordings in vlPAG slices in vitro and hot-plate nociceptive responses in rats after drug microinjection into the vlPAG in vivo.

KEY RESULTS

Capsaicin (1-10 µM) depressed evoked GABAergic inhibitory postsynaptic currents (eIPSCs) in vlPAG slices presynaptically, while increasing miniature excitatory PSC frequency. Capsaicin-induced eIPSC depression was antagonized by cannabinoid CB₁ and metabotropic glutamate (mGlu₅) receptor antagonists, and prevented by inhibiting diacylglycerol lipase (DAGL), which converts DAG into 2-arachidonoylglycerol (2-AG), an endocannabinoid. Capsaicin induced membrane depolarization in 2/3 neurons recorded but, overall, increased neuronal firings by increasing evoked postsynaptic potentials. Intra-vlPAG capsaicin reduced hot-plate responses in rats, effects blocked by CB₁ and mGlu receptor antagonists. Effects of capsaicin were antagonized by SB 366791, a TRPV1 channel antagonist.

CONCLUSIONS AND IMPLICATIONS

Capsaicin activated TRPV1s on glutamatergic terminals to release glutamate which activated postsynaptic mGlu₅ receptors, yielding 2-AG from DAG by DAGL hydrolysis. 2-AG induces retrograde inhibition (disinhibition) of GABA release via presynaptic CB₁ receptors. This disinhibition in the vlPAG leads to anti-nociception by activating the descending pain inhibitory pathway. This is a novel TRPV1 channel-mediated anti-nociceptive mechanism in the brain and a new interaction between vanilloid and endocannabinoid systems.

Local GABAergic modulation of the activity of serotoninergic neurons in the nucleus raphe magnus

Experiments on rat brainstem sections in membrane potential clamping conditions addressed the effects of serotonin and GABA on serotoninergic neurons in the nucleus raphe magnus. Local application of serotonin stimulated inhibitory postsynaptic currents (IPSC) in 45% of the serotoninergic neurons studied. This response was not seen in the presence of the fast sodium channel blocker tetrodotoxin. The GABAA receptor antagonist gabazine blocked IPSC in both serotonin-sensitive and serotonin-insensitive neurons. Application of GABA evoked generation of a membrane current (IGABA), which was completely blocked by gabazine. These results indicate self-regulation of the activity of serotoninergic neurons in the nucleus raphe magnus via a negative feedback circuit involving local GABAergic interneurons.

GABAA receptor signaling in caudal periaqueductal gray regulates maternal aggression and maternal care in mice

Maternal aggression (maternal defense) is exhibited by lactating females towards intruders and contributes to the protection of offspring. Enhancement of Gamma-Aminobutyric acid (GABA)(A) receptor signaling by benzodiazepines elevates maternal aggression, and we previously found indirect evidence (via c-Fos immunohistochemistry) that caudal periaqueductal gray (cPAG) and lateral septum (LS) could be sites where benzodiazepines increase aggression. We recently found that GABA(A) receptor signaling in LS modulates maternal aggression, and in this study, we tested the hypothesis that GABA(A) receptor signaling in cPAG also regulates this behavior. Site-directed injections to cPAG were made in lactating mice using the GABA(A) receptor antagonist, bicuculline (3-9 ng) or the GABA(A) receptor positive modulator, chlordiazepoxide (CDP), a benzodiazepine (2.5-20 microg). Maternal aggression, other maternal behaviors, and anxiety-like measures (using the light-dark box) were then examined. GABA(A) receptor positive modulator did not increase aggression, which could have resulted from a ceiling effect. However, 8 ng and 9 ng of bicuculline in cPAG significantly decreased maternal aggression without altering other maternal behaviors or light-dark box performance, suggesting some GABA(A) receptor signaling in cPAG is required for full maternal aggression expression. Additionally, 7 ng of bicuculline significantly increased licking/grooming of pups, and decreased the number of transitions between the light and dark compartments of the light-dark box without affecting aggression. Given these results indicating that antagonizing GABA(A) receptor in cPAG dose-dependently promotes offspring grooming behavior while impairing aggression, it is possible that the cPAG represents a key site for decision making (aggression versus other behaviors) in the lactating female.

Contribution of dopamine receptors to periaqueductal gray-mediated antinociception

RATIONALE

Morphine relieves pain, in part, by acting on neurons within the periaqueductal gray (PAG). Given that the PAG contains a subpopulation of dopamine neurons, dopamine may contribute to the antinociceptive effects mediated by the PAG.

METHODS

This hypothesis was tested by measuring the behavioral and electrophysiological effects of administering dopamine agonists and antagonists into the ventrolateral PAG (vPAG). An initial histological experiment verified the existence of dopamine neurons within the vPAG using dopamine transporter and tyrosine hydroxylase antibodies visualized with confocal microscopy.

RESULTS

Microinjection of cumulative doses of morphine into the vPAG caused antinociception that was dose-dependently inhibited by the dopamine receptor antagonist alpha-flupenthixol. alpha-Flupenthixol had no effect on nociception when administered alone. Injection of the dopamine receptor agonist (-) apomorphine into the vPAG caused a robust antinociception that was inhibited by the D2 antagonist eticlopride but not the D1 antagonist SCH-23390. The effects of dopamine on GABA(A)-mediated evoked inhibitory post-synaptic potentials (eIPSCs) were measured in PAG slices. Administration of met-enkephalin inhibited peak eIPSCs by 20-50%. Dopamine inhibited eIPSCs by approximately 20-25%. Administration of alpha-flupenthixol (20 muM) attenuated eIPSC inhibition by dopamine but had no effect on met-enkephalin-induced inhibition.

CONCLUSIONS

These data indicate that PAG dopamine has a direct antinociceptive effect in addition to modulating the antinociceptive effect of morphine. The lack of an effect of alpha-flupenthixol on opioid-inhibition of eIPSCs indicates that this modulation occurs in parallel or subsequent to inhibition of GABA release.

A comparative study of excitatory and inhibitory amino acids in three different brainstem nuclei

This study was designed to shed more light onto the three different brainstem regions which are implicated in the pain pathway for the level of various excitatory and inhibitory neurotransmitters before and following neuronal stimulation. The in vivo microdialysis technique was used in awake, freely moving adult Sprague-Dawley rats. The neurotransmitters studied included aspartate, glutamate, GABA, glycine, and taurine. The three brainstem regions examined included the mid-brain periaqueductal gray (PAG), the medullary nucleus raphe magnus (NRM), and the spinal trigeminal nucleus (STN). Neuronal stimulation was achieved following the administration of the sodium channel activator veratridine. The highest baseline levels of glutamate (P < 0.0001), aspartate (P < 0.0001), GABA (P < 0.01), taurine (P < 0.0001), and glycine (P < 0.001) were seen in the NRM. On the other hand, the lowest baseline levels of glutamate, GABA, glycine, and taurine were found in the PAG, while that of aspartate was found in the STN. Following the administration of veratridine, the highest release of the above neurotransmitters except for the aspartate and glycine was found in the PAG where the level of glutamate increased by 1,310 +/- 293% (P < 0.001), taurine by 1,008 +/- 143% (P < 0.01), and GABA by 10,358 +/- 1,920% (P < 0.0001) when comparison was performed among the three brainstem regions and in relation to the baseline levels. The highest release of aspartate was seen in the STN (2,357 +/- 1,060%, P < 0.001), while no significant difference was associated with glycine. On the other hand, the lowest release of GABA and taurine was found in the STN (696 +/- 91 and 305 +/- 25%, respectively), and glutamate and aspartate in the NRM (558 +/- 200 and 874 +/- 315%, respectively). Our results indicate, and for the first time, that although some differences are seen in the baseline levels of the above neurotransmitters in the three regions studied, there are quite striking variations in the level of release of these neurotransmitters following neuronal stimulation in these regions. In our opinion this is the first study to describe the pain activation/modulation related changes of the excitatory and inhibitory amino acids profile of the three different brainstem areas.

c-Fos expression in the periaqueductal gray is induced by electroacupuncture in the rat, with possible reference to GABAergic neurons

Electroacupuncture (EA) delivered to the acupoint (AP) called Zusanli (ST36) was administered on the bilateral hindlimb. This experiment resulted in strong expression of c-Fos immunoreactivity in the ventrolateral to lateral subdivision throughout the periaqueductal gray (PAG) compared to the non-AP and sham cases. On the other hand, it was of particular interest in the experiment of the AP that strong expression of gamma aminobutylic acid (GABA) frequently showed similar pattern of distribution to that of c-Fos in the PAG. This overlapped pattern of distribution, demonstrated in the present study, suggests that the PAG neurons activated by EA at the AP might play an important role in the descending pain control system involving the GABA since the PAG has special reference to the dorsal horn of the spinal cord and function of pain control.

Cholinergic modulation of periaqueductal grey neurons: Does it contribute to epileptogenesis after organophosphorus nerve agent intoxication?

Previous work has shown that a single focal microinjection of the unselective cholinergic agonist, carbachol, into the periaqueductal grey (PAG) of the midbrain is sufficient to induce forebrain seizures in rats. In order to determine the cholinergic mechanisms underlying epileptogenesis at the cellular and network level of the PAG, we performed whole-cell recordings from rat PAG neurons in vitro and examined how the activation of muscarinic and nicotinic receptors modulates cellular excitability and synaptic responses. Stimulation of muscarinic receptors produced either a pirenzepine-sensitive depolarization (40% of PAG neurons), or a gallamine-sensitive hyperpolarization (20%), suggesting the involvement of M1 and M2 receptors, respectively. In the remaining neurons (40%), no change was observed. Voltage-clamp recordings showed that muscarinic depolarization resulted from the inhibition of a resting K(+) current, in part accompanied by simultaneous activation of a presumed non-selective cation current. Muscarinic hyperpolarization was caused by the activation of a G protein-coupled, inwardly rectifying K(+) current. Stimulation of muscarinic receptors enhanced the frequency of spontaneous inhibitory postsynaptic currents (IPSCs), but strongly suppressed evoked IPSCs. In addition, nicotine almost doubled the frequency of miniature IPSCs. Based on our findings and the network properties of the PAG, we advance a scenario in which excessive stimulation of cholinergic receptors would substantially contribute to generalized seizures after organophosphorus nerve agent poisoning.

Opiate anti-nociception is attenuated following lesion of large dopamine neurons of the periaqueductal grey: critical role for D1 (not D2) dopamine receptors

The periaqueductal grey (PAG) area is involved in pain modulation as well as in opiate-induced anti-nociceptive effects. The PAG possess dopamine neurons, and it is likely that this dopaminergic network participates in anti-nociception. The objective was to further study the morphology of the PAG dopaminergic network, along with its role in nociception and opiate-induced analgesia in rats, following either dopamine depletion with the toxin 6-hydroxydopamine or local injection of dopaminergic antagonists. Nociceptive responses were studied through the tail-immersion (spinal reflex) and the hot-plate tests (integrated supraspinal response), establishing a cut-off time to further minimize animal suffering. Heroin and morphine were employed as opiates. Histological data indicated that the dopaminergic network of the PAG is composed of two types of neurons: small rounded cells, and large multipolar neurons. Following dopamine depletion of the PAG, large neurons (not small ones) were selectively affected by the toxin (61.9% dopamine cell loss, 80.7% reduction of in vitro dopaminergic peak), and opiate-induced analgesia in the hot-plate test (not the tail-immersion test) was reliably attenuated in lesioned rats (P < 0.01). After infusions of dopaminergic ligands into the PAG, D(1) (not D(2)) receptor antagonism attenuated opiate-induced analgesia in a dose-dependent manner in the hot-plate test. The present study provides evidence that large neurons of the dopaminergic network of the PAG participate in supraspinal (not spinal) nociceptive responses after opiates through the involvement of D(1) dopamine receptors. This dopaminergic system should be included as another network within the PAG involved in opiate-induced anti-nociception.

GABAergic projection from the ventral tegmental area and substantia nigra to the periaqueductal gray region and the dorsal raphe nucleus

Previous studies have shown that neurons in the ventral tegmental area (VTA) and substantia nigra (SN) project to the ventrolateral periaqueductal gray (PAGvl) and dorsal raphe nucleus (DR). Research has also shown that stimulation of neurons in the VTA/SN elicits cardiovascular depressor responses that are mediated by a projection to the PAGvl/DR. Anatomic and physiological experiments were done in the present study to determine the neurochemical identity of the VTA/SN projection to the PAGvl/DR. Experiments were done to characterize the origin and chemical nature of this projection by combining cholera toxin B tracing with immunofluorescence for the 67K isoform of glutamic acid decarboxylase (GAD) and tyrosine hydroxylase. The PAGvl/DR region was found to receive a substantial input from neurons in the VTA, SN, and deep mesencephalic nucleus. The DR was preferentially innervated by neurons in the VTA, whereas the PAGvl was preferentially innervated by neurons in the SN. A proportion of neurons in the VTA and the reticular portion of the SN found to project to the PAGvl/DR were GAD positive. In addition, experiments were done in urethane-anesthetized rats to determine whether injections of a gamma-aminobutyric acid (GABA) antagonist in the region of the PAGvl/DR attenuated the cardiovascular depressor responses produced by glutamate stimulation of the VTA/SN. Injections of the GABA-blocking agent picrotoxin (2.5 nmol, 500 nl) into the PAGvl/DR eliminated the cardiovascular responses from stimulation of the VTA/SN (0.01 M, 50 nl). The results of the present investigation provide evidence for a GABAergic projection from the VTA/SN to the PAGvl/DR. This projection may be an important regulator of the PAGvl/DR, an area of the midbrain involved in the production of behavioral and physiological responses to pain and stress.

Neurons with spontaneous high-frequency discharges in the central nervous system and chronic pain

Neurons with high-frequency discharge (hyperactive neuron) were recorded in the thalamus of chronic pain patients. We created a chronic pain rat model, and recorded the discharge of high-frequency neurons by using a microelectrode in the thalamic area. The character of these high-frequency discharge neurons was assessed. Deep brain stimulation (DBS) was done in 9 patients, and motor cortical stimulation (MCS) in 2. Prior to implantation of the stimulation electrodes, extracellular unitary activity was recorded and local microstimulation was done by the same microelectrode. From the ventrocaudal (Vc) nucleus of the chronic pain patients, hyperactive neurons were recorded. There were three types of discharge patterns in interval histograms. Some hyperactive neurons showed firing suppression for a short time period by intravenous administration of phenytoin and calcium antagonist (Nicardipine). In animal experiments, unilateral dorsal root sectioning from C5 to Th1 were made in male Wistar rats according to the method of Lombard et al. (1979). A few months after the operation, hyperactive neurons were recorded from the thalamic nuclei and lemniscus medialis. The firing patterns and distribution of hyperactive neurons were very similar to those of humans. Sensorimotor cortical electrical stimulation showed a reduction of firing in the hyperactive neurons. Iontophoretical application of glutamate yielded an increase in firing. In contrast, GABA and NMDA antagonist MK-801 revealed remarkable firing suppression. These results suggest that hyperactive neurons may correlate with the glutamatergic, especially NMDA and GABAergic receptor or fibers.

Behavioral evidence linking opioid-sensitive GABAergic neurons in the ventrolateral periaqueductal gray to morphine tolerance

Tolerance develops to the antinociceptive effects of morphine with repeated microinjections into the ventrolateral periaqueductal gray (PAG). This tolerance could be caused by adaptations within the PAG or anywhere along the descending pathway (rostral ventromedial medulla to spinal cord). If tolerance is caused by a change along the descending pathway, then tolerance should develop to direct activation of PAG output neurons. However, if tolerance is caused by a change to neurons within the PAG, then tolerance will not occur with repeated direct activation of PAG output neurons. This hypothesis was tested by assessing antinociception following repeated microinjections of the GABA antagonist bicuculline and the excitatory amino acid kainate into the ventrolateral PAG. Microinjection of bicuculline and kainate produces antinociception by disinhibition and direct excitation of ventrolateral PAG output neurons, respectively. Repeated administration of these drugs into the ventrolateral PAG produced antinociception with no evidence of tolerance. That is, the hot-plate latency and responsiveness to intraplantar formalin administration was comparable whether rats received the drug for the first or fifth time. Moreover, microinjection of bicuculline or kainate produced comparable antinociception in rats pretreated with these drugs and saline-treated control rats. These data demonstrate that repeated activation of ventrolateral PAG output neurons is not sufficient to produce tolerance. Thus, tolerance must be caused by a change in neurons preceding output neurons in this circuit, presumably opioid-sensitive GABAergic neurons.

Mu- and delta-opioid receptor mRNAs are expressed in periaqueductal gray neurons projecting to the rostral ventromedial medulla

Opioid antinociception appears to be mediated at least in part by a pathway that projects from the periaqueductal gray (PAG) to the rostral ventromedial medulla (RVM), but the relationship between opioid receptors and PAG-RVM projection neurons is unclear. Previous electrophysiological studies have suggested that opioids act directly on some PAG neurons projecting to the RVM. However, immunoreactivity for neither the cloned mu-opioid receptor (MOR1) nor the cloned delta-opioid receptor (DOR1) has been observed in PAG cells retrogradely labeled from the RVM. In the present study, we examined the expression of DOR1 and MOR1 mRNAs in PAG neurons projecting to RVM using quantitative in situ hybridization and retrograde tract-tracing. Mesencephalic neurons were labeled in three male Sprague-Dawley rats by microinjection of Fluoro-Gold into the RVM. Five micrometer cryostat sections were cut and in situ hybridization was performed using full-length cRNA probes labeled with 35S-UTP. Retrogradely labeled neurons that were also labeled for MOR1 or DOR1 mRNA were observed in the dorsomedial, lateral, and ventrolateral portions of the PAG. Quantification was performed in the dorsomedial and ventrolateral PAG using the physical disector. We found that of 219 retrogradely labeled neurons, 50 +/- 14% expressed DOR1 mRNA. In a second set of 120 Fluoro-Gold-labeled neurons, 27 +/- 8% expressed MOR1 mRNA. Significantly more PAG-RVM projection neurons were labeled for MOR1 mRNA in the ventrolateral subregion of the PAG than in the dorsomedial subregion. However, no significant difference was observed in the proportions of retrogradely labeled neurons labeled for DOR1 mRNA in the ventrolateral subregion compared to the dorsomedial subregion. We conclude that opioids are likely to exert direct effects on PAG-RVM projection neurons through both delta- and mu-opioid receptors. In addition, direct effects on PAG-RVM projection neurons from activation of MOR1 appear more likely to be exerted in the ventrolateral PAG than in the dorsomedial PAG.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

The projections of the midbrain periaqueductal grey to the pons and medulla oblongata in rats

It is now established that stimulation of the ventrolateral midbrain periaqueductal grey (PAG) evokes inhibition of nociceptive spinal neurons, which results in analgesia and a powerful attenuation of pain behaviour. It is postulated that the PAG exerts this inhibitory effect on spinal nociceptive functions through the activation of descending serotonergic and noradrenergic pathways that arise from the rostral ventromedial medulla (RVM) and pontine noradrenergic nuclei. To investigate the neuroanatomical substrate of this functional link between the PAG and RVM, as well as the pontine noradrenergic nuclei in the rat, we labelled axons that project from the ventrolateral PAG to various regions of the pons and medulla oblongata using the anterograde tracing substance, Phaseolus vulgaris leucoagglutinin. We demonstrated that some of PAG efferents really do terminate in the RVM and pontine noradrenergic nuclei, but a substantial proportion of them project to the intermediate subdivision of the pontobulbar reticular formation. Combining the axonal tracing with serotonin- and tyrosine-hydroxylase-immunohistochemistry, we also found that, in contrast to previous results, PAG efferents make relatively few appositions with serotonin- and tyrosine-hydroxylase-immunoreactive neurons in the RVM and pontine noradrenergic nuclei; most of them terminate in nonimmunoreactive territories. The results suggest that the ventrolateral PAG may activate a complex pontobulbar neuronal assembly including neurons in the intermediate subdivision of the pontobulbar reticular formation, serotonin- and tyrosine-hydroxylase-immunoreactive and nonimmunoreactive neurons in the RVM and pontine noradrenergic nuclei. This pontobulbar neural circuitry, then, may mediate the PAG-evoked activities towards the spinal dorsal horn resulting in the inhibition of spinal nociceptive functions.

Characterization of synaptic transmission in the ventrolateral periaqueductal gray of rat brain slices

Synaptic transmission evoked by focal stimulation in the ventrolateral periaqueductal gray was characterized using the whole-cell recording technique in rat brain slices. At resting membrane potential (-62+/-1 mV), focal stimulation (0.05-0.1 ms, 0.03 Hz) usually evoked a 6-cyano-7-nitroquinoxaline-2, 3-dione-sensitive fast excitatory postsynaptic potential and a DL-2-amino-5-phosphonopentanoic acid-sensitive slow excitatory postsynaptic potential with a bicuculline-sensitive inhibitory postsynaptic potential in between. In the presence of kynurenic acid, bicuculline-sensitive inhibitory postsynaptic currents recorded in the voltage-clamp mode displayed a reversal potential of -68+/-3 mV, resembling GABA(A) receptor-mediated inhibitory postsynaptic currents. However, no GABA(B) receptor-mediated inhibitory postsynaptic current was evoked, even at stronger stimulating intensity. 6-Cyano-7-nitroquinoxaline-2,3-dione-sensitive fast excitatory postsynaptic currents were isolated by DL-2-amino-5-phosphonopentanoic acid plus bicuculline and DL-2-amino-5-phosphonopentanoic acid-sensitive slow fast excitatory postsynaptic currents by bicuculline plus 6-cyano-7-nitroquinoxaline-2,3-dione. Both types of excitatory postsynaptic current reversed at potentials near 0 mV. The I-V curve of slow fast excitatory postsynaptic currents or N-methyl-D-aspartate currents displayed a negative slope at potentials more negative than -30 mV in an Mg(2+)-sensitive manner. The control postsynaptic currents reversed at potentials between -50 and -35 mV, inclined to the reversal potential of GABA(A), but not glutamate, receptor channels. It is concluded that, in the ventrolateral periaqueductal gray, focal stimulation elicits both inhibitory and excitatory transmission, while the former is dominant. The inhibitory transmission is mediated by GABA(A) but not GABA(B) receptors. The excitatory transmission is mediated by glutamate acting on alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate as well as N-methyl-D-aspartate receptors.

Mechanism underlying increased neuronal activity in the rat ventrolateral periaqueductal grey by a mu-opioid

1. The overall effect of the mu-opioid receptor agonist DAMGO (Tyr-D-Ala-Gly-MePhe-Gly-ol) on ventrolateral periaqueductal grey (PAG) neurons in brain slices was studied using the whole-cell patch-clamp recording technique. 2. Under current-clamp conditions, DAMGO (1 microM) increased cell firing in many PAG neurons even though the opioid induced hyperpolarization and inhibited excitatory postsynaptic potentials (EPSPs) in these cells. 3. The increase in cell activity by DAMGO was observed in both transverse and horizontal slices. The increase persisted when the membrane potential was re-depolarized to the control level. Thus, different planes of sections or the removal of Na+ channel inactivation could not account for the observation. 4. The GABA antagonist bicuculline caused cell firing, mimicking the excitatory effect of DAMGO. Unlike DAMGO, however, bicuculline depolarized PAG cells. 5. Under voltage-clamp conditions, with the same driving force, the evoked inhibitory postsynaptic currents (IPSCs) in these neurons were 2.3 times larger than the evoked excitatory postsynaptic currents (EPSCs). Furthermore, DAMGO inhibited IPSCs by 60.7% while it inhibited EPSCs by 35.3%. 6. We propose that the overall effect of an opioid depends on the dynamic balance of its excitatory and inhibitory actions. In the PAG, the blockade of the inhibitory drive of GABAergic inputs by DAMGO is large. It overcomes the DAMGO-induced hyperpolarization and inhibition of EPSCs and thus results in the excitation of these neurons.

Peripheral inflammation is associated with decreased veratridine-induced release of GABA in the rat ventrocaudal periaqueductal gray: microdialysis study

Systemic administration of opiates or direct injection of opioid peptides into the periaqueductal gray (PAG) produces a profound antinociception which is thought to be associated with inhibition of neuronal activity in the PAG. This inhibitory effect has been postulated to result from opiate inhibition of GABAergic neurons in the PAG. Whether this opioid-GABAergic system is affected in acute pain state has not been investigated. The present study was thus designed to determine the effects of unilateral peripheral inflammation on ventrocaudal PAG gamma-aminobutyric acid (GABA) release in the rat using in vivo microdialysis and subsequent high pressure liquid chromatography (HPLC) analysis. Microdialysis was chosen to perform direct and dynamic studies of amino acid concentrations in the PAG in control rats and in animals subjected to acute and prolonged inflammation caused by injection of 120 microl of Complete Freund's Adjuvant (CFA) into the hind paw. GABA release was significantly decreased in the CFA treated groups both 24 h as well as 7 days post-treatment. GABA release decreased to approximately one-fourth that of the 24 h mineral oil control group. Likewise, veratridine-induced release of GABA was decreased in rats treated with CFA 7 days prior to dialysis. Systemic injection of naloxone (5 mg/kg i.p.) caused selective and significant block in the decrease of veratridine-induced release of GABA in the 24 h CFA-treated rats. Taken together with data from our previous studies, these results suggest that the decrease in veratridine-induced GABA release in this study may be due to an increase opiate inhibition of GABA resulting from the induction of acute or prolonged elevation of nociceptive input.

Quantitative immunoelectron microscopic colocalization of GABA and enkephalin in the ventrocaudal periaqueductal gray of the rat

In the present ultrastructural study in the ventrocaudal periaqueductal gray (PAG) of the rat, the relationship and the association between GABAergic and enkephalinergic neuronal elements were investigated using postembedding colocalization immunogold electron microscopic technique in order to establish the precise relationship between these two important neurotransmitters in this part of the brain stem. The GABA-like neuronal elements were immunoreacted with 20 nm gold particles and the enkephalin (ENK)-like immunoreactive neurons were labeled with 10 nm gold particles. Double labeling of sections with ENK and GABA produced colocalization in 23.3% and 1.2% of axon terminals and dendrites, respectively. Most of the double-labeled terminals contained more GABA-like than ENK-like immunolabeling. Approximately 19.4% of the labeled axon terminals and 8.5% of the labeled dendrites contained only GABA-like immunoreactivity, while 24% of the immunolabeled dendrites were immunoreactive with only ENK-like immunoreactivity. The synapses between the two kinds of immunolabeled neuronal profiles appear to be both asymmetrical and symmetrical. GABA-like immunolabeled terminals contained small, clear, pleomorphic or round vesicles and were found to make synapses with ENK-like immunolabeled and nonimmunolabeled dendrites, whereas most of the ENK-like immunolabeled axon terminals contained dense-cored vesicles. Approximately half of the axon terminals (51%) and dendrites (56%) in the ventrolateral PAG were not labeled for either GABA or for ENK immunoreactivity. The results are discussed in terms of GABAergic inhibition of antinociceptive mechanisms in the ventrolateral PAG and of the activation of these mechanisms by ENK neurotransmitter.

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.

Opioid-induced release of neurotensin in the periaqueductal gray matter of freely moving rats

The midbrain periaqueductal gray matter (PAG) is an important region for endogenous pain suppression. Nerve terminals containing opioid peptides and neurotensin (NT), as well as high densities of opioid- and NT-receptors, have been demonstrated in the ventromedial PAG. Local administration of opioids or NT in this region induces antinociception in experimental animals. In the present microdialysis study, the effect of opioids on the release of NT in the ventromedial PAG was investigated. Perfusion of the microdialysis probe with 10 microM morphine induced a significant increase (P < 0.05; n = 5) of the extracellular level of NT-like immunoreactivity (NT-LI), while perfusion with a 10-fold higher concentration of morphine had no significant effect on the NT-LI release in the PAG. Also perfusion of the dialysis probe with the mu-opioid receptor-specific agonist [D-Ala2-N-Me-Phe4-Gly5-ol]-enkephaline (DAGO) (1 or 100 microM) induced a significant (P < 0.05; n = 7-9) increase of the NT-LI level. The increase in NT-LI release in response to 1 microM DAGO was both calcium-dependent and naloxone-reversible. Since opioid agonists generally inhibit neuronal activity, an indirect mechanism, involving inhibition of tonically active inhibitory neurons, e.g. gamma-aminobutyric acid (GABA) neurons, could be of importance for the opioid induced release of NT. However, local administration in the PAG of the GABA(A) antagonist bicuculline (0.1-10 microM) or the GABA(A) agonist muscimol (1-100 microM) had no significant effect on the extracellular NT-LI level in the PAG, suggesting that GABAergic mechanisms are not involved in the opioid-induced release of NT-LI. In conclusion, the present data provide in vivo evidence that mu-opioid receptors mediate stimulation of neurotensin release in the PAG.

Glutamate-positive neurons and terminals in the cat periaqueductal gray matter (PAG): a light and electron microscopic immunocytochemical study

The morphology, distribution, proportion, size, and synaptic organization of periaqueductal gray matter neurons labeled with immunocytochemical techniques by an anti-glutamate (Glu) polyclonal serum were investigated in six adult cats (PAG-GLU 1-6). At the light microscopic level, numerous Glu-positive neurons were found throughout each subdivision of the periaqueductal gray matter. Their proportion and size, calculated in semi-thin sections (1-microm-thick), varied slightly among the subdivisions of the periaqueductal gray matter. The morphology of Glu-positive neurons was similar to that of the multipolar, triangular, and fusiform cells described in previous Golgi studies. Numerous puncta, interpreted as dendrites, axons, and axon terminals were also present in all subdivisions without preferential distribution. At the electron microscopic level, all synaptic contacts made by Glu-positive axon terminals were of the asymmetric type, but not all presynaptic elements making asymmetric synapses were labeled. The vast majority of postsynaptic elements contacted by Glu-positive axon terminals were labeled and unlabeled dendrites. The present results describe for the first time the presence of both Glu-positive neurons and terminals in the feline periaqueductal gray matter and provide further evidence that Glu is the probable neurotransmitter of numerous excitatory neurons of this structure.

Involvement of GABAB receptors in the antinociception induced by baclofen in the formalin test

1. The effect of GABA receptor antagonists on baclofen-induced antinociception was examined in rats using the formalin test. Intraperitoneal (IP) administration of different doses of baclofen (2.5-10 mg kg-1) to rats induced antinociception in both phases of the test. 2. The response was dose-dependent and the maximum response was observed with 10 mg kg-1 of the drug. Intracerebroventricular (ICV) injection of baclofen (0.5-20 micrograms/rat) also induced dose-dependent antinociception in the second phase of the formalin test. 3. The GABAB antagonist, phaclofen (1 mg kg-1, IP) but not the GABAA antagonists picrotoxin (1 mg kg-1, IP) and bicuculline (1.5 mg kg-1, IP), decreased the antinociception induced by both ICV and IP administration of baclofen. 4. It is concluded that baclofen antinociception in the formalin test is mediated through GABAB receptor activation.

Endogenous opioids acting at a medullary mu-opioid receptor contribute to the behavioral antinociception produced by GABA antagonism in the midbrain periaqueductal gray

This study examined the contribution of endogenous opioids to the antinociception produced by microinjection of the GABAA receptor antagonist, bicuculline, into the rat midbrain ventrolateral periaqueductal gray region. Microinjection of bicuculline (40 ng/0.4 microliter) into the periaqueductal gray produced robust antinociception as measured by the tail-flick latency to noxious heat. This antinociception was partially reversed by intravenous administration of the non-selective opioid antagonist naloxone hydrochloride (1 and 5 mg/kg), indicating that endogenous opioid release is necessary for this effect. To determine whether opioid release in the rostral ventromedial medulla, a major projection target of the periaqueductal gray, contributes to this effect, we microinjected another opioid antagonist, naltrexone, into the rostral ventromedial medulla. Naltrexone in the rostral ventromedial medulla (5 and 10 micrograms/microliter) significantly attenuated bicuculline antinociception elicited from the periaqueductal gray. Cys2, tyr3, orn5, pen7-amide (26.5 nmol), a selective mu-opioid receptor antagonist, also reversed the antinociception when microinjected into the rostral ventromedial medulla. Microinjections of naltrexone (10 micrograms/microliter) or cys2, tyr3, orn5, pen7-amide at sites in the medulla dorsal to the rostral ventromedial medulla were ineffective. None of the antagonists altered baseline tail-flick latencies. These results support the hypothesis that a population of periaqueductal gray neurons produces antinociception through a mu-opioid receptor-mediated action of endogenous opioids in the rostral ventromedial medulla. Thus, two opioid-sensitive pain-modulating brainstem sites are linked by an endogenous opioid synapse in the rostral ventromedial medulla.

Neurones in the midbrain periaqueductal grey send collateral projections to nucleus raphe magnus and the rostral ventrolateral medulla in the rat

Projections to the rostral ventrolateral medulla (RVLM) and nucleus raphe magnus (NRM) appear to originate from neurones with overlapping distributions in the periaqueductal grey (PAG) as demonstrated by the retrograde transport of red and green fluorescent latex microspheres. Furthermore, double-labelling studies demonstrated collateral projections from individual neurones in the PAG to the RVLM and NRM. This anatomical arrangement may allow interactions between descending control systems during specific behaviours.

The medullary reticular formation is a site of muscle relaxant action of diazepam on deep back and neck muscles in the female rat

We tested the hypothesis that the effect of systemic injections of diazepam (DZ, 125 mg/kg) to reduce the quality of the reproductive behavior, lordosis, and to reduce the EMG of lumbar back muscles involved in lordosis (Schwartz-Giblin et al., 1984) is exerted through a reticulospinal pathway with cells of origin in the nucleus gigantocellularis that excites lumbar motoneurons indirectly (Robbins et al., 1990, Robbins et al., 1992). In contrast, DZ facilitates lordosis behavior when infused into the midbrain central gray (McCarthy et al., 1995). Direct deposits of crystalline mixtures of DZ (20-80 ng) in dextrose were delivered to the medullary reticular formation (MRF) by diffusion from a cannula inserted through a guide to which a bipolar stimulating electrode was attached. The multiunit EMG response evoked by 20 (300 ms long) stimulus trains was recorded in back and neck muscles, lateral longissimus and splenius before and 5, 15, 30 and 60 min after local DZ deposits. There was a significant reduction in EMG response over this time period when stimulus intensities were within the range of 1.2-1.5 times threshold (Friedman two-way non-parametric test, P < 0.002). Large amplitude motor units that provide large tensions were the most sensitive to DZ-induced inhibition. Control deposits of dextrose had no significant effect. Systemic injections of progesterone (1 mg, i.p.) 60 min after DZ deposits, but not after dextrose deposits, further reduced the MRF-evoked EMG responses over the course of 1 h. As predicted, DZ infusions into the midbrain central gray did not reduce the reticulospinal-evoked axial muscle response, consistent with the facilitatory effect of midbrain central gray infusions of DZ on the lordosis quotient. The results suggest that benzodiazepine agonists (if endogenous) acting at sites in the MRF would be effective muscle relaxants during pregnancy, prior to the fall in progesterone that precedes labor.

Repeated spinal cord stimulation decreases the extracellular level of gamma-aminobutyric acid in the periaqueductal gray matter of freely moving rats

Most of the previous experimental studies on the antinociceptive effects of electrical spinal cord stimulation (SCS) have focused on short-lasting effects mainly depending on spinal mechanisms. However, patients treated with SCS for chronic pain often report pain relief exceeding the period of stimulation for several hours. The long lasting effect of SCS might not only involve spinal, but also supraspinal mechanisms. A supraspinal region of major importance for the coordination of descending pain inhibition is the periaqueductal grey matter (PAG). The aim of the present microdialysis study, performed in awake freely moving rats, was to investigate if repeated SCS (two 30 min periods separated by a 90 min resting period) alters the extracellular neurotransmitter concentrations in the ventrolateral PAG. In a first series of experiments significantly decreased (-30%; P < 0.05; n = 7) gamma-aminobutyric acid (GABA) levels were detected immediately after the second SCS session. Neither the concentration of serotonin nor that of substance P-like immunoreactivity (SP-LI) was affected by SCS. The decrease of GABA after two SCS sessions was confirmed in a second series of experiments (-30%; P < 0.05; n = 7). No spontaneous decline of GABA was observed in sham-stimulated animals (n = 6). The glutamate concentration was also determined in this latter series of experiments and a significant decrease (-23%; P < 0.05; n = 5) was observed after the second SCS session. As GABA-neurons in the PAG exert a tonic depressive effect on the activity in descending pain inhibitory pathways, a decreased extracellular GABA level in this region, as detected following repeated SCS, might indicate an increased pain inhibition.

Estrogen modulation of mRNA levels for the two forms of glutamic acid decarboxylase (GAD) in female rat brain

Two separate forms of glutamic acid decarboxylase, now termed GAD65 and GAD67, are the rate limiting enzymes for synthesis of gamma-aminobutyric acid (GABA). Because of the significance of GABA to neuroendocrine processes, numerous attempts have been made to determine the impact of gonadal steroids on enzyme functioning with inconclusive results. Therefore, we attempted to determine the impact of estradiol on mRNA levels for each form of GAD by quantitative in situ hybridization histochemistry in various brain regions. Ovariectomized rats were treated with estradiol benzoate or oil vehicle on 2 consecutive days and the brains collected on the third day. DNA probes specific for GAD65 and GAD67 were radiolabeled with CTP32 using asymmetric polymerase chain reaction. Results of in situ hybridizations for each probe on alternate sections from the same animals were analyzed for magnocellular preoptic area (McPOA), dorsal medial nucleus of the hypothalamus (DMN), zona incerta (ZI), and midbrain central gray (MCG). In the McPOA, estradiol exerted opposite effects on the frequency distribution of pixels per cell for two GAD mRNA probes, significantly increasing GAD65 (P < .05) and decreasing GAD67 (P < .01; Kolmogorov-Smirnov). In the DMN, estradiol treatment significantly increased GAD67 by 60% (P < .05; two-way ANOVA) but decreased GAD65 mRNA by 73% (P < .01). Note the direction of effects are opposite between McPOA and DMN. In MCG, analysis showed no estradiol effect on GAD mRNA levels/cells, but the proportion of cells expressing detectable levels of GAD65 or GAD67 increased by 33-40% in estradiol-treated rats (chi 2, P < .001).

Basal release of Met-enkephalin and neurotensin in the ventrolateral periaqueductal gray matter of the rat: a microdialysis study of antinociceptive circuits

The periaqueductal gray (PAG) contains neural circuits that participate in descending antinociception. Anatomical and electrophysiological evidence suggests that these circuits might employ opioid peptides and GABA in series to remove a tonic inhibition of descending PAG output neurons. The present studies examined the release of the antinociceptive peptides Met-enkephalin and neurotensin in the ventrolateral PAG, and investigated the interaction between GABA and Met-enkephalin release. In awake and freely moving rats the ventrolateral PAG was dialysed using 25 ga. concentric probes. Basal release of peptide in 12 min or 40 min fractions was determined using radioimmunoassays. To establish how the ventrolateral PAG responds to nociception, dialysis was performed following unilateral hindpaw inflammation using Complete Freund's Adjuvant. Twenty-four hours after inflammation was induced, neurotensin release was increased 133% and Met-enkephalin release was increased 353% compared to control animals. Seven days after inflammation was induced, neurotensin release declined precipitously, while basal Met-enkephalin release remained elevated 313% above controls. Thus, unlike enkephalin, increased basal neurotensin release is not sustained with persistent tonic nociception. In addition, we confirmed in normal animals that the ventrolateral PAG is induced to release Met-enkephalin by systemic morphine. A 43% increase in basal Met-enkephalin release was observed immediately following a 12 mg/kg i.p. morphine injection. Morphine should have the opposite effect (inhibit peptide release) if it acts directly on the enkephalinergic neurons. Thus, we examined the hypothesis that GABAergic interneurons in the PAG mediated morphine-stimulated enkephalin release. When the GABAantagonist bicuculline (0.25 microM to 25 microM) was co-infused with the dialysis medium, Met-enkephalin release increased in a dose-dependent fashion and peaked 68% above pre-infusion levels. These data elucidate the reciprocal inhibitory relationship between GABA and enkephalin in the ventrolateral PAG. We hypothesize that, when nociception induces Met-enkephalin release within this region, the tonic GABAergic inhibition is overcome, resulting in greater sensitivity of PAG enkephalinergic neurons. Ultimately, this enhanced enkephalin release should result in greater excitability of the descending PAG output neurons that are responsible for antinociception.

Functional characteristics of the midbrain periaqueductal gray

The major functions of the midbrain periaqueductal gray (PAG), including pain and analgesia, fear and anxiety, vocalization, lordosis and cardiovascular control are considered in this review article. The PAG is an important site in ascending pain transmission. It receives afferents from nociceptive neurons in the spinal cord and sends projections to thalamic nuclei that process nociception. The PAG is also a major component of a descending pain inhibitory system. Activation of this system inhibits nociceptive neurons in the dorsal horn of the sinal cord. The dorsal PAG is a major site for processing of fear and anxiety. It interacts with the amygdala and its lesion alters fear and anxiety produced by stimulation of amygdala. Stimulation of PAG produces vocalization and its lesion produces mutism. The firing of many cells within the PAG correlates with vocalization. The PAG is a major site for lordosis and this role of PAG is mediated by a pathway connecting the medial preoptic with the PAG. The cardiovascular controlling network within the PAG are organized in columns. The dorsal column is involved in pressor and the ventrolateral column mediates depressor responses. The major intrinsic circuit within the PAG is a tonically-active GABAergic network and inhibition of this network is an important mechanism for activation of outputs of the PAG. The various functions of the PAG are interrelated and there is a significant interaction between different functional components of the PAG. Using the current information about the anatomy, physiology, and pharmacology of the PAG, a model is proposed to account for the interactions between these different functional components.

Activity changes in rat raphe magnus neurons at different concentrations of fentanyl in vitro

The nucleus raphe magnus (NRM) is an important descending pain inhibitory system. We postulated that the analgesic action of supraspinally administered opiates results from increased descending inhibitory control of the NRM. We tested whether fentanyl activates NRM neurons in the rat slice preparation using extra-cellular recording. Eighty-seven percent of NRM neurons (total number = 68) tested were spontaneously active with firing frequencies of 0.2-4 spikes/s in artificial cerebrospinal fluid. Application of fentanyl (0.25, 0.5, and 1 mumol/L) increased firing frequencies in 12 of 59 (20%) spontaneously active neurons. In 6 of 9 (67%) silent neurons, fentanyl induced firing activities. Naloxone (1-2 mumol/L) antagonized the increased or induced activities by fentanyl in three neurons. In 13 of 59 (22%) spontaneously active neurons, fentanyl decreased the firing frequencies. Although fentanyl was associated with increased activity in a total of 18 NRM neurons, fentanyl at a higher concentration significantly increased the number of inhibited neurons. The results indicate that fentanyl partly activates the descending inhibitory system originating from the NRM; however, at higher concentrations, it appears also to inhibit this same system.

Immunoelectron microscopy of enkephalinergic innervation of GABAergic neurons in the periaqueductal gray

The pre-embedding double immunoreaction method was used to study synaptic relations of enkephalinergic and GABAergic neuronal elements in the ventrolateral part of the periaqueductal gray of the Wistar albino rat. The enkephalin-like neuronal elements were immunoreacted by the silver-gold intensified peroxidase-antiperoxidase method and the GABA-like immunoreactive neurons were immunoreacted by the unintensified peroxidase-antiperoxidase method. GABA-like immunoreactive neuronal somata were post-synaptic to both the enkephalin-like immunoreactive and the non-immunoreactive axon terminals. Enkephalin-like immunoreactive axon terminals were found to make synapses with GABA-like immunoreactive and non-immunoreactive dendrites. The synapses between the two kinds of chemically characterized neurons appeared to be both asymmetrical and symmetrical. Possible functional activity related to pain modulation, and synaptic relations between the enkephalinergic and GABAergic neurons in the periaqueductal gray and the dorsal raphe nucleus, are discussed.

Distribution of cholinergic, GABAergic and serotonergic neurons in the medial medullary reticular formation and their projections studied by cytotoxic lesions in the cat

As part of a larger study concerning the role of neurons in the medial medullary reticular formation in sleep-wake states, the distribution and projections of cholinergic, GABAergic and serotonergic neurons were studied within the lower brainstem of the cat. Cells were plotted with the aid of an image analysis system through the medullary reticular formation and raphe in adjacent sections immunostained for choline acetyltransferase, glutamic acid decarboxylase and serotonin. Immunostained fibres and varicosities were examined and quantified by microdensitometry in regions of the medulla, pons and upper spinal cord in normal and quisqualate-injected animals to assess the loss of local and distant projections following cytotoxic destruction of neurons in the medial medullary reticular formation. Choline acetyltransferase-immunoreactive neurons are unevenly and sparsely distributed, though none the less in significant numbers (estimated at approximately 9080 in total), through the medial medullary reticular formation, and are present in all tegmental fields, including the gigantocellular (approximately 3700) and magnocellular (approximately 1760) rostrally and the ventral (approximately 3240) and paramedian (approximately 380) caudally, and are absent in the midline raphe. Glutamic acid decarboxylase-immunoreactive neurons are more evenly and densely distributed in large numbers (estimated at approximately 18,720) through the medial medullary reticular formation, being present in the gigantocellular (approximately 5960), magnocellular (approximately 8260), ventral (approximately 2280) and paramedian (approximately 2220) tegmental fields, and are also numerous within the raphe magnus and pallidus-obscurus nuclei (approximately 3880). Serotonin-immunoreactive cells are sparse in the medial medullary reticular formation (estimated to total approximately 1540), where they are mainly located in the magnocellular tegmental field (approximately 1340), and are concentrated in larger numbers within the raphe nuclei (approximately 8060). Cholinergic varicose fibres were moderately densely distributed through the medial medullary reticular formation, as well as through more distant lateral, rostral and caudal brainstem and upper spinal regions. After cytotoxic lesions focussed in the gigantocellular and magnocellular tegmental fields, a loss of approximately 55% of the cholinergic neurons in the medial medullary reticular formation was associated with a minor decrease (approximately 35% in optical density measures) of local cholinergic fibres. Small and variable reductions in varicose fibres (and their optical density measures) were detected in distant structures (including the pontine lateral, gigantocellular and subcoerular tegmental fields and the caudal spinal trigeminal nucleus), that were none the less correlated with the number of intact medial medullary cholinergic cells, suggesting that these cells may project to distant brainstem targets, in addition to providing a minor proportion of the local cholinergic innervation of the medial medullary reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)