Differential distribution of (GABA)A receptor subunits on bulbospinal serotonergic and nonserotonergic neurons of the ventromedial medulla of the rat

Harmaline potentiates morphine-induced antinociception via affecting the ventral hippocampal GABA-A receptors in mice

Morphine is one of the most effective medications for treatment of pain, but its side effects limit its use. Therefore, identification of new strategies that can enhance morphine-induced antinociception and/or reduce its side effects will help to develop therapeutic approaches for pain relief. Considering antinociceptive efficacy of harmaline and also highlighted the important role of GABA-A receptors in the pain perception, this research aimed to determine whether the ventral hippocampal (vHip) GABA-A receptors are involved in the possible harmaline-induced enhancement of morphine antinociception. To achieve this, vHip regions of adult male mice were bilaterally cannulated and pain sensitivity was measured in a tail-flick apparatus. Intraperitoneally administration of morphine (0, 2, 4 and 6 mg/kg) or harmaline (0, 1.25, 5 and 10 mg/kg) increased the percentage of maximal possible effect (%MPE) and induced antinociception. Interestingly, co-administration of sub-effective doses of harmaline (5 mg/kg) and morphine (2 mg/kg) induced antinociception. Intra-vHip microinjection of muscimol (0, 200 and 300 ng/mice), a GABA-A receptor agonist, enhanced the anti-nociceptive effects of harmaline (2.5 mg/kg)+morphine (2 mg/kg) combination. Microinjection of the same doses of muscimol into the vHip by itself did not alter tail-flick latency. Intra-vHip microinjection of bicuculline (100 ng/mouse), a GABA-A receptor antagonist, did not cause a significant change in MPE%. Bicuculline (60 and 100 ng/mouse, intra-vHip) was administered with the harmaline (5 mg/kg)+morphine (2 mg/kg), and inhibited the potentiating effect of harmaline on morphine response. These findings favor the notion that GABAergic mechanisms in the vHip facilitate harmaline-induced potentiation of morphine response in the tail-flick test in part through GABA-A receptors. These findings shall provide insights and strategies into the development of pain suppressing drugs.

GABA pharmacology: the search for analgesics

Decades of research have been devoted to defining the role of GABAergic transmission in nociceptive processing. Much of this work was performed using rigid, orthosteric GABA analogs created by Povl Krogsgaard-Larsen and his associates. A relationship between GABA and pain is suggested by the anatomical distribution of GABA receptors and the ability of some GABA agonists to alter nociceptive responsiveness. Outlined in this report are data supporting this proposition, with particular emphasis on the anatomical localization and function of GABA-containing neurons and the molecular and pharmacological properties of GABAA and GABAB receptor subtypes. Reference is made to changes in overall GABAergic tone, GABA receptor expression and activity as a function of the duration and intensity of a painful stimulus or exposure to GABAergic agents. Evidence is presented that the plasticity of this receptor system may be responsible for the variability in the antinociceptive effectiveness of compounds that influence GABA transmission. These findings demonstrate that at least some types of persistent pain are associated with a regionally selective decline in GABAergic tone, highlighting the need for agents that enhance GABA activity in the affected regions without compromising GABA function over the long-term. As subtype selective positive allosteric modulators may accomplish these goals, such compounds might represent a new class of analgesic drugs.

The rostroventromedial medulla is engaged in the effects of spinal cord stimulation in a rodent model of neuropathic pain

The neurobiological mechanisms underlying the suppression of neuropathic pain by spinal cord stimulation (SCS) are still incompletely known. The present study aims at exploring whether the descending pain control system in the rostroventromedial medulla (RVM) exerts a role in the attenuation of neuropathic pain by SCS. Experiments were performed in the rat spared nerve injury (SNI) pain model. The effects of SCS on neuronal activity of pronociceptive ON-like, antinociceptive OFF-like, and neutral cells, including 5-HT-like cells, in the RVM were analyzed in SCS responding and SCS non-responding SNI animals as well as in naïve controls. Decreased spontaneous activities in OFF-like cells and increased spontaneous activities in ON-like cells were observed in SNI animals, whereas the spontaneous activities of 5-HT-like and neutral cells were unchanged. SCS produced a prominent increase in the discharge of OFF- and 5-HT-like cells in SCS responding, but not in non-responding SNI animals or controls. Discharge rates of ON-like and neutral cell were not affected by SCS. In awake SNI animals, microinjection of a GABAA receptor agonist, muscimol, into the RVM significantly attenuated the antihypersensitivity effect induced by SCS while a non-selective opioid receptor antagonist, naltrexone, was ineffective. It is concluded that SCS may shift the reciprocal inhibitory and facilitatory pain modulation balance controlled by the RVM in favor of inhibition. This increase in the descending antinociceptive effect operates in concert with segmental spinal mechanisms in producing pain relief.

Cellular basis for opioid potentiation in the rostral ventromedial medulla of rats with persistent inflammatory nociception

Direct inhibition of pain facilitatory neurons in the rostral ventromedial medulla (RVM) is one mechanism by which mu opioid receptor (MOPr) agonists are proposed to produce antinociception. The antinociceptive and anti-hyperalgesic effects of the MOPr agonist DAMGO are enhanced after intraplantar injection of complete Freund's adjuvant (CFA). This study therefore examined whether CFA treatment similarly enhanced the ability of DAMGO to induce outward currents in spinally projecting RVM neurons. It further examined whether the electrophysiological properties of RVM neurons are altered by CFA treatment. Whole-cell patch clamp recordings were made from three types of serotonergic as well as non-serotonergic spinally projecting RVM neurons obtained from control rats and rats 4h or four days after CFA. Persistent, but not acute inflammatory nociception increased the percentage of Type 2 non-serotonergic neurons that responded to DAMGO from 17% to 57% and the percentage of Type 3 serotonergic neurons that responded to DAMGO from 5% to 55%. These same two populations of RVM neurons exhibited significant differences in their passive membrane properties or spontaneous discharge rate. The outward currents produced by the GABA(B) receptor agonist baclofen were not enhanced, suggesting that the enhancement does not reflect global changes in levels of G(i/o) or activity of G-protein regulated inwardly rectifying potassium channels. These results provide a cellular basis for the enhanced anti-hyperalgesic and antinociceptive effects of MOPr agonists under conditions of persistent inflammatory nociception. These results also provide intriguing, albeit indirect, evidence for two different populations of pain facilitatory neurons in the RVM.

Periaqueductal gray neurons project to spinally projecting GABAergic neurons in the rostral ventromedial medulla

The analgesic effects of morphine are mediated, in part, by periaqueductal gray (PAG) neurons that project to the rostral ventromedial medulla (RVM). Although much of the neural circuitry within the RVM has been described, the relationship between RVM neurons and PAG input and spinal output is not known. The objective of this study was to determine whether GABAergic output neurons from the PAG target RVM reticulospinal neurons. Immunocytochemistry and confocal microscopy revealed that PAG neurons project extensively to RVM neurons projecting to the spinal cord, and two-thirds of these reticulospinal neurons appear to be GABAergic (contain GAD67 immunoreactivity). The majority (71%) of PAG fibers that contact RVM reticulospinal GAD67-immunoreactive neurons also contained GAD67 immunoreactivity. Thus, there is an inhibitory projection from PAG to inhibitory RVM reticulospinal neurons. However, there were also PAG projections to the RVM that did not contain GAD67 immunoreactivity. Additional experiments were conducted to determine whether the heterogeneity in this projection can be explained by the electrophysiological character of the RVM target neurons. PAG projections to electrophysiologically defined and juxtacellularly filled ON, OFF, and Neutral cells in the RVM were examined. Similar to the pattern reported above, both GAD67- and non-GAD67-immunoreactive PAG neurons project to RVM ON, OFF, and Neutral cells in the RVM. These inputs include a GAD67-immunoreactive projection to a GAD67-immunoreactive ON cell and non-GAD67 projections to GAD67-immunoreactive OFF cells. This pattern is consistent with PAG neurons producing antinociception by direct excitation of RVM OFF cells and inhibition of ON cells.

Inflammation-induced changes in rostral ventromedial medulla mu and kappa opioid receptor mediated antinociception

Acute microinjection of mu-, delta-, or kappa-opioid receptor (MOPr, DOPr, KOPr) agonists into the rostral ventromedial medulla (RVM) produces antinociception. Thermal antinociception produced by MOPr and DOPr agonists is potentiated during inflammation [Hurley RW, Hammond DL. The analgesic effects of supraspinal mu and delta opioid receptor agonists are potentiated during persistent inflammation. J Neurosci 2000;20:1249-59]. Whether this potentiation extends to other stimulus modalities or to KOPr agonists is unknown. To examine these issues, rats received a unilateral intraplantar injection of complete Freund's adjuvant (CFA). Antinociception produced by RVM infusion of the KOPr agonist, U69593, and the MOPr agonist, DAMGO, was tested 4h-2 weeks thereafter. Thermal paw withdrawal latencies (PWLs) were assessed using the Hargreaves method. Mechanical thresholds were determined with the Von Frey and Randall-Selitto method. PWLs of the inflamed paw were reduced 4h-2 weeks after CFA injection. Infusion of either U69593 or DAMGO increased PWLs in CFA treated rats. A bilateral enhancement of the response to both agonists was observed 2 weeks relative to 4h post-CFA injection. Mechanical thresholds of the inflamed paw were decreased for >2 weeks post-CFA injection. Infusion of either agonist elevated thresholds of the inflamed and non-inflamed paws of CFA-treated rats. The magnitude of these effects was greater 2 weeks post-CFA injection for DAMGO and increased progressively for U69593. These data demonstrate that RVM infusion of MOPr or KOPr agonists attenuates CFA-evoked thermal and tactile allodynia and that these effects increase during prolonged inflammation. The augmented response of the non-inflamed paw to agonists suggests that inflammation induces centrally-mediated neuroplastic changes which enhance MOPr- and KOPr-mediated antinociception.

Suitability of the retrograde tracer Dil for electrophysiological studies of brainstem neurons: adverse ramifications for G-protein coupled receptor agonists

Despite the acknowledged advantages of studying identified populations of neurons, few studies have convincingly established that fluorescent retrograde tracers do not alter the passive membrane properties, action potential characteristics, or effects of drugs on the labeled neurons. Whole-cell patch clamp recordings were made from spinally-projecting serotonergic neurons in the rostral ventromedial medulla (RVM) and spinally-projecting noradrenergic neurons in the locus coeruleus (LC) that were retrogradely labeled with 1,1'-dioactadecyl-3,3,3',3'-tetramethylindocarbodyanine perchlorate (Dil). The passive membrane and the action potential properties of Dil-labeled (0.2%) and non-labeled serotonergic neurons in the RVM did not differ. Similarly, the passive membrane and action potential properties of non-labeled noradrenergic LC neurons did not differ from neurons labeled with 0.2% or 5% Dil. Although the mu opioid receptor agonist [D-Ala(2)-NMePhe(4)-Gly-ol(5)]enkephalin (DAMGO) produced equivalent outward currents in non-labeled noradrenergic LC neurons and those labeled with 0.2% Dil, significantly smaller currents were recorded in LC neurons labeled with 5% Dil. Baclofen, a gamma-aminobutryic acid(B) receptor agonist, also produced smaller currents in RVM neurons labeled with 5% Dil compared to 0.2% Dil. These results indicate that 0.2% Dil is suitable for retrograde labeling of brainstem neurons in vivo for subsequent in vitro electrophysiological study. However, 5% Dil is likely to confound studies of the postsynaptic actions of G-protein coupled receptor ligands.

Ventilatory effects of muscimol microdialysis into the rostral medullary raphé region of conscious rats

We hypothesized that inhibition of the rostral medullary raphe region (MRR), a putative central chemoreceptor location, with the GABA(A) receptor agonist muscimol would decrease ventilatory responses to hypercapnia and hypoxia in conscious rats, and that its known effect at this site on body temperature might alter its effect upon these ventilatory responses. At ambient temperatures of 24.5-26.5 degrees C (Cool), microdialysis of 1mM muscimol into the MRR significantly decreased body temperature by approximately 0.5 degrees C, increased the ventilatory response to 7% CO(2) and decreased the response to 10% O(2). At ambient temperatures of 29.5-30.5 degrees C (Warm), 1 mM muscimol microdialysis no longer decreased body temperature and increased the ventilatory response to hypercapnia and to hypoxia. Muscimol did not significantly affect the VE/VO2 ratio at either temperature. Muscimol significantly increased the hypercapnic ventilatory responses in Cool and Warm conditions and the hypoxic response in Warm conditions, which indicates the presence of an inhibitory effect of rostral MRR neurons sensitive to muscimol. In the Cool condition the ventilatory response to hypoxia is inhibited but appropriately so for the lower VO2 .

The Role of GABA in the Mediation and Perception of Pain

A great deal of effort has been expended in attempting to define the role of GABA in mediating the transmission and perception of pain. Pursuit of this question has been stimulated by the fact that GABAergic neurons are widely distributed throughout the central nervous system, including regions of the spinal cord dorsal horn known to be important for transmitting pain impulses to the brain. In addition, GABA neurons and receptors are found in supraspinal sites known to coordinate the perception and response to painful stimuli and this neurotransmitter system has been shown to regulate control of sensory information processing in the spinal cord. The discovery that GABA receptor agonists display antinociceptive properties in a variety of animal models of pain has provided an impetus for developing such agents for this purpose. It has been shown that GABA receptor agonists, as well as inhibitors of GABA uptake or metabolism, are clinically effective in treating this symptom. However, even with an enhanced understanding of the relationship between GABAergic transmission and pain, it has proven difficult to exploit these findings in designing novel analgesics that can be employed for the routine management of pain. Work in this area has revealed a host of reasons why GABAergic drugs have, to date, been of limited utility in the management of pain. Chief among these are the side effects associated with such agents, in particular sedation. These limitations are likely due to the simultaneous activation of GABA receptors throughout the neuraxis, most of which are not involved in the transmission or perception of pain. This makes it difficult to fully exploit the antinociceptive properties of GABAergic drugs before untoward effects intervene. The discovery of molecularly and pharmacologically distinct GABAA receptors may open the way to developing subtype selective agents that target those receptors most intimately involved in the transmission and perception of pain. The more limited repertoire of GABAB receptor subunits makes it more difficult to develop subtype selective agents for this site. Nonetheless, a GABAB agonist, CGP 35024, has been identified that induces antinociceptive responses at doses well below those that cause sedation (Patel et al., 2001). It has also been reported that, unlike baclofen, tolerance to antinociceptive responses is not observed with CGP 44532, a more potent GABAB receptor agonist (Enna et al., 1998). While the reasons for these differences in responses to members of the same class remain unknown, these findings suggest it may be possible to design a GABAB agonist with a superior clinical profile than existing agents. Besides the challenges associated with identifying subtype selective GABAA and GABAB receptor agonists, the development of GABA analgesics has been hindered by the fact that the responsiveness of these receptor systems appear to vary with the type and duration of pain being treated and the mode of drug administration. Further studies are necessary to more precisely define the types of pain most amenable to treatment with GABAergic drugs. Inasmuch as the antinociceptive responses to these agents in laboratory animals are mediated, at least in part, through activation or inhibition of other neurotransmitter and neuromodulator systems, it is conceivable that GABA agonists will be most efficacious as analgesics when administered in combination with other agents. The results of anatomical, biochemical, molecular, and pharmacological studies support the notion that generalized activation of GABA receptor systems dampens the response to painful stimuli. The data leave little doubt that, under certain circumstances, stimulation of neuroanatomically discreet GABA receptor sites could be of benefit in the management of pain. Continued research in this area is warranted given the limited choices, and clinical difficulties, associated with conventional analgesics.

Transcutaneous electrical nerve stimulation (TENS) reduces chronic hyperalgesia induced by muscle inflammation

Transcutaneous electrical nerve stimulation (TENS) reduces pain through central mechanisms involving spinal cord and brainstem sites. Since TENS acts through central mechanisms, we hypothesized that TENS will reduce chronic bilateral hyperalgesia produced by unilateral inflammation when applied either ipsilateral or contralateral to the site of muscle inflammation. Sprague-Dawley rats were injected with carrageenan in the left gastrocnemius muscle belly. Mechanical withdrawal threshold was tested bilaterally before and 2 weeks after carrageenan injection. After testing withdrawal thresholds at 2 weeks, rats received TENS treatment either ipsilateral or contralateral to the site of inflammation. In each of these groups, rats were randomized to control (no TENS), low frequency (4 Hz), or high frequency (100 Hz) TENS treatment. TENS was applied for 20 min at sensory intensity under light halothane anesthesia. Mechanical withdrawal thresholds were re-assessed after TENS or 'no TENS' treatment. Unilateral injection of carrageenan to the gastrocnemius muscle significantly reduced the mechanical withdrawal threshold (mechanical hyperalgesia) bilaterally 2 weeks later. Either low or high frequency TENS applied to the gastrocnemius muscle ipsilateral to the site of inflammation significantly reversed mechanical hyperalgesia, both ipsilateral and contralateral to the site of inflammation. Low or high frequency TENS applied to the gastrocnemius muscle contralateral to the site of inflammation also significantly reduced mechanical hyperalgesia, both ipsilateral and contralateral to the site of inflammation. Since ipsilateral or contralateral TENS treatments were effective in reducing chronic bilateral hyperalgesia in this animal model, we suggest that TENS act through modulating descending influences from supraspinal sites such as rostral ventromedial medulla (RVM).

Electrophysiological heterogeneity of spinally projecting serotonergic and nonserotonergic neurons in the rostral ventromedial medulla

This study examined the passive membrane and action potential properties of serotonergic and nonserotonergic neurons in the rostral ventromedial medulla (RVM) of the rat using whole cell patch-clamp recording techniques in the slice. Serotonergic neurons were identified by immunoreactivity for tryptophan hydroxylase (TrpH). Spinally projecting neurons were retrogradely labeled with 1'-dioactadecyl-3,3,3',3'-tetramethylindocarbodyanine perchlorate (DiI). Three types of neurons were identified within both spinally projecting serotonergic and nonserotonergic populations. Type 1 neurons exhibited irregular or sporadic spontaneous activity interspersed with periods of quiescence. Type 2 neurons were not spontaneously active and were additionally discriminated by a more negative resting membrane potential and a larger-amplitude action potential. Type 3 neurons fired repetitively without pause. Serotonergic neurons had a higher membrane resistance and greater action potential half-width than their nonserotonergic counterparts and rarely exhibited a fast afterhyperpolarization. Serotonergic type 3 neurons also fired more slowly and regularly than nonserotonergic type 3 neurons. Comparison of electrophysiological and immunohistochemical characteristics suggested that the smallest type 3 serotonergic neurons had an increased risk of immunohistochemical "misclassification" due to failure to detect TrpH, possibly due to more complete dialysis of intracellular contents during lengthy recordings. This risk was minimal for type 1 or 2 serotonergic neurons. The three different types of spinally projecting serotonergic neurons also differed markedly in their responsiveness to the mu opioid receptor agonist D-Ala2, NMePhe4, Gly5-ol]enkephalin. These results provide important new electrophysiological and pharmacological evidence for a significant heterogeneity among spinally projecting serotonergic RVM neurons. They may also provide a basis for resolving the controversy concerning the role of serotonergic RVM neurons in opioid analgesia.

Separate populations of neurons in the rostral ventromedial medulla project to the spinal cord and to the dorsolateral pons in the rat

Activation of neurons in the rostral ventromedial medulla (RVM) directly modulates spinal nociceptive transmission by projections to the spinal cord dorsal horn and indirectly by projections to neurons in the dorsolateral pons (DLP) that project to the spinal cord dorsal horn. However, it is not known whether the same neurons in the RVM produce both direct and indirect modulation of nociception. Deposits of the retrograde tracers Fluoro-Gold (FG) in the spinal cord dorsal horn and DiI in the DLP were used to determine whether the same RVM neurons project to both of these regions. Only 0.9+/-0.1% of RVM neurons retrogradely labeled with Fluoro-Gold from the spinal cord were also labeled with DiI placed in the DLP. In addition, spinally projecting RVM neurons were significantly larger than RVM neurons that project to the DLP. Finally, spinally projecting neurons were found predominantly on the midline and within the RVM; neurons that project to the DLP had a wider distribution and were present both within and outside of the RVM. Thus, separate and morphologically distinct populations of RVM neurons appear to modulate nociception by direct and indirect descending pathways.

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.

Effect of embryonic knock-down of GABAA receptors on the levels of monoamines and their metabolites in the CNS of the mouse

In vitro evidence indicates that gamma-aminobutyric acid (GABA), acting at GABA(A) receptors, exerts a positive trophic effect on monoaminergic neurons during embryogenesis. To determine whether in vivo antagonism of GABA(A) receptors during embryogenesis interferes with the development of monoaminergic neurons, we used mice in which the number of GABA(A) receptors was decreased by 50% by targeted deletion of the beta(3) subunit gene of the GABA(A) receptor. Levels of serotonin, dopamine, norepinephrine, and the metabolites 3,4-deoxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid were measured in the brainstem, cortex, striatum and spinal cord of female adult homozygous null (beta3-/-) and wild-type (beta3+/+) mice, as well as progenitor C57BL/6J and Strain 129/SvJ mice. The level of norepinephrine in the spinal cord of beta3-/- mice was 44% less than that of beta3+/+ mice, and did not differ in the brainstem, cortex or striatum. This finding suggests that beta3 subunit-containing GABA(A) receptors mediate the trophic effects of GABA on a subpopulation of spinally-projecting noradrenergic neurons. In contrast, the levels of serotonin, dopamine or their metabolites were unaffected, suggesting that the development of serotonergic and dopaminergic neurons may require activation of only a small fraction of GABA(A) receptors or may not be dependent on beta3 subunit-containing GABA(A) receptors. Finally, Strain 129/SvJ and C57BL/6J mice differed with respect to the levels of dopamine and its metabolites in the brainstem, spinal cord and cortex. These differences may need to be considered when assessing the phenotype of gene-targeted mice for which these mice serve as progenitor strains.

GABAergic modulation of descending inhibitory systems from the rostral ventromedial medulla (RVM). Dose-response analysis of nociception and neurological deficits

We have examined the effects of muscimol and bicuculline microinjected in the rostral ventromedial medulla (RVM) on motor function and on nociception in three pain tests. In Exp. 1 microinjection of muscimol (6.25-400 ng in 1 microl) in the RVM dose-dependently decreased pain threshold of rats and the ED(50) for muscimol was the same in both the hot plate and tail immersion pain tests. In the hot plate test, but not in the tail immersion test, paw withdrawal latencies increased again with high doses of muscimol (75-400 ng). High doses also produced catalepsy. Exp. 2 examined the effects of muscimol (50 ng) and bicuculline (50 ng) over a range of formalin concentrations (0.25-4%) in the formalin test. Muscimol increased responsiveness to formalin and reduced the slope of the formalin dose-response relation. Bicuculline decreased responses to formalin and reduced the slope of the formalin dose-response relation. It is suggested that RVM cells with inhibitory projections to the dorsal horn are not subject to strong GABAergic influence under mild noxious stimulation. RVM cells are thus active, and spinal dorsal horn relay neurons are inhibited. On the other hand, intense noxious peripheral stimulation may stimulate the release of GABA onto RVM cells, which in turn shuts off descending inhibitory fibers to allow transmission of nociceptor input through the dorsal horn.

Opioid- and GABA(A)-receptors are co-expressed by neurons in rat brain

Pharmacological data suggest that opioids exert their excitatory action in brain indirectly, by inhibiting release of the inhibitory neurotransmitter GABA. However, it is also possible that single neuron may interact with both opioids and GABA. In the present study, we investigated whether neurons in rat midbrain and medulla express both opioid and GABA(A) receptors. Coronal sections through rat brain were double-stained using antibodies against the alpha 1 subunit of GABA(A) receptor that were combined with antibodies either against the cloned mu-opioid receptor (MOR1) or the cloned kappa-opioid receptor (KOR1). Neurons double-labeled for GABA(A) receptors and either MOR1 or KOR1 were found in many brain regions including inferior colliculus, mesencephalic trigeminal nuclei, pontine reticular nuclei and raphe interpositus nucleus. Neurons double-labeled for GABA(A) and MOR1 were observed less frequently than those labeled for GABA(A) and KOR1. Our findings provide anatomical evidence that GABAergic and opioidergic systems are closely linked and activity of the same neuron may be regulated directly by both GABA and opioids.

The analgesic effects of supraspinal mu and delta opioid receptor agonists are potentiated during persistent inflammation

This study examined the antihyperalgesic and antinociceptive effects of opioid receptor agonists microinjected in the rostral ventromedial medulla (RVM) of rats 4 hr, 4 d, and 2 weeks after the induction of an inflammatory injury by injection of complete Freund's adjuvant (CFA) in one hindpaw. Nociceptive sensitivity of the ipsilateral, inflamed and the contralateral, uninflamed hindpaws was determined by the radiant-heat paw withdrawal test. The antihyperalgesic potency of the mu opioid receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO), determined for the inflamed hindpaw, was enhanced 4 d and 2 weeks after injury. The antinociceptive potency of DAMGO, determined for the contralateral, uninflamed hindpaw, was also progressively enhanced 4 hr, 4 d, and 2 weeks after injury. The magnitude of enhancement paralleled the chronicity of the injury. The greatest potentiation occurred 2 weeks after injury when the ED(50) value of DAMGO in CFA-treated rats was one-tenth that in saline-treated rats. The antihyperalgesic and antinociceptive effects of the delta opioid receptor agonist [D-Ala(2),Glu(4)]deltorphin were also increased 2 weeks after injury. These results indicate that peripheral inflammatory injury alters the pharmacology of excitatory and inhibitory inputs that modulate the activity of RVM neurons in such a manner as to enhance the effects of opioid agonists in this region. These changes have ramifications not only for the alleviation of hyperalgesia at the site of injury but also for opioid-induced antinociception at sites remote to the injury as revealed by increases in the potency of opioid agonists to suppress nociceptive responses of the contralateral, uninflamed hindpaw.