An analgesia circuit activated by cannabinoids

Neural Basis for the Hyperalgesic Action of Cholecystokinin in the Rostral Ventromedial Medulla

The analgesic actions of opioids can be modified by endogenous “anti-opioid” peptides, among them cholecystokinin (CCK). CCK is now thought to have a broader, pronociceptive role, and contributes to hyperalgesia in inflammatory and neuropathic pain states. The aim of this study was to determine whether anti-opioid and pronociceptive actions of CCK have a common underlying mechanism. We showed previously that a low dose of CCK microinjected into the rostral ventromedial medulla (RVM) blocked the analgesic effect of systemically administered morphine by preventing activation of off-cells, which are the antinociceptive output of this well characterized pain-modulating region. At this anti-opioid dose, CCK had no effect on the spontaneous activity of these neurons or on the activity of on-cells (hypothesized to facilitate nociception) or “neutral cells” (which have no known role in pain modulation). In this study, we used microinjection of a higher dose of CCK into the RVM to test whether activation of on-cells could explain the pronociceptive action of this peptide. Paw withdrawal latencies to noxious heat and the activity of a characterized RVM neuron were recorded in rats lightly anesthetized with methohexital. CCK (30 ng/200 nl) activated on-cells selectively and produced behavioral hyperalgesia. Firing of off-cells and neutral cells was unaffected. These data show that direct, selective activation of RVM on-cells by CCK is sufficient to produce thermal hyperalgesia and indicate that the anti-opioid and pronociceptive effects of this peptide are mediated by actions on different RVM cell classes.

Arachidonylethanolamide Induces Apoptosis of Human Glioma Cells through Vanilloid Receptor-1

The anti-tumor properties of cannabinoids have recently been evidenced, mainly with delta9-tetrahydrocannabinol (THC). However, the clinical application of this drug is limited by possible undesirable side effects due to a broad expression of cannabinoid receptors (CB1 and CB2). An attractive field of research therefore is to identify molecules with more selective tumor targeting. This is particularly important for malignant gliomas, considering their poor prognosis and their location in the brain. Here we investigated whether the most potent endogenous cannabinoid, arachidonylethanolamide (AEA), could be a candidate. We observed that AEA induced apoptosis in long-term and recently established glioma cell lines via aberrantly expressed vanilloid receptor-1 (VR1). In contrast with their role in THC-mediated death, both CB1 and CB2 partially protected glioma against AEA-induced apoptosis. These data show that the selective targeting of VR1 by AEA or more stable analogues is an attractive research area for the treatment of glioma.

Evidence for differential modulation of conditioned aversion and fear-conditioned analgesia by CB1 receptors

Fear-conditioned analgesia is an important survival response mediated by substrates controlling nociception and aversion. Cannabinoid(1) (CB(1)) receptors play an important role in nociception and aversion. However, their role in fear-conditioned analgesia has not been investigated. This study investigated the effects of systemic administration of the CB(1) receptor antagonist, SR141716A (1 mg/kg, i.p.), on fear-conditioned analgesia and conditioned aversion in rats. Twenty-four hours after receiving footshock, rats exhibited reduced formalin-evoked nociceptive behaviour, increased freezing and increased defecation when tested in the footshock apparatus, compared with non-footshocked formalin-injected rats. SR141716A attenuated fear-conditioned analgesia, freezing and defecation. Importantly, SR141716A had no effect on formalin-evoked nociceptive behaviour over an equivalent time period in rats not receiving footshock. SR141716A had no effect on contextually induced freezing during the first half of the test trial in rats receiving intra-plantar injection of saline. Administration of SR1417176A did, however, attenuate short-term extinction of contextually induced freezing and ultrasound emission in rats receiving intra-plantar saline, compared with vehicle-treated saline controls. These data suggest an important role for the CB(1) receptor in mediating fear-conditioned analgesia and provide evidence for differential modulation of conditioned aversive behaviour by CB(1) receptors during tonic, persistent pain.

Differential modulation of withdrawal reflexes by a cannabinoid in the rabbit

Inhibition of spinal and trigeminal withdrawal reflexes by morphine and by the cannabinoid agonist HU 210 has been studied in anaesthetized and in decerebrated rabbits. In intact, pentobarbitone-anaesthetized animals, the jaw-depressor reflex (JDR) evoked by stimulation of the tongue, and the reflex elicited in the ankle flexor tibialis anterior (TA) by stimulation of the toes were inhibited to the same extent by morphine (1-30 mg kg(-1) i.v. cumulative). In spinalized, anaesthetized rabbits morphine depressed the JDR to the same level as in non-spinal preparations, but the effect of the opioid on the TA reflex was significantly reduced. All effects of morphine were reversed by naloxone (0.25 mg kg(-1), i.v.). In anaesthetised intact animals, HU 210 depressed the JDR at a dose of 100 nmol kg(-1) i.v. cumulative, reduced reflexes evoked in the knee flexor muscle semitendinosus (ST) by stimulation at the toes at a dose of 30 nmol kg(-1) i.v. cumulative, but had no consistent or significant effects on the TA reflex to toe stimulation. The same results were obtained in spinalized, anaesthetised animals. In decerebrated, spinalized rabbits with no anaesthesia, HU 210 (30 nmol kg(-1)) depressed both ST and TA reflexes evoked by toe stimulation. These data reveal that trigeminal and spinal withdrawal reflexes are equally sensitive to morphine provided the spinal cord is intact, suggesting that at least part of the action of systemic morphine is due to activation of descending inhibition. The present results also show for the first time that cannabinoid agonists can inhibit trigeminal withdrawal reflexes. HU 210 had differential effects on the three reflexes studied depending on the presence or absence of anaesthesia. This is the first occasion on which we have found pharmacological distinctions between withdrawal reflexes, and indicates that spinal sensorimotor processing is more heterogeneous than has been suspected previously.

DREAM ablation selectively alters THC place aversion and analgesia but leaves intact the motivational and analgesic effects of morphine

DREAM (downstream regulatory element antagonistic modulator) is a novel transcriptional repressor for the prodynorphin gene, and genetic deletion of DREAM in mice results in a phenotype of ongoing analgesia by virtue of its effect on opioid gene expression. In the present study, we evaluated the motivational effects of opioids (morphine), cannabinoids [Delta(9)-tetrahydrocannabinol (THC)] and cocaine in mice lacking the dream gene (dream(-/-)). The aversive effects of THC were potentiated in dream(-/-) mice in a kappa-opioid receptor-dependent fashion, whereas morphine reward and the aversive effects of morphine withdrawal remained intact. The rewarding and aversive effects of cocaine were likewise unperturbed in dream(-/-) mice. Moreover, the aversive properties of lithium chloride and naloxone were unaffected by the absence of DREAM, indicating that the effect of DREAM on THC-induced dysphoria is not due to a general involvement in the behavioral response to aversive stimuli. Additionally, physical dependence to morphine and the locomotor-sensitizing effects of cocaine were unaltered in these animals. Finally, whereas the absence of DREAM reduced the analgesic efficacy of THC, morphine analgesia was unaffected in dream(-/-) mice.

Repeated administration with Delta9-tetrahydrocannabinol regulates mu-opioid receptor density in the rat brain

Several studies have demonstrated reciprocal, as well as synergistic interactions between cannabinoid and opioid systems. The aim of this study was to explore the time-related effects of repeated administration of Delta9-tetrahydrocannabinol on mu-opioid receptor autoradiography in various brain regions of the rat. To this aim, the effects of Delta9-tetrahydrocannabinol (Delta9-THC, 5 mg/kg/day; i.p.) were examined after 1, 3, 7 and 14 days of repeated administration on regions containing mu-opioid receptors: (i) forebrain [caudate-putamen, nucleus accumbens (core and shell) and piriform cortex]; (ii) amygdala (medial pars and cortical posteromedial pars), hypothalamus (ventromedial and dorsomedial nuclei, zona incerta), hippocampal regions (CA1, CA2, CA3, dentate girus), hindbrain (substantia nigra and ventral tegmental area); and (iii) thalamus, including 12 thalamic nuclei. In most of these regions, repeated cannabinoid administration increases mu-opioid receptor density; however, the onset, degree of magnitude reached and time-related effects produced by administration with Delta9-tetrahydrocannabinol are dependent upon the brain region examined. It appears that the major increase in mu-opioid receptor density occurs 1 and 3 days after Delta9-THC administration. In some regions, this increase is maintained and, for most of the brain areas examined, this effect is no longer significant by 14 days of administration, suggesting tolerance to cannabinoid treatment. Taken together, the results of this study suggest that cannabinoids produce a time-related differential responsiveness in mu-opioid receptor density in several brain areas that may be relevant to an understanding of the alterations associated with cannabinoid exposure.

Lack of analgesic efficacy of oral delta-9-tetrahydrocannabinol in postoperative pain

We have evaluated the efficacy of delta-9-tetrahydrocannabinol (delta-9-THC), the main psychoactive constituent of cannabis, in postoperative pain. In a randomized double-blind, placebo-controlled, single-dose trial, we investigated 40 women undergoing elective abdominal hysterectomy. Randomization took place when postoperative patient-controlled analgesia was discontinued on the second postoperative day. When patients requested further analgesia, they received a single, identical capsule of either oral delta-9-THC 5 mg (n=20) or placebo (n=20) in a double-blind fashion. The primary outcome measure was summed pain intensity difference (SPID) at 6 h after administration of study medication derived from visual analogue pain scores on movement and at rest. Secondary outcome measures were time to rescue medication and adverse effects of study medication. Mean (SD) VAS pain scores before medication in the placebo and delta-9-THC groups were 6.3(2.6) and 6.4(1.3)cm on movement, and 3.2(1.9) and 3.3(0.9) on rest, respectively. There were no significant differences in mean (95% confidence interval of the difference) SPID at 6 h between the groups [placebo 7.9, delta-9-THC 4.3(-1.8 to 9.0)cm h on movement; placebo 8.8, delta-9-THC 4.9(-0.2 to 8.1)cm h at rest] and time to rescue analgesia [placebo 217, delta-9-THC 163(-22 to 130)min]. Increased awareness of surroundings was reported more frequently in patients receiving delta-9-THC (40 vs 5%, P=0.04). There were no other significant differences with respect to adverse events. This study demonstrates no evidence of an analgesic effect of orally administered delta-9-THC 5 mg in postoperative pain in humans.

Antinociception and modulation of rostral ventromedial medulla neuronal activity by local microinfusion of a cannabinoid receptor agonist

Systemic administration of a cannabinoid agonist produces antinociception through the activation of pain modulating neurons in the rostral ventromedial medulla (RVM). The aim of the present study was to determine how a cannabinoid receptor agonist acting directly within the RVM affects neuronal activity to produce behaviorally measurable antinociception. In lightly anesthetized rats, two types of RVM neurons have been defined based on changes in tail flick-related activity. On-cells increase firing (on-cell burst), whereas off-cells cease firing (off-cell pause), just prior to a tail flick. The cannabinoid receptor agonist WIN55,212-2 was microinfused directly into the RVM while monitoring tail flick latencies and on- and off-cell activity. Microinfusion of WIN55,212-2 (2.0 microg/microl and 0.4 microg/microl) reduced the tail flick-related on-cell burst, decreased the duration of the off-cell pause, and increased off-cell ongoing activity. These changes were prevented by co-infusing the CB1 receptor antagonist, SR141716A (0.35 microg/microl), with WIN55,212-2 (0.4 microg/microl). Furthermore, 2.0 microg/microl WIN55,212-2 delayed the onset of the off-cell pause and increased tail flick latencies. Microinfusion of WIN55,212-2 to brain regions caudal or lateral to the RVM had no effect on RVM neuronal activity or tail flick latencies. These results indicate that cannabinoids act directly within the RVM to affect off-cell activity, providing one mechanism by which cannabinoids produce antinociception.

Synergistic interactions between cannabinoid and opioid analgesics

Cannabinoids and opioids both produce analgesia through a G-protein-coupled mechanism that blocks the release of pain-propagating neurotransmitters in the brain and spinal cord. However, high doses of these drugs, which may be required to treat chronic, severe pain, are accompanied by undesirable side effects. Thus, a search for a better analgesic strategy led to the discovery that delta 9-tetrahydrocannabinol (THC), the major psychoactive constituent of marijuana, enhances the potency of opioids such as morphine in animal models. In addition, studies have determined that the analgesic effect of THC is, at least in part, mediated through delta and kappa opioid receptors, indicating an intimate connection between cannabinoid and opioid signaling pathways in the modulation of pain perception. A host of behavioral and molecular experiments have been performed to elucidate the role of opioid receptors in cannabinoid-induced analgesia, and some of these findings are presented below. The aim of such studies is to develop a novel analgesic regimen using low dose combinations of cannabinoids and opioids to effectively treat acute and chronic pain, especially pain that may be resistant to opioids alone.

The endogenous cannabinoid system and its role in nociceptive behavior

The analgesic properties of exogenous cannabinoids have been recognized for many years and suggest a regulatory role for the endogenous cannabinoid ("endocannabinoid") system in mammalian nociceptive pathways. The endocannabinoid system includes: (1) at least two families of lipid signaling molecules, the N-acyl ethanolamines (e.g., anandamide) and the monoacylglycerols (e.g., 2-arachidonoyl glycerol); (2) multiple enzymes involved in the biosynthesis and degradation of these lipids, including the integral membrane enzyme fatty acid amide hydrolase; and (3) two G-protein coupled receptors, CB1 and CB2, which are primarily localized to the nervous system and immune system, respectively. Here, we review recent genetic, behavioral, and pharmacological studies that have tested the function of the endocannabinoid system in pain sensation. Collectively, these investigations support a role for endocannabinoids in modulating behavioral responses to acute, inflammatory, and neuropathic pain stimuli.

Effects of neurotoxic destruction of descending noradrenergic pathways on cannabinoid antinociception in models of acute and tonic nociception

The effects of neurotoxic destruction of catecholaminergic projections to the spinal cord on cannabinoid antinociception were examined in models of acute and tonic nociception. High performance liquid chromatography was used to quantify monoamine levels in sham-operated and lesioned rats. Intrathecal administration of the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) induced a selective depletion of norepinephrine (by approximately 85% of control) in rat lumbar spinal cord without altering levels of dopamine or serotonin. By contrast, brain levels of monoamines did not differ in sham-operated and lesioned rats. Pain behavior was similar in sham-operated and lesioned rats receiving vehicle in models of both acute and tonic nociception. The cannabinoid agonist WIN55,212-2 (5 or 10 mg/kg, i.p.) produced antinociception in the tail-flick test in sham-operated rats. The antinociceptive effect of WIN55,212-2 was attenuated relative to control conditions in rats depleted of spinal norepinephrine. WIN55,212-2 suppressed tonic pain behavior in the formalin test in sham-operated rats during phase 2 (15-60 min post formalin) of nociceptive responding. By contrast, in lesioned rats, WIN55,212-2 suppressed pain behavior during phase 1 (0-9.9 min) and phase 2A (10-39.9 min), but not during phase 2B (40-60 min). The cannabinoid agonist suppressed formalin-evoked Fos protein expression, a marker of neuronal activity, in the lumbar dorsal horn of sham-operated rats, but no suppression was observed in lesioned rats. The number of formalin-evoked Fos-like immunoreactive (FLI) cells was greater in lamina I and II of lesioned rats relative to sham-operated rats. These data indicate that the suppressive effect of the cannabinoid on formalin-evoked Fos protein expression in the superficial dorsal horn was attenuated following destruction of descending noradrenergic pathways. Our data are consistent with the hypothesis that cannabinoids produce antinociception, in part, by modulating descending noradrenergic systems and support a differential involvement of noradrenergic projections to the spinal cord in cannabinoid modulation of acute versus tonic nociception.

Participation of the opioid system in cannabinoid-induced antinociception and emotional-like responses

Several anatomical, biochemical and pharmacological evidence support the existence of bidirectional interactions between cannabinoid and opioid systems. The present review is focused on the participation of the endogenous opioid system in the antinociceptive and emotional-like responses induced by cannabinoids, and the development of tolerance to cannabinoid pharmacological effects. Cannabinoid and opioid agonists produce antinociception by acting on similar structures within the central nervous system, and a peripheral mechanism has been also proposed for both compounds. Pharmacological studies have suggested that the endogenous opioid system could be involved in cannabinoid antinociception and the development of cannabinoid tolerance. Recent studies using knockout mice have also demonstrated the role of the opioid system in cannabinoid antinociception and tolerance, although some discrepancies with the previous pharmacological results have been reported when using knockout mice. On the other hand, cannabinoid administration can induce anxiolytic-like responses that are mediated at least in part by an endogenous opioid activity on micro- and delta-opioid receptors.

The rodent amygdala contributes to the production of cannabinoid-induced antinociception

The amygdala is a temporal lobe region that is implicated in emotional information processing. The amygdala also is associated with the processing and modulation of pain sensation. Recently, we demonstrated that in nonhuman primates, the amygdala is necessary for the full expression of cannabinoid-induced antinociception [J Neurosci 21 (2001) 8238]. The antinociceptive effect of the cannabinoid receptor agonist (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo(1,2,3-de)-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN55,212-2) was significantly reduced in rhesus monkeys with large bilateral lesions of the amygdaloid complex. In the present study, we investigated the contribution of the amygdala to cannabinoid-induced antinociception in the rat. Using bilateral local microinjections of the GABA(A) receptor agonist muscimol, we inactivated neurons originating from the central nucleus of the amygdala (CeA) or basolateral nucleus of the amygdala (BLA). In rats injected with intra-CeA saline, the cannabinoid receptor agonist WIN55,212-2 produced dose-dependent antinociception on the noxious heat-evoked tail flick assay. In rats treated with intra-CeA muscimol, however, the antinociceptive effect of WIN55,212-2 was significantly reduced. Rats treated with intra-BLA muscimol showed no deficit in WIN55,212-2-induced antinociception. The effect of CeA inactivation on WIN55,212-2-induced suppression of prolonged pain in the formalin test also was tested. In rats treated with intra-CeA saline, WIN55,212-2 reduced the incidence of formalin-induced nociceptive behaviors and also reduced formalin-evoked c-fos expression in both superficial and deep laminae of the spinal cord dorsal horn. In rats treated with intra-CeA muscimol, however, these effects of WIN55,212-2 were significantly reduced. The results constitute the first causal data demonstrating the necessity of descending pain-modulatory circuitry (of which the CeA is a component) for the full expression of cannabinoid-induced antinociception in the rat. Furthermore, the results complement previous findings suggesting an overlap in neural circuitry activated by opioids and cannabinoids.

The cannabinoid agonist WIN 55, 212-2 increases nociception threshold in cholestatic rats: implications for the treatment of the pruritus of cholestasis

Dronabinol, a synthetic agonist at cannabinoid receptors, was reported to decrease the pruritus of cholestasis, in an uncontrolled observation. We hypothesized that the reported antipruritic effect of dronabinol might have resulted from an increased threshold to experience nociception (i.e. pruritus) by the drug. To test this hypothesis, we studied the effect of WIN 55, 212-2, a cannabinoid agonist, on the threshold to experience nociception, using a tail-flick assay in rats with cholestasis secondary to bile duct resection and in sham-resected controls. The administration of WIN 55, 212-2 was associated with a significant increase in the mean tail-flick latency in both groups as compared to baseline. Pruritus is a nociceptive stimulus; accordingly, drugs that increase the threshold to nociception in human beings may be a novel approach to the treatment of this symptom in patients with liver disease.

No evidence for direct modulatory effects of delta 9-tetrahydrocannabinol on human polymorphonuclear leukocytes

The effects of cannabinoids (CB) that have been reported in various leukocyte populations were mainly immunosuppressive or immunomodulatory. Almost nothing is known, however, about direct interactions of cannabinoids with human polymorphonuclear cells (PMN), although m-RNA for the cannabinoid receptor-2 (CB(2)) was found in human PMN. In order to investigate a potential influence of cannabinoids on human PMN, the migration and phagocytosis of PMN were studied in the presence of Delta(9)-Tetrahydrocannabinol (Delta(9)-THC) at final concentrations between 10(-10) and 10(-5) M. No effect was detectable on these essential PMN functions; and besides, no CB(2)-receptor expression could be detected using the Western blotting technique. Thus, circulating human PMN from healthy individuals remain unaffected by Delta(9)-THC due to the absence of functional CB(2)-receptor expression.

The effect of cannabinoid receptor antagonism with SR141716A on antinociception induced by cocaine and the NMDA receptor antagonist, MK-801

In the rat, antinociception of supraspinal origin is observed in response to administration of cocaine or an antagonist of the NMDA receptor for glutamate. The current study was conducted to determine if endocannabinoids are involved in the antinociceptive effect of cocaine, or antagonism of NMDA receptor binding. Intraperitoneal (i.p.) administration to male rats of cocaine, or the NMDA receptor antagonist, MK-801, resulted in a significant antinociceptive response of supraspinal origin, as indicated by a significant increase in reaction time in the hot plate test of analgesia (increase in the amount of time before the animal reacted to the hot plate by licking its paws or jumping). Treatment with SR141716A, a specific antagonist of the cannabinoid (CB1) receptor, resulted in a complete reversal of cocaine-induced antinociception when administered at a dose of 5.0mg/kg. Although the 2.5 and 5.0mg/kg doses of SR141716A produced a significant reduction in the antinociceptive effect of MK-801, the effect was incomplete since the reaction time in the hot plate test remained greater than that observed in vehicle-treated controls. These findings suggest that activation of the CB1 receptor participates significantly in antinociception resulting from treatment with cocaine and with the NMDA receptor antagonist, MK-801. The partial reversal of the antinociceptive effect of MK-801 by CB1 receptor antagonism indicates other mediators of nociception, in addition to the endocannabinoids, appear to be active in the antinociceptive response to NMDA receptor antagonism.

Role of endogenous cannabinoids in synaptic signaling

Research of cannabinoid actions was boosted in the 1990s by remarkable discoveries including identification of endogenous compounds with cannabimimetic activity (endocannabinoids) and the cloning of their molecular targets, the CB1 and CB2 receptors. Although the existence of an endogenous cannabinoid signaling system has been established for a decade, its physiological roles have just begun to unfold. In addition, the behavioral effects of exogenous cannabinoids such as delta-9-tetrahydrocannabinol, the major active compound of hashish and marijuana, await explanation at the cellular and network levels. Recent physiological, pharmacological, and high-resolution anatomical studies provided evidence that the major physiological effect of cannabinoids is the regulation of neurotransmitter release via activation of presynaptic CB1 receptors located on distinct types of axon terminals throughout the brain. Subsequent discoveries shed light on the functional consequences of this localization by demonstrating the involvement of endocannabinoids in retrograde signaling at GABAergic and glutamatergic synapses. In this review, we aim to synthesize recent progress in our understanding of the physiological roles of endocannabinoids in the brain. First, the synthetic pathways of endocannabinoids are discussed, along with the putative mechanisms of their release, uptake, and degradation. The fine-grain anatomical distribution of the neuronal cannabinoid receptor CB1 is described in most brain areas, emphasizing its general presynaptic localization and role in controlling neurotransmitter release. Finally, the possible functions of endocannabinoids as retrograde synaptic signal molecules are discussed in relation to synaptic plasticity and network activity patterns.

Induction of CB2 receptor expression in the rat spinal cord of neuropathic but not inflammatory chronic pain models

Cannabinoids have been considered for some time as potent therapeutic agents in chronic pain management. Central and systemic administration of natural, synthetic and endogenous cannabinoids produce antinociceptive and antihyperalgesic effects in both acute and chronic animal pain models. Although much of the existing data suggest that the analgesic effects of cannabinoids are mediated via neuronal CB1 receptors, there is increasing evidence to support a role for peripheral CB2 receptors, which are expressed preferentially on immune cells. As yet, little is known about the central contribution of CB2 in neuropathic pain states. We report here that chronic pain models associated with peripheral nerve injury, but not peripheral inflammation, induce CB2 receptor expression in a highly restricted and specific manner within the lumbar spinal cord. Moreover, the appearance of CB2 expression coincides with the appearance of activated microglia.

Cannabis and the brain

The active compound in herbal cannabis, Delta(9)-tetrahydrocannabinol, exerts all of its known central effects through the CB(1) cannabinoid receptor. Research on cannabinoid mechanisms has been facilitated by the availability of selective antagonists acting at CB(1) receptors and the generation of CB(1) receptor knockout mice. Particularly important classes of neurons that express high levels of CB(1) receptors are GABAergic interneurons in hippocampus, amygdala and cerebral cortex, which also contain the neuropeptides cholecystokinin. Activation of CB(1) receptors leads to inhibition of the release of amino acid and monoamine neurotransmitters. The lipid derivatives anandamide and 2-arachidonylglycerol act as endogenous ligands for CB(1) receptors (endocannabinoids). They may act as retrograde synaptic mediators of the phenomena of depolarization-induced suppression of inhibition or excitation in hippocampus and cerebellum. Central effects of cannabinoids include disruption of psychomotor behaviour, short-term memory impairment, intoxication, stimulation of appetite, antinociceptive actions (particularly against pain of neuropathic origin) and anti-emetic effects. Although there are signs of mild cognitive impairment in chronic cannabis users there is little evidence that such impairments are irreversible, or that they are accompanied by drug-induced neuropathology. A proportion of regular users of cannabis develop tolerance and dependence on the drug. Some studies have linked chronic use of cannabis with an increased risk of psychiatric illness, but there is little evidence for any causal link. The potential medical applications of cannabis in the treatment of painful muscle spasms and other symptoms of multiple sclerosis are currently being tested in clinical trials. Medicines based on drugs that enhance the function of endocannabinoids may offer novel therapeutic approaches in the future.

The cerebellum: organization, functions and its role in nociception

Our vision of the cerebellum has been gradually transformed throughout the last century from a 'little brain' to a 'neuronal machine' capable of multitasks, all arguably based on a principle computational model. We review here the main functions of the cerebellum in light of its organization and connectivity. In addition to providing a clear and extensive review of the cerebellar literature, we emphasize the role of the cerebellum in nociception, which is novel to the neurophysiology of pain. However, it is premature to conclude that the cerebellum influences sensory experience in the absence of clinical data.

Cannabis use for chronic non-cancer pain: results of a prospective survey

There has been a surge in interest in medicinal cannabis in Canada. We conducted a questionnaire survey to determine the current prevalence of medicinal cannabis use among patients with chronic non-cancer pain, to estimate the dose size and frequency of cannabis use, and to describe the main symptoms for which relief was being sought. Over a 6-week period in mid-2001, 209 chronic non-cancer pain patients were recruited in an anonymous cross-sectional survey. Seventy-two (35%) subjects reported ever having used cannabis. Thirty-two (15%) subjects reported having used cannabis for pain relief (pain users), and 20 (10%) subjects were currently using cannabis for pain relief. Thirty-eight subjects denied using cannabis for pain relief (recreational users). Compared to never users, pain users were significantly younger (P=0.001) and were more likely to be tobacco users (P=0.0001). The largest group of patients using cannabis had pain caused by trauma and/or surgery (51%), and the site of pain was predominantly neck/upper body and myofascial (68% and 65%, respectively). The median duration of pain was similar in both pain users and recreational users (8 vs. 7 years; P=0.7). There was a wide range of amounts and frequency of cannabis use. Of the 32 subjects who used cannabis for pain, 17 (53%) used four puffs or less at each dosing interval, eight (25%) smoked a whole cannabis cigarette (joint) and four (12%) smoked more than one joint. Seven (22%) of these subjects used cannabis more than once daily, five (16%) used it daily, eight (25%) used it weekly and nine (28%) used it rarely. Pain, sleep and mood were most frequently reported as improving with cannabis use, and 'high' and dry mouth were the most commonly reported side effects. We conclude that cannabis use is prevalent among the chronic non-cancer pain population, for a wide range of symptoms, with considerable variability in the amounts used. Discussions between patients and health care providers concerning cannabis use may facilitate education and follow up, and would allow side effects and potential interactions with other medications to be monitored. Clinical trials of cannabis for chronic non-cancer pain are warranted.

The neuronal distribution of cannabinoid receptor type 1 in the trigeminal ganglion of the rat

Cannabinoid compounds have been shown to produce antinociception and antihyperalgesia by acting upon cannabinoid receptors located in both the CNS and the periphery. A potential mechanism by which cannabinoids could inhibit nociception in the periphery is the activation of cannabinoid receptors located on one or more classes of primary nociceptive neurons. To address this hypothesis, we evaluated the neuronal distribution of cannabinoid receptor type 1 (CB1) in the trigeminal ganglion (TG) of the adult rat through combined in situ hybridization (ISH) and immunohistochemistry (IHC). CB1 receptor mRNA was localized mainly to medium and large diameter neurons of the maxillary and mandibular branches of the TG. Consistent with this distribution, in a de facto nociceptive sensory neuron population that exhibited vanilloid receptor type 1 immunoreactivity, colocalization with CB1 mRNA was also sparse (<5%). Furthermore, very few neurons (approximately 5%) in the peptidergic (defined as calcitonin gene-related peptide- or substance P-immunoreactive) or the isolectin B4-binding sensory neuron populations contained CB1 mRNA. In contrast, and consistent with the neuron-size distribution for CB1, nearly 75% of CB1-positive neurons exhibited N52-immunoreactivity, a marker of myelinated axons. These results indicate that in the rat TG, CB1 receptors are expressed predominantly in neurons that are not thought to subserve nociceptive neurotransmission in the noninjured animal. Taken together with the absence of an above background in situ signal for CB2 mRNA in TG neurons, these findings suggest that the peripherally mediated antinociceptive effects of cannabinoids may involve either as yet unidentified receptors or interaction with afferent neuron populations that normally subserve non-nociceptive functions.

Challenges in the federal regulation of pain management technologies

Those who write about pain management have focused almost entirely on delivery issues, paying essentially no attention to the federal regulatory challenges that affect the development of pain relief technologies — namely, pharmaceuticals and medical devices indicated for analgesic uses. The academic literature is strangely devoid of any sophisticated discussion of the difficulties that attend, first, the product approval decisions of the Food and Drug Administration (FDA) and, second, the scheduling decisions made by the Drug Enforcement Administration (DEA). If a “bottleneck” develops upstream, it could have serious repercussions downstream — without pain relief technologies, the issues of access that have preoccupied previous commentators would have little practical consequence.

The modern pharmaceutical industry traces its origins back more than a century, around the time that the German company Bayer first synthesized aspirin (acetylsalicylic acid) and began marketing it as an analgesic.

Spinal and peripheral mechanisms of cannabinoid antinociception: behavioral, neurophysiological and neuroanatomical perspectives

A large body of literature indicates that cannabinoids suppress behavioral responses to acute and persistent noxious stimulation. This review examines behavioral, neurophysiological and neuroanatomical evidence supporting a role for cannabinoids in suppressing nociceptive transmission at spinal and peripheral levels. The development of subtype-selective competitive antagonists and high-affinity agonists provides the pharmacological tools required to study cannabinoid antinociceptive mechanisms. These studies provide insight into the functional roles of cannabinoid receptor subtypes, CB1 and CB2, in cannabinoid antinociceptive mechanisms as revealed in animal models of acute and persistent (somatic inflammatory, visceral inflammatory, neuropathic) pain. Localization studies employing receptor binding and quantitative autoradiography, immunocytochemistry and in situ hybridization are reviewed to examine the distribution of cannabinoid receptors at these levels and provide a neuroanatomical framework with which to understand the roles of endogenous cannabinoids in sensory processing.

Inhibition of improgan antinociception by the cannabinoid (CB)(1) antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A): lack of obligatory role for endocannabinoids acting at CB(1) receptors

Improgan, a nonopioid antinociceptive agent, activates descending, pain-relieving mechanisms in the brain stem, but the receptor for this compound has not been identified. Because cannabinoids also activate nonopioid analgesia by a brain stem action, experiments were performed to assess the significance of cannabinoid mechanisms in improgan antinociception. The cannabinoid CB(1) antagonist N-(piperidin-1-yl)-5-(4-chloro phenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A) induced dose-dependent inhibition of improgan antinociception on the tail-flick test after i.c.v. administration in rats. The same treatments yielded comparable inhibition of cannabinoid [R-(+)-(2,3-dihydro-5-methyl-3-[(4-mor pholinyl)methyl]pyrol[1,2,3-de]-1,4-benzoxazin-6-yl)(1-naphthalenyl)methanone monomethanesulfonate, WIN 55,212-2] analgesia. Inhibition of improgan and WIN 55,212-2 antinociception by SR141716A was also observed in Swiss-Webster mice. Radioligand binding studies showed no appreciable affinity of improgan on rat brain, mouse brain, and human recombinant CB(1) receptors, ruling out a direct action at these sites. To test the hypothesis that CB(1) receptors indirectly participate in improgan signaling, the effects of improgan were assessed in mice with a null mutation of the CB(1) gene with and without SR141716A pretreatment. Surprisingly, improgan induced complete antinociception in both CB(1) (-/-) and wild-type control [CB(1) (+/+)] mice. Furthermore, SR141716A inhibited improgan antinociception in CB(1) (+/+) mice, but not in CB(1) (-/-) mice. Taken together, the results show that SR141716A reduces improgan antinociception, but neither cannabinoids nor CB(1) receptors seem to play an obligatory role in improgan signaling. Present and previous studies suggest that Delta(9)-tetrahydrocannabinol may act at both CB(1) and other receptors to relieve pain, but no evidence was found indicating that improgan uses either of these mechanisms. SR141716A will facilitate the study of improgan-like analgesics.

The role of descending fibers from the rostral ventromedial medulla in opioid analgesia in rats

There has been controversy as to whether the contribution of descending fibers from the rostral ventromedial medulla to opioid analgesia depends on the nature of the noxious stimulus eliciting pain. In the present study, inactivation of descending fibers by microinjection of muscimol (50 ng) in the rostral ventromedial medulla abolished morphine analgesia in the tail immersion and hot plate tests but decreased morphine analgesia by 60% in the formalin test. Analysis of the dose-response relation for morphine after inactivation of descending fibers revealed that, except for the tail immersion test, high doses of morphine could not overcome the block induced by muscimol. Also, morphine analgesia elicited supraspinally was not detectable when descending fibers were inactivated, suggesting that the analgesic effect of morphine in the brain requires a relay via the rostral ventromedial medulla. The analgesic effect of buprenorphine also depends on the integrity of descending fibers from the rostral ventromedial medulla. The results indicate that descending fibers from the rostral ventromedial medulla are critically important to the analgesic effect of opioids, regardless of the type of noxious stimulation eliciting pain. Residual analgesic effects of opioids after inactivation of descending fibers may be due to peripheral effects in the presence of inflammation.

Preliminary observation with dronabinol in patients with intractable pruritus secondary to cholestatic liver disease

Pruritus due to cholestatic liver disease can be particularly difficult to manage and frequently is intractable to a variety of medical therapies. The aim of our study is to evaluate the efficacy of delta-9-tetrahydrocannabinol (delta-9-THC) for intractable cholestatic related pruritus (ICRP) that has failed conventional (and unconventional) remedies. Three patients were evaluated for plasmapheresis because of ICRP. All 3 patients had previously been extensively treated with standard therapies for ICRP including: diphenhydramine, chlorpheniramine, cholestyramine, rifampicin, phenobarbital, doxepin, naltrexone, UV therapy, and topical lotions. Even multiple courses of plasmapheresis were performed without any benefit for the intractable pruritus. All patients reported significant decreases in their quality of life, including lack of sleep, depression, inability to work, and suicidal ideations. All patients were started on 5 mg of delta-9-THC (Marinol) at bedtime. All 3 patients reported a decrease in pruritus, marked improvement in sleep, and eventually were able to return to work. Resolution of depression occurred in two of three. Side effects related to the drug include one patient experiencing a disturbance in coordination. Marinol dosage was decreased to 2.5 mg in this patient with resolution of symptoms. The duration of antipruritic effect is approximately 4-6 hrs in all three patients suggesting the need for more frequent dosing. Delta-9-tetrahydrocannabinol may be an effective alternative in patients with intractable cholestatic pruritus.

Cannabinoid analgesia

During the last decade, rigorous scientific methods have been applied to determine the effects of cannabinoids on nociceptive neurotransmission. Cannabinoids have been observed to markedly decrease signalling in specific neural pathways that transmit messages about pain. These effects were found to be due to the suppression of spinal and thalamic nociceptive neurons, and independent of any actions on either the motor system or sensory neurons that transmit messages related to non-nociceptive stimulation. Spinal, supraspinal, and peripheral sites of cannabinoid analgesia have been identified. The discovery of endocannabinoids raised the question of their natural role in pain. Multiple lines of evidence indicate that endocannabinoids serve naturally to suppress pain. While it is now clear that cannabinoids suppress nociceptive neurotransmission, more work is needed to establish the clinical utility of these compounds. The few human studies conducted to date produced mixed results, with more promising findings coming from studies of clinical pain as compared with experimental pain. The therapeutic potential of cannabinoids remains an important topic for future investigations.

[The pharmacology of cannabinoid derivatives: are there applications to treatment of pain?]

OBJECTIVE

To present the cannabinoid system together with recent findings on the pharmacology of these compounds in the treatment of pain.

DATA SOURCES

Search through Medline database of articles published in French and English since 1966. Also use of other publications such as books on cannabis.

STUDY SELECTION

All the relevant documents within the theme of this review were used.

DATA EXTRACTION

All the data linked to the present topic were searched.

DATA SYNTHESIS

Recent advances have dramatically increased our understanding of cannabinoid pharmacology. The psychoactive constituents of Cannabis sativa have been isolated, synthetic cannabinoids described and an endocannabinoid system identified, together with its component receptors and ligands. Strong laboratory evidence now underwrites anecdotal claims of cannabinoid analgesia in inflammatory and neuropathic pain. Sites of analgesic action have been identified in brain, spinal cord and the periphery, with the latter two presenting attractive targets for divorcing the analgesic and psychotrophic effects of cannabinoids. Clinical trials are now required, but are hindered by a paucity of cannabinoids of suitable bioavailability and therapeutic ratio.

CONCLUSION

The cannabinoid system is a major target in the treatment of pain and its therapeutic potential should be assessed in the near future by the performance of new clinical trials.

Endocannabinoid signaling in the brain

The primary psychoactive ingredient in cannabis, Delta9-tetrahydrocannabinol (Delta9-THC), affects the brain mainly by activating a specific receptor (CB1). CB1 is expressed at high levels in many brain regions, and several endogenous brain lipids have been identified as CB1 ligands. In contrast to classical neurotransmitters, endogenous cannabinoids can function as retrograde synaptic messengers: They are released from postsynaptic neurons and travel backward across synapses, activating CB1 on presynaptic axons and suppressing neurotransmitter release. Cannabinoids may affect memory, cognition, and pain perception by means of this cellular mechanism.

Brain mechanisms of pain affect and pain modulation

Recent animal studies reveal ascending nociceptive and descending modulatory pathways that may contribute to the affective-motivational aspects of pain and play a critical role in the modulation of pain. In humans, a reliable pattern of cerebral activity occurs during the subjective experience of pain. Activity within the anterior cingulate cortex and possibly in other classical limbic structures, appears to be closely related to the subjective experience of pain unpleasantness and may reflect the regulation of endogenous mechanisms of pain modulation.

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.

Endocannabinoids, hormone-cytokine networks and human fertility

Anandamide (N -arachidonoylethanolamine, AEA) is a major endocannabinoid, shown to impair mouse pregnancy and embryo development and to induce apoptosis in blastocysts. Here, we review the roles of AEA, of the AEA-binding cannabinoid (CB) receptors, of the selective AEA membrane transporter (AMT), and of the AEA-hydrolyzing enzyme fatty acid amide hydrolase (FAAH), in human gestation. In particular, we discuss the interplay between the endocannabinoid system and the hormone-cytokine array involved in the control of human pregnancy, showing that the endocannabinoids take part in the immunological adaptation occurring during early pregnancy. In this line, we discuss the critical role of FAAH in human peripheral lymphocytes, showing that the expression of this enzyme is regulated by progesterone, Th1 and Th2 cytokines, which also regulate fertility. Moreover, we show that AEA and the other endocannabinoid, 2-arachidonoylglycerol, inhibit the release of the fertility-promoting cytokine leukemia inhibitory factor from human lymphocytes. Taken together, low FAAH and consistently high blood levels of AEA, but not CB receptors or AMT, can be early (<8 weeks of gestation) markers of spontaneous abortion, potentially useful as diagnostic tools for large-scale, routine monitoring of gestation in humans.

Endocannabinoids in pain modulation

Five major approaches have been employed to determine the role of endocannabinoids in pain modulation: (1) studies of various markers of endocannabinoid action aimed at determining whether the necessary cannabinoid biochemical machinery is present in those brain areas that control pain sensitivity; (2) administration of exogenous cannabinoids to determine whether endocannabinoid action at appropriate sites would lead to a loss of pain sensitivity; (3) administration of compounds that would affect endocannabinoid action such as antagonists and transport inhibitors to determine whether drug-induced preterbation of cannabinoid action would alter pain sensitivity; (4) studies of genetically altered animals aimed at determining whether pain responses or responses to cannabinergic drugs are altered; and (5) studies that measure the release of endocannabinoids. Converging evidence from each of these research areas indicates that endocannabinods function to control pain in parallel with endogenous opioids but via different mechanisms.

Absence of delta -9-tetrahydrocannabinol dysphoric effects in dynorphin-deficient mice

The involvement of dynorphin on Delta-9-tetrahydrocannabinol (THC) and morphine responses has been investigated by using mice with a targeted inactivation of the prodynorphin (Pdyn) gene. Dynorphin-deficient mice show specific changes in the behavioral effects of THC, including a reduction of spinal THC analgesia and the absence of THC-induced conditioned place aversion. In contrast, acute and chronic opioid effects were normal. The lack of negative motivational effects of THC in the absence of dynorphin demonstrates that this endogenous opioid peptide mediates the dysphoric effects of marijuana.

Reduction in opioid- and cannabinoid-induced antinociception in rhesus monkeys after bilateral lesions of the amygdaloid complex

The amygdaloid complex is a prominent temporal lobe region that is associated with "emotional" information processing. Studies in the rodent have also recently implicated the amygdala in the processing and modulation of pain sensation, the experience of which involves a considerable emotional component in humans. In the present study, we sought to establish the relevance of the amygdala to pain modulation in humans by investigating the contribution of this region to antinociceptive processes in nonhuman primates. Using magnetic resonance imaging guidance, the amygdaloid complex was lesioned bilaterally in six rhesus monkeys (Macaca mulatta) through microinjection of the neurotoxin ibotenic acid. This procedure resulted in substantial neuronal cell loss in all nuclear subdivisions of this structure. In awake unoperated control monkeys, systemic administration of the prototypical opioid morphine or the cannabinoid receptor agonist WIN55,212-2 produced dose-dependent antinociception on a warm-water tail-withdrawal assay. The antinociceptive effects of each drug were reversible with an appropriate antagonist. In monkeys with bilateral amygdala lesions, however, the antinociceptive effects of each drug were significantly reduced. These results constitute the first causal data demonstrating the necessity of neurons in a specific brain region for the full expression of opioid- and cannabinoid-induced antinociception in the primate. Because our amygdala-lesioned monkeys exhibited both a reduction in antinociception and a reduction in behavioral indices of fear (Emery et al., 2001), the possibility should be considered that, in the primate, "antinociceptive circuitry" and "fear circuitry" overlap at the level of the amygdala.

Possible mechanisms of cannabinoid-induced antinociception in the spinal cord

Anandamide is an endogenous ligand at both the inhibitory cannabinoid CB(1) receptor and the excitatory vanilloid receptor 1 (VR1). The CB(1) receptor and vanilloid VR1 receptor are expressed in about 50% and 40% of dorsal root ganglion neurons, respectively. While all vanilloid VR1 receptor-expressing cells belong to the calcitonin gene-related peptide-containing and isolectin B4-binding sub-populations of nociceptive primary sensory neurons, about 80% of the cannabinoid CB(1) receptor-expressing cells belong to those sub-populations. Furthermore, all vanilloid VR1 receptor-expressing cells co-express the cannabinoid CB(1) receptor. In agreement with these findings, neonatal capsaicin treatment that induces degeneration of capsaicin-sensitive, vanilloid VR1 receptor-expressing, thin, unmyelinated, nociceptive primary afferent fibres significantly reduced the cannabinoid CB(1) receptor immunostaining in the superficial spinal dorsal horn. Synthetic cannabinoid CB(1) receptor agonists, which do not have affinity at the vanilloid VR1 receptor, and low concentrations of anandamide both reduce the frequency of miniature excitatory postsynaptic currents and electrical stimulation-evoked or capsaicin-induced excitatory postsynaptic currents in substantia gelatinosa cells in the spinal cord without any effect on their amplitude. These effects are blocked by selective cannabinoid CB(1) receptor antagonists. Furthermore, the paired-pulse ratio is increased while the postsynaptic response of substantia gelatinosa neurons induced by alpha-amino-3-hydroxy-5-methylisoxasole-propionic acid (AMPA) in the presence of tetrodotoxin is unchanged following cannabinoid CB(1) receptor activation. These results strongly suggest that the cannabinoid CB(1) receptor is expressed presynaptically and that the activation of these receptors by synthetic cannabinoid CB(1) receptor agonists or low concentration of anandamide results in inhibition of transmitter release from nociceptive primary sensory neurons. High concentrations of anandamide, on the other hand, increase the frequency of miniature excitatory postsynaptic currents recorded from substantia gelatinosa neurons. This increase is blocked by ruthenium red, suggesting that this effect is mediated through the vanilloid VR1 receptor. Thus, anandamide at high concentrations can activate the VR1 and produce an opposite, excitatory effect to its inhibitory action produced at low concentrations through cannabinoid CB(1) receptor activation. This "dual", concentration-dependent effect of anandamide could be an important presynaptic modulatory mechanism in the spinal nociceptive system.

Actions of nociceptin/orphanin FQ and other prepronociceptin products on rat rostral ventromedial medulla neurons in vitro

1. Whole-cell patch clamp recordings were made from rat rostral ventromedial medulla (RVM) neurons in vitro to investigate the cellular actions of the opioid-like receptor ORL1 (NOP), ligand nociceptin/orphanin FQ and other putative prepronociceptin products. 2. Primary and secondary RVM neurons were identified as responding to the kappa-opioid receptor agonist U-69593 (300 nM to 1 microM) and the mu- and delta-opioid receptor agonist met-enkephalin (10 microM), respectively. Both primary and secondary RVM neurons responded to nociceptin (3 nM to 1 microM) with an outward current that reversed polarity at -115 mV in brain slices and with inhibition of Ca(2+) channel currents in acutely isolated cells. 3. The putative ORL1 antagonist J-113397 (1 microM) produced no change in membrane current and abolished the outward current produced by nociceptin (100 nM). In contrast, Phe(1)psi(CH(2)-NH)Gly(2)]-nociceptin-(1-13)NH(2) (300 nM to 1 microM) alone produced an outward current and partially reduced the outward current produced by nociceptin (300 nM) when co-applied. 4. In brain slices nociceptin (300 nM) reduced the amplitude of evoked GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) but not non-NMDA receptor-mediated excitatory postsynaptic currents (EPSCs). 5. Met-enkephalin (10 microM), but not nociceptin (300 nM), reduced the rate of spontaneous miniature IPSCs in normal external potassium solution (K(+) 2.5 mM). In high external potassium (K(+) 17.5 mM), nociceptin reduced the rate of miniature IPSCs in the presence (Ca(2+) 2.4 mM, Mg(2+) 1.2 mM) but not in the absence of external calcium (Ca(2+) 0 mM, Mg(2+) 10 mM, Cd(2+) 10 microM). Nociceptin and met-enkephalin had no effect on the amplitude of miniature IPSCs. 6. The putative nociceptin precursor products nocistatin (rat prepronociceptin(125-132)) and rat prepronociceptin(154-181) had no effect on membrane currents, evoked IPSCs and evoked EPSCs. 7. These results indicate that nociceptin acts via the ORL1 receptor to directly inhibit both primary and secondary RVM neurons by activating a potassium conductance and by inhibiting calcium conductances. In addition, nociceptin inhibits GABA release within the RVM via a presynaptic Ca(2+)-dependent mechanism. Thus, nociceptin has the potential to exert both disinhibitory and inhibitory effects on neuronal action potential firing within the RVM.

Cannabinoid actions on rat superficial medullary dorsal horn neurons in vitro

1. This study examined the cellular actions of cannabinoids on neurons in the substantia gelatinosa of the spinal trigeminal nucleus pars caudalis, using whole-cell and perforated patch recording in brain slices. 2. The cannabinoid agonist WIN55,212-2 (3 microM) decreased the amplitude of both GABAergic and glycinergic electrically evoked inhibitory postsynaptic currents (IPSCs) by 35 and 41 %, respectively. This inhibition was completely reversed by the CB(1) receptor-selective antagonist N-piperidino-5-(4-chlorophenyl)-l-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide) (SR141716A, 3 microM). WIN55,212-2 also produced relative facilitation of the second evoked IPSC to paired stimuli. 3. WIN55,212-2 decreased the rate of both GABAergic and glycinergic miniature IPSCs by 44 and 34 %, respectively, without changing their amplitude distributions or kinetics. 4. WIN55,212-2 did not affect the amplitude of electrically evoked non-NMDA glutamatergic excitatory postsynaptic currents (EPSCs). 5. WIN55,212-2 produced no postsynaptic membrane current and had no significant effect on membrane conductance over a range of membrane potentials (-60 to -130 mV). 6. These results suggest that, within the superficial medullary dorsal horn, cannabinoids presynaptically inhibit GABAergic and glycinergic neurotransmission. At the cellular level, the analgesic action of cannabinoids on these medullary dorsal horn neurons therefore differs from that of mu-opioids, which have both pre- and postsynaptic actions.