Endocannabinoid mechanisms of pain modulation

Cannabinoids are antinociceptive in animal models of acute, tissue injury-, and nerve injury-induced nociception. This review examines the biology of endogenous cannabinoids (endocannabinoids) and behavioral, neurophysiological, and neuroanatomical evidence supporting the notion that cannabinoids play a role in pain modulation. Behavioral pharmacological approaches, in conjunction with the identification and quantification of endocannabinoids through the use of liquid and gas chromatography mass spectrometry, have provided insight into the functional roles of endocannabinoids in pain modulation. Here we examine the distribution of cannabinoid receptors and endocannabinoid-hydrolyzing enzymes within pain modulatory circuits together with behavioral, neurochemical, and neurophysiological studies that suggest a role for endocannabinoid signaling in pain modulation. This review will provide a comprehensive evaluation of the roles of the endocannabinoids 2-arachidonoylglycerol and anandamide in stress-induced analgesia. These findings provide a functional framework with which to understand the roles of endocannabinoids in nociceptive processing at the supraspinal level.     

Nicotine modulation of fear memories and anxiety: Implications for learning and anxiety disorders

Anxiety disorders are a group of crippling mental diseases affecting millions of Americans with a 30% lifetime prevalence and costs associated with healthcare of $42.3 billion. While anxiety disorders show high levels of co-morbidity with smoking (45.3% vs. 22.5% in healthy individuals), they are also more common among the smoking population (22% vs. 11.1% in the non-smoking population). Moreover, there is clear evidence that smoking modulates symptom severity in patients with anxiety disorders. In order to better understand this relationship, several animal paradigms are used to model several key symptoms of anxiety disorders; these include fear conditioning and measures of anxiety. Studies clearly demonstrate that nicotine mediates acquisition and extinction of fear as well as anxiety through the modulation of specific subtypes of nicotinic acetylcholine receptors (nAChRs) in brain regions involved in emotion processing such as the hippocampus. However, the direction of nicotine’s effects on these behaviors is determined by several factors that include the length of administration, hippocampus-dependency of the fear learning task, and source of anxiety (novelty-driven vs. social anxiety). Overall, the studies reviewed here suggest that nicotine alters behaviors related to fear and anxiety and that nicotine contributes to the development, maintenance, and reoccurrence of anxiety disorders.     

The effects of tolerance on state-dependent learning in social drinkers

Sixty drinkers were assigned to three experimental groups which manipulated pre and post drug state. Subjects on learning trials were exposed to 30 slides, composed of geometrical shapes and alcohol-related self-statements. On the day following learning, subjects were required to try and recognise the 30 original slides out of 60 test slides. Signal detection method was used to analyse differences between the groups. Results showed that medium drinkers who consumed an experimental dose similar to their usual dose showed state dependency over both stimuli. Light drinkers showed an impairment on all alcohol-related conditions but alcohol on learning primed the recognition of alcohol-associated stimuli. Heavy drinkers, who drank a dose that was lower than their usual dose, showed little effect. The findings thus showed the importance of controlling for the level of tolerance and the type of stimuli presented in the State Dependent Learning experiments.     

Endocannabinoid system and stress and anxiety responses

Cannabinoid agonists induce complex and often contradictory effects on anxiety in humans and experimental animals. The data from animal tests provide evidence of dose-dependent bidirectional modulation of anxiety by the cannabinoid system and the importance of environmental context. The mechanisms mediating the effects of cannabinoids on anxiety-related responses appear to involve CB1 and non-CB1 cannabinoid receptors. In addition, the CRH, GABA(A), cholecystokinin, opioid and serotonergic systems have also been implicated. Brain regions such as the amygdala, hippocampus and cortex, directly involved in the regulation of emotional behavior, contain high densities of CB1 receptors. Mutant mice lacking CB1 receptors show anxiogenic-like and depressive-like phenotypes in several tests, as well as profound alterations in their adrenocortical activity. Pharmacological blockade of CB1 receptors induces anxiety in rats, and inhibition of anandamide metabolism produces anxiolytic-like effects. Thus, the endocannabinoid system appears to play a pivotal role in the regulation of emotional states and may constitute a novel pharmacological target for anti-anxiety therapy.     

Physostigmine: effects on fear or defense responses in the rat

Previous research had shown that the anticholinergic drug, scopolamine, decreased innate defensive responses of rats to a live cat or mechanical robot, and that the effects of scopolamine were attributable to actions of the drug on the central nervous system. In the present research, the anticholinesterase, physostigmine, which increases central cholinergic activity, caused an increase in the defense responses of male hooded rats. Physostigmine caused significantly more freezing and significantly more suppression of feeding and suppression of time near the aversive stimulus (ROBOT). Dose-response curves showed a positive, linear relationship between dose (0.025, 0.05, 0.1 and 0.2 mg/kg) of physostigmine and defense responses. The present results could not be attributed to general response suppression since the effects of physostigmine were situation-specific, i.e., the drug had no significant effect on behavior in the non-aversive or NO ROBOT condition. The present results were taken as further evidence of the involvement of cholinergic activity in the mediation of defense responses. The effects of cholinergic and anticholinergic drugs on the observable defense response of freezing were thought to have important implications for the large literature relating these drugs and avoidance responding.     

Morphine state-dependent learning: sensitization and interactions with dopamine receptors

In the present study, the effects of morphine sensitization on impairment of memory formation and the state-dependent learning by morphine have been investigated in mice. Pretraining administration of morphine (0.5, 2.5 and 5 mg/kg) dose dependently decreased the learning of a one-trial passive avoidance task. Pretest administration of morphine (0.5, 2.5 and 5 mg/kg) induced state-dependent retrieval of the memory acquired under pretraining morphine influence. Pretraining or pretest administration of naloxone (0.25, 0.5 and 1 mg/kg) reversed both responses to morphine (5 mg/kg). Amnesia induced by pretraining morphine was significantly reversed in morphine-sensitized mice which had previously received once daily injections of morphine [20 and 30 mg/kg, subcutaneously (s.c.)] for 3 days. Morphine sensitization tended to reverse but did not significantly affect morphine state-dependent memory. The inhibition of morphine-induced amnesia in morphine-sensitized mice was decreased by once daily administration of naloxone (0.5, 1 and 2 mg/kg) 30 min prior to injection of morphine (20 mg/kg/day x 3 days). Three-days administration of 1-phenyl-7,8-dihydroxy-2,3,4,5-tetrahydro-1H-3-benzazepine HCL (SKF 38393; 8, 16 and 32 mg/kg) or SCH 23390; R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine HCL (0.01, 0.05 and 0.1 mg/kg) before morphine (for 3 days) and during morphine-sensitization, decreased and increased, the amnesia induced by pretraining morphine, respectively. Similar administration of quinpirole (0.5, 1 and 2 mg/kg) or sulpiride (25, 50 and 100 mg/kg) before morphine also decreased and increased the amnesia induced by pretraining morphine, respectively. The results suggest that morphine sensitization affects the impairment of memory formation, but not the facilitation of retrieval induced by morphine and thus it is postulated that dopamine receptors may play an important role in this effect.     

NMDA antagonists make learning and recall state-dependent

Rats injected with either drug or saline were trained in a food-rewarded lever-pressing task until they could complete an FR10 requirement within the first 120s of the session, and were then tested for the retention of this response under various treatment conditions. In this procedure, state-dependency occurred with ketamine, phencyclidine, MK-801 and CGS 19755; rats trained with any of these NMDA antagonists failed to show response transfer when tested with saline. Also, in rats trained with saline the response failed to transfer in tests with any of these drugs. The doses at which the failure to transfer occurred in drug-to-saline tests, were approximately 3-fold lower than those at which the failure occurred in saline-to-drug tests. Even higher doses of these compounds were required to inhibit acquisition. The state-dependency which NMDA antagonists appear to produce in tests for saline-to-drug transfer, might constitute the mechanism whereby these compounds apparently disorganize the behavior of animals and, perhaps also, of humans. The data do not support the widely held notion that NMDA antagonists produce deficits in memory or retention; instead, at doses considerably lower than those impairing acquisition, NMDA antagonists produce a state upon which the recall of the newly learned response is dependent.     

Drug-induced state-dependent learning: review of an operant procedure in rats

Drug discrimination and drug state dependence are often thought to be based on the same drug actions, and to differ only in the doses needed to produce them, with discrimination occurring at low doses and state dependence at high doses. Testing this hypothesis has been hampered by the use of discrimination and state dependence procedures that differed in many respects. In 1986, Colpaert introduced a procedure to study state dependence in rats that used the same response, the same reinforcer, and the same reinforcement schedule that are commonly used in drug discrimination. Using this procedure, differences between drug state dependence and drug discrimination were found with some drugs (e.g. alcohol), consistent with the hypothesis that the procedures differ in the drug properties they measure, but not with other drugs (e.g. chlordiazepoxide). Thus, state dependence and drug discrimination can generate different outcomes, but the conditions in which they do require further study. However, all the studies conducted with the procedure introduced by Colpaert clearly show that state dependence is not necessarily only a high-dose phenomenon, but can also occur at doses at which many central nervous system drugs produce their characteristic effects. This finding led to the hypothesis that state dependence may be involved in the therapeutic and other effects of psychoactive drugs.     

Opioid modulation of attention-related responses: peripheral-to-central progression and development of mu influence as learning occurs

 Endogenous opioids modulate attention-related bradycardiac responses evoked by novel stimuli and Pavlovian conditioned signals, and these effects are distinct from those of endogenous opioids on memory. We investigated the role of peripheral opioid receptors in modulating attention and Pavlovian learning, in rabbits tested for bradycardiac orienting responses to novel tones, and for Pavlovian conditioning and extinction of cardiac discrimination. Pretraining, IV treatment with the opiate antagonist naloxone-HCl (0.1–0.5 mg/kg) facilitated initial development of Pavlovian conditioned discrimination and delayed its later extinction, compared to saline vehicle, as previously observed. Pretraining treatment with its peripherally acting analog, quaternary naloxone-methiodide (1.29–6.47 mg/kg), also promoted initial development, but not extinction, of discrimination, and it reduced the magnitude of bradycardiac orienting responses and of tachycardiac unconditioned responses. Treatment with the selective μ-antagonist peptide CTOP (10–30 μg/kg) facilitated conditioned responses and reduced unconditioned responses, somewhat later during training, but it did not reliably affect extinction or orienting responses. These results confirm an important role of peripheral opioids in regulating attentional and associative functions involved in orienting and the earliest stage of Pavlovian learning, prior to development of central opioid regulation of later associative, hedonic and mnemonic functions. These findings also suggest that cardiovascular opioid receptors might mediate peripheral opioid influences on attention and early association formation, via modulation of cardiac responses to stimuli and autonomic sensory feedback to the brain. Received: 27 July 1996 / Final version: 13 March 1997     

Morphine state-dependent learning: interactions with alpha2-adrenoceptors and acute stress

The interactions of -adrenoceptors and acute restraint stress with morphine state-dependent memory of passive avoidance were examined in mice. Memory acquired following pre-training morphine administration (5 mg/kg, i.p.) was dose- and time-dependently retrieved by pre-test morphine; this effect was reversible by yohimbine (1 mg/kg). Pre-test clonidine (0.005-0.1 mg/kg) was also effective in restoring morphine-induced memory. Pre-training clonidine (2 mg/kg) induced an amnestic effect that was restorable by pre-test clonidine or morphine; this effect was also blocked by yohimbine. Acute pre-training stress for 2 h induced an amnestic effect that was reversible by pre-test morphine (1 and 5 mg/kg) or clonidine (0.01 and 0.1 mg/kg). Finally, acute pre-test stress could restore the impairment of memory induced by pre-training morphine. The data are suggestive of a functional interaction between -opioid, -adrenergic receptors and stress in modulating state-dependent learning and memory.     

Benzodiazepine induced state-dependent learning: A correlative of abuse potential?

Using a two-way avoidance procedure, the two major benzodiadepines, diazepam and chlordiazepoxide, were tested for state-dependent learning effects in rats. Results showed that compared to saline control animals, subjects receiving diazepam or chlordiazepoxide during extinction displayed significantly more rapid extinction but a significantly greater amount of spontaneous recovery of the conditioned avoidance response. No significant dose-response differences were found. These findings are consistent with Overton's hypothesis that drug state discriminability and state-dependent learning parallel a compound's abuse potential. Drug state discriminability vs state-dependent learning is discussed and a potential relationship between state dependent properties and physical dependence is suggested.     

Incerto-thalamic modulation of fear via GABA and dopamine

Fear generalization and deficits in extinction learning are debilitating dimensions of Post-Traumatic Stress Disorder (PTSD). Most understanding of the neurobiology underlying these dimensions comes from studies of cortical and limbic brain regions. While thalamic and subthalamic regions have been implicated in modulating fear, the potential for incerto-thalamic pathways to suppress fear generalization and rescue deficits in extinction recall remains unexplored. We first used patch-clamp electrophysiology to examine functional connections between the subthalamic zona incerta and thalamic reuniens (RE). Optogenetic stimulation of GABAergic ZI → RE cell terminals in vitro induced inhibitory post-synaptic currents (IPSCs) in the RE. We then combined high-intensity discriminative auditory fear conditioning with cell-type-specific and projection-specific optogenetics in mice to assess functional roles of GABAergic ZI → RE cell projections in modulating fear generalization and extinction recall. In addition, we used a similar approach to test the possibility of fear generalization and extinction recall being modulated by a smaller subset of GABAergic ZI → RE cells, the A13 dopaminergic cell population. Optogenetic stimulation of GABAergic ZI → RE cell terminals attenuated fear generalization and enhanced extinction recall. In contrast, optogenetic stimulation of dopaminergic ZI → RE cell terminals had no effect on fear generalization but enhanced extinction recall in a dopamine receptor D1-dependent manner. Our findings shed new light on the neuroanatomy and neurochemistry of ZI-located cells that contribute to adaptive fear by increasing the precision and extinction of learned associations. In so doing, these data reveal novel neuroanatomical substrates that could be therapeutically targeted for treatment of PTSD.Subject terms: Classical conditioning, Behavioural methods     

Morphine state-dependent learning sensitization and interaction with nitric oxide

In the present study, the effects of nitric oxide (NO) precursor L-arginine and L-NAME, a potent inhibitor of NO synthase (NOS), on the expression of sensitization of morphine were investigated. Pre-training administration of morphine (5 mg/kg) impaired memory retrieval compared to pre-training saline-treated animals. Amnesia due to pre-training morphine (5 mg/kg) was restored by pre-test morphine (5 mg/kg).The retrieval impairment was also inhibited in mice which had received once-daily injections of morphine (20 and 30 mg/kg, s.c.) for 3 days, followed by 5 days of no drug treatment before training (in order to induce morphine sensitization). Administration of L-arginine (60 mg/kg/day - 3 days) or L-NAME (20 mg/kg/day - 3 days) before training did not alter morphine state dependency. During acquisition of sensitization, administration of L-arginine (60 mg/kg) 20 min before morphine (10 mg/kg/day, for 3 days) increased, while injection of L-NAME (20 mg/kg) 20 min before morphine (30 mg/kg/day, for 3 days) decreased morphine state dependency. It is concluded that NO is involved in the morphine-induced sensitization.     

Tricyclic antidepressants: Effects on extinction and fear learning

Rats trained to run an alley for a food reward were extinguished following injections of different antidepressants. When retested several days later, the animals extinguished following pretreatment with the NE reuptake blocker protriptyline showed faster running speeds than did the other groups. Other rats given electrical shocks following pretreatment with protriptyline avoided the compartment in which they had been shocked less than did animals shocked following pretreatment with other antidepressants. This implies an interference with some aspect of the learning or consolidation process which is correlated with the degree of NE reuptake blockage. It is hypothesized that NE terminals are deactivated following frustrative nonreward or punishment by the conversion and reuptake of the released NE to an altered extinction molecule.     

Diazepam and human memory: Influence on acquisition,retrieval, and state-dependent learning

• 1.1. Diazepam's capacity to influence learning and memory processes in man was evaluated using five tasks involving free and cued recall and mental imagery.
• 2.2. State-dependent learning was assessed by manipulating the drug condition (diazepam or placebo) during the learning and recall sessions.
• 3.3. Results indicated that 0.3 mg/kg diazepam administered orally significantly impaired subjects' ability to learn new material using a variety of procedures.
• 4.4. Retrieval of material once learned was not severely impaired by the drug.
• 5.5. Some evidence for state dependency was found, but additional studies are required.

     

Learning in hyperactive children: Are there stimulant-related and state-dependent effects?

A group of 17 hyperactive children participated in a laboratory learning task in order to replicate the findings of Swanson and Kinsbourne (1976) indicating that learning performance is facilitated by a single dose of methylphenidate and retention performance is state-dependent. This study revealed no cross-validation in support of the findings of Swanson and Kinsbourne (1976).     

Fluoxetine protects hippocampal plasticity during conditioned fear stress and prevents fear learning potentiation

Rationale Contextual fear conditioning can produce both changes in hippocampal synaptic efficacy and potentiation of subsequent fear learning. Objectives In this study, we tested whether fluoxetine reverses these effects. Materials and methods In the first experiment, we examined alterations of baseline synaptic efficacy and induction of synaptic plasticity in the CA3 region of the hippocampus during re-exposure of rats, treated with fluoxetine (7 mg/kg) or vehicle, in a context where they previously received 15 eyelid shocks or no shock (controls). In the second experiment, fear learning potentiation was examined in rats that were initially submitted to conditioning (15 eyelid shocks) and extinction training and then re-exposed to a less intense stressor (three eyelid shocks). Results Conditioned fear stress decreased synaptic efficacy and blocked the induction of synaptic potentiation in the fimbria–CA3 pathway. Conditioned rats treated with fluoxetine were protected against these electrophysiological changes and did not differ from controls (i.e., no depression and normal induction of potentiation of synaptic efficacy). However, fluoxetine treatment did not suppress conditioned freezing. After fear extinction, exposure of rats to a subconditioning stressor provoked conditioning (fear learning potentiation) in rats treated with vehicle but not in those treated with fluoxetine. Conclusions These findings indicate that fluoxetine treatment, which is ineffective on conditioned fear stress-induced freezing, may have beneficial effects on conditioned fear stress-induced disturbance of hippocampal plasticity. These data also suggest that restoration of hippocampal functioning may contribute to protection against exaggerated reactions to mild stressors reported in patients with post-traumatic stress disorder.     

Endocannabinoid modulation by FAAH and monoacylglycerol lipase within the analgesic circuitry of the periaqueductal grey

Background and Purpose

Endogenous cannabinoids (endocannabinoids) in the periaqueductal grey (PAG) play a vital role in mediating stress-induced analgesia. This analgesic effect of endocannabinoids is enhanced by pharmacological inhibition of their degradative enzymes. However, the specific effects of endocannabinoids and the inhibitors of their degradation are largely unknown within this pain-modulating region.

Experimental Approach

In vitro electrophysiological recordings were conducted from PAG neurons in rat midbrain slices. The effects of the major endocannabinoids and their degradation inhibitors on inhibitory GABAergic synaptic transmission were examined.

Key Results

Exogenous application of the endocannabinoid, anandamide (AEA), but not 2-arachidonoylglycerol (2-AG), produced a reduction in inhibitory GABAergic transmission in PAG neurons. This AEA-induced suppression of inhibition was enhanced by the fatty acid amide hydrolase (FAAH) inhibitor, URB597, whereas a 2-AG-induced suppression of inhibition was unmasked by the monoacylglycerol lipase (MGL) inhibitor, JZL184. In addition, application of the CB₁ receptor antagonist, AM251, facilitated the basal GABAergic transmission in the presence of URB597 and JZL184, which was further enhanced by the dual FAAH/MGL inhibitor, JZL195.

Conclusions and Implications

Our results indicate that AEA and 2-AG act via disinhibition within the PAG, a cellular action consistent with analgesia. These actions of AEA and 2-AG are tightly regulated by their respective degradative enzymes, FAAH and MGL. Furthermore, individual or combined inhibition of FAAH and/or MGL enhanced tonic disinhibition within the PAG. Therefore, the current findings support the therapeutic potential of FAAH and MGL inhibitors as a novel pharmacotherapy for pain.Table of Links