Functional consequences of the acute administration of the cannabinoid receptor antagonist, SR141716A, in cannabinoid-naive and -tolerant animals: a quantitative 2-[14C]deoxyglucose study

Differential disruption of response alternation by precipitated Δ9-THC withdrawal and subsequent Δ9-THC abstinence in mice

In addition to overt somatic symptoms, cannabinoid withdrawal can also manifest as disruptions in motivation and attention. Experimental animal models using operant-conditioning approaches reveal these differences, in either antagonist-precipitated or spontaneous withdrawal models. However, these processes have yet to be characterized in the same subjects simultaneously. To differentiate between motivational and attentional processes disrupted in cannabinoid withdrawal, the current study used a response alternation task in which a fixed-ratio (FR) schedule repeatedly alternated between two spatially distinct response options throughout daily training sessions. This task yielded traditional measures of motivation (e.g., response latency) as well as attention (e.g., responses to the incorrect side). After two weeks of training, male and female C57BL/6 J mice either received vehicle or Δ9-THC (10 mg/kg, s.c.) twice daily for 5 days. On the 6th day, all mice received their final injection of vehicle or Δ9-THC followed 30 min later by injection of the CB1 receptor selective inverse agonist rimonabant (2 mg/kg, i.p.) to precipitate withdrawal. Testing continued for 3 days post-rimonabant to assess how THC abstinence impacted task performance. Whereas rimonabant decreased response rates to equal degrees in THC-treated and vehicle-treated mice, THC-treated mice showed longer session times, longer response latencies, and more errors per reinforcer. Only THC-treated mice showed a longer latency to switch after committing an error reflecting that precipitated withdrawal impacted measures of both motivation and attention. During the 3-day abstinence window, performance of vehicle-treated mice returned to baseline, but THC-treated mice continued to show disruptions in motivational measures. Importantly, attentional measures (errors and latency to switch after an error) were unaffected by THC abstinence. These data suggest that precipitated and "spontaneous" cannabinoid withdrawal may be qualitatively and quantitatively distinct withdrawal conditions with precipitated withdrawal disrupting both attentional and motivational processes, while abstinence may only affect motivation.

Impaired hippocampal glucoregulation in the cannabinoid CB1 receptor knockout mice as revealed by an optimized in vitro experimental approach

Several techniques exist to study the rate of glucose uptake and metabolism in the brain but most of them are not sufficiently robust to permit extensive pharmacological analysis. Here we optimized an in vitro measurement of the simultaneous accumulation of the metabolizable and non-metabolizable (3)H and (14)C d-glucose analogues; permitting convenient large-scale studies on glucose uptake and metabolism in brain slices. Next, we performed an extensive pharmacological characterization on the putative glucoregulator role of the endocannabinoid system in the hippocampal slices of the rat, and the wild-type and the CB(1) cannabinoid receptor (CB(1)R) knockout mice. We observed that (3)H-3-O-methylglucose is a poor substrate to measure glucose uptake in the hippocampus. (3)H-2-deoxyglucose is a better substrate but its uptake is still lower than that of (14)C-U-d-glucose, from which the slices constantly metabolize and dissipate (14)C atoms. Thus, uptake and the metabolism values are not to be used as standalones but as differences between a control and a treatment. The CB(1)R knockout mice exhibited ∼10% less glucose uptake and glucose carbon atom dissipation in comparison with the wild-type mice. This may represent congenital defects as acute treatments of the rat and mouse slices with cannabinoid agonists, antagonists and inhibitors of endocannabinoid uptake/metabolism failed to induce robust changes in either the uptake or the metabolism of glucose. In summary, we report here an optimized technique ideal to complement other metabolic approaches of high spatiotemporal resolution. This technique allowed us concluding that CB(1)Rs are at least indirectly involved in hippocampal glucoregulation.

Detailed analysis of food-reinforced operant lever pressing distinguishes effects of a cannabinoid CB1 inverse agonist and dopamine D1 and D2 antagonists

Overt similarities exist between the effects of systemic cannabinoid CB1 inverse agonists and dopamine (DA) antagonists on appetitive behavior. The present set of studies was undertaken to apply a fine-grained analysis of food-reinforced operant lever pressing in rats in order to compare the pattern of effects produced by administration of the CB1 inverse agonist AM 251 and those induced by the DA D1 antagonist SKF 83566, and the D2 antagonist raclopride. Three groups of rats were trained on a fixed-ratio 5 (FR5) schedule and administered these compounds over a range of doses expected to suppress responding. All three drugs produced a dose-related suppression of total lever pressing. In addition to main effects of dose, regression analyses were performed to determine which of several response timing- and rate-related variables correlated most strongly with overall responding in each group. It was found that total session time spent pausing from responding was significantly better at predicting responding in the AM 251 group, while both DA antagonists produced significantly stronger regression coefficients (versus AM 251) from fast responding measures. These results suggest that, while several similarities exist, CB1, D1, and D2 antagonists are not identical in their pattern of suppression of food-maintained lever pressing.

Plasticity of central autonomic neural circuits in diabetes

Regulation of energy metabolism is controlled by the brain, in which key central neuronal circuits process a variety of information reflecting nutritional state. Special sensory and gastrointestinal afferent neural signals, along with blood-borne metabolic signals, impinge on parallel central autonomic circuits located in the brainstem and hypothalamus to signal changes in metabolic balance. Specifically, neural and humoral signals converge on the brainstem vagal system and similar signals concentrate in the hypothalamus, with significant overlap between both sensory and motor components of each system and extensive cross-talk between the systems. This ultimately results in production of coordinated regulatory autonomic and neuroendocrine cues to maintain energy homeostasis. Therapeutic metabolic adjustments can be accomplished by modulating viscerosensory input or autonomic motor output, including altering parasympathetic circuitry related to GI, pancreas, and liver regulation. These alterations can include pharmacological manipulation, but surgical modification of neural signaling should also be considered. In addition, central control of visceral function is often compromised by diabetes mellitus, indicating that circuit modification should be studied in the context of its effect on neurons in the diabetic state. Diabetes has traditionally been handled as a peripheral metabolic disease, but the central nervous system plays a crucial role in regulating glucose homeostasis. This review focuses on key autonomic brain areas associated with management of energy homeostasis and functional changes in these areas associated with the development of diabetes.

Augmented acquisition of cocaine self-administration and altered brain glucose metabolism in adult female but not male rats exposed to a cannabinoid agonist during adolescence

Marijuana consumption during adolescence has been proposed to be a stepping-stone for adult cocaine addiction. However, experimental evidence for this hypothesis is missing. In this work we chronically injected male and female Wistar rats with either the cannabinoid agonist CP 55,940 (CP; 0.4 mg/kg) or its corresponding vehicle. Adult acquisition (seven 30 min daily sessions) and maintenance (fourteen 2 h daily sessions) of cocaine self-administration (1 mg/kg), food-reinforced operant learning under conditions of normal (ad libitum access to food), and high motivation (food-restriction schedule) were measured. Additionally, brain metabolic activity was analyzed by means of [(18)F]-fluorodeoxyglucose positron emission tomography. During the acquisition phase, female CP-treated rats showed a higher rate of cocaine self-administration as compared to vehicle-treated females and males; no differences were found between both male groups. This effect disappeared in the maintenance phase. Moreover, no differences among groups were evident in the food-reinforced operant task, pointing to the cocaine-specific nature of the effect seen in self-administration rather than a general change in reward processing. Basal brain metabolic activity also changed in CP-treated females when compared to their vehicle-treated counterparts with no differences being found in the males; more specifically we observed a hyper activation of the frontal cortex and a hypo activation of the amygdalo-entorhinal cortex. Our results suggest that a chronic exposure to cannabinoids during adolescence alters the susceptibility to acquire cocaine self-administration, in a sex-specific fashion. This increased susceptibility could be related to the changes in brain metabolic activity induced by cannabinoids during adolescence.

Increase of cannabinoid CB1 receptor density in the hippocampus of streptozotocin-induced diabetic rats

In the hippocampus, impaired neurophysiology, compromised neurogenesis, and eventually apoptosis accompany cognitive deficits in insulinopenic (type-1) diabetes (T1D). The underlying pathological mechanisms remain to be defined. The hippocampus has a high density of the cannabinoid type 1 receptor (CB(1)R), which has been shown to control several brain functions affected by diabetes, such as synaptic plasticity, glucose utilisation, memory consolidation and cognition, and maturation and survival of hippocampal neurons. However, the role of this receptor has not been investigated yet in diabetic encephalopathy. We report now that in the streptozotocin animal model of T1D, the hippocampal densities of CB(1)R protein and of specific CB(1)R binding sites are significantly increased both in the nerve terminals and in total membranes (changes between 13% and 38%), whereas CB(1)R mRNA expression is decreased by 25%, suggesting that CB(1)Rs might play a role in diabetic encephalopathy.

The endocannabinoid system as an emerging target of pharmacotherapy

The recent identification of cannabinoid receptors and their endogenous lipid ligands has triggered an exponential growth of studies exploring the endocannabinoid system and its regulatory functions in health and disease. Such studies have been greatly facilitated by the introduction of selective cannabinoid receptor antagonists and inhibitors of endocannabinoid metabolism and transport, as well as mice deficient in cannabinoid receptors or the endocannabinoid-degrading enzyme fatty acid amidohydrolase. In the past decade, the endocannabinoid system has been implicated in a growing number of physiological functions, both in the central and peripheral nervous systems and in peripheral organs. More importantly, modulating the activity of the endocannabinoid system turned out to hold therapeutic promise in a wide range of disparate diseases and pathological conditions, ranging from mood and anxiety disorders, movement disorders such as Parkinson's and Huntington's disease, neuropathic pain, multiple sclerosis and spinal cord injury, to cancer, atherosclerosis, myocardial infarction, stroke, hypertension, glaucoma, obesity/metabolic syndrome, and osteoporosis, to name just a few. An impediment to the development of cannabinoid medications has been the socially unacceptable psychoactive properties of plant-derived or synthetic agonists, mediated by CB(1) receptors. However, this problem does not arise when the therapeutic aim is achieved by treatment with a CB(1) receptor antagonist, such as in obesity, and may also be absent when the action of endocannabinoids is enhanced indirectly through blocking their metabolism or transport. The use of selective CB(2) receptor agonists, which lack psychoactive properties, could represent another promising avenue for certain conditions. The abuse potential of plant-derived cannabinoids may also be limited through the use of preparations with controlled composition and the careful selection of dose and route of administration. The growing number of preclinical studies and clinical trials with compounds that modulate the endocannabinoid system will probably result in novel therapeutic approaches in a number of diseases for which current treatments do not fully address the patients' need. Here, we provide a comprehensive overview on the current state of knowledge of the endocannabinoid system as a target of pharmacotherapy.

The role of the endocannabinoid system in the control of energy homeostasis

The endocannabinoid system has recently emerged as an important regulator of energy homeostasis, involved in the control of both appetite and peripheral fat metabolism. We briefly review current understanding of the possible sites of action and cellular mechanisms involved in the central appetitive and peripheral metabolic effects of endocannabinoids. Studies in our laboratory, using leptin-deficient obese rodents and CB1 cannabinoid receptor (CB1)-deficient mice, have indicated that endocannabinoids acting via CB1 are involved in the hunger-induced increase in food intake and are negatively regulated by leptin in brain areas involved in appetite control, including the hypothalamus, limbic forebrain and amygdala. CB1-/- mice are lean and are resistant to diet-induced obesity (DIO) despite similar energy intake to wild-type mice with DIO, suggesting that CB1 regulation of body weight involves additional peripheral targets. Such targets appear to include both adipose tissue and the liver. CB1 expressed in adipocytes has been implicated in the control of adiponectin secretion and lipoprotein lipase activity. Recent findings indicate that both endocannabinoids and CB1 are present in the liver and are upregulated in DIO. CB1 stimulation increases de novo hepatic lipogenesis through activation of the fatty acid biosynthetic pathway. Components of this pathway are also expressed in the hypothalamus where they have been implicated in the regulation of appetite. The fatty acid biosynthetic pathway may thus represent a common molecular target for the central appetitive and peripheral metabolic effects of endocannabinoids.

Cannabinoid tolerance and dependence

The use of marijuana for recreational and medicinal purposes has resulted in a large prevalence of chronic marijuana users. Consequences of chronic cannabinoid administration include profound behavioral tolerance and withdrawal symptoms upon drug cessation. A marijuana withdrawal syndrome is only recently gaining acceptance as being clinically significant. Similarly, laboratory animals exhibit both tolerance and dependence following chronic administration of cannabinoids. These animal models are being used to evaluate the high degree of plasticity that occurs at the molecular level in various brain regions following chronic cannabinoid exposure. In this review, we describe recent advances that have increased our understanding of the impact of chronic cannabinoid administration on cannabinoid receptors and their signal transduction pathways. Additionally, we discuss several potential pharmacotherapies that have been examined to treat marijuana dependence.

Imaging of the brain cannabinoid system

This review covers two major strategies for imaging of the brain cannabinoid system: autoradiography and in vivo neuroimaging. Cannabinoid receptors can be imaged directly with autoradiography in brain slices using radiolabeled cannabinoid receptor ligands. In addition, the effects of pharmacologic doses of unlabeled cannabinoid drugs can be autoradiographically imaged using indicators of blood flow or indicators of metabolism such as glucose analogs. Although cannabinoid imaging is a relatively new topic of research compared to imaging of other drugs of abuse, autoradiographic strategies have produced high-quality information about the distribution of brain cannabinoid receptors and the effects of cannabinoid drugs on brain metabolism. In vivo neuroimaging, in contrast to autoradiography, utilizes noninvasive techniques such as positron emission tomography (PET), single photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI) to image both the binding and the effects of drugs within living brain. These techniques are well developed; however, in vivo imaging of cannabinoid systems is in a very preliminary state. Early results have been promising yet hard to generalize. Definitive answers to some of the most important questions about cannabinoid drugs and their effects await development of suitable in vivo neuroimaging ligands for cannabinoid systems.

Endocannabinoid activation at hepatic CB1 receptors stimulates fatty acid synthesis and contributes to diet-induced obesity

Endogenous cannabinoids acting at CB(1) receptors stimulate appetite, and CB(1) antagonists show promise in the treatment of obesity. CB(1) (-/-) mice are resistant to diet-induced obesity even though their caloric intake is similar to that of wild-type mice, suggesting that endocannabinoids also regulate fat metabolism. Here, we investigated the possible role of endocannabinoids in the regulation of hepatic lipogenesis. Activation of CB(1) in mice increases the hepatic gene expression of the lipogenic transcription factor SREBP-1c and its targets acetyl-CoA carboxylase-1 and fatty acid synthase (FAS). Treatment with a CB(1) agonist also increases de novo fatty acid synthesis in the liver or in isolated hepatocytes, which express CB(1). High-fat diet increases hepatic levels of the endocannabinoid anandamide (arachidonoyl ethanolamide), CB(1) density, and basal rates of fatty acid synthesis, and the latter is reduced by CB(1) blockade. In the hypothalamus, where FAS inhibitors elicit anorexia, SREBP-1c and FAS expression are similarly affected by CB(1) ligands. We conclude that anandamide acting at hepatic CB(1) contributes to diet-induced obesity and that the FAS pathway may be a common molecular target for central appetitive and peripheral metabolic regulation.

Effects of melanocortin receptor activation and blockade on ethanol intake: a possible role for the melanocortin-4 receptor

BACKGROUND

The melanocortin (MC) system is composed of peptides that are cleaved from the polypeptide precursor pro-opiomelanocortin. A growing body of literature suggests that the MC system modulates neurobiological responses to drugs of abuse. Because ethanol has direct effects on central pro-opiomelanocortin activity, it is possible that MC neuropeptides participate in the control of voluntary ethanol consumption. Here we assessed the possibility that MC receptor (MCR) agonists modulate ethanol intake via the MC3 receptor (MC3R) and/or the MC4 receptor (MC4R) and whether the MCR antagonist AgRP-(83-132) controls ethanol consumption.

METHODS

Mc3r-deficient (Mc3r) and wild-type (Mc3r) littermate mice were given intraperitoneal (10 mg/kg) and intracerebroventricular (1.0 microg ICV) doses of melanotan II (MTII), a nonselective MCR agonist. To assess the role of MC4R, C57BL/6J mice were given an ICV infusion of the highly selective MC4R agonist cyclo(NH-CH2-CH2-CO-His-d-Phe-Arg-Trp-Glu)-NH2 (1.0 or 3.0 microg). Finally, naïve C57BL/6J mice were given an ICV infusion of AgRP-(83-132) (0.05 and 1.0 microg).

RESULTS

MTII was similarly effective at reducing ethanol drinking in Mc3r-deficient (Mc3r) and wild-type (Mc3r) littermate mice. Furthermore, ICV infusion of the MC4R agonist significantly reduced ethanol drinking, whereas ICV infusion of AgRP-(83-132) significantly increased ethanol drinking in C57BL/6J mice. Neither MTII nor AgRP-(83-132) altered blood ethanol levels at doses that modulated ethanol drinking.

CONCLUSIONS

The present results suggest that MC4R, and not MC3R, modulates MCR agonist-induced reduction of ethanol consumption and that ethanol intake is increased by the antagonistic actions of AgRP-(83-132). These findings strengthen the argument that MCR signaling controls ethanol consumption and that compounds directed at MCR may represent promising targets for treating alcohol abuse disorders in addition to obesity.

Cocaine- and amphetamine-related transcript is involved in the orexigenic effect of endogenous anandamide

Endocannabinoids acting at CB1 cannabinoid receptors (CB1) increase appetite. In view of the predominant presynaptic localization of CB1 in the brain, we tested the hypothesis that the orexigenic effect of endocannabinoids involves inhibition of the release of a tonically active anorexigenic mediator, such as the peptide product of the cocaine- and amphetamine-related transcript (CART). The CB1 antagonist rimonabant inhibited food intake in food-restricted wild-type mice, but not in their CART-deficient littermates. Mice deficient in fatty acid amide hydrolase (FAAH), the enzyme responsible for the in vivo metabolism of the endocannabinoid anandamide, have reduced levels of CART-immunoreactive nerve fibers and terminals in several brain regions implicated in appetite control, including the arcuate, dorsomedial and periventricular nuclei of the hypothalamus, the amygdala, the bed nucleus of the stria terminalis and the nucleus accumbens, and treatment of FAAH(-/-) mice with rimonabant, 3 mg/kg/day for 7 days, increased CART levels toward those seen in FAAH(+/+) wild-type controls. In contrast, no difference in the density of CART-immunoreactive fibers was observed in the median eminence and the paraventricular nucleus of FAAH(+/+) and FAAH(-/-) mice. Acute treatment of wild-type mice with the cannabinoid agonist HU-210 resulted in elevated CART levels in the dorsomedial nucleus and the shell portion of the nucleus accumbens. These observations are compatible with CART being a downstream mediator of the CB1-mediated orexigenic effect of endogenous anandamide.

Cannabinoid physiology and pharmacology: 30 years of progress

Delta9-Tetrahydrocannabinol from Cannabis sativa is mimicked by cannabimimetic analogs such as CP55940 and WIN55212-2, and antagonized by rimonabant and SR144528, through G-protein-coupled receptors, CB1 in the brain, and CB2 in the immune system. Eicosanoids anandamide and 2-arachidonoylglycerol are the "endocannabinoid" agonists for these receptors. CB1 receptors are abundant in basal ganglia, hippocampus and cerebellum, and their functional activity can be mapped during behaviors using cerebral metabolism as the neuroimaging tool. CB1 receptors couple to G(i/o) to inhibit cAMP production, decrease Ca2+ conductance, increase K+ conductance, and increase mitogen-activated protein kinase activity. Functional activation of G-proteins can be imaged by [35S]GTPgammaS autoradiography. Post-synaptically generated endocannabinoids form the basis of a retrograde signaling mechanism referred to as depolarization-induced suppression of inhibition (DSI) or excitation (DSE). Under circumstances of sufficient intracellular Ca2+ (e.g., burst activity in seizures), synthesis of endocannabinoids releases a diffusible retrograde messenger to stimulate presynaptic CB1 receptors. This results in suppression of gamma-aminobutyric acid (GABA) release, thereby relieving the post-synaptic inhibition. Tolerance develops as neurons adjust both receptor number and cellular signal transduction to the chronic administration of cannabinoid drugs. Future therapeutic drug design can progress based upon our current understanding of the physiology and pharmacology of CB1, CB2 and related receptors. One very important role for CB1 antagonists will be in the treatment of craving in the disease of substance abuse.

Behavioral mechanisms underlying inhibition of food-maintained responding by the cannabinoid receptor antagonist/inverse agonist SR141716A

This study investigated the possible behavioral mechanisms underlying the anorectic effect of the cannabinoid CB(1) receptor antagonist/inverse agonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A). Male or female rats were food-restricted and trained to emit stable responding in daily 10-min, fixed ratio 10 food-reinforced operant sessions. Under these conditions, as well as under free-feeding conditions, SR141716A inhibited food-maintained responding (ED(50) values ranging from 0.92 to 2.52 mg/kg, i.p.). In the same operant procedure, SR141716A suppressed intracranial self-stimulation with a potency which was slightly lower than the anorectic potency (ED(50): 4.50 mg/kg). As assessed during a 10-min test period SR141716A (1-10 mg/kg) did not affect activity counts; suggesting that the observed inhibition of operant behavior is not a direct consequence of impairment of locomotor activity. SR141716A, however, attenuated saccharin-preference in a conditioned taste aversion paradigm (ED(50): 6.45 mg/kg). Although the data support the suggestion that the anorectic effect of SR141716A results from an attenuating effect on the rewarding effect of food, the contribution of drug-induced aversion/malaise cannot be excluded.

The cannabinoid CB1 antagonists SR 141716A and AM 251 suppress food intake and food-reinforced behavior in a variety of tasks in rats

Cannabinoid CB1 receptor agonists, including delta-9-tetrahydrocannabinol (Delta 9-THC) (the main psychoactive ingredient in marijuana) have been shown to increase feeding in rats and humans. Conversely, it has been reported that acute administration of the CB1 receptor antagonist SR 141716A reduces food intake in rats. Based upon this observation, it has been suggested that CB1 antagonists could be useful as appetite suppressant drugs. The present studies were designed to provide a detailed examination of the effects of CB1 antagonists on food intake across a range of paradigms. Two CB1 antagonists (SR 141716A and AM 251) were administered to rats trained on fixed-ratio schedules with two different ratio requirements (fixed-ratio 1 and fixed-ratio 5). Both drugs produced a dose-dependent decrease in lever pressing, and had a relatively long duration of action (T1/2: SR 141716A, 15.1 h; AM 251, 22.0 h). Furthermore, intake of three diets with differing macronutrient composition (lab chow, high fat, high carbohydrate) was studied. Both drugs significantly suppressed intake of all three foods, and there were no significant interactions between drug dose and diet type. These findings support the hypothesis that CB1 receptor antagonists could be useful pharmacological tools for the suppression of appetite.