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Background: In recent years, growing concerns about the effects of cannabis use on mental health have renewed interest in cannabis research. In particular, there has been a marked increase in the number of neuroimaging studies of the effects of cannabinoids. We conducted a systematic review to assess the impact of acute cannabis exposure on brain function in humans and in experimental animals. Methods: Papers published until June 2012 were included from EMBASE, Medline, PubMed and LILACS databases following a comprehensive search strategy and pre-determined set of criteria for article selection. Only pharmacological challenge studies involving the acute experimental administration of cannabinoids in occasional or naïve cannabis users, and naïve animals were considered. Results: Two hundred and twenty-four studies were identified, of which 45 met our inclusion criteria. Twenty-four studies were in humans and 21 in animals. Most comprised studies of the acute effects of cannabinoids on brain functioning in the context of either resting state activity or activation during cognitive paradigms. In general, THC and CBD had opposite neurophysiological effects. There were also a smaller number of neurochemical imaging studies: overall, these did not support a central role for increased dopaminergic activity in THC-induced psychosis. There was a considerable degree of methodological heterogeneity in the imaging literature reviewed. Conclusion: Functional neuroimaging studies have provided extensive evidence for the acute modulation of brain function by cannabinoids, but further studies are needed in order to understand the neural mechanisms underlying these effects. Future studies should also consider the need for more standardised methodology and the replication of findings.

The endocannabinoid system has been involved in the control of several neurophysiological and behavioural responses. Indeed, recent studies have suggested that the cannabinoid system could represent an important substrate for the control of emotional behaviour, and further research would probably help to identify new promising therapeutic targets. This paper reviews the results obtained in different animal models used to investigate emotional states after the manipulation of the endocannabinoid system. Cannabinoid compounds can induce anxiogenic- and anxiolytic-like responses in rodents depending on the experimental conditions. Studies using knockout mice lacking the CB1 cannabinoid receptors have shown the participation of this receptor in several behavioural responses including anxiety- and depressive-like states. Furthermore, the endocannabinoid system regulates the hypothalamic-pituitary adrenal axis, which is involved in providing an appropriate response to stressful situations. Recent studies have also demonstrated that the endocannabinoids can function as retrograde messengers, modulating the release of different neurotransmitter, including opioids, GABA and cholecystokinin that have been classically involved in the control of anxiety-like responses. All this recent information has further clarified the role played by the endogenous cannabinoid system in the control of emotional behaviour and provides data to support a new possible therapeutic use of cannabinoid compounds.

Cannabis is the most widely consumed illicit drug and its consumption is currently associated with tobacco, which contains another psychoactive compound, namely nicotine. Interactions between cannabinoids and other drugs of abuse, such as opioids, have been previously reported. The aim of the present study was to evaluate the possible role of CB1 cannabinoid receptor in responses induced by acute and repeated nicotine administration by using knockout mice lacking the CB1 cannabinoid receptor and their wild-type littermates. Acute nicotine (0.5, 1, 3 and 6 mg/kg, sc) administration decreased locomotor activity and induced antinociceptive responses in the tail-immersion and the hot-plate test, in wild-type animals. The antinociceptive effects in the tail-immersion test were significantly enhanced in CB1 knockout mice. In wild-type mice nicotine (0.5 mg/kg, sc) produced a significant rewarding effect, as measured by a conditioned place preference paradigm. This response was absent in CB1 knockout mice. Finally, a model of mecamylamine-induced abstinence in chronic nicotine-treated mice (10 mg/kg/day, sc) was developed. Mecamylamine (1 and 2 mg/kg, sc) precipitated several somatic signs of nicotine withdrawal in wild-type dependent mice. However, no difference in the severity of nicotine withdrawal was observed in CB1 knockout mice. These results demonstrate that some acute effects and motivational responses elicited by nicotine can be modulated by the endogenous cannabinoid system and support the existence of a physiological interaction between these two systems.

The interaction between cholecystokinin and endogenous opioid systems on rewarding responses was examined. Motivational effects induced by peripheral administration of a complete inhibitor of enkephalin catabolism, RB 101 or the CCKB antagonist PD-134,308, and by both compounds in combination were evaluated in the conditioned place preference test in rats. RB 101 (5, 10, 20, 40 and 80 mg/kg, IP, and 20 mg/kg, IV) given alone produced a bell-shaped dose-effect function. A significant increase of the preference for the drug-associated compartment was only observed at doses of 10 and 20 mg/kg (IP). The effect observed with morphine was stronger, and all the doses used of this compound (1.25, 2.5 and 5 mg/kg, SC) were found to be active. These results suggest that the inhibitor of enkephalin catabolism has weak rewarding properties. Pretreatment with the CCKB antagonist PD-134,308 (0.1, 0.3, 1 and 3 mg/kg, IP) alone failed to produce a reliable aversion or preference on the paradigm studied. When PD-134,308 (0.3 mg/kg, IP) was coadministered with a subthreshold dose of morphine (0.6 mg/kg, SC) or RB 101 (5 mg/kg, IP), a conditioned place preference was observed, indicating that the CCKB antagonist facilitated the motivational responses induced by endogenous enkephalins as compared to morphine. This suggests that endogenous cholecystokinin, acting through CCKB receptors, modulates the rewarding effects of endogenous enkephalins.

The abuse of benzodiazepines by narcotic addicts has been well documented. However, the pharmacological basis of this abuse is not clear. In this study the effects of different benzodiazepines (flunitrazepam: 0.5, 1 and 2 mg/kg; nitrazepam: 0.5, 1, 2.5, 5 and 10 mg/kg; diazepam: 0.5, 1, 2.5, 5 and 10 mg/kg; chlordiazepoxide: 0.5, 1, 2.5, 5 and 10 mg/kg; and triazolam: 0.5, 1 and 2 mg/kg) on the morphine withdrawal syndrome in mice have been compared. Drugs were administered 30 min before naloxone-induced withdrawal. All benzodiazepines tested induced similar changes in some of the signs of morphine abstinence: decreased jumping behavior and increased wet dog shake frequency. Jumping behavior was particularly decreased by triazolam and wet dog shakes were mainly increased by flunitrazepam and nitrazepam. Forepaw treading was reduced by flunitrazepam, diazepam and triazolam, increased by nitrazepam and not changed by chlordiazepoxide. Tremor was effectively reduced by diazepam but less reliably by the other benzodiazepines. Teeth chattering was reduced by flunitrazepam and diazepam. These results indicate that benzodiazepines can interfere with the expression of the morphine withdrawal syndrome.

The effects of pretreatment with the selective cholecystokinin (CCK) B antagonists (3R-(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1, 4-benzodiazepin-3-yl)-N1-(3-methylphenyl urea (L-365,260), 4-([2-[[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[[tricyclo[3.3, 1.1(3.7)]dec-2-yloxy)carbonyl]amino]propyl]amino]-1-phenylethyl] amin)-4-oxo-[R-(R*,R*)]butanoate-N-methyl-D-glucamine (PD-134,308) and N-(2-adamantyloxycarbonyl)-D-alpha-methyltryptophanyl-[N-(2- (4-chlorophenyl)ethyl)]glycine (RB 211), on the naloxone-reversible, antinociceptive responses induced by systemic (i.v.) administration of the complete inhibitor of the enkephalin-catabolizing enzymes, N-((R,S,)-2-benzyl-3[(S)-(2-amino-4-methylthio)butyldithio]-1- oxopropyl)-L-phenylalanine benzyl ester (RB 101), were determined in rat tail-flick and mouse hot-plate tests. L-365,260 (0.12, 0.25 and 0.5 mg/kg s.c.), PD-134,308 (0.3, 1 and 3 mg/kg i.p.) and RB 211 (0.5, 1 and 1.5 mg/kg i.p.) strongly potentiated the antinociceptive effects induced by RB 101 in the rat tail-flick test, in which spinal control of nociception is predominant. Thus, the antinociception observed after the association of L-365,260 (0.5 mg/kg), RB 211 (1.5 mg/kg) or PD-134,308 (3 mg/kg) with RB 101 (5 mg/kg) was, respectively, 300, 500 and 800% higher than that observed with RB 101 given alone. This facilitatory effect was partially blocked by the administration of naloxone (1 mg/kg s.c.). Under the same conditions the potentiation of the antinociceptive response produced by morphine (0.1-4 mg/kg s.c.) was inferior to 250%. In the mouse hot-plate test, L-365,260 (0.02 and 0.1 mg/kg i.p.) and PD-134,308 (0.3, 1 and 3 mg/kg i.p.) also enhanced endogenous enkephalin induced antiociception, but this potentiating effect, completely reversed by administration of naloxone (0.1 mg/kg s.c.), was about 2 times less effective than in the tail-flick assay. The present findings demonstrate an opposing physiological role of endogenous CCK, acting on CCK B receptors, and opioid peptides in the control of pain perception at both spinal and supraspinal levels. These results could have important clinical applications because a combination of a CCK B antagonist and RB 101, which has been showed to be almost devoid of morphine side effects, would increase the overall antinociceptive efficacy into a range that will be more clinically useful.