Cannabidiol is a non-psychotomimetic constituent of Cannabis sativa, which induces central effects in rodents. It has been shown that cannabidiol attenuates cue-induced reinstatement of heroin seeking. However, to the best of our knowledge, its effects on brain stimulation reward and the reward-facilitating effects of drugs of abuse have not yet been examined. Therefore, we investigated the effects of cannabidiol on brain reward function and on the reward-facilitating effect of morphine and cocaine using the intracranial self-stimulation (ICSS) paradigm. Rats were prepared with a stimulating electrode into the medial forebrain bundle (MFB), and a guide cannula into the dorsal raphe (microinjection experiments), and were trained to respond for electrical brain stimulation. A low dose of cannabidiol did not affect the reinforcing efficacy of brain stimulation, whereas higher doses significantly elevated the threshold frequency required for MFB ICSS. Both cocaine and morphine lowered ICSS thresholds. Cannabidiol inhibited the reward-facilitating effect of morphine, but not cocaine. This effect was reversed by pre-treatment with an intra-dorsal raphe injection of the selective 5-HT1A receptor antagonist WAY-100635. The present findings indicate that cannabidiol does not exhibit reinforcing properties in the ICSS paradigm at any of the doses tested, while it decreases the reward-facilitating effects of morphine. These effects were mediated by activation of 5-HT1A receptors in the dorsal raphe. Our results suggest that cannabidiol interferes with brain reward mechanisms responsible for the expression of the acute reinforcing properties of opioids, thus indicating that cannabidiol may be clinically useful in attenuating the rewarding effects of opioids.

Chronic nicotine (NIC) pretreatment has been shown to enhance NIC‐induced locomotor stimulation, an effect that seems critically dependent on activation of brain dopamine (DA) systems. In the present study the effects of chronic, intermittent NIC treatment were examined in the rat to establish whether such behavioral sensitization is associated with specific, regional changes in brain dopaminergic activity. Male rats received daily injections in their home cage with either saline (SAL) or NIC (0.5 mg/kg, s.c.) for 12 days. Twenty‐four hours later, the locomotor activity of the animals subjected to NIC challenge as well as the functional responsiveness of the mesolimbocortical dopaminergic system were assessed. To this end, microdialysis experiments were performed in awake animals, measuring extracellular concentrations of DA and its metabolites in the prefrontal cortex (PFC) and the nucleus accumbens (NAC). Extracellular single cell recordings from DA neurons in the ventral tegmental area (VTA) were also performed in anesthetized animals. NIC (0.5 mg/kg, s.c.) increased all measured parameters of locomotor activity, with the exception of rearing, in SAL‐pretreated animals; these effects were substantially enhanced after pretreatment with NIC. Nicotine (0.5 mg/kg, s.c.) increased DA release in both the PFC and the NAC in SAL‐treated animals. Nicotine pretreatment significantly enhanced this effect in the PFC, whereas it did not affect the response in the NAC. Low doses of intravenously administered NIC dose‐dependently increased burst activity, starting at 12 μg/kg in the SAL pretreated animals and at 6 μg/kg in the NIC‐pretreated animals, and also dose‐dependently increased firing rate in SAL as well as NIC‐pretreated animals, although starting at a higher dose level, i.e., 25 μg/kg. These results demonstrate that behavioral sensitization after chronic NIC treatment is accompanied by an enhanced dopamine release specifically within the PFC. This phenomenon may be highly significant for the dependence‐producing effects of NIC, particularly in association with major psychiatric disorder, such as schizophrenia. © 1996 Wiley‐Liss, Inc.

We have previously provided evidence that the stimulatory action of systemic nicotine on dopamine release in the rat nucleus accumbens is initiated in the ventral tegmental area (VTA), and that it appears to be mediated partly through an indirect, presynaptic mechanism. Thus, it was found that blockade of N‐methyl‐D‐aspartate (NMDA) receptors in the VTA attenuates the enhancing effect of nicotine on extracellular levels of dopamine in the nucleus accumbens. Moreover, the nicotine‐induced dopamine output in the nucleus accumbens was found to be blocked by pretreatment with methyllycaconitine (MLA) in the VTA, indicating a role for α7* nicotinic acetylcholine receptors (nAChRs) in this mechanism. Thus, nicotine may exert its effects in the VTA through stimulation of α7* nAChRs localized on excitatory amino acid (EAA)ergic afferents. To test this hypothesis, we here measured extracellular concentrations of glutamate and aspartate in the VTA in response to systemic nicotine, with or without concurrent infusion of MLA in the VTA, using microdialysis in anaesthetized rats. Since the medial prefrontal cortex is an important source of EAA input to the VTA, we also assessed the density of α‐bungarotoxin binding sites in the VTA in rats lesioned bilaterally in the prefrontal cortex with ibotenic acid and in sham‐lesioned rats by means of quantitative autoradiography. Nicotine (0.5 mg/kg, s.c.) significantly increased extracellular levels of both aspartate and glutamate in the VTA. MLA (0.3 mM) infused locally in the VTA prevented the nicotine‐induced increase in glutamate and aspartate levels. Ibotenic acid lesions of the prefrontal cortex decreased the density of α‐bungarotoxin binding sites in the VTA by about 30%. These data indicate that nicotine increases the extracellular levels of excitatory amino acids in the VTA through stimulation of nAChRs in the VTA and that part of the α7* nAChR population in the VTA is localized on neurons originating in the prefrontal cortex. Synapse 38:375–383, 2000. © 2000 Wiley‐Liss, Inc.

Brain mesolimbic dopamine (DA) neurons are considered critical for the dependence-producing action of nicotine, and its stimulatory effect on behavior and DA neurotransmission appears largely mediated via nicotinic receptors (nAChRs) in the ventral tegmental area (VTA). The nAChR antagonist mecamylamine administered systemically in chronically nicotine-treated rats elicits a behavioral withdrawal syndrome concomitant with a reduced DA output in the nucleus accumbens (NAC). Here, we investigated the behavioral and biochemical consequences of intrategmental administration of mecamylamine in rats chronically infused with nicotine by means of minipumps for 14 days (9 mg/kg/day). Bilateral, intrategmental mecamylamine injections (1, 3 or 9 g/0.5 l/ side) dose-dependently increased abstinence signs such as gasps, teeth chatter, and reduced locomotor activity in nicotine-treated, but not in control animals. Moreover, a unilateral intrategmental injection of 9 g mecamylamine reduced DA output in the ipsilateralThe habit of tobacco smoking is widely considered to represent a form of drug addiction to nicotine (Clarke 1990;Corrigall et al. 1992) and, in fact, constitutes the leading preventable cause of human morbidity and premature mortality in the US (US Department of Health and Human Services 1988). Although smokers are usually aware of the negative health consequences of their habit, the prognosis of successful smoking cessation is relatively poor. Thus, approximately 80% of smokers enrolled in cessation programmes fail to quit (Stitzer and Gross 1988). A nicotine withdrawal reaction, which usually emerges within 24 hrs after the last cigarette, is considered to significantly contribute to the high relapse rate during the early stages of attempted cessation (Shiffman and Jarvik 1976;Hughes et al. 1992;Killen et al. 1991;West et al. 1989), although a lack of correlation Received December 10, 1998; revised April 7, 1999; accepted April 20, 1999. N EUROPSYCHOPHARMACOLOGY 1999 -VOL . 21 , NO . 4 Role of VTA Nicotinic Receptors in Nicotine Withdrawal 561 between withdrawal severity and probability for relapse has been reported in other studies (see Hughes et al. 1991).In rats chronically treated with nicotine, sudden withdrawal precipitated by systemic administration of the non-competitive nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (Malin et al. 1994;Hildebrand et al. 1997), as well as spontaneous withdrawal caused by termination of a chronic nicotine infusion (Malin et al. 1992;Hildebrand et al. 1997), produces behavioral symptoms similar to those described in opiate withdrawal (see Bläsig et al. 1973). Acute as well as chronic administration of nicotine increases locomotor activity in rats (Benwell and Balfour 1992;Clarke and Kumar 1983;Stolerman et al. 1973), whereas in nicotine withdrawal locomotor activity is markedly reduced (Malin et al. 1992(Malin et al. , 1994Hildebrand et al. 1997).Compelling evidence indicates that the mesolimbic dopamine (DA) system plays a pivotal role in mediati...

Δ(9)-tetrahydrocannabinol (Δ(9)-THC), the main psychoactive ingredient of marijuana, has led to equivocal results when tested with the intracranial self-stimulation (ICSS) procedure or the open-field test for motor activity, two behavioural models for evaluating the reward-facilitating and locomotor stimulating effects of drugs of abuse, respectively. Therefore, in the present study, the effects of high and low doses of Δ(9)-THC were compared in the ICSS procedure and the open-field test. Moreover, the involvement of CB(1) receptors in tentative Δ(9)-THC-induced effects was investigated by pre-treating the animals with the CB(1) receptor antagonist SR141716A (rimonabant). The results obtained show that low doses of Δ(9)-THC induce opposite effects from high doses of Δ(9)-THC. Specifically, 0.1 mg/kg Δ(9)-THC decreased ICSS thresholds and produced hyperactivity, whereas 1 mg/kg increased ICSS thresholds and produced hypoactivity. Both effects were reversed by pre-treatment with SR141716A, indicating the involvement of CB(1) receptors on these actions. Altogether, our results indicate that Δ(9)-THC can produce acute activating effects in locomotion that coincide with its reward-facilitating effects in the ICSS paradigm. The present findings provide further support that Δ(9)-THC induces behaviours typical of abuse and substantiate the notion that marijuana resembles other drugs of abuse.

It has been shown that nicotine facilitates intracranial self-stimulation (ICSS) reward and that nicotinic acetylcholine receptors (nAChRs) in the ventral tegmental area (VTA) are of primary importance for its reinforcing and dependence-producing actions. Recently, we have shown that alpha 7 nicotinic receptors in the VTA contribute to both the acute effects of nicotine on the mesolimbic dopamine system, as well as to nicotine withdrawal reactions. However, it is not yet known whether the same receptor conformation is directly involved in the reinforcing actions of nicotine. Here, using the curve-shift method we studied the effects of methyllycaconitine (MLA), a selective alpha 7 receptor antagonist, microinjected (graded doses: 1, 3, 9 micrograms/microliter per side) into the VTA on the rewarding efficacy of lateral hypothalamic self-stimulation and on the systemic nicotine-induced potentiation of brain stimulation reward. MLA did not affect baseline self-stimulation. Nicotine produced a significant reduction in ICSS threshold, without altering maximal rates of responding, while MLA attenuated the effect of nicotine at the two lower doses. Given the reported interaction between nicotine and cocaine at both the neuronal and the behavioral level, we also examined whether alpha 7 receptor antagonism within the VTA can affect the reinforcing action of cocaine, as measured with ICSS. Interestingly, MLA attenuated the reinforcing effect of cocaine in all doses tested, without altering the maximal rate of responding, i.e. the performance of the animals. These results suggest that alpha 7 nAChRs in the VTA are involved in mediating the reinforcing actions of drugs of abuse, such as nicotine and cocaine, and provide evidence that alpha 7 nAChR antagonists may be clinically useful in attenuating the rewarding effects of addictive drugs.

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