Experiments are being conducted to assess the different neuropharmacological and behavioral mechanisms underlying behavior controlled by drugs as discriminative stimuli in rats and monkeys and the ability of pharmacological or behavioral manipulations to modify discrimination, as well as self-administration, of THC or nicotine, to disrupt ongoing food-maintained behavior to alter emotional responses such as anxiety or to modulate attention learning and memory processes. Currently, studies are focusing on nicotine and a series of cannabinoids, including delta-9-tetrahydrocannabinol (THC), the psychoactive ingredient in marijuana, the cannabinoid CB1 receptor antagonists Rimonabant and AM251, the endogenous cannabinoids anandamide and 2AG, the anandamide uptake inhibitor AM404, and the fatty acid amide hydrolase (FAAH) inhibitor URB597. Studies are also being conducted on methamphetamine, cocaine and heroin.? ? There is previous data indicating involvement of the cholinergic system in the psychotropic effects of cannabis and systemic administration of the main active ingredient in cannabis, delta-9-tetrahydrocannabinol (THC), has been reported to increase or to decrease extra-cellular levels of acetylcholine in several brain areas. In a recent study Solinas et al., 2007), we investigated whether drugs acting at either nicotinic or muscarinic receptors could modulate the discriminative effects of THC. In rats that had learned to discriminate 3mg/kg of THC from vehicle injections, the nicotinic agonist nicotine and the muscarinic agonist pilocarpine did not produce THC-like effects but they both potentiated the discriminative effects of low doses of THC. Neither the nicotinic antagonist mecamylamine nor the muscarinic antagonist scopolamine altered the discriminative effects of THC but they blocked the potentiation induced by nicotine and pilocarpine, respectively. In order to further investigate the nicotinic receptor subtypes involved in the discriminative effects of THC we then made use of selective antagonists for beta-2 or alpha-7 subunits. The selective alfa-7 nicotinic receptor antagonist MLA but not the selective beta-2 nicotine antagonist DHBE significantly reduced the discriminative effects of THC. However, both MLA and DHBE reversed the potentiation induced by nicotine. Our results suggest that the cholinergic system is involved in the discriminative effects of THC. Furthermore, our results with selective nicotinic antagonists suggest that potentiation or reduction of THCs effects may depend on different mechanisms. Finally, alpha-7 nicotinic antagonists could be useful in the treatment of cannabis intoxication and abuse.? ? It is known that strong functional interactions exist between endogenous cannabinoid and adenosine systems. The ventral striatum (mostly represented by the nucleus accumbens) forms part of the brain circuitry involved in goal-directed behavior, the conversion of motivation into action. A common denominator in the neurochemical effects of most addictive substances, including opiates, ethanol, amphetamines, cocaine, nicotine in tobacco and G9-tetrahydrocannabinol (THC) in marijuana, is striatal dopamine release, with a preferential effect on the ventral striatum (shell of the nucleus accumbens). Since the ventral striatum contains a high density of adenosine A2A receptors, which strongly modulate excitatory synapses of GABAergic enkephalinergic neurons, adenosinergic modulation of drug-seeking and drug-taking behavior is likely.? ? We have now demonstrated functional and physical interactions between adenosine A2A and cannabinoid CB1 receptors (Carriba et al., 2007; Ferre et al., 2007). Also, adenosine A2A receptor antagonists can potentiate the reinforcing effects of nicotine and the discriminative effects of nicotine, methamphetamine and cocaine in rats (Munzar et al. 2002; Justinova et al. 2003). Recently (Justinova et al., 2008)we investigated the relative involvement of adenosine A1 and A2A receptors in nicotine, cocaine and methamphetamine discrimination, before and/or during chronic caffeine exposure. The non-selective adenosine antagonist caffeine, the A1-receptor antagonist CPT and the A2A-receptor antagonist MSX-3 were evaluated in rats trained to discriminate 0.4 mg/kg nicotine from saline under a fixed-ratio schedule of food delivery. Effects of antagonists were then compared in rats discriminating nicotine, methamphetamine or cocaine during chronic caffeine exposure in their drinking water. Caffeine, CPT and MSX-3 partially generalized to nicotine and shifted nicotine dose-response curves leftwards. During chronic caffeine exposure, however, all three ligands failed to generalize to nicotine and failed to shift nicotine dose-response curves. In previous experiments, CPT and MSX-3 partially generalized to methamphetamine and cocaine and shifted dose-response curves leftwards. In the present experiments, CPT neither generalized nor shifted dose-response curves for methamphetamine or cocaine during chronic caffeine exposure. However, MSX-3 partially generalized to both psychostimulants and shifted their dose-response curves leftwards. Caffeine partially generalized to cocaine, but not methamphetamine, and shifted both dose-response curves leftwards. Both adenosine A1 and A2A receptors are capable of modulating the discriminative-stimulus effects of nicotine. Chronic caffeine exposure produces complete tolerance to both A1- and A2A-mediated effects in nicotine-trained rats. In contrast, chronic caffeine exposure produces tolerance to adenosine A1-mediated, but not A2A-mediated, effects in methamphetamine- and cocaine-trained rats. ? ? ? We have recently reported that nicotine displays high reinforcing efficacy in squirrel monkeys. There is great interest in quantifying brain nicotinic acetylcholine receptors (nAChR) in Squirrel monkeys to study the involvement of these receptors in nicotine dependence. Due to the limitations of existing methods of quantification for PET (need for arterial blood or need of a reference region in the brain), we have now explored the use of an extra-cerebral area for quantification of central receptors (Le Foll et al., 2007). If the ratio between the volume of distribution of the non-displaceable compartment (VDND) in the region of interest and a reference region inside or outside of the brain is constant and known, then theoretically it is possible to use the accumulation of radioactivity over time in a reference region and a region of interest to calculate the binding potential (BPND). To validate this approach, we performed quantitative PET studies of 42* nAChRs with 2 18Ffluoro-A-85380 (2-18FFA) in Squirrel monkeys. VT was measured in several brain areas using a blood input function. BPND values were measured with data from a reference region inside (cerebellum, Cb) or outside (muscle) of the brain using a simplified reference tissue model and PMOD software. When muscle was used as the reference region, BPND values were corrected for the difference between VT in muscle and VDND in brain. VT values were highest in thalamus and lowest in cerebellum and muscle, consistent with the distribution of 42* nAChRs in primates. Limited (94% for Cb) or no displacement (for muscle) of radioligands by nicotine was observed, indicating limited specific binding of 2-18FFA in these regions. A strong correlation (r = 0.9, P<0.001) between BPND values measured using Cb and muscle as reference regions was observed. These results suggest that quantifying central receptors is feasible using an extra-cerebral reference region, providing a novel approach for quantification of brain receptors.
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