ABSTRACT
RATIONALE: The endocannabinoid system plays an important role in conditioned drug seeking, but the neuronal mechanisms involved in this behavior are unclear. OBJECTIVES: Here, we evaluate the role of endogenous cannabinoids in the cortico-limbic circuitry in cue-induced nicotine-seeking behavior in rats. METHODS: Animals were first trained to self-administer nicotine (0.03 mg/kg/injection, IV) under conditions in which responding was reinforced jointly by response-contingent nicotine injections and audiovisual stimuli. During subsequent sessions, nicotine was withdrawn and responding was reinforced by contingent presentation of the stimuli only. One month after nicotine removal, the cannabinoid CB1 receptor antagonist, rimonabant, was injected bilaterally into the shell of the nucleus accumbens (ShNAcc, 0.3, 3, or 30 ng/0.5 microl), the basolateral amygdala (BLA, 30 ng/0.5 microl), or the prelimbic cortex (PLCx, 30 ng/0.5 microl). RESULTS: Rimonabant injected into the ShNAcc dose-dependently reduced nicotine-seeking behavior without modifying spontaneous locomotor activity. Similar results were obtained when the drug (30 ng) was injected into the BLA or the PLCx. The anatomical specificity was confirmed in a control experiment using [(3)H]rimonabant. Fifteen minutes after drug injection, when the behavioral effects of rimonabant were already achieved, radioactivity was detected at the site of injection and had not diffused to adjacent regions. CONCLUSIONS: These findings demonstrate that increased endocannabinoid transmission critically triggers conditioned nicotine-seeking behavior in key cortico-limbic regions.
Subject(s)
Cannabinoid Receptor Modulators/physiology , Cerebral Cortex/physiology , Conditioning, Operant/physiology , Endocannabinoids , Limbic System/physiology , Nicotine/administration & dosage , Signal Transduction/physiology , Amygdala/anatomy & histology , Amygdala/drug effects , Amygdala/physiology , Animals , Behavior, Addictive/physiopathology , Behavior, Addictive/prevention & control , Cannabinoid Receptor Modulators/antagonists & inhibitors , Cerebral Cortex/anatomy & histology , Cerebral Cortex/drug effects , Conditioning, Operant/drug effects , Cues , Dose-Response Relationship, Drug , Ganglionic Stimulants/administration & dosage , Injections, Intravenous , Injections, Intraventricular , Limbic System/anatomy & histology , Limbic System/drug effects , Male , Models, Anatomic , Motor Activity/drug effects , Motor Activity/physiology , Nucleus Accumbens/anatomy & histology , Nucleus Accumbens/drug effects , Nucleus Accumbens/physiology , Piperidines/administration & dosage , Piperidines/therapeutic use , Pyrazoles/administration & dosage , Pyrazoles/therapeutic use , Rats , Rats, Sprague-Dawley , Rimonabant , Self Administration , Signal Transduction/drug effects , Time FactorsABSTRACT
The role of dopamine as a major modulator of CNS function is well-known, and the homeostasis of dopamine is considered to be of major importance in the pathogenesis of several psychiatric and neurological diseases. Few methods are currently available for in vivo study of dopamine transporter function, which regulates extracellular levels of dopamine. Adapting the 'indicator diffusion' method applied to the microdialysis technique, we present here a suitable method for this functional investigation. We measured the cellular extraction of [3H]-MPP+, which is known to accumulate in the dopaminergic neurones through the DAT in the rat striatum, using [14C]-mannitol as reference substance characterized by absence of cellular accumulation. The cellular extraction was 0.41 and was almost abolished in the presence of the dopamine-uptake inhibitor cocaine, reaching 0.07. This suggested that extraction of [3H]-MPP+ was due to cellular uptake by dopamine transporters. Tissue analysis confirmed that [3H]-MPP+ was internalized in cells and that such transport was stopped by cocaine. Moreover, [3H]-MPP+ extraction was dramatically decreased after lesioning the nigro-striatal pathway with 6-hydroxydopamine, whereas [14C]-mannitol extraction was unchanged. It is concluded that the presented method can be used to study the functioning of the dopamine transporter in live animals.
