Modificazioni neuronali nella via abenulo-mesencefalica durante l’astinenza da Δ9-tetraidrocannabinolo

The mesolimbic dopamine (DA) system, which originates in the midbrain ventral tegmental area (VTA), shows a reduction in its spontaneous activity after chronic cannabinoid exposure and withdrawal, the critical phases of the drug addiction cycle. These adaptive changes, which result from imbalances between excitatory and inhibitory afferents onto DA cells, are thought to play a critical role into withdrawal-induced negative affective states that eventually lead to relapse into drug taking. The lateral habenula (LHb) exerts a negative control over the VTA via a γ-aminobutyric acid (GABA) structure, the rostromedial tegmental nucleus (RMTg), encoding aversion-related stimuli. In fact, both RMTg and LHb neurons are excited by aversive/unpleasant events and inhibited by rewarding/positive stimuli. Moreover, RMTg GABA neurons express CB1 receptors on their axon terminals impinging upon VTA DA neurons and are a target for cannabinoid action on DA cells. Indeed, acute administration of these drugs reduces RMTg neuron discharge activity and strongly suppresses the inhibition exerted by RMTg afferents, thus contributing to cannabinoid-induced DA neuronal excitation. Therefore, these nuclei represent a potential convergence point for drug-evoked reward and aversive opponent processes. On these bases, in this thesis we tested the possibility that LHb-RMTg pathway is causally involved in the hypodopaminergic state, which is one hallmark of cannabinoid withdrawal. To this aim, we used single unit extracellular recordings from either VTA, RMTg and LHb neurons in anesthetized male Sprague-Dawley rats. In order to induce Δ9-tetrahydrocannabinol (Δ9-THC) dependence, rats were chronically treated with Δ9-THC (15 mg/kg, i.p.), or its vehicle, twice daily for 6.5 days. Administration of the cannabinoid antagonist SR141716A (5 mg/kg, i.p.) precipitated an intense behavioral withdrawal syndrome, whereas abrupt Δ9-THC suspension caused only milder signs of abstinence. Electrophysiological experiments confirmed that Δ9-THC withdrawal produced a marked decrease in the firing rate and burst firing of VTA DA neurons. We then investigated the inhibitory contribution from RMTg afferents to VTA DA neurons. As expected, RMTg electrical stimulation elicited a complete suppression of spontaneous activity in approximately half of the DA neurons examined. Notably, we found that the duration of RMTg-evoked inhibition was prolonged in Δ9-THC withdrawn rats when compared with controls, suggesting an augmented GABA inhibitory input onto DA cells. By contrast, the spontaneous activity of RMTg GABA neurons was reduced in cannabinoid-withdrawn rats. Consistent with results, we also found that firing rate of RMTg-projecting LHb neurons was markedly suppressed after cannabinoid withdrawal. While further highlighting the role of the RMTg as a new master brake for DA neurons, our data support the hypothesis that enhanced GABA inputs from this nucleus might contribute to the hypodopaminergia induced by cannabinoid withdrawal. They also confirm that the LHb-RMTg pathway takes part in the neuronal circuits underlying drug dependence and addiction.