Modulation by the dorsal, but not the ventral, hippocampus of the expression of behavioural sensitization to amphetamine.

Although the dorsal hippocampus (DH) and the ventral hippocampus (VH) densely innervate the nucleus accumbens, which mediates the expression of behavioural sensitization, the respective and specific contribution of DH and VH in the expression of behavioural sensitization to amphetamine has not been investigated. In the present study, we investigated how lidocaine infused in DH or VH modulated behavioural locomotor sensitization induced by repeated administration of systemic amphetamine. Rats, well habituated to their environmental conditions and experimental protocol, were given repeated administration of systemic amphetamine. Once behavioural sensitization was developed, rats were challenged with amphetamine and infused with saline (controls) or lidocaine into DH or VH. We found that reversible inhibition by lidocaine of DH, but not VH, blocks the expression of behavioural sensitization to amphetamine. Control animals injected with saline solution do express behavioural sensitization. Our results bring new insights on the role of the hippocampus complex in the expression of behavioural sensitization, indicating that, in individuals well habituated to the drug-associated context, DH but not VH would play a key role. The results provide experimental evidence for clinical studies in human addicts that have demonstrated that exposure to environmental stimuli associated with drug-taking behaviour elicits craving and can promote relapse, and further suggest that in drug abusers, once addiction has occurred, the contextual and spatial conditions that are associated with drug consumption may play a critical role in the maintenance of drug abuse.