Effect of isolation-rearing on conditioned dopamine release in vivo in the nucleus accumbens of the rat.

Intracerebral microdialysis in conjunction with HPLC coupled to electrochemical detection was used to investigate the effect of isolation-rearing in the rat on extracellular dopamine (DA) and its metabolites in vivo, in the shell region of the nucleus accumbens, in response to footshock and in relation to a conditioned emotional response. Male Lister hooded rats were reared from weaning for 6-8 weeks in either social isolation or groups of five. In the training phase, rats were exposed to a novel environment for 10 min where they experienced mild footshock. Footshock caused an immediate increase in basal extracellular DA levels in both rearing groups relative to control rats. However, the increase in extracellular DA was prolonged in the case of the isolation-reared rats and significantly greater than in group-reared rats. Exposure to the novel environment without shock (control groups) did not significantly alter basal extracellular DA in the nucleus accumbens shell; 140 min later rats were returned to the testing box (contextual stimulus) without receiving footshock. The contextual stimulus increased basal extracellular DA in the nucleus accumbens of both groups of rats with respect to controls; however, this increase was significantly greater and more prolonged in isolates. Extracellular levels of the metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid did not differ between isolation- and group-reared rats, and they were not significantly affected by either footshock or the contextual stimulus. These results suggest that exposure to footshock and a contextual stimulus are associated with increases in basal extracellular DA levels in the nucleus accumbens shell. The results also support evidence in favour of an isolation-induced enhancement in dopaminergic activity in the nucleus accumbens, which probably underlies aspects of the behavioural syndrome associated with isolation.