DIFFERENTIAL-EFFECTS OF FOREBRAIN 5-HYDROXYTRYPTAMINE DEPLETIONS ON PAVLOVIAN AVERSIVE-CONDITIONING TO DISCRETE AND CONTEXTUAL STIMULI IN THE RAT

The experiments examined the effects of depleting forebrain 5-hydroxyt ryptamine (5HT) on Pavlovian aversive conditioning to discrete and con textual stimuli. Rats were lesioned with intracerebroventricular injec tions of the neurotoxin 5,7-dihydroxytryptamine and then conditioned i n a distinctive environment (termed the context) to a 30 s auditory st imulus. In 50% of animals the interval between the offset of the discr ete auditory stimulus and the reinforcer, a mild foot-shock (0.5 mA, 0 .5 s), was 5 s (the short-trace group) and in the other 50%, 30 s (the long-trace group). Theory predicts that animals in the short-trace co ndition will learn more about the discrete stimulus as a predictor of shock and become strongly conditioned, while those in the long-trace c ondition learn relatively more about the context. The extent of condit ioning to the discrete and contextual stimuli was assessed separately, in extinction, using lick-suppression and place-preference measures r espectively, Under these conditions sham subjects exhibited the expect ed dissociation with respect to trace interval. However, lesioned anim als exhibited a specific impairment in contextual conditioning. The re sults are discussed in terms of the behavioural, neurochemical and neu roanatomical specificity of 5HT function in aversive conditioning and the implications for general theories of the role of 5HT in aversive p rocesses.