Basolateral amygdala lesions do not disrupt latent inhibition

Latent inhibition (LI) is a measure of retarded conditioning to a previously presented non-reinforced stimulus that is impaired in schizophrenic patients and in rats treated with amphetamine, and is restored in both by neuroleptic drugs. In terms of neural substrates, LI depends on the integrity of the nucleus accumbens (NAC) and the inputs to this structure from the hippocampal formation and adjacent cortical areas. The present experiments investigated the effects of electrolytic lesions to the basolateral amygdala (BLA), which is another major source of input to the NAC, on the LI effect. LI was assessed in a conditioned emotional response (CER) procedure in rats licking for water, consisting of three stages: pre-exposure, in which the to-be-conditioned stimulus (a tone) was repeatedly presented without being followed by reinforcement; conditioning, in which the pre-exposed stimulus was paired with reinforcement (a foot shock); and test, in which LI was indexed by the animal's degree of suppression of licking during tone presentation. In Expt. 1, which used a lesion at a more posterior location, no effect on either LI or CER conditioning was seen. In Expt. 2, lesion at a more anterior location reduced the magnitude of CER conditioning in both the pre-exposed and the non-pre-exposed animals, but left the LI effect intact. The latter lesion did not affect spontaneous and amphetamine-induced activity (Expt. 3). These results suggest that the development of LI is not dependent on the amygdalar input to the NAC, but that the input from the anterior aspects of BLA to the NAC is involved in the establishment of stimulus-reinforcement associations.