Long-term attentional deficit in nonhandled males: possible involvement of the dopaminergic system

Latent inhibition (LI) is a behavioral paradigm in which nonreinforced pre-exposure to a stimulus retards subsequent conditioning to that stimulus. The development of LI is considered to reflect learning not to attend to, or ignore, irrelevant stimuli. In our previous studies investigating the effects of early handling on LI, we have shown that normal LI was obtained in handled males and females, as well as in nonhandled females. In contrast, nonhandled males failed to show LI. This finding pointed to a long-term attentional deficit in nonhandled males. Since there is evidence that the development of LI is mediated by the dopaminergic system, the present experiments tested the possibility that the attentional deficit of nonhandled males may be related to a dopaminergic dysfunction. Experiment 1 tested whether the administration of haloperidol, which was shown to enhance LI in normal animals, would reinstate the LI effect in nonhandled males. Infantile handled (Days 1-22) and nonhandled male and female rats were tested in maturity in the LI paradigm, using a conditioned emotional response procedure. Experiment 2 tested the locomotor response of handled and nonhandled males to 0.3, 1 and 2.5 mg/kg D-amphetamine. Experiment 1 showed that handled males, handled females and nonhandled females showed a normal LI effect, whereas nonhandled males failed to develop LI. Haloperidol enhanced LI in all the groups, but this effect was most dramatic in nonhandled males, in which the drug reinstated LI. Experiment 2 showed that nonhandled males exhibited a reduced locomotor response to D-amphetamine.