Latent inhibition in 35-day-old rats is not an "adult" latent inhibition: implications for neurodevelopmental models of schizophrenia.

RATIONALE

Latent inhibition (LI) refers to retarded conditioning to a stimulus as a consequence of its inconsequential preexposure. Amphetamine-induced disruption of LI and its potentiation by antipsychotic drugs (APDs) in the adult rat are well-established models of schizophrenia and antipsychotic drug action, respectively. It is not clear whether LI can be similarly modulated at prepubertal age.

OBJECTIVES

In view of the notion that schizophrenia is a neurodevelopmental disorder whose overt expression depends on postpubertal brain maturational processes, we investigated whether several manipulations known to modulate LI in adult rats, including systemic administration of amphetamine and the atypical APD clozapine, are capable of producing the same effects in prepubertal (35-day-old) rats.

METHODS

LI was measured in a thirst motivated conditioned emotional response (CER) procedure in which rats received 10 or 40 tone preexposures followed by 2 or 5 tone-footshock pairings.

RESULTS

Like in adults, LI was present with 40 preexposures and 2 conditioning trials. In contrast to findings in adults, LI was resistant to disruption by amphetamine at a dose (1 mg/kg) that significantly increased locomotor activity, as well as by reducing the number of preexposures to ten, increasing the number of conditioning trials to five, or changing the context between preexposure and conditioning. Clozapine (5 mg/kg) and the selective 5HT2A antagonist M100907 (0.3 mg/kg) administered in conditioning were without an effect on "persistent" LI with extended conditioning, but were capable of disrupting LI when administered in the preexposure stage, as found in adults.

CONCLUSION

The results point to functionality within brain systems regulating LI acquisition but not those regulating LI expression in periadolescent rats, further suggesting that postpubertal maturation of the latter systems may underlie schizophrenia-mimicking LI disruption reported in adult rats following perinatal manipulations and possibly disrupted LI observed in schizophrenia.