Involvement of Different Types of Dopamine Receptors in the Formation of Latent Inhibition of a Conditioned Passive Avoidance Reaction in Rats

The effects of systemic injections of dopaminergic agents on normal and weak latent inhibition of a conditioned passive avoidance reaction were studied in rats. Formation of normal latent inhibition was induced using 20 pre-exposures to a contextual conditioned stimulus prior to training. Weak latent inhibition was modeled using 10 pre-exposures. The effects of the D2/D3 receptor agonist quinpirole (1 mg/kg) and the D1 receptor agonist SKF 38393 (1 mg/kg) separately and in combination with haloperidol (0.5 mg/kg) were tested. Quinpirole induced the expression of normal latent inhibition but had no effect on weak latent inhibition. Activation of D1 receptors with SKF 38393 had no effect on the formation of latent inhibition regardless of the number of pre-exposures. Haloperidol significantly strengthened weak latent inhibition but impaired normal latent inhibition. Administration of haloperidol in combination with SKF 38393 prevented this impairment. These results suggest that while D1 receptors have no influence as an independent substrate on the formation of latent inhibition, activation of these receptors is required for the complete manifestation of D2-mediated modulation of this process.