. The delayed effects of DTG and MK-801 on latent inhibition in a conditioned taste-aversion paradigm. PHARMACOL BIOCHEM BEHAV 66 (3) 533-539, 2000.-The delayed effects of phencyclidine (PCP) have been shown to disrupt latent inhibition (LI) in a conditioned taste-aversion paradigm. In an attempt to understand the mechanism of this disruption, the delayed effects of the selective sigma receptor agonist 1,3-Di(2-tolyl)guanidine (DTG) and the selective NMDA receptor antagonist MK-801 on latent inhibition were assessed in the same paradigm. Water-deprived male rats were allowed access to either water (nonpreexposed; NPE) or 5% sucrose (preexposed; PE) for 30 min on 2 consecutive days. On the third day, animals were allowed access to sucrose and subsequently injected with lithium chloride. On the forth day, animals were allowed access to both sucrose and water. LI was assessed by comparing the percent sucrose consumed in PE and NPE groups on the fourth day. DTG (1.0, 5.0, or 10.0 mg/kg), MK-801 (0.5, 1.0, or 2.0 mg/kg), or vehicle was administered IP 20 h before preexposure (days 1 and 2) and conditioning (day 3). In vehicle-treated groups, PE animals consumed a significantly higher percent sucrose on the test day than NPE animals, indicating the presence of LI. DTG (10.0 mg/kg) and MK-801 (2.0 mg/kg) decreased the percent sucrose consumed by animals in the PE group to the level observed in the NPE group, indicating disrupted LI. However, this dose of MK-801 was found to produce a decrease in percent sucrose consumed in PE animals not treated with lithium chloride, indicating that the decrease observed in the LI paradigm could be due to MK-801-induced decrease in taste preference for sucrose rather than a disruption of LI. Lower doses of MK-801 that did not produce a decrease in taste preference for sucrose did not significantly disrupt LI. None of the doses of DTG tested altered taste preference for sucrose. These data suggest a role for sigma receptors in the previously observed PCP-induced disruption of LI. LATENT inhibition (LI) is the impaired acquisition of a conditioned response to a stimulus that has been previously presented without reinforcement (35). LI is thought to be a measure of an organism's capacity to ignore irrelevant stimuli, and has been demonstrated in a variety of species including humans (4,35,36). Disruption of LI has been observed in nonmedicated acute schizophrenics (4) as well as animals and humans treated with the psychotomimetic drug amphetamine (14,22,49,(64)(65)(66)(67). Medicated schizophrenic patients do not demonstrate disrupted LI (4,37). In addition, antipsychotic medications reverse amphetamine-induced disruption of LI in animals (49,68), and can enhance LI on their own (11,15). These observations suggest that the disruption of LI is a useful animal model for this specific attentional deficit seen in schizophrenia (12,15). Phencyclidine (PCP) is a drug that can produce a psychotomimetic effect in humans (3,28,38) as well as a number of behavioral and cognitive changes in animals...