Neurotensin-Deficient Mice Have Deficits in Prepulse Inhibition: Restoration by Clozapine but Not Haloperidol, Olanzapine, or Quetiapine

Prepulse inhibition (PPI) of the acoustic startle reflex is a commonly used measure of preattentive sensorimotor gating. Disrupted PPI in rodents represents an animal model of the sensorimotor gating deficits characteristic of schizophrenia. The neurotensin (NT) system is implicated in the pathophysiology of schizophrenia, and NT has been hypothesized to act as an endogenous antipsychotic. In rats, NT receptor agonists restore PPI disrupted by dopamine receptor agonists and N-methyl-D-aspartate receptor antagonists, and pretreatment with an NT receptor antagonist blocks restoration of isolation rearing induced deficits in PPI by some antipsychotic drugs. The current studies further scrutinized the role of the NT system in the regulation of PPI and in antipsychotic drug-induced restoration of PPI using NT-null mutant mice (NT-/-). NT-/- mice exhibited significantly higher pulse alone startle amplitudes and disrupted PPI compared with NT+/+ mice. Haloperidol (0.1 mg/kg) and quetiapine (0.5 mg/kg) administered 30 min before PPI testing significantly increased PPI in NT+/+ mice but had no effect on PPI in NT-/- mice. In contrast, clozapine (1.0 mg/kg) significantly increased PPI in both NT-/- and NT+/+ mice, whereas olanzapine (0.5 mg/kg) had no effect on PPI in either NT-/- or NT+/+ mice. In a separate experiment, amphetamine (2.0 mg/kg i.p.) significantly disrupted PPI in NT+/+ mice but not NT-/- mice. These results provide evidence that the effects of antipsychotic drugs (APDs) may be differentially affected by the state of NT neurotransmission and, moreover, that APDs differ in their dependence on an intact NT system.

Neurotensin-deficient mice show altered responses to antipsychotic drugs

The peptide transmitter neurotensin (NT) exerts diverse neurochemical effects that resemble those seen after acute administration of antipsychotic drugs (APDs). These drugs also induce NT expression in the striatum; this and other convergent findings have led to the suggestion that NT may mediate some APD effects. Here, we demonstrate that the ability of the typical APD haloperidol to induce Fos expression in the dorsolateral striatum is markedly attenuated in NT-null mutant mice. The induction of Fos and NT in the dorsolateral striatum in response to typical, but not atypical, APDs has led to the hypothesis that the increased expression of these proteins is mechanistically related to the production of extrapyramidal side effects (EPS). However, we found that catalepsy, which is thought to reflect the EPS of typical APDs, is unaffected in NT-null mutant mice, suggesting that NT does not contribute to the generation of EPS. We conclude that NT is required for haloperidol-elicited activation of a specific population of striatal neurons but not haloperidol-induced catalepsy. These results are consistent with the hypothesis that endogenous NT mediates a specific subset of APD actions.