Effects of Haloperidol and Risperidone on Neurotensin Levels in Brain Regions and Neurotensin Efflux in the Ventral Striatum of the Rat

Abstract

Neurotensin (NT) may play a role in the pathophysiology of schizophrenia and in the mechanism of action of antipsychotic drugs. Here we studied the effects of a 30-day regimen of haloperidol (1.15 mg/100 g food) and risperidone (1.15 and 2.3 mg/100 g food) on NT-like immunoreactivity (-LI) levels in brain tissue and NT-LI efflux in the ventral striatum (VSTR) of the rat. Haloperidol, but not risperidone, increased NT-LI levels in the striatum. In the occipital cortex, risperidone, but not haloperidol, decreased levels of NT-LI. In the hippocampus and the frontal cortex both haloperidol and risperidone (the higher dose) increased NT-LI levels. In the VSTR, haloperidol and risperidone (the higher dose) decreased NT-LI efflux and abolished the stimulatory effect of d-amphetamine (1.5 mg/kg, s.c.). Thus, changes in NT occur in response toNeurotensin (NT) is a 13-amino acid peptide originally isolated from hypothalamus (Carraway and Leeman 1973). Several anatomical, behavioral and pharmacological studies have demonstrated an interaction between NT and dopamine (DA) in the central nervous system (CNS). For example, high levels of NT and its binding sites have been found in both the cell body regions, i.e. the substantia nigra pars compacta (SN) and the ventral tegmental area (VTA), as well as the major projection areas of the ascending dopaminergic pathways, i.e. the striatum and the nucleus accumbens (Kasckow and Nemeroff 1991;Lambert et al. 1995; Quirion 1983). Within both the VTA and the SN, NT immunoreactive fibers are found in close proximity to DA cell bodies (Hökfelt et al. 1984); within the VTA, NT is colocalized with DA (Hökfelt et al. 1984;Kalivas 1993), while in the striatal complex, the majority of the NT binding sites in rat are located on DA terminals (Cadet et al. 1991). There exists substantial pharmacological evidence showing that NT is involved in the regulation of DA neurotransmission. For example, local perfusion with NT dose-dependently increases striatal extracellular DA levels without affecting DA metabolite levels and modifies the inhibitory effects of DA-D 1 and DA-D 2 receptor agonists on striatal DA release (Fuxe et al. 1992). In contrast to the striatum, in the nucleus accumbens direct infu- NO . 5 sion of NT does not affect extracellular DA levels, but increases extracellular levels of the DA metabolite homovanillic acid (Chapman et al. 1992). Microinjection of NT into the VTA, produces a dose-dependent in vivo DA release in the nucleus accumbens and also causes a dose-dependent circling behavior (Steinberg et al. 1995). Moreover, blockade of NT receptors by the NT receptor antagonist SR 48692 decreases basal extracellular levels of DA and its metabolites in the nucleus accumbens, suggesting that NT modulates the mesolimbic DA system (Azzi et al. 1998). Neurotensin has also been shown to mimic the effects of neuroleptic drugs in behavioral models of antipsychotic drug action. Thus, centrally injected NT antagonizes psychostimulant-induced locomotor hyperactivity (Ervin et al. 1981;Joli...

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