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Chronic use of psychostimulants induces enduringly increased responsiveness to a subsequent psychostimulant injection and sensitivity to drug-associated cues, contributing to drug craving and relapse. Neurotensin (NT), a neuropeptide functionally linked to dopaminergic neurons, was suggested to participate in these phenomena. We and others have reported that SR 48692, an NT receptor antagonist, given in pre- or co-treatments with cocaine or amphetamine, alters some behavioral effects of these drugs in rats. However, its efficacy when applied following repeated cocaine administration remains unknown. We, therefore, evaluated the ability of SR 48692, administered after a cocaine regimen, to interfere with the expression of locomotor sensitization and conditioned place preference (CPP) in rats. We demonstrated that the expression of locomotor sensitization, induced by four cocaine injections (15 mg/kg, i.p.) every other day and a cocaine challenge 1 week later, was attenuated by a subsequent 2-week daily administration of SR 48692 (1 mg/kg, i.p.). Furthermore, the expression of cocaine-induced CPP was suppressed by a 10-day SR 48692 treatment started after the conditioning period (four 15 mg/kg cocaine injections every other day). Taken together, our data show that a chronic SR 48692 treatment given after a cocaine regimen partly reverses the expression of locomotor sensitization and CPP in the rat, suggesting that NT participates in the maintenance of these behaviors. Our results support the hypothesis that targeting neuromodulatory systems, such as the NT systems may offer new strategies in the treatment of drug addiction.

The aim of the present study was to investigate the role of neurotensin in the regulation of NT(1) receptors during postnatal development in the rat brain. Characterization of the ontogeny of neurotensin concentration and [(125)I]neurotensin binding to NT(1) receptors in the brain at different embryonic and postnatal stages showed that neurotensin was highly expressed at birth, reaching peak levels at postnatal day 5 (P5) and decreasing thereafter. The transient rise in neurotensin levels preceded the maximal expression of NT(1) receptors, observed at P10, suggesting that neurotensin may influence the developmental profile of NT(1) receptors. Using primary cultures of cerebral cortex neurons from fetal rats, we showed that exposure to the neurotensin agonist JMV 449 (1 nM) decreased (-43%) the amount of NT(1) receptor mRNA measured by reverse transcription-PCR, an effect that was abolished by the nonpeptide NT(1) receptor antagonist SR 48692 (1 microM). However, daily injection of SR 48692 to rat pups from birth for 5, 9, or 15 days did not modify [(125)I]neurotensin binding in brain membrane homogenates. Moreover, postnatal blockade of neurotensin transmission did not alter the density and distribution of NT(1) receptors assessed by quantitative autoradiography nor NT(1) receptor mRNA expression measured by in situ hybridization in the cerebral cortex, caudate-putamen, and midbrain. These results suggest that although NT(1) receptor expression can be regulated in vitro by the agonist at an early developmental stage, neurotensin is not a major factor in the establishment of the ontogenetic pattern of NT receptors in the rat brain., (Copyright 2000 Wiley-Liss, Inc.)

The purpose of the present study was to investigate the effects of repeated administration of the neurotensin receptor antagonist, SR 48692, on the activity of the mesocortical and mesolimbic dopaminergic (DA) systems. We showed that daily administration of SR 48692 for 15 days (1 mg/kg i.p.) to Wistar rats increased the expression of tyrosine hydroxylase mRNA and protein in the ventral mesencephalon. Simultaneous in vivo microdialysis in the shell part of the nucleus accumbens (AcbSh) and the medial prefrontal cortex (mPFC) revealed that blockade of neurotensin receptors for 15 days decreased basal extracellular levels of DA (approximately 50%) and its metabolites in the AcbSh, whereas no modification in DA levels was observed in the mPFC. In animals submitted to a forced swimming stress, which preferentially enhanced extracellular DA levels in the mPFC, treatment with SR 48692 failed to affect the stress-induced increase in DA. Moreover, given that glucocorticoids can modulate the activity of mesencephalic DA neurons, we examined the effect of the same SR 48692 treatment on corticosterone levels in dialysates from the AcbSh. We found that repeated SR 48692 did not affect the basal levels of free corticosterone, but significantly reduced the increase induced by forced swimming stress. The present results demonstrate that repeated treatment with SR 48692 modulates selectively the DA mesolimbic system when compared with the mesocortical pathway. These findings suggest that long-term treatment with selective neurotensin receptor antagonists could have potential clinical utility in the treatment of neuropsychiatric disorders associated with hyperactivity of the mesolimbic DA systems or the hypothalamic-pituitary-adrenal axis.

Previous studies in our laboratory suggest that neurotensin (NT) acts centrally to modulate adrenocorticotropin hormone (ACTH) and corticosterone release. In the present studies, we examined hypothalamic-pituitary-adrenal (HPA) function under basal conditions and during restraint stress following central administration of the highly specific NT receptor antagonist, SR48692. Chronic delivery of SR48692 to the paraventricular nucleus (PVN) of the hypothalamus via indwelling central cannulae attenuated both the diurnal- and stress-induced elevations in HPA activity. Thus, SR48692 decreased the diurnal increase in plasma ACTH and corticosterone during the evening phase of the cycle, but did not affect morning levels. Restraint-induced increases in plasma ACTH and corticosterone levels were also significantly reduced in the SR48692-implanted animals. This suggests that the inhibitory effects of SR48692 were restricted to periods of stimulated HPA activity. A decrease in corticotropin-releasing hormone (CRH)-like immunoreactivity was observed within the PVN following chronic SR48692, and parallel decreases in CRH-like immunoreactivity were observed within the external zone of the median eminence. These findings suggest that endogenous NT serves to increase HPA activity during periods of enhanced stimulation.

Adaptive changes in brain neurotensin (NT) receptors were investigated in rats after repeated administration of SR 48692, a potent and selective non-peptide NT receptor antagonist. Administration of SR 48692 (1 mg/kg i.p.) for 15 days did not alter NT content in the brain but highly enhanced the expression of NT receptor mRNA as shown by quantitative in situ hybridization. The increase of the signal was observed in numerous areas of the brain, such as the anterior cingulate, perirhinal and retrosplenial cortices, the suprachiasmatic nucleus, the ventral tegmental area, the substantia nigra and the posterior cortical nucleus of the amygdaloid complex. Moreover, the SR 48692 treatment induced the expression of NT receptor mRNA in several nuclei of the diencephalon where it could not be detected in basal conditions. Immunoblot analysis with a specific antibody directed against the rat cloned NT receptor revealed an important increase in NT receptor protein in the brain of SR 48692-treated rats, correlating well with the increase in NT receptor mRNA levels. Surprisingly, the number and the affinity constant of NT binding sites determined on brain membrane homogenates remained unchanged after SR 48692 treatment, even after membrane permeabilization with low concentrations of digitonin. These results suggest that chronic treatment with a specific NT antagonist induces an up-regulation of NT receptors at the level of mRNA and protein. Moreover, they indicate that after a chronic treatment with SR 48692, the number of NT binding sites remains stable in contrast to what is observed after 5-day treatment or with central monoaminergic receptor following their long-term blockade.

When administered as an intravenous injection in the pentobarbitone-anaesthetized rat, neurotensin (NT) elicits a biphasic depressor-pressor effect that can be evaluated by the mean arterial blood pressure (MABP). The first hypotensive phase elicited by low doses of NT is dependent on the interaction of NT with its specific receptors and may be mediated by the release of histamine, since it is prevented by oral pretreatment with the selective NT receptor antagonist SR 48692 and by intravenous pretreatment with a selective H1 receptor antagonist mepyramine. The hypertensive effect evoked by higher doses of NT is histamine-independent but remains NT receptor- mediated. The prevention of the biphasic effect on MABP by oral administration of the NT receptor antagonist SR 48692 validates the implication of NT receptors in the histamine release phenomenon.

SR 48692, a selective nonpeptide antagonist of neurotensin (NT) receptors was developed recently. In the present work we studied the binding properties of the corresponding radioligand, [3H]SR 48692, in the adult guinea pig brain. The characterization of [3H]SR 48692 binding was carried out on brain membrane preparations and the distribution of [3H]SR 48692 binding sites was determined by receptor autoradiography and compared to that of [125I]NT binding sites. In brain homogenates, [3H]SR 48692 bound to a single population of sites with a Kd of 2.19 nM and a maximal binding capacity of 1.15 pmol/mg of protein. This maximal binding capacity value was 20 times higher than that observed for [125I]NT. NT agonists were able to interact competitively with the entire population of binding sites labeled by [3H]SR 48692, but their affinities were much lower than those observed for [125I]NT. By contrast, NT antagonists exhibited similar abilities to inhibit the binding of both radioligands. The addition of unlabeled NT in saturation assays revealed a competitive inhibition of [3H]SR 48692 binding, suggesting that agonist and antagonist ligand bind to overlapping domains of the NT receptor. The autoradiographic distribution of the low-affinity NT binding sites detected by [3H]SR 48692 (96% of the receptors) was very similar to the distribution of high-affinity receptors labeled with [125I]NT (4% of the receptors). In addition, the binding of [3H]SR 48692 was insensitive to guanyl nucleotides. Taken together, these findings suggest that the binding sites detected by [3H]SR 48692 in the guinea pig brain mainly represent the uncoupled form of the NT receptor.

SR 48692 is the first non-peptide antagonist of neurotensin receptors. It is potent and selective vs the high-affinity binding sites and with a small activity on the levocabastine-sensitive binding sites. It is active on several species including man without partial agonist properties. In vivo, it is active by oral route with a long duration of action and it is able to cross the blood-brain barrier. As an antagonist of neurotensin receptors, SR 48692 modulates central dopaminergic neurons. Thus it might be considered as a good tool for investigating the pathophysiological role of neurotensin in psychiatric diseases.

The nonpeptide neurotensin (NT) antagonist, SR 48692, was recently shown to inhibit NT binding to the cloned rat and human NT receptor and to antagonize NT effects in a variety of in vitro and in vivo assays. Here, we show that, in contrast to its antagonistic action on NT-induced hypomotility in the rat, SR 48692 failed to antagonize NT-induced hypothermia and analgesia in the mouse and rat. We suggest that these effects might be mediated through a subtype of SR 48692-insensitive NT receptor.

In the present study, we examined the regulation of neurotensin receptor following a chronic pharmacological blockade of the neurotensin transmission with a nonpeptide neurotensin receptor antagonist, SR 48692. Our results showed that treatment of the rats for five days with SR 48692, at a dose of 1 mg/kg, i.p., induced an increase of both the number of binding sites for 125I-neurotensin to whole brain membrane homogenates and neurotensin receptor mRNA levels in the ventral mesencephalon. This study brings the first evidence for an in vivo up-regulation of neurotensin receptors following their pharmacological blockade, and suggests that endogenous neurotensin exerts a tonic inhibitory control on neurotensin receptor mRNA levels.