Temporal dissociation of neurotensin/neuromedin N mRNA expression in topographically separate subsets of rat striatal neurons following administration of haloperidol

Brain neurotensin, psychostimulants, and stress--emphasis on neuroanatomical substrates

Neurotensin (NT) is a peptide that is widely distributed throughout the brain. NT is involved in locomotion, reward, stress and pain modulation, and in the pathophysiology of drug addiction and depression. In its first part this review brings together relevant literature about the neuroanatomy of NT and its receptors. The second part focuses on functional-anatomical interactions between NT, the mesotelencephalic dopamine system and structures targeted by dopaminergic projections. Finally, recent data about the actions of NT in processes underlying behavioral sensitization to psychostimulant drugs and the involvement of NT in the regulation of the hypothalamo-pituitary-adrenal gland axis are considered.

Neurotensin afferents of the ventral tegmental area in the rat: [1] re-examination of their origins and [2] responses to acute psychostimulant and antipsychotic drug administration

The ventral tegmental area (VTA) is involved in reward-related behaviours and the actions of psychostimulant drugs. It is influenced by afferents expressing a variety of neurotransmitters and neuromodulators; the innervation containing neurotensin is among the densest of these. Intra-VTA neurotensin activates dopaminergic neurons and plays an important role in the development of behavioural sensitization to psychostimulant drugs and possibly in schizophrenia. Using gold-coupled wheatgerm agglutinin as retrograde tracer in combination with nonisotopic in situ hybridization for neurotensin mRNA or neurotensin antibodies after colchicine treatment, the present study was undertaken to demonstrate the neurotensinergic neurons projecting to the VTA and determine whether (and in which subpopulations) neurotensin expression is regulated in VTA-projecting neurons after administrations of the psychostimulant drug methamphetamine or the antipsychotic haloperidol. This study reveals the lateral preoptico-rostral lateral hypothalamic continuum and the medial preoptic area as main sources for the neurotensin afferents of the VTA. Fewer neurotensinergic, VTA-projecting neurons are situated in the dorsal raphe, pedunculopontine and laterodorsal tegmental nuclei, lateral hypothalamic area, ventral endopiriform area, lateral septum, accumbens shell, parabrachial nucleus and different parts of the extended amygdala. The number of neurotensinergic VTA-projecting neurons increased significantly only after methamphetamine administration and exclusively in the accumbens shell. It is concluded that the widespread neurotensinergic VTA-projecting neurons, situated in areas involved in different reward-related behaviours, are well suited to convey distinct reward information to the VTA. The up-regulation of neurotensin expression selectively in VTA-projecting neurons in the accumbens shell following methamphetamine administration may be an important factor in the development of behavioural sensitization.

Typical and atypical antipsychotic drugs target dopamine and cyclic AMP-regulated phosphoprotein, 32 kDa and neurotensin-containing neurons, but not GABAergic interneurons in the shell of nucleus accumbens of ventral striatum

Specific neurons in the brain are the primary targets of the action of antipsychotic drugs. Identification and characterization of the nature of these neurons are important for understanding how antipsychotic drugs produce their effects. In previous studies GABAergic/dynorphinergic neurons were identified as a principal cell target of antipsychotic drugs in the shell of nucleus accumbens. In the present study, we further characterized which subpopulations of GABAergic neurons in this area respond after acute administration of antipsychotic drugs. Rats were treated with the typical antipsychotic haloperidol, or the prototype atypical antipsychotic clozapine and killed two hours after treatment. In appropriate sections of brain, double immunofluorescence labeling was performed with antibodies directed against markers specific to candidate cell types and Fos-like proteins (a marker to identify drug-induced cell activation). We reported here that haloperidol- and clozapine-activated neurons showed the following features: 1) approximately 54-57% of them express dopamine and cyclic AMP-regulated phosphoprotein, 32 kDa (a marker for GABAergic medium spiny projection neurons), 2) they appear rarely to be GABAergic interneurons, marked by the calcium binding proteins, parvalbumin, calretinin or calbindin-D28K, 3) about 84-86% of them express the neuropeptide neurotensin (a neurotransmitter most often associated with projection neurons in the site tested). The results suggest that most of the antipsychotic drug-activated neurons in the shell of nucleus accumbens are likely to be neurotensin containing projection neurons.

The role of neurotensin in the pathophysiology of schizophrenia and the mechanism of action of antipsychotic drugs

It has become increasingly clear that schizophrenia does not result from the dysfunction of a single neurotransmitter system, but rather pathologic alterations of several interacting systems. Targeting of neuropeptide neuromodulator systems, capable of concomitantly regulating several transmitter systems, represents a promising approach for the development of increasingly effective and side effect-free antipsychotic drugs. Neurotensin (NT) is a neuropeptide implicated in the pathophysiology of schizophrenia that specifically modulates neurotransmitter systems previously demonstrated to be dysregulated in this disorder. Clinical studies in which cerebrospinal fluid (CSF) NT concentrations have been measured revealed a subset of schizophrenic patients with decreased CSF NT concentrations that are restored by effective antipsychotic drug treatment. Considerable evidence also exists concordant with the involvement of NT systems in the mechanism of action of antipsychotic drugs. The behavioral and biochemical effects of centrally administered NT remarkably resemble those of systemically administered antipsychotic drugs, and antipsychotic drugs increase NT neurotransmission. This concatenation of findings led to the hypothesis that NT functions as an endogenous antipsychotic. Moreover, typical and atypical antipsychotic drugs differentially alter NT neurotransmission in nigrostriatal and mesolimbic dopamine (DA) terminal regions, and these effects are predictive of side effect liability and efficacy, respectively. This review summarizes the evidence in support of a role for the NT system in both the pathophysiology of schizophrenia and the mechanism of action of antipsychotic drugs.

Neurons of origin of the neurotensinergic plexus enmeshing the ventral tegmental area in rat: retrograde labeling and in situ hybridization combined

The morphological and physiological substrates that underlie the mutual regulatory interactions of neurotensin and dopamine in the rat mesotelencephalic projections and related structures remain to be fully described. A salient candidate for neurotensinergic effects on the mesotelencephalic dopamine projection is the dense plexus of neurotensin immunoreactive axons that enmeshes the ventral tegmental area and substantia nigra, but the locations of the neurons that give rise to this plexus have not been identified and its systemic context remains obscure. To address this, Fluoro-Gold and the cholera toxin beta subunit, retrogradely transported axonal tracers, were injected into the ventral tegmental area of rats and the brains were processed to demonstrate neurons that contained both retrograde tracer immunoreactivity and a probe against neurotensin/neuromedin N messenger RNA. Substantial numbers of double-labeled neurons were observed in the rostral part of the lateral septum, and in a region centered on the shared boundaries of the bed nucleus of stria terminalis, ventromedial ventral pallidum, diagonal band of Broca, lateral preoptic area and rostral lateral hypothalamus. A few double-labeled neurons were also observed in the dorsal raphe nucleus and adjacent periaqueductal gray. Despite the administration of haloperidol and D-amphetamine to elicit and enhance neurotensin/neuromedin N messenger RNA expression in striatum, including the nucleus accumbens and olfactory tubercle, no double-labeled neurons were observed there. These results identify a novel brain substrate for control of midbrain dopamine levels, which affect reward mechanisms and motivation.

Distinct and interactive effects of d-amphetamine and haloperidol on levels of neurotensin and its mRNA in subterritories in the dorsal and ventral striatum of the rat

Striatal tissue concentrations of neurotensin, expression of neurotensin/neuromedin N (NT/N) mRNA, and numbers of neurotensin-immunoreactive neurons are increased by d-amphetamine (amph), which stimulates dopamine release in the striatum, and haloperidol (hal), a dopamine receptor antagonist with high affinity for D2-like receptors. The possibility that the effects of these drugs involve distinct subpopulations of striatal neurons was addressed in this study, in which the relative numbers and distributions of striatal neuron profiles containing neurotensin immunoreactivity and/or NT/N mRNA were compared following administrations of hal, amph, hal and amph co-administered, and vehicle. Fourteen striatal subterritories in caudate-putamen, nucleus accumbens, and olfactory tubercle were evaluated. Amph produced increases in the expression of neurotensin preferentially in the ventromedial and caudodorsal subterritories of the caudate-putamen, the rostrobasal cell cluster and lateral shell of the nucleus accumbens, and the olfactory tubercle. Haloperidol produced increased neurotensin expression in much of dorsal and ventral striatum, most prominently in the rostral, dorsomedial and ventrolateral quadrants of the caudate-putamen, and in the rostrobasal cell cluster, rostral pole, medial and lateral shell of the nucleus accumbens and the olfactory tubercle. The numbers of neurons responding to amph and hal in all subterritories following co-administration of the two drugs were significantly less than the summed numbers responding individually to amph and hal. Furthermore, in the subterritories where immunohistochemically detectable responses elicited by amph exceeded those produced by hal, co-administration of the two drugs resulted in responses comparable to those elicited by hal given alone. It is suggested that some of the reported anti-dopaminergic behavioral effects of basal ganglia neurotensin may be attenuated in conditions of reduced dopamine neurotransmission.

Desensitization and enhancement of neurotensin/neuromedin N mRNA responses in subsets of rat caudate-putamen neurons following multiple administrations of haloperidol

Striatal neurons that respond to blockade of dopamine receptors with altered expression of neurotensin/neuromedin N mRNA were examined. Injections of haloperidol were given to rats at four or 24 h and both four and 24 h prior to sacrifice. Pair-matched controls were injected with equivalent volumes of vehicle at either 4 or 24 h prior to sacrifice. Sections of striatum were processed non-isotopically with a cRNA neurotensin/neuromedin N probe. Massive numbers of neurons exhibited hybridization in the lateral and dorsolateral caudate-putamen at 4 h. At 24 h, hybridized neurons were few in lateral and dorsolateral parts of the caudate-putamen, but more numerous in the dorsomedial and ventrolateral caudate-putamen than in controls. A second injection of haloperidol 4 h prior to sacrifice enhanced the dorsomedial/ventrolateral response, but failed to elicit substantial numbers of lateral and dorsolateral hybrids, as were observed at 4 h after one injection. Resistance of neurotensin expression to a second injection of haloperidol was selective for the lateral and dorsolateral parts of the caudate-putamen and may reflect residual blockade by haloperidol or altered DA receptors or second messengers. Sections subjected to immunohistochemical processing for neurotensin peptide and in situ hybridization with the neurotensin/neuromedin N mRNA probe exhibited numerous neurons in the dorsomedial and ventrolateral quadrants of the caudate-putamen that were double-labeled with immunoperoxidase and hybridization signals. This suggests that peptide synthesis, as opposed to decreased release of peptide, has a role in the accumulation of neurotensin immunoreactivity by dorsomedial and ventrolateral striatal neurons.