Neurotensin receptor localization by light microscopic autoradiography in rat brain

Selection of single domain anti-transferrin receptor antibodies for blood-brain barrier transcytosis using a neurotensin based assay and histological assessment of target engagement in a mouse model of Alzheimer's related amyloid-beta pathology

The blood-brain barrier (BBB) presents a major obstacle in developing specific diagnostic imaging agents for many neurological disorders. In this study we aimed to generate single domain anti-mouse transferrin receptor antibodies (anti-mTfR VHHs) to mediate BBB transcytosis as components of novel MRI molecular contrast imaging agents. Anti-mTfR VHHs were produced by immunizing a llama with mTfR, generation of a VHH phage display library, immunopanning, and in vitro characterization of candidates. Site directed mutagenesis was used to generate additional variants. VHH fusions with neurotensin (NT) allowed rapid, hypothermia-based screening for VHH-mediated BBB transcytosis in wild-type mice. One anti-mTfR VHH variant was fused with an anti-amyloid-beta (Aβ) VHH dimer and labeled with fluorescent dye for direct assessment of in vivo target engagement in a mouse model of AD-related Aβ plaque pathology. An anti-mTfR VHH called M1 and variants had binding affinities to mTfR of <1nM to 1.52nM. The affinity of the VHH binding to mTfR correlated with the efficiency of the VHH-NT induced hypothermia effects after intravenous injection of 600 nmol/kg body weight, ranging from undetectable for nonbinding mutants to -6°C for the best mutants. The anti-mTfR VHH variant M1P96H with the strongest hypothermia effect was fused to the anti-Aβ VHH dimer and labeled with Alexa647; the dye-labeled VHH fusion construct still bound both mTfR and Aβ plaques at concentrations as low as 0.22 nM. However, after intravenous injection at 600 nmol/kg body weight into APP/PS1 transgenic mice, there was no detectible labeling of plaques above control levels. Thus, NT-induced hypothermia did not correlate with direct target engagement in cortex, likely because the concentration required for NT-induced hypothermia was lower than the concentration required to produce in situ labeling. These findings reveal an important dissociation between NT-induced hypothermia, presumably mediated by hypothalamus, and direct engagement with Aβ-plaques in cortex. Additional methods to assess anti-mTfR VHH BBB transcytosis will need to be developed for anti-mTfR VHH screening and the development of novel MRI molecular contrast agents.

An autoradiographic study of neurotensin receptors in the human hypothalamus

The aim of the present investigation was to determine a detailed mapping of neurotensin (NT) in the human hypothalamus, the brain region involved in neuroendocrine control. For this, we investigated the presence and the distribution of neurotensin binding sites in the human hypothalamus, using an in vitro quantitative autoradiography technique and the selective radioligand monoiodo-Tyr3-neurotensin (2000Ci/mM). This study was performed on nine adult human postmortem hypothalami. We first determined the biochemical kinetics of the binding and found that binding affinity constants were of high affinity and do not differ significantly between all cases investigated. Our analysis of the autoradiographic distribution shows that NT binding sites are widely distributed throughout the rostrocaudal extent of the hypothalamus. However, the distribution of NT binding sites is not homogenous and regional variations exist. In general, the highest densities are mainly present in the anterior hypothalamic level, particularly in the preoptic region and the anterior boarding limit (i.e. the diagonal band of Broca). Important NT binding site densities are also present at the mediobasal hypothalamic level, particularly in the paraventricular, parafornical and dorsomedial nuclei. At the posterior level, relatively moderate densities could be observed in the mammillary complex subdivisions, apart from the supramammillary nucleus and the posterior hypothalamic area. In conclusion, the present study demonstrates the occurrence of high concentrations of NT binding sites in various structures in many regions in the human adult hypothalamus, involved in the control of neuroendocrine and/or neurovegetative functions.

Loss of neurotensin receptor-1 disrupts the control of the mesolimbic dopamine system by leptin and promotes hedonic feeding and obesity

Neurons of the lateral hypothalamic area (LHA) control motivated behaviors such as feeding and ambulatory activity, in part by modulating mesolimbic dopamine (DA) circuits. The hormone, leptin, acts via the long form of the leptin receptor (LepRb) in the brain to signal the repletion of body energy stores, thereby decreasing feeding and promoting activity. LHA LepRb neurons, most of which contain neurotensin (Nts; LepRb(Nts) neurons) link leptin action to the control of mesolimbic DA function and energy balance. To understand potential roles for Nts in these processes, we examined mice null for Nts receptor 1 (NtsR1KO). While NtsR1KO mice consume less food than controls on a chow diet, they eat more and become obese when fed a high-fat, high-sucrose palatable diet; NtsR1KO mice also exhibit augmented sucrose preference, consistent with increased hedonic feeding in these animals. We thus sought to understand potential roles for NtsR1 in the control of the mesolimbic DA system and LHA leptin action. LHA Nts cells project to DA-containing midbrain areas, including the ventral tegmental area (VTA) and the substantia nigra (SN), where many DA neurons express NtsR1. Furthermore, in contrast to wild-type mice, intra-LHA leptin treatment increased feeding and decreased VTA Th expression in NtsR1KO mice, consistent with a role for NtsR1 signaling from LHA LepRb neurons in the suppression of food intake and control of mesolimbic DA function. Additionally, these data suggest that other leptin-regulated LHA neurotransmitters normally oppose aspects of Nts action to promote balanced responses to leptin.

Neurotensin inhibits glutamate-mediated synaptic inputs onto ventral tegmental area dopamine neurons through the release of the endocannabinoid 2-AG

Neurotensin (NT), a neuropeptide abundant in the ventral midbrain, is known to act as a key regulator of the mesolimbic dopamine (DA) system, originating in the ventral tegmental area (VTA). NT activates metabotropic receptors coupled to Gq heterotrimeric G proteins, a signaling pathway often triggering endocannabinoid (EC) production in the brain. Because ECs act as negative regulators of many glutamate synapses and have also been shown recently to gate LTP induction in the VTA, we examined the hypothesis that NT regulates glutamate-mediated synaptic inputs to VTA DA neurons. We performed whole cell patch-clamp recordings in VTA DA neurons in TH-EGFP transgenic mouse brain slices and found that NT induces a long-lasting decrease of the EPSC amplitude that was mediated by the type 1 NT receptor. An antagonist of the CB1 EC receptor blocked this decrease. This effect of NT was not dependent on intracellular calcium, but required G-protein activation and phospholipase C. Blockade of the CB1 receptor after the induction of EPSC depression reversed synaptic depression, an effect not mimicked by blocking NT receptors, thus suggesting the occurrence of prolonged EC production and release. The EC responsible for synaptic depression was identified as 2-arachidonoylglycerol, the same EC known to gate LTP induction in VTA DA neurons. However, blocking NT receptors during LTP induction did not facilitate LTP induction, suggesting that endogenously released NT is not a major source of EC production during LTP inducing stimulations.

An Assessment of the Antinociceptive Efficacy of Intrathecal and Epidural Contulakin-G in Rats and Dogs

Contulakin-G is a novel conopeptide with an incompletely defined mechanism of action. To assess nociceptive activity we delivered Contulakin-G as a bolus intrathecally (0.03, 0.1, 0.3, 3 nmol) or epidurally (10, 30, 89 nmol) in rats. Intrathecal Contulakin G significantly decreased Phase II and, to a lesser degree, Phase I paw flinching produced by intradermal formalin. Intrathecal and epidural doses of ED50s were 0.07 nmol and 45 nmol, respectively, giving an epidural/intrathecal ED50 ratio = 647). In dogs, intrathecal Contulakin-G (50-500 nmoL) produced a dose-dependent increase in the thermally evoked skin twitch latency by 30 min after administration, as did morphine (150 and 450 nmol). Epidural morphine (750 and 7500 nmol), but not epidural 1000 nmol Contulakin-G, also significantly decreased skin twitch in dogs. No changes in motor function were seen in any rats or dogs receiving these doses of Contulakin-G. In dogs, no physiologically significant dose-dependent changes in motor function, heart rate, arterial blood pressure, or body temperature were found. Contulakin-G is a potent antinociceptive drug when delivered intrathecally with no observable negative side effects in rats or dogs and may provide an alternative to opioid spinal analgesics.

Neurotensin and pain modulation

Neurotensin (NT) can produce a profound analgesia or enhance pain responses, depending on the circumstances. Recent evidence suggests that this may be due to a dose-dependent recruitment of distinct populations of pain modulatory neurons. NT knockout mice display defects in both basal nociceptive responses and stress-induced analgesia. Stress-induced antinociception is absent in these mice and instead stress induces a hyperalgesic response, suggesting that NT plays a key role in the stress-induced suppression of pain. Cold water swim stress results in increased NT mRNA expression in hypothalamic regions known to project to periaqueductal gray, a key region involved in pain modulation. Thus, stress-induced increases in NT signaling in pain modulatory regions may be responsible for the transition from pain facilitation to analgesia. This review focuses on recent advances that have provided insights into the role of NT in pain modulation.

Potent spinal analgesia elicited through stimulation of NTS2 neurotensin receptors

Intrathecal administration of the neuropeptide neurotensin (NT) was shown previously to exert antinociceptive effects in a variety of acute spinal pain paradigms including hotplate, tail-flick, and writhing tests. In the present study, we sought to determine whether some of these antinociceptive effects might be elicited via stimulation of low-affinity NTS2 receptors. We first established, using immunoblotting and immunohistochemical techniques, that NTS2 receptors were extensively associated with putative spinal nociceptive pathways, both at the level of the dorsal root ganglia and of the superficial layers of the dorsal horn of the spinal cord. We then examined the effects of intrathecal administration of NT or selective NTS2 agonists on acute thermal pain. Both NT and NTS2 agonists, levocabastine and Boc-Arg-Arg-Pro-Tyrpsi(CH2NH)Ile-Leu-OH (JMV-431), induced dose-dependent antinociceptive responses in the tail-flick test. The effects of levocabastine and of JMV-431 were unaffected by coadministration of the NTS1-specific antagonist 2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxy-phenyl)pyrazol-3-yl)carboxylamino]tricyclo)3.3.1.1.(3.7))-decan-2-carboxylic acid (SR48692), confirming that they were NTS2 mediated. In contrast, the antinociceptive effects of NT were partly abolished by coadministration of SR48692, indicating that NTS1 and NTS2 receptors were both involved. These results suggest that NTS2 receptors play a role in the regulation of spinal nociceptive inputs and that selective NTS2 agonists may offer new avenues for the treatment of acute pain.

A method for acetylcholinesterase staining of brain sections previously processed for receptor autoradiography

Receptor autoradiography using selective radiolabeled ligands allows visualization of brain receptor distribution and density on film. The resolution of specific brain regions on the film often can be difficult to discern owing to the general spread of the radioactive label and the lack of neuroanatomical landmarks on film. Receptor binding is a chemically harsh protocol that can render the tissue virtually unstainable by Nissl and other conventional stains used to delineate neuroanatomical boundaries of brain regions. We describe a method for acetylcholinesterase (AChE) staining of slides previously processed for receptor binding. AChE staining is a useful tool for delineating major brain nuclei and tracts. AChE staining on sections that have been processed for receptor autoradiography provides a direct comparison of brain regions for more precise neuroanatomical description. We report a detailed thiocholine protocol that is a modification of the Koelle-Friedenwald method to amplify the AChE signal in brain sections previously processed for autoradiography. We also describe several temporal and experimental factors that can affect the density and clarity of the AChE signal when using this protocol.

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.

Distribution of the neurotensin receptor NTS1 in the rat CNS studied using an amino-terminal directed antibody

The distribution of neurotensin receptor 1 immunoreactivity in the rat brain was studied using an antibody against the amino-terminal of the receptor expressed as a fusion protein with glutathione-S transferase. Affinity purified antibodies detected the fusion protein and the complete neurotensin receptor sequence expressed in Escherichia coli. The immunostaining was abolished by preabsorption with the amino-terminal fusion protein. Immunoreactive neurotensin receptor 1 immunoreactivity was detected on cell bodies and their processes in a number of CNS regions. In agreement with previous binding studies neurotensin receptor 1 immunoreactivity was particularly localised in cell bodies in the basal forebrain, nucleus basalis and substantia nigra. At the electron microscope level immunoreactivity was found both in axonal bouton and dendrites and spines in the basal forebrain indicating that neurotensin may act both pre- and post-synaptically. There were several regions such as the substantia gelatinosa, ventral caudate-putamen and the lateral reticular nucleus where the neurotensin receptor 1 positive cells had not previously been reported, indicating that distribution of this receptor is widespread.

Regional effect of naltrexone in the nucleus of the solitary tract in blockade of NPY-induced feeding

Naltrexone (NLTX) in the nucleus of the solitary tract (NTS) decreases feeding induced by neuropeptide Y (NPY) in the paraventricular nucleus (PVN). We sought to determine the NTS region most sensitive to NLTX blockade of PVN NPY-induced feeding. Male Sprague-Dawley rats were fitted with two cannulas; one in the PVN and one in a hindbrain region: caudal, medial, or rostral NTS or 1 mm outside the NTS. Animals received NLTX (0, 1, 3, 10, and 30 microg in 0.3 microl) into the hindbrain region just prior to PVN NPY (0.5 microg, 0.3 microl) or artificial cerebrospinal fluid (0.3 microl). Food intake was measured at 2 h following injection. PVN NPY stimulated feeding, and NLTX in the medial NTS significantly decreased NPY-induced feeding at 2 h, whereas administration of NLTX in the other hindbrain regions did not significantly influence PVN NPY induced feeding. These data suggest that opioid receptors in the medial NTS are most responsive to feeding signals originating in the PVN after NPY stimulation.

Possible role of plasma neurotensin on growth hormone regulation in neonates

OBJECTIVE

To evaluate secretion of plasma neurotensin (NT) which could be involved as a peripheral signal in growth hormone (GH) regulation, NT release was measured during early postnatal life, a period of striking changes in GH secretion.

METHODS

Blood samples were collected from 19 normal full-term neonates on day 5 and at 3 months of age to evaluate plasma NT concentrations by radioimmunoassay, serum growth hormone (GH) levels using an immunofluorometric assay, and serum insulin-like growth factor-I (IGF-I) values by radioimmunoassay.

RESULTS

Five day-old neonates showed significantly higher (p < 0.001) mean (+/- SEM) plasma NT levels (83.55 +/- 12.07 fmol/ml) compared with those in 11 prepubertal children and those in 14 adults who were studied as control subjects (13.30 +/- 2.90 and 9.70 +/- 1.10 fmol/ml, respectively). In 5 day-old neonates we observed significantly higher (p < 0.001) serum GH levels (29.53 +/- 3.40 ng/ml) compared with those in the prepubertal children (1.26 +/- 0.28 ng/ml). Five day-old neonates showed significantly lower (p < 0.001) serum IGF-I concentrations (27.01 +/- 0.77 ng/ml) than those in the prepubertal children (210 +/- 25 ng/ml). At 3 months of age, plasma NT levels (59.37 +/- 7.47 fmol/ml) and serum GH values (4.40 +/- 0.60 ng/ml) were significantly decreased (p < 0.001). At the 3rd month of life, serum IGF-I levels (44.88 +/- 4.30 ng/ml) were increased significantly (p < 0.001).

CONCLUSIONS

The human neonate showed very high concentrations of NT and GH in comparison with those observed in control subjects. The postnatal rise in IGF-I values is presumed to determine the fall in serum GH concentrations by stimulating somatostatin secretion. Neurotensin could be involved as a peripheral signal in the inhibitory mechanisms mediated by release of somatostatin.

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.

Neuroimmune mechanisms of intestinal responses to stress. Role of corticotropin-releasing factor and neurotensin

Previous studies showed that exposure of experimental animals to immobilization stress increases colonic motility and that these effects are mediated by release of corticotropin-releasing factor (CRF), Studies from our laboratory showed that 30-min immobilization stress of rats caused several not previously described colonic responses to stress, including increased colonic mucin and prostaglandin E2 (PGE2) secretion, increased colonic mucosal levels of cyclooxygenase-2 (COX-2) mRNA, and degranulation of colonic mast cells. These stress-associated colonic changes were reproduced by intravenous or intracerebral injection of CRF in conscious, nonstressed rats. Furthermore, pretreatment of rats with the CRF antagonist alpha-helical CRF9-41, hexamethonium, or the mast cell stabilizer lodoxamide inhibited our observed colon responses to immobilization stress. Our results indicate that CRF released during immobilization stress increases colonic transit via a neuronal pathway and stimulates colonic mucin release via activation of neurons and colonic mast cells. These results provide support for an important role for CRF in stress-mediated colonic responses and a link between the nervous and the immune systems.

Electrophysiological studies on rat dorsal root ganglion neurons after peripheral axotomy: changes in responses to neuropeptides

The effect of three peptides, galanin, sulfated cholecystokinin octapeptide, and neurotensin (NT), was studied on acutely extirpated rat dorsal root ganglia (DRGs) in vitro with intracellular recording techniques. Both normal and peripherally axotomized DRGs were analyzed, and recordings were made from C-type (small) and A-type (large) neurons. Galanin and sulfated cholecystokinin octapeptide, with one exception, had no effect on normal C- and A-type neurons but caused an inward current in both types of neurons after sciatic nerve cut. In normal rats, NT caused an outward current in C-type neurons and an inward current in A-type neurons. After sciatic nerve cut, NT only caused an inward current in both C- and A-type neurons. These results suggest that (i) normal DRG neurons express receptors on their soma for some but not all peptides studied, (ii) C- and A-type neurons can have different types of receptors, and (iii) peripheral nerve injury can change the receptor phenotype of both C- and A-type neurons and may have differential effects on these neuron types.

Opioid-induced release of neurotensin in the periaqueductal gray matter of freely moving rats

The midbrain periaqueductal gray matter (PAG) is an important region for endogenous pain suppression. Nerve terminals containing opioid peptides and neurotensin (NT), as well as high densities of opioid- and NT-receptors, have been demonstrated in the ventromedial PAG. Local administration of opioids or NT in this region induces antinociception in experimental animals. In the present microdialysis study, the effect of opioids on the release of NT in the ventromedial PAG was investigated. Perfusion of the microdialysis probe with 10 microM morphine induced a significant increase (P < 0.05; n = 5) of the extracellular level of NT-like immunoreactivity (NT-LI), while perfusion with a 10-fold higher concentration of morphine had no significant effect on the NT-LI release in the PAG. Also perfusion of the dialysis probe with the mu-opioid receptor-specific agonist [D-Ala2-N-Me-Phe4-Gly5-ol]-enkephaline (DAGO) (1 or 100 microM) induced a significant (P < 0.05; n = 7-9) increase of the NT-LI level. The increase in NT-LI release in response to 1 microM DAGO was both calcium-dependent and naloxone-reversible. Since opioid agonists generally inhibit neuronal activity, an indirect mechanism, involving inhibition of tonically active inhibitory neurons, e.g. gamma-aminobutyric acid (GABA) neurons, could be of importance for the opioid induced release of NT. However, local administration in the PAG of the GABA(A) antagonist bicuculline (0.1-10 microM) or the GABA(A) agonist muscimol (1-100 microM) had no significant effect on the extracellular NT-LI level in the PAG, suggesting that GABAergic mechanisms are not involved in the opioid-induced release of NT-LI. In conclusion, the present data provide in vivo evidence that mu-opioid receptors mediate stimulation of neurotensin release in the PAG.

Neurotensin and the serotonergic system

The serotonergic system, because of very diffuse projections throughout the central nervous system, has been implicated in numerous functions including nociception, analgesia, sleep-wakefulness and autonomic regulation. Despite an abundant literature indicating the presence of neurotensin-containing (neurotensinergic) neurons, fibres and terminals in most areas containing serotonergic neurons, little is known about the possible relationship between serotonergic and neurotensinergic systems. The purpose of this review is (i) to summarize current knowledge on the anatomical relation between neurotensinergic and serotonergic system, (ii) to summarize current knowledge on the action of neurotensin on serotonergic neurons and (iii) to discuss the possible physiological relevance of this action. Neurotensin-containing cell bodies can be found in the most rostral raphe nuclei. There are neurotensin-containing fibres and terminals in all raphe nuclei. Raphe nuclei have also been shown to contain neurotensin-receptor binding sites. In the dorsal raphe nucleus, neurotensin induces a concentration-dependent increase in the firing rate of a subpopulation of serotonergic neurons. The neurotensin-induced excitation, which is selectively blocked by the non-peptide neurotensin receptor antagonist SR 48692, is observed mainly in the ventral part of the nucleus. Most serotonergic neurons show marked desensitization to neurotensin, even at low concentrations. In intracellular experiments, neurotensin induces an inward current, associated in some cases with a decrease in apparent input conductance, which is occluded by supramaximal concentrations of the alpha 1-adrenoceptor agonist phenylephrine. In rare cases, neurotensin induces an excitation of GABAergic or glutamatergic neurons. Since the neurotensinergic system has also been implicated in nociception, analgesia, sleep-wakefulness, and autonomic regulation, the review discusses the possibility that part of this regulation could involve the activation of the serotonergic system.

Neurotensin and neuroendocrine regulation

More than two decades of research indicate that the peptide neurotensin (NT) and its cognate receptors participate to a remarkable extent in the regulation of mammalian neuroendocrine systems, potentially at multiple levels in a given system. NT-synthesizing neurons appear to exert a direct or indirect stimulatory influence on neurosecretory cells that synthesize gonadotropin-releasing hormone, dopamine (DA), somatostatin, and corticotropin-releasing hormone (CRH). In addition, context-specific synthesis of NT occurs in hypothalamic neurosecretory cells located in the arcuate nucleus and parvocellular paraventricular nucleus, including distinct subsets of cells which release DA, CRH, or growth hormone-releasing hormone into the hypophysial portal circulation. At the level of the anterior pituitary, NT stimulates secretion of prolactin and occurs in subsets of gonadotropes and thyrotropes. Moreover, circulating hormones influence NT synthesis in the hypothalamus and anterior pituitary, raising the possibility that NT mediates certain feedback effects of the hormones on neuroendocrine cells. Gonadal steroids alter NT levels in the preoptic area, arcuate nucleus, and anterior pituitary; adrenal steroids alter NT levels in the hypothalamic periventricular nucleus and arcuate nucleus; and thyroid hormones alter NT levels in the hypothalamus and anterior pituitary. Finally, clarification of the specific neuroendocrine roles subserved by NT should be greatly facilitated by the use of newly developed agonists and antagonists of the peptide.

Cholecystokinin and neurotensin inversely modulate excitatory synaptic transmission in the parabrachial nucleus in vitro

Cholecystokinin and neurotensin are present in fibres innervating the parabrachial nucleus and have previously been shown to modulate the flow of visceral afferent information through the parabrachial nucleus to the cortex in the rat. This study examined the effects of cholecystokinin and neurotensin on synaptic transmission in the parabrachial nucleus using a pontine slice preparation and the nystatin perforated-patch recording technique. Stimulation of the ventral, external lateral portion of the parabrachial nucleus elicited glutamate-mediated, excitatory postsynaptic currents in cells recorded in the parabrachial nucleus. Bath application of neurotensin dose-dependently and reversibly enhanced, while cholecystokinin attenuated, the evoked excitatory postsynaptic current. In addition, the frequency of spontaneous, miniature excitatory postsynaptic currents recorded in parabrachial nucleus cells was significantly increased by neurotensin and decreased by cholecystokinin application. Paired-pulse depression was also enhanced and decreased by neurotensin and cholecystokinin, respectively. These synaptic changes induced by neurotensin and cholecystokinin were not accompanied by changes in input resistance of parabrachial nucleus cells over a wide voltage range (although neurotensin reduced an outwardly rectifying conductance at potentials positive to -20 mV), nor did these peptides alter the inward current induced by a brief bath application of the glutamate agonist, alpha-amino-3-hydroxy-methylisoxazole-4-propionate. The neurotensin antagonist, SR48692 (100 microM), completely and reversibly blocked the neurotensin-induced enhancement of the excitatory postsynaptic current. The non-selective cholecystokinin receptor antagonist, proglumide (100 microM), completely and reversibly blocked the cholecystokinin-induced attenuation of the excitatory postsynaptic current. In addition, the selective cholecystokinin-A receptor antagonist, L-364,718 (10 microM), but not the selective cholecystokinin-B receptor antagonist, L-365,260 (100 microM), blocked the effect of cholecystokinin on synaptic transmission. These results suggest that neurotensin and cholecystokinin act at presynaptic neurotensin and cholecystokinin-A receptors, respectively, to modulate excitatory synaptic transmission in the parabrachial nucleus.

EEDQ reduces the striatal neurotensin mRNA response to haloperidol

Haloperidol is believed to induce neurotensin/neuromedin N (NT/ N) gene expression in the dorsolateral striatum (DLST) of the rat brain via dopamine D2 receptor blockade, but is also known to interact with other receptors as well. To further characterize haloperidol's effects, rats were treated with the irreversible monoaminergic receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2 hydroxyquinolone (EEDQ). In situ hybridization was performed for NT/N mRNA. D2-like and sigma receptor autoradiography was performed using 125I-sulpride and 3H-1,3-di-o-tolylguanidine (DTG), respectively. Despite antagonism of D2 receptors, pretreatment with EEDQ failed to significantly reduce the NT/N mRNA response when given 3 days prior to the haloperidol challenge. These data suggest that the acute effects of haloperidol on NT/N mRNA expression in large part result from D2 receptor antagonism. Nonetheless, a contribution of other receptors can not be excluded.

A neurotensin antagonist, SR 48692, inhibits colonic responses to immobilization stress in rats

We previously reported that short-term immobilization stress of rats causes increased colonic mucin release, goblet cell depletion, prostaglandin E2 secretion, and colonic mast cell activation, as well as increased colonic motility. The purpose of this study was to investigate whether neurotensin (NT), a peptide expressed in both brain and digestive tract, participates in these responses. Rats were pretreated with SR 48692 (1 mg/kg, i.p.), an NT antagonist, 15 min before immobilization (30 min). The administration of the antagonist significantly inhibited stress-mediated secretion of colonic mucin, prostaglandin E2, and a product of rat mast cells, rat mast cell protease II (P < 0.05), but did not alter the increase in fecal pellet output caused by immobilization stress. Immobilization stress also resulted in a quantifiable decrease in the abundance of NT receptor mRNA in rat colon compared with that in colonic tissues from nonimmobilized rats as measured by densitometric analysis of in situ hybridization studies (P < 0.03). We conclude that the peptide NT is involved in colonic goblet cell release and mucosal mast cell activation after immobilization stress.

Cellular distribution of neurotensin receptors in rat brain: immunohistochemical study using an antipeptide antibody against the cloned high affinity receptor

Receptors for the neuropeptide, neurotensin, were localized by immunohistochemistry in the rat brain by using an antibody raised against a sequence of the third intracellular loop of the cloned high affinity receptor. Selective receptor immunostaining was observed throughout the brain and brainstem. This immunostaining was totally prevented by preadsorbing the antibody with the immunogenic peptide. The regional distribution of the immunoreactivity conformed for the most part to that of [3H]- or [125I]-neurotensin binding sites previously identified by autoradiography. Thus, the highest levels of immunostaining were observed in the islands of Calleja, diagonal band of Broca, magnocellular preoptic nucleus, pre- and parasubiculum, suprachiasmatic nucleus, anterodorsal nucleus of the thalamus, substantia nigra, ventral tegmental area, pontine nuclei and dorsal motor nucleus of the vagus, all of which had previously been documented to contain high densities of neurotensin binding sites. There were, however, a number of regions reportedly endowed with neurotensin binding sites, including the central amygdaloid nucleus, periaqueductal gray, outer layer of the superior colliculus and dorsal tegmental nucleus, which showed no or divergent patterns of immunostaining, suggesting that they might be expressing a molecularly distinct form of the receptor. At the cellular level, neurotensin receptor immunoreactivity was predominantly associated with perikarya and dendrites in some regions (e.g., in the basal forebrain, ventral midbrain, pons and rostral medulla) and with axons and axon terminals in others (e.g., in the lateral septum, bed nucleus of the stria terminalis, neostriatum, paraventricular nucleus of the thalamus and nucleus of the solitary tract). These data indicate that neurotensin may act both post- and presynaptically in the central nervous system and confirm that some of its effects are exerted on projection neurons. There were also areas, such as the cerebral cortex, nucleus accumbens and para- and periventricular nucleus of the hypothalamus, which contained both immunoreactive perikarya/dendrites and axon terminals, consistent with either a joint association of the receptor with afferent and efferent elements or its presence on interneurons. Taken together, these results also suggest that the neurotensin high affinity receptor protein is associated with a neuronal population that is more extensive than originally surmised from in situ hybridization studies.

Neurotensin levels and receptors in HAS and LAS rat brains: effects of ethanol

Previous studies of neurotensin (NT) levels and NT receptor densities in specific brain regions of mice selectively bred for differences in sensitivity to ethanol have shown that NTergic processes may mediate some actions of ethanol. In the present study, we have determined the levels of NT and NT receptor densities in specific brain regions of HAS and LAS rats that have been selectively bred for differences in sensitivity to ethanol-induced loss of righting response. Regional differences in NT levels were observed in brains from both HAS and LAS rats and values in hypothalamus, ventral midbrain, and nucleus accumbens from female rats were 25 to 75% higher than levels in corresponding regions from male rats. However, there were no significant line differences in NT-ir levels in corresponding regions from HAS and LAS animals. High-affinity binding (NTH Bmax values), measured by Scatchard analyses, were higher in ventral midbrain from HAS males than from LAS males. NTH receptor densities were higher in HAS males than in HAS females; sex differences were not observed in the LAS line. There were no significant line or sex differences between HAS and LAS in low-affinity (NTL) Bmax values in any brain region. In HAS females, subhypnotic doses of ethanol produced a decrease in NT levels in nucleus accumbens, whereas, hypnotic doses caused an increase in NT levels. Likewise, hypnotic doses elicited increases in NT levels in hypothalamus of female HAS and LAS, but not in ventral midbrain or caudate putamen. These results are consistent with low dose activation of mesolimbic and nigrostriatal dopaminergic neurons in which NT is colocalized with dopamine and with high dose inhibition of these pathways.

Peptide changes in the parabrachial nucleus following cervical vagal stimulation

Previous studies in our laboratory have shown that the peptides, neurotensin (NT), cholecystokinin (CCK), substance P (SP), somatostatin (SOM), and calcitonin gene-related peptide (CGRP), have a role in modulating ascending visceral sensory information from the nucleus of the solitary tract to the thalamus via a mandatory synapse in the parabrachial nucleus (PB). In this investigation, we examined the changes in the levels of these peptides detected by immunohistochemistry in response to cervical vagal stimulation in the inactin-anesthetized male Wistar rat. Paired control and experimental animals were instrumented to monitor blood pressure and heart rate. The vagus nerve was stimulated for 0.5, 2, or 4 hours, after which time the animals were perfused and the brains processed immunohistochemically for the Fos protein and the peptides NT, CCK, SP, SOM, and CGRP. Vagal stimulation for 1 hour produced large numbers of Fos-positive cells in the external lateral (el), external medial (em), and central lateral (cl) subnuclei of the PB (N = 3). Vagal stimulation produced a reduction in the level of immunolabeling for NT, SOM, and CCK in the el and em subnuclei of the PB. This depletion was present at 0.5 hour and increased in magnitude with the length of vagal stimulation, reaching a maximum after 4 hours. In contrast, the immunolabeling for SP and CGRP increased after 0.5 hour, reaching a maximum after 2 hours of vagal stimulation in the el and em subnuclei of the PB. After 4 hours of vagal stimulation, the immunolabeling for SP and CGRP was depleted in the two PB subnuclei. Thus, the neuropeptides NT, CCK, SP, SOM, and CGRP, which modulate the visceral sensory information in the PB, are influenced somewhat differentially by the level of activity in the vagus nerve.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Neuropharmacological profile of non-peptide neurotensin antagonists

Neurotensin, an endogenous peptide widely distributed throughout the brain, fulfils neurotransmitter criteria. When administered centrally, neurotensin induces various effects and modulates the activity of the mesolimbic dopamine system. It antagonizes the behavioural action of dopamine in a manner similar, but not identical, to antipsychotic drugs. Neurotensin is even considered to be an endogenous neuroleptic. In fact, microinjection of neurotensin elicits different effects depending on both the dose and the cerebral structures into which the injection is made. Our work on the development of orally-active neurotensin antagonists has led to the identification of SR 48692, the first non-peptide antagonist of the neurotensin receptor, and some analogues. This small molecule reveals a surprising neuropharmacological profile. It antagonizes turning behaviour induced in mice and rats (after striatal or ventral tegmental area administration of neurotensin, respectively), hypolocomotion induced by intracerebroventricular injection of neurotensin in rats, and reverses the inhibitory effect of neurotensin (nucleus accumbens injection) on amphetamine-induced hyperlocomotion in rats. However, SR 48692 cannot reverse either dopamine release in the nucleus accumbens evoked by neurotensin injection in ventral tegmental area, or hypothermia and analgesia induced by intracerebroventricular injection of neurotensin. As direct and indirect dopamine agonists have been reported to promote neurotensin release in the cortex, behavioural studies were performed using injection of apomorphine. In these experiments, SR 48692 inhibited only turning and yawning. It did not antagonize other apomorphine-dependent effects such as climbing, hypothermia, hypo- or hyperlocomotion, penile erection and stereotypies. All together, these data raise the question of the existence of neurotensin receptor subtypes and confirm that the nature of neurotensin and dopamine interactions depends on the brain structures considered.

Effect of muscimol on haloperidol-induced alteration of neurotensin gene expression in the striatum and nucleus accumbens in the rat

Acute neuroleptic administration increases the expression of neurotensin/neuromedin (NT/N) gene in rat dorsolateral striatum and shell sector of the nucleus accumbens. The purpose of this study was to examine modulation of neuroleptic induction of NT/N and the proto-oncogene c-fos expression by the GABAA agonist muscimol. Adult male Sprague-Dawley rats were treated with saline, haloperidol (1 mg/kg); muscimol (3.2 mg/kg); or haloperidol (1 mg/kg) plus muscimol (3.2 mg/kg). Animals were sacrificed 1 h after drug administration. Expression of NT/N and c-fos mRNA was examined by in situ hybridization using 35S-antisense probes. Muscimol alone had no measurable effect on basal levels of NT/N or c-fos mRNA in either the dorsolateral striatum or the nucleus accumbens. However, co-administration of muscimol with haloperidol reduced haloperidol-induced increases in NT/N as well as c-fos mRNA in the dorsolateral striatum. In contrast, NT/N mRNA expression in accumbal shell induced by haloperidol was not modulated by co-administration of muscimol. These data suggest that GABAA receptors may be involved in regulation of NT/N gene expression in the DLSt, but not in the nucleus accumbens.

C-fos mediates antipsychotic-induced neurotensin gene expression in the rodent striatum

The ubiquitous inducibility of the immediate-early gene c-fos in the central nervous system has led to the search for downstream genes which are regulated by its product, Fos. Recent evidence suggests that c-fos induction by a single injection of the classical antipsychotic haloperidol may contribute to the subsequent increase in neurotensin gene expression in the rodent striatum. Consistent with this proposal, in the present study haloperidol-induced Fos-like immunoreactivity and neurotensin/neuromedin N messenger RNA were found to be expressed by the same population of striatal neurons. Moreover, inhibition of haloperidol-induced c-fos expression by intrastriatal injection of antisense phosphorothioate oligodeoxynucleotides complimentary either to bases 109-126 or 127-144 of c-fos attenuated the subsequent increase in neurotensin/neuromedin N messenger RNA. However, injection of a sense phosphorothioate oligodeoxynucleotide corresponding to bases 127-144 of c-fos did not reduce haloperidol-induced c-fos or neurotensin/neuromedin N expression. Furthermore, constitutive expression of Jun-like immunoreactivity in the striatum was not reduced by either the sense or antisense phosphorothioate oligodeoxynucleotides. Similarly, the sense and antisense phosphorothioate oligodeoxynucleotide failed to reduce proenkephalin messenger RNA, which is located in the same striatal neurons that express haloperidol-induced neurotensin/neuromedin N messenger RNA, which is located in the same striatal neurons that express haloperidol-induced neurotensin/neuromedin N messenger RNA. Lastly, haloperidol-induced increases in nerve growth factor I-A-, JunB- and FosB-like immunoreactivity and fosB messenger RNA were not decreased by intrastriatal injection of either the sense or antisense phosphorothioate oligodeoxynucleotides. These results indicate that the antisense phosphorothioate oligodeoxynucleotides attenuated haloperidol-induced neurotensin/neuromedin N expression by selectively reducing c-fos expression and emphasize the potential importance of immediate-early gene induction in the mechanism of action of this antipsychotic drug.

Autoradiographic and electrophysiological evidence for the existence of neurotensin receptors on cultured astrocytes

By means of autoradiography we have studied the cellular localization of binding sites for [3H]neurotensin and its nonpeptide receptor antagonist [3H]SR-48692 in explant cultures of rat neocortex, striatum, brain stem and spinal cord. Binding sites for the peptide and its antagonist were observed on a great number of astrocytes in all CNS regions studied. Simultaneous staining of the cultures with a monoclonal antibody against glial fibrillary acidic protein has shown that the labelled cells in the outgrowth zone of the cultures were glial fibrillary acidic protein-positive and could therefore be identified as astrocytes. In addition to astrocytes, many neurons and outgrowing nerve fibres were labelled by the radioligands. Binding of [3H]neurotensin and [3H]SR-48692 (10(-8)M) to neurons and glial cells was markedly reduced or inhibited by the unlabelled compounds at high concentration (10(-6)M), suggesting "specific" binding of the radioligands. Electrophysiological studies have shown that addition of neurotensin to the bathing solution caused a hyperpolarization of the majority of astrocytes tested. There was a dose-response relationship between the magnitude of the hyperpolarization and the concentration of the peptide (10(-10)-10(-7)M); 10(-10)M being the threshold concentration. The specificity of the action of neurotensin was confirmed by the selective nonpeptide neurotensin receptor antagonist SR-48692 which reversibly blocked or markedly reduced the hyperpolarization by the peptide on all astrocytes tested. Our electrophysiological findings together with our autoradiographic data provide strong evidence for the presence of specific and functional neurotensin receptors on astrocytes.

Effects of neurotensin in a rodent model of tardive dyskinesia

The role of neurotensin (NT) in a putative model of tardive dyskinesia (TD) was examined in the rat. When administered directly into the ventrolateral striatum of neuroleptic-naive animals, NT (2.5 micrograms/side) elicited vacuous chewing movements. This response was not seen following administration of NT into other striatal regions or the substantia nigra and was suppressed by the NT antagonist SR 48692 (100 micrograms/kg i.p.). Vacuous chewing movements were also seen following chronic administration of fluphenazine decanoate. These movements were likewise suppressed by SR 48692 (10-100 micrograms/kg i.p.), which failed to affect other behavioural responses and was without effect in neuroleptic-naive animals. Our data suggest that increased levels of endogenous NT within the ventrolateral striatum may play a critical role in the development of TD following chronic neuroleptic administration and that NT antagonists may be beneficial for the treatment of this disorder.

Distribution of neurotensin-containing neurons in the central nervous system of the dog

The distribution of neurotensin-containing cell bodies and fibers was examined in the central nervous system of the dog using light microscopic immunohistochemistry. A very large population of neurotensin-containing cell bodies was observed in the septal nuclei, nucleus accumbens septi, preoptic areas, bed nucleus of the stria terminalis, olfactory tubercle, entorhinal cortex, ventral subiculum, anterodorsal thalamic nucleus, anteroventral thalamic nucleus, nucleus reuniens, lateral habenular nucleus, parabrachial nucleus, and nucleus of the solitary tract. Extremely dense networks of neurotensin-containing fibers were found in the globus pallidus, hypothalamus, infundibular stalk, ventral tegmental area, periaqueductal gray, interpeduncular nucleus, and spinal nucleus of the trigeminal nerve and substantia gelatinosa. However, the cerebral neocortex and cerebellum were negative for neurotensin in the present study. When the present findings are compared with those in other animals, it is clear that the major species-specific differences in distribution involve three immunonegative regions and four immunopositive regions in the dog: The former are the cerebral neocortex, mammillary body, and hippocampus; the latter are the cell bodies in the pyramidal layer of the olfactory tubercle, the superficial and middle layers of the entorhinal cortex and ventral subiculum, and the nerve fibers in the interpeduncular nucleus. The present study indicates a rather extensive network of neurotensin neurons in the central nervous system of the dog.

Stimulation of CRH-mediated ACTH secretion by central administration of neurotensin: evidence for the participation of the paraventricular nucleus

Central administration of neurotensin (NT) stimulates hypothalamic-pituitary-adrenal (HPA) activity in freely-moving rats. Increases in adrenocorticotropin hormone (ACTH) and corticosterone (B) were observed 15 min following central NT administration and remained elevated for up to 4 h. Of the two NT fragments tested, NT1-8 and NT8-13, only NT8-13 was found to significantly elevate ACTH and B levels. Moreover, NT8-13 activated the HPA axis with a temporal profile similar to NT1-13, suggesting an interaction with the pharmacologically and molecularly characterized NT receptor. Animals pre-treated intravenously with the corticotropin-releasing hormone (CRH) antagonist, alpha-helical CRH, showed attenuated plasma ACTH and B responses to central NT administration. This indicates that CRH receptor activation is necessary for the stimulatory effects of NT on HPA function. Bilateral lesions of the paraventricular nucleus (PVN) of the hypothalamus significantly reduced NT-induced stimulation of ACTH and B release suggesting that the PVN is essential for NT's stimulatory action. Median eminence content studies indicated that acute central NT administration stimulates CRH, but not arginine vassopressin (AVP), release in animals examined 60 min following NT injection. Taken together, these findings suggest that the stimulatory effects of NT on HPA activity occur via specific NT receptors and that one site of action of NT is likely at the level of the PVN where NT elicits the release of CRH.

Antagonization of fentanyl-induced muscular rigidity by neurotensin at the locus coeruleus of the rat

We evaluated the interaction between neurotensin (NT) and mu-opioid receptors at the locus coeruleus (LC), using fentanyl-induced muscular rigidity as our experimental index. Adult, male Sprague-Dawley rats anesthetized with ketamine (120 mg/kg, i.p., with 24 mg/kg/h i.v. infusion supplements) were used. Intravenous injection of fentanyl (100 micrograms/kg) consistently promoted a significant increase in the electromyographic activity recorded from the sacrococcygeus dorsalis lateralis muscle. This implied muscular rigidity was appreciably and dose-dependently antagonized by prior intracerebroventricular (i.c.v.) application of NT (15, 30 or 60 nmol/5 microliter). Microinjection of the tridecapeptide (300 or 600 pmol/100 nl) into the bilateral LC produced similar results. This suppressive effect of NT on fentanyl-induced muscular rigidity was antagonized by simultaneously administered NT antiserum (1:80), or partially blocked by its antagonist, (D-Trp11)-NT (300 pmol), but not by normal rabbit serum (1:80). These results suggest that NT may interact with the mu-opioid receptors at the LC, resulting in the suppression of fentanyl-induced muscular rigidity in the rat.

Neurotensin excitation of rat ventral tegmental neurones

1. Whole-cell patch-clamp recordings were made from ventral tegmental area neurones in rat midbrain slices in vitro. In principal cells, which are presumed to contain dopamine, neurotensin (< or = 1 microM) caused an inward current at -60 mV in thirty of forty-seven neurones and had no effect on the remainder. In secondary neurones, neurotensin caused an inward current in twelve of thirty-three cells. 2. The inward current evoked by neurotensin reached a maximum amplitude of about 80 pA, and declined over several minutes when the application was discontinued. The current was most commonly accompanied by a decrease in membrane conductance and reversed polarity at a strongly hyperpolarized potential; this reversal potential was less negative in a higher extracellular potassium concentration. Neurotensin also caused an inward current even in potassium-free internal and external solutions; this current was accompanied by a conductance increase, reversed close to 0 mV and was inhibited by reduction of the extracellular sodium concentration (from 150 to 20 mM). 3. The inward current was associated with a large increase in noise; this persisted in calcium-free solutions but was inhibited by low sodium concentration. The increase in noise was more prominent at hyperpolarized potentials. The amplitude of the unitary current underlying the increase in noise was estimated from the ratio of the variance to the mean as about 1.5 pA at -100 mV. 4. When the recording was made with an electrode containing guanosine 5'-thio-triphosphate, the steady inward current evoked by neurotensin did not reverse when the application was discontinued. When the recording electrode contained pertussis toxin, the action of neurotensin was not different although outward currents evoked by dopamine and baclofen declined with time. 5. It is concluded that neurotensin excites ventral tegmental area neurones by activating a pertussis toxin-insensitive guanosine nucleotide-binding protein. This leads to a reduction in membrane potassium conductance and an increase in membrane sodium conductance, the relative contribution of which varies from cell to cell.

Autoradiographic characterization of 125I-neurotensin binding sites in human entorhinal cortex

The laminar and rostro-caudal distribution of 125I-neurotensin binding sites is described in human entorhinal cortex using quantitative autoradiography. Specific binding was most prominent over the cell clusters of layer II of the entorhinal cortex throughout its rostro-caudal extent. Dense binding was also observed in the adjacent presubiculum and cortical amygdaloid transition area, whereas minimal binding was detected in the hippocampus and dentate gyrus. 125I-Neurotensin may serve as a selective probe for neurotensin receptor alterations and layer II-specific cytoarchitectural disturbances in the entorhinal cortex in neuropsychiatric diseases associated with abnormalities of the mesial temporal lobe.

Molecular aspects of neuropeptide regulation and function in the corpus striatum and nucleus accumbens

In the corpus striatum and nucleus accumbens, neuropeptides participate along with conventional neurotransmitters such as dopamine, gamma-aminobutyric acid (GABA), acetylcholine and glutamate in the regulation of locomotor activity, stereotyped motor behaviors and neural events related to reward and affective state. The present review concerns itself with four major neuropeptide systems--enkephalin, dynorphin, tachykinins and neurotensin--and it summarizes neuroanatomical and functional studies as well as emphasizing regulatory interactions between neurotransmitters and neuropeptides at the level of neuropeptide gene expression. Dopaminergic transmission emanating from midbrain dopaminergic cell bodies of the substantia nigra and the ventral tegmentum regulates striatal and accumbens neuropeptide levels and their mRNAs. Evidence is presented for D1 or D2 receptor involvement as well as D1-D2 interactions that modulate neuropeptide and mRNA levels in striatum and accumbens neurons. Regulatory influences by GABAergic, serotonergic and cortical (glutamatergic) neurotransmission and via sigma receptors and circulating adrenal steroids are also described. The evidence gathered in many laboratories thus far indicates that these major basal ganglia peptidergic systems are modulated dynamically and sometimes in opposing ways by various neurochemical inputs which alter neuropeptide and neuropeptide mRNA levels over both short- and long-term. Neuropeptide systems are involved in the regulation and execution of motor programs and may also be involved in the control of mood and affect as well as self-administration behavior and behavioral sensitization, especially via the nucleus accumbens and its reciprocal connections with the midbrain, hippocampus and frontal cortex. Glucocorticoids modulate mood as well as self-administration behavior and influence locomotor activity and certain forms of stereotypy. The modulation of striatal proenkephalin and protachykinin mRNA levels by adrenal steroids is described along with distribution of adrenal steroid receptor subtypes. Adrenal steroid regulation of neuropeptide gene expression in striatum, accumbens and midbrain suggests that there may be a wider role for glucocorticoids and for other neuropeptide systems in environmental and drug influences on normal and abnormal behaviors involving the nigrostriatal and mesolimic systems.

Immunohistochemical evidence for a neurotensin striatonigral pathway in the rat brain

The distribution and origin of neurotensin-like immunoreactivity in the substantia nigra pars reticulata of the rat have been analysed using immunohistochemistry combined with different drug treatments and lesioning techniques. In normal rats, a distinct but weakly fluorescent network of neurotensin-immunoreactive fibers was found in the central part of the substantia nigra pars reticulata. When the animals were treated with reserpine, which suppresses dopamine transmission, a similar pattern of immunoreactivity was found, though the intensity of staining was slightly enhanced. However, when rats were treated with methamphetamine, a potent dopamine releaser, the intensity of immunoreactivity was dramatically increased. In particular, densely packed neurotensin-immunoreactive fibers were found at the dorsal border and at the ventral periphery of the substantia nigra pars reticulata. This pattern of immunoreactivity was found to be similar to that displayed by dynorphin. In the nucleus caudatus, several neurotensin-immunoreactive cell bodies were seen after reserpine treatment. Morphologically similar perikarya were observed in methamphetamine-treated rats, but they were less numerous, whereas no cell bodies were detectable in untreated animals. When a unilateral mechanical transection or an ibotenic acid injection was performed in the striatum, the patterns of neurotensin as well as dynorphin and substance P immunoreactivities in the substantia nigra pars reticulata were strongly affected. Both types of lesion caused a marked, parallel depletion of all three immunoreactive substances on the side ipsilateral to the lesion, where a restricted area was virtually devoid of immunoreactive elements. Thus the present study provides evidence for the existence of a unilateral neurotensin striatonigral pathway, terminating in the pars reticulata. The origin of the neurotensin fibers in the pars compacta has not been established but does not appear to be the caudate nucleus. These results together with evidence from the literature suggest that methamphetamine induced a massive release of dopamine from nigral dendrites acting on presynaptic D1 dopamine receptors located on neurotensinergic terminals leading to a marked increase in neurotensin-like immunoreactivity in the pars reticulata.

Effect of neurotensin and immunneutralization with anti-neurotensin-serum on dopaminergic-cholinergic interaction in the striatum

The effect of neurotensin (NT) on the release of acetylcholine (ACh) and dopamine (DA) from striatal slices of the rat brain was studied. Neurotensin, 1-150 nM, was able to release ACh from cholinergic interneurons of the striatum. Like the response to electrical stimulation, the ACh-releasing effect of NT was completely inhibited by tetrodotoxin indicating that neuronal firing is involved in its effect. Immunneutralization reduced the stimulation-evoked release of ACh, an effect that was much marked when the inhibitory dopaminergic input was suspended by sulpiride-selective antagonists of D2 receptors. Sulpiride, 0.1 mM, induced a 2-fold increase in the NT- and electrically-induced release of ACh. A quantitatively similar increase was also observed after degeneration of the nigrostriatal DA pathway with 6-hydroxydopamine (6-OHDA) (2 x 250 micrograms/animal, i.c.v.). However, the D2 receptor agonist quinpirole, 0.01 mM, significantly reduced the NT-induced release of ACh by 77%. Neurotensin enhanced the stimulation-evoked release of [3H]DA. These findings indicate that, using field stimulation when dopaminergic, cholinergic and NT-containing neurons are stimulated in concert, NT is capable of releasing both ACh and DA in the striatum, but its effect on ACh release is masked unless the D2 receptor-mediated tonic inhibitory effect of DA released from the nigro-striatal pathway is attenuated. Thus, in Parkinson's disease where the dopaminergic input is impaired, NT may be involved in producing cholinergic dominance.

Autoradiographic distribution of [3H]neurotensin receptors in the brains of superoxide dismutase transgenic mice

Superoxide dismutase (SOD) plays an important role in the protection of cells against the deleterious effects of free radicals by dismutating the toxic superoxide anion radical. Although oxygen-based radicals have been implicated in the process of aging and in neurodegenerative disorders such as Parkinson's disease, the contribution of these free radicals to the pathology of these entities has yet to be clarified. It is also not certain that increased levels of free radical scavenging enzymes would attenuate the molecular and cellular processes that lead to these pathological states. In order to assess the contribution of increased SOD gene dosage to the pathogenesis of Down's syndrome, transgenic mice have been constructed that overexpress the human CuZnSOD. We are also using this model to evaluate the role of free radicals in age-associated changes in brain neurotransmitters and their receptors. In the present study, transgenic mice and their nontransgenic littermates, aged 6 weeks and 21 months, were used in an autoradiographic receptor study of the distribution of brain neurotensin receptors. At 6 weeks of age, there were no significant differences between the two groups of mice in most brain regions. In addition, [3H]NT binding sites showed parallel age-related decreases in the majority of the areas examined in both groups. However, significant age-related decreases in the septum, the diagonal band of Broca, and in some subdivisions of the caudate-putamen were observed only in SOD-Tg mice. In contrast, significant age-related decreases in the core area of the nucleus accumbens and the dorsal aspect of the dentate gyrus of the hippocampus were seen only in non-Tg mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.