Electrophysiological actions of neurotensin in rat cerebellum

Diverse actions of the modulatory peptide neurotensin on central synaptic transmission

Neurotensin (NT) is a 13 amino acid neuropeptide that is expressed throughout the central nervous system and is implicated in the etiology of multiple diseases and disorders. Many primary investigations of NT-induced modulation of neuronal excitability at the level of the synapse have been conducted, but they have not been summarized in review form in nearly 30 years. Therefore, the goal of this review is to discuss the many actions of NT on neuronal excitability across brain regions as well as NT circuit architecture. In the basal ganglia as well as other brain nuclei, NT can act through diverse intracellular signaling cascades to enhance or depress neuronal activity by modulating activity of ion channels, ionotropic and metabotropic neurotransmitter receptors, and presynaptic release of neurotransmitters. Further, NT can produce indirect effects by evoking endocannabinoid release, and recently has itself been identified as a putative retrograde messenger. In the basal ganglia, the diverse actions and circuit architecture of NT signaling allow for input-specific control of reward-related behaviors.

Effects of neurotensin on visual neurons in the superficial laminae of the hamster's superior colliculus

Autoradiography with 125I-neurotensin in normal and enucleated hamsters was used to define the distribution of receptors for this peptide in the superficial layers of the superior colliculus (SC). Neurotensin binding sites were densely distributed in the stratum griseum superficiale (SGS), and results from the enucleated animals indicated that they were not located on retinal axons. The effects of neurotensin on individual superficial layer cells were tested in single-unit recording experiments. Neurotensin was delivered via micropressure ejection during visual stimulation (n = 75 cells), or during electrical stimulation of either the optic chiasm (OX; n = 47 cells) or visual cortex (CTX; n = 29 cells). In comparison with control values, application of neurotensin decreased visual responses of all SC cells tested to 54.1 +/- 34.9% (mean +/- standard deviation; range of decrement 7.5 to 100%; nine cells showed no effect or an increase in visual activity, which for four of these was > or = 30%). Neurotensin application also reduced responses to electrical stimulation of either OX or CTX, respectively, to 65.8 +/- 36.5% of control values (range of decrement 2.6 to 97.4%; 12 neurons showed a weak increment < or = 30%) and 68.0 +/- 38.5% (range of decrement 3.3 to 100%; five cells showed no effect or an increment, in one case > or = 30%). Of the 25 neurons tested with both OX and CTX stimulation, the correlation of evoked response suppression by neurotensin was highly significant (r = 0.70; P < 0.001). This suggests that the suppressive effects of neurotensin were common to both pathways. To test whether the inhibitory effects of neurotensin were presynaptic or postsynaptic, Mg2+ ions were ejected iontophoretically to abolish synaptic responses, and the neurons (n = 16) were activated by iontophoresis of glutamate and then tested with neurotensin. Neurotensin reduced the glutamate-evoked responses to an average 59.3 +/- 37.9% of control values (range 2.3 to 92.5%; one cell showed an increment > 30%). This result suggests that the site of action of neurotensin is most likely postsynaptic.

Actions of neurotensin: a review of the electrophysiological studies

Three effects of NT were observed on midbrain DA cells. The modulatory effect of NT, that is, the attenuation of DA-induced inhibition, has been most extensively examined. Studies indicate that this effect of NT was not simply due to a nonspecific excitation. NT selectively attenuated DA-induced inhibition without affecting either GABA-induced inhibition or glutamate-induced excitation of the same cells, and the attenuation of DA-induced inhibition could be observed at the doses at which the basal activity of DA cells was not changed by NT. The attenuation of DA-induced inhibition by NT is also unlikely to result from the formation of a DA-NT complex, since neuromedin N, which competes with NT for the same receptor but does not bind to DA, mimicked the effects, and neurotensin(1-11), which forms a complex with DA but is inactive in competing for NT receptors, did not. The similarities between the effects of NT and those of 8-bromo-cAMP and forskolin suggest that intracellular cAMP and protein kinase A may be involved. This suggestion was supported by the findings that IBMX (an inhibitor of phosphodiesterases) potentiated the effect of NT; and SQ22536 (an inhibitor of adenylate cyclase) and H8 (an inhibitor of protein kinase A) antagonized it. Phorbal-12,13-dibutyrate (an activator of protein kinase C) did not mimic the effect of neurotensin, and H7 (an inhibitor of protein kinase C) did not reduce the effect, suggesting that protein kinase C is unlikely to be involved in the modulatory effect of neurotensin. Experiments in vitro indicated that the excitatory effect of NT on DA cells occurred at higher concentrations (> 10 nM) than those needed for producing the modulatory effect. Its persistence during DA receptor blockade by sulpiride suggests that this effect was not entirely mediated by an attenuation of the inhibition induced by endogenously released DA. At even higher concentrations (> 100 nM), a sudden cessation of cell activity preceded by an increase in firing rate was observed. Whether this effect of NT was due to depolarization inactivation or a toxic effect of the peptide at high concentrations remains to be determined. In most other areas studied, the excitatory effect of NT was most commonly observed. In many areas, this excitatory effect was apparently a direct postsynaptic effect of NT. However, different mechanisms may be involved (see Table 1). For example, in some areas NT acted through a decrease in membrane conductance, while in others no change or an increase in the membrane conductance was observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Opposite effects of neurotensin on dopamine inhibition in different regions of the rat brain: An iontophoretic study

Anatomical, biochemical and behavioral data suggest functional interactions between dopamine and neurotensin in regions of the brain receiving a co-existent and/or distinct innervation by these two transmitters. We therefore measured and compared the effects of iontophoretically applied dopamine and neurotensin in the prefrontal and anterior cingulate cortex (co-existent innervation) vs the nucleus accumbens and neostriatum (distinct innervation) of urethane-anesthetized rats. In every region, the firing rate of most spontaneously active neurons was depressed by dopamine. Neurotensin had no effect on the same cells, except for a few nucleus accumbens units which were inhibited by the peptide. When dopamine and neurotensin were concomitantly applied, the magnitude of maximal inhibitions induced by dopamine was modified in the majority of neurons tested. A significant decrease in dopamine inhibition was observed in 100% of anterior cingulate, 74% of prefrontal cortex and 48% of accumbens units. On the contrary, in neostriatum, dopamine inhibition was significantly increased in 60% of the units tested. In every region, the remaining neurons showed less than 30% changes in dopamine responsiveness, and were therefore considered unaffected by neurotensin. In the anterior cingulate cortex, inhibitions, respectively, induced by the dopamine D1 agonist, SKF 38393, and the D2 agonist, LY 171555, were also decreased by simultaneous application of neurotensin. Together with currently available data on the cellular localization of neurotensin receptors in rat brain, these results suggest that the modulation of dopamine inhibition by neurotensin may have opposite effects depending on whether the neurotensin receptors are located postsynaptically on target neurons (antagonistic effects) or presynaptically on dopamine terminals (potentiating effects).

Effects of neurotensin on midbrain dopamine neurons: are they mediated by formation of a neurotensin-dopamine complex?

The effects of neurotensin on midbrain dopamine neuron activity were studied in brain slices using single-unit recording techniques. At low concentrations (0.2-10 nM), neurotensin attenuated dopamine-induced inhibition without a significant effect on the basal firing rate. At higher concentrations (greater than 10 nM), however, it consistently caused an increase in cell activity. At even higher concentrations (greater than 100 nM), a sudden cessation of cell activity preceded by an increase in firing rate was observed. Whether this effect of neurotensin was due to depolarization inactivation or to a toxic effect of the peptide at high concentrations remains to be determined. To determine whether the effects of neurotensin were mediated by formation of a neurotensin-dopamine complex, several neurotensin analogues were studied. Neurotensin (8-13), which binds to both neurotensin receptors and dopamine, mimicked the effects of native neurotensin. Neuromedin N, which competes with neurotensin for the same receptor but does not bind to dopamine, also mimicked the effects. However, neurotensin (1-11), which forms a complex with dopamine but is inactive in competing for neurotensin receptors, was ineffective. In addition, the excitatory effect of neurotensin was not attenuated in the presence of dopamine receptor blockade by sulpiride. These results suggest that formation of a neurotensin-dopamine complex may not account for the action of neurotensin on dopamine cells. When combined with the fact that there is a high density of neurotensin receptors on dopamine cells, our results support the suggestion that the observed effects of neurotensin on dopamine neurons are most likely mediated by an activation of neurotensin receptors.

The electrophysiological actions of neurotensin in the central nervous system

The endogenous neuropeptide, neurotensin (NT) alters the firing frequencies of certain neurons in the central nervous system (CNS). This is one of the findings that support the hypothesis that NT is a neurotransmitter substance. The direct application of NT on CNS neurons causes predominantly excitatory effects. These effects occur in a dose-related fashion via a calcium-dependent postsynaptic mechanism. The C-terminal hexapeptide fragment, NT 8-13 exerts similar electrophysiological effects to NT, while the N-terminal octapeptide fragment, NT 1-8 is devoid of such activity. NT produces a significant increase in the firing rates of individual neurons in the substantia nigra (SN), ventral tegmental area (VTA), medial prefrontal cortex (MPF), hypothalamus, and periaqueductal grey (PAG). This excitation occurs with a rapid onset and is readily reversible after cessation of NT application. In contrast, NT has no effect or weak inhibitory effects on the firing rates of neurons in the locus coeruleus (LC) and cerebellum. These electrophysiological actions of NT appear to be unique and not shared by other neurotransmitter and neuropeptide receptor antagonists and agonists that have been studied via direct co-application. NT attenuates dopamine (DA)-induced inhibition associated with direct application onto neurons in the SN and VTA both in vivo and in vitro. Intracellular recordings suggest that direct application of higher concentrations of NT appears to produce 'depolarization block' on individual neurons in the SN, VTA, MPF, and hypothalamus. The electrophysiological consequences of NT application not only show similarities to clinically efficacious antipsychotic medications, but also demonstrate the ability of NT to modulate the activity of dopamine (DA) neurons at the cellular level via specific NT binding sites. These findings further underscore the possibility that NT may play a pre-eminent role in the pathogenesis of, and psychopharmacological management of neurological and psychiatric disorders purportedly related to perturbation of CNS DA systems including schizophrenia.

Neurotensin-induced excitation of neurons of the rat's frontal cortex studied intracellularly in vitro

The actions of neurotensin (NT) on frontal pyramidal neurons were studied in vitro in slices of rat cerebral cortex using current clamp and single electrode voltage clamp (SEVC) techniques. Bath application of NT (0.1 microM-10 microM) induced a depolarization (2-13 mV) in 88% of the pyramidal cells, this effect was associated with a decrease in input conductance of 5-35% and its reversal potential was estimated at -88 +/ -9.7 mV. Typically, this depolarizing effect of NT was transient, since no cell responded to a second application of the peptide within 20 min after the first one. NT also induced an increase in the rate of firing of pyramidal cells evoked by direct stimulation, even when an hyperpolarizing current was applied to prevent the depolarization induced by NT. This effect could neither be explained by a decrease of the post-spike after-hyperpolarization, nor by an increase of the persistent sodium current which sustains the spiking of pyramidal cells, since the former was not affected consistently by NT and the later was insensitive to the peptide. This excitation of pyramidal neurons by NT persisted after blockade of synaptic transmission. On the other hand, NT also enhanced the synaptic noise recorded in pyramidal cells in standard perfusing medium. Furthermore, dopaminergic antagonists and noradrenergic antagonists failed to block these effects of NT. Finally, the inactive fragment of the peptide, NT(1-8), did not affect membrane properties of pyramidal cells. All together, these results suggest that NT excites frontal cortical neurons through the activation of specific NT receptors.

Localization of neurotensin receptors in the forebrain of the common marmoset and the effects of treatment with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

The distribution of neurotensin binding sites was mapped in the brain of the common marmoset using [3H]neurotensin as the ligand. Autoradiographic techniques show that the density of receptors is particularly high in the substantia nigra, ventral tegmental area, olfactory tubercle and cerebral cortex and that the distribution of neurotensin receptors in the cerebral cortex and striatum is heterogenous. In the cerebral cortex neurotensin receptors are concentrated in layers I, II, III and V, whilst receptor density is generally less in all layers of middle temporal cortex. Striatal neurotensin receptors conformed to a striosomal distribution as defined by acetylcholinesterase staining with the highest density of binding sites in the matrix. The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine selectively destroyed over 80% of the dopaminergic neurons of the marmoset substantia nigra and almost 60% of those in the adjacent ventral tegmental area. The subsequent loss of a large proportion of neurotensin receptors from marmoset substantia nigra and striatum suggests their presence on the dopaminergic nigrostriatal pathway.

Effects of neurotensin on regional concentrations of norepinephrine in rat brain

The effects of intraventricular administration of either 7.5 or 30 micrograms neurotensin on norepinephrine concentrations were examined in several brain regions at 5 or 30 min post-injection. Significant elevations in the level of this amine were found in the hypothalamus, nucleus accumbens and amygdala. Levels in striatum, ventral tegmentum, septum, frontal cortex and substantia nigra were not affected. The effects were most prominent 5 min after injection. Results are discussed in terms of behavioral changes induced by these doses of neurotensin.

Sites of action of brain-gut peptides in cultured neurons of rat brainstem

Using cultures of dissociated neurons from the lower brainstem of 14- to 15-day-old rat embryos, we studied a site of action of a brain-gut peptide by determining whether neuronal responses to a test peptide are abolished or not after replacement of normal medium with low Ca2+- high Mg2+ medium. VIP, secretin and CCK-4 may act on the postsynaptic membrane, while motilin and neurotensin may act on the presynaptic terminal. Somatostatin and bombesin may work either presynaptically or postsynaptically.

Mechanism of acute hypercalcemic hypertension in the conscious rat

Acute hypercalcemia in the conscious, unanesthetized rat, achieved by a 30-minute infusion of CaCl2 (serum calcium level, 12.8 +/- 0.6 mg/dl) resulted in significant elevation of mean arterial pressure (from 112 +/- 2 mm Hg to 129 +/- 3 mm Hg, p less than 0.001). This pressor response was associated with a significant increase in systemic vascular resistance, from 0.45 +/- 0.02 mm Hg/(ml/min)/kg body weight to 0.50 +/- 0.02 mm Hg/(ml/min)/kg body weight (p less than 0.05), but it caused no alteration in cardiac index. The pressor response to acute hypercalcemia does not appear to be mediated by vasopressor hormones or attenuated by vasodepressor hormones since inhibition of the renin-angiotensin system (nephrectomy), catecholamines (central and peripheral 6-hydroxydopamine), vasopressin (vascular antagonist), prostaglandins (indomethacin), and parathyroid hormone (parathyroidectomy) did not significantly alter the pressor response to infusion of CaCl2 in spite of similar serum calcium levels in all groups of animals. Rather, the pressor response to acute hypercalcemia seems to be mediated by a direct action of calcium ion on smooth muscle and perhaps myocardial cell contractility, since pretreatment with the calcium channel blockers verapamil or nifedipine blocked the pressor response to acute hypercalcemia.

Structure-activity studies with carboxy- and amino-terminal fragments of neurotensin on hypothalamic neurons in vitro

The purpose of this study was to determine the structural requirements for the activity of neurotensin (NT1-13) on preoptic/anterior hypothalamic (POAH) neurons in vitro. Standard explant culture electrophysiological techniques were employed. NT was administered to POAH cultures through the superfusion fluid, or, to the vicinity of individual neurons by pressure ejection (0.5-10 psi) from micropipettes. Computer-generated, peri-event histograms were used to quantitate neuronal responses. Pressure ejection of NT1-13 (50 pM to 1 microM) consistently produced an excitatory effect on 30 of 42 neurons. The remaining cells were either inhibited or unaffected. Application of the C-terminal hexapeptide, NT8-13, but not the N-terminal octapeptide, NT1-8 (less than or equal to 1 mM), produced an excitatory response in 21 of 30 neurons, but was less potent than NT1-13. Application of an N-acetylated NT8-13 fragment (NTAC8-13) produced a response that was similar to that produced by NT8-13. The excitatory effects of NT1-13 and NT8-13 were maintained in medium which effectively blocked synaptic transmission (0 mM Ca2+/12 mM Mg2+ 1 mM EGTA). These data indicate that the C-terminal hexapeptide, but not the N-terminal octapeptide, produces a dose-related, excitatory effect on single neurons in the POAH in vitro. The persistence of these effects in Ca2+-free medium supports a postsynaptic site of action for these peptides.

Postsynaptic actions of neurotensin on preoptic-anterior hypothalamic neurons in vitro

This study examines the effects of neurotensin (NT) on single neurons in explants of the preoptic-anterior hypothalamus (POAH) in vitro. Standard in vitro electrophysiological techniques were employed. Cultures were prepared from newborn rats and maintained in roller tubes for 3-4 weeks. NT was administered either through the superfusion fluid or via micropressure ejection (0.5-10 psi). Pressure ejection of NT produced a consistent, dose-related, excitatory effect on 71% of the cells studied. The remaining cells were either unresponsive to NT or inhibited by it. In addition, the excitatory effects of microiontophoretically applied glutamate (10-100 nA) were markedly enhanced by NT applied at concentrations that did not alter spontaneous rate. The effects of NT at higher concentrations were additive with glutamate. The effects of NT persisted in Ca2+-free medium when synaptic activity was suppressed. These data indicate that NT exerts a potent excitatory effect on single neurons in the POAH in vitro. Moreover, this effect proved to be additive with that of glutamate. The persistence of these effects in Ca2+-free medium suggests that the actions of NT are postsynaptic in nature.

Specific binding of tritiated neurotensin to rat brain membranes: characterization and regional distribution

The characteristics of [3H]neurotensin binding were studied using membranes prepared from the rat brain. Binding of [3H]neurotensin was found to be specific, saturable and reversible. Under the conditions of the assay non-specific binding represented less than 20% of the total binding at a radioligand concentration of 2 nM. The specific [3H]neurotensin binding increased linearly with protein concentration and was dependent on the pH and the temperature of the incubation medium. At 25 degrees C equilibrium was reached rapidly and the association kinetics appeared to be monophasic. The dissociation was not monophasic and it could be resolved into two distinct components. Scatchard analysis of the saturation data indicated a single population of binding sites with a density of 432 fmol/mg protein and an equilibrium dissociation constant of 2.85 nM. A Hill transformation of the competitive inhibition of specific [3H]neurotensin binding by increasing concentrations of unlabelled peptide yielded a slope not significantly different from unity. Neurotensin 1-13 and various neurotensin analogues were tested for their ability to compete with [3H]neurotensin for its binding site. Neurotensin 1-13, neurotensin 8-13 and the amphibian skin peptide xenopsin were equipotent and strongly inhibited the specific binding of [3H]neurotensin. Neurotensin 9-13 and the chicken intestinal peptide Lys8,Asn9-neurotensin 8-13 were weakly active, whereas neurotensin 10-13 and the amino-terminal fragments neurotensin 1-6, neurotensin 1-8 and neurotensin 1-11 were inactive. Physiological concentrations of sodium chloride inhibited specific [3H]neurotensin binding, whereas divalent cations and guanyl nucleotides did not produce a significant change in either the equilibrium dissociation constant or the total number of binding sites. There was no apparent correlation between the content of neurotensin-like immunoreactivity in the rat brain and the density of [3H]neurotensin binding sites in the various brain regions. The highest density of binding sites was found in the hypothalamus and the frontal cortex, intermediate levels in striatum, thalamus, midbrain, hippocampus and olfactory bulb, and low levels in cerebellum and pons-medulla oblongata. In general, there was less variation between different brain regions in the number of [3H]neurotensin binding sites than in the content of neurotensin-like immunoreactivity. The characteristics of this binding assay are consistent with [3H]neurotensin binding to a physiological receptor.

Neurotensin affects hyperactivity but not stereotypy induced by pre and post synaptic dopaminergic stimulation

The effects of intraventricular administration of neurotensin (0.9, 3.75 and 15.0 micrograms) on hyperactivity and stereotypy induced by either amphetamine (1 mg/kg), nomifensine (20 mg/kg), apomorphine (0.5 mg/kg) or N-n-propylnorapomorphine (0.5 mg/kg) were examined. Results indicate that for each drug treatment, the effects of neurotensin were identical: hyperactivity was significantly reduced while stereotypy remained unaffected. Results also revealed that neurotensin significantly increased the hypothermia induced by apomorphine and N-n-propylnorapomorphine. Possible mechanisms which could underly neurotensin's selective inhibitory action on hyperactivity produced by both pre and post synaptic dopaminergic stimulation are discussed.