A comparison of the effects of mesencephalic injections of neurotensin(1-13) and neuromedin N on brain electrical self-stimulation

Synthesis and binding characteristics of [(3)H]neuromedin N, a NTS2 receptor ligand

Neurotensin (NT) and its analog neuromedin N (NN) are formed by the processing of a common precursor in mammalian brain tissue and intestines. The biological effects mediated by NT and NN (e.g. analgesia, hypothermia) result from the interaction with G protein-coupled receptors. The goal of this study consisted of the synthesis and radiolabeling of NN, as well as the determination of the binding characteristics of [(3)H]NN and G protein activation by the cold ligand. In homologous displacement studies a weak affinity was determined for NN, with IC50 values of 454nM in rat brain and 425nM in rat spinal cord membranes. In saturation binding experiments the Kd value proved to be 264.8±30.18nM, while the Bmax value corresponded to 3.8±0.2pmol/mg protein in rat brain membranes. The specific binding of [(3)H]NN was saturable, interacting with a single set of homogenous binding sites. In sodium sensitivity experiments, a very weak inhibitory effect of Na(+) ions was observed on the binding of [(3)H]NN, resulting in an IC50 of 150.6mM. In [(35)S]GTPγS binding experiments the Emax value was 112.3±1.4% in rat brain and 112.9±2.4% in rat spinal cord membranes and EC50 values of 0.7nM and 0.79nM were determined, respectively. NN showed moderate agonist activities in stimulating G proteins. The stimulatory effect of NN could be maximally inhibited via use of the NTS2 receptor antagonist levocabastine, but not by the opioid receptor specific antagonist naloxone, nor by the NTS1 antagonist SR48692. These observations allow us to conclude that [(3)H]NN labels NTS2 receptors in rat brain membranes.

Ventral Midbrain NTS1 Receptors Mediate Conditioned Reward Induced by the Neurotensin Analog, D-Tyr[11]neurotensin

The present study was aimed at characterizing the mechanisms by which neurotensin (NT) is acting within the ventral midbrain to induce a psychostimulant-like effect. In a first experiment, we determine which subtype(s) of NT receptors is/are involved in the reward-inducing effect of ventral midbrain microinjection of NT using the conditioned place-preference (CPP) paradigm. In a second study, we used in vitro patch clamp recording technique to characterize the NT receptor subtype(s) involved in the modulation of glutamatergic neurotransmission (excitatory post-synaptic current, EPSC) in ventral tegmental neurons that expressed (Ih+), or do not express (Ih-), a hyperpolarization-activated cationic current. Behavioral studies were performed with adult male Long-Evans rats while electrophysiological recordings were obtained from brain slices of rat pups aged between 14 and 21 days. Results show that bilateral ventral midbrain microinjections of 1.5 and 3 nmol of D-Tyr[¹¹]NT induced a CPP that was respectively attenuated or blocked by co-injection with 1.2 nmol of the NTS1/NTS2 antagonist, SR142948, and the preferred NTS1 antagonist, SR48692. In electrophysiological experiments, D-Tyr[¹¹]NT (0.01-0.5 μM) attenuated glutamatergic EPSC in Ih+ but enhanced it in Ih- neurons. The attenuation effect (Ih+ neurons) was blocked by SR142948 (0.1 μM) while the enhancement effect (Ih- neurons) was blocked by both antagonists (0.1 μM). These findings suggest that (i) NT is acting on ventral midbrain NTS1 receptors to induce a rewarding effect and (ii) that this psychostimulant-like effect could be due to a direct action of NT on dopamine neurons and/or an enhancement of glutamatergic inputs to non-dopamine (Ih-) neurons.

Identification of neurotensin and LANT-6 and localization of mRNA encoding their precursor in the chicken brain

Neurotensin (NT) and neurotensin-related peptide (Lys(8), Asn(9), NT(8-13): LANT-6) have previously been purified from chicken intestine. However, the presence of these peptides and the localization of their precursor mRNA in the brain were not well understood. In the present study, through a comprehensive analysis of bioactive substances, NT and LANT-6 were identified in the chicken brain using tandem mass spectrometry combined with a bioassay of the colon contraction. The effect of NT and LANT-6 on the colon contraction was assessed, and NT was found to be 10 times more potent than LANT-6. Furthermore, the sites of NT/LANT-6 precursor mRNA expression in the brain were investigated using quantitative RT-PCR. The result showed that the mRNA was expressed most in the telencephalon, followed by the diencephalon. In situ hybridization analysis revealed that cells containing NT/LANT-6 precursor mRNA were widely distributed throughout the brain except for the cerebellum. Additionally, these were highly concentrated in the frontal telencephalon, including the nidopallium, hyperpallium, and hippocampus. Collectively, these results indicate that NT and LANT-6 are produced in the chicken brain, and they may participate in multiple functions.

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

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

Neurotensin antagonist acutely and robustly attenuates locomotion that accompanies stimulation of a neurotensin-containing pathway from rostrobasal forebrain to the ventral tegmental area

Neurotensin exerts complex effects on the mesolimbic dopamine system that alter motivation and contribute to neuroadaptations associated with psychostimulant drug administration. Activation of abundant neurotensin receptors in the ventral tegmental area (VTA) enhances dopamine neuron activity and associated release of dopamine in the nucleus accumbens (Acb) and cortex. In view of recent anatomical studies demonstrating that 70% of all neurotensin-containing neurons projecting to the VTA occupy the lateral preoptic area-rostral lateral hypothalamus (LPH) and lateral part of the medial preoptic area (MPOA), the present study examined functionality in the LPH-MPOA neurotensinergic pathway in the rat. Disinhibition (resulting ultimately in stimulation-like effects) of LPH-MPOA neurons with microinjected bicuculline (50 or 100 ng in 0.25 microL) produced locomotor activation that was considerably attenuated by systemic administration of the neurotensin antagonist SR 142948 A (0.03 and 0.1 mg/kg). In contrast, locomotion elicited in this manner was completely blocked by SR 142948 A infused directly into the VTA (5.0 and 15.0 ng in 0.25 microL). Baseline locomotion was unaffected by systemic or intra-VTA administration of SR 142948 A and LPH-MPOA-elicited locomotion was unaffected by infusion of SR 142948 A into the substantia nigra pars compacta and sites rostral and dorsal to the VTA. Locomotion was not elicited by infusions of bicuculline into the lateral hypothalamus at sites caudal to the LPH-MPOA, where neurotensin neurons projecting to the VTA are fewer. The results demonstrate the capacity of a neurotensin-containing pathway from LPH-MPOA to VTA to modulate locomotion. This pathway may be important in linking hippocampal and mesolimbic mechanisms in normal behaviour and drug addiction.

On the retention of neurotensin in the ventral tegmental area (VTA) despite destruction of the main neurotensinergic afferents of the VTA--implications for the organization of forebrain projections to the VTA

Neurotensin (NT) modulates ventral tegmental area (VTA) signaling in a manner relevant to psychostimulant drug actions, thus inviting evaluation of psychostimulant effects in conditions of reduced or absent VTA NT. However, in a preliminary study, NT immunoreactivity (-ir) in the VTA was unaffected following destruction of the main concentration of forebrain neurotensinergic VTA afferents in the lateral preoptic-rostral lateral hypothalamic continuum (LPH) and adjacent lateral part of the medial preoptic area (MPOA). This study attempted to determine what measures are necessary to obtain a significant reduction of VTA NT-ir. Large unilateral ibotenic acid lesions were made in several structures containing NTergic, VTA-projecting neurons, including the LPH-MPOA, nucleus accumbens, VTA itself and dorsal raphe. None of these was associated with substantial ipsilateral loss of NT-ir in the VTA, lateral hypothalamus or lateral habenula. Combinations of lesions, such as LPH-MPOA plus VTA and LPH-MPOA plus dorsal raphe, also failed to substantially reduce NT-ir in these structures. Transections of the medial forebrain bundle (mfb) likewise failed to produce a substantial loss of VTA NT-ir measured with immunohistochemistry and radioimmunoassay. Transections of the mfb were carried out in combination with infusions of retrograde and anterograde axonal tract-tracers, revealing that the routes taken by some forebrain NT-ir VTA afferents circumvent mfb transections. All of these results together are consistent with the hypothesis that the connectional organization of forebrain and brainstem, potentially in combination with limited adaptive synaptogenesis, renders the VTA relatively insensitive to moderate losses of neurotensinergic and, perhaps, other peptidergic afferents.

Presynaptic action of neurotensin on cultured ventral tegmental area dopaminergic neurones

Dopamine-containing neurones of the ventral tegmental area express neurotensin receptors which are involved in regulating cell firing and dopamine release. Although indirect evidence suggests that some neurotensin receptors may be localised on the nerve terminals of dopaminergic neurones in the striatum and thus locally regulate dopamine release, a clear demonstration of such a mechanism is lacking and a number of indirect sites of action are possible. We have taken advantage of a simplified preparation in which cultured rat ventral tegmental area dopaminergic neurones establish nerve terminals that co-release glutamate to determine whether neurotensin can act at presynaptic sites. We recorded glutamate-mediated synaptic currents that were generated by dopaminergic nerve terminals as an index of presynaptic function. The neurotensin receptor agonist NT(8-13) caused an inward current and an enhancement of the firing rate of dopaminergic neurones together with an increase in the frequency of spontaneous glutamate receptor-mediated excitatory postsynaptic currents (EPSCs). Incompatible with a direct excitatory action on nerve terminals, NT(8-13) failed to change the amplitude of individual action potential-evoked EPSCs or the frequency of miniature EPSCs recorded in the presence of tetrodotoxin. However, NT(8-13) reduced the ability of terminal D2 dopamine receptors to inhibit action potential-evoked EPSCs in isolated dopaminergic neurones. Taken together, our results suggest that in addition to its well-known somatodendritic excitatory effect leading to an increase in firing rate, neurotensin also acts on nerve terminals. The main effect of neurotensin on nerve terminals is not to produce a direct excitation, but rather to decrease the effectiveness of D2 receptor-mediated presynaptic inhibition.

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 neurotensin-containing neurons in the central nervous system of the pigeon and the chicken

Neurotensin is widely located in neurons of the central and peripheral nervous systems among mammalian species. To obtain a comparative evaluation, we examined the distribution of neurotensin-containing cell bodies and fibers in the central nervous system of the pigeon and the chicken. The pattern of localization of neurotensin immunoreactivity was similar in the two species. Abundant accumulations of neurotensin-containing cell bodies were found in the dorsolateral corticoid area, the piriform cortex, the parahippocampal area, the medial part of the frontal neostriatum, the lateral part of the caudal neostriatum, nucleus accumbens, the bed nucleus of the stria terminalis, ventral paleostriatum, the preoptic area, the ventromedial hypothalamic nucleus, the inferior hypothalamic nucleus, the infundibular hypothalamic nucleus, and the mammillary nuclei. Extremely dense networks of neurotensin-containing fibers were found in the pallial commissure, the lateral septal nucleus, the preoptic area, the periventricular gray around the third ventricle, the dorsalis hypothalamic area, the hypothalamic nuclei, the parabrachial nucleus, the locus ceruleus, and the dorsal vagal complex. Major differences of immunoreactivity between the two species were as follows. 1) The chicken neurohypophysis contained an extremely large accumulation of immunoreactive fibers, but there were few in the median eminence. The reverse was found in the pigeon. 2) The optic tectum in the pigeon contained immunoreactive cells and fibers in layers 2 and 4, but no immunoreactivity was seen in the chicken optic tectum. 3) The cerebellar cortex in the pigeon contained a small number of immunoreactive fibers, whereas that in the chicken did not. 4) The pigeon spinal cord contained immunoreactive neurons in the subependymal layer, but the chicken spinal cord did not. Our observations suggest the presence of a very wide network of neurotensin-containing neurons in the avian brain and spinal cord, which is also the case in mammals.

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.

Isolation, identification, and synthesis of Mas-MG-MT I, a novel peptide from the larval midgut of Manduca sexta (Lepidoptera: Sphingidae)

A five-residue myotropic peptide, Manduca sexta midgut myotropin I (Mas-MG-MT I), was isolated from an extract of 800 midguts of fifth instar larvae of the tobacco hornworm, Manduca sexta. It was purified by reverse phase and normal phase HPLC. Myotropic activity was screened by a heterologous Locusta migratoria oviduct bioassay. Sequence analysis, amino acid composition analysis, and comparison of candidate synthetic peptides in the amide and acid form revealed the following primary structure: Ala-Glu-Pro-Tyr-Thr-NH2. This is the first fully identified peptide isolated directly from the midgut of an insect species. Few significant sequence homologies with known vertebrate and invertebrate peptides have been found.

Psychostimulant-like effect of central microinjection of neurotensin on brain stimulation reward

The curve shift method and the brain stimulation reward paradigm were used to dissociate reward and performance changes and determine whether unilateral ICV microinjection of neurotensin (3, 10, and 30 micrograms/10 microliters) produces neuroleptic- or psychostimulant-like effect on a dopamine-dependent behavior. At the highest dose tested, neurotensin potentiated brain stimulation reward, producing a significant time-dependent decrease in frequency threshold. Neurotensin also suppressed maximal rate of responding at every dose tested, suggesting that it was more effective at attenuating performance capability. These results suggest that a centrally acting neurotensin receptor agonist may specifically stimulate dopamine-dependent behaviors, producing psychostimulant-like effect that can be attenuated or masked by a suppression of performance capability.

Investigations on the acute effects of neuropeptides on the pituitary-adrenocortical function in normal and cold-stressed rats. II. Neurotensin and neuromedin N

The effects of a subcutaneous bolus injection of 2 micrograms neurotensin (NT) or neuromedin N (NMN) on the function of the hypothalamo-pituitary-adrenocortical axis were investigated in both normal and cold-stressed rats. The blood concentrations of ACTH, corticosterone (B) and aldosterone (ALDO) were measured by specific radioimmunoassays 1, 2 or 4 h after the neuropeptide administration. Cold stress enhanced plasma levels of ACTH, B and ALDO, and these rises lasted unchanged until 4 h. NT did not affect either basal or stress-stimulated plasma levels of ACTH and B, while it lowered the plasma ALDO concentration at 4 h in normal rats and increased it at 1 h in stressed animals. NMN did not change the basal plasma level of ACTH, but it did markedly raise blood levels of both B and ALDO; on the other hand, in cold-stressed rats NMN strongly depressed ACTH response and decreased B plasma concentration at 2 h, without evoking apparent changes in ALDO response. In light of these findings the following conclusions and hypotheses can be drawn and suggested: (i) NT and NMN, when administered at a relatively high dose, do not affect ACTH release in rats under basal conditions; (ii) NMN, but not NT, is able to prevent cold stress-induced stimulation of ACTH secretion, probably by inhibiting hypothalamic thermoregulatory centers; and (iii) NT and NMN exert direct adrenocortical antisecretagogue and secretagogue effects, respectively, which could explain the evident lack of correlation between the levels of circulating ACTH and the plasma concentrations of the main adrenal steroid hormones in both normal and stressed rats after neuropeptide administration.

Proliferogenic effect of neurotensin (NT) and neuromedin-N (NMN) on the rat adrenal cortex: evidence that angiotensin-II mediates the effect of NMN, but not of NT

NT and NMN, two biologically active peptides acting via the same specific receptor, are both able to stimulate in vivo the proliferative activity of rat adrenocortical cells. The present study aimed to investigate whether the mechanism underlying this effect of NT and NMN may involve an enhanced production of angiotensin-II (ANG-II), a potent adrenocortical proliferogenic factor. Metaphases per adrenal section were counted 120 min after vincristine injection. A bolus administration of ANG-II resulted in a marked increase in the number of metaphase-arrested cells 12, 24 and 48 h after the beginning of the experiment; the concomitant administration of saralasin (SAR), a competitive antagonist of ANG-II, completely blocked the proliferogenic effect of ANG-II. NT-evoked rise in the number of metaphases occurred 48 h after administration and was not influenced by the simultaneous SAR injection. On the contrary, NMN injection induced a burst of adrenocortical cell proliferation within 12 h, and this effect was prevented by SAR. These data suggest that ANG-II mediates the proliferogenic effect of NMN, but not that of NT.

Compared binding properties of 125I-labeled analogues of neurotensin and neuromedin N in rat and mouse brain

Neurotensin and neuromedin N are two structurally related peptides that are synthesized by a common precursor. The purpose of the present work was to characterize neuromedin N receptors in rat and mouse brain and to compare these receptors with those of neurotensin. A radiolabeled analogue of neuromedin N has been prepared by acylation of the N-terminal amino group of the peptide with the 125I-labeled Bolton-Hunter reagent. This 125I-labeled derivative of neuromedin N bound to newborn mouse brain homogenate with high affinity (KD = 0.5 nM). Cross-competition experiments between radiolabeled and unlabeled neurotensin and neuromedin N indicated that each peptide was able to displace completely and specifically the other peptide from its interaction with its receptor. Independently of the radioligand used, the affinity of neurotensin was always better than that of neuromedin N. Quantitative radioautographic studies demonstrated that the ratio of labeling intensities obtained with 125I-labeled analogues of neurotensin and neuromedin N remained constant in all the brain areas. Our results do not support the existence of a specific neuromedin N receptor in rat and mouse brain and can be explained by the presence of a common receptor for both peptides.

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.

Mesencephalic microinjections of neurotensin-(1-13) and its C-terminal fragment, neurotensin-(8-13), potentiate brain stimulation reward

Using the curve shift method, we assessed the effects of ventromedial mesencephalic tegmental (VMT) microinjections of an equimolar concentration of neurotensin-(1-13) (NT-(1-13)) and of its C-terminal fragment, neurotensin-(8-13) (NT-(8-13)), on operant responding for rewarding electrical stimulation of the caudal mesencephalic central gray. The effects of NT-(1-13) and NT-(8-13) on brain stimulation reward (BSR) were also compared to those of systemically administered quinpirole (0.1 and 0.2 mg/kg, s.c.), a direct dopamine agonist, and GBR-12909 (10 and 20 mg/kg, i.p.), a selective dopamine uptake blocker. At the concentration injected, NT-(8-13) was as effective as NT-(1-13) at facilitating BSR, producing significant leftward shifts of the function relating the rate of responding to the stimulation frequency (R/F function); neither form of the peptide was effective when injected in regions dorsal to the VMT. Similarly, GBR-12909 produced a parallel leftward shift of the R/F function, but, unlike NT-(1-13), also significantly increased the asymptotic rates of responding. In contrast, the high dose of quinpirole produced non-parallel leftward shifts of the R/F function and suppressed the asymptote. The similarity between the effects of neurotensin and GBR-12909 on one hand, and the differences between those of neurotensin and quinpirole on the other, suggest that activation VMT neurotensin receptors potentiate BSR by enhancing increases in dopamine neurotransmission that are contingent upon operant responding or rewarding brain stimulation, or both.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential catabolic fate of neuromedin N and neurotensin in the canine intestinal mucosa

We have established the peptidase content of a P2 fraction (enriched in synaptosomes) and plasma membranes prepared from canine intestinal mucosa. Fourteen exo- and endopeptidases were assayed with fluorimetric or chromogenic substrates and identified by means of specific peptidase inhibitors. Post-proline dipeptidyl aminopeptidase IV, aminopeptidase M, and carboxypeptidase A were the most abundant exopeptidases, while aminopeptidases A and B, dipeptidyl aminopeptidase, pyroglutamyl peptide hydrolase I, and carboxypeptidase B displayed little, if any, activity. Endopeptidase 24.11 was the only endopeptidase that was detected in high amount. By contrast, proline endopeptidase exhibited a low activity, while angiotensin-converting enzyme, endopeptidase 24.15, endopeptidase 24.16, and cathepsin B and D-like activities were not detected. The catabolic rates of the two related neuropeptides, neurotensin (NT) and neuromedin N (NN), established that NN was inactivated 16 to 24 times faster than NT by plasma membrane and P2 fractions, respectively. Furthermore, the two peptides underwent qualitatively distinct mechanisms of degradation. A phosphoramidon-sensitive formation of NT(1-10) was detected as the major NT catabolite, indicating that NT was susceptible to an endoproteolytic cleavage elicited by endopeptidase 24.11. By contrast, NN was inactivated by the action of an exopeptidase at its N-terminus, leading to the formation of [des-Lys1]NN. The occurrence of this NN metabolite was prevented by bestatin and actinonin, but not by the aminopeptidase B inhibitor, arphamenine B, indicating that the release of the N-terminal residue of NN was likely due to aminopeptidase M.(ABSTRACT TRUNCATED AT 250 WORDS)

Opposite effects of mesencephalic microinjections of cholecystokinin octapeptide and neurotensin-(1-13) on brain stimulation reward

Changes in operant responding for brain stimulation reward were studied before and after a microinjection of 1 nmol of sulphated cholecystokinin octapeptide, neurotensin or saline into the ventral tegmental area. Neurotensin produced a significant and long lasting decrease in the stimulation frequency required to produce a half-maximal rate of responding; cholecystokinin had the opposite effect, attenuating the rewarding efficacy the stimulation during the first 30 min post-injection. It is suggested that the opposite effects of the peptides on reward are due to their differential modulatory effects on a subpopulation of mesencephalic dopamine neurones.