Neurotensin receptors on the ileum of the guinea-pig: evidence for the coexistence of inhibitory and excitatory receptors

Characterization of neurotensin receptors in intestinal smooth muscle using a nonpeptide antagonist

Neurotensin reduced substance P-induced tension in ileal muscle strips and the relaxant effect was inhibited by a nonpeptide antagonist, SR 48692, 2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl)pyrazol-3-yl)car bonylamino]tricyclo(3.3.1.1.(3.7)decan-2-carboxylic acid with a half-maximal concentration (IC50) of 7.4+/-2.1 nM (n = 9) and a dissociation constant (Kb) of 0.9+/-0.2 nM. Neurotensin produced a contractile response in ileal muscle strips pretreated with apamin (50 nM) and in isolated chick rectums and both contractile effects were inhibited by SR 48692 with IC50 of 31.6+/-9.5 nM and Kh of 3.2+/-0.9 nM (n = 6) and with IC50 of 28.9+/-6.9 nM and Kb of 5.4+/-1.0 nM (n = 7), respectively. The Kb values for the contractile effects were not significantly different from each other, but significantly different from that for the relaxant effect, suggesting that both types of effect are mediated via distinct subtypes of neurotensin receptor in the intestinal smooth muscles. Contractile responses and excitatory junction potentials evoked by electrical stimulation of nonadrenergic, noncholinergic (NANC) nerves in isolated chick rectums were not inhibited by SR 48692 (up to 3.3 microM). This does not provide functional evidence for the idea that neurotensin acts as an unidentified excitatory neurotransmitter of NANC nerves in the avian rectum.

Mechanisms of regulation of neurotensin receptors

Since its discovery in 1973, the neuropeptide neurotensin has been demonstrated to be involved in the control of a broad variety of physiological activities in both the central nervous system and in the periphery. Pharmacological studies have shown that the biological effects elicited by neurotensin result from its specific binding to cell membrane neurotensin receptors that have been characterized in various tissue and in cell preparations. In addition, it is now well documented that most of these responses are subject to rapid desensitization. Such desensitization results in transient responses to sustained peptide applications, or to tachyphylaxis during successive stimulations in the same conditions. More recently, desensitization of neurotensin signalling was investigated at the cellular and molecular levels. In cultured cells, regulation at the second messenger level, receptor internalization, and receptor down-regulation processes have been reported. These are proposed to play a critical role in the control of cell responsiveness to neurotensin. This review aims to compile recent data on the different biochemical processes involved in the regulation of the neurotensin receptor and to discuss the physiological consequences of this regulation in vivo.

Modulation of peristalsis by neurotensin in isolated guinea-pig intestinal segments

Neurotensin (1-100 nM) produced an inhibitory effect and an excitatory effect on the peristaltic activity elicited by intraluminal distension using the Trendelenburg method or the intraluminal perfusion method in isolated segments of guinea-pig small intestine. The relative contribution of these effects to the overall effect varied from one region to another of the small intestine. In the Trendelenburg preparation, the excitatory effect was found to be accompanied by a decrease in the threshold intraluminal pressure required to trigger a peristaltic reflex. A substantial difference between the jejunum and the ileum was noted in that neurotensin-induced stimulation of peristaltic activity was observed in a smaller number of the segments in the jejunum than in the ileum. A nonpeptide neurotensin receptor antagonist, SR 48692, 2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl) pyrazol-3-yl)carbonylamino]tricyclo(3.3.1.1.(3.7))decan++ +-2-carboxylic acid (90 nM), abolished both the inhibitory and excitatory effects. Apamin (10 nM) abolished the inhibitory effect. From these results, neurotensin appears to exert both excitatory and inhibitory actions, via its receptors sensitive to SR 48692, on peristaltic activity in guinea-pig small intestine. The excitatory action varies with an increasing gradient toward the terminal end of the small intestine, and the inhibitory action involves apamin-sensitive mechanism.

Collisions and encounters in simulations of receptor/GTP-binding protein interactions via simple diffusion

In two intact cell systems in which GTP-binding protein (G) activity is initiated by the presence of agonist-bound receptors (R), it has been demonstrated that the rate of G activation is influenced by the rate of turnover of agonist occupancy among the receptor population. G activity is reduced when a low concentration of agonist-occupied receptors comprised by low fractional occupancy of a large receptor population is replaced by the presence of the same concentration of 100%-occupied receptors. This effect has been proposed to be due to a time interval of interaction between R and G (an encounter) that is long compared to the time of a single collection between R and G and long compared to the lifetime of an agonist-receptor complex. In a recent simulation study of R-G interaction via diffusion, the effect of agonist occupancy turnover was observed but it was assumed that long encounters were not operative. In this study, encounter intervals in simulations of R-G interaction by simple diffusion were measured in order to address that difference. The results demonstrate that relatively long encounters comprised of multiple, separate collisions are an inherent part of R-G interaction as modelled by diffusion. The implications for further implementation of simulation studies of R-G interaction are discussed.

Motility pattern of isolated rat proximal colon and excitatory action of neurotensin

The investigation concerned the effects of neurotensin on mechanical activity of isolated rat proximal colon. An isometric-isovolumic preparation was used. Colonic segments showed spontaneous contractile activity, consisting of regular changes in both endoluminal and isometric tension. Neurotensin (1 pM to 0.1 microM) induced a concentration-dependent tonic contraction of both circular and longitudinal muscle accompanied by high frequency oscillatory activity. Desensitization of the neurotensin receptors antagonized the contractile activity of neurotensin. The excitatory effects of neurotensin were partially blocked to the same degree by tetrodotoxin and atropine, indicating that a component of the neurotensin-mediated contraction involves the release of endogenous acetylcholine. The tetrodotoxin-resistant component of the neurotensin-induced effect seems to be due to a direct action on the smooth muscle cells.

Effects of neurotensin on rat supraoptic nucleus neurones in vitro

1. The electrophysiological actions of neurotensin on magnocellular neurosecretory cells (MNCs) were examined during intracellular recording from seventy-three supraoptic nucleus neurones in superfused explants of rat hypothalamus. 2. Application of neurotensin tridecapeptide (NT(1-13); 1 nM to 3 microM) caused a membrane depolarization and reversibly attenuated the after-hyperpolarization (AHP) which followed current-evoked spike trains. This effect was accompanied by increased firing frequency during depolarizing current pulses evoked from a fixed potential. 3. The effects of neurotensin could be mimicked by the C-terminal fragment, NT(8-13), but not by the N-terminal fragment, NT(1-8). 4. Depolarizing responses to NT(1-13) or NT(8-13), retained during K+ channel blockade with internal Cs+, were accompanied by increased membrane conductance. Current- and voltage-clamp analyses revealed that neurotensin-evoked depolarizations result partly from the activation of a non-selective cationic conductance reversing near -34 mV. 5. Depolarizing responses to neurotensin were retained in the presence of TTX or in Ca(2+)-free solutions, indicating the involvement of receptors located on the plasma membrane of MNCs themselves. 6. Through these effects endogenously released neurotensin may modulate excitability, activity patterns and secretion from the hypothalamo-neurohypophysial axis.

Mechanical and current responses to neurotensin in the smooth muscle of guinea-pig intestine

1. The effects of neurotensin (NT) on the mechanical activity of the smooth muscle and membrane currents of enzymatically-dispersed single smooth muscle cells of guinea-pig intestine were investigated by the isometric tension recording method and the whole-cell patch clamp technique, respectively. 2. NT (0.3-670 nM) produced an initial relaxation followed by a contraction in segment preparations of the jejunum and ileum. Atropine (0.2 microM) abolished the contraction. In 30% of ileal segments, NT produced a slowly-developed contraction in the presence of atropine. The atropine-resistant contraction was insensitive to tetrodotoxin (TTX, 0.2 microM). The relaxant effect of NT was unaffected by TTX (0.2 microM) and guanethidine (2 microM), but markedly reduced or abolished by apamin (67 nM). 3. In segment preparations of the colon and rectum, NT produced a biphasic response, similar to that in the small intestine, or a triphasic one (relaxation, contraction and relaxation). The contractile effect of NT was affected neither by atropine (0.2 microM) nor by TTX (0.2 microM). The first relaxation in response to NT was similar in pharmacological properties to that in the small intestine. 4. Responses to NT in longitudinal muscle strips were similar to those in segment preparations. The taenia caecum responded to NT with a contraction alone and the effect was unaffected by atropine (0.2 microM) and TTX (0.2 microM). 5. NT had little or no effect on the mechanical activity in the circular muscle of the small intestine. In the circular muscle of the large intestine, NT had a weak inhibitory effect on the spontaneous activity which was followed by a small rise in muscle tone. Apamin (67 nM) converted the biphasic response to a contraction. 6. In cells dialysed with a KCl-based solution under voltage clamp at 0 mV, NT (5 microM) produced a brief, outward current in a very small fraction of cells (4 out of 40 cells). When cells were dialysed with a CsCl-based solution under voltage clamp at -50 or -60 mV, no current response to NT (5 microM) was observed in all 37 cells, but NT increased inward Ca2+ currents evoked upon depolarization. 7. From these results, it appears that there is a regional difference in the mode of action of NT to contract and relax the smooth muscle of guinea-pig intestine, and NT acts on its receptors on the smooth muscle to enhance activation of voltage-dependent Ca2+ channels, which underlies the slow contraction in the longitudinal muscle of the intestine.

Binding and biologic activity of neurotensin in guinea pig ileum

Experiments were performed to relate receptor binding to biologic activity for the contractile effect of neurotensin (NT) in guinea pig ileum. The contractile response was examined on pieces of ileum under 1 g tension in a 5 ml bath of oxygenated Tyrode's at 38 degrees C. NT contracted the longitudinal muscle (ED50, approximately 0.3 nM), the 2-3 g response peaking at 1 min and fading rapidly. In the presence of atropine (1 microM), > or = 50% of the response was blocked and the residual effect gave an ED50 of approximately 1.4 nM. In the presence of atropine and CP-96,345, a substance P receptor antagonist (0.2 microM), no contraction was observed at 20 nM NT. Thus, there were two components to the response, one involving acetylcholine (ED50, 0.3 nM) and one substance P (ED50, 1.4 nM). Using membrane preparations and 125I-labeled NT, specific, high affinity receptors for NT were demonstrated in the muscle and myenteric plexus. Scatchard analyses indicated the presence of two binding sites (Kds: approximately 0.1 nM and approximately 2 nM). Sodium ion and GTP analogs inhibited binding. Binding and biologic activity were similar in regard to dependence on specific groups within NT and sensitivity to metal ions. The high potency of Hg++ was consistent with an involvement of free sulfhydryl group(s) in the binding reaction; this was supported by work with SH-directed agents. The results suggest that two receptor types or configurations may mediate the two components of the contractile effect of NT on guinea pig ileum.

Effect of neurotensin on contractile activity and [3H]acetylcholine release in cat terminal ileum during different postnatal periods

The effect of neurotensin (NT) on the contractile activity of circular and longitudinal strips from the terminal ileum of 15-, 30-, 60-day-old and adult cats as well as on the resting and electrically-evoked release of [3H]acetylcholine (ACh) was studied. Radioactivity was measured by liquid scintillation spectrometry and the effect of NT was evaluated by the S2/S1 ratio. In the circular muscle strips NT (1-100 nM) inhibited spontaneous contractions in all age groups. In the longitudinal strips the effect of NT was concentration- and age-dependent. NT at a concentration of 1 nM had no effect on the spontaneous activity in 15-day-old cats, but in the other age groups in 70-80% of the cats it inhibited spontaneous contractions. The response to 10 and 100 nM NT was either biphasic (relaxation followed by contraction) or inhibitory: in 15-day-old cats the response was biphasic only and with increasing age the percentage of strips responding with inhibition of the contractions increased. Neither substances affecting adrenergic and cholinergic transmission nor TTX changed the inhibitory response to NT. The contractile component of the biphasic response was TTX-resistant in all age groups and was significantly decreased by scopolamine in 60-day-old and adult cats. NT increased both resting and electrically-evoked release of [3H]ACh which was not changed by TTX. In the presence of the peptide the S2/S1 ratio increased as NT-induced [3H]ACh release in the strips of adult cats was higher than that in young cats.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotensin: dual effect on the motor activity of rat duodenum

The effects of neurotensin on mechanical activity of rat duodenum were investigated using an isometric-isovolumic preparation. Neurotensin (1 pM to 10 nM) induced a concentration-dependent, tetrodotoxin (TTX)-insensitive fall in both endoluminal pressure and isometric tension. At higher concentrations of neurotensin (1 nM to 1 microM) the relaxation was followed by a concentration-dependent TTX-insensitive contraction, detected only by an increase in endoluminal pressure. Different concentrations of neurotensin were required to desensitize the relaxant and the contractile actions of the neuropeptide. The relaxation was antagonized by apamin, while the contractile response was blocked by nifedipine. Neurotensin, when tested separately on longitudinal and circular muscular strips, caused relaxation of the longitudinal strips. Circular strips showed contractions in response to neurotensin, following an inhibitory phase, if the strips were spontaneously or pharmacologically activated. The results suggest the presence of two sets of neurotensin receptors with a differential localization between the two muscular layers in rat duodenum.

Characterization of large neuromedin-N using antisera towards regions of the neurotensin/neuromedin-N precursor

Processing of the precursor to neurotensin/neuromedin-N was studied in brain and intestine from four mammalian species (dog, cat, guinea pig and rat) using previously characterized immunoassays for neurotensin and neuromedin-N, as well as newly developed assays towards the 35-44 sequence (P1) and the 70-85 sequence (P2) of the canine precursor. While neurotensin was the major product (approximately 98%) with neurotensin immunoreactivity in brain and ileum, a large molecular form of neuromedin-N was found to comprise 55-91% of the neuromedin-N activity in the ileum of these species and only 2-8% that in brain. Large neuromedin-N, which behaved as a single substance during multiple chromatographic steps, was found to cross-react in the assays for P1 and P2, indicating that this molecule extended at least from residues 40-148, neuromedin-N being located at its C-terminus. Western blots confirmed the results obtained by immunoassay. Partially purified preparations of large neuromedin-N from dog, cat and rat were also found to contract the isolated guinea pig ileum, exhibiting potencies near to that of neuromedin-N. These results indicate that tissue-specific storage of large neuromedin-N, a biologically active molecule with greater than 100 amino acids, occurs in these four mammals.

Neurotensin effects on calcium/calmodulin-dependent protein phosphorylation in rat neostriatal slices

Neurotensin (NT) is an endogenous brain tridecapeptide for which high affinity binding sites exist in the central nervous system. We have investigated the effects of NT incubation with rat neostriatal slices on calcium/calmodulin (Ca/CaM)-dependent protein phosphorylation. Slices were incubated with NT (5 or 50 nM) for 3, 10, 16 or 30 min followed by in vitro phosphorylation, electrophoresis and autoradiography. NT significantly altered the phosphorylation of a 62 kDa protein which is likely the beta subunit of the Ca/CaM dependent protein kinase. These changes may reflect the ability of NT to influence calcium mediated signal transduction.

Neurotensin-related peptides inhibit spontaneous longitudinal contractions of porcine distal jejunum

The tridecapeptide neurotensin (NT) and its C-terminal homologs, including xenopsin (XP) and neuromedin N (NM-N), reduced the amplitude of spontaneous contractions in longitudinal smooth muscle strips from the porcine distal jejunum in vitro. The rank order of potency (IC50 in nM) was XP (0.1) greater than NT (0.9) approximately avian XP (1.0) greater than NM-N (1.6), which could not be explained on the basis of differential peptide degradation. Tachyphylaxis and cross-tachyphylaxis were observed after repeated NT and XP addition to muscle strips. The action of NT was mimicked by norepinephrine (NE), but not by opioid peptides, somatostatin, or vasoactive intestinal peptide. NE was nearly 100-fold less potent than NT and did not produce a state of tachyphylaxis to NT. The effects of NT and NE were unaltered by the neuronal conduction blocker tetrodotoxin (70 nM). However, the actions of NE, unlike those of NT, were reduced by the alpha-adrenoceptor blocker phentolamine (70 nM), the K(+)-channel blocker apamin (7 nM) and the Ca2(+)-channel blocker verapamil (0.7 microM). These results suggest that NT and related peptides, through a nonadrenergic mechanism, interact with smooth muscle receptors to modulate jejunoileal motor function in the pig.

Neurotensin binding sites in porcine jejunum: biochemical characterization and intramural localization

Neurotensin is present in high concentrations in the mammalian gut, especially in enteroendocrine cells of the mucosa. Exogenous neurotensin has been shown to alter ion transport by the mucosa and contractile activity of intestinal smooth muscle. The purpose of this study was to determine the distribution of neurotensin binding sites within the intestinal wall. Initially, biochemical characteristics of [125I]neurotensin binding sites were determined within two preparations of the distal porcine jejunum: (1) the mucosa and submucosa, and (2) the circular and longitudinal muscle with their intramural plexuses. Ligand binding data for the preparation including the mucosa and submucosa indicated that [125I]neurotensin bound specifically to two sites having apparent equilibrium dissociation constants of approximately 0.046 and 0.37 nM. A binding site with a dissociation constant of approximately 0.38 nM was confirmed for the preparation of muscle and associated intramural plexuses. Xenopsin and neurotensin were equipotent to neurotensin in competing for these binding sites; neuromedin N was approximately 40 times less potent in the preparation of mucosa and submucosa. Receptor autoradiography was used to determine the distribution of [125I]neurotensin binding sites within the wall of the jejunum. Autoradiograms of [125I]neurotensin bound to cross sections of the proximal and distal jejunum showed that the highest densities of silver grains were associated with the internal submucosal ganglia, external submucosal plexus and myenteric ganglia. A moderate density of silver grains was associated with the circular muscle. The localization of neurotensin binding sites to submucosal ganglia is consistent with observations that neurotensin effects on active anion secretion by the mucosa are blocked by tetrodotoxin. Immunohistochemical localization of neurotensin in the porcine jejunum demonstrated a limited population of neurotensin immunoreactive cells within the mucosal epithelium. It is possible that neurotensin released from these cells in the mucosa as well as neurotensin-related peptides released from enteric neurons may be the endogenous ligands for the binding sites visualized in this study.

Neurotensin changes the motor pattern in canine ileum from propulsive to segmenting

Modulation of ileal propulsive motility by different doses of neurotensin has been determined in detail. In conscious dogs neurotensin (2.5, 5, and 10 pmol/kg/min) was intravenously infused during the propulsive motor pattern induced by an acaloric viscous meal. Motor patterns were recorded by seven closely spaced strain gauge transducers and analyzed by a computerized method. Luminal transit was measured fluoroscopically. Neurotensin changed the propulsive motor pattern induced by the acaloric meal into a segmenting pattern. Effects were dose-dependent and most striking at a dose of 10 pmol/kg/min. The transit of luminal contents decreased from 20 to 8 cm/min. This was achieved by the increase in the number of stationary contractions (31 vs 55%), the reduction in the length of contraction spread (3.8 vs 1.8 cm), and the decrease in the number of contractions (11.5 vs 9.2 contractions/min). The contraction force was enhanced from 67 to 107 mN. Because of the marked effects on ileal motor patterns, neurotensin might be involved in the regulation of digestive motility in the distal small intestine.

Effects of guinea pig neurotensin ([Ser7]neurotensin) on gastrointestinal smooth muscle

Guinea pig neurotensin differs from other mammalian neurotensin by the substitution of proline by serine at position 7. [Ser7]neurotensin produced a concentration-dependent contraction of the guinea pig oesophagus and rat fundus, a biphasic effect (initial relaxation followed by more sustained contraction) on the guinea pig stomach corpus, duodenum and longitudinal muscle of the ileum and a relaxation of the guinea pig colon and rat ileum. The potencies (EC50) and % maximum contraction compared with the maximum effect produced by either histamine or acetylcholine of [Ser7]neurotensin and neurotensin were not significantly different in any tissue studied.

Release of gamma-aminobutyric acid and acetylcholine by neurotensin in guinea-pig ileum

The release of gamma-aminobutyric acid (GABA) and acetylcholine (ACh) from the strips of guinea-pig ileum was investigated in the presence of neurotensin. Neurotensin evoked the release of [3H]-GABA from the strips preloaded with [3H]-GABA, and the evoked release was Ca2+-dependent and tetrodotoxin-sensitive. Hexamethonium, scopolamine, [D-Pro2,D-Trp7,9] substance P and pretreatment with substance P did not alter the neurotensin-evoked release of [3H]-GABA. Pretreatment with neurotensin inhibited the release of [3H]-GABA evoked by neurotensin but not by high K+, thereby indicating that neurotensin induced a specific desensitization of its own receptor. These observations indicate that neurotensin may stimulate the GABAergic neurone through its own receptor. Neurotensin evoked the release of [3H]-ACh from strips preloaded with [3H]-choline and this release was Ca2+-dependent and tetrodotoxin-sensitive. The evoked release of [3H]-ACh was not affected by hexamethonium, scopolamine and [D-Pro2,D-Trp7,9] substance P. Bicuculline partly inhibited the neurotensin-evoked release of [3H]-ACh; thus neurotensin seems to induce a release of ACh partly through the release of endogenous GABA. All this evidence indicates that neurotensin induces release of GABA as well as ACh from the myenteric neurones of the guinea-pig ileum.

Further investigations on neurotensin as central modulator of intestinal motility in rats

Previous studies have shown that neurotensin (NT) administered intracerebroventricularly (i.c.v.) to rats provokes an inhibition of intestinal propulsion linearly related to the log of administered doses. In the present study it is demonstrated that, in contrast to morphine, repeated i.c.v. administrations of NT (2.5 nmol/rat/day) did not result in tolerance to the intestinal effect. Naloxone (Nx) administered i.c.v. fully antagonized the intestinal inhibition of i.c.v. morphine, but did not significantly alter the NT effect. However, centrally administered thyrotropin-releasing hormone (TRH) inhibited NT-induced (but not morphine-induced) intestinal inhibition. Direct microinjections of NT into the periaqueductal gray matter (PAG) produced complete inhibition of intestinal propulsion when the microinjections were localized in the dorsal portion. Finally, subdiaphragmatic vagotomy totally abolished the inhibition induced by NT into the PAG, while morphine was not affected. Some considerations are put forward concerning the existence in the central nervous system of a peptidergic pathway modulating intestinal function.

Involvement of calcium channels in the contractile activity of neurotensin but not acetylcholine: studies with calcium channel blockers and Bay K 8644 on the rat fundus

The contractile activity of neurotensin and acetylcholine on rat isolated fundus strips was examined in preparations maintained in Tyrode buffer containing 2.5, 1.0 or 0 mM Ca2+. While the neurotensin contractions depended markedly on the external Ca2+ concentration, the acetylcholine-induced muscular responses were not significantly affected by omission of calcium in the superfusion media. Pre-incubation of rat fundus strips with nifedipine (0.03-3.8 microM), diltiazem (0.5-3.5 microM) or methoxyverapamil (0.3-1.3 microM) antagonized in a non-surmountable fashion the contractile activity of neurotensin but not of acetylcholine. Pretreatment with Bay K 8644 potentiated in a concentration-dependent fashion the contractile activity of rat fundus strips to neurotensin without modifying to any significant degree the acetylcholine-induced contractions. Nifedipine blocked in a concentration-dependent manner the Bay K 8644-induced potentiation of the neurotensin contractile responses in the fundus. Results demonstrate the dependence on external calcium of the contractile activity of neurotensin and the resistance of the muscarinic response to external calcium manipulations. The coupling of the neurotensin receptor to calcium channels is discussed.

Gastrointestinal neurotensin receptors: contribution of the aromatic hydroxyl group in position 11 to peptide potency

Neurotensin structural analogues on tyrosine11 were tested in vitro to determine their ability to contract the fundus or relax the intestine. The rank order of potency was: neurotensin greater than [Phe11]-neurotensin greater than [D-Tyr11]-neurotensin greater than [D-Phe11]-neurotensin. All peptides behaved as full agonists. It is concluded that tyrosine11 is part of the neurotensin pharmacophore; the hydroxyl group increases the affinity not the intrinsic activity of the peptide at the receptor.

Excitatory neurotensin receptors on the smooth muscle of the rat fundus: possible implications in gastric motility

Picomolar concentrations of neurotensin caused concentration-dependent contractions of the longitudinal musculature of the fundus of the rat stomach. The EC50 of neurotensin was approximately 1.5 nM. On a molar basis neurotensin was about 5-10 times more potent than 5-hydroxytryptamine (5-HT) and approximately 80 times as active as acetylcholine in producing similar contractions. Studies with structurally related peptides indicated that whereas the carboxy terminal portion of neurotensin was essential for biological activity, a substantial part of its amino terminus end could be removed without affecting its potency. The EC50 for the neurotensin fragment 8-13 was identical to that of neurotensin, however its 1-8 or 1-11 fragments were completely inactive. Tetrodotoxin did not modify the potency of neurotensin or structurally related analogues suggesting that the neurotensin receptor is probably located on the smooth muscle membrane. In addition, the potency of neurotensin in contracting the fundus was not modified by pretreatment with atropine, methysergide or diphenhydramine. Fade to the contractile response of neurotensin was followed by the development of tachyphylaxis; desensitization was concentration-dependent and characterized by a shift in the agonist concentration-response curve to the right and downwards. Desensitization with a priming concentration of neurotensin (approx. EC50) caused a substantial blockade of its excitability. There was cross-desensitization between neurotensin and the contractile activity of neurotensin 8-13 or xenopsin, but not with angiotensin II, bradykinin, substance P, acetylcholine, 5-HT or histamine. Pretreatment of the fundus strip with verapamil 0.3-1 microM antagonized in a concentration-dependent fashion the neurotensin-induced contractions but not the muscular contractions caused by acetylcholine. It is concluded that neurotensin activates a specific excitatory receptor probably located on the cell membrane of the smooth muscles of the rat fundus. In addition, we suggest that this receptor is somehow related to a voltage-dependent calcium channel, sensitive to verapamil.