Lack of mixed agonist-antagonist properties of [Gln8-Gly31]-beta h-EP-Gly-Gly-NH2 and [Arg9,19,24,28,29]-beta h-EP in the rat vas deferens neuroeffector junction: studies with naloxone, beta-funaltrexamine and ICI 174,864

The 1-27 truncated fragment of beta h-endorphin (beta h-EP) as well as [Gln8,Gly31]-beta h-EP-Gly-Gly-NH2 or [Arg9,19,24,28,29]-beta h-EP exhibited opiate agonist activity in the rat vas deferens bioassay; the potency of these peptides was 3 to 6 times less than that of beta h-EP. None of these compounds exhibited any degree of antagonism towards the inhibitory action of beta h-EP. Naloxone antagonized and reversed the inhibitory action of beta h-EP and its analogues though with varying potencies. The apparent naloxone-pA2 value for beta h-EP was 8.94; that for [Gln8-Gly31]-beta h-EP-Gly-Gly-NH2 was 8.08 and that for [Arg9,19,24,28,29]-beta h-EP was 8.38. beta-Funaltrexamine (beta-FNA) potently antagonized the inhibitory action of beta h-EP following non-equilibrium kinetics. Tissue preincubation with 10 nM beta-FNA for 60 min followed by extensive washing caused a 10-fold increase in the beta h-EP IC50. However, 10 nM beta-FNA caused only a 1.2 increase in the IC50 of [Gln8,Gly31]-beta h-EP-Gly-Gly-NH2 and a 4.1-fold increase in the IC50 of [Arg9,19,24,28,29]-beta h-EP. In contrast, preincubation of the tissue with 3 microM ICI 174,864 did not modify the potency of beta h-EP or its structural analogues. However, a 60 min pretreatment with 10 microM beta-FNA followed by the addition of 3 microM ICI 174,864 revealed a further decrease in the potency of the opiopeptins compared with tissues exposed to beta-FNA alone or ICI 174,864 alone. In conclusion, the inhibitory action of these peptides is remarkably sensitive to beta-FNA antagonism; in addition the peptides act as pure opiate agonists in marked contrast with the agonist-antagonist properties described in the CNS.

Involvement of postjunctional purinergic mechanisms in the facilitatory action of bradykinin in neurotransmission in the rat vas deferens

To investigate the nature of the bradykinin-induced potentiation of electrically driven muscle twitches in the isolated vas deferens, bradykinin, noradrenaline and adenosine 5'-triphosphate (ATP) concentration-response curves were made with control, reserpinized and chemically sympathectomized rats. Bradykinin potentiated the ATP- but not the noradrenaline-induced contractions in the epididymal and prostatic segments of the ductus. The epididymal segment of the ductus did not respond to transmural electrical stimulation following reserpine treatment. Bradykinin potentiated the muscular contractions caused by exogenous ATP but not by noradrenaline. In contrast, the transmurally evoked twitches of the prostatic portion of the ductus remained almost unaltered; bradykinin increased the motor effect of ATP without modifying the potency of noradrenaline. All neuronally induced contractile activity was absent in sympathectomized rats; bradykinin potentiated the contractile effect of ATP without altering the noradrenaline-induced contractions. These results suggest that bradykinin potentiates the ATP-evoked contractions by acting postjunctionally.

Purinergic supersensitivity following sympathectomy adds further support to co-transmission in the rat vas deferens

Adrenergic and purinergic compounds contract the longitudinal muscles of the rat vas deferens. Whereas ATP and related purinergic analogs produced contractions of greater magnitude in the prostatic half as compared to that of the epididymal end, the magnitude of the alpha 1-adrenoceptor-induced responses was larger in the epididymal than in the prostatic half of the rat ductus. Chemical sympathectomy following a 48 hr 6-hydroxydopamine-treatment (6-OHDA) caused a leftward displacement of the concentration-response curves for adrenergic and purinergic drugs, this effect being more evident in the prostatic segment. Sympathectomy caused a significant increase in the maximal response induced by ATP and adrenergic compounds which was more evident in the prostatic half of the rat ductus. The denervation-induced supersensitivity was stimulus-specific since angiotensin II and acetylcholine showed no significant change in potency. In the case of bradykinin, there was a manifest increase in the maximal response of the prostatic segment of the ductus of the chemically denervated tissues. In addition, denervation also caused an increase in the potency of prazosin and phentolamine as alpha 1-adrenoceptor blocking agents; denervation did not change the potency of yohimbine as an alpha 2-adrenoceptor blocker.

Neuropeptide Y (NPY), an endogenous presynaptic modulator of adrenergic neurotransmission in the rat vas deferens: structural and functional studies

The role of neuropeptide tyrosine (NPY) on adrenergic neurotransmission was assessed in the rat vas deferens transmurally stimulated with square pulses of 0.15 or 15 Hz. Nanomoles of NPY inhibited the electrically-induced contractions on the prostatic half but not on the epididymal end of the ductus. NPY was at least 200-fold more potent than norepinephrine or adenosine to produce an equivalent inhibition. Complete amino acid sequence of NPY is required for full agonist activity; deletion of tyrosine at the amino terminus, i.e., NPY fragment 2-36 was 3-fold less potent than the native peptide. NPY fragment 5-36, 11-36 or 25-36 were proportionally less potent than NPY. Avian pancreatic polypeptide was inactive. The presynaptic nature of the NPY activity was established measuring the outflow of 3H-norepinephrine from the adrenergic varicosities of the vas deferens electrically stimulated. In this assay, NPY was more potent than NPY 2-36 or NPY fragment 5-36. No inhibitory action of NPY was detected in K+ depolarized tissues. The inhibitory effect of NPY on the rat vas deferens neurotransmission was not significantly modified by yohimbine, theophylline or naloxone, indicating that the effect of NPY is not due to the activation of alpha 2-adrenoceptors, adenosine receptors or opiate receptors respectively. Picrotoxin or apamin did not modify the inhibitory potency of NPY; verapamil or methoxyverapamil significantly reduced its potency. The inhibitory action of NPY is best explained through the activation of presynaptic NPY receptors that regulate norepinephrine release via a negative feedback mechanism. Structure activity studies give support to the notion of NPY receptors.

Co-transmission in the rat vas deferens: postjunctional synergism of noradrenaline and adenosine 5'-triphosphate

In the isolated prostatic half of the rat vas deferens, joint application of noradrenaline (NA) and adenosine 5'-triphosphate (ATP) produced a contractile response whose magnitude was greatly larger than the addition of the tension generated by the application of each agent alone. The effect of ATP was mimicked by two non-hydrolyzable ATP analogs, but not by GTP, AMP or adenosine. In sympathectomized rats, ATP potentiated NA effects, increasing both the peak tension and the duration of the vas deferens contractile response. The synergism was concentration related. Prazosin antagonized the NA synergism but not the ATP response. Likewise, desensitization of the P2-purinoceptor blocked the ATP synergism without modifying the NA-induced contraction.

Bradykinin receptor antagonists used to characterize the heterogeneity of bradykinin-induced responses in rat vas deferens

The application of bradykinin to the isolated, transmurally stimulated rat vas deferens caused two effects: increase of the basal tension of the tissue and potentiation of the magnitude of electrically driven twitches. These bradykinin responses were not evenly distributed along the ductus. The direct contractile action of bradykinin was found to be stronger in the epididymal half of the tissue while the potentiation of the muscle twitches was more pronounced in the prostatic half of the rat ductus. Bradykinin is more potent to potentiate the electrically driven twitches than to act as a postjunctional agonist. Tyr-bradykinin, [Tyr5]bradykinin and [Tyr8]bradykinin exhibited significant differences in the potency ratio to produce each of these responses. [Thi5,8,D-Phe7]bradykinin is a weak postjunctional agonist but was a full agonist to potentiate the electrically induced twitches. Furthermore, this compound antagonized the bradykinin-induced contractions. [Hyp3,Thi5,8,D-Phe7]bradykinin was devoid of agonist activity at either pre- or postjunctional sites; it behaved as a pure antagonist and was more than potent its non-hydroxylated analog. The addition of a D-Arg residue at the amino terminal increased the antagonist potency significantly. The pA2 of D-Arg-[Hyp3,Thi5,8,D-Phe7]bradykinin to antagonize the postjunctional effect of bradykinin was 6.35, a value that differed significantly from the value of 6.93 required to block the prejunctional effect of the peptide. The bradykinin receptor antagonists did not modify significantly the magnitude of the contractile responses caused by angiotensin II, norepinephrine or 5-hydroxy-tryptamine.

Chemical sympathectomy reveals pre- and postsynaptic effects of neuropeptide Y (NPY) in the cardiovascular system

Intravenous injection of neuropeptide Y (NPY) caused short-lasting dose-dependent pressor responses in anesthetized rats. NPY was equipotent with noradrenaline in producing proportional pressor effects. Chemical sympathectomy, following the administration of 100 mg/kg 6-hydroxydopamine (6-OHDA), significantly potentiated the systemic pressor effects elicited by NPY or noradrenaline. Pretreatment with 2 nmol NPY enhanced the noradrenaline-induced pressor response in control rats. NPY did not change the basal tension of isolated rat aortic strips but significantly potentiated the contractile activity induced by 16 nM noradrenaline. This effect of NPY was not observed in aortic strips from rats pretreated with 6-OHDA. The presence of pre- and postsynaptic sites of action for NPY in the cardiovascular system of the rat is discussed.

Synthesis and expression of functional angiotensin II receptors in Xenopus oocytes injected with rat brain mRNA

Xenopus laevis oocytes were injected with poly(A)+ mRNA isolated from rat brain and superfused in a medium containing either serotonin, angiotensin II or bradykinin. Applications of serotonin or angiotensin II to injected oocytes elicited, in a dose-dependent manner, changes in membrane potential. The angiotensin II receptor was desensitized fairly rapidly in the continued presence of the agonist. No response was obtained with bradykinin. The selectivity of the angiotensin II-induced response was demonstrated by the finding that the angiotensin II antagonist [( Sar1,Ala8]angiotensin II, saralasin) blocked the angiotensin II-induced response. It is concluded that an appropriate fraction of brain mRNA is capable of directing the synthesis and correct insertion of functional angiotensin II receptors in the Xenopus oocyte membrane.

Neurochemical actions of anesthetic drugs on the gamma-aminobutyric acid receptor-chloride channel complex

Interaction of intoxicant-anesthetic drugs with the gamma-aminobutyric acid (GABA) receptor-chloride channel complex of mouse brain was studied using the binding of [35S]t-butylbicyclophosphorothionate (TBPS) to isolated membranes and the GABA-stimulated uptake of 36Cl- by membrane vesicles. Anesthetic drugs, including barbiturates, chloroform, diethylether and ethanol, inhibited the binding of TBPS and enhanced the GABA-dependent influx of chloride. In the presence of bicuculline, barbiturates increased the binding of TBPS, but this action was not shared by other anesthetic agents. Inhibition of TBPS binding was found with drug concentrations that produce anesthesia in vivo, whereas augmentation of GABA action occurred at subanesthetic concentrations. Effects of a series of n-alcohols (methanol to decanol) were studied on TBPS binding and membrane fluidity (using 1,6-diphenyl-1,3,5-hexatriene as a fluorescent probe). The potencies of the alcohols for inhibiting TBPS binding and fluidizing synaptic membranes were similar to their anesthetic potencies, but there were differences in the relative potencies of the drugs for inhibition of TBPS binding and membrane fluidization. These results, together with effects of assay temperature, suggest that effects of anesthetics on the GABA receptor-chloride channel complex were not due to changes in bulk membrane fluidity. Correlation of anesthetic potencies of chemically diverse agents with both inhibition of TBPS binding and augmentation of GABA-dependent chloride flux suggests a role for the GABA complex in anesthesia.

Identification of pre- and postsynaptic bradykinin receptor sites in the vas deferens: evidence for different structural prerequisites

The effect of bradykinin on the neuroeffector junction of the isolated rat vas deferens was studied in tissues stimulated transmurally at a frequency of 0.15 Hz. Bradykinin caused two distinct and independent actions: it potentiated the magnitude of the muscular response to the electrically driven twitches and, in addition, contracted the smooth muscle generating an increased muscular tone. The former action is referred to as the neurogenic or presynaptic effect, whereas the latter effect is called the musculotropic or postjunctional action. The neurogenic effect was abolished by tetrodotoxin or tissue denervation either by cold storage or chemical sympathectomy after 6-hydroxydopamine administration. However, these procedures did not significantly modify the musculotropic potency of bradykinin. Both actions of the peptide are receptor-mediated, as minor structural modifications in the amino acid sequence caused significant changes in biological potency. In addition, the peptide analog, [Thi5,8-D-Phe7]-bradykinin, behaved as an agonist at the presynaptic site but as an antagonist at the muscular site. The most potent peptide analog to produce the neurogenic effect was Met-Lys-bradykinin followed by Lys-bradykinin and [Tyr8]-bradykinin. In contrast, the potency of these peptide analogs acting at the postsynaptic site was about the same. des Arg9 bradykinin and des Arg9-[Leu8]-bradykinin were inactive at the pre- and postjunctional site. The neurogenic action of bradykinin was not mimicked by angiotensin II, neurotensin, substance P or vasopressin.(ABSTRACT TRUNCATED AT 250 WORDS)

Kappa-opiate-induced diuresis and changes in blood pressure: demonstration of receptor stereoselectivity using (+)- and (-)-tifluadom

(+)-Tifluadom injected i.p. produced a biphasic response of urine output: within the first hour of its administration the drug produced antidiuresis followed by a diuretic phase. In contrast, the (-) isomer produced a modest reduction in urine output as compared to the output of the saline-treated rats. In addition, (+)-tifluadom markedly reduced the output of urinary Na+ and K+. The effects of (+)-tifluadom were blocked by 7.5 mg/kg naloxone but not by 10 mg/kg of the benzodiazepine antagonist Ro 15-1788. Parallel experiments demonstrated that the i.v. administration of (+)-tifluadom to non-anesthetized rats caused a dose-related pressor response that lasted for at least 15 min. This effect of (+)-tifluadom was blocked and antagonized by naloxone. In contrast, (-)-tifluadom was either inactive on the cardiovascular system or produced short-lasting hypotension. In pentobarbital-anesthetized rats, 100 micrograms/kg (+)-tifluadom caused a precipitous hypotension that was reversed by naloxone but not by Ro 15-1788.

Extracellular calcium dependence of the neurotensin-induced relaxation of intestinal smooth muscles: studies with calcium channel blockers and BAY K-8644

To investigate whether the neurotensin-induced relaxation of the rat duodenum and ileum is dependent on the external concentration of calcium, the effect of the neuropeptide was studied in isolated intestinal segments superfused with Tyrode solution containing no calcium, 1 or 2.5 mM Ca2+. The neurotensin-induced intestinal relaxation was reduced when the extracellular calcium concentration was lowered. In addition, the inhibitory effect of neurotensin was cancelled when the tissues were incubated in the presence of diltiazem, methoxyverapamil or nifedipine. BAY K-8644, a structural analog of nifedipine that functions as an agonist of the calcium channel, potentiated the neurotensin-induced smooth muscle relaxation. The facilitatory effect of BAY K-8644 was antagonized by nifedipine, indicating competition between the 2 dihydropyridines. Apamin, the K+ channel blocker, antagonized the neurotensin-induced visceral relaxation, displacing the concentration-response curve of the peptide to the right. Furthermore, apamin also blocked the effect of neurotensin when the neuropeptide was assayed in the presence of BAY K-8644. It is concluded that the smooth muscle relaxation induced by neurotensin is dependent on external calcium, suggesting that the activation of the neuropeptide receptor causes an influx of calcium which leads to the opening of K+ channels before smooth muscle relaxation is triggered.

Post-tetanic contractile events further support the interaction of multiple neurotransmitters in the neuroeffector junction of the rat vas deferens

Post-tetanic events were recorded in isolated, superfused, epididymal and prostatic halves of the rat vas deferens. Increasing the frequency of nerve stimulation from 0.15 to 15 Hz (1-30 s) and then back to 0.15 Hz produced a post-tetanic potentiation (PTP) of the muscular responses in the epididymal end but a post-tetanic inhibitory response (PTI) in the prostatic half. Both effects were abolished by tetrodotoxin or animal pretreatment with 6-hydroxy dopamine (6-OHDA). PTP was markedly reduced by reserpine treatment or tissue incubation with prazosin. PTI was not altered by adrenergic drugs but partially reduced by tissue application of bicuculline or strychnine, revealing that gamma-aminobutyric acid (GABA) may modulate the motor transmission towards the prostatic half of the rat ductus.

Asymmetric distribution of purinergic and adrenergic neurotransmission cooperates in the motor activity along the rat vas deferens

The distribution of purinergic and adrenergic responses in the epididymal and prostatic segment of the rat vas deferens were studied in vitro. Prazosin antagonizes the twitch elicited by electrical stimulation mainly in the epididymal segment while alpha,beta-methyleneadenosine 5'-triphosphate (alpha,beta-mATP) preferentially inhibits the response of the prostatic segment. Using both prazosin plus alpha,beta-mATP, the response to field stimulation was completely inhibited. Concentration-response curves revealed that adrenergic compounds elicited a greater contraction in the epididymal portion than in the prostatic end of the ductus. Purinergic compounds caused a contraction of greater magnitude in the prostatic portion. The results suggest that adrenergic and purinergic mechanisms are asymmetrically distributed along the vas deferens reflecting a gradient of adrenergic and purinergic receptors along the ductus.

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.

Pre- and postsynaptic bradykinin responses in the rat vas deferens: asymmetric distribution of the postsynaptic effect

The application of bradykinin to isolated segments of the epididymal or prostatic portion of the rat vas deferens increased the basal muscular tension mainly of the epididymal portion of the organ. Upon low frequency transmural electrical stimulation, the epididymal and the prostatic end of the organ both reacted to the application of bradykinin with an increase of twitch height. The increase in muscular tension observed in the epididymal portion of the ductus was resistant to tetrodotoxin but obliterated by 1 microM verapamil or nifedipine. The augmentation of the magnitude of the electrically driven muscle twitches was blocked by tetrodotoxin but not affected by 1 microM verapamil or nifedipine. Preincubation of the segments of the ductus with 10 microM indomethacin significantly reduced the increase in basal muscular tension caused by low concentrations of bradykinin but did not affect the electrically driven muscle twitches. Results suggest the existence of bradykinin receptors in the nerve endings of the adrenergic terminals of the organ and in the smooth muscle membrane. Whereas the neurogenic response found in both segments of the organ was of equal magnitude, the musculotropic response was predominantly found in the epididymal half of the organ.

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.

Reserpine-induced potentiation of the inhibitory action of neuropeptide Y on the rat vas deferens neurotransmission

The inhibitory action of neuropeptide Y (NPY) on the muscular activity of the prostatic end of the rat vas deferens elicited by transmural electrical stimulation was examined in control and in reserpinized rats. Pretreatment with 1 mg/kg reserpine for 48 h induced a 6-fold increase in NPY potency. Likewise, the potency of clonidine to inhibit the electrically induced muscular activity or noradrenaline to contract the ductus musculature was also potentiated. It is hypothesized that reserpine via a denervation super-sensitivity-like process increases the density of the NPY receptors. The functional significance of NPY in the motor activity of the vas deferens is discussed.

Gastrointestinal neurotensin receptors: lack of modulation by thyrotropin releasing hormone

To examine whether thyrotropin releasing hormone (TRH) antagonized gastrointestinal neurotensin receptors in isolated segments of the rat fundus, duodenum and ileum, tissues were superfused, mounted and the isometric tension recorded. Picomoles of neurotensin caused concentration-dependent contractions of the fundus and relaxation of the smooth muscles of the small intestine. Preincubation with 1-10 microM TRH failed to antagonize the activity of neurotensin but potentiated neurotensin-induced relaxation of the ileum. Pretreatment of the tissues with 0.6 microM of neuropeptide fragment 1-11, also failed to block the neurotensin-induced effects but produced a significant potentiation of the relaxant action of neurotensin.

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.

Kappa-opiates and urination: pharmacological evidence for an endogenous role of the kappa-opiate receptor in fluid and electrolyte balance

In prehydrated rats, the administration of kappa-opiate agonists such as bremazocine, ethylketocyclazocine or compound Upjohn-50,488 produced a dose-dependent increase in urine output and decreased the concentration of Na+ and K+ in the urine as compared to that of saline-treated rats. The diuretic effect of bremazocine lasted at least 3 h. The increase in urine output was independent of the hydration state of the rat since in non water-loaded animals, bremazocine produced proportionally as much diuresis and a decrease in the output of urine electrolytes of about the same magnitude as that observed in the prehydrated animals treated with the opioid. In contrast to the diuretic action of kappa-opiate agonists, the administration of antagonists with high affinity for the kappa-opiate receptor (Win 44,441 or Mr 2266) decreased dose dependently the output of urine and reduced very significantly the total output of Na+ and K+. Whereas 2 mg/kg naloxone did not block the bremazocine-induced urinary effects, 1 mg/kg Win 44,441 or Mr 2266 antagonized competitively the renal activity of bremazocine. The results are interpreted to suggest that the kappa-opiate receptor may be involved in the regulation of fluid and electrolyte balance.

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

Neurotensin caused a complex muscular response of the longitudinal muscle-myenteric plexus preparation of guinea-pig ileum: picomoles of neurotensin produced inhibition while larger concentrations caused an inhibitory effect followed by a delayed dose-dependent contraction. The inhibitory phase of the neurotensin-induced muscular activity was not modified by tetrodotoxin but was potently antagonized in a non-competitive manner by apamin, a bee venom toxin. The contractile component was blocked by tetrodotoxin but not by apamin. These toxins were used to dissect the neurotensin muscular response into an inhibitory phase and an excitatory component. It was possible to further characterize the two neurotensin muscular components by their kinetics of desensitization. The inhibitory neurotensin response showed a fast rate of desensitization and presented a relatively low spare receptor capacity. In contrast, desensitization to the excitatory action of neurotensin was much slower, the excitatory receptors apparently having a larger spare receptor capacity. Desensitization to the action of neurotensin was selective for the neuropeptide not altering the contractile activity of substance P, angiotensin II, bradykinin, histamine or acetylcholine. These results strongly suggest the presence of two subsets of neurotensin receptors in the ileum: the inhibitory set probably localized at the postsynaptic effector level and excitatory neurotensin receptors probably of neuronal origin whose function is probably to modulate the release of neurotransmitters. The physiological implications of these two subtypes of neurotensin receptors in the control of gastrointestinal motility are discussed.