Prejunctional modulatory action of neuropeptide Y on peripheral terminals of capsaicin-sensitive sensory nerves.

1. We have determined the effect of neuropeptide Y (NPY) on motor responses produced by activation of capsaicin-sensitive primary afferents in the guinea-pig isolated left atria (reserpine-pretreatment, atropine in the bath) and bronchi (atropine and indomethacin in the bath) using capsaicin itself and electrical field stimulation as stimuli. 2. In both preparations, NPY inhibited or suppressed the response produced by electrical field stimulation while leaving the response to a submaximal concentration of capsaicin unaffected. 3. NPY had no effect on motor responses produced by a submaximal concentration of calcitonin gene-related peptide (atria) or neurokinin A (bronchi), the putative endogenous mediators of the responses produced by activation of the 'efferent' function of sensory fibres in these preparations. 4. We conclude that NPY exerts a prejunctional inhibitory action on transmitter release from peripheral endings of capsaicin-sensitive nerves. Failure of NPY to modulate responses activated by capsaicin provides further evidence for the existence of two independent modes of activation of the 'efferent' function of capsaicin-sensitive sensory nerves.     

Effects of piperine on the motility of the isolated guinea-pig ileum: comparison with capsaicin.

Piperine (1 microM), a congener of capsaicin, produced an initial contraction blocked the capsaicin-sensitive contractile response to mesenteric nerve stimulation and inhibited the twitch response induced by field stimulation in the isolated guinea-pig ileum. These three effects of piperine (1 microM) were rapidly desensitized and significantly antagonized by ruthenium red (0.5-1 microM), an inorganic dye known to antagonize the effects of capsaicin. The contractile effect of piperine was abolished by application of tetrodotoxin plus desensitization with substance P or by extrinsic denervation. The inhibitory effect of piperine (1 microM) on the twitch response was antagonized by desensitization with calcitonin gene-related peptide (CGRP). Moreover, cross-tachyphylaxis between piperine and capsaicin was observed, suggesting that a similar mechanism may be involved in the effects of these agents. The contractile effects induced by piperine (10 microM) and the subsequent inhibitory effects on the twitch response were not desensitized and largely persisted after extrinsic denervation. The contractile effects of piperine (10 microM) were not strongly inhibited by tetrodotoxin plus desensitization with substance P. It was concluded that the lower concentration of piperine caused contraction and inhibited the twitch responses by releasing substance P and CGRP, respectively, from sensory nerves, and blocked the response to mesenteric nerve stimulation by a mechanism similar to that of capsaicin. At higher concentrations, piperine had non-specific direct actions on the smooth muscle.     

Differential effects of neuropeptide Y and opioids on neurogenic responses of the perfused rat mesentery

In perfused rat mesentery transmural nerve stimulation activates both adrenergic and capsaicin-sensitive sensory nerves. When adrenergic nerves were blocked with guanethidine and smooth muscle tone was increased, transmural nerve stimulation caused a dilator response which was attenuated by tetrodotoxin and abolished by capsaicin. Indomethacin increased the vasodilator response to transmural nerve stimulation, but did not affect the dilation to calcitonin gene-related peptide. Neuropeptide Y (NPY) potentiated vasoconstrictor responses to transmural nerve stimulation, but suppressed capsaicin-sensitive vasodilation, an effect which was unaltered by indomethacin. Opioid agonists selective for mu, delta or kappa receptors, DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin), DPDPE ([D-Pen2,D-Pen5]enkephalin) and ethylketocyclazocine, had no effect on the vasoconstrictor response to transmural nerve stimulation. DAMGO and DPDPE significantly inhibited vasodilator responses to transmural nerve stimulation, but ethylketocyclazocine was without effect. After treatment with indomethacin, DAMGO still inhibited the vasodilator response, but DPDPE was no longer effective. Prejunctional control of transmitter release by NPY or opioids is dependent on the specific nerve type as well as, in some cases, the participation of endogenous prostaglandins.     

Nociceptin protects capsaicin-sensitive afferent fibers in the rat urinary bladder from desensitization

Chemical stimulation of primary afferent nerves in the rat urinary bladder in vivo with topical capsaicin (1 microg in 50 microl saline) determines a dual motor response, consisting of a contractile effect mediated by tachykinins released from sensory nerves in the bladder wall and a transient activation of a bladder-to-bladder micturition reflex organized at the supraspinal level (chemoceptive micturition reflex). Both responses undergo complete desensitization upon repeated applications of capsaicin. The i.v. administration of the novel neuropeptide nociceptin (100 nmol/kg) produced a long-lasting protection from capsaicin desensitization of afferent nerves which mediate the chemoceptive micturition reflex. In fact a chemoceptive micturition reflex could be repeatedly evoked by topical capsaicin in nociceptin-pretreated rats. In sharp contrast, nociceptin did not influence the development of desensitization of the local response to capsaicin, corresponding to the 'efferent' function of capsaicin-sensitive afferent neurons. These results suggest that the afferent and 'efferent' function of capsaicin-sensitive primary afferent neurons (CSPANs) in the rat bladder are differentiated by nociceptin. Alternative mechanisms underlying this phenomenon are discussed.     

Involvement of endogenous tachykinins and CGRP in the motor responses produced by capsaicin in the guinea-pig common bile duct

In functional experiments, we have investigated the effect exerted by neurotransmitters released from capsaicin-sensitive primary afferent nerve terminals in the isolated guinea-pig common bile duct. In resting preparations, capsaicin (0.1 microM) produced a quick contraction (45.1+/-4% of KCl 80mM) which was abolished by either atropine (1 microM) or tetrodotoxin (0.5 microM). The tachykinin receptor-selective antagonists GR 82334 (NK1 receptor-selective; 3 microM), MEN 11420 (NK2 receptor-selective; 1 microM) and SR 142801 (NK3 receptor-selective; 0.1 microM) administered separately failed to reduce the capsaicin-evoked contraction, whereas any combination of the three antagonists was effective: GR 82334 plus MEN 11420, 36+/-7% reduction; GR 82334 plus SR 142801, 48+/-4% reduction; MEN 11420 plus SR 142801, 55+/-3% reduction; GR 82334 plus MEN 11420 plus SR 142801, 57+/-5% reduction. Neither the CGRP1 receptor antagonist h-CGRP (8-37) (1.5 microM) nor the P2X purinoceptor antagonist PPADS (50 microM) affected the contractile response to capsaicin. The effect of capsaicin (0.1 microM) was abolished by pretreatment with capsaicin itself (10 microM for 15 min). Human calcitonin gene-related peptide (h-CGRP; 0.1 microM) mimicked the effect of capsaicin on resting preparations (contractile response =28% of KCl 80 mM). In preparations precontracted with a submaximal concentration of KCl (24 mM), and in the presence of atropine (1 microM), GR 82334 (3 microM) and MEN 11420 (3 microM), capsaicin (1 microM) produced a tetrodotoxin-insensitive long-lasting relaxation (45+/-3% reduction of tone, at 4min from administration), which was unaffected by the nitric oxide (NO) synthase inhibitor, L-NOARG (100 microM). h-CGRP (10-50 nM) produced a similar sustained relaxation of precontracted preparations (59+/-4% reduction of tone). h-CGRP (8-37) (1.5 microM) almost completely reversed the relaxations produced by both capsaicin and h-CGRP. Application of electrical field stimulation (EFS: trains of stimuli of 10Hz; 0.25ms pulse width; supramaximal voltage; for 60s) to precontracted preparations produced a sustained, tetrodotoxin (1 microM)-sensitive relaxation (32+/-4% reduction of tone). L-NOARG (100 microM) greatly reduced (69+/-5% inhibition) the EFS-elicited relaxation. A complete reversal of the relaxant response to EFS into a contraction was obtained by administering L-NOARG to preparations in which a functional blockade of capsaicin-sensitive primary afferent neurons had been achieved by incubating the tissue with capsaicin (10 microM) for 15 min. At immunohistochemistry, tachykinin- and CGRP-immunoreactivities (TK-IR/CGRP-IR) were detected in varicose nerve fibers throughout the common bile duct, while TK-IR cell bodies were observed in the terminal portion (ampulla) only. In vivo pretreatment with capsaicin (50 mg/kg; 6-7 days before) decreased the number of CGRP-IR nerves, whereas the TK-IR neural network was apparently unchanged. In conclusion, our data provide functional evidence for the presence of capsaicin-sensitive primary afferent nerve endings in the guinea-pig terminal biliary tract, whose stimulation by capsaicin or EFS produces the release of tachykinins and CGRP. In addition, morphological evidence is provided that the bulk of TK-IR material in the biliary tract is contained in intrinsic neuronal elements, while CGRP in this tissue is of extrinsic origin only. Tachykinins, probably released in small amounts by capsaicin, act by activating receptors of the NK1, NK2 and NK3 type, most probably located on intrinsic cholinergic neurons, which in turn release ACh to produce the final excitatory motor response. The contractile response to capsaicin obtained in the presence of the three tachykinin receptor antagonists could be due to the co-released CGRP and/or to other unknown neurotransmitters. CGRP produces either indirect excitatory or direct inhibitory responses by stimulation of CGRP2 and CGRP1 receptors, respectively.     

Visceromotor responses to calcitonin gene-related peptide (CGRP) in the rat lower urinary tract: evidence for a transmitter role in the capsaicin-sensitive nerves of the ureter

Either intra-arterial or topical administration of calcitonin gene-related peptide (CGRP) had little effect on motility of the urinary bladder in urethane-anaesthetized rats. Only a high concentration (50 microM) of topical CGRP activated the micturition reflex and potentiated the response to exogenous substance P (SP). In the isolated rat bladder CGRP had inconsistent effects on spontaneous or field-stimulated contractions. CGRP neither produced any significant plasma extravasation (Evans blue leakage) in the rat lower urinary tract, nor potentiated the response to exogenous SP. CGRP inhibited motility in the rat isolated proximal urethra and ureters and counteracted the contractile response to neurokinins. An inhibitory effect of capsaicin on stimulated motility of the urethra was observed in all preparations and a small contractile response was evident in about 40% of cases. Lack of desensitization to the action of CGRP prevented the study of its interaction with capsaicin. The inhibitory effect of CGRP in the ureter exhibited a specific desensitization: if the preparations were pre-exposed to exogenous CGRP, the inhibition of motility produced by antidromic activation of the capsaicin-sensitive nerve terminals (field stimulation) as well as the response to capsaicin (1 microM) was prevented but the inhibitory response to isoprenaline was unaffected. These findings indicate that CGRP is able to influence markedly the motility of the rat lower urinary tract, but exhibits marked regional differences in its action. Endogenous CGRP could be the inhibitory transmitter which, when released from capsaicin-sensitive fibers, participate in the control of ureteral motility.     

Selective potentiation of extracellular Ca2+-dependent contraction by neuropeptide Y in rabbit mesenteric arteries

1. Neuropeptide Y (NPY) potentiated the contractile responses induced by electrical transmural stimulation, noradrenaline and KCl in the rabbit mesenteric artery. 2. In preparations treated with noradrenaline or KCl in Ca2+ free medium, NPY also potentiated the contractile response induced by resupplementation of Ca2+. 3. 3H-efflux from the arteries preincubated with [3H]-noradrenaline was not affected by NPY. 4. These results suggest that NPY selectively acts on the postsynaptic membrane and potentiates the contractions mediated through receptor-operated and voltage-dependent Ca channels.     

Actions of calcium antagonists on pre- and postjunctional effects of neuropeptide Y on human peripheral blood vessels in vitro

The mechanisms underlying the contractile effects of neuropeptide Y (NPY) in relation to those of noradrenaline (NA) on small human blood vessels were studied in vitro. NPY caused contractions of mesenteric veins, renal and skeletal muscle arteries but not of mesenteric arteries. NPY was about 5- to 10-fold more potent than NA. The maximal contractile responses to NPY (5 X 10(-7) M) were 38 +/- 4, 37 +/- 8 and 95 +/- 16% of the response evoked by NA 10(-5) M in the mesenteric vein, renal and skeletal muscle arteries respectively. The NPY effects were resistant to adrenoceptor antagonists. The calcium antagonist nifedipine reduced the effect of NA but not the contractile response to NPY on mesenteric veins. Nifedipine and felodipine reduced the contractile response to both NA and NPY on renal and skeletal muscle arteries. In contrast to the contractile effects of K+, the responses to NPY and NA were largely uninfluenced by changes in extracellular Ca2+ concentrations. Nifedipine still inhibited the NPY contractions in a Ca2+-free medium while high extracellular Ca2+ (7.5 mM) partly reduced the nifedipine effect. NPY reduced the nerve stimulation-evoked [3H]NA overflow from the mesenteric veins via a nifedipine resistant mechanism. The stable analogue alpha, beta-methylene adenosine triphosphate (mATP) was more potent than ATP and had nifedipine-sensitive contractile effects similar to those of NA on the human blood vessels without influencing the nerve-evoked [3H]NA efflux. In conclusion, NPY exerts a potent nifedipine-sensitive vasoconstrictor activity, especially on human skeletal muscle arteries in vitro, although the influx of extracellular calcium may not be a crucial mechanism. The NPY-induced contractions of mesenteric veins and the inhibition of nerve-evoked [3H]NA efflux seem to be mediated via nifedipine resistant messenger systems.     

Modulation of cholinergic neurotransmission in guinea-pig trachea by neuropeptide Y.

1. Neuropeptide Y (NPY) is localized to adrenergic nerves in guinea-pig airways but its function is not known. 2. NPY (1 X 10(-10)-3 X 10(-7) M) had no direct effect on guinea-pig tracheal smooth muscle in vitro. 3. NPY produced a concentration- and frequency-dependent inhibition of the cholinergic component of responses elicited by electrical field stimulation (EFS) whilst having no effect on the contractile response to exogenously applied acetylcholine (ACh). 4. Yohimbine was able to reverse significantly the inhibitory effect of noradrenaline on the cholinergic component to EFS without having any significant effect on the inhibition produced by NPY. 5. Neither blockade of beta-adrenoceptors by propranolol, nor depletion of adrenergic nerves by incubation with 6-hydroxydopamine caused any significant alteration in the response to EFS in the presence of 3 X 10(-7) M NPY. Bretylium tosylate incubation to prevent noradrenaline release produced a small but significant enhancement of the inhibitory effect of NPY on EFS at high frequencies. 6. NPY appears to reduce the cholinergic component to EFS via a prejunctional mechanism, acting directly on receptors on cholinergic nerve terminals, rather than affecting adrenergic mechanisms. NPY released by adrenergic nerves may modulate cholinergic neurotransmission in guinea-pig airways.     

Effect of neuropeptide Y on adrenergic and non-adrenergic, non-cholinergic responses in the rat anococcygeus muscle.

1. The effects of neuropeptide Y (NPY) were examined on adrenergic and non-adrenergic, non-cholinergic (NANC) neurotransmission in the rat anococcygeus muscle. 2. NPY (0.1-0.3 microM) greatly potentiated the contractile responses induced by field stimulation. Prazosin (0.1 microM) completely abolished the stimulation-induced responses either in the absence or presence of NPY. 3. NPY (0.1-0.3 microM) enhanced only the contractile responses to low doses of noradrenaline (NA, 0.003-0.01 microM). Responses to tyramine were unaffected by the same concentrations of NPY. 4. In superfused anococcygeus, previously loaded with [3H]-NA, NPY (0.1-0.3 microM) failed to modify the basal, as well as the stimulation-evoked, release of tritium at 2 and 4 Hz. 5. NANC relaxations induced by electrical stimulation were significantly reduced, in a concentration-related manner, by 0.1-0.3 microM NPY. 6. L-NG-nitro-arginine (L-NOARG, 30 microM) enhanced the stimulation (0.25-1 Hz)-induced motor responses. In the presence of L-NOARG (30 microM), NPY (0.1 microM) did not modify the motor responses induced by field stimulation (0.25-0.5 Hz). L-Arginine did not reverse the NPY-induced potentiation of stimulation-induced motor responses. 7. The relaxations of anococcygeus muscle induced by sodium nitroprusside (SNP, 0.01-0.3 microM) were diminished by NPY (0.1-0.3 microM). 8. Our study suggests that NPY, at concentrations devoid of contractile effect, potentiates the motor responses of rat anococcygeus muscle as a consequence, at least in part, of the inhibition of NANC relaxing responses by a different mechanism from L-NOARG.     

Action of histamine on nerve mediated responses of the guinea-pig ileum

The effect of histamine on the responses of the guinea-pig ileum to stimulation of intramural nerves and to some potential nonadrenergic, noncholinergic (NANC) neurotransmitters was analysed. During sustained tonic histamine contraction, electrical stimulation of all intramural nerves elicited a biphasic response (contraction followed by after-relaxation) and application of ATP and bradykinin caused relaxation of the ileum in contrast to their contractile effect on basal tension. Histamine reduced contractile and augmented relaxatory NANC responses, and prevented capsaicin from producing any contractile effect and from significantly influencing the NANC contractions. The present results suggest that, besides its direct effect, histamine activates intramural nerve fibres, mainly the sensory ones, and unmasks NANC relaxation thus modifying the mechanical activity.     

Hydrogen sulfide (H2S) stimulates capsaicin-sensitive primary afferent neurons in the rat urinary bladder

In the rat isolated urinary bladder, NaHS (30 microm-3 mm) and capsaicin (10 nm-3 microm) produced concentration-dependent contractile responses (pEC(50)=3.5+/-0.02 and 7.1+/-0.02, respectively) undergoing dramatic tachyphylaxis. In preparations in which sensory nerves were rendered desensitized (defunctionalized) by high-capsaicin (10 microm for 15 min) pretreatment, neither capsaicin itself nor NaHS produced any motor effect. NaHS-induced contractile effects were totally prevented by the simultaneous incubation with tachykinin NK(1) (GR 82334; 10 microm) and NK(2) (nepadutant; 0.3 microm) receptor-selective antagonists. Tetrodotoxin (1 microm) only partially reduced the response to NaHS. These results provide pharmacological evidence that H(2)S stimulates capsaicin-sensitive primary afferent nerve terminals, from which tachykinins are released to produce the observed contraction by activating NK(1) and NK(2) receptors. While the molecular site of action of H(2)S remains to be investigated, our discovery may have important physiological significance since H(2)S concentrations capable of stimulating sensory nerves overlap those occurring in mammalian tissues under normal conditions.     

Effect of resiniferatoxin on the isolated rabbit iris sphincter muscle: comparison with capsaicin and bradykinin.

The effect of resiniferatoxin on the isolated iris sphincter muscle of the rabbit was compared with the effects of capsaicin and bradykinin. The three compounds all contracted the sphincter muscle in a concentration-dependent manner. The contractions were inhibited by spantide II, a tachykinin antagonist. The concentration-response curve of resiniferatoxin was biphasic, yielding two EC50 values, one about 10,000 times lower than the other, the latter value being similar to that for capsaicin. The responses to resiniferatoxin, capsaicin or bradykinin were progressively reduced upon repeated application. The contraction evoked by either of the three compounds was not affected by thiorphan (an enkephalinase inhibitor) or captopril (an angiotensin-converting enzyme inhibitor). Pretreatment with capsaicin concentration dependently reduced the contractile response to a subsequent application of resiniferatoxin and vice versa. Bradykinin pretreatment reduced the resiniferatoxin response by about 50%; resiniferatoxin pretreatment completely abolished the response to bradykinin. Also, the contractile response to electrical stimulation was reduced concentration dependently by resiniferatoxin, capsaicin and bradykinin pretreatment. The response to electrical stimulation could be completely abolished by pretreatment with large doses of resiniferatoxin or capsaicin; pretreatment with large doses of bradykinin reduced the response by about 50%. Pretreatment with high concentrations of resiniferatoxin, or capsaicin, but not bradykinin, reduced the contractile response to the NK1 receptor agonist, [Sar9, Met(O2)11]SP (10(-8) M). The results suggest that the three compounds produce tachyphylaxis, mainly through partial (bradykinin) or complete (capsaicin, resiniferatoxin) exhaustion of the neuronal pool of releasable tachykinins although desensitization of tachykinin receptors may also contribute.     

Increase in gastric secretion induced by 2-deoxy-D-glucose is impaired in capsaicin pretreated rats.

Gastric acid secretion was determined following intravenous administration of 2-deoxy-D-glucose (2-DG; 60 mg kg-1) or electrical stimulation of the vagus nerve in urethane-anaesthetized rats pretreated when newborn with either capsaicin or the vehicle. The secretory response to 2-DG was substantially reduced in the capsaicin pretreated rats, while that induced by electrical vagal stimulation (1 mA, 1 ms. 3 Hz) was unaffected. These results suggest that capsaicin-sensitive fibres are involved in the afferent branch of the reflex response activated by 2-DG to stimulate gastric acid secretion.     

Selective inhibition by nifedipine of the purinergic component of neurogenic vasoconstriction in the dog mesenteric artery

The neurogenic contractions evoked by perivascular sympathetic nerve stimulation of dog mesenteric artery consist of purinergic and adrenergic components, and these components were selectively inhibited by alpha, beta-methylene ATP and prazosin, respectively. We examined the effects of Ca antagonists on both these components in dog mesenteric arteries. Nifedipine (10(-8)-10(-6) M) inhibited the purinergic and adrenergic contractions evoked by transmural electrical stimulation, and this inhibition was more evident for the purinergic component of the response. Nifedipine was also more potent to inhibit the contractile response to alpha, beta-methylene ATP than it was to inhibit the responses to noradrenaline. Verapamil and diltiazem also inhibited the purinergic and adrenergic responses induced by transmural electrical stimulation, alpha, beta-methylene ATP or noradrenaline, but the extend of the inhibition was less than that seen with nifedipine. These three Ca antagonists had little effect on the 3H efflux evoked by electrical transmural stimulation of arteries that had been preincubated with [3H]noradrenaline. These results show that nifedipine is a selective inhibitor of the purinergic component of contractions evoked by sympathetic nerve stimulation of blood vessels.     

A comparison of capsazepine and ruthenium red as capsaicin antagonists in the rat isolated urinary bladder and vas deferens.

1. The ability of capsazepine, a recently developed capsaicin receptor antagonist, to prevent the effects of capsaicin on the rat isolated urinary bladder (contraction) and vas deferens (inhibition of electrically-evoked twitches) was compared to that of ruthenium red, a dye which behaves as a functional antagonist of capsaicin. 2. In the rat bladder, capsazepine (3-30 microM) produced a concentration-dependent rightward shift of the curve to capsaicin without any significant depression of the maximal response to the agonist. By contrast, ruthenium red (10-30 microM) produced a non-competitive type of antagonism, characterized by marked depression of the maximal response attainable. Similar findings were obtained in the rat isolated vas deferens in which capsazepine (10 microM) produced a rightward shift of the curve to capsaicin while ruthenium red (3 microM) depressed the maximal response to the agonist. 3. At the concentrations used to block the effect of capsaicin, neither capsazepine nor ruthenium red affected the contractile response of the rat urinary bladder produced by either neurokinin A or electrical field stimulation or the twitch inhibition produced by rat alpha-calcitonin gene-related peptide (alpha CGRP) in the vas deferens. 4. These findings provide additional evidence that both capsazepine and ruthenium red are valuable tools for exploration of the function of capsaicin-sensitive primary afferent neurones. The antagonism of the action of capsaicin by capsazepine is entirely consistent with the proposed interaction of this substance with a vanilloid receptor located on primary afferents, while the action of ruthenium red apparently involves a more complex, non-competitive antagonism.     

Effects of opioid drugs on capsaicin-sensitive neurones in guinea-pig atria

Transmural nerve stimulation of isolated guinea-pig atria in the presence of atropine induced a biphasic positive inotropic effect but only a slow increase in contractility (NANC response) in atria obtained from 6-hydroxydopamine-pretreated animals. The latter effect disappeared after exposure of the preparations to capsaicin. The effects of some opioid peptides were investigated on NANC responses. [D-Ala²,D-Leu⁵]enkephalin (DADLE) and [D-Ala²,N-Me-Phe⁴ Gly⁵-ol]enkephalin (DAGO, 0.1–10 μM) inhibited the cardiac response to transmural nerve stimulation in a dose-dependent and naloxone-sensitive manner. Dynorphin-(1–13) and morphine, at 10-fold higher concentrations (1–10 μM), reduced the response in a naloxone-sensitive manner. Naloxone alone however did not affect the response. Opioid peptides were not able to reduce the positive inotropic effect induced by calcitonin gene-related peptide (CGRP), or the increase in cardiac contractility produced by capsaicin. These results suggest that opioid receptors exert a modulatory role on peripheral terminals of capsaicin-sensitive sensory nerves.     

Neurochemical evidence of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) release from capsaicin-sensitive nerves in rat mesenteric arteries and veins

1. The ability of capsaicin and antidromic stimulation of perivascular nerve fibers to release sensory neuropeptides (SP-LI and CGRP-LI) have been investigated in rat mesenteric arteries and veins. 2. Both in mesenteric arteries and veins substantial SP-LI and CGRP-LI tissue levels were measured. A significant reduction in sensory neuropeptides levels was observed in tissues obtained from capsaicin-pretreated animals. 3. Superfusion of isolated vessels with capsaicin (1 microM) produced a prompt and remarkable release of both SP-LI and CGRP-LI, which can be evoked only once in each preparation. 4. Electrical field stimulation (EFS, 20 Hz, 50 V, 0.5 msec, trains of 10 sec every 20 sec for 15 min) of isolated vessels resulted in a significant release of CGRP-LI. This release was significantly greater in veins as compared to arteries. EFS-induced CGRP-LI release was unaffected by atropine or guanethidine and absent in preparations obtained from capsaicin-pretreated rats. 5. These neurochemical findings further suggest that the local release of sensory neuropeptides from capsaicin-sensitive nerve endings might be important in the regulation of mesenteric circulation.     

Different effect of antiulcer agents on rat cysteamine-induced duodenal ulcer after sialoadenectomy, but not gastrectomy

An attempt has been made to pharmacologically isolate cholinergic, P(2) purinoceptor-mediated and peptidergic (capsaicin-sensitive, tachykinin-mediated) contraction of the guanethidine-treated rat bladder detrusor preparation, in vitro. The effect of experimental diabetes was assessed on these types of contraction. Responses were evoked by electrical field stimulation (single shocks or 1 Hz for 30 s or 10 Hz for 40 s). Single shocks and 1-Hz stimulation were applied in the presence of (a). atropine (1 microM) or (b). P(2) purinoceptor antagonists (50 microM pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid) [PPADS] plus 100 microM suramin. Long-term electrical field stimulation (10 Hz for 40 s) (c). was applied with both atropine and the P(2) purinoceptor antagonists present in the organ bath. The effects of capsaicin (d). and ATP (e). were also studied. Three groups of experimental animals were used: streptozotocin-treated (50 mg.kg(-1) i.p., 8 weeks before the experiment), parallel solvent-treated and untreated rats. (a). Responses to electrical field stimulation in the presence of atropine were reduced by half by PPADS plus suramin, but were resistant to capsaicin tachyphylaxis. They were enhanced in preparations taken from diabetic rats. (b). Contractions to electrical field stimulation in the presence of PPADS plus suramin were reduced by 2/3 by atropine, but were left unchanged by capsaicin or diabetes. (c). Contractions to long-term stimulation had a quick and a sustained phase. Especially the latter was inhibited by capsaicin tachypyhlaxis; it was also strongly reduced in preparations taken from diabetic rats. (d). Contractions to capsaicin (30 nM and 1 microM) were resistant to tetrodotoxin, strongly reduced by a combination of tachykinin NK(1) and NK(2) receptor antagonists, and slightly reduced in preparations from diabetic animals. Capsaicin (1 microM) had no acute inhibitory action on cholinergic or purinergic responses, nor did it cause relaxation in precontracted preparations treated with tachykinin receptor antagonists. (e) ATP-induced contractions were strongly reduced by PPADS plus suramin (50 plus 100 microM) and to a similar degree by 100 plus 200 microM, respectively. It is concluded that experimental diabetes selectively impairs peptidergic, capsaicin-sensitive responses (especially those that involve impulse conduction) in the rat detrusor preparation. The contractile response to electrical field stimulation that remains after atropine plus the P(2) purinoceptor antagonists has a yet unknown transmitter background.     

Adenosine-modulation of cholinergic and non-adrenergic non-cholinergic neurotransmission in the rabbit iris sphincter.

The characteristics of smooth muscle responses to transmural nerve stimulation in the rabbit iris sphincter were examined. Transmural stimulation elicited a composite contractile response that could be divided in two phases. Atropine abolished the phase I contraction and inhibited the phase II contraction. The atropine-resistant component of the phase II contraction which was unaltered by sympathetic denervation, was mimicked by substance P and abolished by capsaicin. Adenosine inhibited the phase I contraction. The adenosine analogue L-N6-phenylisopropyladenosine (L-PIA) was more potent than 5'-N-ethylcarboxamideadenosine (NECA) in mimicking this adenosine effect. By contrast, adenosine enhanced the phase II contraction in non-pretreated preparations, as well as the atropine-resistant capsaicin-sensitive part of this contraction. Here, NECA was more potent than L-PIA. Adenosine, NECA, L-PIA and D-PIA also enhanced the atropine-sensitive component of the phase II contraction, as well as the contractile response to exogenous acetylcholine or carbachol, but not to exogenous substance P. In this respect, L-PIA was the most powerful adenosine analogue with at least 10 fold higher potency than D-PIA. The adenosine antagonist 8-p-sulphophenyltheophylline enhanced the phase I contraction and decreased the capsaicin-sensitive non-adrenergic non-cholinergic component of the phase II contraction. We conclude that adenosine inhibited the nerve-induced cholinergic twitch (phase I) responses by action at prejunctional A1-receptors. Furthermore, adenosine enhanced the phase II contractile responses via postjunctional enhancement of the cholinergic transmission by action at A1-receptors, and via enhancement of the non-adrenergic non-cholinergic transmission by action at presumably prejunctional A2 receptors.