Action of morphine on the neuro-effector transmission in the guinea-pig ileum and in the mouse vas deferens

Regulation of neural responses in the canine pyloric sphincter by opioids

1. Regulation of excitatory and inhibitory junction potentials (e.j.ps and i.j.ps) by opioid peptides was studied in isolated muscle strips from the pyloric sphincter of the dog. 2. Methionine enkephalin (MetEnk; 10(-10) to 10(-6) M) and [D-Ala2, D-Leu5] enkephalin (DADLE; 10(-11) to 10(-7) M), a delta-specific opioid agonist, inhibited i.j.ps and e.j.ps recorded from cells in the myenteric and submucosal regions of the circular muscle layer. These compounds had no effect on resting potential or slow wave activity suggesting that the effects on junction potentials were not due to direct effects on smooth muscle cells. 3. MetEnk and DADLE caused similar effects on junction potentials in preparations in which the myenteric plexus was removed, suggesting that opioids inhibit pre-junctional effects on nerve fibres within the muscularis externa. 4. Inhibition of junction potentials by MetEnk and DADLE was blocked by approximately the same extent by naloxone (10(-6) M) and ICI 174,864 (10(-6) M), a delta-specific antagonist. 5. MetEnk and DADLE blocked a portion of the i.j.p. that was sensitive to arginine analogues; after treatment with N omega-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), MetEnk and DADLE had no further effect on i.j.ps. These data suggest that opioids regulate nitric oxide-dependent neurotransmission. 6. Naloxone (10(-6) M) alone had no effect on i.j.ps elicited by short trains of electrical field stimuli. 7. I.j.p. amplitude was reduced after a period of conditioning stimulation (2 min, 30 Hz, 30 V). Naloxone blocked the post-stimulation inhibition. Repetitive stimulation at high frequencies (30 Hz) resulted in sustained hyperpolarization. Naloxone increased the amplitude of the hyperpolarization responses elicited by high frequency stimulation.8. These results show that e.j.ps and i.j.ps in the canine pylorus are inhibited by opioids. A portion of the inhibitory effects appears to be mediated via delta receptors.9. Although pyloric muscles are richly innervated by nerves containing opioid peptides, brief trains of stimuli do not appear to release concentrations of opioids that are effective in regulating junction potentials. Higher frequency stimulation (or longer durations of stimulation) appear to be necessary to release concentrations of opioids that are effective in modulating the amplitude of junction potentials.

Effect of opioid peptides on circular muscle of canine duodenum

1. The effects of opioid peptides on inhibitory transmission in the circular muscle layer of canine duodenum were investigated in vitro using simultaneous mechanical and intracellular electrical recording techniques. 2. Exogenously added [Met5]enkephalin, [Leu5]enkephalin and dynorphin (1-13) decreased the amplitude of non-adrenergic, non-cholinergic inhibitory junction potentials (IJPs) evoked by transmural nerve stimulation. 3. A selective delta-receptor agonist, DPDPE ([D-Pen2, D-Pen5]enkephalin), and a selective mu-receptor agonist, PL017 (Try-Pro-NMePhe-D-Pro-NH2), decreased the amplitude of IJPs whereas a selective kappa-receptor agonist, U-50,488H ([trans-3,4-dichloro-N-methyl-N-(2-91-pyrolidinyl)-cyclohexyl]- benzeneacetamide methanesulphonate), in large doses (1 microM) produced only a small reduction. 4. A selective delta-receptor antagonist, ICI-174,864, blocked the effect of DPDPE but not that of PL017 suggesting the presence of distinct delta- and mu-opioid receptors on inhibitory motor nerves. 5. Exogenously added dynorphin (1-13) decreased the amplitude of IJPs. delta-Opioid receptors appeared to be involved because ICI-174,864, a selective delta-antagonist, blocked the inhibitory effect of exogenously added dynorphin (1-13). 6. The inhibitory effect of the opioid peptides was still observed in preparations of circular muscle devoid of myenteric and submucosal plexuses, indicating that the site of action was on inhibitory motor nerve fibres located within the circular muscle layer and not on neuronal cell bodies in the enteric plexuses. 7. It was concluded that in the canine small intestine, opioid peptides could modulate release of inhibitory transmitter(s) at or near nerve terminals of inhibitory motor nerves innervating circular muscle cells.

Opioid dependence in myenteric neurons innervating the circular muscle of guinea-pig ileum

Guinea-pigs were treated with morphine for 6-8 days by subcutaneous implantation of pellets, each containing a mixture of morphine base (120 mg) and morphine hydrochloride (35 mg). Each guinea-pig received a single pellet. Mechanical activity of the circular muscle was recorded in vitro in preparations comprising the circular muscle and myenteric plexus. Exposure to morphine was maintained by addition of 1 microM morphine to the organ baths. After 90 min, morphine was withdrawn, either by repeatedly washing tissues in morphine-free Krebs' solution, or by addition of naloxone to reduce the occupancy of the opioid receptors by morphine. Withdrawal of morphine resulted in markedly enhanced contractile activity compared with that in circular muscle-myenteric plexus preparations from untreated control guinea-pigs. The withdrawal contractions were abolished by tetrodotoxin (600 nM) and greatly reduced by hyoscine (1 microM), indicating that they resulted from action potential discharge in myenteric neurons that release acetylcholine onto the circular muscle. Activation of the cholinergic excitatory motor neurons was not secondary to synaptic activation by cholinergic interneurons, because hexamethonium (100 microM) did not affect withdrawal contractions. The withdrawal response may therefore arise in the cholinergic excitatory motor neurons themselves, or in neurons that activate them via noncholinergic mechanisms.

Inhibitory effects of opioids in a circular muscle-myenteric plexus preparation of guinea-pig ileum

The actions of opioids were examined in a strip preparation of the external muscle and myenteric plexus of the guinea-pig ileum cut parallel to the circular muscle. Contractions of the circular muscle induced by electrical stimulation of myenteric neurons were depressed in a concentration-dependent manner by the mu agonists, morphine and DAGO, and by the kappa agonist, U-50,488H. The concentrations of morphine, DAGO and U-50,488H which depressed nerve-mediated contractions by 50% (IC50) were 86 nM, 11 nM and 5.0 nM, respectively. The equilibrium dissociation constants (KD) for naloxone as an antagonist of the inhibitory effects of DAGO and of U-50,488H were 5.6 nM and 29.4 nM, respectively. In contrast to the potent inhibitory effects of mu and kappa agonists, the delta-selective agonist, D-Pen-L-Pen, produced only weak inhibition of nerve-mediated contractions. Even at a concentration of 3 microM, there was less than 50% inhibition, which was not antagonised by the delta receptor antagonist, ICI 174864. The experiments indicate that both mu and kappa opioid receptors are present on the myenteric neurons supplying the circular muscle and that delta receptors are either absent or ineffectively activated.

Effects of morphine on electrical activity of the rectum in man

1. The effects of morphine on the electrical activity of the human rectum were investigated. 2. In healthy volunteers, morphine (0.04-0.16 mg/kg, i.v.) induced spike activity that could become cyclical. 3. All the effects of morphine were antagonized by naloxone (0.03-0.2 mg/kg, i.v.), but not blocked by atropine (0.007-0.014 mg/kg, i.v.). 4. In patients with spinal cord injury, morphine was observed to have similar excitatory effects. Spinal cord transection was complete in all patients, so that participation of supraspinal nervous structures in these effects could be ruled out. The sacral parasympathetic nervous centres could not have been involved in two patients in whom the medullary cone was also destroyed. The thoracolumbar sympathetic nervous centres were not completely destroyed in any of the patients, however, so that the possibility that these centres may have been involved cannot be entirely ruled out. 5. Morphine failed to activate an aganglionic rectum in a patient with Hirschsprung's disease, indicating that it had no direct effect on smooth muscle cells. It is therefore probable that morphine may have an effect on the intrinsic innervation.

Actions of morphine and enkephalins on the internal anal sphincter of the cat: relevance for the physiological role of opiates

The effects of Leu-enkephalin, Met-enkephalin and morphine on the electrical activity of the internal anal sphincter were studied in anesthetized spinalized cats and in vitro on sphincteric muscle strips. All the effects of enkephalins and morphine were antagonized by naloxone (2 mg/kg, i.v. in vivo and 10(-6)M in vitro). In vivo, the enkephalins (0.01 mg/kg i.v.) and morphine (2 mg/kg, i.v.) decreased the amplitude of the excitatory responses evoked in the sphincter by stimulation of the hypogastric nerves. Opiates presumably act on the sympathetic nerve endings by reducing the release of noradrenaline. In vitro, the enkephalins (10(-6)M) and morphine (10(-6)M) had a similar inhibitory effect, indicating that opiates act, at least partly, at intramural level. In vivo, the enkephalins and morphine produced an inhibition of the spontaneous electrical activity of the internal anal sphincter. This inhibition occurs also in vitro; it is thus due to a peripheral effect of opiates acting either directly on the sphincteric smooth muscle cells, or through the nervous structures controlling sphincteric motility. In addition, the distribution of nerves containing enkephalin-like immunoreactivity, using whole mount preparations of cat internal anal sphincter, indicates that this area is supplied with a dense Leu- and Met-enkephalinergic innervation. Met- and Leu-enkephalin-like immunoreactive axons were detected within the circular and longitudinal muscles.

Inhibitory action of propranolol on the contractions induced by nerve stimulations or calcium in the smooth muscle of rat vas deferens

Inhibitory effects of propranolol on the contractions to various treatments were investigated in the epididymal half of the rat vas deferens. Reportedly, 10(-5)-3 X 10(-4) M propranolol inhibited 150 mM K-induced contractions dose-dependently; 3 X 10(-4) M propranolol abolished the contractions. The present results showed that propranolol at concentrations up to 10(-4) M did not inhibit the maximal contractions to 10(-3) M norepinephrine (NE) or 10(-2) M methacholine (MCh). Propranolol at 3 X 10(-4) M slightly inhibited contractions to NE and MCh by 11% and 12%, respectively. In contrast, propranolol inhibited twitch components of the contractions induced by nerve stimulations at similar doses to those reported for high K contractions. Propranolol also inhibited contractions to Ca in high K-containing solution and shifted the dose-response curve to the right. Propranolol did not affect the depolarizations by high K measured by microelectrodes. Propranolol at concentrations of 10(-5)-3 X 10(-5) M diminished the magnitude of spikes dose-dependently. Spikes were rarely observed in the presence of 10(-4) M propranolol in spite of generation of e.j.p.s with amplitudes that would be sufficient to induce spikes in the absence of propranolol. These results suggest that propranolol inhibits contractions by decreasing Ca-influx through the potential-operated Ca-channels in the smooth muscle cells of rat vas deferens.

On the opioid receptor subtype inhibiting the evoked release of 3H-noradrenaline from guinea-pig atria in vitro

Guinea-pig isolated atria were incubated and loaded with 3H-(-)-noradrenaline. The intrinsic nerves were stimulated with trains of 5 or 35 field pulses (4 Hz), and the evoked efflux of 3H-noradrenaline and of total tritium was determined in the presence of atropine, corticosterone, desipramine, and phentolamine by liquid scintillation spectrometry. Ethylketocyclazocine (1.4 nmol/l, IC50), MR 2033 (9.1 nmol/l), dynorphin A (1-13) (25 nmol/l, peptidase inhibitors present), etorphine (71 nmol/l), and [D-Ala2, D-Leu5]-enkephalin (greater than 10 mumol/l, peptidase inhibitors present) inhibited the stimulation-evoked efflux of 3H-noradrenaline in a concentration-dependent manner, but not morphine up to 10 mumol/l. The inhibition by ethylketocyclazocine, MR 2033, and etorphine was antagonized by naloxone 1 mumol/l. Similarly, the MR 2033 effect was antagonized by SKF 10047 1 mumol/l. All antagonists investigated failed to affect the evoked 3H-noradrenaline efflux when present in the absence of exogenous agonists. Arunlakshana-Schild plots were calculated for the antagonism between ethylketocyclazocine and a pair of stereoisomers, (-)-MR 2266 (20 nmol/l-5 mumol/l) and (+)-MR 2267 (0.3-10 mumol/l) at the presynaptic opioid receptor, and pA2 values were estimated. The isomeric affinity ratio was 60, with pA2 values of (-)-MR 2266, 9.06, and (+)-MR 2267, 7.28, respectively. The results show that the 3H-noradrenaline release can be inhibited via activation of presynaptic opioid receptors. Under the conditions presently investigated endogenous opioids do not modulate the evoked transmitter release. The results favour the idea that a single population (presumably of the kappa-subtype) of opioid receptors is present at guinea-pig atrial noradrenergic nerves.

Cellular mechanisms of opiate receptor stimulation in cat middle cerebral artery

To determine some of the cellular mechanisms of opiate receptor stimulation in cat middle cerebral arterial muscle, intracellular electrical measurements and force development were monitored before and after addition of morphine. Addition of morphine resulted in a dose-dependent hyperpolarization of the muscle cells in the middle cerebral artery with a concomitant relaxation, indicating a high degree of electromechanical coupling in this preparation. The curve relating membrane potential vs. morphine was shifted to the right and downward by naloxone, demonstrating competitive inhibition at receptor sites. When middle cerebral arteries were studied from animals which had been injected with morphine prior to sacrifice, a significant hyperpolarization of the membrane was recorded when studied in an organ bath. This hyperpolarization was abolished if the animal had been pretreated with naloxone prior to morphine injection, suggesting that morphine may act in vivo as we have observed it to act in vitro. Morphine-induced hyperpolarization could be blocked in the organ bath when potassium conductance (gk) was inhibited. Similarly, the reduction in the slope of the voltage/current curve induced by morphine was blocked by agents which reduced gk. These data suggest the presence of opiate receptors on cat cerebral artery and suggest that morphine relaxes these vessels through a mechanism involving increased gk. These findings suggest a role for opiate-mediated systems in cerebral vascular control.

Effects of a new alpha-adrenoceptor blocking agent, ethyl-7-[4-(2-methoxyphenyl)-1-piperazinyl] heptanoate dihydrochloride (SGB-483), on smooth muscle and neuromuscular transmission in guinea-pig mesenteric artery

The effects of ethyl-7-[4-(2-methoxyphenyl)-1-piperazinyl] heptanoate dihydrochloride (SGB-483) on the smooth muscle of the guinea-pig mesenteric artery were investigated using microelectrodes. The resting membrane potential was -70.3 +/- 2.1 mV.SGB-483 (10(-8)M - 10(-4)M) did not modify the membrane potential or membrane resistance, as estimated from measurement of current-voltage relationships. Noradrenaline (NA; above 10(-5)M) depolarized the membrane. After pretreatment with SBG-483 10(-5)M or prazosin 10(-6)M, the NA-induced depolarization of the membrane was inhibited; yohimbine (10(-5)M) was ineffective. Phenylephrine and NA (greater than 3 X 10(-7)M) but not clonidine (10(-6)M) contracted the artery. These contractions were inhibited by SGB-483. Following repetitive perivascular nerve stimulation, the amplitude of excitatory junction potentials (e.j.ps) increased to a certain steady state value (e.j.p.(s]. The amplitude of e.j.p.(s) was frequency-dependent. Application of SGB-483 (over 10(-8)M) enhanced the amplitude of e.j.p.(s), dose-dependently with no change in the amplitude of the first e.j.p. (e.j.p.(f] evoked by the first stimulus. After pretreatment with NA, the amplitudes of both e.j.p.(f) and e.j.p.(s) were inhibited, dose-dependently. Following pretreatment with SGB-483 (10(-6) - 10(-5)M), the NA-induced reduction in the amplitude of both e.j.p.(f) and e.j.p.(s) were reversed and the control values restored. Clonidine (10(-7)M) inhibited the amplitude of e.j.p.(f) and e.j.p.(s), and SGB-483 (10(-7)M) partially restored the amplitude of both. Yohimbine (10(-7)M) and phentolamine (10(-7)M) enlarged the amplitude of e.j.p.(s); the amplitude of e.j.p.(f) was inhibited by yohimbine and enlarged by phentolamine. Prazosin (10(-6)M) had no effect on the amplitude of either e.j.p.(f) or e.j.p.(s), at any given stimulus frequency. SBG-483 (10(-7)M) did not enhance the amplitudes of either e.j.p.(f) or e.j.p.(s) following pretreatment with phentolamine (10(-7)M) or yohimbine (10(-7)M) but did enhance the amplitude following pretreatment with prazosin (10(-7)M). SGB-483 possesses the property of an alpha 1- and alpha 2-adrenoceptor antagonist. The alpha-antagonistic action was apparent on post-junctional smooth muscle cells. The alpha 2-antagonistic action on neuromuscular transmission was mediated at pre-junctional nerve terminals to enhance the release of NA. The prejunctional actions of SGB-483 were more selective than those of yohimbine or phentolamine.

Block of synaptic vesicle exocytosis without block of Ca2+-influx. An ultrastructural analysis of the paralysing action of Habrobracon venom on locust motor nerve terminals

The venom of the wasp Habrobracon hebetor presynaptically blocks excitatory but not inhibitory neuromuscular transmission at locust skeletal muscle. Its mode of action on excitatory motor nerve terminals has been studied at the retractor unguis muscle of Schistocerca by means of ultrastructural stereology. paralysed and unparalysed preparations, either resting or stimulated for 7 min at 20 Hz, were compared. Paralysis does not cause structural damage to the nerve terminals but prevents the depletion of vesicles occurring upon nerve stimulation in the controls. Prolonged paralysis leads to an increase in the number and the size of vesicles resulting in an increase of total membrane per terminal cross-section by about 33% after 2 days. Stimulation causes swelling of mitochondria both in controls and in paralysed preparations, resulting from a rise of intraterminal [Ca2+] as is indicated by the absence of the swelling if extracellular Ca2+ is replaced by Mg2+. In addition, stimulation leads to a reduction of vesicle size, an increase in the area of axolemma and in the number of cisternae and of profiles of the smooth endoplasmic reticulum in controls but not in paralysed preparations. However, neither in controls nor in paralysed preparations is the total amount of membrane per terminal cross-section affected by stimulation. Under paralysis, vesicles tend to stick to the presynaptic membrane. It is concluded that Habrobracon venom does not block the depolarizing-dependent Ca2+-influx into the nerve terminal and that it is unlikely to interfere with some transmitter-related process. Rather, the venom seems to block vesicle exocytosis itself. The results lend further support to the view that in insect neuromuscular synapses exocytosis is the mechanism whereby transmitter quanta are released.

Multiple actions of cocaine on neuromuscular transmission and smooth muscle cells of the guinea-pig mesenteric artery

1. The effects of cocaine on the neuromuscular transmission and smooth muscle cells of the guinea-pig mesenteric artery were observed using various experimental procedures.2. Cocaine (10(-7) M) depolarized the membrane and increased the membrane resistance of single smooth muscle cells. Outward current pulses produced neither spikes nor graded responses in Krebs solution, but in the presence of 10(-5) M-cocaine, outward current did produce spikes.3. Perivascular nerve stimulation evoked excitatory junction potentials (e.j.p.s). Repetitive stimulation (0.25-1.0 Hz) produced a frequency-dependent facilitation. Application of cocaine (10(-7) M) reduced the amplitude of the first e.j.p. (e.j.p.(f)) and also after completion of facilitation (e.j.p.(s)). However, the facilitation process was not affected by cocaine (10(-5) M).4. On pre-treatment with phentolamine (3 x 10(-7) M), both e.j.p.(f) and e.j.p.(s) were enhanced, but on pre-treatment with yohimbine (3 x 10(-7) M), e.j.p.(f) was inhibited and e.j.p.(s) was enhanced. Both phentolamine and yohimbine accelerated the facilitation of e.j.p.s in the absence and presence of cocaine (10(-7) M).5. The conduction velocity of nerve excitation measured from the latency of generation of e.j.p.s was slightly lowered by cocaine. The number of nerve fibres or varicosities contributing to the generation of an e.j.p. was not reduced in the presence of cocaine (10(-5) M).6. Mechanical responses could be recorded on perivascular nerve stimulation, and direct muscle stimulation on treatment with tetrodotoxin. Cocaine (10(-7) to 10(-4) M) enhanced the contraction evoked by direct muscle stimulation and inhibited the contraction evoked by perivascular nerve stimulation.7. Cocaine (10(-5) M) enhanced the contraction evoked by 5 x 10(-6) M-noradrenaline (NA) and direct muscle stimulation (5 sec pulse) but no effect was observed on the K-induced contraction (39.2 mM-K). On pre-treatment with guanethidine (10(-6) M) these effects of cocaine were not affected.8. In the presence of cocaine (10(-5) M), the depolarization of the membrane induced by NA was additively increased, and the dose response curve for NA was shifted to the left with no change in the maximum amplitude of contraction.9. When 10(-5) M-cocaine was applied during contractions evoked by alternate perivascular nerve stimulation and exogenously applied NA, the contraction evoked by perivascular nerve stimulation was reduced, while that evoked by NA was enhanced.10. In saponin-treated skinned muscles, the pCa-tension relationship was not affected by application of 10(-4) M-cocaine. The effects of cocaine on the Ca accumulation and release from the store site were estimated. It was found that cocaine (10(-4) M) slightly inhibited the Ca accumulation (0.89 times the control) but did not modify the Ca-release mechanism.11. The overflow of NA, 3,4-dihydroxyphenylglycol (DOPEG) and 3-methoxy-4-hydroxyphenylglycol (MOPEG) were measured in the same tissue before and after application of perivascular nerve stimulation in the presence or absence of cocaine (10(-7) to 10(-5) M). Cocaine induced a concentration-dependent increase in the overflow of NA and a reduction in the amounts of DOPEG and MOPEG.12. We conclude from these studies that cocaine mainly inhibits the sensitivity of the intra-junctional adrenoceptor, but increases the sensitivity of the extrajunctional adrenoceptor distributed on the post-junctional muscle membrane, with increase in the overflow of NA. The enhancement of mechanical response in the presence of cocaine is probably due to an increased sensitivity of the extra-junctional adrenoceptor and changes in the post-junctional muscle membrane, without any marked effect on the prejunctional mechanism.

Effects of opiates on synaptosomal calmodulin and calcium uptake

Acute opiate administration in vivo increases the level of cytoplasmic calmodulin in isolated rat brain synaptosomes. These synaptosomes do not, however, display decreased K+-stimulated 45Ca uptake in vitro. Opiates affect neither cytoplasmic calmodulin nor Ca uptake after incubation of synaptosomes with the drugs in vitro. In contrast to the interpretation of electrophysiological data, these results suggest that the observed inhibition by opiates of the release of several transmitters may not be mediated by presynaptic opiate receptors that inhibit Ca uptake.

Non-competitive interaction between normorphine and calcium on the release of noradrenaline

The interaction between calcium and magnesium or normorphine was studied on the electrically evoked outflow of previously incorporated [3H]noradrenaline in the isolated mouse vas deferens. The stimulation-evoked outflow of [3H]noradrenaline increased with increasing concentrations of Ca2+ ions in the medium. Mg2+ counteracted the effect of Ca2+ in a manner compatible with competitive antagonism. In contrast, normorphine interfered with the effect of Ca2+ non-competitively, thus suggesting that its mode of action differed from that of Mg2+.

Effects of enkephalins and morphine on spontaneous electrical activity and on junction potentials elicited by parasympathetic nerve stimulation in cat and rabbit colon

1 The effects of Leu-enkephalin, Met-enkephalin and morphine on excitatory junction potentials (e.j.ps) and inhibitory junction potentials (i.j.ps) elicited by stimulation of efferent parasympathetic nerves were studied in cats and rabbits, anaesthetized and in vitro.2 Enkephalins (0.008 mg/kg in vivo and 10(-6)M in vitro) enhanced e.j.p. amplitude in rabbit proximal colon and decreased it in rabbit distal colon and in cat colon. Enkephalins decreased i.j.p. amplitude in all the three models.3 Morphine (0.2 mg/kg in vivo and 10(-6)M in vitro) had the same effects as enkephalins on e.j.ps. In contrast, morphine decreased i.j.p. amplitude in rabbit proximal and distal colon and increased it in cat colon.4 Enkephalins and morphine induced (especially in the cat) spike activity which was potentiated by atropine (0.1 mg/kg in vivo or 10(-6)M in vitro).5 All the effects of enkephalins and morphine were antagonized by naloxone (0.2 mg/kg in vivo or 10(-6)M in vitro).6 These results suggest that the facilitatory effects of enkephalins and morphine on e.j.ps of rabbit proximal colon are due to the absence of opiate receptors on the excitatory nerve pathway and to a removal of inhibition by blockade of the non-adrenergic, non-cholinergic inhibitory pathway. Enkephalinergic intramural neurones may modulate the activation of either excitatory or inhibitory pathways in intramural reflexes.

The effects of substance P on smooth muscle cells and on neuro-effector transmission in the guinea-pig ileum

1 The effects of substance P (SP) on the membrane and contractile properties of the smooth muscle cell, or on neuro-effector transmission in the guinea-pig ileum were observed by means of microelectrodes, double sucrose gap and tension recording.2 SP (10(-13)-10(-10)M) induced a phasic contraction of longitudinal muscle strips, but did not change the muscle tone of circular muscle strips, in concentrations up to 10(-8)M.3 SP (10(-10)-10(-8)M) evoked three different membrane responses in longitudinal muscle cells: (i) bursts of spike discharges with no significant change in the membrane potential and input membrane resistance; (ii) bursts of spike discharges with a small but clear depolarization of the membrane and increase in the input membrane resistance; (iii) slow waves with no change in the membrane potential.4 In the circular muscle cells, low concentrations of SP (<10(-8)M) did not affect the membrane potential or the spikes, but SP (10(-7)M) increased the spike discharges with no significant change in the membrane potential.5 SP (10(-10)M) reduced the threshold depolarization required for the generation of action potentials with no change in membrane potential of the longitudinal muscle cells.6 Pretreatment with atropine (5 x 10(-6)M), tetrodotoxin (TTX 10(-6)M) or baclofen (4.7 x 10(-6)M) had no effect on the excitatory actions of SP on the smooth muscle cells of longitudinal and circular muscle strips.7 Excitatory actions of SP on the membrane potential or spike activities of longitudinal muscle cells were preserved in NaCl but not in Ca-deficient solution.8 SP (10(-10)-10(-9)M) enhanced the amplitude of the excitatory junction potentials (e.j.ps) evoked by electrical field stimulation in longitudinal muscle cells with no change in the membrane potential and input resistance. SP (10(-10)-10(-9)M), however, did not change the amplitude of inhibitory junction potentials (i.j.ps) recorded from the circular muscle cells.9 These results indicate that SP in relatively low concentrations acts on both smooth muscle cells and on excitatory neuro-effector transmission in the longitudinal muscle; the main site of the action of SP is probably the muscle membrane.

Effects of divalent cations and normorphine on spontaneous excitatory junction potentials in the mouse vas deferens

1 Excitatory junction potentials (e.j.ps) occurring spontaneously or evoked by nerve stimulation were recorded intracellularly from smooth muscle cells of the mouse isolated vas deferens. 2 The amplitude of the evoked e.j.ps and the amplitude and frequency of spontaneous e.j.ps were measured before and during application of normorphine or solutions which might be expected change the influx of calcium ions into the nerve terminals. 3 Spontaneous e.j.ps could be recorded even in solutins which contained tetrodotoxin (1 microM), no added calcium an EGTA (1mM). A four fold increase in calcium concentration from 1.25 to 5 mM greatly increased the amplitude of the evoked e.j.ps but had no effect on the amplitude or frequency of the spontaneous e.j.ps. 4 Magnesium (12mM) and cobalt (4mM) both greatly reduced the evoked e.j.ps and also reduced the frequency of spontaneous e.j.ps. 5 Normorphine (2 microM) reduced the amplitude of the evoked e.j.p by 70% but had no effect on the amplitude or frequency of spontaneous e.j.ps. 6 It is suggested that normorphine inhibits noradrenaline secretion from nerve varicosities by a mechanism different from that of magnesium and cobalt. One possibility is a block of action potential propagation along varicose fibers.

Actions of indomethacin and prostaglandins on neuro-effector transmission in the dog trachea

Neuro-effector transmission in the smooth muscle layer of the dog trachea was studied in vitro using the micro-electrode and double sucrose gap methods.1. Electrical field stimulations with short duration (50-100 musec) applied to the whole tissue produced an excitation of the intrinsic nerves, and evoked excitatory junction potentials (e.j.p.s) followed by twitch tension development and subsequent long lasting relaxation of the smooth muscle tissue.2. The effects of field stimulations were abolished by tetrodotoxin (2 x 10(-7)m), and atropine (1.7 x 10(-5)m) selectively blocked both the e.j.p. and twitch tension. On the other hand, propranolol (1.9 x 10(-5)m) suppressed the generation of the prolonged relaxation evoked by the field stimulations.3. E.j.p.s recorded by the double sucrose gap method showed gradual and continuous reduction in amplitude during prolonged exposure in Krebs solution (1-2 hr), and there were no changes in the membrane potential or in the input membrane resistance.4. With application of indomethacin (10(-5)m), a gradual and continuous reduction in the amplitude of e.j.p. was no longer observed, and (after the initial increase in the amplitude) e.j.p.s with a constant amplitude were obtained during 1-1.5 hr. Indomethacin (10(-5)m) modified neither the resting membrane potential nor the input membrane resistance of smooth muscle cells.5. After pre-treatment with indomethacin, low concentrations (10(-11)-10(-8)m) of prostaglandin E(1) or E(2) (PGE series) markedly suppressed the amplitude of e.j.p. with no changes in the resting membrane potential or in the input membrane resistance.6. During the repetitive field stimulation at the stimulus frequency of 0.1-1 Hz, the amplitude of the e.j.p.s was gradually reduced (the depression process). The depression was not affected by applications of prostaglandins, indomethacin or alpha- and beta-adrenoceptor blockers.7. These results indicate that in the dog tracheal smooth muscles, the endogenous PGE series may play an important role in feed-back inhibitory mechanisms, at the nerve terminals related to acetylcholine release.