An electrophysiological analysis of the effects of morphine on the calcium dependence of neuromuscular transmission in the mouse vas deferens

Effect of chronic morphine treatment on alpha(2)-adrenoceptor mediated autoinhibition of transmitter release from sympathetic varicosities of the mouse vas deferens

1. The effect of chronic morphine treatment (CMT) on sympathetic innervation of the mouse vas deferens and on alpha(2)-adrenoceptor mediated autoinhibition has been examined using intracellular recording of excitatory junction potentials (EJPs) and histochemistry. 2. In chronically saline treated (CST) preparations, morphine (1 microM) and the alpha(2)-adrenoceptor agonist (clonidine, 1 microM) decreased the mean amplitude of EJPs evoked with 0.03 Hz stimulation by 81+/-8% (n=16) and 92+/-6% (n=7) respectively. In CMT preparations, morphine (1 microM) and clonidine (1 microM) decreased mean EJP amplitude by 68+/-8% (n=7) and 79+/-8% (n=7) respectively. 3. When stimulating the sympathetic axons at 0.03 Hz, the mean EJP amplitude recorded from smooth muscles acutely withdrawn from CMT was four times greater than for CST smooth muscles (40.7+/-3.8 mV, n=7 compared with 9.9+/-0.3 mV, n=7). 4. Part of the increase in mean EJP amplitude following CMT was produced by a 31% increase in the density of sympathetic axons and varicosities innervating the smooth muscle. 5. Results from the present study indicate that the effectiveness of alpha(2)-adrenoceptor mediated autoinhibition is only slightly reduced in CMT preparations. Most of the cross tolerance which develops between morphine, clonidine and alpha(2)-adrenoceptor mediated autoinhibition occurs as a consequence of increased efficacy of neuromuscular transmission which is produced by an increase in the probability of transmitter release and an increase in the density of sympathetic innervation.

Stress induced changes in transmitter release from sympathetic varicosities of the mouse vas deferens

Activation of the sympathetic nervous system is an important component of the response to stress, but the effects of prolonged stress on sympathetic neurotransmission have not been assessed. In the present study we have examined the effect of 3 to 10 days exposure to stress induced by frequent handling and sham injections on neurotransmitter release from sympathetic varicosities of the mouse vas deferens. DiOC2(5)-fluorescence was used to visualise the sympathetic varicosities so that extracellular electrodes could be placed over known numbers of varicosities to monitor transmitter release using electrophysiological techniques. The frequency of excitatory junction currents (EJCs) increased with increasing duration of exposure to stress. The mean and maximum EJC amplitude significantly increased by 107% and 43%, respectively after 10 days of exposure to stress. The density of sympathetic varicosities innervating smooth muscle of the mouse vas deferens was not changed throughout the duration of the exposure to stress. The findings from this study demonstrate that the efficacy of transmitter release from the sympathetic varicosities is altered by repeated exposure of mice to stressful stimuli, such as handling and sham injections. Since such procedures are routine in many pharmacological experiments, it is important that investigators are aware of these changes so that due consideration is given when interpreting the data obtained from animals treated in this way.

GABA(B)-mediated presynaptic inhibition of excitatory transmission and synaptic vesicle dynamics in cultured hippocampal neurons

Local recycling of synaptic vesicle membrane at nerve terminals is necessary to maintain a readily releasable pool of transmitter. To what extent are the dynamics of vesicle recycling subject to modulation? We examined the influence of presynaptic GABA(B) receptors on vesicle dynamics at single synapses using optical imaging of FM1-43 in cultured rat hippocampal neurons. The kinetics of FM1-43 destaining indicate that synapses from a single neuron have a unimodal distribution of release probabilities, and GABA(B)-mediated inhibition occurs uniformly at all sites. Electrical and optical recordings from single cells show that the inhibition of excitatory transmission is entirely accounted for by a rapidly reversible reduction of exocytosis. In contrast, GABA(B) receptors do not alter the rate or extent of endocytosis.

Effect of opiates on transmitter release from visualized hypogastric boutons innervating the rat pelvic ganglia

1. The effect of opiates on neurotransmission between visualized hypogastric nerve boutons and postganglionic cell bodies has been examined using extracellular recording of nerve bouton impulses (NBIs) and excitatory postsynaptic currents (e.p.s.cs). 2. Morphine (10 to 40 microM) did not affect neurotransmission in the ganglia. Dynorphin-A (4 microM) and U50488H (1 microM) decreased quantal transmitter release and naloxone (10 microM) reversed these effects. 3. Morphine (10 microM), dynorphin-A (4 microM) and U50488H (1 microM) did not affect either the time course or consistency with which the NBI was recorded. 4. Dynorphin-A (1 to 4 microM) and U50488H (1 microM) decreased the average amplitude of e.p.s.cs by increasing the number of failures to release quanta from single or small groups of 2 to 4 boutons during continuous nerve stimulation at 0.1 Hz. 5. The decrease in quantal release induced by dynorphin-A and U50488H in 0.2 to 0.5 mM [Ca2+]zero was readily reversed by increasing the extracellular calcium ion concentration to 1 mM. 6. It was concluded that kappa-opioid receptors are located on the boutons of the hypogastric nerve and when activated by kappa-opioid receptor agonists reduce quantal release without affecting the NBI.

Effect of chronic morphine treatment on transmitter release from sympathetic varicosities of the mouse vas deferens

1 Transmitter release from sympathetic varicosities of mouse vasa deferentia removed from animals which were chronically treated with morphine for 7 to 9 days has been evaluated. 2 In control preparations increasing the extracellular calcium concentration ([Ca2+]o) from 1 to 2 mM increased transmitter release by 3 fold while increasing [Ca2+]o from 6 to 8 mM increased transmitter release by about 0.9 fold. Introduction of morphine (1.0 microM) produced a uniform decrease in transmitter release, shifting the relationship between transmitter release and [Ca2+]o to the right. 3 Only sympathetic varicosities with probabilities of transmitter release greater than 0.01 were chosen for this study. In these varicosities the decrease in transmitter release induced by morphine in control preparations (bathed in [Ca2+]o 2.0 mM) was not observed following 7 to 9 days of morphine treatment. When the morphine was acutely withdrawn from these preparations transmitter release was more than 6 times the average level of transmitter release from control preparations. 4 The morphine induced increase in facilitation of transmitter release while stimulating with short trains of nerve impulses was not observed when the preparations were removed from animals which had been exposed to morphine for 7 to 9 days. When these preparations were acutely withdrawn from morphine there was a further decrease in the level of facilitation and a significant increase in depression of transmitter release when compared to control. 5 The morphine induced decrease in probability of transmitter release when naive sympathetic varicosities in vitro were bathed with morphine (1 microM) was not observed following chronic morphine treatment of the animals for 7 to 9 days. When the morphine was acutely withdrawn from chronically morphine treated preparations the underlying increase in probabilities of transmitter release of sympathetic varicosities was unmasked.

Effect of morphine on the nerve terminal impulse and transmitter release from sympathetic varicosities innervating the mouse vas deferens

1 The effect of morphine on both the propagation of the nerve terminal impulse along the sympathetic varicose axons as well as the evoked and spontaneous transmitter release has been evaluated. 2 Morphine (1 microM) did not significantly change the shape or the regularity by which the nerve terminal impulse was recorded while evoked transmitter release was greatly reduced. 3 Morphine induced a uniform decrease in evoked transmitter release irrespective of the release probability of individual varicosities of their position along terminal branches. 4 Procedures which are thought to increase intracellular calcium concentration such as increasing the extracellular calcium concentration, stimulation of the nerve with trains of impulses and increasing the duration of the action potential with 4-aminopyridine reduced the ability of morphine to decrease evoked transmitter release. 5 Morphine had to act directly on the varicosities to induce a decrease in evoked transmitter release. 6 The decrease in evoked quantal release does not involve an affect on the nerve terminal impulse or the vesicle release process and morphine may affect the dependence of the secretory process on calcium.

The effect of opiates on the terminal nerve impulse and quantal secretion from visualized amphibian nerve terminals

1. Secretion of transmitter from amphibian motor nerve terminal release sites is intermittent, spatially non-uniform and varies considerably throughout the year and during development. The role of opioid receptors in modulating transmitter secretion from amphibian motor nerve terminals is evaluated in this study. 2. Dynorphin-A (24 microM) and morphine (500 microM) did not significantly change the shape of the nerve impulse or the consistency with which it was observed, but decreased evoked quantal secretion by more than 50%. These effects of dynorphin-A and morphine were largely reversed by naloxone (50 microM). 3. Dynorphin-A and morphine did not significantly change either the amplitude or the frequency of spontaneous quantal secretions. 4. There was a uniform decrease in evoked quantal secretion from release sites along terminal branches, irrespective of the quantal content value before drug treatment, indicating no difference in the susceptibility of proximal vs distal release sites to opiates. 5. Increasing the extracellular calcium concentration (0.3 to 0.4 mM) or trains of conditioning-test impulses (25 to 100 Hz) resulted in smaller dynorphin-A or morphine-induced decreases in evoked quantal secretion. 6. The decrease in evoked quantal secretion occurs as a result of a uniform decrease in the probability of quantal secretion from release sites without any affect on the propagation of the nerve terminal impulse. Low probability release sites become effectively silent.

The effect of opiates on the secretion of transmitter from amphibian motor nerve terminals

The effects of dynorphin-A, dermorphine and morphine on the secretion of transmitter from the toad (Bufo marinus) motor nerve terminal have been determined. Intracellular recordings of miniature end plate potentials (m.e.p.p.s) and evoked end plate potentials (e.p.p.s) were used to estimate quantal content (m) and binomial parameters p and n. Dynorphin-A, and to a lesser extent morphine, decreased (m) while dermorphine had no significant effect on m. Dynorphin-A (ED50 = 24 microM) was 21 times more potent then morphine (ED50 = 510 microM) in decreasing m. The decrease in m produced by dynorphin-A and morphine was accompanied by a greater decrease in the variance (S2) of number of quanta secreted per stimulation over the recording period. The decrease in m produced by dynorphin-A, and to a lesser extent by morphine, is probably mediated by the opiates acting on kappa-opioid receptors.

Effects of acetaldehyde on electrical activity during neuroeffector transmission in guinea-pig vas deferens

The effects of acetaldehyde on electrical activity during sympathetic neuroeffector transmission were studied in the guinea-pig vas deferens. Application of 1 mM acetaldehyde produced a slow depolarization of the smooth muscle membrane. The amplitudes of facilitated excitatory junction potentials (EJPs) evoked by nerve stimulation were slightly decreased. A higher concentration of acetaldehyde (5 mM) initially hyperpolarized and later depolarized the membrane. The decrease in EJP amplitudes was more pronounced during hyperpolarization. Acetaldehyde (5 mM) increased the frequency of the spontaneous EJPs and reduced their amplitudes, whereas action potentials in postganglionic nerves were unaffected. Acetaldehyde (1-5 mM) decreased the amplitudes of EJPs in vasa pretreated with reserpine but did not alter the resting membrane potentials. The decrease in the EJP amplitudes together with the hyperpolarization of the membrane could be responsible for the early inhibitory effect of acetaldehyde on neuroeffector transmission. The slow depolarization, which is presumably mediated by endogenous noradrenaline, may cause the late facilitatory effect.

Importance of frequency and pulse with in field stimulation of the mouse vas deferens: different behaviour of twitch-inhibiting drugs

The effect of six twitch-inhibiting drugs on the stimulus-response relationship in the field-stimulated mouse vas deferens was compared by means of stimulus response curves which were obtained in two ways, that is, variation of frequency at constant pulse width and variation of pulse width at constant frequency. The twitch-inhibiting potency (in the range of maximal twitch responses) differed with the type of stimulation in a way permitting two groups of substances to be defined: group A (tetrodotoxin, procaine, verapamil) was more effective on frequency-response curves and group B (FK 33-824, clonidine, nifedipine) on pulse-width response curves; the latter applied also to subnormal calcium concentrations in the bath solution. The results suggest that the effect of inhibitory drugs on the field-stimulated vas of the mouse varies greatly with the type of field stimulation.

The dependence of excitatory junction potential amplitude on the external calcium concentration in mouse vas deferens during narcotic withdrawal

The dependence of neurotransmitter release on calcium was evaluated in adrenergic terminals from mice that were acutely withdrawn from chronic morphine treatment (CMT). A two fold increase in the number of writhes in response to an i.p. injection of acetylcholine was induced in mice by CMT and subsequent withdrawal. A shift to the left in the relationship between the excitatory junction potential (e.j.p.) amplitude and extracellular calcium concentration ([Ca]o) was induced in vasa deferentia from CMT-withdrawn mice. A reduction in the degree of facilitation of transmitter release during a short low-frequency train of impulses and an increase in the amount of transmitter release during a high-frequency train of impulses was induced in vasa deferentia from CMT-withdrawn mice. The adaptive mechanism of the terminals to the sustained presence of morphine may involve an increase in the probability that the release sites will release transmitter either via increase in calcium influx or an increase in the affinity of calcium to the hypothetical X-receptor.

The dependence of excitatory junction potential amplitude on the external calcium concentration in narcotic tolerant mouse vas deferens

The dependence of neurotransmitter secretion on external calcium ions during development of opiate tolerance in the mouse vas deferens was studied. The writhing response of mice to an i.p. injection of acetylcholine was inhibited by morphine. Reversal of this antinociceptive effect of morphine during chronic treatment signalled the development of tolerance. Tolerance to morphine at the neuromuscular junction was shown as a reversal of the initial shift of the size of the excitatory junction potential (e.j.p.) vs extracellular calcium concentration relationship back towards the control without any change in the power of 2.4. Facilitation in the amplitude of the e.j.p. occurs with low frequency (2 Hz) stimulation. The initial increase in facilitation induced by morphine was reversed by chronic morphine treatment without any change in the plateau e.j.p. amplitude achieved after a long low frequency train of impulses. At high frequencies (10 Hz) the initial increase in e.j.p. amplitude was followed by a depression. Acute morphine administration decreased the size of the e.j.p., this was followed by an increase in facilitation and a decrease in depression. These effects were reversed after chronic morphine treatment. Tolerance to morphine involves a counteradaptive process which restores the normal entry of calcium ions or its actions within the release sites in promoting transmitter release.

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+.

The effect of naloxone on opioid-induced inhibition and facilitation of acetylcholine release in brain slices

1 The effect of morphine, methionine-enkephalin (Met-enkephalin) and D-Ala2-D-Leu5-enkephalin (DADLE) were tested on the spontaneous and electrically-evoked release of acetylcholine (ACh) from superfused slices of guinea-pig thalamus, caudate nucleus and cerebral cortex. 2 At no concentration did morphine, Met-enkephalin or DADLE modify the outflow of ACh at rest but Met-enkephalin in the presence of naloxone, reduced the resting ACh release. 3 Morphine, at a low dose (3 microM) had no effect in slices of cerebral cortex, but it enhanced the evoked release of ACh in thalamic and caudate, slices. At higher doses of morphine (10-30 microM), the ACh release evoked by electrical pulses was significantly inhibited in every area. 4 Met-enkephalin behaved like morphine in thalamic slices, whereas DADLE, a specific delta agonist, produced a slight inhibition of ACh outflow only at 10 microM. 5 Naloxone antagonized the inhibitory effect of morphine in the cerebral cortex and caudate nucleus slices. Naloxone and also spiroperidol blocked the releasing effect of morphine in caudate slices. In contrast naloxone did not affect the increase of ACh release caused by morphine and Met-enkephalin in thalamic slices. The inhibitory effect of both opioids at high doses was reversed by naloxone so that they then enhanced ACh release. 6 A two fold increase of calcium concentration in the Krebs solution prevented the inhibitory effects of morphine 10 microM. 7 It is suggested that two receptors are present in thalamic slices, one of which inhibits and the other facilitates ACh release.

Interactions among the effects of normorphine, calcium and magnesium on transmitter release in the mouse vas deferens

1 Excitatory junction potentials (e. j. ps) were recorded with intracellular electrodes from smooth muscle cells of the mouse vas deferens. 2 The dependence of the e.j.p. amplitude on the extracellular calcium ion concentration was determined in the absence or presence of normorphine (50 nM-1 microM) or magnesium (1.2-4.8 mM). 3 The interaction between normorphine and calcium was non-competitive beyond a dose-ratio of 1.5, whereas the interaction between magnesium and calcium was competitive up to the highest dose-ratio investigated (1.9). 4 It is suggested that inhibition by normorphine occurs at least partly by a mechanism different from that of magnesium.

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.

Calcium ions, morphine tolerance and noradrenergic transmission in the mouse vas deferens

The response of the isolated vas deferens of the mouse to electrical stimulation is inhibited by morphine and levorphanol via an opiate receptor, the inhibition decreasing with increasing stimulation frequency (0.2-16 Hz). Tolerance to the locomotor stimulant effect of morphine was induced over 48 h using a slow release preparation. Vasa from mice similarly treated with the slow release preparation showed a shift to the right of the morphine and levorphanol twitch inhibition curves. The reduction in the fractional release of [3H]noradrenaline by morphine and levorphanol was less in vasa from morphine-pretreated mice. Altering the Krebs solution by reducing the Ca2+ or Na+ or adding Mg2+ increased the effect of opiate agonists in vasa from naive and morphine-tolerant mice. Therefore, tolerance to morphine has not changed the ability of these ions to modulate opiate responses.

The effects of 4-aminopyridine on the isolated vas deferens and its effects on the inhibitory properties of adenosine, morphine, noradrenaline and gamma-aminobutyric acid

1 Adenosine, adenosine 5'-triphosphate (ATP), morphine, noradrenaline, gamma-aminobutyric acid (GABA) phentolamine and amyl nitrite were used to inhibit electrically-evoked contractions of the isolated superfused vas deferens of the mouse. 2 The inhibitory effects of adenosine ATP, morphine, noradrenaline and GABA, which are thought to be due to presynaptic action, were reduced by perfusion with media containing 4-aminopyridine (4AP) or tetraethylammonium (TEA) ions. The inhibitory effects of phentolamine and amyl nitrite were unaffected by 4AP or TEA. 3 Quinidine, which like 4AP and TEA produced some increase to twitch height, did not reduce responses to the various agonists, indicating that an increased muscle contraction was not itself responsible for the reduced responses. 4 It is concluded that antagonism between 4AP and adenosine is not a specific interaction, as had been suggested, but probably reflects an interaction with Ca2+ requiring processes in the presynaptic terminal.