Interaction of forskolin with the effect of atropine on [3H]acetylcholine secretion in guinea-pig ileum myenteric plexus

Pre-junctional effects of oximes on [3H]-acetylcholine release in rat hippocampal slices during soman intoxication

In this study, the non-reactivating effects of oximes in the hippocampus of the rat are investigated. The potassium (51 mM) evoked release of [(3)H]-acetylcholine and the liberation of [(3)H]-choline were determined in hippocampal slices following in vitro exposure to soman and five oximes (toxogonin, HI-6, HLö-7, P2S and 2-PAM) in separate experiments by superfusion. In the absence of soman, toxogonin and HLö-7 in particular induced a concentration dependent significant increase in the evoked release of [(3)H]-acetylcholine. There was also a significant effect of HI-6, but the effect was much smaller. Two pralidoxime salts, P2S (methanesulfonate salt) and 2-PAM (methiodide salt), had similar but lower effects that were only observed at relatively high concentrations. Experiments performed following complete inhibition of the acetylcholinesterase activity by soman (1.0 microM) showed that HI-6 and HLö-7 induced a significant decrease in the potassium-evoked release of [(3)H]-acetylcholine, while the liberation of [(3)H]-choline increased. Toxogonin, P2S and 2-PAM did not reduce significantly the evoked release of [(3)H]-acetylcholine. Only limited reactivation of the acetylcholinesterase activity was observed in superfusion experiments with toxogonin, HI-6, P2S and 2-PAM following exposure of hippocampal slices to soman. However, HLö-7 was proved to be relatively more effective in reactivating the acetylcholinesterase activity at high concentrations (50 and 200 microM). The acetylcholinesterase activity was reactivated to approximately 12% and 40% of control, respectively. It is concluded that HI-6 and HLö-7 have important non-acetylcholinesterase reactivating properties following soman poisoning, as may be seen by the significant reduction in the evoked release of [(3)H]-acetylcholine effected by these oximes. HLö-7 is of particular interest in view of its ability to additionally improve reactivation of the acetylcholinesterase activity.

Depolarization initiates phasic acetylcholine release by relief of a tonic block imposed by presynaptic M2 muscarinic receptors

The role of presynaptic muscarinic autoreceptors in the initiation of phasic acetylcholine (ACh) release at frog and mouse neuromuscular junctions was studied by measuring the dependency of the amount (m) of ACh release on the level of presynaptic depolarization. Addition of methoctramine (a blocker of M2 muscarinic receptors), or of acetylcholinesterase (AChE), increased release in a voltage-dependent manner; enhancement of release declined as the depolarizing pulse amplitude increased. In frogs and wild-type mice the slope of log m/log pulse amplitude (PA) was reduced from about 7 in the control to about 4 in the presence of methoctramine or AChE. In M2 muscarinic receptor knockout mice, the slope of log m/log PA was much smaller (about 4) and was not further reduced by addition of either methoctramine or AChE. The effect of a brief (0.1 ms), but strong (-1.2 microA) depolarizing prepulse on the dependency of m on PA was also studied. The depolarizing prepulse had effects similar to those of methoctramine and AChE. In particular, it enhanced release of test pulses in a voltage-dependent manner and reduced the slope of log m/log PA from about 7 to about 4. Methoctramine + AChE occluded the prepulse effects. In knockout mice, the depolarizing prepulse had no effects. The cumulative results suggest that initiation of phasic ACh release is achieved by depolarization-mediated relief of a tonic block imposed by presynaptic M2 muscarinic receptors.

Use of knockout mice reveals involvement of M2-muscarinic receptors in control of the kinetics of acetylcholine release

We have previously suggested that presynaptic M(2)-muscarinic receptors (M(2)R) are involved in the control of the time course of evoked acetylcholine release in the frog neuromuscular junction. The availability of knockout mice lacking functional M(2)R (M(2)-KO) enabled us to address this issue in a more direct way. Using the phrenic diaphragm preparation, we show that in wild-type (WT) mice experimental manipulations known to affect Ca(2+) entry and removal, greatly affected the amount of acetylcholine released (quantal content). However, the time course of release remained unaltered under all these experimental treatments. On the other hand, in the M(2)-KO mice, similar experimental treatments affected both the quantal content and the time course of release. In general, a larger quantal content was accompanied by a longer duration of release. Similarly, the rise time of the postsynaptic current produced by axon stimulation was sensitive to changes in [Ca(2+)](o) or [Mg(2+)](o) in M(2)-KO mice but not in WT mice. Measurements of Ca(2+) currents revealed that the shorter rise time of the postsynaptic current seen in high [Mg(2+)](o) in M(2)-KO mice was not produced by a shorter wave of the presynaptic Ca(2+) current. These results support our earlier findings and provide direct evidence for the major role that presynaptic M(2)-muscarinic receptors play in the control of the time course of evoked acetylcholine release under physiological conditions.

Can the Ca2+ hypothesis and the Ca2+-voltage hypothesis for neurotransmitter release be reconciled?

It is well established that Ca2+ plays a key role in promoting the physiological depolarization-induced release (DIR) of neurotransmitters from nerve terminals (Ca2+ hypothesis). Yet, evidence has accumulated for the Ca2+-voltage hypothesis, which states that not only is Ca2+ required, but membrane potential as such also plays a pivotal role in promoting DIR. An essential aspect of the Ca2+-voltage hypothesis is that it is depolarization that is responsible for the initiation of release. This assertion seems to be contradicted by recent experiments wherein release was triggered by high concentrations of intracellular Ca2+ in the absence of depolarization [calcium-induced release (CIR)]. Here we show that there is no contradiction between CIR and the Ca2+-voltage hypothesis. Rather, CIR can be looked at as a manifestation of spontaneous release under conditions of high intracellular Ca2+ concentration. Spontaneous release in turn is governed by a subset of the molecular scheme for DIR, under conditions of no depolarization. Prevailing estimates for the intracellular calcium concentration, [Ca2+]i, in physiological DIR rely on experiments under conditions of CIR. Our theory suggests that these estimates are too high, because depolarization is absent in these experiments and [Ca2+]i is held at high levels for an extended period.

Presynaptic M(2) muscarinic receptors are involved in controlling the kinetics of ACh release at the frog neuromuscular junction

1. Macropatch recording was used to study release of acetylcholine in the frog neuromuscular junction evoked by either direct local depolarization or by an action potential. 2. The quantal content was established by directly counting the released quanta. The time course of release was obtained by constructing synaptic delay histograms. 3. Perfusion of the neuromuscular junction with methoctramine, a selective M(2)/M(4) muscarinic antagonist, increased the quantal content and slowed the exponential decay of the synaptic delay histograms. Addition of the agonist muscarine reversed these effects. 4. Addition of acetylcholinesterase prolonged the decay of the delay histogram, and muscarine reversed this effect. 5. Methoctramine slowed the rise time of the postsynaptic current produced by axon stimulation without affecting either the excitatory nerve terminal current or the presynaptic Ca(2+) current. 6. These results show that presynaptic M(2) muscarinic receptors are involved in the process which terminates evoked ACh release.

Autoreceptors, membrane potential and the regulation of transmitter release

It has been suggested that depolarization per se can control neurotransmitter release, in addition to its role in promoting Ca2+ influx. The 'Ca2+ hypothesis' has provided an essential framework for understanding how Ca2+ entry and accumulation in nerve terminals controls transmitter release. Yet, increases in intracellular Ca2+ levels alone cannot account for the initiation and termination of release; some additional mechanism is needed. Several experiments from various laboratories indicate that membrane potential has a decisive role in controlling this release. For example, depolarization causes release when Ca2+ entry is blocked and intracellular Ca2+ levels are held at an elevated level. The key molecules that link membrane potential with release control have not yet been identified: likely candidates are presynaptic autoreceptors and perhaps the Ca2+ channel itself.

Modulation of ACh release from airway cholinergic nerves in horses with recurrent airway obstruction

To evaluate the functional status of neuronal alpha2-adrenoceptors (ARs) and beta2-ARs on ACh release in horses with recurrent airway obstruction (RAO), we examined the effects of the physiological agonists epinephrine (Epi) and norepinephrine (NE) and the beta2-agonists RR- and RR/SS-formoterol on ACh release from airway cholinergic nerves of horses with RAO. Because SS-formoterol, a distomer of the beta2-agonist, increases ACh release from airways of control horses only after the autoinhibitory muscarinic receptors are blocked by atropine, we also tested the hypothesis that if there is an M2-receptor dysfunction in equine RAO, SS-formoterol should increase ACh release even in the absence of atropine. ACh release was evoked by electrical field stimulation and measured by HPLC. Epi and NE caused less inhibition of ACh release in horses with RAO than in control horses. At the catecholamine concentration achieved during exercise (10(-7) M), the inhibition induced by Epi and NE was 10.8 +/- 13.2 and 3.4 +/- 6.8%, respectively, in equine RAO versus 41.0 +/- 6.4 and 27.1 +/- 5.6%, respectively, in control horses. RR- and RR/SS-formoterol (10(-8) to 10(-5) M) increased ACh release to a similar magnitude as that in control horses. These results indicate that neuronal beta2-ARs are functioning; however, the alpha2-ARs are dysfunctional in the airways of horses with RAO in response to circulating catecholamines. SS-formoterol (10(-8) to 10(-5) M) facilitated ACh release in horses with RAO even in the absence of atropine. Addition of atropine did not cause significantly more augmentation of ACh release over the effect of SS-formoterol alone. The magnitude of augmentation in horses with RAO in the absence of atropine was similar to that in control horses in the presence of atropine. The latter observations could be explained by neuronal muscarinic-autoreceptor dysfunction in equine RAO.

Nerve-induced release of nitric oxide exerts dual effects on nicotinic transmission within the coeliac ganglion in the rabbit

The involvement of nitric oxide in the modulation of nicotinic activation was investigated in vitro in isolated rabbit coeliac ganglion. The electrical activity of the ganglionic neurons was recorded using intracellular recording techniques. When a train of pulses of supramaximum intensity was applied to the splanchnic nerves, gradual depression of fast nicotinic activation occurred: the pulses do not systematically elicit action potentials, but very often elicit excitatory postsynaptic potentials only. This phenomenon appeared between 15 and 20 Hz and increased with the frequency of stimulation. It was not related to any change in the membrane potential of the ganglionic neurons. For a given frequency, the depression appeared progressively and it was particularly strong at the end of the train. The use of pharmacological agents that interfere with the nitric oxide pathway, such as L-arginine (precursor of nitric oxide), D-arginine (non-precursor of nitric oxide) N(omega_-nitro-L-arginine and N(omega)-nitro-L-arginine methyl ester (inhibitors of nitric oxide synthase), and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (nitric oxide scavenger), demonstrated that nitric oxide modulated this depression phenomenon by exerting a dual effect on the nicotinic activation, i.e. facilitation or inhibition. Agents interfering with the guanosine 3',5'-cyclic monophosphate pathway, such as oxadiazolo[4,3-a] quinoxalin-1-one (selective inhibitor of the nitric oxide-activated soluble guanylate cyclase) and zaprinast (selective inhibitor of the phosphodiesterases involved in the guanosine 3',5'-cyclic monophosphate pathway) demonstrated that only the facilitatory effect of nitric oxide on the nicotinic activation was mediated through the guanosine 3',5'-cyclic monophosphate pathway. The mechanism sustaining the inhibitory effect remains to be determined. By modulating the nicotinic activation, nitric oxide plays a role in the integrative properties of the prevertebral ganglia. This opens new perspectives with regard to the control of visceral functions by the prevertebral level of regulation.

Nitric oxide-sensitive guanylyl cyclase inhibits acetylcholine release and excitatory motor transmission in the guinea-pig ileum

This study examined the mechanism through which nitric oxide inhibits the release of acetylcholine and excitatory motor neurotransmission in the guinea-pig ileum. The selective inhibitor of nitric oxide-sensitive guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), concentration-dependently enhanced both basal release (-log EC50: 6.8) and electrically (10 Hz)-evoked release (-log EC50: 6.0) of [3H]acetylcholine from longitudinal muscle-myenteric plexus preparations preincubated with [3H]choline. The increase by ODQ of basal release appeared to be exocytotic since it was prevented by tetrodotoxin (300 nM) and absence of calcium from the superfusion medium. In addition, ODQ (1 microM) increased the electrically-evoked tachykininergic and cholinergic muscle contractions as measured in the presence of scopolamine (100 nM) or of the neurokinin-1 receptor antagonist CP 99994 (100 nM), respectively. The nitric oxide synthase inhibitor L-N(G)-nitro-arginine (100 microM) behaved similar to ODQ and increased cholinergic and tachykininergic motor neurotransmission. The nitric oxide-independent activator of soluble guanylyl cyclase, 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole, concentration-dependently inhibited the electrically evoked acetylcholine release (-log EC50: 6.0) and longitudinal muscle contractions (-log EC50: 5.7). ODQ (10 microM) antagonized the effects of 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole. The results suggest that endogenous nitric oxide tonically activates soluble guanylyl cyclase in myenteric neurons which leads to inhibition of the release of the excitatory transmitters acetylcholine and substance P. ODQ prevents the effects of nitric oxide and thus facilitates cholinergic and tachykininergic motor neurotransmission in the guinea-pig ileum.

The effect of trimethyltin on acetylcholine release in the guinea-pig trachea

The purpose of the present work was to characterise the effects of trimethyltin on the release of acetylcholine from parasympathetic nerves and its effect on the postjunctional cholinergic stimulation of a smooth muscle. The guinea-pig trachea has been used as a model. Prejunctionally, trimethyltin (3.0 × 10(-3) M) significantly enhanced in a reversible manner the high K(+) (75 mM) evoked release of endogenous acetylcholine and [(3)H]acetylcholine. The evoked release of endogenous acetylcholine and [(3)H]acetylcholine was released from a pool of acetylcholine being independent of extraneuronal Ca(2+) in the presence, but not in the absence of trimethyltin. The effect of trimethyltin on the release was not inhibited by low Ca(2+) (0 mM and 1.0 × 10(-4) M) or by Ca(2+) channel blockers (verapamil, 1.0 × 10(-4) M, flunarizine, 1.0 × 10(-4) M, ω-conotoxin GVIA, 2.0 × 10(-7) M and ω-agatoxin, 2.0 × 10(-7) M). The present results also demonstrate that trimethyltin induce emptying of a non-vesicular, probably a cytoplasmic storage pool of acetylcholine, since AH5183 (2.0 × 10(-5) M), an inhibitor of the translocation of acetylcholine into synaptic vesicles, and α-latrotoxin (1.0 × 10(-8) M), a toxin from black widow spider venom inducing vesicle depletion, had no inhibitory effects on the release of [(3)H]acetylcholine evoked by trimethyltin (3.0 × 10(-3) M). The release of [(3)H]acetylcholine was moreover enhanced by trimethyltin when the vesicular uptake of [(3)H]acetylcholine was inhibited by AH5183, probably as a result of a higher cytoplasmic concentration of [(3)H]acetylcholine. Trimethyltin also reduced the neuronal uptake of [(3)H]choline and this was probably due to a depolarising effect of trimethyltin on the cholinergic nerve terminals. A similar depolarisation induced by trimethyltin was observed during patch clamping of GH(4) C(1) neuronal cells. Postjunctionally, trimethyltin had no effect by itself or on the carbachol-induced smooth muscle contraction, indicating that trimethyltin did not have a general depolarising effect on smooth muscle cells or an effect on muscarinic receptors. Furthermore, the reduced electrical field-induced contraction and the subsequent increase in the basal smooth muscle tension that was observed by addition of trimethyltin was activity-dependent, and was most probably due to emptying of a nervous non-vesicular storage pool of acetylcholine, followed by rapid hydrolysis of acetylcholine by acetyl- and pseudocholinesterases.

Muscarinic autoinhibition of acetylcholine release in mouse atria is not transduced through cyclic AMP or protein kinase C

1. The present study investigated the second messenger pathways that may mediate muscarinic receptor autoinhibition of acetylcholine release in mouse atria. The stimulation-induced (S-I) outflow of radioactivity from mouse isolated atria incubated with [3H]-choline was Ca(2+)-dependent and tetrodotoxin-sensitive and was used as an index of neuronal acetylcholine release. 2. The cell permeable analogue of cyclic AMP, 8-bromocyclic AMP (1 x 10(-3)M) enhanced the S-I outflow of radioactivity (33%), lower concentrations having no effect. Similarly, the adenylate cyclase activator forskolin (1 x 10(-5)M) had a small facilitatory effect on acetylcholine release. On the other hand the phosphodiesterase inhibitor 3-isobutylmethylxanthine (1 x 10(-4)M) had no effect on the S-I outflow of radioactivity. Together these results suggest that the adenylate cyclase/cyclic AMP system does not have an appreciable role in the modulation of acetylcholine release. 3. The protein kinase C activator phorbol dibutyrate (0.1-3 x 10(-6)M) enhanced the S-I acetylcholine release (maximally by 45%). The effects of phorbol dibutyrate were attenuated by the protein kinase inhibitor staurosporine (1 x 10(-7)M), which by itself had no effect on the S-I outflow of radioactivity. This latter result suggests that there is no tonic activation of protein kinase C during acetylcholine release. 4. Atropine (1 x 10(-7)M) markedly enhanced (232%) the S-I outflow of radioactivity, presumably by preventing feedback inhibition on acetylcholine release through prejunctional muscarinic receptors. This effect is unlikely to involve adenylate cyclase or protein kinase C since it was far greater than the effects of activation of either system with forskolin and phorbol dibutyrate, respectively. Furthermore, the facilitatory effect of atropine was not attenuated by staurosporine, which although a protein kinase C inhibitor, is also an effective inhibitor of cyclic AMP dependent protein kinase (protein kinase A).

3,4-Diaminopyridine and choline increase in vivo acetylcholine release in rat striatum

We investigated the effects of choline, 3,4-diaminopyridine and their combination on acetylcholine release from the corpus striatum of freely moving rats which were treated or not with atropine. Intraperitoneal administration of choline or intrastriatal administration of 3,4-diaminopyridine increased acetylcholine levels in striatal dialysates in a dose-dependent manner. When 3,4-diaminopyridine treatment was combined with choline, the observed effect was considerably greater than the sum of the increases produced by choline or 3,4-diaminopyridine alone. Administration of atropine (1 microM) in the dialysing medium was also found to be effective to stimulate striatal acetylcholine levels. 3,4-Diaminopyridine did not affect acetylcholine levels under these conditions. Whereas the choline-induced increase in acetylcholine release was significantly potentiated by atropine, co-administration of 3,4-diaminopyridine with choline failed to produce a further significant increase in the presence of atropine. These results suggest that a highly effective means for increasing acetylcholine release involves two concurrent treatments that increase neuronal choline levels and inhibition of the negative feedback modulation of acetylcholine release.

Increase by NO synthase inhibitors of acetylcholine release from guinea-pig myenteric plexus

The effects of nitric oxide (NO) synthase inhibitors on the electrically evoked release of [3H]acetylcholine were studied in guinea-pig myenteric plexus preparations preincubated with [3H]choline. NG-monomethyl-L-arginine (EC50 5.3 mumol l-1) and NG-nitro-L-arginine (EC50 1.3 mumol l-1) concentration-dependently increased the evoked release of [3H]acetylcholine without affecting the basal outflow. The facilitatory effect of NG-mono-methyl-L-arginine was prevented by L-arginine but not by D-arginine. The results suggest that endogenous NO inhibits the depolarisation-evoked release of acetylcholine.

Histamine H3-receptor-induced inhibition of duodenal cholinergic transmission is independent of intracellular cyclic AMP and GMP

1. The inhibitory effect of the histamine H3-receptor agonist (R) alpha-methylhistamine on cholinergic neurotransmission was studied in the isolated guinea pig duodenum in the presence of different compounds which alter intracellular levels of cyclic nucleotides and of the G proteins blocker pertussis toxin. 2. The action of (R) alpha-methylhistamine on electrically-evoked contractions was not modified either by forskolin and isobutylmethylxanthine (which increase cyclic AMP) or by zaprinast and methylene blue (which increase and decrease, respectively intracellular cyclic GMP). Drugs affecting cyclic nucleotide levels were also ineffective against the inhibitory effect of the alpha 2 adrenergic agonist clonidine. 3. Pertussis toxin significantly reduced the maximum inhibition induced by (R) alpha-methylhistamine and clonidine, without influencing the effect of low concentrations of the above compounds; conversely it shifted to the right in a parallel way the inhibitory effect of adenosine. 4. These data suggest that H3-receptor-mediated inhibition of cholinergic transmission in the guinea pig duodenum is not linked to intracellular nucleotide changes. Moreover the signal transducing mechanism activated by (R) alpha-methylhistamine involves pertussis toxin both sensitive and insensitive G proteins.

Presynaptic muscarinic receptors and the release of acetylcholine from cerebrocortical prisms: roles of Ca2+ and K+ concentrations

The mechanism by which presynaptic muscarinic autoreceptors inhibit the release of acetylcholine (ACh) from cerebrocortical cholinergic fibres has not been clarified. To test the view that muscarinic autoreceptors act by decreasing Ca2+ influx, we performed experiments in which rat cerebrocortical prisms were preloaded with (14C)choline, washed, depolarized with 14-65 mM K+ in the absence of Ca2+ and then exposed (still under depolarization) to various concentrations of Ca2+ to evoke the release of (14C)ACh. The muscarinic agonist, oxotremorine, used at a 100 microM concentration, inhibited the release of (14C)ACh by 59-86% in experiments with 14 and 26.5 mM K+ but had no significant effect at 65.5 mM K+. No systematic changes in the inhibitory effects of oxotremorine could be found at any of the K+ concentrations used when the concentration of Ca2+ was varied in the range of 0.25-4.0 mM. At 2 mM Ca2+ and K+ concentrations above 14 mM, the inhibitory effect of oxotremorine was inversely related to the concentration of K+. The inhibitory effect of oxotremorine on (14C)ACh release was not blocked by 100 microM 4-amino-pyridine. The fact that the inhibitory effect of oxotremorine could not be overcome by an increase in the concentration of Ca2+ suggests that, under the conditions used, a restriction of the influx of Ca2+ did not play a major role in the muscarinic inhibition of ACh release; rather, oxotremorine appeared to act by decreasing membrane depolarization.2+ of the Ca(2+)-voltage hypothesis of neurotransmitter release, supposing

Role of cyclic AMP in prostaglandin-induced modulation of acetylcholine release from the myenteric plexus of guinea pig ileum

Prostaglandins (PGs) have modulatory effects on spontaneous and nicotine-induced release of acetylcholine (ACh) from the myenteric plexus of guinea pig ileum. To determine whether cyclic AMP is involved in the mechanisms of these effects, we studied ACh release under conditions that inhibit PG synthesis. Indomethacin (IND), a cyclooxygenase inhibitor, inhibited ACh release concentration-dependently. The effect of the maximally inhibitory concentration of IND (2.8 microM) on nicotine-induced ACh release were reversed concentration-dependently by PGE2, forskolin, 3-isobutyl-1-methylxanthine (IBMX) and 8-bromo cyclic AMP. These compounds caused concentration-dependent reversal of the inhibition of spontaneous ACh release by IND, but their concentrations for restoration of spontaneous release were higher than those for restoration of nicotine-induced release. The effects of PGE2 and forskolin or IBMX were not additive in reversing the inhibition of nicotine-induced ACh release by IND. Neither forskolin nor 8-bromo cyclic AMP alone had any significant effect on either release. These results showed that increase in the level of cyclic AMP in myenteric cholinergic neurons restored ACh release from the tissue whose PG level had been lowered by IND and indicated that endogenous PGs may modulate the level of intraneuronal cyclic AMP.

Subtype classification of the presynaptic alpha-adrenoceptors which regulate [3H]-noradrenaline secretion in guinea-pig isolated urethra

1. The following experiments were carried out to investigate the presence and type of functional presynaptic receptors in adrenergic nerves of the guinea-pig urethra. 2. The urethra from male guinea-pigs was incubated with [3H]-noradrenaline and superfused with Tyrode solution in vitro. The fractional secretion of [3H]-noradrenaline evoked by 300 electrical pulses was measured. 3. The [3H]-noradrenaline secretion was positively frequency-dependent, yielding a half-maximal secretion at 8 +/- 5 Hz. Stimulation was usually applied at 5 Hz. 4. The [3H]-noradrenaline secretion was not altered by noradrenaline (1 or 100 microM), norephedrine (1 microM), isoprenaline (0.1 microM), 5-hydroxytryptamine (10 microM), oxotremorine (10 microM), adenosine (0.2 mM), propranolol (1 microM), atropine (1 microM) or 8-phenyltheophylline (10 microM). 5. The [3H]-noradrenaline secretion was enhanced by clonidine (3 microM), chlorpromazine (10 microM), metitepine (1 microM), 4-aminopyridine (0.5 mM), tetraethylammonium (2 mM), 3-isobutyl-1-methylxanthine (4 mM), 8-bromo cyclic AMP (1 mM) and forskolin (25 microM). 6. The alpha-adrenoceptor antagonists rauwolscine, yohimbine, phentolamine, prazosin and AR-C 239 maximally enhanced the [3H]-noradrenaline secretion to about 300% of control. The partial alpha-adrenoceptor agonist oxymetazoline maximally enhanced the secretion to about 200% of control. The order of apparent EC50 values was rauwolscine less than yohimbine less than phentolamine less than oxymetazoline less than prazosin less than AR-C 239.7. The enhancing effects of yohimbine (1 microM) with tetraethylammonium (2mM), 8-bromo cyclic AMP (1 mM), or forskolin (25,microM) were additive, but not those of yohimbine (1 microM) with prazosin (10 microM), 4-aminopyridine (0.5 mM), or 3-isobutyl-1-methylxanthine (4 mM).8. These results suggest that the [3H]-noradrenaline secretion in the guinea-pig urethra is regulated by presynaptic alpha2A-adrenoceptors which may, in a cyclic AMP-independent manner, be coupled to a 4-aminopyridine-sensitive potassium channel. The secretion is not influenced by compounds acting at beta-adrenoceptors, muscarinic cholinoceptors or adenosine receptors.

Non-synaptic cholinergic modulation of neurogenic twitches of the guinea-pig ileum

The effect of cholinergic and anticholinergic compounds on conduction of neuronal excitation has been studied in myenteric plexus-longitudinal muscle strips from the guinea-pig ileum. A preparation in a special triple bath was drawn through two rubber membranes dividing the strip into three segments. Neurogenic stimulation of the oral segment set up nerve action potentials propagating aborally across the middle segment (10 mm) so that the aboral segment might be also invaded, eventually. Drugs were added to the middle segment to affect neuronal propagation (non-synaptic effects) which was monitored by twitch height of the aboral segment. The application of acetylcholine to the middle segment augmented aboral twitches. The effects of nicotine, pilocarpine and oxotremorine were selectively blocked by (+)-tubocurarine, pirenzepine and atropine, respectively. The effect of acetylcholine was suppressed by pirenzepine and atropine and mimicked by doubling of KCl concentration. The effect of acetylcholine may be thus explained by the facilitated propagation of nerve action potentials in partially depolarized cholinergic terminals via stimulation of muscarinic receptors. The adenylate cyclase system is not directly involved in the mechanism of muscarinic facilitation of neuronal propagation in the terminals; however, it may participate in the modulation of a final common effector mechanism.

Determination of levels of cyclic AMP in the myenteric plexus of guinea-pig small intestine

Enzymatically dissociated ganglia from the myenteric plexus of the guinea-pig small intestine were used to investigate changes in levels of cyclic 3',5'-adenosine monophosphate (cAMP) in response to stimulation of adenylate cyclase by forskolin and inhibition of phosphodiesterase by 3-isobutyl-1-methylxanthine (IBMX). A linear relation with a positive correlation coefficient greater than 0.98 was found between: (1) amount of cAMP and number of ganglia; (2) amount of protein and number of ganglia; (3) amount of DNA and amount of protein; (4) amount of DNA and number of ganglia. Basal levels of cAMP were 2.25 +/- 0.21 fmol per ganglion for 900 ganglia. Forskolin stimulated a dose-dependent increase in cAMP over a concentration range of 0.05 to 50 microM, with a level of 18.6 +/- 4.9 fmol/ganglion at 50 microM forskolin. The inactive forskolin analog 1,9-dideoxyforskolin did not elevate cAMP. Addition of IBMX to the incubation medium stimulated a dose-dependent increase in cAMP over a concentration range of 0.1-1000 microM, with a level of 17.58 +/- 3.38 fmol/ganglion at 1000 microM IBMX. Application of 1 mM IBMX strongly potentiated the stimulating action of forskolin on cAMP levels. Our results derived from direct determination of cAMP changes in small intestinal myenteric ganglia are consistent with existing electrophysiological evidence for second messenger function of cAMP in slow synaptic modulation of excitability in AH/Type 2 neurons of the enteric nervous system.

Forskolin modulates acetylcholine release in the hippocampus independently of adenylate cyclase activation

[3H]Acetylcholine release from slices of rabbit hippocampus was elicited by electrical field stimulation (360 pulses/3 Hz). Both forskolin, commonly used as a specific activator of adenylate cyclase, as well as 1,9-dideoxy-forskolin, which fails to activate adenylate cyclase, increased the evoked transmitter release in an almost identical manner. In addition, the phosphodiesterase inhibitor, rolipram, and the membrane-permeable analogue of cAMP, 8-Br-cAMP, did not influence acetylcholine release. These data show that forskolin is not specific to adenylate cyclase and that the increase in acetylcholine release in the rabbit hippocampus occurs through a mechanism other than activation of adenylate cyclase.

Effect of morphine and acetylcholine on contractile activity and cyclic AMP in guinea-pig ileum

Neither acute nor prolonged exposure to morphine altered cAMP content or spontaneous movements of longitudinal muscle-myenteric plexus strips of the guinea-pig ileum. By contrast, exogenous acetylcholine or electrical stimulation of the strips elicited both a decrease of cAMP concentration and a twitch response. Atropine blocked the effects of stimulation on these parameters. Addition of morphine to electrically stimulated strips inhibited the twitch response but did not affect cAMP levels. Incubation with morphine led to the development of tolerance to the inhibitory effect on twitch activity and prevented the fall in cAMP normally elicited by electrical stimulation. These results suggest that muscarinic activation is associated with a reduction of cAMP content, an effect which would be impaired in opiate-tolerant tissues.

Effects of N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate, adenosine, and of oxotremorine and 3-isobutyl-1-methylxanthine on the electrically evoked [3H]acetylcholine secretion in the guinea-pig ileum myenteric plexus

The guinea-pig ileum longitudinal muscle-myenteric plexus preparation, pre-incubated with [3H]choline, was mounted in an organ bath and superfused with Tyrode's solution. [3H]Acetylcholine secretion was evoked by 150 electrical shocks at 0.5 Hz. N6,2'-O-Dibutyryladenosine 3',5'-cyclic monophosphate (dibutyryl cyclic AMP) enhanced the [3H]acetylcholine secretion in the presence of eserine and the adenosine receptor antagonist 8-phenyltheophylline (10 mumol l-1). Conversely, in the absence of 8-phenyltheophylline the [3H]acetylcholine secretion was reduced by dibutyryl cyclic AMP. In the absence and presence of 8-phenyltheophylline (apparent KD = 12 mumol l-1), adenosine reduced the [3H]acetylcholine secretion to 33% of control (IC50 = 8 mumol l-1) and to 48% of control (IC50 = 14 mumol l-1) respectively. Neither butyrate, dibutyryl cyclic GMP nor guanosine altered the [3H]acetylcholine secretion. Interaction experiments with 3-isobutyl-1-methylxanthine and oxotremorine were done in the absence of eserine, i.e. when oxotremorine is effective. Oxotremorine depressed the fractional secretion of [3H]acetylcholine with a 'maximal inhibition' of 13% of control (IC50 = 10 nmol l-1). In the presence of 3-isobutyl-1-methylxanthine (5 mmol l-1) oxotremorine depressed the secretion to 2% of control with an apparent IC50 value of 0.9 mumol l-1. 3-Isobutyl-I-methylxanthine (0.01-4 mmol l-1) enhanced the fractional secretion of [3H]acetylcholine with a 'maximal enhancement' value of 232% of control (EC50 = 0.19 mmol l-1). The presence of oxotremorine (30 nmol l-1) counteracted, and higher concentrations reversed, the enhancement caused by 3-isobutyl-1-methylxanthine.(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic muscarinic receptors in guinea pig superior cervical ganglion

This study characterizes the presynaptic muscarinic cholinergic receptors associated with the modulation of the electrically-evoked acetylcholine output from guinea pig superior cervical ganglion preincubated with [3H]choline. The M1-selective agonist pilocarpine had no effect while carbachol and oxotremorine strongly decreased the evoked outflow of tritium. Atropine increased such evoked release of [3H]acetylcholine whereas the M1-selective antagonist pirenzepine was ineffective. Moreover, atropine but not pirenzepine antagonized the inhibitory effect of carbachol. These results suggest that the guinea-pig superior cervical ganglion is equipped with presynaptic inhibitory muscarinic receptors of the M2 subtype.

Effects of alaproclate, potassium channel blockers, and lidocaine on the release of 3H-acetylcholine from the guinea-pig ileum myenteric plexus

The guinea-pig ileum longitudinal muscle-myenteric plexus preparation, preincubated with 3H-choline or 3H-noradrenaline, was mounted in an organ bath and superfused with Tyrode's solution. Alaproclate (2-(4-chlorophenyl)-1,1-dimethyl 2-aminopropanoate) (0.01-0.5 mmol/l) reduced (IC50 = 0.1 mmol/l) and at about 0.5 mmol/l completely blocked the electrically evoked 3H-acetylcholine secretion. The depressing effect of alaproclate (0.2 mmol/l) was not counteracted by atropine (0.01, 1 or 10 mumol/l), hexamethonium (0.1 mmol/l), phentolamine (1 mumol/l) yohimbine (1 mumol/l), haloperidol (1 mumol/l), 8-phenyltheophylline (10 mumol/l), cyproheptadine (1 mumol/l), metitepine (1 mumol/l), bicuculline (10 mumol/l), picrotoxinin (0.1 mmol/l), forskolin (25 mumol/l), 3-isobutyl-1-methylxanthine (5 mmol/l), nifedipine (1 mumol/l), verapamil (1 mumol/l), dilitiazem (1 mumol/l), high calcium (6 mmol/l), high potassium (10 or 15 mmol/l), tetraethylammonium (2 mmol/l), 4-aminopyridine (0.5 mmol/l), apamin (0.5 mumol/l), barium (0.5 mmol/l) or quinine (0.1 mmol/l). Among the potassium channel blockers tested only quinine (at 0.5 or 1 mmol/l), in the same manner as lidocaine, reduced the evoked secretion of 3H-acetylcholine. The results are in agreement with the hypothesis that the effect of alaproclate on the evoked 3H-acetylcholine secretion is not mediated by a neurotransmitter receptor, or a potassium channel sensitive to tetraethylammonium, 4-aminopyridine, apamin, or barium or quinine, but is due to a local anaesthetic effect. In contrast to the evoked secretion, the spontaneous release of 3H-acetylcholine was enhanced by high concentrations of alaproclate (0.4-1 mmol/l). The mechanism underlying the effect of alaproclate on the spontaneous release remains to be established. Alaproclate (0.25 or 0.5 mmol/l) also enhanced the spontaneous release and reduced the electrically evoked 3H-noradrenaline secretion.

5-Hydroxytryptamine produces presynaptic facilitation of cholinergic transmission in rabbit parasympathetic ganglia

Intracellular recordings were made from neurons of rabbit vesical pelvic (parasympathetic) ganglia (VPG). Application of 5-hydroxytryptamine (5-HT, 0.3-30 microM) produced an initial depression followed by a long-lasting facilitation of the fast excitatory postsynaptic potential (e.p.s.p.) evoked by stimulation of the pelvic preganglionic nerve. The facilitation of nicotinic transmission lasted for 30-120 min, even when 5-HT was removed from the superfusing solution. 5-HT (0.3-30 microM) did not change the depolarization induced by a direct application of acetylcholine (ACh) to the VPG neurons pretreated with 1 microM atropine. 5-HT also caused an initial depression followed by an increase in the quantal content of the fast e.p.s.p. It is, therefore, suggested that diphasic effect of 5-HT on the nicotinic transmission is due mainly to a modulation of the ACh-release from presynaptic nerve terminals. Methysergide (5 microM), mianserin (5-30 microM) and ICS 205-930 (100-300 nM) did not antagonize the presynaptic actions of 5-HT on the nicotinic transmission, suggesting that the presynaptic 5-HT receptor may belong to a class of 5-HT1 subtypes. Spiperone (1 microM), a selective 5-HT1A antagonist, blocked the 5-HT-induced inhibition of the fast e.p.s.p. Under the effect of spiperone, the facilitation appeared soon after application of 5-HT. The facilitation of the fast e.p.s.p. may be mediated through a 5-HT1B or 5-HT1C subtype. Lowering temperature of the external solution eliminated the 5-HT-induced facilitation of the nicotinic transmission. Forskolin produced a presynaptic facilitation of the fast e.p.s.p., without producing an initial depression. 3-Isobutyl-1-methylxanthine (10 microM) potentiated the facilitatory action of 5-HT. Bath-application of dibutyryl cyclic adenosine monophosphate (cAMP) (1-6 mM) and 8-bromo-cyclic AMP (2-5 mM) mimicked the effect of 5-HT in producing the facilitation of the fast e.p.s.p.s. All data presented are consistent with the hypothesis that 5-HT, acting on presynaptic 5-HT1 receptors, causes a facilitation in the release of ACh from preganglionic nerve terminals possibly mediated through an activation of adenylate cyclase.

Effects of Pertussis Toxin Suggest a Role for G-Proteins in the Inhibition of Acetylcholine Release from Rat Myenteric Plexus by Opioid and Presynaptic Muscarinic Receptors

(1) Longitudinal muscle preparations of the rat ileum with the attached myenteric plexuses (LMMPs) were preloaded with (3H)choline and the effects of drugs on the depolarization-evoked release of radioactivity corresponding to (3H) acetylcholine ((3H)ACh) were measured. The release of (3H)ACh was inhibited by morphine and the effect of morphine was blocked by naloxone. Morphine had no effect on the release of (3H)ACh in LMMPs from rats that had been injected with pertussis toxin (PTX) 7 days before experiments. (2) Carbamoylcholine applied in the presence of tetrodotoxin inhibited the release of (3H)ACh evoked by depolarization of LMMPs. The effect of carbamoylcholine was absent in LMMPs from rats pretreated with PTX. (3) The effects of PTX indicate that one or more PTX-sensitive G proteins are involved in the chain of events mediating the action of opioid and muscarinic receptors on the release of ACh from the myenteric plexus. It is suggested that the inhibition of ACh release depends on G-protein-mediated coupling of opiod receptors with K+ channels and of muscarinic receptors with Ca2+ channels, but alternative explanations cannot be excluded.

Effects of oxotremorine and RMI 12330 A on [3H]acetylcholine release and adenylate cyclase activity in guinea pig superior cervical ganglion

There is considerable evidence that adenosine 3',5'-cyclic monophosphate (cAMP) is involved in the modulation of synaptic transmission in the guinea pig superior cervical ganglion (SCG). Presynaptic muscarinic receptors are known to attenuate, when activated, acetylcholine (ACh) release in the periphery as well as in the brain. Thus, the possible relationship between ganglionic adenylate cyclase activity and the output of ACh from electrically stimulated ganglia, preloaded with [3H]choline, was investigated. The muscarinic agonist oxotremorine significantly reduced in a dose-dependent manner the electrically evoked neurotransmitter release. The adenylate cyclase inhibitor N-(cis-2-phenylcyclopentyl)azacyclotridecan-2-imine hydrochloride (RMI 12330 A) also decreased ACh output. The inhibitory effects of these two drugs were additive. In crude ganglion membrane fractions oxotremorine significantly inhibited adenylate cyclase activity. The results indicate that drugs capable of inhibiting adenylate cyclase, significantly decrease ACh output from preganglionic nerve terminals in guinea pig SCG.