The effects of atropine on [3H]acetylcholine secretion from guinea-pig myenteric plexus evoked electrically or by high potassium

A genetically encoded fluorescent acetylcholine indicator for in vitro and in vivo studies

The neurotransmitter acetylcholine (ACh) regulates a diverse array of physiological processes throughout the body. Despite its importance, cholinergic transmission in the majority of tissues and organs remains poorly understood owing primarily to the limitations of available ACh-monitoring techniques. We developed a family of ACh sensors (GACh) based on G-protein-coupled receptors that has the sensitivity, specificity, signal-to-noise ratio, kinetics and photostability suitable for monitoring ACh signals in vitro and in vivo. GACh sensors were validated with transfection, viral and/or transgenic expression in a dozen types of neuronal and non-neuronal cells prepared from multiple animal species. In all preparations, GACh sensors selectively responded to exogenous and/or endogenous ACh with robust fluorescence signals that were captured by epifluorescence, confocal, and/or two-photon microscopy. Moreover, analysis of endogenous ACh release revealed firing-pattern-dependent release and restricted volume transmission, resolving two long-standing questions about central cholinergic transmission. Thus, GACh sensors provide a user-friendly, broadly applicable tool for monitoring cholinergic transmission underlying diverse biological processes.

Voltage effects on muscarinic acetylcholine receptor-mediated contractions of airway smooth muscle

Studies have shown that the activity of muscarinic receptors and their affinity to agonists are sensitive to membrane potential. It was reported that in airway smooth muscle (ASM) depolarization evoked by high K+ solution increases contractility through direct effects on M3 muscarinic receptors. In this study, we assessed the physiological relevance of voltage sensitivity of muscarinic receptors on ASM contractility. Our findings reveal that depolarization by high K+ solution induces contraction in intact mouse trachea predominantly through activation of acetylcholine release from embedded nerves, and to a lesser extent by direct effects on M3 receptors. We therefore devised a pharmacological approach to depolarize tissue to various extents in an organ bath preparation, and isolate contraction due exclusively to ASM muscarinic receptors within range of physiological voltages. Our results indicate that unliganded muscarinic receptors do not contribute to contraction regardless of voltage. Utilizing low K+ solution to hyperpolarize membrane potentials during contractions had no effect on liganded muscarinic receptor-evoked contractions, although it eliminated the contribution of voltage-gated calcium channels. However, we found that muscarinic signaling was potentiated by at least 42% at depolarizing voltages (average -12 mV) induced by high K+ solution (20 mmol/L K+ ). In summary, we conclude that contractions evoked by direct activation of muscarinic receptors have negligible sensitivity to physiological voltages. However, contraction activated by cholinergic stimulation can be potentiated by membrane potentials occurring beyond the physiological range of ASM.

Characterization of energy expenditure in rodents by indirect calorimetry

The need for treatment of obesity and obesity-related diseases, such as type 2 diabetes, has been intensified by the epidemic rise of obesity. Recent advances make possible continuous monitoring of metabolically relevant functions in animals to identify novel thermogenic and anorectic compounds. This unit describes non-invasive in vivo calorimetric assessment of energy expenditure using measurements of oxygen consumption and carbon dioxide production, complemented by telemetric monitoring of body core temperature and locomotor activity in mice and rats. Reference compounds are used to illustrate the determination of substance-specific parameters, such as the dose that produces the half-maximal effect (ED(50)), the maximal effect, as well as the time of onset and duration of compound action. Indirect calorimetry performed at different temperatures provides information on several other well-defined parameters, including resting metabolic rate, basal metabolic rate, lower critical temperature, temperature sensitivity, defended body temperature, and respiratory quotient.

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.

Expression and function of KCNH2 (HERG) in the human jejunum

Previous studies suggest that ether-a-go-go related gene (ERG) KCNH2 potassium channels contribute to the control of motility patterns in the gastrointestinal tract of animal models. The present study examines whether these results can be translated into a role in human gastrointestinal muscles. Messages for two different variants of the KCNH2 gene were detected: KCNH2 V1 human ERG (HERG) (28) and KCNH2 V2 (HERG(USO)) (13). The amount of V2 message was greater than V1 in both human jejunum and brain. The base-pair sequence that gives rise to domains S3-S5 of the channel was identical to that previously published for human KCNH2 V1 and V2. KCNH2 protein was detected immunohistochemically in circular and longitudinal smooth muscle and enteric neurons but not in interstitial cells of Cajal. In the presence of TTX (10(-6) M), atropine (10(-6) M). and l-nitroarginine (10(-4) M) human jejunal circular muscle strips contracted phasically (9 cycles/min) and generated slow waves with superimposed spikes. Low concentrations of the KCNH2 blockers E-4031 (10(-8) M) and MK-499 (3 x 10(-8) M) increased phasic contractile amplitude and the number of spikes per slow wave. The highest concentration of E-4031 (10(-6) M) produced a 10-20 mV depolarization, eliminated slow waves, and replaced phasic contractions with a small tonic contracture. E-4031 (10(-6) M) did not affect [(14)C]ACh release from enteric neurons. We conclude that KCNH2 channels play a fundamental role in the control of motility patterns in human jejunum through their ability to modulate the electrical behavior of smooth muscle cells.

Loss of enteric motor neurotransmission in the gastric fundus of Sl/Sl(d) mice

Studies of W/W(V) mice, which lack intramuscular interstitial cells of Cajal (IC-IM), have suggested that IC-IM act as mediators of enteric motor neurotransmission in the gastrointestinal tract. We have studied Sl/Sl(d) mice, which lack the ability to make membrane-bound stem cell factor, to determine the consequences of inappropriate stem cell factor expression on IC-IM populations and on enteric motor neurotransmission. IC-IM were found within the circular and longitudinal muscles of the gastric fundus of wild-type mice. IC-IM were intimately associated with motor nerve terminals and nerve varicosities formed synaptic structures with these cells. IC-IM were also connected with neighbouring smooth muscle cells via gap junctions. Immunohistochemistry and electron microscopy showed that IC-IM were absent from fundus muscles of Sl/Sl(d) mice, but the density of excitatory and inhibitory nerves was not significantly different than in wild-type muscles. Loss of IC-IM was associated with decreased membrane noise (unitary potentials) and significant reductions in post-junctional excitatory and inhibitory enteric nerve responses. Reductions in neural responses were not due to defects in smooth muscle cells as responses to exogenous ACh and K(+)-induced depolarization were normal in Sl/Sl(d) mice. Responses to neurally released ACh were revealed in Sl/Sl(d) mice by inhibiting ACh breakdown with the acetylcholinesterase inhibitor neostigmine. Inhibitory nerve stimulation elicited inhibitory junction potentials (IJPs) and relaxations in wild-type mice. IJPs were reduced in amplitude and relaxation responses were absent in Sl/Sl(d) mice. These observations suggest that membrane-bound stem cell factor is essential for development of IC-IM and that the close, synaptic-like relationship between nerve terminals and IC-IM may be the primary site of innervation by enteric motor neurons in gastric muscles.

Characterization of a new muscarinic receptor antagonist PNU-171990 in guinea pig, cat and human smooth muscle

The present study was done to characterize a new compound, PNU-171990, 2-diisopropyl aminoethyl 1-phenylcyclopentane carboxylate hydrochloride, with functional smooth muscle selectivity at least as high as tolterodine. In vitro homogenates of guinea pig cerebral cortex, parotid gland, heart, urinary bladder, and Chinese hamster ovary (CHO) cells expressing human muscarinic m(1)-m(5) receptors PNU-171990 did not show selectivity for any subtype (pK(i), 7.72-8.64). PNU-171990 caused a parallel shift in the concentration-response curve for carbachol-induced contraction of smooth muscle from guinea pig bladder (pK(B), 7.65), guinea pig ileum (pK(B), 8.48), and human ileum (pK(B), 7.10). In vivo PNU-171990 inhibited urinary bladder contraction with a significantly lower ID(50) than on the salivary secretion (206 and 706 nmol/kg, respectively, P<0.05). In conclusion, PNU-171990 is a competitive and potent muscarinic receptor antagonist in vitro with a numerically better selectivity ratio for the bladder contraction over salivation in vivo than tolterodine.

Possible role of potassium channels in mu-receptor-mediated inhibition and muscarinic autoinhibition in acetylcholine release from myenteric plexus of guinea pig ileum

It is known that mu-agonists inhibit electrical field stimulation (EFS)-evoked ACh release from longitudinal muscle myenteric plexus (LMMP) preparation of guinea pig ileum when muscarinic autoinhibition does not fully work. In the present study, the possible role of K+ channels in the mechanisms of mu-agonists-induced inhibition and autoinhibition of ACh release was studied. In the presence of atropine, which blocks the autoinhibition, non-selective K+ channel blockers, tetraethylammonium (TEA) and 4-aminopyridine (4-AP), reversed the inhibitory effect of mu-agonists, morphine and [D-Ala2, N-Me-Phe4, Gly5-ol] enkephalin, on EFS-evoked ACh release, but not that of kappa-agonist U-50,488. Apamin, iberiotoxin or glibenclamide did not affect the inhibition of ACh release by morphine. On the other hand, in the absence of atropine (under the autoinhibition working condition), 4-AP increased EFS-evoked ACh release, but atropine did not further increase ACh release in the presence of 4-AP. In contrast, although TEA did not affect EFS-evoked ACh release, atropine increased ACh release in the presence of TEA. These results suggest that the inhibitory effects of mu-agonists and muscarinic autoinhibition on the ACh release are associated with activation of different types of K+ channels in the guinea pig LMMP preparations: the former is associated with 4-AP- and TEA-sensitive K+ channels and the latter is associated with 4-AP- but not TEA-sensitive K+ channels.

Interstitial cells of Cajal mediate cholinergic neurotransmission from enteric motor neurons

Interstitial cells of Cajal (ICC) are interposed between enteric neurons and smooth muscle cells in gastrointestinal muscles. The role of intramuscular ICC (IC-IM) in mediating enteric excitatory neural inputs was studied using gastric fundus muscles of wild-type animals and W/W(v) mutant mice, which lack IC-IM. Excitatory motor neurons, labeled with antibodies to vesicular acetylcholine transporter or substance-P, were closely associated with IC-IM. Immunocytochemistry showed close contacts between enteric neurons and IC-IM. IC-IM also formed gap junctions with smooth muscle cells. Electrical field stimulation yielded fast excitatory junction potentials in the smooth muscle that were blocked by atropine. Neural responses were greatly reduced in muscles of W/W(v) animals. Loss of cholinergic responses in W/W(v) muscles seemed to be caused by the loss of close synaptic contacts between motor neurons and IC-IM, because these muscles were not less responsive to exogenous acetylcholine than were wild-type muscles. W/W(v) muscles also responded to excitatory nerve stimulation when the breakdown of acetylcholine was blocked by neostigmine. The density of cholinergic nerve bundles within the muscles was not significantly different in wild-type and W/W(v) muscles, and similar amounts of (14)[C]choline were released from preloaded wild-type and W/W(v) muscles in response to nerve stimulation. The impact of losing IC-IM on gastric compliance was also evaluated in intact stomachs. Pressure increased as a function of fluid volume and infusion rate in wild-type animals, but W/W(v) animals showed little basal tone and minimal increases in pressure with fluid infusions. These data suggest that IC-IM play a major role in receiving cholinergic excitatory inputs from the enteric nervous system in the murine fundus.

Interstitial cells of Cajal mediate cholinergic neurotransmission from enteric motor neurons

Interstitial cells of Cajal (ICC) are interposed between enteric neurons and smooth muscle cells in gastrointestinal muscles. The role of intramuscular ICC (IC-IM) in mediating enteric excitatory neural inputs was studied using gastric fundus muscles of wild-type animals and W/W(v) mutant mice, which lack IC-IM. Excitatory motor neurons, labeled with antibodies to vesicular acetylcholine transporter or substance-P, were closely associated with IC-IM. Immunocytochemistry showed close contacts between enteric neurons and IC-IM. IC-IM also formed gap junctions with smooth muscle cells. Electrical field stimulation yielded fast excitatory junction potentials in the smooth muscle that were blocked by atropine. Neural responses were greatly reduced in muscles of W/W(v) animals. Loss of cholinergic responses in W/W(v) muscles seemed to be caused by the loss of close synaptic contacts between motor neurons and IC-IM, because these muscles were not less responsive to exogenous acetylcholine than were wild-type muscles. W/W(v) muscles also responded to excitatory nerve stimulation when the breakdown of acetylcholine was blocked by neostigmine. The density of cholinergic nerve bundles within the muscles was not significantly different in wild-type and W/W(v) muscles, and similar amounts of (14)[C]choline were released from preloaded wild-type and W/W(v) muscles in response to nerve stimulation. The impact of losing IC-IM on gastric compliance was also evaluated in intact stomachs. Pressure increased as a function of fluid volume and infusion rate in wild-type animals, but W/W(v) animals showed little basal tone and minimal increases in pressure with fluid infusions. These data suggest that IC-IM play a major role in receiving cholinergic excitatory inputs from the enteric nervous system in the murine fundus.

Neuroregulation by vasoactive intestinal peptide (VIP) of mucus secretion in ferret trachea: activation of BK(Ca) channels and inhibition of neurotransmitter release

1. The aims of this study were to determine: (1) whether vasoactive intestinal peptide (VIP) regulates cholinergic and 'sensory-efferent' (tachykininergic) 35SO4 labelled mucus output in ferret trachea in vitro, using a VIP antibody, (2) the class of potassium (K+) channel involved in VIP-regulation of cholinergic neural secretion using glibenclamide (an ATP-sensitive K+ (K(ATP)) channel inhibitor), iberiotoxin (a large conductance calcium activated K+ (BK(ca)) channel blocker), and apamin (a small conductance K(ca) (SK(ca)) channel blocker), and (3) the effect of VIP on cholinergic neurotransmission using [3H]-choline overflow as a marker for acetylcholine (ACh) release. 2. Exogenous VIP (1 and 10 microM) alone increased 35SO4 output by up to 53% above baseline, but suppressed (by up to 80% at 1 microM) cholinergic and tachykininergic neural secretion without altering secretion induced by ACh or substance P (1 microM each). Endogenous VIP accounted for the minor increase in non-adrenergic, non-cholinergic (NANC), non-tachykininergic neural secretion, which was compatible with the secretory response of exogenous VIP. 3. Iberiotoxin (3 microM), but not apamin (1 microM) or glibenclamide (0.1 microM), reversed the inhibition by VIP (10 nM) of cholinergic neural secretion. 4. Both endogenous VIP (by use of the VIP antibody; 1:500 dilution) and exogenous VIP (0.1 microM), the latter by 34%, inhibited ACh release from cholinergic nerve terminals and this suppression was completely reversed by iberiotoxin (0.1 microM). 5. We conclude that, in ferret trachea in vitro, endogenous VIP has dual activity whereby its small direct stimulatory action on mucus secretion is secondary to its marked regulation of cholinergic and tachykininergic neurogenic mucus secretion. Regulation is via inhibition of neurotransmitter release, consequent upon opening of BK(Ca) channels. In the context of neurogenic mucus secretion, we propose that VIP joins NO as a neurotransmitter of i-NANC nerves in ferret trachea.

Modulation of cholinergic neuromuscular transmission by nitric oxide in canine colonic circular smooth muscle

This study examines the effect of nitric oxide (NO) on cholinergic transmission in strips of canine colonic circular muscle in which neural plexus-pacemaker regions had been removed. Electrical field stimulation gave rise to atropine- and TTX-sensitive excitatory junction potentials (EJPs), the amplitude of which were frequency dependent. In 47% of control muscles, the EJP was followed by an inhibitory junction potential (IJP), whereas in the presence of atropine all preparations exhibited only IJPs. The NO synthase inhibitor Nomega-nitro-L-arginine (L-NNA), the guanylyl cyclase inhibitor 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxaline-1-one (ODQ), and the protein kinase G (PKG) antagonist Rp-8-bromo-PET-cGMPS all significantly increased EJP amplitude and reduced or abolished IJPs. The potentiation of EJPs by L-NNA was reversed by the NO donors sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine in a manner blocked by ODQ. [14C]ACh overflow was also measured to evaluate the possible prejunctional effects of NO. Both norepinephrine and TTX significantly decreased [14C]ACh overflow; however, L-NNA, ODQ, and SNP were without effect. These data suggest that both cholinergic and nitrergic motoneurons functionally innervate the interior of the circular muscle layer. The inhibitory actions of NO on cholinergic transmission appear to be post- rather than prejunctional and to involve guanylyl cyclase as well as possibly PKG.

Inhibitory effect of endomorphin-1 and -2 on acetylcholine release from myenteric plexus of guinea pig ileum

Endomorphin-1 and -2, putative endogenous ligands for the mu-opioid receptor, inhibited acetylcholine (ACh) release evoked by electrical field stimulation (EFS) at 1 Hz, which partially activates muscarinic autoreceptors, but not at 10 Hz, which fully activates muscarinic autoreceptors, in longitudinal muscle with the myenteric plexus (LMMP) preparations of guinea pig ileum. After blockade of autoinhibition by atropine, the peptides also inhibited EFS-evoked ACh release at 10 Hz. The inhibitory effects on ACh release were abolished by the mu-opioid antagonist cyprodime. These results suggest that endomorphin-1 and -2 inhibit ACh release from LMMP preparations of guinea pig ileum and that the mechanism of the inhibition must have a component in common with muscarinic autoinhibition.

Relationship between muscarinic autoinhibition and the inhibitory effect of morphine on acetylcholine release from myenteric plexus of guinea pig ileum

The relationship between muscarinic autoinhibition and the inhibitory effect of morphine on acetylcholine (ACh) release was investigated in a longitudinal muscle with myenteric plexus (LMMP) preparation of guinea pig ileum. Morphine (10 microM) inhibited spontaneous and evoked ACh release by electrical field stimulation (EFS) at 1 Hz but not at 10 Hz. Atropine (1 microM) did not affect the resting ACh release, but it significantly increased EFS-evoked release, suggesting activation of muscarinic autoreceptors by ACh released during EFS. Only when the autoinhibition was weakened by atropine, morphine exhibited an inhibitory effect on the EFS-evoked release at 10 Hz. Bethanechol (300 microM) inhibited the EFS-evoked release at 1 Hz but not 10 Hz, suggesting that muscarinic autoreceptors are partially or almost fully activated at 1 or 10 Hz stimulation, respectively. After bethanechol treatment, morphine did not exhibit its inhibitory effect on the EFS-evoked release at 1 Hz. Naloxone (1 microM) increased spontaneous and EFS-evoked ACh release at 1 Hz but not at 10 Hz. Following treatment with atropine, naloxone also increased ACh release at 10-Hz stimulation. These results suggest that morphine and an endogenous opioid inhibit ACh release from LMMP preparations when muscarinic autoinhibition mechanism does not fully work. This inhibitory effect of morphine is discussed in relation to the calcium sensitivity of the preparations in ACh release.

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.

Hyperpolarization and inhibition of contraction mediated by nitric oxide released from enteric inhibitory neurones in guinea-pig taenia coli

1. Inhibition of nitric oxide synthase by NG-nitro-L-arginine (L-NNA) reduced the neurogenic relaxation of precontracted taenia coli only in the absence of atropine. The membrane hyperpolarization associated with the neurogenic relaxation was also reduced by inhibition of NOS only when atropine was absent. 2. The membrane hyperpolarization associated with the neurogenic relaxation of the taenia coli was inhibited by oxyhaemoglobin only in the absence of atropine. In the presence of atropine, oxyhaemoglobin did not reduce the i.j.p. or nerve evoked relaxation. 3. Inhibition of NOS by L-NNA did not affect the overflow of [3H]-ACh in response to electrical field stimulation (EFS), suggesting that, under the conditions of our experiments, endogenous NO did not modulate release of ACh. Sodium nitroprusside also had no effect on the neurogenic overflow of [3H]-ACh; however, noradrenaline significantly reduced [3H]-ACh overflow. 4. In summary, the postjunctional effects of neurally-released NO are not apparent in guinea-pig taenia coli when atropine is present. This implies muscarinic regulation of NO release or muscarinic regulation of another excitatory substance, such as tachykinin(s), that, when blocked, masks the postjunctional effects of NO. These data, together with previous studies, suggest a possible regulatory role for NO in enteric neurotransmission that may be more prominent in some species or tissues than others.

Site of action of galanin in the cholinergic transmission of guinea pig small intestine

The mode and site of action of galanin were examined in the guinea pig small intestine. Galanin (3 x 10(-9)-10(-7) M) inhibited the twitch contractions of longitudinally and circularly oriented muscle strips mediated by the stimulation of cholinergic neurons, but not the contractions mediated by direct stimulation of smooth muscle cells with carbachol. Galanin (3 x 10(-9)-10(-7) M) inhibited both the electrically stimulated and the tetrodotoxin-resistant high K+ (40 mM)-induced increase of [3H]acetylcholine outflow from the ileal strips preloaded with [3H]choline, in a concentration dependent fashion. The inhibitory effect of galanin was antagonized by galantide and produced self-desensitization. The spontaneous and stimulated outflow of [3H]noradrenaline and [3H]gamma-aminobutyric acid were not affected by galanin even at 10(-7) M. Thus, galanin inhibits the motility of guinea pig ileum by inhibition of acetylcholine release from the enteric cholinergic neurons. Galanin may act on the specific receptor located on soma-dendritic regions and nerve terminals of cholinergic neurons.

Dopamine-induced inhibition of endogenous acetylcholine release from the isolated ileal synaptosomal preparations of guinea-pig mediated via alpha-adrenoceptors

1. The effect of exogenous dopamine on the release of endogenous acetylcholine (ACh) from isolated ileal synaptosomal guinea-pig preparations was examined by means of high pressure liquid chromatography with electrochemical detection. 2. Release of ACh was induced by substance P or by depolarization with high potassium (50 mM) in a medium containing atropine propranolol and naloxone. 3. Dopamine produced a concentration-dependent inhibition of the evoked ACh release induced by substance P or in samples depolarized by high potassium. This action of dopamine was not reversed by the dopamine receptor antagonists either for the DA2 subtype domperidone, or for the DA1 subtype, SCH23390. Fenoldopam, the agonist of dopamine DA1 receptors, or quinpirole, the agonist of dopamine DA2 receptors, reduced the evoked ACh release, although only in high, non-dopamine-specific concentrations. 4. Failure of guanethidine or desipramine to inhibit this effect of dopamine ruled out mediation by endogenous noradrenaline. 5. Idazoxan and yohimbine reversed this dopamine-induced inhibition at concentration sufficient to abolish the action of clonidine. Influx of (45)Ca stimulated by substance P or high potassium into synaptosomal preparations was attenuated in the presence of dopamine. This inhibition by dopamine was also reversed by idazoxan or yohimbine but not by dopamine receptor antagonists. Moreover, the dopamine-induced inhibitions of both the ACh release and the influx of (45)Ca disappeared in the samples treated with pertussis toxin at a dose sufficient to abolish the action of clonidine. 6. It is concluded that dopamine suppresses the influx of calcium ions into cholinergic nerve terminals via an activation of alpha2-adrenoceptors coupled with a pertussis toxin-sensitive GTP-binding protein, resulting in the decrease of ACh release from ileal synaptosomes of guinea-pigs.

Inhibitory effect of octopamine on the release of endogenous acetylcholine from isolated myenteric synaptosomes of guinea-pig

1. The effect of octopamine on the release of endogenous acetylcholine (ACh) from isolated ileal synaptosomal preparations of guinea-pigs was examined using high pressure liquid chromatography with electrochemical detection. Release of ACh was induced by substance P or by depolarization with high potassium (50 mmol/L) in medium containing atropine, propranolol and naloxone. 2. Octopamine produced a dose-dependent inhibition of substance P-induced ACh release. A similar inhibitory action of octopamine was found in the samples depolarized by high potassium as a reference. 3. The action of octopamine was not reversed by the dopamine receptor antagonists either for the DA-2 subtype, domperidone, or for the DA-1 subtype, SCH23390, or by haloperidol. However, idazoxan and yohimbine antagonized this octopamine-induced inhibition at concentrations sufficient to abolish the action of clonidine. 4. Failure of guanethidine or nomifensine to inhibit octopamine ruled out mediation by noradrenergic neurotransmitters. 5. Octopamine decreased the influx of [45Ca] stimulated by substance P into synaptosomal preparations and this was reversed by idazoxan or yohimbine at concentrations sufficient to block the action of clonidine. 6. Pertussis toxin abolished the inhibitory action of octopamine at a dose high enough to block the action of clonidine. 7. These results indicate that octopamine suppresses the influx of calcium ions into cholinergic nerve terminals of ileal synaptosomes of guinea-pigs via an activation of alpha 2-adrenoceptors coupled with a pertussis toxin-sensitive GTP-binding protein which results in a decrease of ACh release.

Effect of loxiglumide (CR 1505) on CCK-induced contractions and 3H-acetylcholine release from guinea-pig gallbladder

Release of [3H]-acetylcholine (3H-ACh) and muscle contractions in response to cholecystokinin (CCK) were measured and recorded simultaneously from isolated guinea-pig gallbladder. Cholecystokinin octapeptide (CCK8) (10(-10)-10(-7) M) enhanced the release of [3H]ACh and the contractions of the muscle. TTX (10(-6) M) inhibited the CCK-induced release of 3H-ACh by only 30%. In Ca(2+)-free medium CCK8 had no effect. Loxiglumide, (CR 1505), a newly synthesized nonpeptide CCK-A-receptor antagonist, D.L-(3,4-dichlorbenzoilamino)-5-/N-(3-methoxypropyl)-pentylamin o-5-oxo-pentanoi c acid, antagonized both the ACh-releasing effect of CCK and the contractions in a dose-dependent manner. The affinity (pA2) of CR 1505 to CCK-receptors, determined by the shift of the concentration-response curves for CCK8 was 8.36. It was 5 logarithmic orders higher than the pA2 of proglumide. The IC50 value of CR 1505 calculated by the CCK-induced release of 3H-ACh was 10 nM. The results suggest the existence not only of muscular CCK receptors but also neuronal receptors for CCK probably located on cholinergic nerves.

Effect of neurotensin on the canine gallbladder motility: in vivo and in vitro experiments

Neurotensin (NT) (10(-8)-10(-6)) exerted a dose-dependent increase in the tone and release of [3H]ACh in the guinea-pig gallbladder muscle strips but was inefficient in the canine gallbladder muscle strips. However, in conscious dogs NT (2.5-20 ng/kg intravenously (i.v.)) dose-dependently increased the gallbladder pressure. Similar was the effect of CCK8 (1-10 ng/kg i.v.) and carbachol (0.5-2 micrograms/kg i.v.). The NT- or CCK8-induced gallbladder pressure was inhibited by atropine (10-50 micrograms/kg i.v.) or hexamethonium (0.5-3 mg/kg i.v.). Somatostatin (1-2 micrograms/kg i.v.) or VIP (0.5-1 microgram/kg i.v.) also reduced or even abolished the NT- or CCK8-induced gallbladder pressure. The NT-induced increase of the tone of guinea-pig gallbladder preparations was accompanied by an increase of [3H]ACh release, suggesting the involvement of cholinergic innervation.

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

Action of myenteric GABAergic neurons in the guinea pig stomach

GABAergic neurons in the guinea pig stomach were localized immunocytochemically using antibodies against its synthesizing enzyme, L-glutamate decarboxylase (GAD). Numerous ganglion cells and nerve bundles in the myenteric plexus were found to be GAD-positive, while the longitudinal muscle, submucosa and mucosa were largely devoid of GABAergic innervation. The distribution of GABAergic neurons and their processes in both myenteric ganglia and circular muscle is rather uneven throughout the stomach. GABA elicited contraction of the longitudinal fashion (LF) strips of the body and antrum of guinea pig stomach in a concentration-dependent fashion with a maximal response at 10(-7) M and 10(-5) M, respectively. GABA-induced contraction appears to be mediated by the GABAA receptor and not the GABAB receptor, since the action is blocked by bicuculline, a GABAA receptor antagonist. Furthermore, GABAA agonists e.g. muscimol produced even stronger responses than GABA itself while GABAB agonist, (-)baclofen had no effect in eliciting muscle contraction. The GABAA receptor is desensitized by prolonged exposure to its agonists such as GABA and muscimol. However, no such desensitization was observed for the muscarinic cholinergic receptor. The GABA action in eliciting smooth muscle contraction seems to involve two components, one is scopolamine and tetrodotoxin (TTX) sensitive, and the other one is insensitive to scopolamine and TTX. It is proposed that the scopolamine- and TTX-sensitive component acts through a direct or indirect interaction between GABAergic and cholinergic neurons whereas the scopolamine- and TTX-insensitive component acts directly upon the smooth muscle.

The Leu13-motilin (KW-5139)-evoked release of acetylcholine from enteric neurones in the rabbit duodenum

1. Involvement of cholinergic mechanisms in the contractile response to Leu13-motilin (LMT, KW-5139) was investigated in rabbit duodenal segments, and longitudinal muscle-myenteric plexus (LM-MP) preparations preincubated wtih [3H]-choline. 2. Contractile response to LMT (0.1 nM-1 microM) consisted of an initial rapid (phasic) contraction and a tonic contraction slowly fading to a sustained plateau. LMT caused a concentration-dependent phasic contraction of rabbit isolated duodenal segments. The EC50 value was 2.5 nM and the maximum amplitude of the contraction was 103% of the response induced by acetylcholine (ACh, 100 microM). Neither tetrodotoxin nor atropine changed the EC50 value or the maximum amplitude of the response to LMT. 3. Both atropine and tetrodotoxin decreased the amplitude and accelerated fading of the tonic contraction produced by LMT. 4. LMT (30 nM-3 microM) induced an increase of 3H-outflow, in a concentration-dependent manner. The LMT-induced increase of 3H-outflow was prevented by removal of external Ca2+ or by the presence of tetrodotoxin. 5. Porcine motilin (10 nM-1 microM) also stimulated the release of 3H at a similar concentration-range to that seen with LMT. 6. Pretreatment with LMT (3 microM for 20 min) decreased LMT- and the porcine motilin-evoked release of 3H but did not alter the high K(+)-evoked release. 7. Our results suggest that LMT and porcine motilin stimulate the release of ACh from enteric neurones through the same receptor, and that the release of ACh plays a role in tonic components of contraction in the rabbit duodenum.

Effect of neurotensin on contractile activity and [3H]acetylcholine release in cat terminal ileum during different postnatal periods

The effect of neurotensin (NT) on the contractile activity of circular and longitudinal strips from the terminal ileum of 15-, 30-, 60-day-old and adult cats as well as on the resting and electrically-evoked release of [3H]acetylcholine (ACh) was studied. Radioactivity was measured by liquid scintillation spectrometry and the effect of NT was evaluated by the S2/S1 ratio. In the circular muscle strips NT (1-100 nM) inhibited spontaneous contractions in all age groups. In the longitudinal strips the effect of NT was concentration- and age-dependent. NT at a concentration of 1 nM had no effect on the spontaneous activity in 15-day-old cats, but in the other age groups in 70-80% of the cats it inhibited spontaneous contractions. The response to 10 and 100 nM NT was either biphasic (relaxation followed by contraction) or inhibitory: in 15-day-old cats the response was biphasic only and with increasing age the percentage of strips responding with inhibition of the contractions increased. Neither substances affecting adrenergic and cholinergic transmission nor TTX changed the inhibitory response to NT. The contractile component of the biphasic response was TTX-resistant in all age groups and was significantly decreased by scopolamine in 60-day-old and adult cats. NT increased both resting and electrically-evoked release of [3H]ACh which was not changed by TTX. In the presence of the peptide the S2/S1 ratio increased as NT-induced [3H]ACh release in the strips of adult cats was higher than that in young cats.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for inhibitory nicotinic and facilitatory muscarinic receptors in cholinergic nerve terminals of the rat urinary bladder

Cholinergic prejunctional modulatory receptors on parasympathetic nerves in the rat urinary bladder were studied by measuring 3H-acetylcholine (ACh) release in muscle strips from the bladder body. Electrical field stimulation markedly increased 3H-ACh overflow in strips preloaded with 3H-choline. Oxotremorine (1 microM), an M2 receptor agonist and DMPP (10 microM) a nicotinic (N) receptor agonist decreased the release of ACh (50% and 55% respectively); whereas McN-A 343 (50 microM) an M1 receptor agonist increased the release (33%), indicating the presence of three types of modulatory receptors. The anticholinesterase agent, physostigmine in concentrations of 1, 5 and 25 microM and neostigmine (5 microM) increased ACh release (44-710%). However a low concentration of physostigmine (0.05 microM) decreased release. Pirenzepine, an M1 muscarinic antagonist or atropine blocked the increased ACh release in physostigmine-treated strips, but in normal strips pirenzepine did not change release and atropine increased release. McN-A 343 or prolonged application (15 min) of DMPP increased ACh release (376% and 391% respectively) in physostigmine-treated strips. The response to McN-A 343 was blocked by pirenzepine. d-Tubocurarine (DTC), a nicotinic receptor blocker, enhanced ACh release in the presence of physostigmine but proved to be ineffective in normal preparations. These findings suggest that all three cholinergic receptors (M1 facilitatory, N inhibitory and M2 inhibitory) are activated by endogenous ACh in physostigmine treated preparations whereas only M2-inhibitory receptors are activated in normal preparations. It will be important in future studies to determine whether M1 and M2 mechanisms can also be activated under more physiological conditions in the bladder and whether they are present at other cholinergic synapses.

Galanin inhibits the potassium-evoked release of acetylcholine and the muscarinic receptor-mediated stimulation of phosphoinositide turnover in slices of monkey hippocampus

In the ventral hippocampus of Cynomologus monkey, galanin, a 29 amino acid long neuropeptide, reduced the potassium-evoked release of [3H]acetylcholine from slices preloaded with [3H]choline and diminished the carbachol-stimulated accumulation of [3H]inositol polyphosphates in hippocampal microprisms preincubated with myo-[2-3H]inositol. Using receptor autoradiography a strong, specific binding of iodinated galanin was observed in the molecular layer of the dentate gyrus. These may thus be the sites where galanin exerts its inhibitory effects on acetylcholine (ACh) release and phosphoinositide breakdown. These data provide evidence that galanin is a modulator of cholinergic function in septo-hippocampal neurons of primates.

Rapid kinetics of potassium-evoked release of acetylcholine from rat brain synaptosomes: analysis by rapid superfusion

The rapid kinetics of spontaneous and evoked [3H]acetylcholine efflux from synaptosomes was investigated using the technique of rapid superfusion. Synaptosomes were isolated from whole rat brain and the intraterminal pool of acetylcholine was radiolabeled by preincubation with [3H]choline. Synaptosomes were retained within the superfusion system on filter disks and superfused with Krebs-bicarbonate buffer, pH 7.4, at flow rates of 0.3-0.5 ml/s. These experimental conditions provided a mixing half-life of 119 ms and efficiency of superfusion of greater than 85%. The kinetics of tritium efflux was followed on the second and subsecond time scales by collection of serial 4.8-s and 50-ms samples for a total of 67.2 and 1.0 s, respectively. Superfusion for 48 s with isoosmotic Krebs buffer containing 10, 20, 30, 50, 75, and 100 mM potassium ion stimulated concentration-dependent tritium release. All of potassium-evoked release, but only 17% of spontaneous release, was calcium-dependent. Kinetic analysis of net (total minus spontaneous) potassium-stimulated release revealed a single calcium-dependent component of release that fit a single exponential function with a half-life of 12.7 s. Analysis of the area under the tritium efflux curves observed on the millisecond time scale revealed that 0.111, 0.550, and 0.614% net tritium release was evoked by superfusion for 750 ms with isoosmotic buffer containing 20, 50, and 100 mM KCl, respectively. Consistent with the results observed on the second time scale, a small fraction of spontaneous release and all of potassium-evoked release observed on the millisecond time scale were calcium-dependent. These data indicate that the technique of rapid superfusion can be utilized for the direct investigation of spontaneous and evoked [3H]acetylcholine release, as well as the factors that regulate this release from brain synaptosomes on the second and millisecond time scales.

Pharmacological differentiation of presynaptic M1 muscarinic receptors modulating acetylcholine release from postsynaptic muscarinic receptors in guinea-pig ileum

1. Effects of three muscarinic antagonists on electrically evoked ACh release and contractile response were investigated in longitudinal muscle strips of guinea-pig ileum suspended in an organ-bath and superfused with Krebs solution. ACh release was determined by a specific radioimmunoassay. 2. Telenzepine, a selective M1 muscarinic antagonist, increased the ACh release at a concentration of 100-fold less than that inhibiting the contractile response (10 vs 1000 nM). 3. AF-DX 116, a cardioselective M2 muscarinic antagonist, inhibited the contractile response at 10 microM, but did not affect the ACh release at this concentration. 4. (-)N-Methylscopolamine (NMS) did not affect the ACh release, but inhibited the contractile response at all concentrations tested (1-1000 nM), indicating (-)NMS can be used as an ileal specific postsynaptic muscarinic antagonist. 5. These data demonstrate that presynaptic muscarinic receptors modulating ACh release are distinct from postsynaptic ones involved in the contractile response and can be classified as M1 subtype.

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)

[3H]acetylcholine release from the guinea-pig distal colon: comparison with ileal [3H]acetylcholine release and effect of adrenoceptor stimulation

To study cholinergic function in the guinea-pig colon, resting and electrically evoked 3H release after preincubation with [3H] choline has been compared in colonic and ileal myenteric plexus preparations. Fractional spontaneous colonic 3H release was significantly higher than ileal 3H release, while the reverse was true for electrically evoked 3H outflow. Electrically evoked 3H outflow in the colon was linearly related to stimulation frequency (0.2-3 Hz range) and current intensity (300-600 mA range), while 3H outflow per pulse was inversely related to stimulation frequency. Electrically evoked 3H outflow was prevented in Ca2(+)-free solution, indicating that it probably mirrored neuronal exocytotic [3H]acetylcholine release. Both noradrenaline and clonidine concentration-dependently inhibited electrically evoked 3H outflow, clonidine being more potent but less efficacious than noradrenaline. For both noradrenaline and clonidine, the potency and efficacy for inhibition of 3H outflow were close to the values previously reported for the inhibition of electrically evoked endogenous acetylcholine output from colonic preparations. In conclusion, these data indicate that 3H release after incubation with [3H]choline is a valid alternative to measurement of endogenous acetylcholine output to study colonic cholinergic neuronal function in the guinea-pig.

Regulation of the substance P-induced contraction via the release of acetylcholine and gamma-aminobutyric acid in the guinea-pig urinary bladder

1. The action of substance P (SP) on the release of gamma-aminobutyric acid (GABA) and acetylcholine (ACh) and on contraction were studied in strips of the guinea-pig urinary bladder. Substance P induced a dose-dependent contraction of strips of guinea-pig urinary bladder (EC50 = 1.2 x 10(-9) M). This contraction was not altered by tetrodotoxin, but with a dose of 10(-9) M and less, there was a complete inhibition by 10(-6) M) atropine. Contractions initiated by 3 x 10(-9) M) SP or more were partly inhibited by atropine. The EC50 value of substance P in the presence of atropine was 7.0 x 10(-9) M. 2. Substance P induced a Ca2+-dependent and tetrodotoxin-resistant release of [3H]-acetylcholine (ACh) from strips of urinary bladder preloaded with [3H]-choline (EC50 = 4.9 x 10(-10) M), and this release was antagonized by [D-Pro2,D-Trp7,9] substance P. 3. Bicuculline increased the substance P-induced contraction and the release of [3H]-ACh from the strips. 4. Substance P induced a Ca2+-dependent and tetrodotoxin-sensitive release of [3H]-gamma-aminobutyric acid (GABA) from strips preloaded with [3H]-GABA (EC50 = 2.6 x 10(-9) M), and this release was antagonized by [D-Pro2,D-Trp7,9] substance P. 5. Therefore, substance P appears to exert excitatory effects on the contractility of urinary bladder predominantly by stimulating its own receptor located on the cholinergic nerve terminals. GABA released by substance P inhibits stimulation of the cholinergic neurone. However, the direct action of substance P on the cholinergic neurone is more potent that the indirect action via GABA release.

Cholinergic neuromodulation by endothelin in guinea pig ileum

The effect of endothelin on cholinergic neuroeffector transmission in guinea pig ileum was investigated. Endothelin was shown to inhibit the nerve-induced contractions and concomitantly to increase the basal muscle tone. Furthermore, endothelin inhibited the nerve-induced release of [3H]acetylcholine whereas the contractile response to exogenous acetylcholine was enhanced. In conclusion, our findings suggest that endothelin is a modulator of cholinergic neuroeffector transmission in guinea pig ileum with possible action via both inhibitory prejunctional and stimulatory postjunctional mechanisms.

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.

Involvement of protein kinase C in the Ca2+-dependent vesicular release of GABA from central and enteric neurons of the guinea pig

The involvement of protein kinase C (PKC) in the release of endogenous gamma-aminobutyric acid (GABA) was studied using slices of deep cerebellar nucleus and strips of small intestine from the guinea pig. 12-O-tetradecanoylphorbol 13-acetate (TPA), but not 4 alpha-phorbol-12,13-didecanoate (4 alpha-PDD), potentiated the high K+-evoked release of GABA from both preparations in the presence of tetrodotoxin. Ouabain evoked the release of GABA from both preparations, and this release was not altered by TPA. Therefore, the activation of protein kinase C potentiates the Ca2+-dependent vesicular release of GABA from nerve terminals of the central and enteric GABAergic neurons of the guinea pig.

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.

Role of protein kinase C in the vesicular release of acetylcholine and norepinephrine from enteric neurons of the guinea pig small intestine

The involvement of protein kinase C in the release of [3H]acetylcholine (ACh) and [3H]norepinephrine (NE) was studied in strips of guinea pig small intestine. 12-O-tetradecanoyl phorbol 13-acetate (TPA), but not 4 alpha-phorbol-12,13-didecanoate (4 alpha-PDD) potentiated the A23187-evoked release of [3H]ACh and [3H]NE from the strips of small intestine preloaded with [3H]choline and [3H]NE, and the potentiating effect of TPA was inhibited by polymyxin B. High K+-evoked releases of [3H]ACh and [3H]NE in the presence of tetrodotoxin were also potentiated by TPA. These TPA-induced potentiations of the evoked release were greater at a low concentration of external Ca2+ (0.5 mM) than at a high concentration (2 mM). Ouabain induced the release of these neurotransmitters both in the absence and presence of the low concentration of external Ca2+. The ouabain-evoked release was not altered by TPA. These results indicate that the activation of protein kinase C potentiates the vesicular release of ACh and NE at low Ca2+ concentration from the nerve terminals of enteric neurons in the guinea pig small intestine.

The toxic effect of an AChE-inhibitor on the cholinergic nervous system in airway smooth muscle

Excessive cholinergic stimulation of presynaptic muscarinic cholinergic receptors, due to complete inhibition of acetylcholinesterase (AChE) by O-(1,2,2-trimethylpropyl)-methyl-phosphonofluoridate (soman), reduced the release of acetylcholine (ACh) from cholinergic nerves in rat bronchi by almost 25%. Furthermore, long-term (40 h) exposure by inhalation of soman (0.45-0.63 mg/m3) reduced the contraction of bronchi induced by ACh by approximately 70%. This is probably due to reduction of the number of muscarinic cholinergic receptors, since there was a reduction in the binding capacity (Bmax) of [3H]QNB by 40%, without any changes in the dissociation constant (Kd).

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

1. Secretion of [3H]acetylcholine was studied in the guinea-pig ileum longitudinal muscle-myenteric plexus preparation. The transmitter stores of the cholinergic nerves were labelled by pre-incubation with [3H]choline. The preparation was mounted in an organ bath and superfused with Tyrode solution containing hemicholinium-3 and eserine. [3H]Acetylcholine secretion was evoked by electrical stimulation (0.5 Hz, 150 shocks). 2. 8-Bromo cyclic AMP, the adenylate cyclase activator forskolin, and the cyclic nucleotide phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine enhanced the [3H]acetylcholine secretion in a concentration-dependent manner. The values of 'maximal enhancement' calculated were similar, viz. 200-300% of control. 3. 8-Bromo cyclic GMP reduced the [3H]acetylcholine secretion. 4. The 'maximal enhancement' of 3-isobutyl-1-methylxanthine was not altered by the presence of forskolin (25 mumol/l) suggesting a common mechanism of action, i.e. elevation of endogenous cyclic AMP levels. 5. The muscarinic acetylcholine receptor antagonist atropine enhanced the [3H]acetylcholine secretion with a 'maximal enhancement' of 506% of control. Presence of neither forskolin (25 mumol/l) nor 3-isobutyl-1-methylxanthine (5 mmol/l) altered the 'maximal enhancement' for atropine. 6. In contrast, atropine (1 mumol/l) and 4-aminopyridine (0.5 mmol/l) additively enhanced the [3H]acetylcholine secretion. 7. The results suggest that neuronal cyclic AMP may be involved in muscarinic acetylcholine receptor-mediated control of [3H]acetylcholine secretion in guinea-pig ileum myenteric plexus.

Galanin inhibits acetylcholine release in the ventral hippocampus of the rat: histochemical, autoradiographic, in vivo, and in vitro studies

A high density of galanin binding sites was found by using 125I-labeled galanin, iodinated by chloramine-T, followed by autoradiography in the ventral, but not in the dorsal, hippocampus of the rat. Lesions of the fimbria and of the septum caused disappearance of a major population of these binding sites, suggesting that a large proportion of them is localized on cholinergic nerve terminals of septal afferents. As a functional correlate to these putative galanin receptor sites, it was shown, both in vivo and in vitro, that galanin, in a concentration-dependent manner, inhibited the evoked release of acetylcholine in the ventral, but not in the dorsal, hippocampus. Intracerebroventricularly applied galanin (10 micrograms/15 microliters) fully inhibited the scopolamine (0.5 mg/kg, s.c.)-stimulated release of acetylcholine in the ventral, but not in the dorsal, hippocampus, as measured by microdialysis technique. In vitro, galanin inhibited the 25 mM K+-evoked release of [3H]acetylcholine from slices of the ventral hippocampus, with an IC50 value of approximately 50 nM. These results are discussed with respect to the colocalization of galanin- and choline acetyltransferase-like immunoreactivity in septal somata projecting to the hippocampus.

Enteric gamma-aminobutyric acid-containing neurons and the relevance to motility of the cat colon

gamma-Aminobutyric acid (GABA)-containing neurons were identified and the functional relevance in the motility of the colon was studied. Autoradiography of the cat colon treated with [3H]GABA demonstrated scattered neurons in the myenteric plexus selectively labeled with [3H]GABA. Electrical transmural stimulation of the isolated cat colon led to an increase in the Ca2+-dependent, tetrodotoxin-sensitive release of endogenous GABA. gamma-Aminobutyric acid increased the amplitude of rhythmic contractions of the circular muscle of the colon and also the release of acetylcholine, which was Ca2+-dependent and tetrodotoxin-sensitive. Scopolamine inhibited the GABA-evoked rhythmic contractions, without effect on the evoked release of acetylcholine. Bicuculline and furosemide reduced the amplitude of spontaneous rhythmic contractions and the tone, which was reversed by GABA. These results suggest that GABA-containing neurons are involved in the control of motility of the cat colon, due to the stimulation of cholinergic neurons.

D2-dopamine receptor-mediated inhibition of intracellular Ca2+ mobilization and release of acetylcholine from guinea-pig neostriatal slices

The effect of dopamine receptor activation on electrically- or high K+ (30 mM)-evoked neurotransmitter release and rise in intracellular Ca2+ concentration was investigated using slices of guinea-pig neostriatum. A specific D2-dopamine receptor agonist, LY-171555 (a laevorotatory enantiomer of LY-141865: N-propyl tricyclic pyrazole) at 10(-6) M inhibited electrical stimulation- and high K+-evoked release of [3H]-acetylcholine ([3H]-ACh) to 47.7 +/- 6.0% and 54.1 +/- 5.0% of control, respectively. The maximal inhibition by LY-171555 at 10(-5) M was 54.8 +/- 5.1% reduction of the control. The half-maximal effective concentration (EC50) of LY-171555 for the inhibition of [3H]-ACh release was 2.3 X 10(-7) M. A specific D2-dopamine receptor antagonist, (-)-sulpiride (10(-7) M) reversed the inhibition of [3H]-ACh release induced by LY-171555. A specific D1-dopamine receptor agonist, SK&F 38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-benzazepine) (10(-5) M) had no effect on the release of [3H]-ACh. LY-171555 (10(-6) M) also inhibited the high K+-evoked endogenous glutamate release, by 47% of control. This inhibitory effect was reversed by (-)-sulpiride (10(-7) M). We used a fluorescent, highly selective Ca2+ indicator, 'quin 2' to measure intracellular free Ca2+ concentrations ([Ca2+]i). Electrical stimulation of slices preloaded with quin 2 led to an elevation of relative fluorescence intensity and this response was reduced by the removal of Ca2+ from the bathing medium. These results indicate that the enhanced elevation in fluorescence intensity in the quin 2-loaded slices reflects the increase of intracellular free Ca2+ concentration, [Ca2+]i. The mixed D1- and D2-receptor agonist, apomorphine and LY-171555 inhibited the increase of [Ca2+]i induced by electrical stimulation or high K+ medium, in a concentration-dependent manner, while SK&F 38393 did not affect the increase of [Ca2+]i. The maximal inhibitory effect of LY-171555 at 3 X 10(-5) M was 35 +/- 3% reduction in control values. The inhibitory effect of LY-171555 was antagonized by (-)-sulpiride (10(-7) M). There was a high correlation (r = 0.997, P less than 0.05) between the D 2-receptor-mediated inhibition of the stimulated rise of [Ca2+]i and [3H]-ACh release. When the slices were superfused with the Ca2+-free medium containing EGTA (10(-4) M) for 5 min, the rise in [Ca2+]i was markedly suppressed to 18.0% of control by LY-171555 (10(-6) M). These data indicate that activation of the D2-dopamine receptor suppresses the elevation of [Ca2+]i induced by depolarizing stimuli. This may be due to inhibition of mobilization of Ca2+ from the intracellullar store. We propose that the D2-receptor-mediated inhibition of transmitter release is probably due to a reduction in intracellular Ca2+ mobilization.