Selective activation of nicotinic and muscarinic transmission in cardiac sympathetic ganglion of the dog.

The myocardial contractile responses to electrical stimulation of different segments of the thoracic sympathetic trunk in the pentobarbital-anesthetized dog were studied. Electrical stimulation of the sympathetic trunk between the second and third thoracic ramus and caudal limb of the ansa subclavia elicited myocardial contractile responses which were not completely antagonized by the ganglionic blocking agent chlorisondamine. Atropine in addition to chlone had no effect. When the sympathetic trunk between the third and fourth ramus was electrically stimulated, the response was partially antagonized by atropine as well as by chlorisondamine alone. From these results, it is suggested that impulses travelling via the third thoracic ramus or above it mediate myocardial contractile responses in which the nicotinic type response predominates whereaas impulses travelling via the fourth thoracic ramus or below it mediate myocardial contractile responses in which the muscarinic type predominates. additionally, evidence presented indicates that a major proportion of both nicotinic and muscarinic receptors mediating myocardial contractile changes are located in the caudal cervical ganglion rather than the stellate ganglion of the dog.     

Presynaptic muscarinic receptors inhibiting active acetylcholine release in the bullfrog sympathetic ganglion.

1 The effects of bethanechol and atropine on the release of acetylcholine (ACh) from bullfrog sympathetic preganglionic nerve terminals were examined electrophysiologically. 2 Bethanechol (1 mM) caused no depolarization of sympathetic preganglionic nerve terminals, whereas carbachol or ACh in the same concentration induced marked depolarizations of these terminals. 3 Bethanechol (10 microM) depressed the amplitude of fast excitatory postsynaptic potentials (e.p.s.ps) recorded in Ca2+-high Mg2+ solution, without depolarizing ganglion cells. The quantal content measured from these fast e.p.s.ps by the variance method showed a significant reduction. 4 Amplitudes of both miniature e.p.s.ps and ACh-potentials induced by iontophoresis of ACh were not affected by addition of bethanechol (10 microM). 5 The depressant effect of bethanechol (10 microM) on fast e.ps.ps disappeared in the presence of atropine (3 microM). 6 Atropine (3 microM) increased the quantal content measured from fast e.p.s.ps recorded in low Ca2+-high Mg2+ solution. 7 The depressant effect of bethanechol (10 microM) on fast e.p.s.ps was unaffected by alpha-adrenoceptor blocking agents (phenoxybenzamine (10 microM) or phentolamine (10 microM). 8 These results suggest that presynaptic nerve terminals in bullfrog sympathetic ganglia possess a muscarinic receptor which inhibits active release of ACh.     

Avoidance facilitation in adult mice by prenatal administration of the nootropic drug oxiracetam

CD-1 mice prenatally exposed to saline solution or to the nootropic drug oxiracetam (50 mg/kg during the whole pregnancy) were tested, when adults, for locomotor activity and for shuttle-box avoidance acquisition. Prenatal drug exposure produced long-lasting effects, evident in mature offspring. At the age of two months, mice prenatally exposed to oxiracetam showed a slight but significant reduction in locomotor activity. At the age of three months, these animals exhibited higher performances than control mice in avoidance acquisition.     

Presynaptic, muscarinic inhibition of non-adrenergic, non-cholinergic neuromuscular transmission in the chicken rectum.

Cholinergic inhibition of the non-adrenergic, non-cholinergic ( NANC ) transmission was investigated in the chicken isolated rectum with Remak's nerve attached. Stimulation of Remak's nerve (RT stimulation) at frequencies higher than 5 Hz elicited a late, slow contraction of the rectum in addition to an initial, fast NANC contraction. The late, slow contraction was blocked by atropine (0.25 microgram ml-1), potentiated by physostigmine (50 ng ml-1) and accompanied by an overflow of acetylcholine into the vascular perfusate, indicating the existence of cholinergic innervation to the rectum via Remak's nerve. RT stimulation (10 pulses at 0.5-1.0 Hz) elicited NANC -mediated excitatory junction potentials (e.j.ps). The e.j.p. amplitude declined at the second stimulus and then increased to reach a plateau. Atropine, by abolishing this decrease in amplitude, increased the mean amplitude of the e.j.ps during trains of stimuli but atropine did not affect the amplitude of the first e.j.p. Physostigmine reduced the mean e.j.p. amplitude, and this action was readily antagonized by atropine. A single intramural nerve stimulation delivered 2s or less before RT stimulation with trains of stimuli, suppressed the amplitude of the first e.j.p. of the train. This effect was abolished by atropine. Atropine in concentrations high enough to affect the e.j.p. amplitude had no effect on the resting membrane potential, the threshold for generating an action potential, or membrane resistance of the smooth muscle. It is concluded that RT stimulation at low frequencies causes the release of acetylcholine simultaneously with the NANC transmitter. The released acetylcholine acts mainly on prejunctional muscarinic receptors and mediates an inhibitory effect on the release of the NANC transmitter.     

Frequency-dependent depression of ganglionic transmission by propranolol and diltiazem in the superior cervical ganglion of the guinea-pig.

1 Effects of propranolol and diltiazem on ganglionic transmission in the superior cervical ganglion of the guinea-pig were investigated with intracellular recording techniques. 2 Propranolol and diltiazem (5 X 10(-6)-10(-5) M) induced a transmission failure in the ganglion upon preganglionic nerve stimulation at high frequency (25-30 Hz) without affecting action potentials induced by direct stimulation of the soma membrane, or potentials induced by iontophoretically applied acetylecholine. 3 The results suggest that propranolol and diltiazem may act on preganglionic nerve terminals to inhibit Ca2+ influx in a frequency-dependent manner. These agents may depress excess sympathetic activity without much affecting normal ganglionic transmission.     

Evidence for sympathetic, purinergic transmission in the mesenteric artery of the dog.

Electrical transmural stimulation of isolated mesenteric artery of the dog produced a transient contraction which consisted of adrenergic and non-adrenergic components. In contrast to the adrenergic component, the nonadrenergic component was resistant to prazosin and other adrenoceptor-blocking agents. However, the nonadrenergic component was completely blocked by guanethidine and by desensitization with alpha,beteta-methylene-ATP (alpha,beta-MeATP). Desensitization induced by alpha,beta-MeATP also inhibited the contractile response to ATP but not the adrenergic responses induced by electrical transmural stimulation and exogenous noradrenaline. These results suggest that the nonadrenergic contraction induced by electrical transmural stimulation is a sympathetic, purinergic response.     

Production of decremental oscillations following spike potential by muscarinic drugs, histamine and bradykinin in cat sympathetic ganglion.

The aim of our study was to analyse drug-induced decremental oscillations following spike potential evoked by a preganglionic stimulus in the superior cervical ganglion of the cat in situ. Ganglionic compound action potentials evoked by preganglionic stimulation and postganglionic asynchronous discharge induced by drugs injected into the ganglionic vascular bed were registered. The muscarine like drugs 4-(m-chlorophenylcarbamoyloxy)-2-butynyl trimethylammonium chloride (McN-A-343), N-benzyl-3-pyrrolidyl acetate methobromide (AHR-602), and pilocarpine, and histamine and bradykinin induced decremental oscillations following spike potential occurring at the time of the late negative wave onset. These drugs evoked slowly developing asynchronous discharge of low amplitude; time of onset and duration of asynchronous discharge and of decremental oscillations were similar. Atropine, which decreases late negative wave amplitude, depressed decremental oscillations induced by McN-A-343, however it did not inhibit histamine-induced decremental oscillations. The ganglion-depolarizing drug isoprenaline, which does not induce asynchronous discharge, evoked increase in the late negative wave amplitude without decremental oscillations. Nicotine induced decrease in the late negative wave amplitude with no concomitant decremental oscillations. Our results indicate that drugs inducing decremental oscillations following spike potential have the common characteristic of causing asynchronous discharge by stimulation of non-nicotinic receptor sites in the cat superior cervical ganglion. It is suggested that a process closely related to the stimulus-evoked release of acetylcholine from presynaptic endings is responsible for the appearance of decremental oscillations by modulating the discharge originating at non-nicotinic receptor sites.     

Effect of tilting on the pressor responses to McN-A-343, a muscarinic sympathetic ganglion stimulant.

1 In anaesthetized cats and dogs treated with mecamylamine, the pressor response to McN-A-343 was increased when the animals were changed from a supine to a head-up, tilted position. This potentiation was not seen in rats. 2 The potentiation of the McN-A-343 pressor response was not affected by propranolol, destruction of the brain, or removal of the intestines, spleen or adrenal glands. It was promptly abolished by applying pressure to the lower half of the tilted animal. No increase in the pressor response to McN-A-343 occurred when cats were tilted head down. The potentiated response in tilted cats was abolished by atropine. 3 The pressor effects of adrenaline, noradrenaline, tyramine and angiotensin in cats treated with mecamylamine were either reduced or unchanged when the animal was changed from the supine to the tilted position. In one cat not treated with mecamylamine in which orthostatic hypotension occurred, tilting potentiated the pressor responses to dimethyl phenylpiperazinium iodide. 4 In cats anaesthetized with chloralose, the reflex pressor response to bilateral carotid occlusion was reduced by tilting. After mecamylamine treatment the residual atropine-sensitive response to carotid occlusion was potentiated when the animal was placed in the tilted position. 5 These results suggest that muscarinic stimulation of sympathetic ganglia by McN-A-343 raises blood pressure by predominantly reducing venous capacity, in contrast to noradrenaline and angiotension which increase blood pressure mainly by arterial vasoconstriction. 6 It is not clear whether this is a general property of sympathetic ganglionic stimulation or is restricted to stimulation of muscarinic sites.     

Inhibition of cardiac sympathetic neurotransmission by histamine in the dog is mediated by H1-receptors.

1 The role of histamine H1- and H2-receptors in mediating prejunctional inhibition of cardiac sympathetic neurotransmission and histamine-induced coronary vasodilatation were investigated in perfused dog hearts in situ. 2 Intra-arterial injections of histamine into the right coronary artery during the resting state caused slightly positive chronotropic responses in doses larger than 1 microgram. 3 Histamine in doses of 0.1 to 10 micrograms into the right coronary artery reduced the tachycardia resulting from electrical stimulation of the cardiac sympathetic nerves. 4 Intra-coronary infusions of chlorpheniramine (300 micrograms/min) significantly reduced the histamine-induced depression of cardiac nerve stimulation. The effects of cimetidine (300 micrograms/min) and metiamide (300 micrograms/min) were less pronounced. 5 Histamine (1 to 10 micrograms) further increased heart rate resulting from the continuous intra-coronary infusion of noradrenaline (1 or 3 micrograms/min). 6 Intra-arterial injections of histamine (0.1 to 10 micrograms) caused an increase in coronary blood flow in a dose-dependent manner. This was partially inhibited by intra-coronary infusion of chlorpheniramine (10 to 300 micrograms/min) and by cimetidine (10 to 300 micrograms/min). The combination of both drugs (10 to 100 micrograms/min of each) caused a larger inhibition. 7 The present results suggest that the histamine-induced depression of heart rate during cardiac sympathetic nerve stimulation is due to a prejunctional effect mediated mainly by H1-receptors. Histamine-induced coronary vasodilatation in the dog is mediated both by H1- and H2-receptors.     

Effects of dopamine receptor and alpha 2-adrenoceptor agonists and antagonists on muscarinic receptor stimulation in cardiac sympathetic ganglia of the dog.

The effects of dopamine (DA)-receptor and alpha 2-adrenoceptor agonists and antagonists on ganglionic muscarinic stimulation were examined in anesthetized dogs. All drugs were injected or infused intra-arterially into the blood supply of the cardiac sympathetic ganglia. The muscarinic agonists McN-A-343 (10, 20, and 30 micrograms) and muscarine (1, 2, and 3 micrograms) increased heart rate. The muscarinic receptor stimulation elicited by McN-A-343 or muscarine was significantly inhibited by infusion of the DA2-receptor agonist quinpirole (3 and 10 micrograms/min) but not by the DA1-receptor agonist SK&F 38393 (10 and 30 micrograms/min). The alpha 2-adrenoceptor agonist BHT 933 (3 and 10 micrograms/min) also inhibited muscarinic receptor stimulation. The DA2-receptor antagonist domperidone (10 micrograms/min) and the alpha 2-adrenoceptor antagonist yohimbine (1 micrograms/min) had no effects on muscarinic receptor stimulation, but they antagonized the inhibitory effects of quinpirole and BHT 933, respectively. The nicotinic transmission elicited by preganglionic cardiac sympathetic nerve stimulation (1 and 2 Hz) was also inhibited by DA-receptor and alpha 2-adrenoceptor agonists and antagonists. These results suggest that DA2-receptors and alpha 2-adrenoceptors suppress muscarinic transmission as well as nicotinic transmission in the cardiac sympathetic ganglia of the dog.     

Effect of nebracetam on nicotinic and muscarinic acetylcholine receptors expressed in Xenopus oocyte by injecting exogenous mRNA.

Using voltage- and current-clamp methods the effects of nebracetam 4-aminomethyl-1-benzylpyrrolidine-2-one hemifumarate, WEB 1881 FU, CAS 118607-07-1), a new agent with nootropic property, on the nicotinic (nAChRs) and muscarinic acetylcholine receptors (mAChRs) were studied, which were expressed in Xenopus oocytes by injecting E. electricus mRNA and rat brain mRNA, respectively. Simultaneous application of nebracetam (0.03-2 mmol/l) with acetylcholine (ACh) (0.01-1 mmol/l) inhibited the ACh-responses of both nAChRs and mAChRs, whereas preapplication of these concentrations of nebracetam for 30 s to 1 min potentiated such inhibition. A simple competitive inhibition model for the effects of both drugs simultaneously applied yielded the inhibition constant, K1 of 0.419 and 0.212 mmol/l for nAChRs and mAChRs, respectively, indicating that the action on mAChRs is a little more potent than on nAChRs. Nebracetam induced a concentration-dependent slight increase in inward currents on mAChRs but not on nAChRs. It is suggested that the direct effects of nebracetam on nAChRs and mAChRS, which were induced only by a rather high concentration, as compared with the clinically expected plasma level, may be a contributing factor to the clinical effectiveness of the drug only if there is some critical change in the sensitivity to the drug.     

Evidence for transmission through sympathetic ganglia mediated by muscarinic receptors in anesthetized guinea pigs

3-Mercaptopropionic acid (3-MP), an inhibitor of the synthesis of GABA, acts in the central nervous system to increase arterial pressure (+50-60 mmHg) in anesthetized guinea pigs, apparently by sympathoadrenal activation. However, blockade of nicotinic receptors in autonomic ganglia with hexamethonium (10 or 20 mg/kg, i.v.) failed to effect any degree of sustained reversal of increases in blood pressure. Infusion of atropine methyl bromide (1 mg/kg, i.v.) likewise was without effect when administered alone, but completely reversed the hypertension induced by 3-MP when given after treatment with hexamethonium. These findings suggest that ganglionic transmission through either the "classical" nicotinic pathway or a muscarinic pathway is sufficient to sustain the sympathetically-mediated pressor response elicited by 3-MP.     

Evidence that 5-HT1D receptors mediate inhibition of sympathetic ganglionic transmission in anaesthetized cats.

In anaesthetized cats, 5-carboxamidotryptamine (5-CT) or 5-hydroxytryptamine (5-HT) (0.3-300 micrograms kg-1,i.v.) inhibited the postganglionic compound action potential evoked by preganglionic electrical stimulation (0.5 Hz) with a similar potency in the stellate and splanchnic ganglia. In the 5-HT experiments transmission thorough the inferior mesenteric ganglia was also recorded. The maximal inhibitory effect of 5-HT was greater on the stellate and splanchnic ganglia (60 +/- 4 and 52 +/- 5%) than on the inferior mesenteric (15 +/- 2%). The effects of 5-HT were unaffected by pretreatment with antagonists (1 mg kg-1;i.v.) for 5-HT2 (BW501C67), 5-HT1A (WAY-100635) and 5-HT3 receptors (ondansetron). However, responses to both 5-HT and 5-CT were attenuated significantly by GR127935 (1 mg kg-1) except the responses to 5-HT at the inferior mesenteric ganglia. These results are consistent with the involvement of 5-HT1D receptors mediating inhibition of sympathetic ganglionic transmission in vivo.     

Specific receptors for bradykinin-induced cardiac sympathetic chemoreflex in the dog

Bradykinin (BK, 0.03-1 microgram), capsaicin (1 microgram) or potassium chloride (KCl, 13 mumol) applied to the epicardium of the left ventricle of anaesthetized, open-chest dogs, caused reflex tachycardia and pressor effects, whereas des-Arg9-BK (1-100 micrograms), a selective bradykinin B1-receptor agonist, failed to produce any cardiovascular response. Superfusion of the epicardium with a selective B1-receptor antagonist, des-Arg9-[Leu8]BK (50-100 micrograms/min) had no effect on reflex responses to epicardial BK (0.03-0.1 microgram). However, the selective B2-receptor antagonist, D-Arg-[Hyp3,Thi5,8,D-Phe7]BK (10-25 micrograms/min) abolished the reflex effects of 0.03 and 0.1 microgram BK and reduced by 50 to 70% the responses to 1 microgram BK. Another B2-receptor antagonist [Thi6,9,D-Phe8]kallidin (10-50 micrograms/min) also reduced (30-70%) responses to 1 microgram BK. The antagonism was reversible and specific for BK since reflex responses to epicardial application of either capsaicin or KCl were not affected. The results indicate that BK interacts with B2-receptors, probably located on terminals and/or axons of sympathetic afferents supplying the dog heart, to activate a cardiac sympathetic chemoreflex.     

Effects of ACTH4-10 on synaptic transmission in frog sympathetic ganglion.

The influence of ACTH4-10, a behaviourally active fragment of adrenocorticotropic hormone (ACTH) devoid of endocrine activity, on synaptic transmission in the paravertebral sympathetic ganglion of the frog was investigated. Postsynaptic potentials evoked by electrical stimulation of pregnanglionic nerves were recorded using a sucrose gap method. Fast excitatory postsynaptic potentials (EPSPs), which are mediated via nicotinic cholinergic synapses, were not affected by 10(-6) M ACTH4-10. Application of ACTH4-10 in a concentration as low as 10(-8) M for 60 min caused a marked augmentation of the amplitude of slow inhibitory postsynaptic potentials (IPSPs) which are mediated via dopaminergic synapses. The increase in amplitude developed gradually after a latency of 60--90 min and outlasted the application of the peptide. In addition, ACTH4-10 at 10(-6) M increased the hyperpolarising response of the ganglion to exogenous dopamine, as studied by a micro-application method. There was no significant effect of ACTH4-10 on the muscarinic cholinergic depolarising response of the ganglion towards exogenous acetylcholine. The behaviourally active vasopressin fragment DG-LVP (10(-6) M) had no effect on slow IPSPs. The results demonstrate that ACTH4-10 specifically affects slow synaptic inhibition in frog sympathetic ganglion, probably by acting upon the postsynaptic membrane. The possibility is discussed that ACTH4-10 affects one of the intermediate steps between dopaminergic receptor interaction and generation of the slow IPSP.