Presynaptic effects of methoctramine on release of acetylcholine

Muscarinic receptor activation is a prerequisite for the endogenous release of nitric oxide modulating nicotinic transmission within the coeliac ganglion in the rabbit

The aim of the present study was to investigate whether the activation of muscarinic receptors is a preliminary step to the endogenous release of nitric oxide modulating nicotinic transmission within the prevertebral ganglia. This work has been performed 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 supramaximal intensity was applied to the splanchnic nerves, gradual depression of fast nicotinic transmission occurred: the pulses do not systematically elicit action potentials, but very often elicit excitatory postsynaptic potentials only. The use of pharmacological agents that interfere with the nitric oxide pathway such as L-arginine (precursor of nitric oxide) or 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (nitric oxide scavenger) demonstrated that nitric oxide modulates this depression phenomenon by facilitating or inhibiting the nicotinic transmission of the ganglionic neurons. A nitric oxide donor (diethylamine/nitric oxide complex) induced an inhibition of the nicotinic synaptic transmission. In the presence of the muscarinic receptors antagonist atropine, L-arginine and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide failed to modify the nicotinic transmission of the ganglionic neurons but diethylamine/nitric oxide complex was still able to inhibit it. These results demonstrate that in the coeliac ganglion, the activation of muscarinic cholinergic receptors is a prerequisite for the activation of neuronal nitric oxide synthase in preganglionic fibres. The nitric oxide released then exerts a facilitation or an inhibition of the nicotinic transmission of the ganglionic neurons. Atropine triggered a facilitation of the nicotinic transmission when superfused alone and an inhibition when superfused in the presence of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. These results confirm that muscarinic receptors activate the nitric oxide pathway modulating the nicotinic transmission of the prevertebral neurons. Our results also demonstrate that when the nitric oxide pathway is blocked, activation of muscarinic receptors leads to facilitation of the nicotinic transmission. Our study brings new insights concerning the modulation by nitric oxide and by muscarinic receptors of the synaptic transmission within the prevertebral ganglia.

Effects of M2 antagonists on in vivo hippocampal acetylcholine levels

There is evidence that muscarinic receptors of the M2 subtype are presynaptic autoreceptors that modify the release of acetylcholine (ACh) through a negative feedback mechanism. Blocking these receptors by selective antagonists may therefore lead to increased ACh release. This in vivo microdialysis study examined the effects of three M2 antagonists, AF-DX 116, AF-DX 384, and AQ-RA 741, on hippocampal cholinergic neurotransmission. Drug (2, 4, 8, or 16 microM) or vehicle (Ringer's solution) was perfused via a microdialysis probe into the CA1 hippocampal region of conscious male Fischer 344 rats. Levels of ACh and choline were assessed by HPLC-EC. When the dose was expressed in K1 multiples, all drugs (except AQ-RA 741 at the two highest concentrations) were found to be on the same linear dose-response curve. Choline levels were not affected by drug administration. All three compounds elevated ACh levels in a similar K1-normalized dose-response fashion, strongly supporting the concept that the proposed presynaptic mechanism of action is indeed based on the same M2 receptor. Such elevations of ACh may not only improve performance on memory tasks, but may also have therapeutic advantages in conditions of cholinergic hypofunction, such as Alzheimer's disease.

Involvement of neurokinins in the non-cholinergic response to activation of 5-HT3 and 5-HT4 receptors in guinea-pig ileum

1. The involvement of neurokinins in the non-cholinergically-mediated contractile response induced by stimulation of 5-HT3 and 5-HT4 receptors has been examined in the longitudinal muscle-myenteric plexus preparation of the guinea-pig ileum. 2. The 5-HT3 receptor agonist, 2-methyl-5-hydroxytryptamine (2-methyl-5-HT), showed a lower potency in this preparation than the more selective 5-HT4 receptor agonist 5-methoxytryptamine. The effect of both drugs was markedly reduced by atropine. 3. Substance P (SP) and neurokinin B (NKB) produced biphasic concentration-response curves in the preparation. Neurokinin A (NKA), the NK1 receptor agonist, [Sar9,Met(O2)11]SP and the NK3 receptor agonist, senktide yielded monophasic concentration-response curves. 4. After desensitization of the NK1 receptor with SP or [Sar9,met(O2)11]SP, in the presence of atropine, the contractile response to 2-methyl-5-HT was entirely blocked. Desensitization of NK3 receptors with NKB, also in the presence of atropine, fully suppressed the 5-HT4 receptor-mediated contraction evoked by 5-methoxytryptamine. 5. In preparations prelabelled with [3H]-choline, SP produced a concentration-dependent increase in tritium overflow, an index of [3H]-acetylcholine release, while an inverse relationship was found with NKB. At low neurokinin concentrations, the releasing effect of NKB was much more marked. 6. It is suggested that in the response to 5-HT3 receptor stimulation, there is a role for SP and acetylcholine. NKB appears to be preferentially involved in the release of acetylcholine elicited by stimulation of 5-HT4 receptors.

Postsynaptic effects of methoctramine at the mouse neuromuscular junction

Functional studies were performed to evaluate the effects of methoctramine at the neuromuscular junction of the mouse. The presynaptic control of acetylcholine release and the postsynaptic activation of the nicotinic receptor have been analysed by means of the extracellular recording with an EPC7 Patch Clamp amplifier. This electrophysiological method revealed a dose-related inhibitory effect of methoctramine on the studied parameters. The dramatic reduction of the kinetics of the quantal conductance change indicates an action at the postsynaptic level. The effects of methoctramine have been compared with those of the muscarinic agonist oxotremorine. Concentration/response curves for the two drugs were obtained and the apparent EC50 values calculated. The effects of oxotremorine were not antagonized by 1 microM methoctramine. These findings suggest an interaction of some muscarinic agents on the postsynaptic receptor-ion-channel complex at the mouse neuromuscular junction.

Elevation of hippocampal extracellular acetylcholine levels by methoctramine

Previous studies suggest that m2 muscarinic receptors serve as presynaptic autoreceptors. Blocking these receptors by selective antagonists may, therefore, lead to increased acetylcholine (ACh) release. This study assessed changes in extracellular ACh levels, via in vivo microdialysis, following administration of the m2 antagonist methoctramine. Drug or placebo (Ringer's solution) was perfused via a microdialysis probe into the CA1 hippocampal region of unrestrained, awake male Fischer rats. HPLC-EC was used for online analysis of the dialysates. Methoctramine significantly enhanced ACh release in a dose-dependent fashion as compared to placebo for the doses employed (0.25-16 microM). The present in vivo data corroborate studies that show increased ACh levels in vitro following application of m2 antagonists.