Nicotinic actions of oxotremorine on murine skeletal muscle. Evidence against muscarinic modulation of acetylcholine release

Immunocytochemical Demonstration of M1 Muscarinic Acetylcholine Receptors at the Presynaptic and Postsynaptic Membranes of Rat Diaphragm Endplates

M1-muscarinic acetylcholine (ACh) receptors (M1R) were directly demonstrated immunocytochemically in electronmicroscopic images of rat diaphragm neuromuscular junctions (NMJ). Specific electron-dense granules were located at presynaptic nerve ending membranes and in the sarcolemma in the depths of postsynaptic folds. This first visualization of M1R on both sides of the NMJ is in agreement with previous pharmacological data on the regulatory role of M1R in quantal and non-quantal ACh release.

Activation of muscle nicotinic acetylcholine receptor channels by nicotinic and muscarinic agonists

1. The dose-response parameters of recombinant mouse adult neuromuscular acetylcholine receptor channels (nAChR) activated by carbamylcholine, nicotine, muscarine and oxotremorine were measured. Rate constants for agonist association and dissociation, and channel opening and closing, were estimated from single-channel kinetic analysis. 2. The dissociation equilibrium constants were (mM): ACh (0. 16)<oxotremorine M (0.6)<carbamylcholine (0.8)<nicotine (2.6). 3. The gating equilibrium constants (opening/closing) were: ACh (45)>carbamylcholine (5.1)>oxotremorine M (0.6)>nicotine (0. 5)>muscarine (0.15). 4. Rat neuronal alpha4beta2 nAChR can be activated by all of the agonists. However, detailed kinetic analysis was impossible because the recordings lacked clusters representing the activity of a single receptor complex. Thus, the number of channels in the patch was unknown and the activation rate constants could not be determined. 5. Considering both receptor affinity and agonist efficacy, muscarine and oxotremorine are significant agonists of muscle-type nAChR. The results are discussed in terms of structure-function relationships at the nAChR transmitter binding site.

Oxotremorine-M activates single nicotinic acetylcholine receptor channels in cultured Xenopus myocytes

Oxotremorine methiodide (oxotremorine-M) is the quaternary amine derivative of oxotremorine and is known to be a potent and oft-reported pure, muscarinic receptor agonist. We report here, for the first time, that oxotremorine-M also has strong nicotinic actions at the single channel level. Although previous reports have suggested that oxotremorine-M has mixed cholinergic properties, its nicotinic actions have only been reported in systems which contain both muscarinic and nicotinic receptors, or in skeletal neuromuscular systems where the site of action of oxotremorine-M may have been ambiguous. We tested the possibility that oxotremorine-M is a nicotinic receptor agonist by examining the responses of single nicotinic acetylcholine receptors in primary cultures of myocytes from skeletal myotomes of Xenopus larvae. Myotomal myocytes are known to express the nicotinic acetylcholine receptor and no evidence exists that muscarinic receptors are expressed in these progenitors of the skeletal musculature. Furthermore, because we used aneural myocyte cultures, the effects of oxotremorine-M cannot be attributed to action on presynaptic receptors. Using cell-attached patches, we compared the responses of the nicotinic acetylcholine receptors to suberyldicholine and oxotremorine-M. Our results show that (1) both agonists activate the receptor channel in nanomolar concentrations; (2) the mean channel open-time is significantly smaller in oxotremorine-M; and (3) activation of the nicotinic acetylcholine receptor by oxotremorine-M is accompanied by a large percentage of short openings and a high frequency of event flickering. We conclude that oxotremorine-M is a mixed function agonist, showing partial blocking behavior, which effectively activates pure nicotinic acetylcholine receptors.

The muscarinic receptor agonist oxotremorine methiodide evokes a nicotinic response in mammalian sympathetic neurons

Oxotremorine methiodide, a congener of oxotremorine, is used as a muscarinic receptor agonist. Responses to oxotremorine methiodide and nicotinic receptor agonists were examined in cultured guinea-pig celiac ganglion neurons using whole-cell voltage clamp techniques. At holding potentials between -30 and -60 mV, a brief application of oxotremorine methiodide produced fast and slow inward current transients, depending upon the concentration applied. Slowly developing inward current transients, characteristic of muscarinic responses, were produced by lower concentrations (EC50: 0.3 microM) and were blocked by atropine. Rapid inward current transients, characteristic of nicotinic responses, were produced by higher concentrations of oxotremorine methiodide (EC50: 168 microM) and were blocked by d-tubocurarine. Thus oxotremorine methiodide, at concentrations of 10 microM and greater, produced an initial nicotinic fast inward current transient followed by a slow muscarinic inward transient. The fast inward transients were similar to responses evoked by the nicotinic receptor agonists acetylcholine, nicotine and 1,1-dimethyl-4-phenyl-piperazinium iodide and were not antagonized by atropine. We conclude that oxotremorine methiodide acts as a nicotinic and muscarinic receptor agonist in celiac sympathetic ganglion neurons.

Possible mechanisms of the effect of physostigmine on the facilitation of acetylcholine release in the guinea pig myenteric plexus

The automodulation of acetylcholine (ACh) release in the guinea pig myenteric plexus-longitudinal muscle preparation was investigated by studying the electric stimulation-evoked release of radiolabeled ACh. When the release associated with neuronal activity was challenged by the muscarinic antagonist atropine, the release was not significantly enhanced. When the acetylcholinesterase (AChE) blocker physostigmine was present, the well-established muscarinic receptor-mediated autoinhibition was operative, i.e., the release was significantly reduced. However, when both drugs were added together, the release was much higher than under control conditions. Therefore, it seems likely that there is also a facilitatory system. We made an effort to clear up the mechanism of this facilitation by blocking possible nicotinic presynaptic receptors, by excluding the alpha 2-adrenoceptor-mediated masking effect of noradrenergic heteromodulation, by preventing a possible ATP-mediated mechanism, and by attempting to prevent the direct effect of physostigmine. None of these manipulations resulted in a decrease of the surplus release. It is concluded, that when the negative feedback modulation of ACh is inhibited and AChE activity is reduced, high levels of ACh facilitates additional release of ACh from the nerve terminals, possibly through a not yet verified class of receptors.

Nicotinic agonists antagonize quantal size increases and evoked release at frog neuromuscular junction

1. Previous studies at the frog neuromuscular junction showed that quantal size can be increased two- to fourfold by a variety of treatments, including prior exposure to hypertonic solution (which activates protein kinase A) and insulin (which acts via an unknown pathway). Size increases largely because quanta contain more acetylcholine (ACh). 2. Now the effects of cholinergic agonists on the increases in quantal size have been studied. One muscle from a frog was kept for 2 h in hypertonic sodium gluconate solution. The miniature endplate potential (MEPP) sizes were measured in saline: they had increased about fourfold. The paired muscle went through the same experimental sequence, except that an agonist was added to the hypertonic gluconate solution. Again MEPP sizes were measured in saline. The increase in quantal size was significantly depressed by 0.2 microM 1,1-dimethyl-4-phenyl- piperazinium (DMPP). The sequence of effectiveness of agonists was: DMPP > carbachol > or = ACh = cytisine > oxytremorine. This sequence suggests that the receptor belongs in the nicotinic class. 3. Quantal size is doubled after 1 h in insulin. One micromolar carbachol largely blocked the size increase. 4. The effects of cholinergic antagonists were tested by keeping the experimental muscle in hypertonic gluconate solution containing 1 microM carbachol plus an antagonist. The controls were paired muscles kept in hypertonic gluconate solution (without carbachol or antagonist). MEPP sizes were measured in saline. The depressing action of carbachol on the increase in MEPP size was blocked by 0.2 microM neuronal-bungarotoxin (nBTX). The sequence of effectiveness of antagonists was: nBTX > trimethaphan > d-tubocurarine (dTC). Ten micromolar atropine (without carbachol) depressed the increase in quantal size. Therefore, the antagonist potency of atropine could not be adequately tested. Carbachol action was not blocked by 10 microM hexamethonium or 10 microM mecamylamine. 5. Once quanta are made large they can be converted back to normal size by cholinergic agonists. Muscles in which quantal size had been enlarged were exposed to hypertonic solutions containing the agonist. Quantal size was reduced to a fraction of its former value when the hypertonic solution contained 1 microM carbachol- or 1 microM DMPP. One micromolar oxytremorine had no effect. Carbachol still reduced quantal size when applied in low-Ca2+ solutions, so it does not appear to act by elevating intracellular [Ca2+]. 6. Previous work suggested that the treatments produce a subpopulation of large quanta that are positioned for release.(ABSTRACT TRUNCATED AT 400 WORDS)

Muscarinic M2 receptor mRNA expression and receptor binding in cholinergic and non-cholinergic cells in the rat brain: a correlative study using in situ hybridization histochemistry and receptor autoradiography

The goal of the present study was to identify the cells containing mRNA coding for the m2 subtype of muscarinic cholinergic receptors in the rat brain. In situ hybridization histochemistry was used, with oligonucleotides as hybridization probes. The distribution of cholinergic cells was examined in consecutive sections with probes complementary to choline acetyltransferase mRNA. Furthermore, the microscopic distribution of muscarinic cholinergic binding sites was examined with a non-selective ligand ([3H]N-methylscopolamine) and with ligands proposed to be M1-selective ([3H]pirenzepine) or M2-selective ([3H]oxotremorine-M). The majority of choline acetyltransferase mRNA-rich (i.e. cholinergic) cell groups (medial septum-diagonal band complex, nucleus basalis, pedunculopontine and laterodorsal tegmental nuclei, nucleus parabigeminalis, several motor nuclei of the brainstem, motoneurons of the spinal cord), also contained m2 mRNA, strongly suggesting that at least a fraction of these receptors may be presynaptic autoreceptors. A few groups of cholinergic cells were an exception to this fact: the medial habenula and some cranial nerve nuclei (principal oculomotor, trochlear, abducens, dorsal motor nucleus of the vagus). Furthermore, m2 mRNA was not restricted to cholinergic cells but was also present in many other cells throughout the rat brain. The distribution of m2 mRNA was in good, although not complete, agreement with that of binding sites for the M2 preferential agonist [3H]oxotremorine-M, but not with [3H]pirenzepine binding sites. Regions where the presence of [3H]oxotremorine-M binding sites was not correlated with that of m2 mRNA are the caudate-putamen, nucleus accumbens, olfactory tubercle and islands of Calleja. The present results strongly suggest that the M2 receptor is expressed by a majority of cholinergic cells, where it probably plays a role as autoreceptor. However, many non-cholinergic neurons also express this receptor, which would be, presumably, postsynaptically located. Finally, comparison between the distribution of m2 mRNA and that of the proposed M2-selective ligand [3H]oxotremorine-M indicates that this ligand, in addition to M2 receptors, may also recognize in certain brain areas other muscarinic receptor populations, particularly M4.