Muscarinic inhibition of quantal transmitter release from the magnesium-paralysed frog sartorius muscle

Muscarinic cholinoreceptors (M1-, M2-, M3- and M4-type) modulate the acetylcholine secretion in the frog neuromuscular junction

Muscarinic cholinoreceptors regulate the neurosecretion process in vertebrate neuromuscular junctions. The diversity of muscarinic effects on acetylcholine (ACh) secretion may be attributed to the different muscarinic subtypes involved in this process. In the present study, the location of five muscarinic receptor subtypes (M1, M2, M3, M4 and M5) on the motor nerve terminals of frog cutaneous pectoris muscle was shown using specific polyclonal antibodies. The modulatory roles of these receptors were investigated via assessment of the effects of muscarine and specific muscarinic antagonists on the quantal content of endplate currents (EPCs) and the time course of secretion, which was estimated from the distribution of "real" synaptic delays of EPCs recorded in a low Ca²⁺/high Mg²⁺ solution. The agonist muscarine decreased the EPC quantal content and synchronized the release process. The depressing action of muscarine on the EPC quantal content was abolished only by pretreatment of the preparation with the M3 blockers 4-DAMP (1,1-Dimethyl-4-diphenylacetoxypiperidinium iodide) and J 104129 fumarate ((αR)-α-Cyclopentyl-α-hydroxy-N-[1-(4-methyl-3-pentenyl)-4-piperidinyl]benzeneacetamide fumarate). Moreover, antagonists of the M1, M2, M3 and M4 receptors per se diminished the intensity of secretion, which suggests a putative up-regulation of the release by endogenous ACh.

Acetylcholine-Induced Inhibition of Presynaptic Calcium Signals and Transmitter Release in the Frog Neuromuscular Junction

Acetylcholine (ACh), released from axonal terminals of motor neurons in neuromuscular junctions regulates the efficacy of neurotransmission through activation of presynaptic nicotinic and muscarinic autoreceptors. Receptor-mediated presynaptic regulation could reflect either direct action on exocytotic machinery or modulation of Ca²⁺ entry and resulting intra-terminal Ca²⁺ dynamics. We have measured free intra-terminal cytosolic Ca²⁺ ([Ca²⁺]_i) using Oregon-Green 488 microfluorimetry, in parallel with voltage-clamp recordings of spontaneous (mEPC) and evoked (EPC) postsynaptic currents in post-junctional skeletal muscle fiber. Activation of presynaptic muscarinic and nicotinic receptors with exogenous acetylcholine and its non-hydrolized analog carbachol reduced amplitude of the intra-terminal [Ca²⁺]_i transients and decreased quantal content (calculated by dividing the area under EPC curve by the area under mEPC curve). Pharmacological analysis revealed the role of muscarinic receptors of M₂ subtype as well as d-tubocurarine-sensitive nicotinic receptor in presynaptic modulation of [Ca²⁺]_i transients. Modulation of synaptic transmission efficacy by ACh receptors was completely eliminated by pharmacological inhibition of N-type Ca²⁺ channels. We conclude that ACh receptor-mediated reduction of Ca²⁺ entry into the nerve terminal through N-type Ca²⁺ channels represents one of possible mechanism of presynaptic modulation in frog neuromuscular junction.

Protein kinase C isoforms at the neuromuscular junction: localization and specific roles in neurotransmission and development

The protein kinase C family (PKC) regulates a variety of neural functions including neurotransmitter release. The selective activation of a wide range of PKC isoforms in different cells and domains is likely to contribute to the functional diversity of PKC phosphorylating activity. In this review, we describe the isoform localization, phosphorylation function, regulation and signalling of the PKC family at the neuromuscular junction. Data show the involvement of the PKC family in several important functions at the neuromuscular junction and in particular in the maturation of the synapse and the modulation of neurotransmission in the adult.

Ketamine-xylazine anesthesia causes hyperopic refractive shift in mice

Mice have increasingly been used as a model for studies of myopia. The key to successful use of mice for myopia research is the ability to obtain accurate measurements of refractive status of their eyes. In order to obtain accurate measurements of refractive errors in mice, the refraction needs to be performed along the optical axis of the eye. This represents a particular challenge, because mice are very difficult to immobilize. Recently, ketamine-xylazine anesthesia has been used to immobilize mice before measuring refractive errors, in combination with tropicamide ophthalmic solution to induce mydriasis. Although these drugs have increasingly been used while refracting mice, their effects on the refractive state of the mouse eye have not yet been investigated. Therefore, we have analyzed the effects of tropicamide eye drops and ketamine-xylazine anesthesia on refraction in P40 C57BL/6J mice. We have also explored two alternative methods to immobilize mice, i.e. the use of a restraining platform and pentobarbital anesthesia. We found that tropicamide caused a very small, but statistically significant, hyperopic shift in refraction. Pentobarbital did not have any substantial effect on refractive status, whereas ketamine-xylazine caused a large and highly significant hyperopic shift in refraction. We also found that the use of a restraining platform represents good alternative for immobilization of mice prior to refraction. Thus, our data suggest that ketamine-xylazine anesthesia should be avoided in studies of refractive development in mice and underscore the importance of providing appropriate experimental conditions when measuring refractive errors in mice.

Modulation of ACh release by presynaptic muscarinic autoreceptors in the neuromuscular junction of the newborn and adult rat

We studied the presynaptic muscarinic autoreceptor subtypes controlling ACh release and their relationship with voltage-dependent calcium channels in the neuromuscular synapses of the Levator auris longus muscle from adult (30-40 days) and newborn (3-6 and 15 days postnatal) rats. Using intracellular recording, we studied how several muscarinic antagonists affected the evoked endplate potentials. In some experiments we previously incubated the muscle with calcium channel blockers (nitrendipine, omega-conotoxin-GVIA and omega-Agatoxin-IVA) before determining the muscarinic response. In the adult, the M1 receptor-selective antagonist pirenzepine (10 micro m) reduced evoked neurotransmission ( approximately 47%). The M2 receptor-selective antagonist methoctramine (1 micro m) increased the evoked release ( approximately 67%). Both M1- and M2-mediated mechanisms depend on calcium influx via P/Q-type synaptic channels. We found nothing to indicate the presence of M3 (4-DAMP-sensitive) or M4 (tropicamide-sensitive) receptors in the muscles of adult or newborn rats. In the 3-6-day newborn rats, pirenzepine reduced the evoked release ( approximately 30%) by a mechanism independent of L-, N- and P/Q-type calcium channels, and the M2 antagonist methoctramine (1 micro m) unexpectedly decreased the evoked release ( approximately 40%). This methoctramine effect was a P/Q-type calcium-channel-dependent mechanism. However, upon maturation in the first two postnatal weeks, the M2 pathway shifted to perform the calcium-dependent release-inhibitory activity found in the adult. We show that the way in which M1 and M2 muscarinic receptors modulate neurotransmission can differ between the developing and adult rat neuromuscular synapse.

Presynaptic effects of muscarine on ACh release at the frog neuromuscular junction

1. Presynaptic effects of muscarine on neurotransmitter release were studied at the frog neuromuscular junction, using focal depolarization of the presynaptic terminal to different levels. 2. Muscarine (10 microM) had a dual effect on ACh release: concomitant inhibition and enhancement of release at the same patch of presynaptic membrane. 3. These two effects were maximal at low depolarizing pulses and diminished as depolarization increased. 4. At low depolarizing pulses, atropine (1 microM) enhanced release, suggesting that ACh in the synaptic cleft causes a net tonic inhibition of ACh release. 5. In the presence of the M2 antagonist methoctramine (1 microM), muscarine (10 microM) enhanced ACh release. 6. In the presence of the M1 antagonist pirenzepine (10 microM), muscarine (10 microM) produced stronger inhibition. 7. These results show that the M2 receptor is responsible for inhibition of ACh release, while the M1 receptor is responsible for its enhancement. 8. The inhibitory effect of muscarine did not depend on extracellular [Ca2+]. Enhancement of release was abolished at low extracellular [Ca2+]. 9. The muscarine inhibitory effect was not associated with a reduction of Ca2+ current, while release enhancement was associated with an increase of Ca2+ current.

Presynaptic inhibition of excitatory synaptic transmission by muscarinic and metabotropic glutamate receptor activation in the hippocampus: are Ca2+ channels involved?

Activation of either muscarinic cholinergic or metabotropic glutamatergic presynaptic receptors inhibits evoked excitatory synaptic responses in the hippocampus. We have investigated two possible mechanisms underlying these actions using whole-cell recording from CA3 pyramidal cells in hippocampal slice cultures. Application of either methacholine (MCh, 10 microM) or trans-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD, 10 microM) was found to reduce the frequency of miniature excitatory postsynaptic currents (mEPSCs) by roughly 50%, without changing their mean amplitude. The voltage-dependent Ca2+ channel blocker Cd2+ (100 microM), in contrast, had no effect on the mEPSC frequency. When the extracellular [K+] was increased from 2.7 to 16 mM, the mEPSC frequency increased from 1.7 to 4.9 Hz. This increase could be completely reversed by applying Cd2+, indicating that it was triggered by voltage-dependent Ca2+ influx. MCh and t-ACPD each decreased the mEPSC frequency by roughly 50% under these conditions. Because the agonists were equally effective in inhibiting spontaneous release whether voltage-dependent channels were activated or not, we conclude that presynaptic cholinergic and glutamatergic inhibition is not mediated by inhibition of presynaptic Ca2+ channels, but rather by a direct interference in the neurotransmitter release process at some point subsequent to Ca2+ influx.

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.

Muscarinic modulation of neurotransmission: the effects of some agonists and antagonists

1. Functional studies were performed to evaluate the effects of some muscarinic agents at the neuromuscular junction of the mouse. 2. The presynaptic control of acetylcholine release and the postsynaptic activation of the nicotinic receptor have been analyzed by means of extracellular recording. The amplitude of spontaneous and of evoked acetylcholine release, the frequency of spontaneous acetylcholine release and the time course of the quantal release have been measured by means of an EPC7 patch clamp amplifier. 3. This electrophysiological method revealed multiple dose-related effects of some agonists and antagonists on the above parameters. Concentration-response curves related to the parameters underlying the function of this cholinergic synapse were obtained and the apparent EC50 values calculated. 4. Many of the interactions of the agonists and antagonists could inhibit neuromuscular transmission. The rank order potencies related to the inhibition of the evoked signals were carbachol > oxotremorine > d,l-muscarine for the agonists and methoctramine > 4-DAMP > l-hyoscyamine > AFDX-116 > ipratropium > pirenzepine for the antagonists. 5. These findings suggest a more complicated pattern related to the muscarinic action at the mouse neuromuscular junction with the involvement of some post-synaptic located sites.

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.

The effects of muscarine and atropine reveal that inhibitory autoreceptors are present on frog motor nerve terminals but are not activated during transmission

The role of presynaptic muscarinic receptors in modulating neuromuscular transmission was studied in the isolated sartorius muscle of the frog using electrophysiological techniques. In low calcium solutions muscarine reduced mEPP frequency and the quantal of EPPs. In solutions containing the normal calcium concentration the effect of muscarine on quantal content, but not the effect on mEPP frequency, was somewhat attenuated. Muscarine-induced reductions in the parameters of ACh release were prevented by atropine. Irrespective of the calcium concentration, atropine had no effect on mEPP frequency except where fibres were pretreated with glycerol. In experiments where evoked acetylcholine release was maintained at physiologically relevant levels, atropine had no effect on the quantal content of EPPs evoked at low frequency or on the extent of rundown in trains of EPPs evoked at high frequency.

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

The effects of oxotremorine, arecoline and muscarine on neuromuscular transmission of mouse or rat phrenic nerve-diaphragm were investigated. For some studies of endplate potentials (e.p.p.s) the preparation was immobilized by cutting muscle fibers. Oxotremorine (0.3-10 microM) depolarized endplate membranes, reduced miniature e.p.p. amplitudes but increased frequency, induced spontaneous neural discharges and muscle fasciculations, and produced contracture of denervated mouse diaphragm. In mouse and young rat preparations pretreated with Mn2+, Co2+, Ni2+, Cd2+ or low Ca2+ Tyrode to depress evoked acetylcholine release, oxotremorine 0.3-1 microM increased indirect twitches as well as amplitudes and quantal contents of e.p.p.s. These increases were not observed when the synaptic transmission was not depressed, nor in adult rat preparations. The augmentation by oxotremorine of evoked acetylcholine release persisted in preparations pretreated with neostigmine (1 microM) and tetrodotoxin (20 nM), which inhibited acetylcholinesterase and oxotremorine-induced spontaneous neural discharges. These effects of oxotremorine were mimicked by arecoline but not by muscarine and were antagonized by tubocurarine (0.3 microM) but not by atropine (0.1-10 microM). Atropine alone did not affect indirect twitches, synaptic transmission, tetanic responses evoked by direct stimulation of diaphragms, nor the durations of muscle action potential. The direct twitch responses were only slightly increased by oxotremorine at 2-3 microM. Oxotremorine at high concentrations (greater than 2 microM), depressed indirect twitches and e.p.p. amplitude, and accelerated the run-down of trains of e.p.p.s. The IC50 on indirect twitches was reduced by pretreatment with diltiazem or proadifen, which are known to promote receptor desensitization.(ABSTRACT TRUNCATED AT 250 WORDS)