Identification of the muscarinic receptor subtypes involved in autoregulation of acetylcholine quantal release from frog motor nerve endings

Cholinesterases in Tripartite Neuromuscular Synapse

The neuromuscular junction (NMJ) is a tripartite synapse in which not only presynaptic and post-synaptic cells participate in synaptic transmission, but also terminal Schwann cells (TSC). Acetylcholine (ACh) is the neurotransmitter that mediates the signal between the motor neuron and the muscle but also between the motor neuron and TSC. ACh action is terminated by acetylcholinesterase (AChE), anchored by collagen Q (ColQ) in the basal lamina of NMJs. AChE is also anchored by a proline-rich membrane anchor (PRiMA) to the surface of the nerve terminal. Butyrylcholinesterase (BChE), a second cholinesterase, is abundant on TSC and anchored by PRiMA to its plasma membrane. Genetic studies in mice have revealed different regulations of synaptic transmission that depend on ACh spillover. One of the strongest is a depression of ACh release that depends on the activation of α7 nicotinic acetylcholine receptors (nAChR). Partial AChE deficiency has been described in many pathologies or during treatment with cholinesterase inhibitors. In addition to changing the activation of muscle nAChR, AChE deficiency results in an ACh spillover that changes TSC signaling. In this mini-review, we will first briefly outline the organization of the NMJ. This will be followed by a look at the role of TSC in synaptic transmission. Finally, we will review the pathological conditions where there is evidence of decreased AChE activity.

Autoregulation of Acetylcholine Release and Micro-Pharmacodynamic Mechanisms at Neuromuscular Junction: Selective Acetylcholinesterase Inhibitors for Therapy of Myasthenic Syndromes

Neuromuscular junctions (NMJs) are directly involved into such indispensable to life processes as respiration and locomotion. However, motor nerve forms only one synaptic contact at each muscle fiber. This unique configuration requires specific properties and constrains to be effective. The very high density of acetylcholine receptors (AChRs) of muscle type in synaptic cleft and an excess of acetylcholine (ACh) released under physiological conditions make this synapse extremely reliable. Nevertheless, under pathological conditions such as myasthenia gravis and congenital myasthenic syndromes, the safety factor can be markedly reduced. Drugs used for short-term symptomatic therapy of these pathological states, cause partial inhibition of cholinesterases (ChEs). These enzymes catalyze the hydrolysis of ACh, thus terminate its action on AChRs. Extension of the lifetime of ACh molecules compensates muscular AChRs abnormalities and, consequently, rescues muscle contractions. In this mini review, we will first outline the functional organization of the NMJ, and then, consider the concept of the safety factor and how it may be changed. This will be followed by a look at autoregulation of ACh release that influences the safety factor of NMJs. Finally, we will consider the morphological features of NMJs as a putative reserve to increase effectiveness of pathological muscle weakness therapy by ChEs inhibitors due to opportunity to use micro-pharmacodynamic mechanisms.

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.

Presynaptic nicotinic cholinoreceptors modulate velocity of the action potential propagation along the motor nerve endings at a high-frequency synaptic activity

Experiments on frog neuromuscular junctions have demonstrated that asynchrony of the acetylcholine quantal release forming the multi-quantal evoked response at high-frequency synaptic activity is caused, in particular, by a decrease in velocity of the action potential propagation along the non-myelinated nerve endings, which is mediated by activation of the α7 and α4β4 nicotinic cholinoreceptors.