Postsynaptic Potentiation in Mouse Motor Synapses Induced by ATP Accumulation in Synaptic Cleft

Delayed increase of acetylcholine quantal size induced by the activity-dependent release of endogenous CGRP but not ATP in neuromuscular junctions

In mouse motor synapses tetanic neuromuscular activity (30 Hz, 2 min) led to a delayed posttetanic potentiation of amplitude and duration of spontaneous miniature endplate potentials (MEPPs). Microelectrode recordings of MEPPs before and after nerve stimulation showed an increase in MEPP amplitude and time course by 30% and 15%, respectively, without changes in their frequency. Peak effect was detected 20 min after tetanic activity and progressively faded throughout the next 40 min of recording. The revealed potentiation of MEPPs was fully preserved in preparations from pannexin 1 knockout mice. It means, that myogenic ATP released via pannexin 1 channels from contracting muscle fibers is not likely to participate in the described phenomenon. But posttetanic potentiation of MEPPs was fully prevented by competitive antagonist of calcitonin gene-related peptide (CGRP) receptors CGRP_8-37 , ryanodine receptors inhibitor ryanodine and by vesicular acetylcholine transporter inhibitor vesamicol. It is suggested that the combination of intensive synaptic and contractile activity in neuromuscular junctions is required to induce Ca²⁺ -dependent exocytosis of endogenous CGRP. The accumulation of CGRP in the synaptic cleft and its presynaptic activity may induce posttetanic potentiation of MEPP amplitude due to CGRP-stimulated acetylcholine loading into vesicles and subsequent increase of quantal size.

Ryanodine- and CaMKII-dependent release of endogenous CGRP induces an increase in acetylcholine quantal size in neuromuscular junctions of mice

OBJECTIVE

The aim of this study was to identify the mechanism responsible for an increase in miniature endplate potentials (MEPPs) amplitude, induced by ryanodine as an agonist of ryanodine receptors in mouse motor nerve terminals.

METHODS

Using intracellular microelectrode recordings of MEPPs and evoked endplate potentials (EPPs), the changes in spontaneous and evoked acetylcholine release in motor synapses of mouse diaphragm neuromuscular preparations were studied.

RESULTS

Ryanodine (0.1 μM) increased both the amplitudes of MEPPs and EPPs to a similar extent (up to 130% compared to control). The ryanodine effect was prevented by blockage of receptors of calcitonin gene-related peptide (CGRP) by a truncated peptide CGRP_8-37 . Endogenous CGRP is stored in large dense-core vesicles in motor nerve terminals and may be released as a co-transmitter. The ryanodine-induced increase in MEPPs amplitude may be fully prevented by inhibition of vesicular acetylcholine transporter by vesamicol or by blocking the activity of protein kinase A with H-89, suggesting that endogenous CGRP is released in response to the activation of ryanodine receptors. Activation of CGRP receptors can, in turn, upregulate the loading of acetylcholine into synaptic vesicles, which will increase the quantal size. This new feature of endogenous CGRP activity looks similar to recently described action of exogenous CGRP in motor synapses of mice. The ryanodine effect was prevented by inhibitors of Ca/Calmodulin-dependent kinase II (CaMKII) KN-62 or KN-93. Inhibition of CaMKII did not prevent the increase in MEPPs amplitude, which was caused by exogenous CGRP.

CONCLUSIONS

We propose that the activity of presynaptic CaMKII is necessary for the ryanodine-stimulated release of endogenous CGRP from motor nerve terminals, but CaMKII does not participate in signaling downstream the activation of CGRP-receptors followed by quantal size increase.

Purinergic Receptors in Neurological Diseases With Motor Symptoms: Targets for Therapy

Since proving adenosine triphosphate (ATP) functions as a neurotransmitter in neuron/glia interactions, the purinergic system has been more intensely studied within the scope of the central nervous system. In neurological disorders with associated motor symptoms, including Parkinson's disease (PD), motor neuron diseases (MND), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), Huntington's Disease (HD), restless leg syndrome (RLS), and ataxias, alterations in purinergic receptor expression and activity have been noted, indicating a potential role for this system in disease etiology and progression. In neurodegenerative conditions, neural cell death provokes extensive ATP release and alters calcium signaling through purinergic receptor modulation. Consequently, neuroinflammatory responses, excitotoxicity and apoptosis are directly or indirectly induced. This review analyzes currently available data, which suggests involvement of the purinergic system in neuro-associated motor dysfunctions and underlying mechanisms. Possible targets for pharmacological interventions are also discussed.