Ryanodine and inositol triphosphate receptors modulate facilitation and tetanic depression at the frog neuromuscular junction

Quantal size increase induced by the endocannabinoid 2-arachidonoylglycerol requires activation of CGRP receptors in mouse motor synapses

In mouse motor synapses, the exogenous application of the endocannabinoid (EC) 2-arachidonoylglycerol (2-AG) increases acetylcholine (ACh) quantal size due to the activation of CB1 receptors and the stimulation of ACh vesicular uptake. In the present study, microelectrode recordings of miniature endplate potentials (MEPP) revealed that this effect of 2-AG is independent of brain-derived neurotrophic factor (BDNF) signaling but involves the activation of calcitonin gene-related peptide (CGRP) receptors along with CB1 receptors. Potentiation of MEPP amplitude in the presence of 2-AG was prevented by blockers of CGRP receptors and ryanodine receptors (RyR) and by inhibitors of phospholipase C (PLC) and Ca2+ /calmodulin-dependent protein kinase II (CaMKII). Therefore, we suggest a hypothetical chain of events, which starts from the activation of presynaptic CB1 receptors, involves PLC, RyR, and CaMKII, and results in CGRP release with the subsequent activation of presynaptic CGRP receptors. Activation of CGRP receptors is probably a part of a complex molecular cascade leading to the 2-AG-induced increase in ACh quantal size and MEPP amplitude. We propose that the same chain of events may also take place if 2-AG is endogenously produced in mouse motor synapses, because the increase in MEPP amplitude that follows after prolonged tetanic muscle contractions (30 Hz, 2 min) was prevented by the blocking of CB1 receptors. This work may help to unveil the previously unknown aspects of the functional interaction between ECs and peptide modulators aimed at the regulation of quantal size and synaptic transmission.

Levator Auris Longus Preparation for Examination of Mammalian Neuromuscular Transmission Under Voltage Clamp Conditions

This protocol describes a technique to record synaptic transmission from the neuromuscular junction under current-clamp and voltage-clamp conditions. An ex vivo preparation of the levator auris longus (LAL) is used because it is a thin muscle that provides easy visualization of the neuromuscular junction for microelectrode impalement at the motor endplate. This method allows for the recording of spontaneous miniature endplate potentials and currents (mEPPs and mEPCs), nerve-evoked endplate potentials and currents (EPPs and EPCs), as well as the membrane properties of the motor endplate. Results obtained from this method include the quantal content (QC), number of vesicle release sites (n), probability of vesicle release (prel), synaptic facilitation and depression, as well as the muscle membrane time constant (τm) and input resistance. Application of this technique to mouse models of human disease can highlight key pathologies in disease states and help identify novel treatment strategies. By fully voltage-clamping a single synapse, this method provides one of the most detailed analyses of synaptic transmission currently available.

Sexual Dimorphism in a Reciprocal Interaction of Ryanodine and IP3 Receptors in the Induction of Hyperalgesic Priming

Hyperalgesic priming, a model of pain chronification in the rat, is mediated by ryanodine receptor-dependent calcium release. Although ryanodine induces priming in both sexes, females are 5 orders of magnitude more sensitive, by an estrogen receptor α (EsRα)-dependent mechanism. An inositol 1,4,5-triphosphate (IP₃) receptor inhibitor prevented the induction of priming by ryanodine. For IP₃ induced priming, females were also more sensitive. IP₃-induced priming was prevented by pretreatment with inhibitors of the sarcoendoplasmic reticulum calcium ATPase and ryanodine receptor. Antisense to EsRα prevented the induction of priming by low-dose IP₃ in females. The induction of priming by an EsRα agonist was ryanodine receptor-dependent and prevented by the IP₃ antagonist. Thus, an EsRα-dependent bidirectional interaction between endoplasmic reticulum IP₃ and ryanodine receptor-mediated calcium signaling is present in the induction of hyperalgesic priming, in females. In cultured male DRG neurons, IP₃ (100 μm) potentiated depolarization-induced transients produced by extracellular application of high-potassium solution (20 mm, K20), in nociceptors incubated with β-estradiol. This potentiation of depolarization-induced calcium transients was blocked by the IP₃ antagonist, and not observed in the absence of IP₃ IP₃ potentiation was also blocked by ryanodine receptor antagonist. The application of ryanodine (2 nm), instead of IP₃, also potentiated K20-induced calcium transients in the presence of β-estradiol, in an IP₃ receptor-dependent manner. Our results point to an EsRα-dependent, reciprocal interaction between IP₃ and ryanodine receptors that contributes to sex differences in hyperalgesic priming.SIGNIFICANCE STATEMENT The present study demonstrates a mechanism that plays a role in the marked sexual dimorphism observed in a model of the transition to chronic pain, hyperalgesic priming. This mechanism involves a reciprocal interaction between the endoplasmic reticulum receptors, IP₃ and ryanodine, in the induction of priming, regulated by estrogen receptor α in the nociceptor of female rats. The presence of this signaling pathway modulating the susceptibility of nociceptors to develop plasticity may contribute to our understanding of sex differences observed clinically in chronic pain syndromes.