Characterization of synaptic transmission at the neuromuscular junction of knock out mice for the acid sensing ion channels (ASICs)

FRANCISCO J. URBANO; CARLOTA GONZÁLEZ INCHAUSPE; OSVALDO D. UCHITEL

Marine Biology Laboratory, Woodshole, Massachusetts, USA

Otro; Marine biology Laboratory summer course: Neurobiology 2009; 2009

Marine Biology Laboratory

Acid-sensing ion channels (ASICs) are neuronal non-voltage-gated cation channels that are activated with the acidification of the extracellular media (that is, when pH falls from normal values). They contribute to sensory function and nociception in the peripheral nervous system, and in the brain they play a role in synaptic transmission, contributing to synaptic plasticity. They are also involved in fear responses, memory and learning. During the course we analyzed the role of ASICs in synaptic transmission using neuromuscular junctions from diaphragm and levator auris muscle from wild-type (WT) and ASIC1a knock-out (KO or ASIC1-/-) mice. Evoked (EPPs) and spontaneous (mEPPs) endplate potentials were recorded with conventional intracellular microelectrodes filled with 3 M KCl. We found that activation of ASICs inhibits neurotransmitter release and have several consequences in synaptic plasticity. The frequency of mEPP is increased at NMJs from ASIC1-/- at low K+ concentrations (2.5mM) compared to those from WT. Accordingly, during an extracellular acidification in NMJs from WT, there is a reversible decrease in the frequency of mEPPs. Paired pulse facilitation (PPF) of EPPs is increased at NMJs from WT with respect to those from ASIC1-/- mice. These results were corroborated using Psalmotoxin (a selective blocker for ASIC1a that increases the apparent affinity for H+ of ASIC1a and so shifts ASIC1a channels into the desensitized state). The enhance in PPF is more significant when the concentration of Hepes in the external solution is increase from 0.8 mM to 10 mM, avoiding the desensitization of ASICs. PPF is also increased when ASICs are activated during a puff of acid solution (pH = 6.0 MES based solution) in WT NMJs while remains unaltered in ASIC1-/- NMJs. Finally, during stimulation at high frequencies (75 Hz), there is a greater inhibition of transmitter release in NMJs from WT compared to those in ASIC1-/-.