Characterization of acid sensing ion channels (ASICs) at the MNTB neurons in auditory brainstem slices

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

San Diego, California

Congreso; 40º Annual Meeting of The Society for Neuroscience.; 2010

The Society for Neuroscience.

              Acid-sensing ion channels (ASICs) are a H+-gated subgroup of the degenerin/epithelial Na+ channel family of non-voltage-gated cation channels that are activated by acidification of the extracellular media. They contribute to sensory function, nociception and mechanosensation in the peripheral nervous system, and in the brain they play a role in synaptic transmission, contributing to synaptic plasticity. They are involved in ischemic stroke, fear responses, memory and learning and are also implicated in pathological conditions including ischemic stroke and multiple sclerosis.              We made whole cell patch clamp recordings of neurons at the Medial Nucleus of the Trapezoid Body (MNTB) in a 0.8 mM Hepes solution at pH = 7.3 and observed that transient extracellular acidification activated ASICs, generating an inward current (IASIC). Local acidification was achieved by a 5 sec duration puff of low pH extracellular solution pressure injected with a micropipette filled with a MES (10 mM) based solution at pH = 5.5-6.3, connected to a Picrospritzer. IASIC had mean amplitudes of 2500 ± 500 pA at pH = 5.5 and 950 ± 200 pA at pH = 6.3. Mean half width was 0.66± 0.07 s, rise time 0.20 ± 0.05 s and decay times 1.45 ± 0.21 s at both pH. The reversal potential of IASIC present at MNTB neurons was at +60 mV, suggesting that ASICs are preferentially permeable to Na+. Local application of a puff with 10 mM Hepes solution at pH = 7.3 did not activate ASICs and evoked no current. IASIC were reduced up to 85 % by external application of amiloride, a non specific inhibitor of all subunits composing ASICs. The same experiments performed in knock out mice for the ASIC1a subunit (ASIC1a -/-) generate no currents, suggesting that ASICs at the MNTB neurons from wild type mice are homomeric ASIC1a channels. These results were corroborated using Psalmotoxin (a selective blocker for ASIC1a), which reduced IASIC by 90 % in wild type mice. ASICs desensitize during successive puffs of acid solutions and recover after 3-5 minutes of washing out at pH = 7.3 Hepes solution. ASICs at MNTB neurons were also activated by the release of H+ by iontophoresis, applying a positive electrical charge to a high resistance micropipette (20 MW) filled with HCl (0.1 M). IASIC evoked by this method have maximum amplitudes of 590 ± 70 pA, half width of 0.72± 0.05 s, rise time of 0.29 ± 0.05 s and decay times of 1.37 ± 0.14s.            The presence of ASIC currents at MNTB neurons modulates short term plasticity of excitatory postsinaptic currents (EPSCs) at the calyx of Held-MNTB synapse, suggesting that they play a role in synaptic transmission during high frequency stimulation.