Negative shift in the glycine reversal potential mediated by a Ca ²⁺- and pH-dependent mechanism in interneurons

Cartwheel cells are glycinergic auditory interneurons which fire Na ⁺- and Ca²⁺-dependent spike bursts, termed complex spikes, and which synapse on both principal cells and one another. The reversal potential for glycine (E_gly) can be hyperpolarizing or depolarizing in cartwheel cells, and many cells are even excited by glycine. We explored the role of spike activity in determining E_gly in mouse cartwheel cells using gramicidin perforated-patch recording. E_gly was found to shift toward more negative potentials after a period of complex spiking or Ca ²⁺ spiking induced by depolarization, thus enhancing glycine's inhibitory effect for ∼30 s following cessation of spiking. Combined perforated patch electrophysiology and imaging studies showed that the negative E_gly shift was triggered by a Ca²⁺-dependent intracellular acidification. The effect on E_gly was likely caused by bicarbonate-Cl^- exchanger-mediated reduction in intracellular Cl ^-, as H₂DIDS and removal of HCO₃ ^-/CO₂ inhibited the negative E_gly shift. The outward Cl^- flux underlying the negative shift in E_gly opposed a positive shift triggered by passive Cl^- redistribution during the depolarization. Thus, a Ca²⁺-dependent mechanism serves to maintain or enhance the strength of inhibition in the face of increased excitatory activity.