Synaptic cleft microenvironment influences potassium permeation and synaptic transmission in hair cells surrounded by calyx afferents in the turtle

Abstract

Key points In central regions of vestibular semicircular canal epithelia, the [K+] in the synaptic cleft ([K+]c) contributes to setting the hair cell and afferent membrane potentials; the potassium efflux from type I hair cells results from the interdependent gating of three conductances. Elevation of [K+]c occurs through a calcium‐activated potassium conductance, GBK, and a low‐voltage‐activating delayed rectifier, GK(LV), that activates upon elevation of [K+]c. Calcium influx that enables quantal transmission also activates IBK, an effect that can be blocked internally by BAPTA, and externally by a CaV1.3 antagonist or iberiotoxin. Elevation of [K+]c or chelation of [Ca2+]c linearizes the GK(LV) steady‐state I–V curve, suggesting that the outward rectification observed for GK(LV) may result largely from a potassium‐sensitive relief of Ca2+ inactivation of the channel pore selectivity filter. Potassium sensitivity of hair cell and afferent conductances allows three modes of transmission: quantal, ion accumulation and resistive coupling to be multiplexed across the synapse. Abstract In the vertebrate nervous system, ions accumulate in diffusion‐limited synaptic clefts during ongoing activity. Such accumulation can be demonstrated at large appositions such as the hair cell–calyx afferent synapses present in central regions of the turtle vestibular semicircular canal epithelia. Type I hair cells influence discharge rates in their calyx afferents by modulating the potassium concentration in the synaptic cleft, [K+]c, which regulates potassium‐sensitive conductances in both hair cell and afferent. Dual recordings from synaptic pairs have demonstrated that, despite a decreased driving force due to potassium accumulation, hair cell depolarization elicits sustained outward currents in the hair cell, and a maintained inward current in the afferent. We used kinetic and pharmacological dissection of the hair cell conductances to understand the interdependence of channel gating and permeation in the context of such restricted extracellular spaces. Hair cell depolarization leads to calcium influx and activation of a large calcium‐activated potassium conductance, GBK, that can be blocked by agents that disrupt calcium influx or buffer the elevation of [Ca2+]i, as well as by the specific KCa1.1 blocker iberiotoxin. Efflux of K+ through GBK can rapidly elevate [K+]c, which speeds the activation and slows the inactivation and deactivation of a second potassium conductance, GK(LV). Elevation of [K+]c or chelation of [Ca2+]c linearizes the GK(LV) steady‐state I–V curve, consistent with a K+‐dependent relief of Ca2+ inactivation of GK(LV). As a result, this potassium‐sensitive hair cell conductance pairs with the potassium‐sensitive hyperpolarization‐activated cyclic nucleotide‐gated channel (HCN) conductance in the afferent and creates resistive coupling at the synaptic cleft.

Keeping your eye on the ball

“…New results in the Contini et al . () report provide evidence that synchronization is probably enhanced in calyx‐bearing afferents by direct resistive coupling between the sensory hair cell and postsynaptic neuron. Depolarization through K + build‐up in the cleft could further contribute to synchronization by poising the afferent on the edge of action potential firing.…”

“…Indeed, elegant and tour‐de‐force paired patch‐clamp recordings from Contini et al . () show that hair cell depolarization causes a slowly activating inward current in the afferent that reverses at the potassium equilibrium Nernst potential. Elevation of cleft K + concentration occurs through the opening of voltage‐ and Ca 2+ ‐activated BK and low‐voltage delayed‐rectifier K + conductances ( G BK and G K(LV) ) on the hair cell basal pole.…”

“…A new report in The Journal of Physiology by Contini, Holstein and Art (Contini et al . ) demonstrates that vestibular calyceal‐type synapses are amazingly versatile, having at least three different modes of synaptic transmission: a fast AMPAR‐based quantal mode of transmission via synaptic ribbons (∼millisecond packets; Rennie & Streeter, ; Lysakowski et al . ; Highstein et al .…”

“…The report by Contini et al . () is focused on the latter two non‐quantal modes of transmission, which depend on the unique morphology of the postsynaptic calyx that engulfs most of the hair cell soma (Fig. ).…”

“…The non‐quantal synaptic mechanisms revealed by Contini et al . () may help explain some long‐standing mysteries in the vestibular system. It has been shown in vivo that calyx bearing vestibular afferents respond to high frequency sound and vibration (>1 kHz) with action potentials locked to a precise phase of the stimulus, with vector strength (synchronization index) often exceeding auditory afferents in the same species (Curthoys et al .…”