Sensory transduction at the frog semicircular canal: how hair cell membrane potential controls junctional transmission

Pre- and Postsynaptic Effects of Glutamate in the Frog Labyrinth

The role of glutamate in quantal release at the cytoneural junction was examined by measuring mEPSPs and afferent spikes at the posterior canal in the intact frog labyrinth. Release was enhanced by exogenous glutamate, or dl-TBOA, a blocker of glutamate reuptake. Conversely, drugs acting on ionotropic glutamate receptors did not affect release; the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R) blocker CNQX decreased mEPSP size in a dose-dependent manner; the NMDA-R blocker d-AP5 at concentrations <200 µM did not affect mEPSP size, either in the presence or absence of Mg and glycine. In isolated hair cells, glutamate did not modify Ca currents. Instead, it systematically reduced the compound delayed potassium current, IKD, whereas the metabotropic glutamate receptor (mGluR)-II inverse agonist, (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid (LY341495), increased it. Given mGluR-II decrease cAMP production, these finding are consistent with the reported sensitivity of IKD to protein kinase A (PKA)-mediated phosphorylation. LY341495 also enhanced transmitter release, presumably through phosphorylation-mediated facilitation of the release machinery. The observed enhancement of release by glutamate confirms previous literature data, and can be attributed to activation of mGluR-I that promotes Ca release from intracellular stores. Glutamate-induced reduction in the repolarizing IKD may contribute to facilitation of release. Overall, glutamate exerts both a positive feedback action on mGluR-I, through activation of the phospholipase C (PLC)/IP₃ path, and the negative feedback, by interfering with substrate phosphorylation through G_i/0-coupled mGluRs-II/III. The positive feedback prevails, which may explain the increase in overall rates of release observed during mechanical stimulation (symmetrical in the excitatory and inhibitory directions). The negative feedback may protect the junction from over-activation.

Forskolin and protein kinase inhibitors differentially affect hair cell potassium currents and transmitter release at the cytoneural junction in the isolated frog labyrinth

The post-transductional elaboration of sensory input at the frog semicircular canal has been studied by correlating the effects of drugs that interfere with phosphorylation processes on: (i) potassium conductances in isolated hair cell and (ii) transmitter release at the cytoneural junction in the intact labyrinth. At hair cells, delayed potassium currents (IKD) undergo voltage- and time-dependent inactivation; inactivation removal requires ATP, is sensitive to kinase blockade, but is unaffected by exogenous application of cyclic nucleotides. We report here that forskolin, an activator of endogenous adenylyl cyclase, enhances IKD inactivation removal in isolated hair cells, but produces an overall decrease in IKD amplitude consistent with the direct blocking action of the drug on several families of K channels. In the intact labyrinth, forskolin enhances transmitter release, consistent with such depression of K conductances. Kinase blockers - H-89 and KT5823 - have been shown to reduce IKD inactivation removal and IKD amplitude at isolated hair cells. In the labyrinth, the effects of these drugs on junctional activity are quite variable, with predominant inhibition of transmitter release, rather than the enhancement expected from the impairment of K currents. The overall action of forskolin and kinase inhibitors on K conductances is similar (depression), but they have opposite effects on transmitter release: this indicates that some intermediate steps between the bioelectric control of hair cell membrane potential and transmitter release are affected in opposite ways and therefore are presumably regulated by protein phosphorylation.

A model of signal processing at the isolated hair cell of the frog semicircular canal

A computational model has been developed to simulate the electrical behavior of the type II hair cell dissected from the crista ampullaris of frog semicircular canals. In its basolateral membrane, it hosts a system of four voltage-dependent conductances (g _A , g _KV , g _KCa , g _Ca ). The conductance behavior was mathematically described using original patch-clamp experimental data. The transient K current, IA, was isolated as the difference between the currents obtained before and after removing IA inactivation. The remaining current, IKD, results from the summation of a voltage-dependent K current, IKV, a voltage-calcium-dependent K current, IKCa, and the calcium current, ICa. IKD was modeled as a single lumped current, since the physiological role of each component is actually not discernible. To gain a clear understanding of its prominent role in sustaining transmitter release at the cytoneural junction, ICa was modeled under different experimental conditions. The model includes the description of voltage- and time-dependent kinetics for each single current. After imposing any starting holding potential, the system sets the pertinent values of the variables and continually updates them in response to variations in membrane potential. The model reconstructs the individual I-V curves obtained in voltage-clamp experiments and simulations compare favorably with the experimental data. The model proves useful in describing the early steps of signal processing that results from the interaction of the apical receptor current with the basolateral voltage-dependent conductances. The program is thus helpful in understanding aspects of sensory transduction that are hard to analyze in the native hair cell of the crista ampullaris.