Ionic currents in cultures horizontal cells isolated from white bass retinas

This dissertation presents an in-depth characterization of the voltage-dependent potassium and calcium currents of cultured horizontal cells (HCs) isolated from the retinas of white bass. The functional characteristics of the sodium current, transient potassium current, sustained calcium current and glutamate-gated current-clamp are also described, in an effort to understand the roles of these current in vivo. In vivo, HCs received input from photoreceptors, and respond to changes in background illumination with graded changed in membrane potential. HCs are responsible for the inhibitory surround of bipolar cells. The data suggest that voltage-dependent potassium and calcium currents make little contribution to changes in HC membrane potential in vivo. The unusually small sustained potassium current in these HCs is important for their ability to maintain two stable membrane potentials. Although this bistable membrane potential phenomenon is not observed in vivo, the small size of the underlying voltage-dependent currents is likely to have important implications for the functions of these cells in vivo. The roles of ionic currents in the membrane response of cultured HCs were examined under current-clamp. Data suggest that HCs are nonspiking in vivo because, although they possess an apparently large transient sodium current, the density of sodium channels in their membranes is actually much lower than in more typical neurons. The sodium current is not sufficiently large to regenerate rapidly enough to produce an action potential before the sodium channels inactivate. The calcium current plays only a small role in helping to maintain the membrane at a depolarized potential. The transient outward potassium current slows the rate at which the membrane can depolarize. Glutamate fails to produce graded changes in membrane potential. In the presence of glutamate, the membrane was never maintained at a potential between the negative resting potential and a level close to the reversal potential of the glutamate-gated current (about + 10 mV). These results call into questions the generally accepted hypothesis that at any moment the HC membrane potential in vivo represents a balance between glutamate-gated and voltage-dependent current, and suggest that a ligand-gated current with a negative reversal potential may play a role.