A surprisingly big role for big K+ channels in retinal horizontal cells

Horizontal cells are lateral inhibitory neurons in the outer part of the retina that receive excitatory synaptic inputs from rod and cone photoreceptors and, in turn, make reciprocal inhibitory contacts back onto photoreceptor synapses (Thoreson & Mangel, 2012). These lateral connections generate inhibitory surrounds in receptive fields of downstream neurons and also contribute to chromatic opponency in the retina. Additionally, horizontal cells can provide positive feedback to cones as well as feedforward inhibition to bipolar cells. In darkness, when glutamate release from photoreceptors is maximal, horizontal cells have a resting membrane potential of ca −30 mV. Horizontal cells hyperpolarize in response to light as glutamate release from photoreceptors is diminished, but various ion currents are also important for shaping their responses. Ion currents have been characterized in horizontal cells from numerous species of fish, reptiles and mammals. Investigators have consistently found five currents: fast sodium currents, delayed rectifier-type outward K+ currents, rapidly inactivating A-type K+ currents, L-type Ca2+ currents (ICa) and inwardly rectifying K+ currents. A study in this issue of The Journal of Physiology by Sun et al. (2017) adds to this long-standing catalogue by showing that rat and mouse horizontal cells exhibit substantial currents mediated by large conductance calcium-activated K+ channels [also known as MaxiK or big K+ (BK) channels]. The findings of Sun et al. are surprising since these currents were not observed in the many earlier studies of horizontal cells despite the fact that BK channels are present in many neurons. The persuasive evidence for BK channels provided by Sun et al. includes the presence of outward currents in isolated rodent horizontal cells whose activation matched the voltage dependence and time dependence of ICa and were blocked by Cd2+; the presence of single channel currents with a large single channel conductance characteristic of BK channels; inhibition by specific BK channel blockers (iberiotoxin and paxilline); and activation by a BK channel activator (NS1619). Why were these channels not observed in earlier studies? Many earlier studies on horizontal cells did not test explicitly for calcium-activated K+ currents and other currents can obscure them. But a study in goldfish horizontal cells that looked for calcium-activated K+ currents also failed to find them (Tachibana, 1983). Most mammals have two types of horizontal cells, A and B (Peichl & Gonzalez-Soriano, 1994), but rats and mice have only a single B-type. Single channel recordings from B-type, but not A-type, horizontal cells from rabbit showed evidence for Ca2+-activated K+ channels (Lohrke & Hoffmann, 1994) so perhaps rather than representing a peculiarity of rodent horizontal cells, BK channels may be a general property of B-type horizontal cells in mammals. Further studies are needed to assess this possibility. The pore forming α subunits of BK channels are encoded by a single gene, but channel properties can be shaped by the presence of splice variants, post-translational modifications, and accessory β and γ subunits (Latorre et al. 2017). Sun et al. found that BK channels in horizontal cells inactivated rapidly and that treating the intracellular membrane surface with a protease reduced inactivation showing that it involves intrinsic properties of the channel complex. Further molecular characterization awaits, but the authors suggest that β2 subunits and β3 splice variants that promote rapid inactivation of BK channels may mediate inactivation in horizontal cells. What roles might these channels play in horizontal cells? BK currents in rodent horizontal cells can be substantial, accounting for half of the outward current when fully active at +50 mV. By comparing effects of the fast and slow Ca2+ buffers BAPTA and EGTA, Sun et al. concluded that BK channels are located in Ca2+ nanodomains quite close to L-type Ca2+ channels in these cells. BK channels should therefore be activated immediately upon opening of a nearby Ca2+ channel and, accordingly, BK current activation closely tracks ICa activation. In other neurons, the close coupling between BK and Ca2+ channels functions to accelerate repolarization during action potentials, but horizontal cells do not generate action potentials because of their low input resistance in situ. Instead, activation of ICa appears to be more important for speeding horizontal cell depolarization at the termination of a hyperpolarizing light response and the coordinated activity of Ca2+ and BK channels is likely to shape the frequency responses of horizontal cells. Activation of BK channels also appears to restrain horizontal cells from depolarizing above their resting potential in darkness. The impact of these channels will be influenced by their locations on dendrites and soma as well as the extent to which BK and Ca2+ channels are co-localized. Incorporation of the properties of BK channels into existing biophysical models of horizontal cell performance could provide further insights into the roles played by these channels. The impact of BK currents on horizontal cells will shape photoreceptor output by influencing feedback and feedforward interactions of horizontal cells with bipolar cells and photoreceptors. The very first intracellular recordings in vertebrate retina were obtained from horizontal cells and these cells have since received considerable study. However, as the study by Sun et al. demonstrates, these well-studied neurons still hold some surprises.