EFFECTS OF CA2-CELLS( CHANNEL BLOCKERS ON DIRECTIONAL SELECTIVITY OF RABBIT RETINAL GANGLION)

(1) Extracellular recordings were made from ON-OFF directionally selec tive ganglion cells in superfused rabbit retinas in order to examine t he effects of voltage-activated Ca2+ channel blockers on the response of these ganglion cells to a moving bar of Light. 2. Bath application of Cd2+ (67-110 mu M) abolished directional selectivity in the ganglio n cells. That is, the cells gave nearly equal responses to the leading and trailing edges of a bar of light moved in the preferred and null directions. This effect of Cd2+ was rapidly reversible. 3. Directional selectivity in the ganglion cells was not affected by Ni2+ (120-440 m u M), Co2+ (180-690 mu M), or the L-type Ca2+ channel blockers nicardi pine (7-29 mu M) and methoxyverapamil (18-60 mu M) These blockers did, however, reduce the responses of the ganglion cells to a bar of light moved in the preferred direction. 4. omega-Conotoxin MVIIC (130 nM-1. 9 mu M), which potently blocks N-type and Q-type Ca2+ channels, abolis hed directional selectivity in the ganglion cells. omega-Conotoxin MVI IC not only brought out large leading and trailing edge responses to m ovement of a bar of light in the null direction, but it also increased the leading and trailing edge responses to movement of the bar of lig ht in the preferred direction. The effect of omega-conotoxin MVIIC was slowly reversible. 5. The N-type Ca2+ channel blocker omega-conotoxin GVIA (1.4-6.3 mu M) did not abolish directional selectivity in the ga nglion cells. This blocker did, however, bring out some response to th e leading edge of a bar of a light moved in the null direction. This e ffect of omega-conotoxin GVIA appeared to be irreversible. 6. omega-Ag atoxin IVA, a potent blocker of P-type Ca2+ channels, when bath applie d at low concentrations (66-83 nM), increased the responses to movemen t of a bar of light in the preferred direction but brought out only sm all responses to movement of the bar of light in the null direction. A t high concentrations (250-280 nM) that reportedly block Q-type Ca2+ c hannels by greater than or equal to 50%, omega-agatoxin IVA nearly abo lished directional selectivity. This effect of omega-agatoxin TVA was slowly reversible. 7. These results indicate that omega-conotoxin MVII C- and omega-agatoxin IVA-sensitive Ca2+ channels (possibly Q-type cha nnels) play an important role in the generation of directional selecti vity in rabbit retinal ganglion cells.