Effects of Ca 2+ channel blockers on directional selectivity of rabbit
retinal ganglion cells.
Jensen, Ralph.
Department of Biomedical Sciences, Southern College of Optometry, Memphis,
Tennessee 38104.
APStracts 2:0038N, 1995.
SUMMARY AND CONCLUSIONS
1. Extracellular recordings were made from ON/OFF directionally selective
ganglion cells in superfused rabbit retinas in order to examine the effects of
voltage-activated Ca 2+ channel blockers on the response of these ganglion
cells to a moving bar of light. 2. Bath application of Cd 2+ (67-110 [mu]M)
abolished directional selectivity in the ganglion cells. That is, the cells
gave near-equal responses to the leading and trailing edges of a bar of light
moved in the preferred and null directions. This effect of Cd 2+ was rapidly
reversible. 3. Directional selectivity in the ganglion cells was not affected
by Ni 2+ (120-440 [mu]M), Co 2+ (180-690 [mu]M), or the L-type Ca 2+ channel
blockers nicardipine (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. _-Conotoxin MVIIC (130 nM -1.9
[mu]M), which potently blocks N-type and Q-type Ca 2+ channels, abolished
directional selectivity in the ganglion cells. _-Conotoxin MVIIC not only
brought out large leading and trailing edge responses to movement 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 light in the preferred direction.
The effect of _- conotoxin MVIIC was slowly reversible. 5. The N-type Ca 2+
channel blocker _- conotoxin GVIA (1.4-6.3 [mu]M) did not abolished
directional selectivity in the ganglion cells. This blocker did however bring
out some response to the leading edge of a bar of a light moved in the null
direction. This effect of _-conotoxin GVIA appeared to be irreversible. 6. _-
Agatoxin IVA, a potent blocker of P-type Ca 2+ channels, when bath-applied at
low concentrations (66-83 nM) increased the responses to movement of a bar of
light in the preferred direction but brought out only small responses to
movement of the bar of light in the null direction. At high concentrations
(250-280 nM) that reportedly block Q-type Ca 2+ channels by at least 50 %, _-
agatoxin IVA nearly abolished directional selectivity. This effect of _-
agatoxin IVA was slowly reversible. 7. These results indicate that _-
conotoxin MVIIC- and _- agatoxin IVA-sensitive Ca 2+ channels (possibly Q-type
channels) play an important role in the generation of directional selectivity
in rabbit retinal ganglion cells.
Received 3 October 1994; accepted in final form 6 March 1995.
APS Manuscript Number J619-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 3 April 1995.