We serendipitously discovered that the preferred-direction
responses of ON–OFF directionally selective (DS)
ganglion cells in the rabbit retina fall as a function
of contrast when the contrast of a moving bar exceeds about
100%. Null-direction responses did not fall for contrasts
up to 400%. Because the non-monotonic (rise-then-fall)
behavior as a function of contrast occurred only for preferred-direction
responses, it must depend on the mechanism of directional
selectivity. It became thus of interest to investigate
how this non-monotonicity depends on the major synapses
involved in directional selectivity. Blockades of nicotinic
acetylcholine (ACh) and NMDA glutamate receptors reduced
responses without eliminating preferred-response non-monotonicity.
Blocking GABAergic inhibition, however, did eliminate non-monotonicity.
These results pose a difficult puzzle, since in the accompanying
paper (Grzywacz et al., 1998), we showed that residual
responses under combined nicotinic and NMDA blockades are
not statistically significantly directionally selective.
How is it possible that null-direction GABAergic inhibition
affects non-nicotinic-non-NMDA residual responses without
generating directional selectivity? This may happen if
there exists an asymmetric GABAergic input to distal dendrites
of the DS cell while the excitatory, non-nicotinic-non-NMDA
input is to proximal dendrites. In support of this hypothesis,
bath-applied GABA reduces responses to exogenous ACh under
synaptic block, providing for the first time in the rabbit's
retina, direct evidence of GABA receptors on DS cells.