Mechanisms of after-hyperpolarization following activation of fly visual motion-sensitive neurons.

In many neurons, strong excitatory stimulation causes an after-hyperpolarization (AHP) at stimulus offset, which might give rise to activity-dependent adaptation. Graded-potential visual motion-sensitive neurons of the fly Calliphora vicina respond with depolarization and hyperpolarization during motion in their preferred direction and their anti-preferred direction, respectively. A prominent after-response, opposite in sign to the response during motion, is selectively expressed after stimulation with preferred-direction motion. Previous findings suggested that this AHP is generated in the motion-sensitive neurons themselves rather than in presynaptic processing layers. However, it remained unknown whether the AHP is caused by membrane depolarization itself or by another process, e.g. a signaling cascade triggered by activity of excitatory input channels. Here we showed by current injections and voltage clamp that the AHP and a corresponding current are generated directly by depolarization. To test whether the generation of an AHP is linked to depolarization via a Ca(2+)-dependent mechanism, we used photoactivation of a high-affinity Ca(2+) buffer. In accordance with previous findings the AHP was insensitive to manipulation of cytosolic Ca(2+). We propose that membrane depolarization presents a more direction-selective mechanism for the control of AHP than other potential control parameters.