Fig. 6. Effects of serotonin on light-induced responses of medulla bilateral neurons (MBNs) recorded from the separated proximal optic stalk. (A,B) Average intensity—response curves showing the effect of serotonin (10 pmol) on MBN photo-responsiveness during the day (A) and the night (B). The number of spikes induced by a light pulse is plotted against the corresponding light intensities. Filled and open circles indicate values either before (filled circles) or after injection (open circles). The effect of serotonin was greater during the night than during the day. (C) Dose—response curve showing the ability of serotonin to suppress light-evoked MBN activity during the day (open squares) and night (filled squares). Note that the sensitivity of MBN to serotonin was always significantly greater during the night than the day (c, P<0.01, t-test). Values are means ± S.E.M. of 5-8 preparations. a and b indicate a significant difference (P<0.01, t-test) compared with Ringer-injected control values for day and night, respectively.