CIRCADIAN ORGANIZATION IN APLYSIA EXPLORED WITH RED-LIGHT, EYE REMOVAL AND BEHAVIORAL RECORDING

We tested the sensitivity of the Aplysia activity rhythm to red light in three ways: first, by measuring the response of the rhythm to 24 h cycles of red light (RD), second, by measuring the effect of constant red background light (RR) on the rhythm driven by superimposed white LD, third, by measuring the effect of RR on freerunning. In several experiments we removed the eyes in order to assess their role in mediating the action of red light. Results for individuals and groups were analyzed by calculating form estimates and periodograms. 1. When tested with RD, intact animals were predominantly, though not exclusively, diurnal. In R D 8:1G activity typically began several hours before dawn (Figs. 1, 3). 2. Eye removal did not prevent diurnal behavior but caused (i) a prominent decrease in activity prior to dawn, (ii) an increase in activity immediately after dawn, (iii) a small decrease of activity during the late lighttime and (iv) a small increase of activity during the mid darktime (Figs. 2, 3). The contribution of the eyes to the entrained locomotor rhythm is thus facilitory at some phases and suppressive at other phases. 3. Continuous red background light reduced the amplitude of the diurnal rhythm driven by superimposed white LD 12:12. Activity during the white-on time was suppressed and activity during the white-off time was enhanced by continuous red background. This effect was greater in eyeless than in intact animals (Figs. 4, 5). 4. In contrast to its effects on the driven rhythm, continuous red background light strengthened freerunning (Figs. 6, 7, Table 1). These results show that Aplysia have photoreceptors outside the eyes that are sensitive to red light and that are coupled to locomotion. It is unlikely that the eyes themselves are red sensitive photoreceptors that can reflexively drive locomotion in red lightcycles. On the other hand, it is quite likely that the eyes are a part of a circadian pacemaking system that can be entrained by extraocular red sensitive photoreceptors. We have drawn a hypothetical flow diagram that relates the known facts about circadian organization in Aplysia (Fig. 8). In the diagram rhythmic behavior arises from the convergence and integration of temporal information from several classes of photoreceptors. Internally timed signals from the eyes both facilitate locomotion and uncouple locomotion from extraocular mechanisms. © 1979 Springer-Verlag.