Circadian Rhythm of Neuron R15 of Aplysia californica: In Vivo Photoentrainment.

(1) The neuron R15 in the parietovisceral ganglion of Aplysia has a circadian rhythm of spiking activity when recorded in the isolated ganglion. The rhythm is entrained in vivo by light-dark cycles. (2) The phase of the R15 rhythm is a function not only of the entraining light schedule, but also of the time of dissection. Changes in the dissection time during the light portion of the light-dark cycle yield little change in the subsequent R15 peak time. Dissections during the dark portion produce peak times that vary with dissection time with a slope that is approximately one. (3) The circadian rhythm of R15 can be phase-shifted in vivo by changes in the phase of the entraining light-dark cycle in one to two weeks. R15 neurons of blinded Aplysia, however, show little or no phase shift in this time. (4) It is concluded that the eyes are important as receptors for the photoentrainment of the R15 rhythm in vivo, but that neural connections from the eyes to R15 are not required.