Part I
The isolated eye of Aplysia californica produces a burstingpattern of spontaneous compound action potentials (CAPs) when recordings are made from the optic nerve in darkness. The CAP frequency varieswith a circadian rhythm. The light response, also composed of CAPs,may be separated into an initial phasic response and a late tonic response similar in form to the dark discharge. Solutions containingLa+++ or high Mg++ with low Ca++, which are expected to block chemicalsynapses, stop the dark discharge and tonic light response but not the phasic light response. The suppression of dark discharge by high Mg++with low Ca++ is usually temporary, lasting about 0.5 to 4 hours.Synchrony of the CAPs is not affected by either La+++ or high Mg++, low Ca++. These results indicate that the dark discharge is driven through chemical synapses, but the light response is not. Replacement of chloride in the bathing medium by propionate, which uncouples electrical junctions in the crayfish septate axon, abolishes all CAPs for varying periods of time, usually several hours. Propionate leaves the ERGintact and the optic nerve electrically excitable. A model for inter-neuronal connections in the Aplysia eye is constructed from these data. It is postulated that the light response is initiated in the photoreceptors, with the receptor depolarization passing through electrical synapses to higher order cells. Spikes are produced in these cells and pass down their axons in the optic nerve. Spontaneous dark activityalso represents spiking in these higher order cells, but is initiatedthrough chemical synapses by pacemaker cell(s). Synchrony of the CAPs is facilitated by electrical synapses between higher order cells. Inlow Ca++ media, these higher order cells may become hyperexcitable tothe point of autoactivity.
Part II
The circadian rhythm of spike output of the single neuron R15 in the isolated PVG of Aplysia californica can be entrained in vivo by light. The timing of the rhythm depends not only on the lightingschedule to which the animal was exposed prior to dissection, but also on the time of dissection relative to that light schedule. Entrainment of the rhythm by light proceeds very slowly, if at all, in Aplysia with their eyes removed. An indirect inhibitory neural pathway isshown to exist between the eyes and R15, but cutting nervous connections containing this and any other neural paths from the eyes to R15 does not prevent entrainment by light in a majority of animals. In vitro experiments show that the eyes can influence the activity of R15 even when the eyes and the PVG are not neurally connected.The eyes therefore must release a water soluble factor which can affectR15, either directly or through some other neurons in the PVG. If the eyes and PVGs from different animals are incubated together forseveral days and then separated, the subsequent spiking behavior of R15 is similar to that observed after in vivo entrainment to a light schedule equivalent in phase to the circadian rhythm of the eyes in vitro. It is a strong possibility that the factor released by theeyes can entrain the circadian rhythm of R15.
