Bag cell electrical activity underlying spontaneous egg laying in freely behaving Aplysia brasiliana

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Abstract
The neuroendocrine bag cells in the abdominal ganglion of Aplysia contain a hormone capable of inducing egg laying. The electrical activity of the bag cells preceding spontaneous egg laying (i.e., no electrical stimulation) was recorded in freely behaving A. brasiliana with surgically implanted cuff electrodes. In normative behavioral studies, isolation of animals (in perforated cages within large aquaria) increased the frequency of egg laying. Amount of eggs laid increased with time after previous egg laying. Spontaneous bag cell activity and normal egg laying always occurred together (latency .apprx. 30 min). In vivo spike trains lasted .apprx. 21 min, and no relation was apparent between duration of bag cell activity and amount of eggs laid. A train of synchronous long-duration spikes by the bag cells normally triggered this behavior. At onset of activity, synchronous spikes increased in amplitude and frequency as they invaded the network of bag cell neurites. Spike potentiation led to high frequency, regular firing. Spike rate remained high for .apprx. 45 s and then declined to irregular bursts. Most bag cell spikes originated in distal neurites and conducted proximally along the connectives, but some propagated in the opposite direction. Conduction failure occurred along bag cell neurites. In vivo electrical activity of the bag cells before natural egg laying appears equivalent to responses previously evoked in vitro by electrical stimulation of a pleurovisceral connective. The duration and temporal pattern of the spike train and the waveforms and conduction of bag cell spikes are similar in vivo and in vitro.