Uptake of [3H]serotonin in the abdominal ganglion of Aplysia californica. Further studies on the morphological and biochemical basis of presynaptic facilitation.

Sensitization of the gill-withdrawal reflex in Aplysia california is mediated, in part, by a group of identified neurons, the L29 cells, which produce presynaptic facilitation of transmitter release from siphon sensory neurons. Physiological and pharmacological studies have provided indirect evidence that the L29 cells are serotonergic. In the present study we have used the specific uptake [3H]serotonin ([3H]5-HT) and electron-microscopic autoradiography in combination with horseradish peroxidase-labeling of identified neurons to characterize the fine structure of Aplysia serotonergic terminals and to examine more directly the transmitter biochemistry of the L29 neurons. Abdominal ganglia were incubated for 2 h in 10(-6) M [3H]5-HT and thick and thin plastic sections examined with the light and electron microscope. L29 varicosities, identified by labeling with HRP, were found to accumulate [3H]5-HT. In addition, [3H]-5-HT was localized to unidentified varicosities within the neuropil as well as to vesicle-filled terminals that formed axosomatic contacts in the cortical regions of the ganglion. The processes that accumulated [3H]5-HT contained conspicuous dense core vesicles identical in morphology to those previously described for L29. Some processes were found to make contact with HRP-labeled varicosities of sensory neurons. Comparison with results obtained from ganglia exposed to [3H]5-HT in the presence of either non-radioactive 5-HT or non-radioactive dopamine indicate that the uptake process is transmitter-specific. These studies provide additional evidence that the L29 cells are serotonergic and are consistent with the notion that aminergic neurons may be preferentially involved in modulatory synaptic actions.