Locomotion in Aplysia: triggering by serotonin and modulation by bag cell extract

Abstract

Locomotion in Aplysia is mediated by a central program. We have found that this program is triggered by the biogenic amine, serotonin (5HT), and modulated by an endogenous source of neuropeptides, bag cell extract. In examining the triggering of locomotion, we found that injecting 5-HT into the bloodstream of quiescent animals elicited locomotion in both intact, freely moving animals and split-foot preparations. Increasing the concentration of 5-HT from lop7 to lop3 M produced both a monotonic increase in the number of steps elicited over a 5-min period after injection and a monotonic decrease in the first interstep interval. The threshold for triggering locomotion was lop7 M in split-foot preparations. Application of a saturated salt solution to the tail-a natural trigger for escape locomotionfacilitated 5-HT locomotion. The effects of 5-HT on locomotion were relatively specific since they were not mimicked by three other biogenic amines known to exist in Aplysia (dopamine, octopamine, and histamine). 5-HT could still trigger locomotion after isolating the pedal and pleural ganglia from the rest of the central nervous system, suggesting that a primary site for the action of 5-HT on locomotion lies in the pedal and pleural ganglia. In examining the modulation of locomotion, we found that injecting bag cell extract, an endogenous source of neuropeptides in Aplysia which elicits egg-laying behavior, reversibly suppressed 5-HTtriggered locomotion. As in the case with serotonin, bag cell extract suppression of locomotion was still observed in animals in which the pleural and pedal ganglia were isolated from the rest of the central nervous system. The suppression of locomotion was specific to bag cell extract since no suppression was produced by extracts of other central nervous tissue (cerebral ganglia). Moreover, the suppressive effect of bag cell extract was abolished by prior treatment of the extract with pronase, suggesting that the active suppressive agent is one or more of the peptides specific to the bag cells. Thus, locomotion in Aplysia provides a useful model system for examining both the triggering and the modulation of a fixed action pattern.

Dispensable, Redundant, Complementary, and Cooperative Roles of Dopamine, Octopamine, and Serotonin inDrosophila melanogaster

“…5-HT is important for regulation of locomotion in some invertebrates (Willard 1981;Mackey and Carew 1983;Nusbaum 1986), and it has been previously suggested that both 5-HT and DA might contribute to the regulation of larval motor function in Drosophila (Mackey and Carew 1983;Neckameyer 1996;Cooper and Neckameyer 1999). We do not detect a significant increase in locomotion in larvae that express DVMAT in 5-HT neurons using TrH-Gal4 (Figure 2A, TrH), in DA neurons using TH-Gal4 (Figure 2A, TH), or, in a separate set of experiments, in both DA and 5-HT neurons using DdcGal4 ( Figure S4).…”

A potentially novel nicotinic receptor inAplysianeuroendocrine cells

“…The smaller-magnitude nicotinic response in bag cell neurons would be insufficient to permit toxin-mediated activation of reproduction. Because egg-laying behavior lowers the defenses of Aplysia (Goldsmith and Byrne 1993;Mackey and Carew 1983), this could protect the animal from predation.…”

Neural Analog of Arousal: Persistent Conditional Activation of a Feeding Modulator by Serotonergic Initiators of Locomotion

“…Moreover, a higher CC9 -10 activity level during programs elicited by P9 vs AT 4 stimulation is associated with a faster cycle frequency of locomotor activity. Together, these findings may account for the behavioral findings that serotonin promotes locomotion (Palovcik et al, 1982;Mackey and Carew, 1983). Notably, CC9 -10 and type I command neurons for locomotion identified earlier (Fredman and Jahan-Parwar, 1983) are similar in their spiking pattern, axonal projections, locations, and roles in locomotion, but CC9 -10 are electrically coupled whereas electrical coupling was not found in type I neurons.…”

“…All statistical tests were performed using GraphPad Prism 4.0. (Palovcik et al, 1982;Mackey and Carew, 1983;Kabotyanski et al, 2000). In the feeding system, serotonin is expressed in the MCC neurons (Eisenstadt et al, 1973;Weinreich et al, 1973), which mediate food-induced arousal of the feeding network Rosen et al, 1989).…”