EFFECT OF A SEROTONERGIC EXTRINSIC MODULATORY NEURON (MCC) ON RADULA MECHANOAFFERENT FUNCTION IN APLYSIA

The serotonergic metacerebral cells (MCCs) and homologous neurons in r elated mollusks have been extensively investigated within the context of feeding. Although previous work has indicated that the MCCs exert w idespread actions, MCC modulation of sensory neurons has not been iden tified. We characterized interactions between the MCCs and a cell that is part of a recently described group of buccal radula mechanoafferen ts. The cell, B21, has a peripheral process in the tissue underlying t he chitinous radula [the subradula tissue (SRT)]. Previous studies hav e shown that B21 can fire phasically during ingestive motor programs a nd provide excitatory drive to the circuitry active during radula clos ing/retraction. We now show that activity of B21 can be modulated by s erotonin (5-HT) and the MCCs. Centrally, although a slow depolarizatio n is typically recorded in B21 as a result of MCC stimulation, this de polarization does not cause B21 to spike. It can, however, increase B2 1 excitability enabling a pulse that was previously subthreshold to el icit an action potential in B21. B21 is in fact rhythmically depolariz ed during the radula closing/retraction phase of ingestive motor progr ams. Thus central effects of the MCCs on radula mechanoafferent activi ty are only likely to be apparent while B21 is receiving input from th e feeding central pattern generator. Peripherally, radula mechanoaffer ent neurons can be activated 1) when a mechanical stimulus is applied to the biting surface of the SRT and 2) when the SRT contracts. MCC st imulation and 5-HT modulate B21 responses to both types of stimuli. Fo r example, MCC stimulation and low concentrations of 5-HT cause subthr eshold mechanical stimuli applied to the SRT to become suprathreshold. 5-HT and MCC stimulation also enhance SRT contractility. Peripheral e ffects of MCC activity are also likely to be phase dependent. For exam ple, MCC stimulation does not cause B21 to respond to peripheral stimu li with an afterdischarge. Consequently, radula mechanoafferents are l ikely to be activated when food is present between the radula halves d uring radula closing/retraction but are not likely to continue to fire as opening/protraction is initiated. In a similar vein, MCC effects o n the contractility of the SRT will only be apparent when contractions are elicited by motor neuron activity. SRT motor neurons are rhythmic ally activated during ingestive motor programs. Thus we have shown tha t radula mechanoafferent activity can be modulated by the MCCs and tha t this modulation is likely to occur in a phase-dependent manner.