Effect of a serotonergic extrinsic modulatory neuron (MCC) on radula mechanoafferent function in Aplysia

The serotonergic metacerebral cells (MCCs) and homologous neurons in related mollusks have been extensively investigated within the context of feeding. Although previous work has indicated that the MCCs exert widespread actions, MCC modulation of sensory neurons has not been identified. We characterized interactions between the MCCs and a cell that is part of a recently described group of buccal radula mechanoafferents. The cell, B21, has a peripheral process in the tissue underlying the chitinous radula [the subradula tissue (SRT)]. Previous studies have shown that B21 can fire phasically during ingestive motor programs and provide excitatory drive to the circuitry active during radula closing/retraction. We now show that activity of B21 can be modulated by serotonin (5-HT) and the MCCs. Centrally, although a slow depolarization is typically recorded in B21 as a result of MCC stimulation, this depolarization does not cause B21 to spike. It can, however, increase B21 excitability enabling a pulse that was previously subthreshold to elicit an action potential in B21. B21 is in fact rhythmically depolarized during the radula closing/retraction phase of ingestive motor programs. Thus central effects of the MCCs on radula mechanoafferent activity are only likely to be apparent while B21 is receiving input from the feeding central pattern generator. Peripherally, radula mechanoafferent 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 stimulation and 5-HT modulate B21 responses to both types of stimuli. For example, MCC stimulation and low concentrations of 5-HT cause subthreshold mechanical stimuli applied to the SRT to become suprathreshold. 5-HT and MCC stimulation also enhance SRT contractility. Peripheral effects of MCC activity are also likely to be phase dependent. For example, MCC stimulation does not cause B21 to respond to peripheral stimuli with an afterdischarge. Consequently, radula mechanoafferents are likely to be activated when food is present between the radula halves during radula closing/retraction but are not likely to continue to fire as opening/protraction is initiated. In a similar vein, MCC effects on the contractility of the SRT will only be apparent when contractions are elicited by motor neuron activity. SRT motor neurons are rhythmically activated during ingestive motor programs. Thus we have shown that radula mechanoafferent activity can be modulated by the MCCs and that this modulation is likely to occur in a phase-dependent manner.