Outputs of radula mechanoafferent neurons in Aplysia are modulated by motor neurons, interneurons, and sensory neurons

Outputs of radula mechanoafferent neurons in Aplysia are modulated by motor neurons, interneurons, and sensory neurons. J. Neurophysiol. 83: 1621-1636 , 2000. The gain of sensory inputs into the nervous system can be modulated so that the nature and intensity of afferent input is variable. Sometimes the variability is a function of other sensory inputs or of the state of mo tor systems that generate behavior. A form of sensory modulation was invest igated in the Aplysia feeding system at the level of a radula mechanoaffere nt neuron (B21) that provides chemical synaptic input to a group of motor n eurons (B8a/b, B15) that control closure and retraction movements of the ra dula, a food grasping structure. B21 has been shown to receive both excitat ory and inhibitory synaptic inputs from a variety of neuron types. The curr ent study investigated the morphological basis of these heterosynaptic inpu ts, whether the inputs could serve to modulate the chemical synaptic output s of B21, and whether the neurons producing the heterosynaptic inputs were periodically active during feeding motor programs that might modulate B21 o utputs in a phase-specific manner. Four cell types making monosynaptic conn ections to B21 were found capable of heterosynaptically modulating the chem ical synaptic output of B21 to motor neurons B8a and B15. These included th e following: I) other sensory neurons, e.g., B22; 2) interneurons, e.g., B1 9; 3) motor neurons, e.g., B82; and 4) multifunction neurons that have sens ory, motor, and interneuronal functions, e.g., B4/5. Each cell type was pha sically active in one or more feeding motor programs driven by command-like interneurons, including an egestive motor program driven by CBI-I and an i ngestive motor program driven by CBI-2. Moreover, the phase of activity dif fered for each of the modulator cells. During the motor programs, shifts in B21 membrane potential were related to the activity patterns of some of th e modulator cells. Inhibitory chemical synapses mediated the modulation pro duced by B4/5, whereas excitatory and/or electrical synapses were involved in the other instances. The data indicate that modulation is due to block o f action potential invasion into synaptic release regions or to alterations of transmitter release as a function of the presynaptic membrane potential . The results indicate that just as the motor system of Aplysia can be modu lated by intrinsic mechanisms that can enhance its efficiency, the properti es of primary sensory cells can be modified by diverse inputs from mediatin g circuitry. Such modulation could serve to optimize sensory cells for the different roles they might play.