Diverse synaptic connections between peptidergic radula mechanoafferent neurons and neurons in the feeding system of Aplysia

Diverse synaptic connections between peptidergic radula mechanoafferent neu rons and neurons in the feeding system of Aplysia. J. Neurophysiol. 83: 160 5-1620, 2000. The buccal ganglion of Aplysia contains a heterogeneous popul ation of peptidergic, radula mechanoafferent (RM) neurons. To investigate t heir function, two of the larger RM cells (B21, B22) were identified by mor phological and electrophysiological criteria. Both are low-threshold, rapid ly adapting, mechanoafferent neurons that responded to touch of the radula, the structure that grasps food during ingestive and egestive feeding movem ents. Sensory responses of the cells consisted of spike bursts at frequenci es of 8-35 Hz. Each cell was found to make chemical, electrical, or combine d synapses with other sensory neurons, motor neurons and interneurons invol ved in radula closure and/or protraction-retraction movements of the odonto phore. Motor neurons receiving input included the following: B8a/b, B15, an d B16, which innervate muscles contributing to radula closing; and B82, a n ewly identified neuron that innervates the anterodorsal region of the I1/I3 muscles of the buccal mass. B21 and B22 can affect buccal motor programs b y way of their connections to interneurons such as B19 and B64. Fast,chemic al, excitatory postsynaptic potentials (EPSPs) produced by RM neurons, such as B21, exhibited strong, frequency-dependent facilitation, a form of homo synaptic plasticity. Firing B21 also produced a slow EPSP in B15 that incre ased the excitability of the cell. Thus a sensory neuron mediating a behavi oral response may have modulatory effects. The data suggest multiple functi ons for RM neurons including 1) triggering of phase transitions in rhythmic motor programs, 2) adjusting the force of radula closure, 3) switching fro m biting to swallowing or swallowing to rejection, and 4) enhancing food-in duced arousal.