Neuromodulation intrinsic to the central pattern generator for escape swimming in Tritonia

Extrinsic neuromodulatory inputs to central pattern generators (CPGs) can alter the properties and synaptic interactions of neurons in those circuits and thereby modify the output of the CPG. Recent work in a number of systems has now demonstrated that neurons intrinsic to CPG can also evoke neuromodulatory actions on other members of the CPG. Such "intrinsic neuromodulation" plays a role in controlling the CPG underlying the escape swim response of the nudibrach mollusc, Tritonia diomedea. The dorsal swim interneurons (DSIs) are a bilaterally represented set of three serotonergic neurons that participate in the generation of the rhythmic swim motor program. Serotonin released from these CPG neurons functions both as a fast neurotransmitter and as a slower neuromodulator. In its modulatory role, serotonin enhances the release of neurotransmitter from another CPG neuron, C2, and also increases C2 excitability by decreasing spike frequency adaptation. These neuromodulatory actions intrinsic to the CPG may be important for the initial reconfiguration of the system into a function CPG and for experience-dependent changes in the output such as behavioral sensitization and habituation.