Coordination of fast and slow rhythmic neuronal circuits

Interactions among rhythmically active neuronal circuits that oscillate at different frequencies are important for generating complex behaviors, yet l ittle is known about the underlying cellular mechanisms. We addressed this issue in the crab stomatogastric ganglion (STG), which contains two distinc t but interacting circuits. These circuits generate the gastric mill rhythm (cycle period, similar to 10 sec) and the pyloric rhythm (cycle period, si milar to 1 sec). When the identified modulatory projection neuron named mod ulatory commissural neuron 1 (MCN1) is activated, the gastric mill motor pa ttern is generated by interactions among MCN1 and two STG neurons [the late ral gastric (LG) neuron and interneuron 1]. We show that, during MCN1 stimu lation, an identified synapse from the pyloric circuit onto the gastric mil l circuit is pivotal for determining the gastric mill cycle period and the gastric-pyloric rhythm coordination Td examine the role of this intercircui t synapse, we replaced it with a computational equivalent via the dynamic-c lamp technique. This enabled us to manipulate better the timing and strengt h of this synapse. We found this synapse to be necessary for production of the normal gastric mill cycle period. The synapse acts, during each LG neur on interburst, to boost rhythmically the influence of the modulatory input from MCN1 to LG and thereby to hasten LG neuron burst onset. The two rhythm s become coordinated because LG burst onset occurs with a constant latency after the onset of the triggering pyloric input. These results indicate tha t intercircuit synapses can enable an oscillatory circuit to control the sp eed of a slower oscillatory circuit, as well as provide a mechanism for int ercircuit coordination.