Monoamine control of the pacemaker kernel and cycle frequency in the lobster pyloric network

The monoamines dopamine (DA), serotonin (5HT), and octopamine (Oct) can eac h sculpt a unique motor pattern from the pyloric network in the stomatogast ric ganglion (STG) of the spiny lobster Panulirus interruptus. In this pape r we investigate the contribution of individual network components in deter mining the specific amine-induced cycle frequency. We used photoinactivatio n of identified neurons and pharmacological blockade of synapses to isolate the anterior burster (AB) and pyloric dilator (PD) neurons. Bath applicati on of DA, 5HT, or Oct enhanced cycle frequency in an isolated AB neuron, wi th DA generating the most rapid oscillations and Oct the slowest. When an A B-PD or AB-2xPD subnetworks were tested, DA often reduced the ongoing cycle frequency, whereas 5HT and Oct both evoked similar accelerations in cycle frequency However, in the intact pyloric network, both DA and Oct either re duced or did not alter the cycle frequency, whereas 5HT continued to enhanc e the cycle frequency as before. Our results show that the major target of 5HT in altering the pyloric cycle frequency is the AB neuron, whereas DA's effects on the AB-2xPD subnetwork are critical in understanding its modulat ion of the cycle frequency. Octopamine's effects on cycle frequency require an understanding of its modulation of the feedback inhibition to the AB-PD group from the lateral pyloric neuron, which constrains the pacemaker grou p to oscillate more slowly than it would alone. We have thus demonstrated t hat the relative importance of the different network components in determin ing the final cycle frequency is not fixed but can vary under different mod ulatory conditions.