Amine modulation of glutamate responses from pyloric motor neurons in lobster stomatogastric ganglion.

The amines dopamine (DA), serotonin (5-HT), and octopamine (Oct) each elicit a distinctive motor pattern from a quiescent pyloric network in the lobster stomatogastric ganglion (STG). We previously have demonstrated that these amines alter the synaptic strength at multiple, distributed sites within the pyloric network that could contribute to the amine-induced motor patterns. Here, we examined the postsynaptic contribution to these changes in synaptic strength by determining how the amines modify responses of pyloric motor neurons to glutamate (Glu), one of the network transmitters, applied iontophoretically into the STG neuropil. Dopamine reduced the Glu responses of the pyloric dilator (PD), ventricular dilator (VD), and inferior cardiac (IC) neurons and enhanced the Glu responses of the lateral pyloric (LP) and pyloric constrictor (PY) neurons. The only effect of 5-HT was to reduce the Glu response of the VD neuron. Oct enhanced the Glu responses of the LP and PY neurons but did not affect the PD, VD, and IC responses. We also examined amine effects on the depolarizing responses to iontophoresed acetylcholine (ACh) in the PD and VD and found that they paralleled the amine effects on Glu responses in these neurons. This suggests that amine modulation of PD and VD responses to Glu and ACh may be explained by general changes in the ionic conductance of these neurons. We compare our results with our earlier work describing amine effects on synaptic strength and input resistance to show that amines act at both pre- and postsynaptic sites to modify graded synaptic transmission in the pyloric network.