The intrinsic rhythmicity of the spiny lobster stomatogastric ganglion (STG) is strongly influenced by the strengths of the graded synapses between identified cells within the neural network. These synaptic strengths can be powerfully influenced by chemical neuromodulators such as dopamine and serotonin. Most of the intraganglionic chemical synapses in the STG are mediated by postsynaptic inhibitory glutamate receptors (IGluRs). To determine whether or not direct effects on these IGluRs contribute to the modulation of synaptic strength, unidentified STG neurons were extracted into primary culture and the effects of these aminergic neuromodulators on the glutamate-evoked membrane current were assessed. Dopamine (100 microM) reliably and significantly reduced the whole cell slope conductance of all IGluRs tested. Serotonin (20 microM) never affected the IGlu response, although it clearly altered other cellular membrane properties. Although all identified STG neurons may not conform to these observations, the data reveal a specific dopamine-activated modulatory pathway within cultured neurons that reduces IGluR slope conductance. The relationship between IGluR modulation and net synaptic modulation in situ contributes to an emerging model in which synaptic strengths can be multiply modulated at different functional sites, yielding a complex, distributed, and state-dependent regulatory structure.