Network consequences of convergent modulation in the stomatogastric nervous system of the crab, Cancer borealis

Modulators play an important role regulating neuronal circuit activity. The complexity of most nervous systems, however, makes a detailed understanding of modulator actions difficult. In this thesis, I address the role of convergent modulation in the functioning of a small neuronal circuit in the crustacean stomatogastric nervous system.; Proctolin activates a voltage-dependent inward current in stomatogastric ganglion (STG) neurons of the crab, Cancer borealis. I found that five additional modulators (Cancer borealis tachykinin-related peptide Ia (CabTRP), crustacean cardioactive peptide (CCAP), red pigment concentrating hormone (RPCH), TNRNFLRFamide and pilocarpine) activated the same ionic current as proctolin.; Despite this convergence, each substance elicits a distinct effect on the pyloric rhythm. How can a network produce divergent outputs using convergent modulators? One possibility is that each modulator targets a different subset of cells. I assayed the pyloric neurons for responses to the modulators and found that each convergent modulator does act on a different subset of neurons.; Can this differential distribution of modulator receptors account for the changes observed at the network level? Proctolin and CCAP act on the Anterior Burster neuron, Lateral Pyloric neuron and the Inferior Cardiac neuron. Additionally, proctolin acts on the Pyloric Dilator (PD) neurons, the Pyloric (PY) neurons and the Ventricular Dilator neuron. Using the dynamic clamp, I introduced an artificial peptide-elicited current into the PD and PY neurons, in the presence of CCAP, to see if I could convert the CCAP rhythm into a proctolin-like rhythm. Using a set of criteria I defined to distinguish quantitatively between proctolin and CCAP-elicited rhythms, I determined that the rhythm produced in CCAP during the artificial peptide current injections was proctolin-like. This suggests that differences in the network effects of these two modulators can largely be attributed to differences in their cell targets.; GABA is colocalized with proctolin in two inputs to the STG. Therefore, I also characterized GABA's effects on STG neurons. GABA evoked two types of responses; an inhibitory response and a Na+-dependent depolarizing response. I also described the pharmacology of these responses.