Dynamic modulation of a rhythmically active neural network in the stomatogastric nervous system of the crab, Cancer borealis

Rhythmically active neuronal networks are influenced by neuromodulators that are released from neurons whose somata are often located in distant regions of the nervous system. Although some information is available regarding the influence of these projection neurons on their neural network targets, little is known regarding the presence and function of influences occurring at their distant terminals. Such influences can play an important role in sculpting the motor pattern produced by these projection neurons. To address this issue directly, biochemical and synaptic interactions are examined within a well-characterized neuronal network of the stomatogastric ganglion (STG), in the stomatogastric nervous system of the crab, Cancer borealis.; The anatomical location of projection neurons to the STG is determined using biocytin backfills of the single STG input nerve. Combining these backfills with immunocytochemistry, the peptide content of a subset of these neurons is determined.; The possible post-release fate of proctolin, one of the neuropeptide modulators of STG activity, is examined using HPLC and electrophysiology. Aminopeptidase activity is found to degrade and functionally inactivate extracellular proctolin in the STG.; One of the proctolin-containing neurons that projects to the STG from the commissural ganglion (CoG) is identified and characterized, electrophysiologically. Stimulation of this neuron, modulatory commissural neuron 1 (MCN1), elicits a gastric mill rhythm that includes alternating impulse bursts between two STG neurons, DG and LG. This alternation is not due to interactions between these two neurons, but results from LG-mediated presynaptic inhibition of the STG terminals of MCN1.; LG is also electrically coupled to the STG terminals of MCN1. Electrical excitation from MCN1 to LG contributes to the generation of each LG burst during an MCN1-elicited gastric mill rhythm. During each burst, LG presynaptically inhibits MCN1 chemical transmission.; Finally, the synaptic mechanisms by which MCN1 elicits the full gastric mill rhythm is presented. The STG interactions between MCN1 and its targets is critical to the production of this rhythm. These interactions are not evident at the MCN1 cell body. Thus, local synaptic events at the electrically distant terminals of projection neurons can be essential to motor pattern generation by these neurons.