Motor pattern selection from a multifunctional neural network

Neural networks underlying rhythmic behaviors produce different motor patterns in response to different modulatory inputs, even in the isolated nervous system. This modulation of network activity has been studied in various systems by neurotransmitter bath application and stimulation of sensory and/or projection neuron inputs. However, little is known about the mechanisms whereby distinct inputs select distinct motor patterns from a single neural network. One possibility is that they use different transmitters, or transmitter complements. While many neurons contain cotransmitters, there is little information regarding the identity and roles of cotransmitters used by modulatory inputs in motor pattern selection. These issues were addressed in the stomatogastric nervous system of the crab Cancer borealis, focusing on the stomatogastric ganglion (STG) neural network. The STG is a well characterized multifunctional network that produces multiple versions of the gastric mill and pyloric rhythms. There are three projection neuron pairs, MPN, MCN1 and MCN7, which innervate the STG and contain the same peptide transmitter, proctolin. Dye-fills and immunocytochemistry were used to document that these projection neurons contain distinct transmitter complements. This documentation included identifying the presence and physiological actions of two tachykinin-like peptides. One of these peptides is related to the crab neuropeptide that is an MCN1 cotransmitter. Standard electrophysiological techniques demonstrated that each proctolin neuron elicits a distinct STG motor pattern. The MCN7 motor pattern results partly from a selective enhancement of activity in one STG network neuron. The MPN motor pattern results partly from its actions on parallel pathways originating in the commissural ganglia (CoGs). MPN stimulation inhibits the gastric mill rhythm by inhibiting these projection neurons. Previous work showed that proctolin application mimics MPN excitation of the pyloric rhythm in the STG. Pharmacological techniques and transmitter application were used here to document that MPN uses only GABA to influence the CoG projection neurons. The separate actions of the MPN cotransmitters demonstrate a functional cotransmitter segregation in an identified modulatory neuron. This work also documents the use of distinct cotransmitter complements, interactions among parallel pathways and selective alteration of network neuron function to select distinct motor patterns from a multifunctional neural network