G Protein Signaling in a Neuronal Network is Necessary for Rhythmic Motor Pattern Production

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Abstract
G protein-coupled receptors are widely recognized as playing important roles in mediating the actions of extrinsic neuromodulatory inputs to motor networks. However, the potential for their direct involvement in rhythmic motor pattern generation has received considerably less attention. Results from this study indicate that G protein signaling appears to be integral to the operation of the central pattern generator (CPG) underlying the escape swim of the mollusk Tritonia diomedea. Blocking G protein signaling in a single CPG neuron, cerebral neuron C2, with intracellular iontophoresis of the guanine nucleotide analogue guanosine 5′-O-(2-thiodiphosphate) (GDP-β-S), prevented the production of the swim motor program. Moreover, tonic activation of G protein signaling in this neuron by iontophoresis of the GTP analogues guanosine 5′-O-(3-thiotriphosphate) (GTP-γ-S) and 5′-guanylyl-imidodiphosphate also inhibited motor pattern production. The possible sites of action of these guanine nucleotide analogues were examined to assess potential mechanisms by which they interfered with motor pattern production. Intracellular iontophoresis of GDP-β-S into C2 did not affect C2 basal synaptic strength. However, it did reduce heterosynaptic facilitation of C2 synapses caused by the dorsal swim interneurons (DSIs), a set of serotonergic swim CPG neurons. In contrast, GTP-γ-S directly enhanced C2 synaptic strength onto DFN, mimicking the neuromodulatory effect of the DSIs. GDP-β-S, but not the GTP analogues, decreased C2 excitability, whereas both GTP analogues, but not GDP-β-S, blocked the ability of DSI stimulation to increase C2 excitability. The decrease in C2 excitability caused by GDP-β-S is not likely to be responsible for the inhibition of the swim motor pattern because decreasing C2 firing rate, by injecting hyperpolarizing current, did not prevent the production of the rhythmic motor pattern. Taken together, these data suggest that G protein signaling is a necessary and integral component of the escape swim CPG in Tritonia and that G protein signaling mediates DSI heterosynaptic facilitation of C2 but may not mediate the DSI-evoked enhancement of C2 excitability.