Dual modulatory effects on feedback from a proprioceptor in the crustacean stomatogastric nervous system.

Neuromodulatory actions that change the properties of proprioceptors or the muscle movements to which they respond necessarily affect the feedback provided to the central network. Here we further characterize the responses of the gastropyloric receptor 1 (GPR1) and gastropyloric receptor 2 (GPR2) neurons in the stomatogastric nervous system of the crab to movements and contractions of muscles, and we report how neuromodulation modifies those responses. We observed that the GPR1 response to contractions of the gastric mill 4 muscle (gm4) was absent, or nearly so, when the neuron was quiescent but robust when it was spontaneously active. We also found that the effects of four neuromodulatory substances (GABA, serotonin, proctolin, and TNRNFLRFamide) on the GPR1 response to muscle stretch were similar to those previously reported for GPR2. Finally, we showed that an excitatory action on gm4 due to proctolin combined with an inhibitory action on GPR2 due to GABA can allow for larger muscle contractions without increased proprioceptive feedback. We report that the combination of GABA and the peptide proctolin increases contraction of a stomatogastric muscle while decreasing the corresponding response of the proprioceptor that reports on it. These results suggest a general mechanism by which muscle movements can be modified while sensory feedback is conserved, one that may be particularly well suited for providing flexibility to central pattern generator networks.