State-dependent presynaptic inhibition regulates central pattern generator feedback to descending inputs

Area of Science:

• Neuroscience
• Computational Neuroscience
• Systems Neuroscience

Background:

• Central pattern generators (CPGs) are neural circuits that produce rhythmic motor patterns.
• Feedback from CPGs to their projection neuron inputs is known, but its regulation and impact on motor output are unclear.
• The stomatogastric nervous system (STNS) in crabs, with its pyloric and gastric mill CPGs, provides a model to study CPG feedback regulation.

Purpose of the Study:

• To investigate whether feedback from the pyloric CPG to projection neurons regulating the gastric mill CPG is regulated.
• To determine how such regulation shapes CPG output.
• To understand the mechanisms underlying state-dependent modulation of CPG feedback.

Main Methods:

• Electrophysiological recordings in the crab STNS.
• Studying identified projection neurons (MCN1, CPN2) and CPG interneurons (AB).
• Analyzing activity patterns during different extrinsic inputs (VCN, POC) triggering distinct gastric mill rhythms.

Main Results:

• Pyloric CPG feedback (via AB neuron) to MCN1/CPN2 is pyloric-timed during the retraction phase of both VCN- and POC-triggered rhythms.
• During VCN-triggered gastric mill protraction, MCN1/CPN2 exhibit tonic firing instead of pyloric-timed activity.
• This tonic firing results from the elimination of AB inhibition, likely due to presynaptic inhibition of AB in the commissural ganglia (CoGs).

Conclusions:

• Rhythmic CPG feedback can be locally and state-dependently regulated.
• Suppression of pyloric CPG feedback during specific motor patterns allows the same projection neurons to drive distinct outputs.
• This provides a mechanism for generating multiple motor patterns from a single neuronal circuit.