Graded Transmission without Action Potentials Sustains Rhythmic Activity in Some But Not All Modulators That Activate...

Area of Science:

• Neuroscience
• Neurophysiology
• Crustacean neurobiology

Background:

• Central pattern-generating circuits, like the stomatogastric ganglion (STG), control rhythmic motor behaviors.
• Neurons in the STG release neurotransmitters via graded potentials and action potentials.
• Neuromodulators activate common ionic currents but can produce distinct network outputs.

Purpose of the Study:

• To investigate whether graded synaptic transmission, in the absence of action potentials, is sufficient to sustain pyloric rhythms.
• To compare the effects of various neuromodulators on rhythmic activity when voltage-gated sodium channels are blocked.

Main Methods:

• Experiments were conducted on the STG of the crab *Cancer borealis*.
• Tetrodotoxin (TTX) was used to block voltage-gated sodium channels and action potentials.
• Neuromodulators (oxotremorine, CabTRP1a, RPCH, proctolin, TNRNFLRFamide, CCAP) were applied to assess their effects on pyloric rhythm generation.

Main Results:

• Oxotremorine and CabTRP1a sustained rhythmic activity with preserved neuronal phase relationships in the presence of TTX.
• RPCH also elicited rhythmic activity in TTX, though cycle frequency was altered.
• Proctolin, TNRNFLRFamide, and CCAP failed to produce rhythmic activity in TTX, indicating a dependence on action potentials.

Conclusions:

• Graded synaptic transmission can support rhythmic motor activity under specific neuromodulatory conditions, even without action potentials.
• Different neuromodulators, despite targeting the same ionic current, utilize distinct mechanisms for burst generation.
• This study reveals fundamental differences in how neural circuits generate rhythmic patterns, depending on the presence or absence of action potentials.