The role of Ih in differentiating the dynamics of the gastric mill and pyloric neurons in the stomatogastric ganglion of the lobster, Homarus americanus

The hyperpolarization-activated inward cationic current (I_h) is known to regulate the rhythmicity, excitability, and synaptic transmission in heart cells and many types of neurons across a variety of species, including some pyloric and gastric mill neurons in the stomatogastric ganglion (STG) in Cancer borealis and Panulirus interruptus. However, little is known about the role of I_h in regulating the gastric mill dynamics and its contribution to the dynamical bifurcation of the gastric mill and pyloric networks. We investigated the role of Ih in the rhythmic activity and cellular excitability of both the gastric mill neurons (MG, GM) and pyloric neurons (PD, LP) in Homarus americanus. Through testing the burst period between 5mM and 50mM CsCl, and elimination of post-inhibitory rebound and voltage sag, we found that 30mM CsCl can sufficiently block I_h in both the pyloric and gastric mill neurons. Our results show that I_h maintains the excitability of both the pyloric and gastric mill neurons. However, I_h regulates slow oscillations of the pyloric and gastric mill neurons differently. Specifically, blocking I_h diminishes the difference between the pyloric and gastric mill burst periods by increasing the pyloric burst period and decreasing the gastric mill burst period. Moreover, the phase-plane analysis shows that blocking I_h causes the trajectory of slow oscillations of the gastric mill neurons to change towards the pyloric sinusoidal-like trajectories. In addition to regulating the pyloric rhythm, we found that I_h is also essential for the gastric mill rhythms and differentially regulates these two dynamics.

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