Contributions of h- and Na⁺/K⁺ pump currents to the generation of episodic and continuous rhythmic activities

AbstractDeveloping spinal motor networks produce a diverse array of outputs, including episodic and continuous patterns of rhythmic activity. Variation in excitability state and neuromodulatory tone can facilitate transitions between episodic and continuous rhythms; however, the intrinsic mechanisms that govern these rhythms and their transitions are poorly understood. Here, we tested the capacity of a single central pattern generator (CPG) circuit with tunable properties to generate multiple outputs. To address this, we deployed a computational model composed of an inhibitory half-centre oscillator (HCO). Following predictions of our computational model, we tested the contributions of key properties to the generation of an episodic rhythm produced by isolated spinal cords of the newborn mouse. The model recapitulates the diverse state-dependent rhythms evoked by dopamine. In the model, episodic bursting depended predominantly on the endogenous oscillatory properties of neurons, with Na+/K+ ATPase pump (IPump) and hyperpolarization-activated currents (Ih) playing key roles. Modulation of either IPumpMax or Ih produced transitions between episodic and continuous rhythms and silence. As IPump increased, the episode duration and period increased along with a reduction in interepisode interval. Increasing Ih increased the episode period along with an increase in episode duration. Pharmacological manipulations of Ih with ZD7288 and IPump with ouabain or monensin in isolated spinal cords produced findings consistent with the model. Our modelling and experimental results highlight key roles of Ih and IPump in producing episodic rhythms and provide insight into mechanisms that permit a single CPG to produce multiple patterns of rhythmicity.Significance statementThe ability of a single CPG to produce and transition between multiple rhythmic patterns of activity is poorly understood. We deployed a complementary computational half-centre oscillator model and an isolated spinal cord experimental preparation to identify key currents whose interaction produced episodic and continuous rhythmic activity. Together, our experimental and modelling approaches suggest mechanisms in spinal networks that govern diverse rhythms and transitions between them. This work sheds light on the ability of a single CPG to produce episodic bouts observed in behavioural and pathological contexts.