Plasticity in voltage-gated ion channels following overwintering in respiratory motoneurons of American bullfrogs

Many animals undergo prolonged dormancy periods to survive cold or dry environments. While humans and most laboratory-based mammals experience a loss of neuromuscular function during inactivity, hibernators possess physiological mechanisms to mitigate this loss. The American bullfrog provides an extreme model of this phenomenon, as brainstem circuits that generate breathing are completely inactive during underwater hibernation, during which motoneurons employ various types of synaptic plasticity to ensure adequate respiratory motor output in the spring. In addition to synapses, voltage-gated ion channels may undergo plasticity to boost neuronal output. Therefore, we hypothesized that motoneuron excitability would also be enhanced after hibernation via alterations in voltage-gated ion channels. We used whole-cell patch clamp electrophysiology to measure membrane excitability and activities of several voltage-gated channels (K⁺, Ca²⁺, Na⁺) from motoneurons that innervate muscles of the buccal pump (hypoglossal) and glottal dilator (vagal). Surprisingly, compared to controls, overwintered hypoglossal motoneurons displayed multiple indices of reduced excitability (hyperpolarized resting membrane potential, lower firing rates, greater lag to first spike). Mechanistically, this occurred via enhanced voltage-gated K⁺ and reduced Ca²⁺ channel activity. In contrast, vagal motoneurons excitability was unaltered, but exhibited altered ion channel profiles which seemed to stabilize neuronal output, involving either reduced Ca²⁺ or K⁺ currents. Therefore, different motoneurons of the same neuromuscular behavior respond differently to overwintering by altering the function of voltage-gated channels. We suggest divergent responses may reflect different energetic demands of these neurons and/or their specific contribution to breathing and other orofacial behaviors.