Inactivity and Ca2+ signaling regulate synaptic compensation in motoneurons following hibernation in American bullfrogs

Abstract

Neural networks tune synaptic and cellular properties to produce stable activity. One form of homeostatic regulation involves scaling the strength of synapses up or down in a global and multiplicative manner to oppose activity disturbances. In American bullfrogs, excitatory synapses scale up to regulate breathing motor function after inactivity in hibernation, connecting homeostatic compensation to motor behavior. In traditional models of homeostatic synaptic plasticity, inactivity is thought to increase synaptic strength via mechanisms that involve reduced Ca2+ influx through voltage-gated channels. Therefore, we tested whether pharmacological inactivity and inhibition of voltage-gated Ca2+ channels are sufficient to drive synaptic compensation in this system. For this, we chronically exposed ex vivo brainstem preparations containing the intact respiratory network to tetrodotoxin (TTX) to stop activity and nimodipine to block L-type Ca2+ channels. We show that hibernation and TTX similarly increased motoneuron synaptic strength and that hibernation occluded the response to TTX. In contrast, inhibiting L-type Ca2+ channels did not upregulate synaptic strength but disrupted the apparent multiplicative scaling of synaptic compensation typically observed in response to hibernation. Thus, inactivity drives up synaptic strength through mechanisms that do not rely on reduced L-type channel function, while Ca2+ signaling associated with the hibernation environment independently regulates the balance of synaptic weights. Altogether, these results point to multiple feedback signals for shaping synaptic compensation that gives rise to proper network function during environmental challenges in vivo.

Hibernation reduces GABA signaling in the brainstem to enhance motor activity of breathing at cool temperatures

“…Here we tested the hypothesis that GABAergic inhibitory neurotransmission would decrease in the respiratory network to compensate for inactivity during hibernation, much like we observed for excitatory synapses within motoneurons (Santin et al, 2017, Zubov et al, 2022). Indeed, we found that GABAergic inhibitory tone was decreased in the respiratory network, but this decrease occurred through presynaptic, and not postsynaptic mechanisms.…”

“…Neural systems use homeostatic forms of plasticity to maintain function in a changing environment. Prior studies have demonstrated that excitatory synapses are up-scaled following hibernation (Zubov et al, 2022; aligning with classic mechanisms of inactivity-induced homeostatic plasticity (Fong et al, 2015;Lambo and Turrigiano, 2013;Turrigiano et al, 1998). Although there is an abundance of research on excitatory mechanisms of homeostatic plasticity, much less is known about the role of inhibitory neurotransmission.…”

“…Instead of atrophying like other muscles and motoneurons during periods of inactivity (Banzrai et al, 2016; Cormery et al, 2006; Langlet et al, 2012; Fahim, 1989), including mammals on mechanical ventilation (Smuder et al, 2023), a suite of compensatory mechanisms ensures functional breathing after hibernation in the spring (Santin & Hartzler, 2016, Santin, 2019). Specifically, motoneurons increase the strength of AMPA-glutamate receptor currents and maintain the capacity for high frequency firing to promote respiratory motor outflow (Santin et al, 2017, Zubov et al, 2022). Given the complex role that inhibition plays within the respiratory network, and the typical trend toward reduced inhibition with inactivity, we hypothesized that compensatory decreases in inhibition would occur where the loss of inhibition is tolerated functionally.…”