Regulation of compensatory synaptic plasticity in inactive motor neurons of overwintering bullfrogs, Lithobates catesbeianus

Abstract

Circuits must maintain stable electrophysiological function to produce behaviors. If neurons face a disturbance, such as changes in activity or environmental perturbations, they often compensate by altering synaptic and intrinsic properties to oppose the challenge, a type of plasticity termed “homeostatic plasticity.” Although homeostatic forms of plasticity are thought to maintain stability in the healthy nervous system, abnormal challenges using drugs, sensory deprivation, and injury have generated most of our mechanistic understanding of these processes. This has led to questions of how animals regulate these mechanisms physiologically in vivo. American bullfrogs, Lithobates catesbeianus, normally breathe often, but during months of aquatic overwintering the respiratory network is completely silent, representing a large and natural challenge to neuronal function. We recently showed that respiratory motor neurons use a classic type of synaptic homeostasis that compensates for inactivity during overwintering which regulates motor output from the network after months under water (Santin et al. 2017, eLife). Current models of homeostatic plasticity predict that neurons continuously increase synaptic strength during inactivity, but whether synaptic strength increases in a graded fashion or rises quickly and then plateaus during winter inactivity is unknown. To test this, we measured spontaneous postsynaptic AMPA receptor currents in respiratory motor neurons in brainstem slices that innervate the laryngeal dilator from frogs maintained at ~21°C (n=13) or those that had been cold (~1–2°C) and submerged for 2 (n=21) and 4 weeks (n=20). Consistent with our previous work, we found that cold submergence increased the postsynaptic amplitude and charge transfer (amplitude, p=0.004; charge transfer, p=0.0004; Kruskal‐Wallis test); however, increases in postsynaptic function did not differ between frogs cooled for 2 and 4 weeks (p>0.05 for amplitude and charge transfer; Dunn’s multiple comparisons test). Frequency of postsynaptic events did not differ between warm and cold‐submerged frogs, indicating that cold‐acclimation does not influence spontaneous and/or action potential‐driven transmitter release. Thus, postsynaptic strength increases relatively soon after entry into the overwintering environment and is likely maintained at a single, compensated level over long time scales. In contrast to theoretical work that suggests neurons may continue to compensate until they return to a homeostatic set point, our work shows that the amount of compensation may be tuned to meet future demands of the network (i.e., appropriate breathing) or constrained by limitations imposed by the environment.Support or Funding InformationThis work was funded by start‐up funds from UNC‐Greensboro to JS.