Overwintering Induces Ischemia-Tolerance of Synaptic Function in the Forebrain of Bullfrogs

Healthy brain function requires the continuous delivery of oxygen and glucose to nerve cells. Interruptions result in rapid cessation of normal circuit function in most vertebrates. The American bullfrog ( Lithobates catesbeianus) is an interesting model, as we have shown that brainstem synapses increase their capacity to function during hypoxia and ischemia by roughly 20-30 fold following aquatic overwintering (cold-acclimated, CA) compared to control conditions (warm-acclimated, WA). Here, we tested the hypothesis that other synapses, specially those in the forebrain, also improve their function to determine if extreme metabolic plasticity is specific to the brainstem, or a generalized response throughout the central nervous system. We used the synapse that carries thalamic input to dorsal pallium, which is involved in sensory processing and associative memory. To measure synaptic function, we stimulated thalamic axons and recorded evoked excitatory postsynaptic currents (eEPSCs) in the medial dorsal pallium, the proposed homolog to the mammalian hippocampus, in normoxic conditions and in oxygen and glucose deprivation (OGD). eEPSC amplitude at a baseline frequency of 0.5 Hz was depressed at OGD onset in WA frogs (paired t-test, p=0.0002, n=12) but not in CA animals (paired t-test, p=0.1602, n=6). This suggests that unlike WA frogs, overwintering allows maintained synaptic function in OGD within the forebrain, as well as the brainstem. Mechanistically, the maintenance of synaptic function in OGD appeared to be associated with dynamic increase in the probability of neurotransmitter release. Paired-pulse experiments to assess dynamics in neurotransmitter release in response to high-frequency stimulus pairs showed that synapses depressed in normoxic conditions in both WA (9 of 11 cells) and CA (4 out of 6 cells) groups. In OGD, synaptic depression was maintained in WA frogs (paired t-test, p=0.9192); however, synaptic depression reversed, whereby paired-pulse stimulation instead led to synaptic facilitation in 5 out of 6 neurons (paired t-test, p=0.0113). This suggests that synapses may alter the probability of glutamate release to act as a countermeasure to maintain forebrain function during energetic stress associated with emergence. Overall, these results show that metabolic plasticity to restart critical behaviors in hypoxia following hibernation is preserved across the brain and that altered synaptic dynamics may play a role in this response. This research was funded by the National Institutes of Health (R01NS114514, R15NS112920-01A1). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.