Homeostatic recovery of embryonic spinal activity initiated by compensatory changes in resting membrane potential.

When baseline activity in a neuronal network is modified by external challenges, a set of mechanisms is prompted to homeostatically restore activity levels. These homeostatic mechanisms are thought to be profoundly important in the maturation of the network. We have previously shown that 2-day blockade of either excitatory GABAergic or glutamatergic transmission in the living embryo transiently blocks the movements generated by spontaneous network activity (SNA) in the spinal cord. However, by 2 hours of persistent receptor blockade embryonic movements begin to recover, and by 12 hours we observe a complete homeostatic recovery in vivo. Compensatory changes in voltage-gated conductances in motoneurons were observed by 12 hours of blockade, but not changes in synaptic strength. It was unclear whether changes in voltage-gated conductances were observed by 2 hours of blockade when the recovery actually begins. Further, compensatory changes in voltage-gated conductances were not observed following glutamatergic blockade where embryonic movements were blocked but then recovered in a similar manner to GABAergic blockade. In this study, we discover a mechanism for homeostatic recovery in these first hours of neurotransmitter receptor blockade. In the first 6 hours of GABAergic or glutamatergic blockade there was a clear depolarization of resting membrane potential in both motoneurons and interneurons. These changes reduced action potential threshold and were mainly observed in the continued presence of the antagonist. Therefore, it appears that fast changes in resting membrane potential represent a key fast homeostatic mechanism for the maintenance of network activity in the living embryonic nervous system.\n\nSignificanceHomeostatic plasticity represents a set of mechanisms that act to recover cellular or network activity following a challenge to that activity and is thought to be critical for the developmental construction of the nervous system. The chick embryo afforded us the opportunity to observe in a living developing system the timing of the homeostatic recovery of network activity following 2 distinct perturbations. Because of this advantage, we have identified a novel homeostatic mechanism that actually occurs as the network recovers and is therefore likely to contribute to nervous system homeostasis. We found that a depolarization of the resting membrane potential in the first hour of the perturbations enhances excitability and supports the recovery of embryonic spinal network activity.