The Uniform and Nonuniform Nature of Slow and Rapid Scaling in Embryonic Motoneurons.

Neurons regulate the strength of their synapses in response to a perturbation to stabilize neuronal signaling through a form of homeostatic plasticity known as synaptic scaling. The process of scaling has the potential to alter all of a cell's miniature postsynaptic current (mPSC) amplitudes by a single multiplicative factor (uniform scaling), and in doing so could change action potential-dependent or evoked synaptic strength by that factor. However, recent studies suggest that individual synapses scale with different scaling factors (nonuniform). This could complicate the simple multiplicative transform from mPSC scaling to the evoked response. We have previously identified a slow AMPAergic and GABAergic synaptic scaling in chick embryo motoneurons following 2 d in vivo perturbations inhibiting neuronal activity or GABAAR function, and now show a rapid form of scaling following NMDAR blockade in vitro Slow GABAergic scaling appeared to be of a classical uniform pattern. Alternatively, other forms of rapid and slow scaling demonstrated a uniform and nonuniform component in their mPSC amplitude distributions. Slow and rapid AMPAergic scaling was mediated by insertion of GluA2-lacking AMPA receptors. The nonuniform pattern of scaling may contribute to the observed complexity of the changes in evoked responses. Scaling-induced changes in mPSC amplitudes were not associated with changes in probability of release (Pr). Together, our results demonstrate a new rapid form of scaling in embryonic motoneurons, that slow and rapid scaling is not purely uniform, and that upscaling does not translate to an increase in evoked responses in a simple manner.SIGNIFICANCE STATEMENT Different forms of homeostatic plasticity are thought to play a critical role in maintaining neural function. For example, altering the amplitudes of spontaneous currents through a form of homeostatic plasticity known as synaptic scaling could affect evoked transmission; however, this is rarely tested. Here we demonstrate two forms of scaling and show that in many cases synaptic strength scales differently for distinct synapses within an embryonic motoneuron. These results have functional consequences for evoked synaptic strength and suggest that, like Hebbian plasticity, scaling can change relative synaptic strengths within a cell. Furthermore, our results demonstrate how different forms of homeostatic plasticity influence neuronal communication as the nascent spinal network is first established in the embryonic period.