Homeostasis of neuronal network firing rate during the induction of plasticity

Neurons modify their activity through synaptic and intrinsic plasticity mechanisms induced by afferent input and temper these changes through homeostatic mechanisms. Consequently, most cortical neurons tend to fire within a frequency range of a few Hertz. While comparisons of firing rates have been used to show plasticity in intracellular recordings of individual neurons, we previously found that firing rate statistics alone less reliably depicted plasticity throughout networks of neurons. Influencing factors may involve: synaptic noise and unreliable action potential transmission across a chain of neurons, convergent and divergent neural pathways, and homeostatic mechanisms that re-adjust firing rates in response to plasticity, all of which can obscure firing rate measures of plasticity. Here, we investigated the relationship between the homeostasis of firing rate in our cultures and long-term plasticity, as measured from activity-dependent changes in action potential propagation in cortical neurons cultured over a multi-electrode array. We found that network firing rate was perturbed by application of a patterned stimulation, and as plasticity accumulated, network firing rate re-approached a homeostatic ldquoset-pointrdquo. This suggests that metrics based on firing rate alone could obscure the observation of long-term plasticity in networks of neurons.