Time course of homeostatic structural plasticity in response to optogenetic stimulation in mouse anterior cingulate cortex
AbstractSynapse formation and network rewiring is key to build neural circuits during development and has been widely observed in adult brains. Maintaining neural activity with the help of synaptic plasticity is essential to enable normal brain function. The model of homeostatic structural plasticity (HSP) was proposed to reflect the homeostatic regulation of neural activity and explain structural changes seen after perturbations. However, the specific temporal profile of such plastic responses has not yet been elucidated in experiments. To address this issue, we combined computational modeling and mouse optogenetic stimulation experiments. Our model predicted that within 48 h post-stimulation, neural activity returns to baseline, while the connectivity among stimulated neurons follows a very specific transient increase and decrease. To capture such dynamics experimentally in vivo, we activated the pyramidal neurons in the anterior cingulate cortex of mice and harvested their brains at 1.5 h, 24 h, and 48 h post-stimulation. Cortical hyperactivity as demonstrated by robust c-Fos expression persisted up to 1.5 h and decayed to baseline after 24 h. However, spine density and spine head volume were increased at 24 h and decreased at 48 h. Synaptic proteins VGLUT1 and PSD-95 were also upregulated and downregulated at 24 h and 48 h, respectively, while the calmodulin-binding protein neurogranin was translocated from the soma to the dendrite. Additionally, lasting astrocyte reactivation and microglia proliferation were observed, suggesting a role of neuron-glia interaction. All this corroborates the interpretation of our experimental results in terms of homeostatic structural plasticity. Our results bring important insights of how external stimulation modulates synaptic plasticity and behaviors.Significance StatementWe combined both computational modeling and mouse experiments to clarify the temporal dynamics of structural and functional homeostatic plasticity in response to external stimulation. We observed the biphasic regulation of spine density, spine head volume, and synaptic proteins at 24 h and 48 h after the optogenetic stimulation of the anterior cingulate cortex, when the neural activity was restored to the homeostatic level. The orchestrated regulation of presynaptic VGLUT1 and postsynaptic PSD-95, as well as the soma-dendrites translocation of neurogranin, suggested an elaborate molecular mechanism underlying homeostatic structural plasticity. Our experimental results thus corroborated the theoretical concept of homeostatic structural plasticity and revealed the temporal evolution of structural and functional plasticity.
