mGlu₁ receptor mediates homeostatic control of intrinsic excitability through I_h in cerebellar Purkinje cells

Homeostatic intrinsic plasticity is a cellular mechanism for maintaining a stable neuronal activity level in response to developmental or activity-dependent changes. Type 1 metabotropic glutamate receptor (mGlu₁ receptor) has been widely known to monitor neuronal activity, which plays a role as a modulator of intrinsic and synaptic plasticity of neurons. Whether mGlu₁ receptor contributes to the compensatory adjustment of Purkinje cells (PCs), the sole output of the cerebellar cortex, in response to chronic changes in excitability remains unclear. Here, we demonstrate that the mGlu₁ receptor is involved in homeostatic intrinsic plasticity through the upregulation of the hyperpolarization- activated current (I_h) in cerebellar PCs. This plasticity was prevented by inhibiting the mGlu₁ receptor with Bay 36-7620, an mGlu₁ receptor inverse agonist, but not with CPCCOEt, a neutral antagonist. Chronic inactivation with tetrodotoxin (TTX) increased the components of I_h in the PCs, and ZD 7288, a hyperpolarizationactivated cyclic nucleotide-gated channel selective inhibitor, fully restored reduction of firing rates in the deprived neurons. The homeostatic elevation of I_h was also prevented by BAY 36-7620, but not CPCCOEt. Furthermore, KT 5720, a blocker of protein kinase A (PKA), prevented the effect of TTX reducing the evoked firing rates, indicating the reduction in excitability of PCs due to PKA activation. Our study shows that both the mGlu₁ receptor and the PKA pathway are involved in the homeostatic intrinsic plasticity of PCs after chronic blockade of the network activity, which provides a novel understanding on how cerebellar PCs can preserve the homeostatic state under activity-deprived conditions.