Functions of group 1 metabotropic glutamate receptors (mGluRs) should be restricted within normal range, otherwise brain disorders could develop. Here we report that neuronal protein kinase N1 (PKN1) normalizes group 1 mGluR functions by upregulating neuronal glutamate transporters. Using our generated PKN1a knockout (KO) mice, we found that deleting PKN1 unmasked abnormal mGluR-dependent homosynaptic long-term depression (LTD) that spread also to inactive synapses (heterosynaptic LTD), and resulted in robust AMPA receptor silencing in CA1 region of the developing hippocampus. Knocking out PKN1 in the mature dentate gyrus of the hippocampus also induced group 1 mGluR-dependent hyperexcitability that was also observed in stressed dentate granule cells in the wild-type hippocampus. Overexpression of PKN1 in SH-SY5Y cells elevated glutamate uptake by excitatory amino acid transporter 3 (EAAT3), a neuronal glutamate transporter. Inhibiting glutamate transporter by DL-TBOA resulted in enhancement of homosynaptic LTD and induction of heterosynaptic LTD in wild-type mice, however, DL-TBOA did not cause any changes in LTD in PKN1a KO mice. DL-TBOA also elevated neuronal excitability in wild-type mice, but not in KO mice, suggesting that all the electrophysiological phenotypes in PKN1 KO mice were mimicked and occluded by inhibition of glutamate transporters. Thus, we propose that PKN1 is critical for regulating synapse maturation and neuronal excitability through normalization of mGluR functions.