NMDA receptor hypofunction on GABAergic interneurons results in input-specific Excitatory/Inhibitory imbalance in pyramidal neurons of medial prefrontal cortex
Excitation/inhibition (E/I) balance in cortical circuits is typically treated as a global neuronal property, yet pyramidal neurons integrate synaptic inputs from anatomically distinct afferent pathways whose regulation may not be uniform. Using a mouse model with early postnatal NMDA receptor (NMDAR) ablation from corticolimbic GABAergic interneurons associated E/I dysfunction, we tested whether interneuron NMDAR hypofunction disrupts the mPFC E/I balance globally or in a pathway-specific manner. Here, we use pathway-specific anatomical labeling, optogenetic circuit mapping, paired pyramidal neuron-fast spiking interneuron recordings, and analysis of synaptic integration and found that both structural and functional E/I imbalance emerged selectively at ventral hippocampal (vHPC) inputs onto mPFC pyramidal neurons, while callosal (contralateral mPFC) inputs remained unaffected. Structurally, this imbalance was restricted to vHPC-derived synapses on apical, but not basal, dendrites. Functionally, mutant mice showed an excitation-shifted E/I ratio specific to vHPC-driven responses, arising from a marked failure to recruit feedforward inhibition via fast-spiking interneurons, whose preferential excitatory drive from vHPC inputs was selectively lost. Short-term synaptic plasticity of vHPC and callosal inputs onto both pyramidal neurons and fast spiking interneurons was unchanged, indicating that presynaptic release dynamics could not account for the deficit. Consistent with impaired feedforward inhibition, pharmacological GABA-A receptor blockade failed to prolong vHPC-evoked EPSPs in mutant pyramidal neurons, in contrast to its clear effect on callosal-evoked responses, demonstrating that inhibitory control over the temporal integration of hippocampal, but not callosal, inputs was lost. Altogether, these findings establish pathway-specific E/I imbalance as a consequence of interneuron NMDAR hypofunction, and thus indicating that circuit dysfunction can selectively bias processing of specific afferent pathways rather than producing a uniform disruption of cortical excitability, with direct relevance for understanding hippocampal-prefrontal dysconnectivity in neuropsychiatric disorders.