Midbrain dopamine loss drives parvalbumin interneuron vulnerability through tissue plasminogen activator-linked perineuronal-net breakdown and hippocampal disinhibition.

Midbrain dopaminergic degeneration is an early feature of Alzheimer's Disease (AD), dementia with Lewy bodies (DLB), and AD-Parkinson's disease overlap (AD-PD). However, its direct contribution to the failure of hippocampal inhibitory-circuits, a pathological feature shared across these conditions, remains unresolved. Parvalbumin-positive interneurons (PV-INs) regulate hippocampal excitation-inhibition balance and are directly modulated by dopamine (DA). These neurons are protected by perineuronal nets (PNNs), extracellular-matrix structures supporting fast GABAergic signaling and neuronal resilience. We tested whether midbrain-derived DA loss is sufficient to destabilize hippocampal PV-IN function, potentially promoting their vulnerability or affecting PNN integrity. Through stereotaxic unilateral 6-hydroxy-dopamine lesion of the Ventral Tegmental Area/Substantia Nigra pars compacta in C57BL/6 N mice, we reduced the hippocampal DA tone and midbrain-derived synaptic input onto PV-INs. At 1-month post-lesion, PV-IN numbers were preserved, but the PNN integrity was reduced, accompanied by increased expression of tissue plasminogen activator (tPA), a PNN-remodeling protease. In CA1 pyramidal neurons, spontaneous inhibitory postsynaptic currents showed reduced frequency with faster decay, and bicuculline unmasked heightened population-spike excitability. By 6-months post-lesion, PV-IN numbers declined significantly, especially in CA1, demonstrating progressive vulnerability. D2/D3 receptor (D2/D3R) activation with quinpirole normalized tPA levels in PV-INs ex vivo, restored PNN integrity after sub-chronic treatment in vivo and increased inhibitory postsynaptic-event frequency, indicating functional recovery of GABAergic drive. These findings support the involvement of a DA-D2/D3R-tPA axis contributing to PV-IN extracellular-matrix integrity and hippocampal inhibitory tone. They also demonstrate that DA depletion is sufficient to trigger PNN breakdown, reduce GABAergic inhibition, network hyperexcitability, and cause progressive PV-IN loss independently of canonical protein aggregates like Aβ, tau or α-synuclein, characteristic of AD, DLB or AD-PD. This mechanism links midbrain degeneration to hippocampal circuit failure, highlighting D2/D3R signaling and extracellular proteolysis as actionable targets for early circuit stabilization across AD, DLB, and AD-PD.