Evidence for age-related vulnerability in dopamine-glutamate projections to the lateral entorhinal cortex

The lateral entorhinal cortex (LEC) supports novelty detection and episodic memory and is selectively vulnerable to aging. Dopamine signals novelty in LEC, but how aging alters this input is unclear. Using intersectional viral strategies to distinguish ventral tegmental area (VTA) dopamine neurons with or without glutamate co-release, we find that co-releasing neurons are a minority ([~]30%) in the VTA, yet provide [~]93% of the dopaminergic projections to LEC. In aged mice, dopamine-glutamate labeling in VTA and LEC decreases [~]80%, whereas pan-dopaminergic labeling of LEC axons remains, consistent with functional silencing rather than degeneration. In LEC dopaminergic axons, dopamine synthesis is reduced while glutamate vesicular packaging is relatively spared. Optogenetic stimulation combined with a dopamine sensor reveals diminished dopamine release at high frequencies, when synthesis demand is greatest. These findings identify selective vulnerability of dopamine-glutamate neurons as a cell- and circuit-specific mechanism that weakens dopamine signaling within memory circuits.