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

The lateral entorhinal cortex (LEC) is selectively vulnerable to age-related decline and is essential for novelty detection and episodic memory. While DAergic (DAergic) input is known to modulate LEC function, how aging impacts this circuitry remains unclear. Here, we used two viral labeling strategies to investigate projections from the ventral tegmental area (VTA) to the LEC. First, we employed an INTRSECT dual-recombinase approach in TH-Flp::VGLUT2-Cre mice to selectively label dopamine-only (DA-only) and dopamine-glutamate co-releasing (DA-GLU) neurons. Next, we used a DAT-Cre-driven ChR2-YFP strategy to broadly label all DA axons. We found that both DA-only and DA-GLU populations innervate the LEC. With age, we observed a selective reduction in tyrosine hydroxylase (TH) signal within DA axons in the LEC, despite preserved axonal structure as revealed by YFP labeling. VGLUT2 signal within DA-GLU terminals appeared less affected. In the VTA, TH+ neuron density declined with age, with distinct spatial patterns along the anterior-posterior axis. These findings reveal an age-related vulnerability of DAergic projections to the LEC and suggest a circuit-level mechanism may contribute to memory impairments in aging.