Differential regulation of forebrain glutamic acid decarboxylase mRNA expression by aging and stress

In aging brain, degeneration or functional impairment of the hippocampus has been connected with stress dysregulation, serving to disinhibit stress responses and allow for glucocorticoid hypersecretion and its attendant pathophysiology. Hippocampal dysfunction appears to be communicated to paraventricular hypothalamic corticotropin-releasing hormone neurons by way of subcortical GABAergic neurons. As such, hippocampal-hypothalamic relays are likely to play an important role in age-related stress dysfunction. To test this hypothesis, regulation of glutamic acid decarboxylase isoform mRNA was studied in young (3 months), middle aged (15 months) and aged (30 months) Fischer 344/Brown Norway F1 hybrid rats. Basal expression of glutamic acid decarboxylase (GAD) 65 mRNA was increased in the medial preoptic area and posteromedial bed nucleus of the stria terminalis (BST) in aged rats relative to both middle-aged and young groups. Unlike young or middle-aged animals, exposure to chronic intermittent stress decreased GAD65 mRNA levels in the medial preoptic area and posteromedial BST of aged rats. Thus, while aged rats show evidence of elevated basal GABA synthesis, chronic stress causes differential loss of GAD in hippocampal-PVN relays, consistent with reduced PVN inhibition.