Age-related loss of the GABA synthetic enzyme glutamic acid decarboxylase in rat primary auditory cortex

Age-related changes within the auditory brainstem typically include alterations in inhibitory neurotransmission and coding mediated by GABA and glycinergic circuits. As part of an effort to evaluate the impact of aging on neurotransmission in the higher auditory centers, the present study examined age-related changes in the GABA synthetic enzyme, glutamic acid decarboxylase (GAD), in rat primary auditory cortex (AI), which contains a vast network of intrinsic and extrinsic GABAergic circuits throughout its layers. Message levels of the two GAD isoforms found in brain, GAD(65) and GAD(67), and GAD(67) protein levels were compared in young adult, middle-aged and aged rats using in situ hybridization and quantitative immunocytochemistry, respectively. For comparison, age-related GAD changes were also assessed in the parietal cortex and hippocampus. Significant age-related decreases in GAD(65&67) messages were observed in AI layers II-VI of aged rats relative to their young adult cohorts. The largest changes were identified in layer II (GAD(65): -26.6% and GAD(67): -40.1%). GAD(67) protein expression decreased significantly in parallel with mRNA decreases in all layers of AI. Adjacent regions of parietal cortex showed no significant GAD(67) protein changes among the age groups, except in layer IV. As previously described, GAD(67) message and protein levels in selected hippocampal regions were significantly reduced in aged rats. Age-related GAD reductions likely reflect decreases in both metabolic and pre-synaptic GABA levels suggesting a plastic down-regulation of normal adult inhibitory GABA neurotransmission. Consistent with the present findings, functional studies in primate visual cortex and preliminary studies in AI find coding changes suggestive of altered inhibitory processing in aged animals. An age-related loss of normal adult GABA neurotransmission in AI would likely alter temporal coding properties and could contribute to the loss in speech understanding observed in the elderly.