Immunocytochemical and neurochemical evidence for age-related loss of GABA in the inferior colliculus: implications for neural presbycusis

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Abstract
The present study describes substantial, selective, age-related loss of the putative inhibitory neurotransmitter GABA in the central nucleus of the inferior colliculus (CIC) of rat based on immunocytochemical and neurochemical data. For immunocytochemistry, neurons in the CIC were immunolabeled using an antibody against a GABA conjugate in young adult (2- to 7-month-old) and aged (18- to 29-month-old) Fischer-344 rats. Computer-assisted morphometry was then used to generate maps of GABA- immunoreactive neurons in the CIC. The number of GABA-positive neurons was reduced 36% in the ventrolateral portion of the CIC of aged animals (93 neurons/mm2) compared to their matched young adult cohorts (145 neurons/mm2; p less than 0.01). For neurochemistry, basal and K(+)- evoked release of the endogenous amino acids GABA, glutamate (Glu), aspartate (Asp), and tyrosine (Tyr) from micropunches of the CIC were measured in 8 age-paired animals from the 2 age groups using high- performance liquid chromatography. Overflow of radiolabeled acetylcholine (3H-ACh) was also determined. In both age groups, K(+)- evoked release of GABA, Glu, Asp, and 3H-ACh from CIC punches was significantly enhanced above basal efflux (+200, +215, +163, and +309%, respectively), while Tyr release was unchanged. Evoked release of 3H- ACh and all amino acids except Tyr showed substantial Ca2+ dependence. A significant (p less than 0.05) age-related reduction in both basal (- 35%) and K(+)-stimulated (-42%) efflux of GABA from the CIC was observed. A corresponding decrease in postrelease tissue content of GABA in CIC of aged rats was observed (-30%, p less than 0.05). In contrast, tissue content as well as basal and evoked release of Glu, Asp, Tyr, and 3H-ACh was similar between the 2 age groups. Age-related GABA neurochemical changes described in the CIC were not observed in the release of the other amino acids or 3H-ACh from either the rostral ventrolateral medulla or the somatosensory cortex, 2 brain regions involved in processing non-auditory sensory input. These data support previous findings that GABA, Glu, Asp, and ACh may subserve neurotransmission in the CIC. Additionally, these data provide clear evidence for a pronounced, region- and neurotransmitter-selective, age- related reduction of GABA in the CIC. These findings support the hypothesis that impairment of inhibitory GABAergic neurotransmission in the CIC may contribute to abnormal auditory perception and processing seen in neural presbycusis.