Affects of aging on receptive fields in rat primary auditory cortex layer V neurons

Advanced age is commonly associated with progressive cochlear pathology and central auditory deficits, collectively known as presbycusis. The present study examined central correlates of presbycusis by measuring response properties of primary auditory cortex ( AI) layer V neurons in the Fischer Brown Norway rat model. Layer V neurons represent the major output of AI to other cortical and subcortical regions ( primarily the inferior colliculus). In vivo single-unit extracellular recordings were obtained from 114 neurons in aged animals ( 29 - 33 mo) and compared with 105 layer V neurons in young-adult rats ( 4 - 6 mo). Three consecutive repetitions of a pure-tone receptive field map were run for each neuron. Age was associated with fewer neurons exhibiting classic V/U-shaped receptive fields and a greater percentage of neurons with more Complex receptive fields. Receptive fields from neurons in aged rats were also less reliable on successive repetitions of the same stimulus set. Aging was also associated with less firing during the stimulus in V/U-shaped receptive field neurons and more firing during the stimulus in Complex neurons, which were generally associated with inhibited firing in young controls. Finally, neurons in aged rats with Complex receptive fields were more easily driven by current pulses delivered to the soma. Collectively, these findings provide support for the notion that age is associated with diminished signal-to-noise coding by AI layer V neurons and are consistent with other research suggesting that GABAergic neurotransmission in AI may be compromised by aging.