Neurophysiological comparison of dendritic cable properties in adolescent, middle-aged, and senescent rats

Relative dendritic atrophy has been reported in hippocampal granule cells of senescent (25 month) as compared to young (3 month) rats. An electrophysiological study of synaptic response waveform was undertaken to determine whether these anatomical changes have physiological correlates of functional significance to the integrative capacity of the granule cells. The variable of interest was the rise time of the extracellularly recorded population synaptic response because this parameter (under appropriate conditions) has been shown to reflect relative electrotonic distance of active synapses from the spike generating zone located near the cell body. By selectively stimulating small subsets of perforant path fibers along its medio-lateral axis, synapses at increasing distances from the soma may be activated and the response waveform recorded. Systematic observations were made on both hemispheres of 3 adolescent (2 months), three middle-aged (12 months), and 3 senescent (28 months) rats. There were no significant differences in the mean, variance, or range of the frequency distributions of synaptic response rise times. However, the medians of the distributions revealed a small shift towards lower values with advanced age. Although the latter finding is consistent with the anatomical changes, the magnitude of the differences were so small as to make their functional significance dubious.