Physiological compensation for loss of afferent synapses in rat hippocampal granule cells during senescence.

Abstract

The effects of senescence on the input-output characteristics of the perforant path projection to granule cells of the fascia dentata were studied in rats using extracellular techniques in vivo, and extra- and intracellular recording in vitro. Senescent animals exhibited a significant reduction in the perforant path excitatory synaptic field potential at all stimulus intensities tested. This was associated with a reduction in the size of the afferent fiber response, although there was no apparent change in the threshold for fiber activation. Anatomical loss of afferent synapses with advanced age is supported. For a given magnitude of afferent fiber response the old animals exhibited a larger synaptic field potential implying the remaining synapses were in fact more powerful. The magnitude of the extracellular population spike, an index of the number of discharging granule cells, was greater in the old animals when plotted as a function of extracellular e.p.s.p. [excitatory postsynaptic potential] amplitude. Intracellular recording from a total of 190 granule cells in the transverse hippocampal slice preparation revealed a 17% reduction in the voltage threshold for synaptically elicited granule cell discharge, and a 13% reduction in the latency of the action potential in old compared to young rats. Resting potentials, action potential amplitudes, whole neuron time constants, the relations between applied current and input resistance, and the discharge threshold following depolarizing current pulses were not different between age groups. Granule cells may partly compensate for a loss of synapses during senescence by an increase in their electrical responsiveness to synaptic activation and possibly by an increase in synaptic efficacy.