Alterations of group Ia-motoneuron monosynaptic EPSPs in aged cats.

The fashion in which the aging process affects synaptic transmission was explored in old cats. The synapses between Ia-afferent fibers and spinal cord alpha-motoneurons were studied employing intracellular recording techniques. The amplitude and time course of Ia-motoneuron monosynaptic excitatory postsynaptic potentials (Ia-EPSPs) recorded from motoneurons of the gastrocnemius medialis motor pool were determined in adult (1-3 years of age) and in old (14-15 years of age) cats. These Ia-EPSPs were evoked by the stimulation of the gastrocnemius medialis nerve in pentobarbital-anesthetized cats. No statistically significant change was found in the amplitude of these synaptic potentials in the old cats compared with the adult controls. In contrast, their time course was markedly modified; both rise-time and half-width were significantly longer in the old cats. In addition, the rate-of-rise of these Ia-EPSPs was significantly slower in the old cats. No breaks or inflections were observed on the rising phase of the potentials in the adult cats, whereas they were present in a subpopulation of potentials in the old cats. There was also a reduction, in the old cats, in the conduction velocity of the peripheral portion of the Ia-afferent fibers. These data indicate that Ia synaptic transmission is considerably affected during old age. It is suggested that these effects in aged cats may be due to modifications in presynaptic as well as postsynaptic processes.