Age-dependent behavioral changes and physiological changes in identified neurons in Aplysia californica.

The gill withdrawal reflex (GWR) to direct gill stimulation was studied in sexually mature Aplysia and in those older by at least two months. The GWR threshold in old Aplysia was five- to sevenfold higher than that in mature animals. In the habituation paradigm, the GWR amplitude decremented rapidly to zero in old animals whereas in mature animals it persisted for at least ten trials. The GWR could not be dishabituated in old animals. The GWR is an age-dependent behavior in that parieto-visceral ganglion suppression of the GWR appears to increase with age. Also the electrophysiological properties of two neurons in the parieto-visceral ganglion were compared in the two age groups: L7 a neuron which dishabituates the GWR in mature and not in old animals; and R2 which manifests cytological changes with age. In old animals L7's input resistance was lower, the time constant was increased, and the size of the psp evoked by gill stimulation was smaller than those of mature L7s. Similar membrane changes with age were measured in R2. Soma size of L7 was approximately the same in the two age groups as was that of R2. The physiological parameters of neurons of known function continue to change during postmetamorphic life of Aplysia.