Age-related changes in rhythmically bursting activity in the medial septumof rats

The effects of aging on the firing: of septohippocampal neurons were estima ted in unanesthetized, restrained young, old and very old rats (respectivel y 3, 23 and 30 months). Extracellular recordings were obtained during vario us states of arousal. The mean spontaneous activity for the overall neurona l population was not modified by aging. In contrast, the percentage of rhyt hmically bursting neurons was significantly lower in aged rats. During wake fulness, decrease of bursting activity was observed in old and very old rat s (P<0.01 and P<0.001) whereas during rapid eye movement sleep it appeared only in the oldest group (P<0.01). The frequency of the bursts decreased in 30-month-old rats during wakefulness while it remained unchanged in both a ged groups during rapid eye movement sleep. In old rats, at a time when the cholinergic septal neurons already deteriorated, a third of neurons record ed during rapid eye movement sleep exhibited a pattern of activity composed of long duration bursts with higher intraburst frequency than in young or very old rats. Our study shows that rhythmically bursting septal activity i s impaired in aged rats and that the amplitude of the changes depends on ad vancing age and on states of arousal. Our findings suggest that age-induced loss and atrophy of cholinergic septal neurons contribute to the disorgani zation of the rhythmic activity but that functional alterations, influenced by the states of arousal, may also be considered. (C) 2000 Elsevier Scienc e BN. All rights reserved.