Age-dependent loss and compensatory changes of septohippocampal cholinergic neurons in two rat strains differing in longevity and response to stress

Abstract

Inbred Wistar-Kyoto rats which are behaviorally more reactive to stress have a shorter life span than Brown-Norway rats. This is paralleled by higher basal activity and more pronounced changes in the septohippocampal cholinergic system of Wistar-Kyotos after stress. Age- and strain-dependent differences were therefore characterized in the septohippocampal system of 3- and 24-month-old (aged) Wistar-Kyotos and Brown-Norways, and in 30-month-old Brown-Norways. High affinity [ 3H]choline uptake and newly synthesized [ 3H]acetylcholine release served as markers for cholinergic terminals in the hippocampus. [ 3H]Quinuclidinylbenzylate binding served as a marker of muscarinic receptors in the hippocampus. Choline acetyltransferase activity served as a marker for cholinergic neurons and their terminals in the septum and hippocampus respectively. Acetylcholinesterase histochemical staining served to localize cholinergic neurons and their terminals in the septum and hippocampus respectively. In the hippocampus of aged Wistar-Kyotos choline uptake and acetylcholine release were release were reduced by ≈50% compared to their young counterparts, but remained unchanged in aged Brown-Norways. Hippocampal choline acetyltransferase activity, acetylcholinesterase staining and muscarinic binding were unchanged in aged rats of both strains. Pyramidal cell loss (observed in Cresyl violet stained sections) was detected in hippocampus of 24-month-old Wistar-Kyotos and 30-month-old, but not younger Brown-Norways. Numbers of acetylcholinesterase-stained cells in the septum were reduced by 45 and 25% in 24-month-old Wistar-Kyotos and Brown-Norways respectively, and by 50% in 30-month-old Brown-Norways. Mean diameter of these cells was increased only in aged Wistar-Kyotos (≈46%) and in 30-month-old Brown-Norways (40%). The results indicate: (1) there is an ongoing age-dependent degeneration of septohippocampal cholinergic neurons which is associated with two principal compensatory changes in remaining cholinergic neurons: (a) hypertrophy of perikarya and (b) relative increase in activity of presynaptic markers in terminals with unchanged regional distribution, suggesting possible collateral sprouting; (2) age-dependent loss of septal cholinergic neurons precedes loss of hippocampal pyramidal neurons and (3) loss of pyramidal neurons in the hippocampus is associated with a compensatory increased muscarinic binding by remaining target hippocampal neurons. The results imply that higher basal and stress-induced activity of septohippocampal cholinergic neurons may be correlated with an accelerated and more pronounced age-dependent degeneration of this cholinergic system. And, in turn, accelerated age-dependent degeneration of susceptible septal cholinergic neurons may lead to a secondary loss of cholinoceptive pyramidal neurons.