Adenylate Cyclase-Mediated Forms of Neuronal Plasticity in Hippocampal Area CA1 Are Reduced With Aging

Abstract

Beta-adrenergic receptors and the cyclic AMP signaling pathway play an important role in neuronal plasticity and in learning and memory and are known to change with aging. We examined the effects of beta-adrenergic stimulation paired with 5-Hz low frequency stimulation (LFS) of Schaffer collateral-commissural afferents on population spike amplitude in area CA1 of hippocampal slices from young (3 mo) and aged (22 mo) Fischer 344 rats. Application of the beta-adrenergic agonist isoproterenol (1 microM) for 10 min followed immediately by 3 min LFS produced long-lasting potentiation in young hippocampi, but the magnitude of potentiation in aged rats was significantly attenuated and was not long-lasting. In slices prepared from young rats, long-term potentiation (LTP) induced by this protocol occludes subsequent attempts to produce conventional high frequency stimulation-induced LTP, and vice versa, suggesting that these two forms of potentiation share one or more molecular mechanisms. Age-related differences in response to LFS alone were not observed, but significant differences in response to beta-adrenergic stimulation were apparent. Similarly, significant age-related differences in response to direct activation of adenylate cyclase with forskolin (10 microM) were observed. In both age groups, this enhancement produced by isoproterenol or forskolin is only transient, returning to baseline within 60 or 90 min, respectively. Taken together, these studies of adenylate cyclase-mediated forms of potentiation in area CA1 suggest that there is an age-related defect, either upstream or downstream of adenylate cyclase activation, in this important signaling system. Such changes may contribute to the compromised performance on memory tasks that is often observed with normal aging.