Normal aging and cognition: System-specific changes to G-protein coupled receptor-mediated signal transduction within the hippocampus

Aging is associated with a general deterioration in the ability to form new memories, a neural process that depends upon the hippocampus and its interconnected brain regions. However, the magnitude of age-related cognitive decline is highly variable, a portion of older humans exhibit no or little memory impairment whereas others exhibit obvious memory deficits that may or may not progress to more severe neurological illness such as Alzheimer’s disease. Thus, there is a pressing need to understand the effects of normal aging separate from and before the manifestation of neuropathological disorders. Aged rats exhibit a qualitatively similar cognitive profile when tested for spatial learning ability in the Morris water maze; ∼50% of aged rats exhibit robust cognitive impairment compared to young controls while the remaining 50% of aged rats perform on par with young. Using this naturally-occurring model, it is possible to examine neurobiological processes that not only changed with age, but also significantly associated with learning and memory. Parameters associated with neurotransmission are logical candidates for neurobiological analysis; including the numbers of cells that produce specific neurotransmitters, the functionality of receptors and associated proteins that transduce extracellular signals and the modulation of intracellular Ca2+, a key molecular messenger that modifies synaptic strength. In Chapter II it is determined that F344xBrown Norway F1 (FBNF1) hybrid rats are impaired at spatial learning by 24 months of age, relative to 6 month-old controls. This finding is novel because it was previously assumed that this strain of rat maintains cognitive function until somewhat older ages (i.e. 28-30 months). The basis for this impairment relates to inferior spatial learning in a subset of aged rats (aged-impaired) relative to young as well as aged rats that are behaviorally similar to young (aged-unimpaired). Importantly, aged-impaired rats exhibit deficits in acquisition, but retention of learned information is not affected. Chapter III presents the stereologically-determined estimates of the total number of cholinergic (ChAT+) neurons and activated (CD68+) microglia in the MS/VDB of young rats and aged-impaired rats. These data show that there is no loss of cholinergic neurons in the aged-impaired MS/VDB, despite increased numbers of activated microglia, suggesting a substantial elevation of the local, basal inflammatory state. In Chapter IV, baclofen-stimulated GTP-Eu binding and western blotting were used to measure the functionality and expression of GABABR proteins in the hippocampus and PFC of young and aged rats. Results of this study reveal that aged-impaired rats express lower levels of GABABR1 protein in the hippocampus compared to young and aged-unimpaired rats while GABABR2 protein level was not changed. Significantly, loss of GABABR1 did not impair baclofen-stimulated GTP-Eu binding in the hippocampus. However, there is clear evidence for reduced GTP-Eu binding and expression of GABABR1 and GABABR2 in the PFC of all aged rats, suggesting an age-, but not spatial-learning, related loss of GABABR substrates and activity in this region. Chapter V describes the use of a modified version of the GTPγS-binding assay to determine that aging is associated with greater basal binding of GTPγS-binding to Gαq/11 across all hippocampal subregions, while oxotremorine-M-stimulated GTPγS-binding to Gαq/11 tended to be lower in the aged hippocampus and was inversely related to basal activity. Also, in Chapter V, confocal imaging of [Ca2+]i demonstrates oxotremorine-M-stimulated elevation of [Ca2+]i is lower in the aged CA1 area compared to young while DHPG-stimulated changes to [Ca2+] i are potentially dysregulated in the aged CA1. Despite these receptor system-specific effects, aged CA1 cells in both experiments utilized ICS to a greater degree than young, suggesting that Ca2+ source, not simply magnitude, is an important factor in neuron aging. In conclusion, these studies demonstrate that changes to GPCRs and their associated physiologic functions are significant targets for further evaluation in the context of aging and hippocampal-dependent cognition.