Authors ’ response to commentaries Analyses of hippocampal circuitry in aging

The commentaries by Barnes [1], Geinisman [5], and Smith et al. [8] reflect the changing landscape in research on the neurobiological and cognitive effects of aging. All agree on several points. Frank neurodegeneration, which was formerly thought to cause cognitive impairment, does not occur among the principal neurons of the hippocampal formation. Instead, other changes within this circuitry are candidate substrates of brain aging. A current emphasis on the structural and functional integrity of synapses signals a shift away from a prior focus on neuron loss that has fundamental implications for understanding the process of brain aging. Further, all agree on the need for behavioral assessment as a background for neurobiological studies to determine the cognitive status of subjects in the study population. Ultrastructural studies provide the most definitive assessment of structural change in synapses. Geinisman evaluates the available data on the aged hippocampus and finds that the results are largely inconclusive. The application of appropriate ultrastructural methods for determining synapse number using sufficient samples of behaviorally characterized subjects is needed to provide a solid quantitative basis for judging whether synaptic connections are preserved or diminished. His commentary, along with that of Barnes, also raises the broad issue of whether changes in synaptic function can be adequately captured by anatomical investigations. We fully agree with these cautionary comments. Our view, which we are confident is shared by the commentators, is that a comprehensive definition of the effects of aging on hippocampal circuitry will require multiple methodological approaches. As noted by Geinisman, the careful application of methods for ultrastructural quantitation could fail to detect alterations in specific receptor proteins at synapses that lead to changes in functional connections, i.e., silent synapses. Barnes points to another example where anatomical studies alone provide limited information about the functional status of a circuit. Her physiological studies have shown that a decrease in synaptic input to the dentate gyrus from the entorhinal cortex in aged rats [1,3] is accompanied by increases in the strength of existing connections. We could add to these examples other findings from our own research. For example, we reported that the levels of metabotropic glutamate and muscarinic cholinergic receptor proteins in hippocampus are not different between young and aged rats or altered as a function of learning ability [4,6]. However, these receptors do not stimulate phosphoinositide turnover to the same extent in the aged rats as compared to the young rats, and the aged rats with the more severe learning deficits were those with more blunting of phosphoinositide turnover. That finding is consistent with other electrophysiological evidence that the response of aged hippocampal neurons to cholinergic stimulation is blunted [7] and further appears to indicate that the defect exists in the biochemical properties of cells that might not be reflected in morphological measures of synaptic connectivity. Although functional studies are undoubtedly important, all would agree that such studies are informed by evidence concerning structural integrity or a lack thereof. The approach used in our report, while providing a method for assessing large global losses in synaptic integrity (a 50% loss in synaptophysin immunoreactivity has been observed in the hippocampus of AD patients [9]), falls far short of the detailed anatomical analysis that is ultimately needed. Our more limited goal was to assess whether any detectable loss of synaptic markers, using three different target proteins, would be evident in a rigorously quantitative assay using an ample number of young and aged behaviorally characterized rats. Acknowledging the limitations of this approach, the data support the interpretation that there is no substantial overall change in synaptic markers in the hippocampus of the aged rat, with or without cognitive impairment. Our method of using protein extracted from the total hippocampus could indeed obscure changes limited to a circumscribed region of this structure, e.g., decreases confined to dorsal hippocampus as noted by Geinisman or decreases confined to a specific connectional zone. The * Corresponding author. Tel. : 11-904-953-1067; fax:11-904-9537117. E-mail address:nicolle.michelle@mayo.edu (M. Nicolle) Neurobiology of Aging 20 (1999) 359–360