Increased electrotonic coupling in aged rat hippocampus: a possible mechanism for cellular excitability changes.

The effects of aging on the intercellular transfer of the low molecular weight fluorescent dye 5,6-carboxyfluorescein was studied in subfields fascia dentata, CA1, and CA3 of rat hippocampal slices maintained in vitro. All three areas exhibited a statistically significant increase in dye coupling with age. The increased dye coupling was accompanied by an apparent increase in postsynaptic excitability as assessed by the ratio of the population spike to EPSP components of the extracellulary recorded field potential. The possibility that artifactual dye coupling due to cell fusion contributed significantly to these results was ruled out by the demonstrations that a high molecular weight, dextran-coupled fluorescein compound did not produce multiple fills and that dye coupling with carboxyfluorescein could be prevented by prior intracellular loading with Ca++, a procedure that decouples gap junctions in other tissue. The increase in extent of electrical coupling suggested by these data may largely account for the apparent increase in cellular excitability of this tissue with age and may reflect the mechanism by which the senescent hippocampus compensates for the loss of afferent input during the course of normal aging. The additional possibility is raised that increased electrical coupling may reflect a mechanism for permanent associative storage of information in this system.