Aged rats need a preserved complement of perforated axospinous synapses per hippocampal neuron to maintain good spatial memory

Abstract

Spatial working memory, which crucially depends on the structural integrity of the hippocampal formation and its afferent connections, is impaired in the most, but not all, of aged rats. This study was designed to verify whether aged animals that do not exhibit the spatial memory deficit are the ones in which the hippocampal synaptic connectivity remains preserved with advancing chronological age. Young adult rats with good spatial memory, aged rats with impaired spatial memory and equally aged rats with intact spatial memory were compared. The number of synapses per neuron was estimated in the hippocampal dentate gyrus. The most important results were obtained when axospinous synapses were divided into perforated and non-perforated ones according to the appearance of their postsynaptic density. A significant decrease in the number of perforated synapses was found in memory-impaired aged rats as compared to either young adults or aged animals without memory deficits. The number of non-perforated synapses per neuron was diminished in memory-deficient aged rats relative to young adults, but not to memory-intact aged rats. However, it was only the loss of perforated synapses which correlated with the degree of spatial memory impairment. Thus, aged rats need a preserved complement of hippocampal perforated synapses to maintain good spatial memory.