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The Calretinin‐containing Mossy Cells Survive Excitotoxic Insult in the Gerbil Dentate Gyrus. Comparison of Excitotoxicity‐induced Neuropathological Changes in the Gerbil and Rat

Our preliminary results showed that mossy fibres do not undergo sprouting after global ischaemia in gerbils, although the pattern of hippocampal cell damage resembled that seen in ischaemic and epileptic rats, where mossy fibre sprouting is known to occur. In order to investigate whether the observed differences in the appearance of mossy fibre sprouting are related to the animal model or species used, this study was undertaken to compare the neuropathological changes induced in gerbils by systemic injection of kainate or by occlusion of carotid arteries with the changes induced in rats by injection of kainate. The pattern of pyramidal cell damage was very similar in each group. Mossy fibre sprouting was present in epileptic rats but not in ischaemic or epileptic gerbils. The number of somatostatin‐immunoreactive neurons was decreased in the hilus of epileptic rats and ischaemic gerbils, but not in epileptic gerbils. The analysis of calretinin immunoreactivity in the dentate gyrus revealed differences between the rat and gerbil. The most striking difference between these species was that the mossy cells contained calretinin in gerbils but not in rats. Cell counting showed that the calretinin‐containing mossy cells had survived both in epileptic and ischaemic gerbils. Therefore, since the mossy cells are known to be highly susceptible to excitotoxic insult in rats and degeneration of these cells is thought to be a key element in the induction of mossy fibre sprouting, we propose that the absence of mossy fibre sprouting in gerbils is related to the survival of the mossy cells.