Comparative immunohistochemical study of the effects of pilocarpine on the mossy cells, mossy fibres and inhibitory neurones in murine dentate gyrus

Treatment with pilocarpine (PILO) induces variable degrees of loss of mossy cells (MCs) and mossy fibre (ME) sprouting in rodents, the relationships of which have not been examined in individual animals. Our aim was to test whether the loss of MCs and ME sprouting are coupled processes in PILO-treated rodents. Animals which exhibited intense PILO-induced convulsions for at least 30 min were used in this study. After a 2-month survival period, the incidence of epileptic seizures was checked individually by neuropeptide-Y (NPY) immunohistochemistry, and the numbers of MCs were counted by means of immunohistochemistry, for calretinin (CR) in mice and calcitonin gene-related peptide (CGRP) in rats. ME sprouting was checked by using Timm's silver-sulphide method for zinc. In our comparative studies, NPY immunohistochemistry resulted in more positive animals than on zinc staining. The CR immunoreactivity remained unchanged even in those mice that displayed ME sprouting and greatly increased NPY immunoreactivity. CR immunoreactivity was also verified after transection of the fornix to exclude the extrahippocampal source of this peptide. However, the CGRP immunoreactivity was severely reduced in those rats that exhibited simultaneous increases in zinc content and NPY immunoreactivity in the supragranular layer and stratum lucidum. Our findings suggest that the MCs survive PILO treatment in mice, but not in rats. There is direct evidence of a close relationship between the loss of MCs and MF sprouting in rats, but not in mice. Thus, similar PILO seizures may result from different changes in the neuronal circuits of rodents.