Transient increases in neuropeptide Y-like immunoreactivity in dentate hilar neurons following fimbria/fornix transection.

Neurons containing neuropeptide Y (NPY) are numerous in those hippocampal regions that receive septal and monoaminergic afferents. To assess the role of these afferents in the expression of NPY in hippocampal neurons, the number and distribution of perikarya with NPY-like immunoreactivity (NPY-LI) was examined quantitatively in the dentate gyrus of adult male rats following unilateral transection of the right fimbria/fornix. In unlesioned rats, immunoperoxidase labeling for the antibody to NPY was detected mostly in fibers and only a few perikarya in the dentate gyrus. Following fornix transections, the number of detectable NPY-containing neurons in the hilus of the dentate gyrus ipsilateral to the lesion increased at 3 days post-lesion (dpl), peaked at 6 and 9 dpl, then returned to basal levels at 14 dpl and 1 and 6 months post-lesion. This elevation followed a rostral to caudal gradient. No apparent changes were found in the distribution of NPY-labeled neurons at any post-lesion interval studied. Moreover, no significant changes at any of the post-lesion times were found in the number or distribution of neurons with NPY-LI in the hilus of sham lesioned (i.e., ablations of the cortex and anterior hippocampal formation sparing the fornix) rats. The observed increases in the number of hippocampal neurons containing detectable NPY suggests that the cellular levels of this peptide are dependent on pathways travelling through the fornix. The rapid and transient increases in NPY are not due exclusively to changes in cholinergic pathways but may involve changes in other pathways within the fornix or even indirect neurotrophic effects.