Substance P receptor in the rat indusium griseum during postnatal development

The presence of substance P (SP) receptor (Neurokinin-1 receptor, NK1R) in the indusium griseum (IG) and anterior hippocampal continuation (AHC) during postnatal development was studied by immunocytochemistry (ICC). NK1R-immunopositive neurons (NK1R_IP-n) first appeared in both areas on postnatal day (P) 5. From P5 onward, their distribution pattern was adult-like. In sagittal sections NK1R_IP-n formed a narrow strip of neurons and dendrites that were located over the corpus callosum (cc); in coronal sections they were found in a roughly triangular area at the base of the cingulate cortex (Cg) on the dorsal surface of the cc. NK1R_IP-n were also found in the AHC, which is considered as a subcallosal extension of the IG, located ventral to the genu of the cc. At all ages studied, IG NK1R_IP-n sent dendrites to the contralateral IG, the underlying cc, and the Cg. Moreover, NK1R_IP-n located in the Cg and the cc sent dendrites to the IG. The present findings are in line with previous ICC studies describing dopaminergic and serotoninergic afferents to the IG. Together these data suggest that, through NK1R, SP could play an important role in regulating the release mechanisms of these afferents and that it could be an important developmental factor. Notably, IG neurons could be activated by cortical and intracallosal afferents.