Neurokinin B and substance P genes are co-expressed in a subset of neurons in the rat habenula.

Colocalization of neurokinin B (NKB) and substance P (SP) mRNAs in neurons of the habenula was examined on thin, adjacent sections in the rat. Extensive colocalization was found in the medial habenula. In its dorsolateral part, most of the neurons contained both transcripts, with high levels of SP, while in the dorsomedial part fewer instances of colocalization were found. The ventral half could be divided into two parts, a dorsal part with most of the neurons containing both messages, having low levels of SP mRNA, and a ventral part with most of the cells containing only NKB mRNA. Cells in the medial habenula had low levels of NKB mRNA. These results suggest a structural and functional heterogeneity of the medial habenula.