DISTRIBUTION OF NEURONS EXPRESSING SUBSTANCE P-RECEPTOR MESSENGER-RNAIN IMMATURE AND ADULT CAT VISUAL-CORTEX

We have investigated the expression of messenger RNA (mRNA) encoding s ubstance P receptor (SPR) in the visual cortex of adult cats and 17-da y-old kittens, using in situ hybridization histochemistry with two dig oxigenin-labeled oligodeoxynucleotides complementary to the SPR mRNA. In the adult cortex, a subset of large pyramidal neurons of layer V an d layer III is heavily labeled. Other, mainly pyramidal neurons in lay ers II, III and V are less intensely labeled, but most neurons in thes e layers appear unlabeled. Neurons in layer IV and VI, and in the whit e matter do not show hybridization signals above background levels. In the 17-day-old kitten, SPR mRNA-expressing cells are confined to laye r V and to the upper white matter (subplate zone), whereas supragranul ar neurons do not yet contain SPR mRNA. A few neurons in layer VI disp lay moderate labeling. Astrocytes, identified with anti-glial fibrilla ry acid protein antibodies, did not express detectable levels of SPR m RNA in both adult and kitten visual cortex. These results indicate tha t SPR mRNA expression is transient in neurons of the white matter, and is developmentally regulated in supragranular layers. In addition, th e localization of SPR mRNA in a subset of pyramidal cells suggests tha t substance P modulates the excitability of certain projection neurons which are the origin of extrinsic connections.