A comparison of the anatomical distribution of substance P and substance P receptors in the rat central nervous system.

A comparison of anatomical distributions of substance P (SP) and substance P receptors in the rat central nervous system was performed. SP was localized by microdissection and radioimmunoassay and SP fibers and cell bodies by immunohistochemistry. Receptors for 125I-Bolton Hunter labelled SP (125I-BH-SP) were characterized pharmacologically by a slice binding technique in sections that contained primarily striatum. The receptor was saturable and had an equilibrium dissociation constant (KD) of 0.30 nM and maximum number of binding sites (Bmax) of 37.8 fmol/mg protein. Pharmacological characterization using C terminal fragments and naturally occurring analogues of SP reflected characteristics of the receptor which had been shown previously in bioassays and biochemical assays. Comparison of distribution of SP fibers and cell bodies and SP receptors indicated that there is no consistent relationship between the amount of SP receptor and density of SP fibers or cell bodies in a given region of the brain.