Substance P receptors: Localization by light microscopic autoradiography in rat brain using [³H]SP as the radioligand

Substance P (SP) is a putative neurotransmitter in both the peripheral and central nervous systems. In the present report we have used a modification of the Young and Kuhar technique to investigate some of the SP receptors binding properties and the distribution of SP receptors in rat brain. Tritiated SP ([³H]SP) adsorbed extensively to glass but this adsorption was greatly reduced by preincubating the slide-mounted tissue sections in a solution containing the cationic polymer polyethylenimine. [³H]SP was found to bind to rat tissue in a saturable fashion with a B_max of 14.7 fmol/mg tissue wet weight and a K_d of 1.1 nM. The rank order of potencies for displacing [³H]SP binding from rat tissue sections was SP > SP sulphoxide > DiMeC₇ > Eledoisin >0. SP(5-11) > SP(COOH) > SP(1-9) amide. Using autoradiography coupled with LKB tritium-sensitive Ultrofilm or the dry emulsion-coated coverslip technique the distribution of [³H]SP binding sites was found to be very dense within the olfactory bulb, amygdalo-hippocampal area and the nucleus of the solitary tract. Heavy concentrations of receptors were observed in the septum, diagonal band of Broca, striatum, subiculum, hypothalamus, locus coeruleus, parabrachial nucleus and lobule 9 and 10 of the cerebellum. Moderate to low concentrations of receptors were observed in the cerebral cortex, globus pallidus, raphe nuclei and the trigeminal nucleus. Very low densities were observed in most aspects of the dorsal thalamus, substantia nigra and cerebellum (other than lobule 9 and 10). Comparisons of the present data with SP peptide levels indicate that in some areas of the brain there is a rough correlation between peptide and receptor levels. However, in other brain areas (olfactory bulb, globus pallidus and substantia nigra) there is little obvious correlation between the two.