IMMUNOCYTOCHEMICAL LOCALIZATION OF SUBSTANCE-P RECEPTOR IN RAT PERIAQUEDUCTAL GRAY-MATTER - A LIGHT AND ELECTRON-MICROSCOPIC STUDY

The distribution of substance P receptor (SPR) protein in the rat peri aqueductal gray matter (PAG) was investigated with a polyclonal antibo dy in the four subdivisions obtained by cytochrome-oxidase histochemis try (Co-hi). At light microscopic analysis, immunoreactivity appeared particularly dense in the dorsal subdivision of the PAG, was less inte nse in the other subdivisions, and formed several longitudinally organ ized columns. SPR-like immunoreactivity (SPR-i) was localized mostly t o cell bodies and dendrites of small and medium-sized neurons, which c onstituted about 6% of the total neuronal population of the PAG. At th e electron microscopic level, SPR-i could be observed on postsynaptic as well as on nonsynaptic regions of both cell bodies and dendrites. A small proportion of axons (4.2%) and axon terminals (5.3%) showed SPR -i, the majority of labeled axon terminals, amounting to about 70% of synapsing elements, formed asymmetric synapses with dendrites. Rare as troglial processes displaying SPR-i were also observed scattered throu ghout the neuropil of all PAG subdivisions. Our observations suggest t hat 1) also in the PAG, SP may act in a diffuse, nonsynaptic manner, p robably on targets that are distant from its sites of release; and 2) SP may modulate excitatory neurotransmission acting presynaptically on those labeled axons that form asymmetric synapses. J. Comp. Neurol. 3 98:473-490, 1998. (C) 1998 Wiley-Liss, Inc.