NITRIC-OXIDE SYNTHASE-IMMUNOREACTIVE AXONS INNERVATING THE GUINEA-PIG LINGUAL ARTERY - AN ULTRASTRUCTURAL IMMUNOHISTOCHEMICAL STUDY USING ELASTIC BRIGHTFIELD IMAGING

The ultrastructure of nitric oxide synthase-immunoreactive (NOS-IR) axons innervating the guinea-pig lingual artery was investigated by means of pre-embedding immunohistochemistry using an indirect peroxidase technique and diaminobenzidine. Sections ranging in thickness from 60 to 500 nm were ultrastructurally evaluated in elastic brightfield imaging mode. Thick sections (optimum at 300 nm) were advantageous for enhancement of the labelling intensity, whilst some subcellular details were better revealed by thin sections. NOS-IR axon terminals often contained aggregations of large, dense-cored vesicles, consistent with a previous light microscopical report on colocalization of NOS and vasoactive intestinal peptide-immunoreactivity in these fibres. NOS-IR axons formed direct neuro-muscular junctions (width less than 50 nm) at the outer surface of the tunica media, thus providing a structural basis for ''nitrergic'' vasodilation. In addition, NOS-IR axons made direct contacts with non-varicose and varicose segments of non-reactive axons, suggesting interneuronal communication between these elements.