Dopaminergic nerve endings visualised by high-resolution autoradiography in adult rat neostriatum

Abstract

Nerve cells containing dopamine (DA), noradrenaline (NA) or serotonin possess powerful physiological mechanisms for the reuptake and storage of their own transmitter. Monoaminergic neurones may therefore be visualised by high-resolution autoradiography¹, provided that tritiated biogenic amines are retained in their sites of uptake and/or storage during suitable preparative histological procedures. Using this method, noradrenergic and serotonergic neuronal cell bodies and axon terminals have been specifically identified and examined in various parts of mammalian central nervous system². However, although the nerve cell bodies of central dopaminergic neurones have been similarly investigated after in vivo labelling with either ³H-DA or ³H-NA^3–7, the autoradiographic detection of dopaminergic nerve endings has remained problematic. Indeed, in previous studies after cerebroventricular infusion, super-fusion or local instillation of ³H-DA or ³H-NA, light and electron microscope autoradiographs of neostriatum, the brain region richest in dopaminergic nerve endings, failed to exhibit the small clusters of silver grains typical of monoamine-labelled axonal varicosities^8–10. This suggested tracer displacement during histological processing, so we tested, after intraventricular administration of ³H-DA, a technique of tissue preparation developed in our laboratory for high-resolution autoradiographic visualisation of deoxyglucose-6-phosphate, a diffusible, water-soluble, unbound substrate¹¹. We report here that the rapid successive vascular perfusion of both a glutaraldehyde primary fixative and osmium tetroxide postfixative, which may be followed by dehydration and resin-embedding also carried out by perfusion, maintains a sufficient amount of ³H-DA in situ to ensure light and electron microscopic identification of DA axon terminals in rat brain¹².