In vivo labeling of dopamine receptors: light microscopic localization at the cellular level by means of dipping autoradiography with the agonist (3H)N-n-propylnorapomorphine.

A convenient method is described for light microscopic autoradiography at the cellular level for the visualization of in vivo labeled dopamine receptors. (3H)N-n-propylnorapomorphine [(3H)NPA] is administered to rats under conditions that are known to give specific and saturable accumulation in the striatum. Seventy minutes after intravenous administration, the brain is rapidly frozen and cut on a cryostate microtome. The sections are treated with formaldehyde vapor, defatted for 1 hour in xylene and dipped in liquid nuclear emulsion, exposed for 2 weeks and developed. Autoradiograms obtained in this way show a high silver grain density over the striatum and a low density over adjacent external capsule and neocortex. There was a sharp delineated boundary between striatum and adjacent external capsule. Since low energy radiation can be quantified in liquid emulsion autoradiograms, we counted silver grains in a number of regions. The distribution of silver grains appeared to be identical with the distribution of radioactive label after similar in vivo administration of (3H)NPA in other studies.