Central nervous system neurons which have access to the general circulation were identified by injecting the retrograde tracer Fluoro-Gold peripherally. Fluoro-Gold does not penetrate the blood-brain barrier but is taken up by nerve terminals which project to areas supplied by fenestrated capillaries or to the periphery. Fluoro-Gold-accumulating neurons were present in the following regions or cell groups of the central nervous system: diagonal band of Broca; medial preoptic area; organum vasculosum of the lamina terminalis; subfornical organ; anterior periventricular area; paraventricular nucleus; arcuate nucleus; accessory magnocellular nuclei of the hypothalamus; motor neurons of cranial nerves III–VII, and IX–XII in the brainstem and spinal cord; autonomic ganglionic cells of cranial nerve III (Westphal-Edinger nucleus) in the mesencephalon and the intermediolateral column of the spinal cord; sensory ganglia of the cranial nerve V (mesencephalic trigeminal nucleus); and the C1–C2 and A2 adrenergic cell groups in the medulla. In addition, Fluoro-Gold-accumulating neurons were seen in the sensory ganglia of cranial and spinal nerves.

Retrograde labeling with Fluoro-Gold can be combined with immunocytochemistry to identify the chemical messengers within Fluoro-Gold-labeled perikarya.

Although a large number of neurons are labeled in the central nervous system with Fluoro-Gold when it is administered peripherally, this technique in combination with immunocytochemistry can be a powerful tool to identify selected neuronal systems in the central nervous system.