Intracellularly injected cobaltous ions accumulate at synaptic densities.

Physiologically identified neurons in the locust were iontophoretically injected with a mixture of cobaltous and potassium ions. After being fixed for electron microscopy, 2.5-micrometer sections of the epoxy-embedded ganglia were intensified with silver. The intensified material was resectioned and examined in the electron microscope. The cobalt-silver precipitate appeared as discrete densities. Localized accumulations of the precipitate were seen within the injected cell along the neuronal membranes and especially at synapses. Location and recognition of the stained neuron in the electron microscope was facilitated by the tendency of the cobaltous ions to aggregate at the synaptic sites.