Serial reconstruction of Lucifer yellow-labeled supraoptic nucleus neurons in perfused rat hypothalamic explants

Intracellular recordings from supraoptic nucleus neurons in perfused explants of rat hypothalamus were followed by intracellular injections of the fluorescent dye, Lucifer yellow. Following fixation, 40 microns sections were processed for whole cell light-microscopic reconstruction in the horizontal or coronal plane. The somata of most supraoptic neurons were elongated (mean 25 X 13 microns) with 1-3 sparsely branched dendrites (length 30-725 microns) that displayed numerous spines. Most (95%) dendrites turned in the ventral direction to end in the glial lamina along the base of the nucleus. Each neuron had one axon: in 60% of cells, the axon arose from a dendritic profile and immediately assumed a varicose appearance; in the other 40% of cells, the axon appeared to arise directly from the soma and demonstrated in its initial 80-200 microns numerous spines and few varicosities, i.e. the morphological features of a dendrite. All axons coursed in a dorsomedial direction over the optic tract. At this point, most axons revealed smaller secondary processes 2-15 microns in length. Axons then turned ventrally towards the basal hypothalamus; some could be followed for up to 2100 microns from the cell somata. This approach to the light microscope morphology of supraoptic neurons provides a surprising array of detail on soma, dendrite and axon characteristics, while retaining the overall relationship between individual neurons and neighboring structures, including the boundaries of the nucleus itself.