Intravenous injection of Evans Blue labels magnocellular neuroendocrine cells of the rat supraoptic nucleus in situ and after dissociation.

Previous work has demonstrated that intravenous injection of neuronal tracers, e.g. horseradish peroxidase or Fast Blue, can retrogradely label neurons in brain areas that project outside the blood-brain barrier, e.g. magnocellular neuroendocrine neurons of the hypothalamus. Here we have shown that 24 h after intravenous injection of the fluorescent retrograde tracer Evans Blue, the same population of magnocellular neuroendocrine neurons is labeled in the paraventricular, supraoptic and accessory magnocellular nuclei. Parvicellular neuroendocrine cells in the paraventricular nuclei are also labeled. Most Evans Blue-labeled magnocellular neuroendocrine cells in the supraoptic nucleus could be stained immunocytochemically for neurophysins, suggesting that these neurons continue to produce their peptide hormones after taking up the fluorescent dye. Ultrastructural observation of supraoptic cells retrogradely labeled with Evans Blue shows that 95% of the neurons appeared healthy. There was no ultrastructural evidence of degeneration, hyperstimulation, or interruption of the axoplasmic flow. Labeling the neuroendocrine cells with Evans Blue did not alter the size of magnocellular cells, the animal's fluid balance or ingestive behavior. Following enzymatic/mechanical dissociation of the supraoptic nucleus from animals that had been injected with Evans Blue 24 h previously, phase-bright neurons that often contained fluorescent material were observed, thus identifying these neurons as neuroendocrine. Recording from identified neuroendocrine cells showed that these neurons generated spontaneous or current-evoked overshooting action potentials with an afterhyperpolarization and had negative resting membrane potentials. Action potential broadening, a feature of magnocellular neurons, was observed during bursts of action potentials elicited by depolarizing current injection. Taken together, this work would suggest that Evans Blue is non-toxic at the doses used and that it provides a method to identify single neuroendocrine cells in primary cell cultures made from adult hypothalamus for voltage-clamp recordings.