Nucleus circularis: is it an osmoreceptor in the brain?

The nucleus circularis, in the anterior hypothalamus, is a group of magnocellular elements arranged in a ring around a capillary bed. The cells are predominantly monopolar, tightly packed, and are flattened at the outer border of the ring. The entire nucleus is surrounded or encapsulated by myelinated fibers. Electrical stimulation of this nucleus produced a short-latency, long-lasting and substantial antidiuresis in ethanol anesthetized rats. Water deprivation induced changes in numbers of nucleoli and cell size increases in these cells. The multiplication of nucleoli in this nucleus during water deprivation was more profound than that previously observed in the supraoptic nucleus. Decreases in multiple nucleoli accompanied voluntary rehydration. Seven criteria for status as an osmoreceptor are listed and the nucleus circularis was found to meet 6 of these criteria, the seventh being the demonstration of receptor potentials which has not yet been attempted.