Osmoregulatory Circuits in Slices and En Bloc Preparations of Rodent Hypothalamus

Osmoregulatory neural networks in the mammalian hypothalamus are responsible for keeping plasma osmolality near a constant set point. This process involves primary osmosensory neurons located in the organum vasculosum lamina terminalis (OVLT) and effector neurons located elsewhere in the hypothalamus and in other parts of the brain. Hypothalamic effector neurons include vasopressin (VP)- and oxytocin (OT)-releasing magnocellular neurosecretory cells (MNCs) in the supraoptic (SON) and paraventricular nuclei. Osmotically induced changes in action potential discharge by MNCs cause proportional changes in VP and OT release into the blood to modulate water and sodium excretion at the kidney. This osmotic regulation of firing rate in MNCs plays a critical role in the maintenance of body fluid balance. In this chapter, we describe spontaneously active in vitro preparations of rodent hypothalamus that have been developed for the specific purpose of studying how osmosensory neurons in the OVLT and other hypothalamic structures regulate MNCs in the SON.