The central controller for water balance is the hypothalamus but there is no single anatomically defined center which is solely responsible for producing an integrated response to changes in water balance. There are numerous pathways interconnecting the various centers or areas in the hypothalamus. The osmoreceptors are located in the area known as the AV3V (anteroventral 3rd ventricle). Lesions in the AV3V region in rats cause acute adipsia.
The thirst centre is located in the lateral hypothalamus. It receives input from osmoreceptors in the AV3V region and from the subfornical organ and the organum vasculosum of the lamina terminalis (OVLT) which are sites for angiotensin II action. The OVLT is in the AV3V region.
ADH is formed predominantly in the neurones of the supraoptic and paraventricular nuclei. These nuclei receive input from the osmoreceptors and also from ascending adrenergic pathways from the low and the high pressure baroreceptors. Aquaporin 4 has recently been identified in cells in the hypothalamus particularly in the paraventricular and supraoptic nuclei.
The key parts of the hypothalamus involved in water balance are:
- Thirst centre
- OVLT & SFO (respond to angiotensin II)
- Supraoptic & paraventricular nuclei (for ADH synthesis)