The role of interconnection between supraoptic nucleus and anterior third ventricular region in osmoregulation in the rat.

1. To determine the importance of the region of the hypothalamus anterior and ventral to the third ventricle (AV3V) in the osmotic regulation of neurohypophysial hormone release, extracellular recordings were made in the urethane-anaesthetized rat from neurones in the supraoptic nucleus (SON) and in the AV3V using a ventral surgical approach. 2. In the first series of experiments, recordings were made from SON cells (n = 41) which could be antidromically activated from the neural stalk in control rats and rats 2 weeks after electrolytic destruction of the AV3V (n = 74). Destruction of AV3V significantly (P less than 0.01) reduced the responsiveness of SON cells to an osmotic challenge (1 ml of 1.5 M-NaCl I.P.). 3. In the second series of experiments recordings were made in the AV3V region while stimulus pulses were applied to the region of the SON. The cells recorded could be divided into five groups; those which were inhibited (presumably synaptically; OD-; n = 97), those which were excited (also presumably synaptically; OD+; n = 73), those which could be antidromically activated (AD; n = 50), those which showed a complex cycle of excitation and inhibition after the stimulus (n = 40) and those which did not respond to the stimulus (n = 47). 4. The characteristic response of OD- cells to an osmotic challenge was inhibition (six out of ten cells) and that of OD+ cells was excitation (thirteen out of seventeen cells). No AD cells showed excitation following I.P. NaCl injection (nine out of nine). 5. Our experiments provide clear evidence that the AV3V region is important for the normal responsiveness of the cells of the SON to raised plasma osmolality and for an interaction between the SON and the AV3V region. 6. Our results also suggest, contrary to our expectation, that the SON exerts both excitatory and inhibitory influences on the AV3V. Since the cells which project from the AV3V to the SON region are not osmoresponsive, the importance of the direct projection from the AV3V to SON may be to maintain the responsiveness of SON neurones rather than to provide osmotic drive.