Abstract
The effect of hyperosmolality on the midbrain reticular formation of rabbits was examined electrophysiologically. Hyperosmolality was induced in rabbits weighing 1.5-2.0 kg by continuous intravenous injection of 7% sodium bicarbonate (1666 mOsm/L) and continued until the death of the animal. An electrophysiological study was made of the effects of hyperosmolality on the unit discharges of the midbrain reticular formation (RF), photopalpebral reflex (PPR) and photo-evoked eyelid microvibration (MV).
1) The frequency of the unit discharges of the RF increased temporarily when the plasma osmolality was 300-310 mOsm/L, and then gradually decreased following the increase of the plasma osmolality over 310 mOsm/L.
2) The MV response induced by photic stimulation consisted of MV1, MV2, MV3, MV4 and MV5 components. MV showed a decrease in their amplitude following the osmolality increase. But at a level of about 400 mOsm/L in plasma osmolality, the amplitude of MV recovered temporarily, though over 400 mOsm/L, it virtually disappeared.
3) PPR consisted of PPR1, PPR2, PPR3, PPR4, PPR5 and PPR6 components. The late components of PPR (PPR5 and PPR6) showed a decrease in amplitude following the osmolality increase and disappeared over 400 mOsm/L. PPR6 showed a temporary recovery in amplitude at 400 mOsm/L in plasma osmolality, just like the MV.
4) The changes in the amplitude of MV and the late components of PPR following the osmolality increase were similar to changes of the frequency of the unit discharges of the RF and arousal reaction. Therefore it is considered that MV and the late components of PPR reflected the RF function.
From these results, it was concluded that the hyperosmolality induced a disfunction of the RF and the MV and PPR can be used as a test of the RF function.
