In med school we had to memorize the major role of both of these regions, and my mnemonics were that the suprachiasmatic nucleus makes you charismatic (because you are well-rested), while the OVLT controls how much ovaltine you should drink.
Rodents are known to increase their fluid intake 1-2 hours before sleeping, which is called anticipatory thirst because the rodents want to make sure that they will have enough water in them to make it through the night (presumably without a dry mouth!).
Gizowski et al’s definitive experiment to show the interplay between these brain regions involved expressing two types of channelrhodopsins in vasopressin-expressing neurons in the suprachiasmatic nucleus in two groups of mice.
This manipulation allowed them to shine blue light in some mice to cause activation of the SCN -> OVLT pathway (G below), and yellow light in others to inhibit this pathway (H below). Here’s what they found:
As you can see, blue light in the blue-responsive mice caused increased water intake PRIOR to the normal anticipatory thirst at levels ~ 3x above baseline.
On the other hand, yellow light in the yellow-responsive mice caused decreased water intake at the expected anticipatory thirst to an insane degree. Basically, responsive mice didn’t drink at all when they shined yellow light and inactivated the SCN -> OVLT pathway.
This effect of yellow light seems too strong to me. Shouldn’t it just be stopping the increase that is typically seen pre-sleep? Instead, it seems to be completely eliminating drinking behavior entirely.
One way to explain this finding is that the SCN -> OVLT pathway might be active at other times other than pre-sleep, and it is just MORE active in the hour or two before sleep.
Despite a pretty extensive search, I can’t figure out whether humans have also been shown to have increased thirst prior to sleep.
On one hand, there are plenty of anecdotal reports of this, while on the other hand, rodents and humans have pretty different life histories. Plus, the authors probably would’ve mentioned it if there was good evidence of pre-sleep anticipatory thirst in humans.
Even if humans don’t have pre-sleep anticipatory thirst, this is still quite an interesting study, as this system is likely a good model of how suprachiasmatic nucleus axons project (with vasopressin-producing neurons?) to several other brain regions to control activities that are regulated by the perceived time of day.