That's a really good find because the molecular weight of cuminaldehyde is only 148, which is a lot less than all these other compounds that are able to cross the blood-brain barrier, so it should cross. Now the question is does it make it through the intestines and past the liver's glucuronation/sulfuronation? It is oxidized by one of the liver's P450 enzymes, but I do not know which one so I do not know if grapefruit or peperine could block it's degradation. It does not appear anyone knows if it can make it past the liver and therefore to the brain.
A way to take a guess about its bioavailability and even brain-availability is to look at what traditional medicine says. Especially if it has noticeable mental effects. "Āyurveda and Siddha regard jeeraka [Cuminum cyminum ] as having a bitter taste with a hot property, capable of removing vāta and kapha doṣas but causing pitta. It is dry, astringent, appetising, digestive, strengthening, light for digestion, good for the eyes and an aphrodisiac. It is used in the treatment of indigestion, dysentery, enlarged spleen, flatulence and vomiting."
Here's the paper :
It's about 1% in light colored cumin seed and half as much in black cumin. It's the ingredient that gives cumin its flavor. It's about 30% in cumin essential oil. It's the primary aldehyde.
It is not available as a supplement. It would take 2 heaping teaspoons cumin to get 100 mg cuminaldehyde which is minimum dose for most of these supplements, except for nicotine (people getting only 50 mg from a pack a day are strongly protected against PD, although 3 packs in one study wa a lot better). 6 cups of coffee is better than 3 cups and that's 600 mg of caffeine. The response in animals is almost always dose-dependent. You can't just take a supplement. It has to be in a strong dose.
It also prevents oxidation of L-DOPA, is antidiabetic, and protects against superoxide oxidation.