While undoubtedly important, the pineal gland does not possess mystical or supernatural properties no matter how much some people would like to believe it does. Even some great, renowned thinkers have fallen into this way of thinking. One example being the scientist and philosopher Descartes, who described the pineal gland as “the seat of the soul”. It is just one gland among many that comprise the endocrine system in vertebrate animal species.
This system’s function is heavily involved in the regulation of circadian (daily) rhythm and the production of hormones – the most important being melatonin, the “sleep hormone”. However, there are a few things that make the pineal gland stand out as unique and interesting. The idea of the pineal gland being a primordial “third eye” has some basis in fact. The gland is made up of cells known as pinealocytes, which in some non-mammal vertebrate species directly respond to light. This ability makes them very similar to the cells of the retina, the part of the eye that receives light from the lens opening. In some fossil species, scientists have even found holes just like eye sockets in the centre-rear part of the skull, which allowed the pineal gland to receive light directly, just like an eye.
In fact, several modern species of reptile and fish still retain a functional “third eye”, such as the New Zealand reptile species the tuatara . The tuatara’s extra eye actually has a lens, a retina and a cornea of its own! It is thought that these functional third eyes are involved in maintaining daily and seasonal cycles of hormone production. In mammals, the pinealocytes aren’t known to directly receive light and there is no evidence of functional “third eyes” existing. However, the pinealocytes of mammals are known to be directly linked to the retina itself, which sends signals in response to changes in light levels in order to regulate circadian rhythms. So in some respects, if one stretches the definition of what constitutes an eye, one could still say that the pineal gland functions somewhat like a rudimentary third eye even in mammals. One interesting aspect of the pineal gland that certainly does apply to mammals, including humans, is this: unlike much of the brain, the pineal gland is not separated from the rest of the body by the blood-brain barrier. It receives abundant blood flow directly from the posterior cerebral artery, which may have something to do with its receptiveness to psychoactive substances. Why do psychoactive substances often affect the pineal gland? The pineal gland is in the perfect position to receive all kinds of substances not filtered by the blood-brain barrier. But it is also at the heart of a “cascade” of reactions which fire off when norepinephrine, a well-known neurotransmitter responsible for regulating the efficacy of many wake- and sleep-promoting medications, binds to its receptors in the pineal gland. When norepinephrine binds to the receptors (known as adrenergic receptors), it results in a chain of hormonal and enzymatic interactions. These reactions are responsible for signalling when it is time to sleep and when it is time for the individual to wake and become active. Thus, this cascade is deeply involved in setting up one’s “mood” and how one perceives and responds to the challenges of the coming day. It also ensures that sleep of sufficient quality and duration is regularly achieved. Clearly, the pineal gland is essential to maintaining a healthy, positive mind state, and is deeply involved with emotional states in general. When humans consume psychiatric drugs, it affects this complex cascade of activity in the pineal gland, in conjunction with various other parts of the brain. This gives a subjectively altered state of perception. One example of the importance of the pineal gland in terms of psychiatric good-health is its relationship with the “Winter Blues” illness, seasonally affective disorder (often abbreviated to SAD).
The fact that bright light is a common treatment for SAD suggests that the pineal gland and its associated light-sensitive hormone, melatonin, is involved. And the mere fact that low light levels can cause such a dramatic set of psychological symptoms indicate that the pineal gland may be fundamentally linked with psychiatric good health in general. If that is the case, it is easy to assume its dysfunction may be behind other mental disorders too. Research on rats has shown that the pineal gland itself contains a functional endocannabinoid system. The cannabinoid receptors type 1 and 2, and the endogenous ligands that bind to them, anandamide and 2-AG, are all present. The study showed that the activity of the CB₁-receptors varied according to a daily cycle, with lowest activity levels occurring at the end of the daylight period. It also showed that levels of an enzyme responsible for synthesizing new endocannabinoid molecules, NAPE-PLD, was reduced during the middle of the dark period. Furthermore, the study showed that presence of THC reduced the activity of an enzyme known as AANAT, and in doing so reduces the synthesis of melatonin itself. An earlier study on rats also showed that THC reduced the activity of AANAT and suggested that the mechanism that occurred was as follows: the neurotransmitter norepinephrine starts a cascade of reactions, the end result of which is the production of melatonin.
THC disrupts this norepinephrine cascade and thereby reduces the production of melatonin. It then seems likely that THC’s endogenous analogue anandamide performs the same action of reducing the norepinephrine-induced release of melatonin and is therefore fundamentally involved in pineal gland function. It seems that low levels of anandamide or THC are therefore needed for melatonin production to increase. As melatonin levels increase, as they should normally do at the end of the daylight period, feelings of sleepiness should occur. If low levels of anandamide are required for melatonin production to increase, and levels of cannabinoid receptor activity are lowest at the end of the daylight period, that seems to imply that using cannabis would cause melatonin to be reduced If that is the case, sleepiness should also be reduced.