“The essence of our study is that at a time of day when we know that light has no effect [on the circadian clock], serotonin will shift the clock,” Turek told Psychiatric News. “If we give light and serotonin [an agonist] at the same time, [light] blocks the effect of serotonin. If light can alter the way that neural tissue, the brain of a hamster, can respond to exogenous serotonergic stimulation, it raises the hypothesis that light may alter the way the brain is responding to serotonin in the brain.” It is clear that light impulses have a direct impact on the brain through the “optic nerves” into the suprachiasmatic nucleus of the hypothalamus, said Turek.
“That light information is somehow able to synchronize the endogenous neuroclock” from within the suprachiasmatic nucleus. Light is effective in treating SAD, observed Turek, yet understanding of its actions remains rudimentary. Similarly, there is but imprecise understanding of how serotonin-enhancing drugs like Prozac (fluoxetine) and Zoloft (sertraline) exert their effects, Turek observed. While they inhibit reuptake of serotonin at the receptor, “we don’t know how that translates into alleviating depression. We need to begin to address physiological mechanisms here.”
So it was logical to ask whether light would affect serotonergic function, he added. Norman Rosenthal, M.D., chief of the Section on Environmental Psychiatry at the National Institute of Mental Health (NIMH), spoke with Psychiatric News from Tromso, where he was chairing the SAD conference. “I think what the Turek article does is indicate interaction between light and a serotonergic drug in an animal model.
It’s not the first time in animals that light and serotonin have somehow been connected with each other.” Earlier work has shown that light exposure alters the firing rate of serotonergic neurons in rats. Another study found that the amount of light influences the concentration of serotonin in the rat brain.