Neurotransmitters

 
 

   Although the cognitive deficits of sleep deprivation have been well established, the neurochemical mechanisms have yet to be elucidated. Research has indicated that these deficits may be mediated by various neurotransmitters, including the catecholamines, serotonin, and acetylcholine.
 

Serotonin

        Scientists way back in the '50s realized a connection between serotonin and sleep.  It is known that stimulation of the raphe nuclei, a region of the brain that contains most of the brain's serotonergic neurons, increases cortical EEG readings.  Additionally, like norepinephrine, serotonergic neurons become virtually zero during REM sleep.

    Despite our understanding of serotonin in REM sleep, we are unclear in whether serotonin induces/maintains sleep or whether sleep maintains/restores the serotonin system.  We can test these theories by examining the effects of sleep deprivation on serotonin levels.  If the former is true, then sleep deprivation should increase serotonin levels.  On the contrary, if the latter is correct, deprivation should decrease the serotonin system.  Findings of the effects of total- and REM-sleep deprivation on serotonin levels have been inconsistent.

    Support for an increase in serotonin function during total sleep deprivation comes from the finding that sleep deprivation produces an antidepressant response in humans.  Most antidepressant drugs increase serotonergic activity.   Additionally, findings of increases in serotonin and 5-HIAA (a metabolite of serotonin) levels in the whole brain, cortex, and midbrain, in both rats and cats, have been reported following REM and total sleep deprivation.   Moreover, serotonergic neurons have been shown to fire more frequently during total sleep deprivation. Finally, it has been suggested that sleep deprivation increases serotonin turnover, as determined by the 5-HIAA/serotonin ratio, in the frontal cortex, hippocampus, hypothalamus and brainstem.
    However, not all studies have confirmed an increase in serotonin during sleep deprivation.  Studies have reported no changes in serotonin or 5-HIAA in the cortex, diencephalon, telecephalon, brainstem, or whole brain after REM sleep deprivation and total sleep deprivation have been reported.