Rhesus macaques with high CSF 5-HIAA concentrations exhibit early sleep onset

The relationship between central nervous system serotonergic activity, as reflected by cerebrospinal fluid (CSF) concentrations of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA), and sleep/wakefulness behavior was investigated in socially housed, juvenile rhesus macaques. Two cohorts of rhesus monkeys (Macaca mulatta), numbering 42 subjects (seventeen 39-month-olds and twenty-five 20-month-olds) were observed in their home cages between 21.30 h and 23.30 h for 10 nights using an infrared night scope. Over each 90-min observation period, the following states were recorded every 5 min using a scan sampling procedure: Sleep, Drowsy, Passive-awake and Active. After more than one quarter of the animals in the group had fallen asleep, states were recorded as they occurred. Six weeks prior to the collection of the behavioral data, a sample of cisternal CSF was obtained to assay for 5-HIAA concentrations. With cohort effects statistically controlled, there was a negative correlation between latency to fall asleep and CSF 5-HIAA concentrations (i.e., subjects with high CSF 5-HIAA concentrations were more likely to fall asleep early). Subjects with low CSF 5-HIAA concentrations were also more active during the daytime hours. Subjects who fell asleep first were, on average, also less active during nighttime hours. The positive correlation between CSF 5-HIAA and sleep onset was not a result of social status since there was no correlation between social dominance rank and time of sleep onset. These results support the hypothesis that the serotonergic system may play a role in sleep onset and possibly in the regulation of diurnal activity rhythms in non-human primates.