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 serot onin metabolite 5-hydroxyindoleacetic acid (5-HIAA), and sleep/wakeful ness behavior was investigated in socially housed, juvenile rhesus mac aques. Two cohorts of rhesus monkeys (Macaca mulatta), numbering 42 su bjects (seventeen 39-month-olds and twenty-five 20-month-olds) were ob served in their home cages between 21.30 h and 23.30 h for 10 nights u sing an infrared night scope. Over each 90-min observation period, the following states were recorded every 5 min using a scan sampling proc edure: Sleep, Drowsy, Passive-awake and Active. After more than one qu arter of the animals in the group had fallen asleep, states were recor ded as they occurred. Six weeks prior to the collection of the behavio ral data, a sample of cisternal CSF was obtained to assay for 5-HIAA c oncentrations. With cohort effects statistically controlled, there was a negative correlation between latency to fall asleep and CSF S-HIAA concentrations (i.e., subjects with high CSF 5-HIAA concentrations wer e more likely to fall asleep early). Subjects with low CSF 5-HIAA conc entrations were also more active during the daytime hours. Subjects wh o fell asleep first were, on average, also less active during nighttim e hours. The positive correlation between CSF 5-HIAA and sleep onset w as not a result of social status since there was no correlation betwee n 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 no n-human primates. Published by Elsevier Science Ireland Ltd.