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.