Temporal changes in growth hormone, cortisol, and glucose: relation to light onset and behavior

Area of Science:

• Neuroendocrinology
• Primate Behavior

Background:

• Cortisol is traditionally considered a sensitive marker of behavioral arousal.
• The relationship between light cycles, hormones, and primate behavior requires further investigation.

Purpose of the Study:

• To investigate the effects of light cycle onset on plasma growth hormone (GH), cortisol, and glucose levels in rhesus monkeys.
• To assess the correlation between these hormonal fluctuations and behavioral arousal.

Main Methods:

• Eight rhesus monkeys were housed in controlled light/dark cycles.
• Plasma GH, cortisol, and glucose were measured every 20 minutes.
• Behavioral arousal was scored concurrently with hormonal measurements.

Main Results:

• Light onset significantly increased GH secretory bursts and plasma glucose levels.
• GH and glucose levels positively correlated with increased behavioral alertness.
• Cortisol levels exhibited minor fluctuations unrelated to light or arousal.

Conclusions:

• Light exposure is a potent stimulus for GH secretion and associated with increased alertness in rhesus monkeys.
• Cortisol is not a reliable indicator of behavioral arousal in this context.
• Findings challenge the established role of cortisol in reflecting arousal states.