Beta-endorphin and dynorphin boosted prolactin release more in the afternoon than morning, and this daily rhythm disappeared when adrenal glands were removed — linking the body's cortisol clock to opioid hormone effects.
Afternoon > morning prolactin responseBeta-endorphin and dynorphin stimulated significantly more prolactin release in the afternoon, a rhythm abolished by adrenalectomy
What the researchers found
Beta-endorphin (0.5 microgram injected into the brain), dynorphin (1 microgram), and U50-488H (a kappa opioid drug, 10 mg/kg) all stimulated prolactin (PRL) release more in the afternoon (4-5 PM) than in the morning (8-9 AM).
Morphine (10 mg/kg subcutaneous), met-enkephalin (200 micrograms), and D-Met-Pro-Enk (0.5 microgram) did not show this daily rhythm. Their prolactin response was the same regardless of time.
For corticosterone (a stress hormone), the pattern was different. Morphine, D-Met-Pro-Enk, met-enkephalin, and dynorphin could only increase corticosterone in the morning, when baseline levels were low. In the afternoon, when baseline corticosterone was already high, there was no room for further increase.
Removing the adrenal glands eliminated the circadian rhythm in prolactin responses, suggesting that natural cortisol cycling drives the time-of-day differences.
Why it matters
This study showed that the time of day you give an opioid peptide affects how the body responds. This has implications for anyone studying opioid effects on hormones. The finding that adrenal hormones control this rhythm connects the stress system to the opioid system in a time-dependent way.
How the study worked
Rats were tested at two time points (8-9 AM and 4-5 PM). Opioid peptides were given by intracerebroventricular injection (into the brain) or subcutaneous injection. Plasma prolactin, ACTH, and corticosterone were measured. Some rats had their adrenal glands removed to test the role of cortisol in the circadian rhythm. Tested in rats, not people.
What this study cannot tell us
Tested in rats, not people. Brain injections bypass normal routes. The study used only two time points; a full 24-hour profile would be more informative. Adrenalectomy is a drastic intervention that changes many systems beyond just removing cortisol cycling.
How to read the evidence
Preliminary animal study using brain injections in rats. Demonstrates a biological principle but results may not directly translate to humans or natural peptide exposure.
When this study was published
Published in 1987. The chronopharmacology principles demonstrated here have been broadly confirmed and expanded in subsequent research.
The bigger picture
This study demonstrated that peptide hormones don't work in isolation — their effects depend on the body's circadian rhythms. This principle is now recognized across pharmacology: the timing of drug and supplement administration can significantly affect outcomes, a concept known as chronopharmacology.
Questions still open
- Does timing of opioid peptide exposure matter for pain management outcomes?
- Do these circadian patterns apply to humans as well as rats?
Common questions
Why did time of day affect opioid peptide responses?
Does this mean peptide supplements should be taken at specific times?
Read the original research
Diurnal variation in prolactin, adrenocorticotropin and corticosterone release induced by opiate agonists in intact and adrenalectomized rats.
Neuroendocrinology, 46(6), 475-80
Citation
Kiem, D T; Kanyicska, B; Stark, E; Fekete, M I. (1987). Diurnal variation in prolactin, adrenocorticotropin and corticosterone release induced by opiate agonists in intact and adrenalectomized rats.. Neuroendocrinology, 46(6), 475-80.