Melatonin responses to clonidine and yohimbine challenges

Area of Science:

• Neuroendocrinology
• Pharmacology
• Psychiatry

Background:

• Melatonin (MT) release from the pineal gland is a marker for central noradrenergic function in major depression.
• Norepinephrine influences MT release via alpha and beta adrenergic receptors, but human contributions are unclear.
• The role of alpha-2 adrenergic receptors in human MT release requires further investigation.

Purpose of the Study:

• To explore the effect of alpha-2 adrenergic receptors on melatonin release in humans.
• To assess the impact of yohimbine (alpha-2 antagonist) and clonidine (alpha-2 agonist) on melatonin and its metabolite.
• To evaluate neuroendocrine and physiological responses to these challenges.

Main Methods:

• 10 female subjects received oral yohimbine (10.8 mg) or clonidine (200 or 300 micrograms) in placebo-controlled trials.
• Serum melatonin and urinary 6-sulphatoxy-melatonin (aMT6s) were measured post-challenge.
• Growth hormone (GH), blood pressure, pulse rate, and side effects were also monitored.

Main Results:

• Neither yohimbine nor clonidine significantly altered nocturnal serum melatonin levels compared to placebo.
• Yohimbine significantly increased urinary aMT6s between 18:00-22:00 h.
• Clonidine reduced pulse rate and blood pressure, and increased sedation; only high-dose clonidine elevated GH.

Conclusions:

• Oral alpha-2 adrenergic challenges have limitations in assessing their direct impact on human melatonin secretion.
• Urinary melatonin metabolite changes suggest some alpha-2 receptor influence, but serum levels remain unaffected.
• Further research is needed to clarify the complex interplay of adrenergic receptors in human neuroendocrine regulation.