Exam 2 (Lecture 15) - Melatonin, EPO

Question 1

Which gland secretes melatonin, where is the gland located, what is the chemical nature of this hormone, and what is its main physiological function in seasonal breeders?

Answer 1

1) Pineal gland (epiphysis)

2) caudo-dorsal to the thalamus

3) Peptide (synthesized from tryptophan)

4) Controls reproduction in seasonal breeders
- antigonadal in most species and influences reproductive cycle of seasonal breeders, most likely via regulation of GnRH
- light inhibits melatonin secretion and this activates gonads in long-day breeders

Question 2

Which stimulus initiates secretion of melatonin and how is it conveyed to the epiphysis?

Answer 2

1) Darkness is seen in the eye and transmitted to the suprachiasmatic nucleus to the brain stem to the cranial cervical ganglion which upregulates beta-adrenergic receptors and alpha-adrenergic receptors to stimulate melatonin production
- the postganglionic parasympathetic neuron stimulates these receptors in the epiphysis

Question 3

What is the effect of melatonin on gonads in most seasonal breeders? What is its effect on gonads in goats and sheep?

Answer 3

1) It usually suppresses reproduction

2) It has the opposite effect in sheep and goats

Question 4

Race horse breeders like to prepare mares for breeding by putting them under artificial light in winter. What is the physiological reason for this practice?

Answer 4

The artificial light inhibits the melatonin production activating gonads in long-day breeders

Question 5

Name several other functions of melatonin.

Answer 5

1) Antagonizes alpha-MSH (white winter coat)

2) Reduces CnRH secretion

3) Enhances immune function

4) Reduces free radical formation

5) Inhibits hair growth in mice

6) Inhibits cortisol release, may stimulate aldosterone release

7) Reduces thyroid function

Question 6

What is erythropoietin, which cells secrete it, what are the main triggers for its secretion and what is its main physiological effect?

Answer 6

1) Glycoprotein produced by fetal liver and adult kidney (peritubular capillary endothelial cells)

2) Stimulates erythropoiesis and iron uptake in small intestine

3) Controlled by PO2 in blood perfusing the kidney

4) Triggers:
- renal hypoxemia
- decreased renal blood flow
- cardiopulmonary disease
- decreased hemoglobin concentration
- hemorrhage
- high altitude
- hypotension