Testing the TRH hypothesis of pineal function

In the Syrian hamster gonadal involution may be induced by exposure of the animal to darkness, short photoperiod, or blindness for several weeks. These conditions also influence body metabolism by inducing a substantial decrease in thyroxin concentrations. The role of the pineal gland in these phenomena has been demonstrated by experiments showing that if pinealectomized hamsters are subjected to darkness, short photoperiod, or blindness, their gonads do not involute and plasma thyroxin levels remain normal. Since thyrotrophin releasing hormone (TRH) causes the release of both prolactin and thyrotrophin from the pituitary we can predict on theoretical grounds that TRH mediates both pineal-gonad and pineal-thyroid phenomena. Melatonin may be the pineal hormone regulating hypothalamic TRH levels.