Introduction
This topic covers three groups of signalling: the pineal gland (epiphysis), the thymus, and the organs that are not classical endocrine glands but secrete hormones. It also covers tissue hormones, which act locally. These systems differ from the classical axes because they usually lack feedback from a higher centre and are controlled by local and neural stimuli.
The pineal gland (epiphysis)
- The pineal gland lies in the roof of the third ventricle. Its pinealocytes make melatonin from tryptophan, through serotonin and N-acetylserotonin.
- The rate-limiting step is arylalkylamine N-acetyltransferase (AANAT). The final step is catalysed by hydroxyindole-O-methyltransferase.
- Light reaching the retina is relayed through the retinohypothalamic tract to the suprachiasmatic nucleus. From there sympathetic fibres from the superior cervical ganglion release noradrenaline onto beta-1 receptors. This raises cyclic AMP and AANAT activity, so melatonin is made in darkness and suppressed by light.
- Melatonin entrains the circadian rhythm, promotes sleep onset, signals day length in seasonal mammals, and has antioxidant effects.
- Clinically, melatonin is used for jet lag and some sleep disorders. Tumours of the pineal region can compress the tectum and cause Parinaud syndrome, with impaired upward gaze.