Presence of a Distinct 24-Hour Melatonin Rhythm in the Ventricular Cerebrospinal Fluid of the Goat

Melatonin profiles in the cerebrospinal fluid (CSF) of conscious goats were examined under long‐day (16L:8D) and short‐day (8L:16D) environments. CSF melatonin, collected from the lateral ventricle, showed distinct 24 h rhythms with high concentrations being restricted to the dark phase, which averaged 1,320.6 pg/ml under 16L:8D and 660.6 pg/ml under 8L:16D. On the contrary, the nocturnal rise in CSF melatonin was totally absent in the pineal sympathetically denervated animals. Night interruption by 1 h lighting (about 400 lux at the height of goat's head) resulted in an abrupt decline of CSF melatonin to the basal level within 30 min and immediate recovery to the previous high level after reestablishment of the dark phase. The CSF/plasma ratio was 10.8–18.4 during the dark phase and 1.7–1.8 during the light phase. CSF and plasma melatonin levels were also examined after exogenous melatonin given either peripherally or intraventricularly. Continuous subcutaneous infusion of melatonin (5 μg/h) maintained melatonin levels in the plasma about 3 times higher than that in the CSF during its application. On the other hand, an intraventricular injection of 2 μg melatonin elevated plasma melatonin by 100 pg/ml within 1 min. These results indicate that turnover of CSF melatonin is fairly rapid and favor a hypothesis for direct access of pineal melatonin to the brain ventricular system in the goat.