Pineal Microdialysis of the Melatonin in Conscious Sheep: Methodology, Application to a Diurnal Rhythm and Effect of Isoproterenol

This paper describes the development of a new technique to measure melatonin contents in the pineal gland of moving sheep: the microdialysis. A dialysis probe was used to collect extracellular fluid in the sheep pineal gland, but also to inject directly into it different drugs such as isoproterenol at a very low concentration. The probe was implanted the day before the beginning of the experiment in order to obtain low levels of melatonin. This technique makes it possible to measure melatonin in the dialysate and plasma of rams submitted to 8L:16D. No melatonin either in the dialysate or in the plasma was found during the light phase. Shortly after lighting off, the melatonin concentration increased in the dialysate and plasma and remained stable during the dark phase. Melatonin concentrations began to decrease before lighting on and no detectable levels were found during the following light phase. The secretion of melatonin is, at least, under adrenergic regulation. Local infusion of isoproterenol (90 μl at 10 −6 M), an agonist of β adrenergic receptor, through the probe, increased melatonin levels during 2 h, even when infusions were repeated 3 times. This demonstrates the presence of β adrenergic receptors. The technique presented in this paper could be of considerable interest for studying sheep pineal gland and its main secretion, melatonin, for example during diurnal rhythms or for studying its regulation.