MICRODIALYSIS OF MELATONIN IN THE RAT PINEAL-GLAND - METHODOLOGY AND PHARMACOLOGICAL APPLICATIONS

The present study describes the development of a new technique to meas ure melatonin contents in the pineal gland of freely moving rats, by m eans of on-line microdialysis. The transcerebral cannula was modified, and a sensitive assay of melatonin, using HPLC with fluorimetric dete ction, was set up. With this system it is possible to monitor the mela tonin levels on-line in the pineal gland during day- and nighttime. Th e nightly increase in melatonin release was recorded. Tetrodotoxin had an inhibitory effect on nighttime levels, whereas even high concentra tions did not alter the daytime level. From this we conclude that neur onal activity is necessary to synthesize melatonin and that during day time no net neuronal activity is present. Melatonin levels could be gr eatly enhanced by systemic administration of the beta-agonist isoprena line (ISO). Also, local infusion of ISO or 8-bromoadenosine 3',5'-cycl ic monophosphate, an analogue of the second messenger cyclic AMP, resu lted in increased melatonin levels, demonstrating the presence of beta -adrenergic receptors, coupled to a cyclic AMP-based second messenger system, on the pineal gland. Injection of phenylephrine had no effect on daytime levels. Only when administered during ISO-induced stimulati on of melatonin release did it enhance this stimulated release. This p roved the regulatory role of alpha1-receptors on pinealocytes. The met hod presented is of special interest for investigating the innervation of the pineal gland and the biochemical processes that regulate the b iosynthesis of melatonin. Also, for studies on the diurnal rhythms of melatonin release and factors that influence these rhythms in freely m oving animals, this model will be of great value.