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5-Methoxytryptamine is a potent agonist of presynaptic 5-hydroxytryptamine autoreceptors modulating serotonin release in the central nervous system. This methoxyindole can be synthesized in the pineal gland, but its presence in vivo is still controversial, probably because of rapid catabolism by monoamine oxidase. An improved high-pressure liquid chromatography method, with coulometric detection, has been developed for the simultaneous measurement of melatonin, 5-methoxytryptamine, 5-methoxytryptophol and 5-methoxyindolacetic acid. We have demonstrated a day-night rhythmicity in the amount of 5-methoxytryptamine in the pineal gland of golden hamsters (Mesocricetus auratus) maintained under a long photoperiod (14 h light: 10 h darkness) and pretreated with the monoamine oxidase inhibitor pargyline. Levels of 5-methoxytryptamine were highest at 16.30 h and lowest at 00.30 h. The rhythm for 5-methoxytryptamine appears to be the same as for serotonin (opposite in phase to that of melatonin). The identification of 5-methoxytryptamine has been confirmed by analysis with gas chromatography-mass spectrometry.

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1.Two presynaptic mechanisms appear to be involved in the regulation of noradrenaline release during nerve stimulation. The first one, mediated by 8-adrenoceptors, operates at low frequencies of nerve stimulation, leading to an increase in transmitter release. The second one, mediated through a-adrenoceptors, is triggered when higher concentrations of the transmitter are reached in the synaptic cleft, leading to inhibition of transmitter release, probably through a restriction in the availability of calcium for the secretory process.2. It is postulated that part of the anti-hypertensive effects of drugs like clonidine, a-methyldopa and 8-receptor-blocking agents may be related to their long-term effects on presynaptic adrenoceptors.The release of noradrenaline elicited by nerve stimulation in the peripheral sympathetic system is regulated through a negative feedback mechanism mediated by presynaptic a-adrenoceptors (Langer,

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