Comparison has been made between the activity of the pineal hormone melatonin, and several analogues and metabolites in inhibiting sexual development in a protein-restricted prepubertal rat model. Eleven melatonin analogues or metabolites were tested with the aim of evaluating the model as a test of the hypothesis that melatonin acts as a prohormone and that the ring schism metabolites (kynurenamines) mediate many of the effects attributable to melatonin. Although the hypothesis could not be confirmed, modification of the melatonin structure by lengthening the acylamide side chain or by replacing the 5 methoxy function with fluorine resulted in loss of biological potency. Modification of the melatonin structure to block the two known points of metabolism resulted in no significant alteration in biological activity. Thus 6-chloromelatonin (blocking 6-hydroxylation) and 2,3-dihydromelatonin (blocking oxidative cleavage of the C2-C3 bond) and 6-chloro-2,3-dihydromelatonin remained biologically active. The metabolic products of brain indolearnine-2,3-dioxygenase, N-acetyl-N 2 -formyl-5-methoxy kynurenamine (aFoMK) and N-acetyl-5-methoxy kynurenamine (aMK), paradoxically were also biologically active.

Abstract. Urinary excretion of 6-suphatoxy melatonin, cortisol, potassium and sodium was monitored at four hourly intervals for 24 h in 30 normal subjects at the summer and winter solstices. The 24 h profiles were fitted to sine curves and mean 24-h excretion, time of maximum excretion and amplitude of the curves compared. The excretion of 6-sulphatoxy melatonin was remarkably stable at the two times of the year (24-h excretion 108 ± 6.3 nmol in summer and 105 ± 6.3 nmol in winter, mean ± sem). The time of maximum excretion was significantly delayed in winter by 1 h and 40 min. Urinary cortisol excretion was significantly higher in winter, however, the amplitude was unaltered. The time of maximum excretion of cortisol was significantly delayed by 1 h and 34 min. Potassium and sodium excretion were both unaffected by seasonal influences. These results contrast with results in some animal species in which the duration of the melatonin signal is thought to be the key determinant in subsequent melatonin action. In humans it is likely that the phasing of the melatonin rhythm is of prime importance.

The purpose of this study was to investigate the effect of bright artificial light exposure on the rhythms of 6-sulphatoxy melatonin and cortisol excretion in urine. Six healthy males were exposed to light (greater than 3,000 lux) from 1900 to 0200 h (sunset 1928 h) on one occasion. The artificial light delayed the onset of 6-sulphatoxy melatonin excretion. On the next evening the onset of 6-sulphatoxy melatonin excretion in normal light/darkness was delayed by 1 h. The timing of the peak excretion of cortisol was not affected by the light treatment; however, cortisol excretion rate was maintained at a significantly higher rate in the morning and afternoon after the treatment. These results demonstrate the inhibitory action of high intensity light in humans and suggest that one 6-h period of extra light in the evening can phase delay the melatonin onset.

Prepubertal ewe lambs were treated with empty or filled melatonin implants. The implants were placed s.c. at birth and pituitary responsiveness to various doses of LHRH, LH/FSH pulsatility and prolactin and melatonin secretion were examined at 10, 19, 28, 36 and 45 weeks of age. Control animals (N = 10) showed no consistent alteration in pituitary responsiveness to LHRH during development. Ewes treated with melatonin (N = 10) had puberty onset delayed by 4 weeks (P less than 0.03) but no effect of melatonin on LH or FSH response to LHRH injection was observed at any stage of development. In the control and melatonin-treated ewe lambs the responses to LHRH injection were lower during darkness than during the day at all stages of development. No consistent differences in LH or FSH pulsatility were observed between treatment groups or during development. Prolactin concentrations, however, failed to decrease at the time of puberty (autumn) in the melatonin-treated group. Melatonin-treated ewe lambs maintained normal rhythmic melatonin production which was superimposed on a higher basal concentration and showed the same increase in melatonin output with age as the control ewes. These results indicate that the delayed puberty caused by melatonin implants is not due to decreased pituitary responsiveness to LHRH or to dramatic changes in basal LH or FSH secretion.

The possibility that there are changes in brain benzodiazepine binding sites controlled by photoperiod was investigated in two strains of male rats. The hypothesis was tested by 3H-diazepam binding studies in various brain regions of prepubertal rats maintained in 14 or 10 h of light or treated with late-afternoon injections of melatonin (50 micrograms/day). Protein restriction was applied during the experiment to sensitise the animals to the treatments. Under the conditions employed, rats kept in short daylength throughout or kept on long photoperiod and given late-afternoon melatonin injections showed evidence of delayed puberty (seminal vesicle, ventral prostate, and testis weight decreased by 45%, 55%, and 60% respectively, compared to control rats). Binding measurements were made 1 h before and 2 and 5 h after the onset of darkness in the pubertal (42-day-old) or experimentally prepubertal rats. In the rats of the Porton strain (for which protein restriction was obligatory for the gonadal response) there was no consistent treatment or time effects on specific binding of 3H-diazepam to washed membranes of the hypothalamus, midbrain, or striatum. Similarly, there were no differences in the stimulation of 3H-diazepam binding by 100 microM GABA or the inhibition of binding by 50 microM N-acetyl 5 methoxy kynurenamine. By contrast, in Wistar rats, specific binding to midbrain membranes was reduced 5 h after dark compared to 2 h (37% saline; 20% melatonin) and the extent of stimulation by GABA in the hypothalamus was increased 5 h after darkness (35.6% to 46.7% saline; 37.4% to 50% melatonin). Melatonin treatment resulted in significantly higher specific binding in the hypothalamus 2 h after dark (10%, control fed; 20%, protein restricted) but reduced the GABA induced stimulation of binding in the midbrain (35.5% to 25%, control fed; 33.7% to 23.5%, protein restricted). The Bmax of benzodiazepine binding to unwashed cortical P2 synaptosomal membranes has been reported to increase twofold in adult Wistar rats at mid-dark. By contrast the Bmax of juvenile Wistar rats in this study increased only 17% (116 +/- 2.4 fmol/mg protein to 140 +/- 3 fmol/mg protein) between 2 and 5 h after darkness. In melatonin-treated animals the increase in Bmax of 3H-diazepam binding was blocked (124 +/- 5 fmol/mg protein at 2 h; 127 +/- 3 fmol/mg protein at 5 h) and the Kd reduced (4.5 +/- 0.5 to 4.0 +/- 0.2 nM).(ABSTRACT TRUNCATED AT 400 WORDS)