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 acrylamide 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 indoleamine-2,3-dioxygenase, N-acetyl-N2-formyl-5-methoxy kynurenamine (aFoMK) and N-acetyl-5-methoxy kynurenamine (aMK), paradoxically were also biologically active.

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 3 H‐diazepam binding studies in various brain regions of prepubertal rats maintained in 14 or 10h of light or treated with late‐afternoon injections of melatonin (50 μg/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 3 H‐diazepam to washed membranes of the hypothalamus, midbrain, or striatum. Similarly, there were no differences in the stimulation of 3 H‐diazepam binding by 100 μM GABA or the inhibition of binding by 50 μM 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%, controd 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 P 2 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 3 H‐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). When the cortical membranes from these animals were extensively washed and analysed for binding using a single concentration of ligand (3 nM) the rise in binding during the night was still observed (430 ± 27 fmol/mg protein to 505 ± 60 fmol/mg protein), but there was no blocking effect of melatonin treatment. These results indicate that some aspects of benzodiazepine binding may be influenced by melatonin treatment during the prepubertal period in the rat.

The purpose of this study was to investigate the effect of bright artificial light exposure on the rhythms of 6‐sulphatoxy melatonin cortisol excretion in urine. Six healthy males were exposed to light (> 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 signficantly higher rate in the morning afternoon after the treatment. These results demonstrate the inhibitory action of high intensity light in humans suggest that one 6‐h period of extra light in the evening can phase delay the melatonin onset.