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2-[125I]iodomelatonin was used to study the distribution and properties of the melatonin receptor in the Japanese quail brain. High receptor density was detected in the major targets of direct retinal input (optic tectum, nucleus of the optic basal rout, ventrolateral geniculate nucleus), as well as areas representing terminals in the visual pathways (nucleus rotundus, ectostriatum, thalamo-hyperstriatal pathway). Binding was also found in the piriform cortex, the hypophyseal pars tuberalis, the oculomotorius nucleus and the associated Edinger-Westphal nucleus, and in the nuclei of the third, fourth and sixth cranial nerves. A comparison of the receptor pharmacological profile to that of the mammalian brain demonstrated pharmacological identity of the two binding sites. In the saturation experiments, GPT gamma S decreased the binding affinity, numerical Kd values increasing from approximately 35 pM to approximately 150 pM.

Male Syrian hamsters that were exposed for 8 weeks to short photoperiod (LD 10:14) or treated with melatonin in the late afternoon under long photoperiod conditions (LD 14:10) had a significantly higher content of androgen receptors in the Lipidex-purified soluble fractions isolated from the Harderian glands as compared to the long photoperiod (LD 14:10) exposed controls. Simultaneous computer-assisted analyses of all series of saturation and competition experiments revealed that the numerical value of the apparent Kd, as determined by using the synthetic androgen R-1881 (methyltrienolone), was not different between the experimental groups, and ranged from 0.050 to 0.067 nM. Of the principal natural androgens, testosterone (T) was most potent in inhibiting methyltrienolone binding to the receptor (Ki values from 0.33 to 0.55 nM), and 5 alpha-dihydrotestosterone (DHT) and delta 4-androstenedione (AD) were less effective (Ki values between 1 and 1.9 nM). In the hypothalami and pituitaries of the same animals, used in parallel control assays, DHT was twice as potent as T. Short-term castration (24 hr post-orchidectomy) did not result in significant changes in the receptor binding characteristics. Following 8 weeks exposure to a long photoperiod (LD 14:10) the Bmax values demonstrated a four-fold increase in castrated animals (179 fmoles/mg protein vs. 47 fmoles/mg protein) over intact controls. The relative binding affinity of the major androgens under these conditions remained unchanged, with the exception of AD, where a five-fold increase in the numerical Ki values (decrease in the binding affinity) was recorded (Ki = 9.6 nM).(ABSTRACT TRUNCATED AT 250 WORDS)

The distribution and the properties of the melatonin binding sites were characterized in the brain of the rabbit by combined use of autoradiography and in vitro ligand-receptor binding. Autoradiography revealed widespread specific binding in the brain. The pars tuberalis of the pituitary gland, suprachiasmatic nuclei, ventromedial hypothalamic nuclei, tapetum, hippocampus, indusium griseum, cingulate gyrus, cortex and the choroid plexus were intensely labelled. Diffuse specific binding was recorded in the olfactory bulb and the anterior hypothalamus. Series of in vitro ligand-receptor binding experiments, using the anterior hypothalamus, confirmed that the binding was of high affinity and specificity. Coincubation with a non-hydrolyzable GTP analogue provoked a shift in the binding affinity, the numerical values of the Kd increasing from 20-30 pM to 280-300 pM. Apparently the melatonin receptor in the rabbit brain is linked to its second messenger via a G protein, similarly to what has been described for the brain of other vertebrates.

Melatonin receptors were characterized in the brains of three mammals (rabbit, horse and sheep) by an in vitro binding technique, using 2-[125I]iodomelatonin as labelled ligand. Although binding sites for melatonin have been described recently in several vertebrate species (including the sheep), the rabbit and the horse have not been the subject of investigation so far. Apart from characterization, the present report describes receptor distribution in a number of brain regions, thus allowing for direct interspecies comparison under the same methodological conditions. 2-[125I]iodomelatonin labelled high-affinity binding sites in crude membrane preparations from these species. A series of kinetic and saturation experiments revealed that the binding was rapid, stable, saturable, reversible, of high affinity (Kd in the low picomolar range) and low capacity (Bmax between 1 and 20 fmol/mg protein). The competition studies showed that the relative order of potency of a variety of indoles for inhibition of 2-[125I]iodomelatonin binding was as follows: 2-iodomelatonin greater than 6-chloromelatonin greater than melatonin much much greater than 5-methoxytryptophol greater than 5-methoxytryptamine, and that it was similar in the different brain regions. Prazosin, which has been reported as an extremely potent melatonin analog in the hamster brain, possessed no potency in all preparations from different regions in the three species under investigation. The regional distribution of the receptor showed insignificant species differences. Highest density was always recorded in the median eminence/pars tuberalis (ME/PT) area. Other regions (SCN, POA and certain cortical areas), showed lower, but significant, receptor content. Saturation and competition studies revealed that these binding sites were also of high affinity, low capacity and high specificity.(ABSTRACT TRUNCATED AT 250 WORDS)

The nocturnal peak in pineal and serum melatonin content was reduced following administration of the selective alpha-1 adrenoceptor antagonist prazosin in a dose of 0.25 mg/kg. The effect was pronounced one hour post treatment during the late dark phase of the daily photocycle. These data confirm the reported findings that the hamster pineal sensitivity to adrenergic challenge is confined to the second part of the dark phase and indicate that postsynaptically located alpha-1 adrenoceptors are also involved in the physiological control of melatonin synthesis and/or release in that species.

The gonads of male hamsters exposed to short photoperiod (LD, 10:14) or treated with melatonin in the late afternoon under long photoperiod (LD, 14:10) had undergone complete regression by the end of the treatments (8 weeks). Animals treated for the same period of time with prazosin (a putative melatonin analogue) under the same conditions failed to show a difference in their gonadal status as compared to the long photoperiod controls. Prazosin was unable to prevent melatonin-induced gonadal atrophy when injected either in the morning or 30 min before melatonin. Moreover, prazosin was without any effect on (and unable to prevent the melatonin-stimulated) progesterone production by rat adrenals under in vitro conditions. These data demonstrate that prazosin, which reportedly inhibits 2-[125I]iodomelatonin binding in the hamster brain in an affinity-related manner, does not possess properties of a biological melatonin analogue under conditions of two different model systems in two species.

The metabolism of testosterone to reduced derivatives was studied in the pituitary gland, the hypothalamus, and the hyperstriatum dorsale of thyroidectomized, sham-operated, and thyroxine (T4)-injected immature cockerels. The levels of plasma thyroid hormones were markedly reduced (P less than 0.001) in thyroidectomized cockerels whereas thyroidectomized or sham-operated birds injected daily with 100 micrograms/kg thyroxine had significantly elevated (P less than 0.001) levels in comparison with sham-operated control birds. Each tissue was found to produce significant amounts of 5 beta-androstane-17 beta-ol-3-one (5 beta-DHT), 5 beta-androstane-3 alpha, 17 beta-diol (5 beta-3 alpha-diol), and androstenedione. Irrespective of thyroid state 5 beta-DHT and 5 beta-3 alpha-diol were produced to the greatest extent by the hyperstriatum dorsale whereas androstenedione was maximally produced in the pituitary gland. In comparison with the hyperstriatum dorsale and the hypothalamus only small quantities of 5 beta-DHT were produced in the pituitary gland. In the hyperstriatum dorsale of thyroidectomized birds both 5 beta-DHT (P less than 0.05) and 5 beta-3 alpha-diol (P less than 0.1) were formed to a greater extent than in sham-operated birds. This effect was reversed by administration of T4 to the operated birds which reduced the levels to those measured in the sham-operated controls. Similarly, injection of T4 into sham-operated birds decreased (P less than 0.05) the production of 5 beta-DHT in the hypothalamus while in the pituitary gland injection of T4 into thyroidectomized birds reduced the production of androstenedione (P less than 0.05). It was concluded that in the cockerel thyroid hormone is likely to play a role in the metabolism of testosterone. The physiological significance of 5 beta-reductase activity in the neuroendocrine tissues is discussed.

It is known that the pineal gland has some antitumor activity. Melatonin, its most important hormone, has been shown to inhibit tumor growth in vivo and in vitro. Moreover, some investigations have demonstrated an altered melatonin secretion in cancer patients. Despite these interesting data, clinical trials have never been carried out to evaluate the effects of melatonin on human neoplasms. The aim of this study was to draw some preliminary conclusions on melatonin therapy in advanced human neoplasms. Nineteen patients suffering from advanced solid tumors, which did not respond to standard therapies, entered the study. Performance status (PS) was 20 or less in 9 cases, and more than 20 in the other 10. Melatonin was given intramuscularly at a daily dose of 20 mg at 3.00 p.m., followed by a maintenance period with lower doses in patients who had a remission, a stabilization of disease or an improvement in PS. Among patients with a PS higher than 20, a partial response was achieved in one case with cancer of the pancreas; moreover, 5 of 10 had stable disease, but the other 4 cases had a progression; an evident improvement of PS was obtained in 6 of the 10 cases. In contrast, among patients with a very poor PS, 7 of 9 died within the first 2 months of therapy. This preliminary study would suggest that melatonin may be of some value in treating cancer patients in whom standard antitumor therapies have failed, particularly in improving their PS and quality of life.

Hamsters exposed for eight weeks to short photoperiod (LD 10:14) or treated with melatonin in the late afternoon under long photoperiod (LD 14:10) had significantly higher number of cytosolic androgen receptors in the pituitaries, hypothalami and harderian glands, as compared to the long photoperiod (LD 14:10) exposed controls. The numerical value of the apparent Kd was two to three times lower in the hypothalami and pituitaries, but not in the harderian glands of the animals from these groups. These results indicate that alterations in receptor numbers and affinity constants may be responsible for the dramatic changes in the sensitivity of the hypothalamo-pituitary axis to the negative feedback actions of the gonadal steroids, observed under inhibitory photoperiods and that this effect could be duplicated by late afternoon melatonin treatment.

The reduced metabolites of testosterone produced in the central nervous system of birds are known to be involved in the regulation of male sexual behavior. Since progesterone may compete with testosterone for 5 alpha- and 5 beta-reduction, it may also interfere with the sexual behavior of birds. In order to test this hypothesis, progesterone was administered to male quail either transferred from short days to long days or kept in short days and treated with testosterone. Sexual behavior and crowing were scored at intervals for 21 days and the size of the cloacal gland was measured at the same times. On Day 21, the birds were killed and their testes were weighed. The administration of a large dose (1 mg/day) of progesterone depressed the sexual behavior of the birds stimulated either by long days or by the administration of testosterone. It is suggested that progesterone may compete with testosterone for the active sites of 5 alpha- and 5 beta-reductase; alternatively, its effect may be due to an antiandrogenic activity.