Nutritional status, time of day and pinealectomy: factors influencing the sensitivity of the neuroendocrine-reproductive axis of the rat to melatonin

Correlation between Changes in Prolactin and Melatonin Serum Levels after Radical Mastectomy

Both prolactin (PRL) and melatonin (MLT) (the most important pineal hormone) have been shown to play a role in regulating breast cancer growth. The present study was carried out to investigate the relationship between PRL and MLT secretions in human breast cancer. Twenty-four women with breast cancer, at clinical stage T1-2 N0-2 M0, were evaluated before and after radical mastectomy. As controls, 14 women who underwent surgery for reasons other than neoplastic disease were included in the study. PRL and MLT serum levels were measured by RIA before and 15 days after surgery. There were no significant differences in mean PRL serum levels between patients and controls; mean MLT serum values were significantly higher in patients than in controls. In no control subject was PRL affected by surgery. In contrast, 13/24 breast cancer women showed high PRL levels after mastectomy; the PRL rise induced by surgery was significantly higher in patients without axillary node involvement. MLT was not affected by mastectomy in 13 patients, whereas it was enhanced in 5 women and decreased in the last 6 cases. No significant correlation was seen between PRL and MLT changes induced by mastectomy. The present study shows that radical mastectomy influences PRL and MLT secretions, however, its clinical significance remains to be established.

Regression of NMU-induced mammary tumors with the combination of melatonin and 9-cis-retinoic acid

A significant increase in tumor regression was induced in N-nitroso-N-methylurea-induced mammary tumors in rats treated with the combination of melatonin and 9-cis-retinoic acid (9cRA). Treatment groups included: control (ethanolic saline), 9cRA (30 mg/kg chow/day), melatonin 500 microg/day, melatonin 1000 microg/day, melatonin 500 microg/day+9cRA and melatonin 1000 microg/day+9cRA. Rats treated with the lower dose of melatonin 500 microg+9cRA show the greatest degree of tumor regression (78%), with 54% undergoing complete regression and a significant increase in apoptotic cells observed by TUNEL Assay. Furthermore, tumor multiplicity and burden were significantly decreased by the combination of melatonin and 9cRA.

Chemoprevention of NMU-induced rat mammary carcinoma with the combination of melatonin and 9-cis-retinoic acid

In experimental trials using the N-nitroso-N-methylurea (NMU)-induced rat mammary tumor model, a significant decrease in tumor incidence (to 5%) was observed in rats treated with melatonin and 9-cis-retinoic acid (9 cRA) compared to controls (55%). Although 9cRA alone decreased tumor incidence to 26%, this response did not reach statistical significance. Tumor incidence was significantly inhibited to 20% in the animals that received melatonin and 9cRA on alternating days. Latency to tumor onset was prolonged in animals receiving either of the combination treatments compared with controls, and tumor multiplicity was also significantly decreased.

Pinealectomy but not melatonin supplementation affects the diurnal variations in 125I-melatonin binding sites in the rat brain

The distribution of melatonin binding sites in synaptosomal preparations from five brain areas of sham-operated and pinealectomized young male rats (maintained in a 14 h light: 10 h darkness cycle; lights on at 5.00 h) was recorded at 10.00, 18.00 and 24.00 h, 18 days after surgery, using 125I-melatonin as a probe. The densities of 125I-melatonin binding sites in the medulla-pons, hippocampus and hypothalamus of the pinealectomized rats, exhibited clear diurnal variations. However, the densities of binding sites in these brain areas at 18.00 h were lower in the pinealectomized animals than at the other times of day tested, whereas in the sham-operated controls, the binding at 18.00 h was higher than at the other times of day. No diurnal variations were evident in the midbrain and cerebellum of the pinealectomized animals. The apparent affinities of the binding sites toward the ligand in the various brain areas were similar in the pinealectomized and sham-operated animals and did not significantly vary at any of the times recorded. Oral supplementation of melatonin to the rats via drinking water had no effect on the diurnal variations in 125I-melatonin binding in the pinealectomized rat brain. The results indicate that the diurnal variations in 125I-melatonin binding sites in the rat brain are not generated by the pineal but are affected by removal of the gland.

Undernutrition potentiates melatonin effects in maturing female rats

Prepubertal (21-22 days) female Sprague-Dawley rats were caged singly in either long (LP; 14:10 LD) or short (SP; 8:16 LD) photoperiod and fed ad libitum or underfed (1/2 the food intake of controls). Additionally, a fed and underfed group in LP received a daily sc injection of saline or 100 micrograms melatonin at 1700 h. Food restriction delayed vaginal opening and resulted in a reduction in body weight and in the weights of the pituitary, ovary and uterus in all underfed groups. Melatonin treatment (but not SP exposure) significantly enhanced the reduction in pituitary, ovarian and uterine weight compared to the underfed saline-treated controls. Thyroid weights were significantly increased in underfed LP and SP groups compared to their respective controls where melatonin treatment in either fed or underfed animals was ineffective. Underfeeding caused a significant rise in pituitary LH (except for SP-underfed group) and FSH concentrations and a fall in pituitary prolactin concentrations and plasma T3 levels. Melatonin injections in underfed rats significantly increased pituitary LH and FSH and decreased prolactin concentrations compared to underfed saline-treated animals. Plasma prolactin levels increased after melatonin administration in both fed and underfed rats. These observations emphasize that environmental influences such as undernutrition can alter the physiological status of immature animals and enhance the sensitivity of the neuroendocrine axis to the pineal and one of its hormones, melatonin.

Effect of a light pulse during the dark on photoperiodic regulation of the rate of thyroxine-induced, spontaneous, and prolactin-inhibited metamorphosis in Rana pipiens tadpoles

Since Rana pipiens tadpoles injected with thyroxine (T4) early in the dark develop more slowly than those injected in the light, we studied the effect of giving a light pulse of 1 hr early in the dark. Tadpoles injected under a 7.5-W red light bulb in a darkened room with 0.2 microgram T4 daily at 2200 hr went through metamorphosis faster on a 12L:3D:1L:8D cycle with a light pulse after injection than on a 12L:12D cycle without a light pulse, and even faster on a 12L:1.5D:1L:9.5D cycle with a light pulse before the injection. Thus a 1-hr light pulse counteracted the metamorphic delay resulting from administration of T4 in the dark, and set in motion the conditions that resulted in a more rapid response to an injection of T4. However, a 1-hr light pulse in the early dark had no effect on growth and development of older or younger untreated tadpoles or those constantly immersed in 30 micrograms/liter T4. Larvae on 21L:3D with T4 injection in the dark and on 12L:3D:1L:8D with T4 injection at 0700 hr just before the start of the main light phase progressed faster than 12L:3D:1L:8D with injection at 2200 hr in the dark before only a 1-hr light pulse. Thus the length of the light phase immediately after T4 injection was significant. There was no difference on 12L:12D and 12L:3D:1L:8D cycles in the effectiveness of daily injections of 10 micrograms prolactin (PRL) in the early dark at 2200 hr in promoting tail growth or antagonizing tail resorption induced by T4 immersion. Under these conditions, PRL utilization did not appear to be inhibited by the light pulse.

Inhibitory effects of the pineal hormone melatonin and underfeeding during the promotional phase of 7,12-dimethylbenzanthracene-(DMBA)-induced mammary tumorigenesis

The effects of melatonin (Mel) and/or underfeeding (30% food restriction) on 7,12-dimethylbenzanthracene (DMBA)-induced mammary tumorigenesis were examined in female Sprague-Dawley rats fed a semipurified diet. During the promotional phase of tumorigenesis, the animals began receiving either daily afternoon Mel (250 micrograms) or saline vehicle injections s.c. for 15 weeks. As compared with fed animals, underfed rats had a lower tumor incidence, tumor number and size while the latency to onset and regression of tumors was increased. Melatonin in fed rats moderately suppressed tumor incidence and number. However, the combination of Mel treatment and underfeeding caused the most marked inhibition of tumorigenesis as compared with either treatment alone. These results indicate that Mel administration and/or underfeeding during the promotional phase inhibit DMBA-induced mammary tumorigenesis perhaps via neuroendocrine and/or peripheral endocrine mechanisms.

Pineal gland morphology in rats with experimentally induced protein-calorie malnutrition

The morphology of the pineal gland was studied in protein-calorie-malnourished (PCM) rats. Twenty-day-old male Sprague-Dawley rats were placed in a 14:10 photoperiod and fed either an 8% low protein diet (LPD) or a standard laboratory diet (SLD) containing 27% protein for 30 d. At 50 d of age, rats from both animal groups were sacrificed at 0900 h and at 2400 h, and the pineal glands were immersion-fixed for either light or electron microscopic analysis. The cytoplasm and nuclei of the pinealocytes from the SLD-fed rats were consistently larger than those of the animals maintained on the LPD. Additionally, the lipid droplets were larger and more prominent in the controls at both 0900 h and 2400 h. Even though the size of these inclusions did not vary among animals given the same diet as a function of the time of sacrifice, they were more numerous in both the well-fed and malnourished rats during the dark phase of the photoperiod. In contrast neither diet nor sampling time affected the size or number of pinealocyte mitochondria. These morphological observations lend further support to the premise than PCM impairs the cellular activity of the pinealocytes.

Effects of daily melatonin injections on the photoperiodic gonadal response of the female frog Rana ridibunda

The effects of daily melatonin injections on female gonads in Rana ridibunda were examined. Melatonin-treated R. ridibunda maintained on a long photoperiod (18L:6D) had significantly smaller gonads than control animals. Thus, frogs injected daily with melatonin exhibited a substantial decrease in ovary weight when compared to saline-injected animals. Our findings suggest that melatonin exerts antigonadal effects in female Rana ridibunda.