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Summary. Male golden hamsters were transferred from long to short days and castrated on the day of transfer (Group SPO) or 1, 2 or 3 weeks afterwards (Groups SP1, SP2 & SP3). Animals in Group LP were castrated and maintained on long days. After 7 weeks of short days, plasma levels of LH and FSH were low in Groups SP1, SP2, and SP3. In Group SPO LH output was also low and FSH secretion, although greater than in Groups SP1, SP2 and SP3, was lower than in the long-day controls (Group LP). This photoperiodic control of gonadotrophin secretion in castrated hamsters raises doubts about the significance of hypothetical alterations in hypothalamo—pituitary sensitivity to sex steroids.
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Summary. Castrated hamsters which were transferred from long (14L:10D) to short (9L:15D) days and received testosterone-filled capsules for 1 week after transfer failed to show a significant suppression in the plasma levels of FSH and LH after capsule removal. In contrast, gonadotrophin concentrations were suppressed in hamsters in which the long-day castration response had been blocked with exogenous testosterone. After castration on long days and exposure to 10 weeks of short days pituitary gland weight and gonadotrophin content, as well as plasma FSH titres, were higher in control animals than in those that had received testosterone implants for 7 weeks of short days. The results suggest that failure of castrated hamsters to respond to the suppressive effects of short days reflects castration-induced changes in hypothalamo—pituitary physiology rather than a neuroendocrine mechanism by which photoperiod modulates gonadotrophin secretion.
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Although the excitatory amino acid (EAA) receptor agonist N-methyl-d-aspartate (NMDA) can exert profound stimulatory effects on the neuroendocrine reproductive axis of Syrian hamsters, the exact relationship between NMDA receptors and LHRH neurones is unclear. In the present study, in situ hybridization histochemistry was performed on sections of hamster brain using an 35S-labelled riboprobe to the EAA receptor gene, NMDAR1. A high content of NMDA receptor mRNA was detected not only in brain areas classically associated with specific NMDA binding (for example, hippocampus and cerebral cortex) but also in the hypothalamus, in particular the ventromedial–arcuate area; diffuse hybridization of the riboprobe also occurred in the medial–septal area and diagonal band of Broca, regions of the hamster brain in which the LHRH neuronal perikarya are primarily located. In a separate experiment, RNA was extracted from immortalized LHRH neurones (GT1–1 and GT1–7 cells) and used for northern analysis with a 32P-labelled NMDAR1 riboprobe. Clear-cut hybridization occurred with RNA bands of approximately 4.2 and 4.4 kb from the two LHRH neuronal subtypes. These findings suggest that at least some of the stimulatory action of EAAs on LHRH secretion is likely to be exerted directly at the level of the LHRH neurones rather than being mediated through interneurones. Furthermore, the demonstration of abundant NMDA receptor gene expression within hypothalamic areas that lie outside the blood–brain barrier adds plausibility to the concern that EAAs of dietary origin, such as monosodium glutamate, have the capacity to perturb the normal secretory activity of neuroendocrine circuits of the hypothalamus.
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Summary. Adult male Syrian hamsters of the inbred LSH/Ss Lak strain were maintained under a 14L:10D light cycle until 13 weeks of age. At this point, they were implanted s.c. with elastomer capsules that were either empty or packed with 30–40 mg of 6-methoxybenzoxazolinone (6-MBOA), a compound found naturally in some monocotyledonous plants; half of the animals from each treatment group were then kept in long days (14L:10D) or transferred to short days (9L:15D). Testicular size was measured and blood samples collected from each hamster immediately before capsule implantation and again 2, 4, 6 and 8 weeks later. Within just 2 weeks of exposure to short days the mean plasma levels of LH and FSH had significantly declined, in both the control and 6-MBOA-treated animals, and were basal within 4 weeks. Testicular size closely followed these gonadotrophin changes; within 4–6 weeks the testes from all of the short-day hamsters had completely regressed to a prepubertal size. At the end of the experiment, at Week 8, the animals were killed and various components of the hypothalamo–pituitary–testicular axis were compared between the treatment groups. The pituitary content of FSH and LH, testicular weight, mean serum level of testosterone, but not hypothalamic LHRH content or pituitary gland weight, were considerably lower in the short-day than in the long-day hamsters, regardless of whether or not they had been chronically treated with 6-MBOA.
In conclusion, these results demonstrate a short-day response in male LSH/Ss Lak hamsters which is more rapid and uniform than that typically shown by outbred hamster stocks. This inbred strain is, therefore, potentially very valuable for studies of neuroendocrine mechanisms involved in mammalian photoperiodism. Additionally, although 6-MBOA can stimulate testicular growth and reproduction in microtine rodents the present results fail to demonstrate any significant effect on reproductive function in the male Syrian hamster. The study also reports on the characterization of a new conformational antiserum to LHRH which can be used in radioimmunoassays for the measurement of LHRH from three vertebrate classes (mammalian LHRH, chicken I LHRH and salmon LHRH).
Keywords: 6-MBOA; LH; FSH; LHRH; testosterone; LHRH antibody; hamster
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Potential maturational changes in patterns of LH secretion during feed restriction and the LH response to realimentation were examined. Ten sets of four growth-matched littermate gilts were fed ad libitum, to reach either 55, 65, 75 or 85 kg target body weights (one littermate per weight). Gilts were then maintenance fed for 8 days (days 1–8), and to appetite on day 9, and kept in a 9 h light (07:00–16:00 h):15 h dark photoperiod. Blood samples were obtained every 10 min from 07:00 to 16:00 h, and from 16:00 to 24:00 h on day 8 to evaluate any nocturnal rise in LH, and from 07:00 to 16:00 h on day 9 to evaluate the LH response to realimentation. Radioimmunoassays for LH, FSH, melatonin and oestradiol were performed on selected plasma samples. In all three sampling periods, LH and FSH secretion decreased as gilt weight increased, in a quadratic manner. During feed restriction, daytime LH, but not FSH, secretion was lower than at nighttime, irrespective of weight. Daytime LH secretion increased in response to realimentation, irrespective of weight. Plasma oestradiol concentrations were increased by feed restriction and decreased by realimentation but showed little relationship to gilt weight. No consistent relationship was established between plasma melatonin concentrations and ambient illumination. In conclusion, LH secretion exhibits a diurnal rhythm during feed restriction in the prepubertal gilt and, in a similar way to FSH, decreases as puberty is approached. The plane of nutrition influences circulating oestrogen concentrations. The increase of LH secretion following realimentation is not influenced by maturation over the prepubertal period.
