Lesions of the melatonin- and androgen-responsive tissue of the dorsomedial nucleus of the hypothalamus block the gonadal response of male Syrian hamsters to programmed infusions of melatonin.
The objective of this study was to characterize a site at which it is likely that melatonin mediates photoperiodic control of reproduction in the male Syrian hamster. The first experiment was a comparison of the distributions of iodomelatonin (IMEL)-binding sites and cells immunoreactive to androgen receptors (AR-ir) in the medio-basal hypothalamus (MBH). AR-ir cells extended throughout the MBH, whereas IMEL binding was restricted to the dorsomedial nucleus (DMN). Comparisons between IMEL binding and AR-ir on adjacent cryostat sections revealed a clear overlap between the IMEL-binding sites and a distinct subpopulation of AR-ir cells within the DMN. The second experiment examined whether lesions of these IMEL- and androgen-responsive cells affected the response of the hamsters to short-day (SD)-like infusions of melatonin. Animals received sham or bilateral electrolytic lesions of the IMEL-binding sites within the DMN of the hypothalamus (MBH-X). Animals were pinealectomized and 4 wk later fitted with an s.c. cannula for the daily infusion of either melatonin (50 ng/h) or saline (500 microliters/10 h). After 6 wk the animals with sham lesions showed gonadal atrophy and lower serum concentrations of LH and prolactin (PRL) after infusions with melatonin. In contrast, MBH-X animals given melatonin had large testes and long-day (LD)-like serum LH concentrations. Infusions of melatonin did, however, cause a significant decline in serum PRL level. This study shows that an intact MBH is essential for the expression of gonadotrophic but not lactotrophic responses to melatonin and/or photoperiod. It also suggests that cells responsive to both gondal steroids and melatonin may be involved in the seasonal variation in GnRH release, and indicates a site at which melatonin might influence sensitivity to steroid feedback, a hypothalamic function known to be regulated by photoperiod.
Maywood
E S
ES
Department of Anatomy, University of Cambridge, United Kingdom. esm1000@hermes.cam.ac.uk
Bittman
E L
EL
Hastings
M H
MH
D000728:Androgens / Q000494:pharmacology*; D000818:Animals; D001665:Binding Sites; D006224:Cricetinae; D007033:Hypothalamus, Middle / Q000187:drug effects / Q000502:physiology*; D007457:Iodine Radioisotopes; D007986:Luteinizing Hormone / Q000097:blood; D008297:Male; D008550:Melatonin / Q000008:administration & dosage / Q000378:metabolism / Q000494:pharmacology*; D008647:Mesocricetus; D017440:Photoperiod; D010870:Pineal Gland / Q000502:physiology / Q000601:surgery; D011388:Prolactin / Q000097:blood; D013737:Testis / Q000187:drug effects* / Q000502:physiology*
D000728:Androgens; D007457:Iodine Radioisotopes; D011388:Prolactin; D007986:Luteinizing Hormone; D008550:Melatonin