Neuroscience Vol. 24, No. 3, pp. 987-991, 1988 Printed in Great Britain 03064522/88 $3.00 + 0.00 Pergamon Press plc 0 1988 IBRO INTRA-HYPOTHALAMIC MELATONIN BLOCKS PHOTOPERIODIC RESPONSIVENESS IN THE MALE SYRIAN HAMSTER M. H. HASTINGS, A. P. WALKER, A. C. ROBERTAand J. HERBERT Department of Anatomy, University of Cambridge, Downing St, Cambridge CB2 3DY, U.K. Ahstraet-Exposure of male Syrian hamsters to a short daylength of 8L: 16D leads to gonadal regression. This effect of photoperiod was prevented by pinealectomy or chronic exposure of the brain to exogenous melatonin delivered from in-dwelling cannulae. However, the effect of melatonin was dependent on the neural site of application. Melatonin delivered into the mid-brain, lateral hypothalamus or amygdala was ineffective. In contrast, bilateral administration of melatonin to the medial hypothalamus prevented testicular regression and maintained high circulating levels of luteinixing hormone and prolactin. These findinas sum-rest that the medial hvnothalamus contains target sites for melatonin involved in pineal-mediated phc&eriodic responses. _ _ The seasonal rhythms in reproductive activity dis- played by photoperiodic mammals are controlled by the pineal hormone, melatonin.2~12~1* The synthesis of melatonin is rhythmic and is precisely regulated by the circadian system. i6 Exposure to shorter day- lengths leads to re-entrainment of the circadian sys- tem and a consequent increase in the duration of the nocturnal peak of melatonin synthesis and secre- tion.2*‘2,‘4 In the Syrian hamster this leads to a reduction in the release of gonadotrophins (lu- teinizing hormone and follicle stimulating hormone, LH and FSH) and prolactin by the pituitary gland and, as a consequence, gonadal involution.‘4*2’*22 It is clear that the effects of melatonin are mediated through neuronal pathways. Photoperiodic control of LH secretion is not effected by changes in pituitary responsiveness to luteinizing hormone-releasing hor- mone (LHRH)29 but is a consequence. of changes in the frequency of the hypothalamic circhoral oscillator which drives LHRH pulsatility.‘0J8 Furthermore, although systemic immunization against melatonin has little effect upon seasonality in sheep,’ central immunization against melatonin releases photo- inhibition of gonadotrophin secretion in hamsters (Walker, unpublished observations). However, the central site(s) and mechanism of action of melatonin are completely unknown. Chronic exposure of intact hamsters to high levels of exogenous melatonin, delivered from sub- cutaneous implants, prevents photoinhibition of the gonadal axis,” probably because the implants ob- scure the phasic nature of the endogenous signal. The Abbreviations: AHA, anterior hypothalamus; AMY, amyg- dala; FSH, follicle-stimulating hormone; LH, luteinizing hormone; LHA, lateral hypothalamus; LHRH, luteinixing hormone-releasing hormone; MBH, medio- basal hypothalamus; MID, mid-brain; FDA, pre-optic area; RIA, radioimmunooassay; SNK, Student- Newman-Keuls test. aim of the experiments reported here was to deter- mine whether this blockade of the photoperiodic response could also be demonstrated following chronic intra-cerebral administration of melatonin. The effects of melatonin-filled cannulae would be expected to be greatest when located in, or near, neural sites with the greatest sensitivity to melatonin. EXPERIMENTAL PROCEDURES Cannulae The lower ends’of stainless steel cannulae (15 mm length, 180um o.d.. 25 urn id.: Cooners Needle Works. Aston Lank, Birmihgha’m, U.K.) were dipped into molten mel- atonin (Sigma) at 150°C and left in position for 30 min to allow melatonin to fill the lumen by capillary action. The cannulae were removed and cooled to allow the melatonin to solidify. The outer surface was wiped with ethanol to remove excess melatonin and the upper end was sealed with adhesive. Thin-layer chromatographic analysis revealed that heating to 150°C did not affect melatonin. In a second experiment, cannulae were prepared from glass micro- pipettes with similar tip dimensions to the metal cannulae (length 12 mm, id. 25 bm). The daily release rate of mel- atonin from a sample of cannulae was determined by incubation in saline at 37°C. The saline was changed daily and its melatonin content was determined by radio- immunoassay (RIA). The release was high (> 3 pmole/day) for the first 6 days but then stabilized at 0.81 _+ 0.13 pmole/day. It was assumed that this also represented the release rate in uivo. Animals and surgery Adult male Syrian hamsters (Wrights of Essex, Chelms- ford, U.K.) were housed for 4 week-under a photoschedule of 16h light. 8 h darkness (LD. 16L:8D. liahts off 17.00) in individus cages with food and water avaiable ad libithm. Animals were then anaesthetized with Avertin (tri- bromoethanol: isopropyl alcohol) and implanted with bilat- eral metal cannulae directed either at the pre-optic area (POA); 0.35 cm anterior to bregma, 0.70 cm below dural surface, f 0.05 cm from midline, incisor bar 0.50 cm above ear bar, n = 6), the anterior hypothalamus (+0.24, -0.74, f0.05, n = 1l), the mediobasal hypothalamus (+0.05, -0.80, +0.05, n = 5), the lateral hypothalamus (+0.20, -0.70, f0.25,n =6),theamygdala(+0.15, -0.90, kO.35, 987