Lesions of the iodomelatonin-binding sites of the mediobasal hypothalamus spare the lactotropic, but block the gonadotropic response of male Syrian hamsters to short photoperiod and to melatonin.

The aim of this study was to determine whether the iodomelatonin-binding sites identified within the preoptic area (POA) or mediobasal hypothalamus (MBH) are essential for the photoperiodic control of seasonal reproduction in male Syrian hamsters. Animals received sham or bilateral electrolytic lesions directed toward either the POA (POA-X; n = 11) or MBH (MBH-X; n = 12) and were then maintained on long days (16 h of light and 8 h of darkness) for at least 4 weeks before transfer to a short photoperiod (SD; 8 h of light and 16 h of darkness). The transscrotal width of the left testis and serum testosterone (Exp 1), PRL, and LH (Exp 2) levels were recorded every 4 weeks in lesioned and intact hamsters to monitor their reproductive state. Lesions of the MBH, but not the POA, abolished the SD-induced gonadal responses (transscrotal width of the left testis after 12 weeks of SD: MBH-X, 10.0 +/- 0.2 mm; sham, 4.6 +/- 0.1 mm; POA-X, 4.0 +/- 0.1 mm; sham, 4.1 +/- 0.1 mm). Similarly, the decrease in serum LH concentrations was prevented by lesions of the MBH (serum LH after 12 weeks SD: MBH-X, 0.74 +/- 0.2 ng/ml; sham, 0.25 +/- 0.1 ng/ml). However, neither lesion prevented the SD-induced decline in serum PRL (serum PRL after 12 weeks SD: MBH-X, 4.7 +/- 1.0 ng/ml; sham, 3.1 +/- 0.1 ng/ml; POA-X, 2.0 +/- 0.1 ng/ml; sham, 2.0 +/- 0.1 ng/ml). To exclude the possibility that the lesion to the MBH prevented gonadal regression through disruption of nocturnal melatonin production, MBH-X animals were switched to a long day photoperiod, pinealectomized, and fitted with a sc cannula for the infusion of either melatonin (500 ng/10 h) or saline (50 microliters/h) once daily for 6 weeks. A group of neurally intact, pinealectomized control animals that received the same infusions showed the expected gonadal regression with melatonin treatment, whereas those receiving saline vehicle had large testes (melatonin, 0.5 +/- 0.1 g; saline, 3.3 +/- 0.3 g. Furthermore, after 6 weeks of infusions, serum LH and PRL concentrations in intact melatonin-infused hamsters were significantly reduced (LH: melatonin, 0.2 +/- 0.04 ng/ml; saline, 1.3 +/- 0.1 ng/ml; PRL: melatonin, 2.2 +/- 0.2 ng/ml; saline, 16.9 +/- 3.1 ng/ml). In contrast to the intact controls, none of the MBH-X animals infused with melatonin exhibited gonadal regression (MBH-X: melatonin, 2.8 +/- 0.5 g; saline, 2.9 +/- 0.5 g).(ABSTRACT TRUNCATED AT 400 WORDS)