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PubMed1975

Photoperiodic control of LH release in the ram. I. Influence of increasing and decreasing light photoperiods.

The influence of variations in the duration of light photoperiod on LH release has been studied in intact and castrated rams submitted to a 6 months "annual" light rhythm. Blood was collected once a month and the plasma LH levels were determined by radioimmunoassay. In intact rams, the LH level is decreased when the light photoperiod is increased, but as soon as this photoperiod begins to decrease, a sharp increase (about 80%) in the LH level is observed. Thereafter a slow decline in plasma LH occurs until the photoperiod reaches a minimum. In the castrated animals, the plasma LH level is three to four times higher than in the intact animals but the pattern of the variations with regard to the light photoperiod duration, is essentially the same. Thus an increase of 59 and 43% in plasma LH occurs in the two breeds used when the light photoperiod begins to decrease. These results indicate that hypothalamo-hypophyseal activity is modulated by gradual changes in the duration of light photoperiods either in the presence or absence of androgens.

Animals↗
PubMed1975

Photoperiodic control of LH release in the ram. II. Light-androgens interaction.

The intensity of negative feedback of androgens on hypothalamo-hypophyseal activity was compared in two groups of castrated rams under 8.00 or 16.00 h of daily illumination respectively following a conditioning period. In experiment I, two groups of 15 rams under each light photoperiod were slaughtered at different times after a 200 mg testosterone propionate (TP) intramuscular injection. Animals receiving the vehicle only acted as controls. The hypophyseal LH concentration, which increases when there is temporary inhibition of the release into the blood, was taken as a criterion of the inhibition due to TP. In both groups of rams the LH concentration increased in the pituitary following TP treatment. A return to the pre-injection level was observed 48 h later only in animals under a 8.00 h light photoperiod, indicating a shorter negative effect of the injection than in the case of rams under 16.00 h light photoperiod. In experiment II, the inhibitory effect of TP was assessed at two doses, i. e. 300 and 600 mg, by the decrease in the plasma LH level following an intramuscularly injection. In both cases the decrease in LH release was more pronounced in rams under 16.00 h of daily illumination than in those under 8.00 h (mean decrease in LH release: 21.2 and 33.2% respectively in 300 and 600 mg treated rams under 16.00 h daylight as against 9.2 and 14.8% in rams under 8.00 h). Thus it appears that to the intrinsic action of the photoperiod is superposed on a modulatory effect of steroid action on the hypothalamo-hypophyseal activity. The hypothesis is therefore presented that, in rams a decreasing light photoperiod acts in two ways: 1) by stimulating the activity of the hypothalamo-hypophyseal system, 2) by decreasing the intensity of the negative feedback effect of steroids.

Animals↗