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Summary.

The pattern of plasma LH levels in castrated rams after injection of testosterone propionate (TP) was biphasic with significant decreases at 12 and 72 hr after injection. The total LH response of similarly treated rams injected with LH-RF at critical times in the biphasic LH pattern was of three types regardless of the dose of LH-RF: (i) a similar response to that of control animals at 12, 72 and 120 hr after TP, (ii) a greater response than that of control animals at 36 hr after TP (this is regarded as a rebound effect after a partial inhibition of the release), and (iii) a lower response than that of control animals 6 hr after TP. The third type of response suggests that in the ram testosterone may act directly at the pituitary level to decrease the responsiveness to LH-RF.

Prolactin is known to increase the effect of testosterone on male accessory sex glands (Segaloff, Steelman & Flores, 1956), and to have a synergistic action with LH on spermatogenesis (Bartke, 1971) and on testicular androgen secretion (Hafiez, Bartke & Lloyd, 1972). It has been shown also that the male pituitary releases prolactin into the blood in the rat (Amenomori, Chen & Meites, 1970) and the human (Jacobs, Mariz & Daughaday, 1972). Little is known, however, about factors which regulate prolactin secretion in this sex.

In the female rat, antagonism between prolactin and gonadotrophin secretion has been shown following drug treatment (Ben-David, Danon & Sulman, 1971). It is also suggested by the fact that ovariectomy produces a decrease in serum prolactin (Amenomori et al., 1970), an effect which is the reverse of the classical increase of FSH and LH

Summary. Sexually mature Ile-de-France rams were exposed to an 8-month light regimen in which the daily light increment and decrement were constant and equal to 7 min/day. Daylength therefore varied from 6 to 20 h. The animals were allotted to two groups of 12 rams each and submitted to the same light regimen but 4 months out of phase. Blood was collected every 40 min for 10 h, on 15 occasions at various intervals during the second light cycle. Plasma LH and on some occasions plasma testosterone concentrations were measured by radioimmunoassay. In both groups the number of LH pulses increased significantly as daylength increased from 11:40 to 20:00 h (P < 0·01) but because their amplitude was low the mean plasma LH increase, although significant, was moderate. As daylength started to decrease (from 20:00 to 18:30 h), the frequency of LH pulses further increased (P < 0·05) and the pulse amplitude doubled so that mean plasma LH values increased abruptly and remained high until the decreasing light photoperiod reached 11:40 h. Thereafter, the frequency and amplitude of LH pulses decreased and the mean plasma LH dropped to one-fourth of its maximal value. Mean plasma testosterone concentrations were low during most of the 8-month cycle but increased steeply when daylength decreased from 11:40 to 6:00 h. These results indicate that increasing daylength provides a slow stimulation of LH release by an increase in the number of LH pulses, while a long decreasing photoperiod (20:00 to 11:40 h) provides a further and strong stimulation of both LH pulse frequency and amplitude in rams submitted to this regimen. The abrupt increase of LH release before and after the shift from increasing to decreasing daylength indicates that this increase is largely dependent on the decreasing daylength per se and not just the result of a photostimulation initiated in increasing daylength. The opposite patterns of plasma LH and testosterone secretions when daylength decreased from 11:40 to 6:00 h also suggest a possible cause-to-effect relationship between the high testosterone and the low plasma LH values recorded at that time.

During seasonal anoestrus in Île-de-France sheep, progestagen treatment with vaginal sponges associated with an intramuscular injection of PMSG induces oestrus and ovulation in dry and lactating females (42 days post partum).

The fertility after artificial insemination, expressed as the percentage of treated ewes which subsequently lamb, is lower in lactating ewes at the induced oestrus than in the non-lactating ewes (35% versus 70%, Colas, Brice, Courot & Cottier, 1971).

The present experiment was designed to investigate the effect of lactation on the induced preovulatory LH discharge since it may have an effect on fertility.

In April, during the period of seasonal anoestrus, twelve non-lactating ewes (Groups 1 and 2) and twelve lactating ewes (Groups 3 and 4) were submitted to the various treatments shown in Table 1. The conditions of treatment for animals in Groups 1

Summary. Two groups of 12 adult Ile-de-France rams were exposed to artificial 6-month light cycles in which daily illumination was provided in one or two photofractions. In Group I, daylength increased linearly from 8 to 16 h in 3 months and decreased similarly from 16 to 8 h. The daily increment or decrement (5.33 min/day) was constant. In Group II, 8 h of light were given in two parts: the main one, 7 h, began at the time of dawn in Group I and an additional 1 h light pulse was coincident with the last hour of the former group. The onset of the pulse changed therefore each day and the interval between dawn of the first block and dusk of the second block of light increased from 8 to 16 h in 3 months and declined from 16 to 8 h the next 3 months. Testicular weight was estimated by an orchidometer every 2 weeks for 2 (N = 12/group) or 3 consecutive light cycles (N = 6/group). The testicular weight variations were identical in both groups. In the 6 rams of each group studied during 3 light cycles, variations of testicular weight were submitted to an harmonic regression analysis according to time and the computed values for the mean, amplitude, period and phase were, respectively, 260 g, 66 g, 185 days and 120 days in Group I and 262 g, 65 g, 181 days and 111 days in Group II. Analysis performed for each ram gave very similar values in all individuals. It is concluded (1) that daylength is not measured by the total duration of light exposure but between two limits represented here by dawn and the pulse of light and (2) that the measurement is not limited to a particular photosensitive phase but is effective throughout the entire light cycle.

Summary. LH and testosterone levels in bull calves were studied in the plasma samples collected sequentially at 15-min intervals every month during the first year of life. An episodic pattern of LH release occurred after birth and the frequency and magnitude of the LH peaks increased up to 4 months of age and decreased thereafter. A testicular response was not observed before this age. It is suggested that this episodic LH activity is responsible for the testicular development which then initiates puberty.

The melatonin1a (Mel1a) receptor gene was cloned in two breeds of goat, one with marked seasonal ovarian activity (Alpine breed) and the other with low seasonal variations in ovulatory activity (Creole breed), to determine whether reproductive seasonality is related to the structure of the Mel1a gene. The main part of exon II was amplified by PCR using sheep sense and antisense primers in 17 Alpine and 13 Creole goats, and cDNAs were subcloned and sequenced in both directions. The results indicate the presence of an identical sequence in 12 of the 30 animals, that is, six Alpine and six Creole goats. The greatest similarity in the 784 nucleotides of exon II (primers excluded) that was obtained was found with ovine Mel1a receptor sequence (98.4%) and the differences consisted of 12 nucleotide and four amino acid changes. The presence of seven mutations compared with the previous reference sequence was observed and their combinations indicated the presence of at least five other alleles; one mutation resulted in a change in one amino acid in three Alpine goats. No difference in allelic distribution was observed between the two breeds. The results indicate that no relationship could be established between the Mel1a receptor gene structure and the expression of seasonality of reproduction in goats.

I.N.R.A.-Laboratoire de Physiologie de la Reproduction, Nouzilly, 37380 Monnaie, France

(Received 29th May 1974)

The concentrations of a number of reproductive hormones found in the peripheral plasma of the cow around the time of oestrus have been measured by numerous authors: plasma concentrations of progesterone throughout the oestrous cycle (Stabenfeldt, Ewing & McDonald, 1969), LH (Schams & Karg, 1969), oestrogen and progesterone (Henricks, Dickey & Hill, 1971) and LH and progesterone (Henricks, Dickey & Niswender, 1970). These and other reports have paid little attention to the precise onset and duration of oestrus, behaviour being checked at various intervals (usually two or three times a day) before the expected onset.

To obtain a more precise pattern of hormone concentrations around oestrus, we have measured simultaneously LH, oestradiol-17β and progesterone in samples of plasma taken at 2-hr intervals from cows over a period of 5 days. The cows were maintained with a

Summary. The effects of testosterone on cytosol and nuclear androgen receptors of ram pituitary were examined in two experiments. In Exp. I, 500 μg testosterone were injected intravenously and groups of 4 rams were slaughtered at 0, 15, 30, 45, 90 and 360 min after injection. Cytosolic receptor concentration decreased from 21 ± 0·9 to 6 ± 0·9 fmol/mg protein 30 min after the testosterone injection (P < 0·001), and then returned towards the preinjection level after 90 min. The pattern of nuclear receptor concentration was the opposite; a maximal increase (12 ± 3·5 to 32 ± 5·7 fmol/mg protein) was observed 30 min after injection (P < 0·001), followed by a progressive but incomplete decrease by 360 min. In Exp. II, blood was collected every 20 min for 17 h in three successive series, each of 12 rams, which were then slaughtered. Plasma LH and testosterone concentrations were measured by radioimmunoassay. No changes were observed in cytosol receptor concentration, but nuclear receptor concentration was negatively correlated with the interval elapsed since the beginning of the last testosterone pulse (r = −0·62; P < 0·001). The highest values for nuclear receptor concentrations were observed at an interval equal to or less than 120 min. These results indicate that natural pulses are associated with androgen binding particularly in the pituitary nuclei.

Summary. After injection of castrated rams with 600 mg testosterone or testosterone propionate, plasma LH levels were biphasic, with significant decreases at 6–10 and 96 h after treatment. These results support the suggestion that testosterone acts at the hypothalamo-pituitary level by two different mechanisms.