Effects of melatonin on salsolinol-induced prolactin secretion in goats.

The aim of the present study was to clarify the effect of melatonin (MEL) on the salsolinol (SAL)-induced release of prolactin (PRL) in goats. Female goats were kept at 20°C with 16 h of light, 8 h of darkness, and orally administered saline or MEL for 5 weeks. A single intravenous (i.v.) injection of saline (controls), SAL, thyrotropin-releasing hormone (TRH) or a dopamine receptor antagonist, sulpiride, was given to the goats 3 weeks after the first oral administrations of saline or MEL, and the responses were compared. The mean basal plasma PRL concentrations in the control group were higher for the saline treatments than MEL treatments (P < 0.05). SAL as well as TRH and sulpiride stimulated the release of PRL promptly after each injection in both the saline- and MEL-treated groups (P < 0.05). The area under the response curve of PRL for the 60-min period after the i.v. injection of SAL, TRH and sulpiride in the saline-treated group was greater than each corresponding value in the MEL-treated group (P < 0.05). These results show that daily exposure to MEL under a long day length reduces the PRL-releasing response to SAL as well as TRH and sulpiride in goats.

Effects of photoperiod on the secretion of growth hormone and prolactin during nighttime in female goats.

The aim of the present study was to clarify the effect of photoperiod on nighttime secretion of growth hormone (GH) in goats. Adult female goats were kept at 20°C with an 8 h or 16 h dark photoperiod, and secretory patterns of GH for 8 h in the dark period were examined with the profile of prolactin (PRL) secretion. GH was secreted in a pulsatile manner in the dark period. There were no significant differences in pulse frequency between the 8- and 16-h dark photoperiods; however, pulse amplitude tended to be greater in the group with the 16-h dark photoperiod (P = 0.1), and mean GH concentrations were significantly greater in the same photoperiod (P < 0.05). PRL secretion increased quickly after lights off under both photoperiods. The PRL-releasing responses were weaker in the 8-h than 16-h dark photoperiod. The secretory response to photoperiod was more obvious for PRL than GH. The present results show that a long dark photoperiod enhances the nighttime secretion of GH in female goats, although the response is not as obvious as that for PRL.

Effects of photoperiod on the secretion of growth hormone in female goats.

The aim of the present study was to clarify the effect of photoperiod on the secretion of growth hormone (GH) in goats. Adult female goats were kept at 20°C with an 8-h or 16-h photoperiod, and secretory patterns of GH for 4 h (12.00 to 16.00 hours) were compared. In addition, the goats were kept under a 16-h photoperiod and orally administered saline (controls) or melatonin, and the effects of melatonin on the secretion of GH were examined. GH was secreted in a pulsatile manner. There were no significant differences in pulse frequency between the 8- and 16-h photoperiods; however, GH pulse amplitude tended to be greater in the group with the 16-h photoperiod (P = 0.1), and mean GH concentrations were significantly greater in the 16-h photoperiod (P < 0.05). The GH-releasing response to GH-releasing hormone (GHRH) was also significantly greater for the 16-h photoperiod (P < 0.05). There were no significant differences in GH pulse frequency between the saline- and melatonin-treated groups. However, GH pulse amplitude and mean GH concentrations were significantly greater in the saline-treated group (P < 0.05). The present results show that a long photoperiod enhances the secretion of GH, and melatonin modifies GH secretion in female goats.

Kisspeptin-10 stimulates the release of luteinizing hormone and testosterone in pre- and post-pubertal male goats.

The aims of the present study were to clarify the effect of kisspeptin-10 (Kp10) on the secretion of luteinizing hormone (LH) and testosterone (T) in pre-pubertal and post-pubertal male ruminants. Four male goats (Shiba goats) were given an intravenous (i.v.) injection of Kp10 (5 µg/kg body weight (b.w.)), gonadotoropin-releasing hormone (GnRH, 1 µg/kg b.w.), or 2 mL of saline as a control at the ages of 3 (pre-pubertal) and 6 (post-pubertal) months. A single i.v. injection of Kp10 significantly stimulated the release of LH and T in both groups. The area under the response curve (AUC) of LH for a 60-min period after the i.v. injection of Kp10 was significantly greater in the pre-pubertal goats (P < 0.05). The AUC of T for a 120 min period post-injection did not differ between the two age groups. A single i.v. injection of GnRH also significantly stimulated the release of LH and T in both groups (P < 0.05). The secretory pattern of LH and T in response to GnRH resembled that in response to Kp10. These results show that the LH-releasing response to Kp10 is greater in pre-pubertal than post-pubertal male goats. They also show that Kp10, as well as GnRH, is able to stimulate the release of T in male goats.

Effects of photoperiod on salsolinol-induced prolactin secretion in goats.

The aim of the present study was to clarify the relation between salsolinol (SAL)-induced prolactin (PRL) release and photoperiod in goats. A single intravenous (i.v.) injection of SAL was given to adult female goats under short (8 h light, 16 h dark) or long (16 h light, 8 h dark) photoperiod conditions at two different ambient temperatures (20°C or 5°C), and the PRL-releasing response to SAL was compared to that of thyrotropin-releasing hormone (TRH) or a dopamine (DA) receptor antagonist, sulpiride. SAL, as well as TRH or sulpiride, stimulated the release of PRL promptly after each injection in both 8- and 16-h daily photoperiods at 20°C (P<0.05). The area under the response curve (AUC) of PRL for the 60-min period after injections of saline (controls), SAL, TRH and sulpiride in the 16-h daily photoperiod group was greater than each corresponding value in the 8-h daily photoperiod group (P<0.05). There were no significant differences in the AUC of PRL among the values produced after the injection of SAL, TRH and sulpiride in 16-h daily photoperiod group; however, the values produced after the injection of TRH were smallest among the three in the 8-h daily photoperiod group (P<0.05). The PRL-releasing responses to SAL, TRH and sulpiride under a short and long photoperiod condition at 5°C resembled those at 20°C. These results show that a long photoperiod highly enhances the PRL-releasing response to SAL as well as TRH or sulpiride in either medium or low ambient temperature in goats.

Bromocriptine inhibits salsolinol-induced prolactin release in male goats.

The secretion of prolactin (PRL) is under the dominant and tonic inhibitory control of dopamine (DA); however, we have recently found that salsolinol (SAL), an endogenous DA-derived compound, strongly stimulated the release of PRL in ruminants. The aim of the present study was to clarify the inhibitory effect of DA on the SAL-induced release of PRL in ruminants. The experiments were performed from late June to early July. Male goats were given a single intravenous (i.v.) injection of SAL (5mg/kg body weight (BW)), a DA receptor antagonist (sulpiride, 0.1mg/kg BW), or thyrotropin-releasing hormone (TRH, 1µg/kg BW) before and after treatment with a DA receptor agonist (bromocriptine), and the effect of DA on SAL-induced PRL release was compared to that on sulpiride- or TRH-induced release. Bromocriptine completely inhibited the SAL-induced release of PRL (P<0.05), and the area under the response curve (AUC) for a 120-min period after the treatment with bromocriptine was 1/28 of that for before the treatment (P<0.05). Bromocriptine also completely inhibited the sulpiride-induced release (P<0.05). The AUC post-treatment was 1/17 that of pre-treatment with bromocriptine (P<0.05). Bromocriptine also inhibited the TRH-induced release (P<0.05), though not completely. The AUC post-treatment was 1/3.8 that of pre-treatment (P<0.05). These results indicate that DA inhibits the SAL-induced release of PRL in male goats, and suggest that SAL and DA are involved in regulating the secretion of PRL. They also suggest that in terms of the regulatory process for the secretion of PRL, SAL resembles sulpiride but differs from TRH.

Sex steroid hormones do not enhance the direct stimulatory effect of kisspetin-10 on the secretion of growth hormone from bovine anterior pituitary cells.

The aims of the present study were to clarify the effect of kisspeptin10 (Kp10) on the secretion of growth hormone (GH) from bovine anterior pituitary (AP) cells, and evaluate the ability of sex steroid hormones to enhance the sensitivity of somatotrophic cells to Kp10. AP cells prepared from 8-11-month-old castrated calves were incubated for 12 h with estradiol (E(2), 10(-8) mol/L),progesterone (P(4), 10(-8) mol/L), testosterone (T, 10(-8) mol/L), or vehicle only (control), and then for 2 h with Kp10. The amount of GH released in the medium was measured by a time-resolved fluoroimmunoassay. Kp10 (10(-6) or 10(-5) mol/L) significantly stimulated the secretion of GH from the AP cells regardless of steroid treatments (P < 0.05), and E(2), P(4), and T had no effect on this response. The GH-releasing response to growth hormone-releasing hormone (GHRH, 10(-8) mol/L) was significantly greater than that to Kp10 (P < 0.05). The present results suggest that Kp10 directly stimulates the release of GH from somatotrophic cells and sex steroid hormones do not enhance the sensitivity of these cells to Kp10. Furthermore, they suggest that the GH-releasing effect of Kp10 is less potent than that of GHRH.

Characteristics of the stimulatory effect of kisspeptin-10 on the secretion of luteinizing hormone, follicle-stimulating hormone and growth hormone in prepubertal male and female cattle.

The aims of the present study were to clarify the effect of kisspeptin-10 (Kp10) on the secretion of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and growth hormone (GH) in prepubertal male and female cattle. The experiments were performed from May to June using five male (4-6 months old) and five female (5-6 months old) Japanese Black calves. A single intravenous (iv) injection of Kp10 (5 microg/kg body weight (b.w.): 3.85 nmol/ kg b.w.) significantly stimulated the release of LH and FSH in male and female calves (P<0.05). A single intramuscular injection of Kp10 (5 microg/kg b.w.) also significantly stimulated the release of LH and FSH in male calves (P<0.05), though the response was smaller than that to the iv injection. The injection of Kp10 did not alter the basal plasma concentration of GH in male or female calves. The area under the curve (AUC) of both LH and FSH for a 120-min period after the iv injection of Kp10 was significantly greater in the males than females (P<0.05). These results show that Kp10 can stimulate the release of LH and FSH in calves of both sexes and that the response to the peptide is greater in males at this age. They also show that Kp10 has no effect on the release of GH in male and female calves and that the LH- and FSH-releasing effect of Kp10 is greater after an iv injection than after an im injection in calves.