Search

RT-PCR followed by 5'- and 3'- rapid amplification of cDNA ends was used to clone and sequence ovine prolactin-releasing peptide (PrRP). The cDNA was characterised by short 5'- and 3'-untranslated regions and a GC-rich (71%) coding region. The nucleotide and deduced amino acid sequences for the coding region showed 95.6 and 94.9% identity with bovine PrRP but the amino acid sequence of PrRP31 was conserved between these species. Northern blot analysis and RT-PCR showed that, as in the rat, the peptide was more abundantly expressed in the brainstem than the hypothalamus. However, in the ovine hypothalamus, PrRP mRNA expression was more widespread than in the rat, with expression detected in both rostral and caudal parts of the mediobasal hypothalamus. The effects of synthetic ovine PrRP on prolactin secretion both in vitro and in vivo were also examined. In primary cultures of sheep pituitary cells, PrRP significantly (P<0.01) increased prolactin concentrations in the culture medium but the response was not observed in every experiment and was only seen when pituitary glands were dispersed with collagenase rather than trypsin. PrRP was much less potent than TRH which caused a significant (P<0.01) two- to threefold increase in prolactin concentrations in every experiment. Intravenous (10 and 50 nmol) or intracerebroventricular (10 and 50 nmol) injection of PrRP had no significant effect on either plasma prolactin concentration or pulsatile LH secretion whereas intravenous injection of TRH (10 nmol) produced a highly significant (P<0.01) and more than sevenfold stimulation of plasma prolactin concentrations. In conclusion, these results suggest that PrRP is unlikely to be an important prolactin-releasing factor in this species.

In sheep, injection of noradrenaline suppresses prolactin secretion by a direct effect at the pituitary gland. The aims of this study were to use primary cultures of ovine pituitary cells to examine the receptor subtypes that mediate the inhibitory effect of noradrenaline on prolactin secretion and, by using receptor antagonists in vivo, determine whether noradrenaline acts as a prolactin release-inhibiting factor (PIF). Noradrenaline and dopamine suppressed prolactin secretion from ovine pituitary cells with ED50s of 60.9+/-46.6 and 1.5+/-1.0x10-9 mol/l, respectively (P<0.05). The in-vitro prolactin release-inhibiting effect of noradrenaline (10-7 mol/l) was not blocked by the dopamine antagonists pimozide (D2) or SCH23390 (D1) but was blocked by each of the adrenoceptor antagonists (alpha1-adrenoceptor antagonists prazosin and WB4101, the alpha2-adrenoceptor antagonist yohimbine and the beta-adrenoceptor antagonist propranolol). The response to adrenoceptor agonists was also tested in vitro. The alpha1-adrenoceptor agonists phenylephrine and cirazoline significantly suppressed prolactin. Of the alpha2-agonists, clonidine had no effect whereas oxymetazoline and p-aminoclonidine both suppressed prolactin. The beta-adrenoceptor agonist isoproterenol also suppressed prolactin while the specific beta3-antagonist BRL37344 had no effect. When the adrenoceptor antagonists were tested in vivo in ewes manipulated to be in the luteal phase, only WB4101 significantly (P<0.05) increased plasma prolactin concentrations but this response was small and only observed in one of two experiments. In summary, these experiments suggest that adrenoceptors and not dopamine receptors are responsible for the inhibitory effect of noradrenaline on prolactin secretion in vitro but do not implicate a particular adrenoceptor subtype. The in-vivo experiments do not provide convincing evidence for a role for noradrenaline as a physiologically important PIF.

Recent evidence suggests that dopamine, acting via its D1 receptors, may function as a neurotransmitter in intrahypothalamic pathways involved in the stimulation of prolactin secretion. Functional dopamine D1 receptors are present in the ventromedial hypothalamic nucleus (VMH) and we hypothesized that they might be part of a prolactin-stimulatory pathway activated by stress. We tested this hypothesis in a series of experiments on sheep involving two different forms of stressors, audiovisual (barking dog) and high environmental temperature. We attempted to block the stimulation of prolactin secretion by infusion into the VMH of an antagonist specific for the D1 receptor. Ovariectomised, oestradiol-implanted merino ewes were surgically implanted with bilateral guide tubes directed at the VMH. After a 180 min pretreatment period, the ewes either were or were not exposed to a stressor (30 min of barking dog or 120 min at 35 degrees C, 65% relative humidity). D1 receptor antagonist, SCH23390 or vehicle (0.9% saline) was infused into the VMH (1.7 microliters/h, 120 nmol/h) for 60 min prior to and during the stressor period. Blood was sampled every 15 min via jugular cannulae and the plasma was assayed for prolactin, cortisol and growth hormone (GH). Both stressors significantly increased prolactin concentrations over control levels. SCH23390 infusion significantly attenuated the prolactin response to high environmental temperature, but had no effect on the prolactin response to audiovisual stress. Cortisol concentrations were significantly increased by audiovisual stress only and were not affected by SCH23390. GH concentrations were not changed by either stressor or infusion. Drug infusion alone did not affect the concentration of the hormones. The data suggest that the VMH D1 receptors are involved in a prolactin stimulatory pathway in response to high environmental temperature. The inability of the D1 antagonist to affect the response to the barking dog indicates that this pathway is stress-specific, implying that there is more than one mechanism or pathway involved in the prolactin response to different stressors.

In sheep intracerebroventricular injection of PACAP (10 nmol) significantly (P<0.01) stimulated the levels of the dopamine metabolite DOPAC within the medial basal hypothalamus (as measured by in vivo microdialysis) and this effect was temporally correlated with a significant (P<0.05) suppression in peripheral prolactin concentrations. This result is in accord with the hypothesis that PACAP suppresses prolactin secretion from the anterior pituitary gland by stimulating dopamine release from tuberoinfundibular dopaminergic neurons., (Copyright 1998 Elsevier Science B.V.)

Interactions between testosterone, estradiol, and inhibin in the control of gonadotrophin secretion in males are poorly understood. Castrated rams were treated with steroid-free bovine follicular fluid (bFF), testosterone, or estradiol and for 7 d (2 x 2 x 2 factorial design). Given independently, none of the exogenous hormones affected follicle-stimulating hormone (FSH) concentrations, but the combination of one or both steroids with bFF reduced FSH secretion. Testosterone and estradiol reduced luteinizing hormone (LH) pulse frequency (there was no synergism), and bFF had no effect. Plasma prolactin concentrations were not affected by any treatment. To locate the central sites of steroid action, castrated rams were bilaterally implanted in the preoptic area (POA), ventromedial nucleus (VMH), or arcuate nucleus (ARC). These implants did not affect FSH or prolactin concentrations, or LH pulse amplitude. The frequency of the LH pulses was not affected by testosterone in any site. Estradiol located in the ARC, but not the POA or VMH, decreased LH pulse frequency. In summary, FSH secretion is controlled by synergistic interactions between inhibin and estradiol or testosterone, whereas GnRH/LH pulse frequency is controlled by testicular steroids. Estradiol acts partly, at least, in the ARC, but the central site of action, testosterone remains unknown.

In this study, we investigated the hypothalamic regulatory role of pituitary adenylate cyclase-activating polypeptide (PACAP) in the control of LH and PRL secretion. Overiectomized ewes were surgically prepared with bilateral guide tubes directed at the preoptic area (POA) or medial basal hypothalamus (MBH). After recovery from surgery, PACAP38 (0.1 nmol in 2.5 microliters over 1 h) or vehicle was bilaterally infused into each site in separate trials. Infusion of PACAP38 into the POA had no effect on either LH or PRL secretion. However, infusion of the peptide into the MBH suppressed PRL secretion during the 3-h postinfusion period; the responding animals (n = 9) had injectors located in the arcuate nucleus. In the three nonresponding animals, both injectors were outside the arcuate nucleus. Mean LH concentration, LH pulse frequency, and pulse amplitude were also significantly suppressed, with LH pulsatility declining in seven of eight animals during infusion of the peptide in the MBH. These results suggest that PACAP acts in the arcuate nucleus region of the MBH, and not the rostral POA, to inhibit both LH and PRL secretion.

Plasma prolactin (PRL) concentrations in captive koalas during lactation were determined by serial blood sampling. PRL concentrations were low (1.3 ± 0.1 ng mL-1; n = 5) during early lactation until pouch young (PY) began to emerge from the pouch (around Day 130) before significantly (P < 0.05) increasing between Day 161 and Day 175 (5.3 ± 1.0 ng mL-1). A significant (P < 0.001) peak in PRL (7.7 ± 0.6 ng mL-1) coincided with maturing young between Day 189 and Day 231. All females failed to exhibit any signs of oestrous behaviour until Day 268.8 ± 8.5 (n = 4), some 102 ± 19 days before PY were weaned following achieving target weights of 2.5-2.7 kg. Throughout lactation, plasma LH concentrations were relatively high (range 4.9-8.7 ng mL-1) and LH responses to exogenous gonadotrophin-releasing hormone were observed in all koalas at all times during lactation. [ABSTRACT FROM AUTHOR]