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Daily GnRH and LH secretion in ewes is not modified by exogenous melatonin during seasonal anestrus

100%
Romanowicz K., Misztal T., Gajewska A., Barcikowski B.
Acta Neurobiologiae Experimentalis
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2001
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vol. 61
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issue 4
289-297
EN
The effect of central, short-term melatonin administration on daily GnRH and LH secretion was studied in ewes during seasonal anestrus. Melatonin, in a total dose of 32 g and the vehicle were perfused for 4 hours into the mediobasal hypothalamus/median eminence (MBH/ME). The mean GnRH concentration during perfusion with melatonin decreased significantly (P0.05), as compared to the concentration during the preceding perfusion with the vehicle only. This change resulted from high variations in GnRH concentration noted during the initial phase of perfusion rather than from an action of melatonin. Melatonin perfused into the MBH/ME did not significantly affect LH secretion. A higher dose of melatonin and vehicle were also infused intracerebroventricularly (icv.) in either intact (300 g for 3 hours) or ovariectomized (OVX) ewes (400 g for 4 hours, 100 g/100 l/h). In the intact animals, melatonin did not significantly affect LH secretion. Interestingly, melatonin significantly decreased (P0.05) the number of LH peaks in OVX ewes. These results demonstrate that melatonin delivered for a few hours directly into the central nervous system did not affect either daily hypothalamic GnRH release or pituitary LH secretion in intact ewes during seasonal anestrus, but did modify pulsatile LH secretion in ewes deprived of the negative feedback of estradiol.
2
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The effect of corticotropin releasing factor (CRF) on the gonadotropin hormone releasing hormone (GnRH) hypothalamic neuronal system during preovulatory period in the ewe

100%
Polkowska J., Przekop F.
Acta Neurobiologiae Experimentalis
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1997
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vol. 57
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issue 2
91-99
EN
Effects of infusions of corticotropin releasing factor (CRF) into the 3rd ventricle of the brain of ewes during the proestrus on the immunoreactive (ir) gonadotropin hormone releasing hormone (GnRH) neuronal system, pituitary luteinizing hormone(LH) producing cells and LH concentrations in the blood plasma were studied. None of the CRF treated sheep displayed the estrous activity nor ovulated on the day of estrus (17th day of the cycle), and two days later when they were slaughtered. The GnRH center of CRF treated ewes situated in the preoptico septal area was well organized , but irGnRH stores in the median eminence were low in comparison to the controls (sheep from the late follicular phase of the estrous cycle). The feature and the number of LH cells in CRF treated ewes were typical for the preovulatory phase of the cycle but the plasma concentrations of LH did not exceed basal levels. These results suggest that CRF induced decrease of irGnRH stores in the nerve terminals of the ME can be responsible for the blockade of the preovulatory surge of GnRH/LH in the sheep.
3
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Effect of stress on the expression of GnRH and GnRH receptor (GnRH-R) genes in the preoptic area-hypothalamus and GnRH-R gene in the stalk/median eminence and anterior pituitary gland in ewes during follicular phase of the estrous cycle

88%
Ciechanowska M., Lapot M., Malewski T., Misztal T., Mateusiak K., Przekop F.
Acta Neurobiologiae Experimentalis
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2007
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vol. 67
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issue 1
1-12
EN
The RT-PCR (reverse transcription polymerase chain reaction) technique was used to analyze GnRH mRNA and GnRH-R mRNA in the preoptic area, anterior and ventromedial hypothalamus, and GnRH-R mRNA in the stalk/median eminence and anterior pituitary gland of follicular ewes subjected to short (3 h during one day) or prolonged (5 h daily during four consecutive days) footshock stimulation. To analyze relationship between expression of GnRH and GnRH-R genes with LH secretion the blood samples were collected at 10 min intervals to determine LH levels in control and stressed animals. The concentration of GnRH mRNA increased significantly in the preoptic area, anterior and ventromedial hypothalamus of ewes subjected to short stress. The prolonged stressful stimuli significantly decreased GnRH mRNA levels in all analyzed structures. In short stressed ewes the significant augmentation of mRNA encoding GnRH-R was detected in the preoptic area, entire hypothalamus, stalk/median eminence and anterior pituitary gland. The GnRH-R mRNA was significantly reduced in all tested structures of animals subjected to prolonged footshocking except for the preoptic area, where GnRH-R mRNA did not differ from control values. The changes in GnRH mRNA and GnRH-R mRNA levels under short or prolonged stress were associated with an increase or decrease of LH concentration in blood plasma, suggesting the existence of a direct relationship between GnRH mRNA and GnRH-R mRNA expression with LH secretion. The results indicate that the expression of both GnRH gene and GnRH-R gene, as well as LH secretion in ewes during the follicular phase of the estrous cycle, are dependent upon the kind of stress.
4
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Stress and nutritional influences on GnRH and somatostatin neuronal system in the ewe

75%
Polkowska J.
Acta Neurobiologiae Experimentalis
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1996
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vol. 56
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issue 3
797-806
EN
The efect of stressful stimulation and protein malnutration on the gonadotrophic and somatotrophic axis of sheep is discussed with special references to the relatioship between these stimuli and the GnRH and somatostatin neuronal system in the hypothalmus.Generally, long-term stimulation and chronic underfeeding reduce gonadal functions in the sheep.There is evidence for the GnRH-dependent pathway for the mechanism of these phenomena in femle sheep.GnRh neurons respond to long-term stress in diminishing of neuropeptide release from the nerve terminals due to the depression of its axonal transport.Chronic restriction of dietary proteins in lambs reduces the plasma LH concentration but does not impair the development of GnRH neurons nor the synthesis and processing of GnRH.It is suggested that malnutrition delays the first ovulation probably due to the neural mechanism responsible for the preovulatory GnRH/LH output.Stress has rather unclear effect on growth hormone (GH) secretion in the sheep.Prolonged, but not short stressful stimulation provokes the rapid release of somatostatin, which is sustained during long-term stimulation.These results suggest that effect of stress on somatotrophic axis depends on the period of stressful stimulation.VChronic malnutrition enhances secretion of GH by an increse in amplitude of GH pulses and reduces the secretory activity of somatostatin neurons.It is postulated that nutrients can influence GH secretion in the sheep by mechanism dependent on the hypothalmic somatostatin.
5
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Neural systems mediating the negative feedback actions of estradiol and progesterone in the ewe

63%
Goodman R. L.
Acta Neurobiologiae Experimentalis
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1996
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vol. 56
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issue 3
727-741
EN
The ewe shows a marked seasonal variation in the effect of ovarian steroids on pulsatile GnRH secretion.In the breeding season progesterone inhibits GnRH pulse frequency, while estradiol suppresses puilse amplitude.In anestrus, both steriods inhibit pulse frequency.The effects of progestrone in both seasons are mediated by endogenous opioid peptides (EOP) that act in the preoptic area (POA) and medial basal hypothalmus (MBH).However, knife cut studies indicate that actions in the MBH are most important.Moreover, blockade of EOP receptors activates GnRH perikarya in the MBH, but not those in the POA.Thus interactions between EOP and GnRH neurons within the MBH may be critical for progesterone negative feedback.The neural systems mediating estradiol suppression of GnRH pulse amplitude in the breeding season are largely unknown, although alpha-adrenergic neurons nay be involved.The seasonal variation in inhibition of GnRH pulse frequency by estradiol is postulated to be mediated by a group of dopaminergic (DA) neurons that have three important properties: (1)they inhibit GnRH pulse frequency; (2) their activity is stimulated by estradiol; and (3) they are functional in anestrus, but not the breeding season.Recent work examining the effects of lesions of DA neyrone and the ability of estradiol to induce Fos inDA cells srongly suggest that DA neurons in the retrochiasmatic area (A15) and POA (A14) have all three characteristics.We thus propose that these DA neurons are responsible for the seasonal variation in the ability of estradiol to inhibit GnRH pulse frequency.
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