Graduate Thesis Or Dissertation
 

Role of hypothalamic biogenic amines in the release of luteinizing hormone in the ewe

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  • The role of hypothalamic biogenic amines (norepinephrine, dopamine and serotonin) in regulating the release of luteinizing hormone (LH) in the ewe was investigated. Eleven experiments utilizing mature crossbred or Hampshire ewes were conducted to determine the ability of exogenous 17β-estradiol and various pharmacological agents to alter the levels of hypothalamic biogenic amines and affect the release of LH in the ewe. The stalk median eminence (SME) and hypothalamus proper (HP) were chemically analyzed for norepinephrine (NE), dopamine (DA) and serotonin (5-HT). Serum LH concentrations were determined using radioimmunoassay. A single intramuscular injection of 750 μg of 17β-estradiol into each of four ewes on day 3 of the estrous cycle (first day of estrus = day 0 of the cycle) caused an increase in serum LH concentration in each ewe (range, 20 to 30 ng/ml) within 16 hours following treatment. A similar injection of 17β-estradiol into five ewes 8 hours prior to necropsy on day 3 of the cycle was without affect on the levels of biogenic amines in the SME or HP. In both treated and control ewes, NE levels were higher (P < . 01) in the HP than concentrations of NE in the SME. Conversely, DA and 5-HT concentrations were found to be higher (P < . 01) in the SME than in the HP. Treatment of six ovariectomized ewes with 750 μg of 17 β-estradiol 3 hours prior to necropsy elevated NE levels in the SME P ≡ .07) but failed to alter NE concentrations in the HP. Serotonin concentrations in both the SME and HP tended to increase after treatment with estradiol. One hour after intravenous injection of six ovariectomized ewes with L-dihydroxyphenylalanine (L-dopa, 12 mg/kg) biogenic amine levels qualitatively resembled those detected following injection of estradiol. Norepinephrine levels in the SME tended to increase while only a slight change in the concentration of NE in the HP was detected. Serotonin concentrations in both the SME and HP were higher after treatment with L-dopa than those observed in control ewes. In an attempt to inhibit the ovulatory surge of LH 12 ewes were injected intravenously with α-methyltyrosine (α-MT), α-methyldopa (α-MD), 5-hydroxytryptophan (5-HTP) or vehicle as soon as they were detected in estrus and again 4 hours later. Three ewes each were injected with α-MT (12 mg/kg), α-MD (6 mg/kg), 5-HTP (6 mg/kg) or vehicle. Elevated serum LH levels indicative of the ovulatory release of LH were detected in at least one serum sample from each ewe. Similarly, pretreatment of four anestrous ewes with α-MT (15 mg/kg) did not block or modify the ability of exogenous estradiol (20 μg) to induce a release of LH. Pretreatment of five ovariectomized ewes with p-chlorophenylalanine (p-cpa) did not inhibit the release of LH elicited by injection of estradiol (50 μg) but did increase the interval from injection of estradiol until the onset of LH release by 4 hours. Experiments were conducted to investigate the effects of L-dopa and p-cpa on the release of LH. L-Dihydroxyphenylalanine was intravenously injected into four ewes on day 3 of the cycle. Forty seven ewes in various stages of anestrus (early, deep or late) were intravenously injected with graded doses of L-dopa (3 to 18 mg/kg), p-cpa (11 to 60 mg/kg) or vehicle. Treatment of ewes with L-dopa during the estrous cycle resulted in a release of LH from only one animal at 24 hours (20 ng /ml) and again at 32 hours (91 ng/ml) post-injection. Elevated serum LH concentrations were detected in three of the 24 ewes treated with L-dopa and in one of nine ewes injected with p-cpa. Each of the ewes that responded to treatment with a release of LH was in late anestrus.
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