Effects of Acute, Intracerebroventricular Administration of Insulin on Serum Concentrations of Luteinizing Hormone, Insulin, and Glucose in Ovariectomized Lambs during Restricted and Ad Libitum Feed Intake1

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Abstract
To test the hypothesis that the increase in pulsatile LH secretion associated with increased feed intake in growth-restricted lambs is due to elevated insulin concentrations, we evaluated the effects of intracerebroventricular (ICV) administration of insulin on patterns of LH in ovariectomized ewe lambs. After weaning (10 wk of age), 12 lambs were fed to maintain a mean body weight of 18.3 kg. At approximately 32 wk of age a permanent cannula was inserted into the lateral ventricle of each lamb. For the first experiment, animals received 3 ICV injections of either 500 ng (n = 6) or 500 micrograms (n = 6) insulin. Blood samples were collected every 10 min for 8 h, with animals receiving injections at 2, 4, and 6 h. Patterns of LH during the 2-h preinjection period were compared to those in the subsequent 2-h periods following each insulin injection. Insulin did not affect mean LH, LH pulse frequency, or LH pulse amplitude. Only the 500-micrograms injections increased (p < 0.0001) peripheral insulin and decreased (p < 0.001) peripheral glucose. The experimental protocol was repeated during a second experiment conducted after 2 wk of ad libitum feeding, when animals weighed an average of 21.4 kg. Increased feed intake was associated with increases in mean LH, LH pulse frequency, and insulin (p < 0.05). Both doses of insulin decreased (p < 0.01) mean LH and LH pulse frequency. The 500-micrograms injections increased (p < 0.0001) peripheral insulin and decreased (p < 0.001) glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

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