Effect of serotonin on LH secretory patterns in ovariectomized cows

Abstract

The effect of serotonin (5-HT) infusion at concentrations of 0.00, 0.07, 0.79 and 7.55 μg/kg/min upon mean concentration, episodic release pattern and the gonadotropin releasing hormone (GnRH)-induced release of luteinizing hormone (LH) was investigated. Four Guernsey cows were ovariectomized at least 30 days prior to the initiation of the experiment. Treatments were given on alternate days until each cow had received the four concentrations of 5-HT, completing a Latin Square Design. Treatment days were composed of 3 consecutive 4 hr periods (Period I, II, III) with blood for plasma LH determination collected via jugular cannula every 15 min during each 12 hr treatment day. Period I was to verify that no carryover of previous treatment had occurred. Infusion of saline or 5-HT, via carotid cannula, was initiated at the start of Period II and continued throughout Period III. The effect of 5-HT on the episodic release of LH was assessed during Period II. At the beginning of Period III 50 μg of GnRH was injected (im) to compare the GnRH-induced LH release among treatments. Mean concentrations of LH did not vary among treatments (× = 2.9 ng/ml) either prior to or during 5-HT infusion for the 4 hr period. Pulse frequency, amplitude and duration did not differ among any groups prior to infusion of 5-HT. However, pulse frequency decreased (P<.05) in animals receiving 7.55 μg/kg/min 5-HT (× = 2.2 peaks/4 hr) compared to cows receiving 0 (× = 3.2 peaks/4 hr), .07 (× = 4.0 peaks/4 hr) or .79 (× = 4.0 peaks/4 hr) μg/kg/min 5-HT. No pulses were observed for 2–4 hr of infusion in 3 of 4 animals receiving the highest dose of 5-HT. In addition, when the initial 4 hr infusion period (Period II) was divided into two 2 hr segments, the mean LH concentration during the last 2 hr of infusion was less (P<.05) than that observed during the first two hours of infusion. Amplitude and duration of LH peaks were not affected by treatment. The GnRH-induced LH release was not affected by treatment, but response was highly variable among cows and average response among cows was different (P<.05). In conclusion, 5-HT infusion at a level of .07 and 0.79 μg/kg/min did not alter plasma LH secretion. However, 5-HT infused at 7.55 μg/min/kg bodyweight decreased the mean level and pulse frequency of LH release but not the pulse amplitude or duration. This effect was observed 2–4 hr following initiation of 5-HT. None of the 5-HT doses used decreased the GnRH-induced LH release.