Medial Preoptic Area Involvement in Norepinephrine-Induced Suppression of Pulsatile Luteinizing Hormone Release in Ovariectomized Rats

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Abstract
This study examined whether the medial preoptic area (MPOA) is a site which mediates the inhibitory effects of norepinephrine (NE) on pulsatile luteinizing hormone (LH) secretion in ovariectomized rats. Animals were bled continuously at a rate of 50 µl whole blood/7 min for 2 h prior to push-pull perfusion in the MPOA, and during a 2–3 h period of perfusion of the MPOA (20 µl/min) with artificial CSF, or 2 or 20 pg NE/min. In another group of rats LH levels were only determined during a 2–3 h period of MPOA perfusion with CSF. Pulsatile LH release was not affected by push-pull perfusion with CSF when a comparison was made to preperfusion LH values in the same rats. Moreover, LH levels obtained from rats only bled during MPOA perfusion with CSF were not different from LH values obtained during the preperfusion periods in the other groups. However, push-pull perfusion of the MPOA with 2 or 20 pg NE/min significantly suppressed mean LH levels by causing a 35–45% reduction in pulse frequency. No decrease occurred in LH pulse amplitude. Therefore, these studies demonstrate that NE acting at the level of the MPOA can suppress pulsatile LH release solely by decreasing LH pulse frequency.