Midbrain LHRH infusions enhance lordotic behavior in ovariectomized estrogen-primed rats independently of a hypothalamic responsiveness to LHRH.

Lordotic behavior of ovariectomized estrone-primed rats was measured after infusions of luteinizing hormone-releasing hormone (LHRH), thyrotropin (TRH) or saline into the ventrolateral midbrain central gray (VL-MCG), or after LHRH or saline infusions into a lateral control site, the dorsolateral reticular formation (DL-RF). Infusion of LHRH, but not TRH, into the VL-MCG increased lordotic behavior. LHRH had no effect in the DL-RF. In a second experiment, rats were fitted with cannulae in the VL-MCG and in the arcuate-ventromedial nucleus area (ARC-VM). Serotonin was infused into the ARC-VM, and LHRH was infused into either the ARC-VM or the VL-MCG. Serotonin blocked the behavioral effect of LHRH infusion into the ARC-VM, but did not prevent enhancement of lordosis by LHRH infusions into the VL-MCG. These results suggest that LHRH infusions into the midbrain do not require hypothalamic responsiveness to LHRH for their effect, and are therefore unlikely to act by diffusion of LHRH rostrally. The effect of LHRH in the midbrain is site-specific, since lateral infusions (1.75 mm away) were ineffective.