Facilitation of lordosis in female rats by CNS-site specific infusions of an LH-RH fragment, Ac-LH-RH-(5-10).

Structural alterations of the luteinizing hormone-releasing hormone (LH-RH) molecule have been performed to yield analogs which are more potent than, or which compete with, the parent hormone to increase the release of LH from the pituitary gland. The effects of these analogs on mating behavior, however, do not always parallel their effects on LH release. The present study tested the effectiveness of a pituitary-inactive fragment of LH-RH, namely Ac-LH-RH-(5-10), in potentiating mating behavior in the ovariectomized, estrogen-primed female rat. This fragment, when infused bilaterally into the medial preoptic area (POA), the ventromedial hypothalamus (VMH), or the midbrain central gray (MCG), significantly enhanced lordosis. Infusion of the fragment into the cerebral cortex was ineffective. Elevated lordotic responding was first apparent in the POA at 15 min postinfusion and was maintained for the duration of the testing session (180 min). Ac-LH-RH-(5-10) infused into the VMH or MCG enhanced lordotic behavior at 90 and 180 min postinfusion. The results indicate that only a portion of the LH-RH molecule may be required for behavioral activity and suggest that degradation of the LH-RH molecule is physiologically relevant.