Facilitation of sexual receptivity in the female rat by C-terminal fragments of LHRH.

Ovariectomized female rats implanted with cannulae directed bilaterally towards the ventromedial nucleus of the hypothalamus (VMH) were primed with estrogen (2 micrograms) and tested for sexual receptivity before and after infusion of the C-terminal LHRH fragment, Ac-LHRH5-10. Animals responsive to the Ac-LHRH5-10 fragment were tested at two-week intervals for responsiveness to other LHRH fragments modified in the C-terminal or saline in the following order: LHRH5-10, des-Gly6LHRH, LHRH5-9OH, saline, and LHRH7-9OH. The lordosis-to-mount (L/M) ratio was used as an index of sexual receptivity. The quality of lordotic posturing and the number of proceptive and resistive behaviors were also recorded for each test. A one-way ANOVA performed on the difference in L/M between pre- and postinfusion tests revealed a significant overall treatment effect. The nonacetylated LHRH5-10 fragment and the LHRH5-9OH fragment were effective in enhancing the L/M ratio, whereas des-Gly6LHRH and LHRH7-9OH were not. The enhancement of sexual behavior by C-terminal fragments of LHRH was associated with a lack of proceptive behavior and moderate levels of resistive behavior. The results indicate that several C-terminal fragments of LHRH are capable of elevating the L/M ratio and suggest that amino acids in positions 6 through 9 may be important for this effect.