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In this study either substance P (SP), SP 5-11, an analogue resistant to enzymatic degradation, or acidified saline was injected bilaterally into the medial preoptic nucleus (MPN) of the medial preoptic area (MPOA) in ovariectomized (ovx) estrogen-primed female rats. The lordosis quotient (LQ) and the lordosis intensity scale (LIS) were recorded using a steroid regime that produced a slow increase of sexual receptivity. In experiment 1, when receptivity was low, bilateral injections of 2 different doses of SP (100 ng,1000 ng/ cannula), had no effect on lordosis behavior when compared to those injected with acidified saline. In experiment , however, when receptivity was maximal, SP 5-11 (1000 ng/cannula) produced a marked decrease in lordosis responding. The results indicate t.hat the effects of bilateral injections of SP on lordosis behavior is dependent on the level of sexual receptivity, and therefore, estrogen levels.



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