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Nitric oxide (NO), an active free radical formed during the conversion of arghnine to citrulline by the enzyme NO synthase (NOS), mediates vasorelaxation, cytotoxicity, and neurotransmission. The hypothalamic hypophysiotropic peptide hormone, LHRH, not only releases LH from the pituitary but also induces sexual behavior (14,17). LHRH induces lordosis after mounting by the male in estrone-primed ovariectomized (17) and estradiol benzoate-primed adrenalectomized female rats (18). These observations raise the possibility that NO might also be involved in the mediation of sexual behavior in female rats. In the present experiments, we investigated the effect(s) of agents that enhance or inhibit NO formation on the sexual behavior of female rats. The results indicate a crucial role for NO in LHRH-potentiated mating behavior as measured by the lordosis response in estrogen (E)-primed ovariectomized female rats. MATERIALS AND METHODSOvariectomized Sprague-Dawley rats (160-180 g body weight) obtained from Sasco (Houston) were housed with a 12/12-h light/dark cycle and given food and water ad libitum. All the animals were administered hormones and tested for sex behavior. 173-Estradiol benzoate (herein used as E; 10 ,ug in sesame oil) was injected subcutaneously (s.c.) followed by progesterone (P) s.c. (100 ug in sesame oil) 48 h later. Four hours after P administration, the animals were tested for sex behavior in the presence of sexually active males in a 50 x 45 x 24 cm polystyrene arena. Both proceptive (hop-darting, ear wiggling, and approaches to the male) and receptive behavior (the number of lordoses, acceptance behavior on being mounted by the male, and the number of mounts by the male) of each female rat in the presence of a male was scored and recorded. The results of all the experiments were converted to lordosis quotient (LQ), defined as a percentage of full lordosis response (perineum elevated, all four legs extended Abbreviations: NO, nitric oxide; NOS, NO synthase; LHRH, luteinizing hormone-releasing hormone; L-NMMA, NG-monomethyl-L-argliine; D-NMMA, NG-monomethyl-D-arginine; 3V, third cerebral ventricle; E, estrogen; P, progesterone; SNP, sodium nitroprusside; NE, norepinephrine; Hb, hemoglobin; NRS, normal rabbit serum; LQ, lordosis quotient; i.c.v., intracerebroventricularly. §To whom reprint requests should be addressed. 6468The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

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Mucin-1 (Muc1), an integral membrane mucin, is expressed on the apical surface of uterine epithelial cells (UE) of various species. Loss of Muc1 is believed to be necessary for embryo attachment. Muc1 expression is markedly reduced in luminal epithelia during the receptive phase in mice, baboons, and pigs. In the present study, we examined Muc1 expression during the rat estrous cycle and at Day 5 of pregnancy, the time of embryo attachment. In contrast to findings in the mouse, indirect immunofluorescence revealed that uterine Muc1 protein expression was unaltered during the estrous cycle. However, similar to what is observed in the mouse and other species, Muc1 protein decreased at Day 5 of pregnancy in luminal UE. The decrease in Muc1 expression was specific to luminal UE and did not occur in glandular UE. A partial cDNA corresponding to the cytoplasmic tail region of rat Muc1 was generated by a reverse transcription-polymerase chain reaction (RT-PCR) strategy. This cDNA sequence is 89% and 91% identical to the corresponding region of mouse Muc1 at the nucleotide and amino acid levels, respectively. The predicted sequence of rat Muc1 protein has 70-90% identity to the Muc1 protein sequence obtained in other species. Semiquantitative RT-PCR experiments indicated that the mRNA encoding rat Muc1 decreased 57% at Day 5 as compared with the levels found at estrus. This value included mRNA from both luminal and glandular UE and so may underestimate the relative decrease in mRNA in the luminal compartment. In conclusion, we have determined that the levels of rat Muc1 protein and mRNA decrease in the luminal UE at the time of implantation, a pattern similar to that seen in the mouse, baboon, and pig. This supports the general theory that reduction of Muc1 expression is necessary for embryo implantation.

Although several studies have reported the localization of membrane progesterone (P(4)) receptors (mPR) in various tissues, few have attempted to describe the distribution and regulation of these receptors in the brain. In the present study, we investigated expression of two mPR subtypes, mPRα and mPRβ, within regions of the brain, known to express estradiol (E(2))-dependent [preoptic area (POA) and hypothalamus] and independent (cortex) classical progestin receptors. Saturation binding and Scatchard analyses on plasma membranes prepared from rat cortex, hypothalamus, and POA demonstrated high-affinity, specific P(4)-binding sites characteristic of mPR. Using quantitative RT-PCR, we found that mPRβ mRNA was expressed at higher levels than mPRα, indicating that mPRβ may be the primary mPR subtype in the rat brain. We also mapped the distribution of mPRβ protein using immunohistochemistry. The mPRβ-immunoreactive neurons were highly expressed in select nuclei of the hypothalamus (paraventricular nucleus, ventromedial hypothalamus, and arcuate nucleus), forebrain (medial septum and horizontal diagonal band), and midbrain (oculomotor and red nuclei) and throughout many areas of the cortex and thalamus. Treatment of ovariectomized female rats with E(2) benzoate increased mPRβ immunoreactivity within the medial septum but not the medial POA, horizontal diagonal band, or oculomotor nucleus. Together, these findings demonstrate a wide distribution of mPRβ in the rodent brain that may contribute to functions affecting behavioral, endocrine, motor, and sensory systems. Furthermore, E(2) regulation of mPRβ indicates a mechanism through which estrogens can regulate P(4) function within discrete brain regions to potentially impact behavior.