The roles of testicular nuclear receptor 4 (TR4) in male fertility-priapism and sexual behavior defects in TR4 knockout mice
- Loretta L Collins†1,
- Yi-Fen Lee†1,
- Huei-Ju Ting1,
- Wen-Jye Lin1,
- Ning-Chun Liu1,
- Charles K Meshul2,
- Hideo Uno3,
- Bo-Ying Bao1, 4,
- Yen-Ta Chen1, 5 and
- Chawnshang Chang1, 4Email author
© Collins et al; licensee BioMed Central Ltd. 2011
Received: 27 June 2011
Accepted: 13 October 2011
Published: 13 October 2011
Successful reproductive efforts require the establishment of a situation favorable for reproduction that requires integration of both behavior and internal physiological events. TR4 nuclear receptor is known to be involved in male fertility via controlling spermatogenesis, yet its roles in regulating other biological events related to reproduction have not been completely revealed.
Male TR4 knockout (TR4-/-) and wild type mice were used for the sexual behavior and penile dysfunction studies. Mice were sacrificed for histological examination and corresponding genes profiles were analyzed by quantitative RT-PCR. Reporter gene assays were performed.
We describe an unexpected finding of priapism in TR4-/- mice. As a transcriptional factor, we demonstrated that TR4 transcriptionally modulates a key enzyme regulating penis erection and neuronal nitric oxide synthese NOS (nNOS). Thereby, elimination of TR4 results in nNOS reduction in both mRNA and protein levels, consequently may lead to erectile dysfunction. In addition, male TR4-/- mice display defects in sexual and social behavior, with increased fear or anxiety, as well as reduced mounting, intromission, and ejaculation. Reduction of ER alpha, ER beta, and oxytocin in the hypothalamus may contribute to defects in sexual behavior and stress response.
Together, these results provide in vivo evidence of important TR4 roles in penile physiology, as well as in male sexual behavior. In conjunction with previous finding, TR4 represents a key factor that controls male fertility via regulating behavior and internal physiological events.
KeywordsTR4 priapism sexual behavior
Members of the nuclear receptor superfamily are known to play important roles in differentiation, development, homeostasis, and metabolism, as well as in disease development and progression . As a member of the nuclear receptor superfamily with several known regulatory targets , Testicular nuclear receptor 4 (TR4) may affect many signaling pathways and thus have a major impact on physiological functions . Creation of mammalian gene knockout models has become a successful strategy to study the physiological roles of orphan receptors in vivo [3, 4]. As a nuclear receptor with wide tissue distribution [5–10] and a relative mystery with respect to physiological function, TR4 was a good candidate for analysis using a knockout mouse model. A complex set of phenotypic abnormalities were found to exist in the TR4 knockout (TR4-/-) mouse, including significant growth retardation , defects in female reproductive function  and maternal behavior , impaired cerebella function [13, 14], and reduced myelination . In addition, recent studies found that TR4 might be a master regulator controlling glucose and lipid metabolism [16–18] as well as foam cell formation .
Our previous study revealed that TR4 expresses predominantly in the testis in a stage-dependent manner and deletion of TR4 in male mice reduced sperm production and disrupted spermatogenesis, thereby impaired male fertility . Successful reproductive efforts require the establishment of a situation favorable for reproduction that requires integration of both behavior and internal physiological events. Following continue studies of TR4's roles in male reproduction, we describe an unexpected finding of priapism in some TR4-/- mice. Penile priapism, or persistent erection, is characterized by trapped blood within the corpus cavernosa, which leads to reduced tissue oxygenation, increased blood viscosity, disruption of tissue elasticity, fibrosis, and finally irreversible failure to achive erection [21, 22]. It is a significant urologic emergency that can lead to irreversible loss of erectile function if not promptly resolved. It has been linked to sickle cell disease as well as the use of vasoactive drugs, yet its molecular mechanism remains unclear. The etiology of this disorder is obscure. The cyclic nucleotide second messenger cGMP generated by activated guanylyl cyclase in penile smooth muscle cells regulates penile erection. These includes activation of guanylyl cyclase requires nitric oxide (NO), release in the penis upon sexual stimulation from neuronal and endothelial sources containing NO synthase (NOS), and respectively termed nNOS and eNOS. The crucial role of NOS in mediating erectile function was further supported by the finding that the mice lacking the gene for eNOS, nNOS, or both have a tendency for priapic activity . To our surprise, in the mice with TR4 knockout , penile priapism was one of the most striking phenotypes observed.
In characterizing the fertility defects, in addition to partial penile priapism among TR4-/- males, we also found that TR4-/- mice displayed delayed sexual maturity, reduced sperm production , priapism, and abnormal sexual behavior, all of which contribute to significant reduction of TR4-/- male fertility. Furthermore, the expressions of nNOS, enzyme involved in production of the erectile mediator NO, were reduced in penis tissue from TR4-knockout mice, where a transcriptional regulation of nNOS by TR4 was also presented.
Experimental animals and genotyping
TR4-/- mice were produced as described , and housed in the vivarium facility of the University of Rochester Medical Center. The animals were provided a standard diet with constant access to food and water, and exposed to a 12-hour light/dark cycle. All experimental protocols were approved by the University Committee on Animal Resources and the office of Environmental Health and Safety prior to implementation.
Genotyping was carried out as described . Briefly, genomic DNA was isolated from mouse tail samples and used as template for polymerase chain reaction (PCR). Primers for amplification of the wildtype and targeted alleles are TR4-107 (wildtype, forward): 5'-GGAGACACACTGCACATGTTCGAATAC-3', TR4-111 (wildtype, reverse): 5'-CACAGCTCATTTCTCTGCTCACTTACTC-3', Neo-3a (targeted allele, forward): 5'-GCAGCGCATCGCCTTCTATC-3', and TR4-34 (targeted allele, reverse): 5'-TGCAAGCATACTTCTTGTTCC-3'.
Tissue preparation, histology, and immunostaining
Mice were anesthetized with an overdose of pentobarbital and perfused through the left ventricle with 20 ml saline (pH 7.3), followed by 20 ml of 4% paraformaldehyde or 10% neutral buffered formalin. Tissues were removed and post-fixed by submersion in 10% formalin. Alternatively, fresh tissues were fixed by direct submersion, in 10% neutral buffered formalin, prior to processing. Tissues were processed for embedding in paraffin using an RHS Tissue Processing System (Hacker Instruments & Industries), or processed manually. Penis tissues from 4.5-7.5 mo. old mice were cut in 5 μm sections, deparaffinized, and stained with Accustain Masson Trichrome (Sigma-Aldrich) reagents, according to the manufacturer's instructions. Additionally, penis sections were stained using a polyclonal antibody against nNOS (BD Transduction Laboratories) at a 1:200 dilution, a polyclonal antibody against S100 (DAKO) at a 1:500 dilution, or a polyclonal antibody against TR4 (Santa Cruz) at a dilution of 1:200, followed by the appropriate biotinylated secondary antibody (1:1000 dilution), VECTASTAIN Elite ABC reagent (Vector Laboratories) and DAB substrate (Vector Laboratories). Hematoxylin was used as a nuclear counterstain following immunostaining.
RT-PCR and Western Blot analysis of gene expression
For RT-PCR analysis, total RNA was isolated from penis tissue using TRIzol® Reagent (Invitrogen). First strand cDNA synthesis was achieved using the Superscript™II RNase H Reverse Transcriptase kit (Invitrogen).
Real time quantitative RT-PCR was carried out, using SYBR Green PCR MasterMix, on the iCycler iQ™ PCR cycler and detection system (Bio-Rad Laboratories). Analysis of data obtained and calculation of relative gene expression was performed using the 2-ΔΔCT method . For each gene analyzed, the expression level of the TR4+/+ was set at 1 and relative expression in TR4-/- samples was calculated.
For Western blot analysis, total protein was isolated from penis tissue using TRIzol® Reagent (Invitrogen). A total of 75 μg protein per sample was separated on either a 6% or a 10% SDS-PAGE gel. After transfer to PVDF membrane, the blots were probed with either a polyclonal antibody against nNOS (BD Transduction Laboratories) at a 1:1000 dilution, or an antibody against β-actin, followed by the appropriate alkaline phosphatase-conjugated secondary antibody at a 1:2000 dilution. Signals were detected using a chromogenic substrate.
Transient transfection/reporter gene assay
The mouse nNOS promoter plasmid pEx2  was provided by Dr. Ted Dawson (Johns Hopkins University). CV1 cells were grown in Dulbecco's Modified Eagle's Medium (DMEM) with 10% fetal bovine serum, at 37°C, and with 5% CO2. Cells were cultured in 24-well plates (Corning) for 24 h. Transient transfection was carried out as described previously . The ß-gal activity in whole cell lysate was measured with a luminometer using a chemiluminescence-based detection system (Reaction Buffer, Galacto-Star, TROPIX) according to the manufacturer's protocol.
Electrophoretic mobility shift assay (EMSA)
TR4 proteins were synthesized in rabbit reticulocyte lysate (Promega), according to the manufacturer's instructions. The oligonucleotide probe (nNOS-NHR, bp -192 to -211, 5'-CTGGTCAACCTTGACTTCCT-3') was end-labeled with [γ]32P in a T4 polynucleotide kinase reaction (New England Biolabs). The EMSA assay was performed as described previously .
Continuous mating and male sexual behavior analysis
For the continuous mating study, 5 TR4+/+ and 5 TR4-/- males at 5 months of age were each paired with an adult TR4+/+ female for 4 months. The number of litters produced and the number of pups per litter were recorded. For male sexual behavior analysis, ovariectomized female mice of strain ICR, at 8-9 weeks of age, were injected with 12 μg estradiol valerate one day before mating and with 500 μg progesterone 4-7 hours before mating. At the time of mating, one primed female was paired with a 7-8 month-old TR4+/+ or TR4-/- mouse for either 90 min or 6 h, beginning between 18:00-22:00. Mating behavior was videotaped and subsequently scored for latency to, and number of mounts, intromissions, and ejaculations [27, 28]. After the 6 h taped mating trial, experimental pairs remained together until the following morning. Female mice were then visually examined for the presence of a vaginal plug.
For sexual behavior data, means and standard deviations for behavior latencies and mean number of behavior displays, with associated ranges, are shown. Extremely high variation in both latencies and numbers of specific behavioral displays was observed both within and between groups. Differences in numbers of mice of a particular genotype displaying a particular behavior were analyzed by one-tailed, independent sample t-tests, assuming equal variances, after coding for the presence (scored as 1) or absence (scored as 0) of the behavior.
TR4-/-males exhibit penile priapism
Regulation of nNOS transcriptional activation by TR4
Reduced fertility and aberrant sexual behavior in male TR4-/-mice
Fertility rates of 5 month old TR4+/+ and TR4-/- males
+/+ (N = 5)
-/- (N = 5)
Male sexual behavior
TRIAL, 90 min
TR4+/+(N = 9)
TR4-/- (N = 11)
TR4+/+(N = 4)
TR4-/- (N = 6)
No. of mice
Mean no. of mounts
Latency to behavior ++
21.5 ± 20.0
136.8 ± 111.1*
No. of mice
Mean no. of mounts
Latency to behavior ++
44.3 ± 32.1
89.8 ± 89.5
No. of mice
Latency to behavior ++
71.2 ± 58.6
Given extended pairing time, up to 6 h (Trial 2), TR4-/- mice did begin to display sexual behavior. Again, proportionally fewer TR4-/- than TR4+/+ mice displayed each defined sexual behavior, with increased latencies to each behavior in TR4-/- males (Table 2). After the videotaped 6 h pairing, male and female mice were left paired, and female mice were examined for vaginal plugs the following morning. Vaginal plugs were discovered in each of the female mice paired with TR4+/+ males, whereas none of the females paired with TR4-/- males had plugs (Table 2).
Expression levels of several genes known to affect behavior, particularly sexual behavior, were determined via semi-quantitative RT-PCR and quantitative real time PCR, using hypothalamic tissue from TR4-/- and TR4+/+ mice. RT-PCR results demonstrate that hypothalamic expression of estrogen receptor (ER) alpha (α), ER beta (β), and oxytocin is reduced in TR4-/- mice (Additional File 2 Figure S2). No differences in levels of the androgen receptor (AR) or vasopressin (VP) were apparent. After quantitative real time PCR analysis, it was clear that the levels of ERα, ERβ, and oxytocin were reduced by approximately 50-60% in the hypothalamic region of TR4-/- mice compared to TR4+/+ (Additional File 2 Figure S2). More detailed mechanism dissection as to how TR4 regulates the expression of those genes involved in the sexual behavior and consequently affects reproductive function need further investigation.
In addition to the inability of the TR4-/- mice with priapism to retract the penis back within the sheath (Figure 1A), histological evidence of priapism was observed (Figure 2). Trapped blood within the corpus cavernosum of the penis leads to pathology resulting from reduced tissue oxygenation and, without treatment, could result in permanent erectile dysfunction [21, 22, 32]. Compared with TR4+/+ mice, both TR4-/- animals with and without priapism have increased presence of red blood cells observed in penile cross section, with the largest amount present in sections of penis tissue from TR4-/- mice with priapism (Figure 2L). Histology also revealed more densely packed tissue in the TR4-/- mice with priapism, compared with TR4+/+ mice and TR4-/- mice without priapism, suggesting loss of elasticity of the normally flexible penile structures (e.g. corpus cavernosum, corpus spongiosum) (Figure 2C).
The neurotransmitter NO is a known mediator of erectile function that promotes vasodilation and the inflow of blood, resulting in penile tumescence . nNOS is an enzyme responsible for production of NO in the penis , and expression of both nNOS mRNA and protein were reduced in the TR4-/- mouse penis (Figure 3A and 3B). It was also found that TR4 is able to bind the NHR of exon 2 (Figure 3D), as well as upregulate nNOS gene expression in a reporter gene assay (Figure 3C). Immunostaining of penis tissue for TR4 and nNOS resulted in localization of both proteins in smooth muscle surrounding penile vasculature, further implicating TR4 in the regulation of vascular function via nNOS. As reported by Champion that NOS mutant mice: eNOS-/- and eNOS-/-, nNOS-/- displayed priapism where the expression of phosphodiesterase type 5 expression (PDE5A) was reduced, and transfection of eNOS normalized PDE5A expression and corrected priapic activity and there is less impact on loss of nNOS . Therefore, in addition to nNOS, we also checked the expression of eNOS and PED5A in TR4-/- penis, and found no significant difference from TR4+/+ (data not shown). These results suggest that erectile function may be negatively affected by the loss of TR4 function in TR4-/- mice, with or without priapism.
In addition to the possible loss of erectile function in TR4-/- mice via reduced nNOS expression and/or resulting from the development of priapism, as well as the detrimental effects of loss of TR4 on male germ cell development , abnormal sexual behavior is a significant contributor to the reduced fertility of male TR4-/- mice. For each sexual behavior demonstrated by TR4-/- mice, the latency to behavioral display was increased compared to TR4+/+ (Table 2). These results indicate that TR4-/- mice still retain sexual motivation, yet take longer to become acclimated to the pairing situation and begin showing sexual behaviors. Furthermore, TR4-/- males are less successful in achieving ejaculation. In fact, a large proportion of TR4-/- males tested for 6 h did not even achieve intromission. From observation of the TR4-/- mice during the mating trials, it became evident that they had difficulty in maintaining the appropriate mounting position through which intromission and ejaculation could be achieved.
TR4 ubiquitously expresses in almost all the tissues we examined, with high abundance in testis, and that leads to our first discovery of TR4 roles in male fertility via controlling spermatogenesis . This current study further extends TR4's role as a master regulator in male fertility via controlling both sexual/social behavior and internal physiological events. This discovery points to the development of novel strategies for male fertility therapy that specifically target TR4. We have identified that polyunsaturated fatty acid could potentiate TR4 activity to control the lipid metabolism , and it will be of great interest to identify TR4 upstream modulators that can promote TR4 activity in male reproductive organs.
Collectively, the data reported here describe severe reproductive malfunctions in TR4-/- male mice, with significant effects stemming from abnormal sexual behavior, as TR4-/- mice rarely achieve intromission or ejaculation. In TR4-/- mice with priapism, erectile function may eventually be lost due to loss of collagenous tissue elasticity in the corpora cavernosa and corpus spongiosum, and account for the lack of intromission or ejaculation. In TR4-/- males with or without priapism, disruptions of signaling pathways involving nNOS (Figures 3 and Additional File 1 Figure S1) may also affect erectile function. These in vivo data demonstrate that TR4 plays an important part in various aspects of reproduction and behavior, and future studies to more finely dissect the roles of TR4 in penile physiology and in brain/behavioral function may lead to identification of additional TR4 target genes, greater knowledge of the involvement of the receptor in various signaling pathways, or the discovery of new physiological ligand for TR4.
testicular orphan nuclear receptor 4
nitric oxide synthase
reverse transcription-polymerase chain reaction
nuclear hormone receptor response element.
This work was supported by NIH (DK063212 and CA127548). Taiwan Department of Health Clinical Trial and Research Center of Excellence grant DOH99-TD-B- 111-004 (China Medical University, Taichung, Taiwan).
- Evans RM: The steroid and thyroid hormone receptor superfamily. Science. 1988, 240: 889-895. 10.1126/science.3283939.View ArticlePubMedGoogle Scholar
- Lee YF, Lee HJ, Chang C: Recent advances in the TR2 and TR4 orphan receptors of the nuclear receptor superfamily. Journal of Steroid Biochemistry & Molecular Biology. 2002, 81: 291-308. 10.1016/S0960-0760(02)00118-8.View ArticleGoogle Scholar
- Qiu Y, Pereira FA, DeMayo FJ, Lydon JP, Tsai SY, Tsai MJ: Null mutation of mCOUP-TFI results in defects in morphogenesis of the glossopharyngeal ganglion, axonal projection, and arborization. Genes Dev. 1997, 11: 1925-1937. 10.1101/gad.11.15.1925.PubMed CentralView ArticlePubMedGoogle Scholar
- Shyr C, Collins LL, Mu X, Platt KA, Chang C: Spermatogenesis and testis development are normal in mice lacking testicular orphan nuclear receptor 2. Mol Cell Biol. 2002, 22: 4661-4666. 10.1128/MCB.22.13.4661-4666.2002.PubMed CentralView ArticlePubMedGoogle Scholar
- Young WJ, Smith SM, Chang C: Induction of the intronic enhancer of the human ciliary neurotrophic factor receptor (CNTFRa) gene by the TR4 orphan receptor. J Biol Chem. 1997, 272: 3109-3116. 10.1074/jbc.272.5.3109.View ArticlePubMedGoogle Scholar
- Young WJ, Lee YF, Smith SM, Chang C: A bidirectional regulation between the TR2/TR4 orphan receptors (TR2/TR4) and the ciliary neurotrophic factor (CNTF) signaling pathway. J Biol Chem. 1998, 273: 20877-20885. 10.1074/jbc.273.33.20877.View ArticlePubMedGoogle Scholar
- Lee YF, Young WJ, Burbach JPH, Chang C: Negative feedback control of the retinoid-retinoic acid/retinoid × receptor pathway by the human TR4 orphan receptor, a member of the steroid receptor superfamily. J Biol Chem. 1998, 273: 13437-13443. 10.1074/jbc.273.22.13437.View ArticlePubMedGoogle Scholar
- Lee CH, Chinpaisal C, Wei LN: A novel nuclear receptor heterodimerization pathway mediated by orphan receptors TR2 and TR4. Journal of Biological Chemistry. 1998, 273: 25209-25215. 10.1074/jbc.273.39.25209.View ArticlePubMedGoogle Scholar
- Chang C, Da Silva SL, Ideta R, Lee YF, Yeh S, Burbach JPH: Human and rat TR4 orphan receptors specify a subclass of the steroid receptor superfamily. Proceedings of the National Academy of Sciences USA. 1994, 1994: 6040-6044.View ArticleGoogle Scholar
- van Schaick HSA, Rosmalen JGM, Lopes da Silva S, Chang C, Burbach JPH: Expression of the orphan receptor TR4 during brain development of the rat. Molecular Brain Research. 2000, 77: 104-110. 10.1016/S0169-328X(00)00046-2.View ArticlePubMedGoogle Scholar
- Collins LL, Lee YF, Heinlein CA, Liu NC, Chen YT, Shyr CR, Meshul CK, Uno H, Platt KA, Chang C: Growth retardation and abnormal maternal behavior in mice lacking testicular orphan nuclear receptor 4. Proc Natl Acad Sci USA. 2004, 101: 15058-15063. 10.1073/pnas.0405700101.PubMed CentralView ArticlePubMedGoogle Scholar
- Chen LM, Wang RS, Lee YF, Liu NC, Chang YJ, Wu CC, Xie S, Hung YC, Chang C: Subfertility with defective folliculogenesis in female mice lacking testicular orphan nuclear receptor 4. Mol Endocrinol. 2008, 22: 858-867. 10.1210/me.2007-0181.PubMed CentralView ArticlePubMedGoogle Scholar
- Chen YT, Collins LL, Uno H, Chang C: Deficits in motor coordination with aberrant cerebellar development in mice lacking testicular orphan nuclear receptor 4. Molecular & Cellular Biology. 2005, 25: 2722-2732. 10.1128/MCB.25.7.2722-2732.2005.View ArticleGoogle Scholar
- Chen YT, Collins LL, Chang SS, Chang C: The roles of testicular orphan nuclear receptor 4 (TR4) in cerebellar development. Cerebellum. 2008, 7: 9-17. 10.1007/s12311-008-0006-3.View ArticlePubMedGoogle Scholar
- Zhang Y, Chen YT, Xie S, Wang L, Lee YF, Chang SS, Chang C: Loss of testicular orphan receptor 4 impairs normal myelination in mouse forebrain. Mol Endocrinol. 2007, 21: 908-920.View ArticlePubMedGoogle Scholar
- Liu NC, Lin WJ, Kim E, Collins LL, Lin HY, Yu IC, Sparks JD, Chen LM, Lee YF, Chang C: Loss of TR4 orphan nuclear receptor reduces phosphoenolpyruvate carboxykinase-mediated gluconeogenesis. Diabetes. 2007, 56: 2901-2909. 10.2337/db07-0359.View ArticlePubMedGoogle Scholar
- Kim E, Liu NC, Yu IC, Lin HY, Lee YF, Sparks JD, Chen LM, Chang C: Metformin inhibits nuclear receptor TR4-mediated hepatic stearoyl-CoA desaturase 1 gene expression with altered insulin sensitivity. Diabetes. 60: 1493-1503.
- Kang HS, Okamoto K, Kim YS, Takeda Y, Bortner CD, Dang H, Wada T, Xie W, Yang XP, Liao G, Jetten AM: Nuclear orphan receptor TAK1/TR4-deficient mice are protected against obesity-linked inflammation, hepatic steatosis, and insulin resistance. Diabetes. 60: 177-188.
- Xie S, Lee YF, Kim E, Chen LM, Ni J, Fang LY, Liu S, Lin SJ, Abe J, Berk B, et al: TR4 nuclear receptor functions as a fatty acid sensor to modulate CD36 expression and foam cell formation. Proceedings of the National Academy of Sciences of the United States of America. 2009, 106: 13353-13358. 10.1073/pnas.0905724106.PubMed CentralView ArticlePubMedGoogle Scholar
- Mu X, Lee Y, Liu N, Chen Y, Kim E, Shyr C, Chang C: Targeted inactivation of Testicular nuclear Orphan Receptor 4 delays and disrupts late meiotic prophase and subsequent meiotic divisions of spermatogenesis. Mol Cell Biochem. 2004, 24: 5887-5899. 10.1128/MCB.24.13.5887-5899.2004.View ArticleGoogle Scholar
- Hauri D, Spycher M, Bruhlmann W: Erection and priapism: A new physiopathological concept. Urol Int. 1983, 88: 138-145.View ArticleGoogle Scholar
- Panteleo-Gandais M, Chalbaud R, Chacon O, Plaza N: Priapism: evaluation and treatment. Urology. 1984, 24: 345-346. 10.1016/0090-4295(84)90207-3.View ArticleGoogle Scholar
- Champion HC, Bivalacqua TJ, Takimoto E, Kass DA, Burnett AL: Phosphodiesterase-5A dysregulation in penile erectile tissue is a mechanism of priapism. Proceedings of the National Academy of Sciences of the United States of America. 2005, 102: 1661-1666. 10.1073/pnas.0407183102.PubMed CentralView ArticlePubMedGoogle Scholar
- Livak KJ, Schmittgen TD: Analysis of relative gene expression data using real-time quantitative PCR and the 2-DDCT method. Methods. 2001, 25: 402-408. 10.1006/meth.2001.1262.View ArticlePubMedGoogle Scholar
- Wei X, Sasaki M, Huang H, Dawson VL, Dawson TM: The orphan nuclear receptor, Steroidogenic Factor 1, regulates neuronal Nitric Oxide Synthase gene expression in pituitary gonadotropes. Mol Endocrinol. 2002, 16: 2828-2839. 10.1210/me.2001-0273.View ArticlePubMedGoogle Scholar
- Lee YF, Pan HJ, Burbach PH, Morkin E, Chang C: Identification of direct repeat 4 as a positive regulatory element for the human TR4 orphan receptor: a modulator for the thyroid hormone target genes. J Biol Chem. 1997, 272: 12215-12220. 10.1074/jbc.272.18.12215.View ArticlePubMedGoogle Scholar
- Kriegsfeld LJ, Demas GE, Huang PL, Burnett AL, Nelson RJ: Ejaculatory abnormalities in mice lacking the gene for endothelial nitric oxide synthase (eNOS-/-). Physiol Behav. 1999, 67: 561-566. 10.1016/S0031-9384(99)00100-6.View ArticlePubMedGoogle Scholar
- Ogawa S, Chan J, Chester AE, Gustafsson J, Korach KS, Pfaff DW: Survival of reproductive behaviors in estrogen receptor b gene-deficient (bERKO) male and female mice. Proc Natl Acad Sci USA. 1999, 96: 12887-12892. 10.1073/pnas.96.22.12887.PubMed CentralView ArticlePubMedGoogle Scholar
- Segal SS, Brett SE, Sessa WC: Codistribution of NOS and caveolin throughout peripheral vasculature and skeletal muscle of hamsters. Am J Physiol. 1999, 277: H1167-H1177.PubMedGoogle Scholar
- Rampin O, Guiliano F: Brain control of penile erection. World J Urol. 2001, 19: 1-8. 10.1007/PL00007089.View ArticlePubMedGoogle Scholar
- Steers WD: Neural pathways and central sites involved in penile erection: neuroanatomy and clinical implications. Neurosci Biobehav Rev. 2000, 24: 507-516. 10.1016/S0149-7634(00)00019-1.View ArticlePubMedGoogle Scholar
- Winter CC: Priapism. Urological Surgery. 1978, 28: 163-Google Scholar
- Burnett AL: Nitric oxide in the penis: physiology and pathology. J Urol. 1995, 157: 320-324.View ArticleGoogle Scholar
- Yun HY, Dawson VL, Dawson TM: Neurobiology of nitric oxide. Crit Rev Neurobiol. 1996, 10: 291-316.View ArticlePubMedGoogle Scholar
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