Tissue-specific Knockout Mice Research Articles

Hyperleptinemia without Obesity in Male Mice Lacking Androgen Receptor in Adipose Tissue

Insulin resistance occurs through an inadequate response to insulin by insulin target organs such as liver, muscle, and adipose tissue with consequent insufficient glucose uptake. In previous studies we demonstrated that whole body androgen receptor (AR) knockout (AR(-/y)) mice develop obesity and exhibit insulin and leptin resistance at advanced age. By examining adipose tissue-specific AR knockout (A-AR(-/y)) mice, we found A-AR(-/y) mice were hyperleptinemic but showed no leptin resistance, although body weight and adiposity index of A-AR(-/y) mice were identical with those of male wild-type control mice. Hypotriglyceridemia and hypocholesterolemia found in nonobese A-AR(-/y) mice suggested a beneficial effect of high leptin levels independent of fat deposition. Further examination showed that androgen-AR signaling in adipose tissue plays a direct regulatory role in leptin expression via enhanced estrogen receptor transactivation activity due to elevated intraadipose estrogens. The present study in A-AR(-/y) mice suggests a differential tissue-specific role of AR in energy balance control in males.

Infertility with defective spermatogenesis and steroidogenesis in male mice lacking androgen receptor in Leydig cells

Androgen and the androgen receptor (AR) have been shown to play critical roles in male fertility. Our previous data demonstrated that mice lacking AR (AR(-/y)) revealed incomplete germ cell development and lowered serum testosterone levels, which resulted in azoospermia and infertility. However, the consequences of AR loss in Leydig cells remain largely unknown. Using a Cre-LoxP conditional knockout strategy, we generated a tissue-specific knockout mouse (L-AR(-/y)) with the AR gene deleted by the anti-Müllerian hormone receptor-2 (Amhr2) promoter driven Cre expressed in Leydig cells. Phenotype analyses show that the outside appearance of L-AR(-/y) mice was indistinguishable from wild type mice (AR(+/y)), but with atrophied testes and epididymis. L-AR(-/y) mice were infertile, with spermatogenic arrest predominately at the round spermatid stage and no sperm could be detected in the epididymis. L-AR(-/y) mice also have lower serum testosterone concentrations and higher serum leuteinizing hormone and follicle-stimulating hormone concentrations than AR(+/y) mice. Further mechanistic studies demonstrated that hypotestosteronemia in L-AR(-/y) mice is not caused by reducing numbers of Leydig cells, but instead by the alterations of several key steroidogenic enzymes, including 17beta-HSD3, 3beta-HSD6, and P450c17. Together, L-AR(-/y) mice provide in vivo evidence that functional AR in Leydig cells is essential to maintain normal spermatogenesis, testosterone production, and required for normal male fertility.

Toxic Mother's Milk

Wan et al . created tissue-specific knockout mice deficient in peroxisome proliferator-activated receptor γ (PPARγ), which is a transcription factor regulated by fatty acids, some prostaglandins, and components of oxidized low-density lipoprotein particles and which regulates lipid biosynthesis. The mice were deficient in PPARγ in the hematopoietic and endothelial tissues. Pups nursed from mothers deficient in PPARγ exhibited growth retardation and hair loss that was completely reversed after the pups were weaned. Furthermore, pups deficient in PPARγ fostered by a wild-type mother did not exhibit these abnormalities, and wild-type pups fostered with the PPARγ-deficient mothers exhibited hair loss and low weight until weaning, confirming that the problem was maternal. Investigation of the skin of the pups nursed by the PPARγ-deficient mothers showed the formation of follicular cysts, which appeared to result from an inflammatory response that was characterized by leukocyte infiltration, elevated expression, and abundance of inflammatory cytokines and cyclooxygenase (COX-1 and COX-2). Hair loss was partially prevented if the pups were treated with aspirin, a COX inhibitor. Inflammation was also noted in the livers of the pups nursed by the PPARγ-deficient mothers. Histological analysis of the mammary tissue of the PPARγ-deficient mice showed increased lipid accumulation. Transcription analysis revealed that the PPARγ-deficient mammary glands exhibited decreased expression of homeobox genes and matrix metalloproteinases involved in mammary gland development. The expression of genes encoding two lipid oxidation enzymes that are involved in the production of inflammatory lipids was elevated in the mutant mice. When a mouse macrophage cell line was exposed to milk from the PPARγ-deficient mice in the presence of lipoprotein lipase, which is involved in lipid digestion, the cells increased production of inflammatory mediators. Furthermore, the skin from the mouse pups fed by the PPARγ-deficient mice exhibited an altered lipid profile compared with pups fed by wild-type mice, and mass analysis suggested that one group of lipids present in the skin of pups fed by the PPARγ-deficient mice was oxidized free fatty acids. Investigation of the gene expression of the macrophages from the mutant mice, which do not express PPARγ in the macrophages but do in the epithelial cells and adipocytes in the mammary gland, revealed that the macrophages had elevated expression of genes encoding enzymes that produce PPAΡγ ligands. The authors propose that these ligands activated PPARγ in the epithelial and adipocyte tissue of the mammary gland, leading to excessive production of inflammatory lipids. Y. Wan, A. Saghatelian, L.-W. Chong, C.-L. Zhang, B. F. Cravatt, R. M. Evans, Maternal PPARγ protects nursing neonates by suppressing the production of inflammatory milk. Genes Dev. 21 , 1895-1908 (2007). [Abstract] [Full Text]

Rolle von Muskulatur und Fettgewebe in der Pathogenese des Typ-2-Diabetes

In the last years type 2 diabetes has reached almost epidemic proportions. More than 170 million individuals are affected worldwide, about 6 million in Germany. In the pathogenesis of type 2 diabetes, insulin resistance in liver, fat and muscle as well as the inability of the pancreatic beta-cell to fully compensate for this insulin resistance are the central pathophysiological events. Both genetic and environmental factors, such as lack of physical exercise and hypercaloric nutrition play a major role in this process, although the precise mechanisms for type 2 diabetes development remain largely unknown. In the characterization of the role of liver, adipose tissue and skeletal muscle in the pathogenesis of type 2 diabetes, tissue specific knockout mouse models have challenged our concepts of glucose homeostasis.

Respiratory chain dysfunction in skeletal muscle does not cause insulin resistance

Insulin resistance in skeletal muscle is a characteristic feature of diabetes mellitus type 2 (DM2). Several lines of circumstantial evidence suggest that reduced mitochondrial oxidative phosphorylation capacity in skeletal muscle is a primary defect causing insulin resistance and subsequent development of DM2. We have now experimentally tested this hypothesis by characterizing glucose homeostasis in tissue-specific knockout mice with progressive respiratory chain dysfunction selectively in skeletal muscle. Surprisingly, these knockout mice are not diabetic and have an increased peripheral glucose disposal when subjected to a glucose tolerance test. Studies of isolated skeletal muscle from knockout animals show an increased basal glucose uptake and a normal increase of glucose uptake in response to insulin. In summary, our findings indicate that mitochondrial dysfunction in skeletal muscle is not a primary etiological event in DM2.

Cutting Edge: The AP-1 Subunit JunB Determines NK Cell-Mediated Target Cell Killing by Regulation of the NKG2D-Ligand RAE-1ε

The activating receptor NKG2D and its ligands RAE-1 play an important role in the NK, gammadelta+, and CD8+ T cell-mediated immune response to tumors. Expression levels of RAE-1 on target cells have to be tightly controlled to allow immune cell activation against tumors but to avoid destruction of healthy tissues. In this study, we report that cell surface expression of RAE-1epsilon is greatly enhanced on cells lacking JunB, a subunit of the transcription complex AP-1. Furthermore, tissue-specific junB knockout mice respond to 12-O-tetradecanoyl-phorbol-13-acetate, a potent AP-1 activator, with markedly increased and sustained epidermal RAE-1epsilon expression. Accordingly, junB-deficient cells are efficiently killed via NKG2D by NK cells and induce IFN-gamma production. Our data indicate that the transcription factor AP-1, which is involved in tumorigenesis and cellular stress responses, regulates RAE-1epsilon. Thus, up-regulated RAE-1epsilon expression due to low levels of JunB could alert immune cells to tumors and stressed cells.

Animal models of diabetes mellitus

Animal models have been used extensively in diabetes research. Early studies used pancreatectomised dogs to confirm the central role of the pancreas in glucose homeostasis, culminating in the discovery and purification of insulin. Today, animal experimentation is contentious and subject to legal and ethical restrictions that vary throughout the world. Most experiments are carried out on rodents, although some studies are still performed on larger animals. Several toxins, including streptozotocin and alloxan, induce hyperglycaemia in rats and mice. Selective inbreeding has produced several strains of animal that are considered reasonable models of Type 1 diabetes, Type 2 diabetes and related phenotypes such as obesity and insulin resistance. Apart from their use in studying the pathogenesis of the disease and its complications, all new treatments for diabetes, including islet cell transplantation and preventative strategies, are initially investigated in animals. In recent years, molecular biological techniques have produced a large number of new animal models for the study of diabetes, including knock-in, generalized knock-out and tissue-specific knockout mice.

PPARγ-mediated insulin sensitization: the importance of fat versus muscle

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a nuclear hormone receptor that functions as a transcriptional regulator in a variety of tissues. PPARgamma activation, e.g., through binding of the synthetic glitazones or thiazolidinediones (TZD), results in a marked improvement in type 2 diabetic patients of insulin and glucose parameters resulting from an improvement of whole body insulin sensitivity. The role of different metabolic tissues (fat, skeletal muscle, liver) in mediating PPARgamma function in glucose and insulin homeostasis is still unclear. Recently, the function of PPARgamma in adipose tissue and skeletal muscle has been intensively characterized by using targeted deletion of PPARgamma in those tissues. In those studies, adipose PPARgamma has been identified as an essential mediator for the maintainance of whole body insulin sensitivity. Two major mechanisms have been described. 1) Adipose PPARgamma protects nonadipose tissue against excessive lipid overload and maintains normal organ function (liver, skeletal muscle); and 2) adipose PPARgamma guarantees a balanced and adequate production of secretion from adipose tissue of adipocytokines such as adiponectin and leptin, which are important mediators of insulin action in peripheral tissues. In contrast to studies in adipose-specific PPARgamma-deficient mice, the data in muscle-specific PPARgamma(-/-) mice demonstrate that whole body insulin sensitivity is, at least in part, relying on an intact PPARgamma system in skeletal muscle. Finally, these early and elegant studies using tissue-specific PPARgamma knockout mouse models pinpoint adipose tissue as the major target of TZD-mediated improvement of hyperlipidemia and insulin sensitization.

Role of the BMK1/ERK5 signaling pathway: lessons from knockout mice

Mitogen-activated protein (MAP) kinase cascades play a central role in mediating extracellular stimuli-induced intracellular signaling during cell activation. The fourth and least studied mammalian MAP kinase pathway, big MAP kinase 1 (BMK1), also known as extracellular signal regulated kinase 5 (ERK5), is activated in response to growth factors and stress. Activation of this signaling pathway has been implicated not only in physiological functions such as cell survival, proliferation and differentiation but also in pathological processes such as carcinogenesis, cardiac hypertrophy and atherosclerosis. In recent years a series of gene-targeted mice lacking components within the BMK1 cascade have been generated, which have enabled us to investigate the role of the BMK1 pathway within different tissues. Analyses of these knockout mice have led to major discoveries in the role of BMK1 signaling in angiogenesis and in cardiac development. Moreover, studies using conditional BMK1 knockout mice, which circumvent the early embryonic lethality of BMK1 knockouts, have unveiled the importance of BMK1 in endothelial survival and maintenance of vascular integrity during adulthood. Here we summarize current understanding of the function of BMK1, as well as include new data generated from a series of tissue-specific BMK1 knockout mice in an attempt to dissect the role of the BMK1 pathway in various cell types in animals.

Overexpression of PACAP in the pancreas failed to rescue early postnatal mortality in PACAP-null mice

PACAP exerts multiple activities as a hormone and neurotransmitter, and has been proposed to play vital roles in a variety of neuronal functions. PACAP is also involved in insulin secretion from pancreatic β-cells. Recently, we and other groups demonstrated that PACAP-deficient mice (PACAP(−/−)) are viable, but suffer from increased postnatal mortality. To ascertain whether this high mortality is rescued by overexpression of PACAP in peripheral tissue (such as pancreas), we performed a genetic cross between PACAP(−/−) and our recently developed transgenic mice overexpressing PACAP in pancreatic β-cells; and then examined the survival rate of their F2 progeny. PACAP(−/−) mice were segregated into two groups based on mortality as well as body weight gain: PACAP(−/−) that survived >20 days of age with normal weight gain and PACAP(−/−) that died before 20 days with a marked weight loss. Kaplan–Meier survival analysis demonstrated that PACAP(−/−) mice and those carrying the PACAP transgene have similarly lower survival probability compared with their heterozygous littermates that served as positive controls. Further study using additional tissue-specific transgenic or knockout mouse models will be required to determine the causative defects underlying the high mortality of PACAP(−/−) mice.

Methods in Nutrition Science: Cre/loxP System for Generating Tissue-specific Knockout Mouse Models

Editor's note: From time to time, we take the opportunity in Nutrition Reviews to highlight a particularly exciting application of sophisticated methodological advances that are relevant to the nutrition research community. In the current issue of Nutrition Reviews , Dr. Claudine Kos has provide a brief review of some of the salient features of the Cre/loxP system for generating tissue-specific knockout mouse models. Hopefully, this review will provide additional background to Dr. George Wolf's Brief Critical Review (page 253) of the use of the Cre/loxP technique by investigators to gain further insight into the function of the peroxysome proliferators-activated receptor-gamma (PPAR-γ), as well as promote its further use within experimental nutrition. Alteration of the mouse genome by conventional transgenic and gene-targeted approaches has greatly facilitated studies of gene function. However, a gene alteration expressed in the germ line may cause an embryonic lethal phenotype resulting in no viable mouse to study gene function. Similarly, a gene alteration may exert its effect in multiple different cell and tissue types, creating a complex phenotype in which it is difficult to distinguish direct function in a particular tissue from secondary effects resulting from altered gene function in other tissues. Therefore, methods have been developed to control conditions such as the timing, cell-type, and tissue specificity of gene activation or repression. This brief review provides an overview of the Cre/LoxP system for generating tissue-specific knockout mouse models .

Cre/loxP system for generating tissue-specific knockout mouse models.

Alteration of the mouse genome by conventional transgenic and gene-targeted approaches has greatly facilitated studies of gene function. However, a gene alteration expressed in the germ line may cause an embryonic lethal phenotype resulting in no viable mouse to study gene function. Similarly, a gene alteration may exert its effect in multiple different cell and tissue types, creating a complex phenotype in which it is difficult to distinguish direct function in a particular tissue from secondary effects resulting from altered gene function in other tissues. Therefore, methods have been developed to control conditions such as the timing, cell-type, and tissue specificity of gene activation or repression. This brief review provides an overview of the Cre/LoxP system for generating tissue-specific knockout mouse models.

Infertility with defective spermatogenesis and hypotestosteronemia in male mice lacking the androgen receptor in Sertoli cells.

Androgens and the androgen receptor (AR) play important roles in male fertility, although the detailed mechanisms, particularly how androgen/AR influences spermatogenesis in particular cell types, remain unclear. Using a Cre-Lox conditional knockout strategy, we generated a tissue-specific knockout mouse with the AR gene deleted only in Sertoli cells (S-AR(-/y)). Phenotype analyses show the S-AR(-/y) mice were indistinguishable from WT AR mice (B6 AR(+/y)) with the exception of testes, which were significantly atrophied. S-AR(-/y) mice were infertile, with spermatogenic arrest predominately at the diplotene premeiotic stage and almost no sperm detected in the epididymides. S-AR(-/y) mice also have lower serum testosterone concentrations and higher serum leuteinizing hormone concentrations than B6 AR(+/y) mice. Further mechanistic studies demonstrated that S-AR(-/y) mice have defects in the expression of anti-Müllerian hormone, androgen-binding protein, cyclin A1, and sperm-1, which play important roles in the control of spermatogenesis and/or steroidogenesis. Together, our Sertoli cell-specific AR knockout mice provide in vivo evidence of the need for functional AR in Sertoli cells to maintain normal spermatogenesis and testosterone production, and ensure normal male fertility.

Getting Specific with RNAi-Transgenic Mice

RNA interference, mediated by 21- to 22-nucleotide double-stranded (ds)-RNA (called small interfering RNA, or siRNA), has proven effective in determining gene function in cultured mammalian cells and in mice by targeting a specific mRNA and reducing or abolishing its translation into protein. Shinagawa and Ishii have now modified this method to generate the equivalent of a tissue-specific knockout mouse model. Longer (about 500 nucleotides) ds-RNA was expressed from a vector using an RNA polymerase II-dependent promoter that is active in only a subset of mouse tissues. In addition, the long ds-RNA generated lacked the 7-methylguanosine cap structure at its 5′ end and poly(A) tail from its 3′ end, thereby blocking its export from the nucleus to the cytosol. By remaining in the nucleus, the long ds-RNA presumably does not trigger an antiviral interferon response that mammalian cells can elicit to this type of RNA, thus causing a general block in translation. Restricted localization to the nucleus would allow ribonucleases, such as dicer, to cleave the long ds-RNA into siRNA, which can then move into the cytosol to degrade target mRNA. An expression vector for long ds-RNA that targets mRNA encoding Ski, a protein that associates with histone deacetylases and operates in transcriptional repression, was injected into fertilized mouse oocytes. siRNA of 21 to 22 nucleotides derived from the long ds-RNA was detected in specific mouse embryonic tissue and corresponded to decreased levels of endogenous Ski mRNA. Developing embryos exhibited many of the phenotypes observed in Ski -deficient mice generated by conventional transgenic techniques. The use of expression vectors with various tissue-specific promoters could allow for more efficient generation of tissue-specific knockout mice. T. Shinagawa, S. Ishii, Generation of Ski -knockdown mice by expressing a long double-stranded RNA from an RNA polymerase II promoter. Genes Dev . 17 , 1340-1345 (2003). [Abstract] [Full Text]

The parathyroid hormone-related protein system: more data but more unsolved questions.

The present review focuses on recent studies that might be considered as the most relevant advances in the parathyroid hormone-related protein field, with special emphasis on proven functions in renovascular and cardiovascular systems, in physiological as well as pathological conditions. Thus, the questions as to whether and how parathyroid hormone-related protein intervenes in vascular development and homeostasis and in vascular diseases such as hypertension, atherosclerosis, restenosis and heart failure have begun to be unraveled. Since its discovery from hypercalcemia-associated tumors in 1987, it has become clear that parathyroid hormone-related protein is a ubiquitously expressed poly-hormone and plays crucial roles in normal life. The early lethality to parathyroid hormone-related protein knockout mice emphasizes the crucial roles of the protein in development but has limited the use of these models. However, data accumulated from transgenic animals overexpressing the protein in particular cells have provided considerable support to its physiological and pathological relevance. The recent demonstration that nascent parathyroid hormone-related protein not only follows the secretory pathways, but also directly translocates to the nucleus, is beginning to uncover new actions for the protein in a number of physiological systems such as bone, mammary gland and vascular smooth muscle, as well as in pathological situations, such as cancer, osteoporosis, sepsis, atherosclerosis and hypertension. The development of mice with conditionally deleted parathyroid hormone-related protein or parathyroid hormone-1 receptor alleles will allow the creation of cell- or tissue-specific parathyroid hormone-related protein knockout mice which will greatly facilitate the determination of the biological relevance of this protein in a specific cell or tissue type, particularly in the cardiovascular system.

Model mice for tissue-specific deletion of the manganese superoxide dismutase (MnSOD) gene

Manganese superoxide dismutase (MnSOD) is the enzyme that converts toxic O 2 − to H 2O 2 in mitochondria. Previous reports showed that a deficiency of MnSOD in mice was neonatal lethal. Therefore, a model mouse was not available for the analysis of the pathological role of O 2 − injuries in adult tissues. To explore an adult-type model mouse, we designed tissue-specific MnSOD conditional knockout mice using a Cre–loxp system. First, we crossbred MnSOD flox mice with transgenic mice expressing Cre recombinase under the control of the chicken actin promoter (CAG). We confirmed that CAG MnSOD knockout mice were completely deficient in MnSOD and died as neonates, validating the use of the Cre–loxp system. Next, we generated liver-specific MnSOD-deficient mice by crossbreeding with Alb–Cre transgenic mice. MnSOD activity and protein were both significantly downregulated in the liver of liver-specific MnSOD knockout mice. However, no obvious morphological abnormality was observed in the liver when biochemical alterations such as lipid peroxidation were not detectable, suggesting a redundant or less important physiological role for MnSOD in the liver than previously thought. In the present study, we successfully generated tissue-specific MnSOD conditional knockout mice that would provide a useful tool for the analysis of various age-associated diseases such as diabetes mellitus, Parkinson's disease, stroke, and heart disease, when crossbred with tissue-specific transgenic Cre mice.

Angiotensin mutant mice: A focus on the brain renin-angiotensin system

With advances in genetic manipulation and molecular biological and physiological techniques, the mouse has become the animal model of choice for studying the genetic basis of human diseases. The two most commonly used methods for analyzing the function of a gene in vivo, overexpression (transgenic mouse) and deletion (knockout mouse), have been extremely useful in establishing the importance of genes in genetic disorders. The renin-angiotensin system (RAS) is one of the most widely studied systems controlling blood pressure. Although the primary site of Ang-II production is the plasma, all the components of the RAS cascade are expressed in many tissues, including the brain. This review briefly summarizes systemic and tissue-specific transgenic and knockout mouse models of the RAS for determining the role of this system in the regulation of blood pressure and in the pathogenesis of hypertension, with a focus on the RAS in the brain.

Impaired insulin secretion and β-cell loss in tissue-specific knockout mice with mitochondrial diabetes

Mitochondrial dysfunction is an important contributor to human pathology and it is estimated that mutations of mitochondrial DNA (mtDNA) cause approximately 0.5-1% of all types of diabetes mellitus. We have generated a mouse model for mitochondrial diabetes by tissue-specific disruption of the nuclear gene encoding mitochondrial transcription factor A (Tfam, previously mtTFA; ref. 7) in pancreatic beta-cells. This transcriptional activator is imported to mitochondria, where it is essential for mtDNA expression and maintenance. The Tfam-mutant mice developed diabetes from the age of approximately 5 weeks and displayed severe mtDNA depletion, deficient oxidative phosphorylation and abnormal appearing mitochondria in islets at the ages of 7-9 weeks. We performed physiological studies of beta-cell stimulus-secretion coupling in islets isolated from 7-9-week-old mutant mice and found reduced hyperpolarization of the mitochondrial membrane potential, impaired Ca(2+)-signalling and lowered insulin release in response to glucose stimulation. We observed reduced beta-cell mass in older mutants. Our findings identify two phases in the pathogenesis of mitochondrial diabetes; mutant beta-cells initially display reduced stimulus-secretion coupling, later followed by beta-cell loss. This animal model reproduces the beta-cell pathology of human mitochondrial diabetes and provides genetic evidence for a critical role of the respiratory chain in insulin secretion.

Genetic modification of survival in tissue-specific knockout mice with mitochondrial cardiomyopathy.

We recently described a mouse model that reproduces important pathophysiological features of mitochondrial DNA (mtDNA) mutation diseases. The gene for mouse mitochondrial transcription factor A, Tfam (also called mtTFA), a nucleus-encoded key regulator of mtDNA expression, was targeted with loxP sites (Tfam(loxP)) and disrupted in vivo by transgenic expression of cre-recombinase from the muscle creatinine kinase (Ckmm) promoter. This promoter is active from embryonic day 13, and the knockouts had normal respiratory chain function in the heart at birth and developed mitochondrial cardiomyopathy postnatally. In this paper, we describe a heart-knockout strain obtained by mating Tfam(loxP) mice to animals expressing cre-recombinase from the alpha-myosin heavy chain (Myhca) promoter. This promoter is active from embryonic day 8, and the knockouts had onset of mitochondrial cardiomyopathy during embryogenesis. The age of onset of cardiac respiratory chain dysfunction can thus be controlled by temporal regulation of cre-recombinase expression. Further characterization demonstrated that approximately 75% of the knockouts died in the neonatal period, whereas, surprisingly, approximately 25% survived for several months before dying from dilated cardiomyopathy with atrioventricular heart conduction blocks. Modifying gene(s) affect the life span of the knockouts, because approximately 95% of the knockout offspring from an intercross of the longer-living knockouts survived the neonatal period. Thus, the tissue-specific knockouts we describe here not only reproduce important pathophysiological features of mitochondrial cardiomyopathy but also provide a powerful system by which to identify modifying genes of potential therapeutic value.

Altered Emotional States in Knockout Mice Lacking 5-HT1A or 5-HT1B Receptors

Dysfunctions of the serotonergic system have been implicated in a number of psychiatric disorders including depression, anxiety and disorders of impulse control. To model these disorders we have generated mice with altered serotonergic systems. Specifically, we have created mice that lack or express reduced levels of two serotonin receptors: 5-HT1A and 5-HT1B receptors. These receptors are localized both on serotonergic neurons where they act as autoreceptors and on non-serotonergic neurons. As a result, the 5-HT1A and 5-HT1B receptors control the tone of the serotonergic system and mediate some of the postsynaptic effects of serotonin. Agonists of these receptors are currently used in the treatment of migraine and anxiety disorders. Mice lacking these receptors develop, feed, and breed normally and do not display any obvious abnormalities. However, when analyzed in a number of behavioral paradigms, the 5-HT1A and 5-HT1B knockout mice display a number of contrasting phenotypes. While the 5-HT1B knockout mice are more aggressive, more reactive, and less anxious than the wild-types, the 5-HT1A knockouts are less reactive, more anxious, and possibly less aggressive than the wild-types. We are currently investigating with tissue-specific knockout mice which neural circuits are responsible for these phenotypes.