GABA(A) receptor changes in delta subunit-deficient mice: altered expression of alpha4 and gamma2 subunits in the forebrain

The delta subunit is a novel subunit of the pentameric gamma-aminobutyric acid (GABA)(A) receptor that conveys special pharmacological and functional properties to recombinant receptors and may be particularly important in mediating tonic inhibition. Mice that lack the delta subunit have been produced by gene-targeting technology, and these mice were studied with immunohistochemical and immunoblot methods to determine whether changes in GABA(A) receptors were limited to deletion of the delta subunit or whether alterations in other GABA(A) receptor subunits were also present in the delta subunit knockout (delta-/-) mice. Immunohistochemical studies of wild-type mice confirmed the restricted distribution of the delta subunit in the forebrain. Regions with moderate to high levels of delta subunit expression included thalamic relay nuclei, caudate-putamen, molecular layer of the dentate gyrus, and outer layers of the cerebral cortex. Virtually no delta subunit labeling was evident in adjacent regions, such as the thalamic reticular nucleus, hypothalamus, and globus pallidus. Comparisons of the expression of other subunits in delta-/- and wild-type mice demonstrated substantial changes in the alpha4 and gamma2 subunits of the GABA(A) receptor in the delta-/- mice. gamma2 Subunit expression was increased, whereas alpha4 subunit expression was decreased in delta-/- mice. Importantly, alterations of both the alpha4 and the gamma2 subunits were confined primarily to brain regions that normally expressed the delta subunit. This suggests that the additional subunit changes are directly linked to loss of the delta subunit and could reflect local changes in subunit composition and function of GABA(A) receptors in delta-/- mice.

The orphanin FQ/nociceptin knockout mouse: a behavioral model for stress responses

Transgenic mice lacking expression of the OFQ/N precursor protein have provided exciting insights in the physiological functions of this neuropeptide system. While injection of OFQ/N or selective synthetic agonists produces anxiolytic effects in rodents, OFQ/N knockout mice display increased anxiety and impaired adaptation to repeated stress. On the other hand, mice lacking the cognate OFQ/N receptor, ORL1, show improved spatial attention and memory but appear to have normal anxiety and stress behavior. Availability of a selective small molecule OFQ/N antagonist might help clarify this discrepancy.

Similar target, different effects: late-onset ataxia and spatial learning in prion protein-deficient mouse lines

Several lines of mice with targeted deletion of the prion protein gene (Prnp) have been produced, some of them appearing phenotypically normal, others developing late-onset ataxia. This has been tentatively attributed to the size of the targeted deletion in the Prnp gene. but a masking role of genetic background could not be excluded. Thus, we have crossed an ataxic mutant line with large deletion of Prnp (Ngsk Prnp0/0) with a knockout line showing only partial deletion of Prnp and no neurological deficits (Zrchl Prnp0/0). A F2 generation was then studied for up to 70 weeks for co-segregation of lesion size and behavioral phenotype, including cognitive and neurological anomalies. These mice were later crossed with a recently generated PrP-deficient line also having a large deletion and late-onset ataxia (Zrch2 Prnp0/0). They underwent similar testing for up to 90 weeks. The ataxic phenotype always co-segregates with large homozygous deletions involving either the Ngsk or the Zrch2 allele, independent of genetic background or sex. Compound heterozygous Zrchl/Ngsk mice or Zrch1/Zrch2 mice showed intermediate neurological phenotypes, suggesting a gene-dosage effect of large deletions. At 12 weeks of age, large deletions were also associated with minor non-cognitive impairments in water maze learning, and hyperactivity in open field and elevated zero maze. These impairments were not predictive for the development of ataxia. Thus, the neurological deficits are closely associated with large deletions, which entail an upregulation of the recently discovered prion Doppel protein (Dpl), while genetic background factors seem to be responsible for shifting the onset of neurological symptoms.

Testicular endocrine function in GH receptor gene disrupted mice

The consequences of disruption of GH receptor gene in GH receptor knockout mice on testicular function were evaluated. Adult male GH receptor knockout mice and their normal siblings were divided in to two subgroups and treated with either saline or ovine LH (0.3 microg/g BW) in saline. One hour after saline or LH administration, blood was obtained via heart puncture. Plasma IGF-I, LH, FSH, PRL, androstenedione, and testosterone levels were measured by RIAs. Testicular LH and PRL receptor numbers as well as pituitary LHbeta-subunit and testicular sulfated glycoprotein-2 mRNA levels were measured. Also, testicular morphometric analysis was performed. Unlike in normal, wild-type mice, the circulating IGF-I was undetectable in GH receptor knockout mice. The plasma PRL levels were (P<0.01) higher in GH receptor knockout mice than in their normal siblings. The basal LH secretion was similar in normal and GH receptor knockout mice. However, the circulating FSH levels were lower (P<0.001) in GH receptor gene disrupted mice. Administration of LH resulted in a significant (P<0.001) increase in plasma testosterone levels in both GH receptor knockout and normal mice. However, this testosterone response was attenuated (P < 0.01) in GH receptor knockout mice. Plasma androstenedione responses were similar in both GH receptor knockout and normal mice. Testicular LH and PRL receptor numbers were significantly decreased in GH receptor knockout mice. The results of the morphometric analysis of the testis revealed that the Leydig cell volume per testis was reduced in mice with GH receptor gene disruption. The steady-state of LHbeta-subunit and testicular sulfated glycoprotein-2 mRNA levels were not different in GH receptor knockout mice relative to their normal siblings. The present in vivo study demonstrates that in GH receptor knockout mice, LH action on the testis in terms of testosterone secretion is significantly attenuated and suggests that this is due to a decrease in the number of testicular LH receptors. The reduced number of PRL receptors may contribute to the diminished responsiveness of testicular steroidogenesis to LH by decreased ability to convert androstenedione to testosterone. These changes are most likely due to the absence of circulating IGF-I. These findings provide evidence that systemic IGF-I plays a major modulatory role in testicular endocrine function.

Studying TGF-beta superfamily signaling by knockouts and knockins

The transforming growth factor beta (TGF-beta) superfamily has profound effects on many aspects of animal development. In the last decade, our laboratory and others have performed in vivo functional studies on multiple components of the TGF-beta superfamily signal transduction pathway, including upstream ligands, transmembrane receptors, receptor-associated proteins and downstream Smad proteins. We have taken gene knockout approaches to generate null alleles of the genes of interest, as well as a gene knockin approach to replace the mature region of one TGF-beta superfamily ligand with another. We found that activin betaB, expressed in the spatiotemporal pattern of activin betaA, can function as a hypomorphic allele of activin betaA and rescue the craniofacial defects and neonatal lethal phenotype of activin betaA-deficient mice. With the knockout approach, we have shown that the expression pattern of a component in the TGF-beta superfamily signal transduction cascade does not necessarily predict its in vivo function. Two liver-specific activins, activin betaC and activin betaE are dispensable for liver development, regeneration and function, whereas ubiquitously expressed Smad5 has specific roles in the development of multiple embryonic and extraembryonic tissues.

Evidence of altered inhibition in layer V pyramidal neurons from neocortex of Kcna1-null mice

Mice lacking the potassium channel subunit KCNA1 exhibit a severe epileptic phenotype beginning at an early postnatal age. The precise cellular physiological substrates for these seizures are unclear, as is the site of origin. Since KCNA1 mRNA in normal mice is expressed in the neocortex, we asked whether neurons in the neocortex of three to four week-old Kcna1-null mutants exhibit evidence of hyperexcitability. Layer V pyramidal neurons were directly visualized in brain slices with infrared differential-interference contrast microscopy and evaluated with cellular electrophysiological techniques. There were no significant differences in intrinsic membrane properties and action potential shape between Kcna1-null and wild-type mice, consistent with previous findings in hippocampal slice recordings. However, the frequency of spontaneous post-synaptic currents was significantly higher in Kcna1-null compared to wild-type mice. The frequency of spontaneous inhibitory post-synaptic currents and miniature (action-potential-independent) inhibitory post-synaptic currents was also significantly higher in Kcna1-null compared to wild-type mice. However, the frequency of spontaneous and miniature excitatory post-synaptic currents was not different in these two groups of animals. Comparison of the amplitude and kinetics of miniature inhibitory and excitatory post-synaptic currents revealed differences in amplitude, rise time and half-width between Kcna1-null and wild-type mice. Our data indicate that the inhibitory drive onto layer V pyramidal neurons is increased in Kcna1 knockout mice, either directly through an increased spontaneous release of GABA from presynaptic terminals contacting layer V pyramidal neurons, or an enhanced excitatory synaptic input to inhibitory interneurons.

An animal model to assess aversion to intra-oral capsaicin: increased threshold in mice lacking substance p

Despite the widespread consumption of products containing chemicals that irritate the oral mucosa, little is known about the underlying neural mechanisms nor is there a corresponding animal model of oral irritation. We have developed a rodent model to assess aversion to capsaicin in drinking water, using a paired preference paradigm. This method was used to test the hypothesis that the neuromodulator substance P (SP) plays a role in the detection of intra-oral capsaicin. 'Knockout' (KO) mice completely lacking SP and neurokinin A due to a disruption of the preprotachykinin A gene and a matched population of wild-type (WT) mice had free access to two drinking bottles, one containing water and the other capsaicin at various concentrations. Both KO and WT mice showed a concentration-dependent aversion to capsaicin. KO mice consumed significantly more capsaicin than WT at a single near threshold (1.65 microM) concentration, indicating that SP plays a limited role in the detection and rejection of oral irritants.

Implications of multiple phenotypes observed in prolactin receptor knockout mice

The development of a mouse line deficient in the PRL receptor (PRLR) would be an ideal means to better understand the multiple functions of prolactin. We were worried initially that removal of the PRLR from the mouse genome might be lethal and were surprised to find this not to be the case. We identified numerous deficiencies in PRLR knockout (KO) animals. Female homozygous mice are completely infertile and lack normal mammary development, while hemizygotes are unable to lactate following their first pregnancy. PRLR KO males and females have markedly elevated (30- to 100-fold) serum prolactin levels and in some instances pituitary hyperplasia is present. Maternal behavior is severely affected in both hemizygous and heterozygous animals. Bone formation is reduced in young animals and adults (males and females). Recently, we noticed that older KO animals show a slight reduction in body weight which appears to be due to reduced abdominal fat deposition.

Molecular pathways of anxiety revealed by knockout mice

Anxiety is a normal reaction to threatening situations, and serves a physiological protective function. Pathological anxiety is characterized by a bias to interpret ambiguous situations as threatening, by avoidance of situations that are perceived to be harmful, and/or by exaggerated reactions to threat. Although much evidence indicates the involvement of the gamma-aminobutyric acid, serotonin, norepinephrine, dopamine, and neuropeptide transmitter systems in the pathophysiology of anxiety, little is known about how anxiety develops and what genetic/environmental factors underlie susceptibility to anxiety. Recently, inactivation of several genes, associated with either chemical communication between neurons or signaling within neurons, has been shown to give rise to anxiety-related behavior in knockout mice. Apart from confirming the involvement of serotonin, gamma-aminobutyric acid, and corticotrophin-releasing hormone as major mediators of anxiety and stress related behaviors, two novel groups of anxiety-relevant molecules have been revealed. The first group consists of neurotrophic-type molecules, such as interferon gamma, neural cell adhesion molecule, and midkine, which play important roles in neuronal development and cell-to-cell communication. The second group comprises regulators of intracellular signaling and gene expression, which emphasizes the importance of gene regulation in anxiety-related behaviors. Defects in these molecules are likely to contribute to the abnormal development and/or function of neuronal networks, which leads to the manifestation of anxiety disorders.

Prolonged swim-test immobility of serotonin N-acetyltransferase (AANAT)-mutant mice

Serotonin N-acetyltransferase (AANAT; EC 2.3.1.87) metabolizes serotonin into N-acetylserotonin (NAS). AANAT mRNA is expressed in the pineal gland and retina, and also in the rat brain. It was proposed that NAS could be a mediator of the antidepressant action of drugs, and it was shown that chronic but not acute treatment of rats with the antidepressant fluoxetine increases the content of AANAT mRNA in the rat hippocampus. Consequently, AANAT deficiency might be involved in the pathobiology of depression. C57BL/6J mice have a mutant AANAT gene and are considered AANAT-deficient, i.e., "knocked down" (compared with their normal counterparts, C3H/HeJ mice). In this study, we investigated whether AANAT mRNA is expressed in the brain of C57BL/6J and C3H/HeJ mice and whether those mice differ behaviorally, i.e., in a forced swimming test which is used to evaluate antidepressant drugs (such drugs shorten the time of immobility). We found that C3H/HeJ mice express in the brain normal AANAT mRNA, whereas C57BL/6J mice express mutated AANAT mRNA. The mutant, AANAT knocked down C57BL/6J mice displayed significantly longer times of immobility ("depression"). This difference was evident regardless of the circadian rhythm, i.e., both during the day and in the dark at night. Further studies are needed to fully characterize the behavioral significance of AANAT mutation and its possible link to depression.

KO's and organisation of peptidergic feeding behavior mechanisms

Feeding behavior results from complex interactions arising between numerous neuromediators, including classical neurotransmitters and neuropeptides present in hypothalamic networks. One way to unravel these complex mechanisms is to examine animal models with a deletion of genes coding for the different neuropeptides involved in the regulation of feeding. The aim of this review is to focus on feeding and body weight regulation in mice lacking neuropeptide Y (NPY), melanocortins (POMC), corticotropin-releasing hormone, melanin-concentrating hormone, or bombesin-like peptides respectively. The phenotypes, which relate to the deletion of gene coding for the peptides, rarely include changes in body weight and food intake, indicating therefore the existence of redundant mechanisms to compensate for the loss of the peptide. The phenotype is much more marked when the gene deletion is targeted towards the functioning of the peptidergic machinery, e.g. the receptors and especially the POMC and NPY receptors, as well as one subtype of bombesin receptor (BRS-3). These knockout models are also interesting when examining the role of environmental and social factors in the determination of feeding behavior. They have granted us better knowledge of all these integrated and complex mechanisms. Moreover, they are also valuable tools for pharmacological studies when specific antagonists are lacking. From the information obtained by the study of knockouts, it is possible to determine certain targets for selective drugs that could be efficient for the pharmacological treatment of obesity. However, at the present state of our knowledge, it seems necessary to target several peptides in order to get good results with weight loss. It will also be imperative to associate these multitherapies with changes in eating and behavioral habits, in order to obtain complete effectiveness and long-lasting results.

Loss of cortical and thalamic neuronal tenascin-C expression in a transgenic mouse expressing exon 1 of the human Huntington disease gene

A transgenic mouse containing the first exon of the human Huntington's disease (HD) gene has revealed a variety of behavioral and pathophysiological anomalies reminiscent of certain aspects of human Huntington's disease (HD). The present study has found that expression of the extracellular matrix glycoprotein tenascin-C appears to be unaffected in astroglial cells in wild-type and R6/2 transgenic mice that express the mutant huntingtin protein but that it is conspicuously absent in two neuronal populations within the cerebral cortex and thalamus of the R6/2 mice. Loss of tenascin-C expression begins between the fourth and eighth postnatal weeks, coincidental with the onset of abnormal behavioral phenotype and the appearance of intranuclear inclusion bodies and neuropil aggregates. By 12 weeks, R6/2 mice exhibit a complete absence of tenascin-C neuronal immunolabeling, a disappearance of cRNA probe-positive neurons across discrete cytoarchitectonic regions of the dorsal thalamus (e.g., the ventromedial, parafascicular, lateral posterior, and posterior thalamic groups) and frontal cortex, and an accompanying thalamic astrogliosis. The loss of neuronal tenascin-C expression includes structures that are known to send converging excitatory axonal projections to the caudate-putamen, the structure that is most at risk for neurodegeneration in HD. Altered neuronal expression of tenascin-C in R6/2 mice implicates altered transcriptional activities of the mutant huntingtin protein. The abnormal biochemistry and possibly abnormal activity of thalamostriate and corticostriate projection neurons may also affect abnormal neuronal activities in their primary connectional target, the neostriatum, which is severely compromised in HD.

Startle responses, heart rate, and temperature in 5-HT1B receptor knockout mice

Relative to wildtype mice, mice lacking 5-HT1B receptors (5-HT1B KO) exhibit exaggerated heart rate and body temperature responses to environmental stimuli. In contrast, acoustic startle reactivity is reduced in 5-HT1B KO mice. We combined heart rate and temperature measurement with startle response paradigms in order to elucidate this apparent contradiction. Habituation and footshock-induced sensitization paradigms modulate startle reactivity. Reduced startle reactivity and unaltered habituation in 5-HT1B KO mice were replicated. Heart rate and temperature were unaffected by startle stimuli, but increased markedly in response to transportation and handling procedures. Footshocks caused a mild startle-sensitization and tachycardia in both genotypes. The physiological hyper-reactivity in 5-HT1B KO mice is a subtle phenotypic difference that contrasts with the phenotypic decrease in startle reactivity.