To investigate the contribution of individual serotonin (5-hydroxytryptamine; 5-HT) receptors to mood control, we have used homologous recombination to generate mice lacking specific serotonergic receptor subtypes. In the present report, we demonstrate that mice without 5-HT1A receptors display decreased exploratory activity and increased fear of aversive environments (open or elevated spaces). 5-HT1A knockout mice also exhibited a decreased immobility in the forced swim test, an effect commonly associated with antidepressant treatment. The serotonin (5-hydroxytryptamine; 5-HT) receptor 1A is found on serotonergic neurons, where it acts as an autoreceptor, and on nonserotonergic neurons (1). 5-HT1A receptor agonists are currently used in the treatment of anxiety disorders (2), and antagonists of this receptor have been suggested to improve the efficacy of certain antidepressant drugs (3). However, the clinical value of these drugs, as well as their mechanism of action, is still unclear. To study the role of the 5-HT1A receptor in mood control, we have generated mice lacking this receptor by homologous recombination (see Methods). The disruption of the 5-HT1A receptor gene was verified by Southern blot analysis (not shown). Despite suggestions that 5-HT1A receptors play a role in development (4, 5), these knockout mice had normal growth and viability and did not display any obvious anatomical or behavioral abnormalities. MATERIALS AND METHODS5-HT1A Gene Targeting. The 5-HT1A gene was cloned from a 129͞Sv mouse genomic library (Lambda EMBL3, Stratagene) by using the human 5-HT1A gene as a probe (6). A KpnI fragment containing the 5-HT1A gene was cloned in pGEM-13(ϩ). The PGK-neo gene was inserted into an AscI site located after the third transmembrane domain of the 5-HT1A gene. W9.5 embryonic stem cells were electroporated (Bio-Rad Gene Pulser; 800 V and 3 F) with 30 g of the targeting construct. These embryonic stem cells were then plated onto mitomycin treated mouse embryonic fibroblasts for 1 week in the presence of G418 (150 g per ml of active substance). The G418-resistant clones were screened by Southern blot with a BglII digest and a 32 P-labeled outside probe (600-bp HindIII-KpnI fragment). Positive cells for the targeting event were injected into C57BL6͞J blastocysts. These blastocysts were reimplanted in B6CBAF1͞J foster mothers, which gave birth to chimeric mice. Chimeras were mated with 129͞Sv females to generate heterozygous mutant (ϩ͞Ϫ) mice on a pure 129͞Sv genetic background. The resulting heterozygous mice were bred and generated 25% homozygous mutant mice.Autoradiography Studies. Coronal cryostat sections (20 m) from three brains of adult male mice were thaw-mounted on gelatin-coated slides and stored at Ϫ20°C. 8-Hydroxy-2-(di-n-[ 3

Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder characterized by motor, cognitive and psychiatric manifestations. Since the mutation responsible for the disease was identified as an unstable expansion of CAG repeats in the gene encoding the huntingtin protein in 1993, numerous mouse models of HD have been generated to study disease pathogenesis and evaluate potential therapeutic approaches. Of these, knock-in models best mimic the human condition from a genetic perspective since they express the mutation in the appropriate genetic and protein context. Behaviorally, however, while some abnormal phenotypes have been detected in knock-in mouse models, a model with an earlier and more robust phenotype than the existing models is required. We describe here for the first time a new mouse line, the zQ175 knock-in mouse, derived from a spontaneous expansion of the CAG copy number in our CAG 140 knock-in colony [1]. Given the inverse relationship typically observed between age of HD onset and length of CAG repeat, since this new mouse line carries a significantly higher CAG repeat length it was expected to be more significantly impaired than the parent line. Using a battery of behavioral tests we evaluated both heterozygous and homozygous zQ175 mice. Homozygous mice showed motor and grip strength abnormalities with an early onset (8 and 4 weeks of age, respectively), which were followed by deficits in rotarod and climbing activity at 30 weeks of age and by cognitive deficits at around 1 year of age. Of particular interest for translational work, we also found clear behavioral deficits in heterozygous mice from around 4.5 months of age, especially in the dark phase of the diurnal cycle. Decreased body weight was observed in both heterozygotes and homozygotes, along with significantly reduced survival in the homozygotes. In addition, we detected an early and significant decrease of striatal gene markers from 12 weeks of age. These data suggest that the zQ175 knock-in line could be a suitable model for the evaluation of therapeutic approaches and early events in the pathogenesis of HD.