Serotonin is implicated in mood regulation, and drugs acting via the serotonergic system are effective in treating anxiety and depression. Specifically, agonists of the serotonin1A receptor have anxiolytic properties, and knockout mice lacking this receptor show increased anxiety-like behaviour. Here we use a tissue-specific, conditional rescue strategy to show that expression of the serotonin1A receptor primarily in the hippocampus and cortex, but not in the raphe nuclei, is sufficient to rescue the behavioural phenotype of the knockout mice. Furthermore, using the conditional nature of these transgenic mice, we suggest that receptor expression during the early postnatal period, but not in the adult, is necessary for this behavioural rescue. These findings show that postnatal developmental processes help to establish adult anxiety-like behaviour. In addition, the normal role of the serotonin1A receptor during development may be different from its function when this receptor is activated by therapeutic intervention in adulthood.

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