5-HT1A-iCre, a new transgenic mouse line for genetic analyses of the serotonergic pathway.

The 5-HT1A receptor not only plays an important role in brain physiology but it may be also implicated in the etiology of behavioral disorders such as pathological anxiety. To further define the role of 5-HT1A receptor-expressing neurons, we generated a transgenic mouse line expressing Cre recombinase in these cells. The 5-HT1A receptor open reading frame was substituted for that of Cre recombinase in a BAC containing the 5-HT1A receptor gene. In adult transgenic brain, Cre expression perfectly matched the distribution of 5-HT1A receptor mRNA. Additionally, Cre-mediated DNA recombination was restricted to neuronal populations that express the receptor, e.g., cerebral cortex, septum, hippocampus, dorsal raphe, thalamic, hypothalamic and amygdaloid nuclei, and spinal cord. Recombination occurred as early as E13 in trigeminal nerve, spinal ganglia and spinal cord. This transgenic line will allow the generation of conditional mutant mice that lack specific gene products along the serotonergic pathways and represents a unique tool for studying 5-HT1A-mediated serotonin signaling in the developing and adult brain.

Analysis of dopamine transporter gene expression pattern -- generation of DAT-iCre transgenic mice.

The dopamine transporter is an essential component of the dopaminergic synapse. It is located in the presynaptic neurons and regulates extracellular dopamine levels. We generated a transgenic mouse line expressing the Cre recombinase under the control of the regulatory elements of the dopamine transporter gene, for investigations of gene function in dopaminergic neurons. The codon-improved Cre recombinase (iCre) gene was inserted into the dopamine transporter gene on a bacterial artificial chromosome. The pattern of expression of the bacterial artificial chromosome-dopamine transporter-iCre transgene was similar to that of the endogenous dopamine transporter gene, as shown by immunohistochemistry. Recombinase activity was further studied in mice carrying both the bacterial artificial chromosome-dopamine transporter-iCre transgene and a construct expressing the beta-galactosidase gene after Cre-mediated recombination. In situ studies showed that beta-galactosidase (5-bromo-4-chloroindol-3-yl beta-D-galactoside staining) and the dopamine transporter (immunofluorescence) had identical distributions in the ventral midbrain. We used this animal model to study the distribution of dopamine transporter gene expression in hypothalamic nuclei in detail. The expression profile of tyrosine hydroxylase (an enzyme required for dopamine synthesis) was broader than that of beta-galactosidase in A12 to A15. Thus, only a fraction of neurons synthesizing dopamine expressed the dopamine transporter gene. The bacterial artificial chromosome-dopamine transporter-iCre transgenic line is a unique tool for targeting Cre/loxP-mediated DNA recombination to dopamine neurons for studies of gene function or for labeling living cells, following the crossing of these mice with transgenic Cre reporter lines producing fluorescent proteins.