5-HT1B receptors are expressed in diverse neuron types throughout the brain where they act as inhibitory receptors on presynaptic terminals. When expressed by serotonergic neurons they are autoreceptors whereas in other types of neurons they act as heteroreceptors. It has been difficult to sort out the role of 5-HT1B autoreceptors and heteroreceptors because they are intermixed in most brain regions, even though they are regulating the release of different neurotransmitters. Furthermore, constitutive knockout of 5-HT1B receptors have a complex phenotype that may reflect developmental compensations predominantly, instead of informing about the role of these receptors in adult brain function. Therefore, two types of conditional expression are needed as tools to investigate the contribution of 5-HT1B receptors in different neurons to complex emotional behavior: temporal and phenotype specificity of gene knockout. This proposal intends to solve this problem by creating a new transgenic mouse that will allow conditional and cell type-specific deletion or protection of 5-HT1B receptor expression using available Cre and Flp driver lines. The proposed transgenic mouse is innovative for several reasons. 1. Conditional, cell type-specific expression of Cre will delete the gene in the targeted neurons. 2. Conditional, cell type-specific expression of Flp will excise a loxP site thereby selectively protecting the 5-HT1B gene in those neurons while remaining 5-HT1B receptors elsewhere can be subsequently knocked out by Cre. 3. The targeting construct is designed to minimize the chances of baseline reduction in 5-HT1B expression prior to deletion (i.e. hypomorphism). 4. This strategy can be applied to many different situations to target (or preserve) 5-HT1B receptors with any available Cre and Flp driver lines. For this revised R21 proposal we will focus our characterization on the conditional knockout of 5-HT1B autoreceptors just in serotonergic neurons.
In Aim 1 we will construct the targeting construct, express it via homologous recombination, derive transgenic mice, and characterize the behavioral phenotype of the mice.
In Aim 2 we will examine the impact of the selective knockout of 5-HT1B autoreceptors (in serotonin neurons) on serotonin transporter function and conditioned fear, an animal model relevant to a number of psychiatric disorders. This will allow us to probe our hypothesis that 5-HT1B autoreceptors are the primary site of 5-HT1B-mediated reductions in fear behavior definitively. In the future it will be possible to examine the site of action of 5-HT1B drugs by using a wide range of other Cre and Flp driver lines to investigate other important neurobiological problems including models of drug addiction, regulation of eating behaviors, and control of respiration in animal models of Sudden Infant Death Syndrome.
5-HT1B receptors are key regulators of neurotransmitter release in many types of neurons in the brain. This project will develop a transgenic mouse line allowing conditional, tissue-specific knockout of this receptor so that it's contribution to emotional disorders such as depression, anxiety, fear, and addiction can be examined with much greater precision and leverage.