N-Ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) administration for studies of 5-HT 1A receptor binding site inactivation and turnover

Abstract

Alterations in the function of the neurotransmitter serotonin (5-HT) have been implicated in several neurobehavioral disorders, including depression, anxiety as well as a well-known disorder of aging, Alzheimer's disease [5, 18, 19]. Age-dependent changes in the serotonergic system include a loss of 5-HT-containing fibers in brain areas which contain high levels of 5-HT 1A receptors [27]. Other changes with aging include decreased 5-HT levels [9], increases in monoamine oxidase (the major 5-HT degrading enzyme) [4], and decreases in the density of 5-HT receptors [12]. While age-related declines in the number of 5-HT 1B and 5-HT 2 receptors have been reported [9], little information is available describing the region-specific effects of aging on the functional dynamics of equilibrium binding at 5-HT receptors, including the 5-HT 1A receptor subtype. For example, there are limited data showing a decrease in the maximal binding capacity ( B max) of 5-HT 1A receptors in the aging cortex of humans. However, changes in affinity ( K d) for this receptor subtype as a function of age and brain region have not been fully investigated [19]. Other reports have failed to indicate age-related modifications in human [2]and rat brain tissue [9]5-HT 1A binding parameters. In contrast, electrophysiological studies suggest that the physiological function of the 5-HT 1A receptor population is altered with aging [25]. Therefore, to elucidate region-specific 5-HT 1A receptor binding characteristics in aging rats, we have utilized a neurotoxic agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) to irreversibly inactivate 5-HT 1A receptors. In this way, subsequent age-related changes that occur in 5-HT 1A receptor binding characteristics may be investigated. EEDQ is an alkylating agent which irreversibly inactivates serotonergic receptors which are coupled to G proteins [3, 11, 14, 21]. This compound is appropriate for examining the binding profile of several 5-HT receptors, including the 5-HT 1A receptor. The 5-HT 1A binding site is among the most sensitive of the serotonergic receptor subtypes to inactivation by EEDQ [10, 11]and is also negatively coupled to adenylate cyclase via interaction with a G i protein [24]. Thus, EEDQ administration is a useful neurotoxicant to examine the relationship between aging and binding characteristics of 5-HT 1A receptors [14]. In addition, using EEDQ to inactivate 5-HT 1A binding sites, we can further investigate the extent to which receptor binding characteristics ( B max and K d) return to baseline levels (i.e., recover) in an age- and brain region-dependent manner following a neurotoxic insult. That is, the age- and region-dependent recovery of 5-HT 1A receptors may be monitored in a time-dependent manner to determine receptor turnover parameters, including receptor synthesis and degradation rate constants, and half-life values. Following receptor inactivation by EEDQ, 5-HT 1A receptors repopulate (i.e., return to baseline levels) with time and exhibit region-specific turnover rates [15]. Therefore, EEDQ administration is an effective pharmacological tool to investigate region-specific differences in 5-HT 1A receptor turnover characteristics. Likewise, by utilizing this neurotoxicant the cellular mechanisms by which pharmacological agents interact with central 5-HT receptors and produce their effects in the aging brain can be addressed. We will illustrate the application of the neurotoxicant EEDQ to irreversibly inactivate 5-HT 1A receptors. Following EEDQ administration, region-specific changes in 5-HT 1A binding characteristics, including receptor density and drug affinity, and kinetics of receptor recovery will be demonstrated by Scatchard analyses and calculations of the recovery of these receptor populations illustrated. Based on the presence of high densities of 5-HT 1A receptors in the hippocampus and frontal cortex [23], these brain regions will be studied for comparisons of both age- and region-specific alterations in receptor binding characteristics.