5-HT3 receptor function and potentiation by alcohols in frontal cortex neurons from transgenic mice overexpressing the receptor
Introduction
Alcohols and halogenated anesthetics have been shown to depress neuronal function. A growing body of evidence indicates that alcohols and anesthetics act on a number of the ligand-gated ion channel-type neurotransmitter receptors (Grant and Lovinger, 1995, Lovinger, 1997, Faingold et al., 1998). Acute alcohol exposure at concentrations that induce intoxication has been shown to potentiate 5-hydroxytryptamine (5-HT)3 receptor function in isolated nodose ganglion neurons (Lovinger and White, 1991), NCB-20 neuroblastoma cells (Zhou and Lovinger, 1996), as well as human embryonic kidney (HEK) 293 cells and Xenopus oocytes expressing recombinant 5-HT3 receptors (Lovinger and Zhou, 1994, Machu and Harris, 1994, Downie et al., 1995).
The 5-HT3 receptor is the only serotonin-activated ligand-gated ion channel in the mammalian nervous system (Derkach et al., 1989, Lambert et al., 1989, Lovinger, 1991, Maricq et al., 1991, Yakel and Jackson, 1988). This receptor has been implicated in functions of both the peripheral and central nervous systems, including nociception, emesis, cardiovascular regulation, anxiety disorder, and schizophrenia (Costall and Naylor, 1992, Gyermek, 1995, Hagan et al., 1993). In addition, it has been suggested that the 5-HT3 receptor plays an important role in the neural response to drugs of abuse, including alcohol (Grant, 1995). However, little is known about 5-HT3 receptor function in neurons within the brain. In fact, 5-HT3 receptors are expressed throughout the forebrain in relatively low densities (Kilpatrick et al., 1987, Gehlert et al., 1991, Laporte et al., 1992) and thus it has been difficult to study 5-HT3 receptor function in the forebrain. Recently, Engel et al. (1998) have developed transgenic mice that express 5-HT3 receptors in the forebrain at a high level. Overexpression of the 5-HT3 receptor in these mice resulted in a decrease in ethanol (EtOH) consumption. These results and an increase in behavioral activation in response to low doses of EtOH (Engel and Allan, 1999) strongly suggest that 5-HT3 receptors in forebrain can influence the neurobehavioral effects of ethanol.
To ensure that these transgenic mice overexpress functional receptors, and to determine the alcohol sensitivity of these receptors, we have used electrophysiological techniques to examine characteristics of 5-HT3 receptors in dissociated frontal cortex neurons from the 5-HT3 receptor-overexpressing transgenic mice.
Section snippets
Methods
Transgenic mice were designed to overexpress the 5-HT3 receptor in the forebrain. Receptor expression in these animals is driven by the CAM kinase II promotor. The method for creating the animals has been described in detail elsewhere (Engel et al., 1998). Tails of wild-type and transgenic mice used in experiments were kept at −20°C and screened for the transgene by polymerase chain reaction (PCR). All wild-type mice were negative (six animals). Line 13 mice showed one negative PCR result among
Results
Application of 5-HT elicited a rapidly activating inward current in the majority of frontal cortex neurons isolated from the transgenic mice we examined. Fig. 1 shows that the ion current evoked by 5-HT application in dissociated frontal cortex neurons from transgenic mice displays properties consistent with 5-HT3 receptor-mediated responses reported in previous studies. The ion current is rapid in onset (within 0.34±0.02 s), inward-going at negative potentials and decays within seconds in the
Discussion
5-HT application consistently induced rapidly-rising and rapidly-desensitizing inward current in dissociated frontal cortex neurons from 5-HT3 receptor overexpressing mice, but did not consistently induce such current in wild-type mice. KA-evoked inward current was similar in both types of mice. These results suggest that functional 5-HT3 receptors are overexpressed in the frontal cortex neurons of transgenic mice without changes in function of another prominent receptor. Ion current activated
Acknowledgements
This study was supported by Grants AA08986 and AA00176 and by a fellowship from KOSEF to K.-W. Sung.
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