Evidence that 5-HT2A receptor function is linked to genetic variation in 5-HT transporter expression

K.A. Jennings1, M.E. Sprakes1, W.J. Sheward2, A.J. Harmar2 & T. Sharp1 1University Department of Pharmacology, Mansfield Rd, Oxford OX1 3QT, 2Centre for Neuroscience Research, School of Biomedical Sciences , Edinburgh EH8 9JZ

5-HT2A receptors are important in the effects of 5-hydroxytryptamine (5-HT) on neural circuitry and linked to key brain functions and neuropsychiatric disorders. Molecular imaging studies report large inter-subject variability in 5-HT2A receptor levels that may underlie individual differences in brain function and disorder vulnerability (Adams et al., 2004; Bhagwagar et al., 2006). The origin of this variability is unclear but genetic factors are likely. One factor may be expression of the 5-HT transporter (5-HTT) gene as 5-HT2A receptor function and expression are decreased in 5-HTT null mice (Qu et al., 2005). Here we measured 5-HT2A receptor function and expression in transgenic 5-HTT overexpressing mice (Jennings et al., 2006).

5-HT2A receptor function was tested in transgenic and wildtype mice ( CBAxC57Bl6J, 3-9 months) by measuring i) regional brain expression of the immediate early gene Arc and ii) head twitches, elicited by the 5-HT2 agonist, DOI ( 2,5-dimethoxy-4-iodoamphetamine hydrobromide; 2 mg kg-1 i.p.). Wildtype mice (male C57Bl6J) were used to test the effect of the 5-HT2A antagonist MDL 100907 (0.3 mg kg-1) on DOI responses. For Arc, DOI injected mice were killed by cervical dislocation after 1 h, prior to measuring Arc mRNA using in situ hybridisation (Pei et al, 2004). Head twitches were counted over 15 min immediately after DOI. 5-HT2A receptor binding and mRNA were measured in brain regions (including frontal cortex, hippocampus, striatum) of treatment naïve transgenic and wildtype mice using receptor autoradiography ( 3H-ketanserin) and in situ hybridisation. Data were analysed using Student’s unpaired t-test or ANOVA with Bonferroni post hoc testing as appropriate.

In wildtype mice, both the increase in regional brain Arc mRNA and head twitches elicited by DOI, were abolished by MDL 100907 (P<0.05). DOI evoked a significantly greater (P<0.05) increase in Arc mRNA in transgenic compared to wildtype mice in several cortical regions (cingulate 178 ±19 vs 121 ±10; motor 289 ±44 vs 170 ±26; orbital 230 ±25 vs 134 ±16 %). DOI also induced a greater increase in head-twitches in transgenic compared to wildtype mice (44 ±4 vs 27 ±4 twitches/15 min). The regional distribution of 3H-ketanserin binding and 5-HT2A mRNA was similar between transgenic and wildtype mice and levels did not differ between genotype in any region.

In summary, these data show that 5-HTT overexpressing mice exhibit greater 5-HT2A receptor-mediated responses compared to wildtype mice, but these changes were not accompanied by increased 5-HT2A receptor expression . The data support the idea that 5-HTT expression is a genetic determinant of variability in 5-HT2A receptor function but matching changes in 5-HT2A receptor expression were not detected in the model used.

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Bhagwagar Z. et al., (2006). Am J Psychiatry, 163, 1580-1587.
Jennings K. et al., (2006). J Neurosci, 26, 8955-8964.
Qu Y. et al., (2005). Psychopharmacol, 180, 12-20.
Pei Q. et al., (2004). Neuropharmacol ogy , 46 , 331-339.

Funded by a MRC studentship (KJ) and EC FP6 Integrated network ( LMSH-CT-2004-503474).