D-serine deficiency attenuates the behavioral and cellular effects induced by the hallucinogenic 5-HT(2A) receptor agonist DOI

Abstract

Both the serotonin and glutamate systems have been implicated in the pathophysiology of schizophrenia, as well as in the mechanism of action of antipsychotic drugs. Psychedelic drugs act through the serotonin 2A receptor (5-HT2AR), and elicit a head-twitch response (HTR) in mice, which directly correlates to 5-HT2AR activation and is absent in 5-HT2AR knockout mice. The precise mechanism of this response remains unclear, but both an intrinsic cortico-cortical pathway and a thalamo-cortical pathway involving glutamate release have been proposed. Here, we used a genetic model of NMDAR hypofunction, the serine racemase knockout (SRKO) mouse, to explore the role of glutamatergic transmission in regulating 5-HT2AR-mediated cellular and behavioral responses. SRKO mice treated with the 5-HT2AR agonist (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) showed a clearly diminished HTR and lower induction of c-fos mRNA. These altered functional responses in SRKO mice were not associated with changes in cortical or hippocampal 5-HT levels or in 5-HT2AR and metabotropic glutamate-2 receptor (mGluR2) mRNA and protein expression. Together, these findings suggest that D-serine-dependent NMDAR activity is involved in mediating the cellular and behavioral effects of 5-HT2AR activation.

Keywords: 5-HT(2A) Receptor; D-Serine; Metabotropic glutamate 2 receptor; NMDA Receptor; Serotonin.

Fig. 1

DOI-induced HTR and c-fos mRNA expression are reduced in SRKO mice. (A) Two-way ANOVA showed a significant effect of treatment [F(1, 19) = 57.16 ; p < 0.001] in animals treated with 0.5 mg/kg DOI or saline. For both WT (n = 5-6; light gray bars) and SRKO (n = 5-6; dark gray bars) mice, 0.5 mg/kg DOI highly induced head-twitches (HTR; ***p < 0.001). In addition, two-way ANOVA showed a significant interaction between treatment [F(1,19) = 131.4; p < 0.0001] and genotype [F(1,19) = 10.06; p < 0.0050] in animals treated with 2 mg/kg DOI or saline. For both WT (n = 6; light gray bars) and SRKO (n = 6; dark gray bars) animals, 2 mg/kg DOI highly induced head-twitches (HTR; ^***p < 0.001). Post hoc analysis showed that SRKO mice displayed a significantly lower induction of HTR compared to WT mice (^##p < 0.005). (B) Two-way ANOVA showed an interaction between the effects of treatment [Fig. 2; F(1,26) = 19.81; *** p = 0.0001] and genotype [F(1,26) = 4.651; * p = 0.0405]. Data are expressed as geometric means ± SEM of individual expression values normalized to the housekeeping gene GAPDH using the comparative 2 ^−ΔΔCt method. DOI (light gray bars) induced c-fos mRNA in both WT (n = 9) and SRKO (n = 9) mice. Post hoc analysis revealed that DOI induced c-fos mRNA to a lesser extent in SRKO mice compared to wild-type mice (# p = 0.005). All values represent the mean ± SEM.

Fig. 2

Regional protein expression of 5-HT_2A and mGluR2/3 receptors. No difference was observed in the 5-HT_2AR antagonist [³H]-MDL100907 binding in (A) M2/CG1/PrL (motor/cingulate/prelimbic) cortex, (B) hippocampus and (C) S1 cortex between WT (dark gray; n = 8) and SRKO (light gray; n = 8) animals. (D-F) No difference was observed in mGluR2/3 antagonist [³H]-LY341495 binding in (D) M2/CG1/PrL cortex, (E) hippocampus and (F) S1 cortex WT (dark gray; n = 8) and SRKO (light gray; n = 8) animals. Unpaired Student’s t-test. All values represent the mean ± SEM.

Fig. 3

mRNA expression of 5-HT_2A and mGluR2 receptors in S1 cortex and hippocampus. No difference was observed in 5-HT_2AR mRNA expression between WT (dark gray; n = 8) and SRKO (light gray; n = 8) animals in (A) S1 cortex and (B) hippocampus. No difference was observed in mGluR2 mRNA expression in (C) S1 cortex and (D) hippocampus. Unpaired Student’s t-test. All values represent the mean ± SEM.

Fig. 4

Tissue levels of 5-HT or 5-HIAA in the frontal cortex and S1 cortex. No differences were observed in the tissue levels of 5-HT or 5-HIAA in the (A) frontal cortex or (B) S1 cortex of WT (dark gray) and SRKO (light gray) animals. Unpaired Student’s t-test. n = 5-6. All values represent the mean ± SEM.