Metabotropic Glutamate2 Receptors Play a Key Role in Modulating Head Twitches Induced by a Serotonergic Hallucinogen in Mice

Abstract

There is substantial evidence that glutamate can modulate the effects of 5-hydroxytryptamine_2A (5-HT_2A) receptor activation through stimulation of metabotropic glutamate_2/3 (mGlu_2/3) receptors in the prefrontal cortex. Here we show that constitutive deletion of the mGlu₂ gene profoundly attenuates an effect of 5-HT_2A receptor activation using the mouse head twitch response (HTR). MGlu₂ and mGlu₃ receptor knockout (KO) as well as age-matched ICR (CD-1) wild type (WT) mice were administered (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and observed for head twitch activity. DOI failed to produce significant head twitches in mGlu₂ receptor KO mice at a dose 10-fold higher than the peak effective dose in WT or mGlu₃ receptor KO mice. In addition, the mGlu_2/3 receptor agonist LY379268, and the mGlu₂ receptor positive allosteric modulator (PAM) CBiPES, potently blocked the HTR to DOI in WT and mGlu₃ receptor KO mice. Conversely, the mGlu_2/3 receptor antagonist LY341495 (10 mg/kg) increased the HTR produced by DOI (3 mg/kg) in mGlu₃ receptor KO mice. Finally, the mGlu₂ receptor potentiator CBiPES was able to attenuate the increase in the HTR produced by LY341495 in mGlu₃ receptor KO mice. Taken together, all of these results are consistent with the hypothesis that that DOI-induced head twitches are modulated by mGlu₂ receptor activation. These results also are in keeping with a critical autoreceptor function for mGlu₂ receptors in the prefrontal cortex with differential effects of acute vs. chronic perturbation (e.g., constitutive mGlu₂ receptor KO mice). The robust attenuation of DOI-induced head twitches in the mGlu₂ receptor KO mice appears to reflect the critical role of glutamate in ongoing regulation of 5-HT_2A receptors in the prefrontal cortex. Future experiments with inducible knockouts for the mGlu₂ receptor and/or selective mGlu₃ receptor agonists/PAMs/antagonists could provide an important tools in understanding glutamatergic modulation of prefrontal cortical 5-HT_2A receptor function.

Keywords: 5-hydroxytryptamine2A (5-HT2A) receptors; CBiPES; DOI; LY341495; LY379268; head twitches; prefrontal cortex (PFC); transgenic mice.

FIGURE 1

(A) The effect of (±)-DOI (0.3–10 mg/kg) on head twitches in CD-1 WT mice observed for 15 min following drug administration. Each bar represents the mean (±SEM) of six animals after i.p. dosing. Significantly different from 0, ^∗p < 0.05. (B) The effect of CBiPES (10–30 mg/kg) on head twitches induced by DOI (3 mg/kg) in CD-1 WT mice observed for 30 min following drug administration. CBiPES was administered 30 min prior to DOI. Each bar represents the mean (±SEM) of eight animals after i.p. dosing. Significantly different from 0 or the vehicle condition by Fisher Exact Test, ^#p < 0.05. (C) The effect of LY379268 (0.01–1 mg/kg) on head twitches induced by DOI (3 mg/kg) in CD-1 WT mice observed for 30 min following drug administration. LY379268 was administered 30 min prior to DOI. Each bar represents the mean (±SEM) of six animals after i.p. dosing. Significantly different from 0 (vehicle) by the Fisher Exact Test, ^#p < 0.05. (D) The effect of LY341495 (1–3 mg/kg) on head twitches induced by DOI (3 mg/kg) in CD-1 WT mice observed for 30 min following drug administration. LY341495 was administered 30 min prior to DOI. Each bar represents the mean (±SEM) of eight animals after i.p. dosing. Significantly different from 0 (vehicle condition), ^∗p < 0.05. Note that the DOI dose-response relationship is the only experiment in this paper where a 15 min observation period was employed, unlike all other experiments using a 30 min observation period.

FIGURE 2

(A) The effect of DOI (3 mg/kg) on head twitches in WT and DOI (3–30 mg/kg) in mGlu₂ receptor KO mice for 30 min following drug administration. Each bar represents the mean (±SEM) of eight animals after i.p. dosing. Significantly different from WT performance, ^∗p < 0.05. WT, wild type; KO, knockout. (B) The effect of DOI (1–3 mg/kg) on head twitches in WT and the effect of DOI (1–3 mg/kg) on head twitches in mGlu₃ receptor KO mice for 30 min following drug administration. Each bar represents the mean (±SEM) of 10 animals after i.p. dosing. WT, wild type; KO, knockout. The horizontal line is shown above the data for the mGlu₂ receptor KO mice and for the mGlu₃ receptor KO mice in the top (A) and lower figures(B), respectively.

FIGURE 3

(A) Effect of CBiPES (1–10 mg/kg) on head twitches induced by DOI in mGlu₃ receptor KO mice for 30 min following drug administration. CBiPES was administered 30 min prior to DOI. Each bar represents the mean (±SEM) of eight animals after i.p. dosing. Significantly different from 0 or the vehicle condition, ^∗p < 0.05. Significantly different from 0 (vehicle) by the Fishers Exact Test, ^#p < 0.05. (B) Effect of LY379268 (1 mg/kg) on head twitches induced by DOI in mGlu₃ receptor KO mice for 30 min following drug administration. LY379268 was administered 30 min prior to DOI. Each bar represents the mean (±SEM) of eight animals after i.p. dosing. Significantly different from respective WT, ^∗p < 0.05 or KO, ^∗∗∗p < 0.05 treated only with DOI. Significantly different from respective WT and KO treated only with DOI by Fishers Exact Test, ^#p < 0.05. (C) The effect of LY341495 (1–10 mg/kg) on head twitches induced by DOI in mGlu₃ receptor KO mice for 30 min following drug administration. LY341495 was administered 30 min prior to DOI. Each bar represents the mean (±SEM) of 16 animals after i.p. dosing. Significantly different from DOI alone (“0” condition in the figure), ^∗p < 0.05.

FIGURE 4

(A) The effect of vehicle or CBiPES (10–30 mg/kg) on head twitches induced by the administration of DOI (3 mg/kg) and LY341495 (3 mg/kg) in CD-1 WT mice for 30 min following drug administration. CBiPES was administered 5 min prior to LY341495, which was administered 30 min prior to DOI. Each bar represents the mean (±SEM) of eight animals after i.p. dosing. (B) The effect of vehicle or CBiPES (30 mg/kg) on head twitches induced by DOI (3 mg/kg), with or without the presence of LY341495 (3 mg/kg), in WT and mGlu₃ receptor KO mice for 30 min following drug administration. Each bar represents the mean (±SEM) of eight animals after i.p. dosing. KO-LY341495 significantly different from KO without LY341495, ^∗p < 0.05 or KO-DOI-LY341495-CBiPES significantly different from KO-DOI-LY341495, ^∗p < 0.05.

FIGURE 5

The range of serotonergic and glutamatergic modulation in the layer V pyramidal neuron apical dendritic field from the prefrontal cortex/neocortex is shown. 5-HT_2A receptors are present at postsynaptic sites to glutamatergic axon terminals and may also be present at presynaptic thalamocortical axon terminals (arising from the midline and thalamic nuclei) relative to the layer V pyramidal neuron apical dendrites. 5-HT_2A receptor activation appears to induce glutamate release from these midline and intralaminar thalamic nuclei axons. Due to the presence of both mGlu₂ receptors acting as a critical autoreceptor on these thalamocortical terminals, mGlu_2/3 receptor agonists and mGlu₂ receptor PAMs suppress glutamate release from these terminals in a homeostatic fashion. MGlu₄ receptors also may be present on these glutamatergic terminals. Glutamate released by these thalamocortical axons activates AMPA receptors on the layer V pyramidal neuron apical dendrite. The PSD-95 protein is a scaffolding protein that may keep AMPA and postsynaptic 5-HT_2A receptors in close physical proximity. The lower insert shows a coronal slice of the mouse brain including the prelimbic region of the medial prefrontal cortex. This region appears to support DOI-induced head twitches in rodents. Thus, this microcircuitry appears very salient for rodent behavior, at least with respect to DOI-induced head twitches.