Hallucinogens, including mescaline, psilocybin, and lysergic acid diethylamide (LSD), profoundly affect perception, cognition, and mood. All known drugs of this class are 5-HT(2A) receptor (2AR) agonists, yet closely related 2AR agonists such as lisuride lack comparable psychoactive properties. Why only certain 2AR agonists are hallucinogens and which neural circuits mediate their effects are poorly understood. By genetically expressing 2AR only in cortex, we show that 2AR-regulated pathways on cortical neurons are sufficient to mediate the signaling pattern and behavioral response to hallucinogens. Hallucinogenic and nonhallucinogenic 2AR agonists both regulate signaling in the same 2AR-expressing cortical neurons. However, the signaling and behavioral responses to the hallucinogens are distinct. While lisuride and LSD both act at 2AR expressed by cortex neurons to regulate phospholipase C, LSD responses also involve pertussis toxin-sensitive heterotrimeric G(i/o) proteins and Src. These studies identify the long-elusive neural and signaling mechanisms responsible for the unique effects of hallucinogens.

The psychosis associated with schizophrenia is characterized by alterations in sensory processing and perception 1,2 . Some antipsychotic drugs were identified by their high affinity for serotonin 5-HT 2A receptors (2AR) 3,4 . Drugs that interact with metabotropic glutamate receptors (mGluR) also show potential for the treatment of schizophrenia [5][6][7] . The effects of hallucinogenic drugs, such as psilocybin and lysergic acid diethylamide (LSD), require the 2AR [8][9][10] and resemble some of the core symptoms of schizophrenia [10][11][12] . Here we show that the mGluR2 interacts via specific transmembrane helix domains with the 2AR, a member of an unrelated G protein-coupled receptor (GPCR) family, to form functional complexes in brain cortex. The 2AR/mGluR2 complex triggers unique cellular responses when targeted by hallucinogenic drugs, and activation of mGluR2 abolishes hallucinogen specific signalling and behavioural responses. In postmortem human brain from untreated schizophrenic subjects, the 2AR is up-regulated and the mGluR2 is down-regulated, a pattern that could predispose to psychosis. These regulatory changes suggest that the 2AR/mGluR2 complex may be involved in the altered cortical processes of schizophrenia, and represents a promising new target for the treatment of psychosis.Correspondence and requests for materials should be addressed to: J.G.M (e-mail: Javier.Maeso@mssm.edu) S.C.S. (e-mail: Stuart.Sealfon@mssm.edu). The 2AR and mGluR2/3 show an overlapping distribution in brain cortex in autoradiography studies 13 . The mGluR2 and mGluR3 are not distinguished by autoradiographic ligands. We used fluorescent in situ hybridization (FISH) to determine whether either of these receptor subtypes are co-expressed by the same neurons. In layer V mouse somatosensory cortex (SCx), 2AR mRNA positive cells were mostly mGluR2 mRNA positive. The level of expression in SCx was much lower for mGluR3 mRNA, which rarely co-localized with 2AR mRNA (Fig. 1a). Control studies validated assay sensitivity and specificity, and similar 2AR/mGluR2 mRNA co-localization was found in cortical primary cultures (Figs. 1a,b,c, and Supplementary Fig. S1). Translation of 2AR protein in cortical pyramidal neurons was found to be necessary for normal mGluR2 expression. Mice with globally disrupted 2AR expression (htr2A−/− mice) showed reduced cortical mGluR2 binding and expression, while mice in which 2AR expression was selectively restored in cortical pyramidal neurons 8,14 showed control expression levels (Supplementary Table S1, and Supplementary Fig. S2). The effects of mGluR2/3 activation on 2AR responses have been generally attributed to synaptic mechanisms 5,6,13,15 . However, the co-localization of 2AR and mGluR2 and the reduction of mGluR2 expression levels in htr2A−/− mice motivated us to examine whether a direct mechanism contributed to cortical crosstalk between these two receptor systems. NIH Public AccessRecent studies have demonstrated that some GPCRs belonging to the same sequence classes can form ...

Most neuropharmacological agents and many drugs of abuse modulate the activity of heptahelical G-protein-coupled receptors. Although the effects of these ligands result from changes in cellular signaling, their neurobehavioral activity may not correlate with results of in vitro signal transduction assays. 5-Hydroxytryptamine 2A receptor (5-HT2AR) partial agonists that have similar pharmacological profiles differ in the behavioral responses they elicit. In vitro studies suggest that different agonists acting at the same receptor may establish distinct patterns of signal transduction. Testing this hypothesis in the brain requires a global signal transduction assay that is applicable in vivo. To distinguish the cellular effects of the different 5-HT2AR agonists, we developed an assay for global signal transduction on the basis of high throughput quantification of rapidly modulated transcripts. Study of the responses to agonists in human embryonic kidney 293 cells stably expressing 5-HT2ARs demonstrated that each agonist elicits a distinct transcriptome fingerprint. We therefore studied behavioral and cortical signal transduction responses in wild-type and 5-HT2AR null-mutant mice. The hallucinogenic chemicals (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) and lysergic acid diethylamide (LSD) stimulated a head-twitch behavioral response that was not observed with the nonhallucinogenic lisuride hydrogen maleate (LHM) and was absent in receptor null-mutant mice. We also found that DOI, LSD, and LHM each induced distinct transcriptome fingerprints in somatosensory cortex that were absent in 5-HT2AR null-mutants. Moreover, DOI and LSD showed similarities in the transcriptome fingerprints obtained that were not observed with the behaviorally inactive drug LHM. Our results demonstrate that chemicals acting at the 5-HT2AR induce specific cellular response patterns in vivo that are reflected in unique changes in the somatosensory cortex transcriptome.

SUMMARY G protein-coupled receptors form hetero-dimers and higher order hetero-oligomers, yet the significance of receptor heteromerization in cellular and behavioral responses is poorly understood. Atypical antipsychotic drugs, such as clozapine and risperidone all have in common a high affinity for the serotonin 5-HT2A receptor (2AR). However, closely related nonantipsychotic drugs, such as ritanserin and methysergide, while blocking 2AR function, lack comparable neuropsychological effects. Why some but not all drugs that inhibit 2AR-dependent signaling exhibit antipsychotic properties remains unresolved. We found that a heteromeric complex formed between the metabotropic glutamate 2 receptor (mGluR2) and the 2AR critically integrates the action of drugs affecting signaling and behavioral outcomes. Acting through the mGluR2/2AR heterocomplex, both glutamatergic and serotonergic drugs achieve a balance between Gi- and Gq-dependent signaling that predicts their psychoactive behavioral effects. These observations provide a novel mechanistic insight into antipsychotic action that may advance therapeutic strategies for schizophrenia.