Existence of Brain 5-HT1A–5-HT2A Isoreceptor Complexes with Antagonistic Allosteric Receptor–Receptor Interactions Regulating 5-HT1A Receptor Recognition

Abstract

Studies on serotonin-selective reuptake inhibitors have established that disturbances in the ascending 5-HT neuron systems and their 5-HT receptor subtypes and collateral networks to the forebrain contribute to the etiology of major depression and are targets for treatment. The therapeutic action of serotonin-selective reuptake inhibitors is of proven effectiveness, but the mechanisms underlying their effect are still unclear. There are many 5-HT subtypes involved; some need to be blocked (e.g., 5-HT2A, 5-HT3, and 5-HT7), whereas others need to be activated (e.g., postjunctional 5-HT1A and 5-HT4). These state-of-the-art developments are in line with the hypothesis that the development of major depression can involve an imbalance of the activity between different types of 5-HT isoreceptors. In the current study, using in situ proximity ligation assay (PLA), we report evidence for the existence of brain 5-HT1A–5-HT2A isoreceptor complexes validated in cellular models with bioluminescence resonance energy transfer (BRET2) assay. A high density of PLA-positive clusters visualizing 5-HT1A–5-HT2A isoreceptor complexes was demonstrated in the pyramidal cell layer of the CA1–CA3 regions of the dorsal hippocampus. A marked reduction in the density of PLA-positive clusters was observed in the CA1 and CA2 regions 24 h after a forced swim test session, indicating the dynamics of this 5-HT isoreceptor complex. Using a bioinformatic approach, previous work indicates that receptors forming heterodimers demonstrate triplet amino acid homologies. The receptor interface of the 5-HT1A–5-HT2A isoreceptor dimer was shown to contain the LLG and QNA protriplets in the transmembrane and intracellular domain, respectively. The 5-HT2A agonist TCB2 markedly reduced the affinity of the 5-HT1A agonist ipsapirone for the 5-HT1A agonist binding sites in the frontal lobe using the 5-HT1A radioligand binding assay. This action was blocked by the 5-HT2A antagonist ketanserin. It is proposed that the demonstrated 5-HT1A–5-HT2A isoreceptor complexes may play a role in depression through integration of 5-HT recognition, signaling and trafficking in the plasma membrane in two major 5-HT receptor subtypes known to be involved in depression. Antagonistic allosteric receptor–receptor interactions appear to be involved in this integrative process.

Dimers of serotonin receptors: Impact on ligand affinity and signaling

“…The existence of heterodimers between the Gi/o-coupled 5-HT 1A receptor and the Gq-coupled 5-HT 2A receptor has been shown by BRET saturation assay in cotransfected cells [62]. In-vivo, PLA-positive clusters of 5-HT 1A -5-HT 2A receptors were identified in pyramidal cells of the CA1-CA3 regions in the rat hippocampus [62].…”

“…The existence of heterodimers between the Gi/o-coupled 5-HT 1A receptor and the Gq-coupled 5-HT 2A receptor has been shown by BRET saturation assay in cotransfected cells [62]. In-vivo, PLA-positive clusters of 5-HT 1A -5-HT 2A receptors were identified in pyramidal cells of the CA1-CA3 regions in the rat hippocampus [62]. Upon prior forced swimming test, these clusters were reduced in the mouse cortex [21], whereas they were increased by 5-HT 2A -receptor antagonist and by low dose of the antipsychotic clozapine.…”

“…Indeed, 5-HT 1A -receptor radioligand binding assays showed that the 5-HT 2A -receptor agonist TCB2 markedly reduced the binding of the 5-HT 1A -receptor agonist ipsapirone in membranes of the frontal lobe of the cortex. This suggests allosteric-like inhibitory interactions in the 5-HT 1A -5-HT 2A heterodimers, with a dominant effect of 5-HT 2A over 5-HT 1A protomer since activation of the 5-HT 2A protomer reduces the affinity of the 5-HT 1A protomer for its usual ligands [62] (Figure 2 Right).…”

<p>Galanin Receptors as Drug Target for Novel Antidepressants: Review</p>

“…46,49,81 In line with this, the existence of GAL-5HT1AR heterorecptor complex dysfunction leads to disturbance in mesolimbic neurotransmission of 5-HT. 82,83 Indeed, the modulation of auto-receptor function is distinctly regulated by the GalR1-GalR2-5-HT1AR heterotrimeric complex to elicit antidepressant effects. 46,83 Besides increasing hippocampal mRNA levels of post junctional serotonin receptors, co-administration of GAL1-15 and fluoxetine (FLX) help to enhance the agonist binding affinity of FLX in the dentate gyrus.…”

Receptor-Receptor Interactions in the Central Nervous System

“…These changes in the receptor protomers produce marked increases in diversity and bias of GPCR function. One emerging concept in neuropsychopharmacology is that a dysfunction of the allosteric receptor-receptor interactions contributes to disease progression in mental and neurological disorders [4][5][6][7][8][9][10]. So far their stoichiometry and topology are unknown within the heteromer formed as well as the number of adapter proteins participating, including their architecture.…”