Antidepressants promote formation of heterocomplexes of dopamine D2 and somatostatin subtype 5 receptors in the mouse striatum

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

Understanding GPCR dimerization

Initially G protein-coupled receptors, GPCRs, were thought to act as monomers, but recently strong evidence has been gathered indicating that they are capable of forming homo- and heterodimers or higher order oligomeric complexes, and that the dimerization phenomenon can modulate the pharmacological response and function of these receptors. In this chapter we point to the great potential of alternative therapeutic approach targeted at GPCR dimers, which is especially important in the field of neuropsychopharmacology. We also included a brief description of methods used for studying the phenomenon of GPCR oligomerization, with particular attention paid to the proximity ligation assay, PLA, the procedure which allows the study of interactions between receptors not only in vitro but also in vivo, with good anatomical resolution, what is especially important in the studies of various GPCRs involved in central neurotransmission.

Repeated Clozapine Increases the Level of Serotonin 5-HT1AR Heterodimerization with 5-HT2A or Dopamine D2 Receptors in the Mouse Cortex

G-protein–coupled receptor (GPCR) heterodimers are new targets for the treatment of schizophrenia. Dopamine D₂ receptors and serotonin 5-HT_1A and 5-HT_2A receptors play an important role in neurotransmission and have been implicated in many human psychiatric disorders, including schizophrenia. Therefore, in this study, we investigated whether antipsychotic drugs (clozapine (CLZ) and haloperidol (HAL)) affected the formation of heterodimers of D₂–5-HT_1A receptors as well as 5-HT_1A–5-HT_2A receptors. Proximity ligation assay (PLA) was used to accurately visualize, for the first time, GPCR heterodimers both at in vitro and ex vivo levels. In line with our previous behavioral studies, we used ketamine to induce cognitive deficits in mice. Our study confirmed the co-localization of D₂/5-HT_1A and 5-HT_1A/5-HT_2A receptors in the mouse cortex. Low-dose CLZ (0.3 mg/kg) administered repeatedly, but not CLZ at 1 mg/kg, increased the level of D₂–5-HT_1A and 5-HT_1A–5-HT_2A heterodimers in the mouse prefrontal and frontal cortex. On the other hand, HAL decreased the level of GPCR heterodimers. Ketamine affected the formation of 5-HT_1A–5-HT_2A, but not D₂–5-HT_1A, heterodimers.

Regulation of somatostatin receptor 2 in the context of antidepressant treatment response in chronic mild stress in rat

RATIONALE

The role of somatostatin and its receptors for the stress-related neuropsychiatric disorders has been widely raised. Recently, we have also demonstrated the involvement of somatostatin receptor type 2-sst2R and dopamine receptor type 2-D2R in stress.

OBJECTIVE

In this context, we decided to find if these receptors are involved in response to antidepressant treatment in animal model of depression-chronic mild stress (CMS).

METHODS

Here, we report data obtained following 7-week CMS procedure. The specific binding of [125I]Tyr3-Octreotide to sst2R and [3H]Domperidone to D2R was measured in the rat brain, using autoradiography. Additionally, the level of dopamine and metabolites was measured in the rat brain.

RESULTS

In the final baseline test after 7 weeks of stress, the reduced consumption of sucrose solution was observed (controls vs the stressed animals (6.25 0.16 vs. 10.39 0.41; p < 0.05). Imipramine was administered for the next 5 weeks, and it reversed anhedonia in majority of animals (imipramine-reactive); however, in some animals, it did not (imipramine-non-reactive). Two-way repeated measures ANOVA revealed significant effects of stress and treatment and time interaction [F(16, 168) = 3.72; p < 0.0001], n = 10 per groups. We observed decreased binding of [125I]Tyr3-Octreotide in most of rat brain regions in imipramine non-reactive groups of animals. The decrease of D2R after stress in striatum and nucleus accumbens and no effect of imipramine were observed. In the striatum and prefrontal cortex, the significant role of stress and imipramine in dopamine levels was observed.

CONCLUSIONS

The results obtained in binding assays, together with dopamine level, indicate the involvement of sst2R receptors for reaction to antidepressant treatment. Besides, the stress context itself changes the effect of antidepressant drug.