Selective 5-HT7 receptor agonists LP 44 and LP 211 elicit an analgesic effect on formalin-induced orofacial pain in mice

Differential Contribution of 5-HT4, 5-HT5, and 5-HT6 Receptors to Acute Pruriceptive Processing Induced by Chloroquine and Histamine in Mice

The involvement of serotonin (5-HT) and/or noradrenaline in acute pruriceptive processing in the central nervous system (CNS) has been reported using antidepressants, such as milnacipran, a serotonin and noradrenaline reuptake inhibitor, and mirtazapine, a noradrenergic and specific serotonergic antidepressant; however, the roles of 5-HT receptor family in acute pruriceptive processing have not been fully elucidated in the CNS. In the present study, scratching behavior induced by chloroquine (CQ) was ameliorated by milnacipran or mirtazapine, and these effects were reversed by SB207266, a 5-HT4 antagonist, or SB258585, a 5-HT6 antagonist, but not by SB258585, a 5-HT5 antagonist. Moreover, CQ-induced scratches were mitigated by intrathecal injection of 5-HT4 agonists, such as BIMU8 and ML10302, and the 5-HT6 agonist, WAY208466. Conversely, histamine-induced scratches were not affected by the 5-HT4 agonists or a 5-HT6 agonist. Similarly, the amelioration of histamine-induced scratches by these antidepressants was not reversed by the 5-HT4, 5-HT5, or 5-HT6 receptor antagonist. Therefore, 5-HT is involved in the amelioration of CQ-induced scratches by milnacipran and mirtazapine, and 5-HT4, 5-HT5, and 5-HT6 receptors play differential roles in acute pruriceptive processing after administration of CQ or histamine.

Serotonergic 5-HT7 Receptors as Modulators of the Nociceptive System

The biogenic amine serotonin modulates pain perception by activating several types of serotonergic receptors, including the 5-HT7 type. These receptors are widely expressed along the pain axis, both peripherally, on primary nociceptors, and centrally, in the spinal cord and the brain. The role of 5-HT7 receptors in modulating pain has been explored in vivo in different models of inflammatory and neuropathic pain. While most studies have reported an antinociceptive effect of 5-HT7 receptor activation, some authors have suggested a pronociceptive action. Differences in pain models, animal species and gender, receptor types, agonists, and route of administration could explain these discrepancies. In this mini-review, some of the main findings concerning the function of 5-HT7 receptors in the pain system have been presented. The expression patterns of the receptors at the different levels of the pain axis, along with the cellular mechanisms involved in their activity, have been described. Alterations in receptor expression and/or function in different pain models and the role of 5-HT7 receptors in controlling pain transmission have also been discussed. Finally, some of the future perspectives in this field have been outlined.

5-HT7 Receptors Regulate Excitatory-Inhibitory Balance in Mouse Spinal Cord Dorsal Horn

Serotonergic receptors of the 5-HT₇ type (5-HT₇Rs) are widely expressed in the central nervous system (CNS), where they modulate several functions, such as pain. Behavioral experiments in vivo have shown both anti- and pro-nociceptive actions of 5-HT₇Rs, although an analgesic effect seems to be prevalent. In the spinal cord dorsal horn, the mechanisms involved in 5-HT₇R-mediated synaptic modulation are still poorly understood, especially those regarding the control of synaptic inhibition. The present study investigated the modulation exerted by 5-HT₇Rs on dorsal horn excitatory and inhibitory synaptic circuits, by performing patch-clamp recordings from lamina II neurons in mouse spinal cord slices. Our results show that applying the selective 5-HT₇ agonist LP-211 facilitates glutamatergic release by enhancing the frequency of spontaneous postsynaptic currents (sEPSCs) and increasing the peak amplitude of excitatory postsynaptic currents (EPSCs) evoked by dorsal root stimulation. The effects on sEPSCs were still observed in the presence of the 5-HT_1A antagonist WAY-100635, while the 5-HT₇ antagonist SB-269970 blocked them. LP-211 was also able to increase the release of gamma-aminobutyric acid (GABA) and glycine, as shown by the increase of spontaneous inhibitory currents (sIPSC) frequency and evoked inhibitory postsynaptic currents (IPSC) amplitude. LP-211 was proved to be more effective in potentiating synaptic inhibition as compared to excitation: consistently, 5-HT₇R activation significantly enhanced the excitability of tonic firing neurons, mainly corresponding to inhibitory interneurons. Our data bring new insights into the mechanisms of synaptic modulation mediated by 5-HT₇Rs in the dorsal horn. Stronger impact on synaptic inhibition supports the hypothesis that these receptors may play an anti-nociceptive role in the spinal cord of naïve animals.

Central nervous system effects of 5-HT7 receptors: a potential target for neurodegenerative diseases

5-HT₇ receptors (5-HT₇R) are the most recently identified among the family of serotonin receptors. Their role in health and disease, particularly as mediators of, and druggable targets for, neurodegenerative diseases, is incompletely understood. Unlike other serotonin receptors, for which abundant preclinical and clinical data evaluating their effect on neurodegenerative conditions exist, the available information on the role of the 5-HT₇R receptor is limited. In this review, we describe the signaling pathways and cellular mechanisms implicated in the activation of the 5-HT₇R; also, we analyze different mechanisms of neurodegeneration and the potential therapeutic implications of pharmacological interventions for 5-HT₇R signaling.

Combined approach of homology modeling, molecular dynamics, and docking: computer-aided drug discovery

With the continuous development in software, algorithms, and increase in computer speed, the field of computer-aided drug design has been witnessing reduction in the time and cost of the drug designing process. Structure based drug design (SBDD), which is based on the 3D structure of the enzyme, is helping in proposing novel inhibitors. Although a number of crystal structures are available in various repositories, there are various proteins whose experimental crystallization is difficult. In such cases, homology modeling, along with the combined application of MD and docking, helps in establishing a reliable 3D structure that can be used for SBDD. In this review, we have reported recent works, which have employed these three techniques for generating structures and further proposing novel inhibitors, for cytoplasmic proteins, membrane proteins, and metal containing proteins. Also, we have discussed these techniques in brief in terms of the theory involved and the various software employed. Hence, this review can give a brief idea about using these tools specifically for a particular problem.

Psychotropic Drugs for the Management of Chronic Pain and Itch

Clinical observations have shown that patients with chronic neuropathic pain or itch exhibit symptoms of increased anxiety, depression and cognitive impairment. Such patients need corrective therapy with antidepressants, antipsychotics or anticonvulsants. It is known that some psychotropic drugs are also effective for the treatment of neuropathic pain and pruritus syndromes due to interaction with the secondary molecular targets. Our own clinical studies have identified antipruritic and/or analgesic efficacy of the following compounds: tianeptine (atypical tricyclic antidepressant), citalopram (selective serotonin reuptake inhibitor), mianserin (tetracyclic antidepressant), carbamazepine (anticonvulsant), trazodone (serotonin antagonist and reuptake inhibitor), and chlorprothixene (antipsychotic). Venlafaxine (serotonin-norepinephrine reuptake inhibitor) is known to have an analgesic effect too. The mechanism of such effect of these drugs is not fully understood. Herein we review and correlate the literature data on analgesic/antipruritic activity with pharmacological profile of these compounds.

Improvising 5-HT7R homology model for design of high affinity ligands: model validation with docking, embrace minimization, MM-GBSA, and molecular dynamic simulations

The subtype, 5-HT₇R has been implicated in neurological disorders and presents itself as a promising target for antidepressant drugs. Design of targeted selective ligands, require a sound knowledge of 3D-receptor structure. In absence of receptor structure, structure-based design of targeted ligands relies on generation of 5-HT₇R homology model. In this study, the impact of template choice, alignment, and model building methods on the homology model of 5-HT₇R is addressed. The compactness and model quality due to the presence of cholesterol (lipidic receptor) have also been observed. The results suggest good stereochemical quality of the final model. Ramachandran Plot Analysis indicated more than 97.5% amino acid residues in the favorable region. The overall quality factor was 91.8% using ERRAT. The Z-score for backbone confirmation and packing quality were -1.248 and -1.427, respectively, using WHATCHECK. The RMS Z-score for side chain planarity was .711. Other validation results for the final model include binding site analysis in which Asp162, Val163, Phe343, Phe344, Arg350, Arg367, and Leu370 conserved residues were found in the active site, correlation coefficient of .82 in ligand-based screening and .85 in embrace minimization. Further, the model showed good correlation for agonist and antagonist in docking ([Formula: see text] ≈ .76, [Formula: see text] ≈ .82), embrace minimization ([Formula: see text] ≈ .73, [Formula: see text] ≈ .72), and MM-GBSA ([Formula: see text] ≈ .69, [Formula: see text] ≈ .75) studies. The model was subjected to Molecular Dynamics (MD) simulation of 20 ns both in ligand-free and ligand-bound receptor (agonist and antagonist) system in order to assess its stability.

Liver 5-HT7 receptors: A novel regulator target of fibrosis and inflammation-induced chronic liver injury in vivo and in vitro

BACKGROUND AND AIM

Hepatocellular cancer (HCC) is the sixth most common cancer and liver fibrosis is strongly associated with HCC. Treatment options are limited, and preventive strategies should be developed. An important step in the beginning of liver fibrosis is a strong inflammatory response. 5-HT7 is the last recognized member of the serotonin receptor family and is expressed in both central nerve system and peripheral system and have a lot of functions like learning, memory, smooth muscular relaxation, in the control of circadian rhythms and thermoregulation, pain and migraine, schizophrenia, anxiety, cognitive disturbances, and even inflammation.

METHODS

We therefore examined the biochemical, histopathological and molecular effects of the 5-HT7 receptor agonist and antagonist on inflammatory liver fibrogenesis in animal models of progressive cirrhosis: a mouse model induced by carbon tetrachloride (CCl4) and in Hep3b cells.

RESULTS

5-HT7 expression was observed in the liver in vivo and in vitro in CCl4-induced damage. 5-HT7 receptor agonist but not the antagonist reduced liver markers in mice and in Hep3b cells in carbon tetrachloride (CCl4) induced damage. 5-HT7 agonist, but not antagonist, protected liver tissue from oxidative stress in fibrosis. 5-HT7 agonist but not antagonist induces anti-inflammatory, anti-fibrinotic and anti-cytokine features in liver fibrosis in vivo and in vitro.

CONCLUSIONS

5-HT7 receptors have modulatory function and are an up-and-coming pharmacological target in the inflammatory fibrotic process. 5-HT7 receptor agonist LP-44 showed significant hepatoprotective effects against liver fibrosis, and LP-44 might become a useful therapeutic target for chronic liver inflammation and fibrosis.