Magnitude of 5-HT1B and 5-HT1A receptor activation in guinea-pig and rat brain: evidence from sumatriptan dimer-mediated [35S]GTPgammaS binding responses

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

5-Hydroxytryptamine 1A receptors inhibit glutamate release in rat medullary dorsal horn neurons

We examined 5-hydroxytryptamine 1A (5-HT1A) receptor-mediated modulation of glutamatergic transmission in rat medullary dorsal horn neurons using a conventional whole-cell patch clamp technique. 5-HT reversibly and concentration dependently decreased the amplitude of glutamatergic excitatory postsynaptic currents and increased the paired-pulse ratio, indicating that 5-HT acts presynaptically to reduce glutamate release from primary afferents. The 5-HT-induced inhibition of excitatory postsynaptic currents was partially occluded by NAN-190, a 5-HT1A receptor antagonist, and mimicked by 8-OH-DPAT, a 5-HT1A receptor agonist. Our results suggest that presynaptic 5-HT1A receptors inhibit glutamate release from trigeminal primary afferents onto medullary dorsal horn neurons, and thus in addition to other 5-HT1 receptor subtypes, 5-HT1A receptors could be a potential target for treatment of pain from orofacial tissues.

Functional significance of serotonin receptor dimerization

The original model of G-protein activation by a single G-protein-coupled receptor (GPCR) is giving way to a new model, wherein two protomers of a GPCR dimer interact with a single G-protein. This article will review the evidence suggesting that 5-HT receptors form dimers/oligomers and will compare the findings with the results obtained from the studies with other biogenic amine receptors. Topics to be covered include the origin or biogenesis of dimer formation, potential dimer interface(s), and oligomer size (dimer vs. tetramer or higher order). The functional significance will be discussed in terms of G-protein activation following ligand binding to one or two protomers in a dimeric structure, the formation of heterodimers, and the development of bivalent ligands.

5-HT(1B) receptors inhibit glutamate release from primary afferent terminals in rat medullary dorsal horn neurons

BACKGROUND AND PURPOSE

Although 5-HT(1B) receptors are expressed in trigeminal sensory neurons, it is still not known whether these receptors can modulate nociceptive transmission from primary afferents onto medullary dorsal horn neurons.

EXPERIMENTAL APPROACH

Primary afferent-evoked EPSCs were recorded from medullary dorsal horn neurons of rat horizontal brain stem slices using a conventional whole-cell patch clamp technique under a voltage-clamp condition.

KEY RESULTS

CP93129, a selective 5-HT(1B) receptor agonist, reversibly and concentration-dependently decreased the amplitude of glutamatergic EPSCs and increased the paired-pulse ratio. In addition, CP93129 reduced the frequency of spontaneous miniature EPSCs without affecting the current amplitude. The CP93129-induced inhibition of EPSCs was significantly occluded by GR55562, a 5-HT(1B/1D) receptor antagonist, but not LY310762, a 5-HT(1D) receptor antagonist. Sumatriptan, an anti-migraine drug, also decreased EPSC amplitude, and this effect was partially blocked by either GR55562 or LY310762. On the other hand, primary afferent-evoked EPSCs were mediated by the Ca(2+) influx passing through both presynaptic N-type and P/Q-type Ca(2+) channels. The CP93129-induced inhibition of EPSCs was significantly occluded by ω-conotoxin GVIA, an N-type Ca(2+) channel blocker.

CONCLUSIONS AND IMPLICATIONS

The present results suggest that the activation of presynaptic 5-HT(1B) receptors reduces glutamate release from primary afferent terminals onto medullary dorsal horn neurons, and that 5-HT(1B) receptors could be, at the very least, a potential target for the treatment of pain from orofacial tissues.

In vivo expression of GLP-1/IgG-Fc fusion protein enhances beta-cell mass and protects against streptozotocin-induced diabetes

Glucagon-like peptide 1 (GLP-1) and its analogue exendin-4 (Ex4) have displayed potent glucose homeostasis-modulating characteristics in type 2 diabetes (T2D). However, there are few reports of effectiveness in type 1 diabetes (T1D) therapy, where there is massive loss of beta cells. We previously described a novel GLP-1 analogue consisting of the fusion of active GLP-1 and IgG heavy chain constant regions (GLP-1/IgG-Fc), and showed that in vivo expression of the protein, via electroporation-enhanced intramuscular plasmid-based gene transfer, normalized blood glucose levels in T2D-prone db/db mice. In the present study, GLP-1/IgG-Fc and Ex4/IgG-Fc were independently tested in multiple low-dose streptozotocin-induced T1D. Both GLP-1/IgG-Fc and Ex4/IgG-Fc effectively reduced fed blood glucose levels in treated mice and ameliorated diabetes symptoms, where as control IgG-Fc had no effect. Treatment with GLP-1/IgG-Fc or Ex4/IgG-Fc improved glucose tolerance and increased circulating insulin and GLP-1 levels. It also significantly enhanced islet beta-cell mass, which is likely a major factor in the amelioration of diabetes. This suggests that GLP-1/IgG-Fc gene therapy may be applicable to diseases where there is either acute or chronic beta-cell injury.

Gene therapy of diabetes using a novel GLP-1/IgG1-Fc fusion construct normalizes glucose levels in db/db mice

Glucagon-like peptide (GLP-1), a major physiological incretin, plays numerous important roles in modulating blood glucose homeostasis and has been proposed for the treatment of type 2 diabetes. The major obstacles for using native GLP-1 as a therapeutic agent are that it must be delivered by a parenteral route and has a short half-life. In an attempt to develop a strategy to prolong the physiological t(1/2) and enhance the potency of GLP-1, a fusion protein consisting of active human GLP-1 and mouse IgG(1) heavy chain constant regions (GLP-1/Fc) was generated. A plasmid encoding an IgK leader peptide-driven secretable fusion protein of the active GLP-1 and IgG(1)-Fc was constructed for mammalian expression. This plasmid allows for expression of bivalent GLP-1 peptide ligands as a result of IgG-Fc homodimerization. In vitro studies employing purified GLP-1/Fc indicate that the fusion protein is functional and elevates cAMP levels in insulin-secreting INS-1 cells. In addition, it stimulates insulin secretion in a glucose concentration-dependent manner. Intramuscular gene transfer of the plasmid in db/db mice demonstrated that expression of the GLP-1/Fc peptide normalizes glucose tolerance by enhancing insulin secretion and suppressing glucagon release. This strategy of using a bivalent GLP-1/Fc fusion protein as a therapeutic agent is a novel approach for the treatment of diabetes.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

In vitro profile of the antidepressant candidate OPC-14523 at rat and human 5-HT1A receptors

This study determined the in vitro functional profile of 1-[3-[4-(3-chlorophenyl)-1-piperazinyl]propyl]-5-methoxy-3,4-dihydro-2-quinolinone monomethanesulfonate (OPC-14523) at rat and human serotonin (5-HT) 5-HT1A receptors and binding affinity of OPC-14523 at human frontocortical 5-HT1A receptors. OPC-14523 (1 microM) increased guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]GTPgammaS) binding to 5-HT1A receptor-containing regions of rat brain tissue sections (approximately 53% of the effect of 1 microM (+)8-hydroxy-2-(di-n-propylamino)tetralin ((+)8-OH-DPAT) that were blocked by the selective 5-HT1A receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY-100635). OPC-14523 also behaved as a partial agonist in its stimulation of [35S]GTPgammaS binding to membranes from rat hippocampus (pEC50=7.60+/-0.23, Emax=41.1% of the effect of 10 microM (+)8-OH-DPAT), human frontal cortex (pEC50=7.89+/-0.08; Emax=64% of the effect of 10 microM (+)8-OH-DPAT), and Chinese Hamster Ovary cells expressing cloned human 5-HT1A receptors (pEC50=8.0+/-0.11; Emax=85.5% of the effect of 10 microM 5-HT), and all of these effects of OPC-14523 were blocked by WAY-100635. Taken together, these data support the development of OPC-14523 as an antidepressant whose mechanism of action involves potent partial agonist activity at 5-HT1A receptors.

Detailed pharmacological characterization of 5-HT1A-receptor-mediated [35S]GTP gamma S binding in rat hippocampal membranes

5-HT-stimulated [(35)S]GTPgammaS binding to rat hippocampal membranes was pharmacologically characterized. Signal/noise ratio or percent increase over basal was optimized with 100 microM GDP, 2-10 mM MgCl(2), and 150-200 mM NaCl. However, we preferred the standard condition (20 microM GDP, 5 mM MgCl(2), and 100 mM NaCl: Condition I) to the alternative one (100 microM GDP, 5 mM MgCl(2), and 150 mM NaCl: Condition II) because 1) absolute values of basal and 5-HT-sensitive bindings decreased with higher concentrations of GDP and NaCl; 2) EC(50) values determined under Condition II were 2 - 6 fold higher than those under Condition I; 3) some partial agonists had less intrinsic activities in the presence of higher concentrations of GDP; and 4) Inhibitory effects of WAY100635 were complete under Condition I, while incomplete under Condition II. Pharmacological profile of concentration-dependent stimulation by a series of 5-HT ligands and concentration-dependent inhibition of 5-HT-stimulated binding by several 5-HT-receptor antagonists clearly indicated that this response under Condition I was mediated solely through 5-HT(1A) receptors. Although caution should be paid especially to the apparent intrinsic activities susceptible to the assay conditions, this method appears useful to investigate functional coupling between 5-HT(1A) receptors and their coupled G proteins in native hippocampal membranes.

Autoradiographic characterisation of [35S]GTPgammaS binding stimulation mediated by 5-HT1B receptor in postmortem human brain

G-protein activation mediated by 5-HT1B receptors was studied in human brain by [35S]GTPgammaS autoradiographic methods. 5-HT (10 microM) increased [35S]GTPgammaS binding in caudate-putamen nucleus, globus pallidus, dentate gyrus, CA1, entorhinal cortex and substantia nigra. In basal ganglia and midbrain, this effect was blocked by GR 127935 (5-HT(1B/1D) antagonist). In contrast, WAY 100635 (selective 5-HT1A antagonist) reversed the effect of 5-HT in hippocampus and entorhinal cortex. Therefore, a detailed pharmacological study was carried out in basal ganglia and substantia nigra using 5-HT and the 5-HT(1B/1D) agonists GTI and CP 93129. In these areas, these agonists stimulated [35S]GTPgammaS binding in a concentration-dependent manner, with no significant differences in the potency for a given structure. Furthermore, GTI was more potent in the putamen than in globus pallidus. In caudate-putamen, the three agonists showed the same efficacy, while in globus pallidus and substantia nigra the efficacy of 5-HT was higher than GTI and CP 93129. The selective 5-HT1B antagonist SB-224289 inhibited GTI- and CP 93129-stimulated [35S]GTPgammaS binding in basal ganglia and substantia nigra, while coincubation with BRL 15572 (selective 5-HT1D antagonist) did not result in any significant change. Here we report the anatomical pattern of distribution of 5-HT1B-dependent functionality by using specific pharmacological tools in human brain sections.

G-protein-coupled receptor oligomerization and its potential for drug discovery

G-protein-coupled receptors (GPCRs) represent by far the largest class of targets for modern drugs. Virtually all therapeutics that are directed towards GPCRs have been designed using assays that presume that these receptors are monomeric. The recent realization that these receptors form homo-oligomeric and hetero-oligomeric complexes has added a new dimension to rational drug design. However, this important aspect of GPCR biology remains largely unincorporated into schemes to search for new therapeutics. This review provides a synopsis of the current thinking surrounding GPCR homo-oligomerization and hetero-oligomerization and shows how new models point towards unexplored avenues in the development of new therapies.