Pharmacological characterization of regulation of phosphoinositide metabolism by recombinant 5-HT2 receptors of the rat

Serotonin and CGRP in migraine

Migraine is defined as recurrent attack of headache that are commonly unilateral and accompanied by gastrointestinal and visual disorders. Migraine is more prevalent in females than males with a ratio of 3:1. It is primarily a complex neurovascular disorder involving local vasodilation of intracranial, extracerebral blood vessels and simultaneous stimulation of surrounding trigeminal sensory nervous pain pathway that results in headache. The activation of 'trigeminovascular system' causes release of various vasodilators, especially calcitonin gene-related peptide (CGRP) that induces pain response. At the same time, decreased levels of neurotransmitter, serotonin have been observed in migraineurs. Serotonin receptors have been found on the trigeminal nerve and cranial vessels and their agonists especially triptans prove effective in migraine treatment. It has been found that triptans act on trigeminovascular system and bring the elevated serum levels of key molecules like calcitonin gene related peptide (CGRP) to normal. Currently CGRP receptor antagonists, olcegepant and telcagepant are under consideration for antimigraine therapeutics. It has been observed that varying levels of ovarian hormones especially estrogen influence serotonin neurotransmission system and CGRP levels making women more predisposed to migraine attacks. This review provides comprehensive information about the role of serotonin and CGRP in migraine, specifically the menstrual migraine.

Laboratory of molecular neurobiology (1988-1994)

While Dr. Costa was the Director of FGIN he became interested in the regulation of genes encoding various neurotransmitter receptors. More specifically, there was increasing evidence supporting a role for tetanic stimulation of the NMDA-subtype of ionotropic glutamate receptors in the development of long term potentiation and long term depression. Moreover, the protein products of the immediate early gene family, such as c-fos and c-jun, were known modulators of downstream signaling events that facilitated changes in neuronal transcriptomes in response to incoming afferent stimulation. The immediate early gene products were known transcriptional factors that activated gene expression in response to excitatory stimulation. In fact, the expression of c-fos/c-jun was often used to map neuronal circuits linked through a common initiation point such as occurs in focally-evoked seizures. Dr. Costa firmly believed that excitatory and inhibitory transmission was balanced in the central nervous system and that this might come about through changes in the expression of the genes encoding these neurotransmitter receptors. In other words, persistent stimulation of NMDA receptors would be expected to increase expression of the inhibitory GABAA receptors to accommodate the increased excitation. That this receptor crosstalk might occur through the products of the immediate early genes was testable and the focus of the Molecular Neurobiology Laboratory from 1988 to 1994. In a broader sense, stimulation of ionotropic NMDA-selective glutamate receptors has been associated with numerous downstream molecular and cellular processes. How these processes are linked to changes in gene expression has been the focus of studies in the neurosciences for many years.

Characterisation of 5-HT2 receptor subtypes in the Suncus murinus intestine

The involvement of 5-HT2 receptor subtypes in mediating a contraction response in the isolated intestine of Suncus murinus was investigated using DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-amino-propane, a 5-HT2 receptor agonist) which produced a bell-shaped concentration response curve that was significantly (p < 0.05) reduced by methysergide (a 5-HT1/2 receptor antagonist, 1 microM) but not ketanserin (a 5-HT2A receptor antagonist, 1 microM), yohimbine (a 5-HT2B receptor antagonist, 1 microM) or a combination of ondansetron (a 5-HT3 receptor antagonist, 1 microM) plus SB204070 (8-amino-7-chloro(N-butyl-4-piperidyl) methylbenzo-1,4-dioxan-5-carboxylate hydrochloride, a 5-HT4 receptor antagonist, 1 nM). The contraction response to the lower concentrations of DOI (10 nM-0.3 microM) was reduced in the presence of SB206553 (5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2 ,3-f]indole, a 5-HT2B/2C receptor antagonist, 1 microM), whilst conversely, the reducing response to the higher concentrations of DOI (1-30 microM) was prevented. A repeated challenge with 3 microM DOI produced a smaller response (desensitisation) and also reduced the response to 5-HT (5-hydroxytryptamine, 0.3 microM) that was inhibited by SB206553 (1 microM). Data indicate that 5-HT2C receptors are likely candidates to mediate the contractile response to DOI and demonstrate desensitisation to repeated challenges.

Dopamine and serotonin interactions in the modulation of the expression of the immediate-early transcription factor, nerve growth factor-inducible B, in the striatum

Nerve growth factor-inducible B is a closely related member of the steroid-thyroid hormone receptor family of ligand-activated transcription factor. Recent evidence suggests a close relationship between nerve growth factor-inducible B and the dopamine system. Basal expression of messenger RNA for nerve growth factor-inducible B is relatively high in the striatum. The aims of the present study were: (i) to study the basal distribution and the modulation of striatal nerve growth factor-inducible B messenger RNA expression by dopamine and serotonin agonists, and (ii) to investigate the effects of combined administration of dopamine (D) and serotonin (5-HT) agonists. First, we investigated the effects of SKF38393 (D1), quinpirole (D2), 8-hydroxy-2-(di-n-propylaminotetralin) (5-HT1A) and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (5-HT2A/2C) administered alone on striatal nerve growth factor-inducible B messenger RNA expression. In a second series of experiments, the effects of a combined administration of dopamine D1 and serotonin 5-HT1A or 5-HT2A/2C agonists were studied. The goal of the last series of experiments was to determine the effects of a combined administration of the dopamine D2 agonist and either serotonin 5-HT1A or 5-HT2A/2C agonists. Our results show that: (i) striatal nerve growth factor-inducible B messenger RNA expression exhibited a lateral-medial gradient in drug-naive rats, (ii) quinpirole and 8-hydroxy-2-(di-n-propylaminotetralin) administered alone induced a significant decrease in striatal nerve growth factor-inducible B messenger RNA expression while 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane significantly increased it, (iii) complex interactions were found when dopamine D1 and serotonin 5-HT1A or 5-HT2A/2C agonists were administered in combination, and (iv) combined administration of quinpirole and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane resulted in a significant decrease in nerve growth factor-inducible B expression. Taken together, these results demonstrate that striatal nerve growth factor-inducible B messenger RNA expression can be modulated by both dopamine and serotonin agonists. They also point out the existence of complex interactions between dopamine and serotonin in regard to striatal expression of the immediate-early transcription factor nerve growth factor-inducible B.

Serotonin 5HT2A receptor activation inhibits inducible nitric oxide synthase activity in C6 glioma cells

C6-glioma cells endogenously express both 5HT2A receptors and inducible nitric oxide synthase (iNOS). iNOS can be induced by transcriptional activation to produce nitric oxide (NO) in response to a challenge with lipopolysaccharide (LPS). Experiments were conducted to determine whether 5HT2A receptor activation could modify the production of NO in response to LPS. Incubation of 10 microg/ml LPS with C6-glioma cells for a period of 24 hours resulted in a 2.6 fold increase in nitrite levels, as a measure of NO levels, over vehicle treated controls. Co-incubation with the selective 5HT2A receptor partial agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) produced a dose-dependent inhibition of the LPS-induced nitrite levels of 22% with an IC50 of 16 nM. The full agonists serotonin (5HT) and alpha-methyl-5HT produced an inhibition of approximately 30% at a concentration of 1 microM. The inhibitory effect of 1 microM DOI was blocked by the 5HT2A receptor antagonists spiperone and ritanserin (10 nM). Inhibition of protein kinase C (PKC) using 100 nM chelerythrine prevented the DOI-mediated decrease in LPS-induced nitrite levels. Addition of DOI to the cells after 1 hr following the LPS addition did not produce a decrease in nitrite levels indicating iNOS was not modified post-translationally. The data demonstrate that iNOS activity can be modulated by serotonin 5HT2A receptor activation, most likely at the initiation of the induction process, via PKC. We therefore suggest that there may be a link between the serotonergic system and NO-mediated immune responses in the brain.

Functional characteristics of heterologously expressed 5-HT receptors

Over the past 10 years, molecular cloning has revealed the presence of 15 serotonin (5-hydroxytryptamine; 5-HT) receptor subtypes, which can be subdivided in seven subfamilies. Except for the 5-HT3 receptors, which are ligand-gated ion channels, all 5-HT receptors belong to the superfamily of G-protein-coupled receptors. The large multiplicity of 5-HT receptor subtypes has been suggested to be a direct result of the evolutionary age of the 5-HT system. Molecular information on G-protein-coupled 5-HT receptors is currently available for several mammalian species as well as for a limited number of invertebrate species (insects, molluscs). The aim of this review is to give an overview of all cloned 5-HT receptor subtypes belonging to the superfamily of G-protein-coupled receptors with specific emphasis on the pharmacological and signaling properties of the receptors upon expression in several heterologous expression systems.

Multiple serotonin receptors: too many, not enough, or just the right number?

In this manuscript, current knowledge about central nervous system serotonin (5-HT) receptors is discussed with an emphasis toward describing the functional significance of the multiple 5-HT receptors. Five characteristics of 5-HT receptors, which are hypothesized to contribute to this functional significance, are discussed: (a) 5-HT has varying affinity and potency for the different receptor subtypes; (b) multiple transduction pathways are used by the different receptor subtypes; (c) receptor subtypes differ in their susceptibility to agonist-mediated desensitization/downregulation; (d) receptor subtypes interact in mediating cellular responses to the neurotransmitter; and (e) receptor subtypes respond differently to changes in the physiological environment. It is hypothesized that these characteristics of the multiple neurotransmitter receptors provide the nervous system with a capacity for coding and decoding of 5-HT-mediated neuronal transmission that could not take place with a single neurotransmitter receptor. Serotonergic regulation of female reproduction and regulation of glucocorticoid release are used to illustrate the integrative potential deriving from the existence of multiple 5-HT receptors.

Preprotachykinin and preproenkephalin mRNA expression within striatal subregions in response to altered serotonin transmission

The effects of lowered serotonin (5-hydroxytryptamine; 5-HT) neurotransmission on preprotachykinin (PPT) and preproenkephalin (PPE) mRNA levels were examined in subregions of the striatum. Adult male rats were treated systemically with para-chlorophenylalanine (pCPA; 350 mg/kg single i.p. injection) which reduced forebrain 5-HT amounts to approximately 20% of saline-injected controls at 24 and 48 h. As measured by Northern analysis, PPT and PPE mRNA levels were elevated 50% and 160% respectively in the anterior ventromedial striatum (region included nucleus accumbens). PPT mRNA levels were raised 90% in posterior striatum (at the level of the globus pallidus) by 48 h post-pCPA injection. To determine if increased PPT and PPE mRNA levels represented a transient response to brief 5-HT inhibition, additional experiments were performed to provide continual suppression of 5-HT within the striatum. First, rats received daily intraperitoneal injections of saline or the 5-HT1A receptor agonist, 8-OH-DPAT (1 mg/kg), for 7 days to reduce 5-HT release from raphestriatal terminals. In a parallel experiment, the serotonin neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT, 5 micrograms), was stereotaxically injected into the striatum as a means to permanently remove 5-HT terminals. Although levels of each mRNA species were differentially sensitive to 5,7-DHT or 8-OH-DPAT, PPT and PPE mRNAs were lowered between 30-55% within the anterior dorsolateral and ventromedial striatum. Although these results support previous studies suggesting an overall positive regulatory role of serotonin on striatal tachykinin biosynthesis, PPT and PPE gene regulation in certain striatal subregions may by differentially sensitive to lowered 5-HT neurotransmission. This suggestion is supported by observations that acute systemic stimulation of 5-HT2A/C receptors with DOI (7 mg/kg single i.p. injection) raised PPT and PPE mRNA levels within anterior dorsolateral (30-60%) and posterior (100-200%) striata, but not within the anterior ventromedial striatum.

Activities of novel aryloxyalkylimidazolines on rat 5-HT2A and 5-HT2C receptors

Using transfected NIH 3T3 mouse fibroblast cell lines expressing the rat 5-HT2A and rat 5-HT2C receptor subtypes, and techniques of 2-[125I](+)-iodolysergic acid diethylamide ([125I]LSD) binding and serotonin (5-hydroxytryptamine, 5-HT)-stimulated phosphoinositide hydrolysis, we have characterized a new structural class of 5-HT receptor ligands, the aryloxyalkylimidazolines. These compounds were found to be potent competitors of [125I]LSD binding at both receptor subtypes (Ki approximately 5-200 nM) and to have efficacy ranging from potent competitive antagonists (IC50 approximately 25 nM) to moderately potent full agonists (EC50 approximately 200 nM). Some of these compounds are agonists at both receptor subtypes, while others are 5-HT2C receptor agonists with 5-HT2A receptor antagonist activity. None of the aryloxyalkylimidazolines reported here have 5-HT2A or 5-HT2C receptor selective antagonist activity. Since these compounds are novel structures, we compared them with a variety of reference 5-HT receptor ligands selected from other chemical classes that have previously been studied at 5-HT2A and 5-HT2C receptors in native tissues.

Effect of citalopram on the desensitization of serotonin-2A receptor-mediated calcium mobilization in rat glioma cells

1. The authors have investigated the effect of citalopram, an effective antidepressant drug with selective serotonin (5-HT) uptake inhibition, on 5-HT-2A receptor-mediated intracellular calcium (Ca2+) rise in C6 cultured cells. 2. Citalopram, at concentrations of 10 and 30 mu M, did not significantly reduce the Ca2+ mobilization induced by 10 mu M 5-HT, indicating that citalopram has little affinity for 5-HT-2A receptors. 3. Citalopram did not alter a subsequent response to 5-HT after citalopram was pre-applied to the cells. 4. However, citalopram inhibited the desensitization of 5-HT-2A receptors. When the cells were pretreated with citalopram and 5-HT, the subsequent response to 5-HT was significantly greater than that obtained following pretreatment with 5-HT alone. 5. To investigate the mechanism of action of citalopram on the desensitization of 5-HT-2A receptors, NaF-induced cGMP generation was measured. Citalopram inhibited the generation of cGMP induced by NaF in C6 cells as well as W-7. 6. These results indicate that citalopram antagonized the desensitization of 5-HT-2A receptor-mediated Ca2+ mobilization and this antagonism may be mediated by a calmodulin-dependent pathway in C6 glioma cells.

Cyclic GMP generation mediated by 5-HT-2 receptors via nitric oxide-dependent pathway and its effect on the desensitization of 5-HT-2 receptors in C6 glioma cells

Serotonin (5-HT)-2 receptor-mediated cGMP generation was investigated in comparison with calcium (Ca2+) mobilization in C6 glioma cells. 5-HT enhanced cGMP generation, and risperidone and ketanserin potently blocked the response. These results indicate that 5-HT-2 receptors are responsible for the cGMP generation. 5-HT-induced cGMP production was completely abolished by BAPTA, an intracellular Ca2+ chelating agent, or NG-mono-methyl-L-arginine(NMMA), a nitric oxide synthase (NOS) inhibitor, suggesting that 5-HT-induced cGMP generation was through nitric oxide (NO)-dependent pathway. 5-HT (10 microM)-elicited Ca2+ mobilization and cGMP generation were reduced to 40 and 15% after pretreatment with 10 microM 5-HT for 4 hours. NMMA did not modify 5-HT-induced desensitization of either Ca2+ mobilization or cGMP generation, suggesting that NO pathway is independent of the desensitization. The present study has demonstrated the nature of 5-HT-2 receptor-mediated cGMP generation in C6 glioma cells.

Molecular biology of serotonin (5-HT) receptors

The hypothesis that multiple serotonin (5-hydroxytryptamine; 5-HT) receptors exist was first developed in the 1950s. However, unequivocal proof of 5-HT receptor multiplicity required the availability of molecular biological technologies. Indeed, the multiplicity of 5-HT receptor subtypes, both within and between species, has far exceeded most of the predictions that might have been made on the basis of pharmacological data. Over the past few years, and especially in 1992 and 1993, numerous "new" 5-HT receptors were reported. In this review, the extensive data generated in the past few years are summarized in an evolutionary context.

Maternal ethanol consumption: effects on G proteins and second messengers in brain regions of offspring

Previous work in this and other laboratories has demonstrated that in utero ethanol exposure adversely affects the development of the serotonergic, dopaminergic, cholinergic, and other neurotransmitter systems. In several of these systems, receptor number is significantly altered. To determine whether the altered number of two G protein-linked receptors is reflected in changes in cell function, we examined dopamine-stimulated adenylate cyclase in the striatum and cortex and carbachol-stimulated phosphoinositide (PI) hydrolysis in the cortex. Serotonin-stimulated cortical PI hydrolysis was assessed for comparison. We also studied G proteins that link adenylate cyclase and other second messenger systems to their receptors. The G proteins that were analyzed include the alpha-subunits for Gs, G0, Gi1, Gi2, and Gi3. G proteins were analyzed in the cortex and cortical regions, as well as in the brain stem. The results of these experiments demonstrated that dopamine-stimulated adenylate cyclase activity was comparable in the striatum of 5- and 19-day offspring of control and ethanol-fed rats and in the motor cortex of 19-day offspring. We also found that carbachol- and serotonin-stimulated hydrolysis of cortical phosphoinositides was unchanged in ethanol-exposed offspring on gestational day 19, and on postnatal days 5 and 19. G protein content was examined by Western blot analysis, using antibodies directed against the alpha-subunits of Gs, G0, and the Gi1/Gi2 and Gi3/G0 combinations. These investigations indicated that, with two minor exceptions (approximately 10% change in the proteins detected by antibodies against the alpha-subunits of the Gi1/Gi2 and Gi3/G0 combinations), there were no significant differences in the content of any of the G proteins analyzed.

Homologous desensitization of serotonin 5-HT2 receptor-stimulated intracellular calcium mobilization in C6BU-1 glioma cells via a mechanism involving a calmodulin pathway

Serotonin 5-HT2 receptor-mediated intracellular Ca2+ mobilization was investigated in rat glioma C6BU-1 cells. The receptors became desensitized after previous exposure to 5-HT in a time- and concentration-dependent manner. The desensitization of 5-HT2 receptor-mediated intracellular signaling appeared to be homologous because previous exposure to 5-HT did not alter the response to other transmitters such as thrombin or isoproterenol and because previous exposure to thrombin or isoproterenol did not diminish the response to 5-HT. The desensitization induced by pretreatment with 5-HT was potently prevented by the naphthalenesulfonamide derivative W-7, a calmodulin antagonist, when it was cosupplied with 5-HT. Furthermore, the preventive effect of W-7 was greater than that of W-5, a weak analogue of W-7, and than that of H-7, a nonselective inhibitor of protein kinases. These results suggest that 5-HT2 receptor-mediated Ca2+ mobilization can be desensitized homologously after prolonged exposure to 5-HT in a calmodulin-dependent manner in rat glioma C6BU-1 cells.