Serotonin 2C (5-HT 2C ) receptor pre-mRNA is a substrate for RNA editing enzymes that convert five adenosines (named A, B, CЈ, C, and D editing sites) to inosines. Editing of two of these sites (CЈ and C) is crucial for decreasing the efficiency of the receptor to activate G-protein. Nucleotide sequence analysis of mouse forebrain neocortical 5-HT 2C mRNA isoforms revealed that editing at these two sites is regulated in a serotonindependent manner. In serotonin-depleted mice, CЈ-and C-site editing is significantly decreased. This results in an increased expression of 5-HT 2C mRNA isoforms encoding receptors with higher sensitivity to serotonin. In contrast, a 4 d treatment with the 5-HT 2A/2C agonist (Ϯ)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane significantly increases the editing frequency at the CЈ site and leads to increased expression of 5-HT 2C mRNA isoforms encoding receptors that activate G-protein least efficiently. None of the drug treatments led to alterations in cytoplasmic 5-HT 2C mRNA levels. These data indicate that editing of 5-HT 2C pre-mRNA is a mechanism that retains basic response properties of 5-HT 2C receptors in the face of changing synaptic input to keep receptor activation within an optimal range for information processing. Key words: serotonin; 5-HT 2C receptor; RNA editing; forebrain neocortex; 5-HT depletion; 5-HT 2A/2C agonistThe conversion of adenosine (A) to inosine (I) by ADARs (adenosine deaminases that act on RNA) is the most widespread editing in higher eukaryotes (Bass, 2002). Although the total inosine content of rat brain poly(A ϩ ) RNA predicts that one inosine occurs approximately once every 17,000 nucleotides (Paul and Bass, 1998), only a few neuronal substrates for A to I editing have been identified. They include the serotonin 2C (5-HT 2C ) receptor, the first G-protein-coupled receptor known to be edited. In 5-HT 2C pre-mRNA, a total of five closely spaced adenosines (named A, B, CЈ, C, and D editing sites) located within a sequence that encodes the second intracellular loop of the receptors protein can be converted to inosines (Burns et al., 1997;Niswender et al., 1999). This editing can change up to three triplet codons and has the potential to generate 24 different protein isoforms. Compared with nonedited 5-HT 2C receptors, the receptor isoform that results from editing at the ABCD sites and other partially edited isoforms that are edited at the CЈsite but not the C site exhibit a fourfold reduction in the efficiency to activate G-protein in response to agonist stimulation. This reduction is even higher (15-to 25-fold) for completely edited isoforms and other partially edited isoforms that are edited at both CЈ and C sites. Other partially edited isoforms that are not edited at the CЈ and/or C site appear to be fully functional (Niswender et al., 1999;Wang et al., 2000).At present, the significance of 5-HT 2C pre-mRNA editing in vivo is still unknown. However, a recent study indicates that this editing is regulated and suggests that this regulation is sensitive ...

Positron emission tomography studies in major depression show reduced serotonin (5-HT) 1A receptor antagonist-binding potentials in many brain regions including occipital cortex. The functional meaning of this observation in terms of signal transduction is unknown. We used postmortem brain samples from depressed suicide victims to examine the downstream effectors of 5-HT 1A receptor activation. The diagnosis was established by means of psychological autopsy using Diagnostic and Statistical Manual of Mental Disorders (DSM) III-R criteria. Measurements of [ 35 S]GTPcS binding to Gai/o in the occipital cortex of suicide victims and matched controls revealed a blunted response in suicide subjects and a decrease in the coupling of 5-HT 1A receptor to adenylyl cyclase. No significant group differences were detected in the expression levels of Gai/o, Gaq/11 or Gas proteins, or in the activity of cAMP-dependent protein kinase A. Studies of a parallel transduction pathway downstream from 5-HT 1A receptor activation demonstrated a decrease in the activity of phosphatidylinositol 3-kinase and its downstream effector Akt, as well as an increase in PTEN (phosphatase and tensin homolog deleted on chromosome 10), the phosphatase that hydrolyzes phosphatidylinositol 3,4,5-triphosphate. Finally, the activation of extracellular signal-regulated kinases 1 and 2 was attenuated in suicide victims. These data suggest that the alterations in agonist-stimulated 5-HT 1A receptor activation in depressed suicide victims are also manifest downstream from the associated G protein, affecting the activity of second messengers in two 5-HT 1A receptor transduction pathways that may have implications for cell survival. Keywords: adenylyl cyclase, Akt, G protein, mitogen-activated protein kinase, phosphatidylinositol 3-kinase, suicide. J. Neurochem. (2003) 87, 182-194. Although several findings indicate that levels of the neurotransmitter serotonin (5-HT) are reduced in depressed suicide victims (Mann 1998), the physiological consequences of such a reduction for 5-HT-receptor-activated downstream signaling are less well described. Autoradiographic studies on brains obtained from suicide victims revealed an increased density of postsynaptic 5-HT 1A receptor agonist

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Parafollicular (PF) cells of the thyroid gland are neural crest derivatives. These cells remain plastic even in adult animals and can be induced to exhibit neural properties when exposed to NGF in vitro. A human cell line derived from PF cells, medullary thyroid carcinoma (MTC), has previously been shown to synthesize and store 5-HT, a serotonin-binding protein (SBP), and several neuropeptides; moreover, when grown in impoverished media, MTC cells display neural properties. The purpose of the current study was to utilize MTC cells as a neurally relevant model system to investigate factors involved in mediating 5-HT secretion. Electron microscopic immunocytochemistry revealed that secretory vesicles of MTC cells costore immunoreactive 5-HT with SBP and calcitonin. The cAMP derivative, N6-2'-O-dibutyryl-adenosine 3',5'-cyclic monophosphate (dibutyryl-cAMP; 1.0 mM) increased the concentration of 5-HT in MTC cells and almost doubled the rate of synthesis of 5-HT from L-tryptophan. Dibutyryl-cAMP also significantly increased the secretion of 5-HT. Cycloheximide (20 micrograms/ml) and anisomycin (20 microM) inhibited the dibutyryl-cAMP-induced increase of 5-HT release, suggesting that this action of dibutyryl-cAMP requires protein synthesis. Cholera toxin (1.0 microgram/ml) and forskolin (0.05 mM) in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (1.0 mM) both increased 5-HT biosynthesis and secretion. Attempts were made to identify a ligand that stimulates cAMP-mediated secretion of 5-HT. Both thyroid-stimulating hormone (TSH: 50 mU/ml) and elevated [Ca2+]e (7.0 mM), each of which acts as a secretogogue for PF cells, stimulated the secretion of 5-HT. The effect of TSH was Ca2(+)-dependent. Immunocytochemistry with monoclonal antibodies to the TSH receptor confirmed that these receptors are present on MTC cells. Neither TSH nor elevated [Ca2+]e elevated cAMP levels. Measurements of Fura-2 fluorescence, however, indicated that both TSH and elevated [Ca2+]e increased cytosolic calcium ([Ca2+]i), as did elevation of [K+]e. It is concluded that exocytosis can be triggered in MTC cells by multiple signal transduction mechanisms. Either cAMP or elevated [Ca2+]i can stimulate secretion; however, a secretogogue that increases cAMP has yet to be identified.

Parafollicular (PF) cells secrete 5-HT in response to stimulation of a G-protein-coupled Ca 2ϩ receptor (CaR) by increased extracellular Ca 2ϩ (1[Ca 2ϩ ] e ). We tested the hypothesis that protein kinase C (PKC) participates in stimulus-secretion coupling. Immunoblots from membrane and cytosolic fractions of isolated PF cells revealed conventional (␣, ␤I, and ␥), novel (␦ and ⑀), and atypical ( / and ) PKCs. Only PKC␥ was found to have been translocated to the membrane fraction when secretion of 5-HT was evoked by 1[Ca 2ϩ ] e or phorbol esters. Although phorbol downregulation caused PKC␥ to disappear, secretion was only partially inhibited. A similar reduction of 1[Ca 2ϩ ] e -evoked secretion was produced by inhibitors of conventional and/or novel PKCs (Gö 6976, calphostin C, and pseudoA), and these compounds did not inhibit secretion at all when applied to phorbol-downregulated cells. In contrast, the phorbol downregulation-resistant component of secretion was abolished by pseudoZ, which inhibits the atypical PKC . Stimulation of PF cells with 1[Ca 2ϩ ] e increased the activity of immunoprecipitated PKC (but not PKC / ), and the activity of this PKC was inhibited by pseudoZ. PF cells were found to express regulatory (p85) and catalytic (p110␣ and p110␤) subunits of phosphatidylinositol 3Ј-kinase (PI3Ј-kinase). 1[Ca 2ϩ ] e increased the activity of immunoprecipitated PI3Ј-kinase; moreover, PI3Ј-kinase inhibitors (wortmannin and LY294002) antagonized secretion. We suggest that PKC isoforms mediate secretion of 5-HT by PF cells in response to stimulation of the CaR. PKC involvement can be accounted for by PKC␥ and an isoform sensitive to inhibition by pseudoZ, probably PKC , which is activated via PI3Ј-kinase.