Type-3 metabotropic glutamate receptors regulate chemoresistance in glioma stem cells, and their levels are inversely related to survival in patients with malignant gliomas.

Drug treatment of malignant gliomas is limited by the intrinsic resistance of glioma stem cells (GSCs) to chemotherapy. GSCs isolated from human glioblastoma multiforme (GBM) expressed metabotropic glutamate receptors (mGlu3 receptors). The DNA-alkylating agent, temozolomide, killed GSCs only if mGlu3 receptors were knocked down or pharmacologically inhibited. In contrast, mGlu3 receptor blockade did not affect the action of paclitaxel, etoposide, cis-platinum, and irinotecan. mGlu3 receptor blockade enabled temozolomide toxicity by inhibiting a phosphatidylinositol-3-kinase/nuclear factor-κB pathway that supports the expression of O(6)-methylguanine-DNA methyltransferase (MGMT), an enzyme that confers resistance against DNA-alkylating agents. In mice implanted with GSCs into the brain, temozolomide combined with mGlu3 receptor blockade substantially reduced tumor growth. Finally, 87 patients with GBM undergoing surgery followed by adjuvant chemotherapy with temozolomide survived for longer time if tumor cells expressed low levels of mGlu3 receptors. In addition, the methylation state of the MGMT gene promoter in tumor extracts influenced survival only in those patients with low expression of mGlu3 receptors in the tumor. These data encourage the use of mGlu3 receptor antagonists as add-on drugs in the treatment of GBM, and suggest that the transcript of mGlu3 receptors should be measured in tumor specimens for a correct prediction of patients' survival in response to temozolomide treatment.

Protective role for type-1 metabotropic glutamate receptors against spike and wave discharges in the WAG/Rij rat model of absence epilepsy.

Eight-month old WAG/Rij rats, which developed spontaneous occurring absence seizures, showed a reduced function of mGlu1 metabotropic glutamate receptors in the thalamus, as assessed by in vivo measurements of DHPG-stimulated polyphosphoinositide hydrolysis, in the presence of the mGlu5 antagonist MPEP as compared to age-matched non-epileptic control rats. These symptomatic 8-month old WAG/Rij rats also showed lower levels of thalamic mGlu1α receptors than age-matched controls and 2-month old (pre-symptomatic) WAG/Rij rats, as detected by immunoblotting. Immunohistochemical and in situ hybridization analysis indicated that the reduced expression of mGlu1 receptors found in symptomatic WAG/Rij rats was confined to an area of the thalamus that excluded the ventroposterolateral nucleus. No mGlu1 receptor mRNA was detected in the reticular thalamic nucleus. Pharmacological manipulation of mGlu1 receptors had a strong impact on absence seizures in WAG/Rij rats. Systemic treatment with the mGlu1 receptor enhancer SYN119, corresponding to compound RO0711401, reduced spontaneous spike and wave discharges spike-wave discharges (SWDs) in epileptic rats. Subcutaneous doses of 10 mg/kg of SYN119 only reduced the incidence of SWDs, whereas higher doses (30 mg/kg) also reduced the mean duration of SWDs. In contrast, treatment with the non-competitive mGlu1 receptor antagonist, JNJ16259685 (2.5 and 5 mg/kg, i.p.) increased the incidence of SWDs. These data suggest that absence epilepsy might be associated with a reduction of mGlu1 receptors in the thalamus, and that compounds that amplify the activity of mGlu1 receptors might be developed as novel anti-absence drugs. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Activation of mGlu2/3 metabotropic glutamate receptors negatively regulates the stimulation of inositol phospholipid hydrolysis mediated by 5-hydroxytryptamine2A serotonin receptors in the frontal cortex of living mice.

The interaction between 5-hydroxytryptamine(2A) (5-HT(2A)) serotonin receptors and metabotropic glutamate (mGlu) 2/3 receptors underlies the antipsychotic activity of mGlu2/3 receptor agonists in experimental animals and humans. The molecular nature of this interaction is only partially known. We here report for the first time that pharmacological activation of mGlu2/3 receptors attenuates the stimulation of polyphosphoinositide (PI) hydrolysis mediated by 5-HT(2A) receptors in the frontal cortex of living mice. Mice were injected intracerebroventricularly with [myo-(3)H]inositol and treated with drugs 1 h after a pretreatment with lithium, which blocks the conversion of inositol monophosphate into free inositol. Systemic injection of the mGlu2/3 receptor agonist (-)-2-oxa-4-aminocyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY379268) inhibited the stimulation of PI hydrolysis induced by the hallucinogenic 5-HT(2A) receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) without affecting the stimulation by mGlu1/5 or muscarinic receptors. The action of LY379268 was prevented by the preferential mGlu2/3 receptor antagonist (2S,1'S,2'S)-2-(9-xanthylmethyl)-2-(2'-carboxycyclopropyl)glycine (LY341495). N-(4'-cyano-biphenyl-3-yl)-N-(3-pyridinylmethyl)-ethanesulfonamide hydrochloride (LY566332), a selective mGlu2 receptor enhancer, also reduced DOI-stimulated PI hydrolysis when combined with subthreshold doses of LY379268. Systemic LY379268 inhibited DOI-stimulated PI hydrolysis in mice lacking either mGlu2 or mGlu3 receptors but was inactive in double mGlu2/mGlu3 receptor knockout mice, suggesting that both mGlu2 and mGlu3 receptors interact with 5-HT(2A) receptors. Surprisingly, contrasting results were obtained in cortical slice preparations, where LY379268 amplified both DOI- and 3,5-dihydroxyphenylglycine-stimulated PI hydrolysis. Amplification was abrogated by the mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)pyridine, suggesting that experiments in brain slices are biased by an additional component of receptor-stimulated PI hydrolysis. This highlights the importance of in vivo models for the study of the interaction between 5-HT(2A) and mGlu2/3 receptors.

Extra-nuclear estrogen receptor GPR30 regulates serotonin function in rat hypothalamus.

Selective serotonin reuptake inhibitors (SSRIs), such as Prozac, are used to treat mood disorders. SSRIs attenuate (i.e. desensitize) serotonin 1A (5-HT(1A)) receptor signaling, as demonstrated in rats through decreased release of oxytocin and adrenocorticotropin hormone (ACTH) following 5-HT(1A) receptor stimulation. Maximal therapeutic effects of SSRIs for treatment of mood disorders, as well as effects on hypothalamic 5-HT(1A) receptor signaling in animals, take 1 to 2 weeks to develop. Estradiol also attenuates 5-HT(1A) receptor signaling, but, in rats, these effects occur within 2 days; thus, estrogens or selective estrogen receptor modulators may serve as useful short-term tools to accelerate desensitization of 5-HT(1A) receptors in response to SSRIs if candidate estrogen receptor targets in the hypothalamus are identified. We found high levels of GPR30, which has been identified recently as a pertussis-toxin (PTX) sensitive G-protein-coupled estrogen receptor, in the hypothalamic paraventricular nucleus (PVN) of rats. Double-label immunohistochemistry revealed that GPR30 co-localizes with 5-HT(1A) receptors, corticotrophin releasing factor (CRF) and oxytocin in neurons in the PVN. Pretreatment with PTX to the PVN before peripheral injections of 17-beta-estradiol 3-benzoate completely prevented the reduction of the oxytocin response to the 5-HT(1A) receptor agonist, (+)-8-hydroxy-2-dipropylaminotetralin (DPAT). Treatment with the selective GRP30 agonist, G-1, attenuated 5-HT(1A) receptor signaling in the PVN as measured by an attenuated oxytocin (by 29%) and ACTH (by 31%) response to DPAT. This study indicates that a putative extra-nuclear estrogen receptor, GPR30, may play a role in estradiol-mediated attenuation of 5-HT(1A) receptor signaling, and potentially in accelerating the effects of SSRIs in treatment of mood disorders.

Metabotropic glutamate receptors regulate differentiation of embryonic stem cells into GABAergic neurons.

Mouse embryonic stem (ES) cells were stimulated to differentiate either as adherent monolayer cultures in DMEM/F12 supplemented with N2/B27, or as floating embryoid bodies (EBs) exposed to 1 microM retinoic acid (RA) for 4 days, starting from 4 DIV, and subsequently re-plated in DMEM/F12 medium. Adherent monolayer cultures of ES cells expressed mGlu5 receptors throughout the entire differentiation period. Selective pharmacological blockade of mGlu5 receptors with methyl-6-(phenylethynyl)-pyridine (MPEP) (1 microM, added once a day) accelerated the appearance of the neuronal marker, beta-tubulin. In addition, treatment with MPEP increased the number of cells expressing glutamate decarboxylase-65/67 (GAD(65/67)), a marker of GABAergic neurons. In floating EBs, mGlu5 receptors are progressively replaced by mGlu4 receptors. The orthosteric mGlu4/6/7/8 receptor agonist, L-2-amino-4-phosphonobutanoate (L-AP4), or the selective mGlu4 receptor enhancer, PHCCC,--both combined with RA at concentrations of 30 microM--increased the expression of both beta-tubulin and GAD(65/67), inducing the appearance of fully differentiated neurons that released GABA in response to membrane depolarization. We conclude that mGlu receptor subtypes regulate neuronal differentiation of ES cells in a context-dependent manner, and that subtype-selective ligands of these receptors might be used for the optimization of in vitro protocols aimed at producing GABAergic neurons from ES cells.

Synergism between fluoxetine and the mGlu2/3 receptor agonist, LY379268, in an in vitro model for antidepressant drug-induced neurogenesis.

We examined the interaction between the selective serotonin reuptake inhibitor, fluoxetine, and group-II metabotropic glutamate (mGlu) receptors using progenitor cells isolated from cultured cerebellar granule cells, considered as an in vitro model of antidepressant-drug induced neurogenesis. These cells expressed mGlu3 receptors negatively coupled to adenylyl cyclase. A 72-h treatment with either fluoxetine or low concentrations of mGlu2/3 receptor agonists (LY379268 or 2R,4R-APDC) enhanced cell proliferation. The action of fluoxetine was mediated by the activation of 5-HT(1A) receptors. We found a strong synergism between fluoxetine and LY379268 in enhancing cell proliferation and inhibiting cAMP formation. The increased cell proliferation induced by fluoxetine+LY379268 was abrogated by the cAMP analogue, 8-Br-cAMP, as well as by drugs that inhibit the mitogen-activated protein kinase and phosphatidyilinositol-3-kinase pathways. Interestingly, fluoxetine and LY379268 also acted synergistically in promoting neuronal differentiation when progenitor cells were incubated in the presence of serum. These data support the hypothesis that a combination between classical antidepressants and mGlu2/3 receptor agonists may be helpful in the experimental treatment of depression.

Agonist-induced serotonin 2A receptor desensitization in the rat frontal cortex and hypothalamus.

This study examined the time course and possible mechanisms of agonist-induced desensitization of 5-hydroxytryptamine serotonin 2A receptors in the rat frontal cortex and hypothalamic paraventricular nucleus after 1, 4, and 7 days of treatment with (-)-1-(2,5-dimethoxy-4-iodophenyl)2-aminopropane HCl [(-)-DOI] (1 mg/kg i.p.), a selective 5-HT(2A/2C) receptor agonist. In the frontal cortex, 5-HT-mediated phospholipase C (PLC) enzyme activity decreased by 24 to 30% after 4 to 7 days of (-)-DOI treatment without any significant changes in the guanosine 5'-3-O-(thio)triphosphate-mediated PLC enzyme activity. Additionally, treatment with (-)-DOI did not significantly change the levels of G(alpha11), regulator of G protein signaling (RGS)4, or RGS7 proteins in the frontal cortex, whereas G(alphaq) protein levels in the frontal cortex decreased (47%) only after 7 daily (-)-DOI injections. The functional status of 5-HT(2A) receptors in the hypothalamic paraventricular nucleus was examined using 5-HT(2A) receptor-mediated increases in plasma hormone levels. Plasma adrenocorticotrophic hormone (ACTH) and oxytocin measurements showed that 5-HT(2A) receptor desensitization began after only 1 day of (-)-DOI treatment, and the desensitization continued to increase after 4 and 7 days of treatment (ACTH response decreased 64.2-67.7%; oxytocin response decreased 82.3-90.1%). There were no significant alterations in levels of G(alphaq) or G(alpha11) lamic paraventricular proteins in the hypothanucleus. In conclusion, these results suggest that chronically administered (-)-DOI induces desensitization of 5-HT(2A) receptors in vivo, via a reduction in the ability of 5-HT(2A) receptors to activate G proteins without consistently altering levels of G(alpha) proteins or RGS proteins.

Endogenous activation of group-II metabotropic glutamate receptors inhibits the hypothalamic-pituitary-adrenocortical axis.

Systemic injection of the mGlu2/3 receptor antagonist, LY341495 (1 mg/kg, i.p.), increased plasma corticosterone in mice to an extent similar to that induced by the despair test. Treatment with the mGlu2/3 receptor agonist, LY379268 (1 mg/kg, i.p.), or the non-competitive mGlu5 receptor antagonist, MPEP (5 mg/kg, i.p.), failed to induce significant changes in corticosterone levels. Searching for a site of action of LY341495, we examined the expression of mGlu receptor subtypes in the various anatomical regions of the mouse hypothalamic-pituitary-adrenal (HPA) axis. Only mGlu5 and -7 receptor mRNAs were detected in the adrenal gland by RT-PCR, whereas mGlu -1, -3, -4, -5, -7 and -8 receptor mRNAs were detected in the anterior pituitary. All transcripts (with the exception of mGlu5 and mGlu6 receptor mRNAs) were detected in the hypothalamus. However, Western blot analysis showed the presence of mGlu2/3 receptor proteins only in the hypothalamus and not in the anterior pituitary. This was consistent with functional data showing that LY341495 (0.1 and 1 microM) failed to affect ACTH secretion from isolated mouse anterior pituitaries. Moving from these observations, we examined whether LY341495 could activate the HPA axis by inhibiting mGlu2/3 receptors at hypothalamic level. We measured the release of corticotropin releasing hormone (CRH) in isolated mouse hypothalami incubated in the presence of subtype-selective mGlu receptor agonists or antagonists. Among all the drugs we have tested, only LY341495 was able to increase CRH secretion. With high concentrations of LY341495 (1 microM) this increase was similar to that induced by 50 mM K(+). The action of LY341495 was prevented by the combined application of the mGlu2/3 receptor agonist, LY379268. We conclude that group-II mGlu receptors tonically regulate the HPA axis by controlling CRH secretion at hypothalamic level.

Destruction of serotonergic nerve terminals prevents fluoxetine-induced desensitization of hypothalamic 5-HT(1A) receptors.

RATIONALE: Selective serotonin (5-HT, 5-hydroxytryptamine) reuptake inhibitors (SSRIs) such as fluoxetine produce a gradual desensitization of hypothalamic post-synaptic 5-HT(1A) receptor systems. It is assumed that the effects of SSRIs on post-synaptic 5-HT(1A) receptors are mediated by 5-HT reuptake inhibition, leading to an increase of 5-HT in the synapse. However, there is no direct evidence to support this hypothesis. OBJECTIVES: The present study determined whether 5-HT(1A) receptor desensitization was mediated by fluoxetine's effects on serotonergic nerve terminals. METHODS: Serotonergic nerve terminals were destroyed by intracerebroventricular (i.c.v.) injection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) combined with injection of the norepinephrine/dopamine reuptake inhibitor nomifensine. 5,7-DHT-induced loss of serotonergic terminals was confirmed by a 95% reduction in the density of [(3)H]paroxetine-labeled 5-HT transporters in the hypothalamus and a 97% reduction in the levels of 5-HT and 5-hydroxyindoleacetic acid in the cortex. Two weeks after the 5,7-DHT injections, rats were injected daily with fluoxetine (5 mg/kg or 10 mg/kg, i.p.) or saline for 14 days. RESULTS: Injections of 10 mg/kg fluoxetine produced a significant decrease in body weight gain. Destruction of serotonergic nerve terminals reduced body weight and potentiated the ability of fluoxetine to further inhibit body weight gain. Increases in plasma levels of adrenal corticotrophic hormone (ACTH, corticotropin), corticosterone, and oxytocin after injection of the 5-HT(1A) agonist 8-hydroxy-2-dipropylaminotetralin [(+/-)8-OH-DPAT] were used as peripheral markers of 5-HT(1A) receptor function in the hypothalamus. In vehicle-pretreated rats, fluoxetine produced a dose-dependent reduction in the (+/-)8-OH-DPAT-induced increase in plasma ACTH and oxytocin levels. Destruction of serotonergic nerve terminals blocked the ability of fluoxetine to produce a desensitization in the ACTH, corticosterone, and oxytocin responses to (+/-)8-OH-DPAT. CONCLUSIONS: The ability of fluoxetine to induce a desensitization of hypothalamic post-synaptic 5-HT(1A) receptor systems is dependent on the integrity of serotonergic nerve terminals in the hypothalamus, while its effect on body weight is not mediated by serotonergic nerve terminals in the hypothalamus.

Treatment of cycling female rats with fluoxetine induces desensitization of hypothalamic 5-HT(1A) receptors with no change in 5-HT(2A) receptors.

Although women constitute the majority of patients who receive treatment with selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine, most animal studies of SSRIs are conducted on males. The present study investigated whether long-term treatment of cycling female rats with fluoxetine alters their estrous cycle and the sensitivity of hypothalamic serotonin (5-HT) 5-HT(1A) and 5-HT(2A) receptor systems. Adult female rats received daily injections of fluoxetine (10 mg/kg, i.p.) for three consecutive estrous cycles (15.2+/-0.2 days) with the first injection beginning on metestrus (when circulating estrogen levels are low and stable). Fluoxetine did not alter basal plasma estradiol levels at metestrus, nor did it alter the pattern of estrous cyclicity. Rats treated with fluoxetine showed a loss in body weight. On the morning of metestrus of the fourth cycle (18 h after the last fluoxetine injection), the rats were injected with a sub-maximal dose of the 5-HT(1A) agonist (+/-)-8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT, 50 MICRO/kg, s.c.) or a maximal dose of the 5-HT(2A) agonist [(+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl] (DOI). Plasma levels of oxytocin, ACTH and corticosterone were measured as peripheral indicators of hypothalamic 5-HT(1A) and 5-HT(2A) receptor sensitivity. Injecting 8-OH-DPAT to saline pretreated rats produced a significant increase in plasma oxytocin (299%), ACTH (1456%) and corticosterone (170%) levels but not in plasma prolactin or renin concentrations. Greater increases in plasma levels of these hormones were observed after injecting DOI. Fluoxetine treatment completely blocked the oxytocin, ACTH and corticosterone responses to 8-OH-DPAT, but did not inhibit the effect of DOI on any hormone, thus confirming that fluoxetine treatment did not produce a deficit in the functioning of corticotropin releasing hormone or oxytocin containing neurons. These results indicate that in cycling female rats, fluoxetine treatment desensitizes hypothalamic post-synaptic 5-HT(1A) receptor signaling. Understanding the pharmacological effects of fluoxetine in females may lead to more effective treatment of women with mood disorders.

High-fiber diet supplementation in patients with irritable bowel syndrome (IBS): a multicenter, randomized, open trial comparison between wheat bran diet and partially hydrolyzed guar gum (PHGG).

High-fiber diet supplementation is commonly used in IBS, although it poses several management problems. Partially hydrolyzed guar gum (PHGG) has shown beneficial effects in animal and human studies, but its potential role in IBS symptom relief has not been evaluated yet. We investigated PHGG in IBS patients and compared it to a wheat bran diet. Abdominal pain, bowel habits, and subjective overall rating were longitudinally evaluated in 188 adult IBS patients (139 women and 49 men) for 12 weeks. Patients were classified as having diarrhea-predominant, constipation-predominant, or changeable bowel habits and were randomly assigned to groups receiving fiber (30 g/day of wheat bran) or PHGG (5 g/day). After four weeks, patients were allowed to switch group, depending on their subjective evaluation of their symptoms. Significantly more patients switched from fiber to PHGG (49.9%) than from PHGG to fiber (10.9%) at four weeks. Per protocol analysis showed that both fiber and PHGG were effective in improving pain and bowel habits, but no difference was found between the two groups. Conversely, intention-to-treat analysis showed a significantly greater success in the PHGG group (60%) than in the fiber group (40%). Moreover, significantly more patients in the PHGG group reported a greater subjective improvement than those in the Fiber group. In conclusion, improvements in core IBS symptoms (abdominal pain and bowel habits) were observed with both bran and PHGG, but the latter was better tolerated and preferred by patients, revealing a higher probability of success than bran and a lower probability of patients abandoning the prescribed regimen, suggesting that it can increase the benefits deriving from fiber intake in IBS, making it a valid option to consider for high-fiber diet supplementation.

Characterization of the functional heterologous desensitization of hypothalamic 5-HT(1A) receptors after 5-HT(2A) receptor activation.

Desensitization of 5-HT(1A) receptors could be involved in the long-term therapeutic effect of anxiolytic and antidepressant drugs. Pretreatment of rats with the 5-HT(2A/2C) agonist DOI induces an attenuation of hypothalamic 5-HT(1A) receptor-G(z)-protein signaling, measured as the ACTH and oxytocin responses to an injection of the 5-HT(1A) agonist 8-OH-DPAT. We characterized this functional heterologous desensitization of 5-HT(1A) receptors in rats and examined some of the mechanisms that are involved. A time course experiment revealed that DOI produces a delayed and reversible reduction of the ACTH and oxytocin responses to an 8-OH-DPAT challenge. The maximal desensitization occurred at 2 hr, and it disappeared 24 hr after DOI injection. The desensitization was dose-dependent, and it shifted the oxytocin and ACTH dose-response curves of 8-OH-DPAT to the right (increased ED(50)) with no change in their maximal responses (E(max)). The 5-HT(2A) receptor antagonist MDL 100,907 prevented the DOI-induced desensitization, indicating that 5-HT(2A) receptors mediate the effect of DOI. Analysis of the components of the 5-HT(1A) receptor-G(z)-protein signaling system showed that DOI did not alter the level of membrane-associated G(z)-proteins in the hypothalamus. Additionally, DOI did not alter the binding of [(3)H]8-OH-DPAT or the inhibition by GTPgammaS of [(3)H]8-OH-DPAT binding in the hypothalamus. In conclusion, the activation of 5-HT(2A) receptors induces a transient functional desensitization of 5-HT(1A) receptor signaling in the hypothalamus, which may occur distal to the 5-HT(1A) receptor-G(z)-protein interface.

Estrogen desensitizes 5-HT(1A) receptors and reduces levels of G(z), G(i1) and G(i3) proteins in the hypothalamus.

The present study investigated whether estrogen would desensitize hypothalamic serotonin(1A) (5-HT(1A)) receptors by examining the neuroendocrine response to 8-OH-DPAT, a 5-HT(1A) agonist. Rats were ovariectomized, allowed to recover for 5 days, then given 2 daily injections of estradiol benzoate or vehicle (10 microg/day, s.c.). Twenty-four hours after the second injection, rats were challenged with a sub-maximal dose of 8-OH-DPAT (50 microg/kg, sc) or saline 15 min prior to sacrifice. 8-OH-DPAT produced a significant increase in plasma oxytocin, ACTH and corticosterone levels in ovariectomized rats. While estrogen treatment for 2 days did not alter basal hormone levels, it did significantly reduce the magnitude of oxytocin, ACTH and corticosterone responses to 8-OH-DPAT. The reduction in hormone responses was accompanied by a significant reduction in hypothalamic levels of G(z), G(i1) and G(i3) proteins (by 50%, 30% and 50%, respectively). These findings suggest that a reduction in these G proteins may contribute to the mechanisms underlying estrogen-induced desensitization of 5-HT(1A) receptors. The desensitization of 5-HT(1A) receptors has been suggested to underlie the therapeutic effects of antidepressant 5-HT uptake inhibitors (SSRIs). Thus, the present results suggest that estrogen or estrogen-like substances in combination with SSRIs may prove effective in developing novel therapeutic strategies for neuropsychiatric disorders in women.

Coadministration of 5-hydroxytryptamine(1A) antagonist WAY-100635 prevents fluoxetine-induced desensitization of postsynaptic 5-hydroxytryptamine(1A) receptors in hypothalamus.

Treatment with selective serotonin reuptake inhibitors induces a desensitization of hypothalamic postsynaptic 5-hydroxytryptamine (5-HT)(1A) receptors in humans and rats. This study investigated whether fluoxetine-induced desensitization is due to overactivation of postsynaptic 5-HT(1A) receptors; whether blockade of somatodendritic 5-HT(1A) autoreceptors accelerates this desensitization; and whether desensitization is associated with a reduction of Gz proteins, which couple to 5-HT(1A) receptors. WAY-100635 was tested at low doses (0.03-0.3 mg/kg), which antagonize somatodendritic 5-HT(1A) autoreceptors in the raphe nuclei, and at a higher dose (1 mg/kg), which completely blocks postsynaptic 5-HT(1A) receptors. Plasma levels of oxytocin and adrenal corticotrophic hormone (corticotropin) were measured as peripheral indicators of hypothalamic 5-HT(1A) receptor function. Daily injections of fluoxetine (10 mg/kg/day i.p.) for 2 days did not desensitize 5-HT(1A) receptors but three daily injections of fluoxetine produced a partial desensitization of the hormone responses to (+/-)-8-hydroxy-2-dipropylaminoetetralin (50 microg/kg s.c.). WAY-100635 (0.03-0.3 mg/kg) did not accelerate or potentiate the fluoxetine-induced desensitization of 5-HT(1A) receptors. However, WAY-100635 at a dose that completely blocks postsynaptic 5-HT(1A) receptors (1.0 mg/kg) completely prevented the fluoxetine-induced desensitization of 5-HT(1A) receptors. These data demonstrate that at least 3 days of fluoxetine exposure is required to produce a homologous desensitization of hypothalamic 5-HT(1A) receptors. Although previous studies indicate that injections of fluoxetine for 14 days produce a reduction of Gz protein levels in the hypothalamus, the levels of Gz proteins were not affected by either fluoxetine or WAY-100635. Alternative mechanisms mediating the initial stages of 5-HT(1A) receptor desensitization could involve post-translational modifications in the 5-HT(1A) receptor-Gz protein-signaling cascade.

Evidence that G(z)-proteins couple to hypothalamic 5-HT(1A) receptors in vivo.

Using in situ hybridization and immunoblot analysis, the present studies identified G(z) mRNA and G(z)-protein in the hypothalamic paraventricular nucleus. The role of G(z)-proteins in hypothalamic 5-HT(1A) receptor signaling was examined in vivo. Activation of 5-HT(1A) receptors increases the secretion of oxytocin and ACTH, but not prolactin. Intracerebroventricular infusion (3-4 d) of G(z) antisense oligodeoxynucleotides, with different sequences and different phosphorothioate modification patterns, reduced the levels of G(z)-protein in the hypothalamic paraventricular nucleus, whereas missense oligodeoxynucleotides had no effect. Neither antisense nor missense oligodeoxynucleotide treatment altered basal plasma levels of ACTH, oxytocin, or prolactin, when compared with untreated controls. An antisense-induced decrease in hypothalamic G(z)-protein levels was paralleled by a significant decrease in the oxytocin and ACTH responses to the 5-HT(1A) agonist 8-hydroxy-dipropylamino-tetralin (8-OH-DPAT). In contrast, the prolactin response to 8-OH-DPAT (which cannot be blocked by 5-HT(1A) antagonists) was not inhibited by G(z) antisense oligodeoxynucleotides. G(z)-proteins are the only members of the G(i)/G(o)-protein family that are not inactivated by pertussis toxin. In a control experiment, pertussis toxin treatment (1 microgram/5 microliter, i.c.v.; 48 hr before the 8-OH-DPAT challenge) did not inhibit the ACTH response, potentiated the oxytocin response, and eliminated the prolactin response to 8-OH-DPAT. Thus, pertussis toxin-sensitive G(i)/G(o)-proteins do not mediate the 5-HT(1A) receptor-mediated increase in ACTH and oxytocin secretion. Combined, these studies provide the first in vivo evidence for a key role of G(z)-proteins in coupling hypothalamic 5-HT(1A) receptors to effector mechanisms.

Sustained desensitization of hypothalamic 5-Hydroxytryptamine1A receptors after discontinuation of fluoxetine: inhibited neuroendocrine responses to 8-hydroxy-2-(Dipropylamino)Tetralin in the absence of changes in Gi/o/z proteins.

Long-term exposure to fluoxetine produces a desensitization of hypothalamic postsynaptic 5-hydroxytryptamine (5-HT)1A receptors, indicated by a substantial inhibition of the 5-HT1A receptor-mediated stimulation of oxytocin and adrenocorticotropic hormone (ACTH) secretion. The present study investigated the time course and mechanism of this desensitization after discontinuation of fluoxetine administration. Male rats were injected with saline or fluoxetine (10 mg/kg/day, i.p.) for 14 days and were challenged with a 5-HT1A agonist, [8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) 50 microg/kg, s.c.] 2, 4, 7, 14, 28, or 60 days post-treatment. In control animals, 8-OH-DPAT significantly increased (approximately 15-fold) plasma levels of oxytocin and ACTH. At 2 days post-treatment, oxytocin and ACTH responses to 8-OH-DPAT were reduced by 74% and 68%, respectively. During further withdrawal from fluoxetine, there was a gradual increase in the oxytocin response toward control levels. However, even 60 days after discontinuation of fluoxetine, the oxytocin response was still significantly reduced by 26% compared with controls. In contrast, the suppressed ACTH response to 8-OH-DPAT (a less-sensitive indicator of desensitization) gradually returned to control levels by day 14 of withdrawal from fluoxetine. Interestingly, the sustained reductions in the hormone responses occurred in the absence of reductions in Gz or Gi protein levels in the hypothalamus. Furthermore, this desensitization was sustained in the absence of detectable levels of fluoxetine and norfluoxetine in plasma and brain tissue. These findings suggest that the sustained desensitization of hypothalamic 5-HT1A receptor systems, observed during fluoxetine withdrawal, may be due to altered interactions among the protein components of the 5-HT1A receptor system, rather than their absolute levels.

Daily injections of fluoxetine induce dose-dependent desensitization of hypothalamic 5-HT1A receptors: reductions in neuroendocrine responses to 8-OH-DPAT and in levels of Gz and Gi proteins.

The present studies examined the dose-response relationship of fluoxetine-induced desensitization of hypothalamic postsynaptic 5-HT1A receptors, as measured from the reduced neuroendocrine responses to a 5-HT1A agonist. Because hypothalamic Gz proteins mediate the ACTH and oxytocin responses to 5-HT1A receptor activation, we also determined the effect of fluoxetine on the levels of Gz proteins in the hypothalamus. Rats were injected daily for 14 days with saline or with fluoxetine doses of 0.3, 1, 3, 5, 7. 5, or 10 mg/kg/day. Fluoxetine produced a dose-dependent reduction in the oxytocin, ACTH, and corticosterone responses to the 5-HT1A agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT, 50 micrograms/kg, s.c.). The lowest fluoxetine dose that significantly, although incompletely, reduced the neuroendocrine responses to 8-OH-DPAT was 5 mg/kg/day. The 10 mg/kg/day dose of fluoxetine maximally inhibited all neuroendocrine responses to 8-OH-DPAT. Hypothalamic levels of Gz protein were reduced by both the 7.5 and 10 mg/kg/day doses of fluoxetine, whereas Gi1 protein levels were reduced only after the highest dose (10 mg/kg/day) of fluoxetine. Gi2, Gi3, and Go levels were not reduced by any fluoxetine dose. Cytosolic levels of Gi1 and Gz proteins were unaltered, indicating that reductions in Gz and Gi1 proteins are not caused by a redistribution of the proteins from the membrane into the cytosol. The results from the present study indicate that fluoxetine-induced desensitization of hypothalamic postsynaptic 5-HT1A receptor systems is dose-dependent and may be caused in part by reductions in the hypothalamic levels of Gz proteins.

Neuroprotective actions of novel and potent ligands of group I and group II metabotropic glutamate receptors.

The role of group I metabotropic glutamate (mGlu) receptors in neurodegeneration is controversial because of the contradictory effects of mGlu1/5 agonists in in vitro models of neuronal cell death. In this study, novel and selective antagonists of mGlu1 and mGlu5: LY367385 and LY367366 were found to show consistent neuroprotective effects against N-methyl-D-aspartate (NMDA)-induced excitotoxicity in vitro and in vivo. Furthermore, intraventricular administration of LY367385 reduced hippocampal cell death in gerbils subjected to transient global ischemia. Previous studies have also shown that activation of group II mGlu receptors may contribute to neuroprotective mechanisms in vitro and in vivo. Three potent group II mGlu agonists--LY354740, LY379268 and LY389795--were found to attenuate both NMDA excitotoxicity and staurosporine-induced neuronal cell death. LY354740 and LY379268 were protective against transient global ischemia in gerbils when dosed intraperitoneally. These results support the view that antagonists of mGlu1 and mGlu5 and agonists of group II mGlu receptors may be useful agents in the therapeutic treatment of neurodegenerative disease.

Otilonium bromide in irritable bowel syndrome: a double-blind, placebo-controlled, 15-week study.

AIM: To evaluate the efficacy of otilonium bromide, a spasmolytic agent, in the treatment of irritable bowel syndrome using modern and validated diagnostic criteria. METHODS: Three hundred and seventy-eight patients with irritable bowel syndrome were enrolled in the study. At entry, endoscopy/barium enema, clinical examination and laboratory tests were used to rule out organic diseases. After a 2-week placebo run-in, 325 patients were randomly assigned to receive either otilonium bromide 40 mg t.d.s. or placebo for 15 weeks. Abdominal pain, abdominal distension and disturbed defecation were scored at the beginning of the study and every 5 weeks. A global determination of well-being by visual analogue scale and the tenderness of the sigmoid colon were also scored. RESULTS: The reduction in the number of abdominal pain episodes was significantly higher (P < 0.01) in otilonium bromide patients (55.3%) than in those taking placebo (39.9%) as was the severity of abdominal distension (42.0%, vs. 30.2%; P < 0.05). Bowel disturbance improved in both groups. but without any statistically significant difference. The visual analogue scale of well-being revealed a significant improvement (P < 0.05) in patients taking otilonium bromide. The investigators' global positive assessment was in favour of otilonium bromide (65.2%) compared with placebo (49.6%) (P < 0.01). CONCLUSIONS: Otilonium bromide may represent an effective treatment for irritable bowel syndrome because it reduces its predominant symptom (abdominal pain/ discomfort) more than placebo does.

Alterations in 8-hydroxy-2-(dipropylamino)tetralin-induced neuroendocrine responses after 5,7-dihydroxytryptamine-induced denervation of serotonergic neurons.

In the present study, we examined denervation-induced changes in the sensitivity of hypothalamic postsynaptic serotonin1A (5-HT1A) receptor function with respect to changes in the dose-dependent elevation in plasma hormones [adrenocorticotropic hormone (ACTH), corticosterone, prolactin, oxytocin, prolactin, renin and vasopressin] by the 5-HT1A agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT). Rats received intracerebroventricular (i.c.v.) injections of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or vehicle (0.1% ascorbate in saline) 3 weeks before challenge with increasing doses of 8-OH-DPAT (0, 10, 50 or 200 micrograms/kg s.c.). The effectiveness of 5,7-DHT-induced destruction of serotonergic neurons was confirmed by a 93% reduction in [3H]paroxetine-labeled 5-HT uptake sites in the hypothalamus. No changes in basal levels of ACTH, corticosterone, oxytocin, prolactin, renin and vasopressin were observed in rats that received i.c.v. 5,7-DHT injections. The dose-response curves for 8-OH-DPAT-induced elevations of plasma corticosterone and prolactin levels were shifted to the left in rats treated with 5,7-DHT, whereas no significant difference in the ACTH dose-response curve was observed between rats treated with vehicle and rats treated with 5,7-DHT. In contrast, the maximal oxytocin response to 8-OH-DPAT was attenuated in rats treated with 5,7-DHT. A 5,7-DHT-induced decline in the synthesis of oxytocin could explain this phenomenon. Although 8-OH-DPAT did not increase plasma levels of renin or vasopressin in rats treated with vehicle, 8-OH-DPAT produced an elevation (75%) in plasma renin concentration but not in vasopressin levels in rats that received i.c.v. injections of 5,7-DHT. No change was observed in [3H]8-OH-DPAT labeled 5-HT1A receptors in the hypothalamus. In summary, denervation of hypothalamic serotonergic nerve terminals produces supersensitivity of some neuroendocrine responses to 8-OH-DPAT independent of changes in the density of hypothalamic 5-HT1A receptors.