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

Gα Protein Signaling Bias at Serotonin 1A Receptor

Serotonin 1A receptor (5-HT1AR) is a clinically relevant target because of its involvement in several central and peripheral functions, including sleep, temperature homeostasis, processing of emotions, and response to stress. As a G protein coupled receptor (GPCR) activating numerous Gα i/o/z family members, 5-HT1AR can potentially modulate multiple intracellular signaling pathways in response to different therapeutics. Here, we applied a cell-based bioluminescence resonance energy transfer assay to quantify how ten structurally diverse 5-HT1AR agonists exert biased signaling by differentially stimulating Gα i/o/z family members. Our concentration-response analysis of the activation of each Gα i/o/z protein revealed unique potency and efficacy profiles of selected agonists when compared with the reference 5-hydroxytryptamine, serotonin. Overall, our analysis of signaling bias identified groups of ligands sharing comparable G protein activation selectivity and also drugs with unique selectivity profiles. We observed, for example, a strong bias of F-15599 toward the activation of Gα i3 that was unique among the agonists tested: we found a biased factor of +2.19 when comparing the activation of Gα i3 versus Gα i2 by F-15599, while it was -0.29 for 8-hydroxy-2-(di-n-propylamino) tetralin. Similarly, vortioxetine showed a biased factor of +1.06 for Gα z versus Gα oA, while it was -1.38 for vilazodone. Considering that alternative signaling pathways are regulated downstream of each Gα protein, our data suggest that the unique pharmacological properties of the tested agonists could result in multiple unrelated cellular outcomes. Further investigation is needed to reveal how this type of ligand bias could affect cellular responses and to illuminate the molecular mechanisms underlying therapeutic profile and side effects of each drug. SIGNIFICANCE STATEMENT: Serotonin 1a receptor (5-HT1AR) activates several members of the Gi/o/z protein family. Here, we examined ten structurally diverse and clinically relevant agonists acting on 5-HT1AR and identified distinctive bias patterns among G proteins. Considering the diversity of their intracellular effectors and signaling properties, this data reveal novel mechanisms underlying both therapeutic and undesirable effects.

Regulation of Serotonin 1A Receptor SUMOylation by SENP2 and PIASxα

Serotonin 1A receptors (5-HT1ARs) are implicated in the control of mood, cognition, and memory and in various neuropsychiatric disorders such as depression and anxiety. As such, understanding the regulation of 5-HT1ARs will inform the development of better treatment approaches. We previously demonstrated 5-HT1ARs are SUMOylated by SUMO1 in the rat brain. Agonist stimulation increased SUMOylation and was further enhanced when combined with 17β-estradiol-3-benzoate (EB), which are treatments that cause the transient and prolonged desensitization of 5-HT1AR signaling, respectively. In the current study, we identified the protein inhibitor of activated STAT (PIAS)xα as the enzyme that facilitates SUMOylation, and SENP2 as the protein that catalyzes the deSUMOylation of 5-HT1ARs. We demonstrated that PIASxα significantly increased in the membrane fraction of rats co-treated with EB and an agonist, compared to either the EB-treated or vehicle-treated groups. The acute treatment with an agonist alone shifted the location of SENP2 from the membrane to the cytoplasmic fraction, but it has little effect on PIASxα. Hence, two separate mechanisms regulate SUMOylation and the activity of 5-HT1ARs by an agonist and EB. The effects of EB on 5-HT1AR SUMOylation and signaling may be related to the higher incidence of mood disorders in women during times with large fluctuations in estrogens. Targeting the SUMOylation of 5-HT1ARs could have important clinical relevance for the therapy for several neuropsychiatric disorders in which 5-HT1ARs are implicated.

Time-Restricted G-Protein Signaling Pathways via GPR176, Gz, and RGS16 Set the Pace of the Master Circadian Clock in the Suprachiasmatic Nucleus

G-protein-coupled receptors (GPCRs) are an important source of drug targets with diverse therapeutic applications. However, there are still more than one hundred orphan GPCRs, whose ligands and functions remain unidentified. The suprachiasmatic nucleus (SCN) is the central circadian clock of the brain, directing daily rhythms in activity–rest behavior and physiology. Malfunction of the circadian clock has been linked to a wide variety of diseases, including sleep–wake disorders, obesity, diabetes, cancer, and hypertension, making the circadian clock an intriguing target for drug development. The orphan receptor GPR176 is an SCN-enriched orphan GPCR that sets the pace of the circadian clock. GPR176 undergoes asparagine (N)-linked glycosylation, a post-translational modification required for its proper cell-surface expression. Although its ligand remains unknown, this orphan receptor shows agonist-independent basal activity. GPR176 couples to the unique G-protein subclass G_z (or G_x) and participates in reducing cAMP production during the night. The regulator of G-protein signaling 16 (RGS16) is equally important for the regulation of circadian cAMP synthesis in the SCN. Genome-wide association studies, employing questionnaire-based evaluations of individual chronotypes, revealed loci near clock genes and in the regions containing RGS16 and ALG10B, a gene encoding an enzyme involved in protein N-glycosylation. Therefore, increasing evidence suggests that N-glycosylation of GPR176 and its downstream G-protein signal regulation may be involved in pathways characterizing human chronotypes. This review argues for the potential impact of focusing on GPCR signaling in the SCN for the purpose of fine-tuning the entire body clock.

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

5-HT1A receptor-mediated activation of neuroendocrine responses and multiple protein kinase pathways in the peripubertal rat hypothalamus

Increasing evidence suggests that multiple factors can produce effects on the immature brain that are distinct and more long-lasting than those produced in adults. The hypothalamic paraventricular nucleus (PVN) is a region integral to the hypothalamic-pituitary-adrenal axis and is affected by anxiety, depression, and drugs used to treat these disorders, yet receptor signaling mechanisms operative in hypothalamus prior to maturation remain to be elucidated. In peripubertal male rats, systemic injection of the selective serotonin 1A (5-HT_1A) receptor agonist (+)8-OH-DPAT (0.2 mg/kg) markedly elevated plasma levels of oxytocin and adrenocorticotropic hormone (ACTH) at 5 and 15 min post-injection. The 5-HT_1A receptor selectivity was demonstrated by the ability of the 5-HT_1A receptor selective antagonist WAY100635 to completely block both oxytocin and ACTH responses at 5 min, with some recovery of the ACTH response at 15 min. At 15 min post-injection, (+)8-OH-DPAT also increased levels of phosphorylated extracellular signal-regulated kinase (pERK) and phosphorylated protein kinase B (pAkt) in the PVN. As previously observed in adults, (+)8-OH-DPAT reduced levels of pERK in hippocampus. WAY100635 also completely blocked (+)8-OH-DPAT-mediated elevations in hypothalamic pERK and pAkt and the reductions in hippocampal pERK, demonstrating 5-HT_1A receptor selectivity of both kinase responses. This study provides the first demonstration of functional 5-HT_1A receptor-mediated ERK and Akt signaling pathways in the immature hypothalamus, activated by a dose of (+)8-OH-DPAT that concomitantly stimulates neuroendocrine responses. This information is fundamental to identifying potential signaling pathways targeted by biased agonists in the development of safe and effective treatment strategies in children and adolescents.

G-protein-coupled receptor signaling through Gpr176, Gz, and RGS16 tunes time in the center of the circadian clock [Review]

G-protein-coupled receptors (GPCRs) constitute an immensely important class of drug targets with diverse clinical applications. There are still more than 120 orphan GPCRs whose cognate ligands and physiological functions are not known. A set of circadian pacemaker neurons that governs daily rhythms in behavior and physiology resides in the suprachiasmatic nucleus (SCN) in the brain. Malfunction of the circadian clock has been linked to a multitude of diseases, such as sleeping disorders, obesity, diabetes, cardiovascular diseases, and cancer, which makes the clock an attractive target for drug development. Here, we review a recently identified role of Gpr176 in the SCN. Gpr176 is an SCN-enriched orphan GPCR that sets the pace of the circadian clock in the SCN. Even without known ligand, this orphan receptor has an agonist-independent basal activity to reduce cAMP signaling. A unique cAMP-repressing G-protein subclass Gz is required for the activity of Gpr176. We also provide an overview on the circadian regulation of G-protein signaling, with an emphasis on a role for the regulator of G-protein signaling 16 (RGS16). RGS16 is indispensable for the circadian regulation of cAMP in the SCN. Developing drugs that target the SCN remains an unfulfilled opportunity for the circadian pharmacology. This review argues for the potential impact of focusing on GPCRs in the SCN for the purpose of tuning the body clock.

Regulators of G-protein-signaling proteins: negative modulators of G-protein-coupled receptor signaling

Regulators of G-protein-signaling (RGS) proteins are a category of intracellular proteins that have an inhibitory effect on the intracellular signaling produced by G-protein-coupled receptors (GPCRs). RGS along with RGS-like proteins switch on through direct contact G-alpha subunits providing a variety of intracellular functions through intracellular signaling. RGS proteins have a common RGS domain that binds to G alpha. RGS proteins accelerate GTPase and thus enhance guanosine triphosphate hydrolysis through the alpha subunit of heterotrimeric G proteins. As a result, they inactivate the G protein and quickly turn off GPCR signaling thus terminating the resulting downstream signals. Activity and subcellular localization of RGS proteins can be changed through covalent molecular changes to the enzyme, differential gene splicing, and processing of the protein. Other roles of RGS proteins have shown them to not be solely committed to being inhibitors but behave more as modulators and integrators of signaling. RGS proteins modulate the duration and kinetics of slow calcium oscillations and rapid phototransduction and ion signaling events. In other cases, RGS proteins integrate G proteins with signaling pathways linked to such diverse cellular responses as cell growth and differentiation, cell motility, and intracellular trafficking. Human and animal studies have revealed that RGS proteins play a vital role in physiology and can be ideal targets for diseases such as those related to addiction where receptor signaling seems continuously switched on.

How much do we know about the coupling of G-proteins to serotonin receptors?

Serotonin receptors are G-protein-coupled receptors (GPCRs) involved in a variety of psychiatric disorders. G-proteins, heterotrimeric complexes that couple to multiple receptors, are activated when their receptor is bound by the appropriate ligand. Activation triggers a cascade of further signalling events that ultimately result in cell function changes. Each of the several known G-protein types can activate multiple pathways. Interestingly, since several G-proteins can couple to the same serotonin receptor type, receptor activation can result in induction of different pathways. To reach a better understanding of the role, interactions and expression of G-proteins a literature search was performed in order to list all the known heterotrimeric combinations and serotonin receptor complexes. Public databases were analysed to collect transcript and protein expression data relating to G-proteins in neural tissues. Only a very small number of heterotrimeric combinations and G-protein-receptor complexes out of the possible thousands suggested by expression data analysis have been examined experimentally. In addition this has mostly been obtained using insect, hamster, rat and, to a lesser extent, human cell lines. Besides highlighting which interactions have not been explored, our findings suggest additional possible interactions that should be examined based on our expression data analysis.

Gαz regulates BDNF-induction of axon growth in cortical neurons

The disruption of neurotransmitter and neurotrophic factor signaling in the central nervous system (CNS) is implicated as the root cause of neuropsychiatric disorders, including schizophrenia, epilepsy, chronic pain, and depression. Therefore, identifying the underlying molecular mechanisms by which neurotransmitter and neurotrophic factor signaling regulates neuronal survival or growth may facilitate identification of more effective therapies for these disorders. Previously, our lab found that the heterotrimeric G protein, Gz, mediates crosstalk between G protein-coupled receptors and neurotrophin signaling in the neural cell line PC12. These data, combined with Gαz expression profiles--predominantly in neuronal cells with higher expression levels corresponding to developmental times of target tissue innervation--suggested that Gαz may play an important role in neurotrophin signaling and neuronal development. Here, we provide evidence in cortical neurons, both manipulated ex vivo and those cultured from Gz knockout mice, that Gαz is localized to axonal growth cones and plays a significant role in the development of axons of cortical neurons in the CNS. Our findings indicate that Gαz inhibits BDNF-stimulated axon growth in cortical neurons, establishing an endogenous role for Gαz in regulating neurotrophin signaling in the CNS.

Estradiol potentiates 8-OH-DPAT-induced sumoylation of 5-HT₁A receptor: characterization and subcellular distribution of sumoylated 5-HT₁A receptors

Sumoylation is a recently described post-translational modification and only a few sumoylated neurotransmitter receptors are known. Through the present studies, we discovered that serotonin1A receptors (5-HT1A-Rs) can be sumoylated by SUMO1 (small-ubiquitin-related modifier 1) protein. The SUMO1-5-HT1A-R is ∼55kDa, is located in the membrane fraction, but not the cytosol, and is distributed in all of the brain regions expressing 5-HT1A-Rs examined. Acute stimulation of 5-HT1A-Rs significantly increased SUMO1-5-HT1A-R in rat hypothalamus. Pre-treatment with estradiol for 2 days, which causes a partial desensitization of 5-HT1A-R signaling, potentiated agonist-induced increases in SUMO1-5-HT1A-Rs in the hypothalamus of ovariectomized rats. Using discontinuous gradient centrifugation followed by digitonin treatment, we found that the majority of SUMO1-5-HT1A-Rs is co-localized with endoplasmic-reticulum and trans-Golgi-network markers. Although a small proportion of SUMO1-5-HT1A-Rs are located in the detergent resistant microdomain (DRM) that contain active G-protein coupled receptors, their distribution was different from that of the Gαz protein that couples to the receptors. These data suggest that the SUMO1-5-HT1A-Rs are an inactive form of 5-HT1A-Rs, a finding further supported by results showing minimal 5-HT1A-R agonist binding to SUMO1-5-HT1A-Rs. Furthermore, SUMO1-5-HT1A-Rs in the DRM were increased by treatment with a 5-HT1A-R agonist, 8-OH-DPAT ((+)8-hydroxy-2-dipropylaminotetralin). Together, these data suggest that sumoylation of 5-HT1A-Rs may be related to 5-HT1A-R trafficking and internalization, which may contribute to 5-HT1A-R desensitization. Since 5-HT1A-Rs play an important role in mood regulation, the present results significantly impact on the understanding of the pathogenesis of affective disorders and development of better therapeutic approaches for these diseases.

Estradiol accelerates the effects of fluoxetine on serotonin 1A receptor signaling

A major problem with current anti-depressant therapy is that it takes on average 6-7 weeks for remission. Since desensitization of serotonin (5-HT)1A receptor signaling contributes to the anti-depressive response, acceleration of the desensitization may reduce this delay in response to antidepressants. The purpose of the present study was to test the hypothesis that estradiol accelerates fluoxetine-induced desensitization of 5-HT1A receptor signaling in the paraventricular nucleus of the hypothalamus (PVN) of rats, via alterations in components of the 5-HT1A receptor signaling pathway. Ovariectomized rats were injected with estradiol and/or fluoxetine, then adrenocorticotropic hormone (ACTH) and oxytocin responses to a 5-HT1A receptor agonist (+)-8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT) were examined to assess the function of 5-HT1A receptors in the PVN. Treatment with estradiol for either 2 or 7 days or fluoxetine for 2 days produced at most a partial desensitization of 5-HT1A receptor signaling, whereas 7 days of fluoxetine produced full desensitization. Combined treatment with estradiol and fluoxetine for 2 days produced nearly a full desensitization, demonstrating an accelerated response compared to either treatment alone. With two days of combined treatments, estradiol prevented the fluoxetine-induced increase in 5-HT1A receptor protein, which could contribute to the more rapid desensitization. Furthermore, EB treatment for 2 days decreased the abundance of the 35 kD Gαz protein which could contribute to the desensitization response. We found two isoforms of Gαz proteins with molecular mass of 35 and 33 kD, which differentially distributed in the detergent resistant microdomain (DRM) and in Triton X-100 soluble membrane region, respectively. The 35 kD Gαz proteins in the DRM can be sumoylated by SUMO1. Stimulation of 5-HT1A receptors with 8-OH-DPAT increases the sumoylation of Gαz proteins and reduces the 33 kD Gαz proteins, suggesting that these responses may be related to the desensitization of 5-HT1A receptors. Treatment with estradiol for 2 days also reduced the levels of the G-protein coupled estrogen receptor GPR30, possibly limiting to the ability of estradiol to produce only a partial desensitization response. These data provide evidence that estradiol may be effective as a short-term adjuvant to SSRIs to accelerate the onset of therapeutic effects.

GPR30 is necessary for estradiol-induced desensitization of 5-HT1A receptor signaling in the paraventricular nucleus of the rat hypothalamus

Estrogen therapy used in combination with selective serotonin reuptake inhibitor (SSRI) treatment improves SSRI efficacy for the treatment of mood disorders. Desensitization of serotonin 1A (5-HT(1A)) receptors, which takes one to two weeks to develop in animals, is necessary for SSRI therapeutic efficacy. Estradiol modifies 5-HT(1A) receptor signaling and induces a partial desensitization in the paraventricular nucleus (PVN) of the rat within two days, but the mechanisms underlying this effect are currently unknown. The purpose of this study was to identify the estrogen receptor necessary for estradiol-induced 5-HT(1A) receptor desensitization. We previously showed that estrogen receptor β is not necessary for 5-HT(1A) receptor desensitization and that selective activation of estrogen receptor GPR30 mimics the effects of estradiol in rat PVN. Here, we used a recombinant adenovirus containing GPR30 siRNAs to decrease GPR30 expression in the PVN. Reduction of GPR30 prevented estradiol-induced desensitization of 5-HT(1A) receptor as measured by hormonal responses to the selective 5-HT(1A) receptor agonist, (+)8-OH-DPAT. To determine the possible mechanisms underlying these effects, we investigated protein and mRNA levels of 5-HT(1A) receptor signaling components including 5-HT(1A) receptor, Gαz, and RGSz1. We found that two days of estradiol increased protein and mRNA expression of RGSz1, and decreased 5-HT(1A) receptor protein but increased 5-HT(1A) mRNA; GPR30 knockdown prevented the estradiol-induced changes in 5-HT(1A) receptor protein in the PVN. Taken together, these data demonstrate that GPR30 is necessary for estradiol-induced changes in the 5-HT(1A) receptor signaling pathway and desensitization of 5-HT(1A) receptor signaling.

Estradiol induces partial desensitization of serotonin 1A receptor signaling in the paraventricular nucleus of the hypothalamus and alters expression and interaction of RGSZ1 and Gαz

Hyperactivity of hypothalamic-pituitary mediated hormone responses, such as to stimulation with a serotonin 1A (5-HT(1A)) receptor agonist, are a feature of depression which are normalized with clinical improvement during drug therapy. We previously reported that SSRIs induce desensitization of 5-HT(1A) receptor signaling in the paraventricular nucleus of the hypothalamus (PVN) while estradiol benzoate (EB) produces a more rapid, partial desensitization. In the current study, time course and dose-response experiments demonstrated that two once daily doses of EB is the minimum needed to induce the desensitization response as indicated by 5-HT(1A) receptor-stimulated release of oxytocin and that 10 μg/kg/day EB produces the maximal response, a partial desensitization of approximately 40%. The effects of two once daily injections of 10 μg/kg/day EB on Gαz and RGSZ1 proteins were examined as components of the 5-HT(1A) receptor signaling system, which mediates the release of oxytocin and adrenocorticotropic hormone. RGSZ1 appears to be a major target for EB-mediated responses in the 5-HT(1A) receptor signaling system. A 55 kD membrane-associate RGSZ1 protein was greatly increased in the PVN and rest of the hypothalamus and moderately increased in the dorsal hippocampus and amygdala after EB treatment as well as after an acute dose of a 5-HT(1A) receptor agonist. These results suggest that EB is a candidate for adjuvant therapy with SSRIs to hasten the therapeutic response and that RGSZ1 is a major target of EB therapy which could be explored as a target for novel therapeutic approaches for the treatment of depression.

Chronic treatment with the opioid antagonist naltrexone favours the coupling of spinal cord μ-opioid receptors to Gαz protein subunits

Sustained administration of opioid antagonists to rodents results in an enhanced antinociceptive response to agonists. We investigated the changes in spinal μ-opioid receptor signalling underlying this phenomenon. Rats received naltrexone (120 μg/h; 7 days) via osmotic minipumps. The antinociceptive response to the μ-agonist sufentanil was tested 24 h after naltrexone withdrawal. In spinal cord samples, we determined the interaction of μ-receptors with Gα proteins (agonist-stimulated [(35)S]GTPγS binding and immunoprecipitation of [(35)S]GTPγS-labelled Gα subunits) as well as μ-opioid receptor-dependent inhibition of the adenylyl cyclase (AC) activity. Chronic naltrexone treatment augmented DAMGO-stimulated [(35)S]GTPγS binding, potentiated the inhibitory effect of DAMGO on the AC/cAMP pathway, and increased the inverse agonist effect of naltrexone on cAMP accumulation. In control rats, the inhibitory effect of DAMGO on cAMP production was antagonized by pertussis toxin (PTX) whereas, after chronic naltrexone, the effect became resistant to the toxin, suggesting a coupling of μ-receptors to PTX-insensitive Gα(z) subunits. Immunoprecipitation assays confirmed the transduction switch from Gα(i/o) to Gα(z) proteins. The consequence was an enhancement of the antinociceptive response to sufentanil that, in consonance with the neurochemical data, was prevented by Gα(z)-antisense oligodeoxyribonucleotides but not by PTX. Such changes in opioid receptor signalling can be a double-edged sword. On the one hand, they may have potential applicability to the optimisation of the analgesic effects of opioid drugs for the control of pain. On the other hand, they represent an important homeostatic dysregulation of the endogenous opioid system that might account for undesirable effects in patients chronically treated with opioid antagonists. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Estradiol-induced desensitization of 5-HT1A receptor signaling in the paraventricular nucleus of the hypothalamus is independent of estrogen receptor-beta

Estradiol regulates serotonin 1A (5-HT(1A)) receptor signaling. Since desensitization of 5-HT(1A) receptors may be an underlying mechanism by which selective serotonin reuptake inhibitors (SSRIs) mediate their therapeutic effects and combining estradiol with SSRIs enhances the efficacy of the SSRIs, it is important to determine which estrogen receptors are capable of desensitizating 5-HT(1A) receptor function. We previously demonstrated that selective activation of the estrogen receptor, GPR30, desensitizes 5-HT(1A) receptor signaling in rat hypothalamic paraventricular nucleus (PVN). However, since estrogen receptor-beta (ERbeta), is highly expressed in the PVN, we investigated the role of ERbeta in estradiol-induced desensitization of 5-HT(1A) receptor signaling. We first showed that a selective ERbeta agonist, diarylpropionitrile (DPN) has a 100-fold lower binding affinity than estradiol for GPR30. Administration of DPN did not desensitize 5-HT(1A) receptor signaling in rat PVN as demonstrated by agonist-stimulated hormone release. Second, we used a recombinant adenovirus containing ERbeta siRNAs to decrease ERbeta expression in the PVN. Reductions in ERbeta did not alter the estradiol-induced desensitization of 5-HT(1A) receptor signaling in oxytocin cells. In contrast, in animals with reduced ERbeta, estradiol administration, instead of producing desensitization, augmented the ACTH response to a 5-HT(1A) agonist. Combined with the results from the DPN treatment experiments, desensitization of 5-HT(1A) receptor signaling does not appear to be mediated by ERbeta in oxytocin cells, but that ERbeta, together with GPR30, may play a complex role in central regulation of 5-HT(1A)-mediated ACTH release. Determining the mechanisms by which estrogens induce desensitization may aid in the development of better treatments for mood disorders.

5-HT1A receptors mediate detrimental effects of cocaine on long-term potentiation and expression of polysialylated neural cell adhesion molecule protein in rat dentate gyrus

The present study investigated the involvement of 5-HT(1A) receptors in the inhibitory effect of single administration of cocaine (COC, 15 mg/kg i.p.) on the induction of long-term potentiation (LTP) in slices of rat dentate gyrus (DG), prepared 30 min and 2 days after COC administration. These effects of COC were blocked by an antagonist of 5-HT(1A) receptors, WAY 100635 (0.4 mg/kg i.p.), which had been administered 20 min before COC. The detrimental effect of COC on LTP in slices prepared 30 min after COC administration could be prevented by blocking glucocorticoid receptors (GRs) using mifepristone (RU 38486, 10 mg/kg s.c. given 1 h before COC), similar as in slices obtained 2 days after COC as reported previously [Maćkowiak et al. (2008) Eur J Neurosci 27:2928-2937]. After a single administration of an agonist of 5-HT(1A) receptors, 8-OH-DPAT, (0.5 mg/kg i.p.), the level of LTP in slices prepared 2 days later was significantly decreased resembling the effect of COC. This effect of 8-OH-DPAT was antagonized by WAY 100635 (0.4 mg/kg i.p.), administered 20 min before 8-OH-DPAT and by RU 38486, given 1 h before 8-OH-DPAT. COC-induced inhibition of LTP could be blocked by the inhibitor of mitogen-activated protein kinase kinase 1/2 (MEK1/2), SL 327 (50 mg/kg i.p.), administered 1 h before COC, but not by the inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), LY 294002 (80 mg/kg i.p.). COC-induced reduction in the number of polysialylated neural cell adhesion molecule (PSA-NCAM)-positive neurons in rat dentate gyrus could also be prevented by WAY 100635, given 20 min before COC. These data indicate that the indirect 5-HT(1A) receptor activation by a single COC administration and subsequent stimulation of extracellular signal-regulated kinases (ERK 1/2) signaling pathway result in a decrease of the potential for long-term increase in synaptic efficacy in rat DG lasting at least two but less than 7 days, most likely via activation of the hypothalamic-pituitary-adrenal (HPA) axis.

Signal transduction and functional selectivity of F15599, a preferential post-synaptic 5-HT1A receptor agonist

BACKGROUND AND PURPOSE

Activation of post-synaptic 5-HT(1A) receptors may provide enhanced therapy against depression. We describe the signal transduction profile of F15599, a novel 5-HT(1A) receptor agonist.

EXPERIMENTAL APPROACH

F15599 was compared with a chemical congener, F13714, and with (+)8-OH-DPAT in models of signal transduction in vitro and ex vivo.

KEY RESULTS

F15599 was highly selective for 5-HT(1A) receptors in binding experiments and in [(35)S]-GTPgammaS autoradiography of rat brain, where F15599 increased labelling in regions expressing 5-HT(1A) receptors. In cell lines expressing h5-HT(1A) receptors, F15599 more potently stimulated extracellular signal-regulated kinase (ERK1/2) phosphorylation, compared with G-protein activation, internalization of h5-HT(1A) receptors or inhibition of cAMP accumulation. F13714, (+)8-OH-DPAT and 5-HT displayed a different rank order of potency for these responses. F15599 stimulated [(35)S]-GTPgammaS binding more potently in frontal cortex than raphe. F15599, unlike 5-HT, more potently and efficaciously stimulated G(alphai) than G(alphao) activation. In rat prefrontal cortex (a region expressing post-synaptic 5-HT(1A) receptors), F15599 potently activated ERK1/2 phosphorylation and strongly induced c-fos mRNA expression. In contrast, in raphe regions (expressing pre-synaptic 5-HT(1A) receptors) F15599 only weakly or did not induce c-fos mRNA expression. Finally, despite its more modest affinity in vitro, F15599 bound to 5-HT(1A) receptors in vivo almost as potently as F13714.

CONCLUSIONS AND IMPLICATIONS

F15599 showed a distinctive activation profiles for 5-HT(1A) receptor-mediated signalling pathways, unlike those of reference agonists and consistent with functional selectivity at 5-HT(1A) receptors. In rat, F15599 potently activated signalling in prefrontal cortex, a feature likely to underlie its beneficial effects in models of depression and cognition.

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.

5-HT1A gene variants and psychiatric disorders: a review of current literature and selection of SNPs for future studies

5-HT1A receptors are key components of the serotonin system, acting both pre- and post- synaptically in different brain areas. There is a growing amount of evidence showing the importance of 5-HT1A in different psychiatric disorders, from mood to anxiety disorders, moving through suicidal behaviour and psychotic disorders. Findings in the literature are not consistent with any definite 5-HT1A influence in psychiatric disorders. 5-HT1A gene variants have been reported to play some role in mood disorders, anxiety disorders and psychotic disorders. Again, the literature findings are not unequivocal. Concerning response to treatment, the C(-1019)G variant seems to be of primary interest in antidepressant response: C allele carriers generally show a better response to treatment, especially in Caucasian samples. Together with the C(-1019)G (rs6295) variant, the Ile28Val (rs1799921), Arg219Leu (rs1800044) and Gly22Ser (rs1799920) variants have been investigated in possible associations with psychiatric disorders, also with no definitive results. This lack of consistency can be also due to an incomplete gene investigation. To make progress on this point, a list of validated single nucleotide polymorphisms (SNPs) covering the whole gene is proposed for further investigations.

Sustained treatment with a 5-HT(2A) receptor agonist causes functional desensitization and reductions in agonist-labeled 5-HT(2A) receptors despite increases in receptor protein levels in rats

Adaptive changes in serotonin2A (5-HT(2A)) receptor signaling are associated with the clinical response to a number of psychiatric drugs including atypical antipsychotics and selective serotonin reuptake inhibitors. The present study examined possible mechanisms of agonist-induced desensitization of 5-HT(2A) receptors in rat hypothalamic paraventricular nucleus (PVN) after 4 and 7 days of treatment with 1mg/kg (-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI). The magnitude of 5-HT(2A) receptor-mediated oxytocin release decreased 78% after 4 days and 61% after 7 days of DOI treatment. Similarly, the magnitude of ACTH release following 1mg/kg DOI decreased by 31% after 4 days and 38% after 7 days of DOI treatment. Treatment with DOI for either 4 or 7 days caused a significant decrease (by approximately 50%) in the high-affinity 5-HT(2A) receptor binding as measured by (125)I-DOI binding compared to saline-treated control rats. In contrast, western blot analysis demonstrated a significant increase in 5-HT(2A) receptor protein levels with 4 or 7 days of DOI treatment to 167% and 191% of control levels, respectively. Real time quantitative RT-PCR analysis revealed a small but nonsignificant increase in the levels of 5-HT(2A) mRNA following treatment with DOI for 4 or 7 days. Taken together, the 5-HT(2A) receptor-stimulated hormone responses, agonist binding data and western blot data suggest that although agonist treatment increases the levels of 5-HT(2A) receptor protein in the cell membrane, there is a reduction in the population of 5-HT(2A) receptors capable of high-affinity binding and mediating a functional response.

5-HT1A receptors mediate (+)8-OH-DPAT-stimulation of extracellular signal-regulated kinase (MAP kinase) in vivo in rat hypothalamus: time dependence and regional differences

Brain serotonin 1A (5-HT1A) receptors play an important role in mood disorders and can modulate various intracellular signaling mechanisms. We previously reported that systemic administration of either full or partial 5-HT1A agonists increases neuroendocrine responses and that tandospirone, an azapirone partial agonist, can activate (phosphorylate) extracellular signal-regulated kinase (ERK) in the hypothalamic paraventricular nucleus (PVN). In contrast, decreased levels of phosphoERK (pERK) have been reported in hippocampus following in vivo administration of either azapirone or aminotetralin 5-HT1A agonists, such as 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT). The present study investigated the time-dependent activation of MAP kinase in hypothalamus by (+)8-OH-DPAT to determine the regional differences and receptor specificity of the changes in pERK. Adult male rats received a systemic injection of (+)8-OH-DPAT (200 microg/kg, s.c.). The time-dependent changes in ERK activation were examined in hypothalamic nuclei as well as other brain regions associated with modulation of mood. (+)8-OH-DPAT produced a rapid increase (at 5 min) and transient return (at 15 min) of pERK levels in PVN and medial basal hypothalamus. In contrast, pERK levels in hippocampus were reduced at both 5 and 15 min after (+)8-OH-DPAT. Pretreatment with the 5-HT1A receptor-specific antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide (WAY100635) completely blocked the (+)8-OH-DPAT-mediated changes in pERK levels in PVN, medial basal hypothalamus, and hippocampus. No significant (+)8-OH-DPAT-induced changes in pERK were observed in dorsal raphe or amygdala. In conclusion, these results demonstrate that 8-OH-DPAT activation of MAP kinase signaling in vivo is a transient and region-specific phenomenon and in rat hypothalamus and hippocampus is mediated by 5-HT1A receptors.

Gz proteins are functionally coupled to dopamine D2-like receptors in vivo

The receptors that couple to the G protein Gz in vivo are still relatively unknown. In this study, we investigated the effects of various dopamine receptor agonists in a mouse deficient in the alpha subunit of Gz. The dopamine D1-like receptor agonist SKF38393 stimulated comparable locomotor activity in both wildtype mice and mice lacking Galphaz. In contrast, the dopamine D2-like receptor agonist quinpirole suppressed locomotor activity in both groups of mice, but this suppression was significantly smaller in Galphaz knockout mice. Consistent with these behavioural observations, quinpirole inhibition of dopamine release in the forebrain nucleus accumbens evoked by electrical stimulation of dopamine axons was significantly attenuated in mice lacking Galphaz. In addition, hypothermia and adrenocorticotropic hormone release resulting from activation of dopamine D2-like receptors were also significantly reduced in Galphaz knockout mice. However, adrenocorticotropic hormone secretion induced by corticotrophin releasing hormone and the serotonin 1A receptor agonist 8-hydroxy-dipropylamino-tetralin were similar between wildtype and Galphaz knockout mice. Western blot analysis showed that the expression levels of Galphai, Galphao, Galphas, Galphaq and Gbeta were the same in the brains of mice of both genotypes. Overall, our data suggest that Gz proteins are functionally coupled to dopamine D2-like receptors in vivo.

Reduced density of functional 5-HT1A receptors in the brain, medulla and spinal cord of monoamine oxidase-A knockout mouse neonates

Abnormally high brain 5-HT levels in monoamine oxidase-A knockout (MAO-A KO) mouse neonates raise the question of whether the distribution and density of the 5-HT1A receptors (5-HT1AR) expressed in the brain by postnatal day P7 are affected and, if so, whether the 5-HT1A autoreceptors in the dorsal raphe are modified in the same way as the postsynaptic 5-HT1AR present in raphe target structures. [3H]8-OH-DPAT binding and quantitative autoradiography were performed to answer these questions. Binding specificity was first confirmed in adult wild-type mice and rat brain sections. 5-HT1AR binding was then analyzed in four MAO-A mutant vs. five wild-type neonatal brains, from olfactory bulb to cervical cord. Among 12 structures expressing postsynaptic 5-HT1AR in wild-type neonates, the highest densities involved the retrosplenial cortex, entorhinal cortex, and septum (52-46 fmol/mg tissue); low densities occurred in the hippocampus and spinal cord (24 fmol/mg tissue); in addition, the raphe autoreceptor density was only 20 fmol/mg tissue. In mutants, the distribution of postsynaptic 5-HT1AR was unchanged, but an overall decrease in density occurred (-32% to -63%); the raphe autoreceptors decreased in mutants by at least -79%. Data are discussed with reference to the ectopic 5-HT uptake and accumulation reported to occur during the first 10 postnatal days in wild-type and MAO-A KO mice. As previously suggested to explain the raphe autoreceptor loss in 2-month-old MAO-A KO mice, the overall 5-HT1AR down-regulation in mutant pups probably results from extracellular 5-HT excess in both raphe and target structures. The greater the 5-HT excess, the more the functional receptor density decreases.

Regional differences in the coupling of 5-hydroxytryptamine-1A receptors to G proteins in the rat brain

Numerous data showed that 5-hydroxytryptamine-1A (5-HT1A) receptors couple to Galpha(o)/alpha(i) proteins for signal transduction. However, the alpha subunit isoforms really involved in 5-HT1A receptor coupling in brain remain to be identified. Moreover, regional differences in the functional characteristics of brain 5-HT1A receptors have been evidenced repeatedly. Because such differences could be due to variations in G proteins interacting with the same receptor, relevant approaches were used for identifying alpha subunits physically coupled to 5-HT1A receptors in different regions of the rat brain. Using immunoaffinity chromatography coupled to Western blot detection, 5-HT1A receptors were found to interact equally with Galpha(o) and Galpha(i3) in the cerebral cortex, mainly with Galpha(o) and weakly with Galpha(i3) in the hippocampus and exclusively with Galpha(i3) in the anterior raphe area. In the hypothalamus, 5-HT(1A) receptors seemed to be coupled to the latter two G proteins plus Galpha(i1) and Galpha(z). Complementary experiments based on an antibody capture technique coupled to both classic radioactivity and scintillation proximity assay detections showed that hippocampal 5-HT1A receptor stimulation induced 5'-O-(3-[35S]thio)triphosphate binding to immunoprecipitates with Galpha(i3) and Galpha(o) antisera. In the anterior raphe, such 5-HT1A receptor-mediated effect was obtained with Galpha(i3) antiserum only. These results demonstrated the existence of regional differences in the coupling of 5-HT1A receptors to G proteins in the rat brain. In the anterior raphe, 5-HT1A receptors seem to interact specifically with Galpha(i3), whereas in the hippocampus, they are mainly coupled to Galpha(o) proteins. Such a disparity in G-protein coupling might explain regional differences in adaptive regulations of brain 5-HT1A receptors.

Enhanced serotonin response in the hippocampus of Galphaz protein knock-out mice

The serotonin-1A [5-hydroxytryptamine 1A (5HT1A)] receptor is important for emotional and homeostatic processes in the central nervous system. In the hippocampus, the 5HT1A receptor couples to inhibitory Gi/o proteins to decrease pyramidal cell excitability. Here we investigate the 5HT1A receptor in a mouse deficient in the alpha-subunit of Gz protein (Galphaz knock-out). Behavioural tests showed heightened anxiety and depression-like behaviour in the Galphaz knock-out mice. Whole-cell recording in CA1 pyramidal neurons showed a significantly greater 5HT1A receptor-mediated potassium current in Galphaz knock-out mice. The effect was independent of 5HT4 receptors as the slow after-hyperpolarization was unaffected and a slow depolarization was absent in the Galphaz knock-out mice. Other receptors linked to Gi/o proteins [gamma-aminobutyric acid type B receptor (GABAB), adenosine A1 and muscarinic acetylcholine receptors] were not affected in Galphaz knock-out mice. These results suggest that the 5HT1A receptor may be linked to Galphaz protein, as reported previously in cell culture but shown here in an intact neural network.

The serotonin1A receptor: a representative member of the serotonin receptor family

1. Serotonin is an intrinsically fluorescent biogenic amine that acts as a neurotransmitter and is found in a wide variety of sites in the central and peripheral nervous system. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions. 2. Serotonin exerts its diverse actions by binding to distinct cell surface receptors which have been classified into many groups. The serotonin1A (5-HT1A) receptor is the most extensively studied of the serotonin receptors and belongs to the large family of seven transmembrane domain G-protein coupled receptors. 3. The tissue and sub-cellular distribution, structural characteristics, signaling of the serotonin1A receptor and its interaction with G-proteins are discussed. 4. The pharmacology of serotonin1A receptors is reviewed in terms of binding of agonists and antagonists and sensitivity of their binding to guanine nucleotides. 5. Membrane biology of 5-HT1A receptors is presented using the bovine hippocampal serotonin1A receptor as a model system. The ligand binding activity and G-protein coupling of the receptor is modulated by membrane cholesterol thereby indicating the requirement of cholesterol in maintaining the receptor organization and function. This, along with the reported detergent resistance characteristics of the receptor, raises important questions on the role of membrane lipids and domains in the function of this receptor.

5-HT receptors couple to activation of Akt, but not extracellular-regulated kinase (ERK), in cultured hippocampal neurons

5-HT(1A) receptors have been hypothesized to mediate some of the neuronal plasticity and behavioral responses stimulated by serotonin selective reuptake inhibitors. Although the cellular signaling pathways required for inducing these actions have not yet been determined, roles for the neuroprotective extracellular-regulated kinase (ERK) mitogen-activated protein (MAP) kinase and Akt pathways have been suggested. In the current studies we have utilized primary cultures to directly determine whether hippocampal 5-HT(1A) receptors couple to activation of Akt and ERK. We found that E18 hippocampal neurons exhibit a twofold activation of Akt when exposed to nanomolar concentrations of 5-HT. The 5-HT(1/7) receptor-selective agonist 5-carboxamidotryptamine maleate (5-CT) and the 5-HT(1A/7) receptor-selective agonist 8-hydroxy-N,N-dipropyl-aminotetralin (8-OH-DPAT) maleate were found to activate Akt with equal efficacy, and similar potency, to 5-HT. p-MPPI and WAY-100635, antagonists selective for 5-HT(1A) receptors, completely inhibited 5-CT- stimulated Akt activation. Activation of Akt was also inhibited by pretreatment with pertussis toxin as well as the phosphatidylinositol 3-kinase inhibitors, wortmannin and LY294002. In contrast, the 5-HT selective antagonist, SB269970, caused no inhibition. Although the density of 5-HT(1A) receptors expressed by cultured neurons was sufficient to activate Akt, no activation of ERK was observed. These findings suggest that Akt, and not ERK, may be relevant to previous reports of hippocampal 5-HT(1A) receptors mediating neurotrophic responses.

Endothelin-induced, Long Lasting, and Ca2+ Influx-independent Blockade of Intrinsic Secretion in Pituitary Cells by Gz Subunits

The G protein-coupled receptors in excitable cells have prominent roles in controlling Ca2+-triggered secretion by modulating voltage-gated Ca2+ influx. In pituitary lactotrophs, spontaneous voltage-gated Ca2+ influx is sufficient to maintain prolactin release high. Here we show that endothelin in picomolar concentrations can interrupt such release for several hours downstream of spontaneous and high K+-stimulated voltage-gated Ca2+ influx. This action occurred through the Gz signaling pathway; the adenylyl cyclase-signaling cascade could mediate sustained inhibition of secretion, whereas rapid inhibition also occurred at elevated cAMP levels regardless of the status of phospholipase C, tyrosine kinases, and protein kinase C. In a nanomolar concentration range, endothelin also inhibited voltage-gated Ca2+ influx through the G i/o signaling pathway. Thus, the coupling of seven-transmembrane domain endothelin receptors to Gz proteins provided a pathway that effectively blocked hormone secretion distal to Ca2+ entry, whereas the cross-coupling to G i/o proteins reinforced such inhibition by simultaneously reducing the pacemaking activity.

Cloned human 5-HT1A receptor pharmacology determined using agonist binding and measurement of cAMP accumulation

Twenty agonists and nine antagonists were evaluated for their ability to compete for [3H]-8-hydroxy-2-(di-n-propylamino)tetralin ([3H]-8-OH-DPAT) binding to the cloned human serotonin-1A (ch-5-HT1A) receptor expressed in Chinese hamster ovary cells and for their ability to alter adenylyl cyclase activity in the same cells. The most potent full agonists of high affinity included N,N-dipropyl-5-carboxamidotryptamine (pEC50=9.6 +/- 0.1), MDL 73005EF (pEC50=9.3 +/- 0.2), 5-methyl-urapidil (pEC50=9.2 +/- 0.1), 5-carboxamidotryptamine (pEC50=9.1 +/- 0.2), R(+)-8-OH-DPAT (pEC50=8.6 +/- 0.1) and BMY-7378 (pEC50=8.6 +/- 0.1). WB-4101 (pEC50=8.3 +/- 0.2; IA=79%), clozapine (pEC50=8.1 +/- 0.3; IA=29%), (buspirone (pEC50=7.6 +/- 0.2; IA=79%), quipazine (pEC50 <5; IA=45%) and R-DOI (pEC50 < 5; IA=31%) were weaker agonists with partial agonist properties. The most potent antagonists were WAY-100,635 (pKi=10.2 +/- 0.1), methiothepin (pKi=8.8 +/- 0.2), spiperone (pKi=8.7 +/- 0.2) and NAN-190 (pKi=8.5 +/- 0.2). The receptor affinities and functional potencies were well correlated (r=0.88; P <0.0001). Our binding data correlated well with the pharmacology of endogenous 5-HT1A receptors in the rabbit iris-ciliary body (r=0.91; P <0.001) and rat hippocampus (r=0.93, P <0.0001). Our functional cAMP data correlated well with other cAMP accumulation data (r=0.8, P <0.01 vs calf hippocampus) but less so with [35S]-GTPgammaS binding to the ch-5-HT(1A) receptor as a functional activity read-out (r=0.58, P <0.05). The present study provides a detailed pharmacological characterization of the ch-5-HT1A receptor using binding and functional assays.

Tandospirone activates neuroendocrine and ERK (MAP kinase) signaling pathways specifically through 5-HT1A receptor mechanisms in vivo

Tandospirone, an azapirone, is a selective serotonin(1A) (5-HT(1A)) receptor agonist. The effects of tandospirone on plasma hormones and on mitogen-activated protein (MAP) kinase activity in the brain of male rats were studied. Tandospirone produced a time- and dose-dependent increase in plasma levels of oxytocin, adrenocorticotropin (ACTH), corticosterone, and prolactin. The minimal dose of tandospirone that led to a significant elevation of plasma oxytocin, ACTH, and prolactin levels was 1.0 mg/kg (s.c.), while the minimal dose for corticosterone release was 3.0 mg/kg (s.c.). The ED(50) of tandospirone was 1.3 mg/kg for oxytocin, 1.2 mg/kg for ACTH, 3.0 mg/kg for corticosterone, and 0.24 mg/kg for prolactin. Pretreatment with the specific 5-HT(1A) receptor antagonist WAY 100,635 (0.3 mg/kg, s.c.) completely blocked the effects of tandospirone on plasma levels of oxytocin, ACTH, and corticosterone but shifted the dose-response curve for prolactin to the right. Tandospirone injection (10 mg/kg, s.c.) stimulated the MAP kinase signaling cascade, specifically the phosphorylation of p42/44 extracellular signal-regulated kinase (ERK). Western blot analysis revealed a significant increase in phosphorylated ERK (p-ERK) levels in the hypothalamic paraventricular nucleus (PVN) as well as the dorsal raphe nucleus 5 min following tandospirone injection. These increases were blocked by pretreatment with WAY 100,635 (0.3 mg/kg). The results are the first evidence that systemic 5-HT(1A) receptor agonist administration produces a rapid increase in p-ERK levels in vivo, providing further insight into the signaling mechanisms of the 5-HT(1A) receptor.

Estrogen treatment increases the levels of regulator of G protein signaling-Z1 in the hypothalamic paraventricular nucleus: possible role in desensitization of 5-hydroxytryptamine1A receptors

Desensitization of post-synaptic serotonin1A (5-HT1A) receptors may underlie the clinical improvement of neuropsychiatric disorders. In the hypothalamic paraventricular nucleus, Galphaz proteins mediate the 5-HT1A receptor-stimulated increases in hormone release. Regulator of G protein signaling-Z1 (RGSZ1) is a GTPase-activating protein selective for Galphaz proteins. RGSZ1 regulates the duration of interaction between Galphaz proteins and effector systems. The present investigation determined the levels of RGSZ1 in the hypothalamic paraventricular nucleus of rats subjected to four different treatment protocols that produce desensitization of 5-HT1A receptors. These protocols include: daily administration of beta estradiol 3-benzoate (estradiol) for 2 days; daily administration of fluoxetine for 3 and 14 days; daily administration of cocaine for 7 or 14 days; and acute administration of (+/-)-1-(2,5 dimethoxy-4-iodophenyl)-2-amino-propane HCl (DOI; a 5-HT2A/2C receptor agonist). Estradiol treatment was the only protocol that increased the levels of RGSZ1 protein in the hypothalamic paraventricular nucleus in a dose-dependent manner (46%-132% over control). Interestingly, previous experiments indicate that only estradiol produces a decreased Emax of 5-HT1A receptor-stimulation of hormone release, whereas fluoxetine, cocaine and DOI produce a shift to the right (increased ED50). Thus, the desensitization of 5-HT1A receptors by estradiol might be attributable to increased levels of RGSZ1 protein. These findings may provide insight into the adaptation of 5-HT1A receptor signaling during pharmacotherapies of mood disorders in women and the well-established gender differences in the vulnerability to depression.

Regulation of affect by the lateral septum: implications for neuropsychiatry

Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.

Serotonergic Stimulation of the Rat Hypothalamo-Pituitary-Adrenal Axis: Interaction between 5-HT1Aand 5-HT2AReceptors

Acute stimulation of the hypothalamo-pituitary-adrenal (HPA) axis by selective serotonin reuptake inhibitors (SSRIs) is mediated by several postsynaptic 5-HT receptor subtypes. Activation of 5-HT(1A) and 5-HT(2A) receptors increases plasma corticosterone levels, and it is likely that these receptor subtypes are central to mediating the effects of SSRIs. To study the interaction of these receptors, rats were administered with the 5-HT(1A/7) agonist 8-OH-DPAT (0.05 to 1.25 mg/kg), the 5-HT(2A/C) agonist DOI (0.25 to 5 mg/kg), or a mixture of both compounds, and trunk blood was taken 60 min later. The two compounds given in combination produced a lower increase in corticosterone than DOI does alone. DOI and 8-OH-DPAT also produced a marked induction of c-Fos in the paraventricular nucleus (PVN), but the induction was not different if the two compounds were given together. These data show that the two serotonin receptor subtypes affect the HPA axis via a central target. In conclusion, 5-HT(1A) and 5-HT(2A) receptors regulate corticotrophin-releasing hormone (CRH) neurons via distinct but strongly interacting pathways, probably converging on the same neurons in the hypothalamus.

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.

Fluoxetine-induced changes in body weight and 5-HT1Areceptor-mediated hormone secretion in rats on a tryptophan-deficient diet

Tryptophan depleting protocols are commonly used to study the role of serotonin in mood disorders. The present study examined the impact of a tryptophan-deficient diet and fluoxetine on the serotonergic regulation of neuroendocrine function and body weight. We hypothesized that the regulation of postsynaptic 5-HT1Areceptors is dependent on the levels of 5-HT in the synapse. Rats on a control or a tryptophan-deficient diet received daily injections of saline or fluoxetine (5 or 10 mg·kg-1·day-1ip) from day 7 to day 21. The tryptophan-deficient diet produced a 41% reduction in the level of 5-HT but no change in the density of [3H]paroxetine-labeled 5-HT transporters. Treatment with fluoxetine inhibited the gain in weight in rats maintained on the control diet. The tryptophan-deficient diet produced a significant loss in body weight that was not significantly altered by treatment with fluoxetine. Treatment with fluoxetine produced a dose-dependent desensitization of hormone responses to injection of the 5-HT1Areceptor agonist (±)8-hydroxy-2-(di- n-propylamino)tetralin ((±)8-OH-DPAT). The tryptophan-deficient diet produced an increase in the basal levels of corticosterone but did not alter the basal levels of ACTH or oxytocin. Also, this diet inhibited the magnitude of 8-OH-DPAT-induced increase in plasma levels of ACTH and oxytocin but did not impair the ability of fluoxetine to desensitize the 5-HT1Areceptor-mediated increase in plasma hormones. These data suggest that a reserve of 5-HT enables fluoxetine to desensitize postsynaptic 5-HT1Areceptors in the hypothalamus. In conclusion, the profound physiological changes induced by tryptophan depletion may complicate the interpretation of studies using this experimental approach.

Phosphorylation-regulated inhibition of the Gz GTPase-activating protein activity of RGS proteins by synapsin I

Synapsins are neuronal proteins that bind and cluster synaptic vesicles in the presynaptic space, presumably by anchoring to actin filaments, but specific regulatory functions of the synapsins are unknown. We found that a sub-population of brain synapsin Ia, a splice variant of one of three synapsin isoforms, inhibits the GTPase-activating protein (GAP) activity of several RGS proteins. Inhibition is highly selective for Galphaz, a member of the Gi family that is found in neurons, platelets, adrenal chromaffin cells, and a few other neurosecretory cells. Gz has been indirectly implicated in the regulation of secretion. Synapsin Ia constitutes a major fraction of the total GAP-inhibitory activity in brain, and its inhibitory activity is absent from the brains of synapsin I(-/-)/II(-/-) mice. Inhibition depends on the cationic D/E domain of synapsin. Phosphorylation of synapsin Ia at serine 9 by either cyclic AMP-dependent protein kinase or p21-activated protein kinase (PAK1) attenuates its potency as a GAP inhibitor more than 7-fold. Synapsin can thus act as a phosphorylation-modulated mediator of feedback regulation of Gz signaling by the synaptic machinery.

A Region-Specific Increase in Gαq And Gα11 Proteins in Brains of Rats during Cocaine Withdrawal

Serotonin 2A (5-HT2A) receptor-mediated increases in plasma hormone levels become supersensitive after 42 h of withdrawal from cocaine treatment. The present study investigated which components of the 5-HT2A receptor signaling system are associated with this supersensitivity. Rats were injected daily for 14 days with either saline or cocaine (15 mg/kg i.p.) twice a day or were injected using a "binge" protocol (three injections per day, 1 h apart). Rats were sacrificed 2 or 7 days after the last cocaine injection, and the levels of membrane and cytosol-associated 5-HT2A receptors, Galphaq, Galpha11, regulators of G protein signaling (RGS)4, and RGS7 proteins were assayed in the hypothalamic paraventricular nucleus, amygdala, and frontal cortex using Western blot analysis. Two days of withdrawal from cocaine, administered twice a day or using a binge protocol, produced an increase in membrane-associated Galphaq and Galpha11 proteins in the paraventricular nucleus and the amygdala (but not in the frontal cortex). This effect was reversible after 7 days of withdrawal. The protein levels of the 5-HT2A receptor, Galphaz protein, and RGS4 or RGS7 proteins were not altered by cocaine withdrawal in any of the above-mentioned brain regions. These findings suggest that the supersensitivity of the 5-HT2A receptors, during withdrawal from chronic cocaine, is associated with an increase in membrane-associated Galphaq and Galpha11 proteins and not with changes in the expression of 5-HT2A receptors.

Attenuated 5-HT1A receptor signaling in brains of suicide victims: involvement of adenylyl cyclase, phosphatidylinositol 3-kinase, Akt and mitogen-activated protein kinase

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-HT1A 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 [35S]GTPgammaS binding to Galphai/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-HT1A receptor to adenylyl cyclase. No significant group differences were detected in the expression levels of Galphai/o, Galphaq/11 or Galphas proteins, or in the activity of cAMP-dependent protein kinase A. Studies of a parallel transduction pathway downstream from 5-HT1A 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-HT1A receptor activation in depressed suicide victims are also manifest downstream from the associated G protein, affecting the activity of second messengers in two 5-HT1A receptor transduction pathways that may have implications for cell survival.

Serotonin and the neuroendocrine regulation of the hypothalamic--pituitary-adrenal axis in health and disease

Serotonin (5-hydroxytryptamine, 5-HT)-containing neurons in the midbrain directly innervate corticotropin-releasing hormone (CRH)-containing cells located in paraventricular nucleus of the hypothalamus. Serotonergic inputs into the paraventricular nucleus mediate the release of CRH, leading to the release of adrenocorticotropin, which triggers glucocorticoid secretion from the adrenal cortex. 5-HT1A and 5-HT2A receptors are the main receptors mediating the serotonergic stimulation of the hypothalamic-pituitary-adrenal axis. In turn, both CRH and glucocorticoids have multiple and complex effects on the serotonergic neurons. Therefore, these two systems are interwoven and communicate closely. The intimate relationship between serotonin and the hypothalamic-pituitary-adrenal axis is of great importance in normal physiology such as circadian rhythm and stress, as well as pathophysiological disorders such as depression, anxiety, eating disorders, and chronic fatigue.

Metabotropic glutamate receptor 5 antagonist-induced stimulation of hypothalamic-pituitary-adrenal axis activity: interaction with serotonergic systems

The mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine (MPEP) produces anxiolytic or antidepressant effects in several rodent models through incompletely described mechanisms. Anxiolytics and antidepressants share several neuroendocrine features, including acute activation of the hypothalamic-pituitary-adrenal (HPA)-axis, desensitization of neuroendocrine responses with repeated dosing, and desensitization of the HPA axis to 5-HT1A agonist stimulation. We characterized these neuroendocrine parameters in rats treated systemically with MPEP and compared them to those induced by the anxiolytic buspirone. Acutely, MPEP dose-dependently (0.1-10 mg/kg i.p.) increased plasma corticosterone concentrations. These responses were blocked by 50% with the 5-HT1A antagonist WAY100635. The corticosterone responses to both 3 mg/kg MPEP and buspirone were decreased by 80% after 5 days of twice-daily injections. Repeated injection with MPEP decreased HPA-axis sensitivity to buspirone challenge by 75%. This desensitization was not associated with changes in mGluR5 or 5-HT1A receptor binding properties, expression of G-protein subunits coupled to these receptors, or in 5-HT-stimulated binding of [(3)H]-GTPgammaS to membranes. We conclude that MPEP acutely disinhibits the HPA axis, in part through uncharacterized changes in serotonergic signaling. Desensitization of 5-HT1A responses after repeated MPEP administration may indicate that, like other anxiolytics and antidepressants, plasticity in 5-HT signal transduction pathways has occurred.

Neuroendocrine pharmacology of stress

Exposure to hostile conditions initiates responses organized to enhance the probability of survival. These coordinated responses, known as stress responses, are composed of alterations in behavior, autonomic function and the secretion of multiple hormones. The activation of the renin-angiotensin system and the hypothalamic-pituitary-adrenocortical axis plays a pivotal role in the stress response. Neuroendocrine components activated by stressors include the increased secretion of epinephrine and norepinephrine from the sympathetic nervous system and adrenal medulla, the release of corticotropin-releasing factor (CRF) and vasopressin from parvicellular neurons into the portal circulation, and seconds later, the secretion of pituitary adrenocorticotropin (ACTH), leading to secretion of glucocorticoids by the adrenal gland. Corticotropin-releasing factor coordinates the endocrine, autonomic, behavioral and immune responses to stress and also acts as a neurotransmitter or neuromodulator in the amygdala, dorsal raphe nucleus, hippocampus and locus coeruleus, to integrate brain multi-system responses to stress. This review discussed the role of classical mediators of the stress response, such as corticotropin-releasing factor, vasopressin, serotonin (5-hydroxytryptamine or 5-HT) and catecholamines. Also discussed are the roles of other neuropeptides/neuromodulators involved in the stress response that have previously received little attention, such as substance P, vasoactive intestinal polypeptide, neuropeptide Y and cholecystokinin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABA(A), histamine and serotonin receptors have been used to attenuate the neuroendocrine response to stressors. The neuroendocrine information for these drugs is still incomplete; however, they are a new class of potential antidepressant and anxiolytic drugs that offer new therapeutic approaches to treating anxiety disorders. The studies described in this review suggest that multiple brain mechanisms are responsible for the regulation of each hormone and that not all hormones are regulated by the same neural circuits. In particular, the renin-angiotensin system seems to be regulated by different brain mechanisms than the hypothalamic-pituitary-adrenal system. This could be an important survival mechanism to ensure that dysfunction of one neurotransmitter system will not endanger the appropriate secretion of hormones during exposure to adverse conditions. The measurement of several hormones to examine the mechanisms underlying the stress response and the effects of drugs and lesions on these responses can provide insight into the nature and location of brain circuits and neurotransmitter receptors involved in anxiety and stress.

Regulation of 5-HT1A receptor function in brain following agonist or antidepressant administration

Adaptive changes in the serotonergic system are generally believed to underlie the therapeutic effectiveness of the azapirone anxiolytics and a variety of antidepressant drugs. The serotonin-1A (5-HT(1A)) receptor has been implicated in affective disorders. Thus, studies of the regulation of 5-HT(1A) receptor function may have important implications for our understanding the role of this receptor in the mechanism of action of these therapeutic agents. This review focuses on the regulation of central 5-HT(1A) receptor function following administration of 5-HT(1A) receptor agonists or antidepressant drugs expected to increase the synaptic concentration of the neurotransmitter 5-HT. The majority of evidence supports regional differences in the regulation of central 5-HT(1A) receptor function following repeated agonist or antidepressant administration, which may be due to differences in processes involved in desensitization of the receptor at the cellular level. Region-specific differences in the regulation of 5-HT(1A) receptor function may be based on compensatory changes distal to the receptor, such as regulatory changes at the level of effector (e.g. adenylyl cyclase or ion channel), or at the level of the G protein such as changes in G protein expression, or phosphorylation of the G protein. It may be that the increase in serotonin neurotransmission, due to somatodendritic autoreceptor desensitization following agonist or antidepressant treatment, to normo-sensitive 5-HT(1A) receptors in certain brain regions (e.g. hippocampus or cortex) and to sub-sensitive 5-HT(1A) receptors in other brain regions (e.g. amygdala or hypothalamus) underlies the therapeutic efficacy of these drugs.

The C-terminus of Gi family G-proteins as a determinant of 5-HT(1A) receptor coupling

Using a universal signaling assay employing G-protein chimeras comprising the C-terminal five amino acids of Gi1/2, Gi3, Go, and Gz fused to Gq, the calcium mobilizing G-protein, we explored the role of the C-terminus of Gi family G-proteins as a determinant for 5-HT(1A) receptor functional coupling. Co-expression of the 5-HT(1A) receptor with each of the Gq/Gi family chimeras resulted in a concentration-dependent increase in calcium upon addition of 5-HT, although the coupling efficiency differed dramatically. Gq/Gi3 resulted in the most efficient coupling based on both potency and relative maximum response to 5-HT. Gq/Go also produced efficient coupling in terms of relative 5-HT efficacy (76% of the Gq/Gi3 maximum response), although 5-HT exhibited 4-fold lower agonist potency, and Gq/Gz and Gq/Gi1/2 conferred poor functional coupling. Agonist potencies and relative efficacies determined for a number of 5-HT(1A) receptor agonists using Gq/Gi3 coupling were significantly weaker than those described previously for coupling through the native G-protein. These results indicate the C-terminus of Gi3 as an important determinant for coupling to the 5-HT(1A) receptor, while the reduced functional agonist activities suggest additional motifs participate in receptor/G-protein coupling.

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.

Sustained blockade of neurokinin-1 receptors enhances serotonin neurotransmission

BACKGROUND

Antagonists of neurokinin-1 (NK(1)) receptors, through which substance P acts, have been proposed to belong to a new class of antidepressants with a unique mode of action. It was postulated that they exert this putative therapeutic effect independently of the serotonin (5-HT) neurons.

METHODS

The aim of the present study was to assess, using in vivo electrophysiological paradigms, the effects of sustained administration of the nonpeptidic NK(1) antagonist CP-96,345 on the firing activity of rat dorsal raphe 5-HT neurons, the responsiveness of pre- and postsynaptic 5-HT(1A) receptors, and overall 5-HT neurotransmission in the hippocampus.

RESULTS

Both short- and long-term treatments with CP-96,345 significantly increased the spontaneous firing activity of dorsal raphe 5-HT neurons, and this increase was associated with an attenuation of somatodendritic 5-HT(1A) autoreceptor responsiveness. In contrast, the inactive enantiomer of CP-96,345 at NK(1) receptors, CP-96,344, did not alter these parameters after short-term administration. Because 5-HT(1A) receptor activation inhibits the firing activity of dorsal hippocampus CA(3) pyramidal neurons, the degree of disinhibition produced by the selective 5-HT(1A) receptor antagonist WAY 100635 was determined to assess the net change in 5-HT neurotransmission. Intravenous injection of WAY 100635 did not disinhibit CA(3) pyramidal neuron firing in rats given saline, CP-96,345 for 2 days, or CP-96,344 for 14 days, but produced a significant enhancement of firing in rats treated with CP-96,345 for 2 weeks. Therefore, only long-term treatment with CP-96,345 enhanced the tonic activation of postsynaptic 5-HT(1A) receptors.

CONCLUSIONS

Similar to all other major types of antidepressant treatments, these data indicate that substance P antagonists might alleviate anxiety and major depression, at least in part, by enhancing the degree of activation of some 5-HT receptors in the forebrain.

G(z) signaling: emerging divergence from G(i) signaling

A large variety of neurotransmitters, hormones, and chemokines regulate cellular functions via cell surface receptors that are coupled to guanine nucleotide-binding regulatory proteins (G proteins) belonging to the G(i) subfamily. All members of the G(i) subfamily, with the sole exception of G(z), are substrates for the pertussis toxin ADP-ribosyl transferase. G(z) also exhibits unique biochemical and regulatory properties. Initial portrayals of the cellular functions of G(z) bear high resemblance to those of other G(i) proteins both in terms of the receptors and effectors linked to G(z). However, recent discoveries have begun to insinuate a distinct role for G(z) in cellular communication. Functional interactions of the alpha subunit of G(z) (Galpha(z)) with the NKR-P1 receptor, Galpha(z)-specific regulator of G protein signaling, p21-activated kinase, G protein-regulated inducers of neurite outgrowth, and the Eya2 transcription cofactor have been demonstrated. These findings provide possible links for G(z) to participate in cellular development, survival, proliferation, differentiation and even apoptosis. In this review, we have drawn a sketch of a signaling network with G(z) as the centerpiece. The emerging picture is one that distinguishes G(z) from other members of the G(i) subfamily.

S 14506: novel receptor coupling at 5-HT(1A) receptors

S 14506 is chemically related to the inverse agonist at 5-HT(1A) receptors, spiperone, but S 14506 behaves as one of the most potent agonists known at these receptors, both in vitro and in vivo. In hippocampal membranes, the specific binding of [(3)H]-S 14506 (K(d)=0.79+/-0.2 nM; B(max)=400+/-32 fmol/mg protein) to 5-HT(1A) receptors resembled that of an antagonist in that it was increased by GppNHp, whereas GppNHp reduced the binding of the classic agonist [(3)H]-8-OH-DPAT (K(d)=1.5+/-0.5 nM; B(max)=303+/-20 fmol/mg protein). Manganese, magnesium and calcium reduced the binding of [(3)H]-S 14506 to 5-HT(1A) receptors whereas the binding of [(3)H]-8-OH-DPAT was increased. Further, sodium markedly reduced the binding of [(3)H]-8-OH-DPAT, without affecting the binding of [(3)H]-S 14506. [(3)H]-S 14506 also bound with high affinity to h 5-HT(1A) receptors stably expressed in membranes of CHO cells (K(d)=0.13+/-0.05 nM; B(max)=2.99+/-0.60 pmol/mg protein): the B(max) was double that of [(3)H]-8-OH-DPAT. GppNHp strongly decreased [(3)H]-8-OH-DPAT binding but scarcely changed [(3)H]-S 14506 binding; calcium, magnesium and manganese had little effect on [(3)H]-S 14506 binding in CHO cells. Antagonists (WAY 100635, WAY 100135) and inverse agonists (spiperone and metitepine) displaced [(3)H]-S 14506 binding with high affinity and Hill slopes close to unity, whereas agonists (5-HT and 5-CT) displayed low affinity with low Hill slopes: partial agonists (buspirone, ipsapirone) showed intermediate properties. In fusion proteins of h 5-HT(1A) receptors with G(ialpha1) the compound potently increased high-affinity GTPase, with a steeper Hill slope than for 5-HT, which may indicate positive cooperativity. The maximum response for S 14506 in these assays was equivalent to 5-HT, indicating it to be a full agonist.In molecular modelling studies, using a three-site model of the 5-HT(1A) receptor, S 14506 spanned between the 5-HT recognition site and the "arginine switch" (DRY microdomain) postulated to activate the interaction of the receptor with the G protein. Thus it is possible to synthesise ligands at G-protein-coupled receptors which are highly potent agonists, but which are structurally related to inverse agonists and show some features of antagonist/inverse agonist binding.