Chromatographic analyses of the serotonin 5-HT1A receptor solubilized from the rat hippocampus

Neuronal Circuitry Mechanisms Regulating Adult Mammalian Neurogenesis

The adult mammalian brain is a dynamic structure, capable of remodeling in response to various physiological and pathological stimuli. One dramatic example of brain plasticity is the birth and subsequent integration of newborn neurons into the existing circuitry. This process, termed adult neurogenesis, recapitulates neural developmental events in two specialized adult brain regions: the lateral ventricles of the forebrain. Recent studies have begun to delineate how the existing neuronal circuits influence the dynamic process of adult neurogenesis, from activation of quiescent neural stem cells (NSCs) to the integration and survival of newborn neurons. Here, we review recent progress toward understanding the circuit-based regulation of adult neurogenesis in the hippocampus and olfactory bulb.

Activity-dependent signaling mechanisms regulating adult hippocampal neural stem cells and their progeny

Adult neural stem cells (NSCs) reside in a restricted microenvironment, where their development is controlled by subtle and presently underexplored cues. This raises a significant question: what instructions must be provided by this supporting niche to regulate NSC development and functions? Signaling from the niche is proposed to control many aspects of NSC behavior, including balancing the quiescence and proliferation of NSCs, determining the cell division mode (symmetric versus asymmetric), and preventing premature depletion of stem cells to maintain neurogenesis throughout life. Interactions between neurogenic niches and NSCs also govern the homeostatic regulation of adult neurogenesis under diverse physiological, environmental, and pathological conditions. An important implication from revisiting many previously-identifi ed regulatory factors is that most of them (e.g., the antidepressant fluoxetine and exercise) affect gross neurogenesis by acting downstream of NSCs at the level of intermediate progenitors and neuroblasts, while leaving the NSC pool unaffected. Therefore, it is critically important to address how various niche components, signaling pathways, and environmental stimuli differentially regulate distinct stages of adult neurogenesis.

Serotonin receptors in hippocampus

Serotonin is an ancient molecular signal and a recognized neurotransmitter brainwide distributed with particular presence in hippocampus. Almost all serotonin receptor subtypes are expressed in hippocampus, which implicates an intricate modulating system, considering that they can be localized as autosynaptic, presynaptic, and postsynaptic receptors, even colocalized within the same cell and being target of homo- and heterodimerization. Neurons and glia, including immune cells, integrate a functional network that uses several serotonin receptors to regulate their roles in this particular part of the limbic system.

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.

Effects of chronic antidepressants and electroconvulsive shock on serotonergic neurotransmission in the rat hippocampus

The hippocampus may play a critical role in the pathophysiology and treatment of depression. There are two main lines of evidence for this: firstly, many of its functions correspond to those altered in depression, and secondly, many hippocampal functions are regulated by the serotonergic (5-HT) system, which is a common target of antidepressant treatments. Chronic effects of antidepressants and electroconvulsive shock (ECS) have been studied by various methods using electrophysiology, in vivo microdialysis or ex vivo neurochemical measurements. The aim of the current review is to point out possible correlations between these studies based on different methods and to suggest neurochemical mechanisms that result in the observed changes in hippocampal physiology and neurogenesis. These changes in hippocampal neurochemistry are reviewed and compared with the abnormalities associated with stress, corticosterone or depression.

Production and characterization of polyclonal antibodies recognizing the intracytoplasmic third loop of the 5-hydroxytryptamine1A receptor

The portion of the complementary DNA encoding the third intracellular loop of the rat 5-hydroxytryptamine1A (serotonin) receptor was subcloned into the vector pGEX-KG and expressed in Escherichia coli as a fusion protein coupled with the glutathione S-transferase of Schistosoma japonicum. The fusion protein was purified on a glutathione-agarose affinity column and used to immunize rabbits for the production of polyclonal anti-5-hydroxytryptamine1A receptor antibodies. Enzyme-linked immunosorbent assay revealed that antibodies were produced as early as one month after the first injection of the fusion protein, and immune response plateaued at a maximum after the third (monthly) booster injection. These antibodies only marginally affected the specific binding of [3H]8-hydroxy-2-(di-n-propyl-amino) tetralin to solubilized and membrane bound 5-hydroxytryptamine1A receptors, and did not interfere with serotonin-induced inhibition of forskolin-stimulated adenylate cyclase negatively coupled to 5-hydroxytryptamine1A receptors in rat hippocampal membranes. However, antibodies were able to immunoprecipitate 5-hydroxytryptamine1A receptor binding sites solubilized from rat hippocampal membranes. The distribution of immunoautoradiographic labelling and immunohistochemical staining of rat brain sections exposed to the antibodies raised against the fusion protein superimposed to that of 5-hydroxytryptamine1A receptor binding sites labelled by specific radioligands, with marked enrichment in the limbic areas (dentate gyrus and CA1 area in the hippocampus, lateral septum, entorhinal cortex) and the anterior raphe nuclei. The differential cellular location of immunoreactivity within the hippocampus (where dendritic fields but not pyramidal cell somas were immunostained) and the median raphe nucleus (where the plasmic membrane of somas was strongly immunoreactive) suggests that the addressing of 5-hydroxytryptamine1A receptors might differ from one neuronal cell type to another.

Human serotonin1B receptor expression in Sf9 cells: phosphorylation, palmitoylation, and adenylyl cyclase inhibition

Analysis of the primary protein structure of the human serotonin1B (5-HT1B) receptor reveals consensus sites for phosphorylation and a putative site for palmitoylation. To investigate these posttranslational modifications, we have expressed a c-myc epitope-tagged 5-HT1B (m5-HT1B) receptor in Sf9 cells. This strategy enabled receptors to be detected by immunoblot analysis and purified by immunoprecipitation using a monoclonal antibody, 9E10, specific for the c-myc epitope. Agonist radioligand [3H]5-HT binding studies showed that the expressed 5-HT1B and m5-HT1B receptors displayed the characteristic pharmacological profile of the neuronal 5-HT1B receptor. The expressed receptors displayed both high- and low-affinity states for [3H]5-HT, suggesting that the receptors were coupled to endogenous G-proteins. Indeed, agonist binding to the high-affinity receptor state was regulated in the presence of GTP gamma S, Gpp(NH)p, and pertussis toxin. [32P]ADP-ribosylation experiments identified a major approximately 41-kDa ADP-ribosylated protein present in Sf9 membranes that comigrated with partially purified bovine brain Gi alpha/G(o) alpha subunits. Measurements of adenylyl cyclase activity in membranes from cells expressing m5-HT1B receptors showed that serotonergic agonists mediated the inhibition of adenylyl cyclase activity with a rank order of potency comparable to their affinity constants. Immunoblot analysis of membranes prepared from cells expressing m5-HT1B receptors and photoaffinity labeling of the immunoprecipitated material revealed photolabeled species at approximately 95 and at approximately 42 kDa.(ABSTRACT TRUNCATED AT 250 WORDS)

The GTP-insensitive component of high-affinity [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding in the rat hippocampus corresponds to an oxidized state of the 5-hydroxytryptamine1A receptor

Previous studies on central 5-hydroxytryptamine1A (5-HT1A) receptors have consistently shown the existence of a GTP-insensitive component of agonist binding, i.e., binding of [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) that persists in the presence of 0.1 mM GTP or guanylylimidodiphosphate (GppNHp). The molecular basis for this apparent heterogeneity was investigated pharmacologically and biochemically in the present study. The GppNHp-insensitive component of [3H]8-OH-DPAT binding increased spontaneously by exposure of rat hippocampal membranes or their 3-[3-(cholamidopropyl)dimethylammonio]-1-propane sulfonate-soluble extracts to air; it was reduced by preincubation of solubilized 5-HT1A binding sites in the presence of dithiothreitol and, in contrast, reversibly increased by preincubation in the presence of various oxidizing reagents like sodium tetrathionate or hydrogen peroxide. In addition, exposure of hippocampal soluble extracts to short-cross-linking reagents specific for thiols produced an irreversible increase in the proportion of GppNHp-insensitive over total [3H]8-OH-DPAT binding. The pharmacological properties of this GppNHp-insensitive component of [3H]8-OH-DPAT binding were similar to those of 5-HT1A sites in the absence of nucleotide. Sucrose gradient sedimentation of solubilized 5-HT1A binding sites treated by dithiothreitol or sodium tetrathionate showed that oxidation prevented the dissociation by GTP of the complex formed by the 5-HT1A receptor binding subunit (R[5-HT1A]) and a guanine nucleotide-binding protein (G protein). Moreover, the oxidation of -SH groups by sodium tetrathionate did not prevent the inactivation of [3H]8-OH-DPAT specific binding by N-ethylmaleimide, in contrast to that expected from an interaction of both reagents with the same -SH groups on the R[5-HT1A]-G protein complex. These data suggest that the appearance of GTP-insensitive [3H]8-OH-DPAT specific binding occurs as a result of the (spontaneous) oxidation of essential -SH groups (different from those preferentially inactivated by N-ethylmaleimide) on the R[5-HT1A]-G protein complex.

The 5-HT1A receptor: an overview of recent advances

Progress in the field of neuronal receptor research has accelerated during the last few years due to developments in pharmacology and molecular biology. This is particularly true in the case of the serotonin 5-HT1A receptor. In 1983 the very selective, high affinity 5-HT1A agonist 8-OH-DPAT was developed which allowed the pharmacology and distribution of the 5-HT1A receptor in the central nervous system of the rat and man to be extensively characterized. By 1987, the gene encoding this receptor protein was cloned and sequenced, allowing not only elucidation of its structure, but also better insight into the nature of its coupling to transmembrane signal transduction systems. Thus in a short period of time considerable knowledge has accumulated on how serotonin exerts its functions in the central nervous system via the 5-HT1A receptor. In the present review we will briefly discuss some of the latest developments regarding the 5-HT1A receptor.

Production of specific anti-rat 5-HT1A receptor antibodies in rabbits injected with a synthetic peptide

Polyclonal antibodies were raised by the repeated injection of rabbits with a synthetic peptide corresponding to a highly selective portion (amino acid residues 243 to 268) of the amino acid sequence of the rat 5-HT1A receptor. The anti-peptide antiserum allowed the immunoprecipitation of 5-HT1A receptors but not of other 5-HT1 sites solubilized from rat hippocampal membranes. Immunoautoradiographic labelling of rat brain sections with the anti-peptide antiserum was superimposed with the autoradiographic distribution of 5-HT1A sites labelled by the selective radioligand [3H]8-OH-DPAT.