Constitutive Activity and Structural Instability of the Wild-Type Human H2 Receptor

Abstract: Stable expression of the human H₂ receptor in Chinese hamster ovary cells resulted in an increase in basal cyclic AMP (cAMP) production, which was inhibited by the inverse agonists cimetidine, famotidine, and ranitidine with potencies similar to those found for the rat H₂ receptor. Burimamide, a neutral antagonist at the rat H₂ receptor, behaved as a weak partial agonist at the human H₂ receptor. Burimamide competitively antagonized both the histamine-induced increase in cAMP and the cimetidine-induced reduction of the basal cAMP level with apparent K _B values that were similar to its H₂ receptor affinity. Investigation of the modulation of receptor expression after long-term drug treatment revealed that at low concentrations histamine induced a significant reduction in H₂ receptor expression, whereas at high concentrations receptor expression was slightly increased. The partial agonist burimamide induced, like inverse agonists, an upregulation of the human H₂ receptor after prolonged treatment. These findings suggest a structural instability of the constitutively active human H₂ receptor in transfected Chinese hamster ovary cells. Occupation of the H₂ receptor by any ligand reduces the instability, thus resulting in higher cellular expression levels.     

Histamine H1 and Endothelin ETB Receptors Mediate Phospholipase D Stimulation in Rat Brain Hippocampal Slices

Abstract: Different neurotransmitter receptor agonists [carbachol, serotonin, noradrenaline, histamine, endothelin-1, and trans -(1 S ,3 R )-aminocyclopentyl-1,3-dicarboxylic acid ( trans -ACPD)], known as stimuli of phospholipase C in brain tissue, were tested for phospholipase D stimulation in [³²P]P_i-prelabeled rat brain cortical and hippocampal slices. The accumulation of [³²P]phosphatidylethanol was measured as an index of phospholipase D-catalyzed transphosphatidylation in the presence of ethanol. Among the six neurotransmitter receptor agonists tested, only noradrenaline, histamine, endothelin-1, and trans -ACPD stimulated phospholipase D in hippocampus and cortex, an effect that was strictly dependent of the presence of millimolar extracellular calcium concentrations. The effect of histamine (EC₅₀ 18 µ M ) was inhibited by the H₁ receptor antagonist mepyramine with a K _i constant of 0.7 n M and was resistant to H₂ and H₃ receptor antagonists (ranitidine and tioperamide, respectively). Endothelin-1-stimulated phospholipase D (EC₅₀ 44 n M ) was not blocked by BQ-123, a specific antagonist of the ET_A receptor. Endothelin-3 and the specific ET_B receptor agonist safarotoxin 6c were also able to stimulate phospholipase D with efficacies similar to that of endothelin-1, and EC₅₀ values of 16 and 3 n M , respectively. These results show that histamine and endothelin-1 stimulate phospholipase D in rat brain through H₁ and ET_B receptors, respectively.     

Epidermal Growth Factor Promotes Uncoupling from Adenylyl Cyclase of the Rat D2S Receptor Expressed in GH4C1 Cells

Abstract: In anterior pituitary cells or when transfected into host cell lines, the D₂ dopamine receptor inhibits adenylyl cyclase and activates potassium channels. The GH-3 pituitary tumor cell line, which lacks functional D₂ receptors, responds to epidermal growth factor (EGF) by expressing a D₂ receptor that, paradoxically, couples to potassium channel activation but poorly inhibits adenylyl cyclase; this was correlated with a pronounced increase in α subunit of the G protein G₁₃. In this study we have investigated the effects of EGF on the transduction mechanisms of D₂ receptors in GH4C1 cells transfected and permanently overexpressing the rat short D₂ receptor. Activation of D₂ receptors in these cells resulted in both inhibition of adenylyl cyclase and opening of potassium channels and inhibition of prolactin release by both cyclic AMP-dependent and independent mechanisms. Exposure of the transfected GH4C1 cells to EGF caused a dramatic decrease in the coupling efficiency of the D₂ receptor to inhibit cyclic AMP-dependent responses, leaving its activity toward potassium channels unchanged. The EGF treatment led to the concomitant increase in the membrane content of G₁₃ protein. These results suggest that the transmembrane signaling specificity of G protein-coupled receptors can be modulated by the relative amounts of different G proteins at the cell membrane.     

Ontogenetic Development of Histamine H1- Receptor Binding in Rat Brain

Abstract: Histamine H_1- receptors labeled with [³H]mepyramine in rat brain show an age-dependent development. [³H]Mepyramine receptor density and histidine decarboxylase activity in whole rat brain reach adult levels at 25–30 days after birth and they attain 50% of adult level at day 10 and 17, respectively. The apparently later development of histidine decarboxylase activity in whole rat brain is partly accounted for by a biphasic developmental increase of this enzymatic activity in cerebral cortex. For all other brain regions examined, the development of histamine H_1- receptors parallels that of histidine decarboxylase. The increase in [³H]mepyramine binding can be accounted for by an absolute increase in the numbers of the receptor sites, with no change in affinity. Subcellular fractionation studies indicate that histamine H_1- receptors are predominantly associated with synaptosomal fractions derived from both newborn and adult rat.     

Histamine induces neural stem cell proliferation and neuronal differentiation by activation of distinct histamine receptors

Histamine has neurotransmitter/neuromodulator functions in the adult brain, but its role during CNS development has been elusive. We studied histamine effects on proliferation, cell death and differentiation of neuroepithelial stem cells from rat cerebral cortex in vitro . RT-PCR and Western blot experiments showed that proliferating and differentiated cells express histamine H₁, H₂ and H₃ receptors. Treatments with histamine concentrations (100 nM–1 mM) caused significant increases in cell numbers without affecting Nestin expression. Cell proliferation was evaluated by BrdU incorporation; histamine caused a significant increase dependent on H₂ receptor activation. Apoptotic cell death during proliferation was significantly decreased at all histamine concentrations, and cell death was promoted in a concentration-dependent manner by histamine in differentiated cells. Immunocytochemistry studies showed that histamine increased 3-fold the number of neurons after differentiation, mainly by activation of H₁ receptor, and also significantly decreased the glial (astrocytic) cell proportion, when compared to control conditions. In summary, histamine increases cell number during proliferative conditions, and has a neuronal-differentiating action on neural stem cells, suggesting that the elevated histamine concentration reported during development might play a role in cerebrocortical neurogenesis, by activation of H₂ receptors to promote proliferation of neural precursors, and favoring neuronal fate by H₁-mediated stimulation.     

Ca2+/Calmodulin-Dependent Protein Kinase II and Protein Kinase C Phosphorylate a Synthetic Peptide Corresponding to a Sequence That Is Specific for the γ2L Subunit of the GABAA Receptor

Abstract: The γ₂ subunit of the GABA_A receptor (GABA_A-R) is alternatively spliced. The long variant (γ_2L) contains eight additional amino acids that possess a consensus sequence site for protein phosphorylation. Previous studies have demonstrated that a peptide or fusion protein containing these eight amino acids is a substrate for protein kinase C (PKC), but not cyclic AMP-dependent protein kinase A (PKA)-stimulated phosphorylation. We have examined the ability of PKA, PKC, and Ca²⁺/calmodulin-dependent protein kinase (CAM kinase II) to phosphorylate a synthetic peptide corresponding to residues 336–351 of the intracellular loop of the γ_2L subunit and inclusive of the alternatively spliced phosphorylation consensus sequence site. PKC and CAM kinase II produced significant phosphorylation of this peptide, but PKA was ineffective. The K _m values for PKC-and CAM kinase II-stimulated phosphorylation of this peptide were 102 and 35 μM , respectively. Maximal velocities of 678 and 278 nmol of phosphate/min/mg were achieved by PKC and CAM kinase II, respectively. The phosphorylation site in the eight-amino-acid insert of the γ_2L subunit has been shown to be necessary for ethanol potentiation of the GABA_A-R. Thus, our results suggest that PKC, CAM kinase II, or both may play a role in the effects of ethanol on GABAergic function.     

Cloning, Expression, and Pharmacology of Four Human 5-Hydroxytryptamine4 Receptor Isoforms Produced by Alternative Splicing in the Carboxyl Terminus

Abstract: We report here the molecular cloning of three new splice variants of the human serotonin 5-hydroxytryptamine₄ (h5-HT₄) receptor, which we named h5-HT_4(b), h5-HT_4(c), and h5-HT_4(d). The sequence following the splicing site at Leu₃₅₈ in the C-terminal tail of h5-HT_4(b) displays a 74% protein identity with the same region in the long form of the rat 5-HT₄ receptor (r5-HT_4L) but is shorter by 18 amino acids compared to its rat counterpart. The splice variants h5-HT_4(c) and h5-HT_4(d) are the first of their kind to be described in any animal species. The C terminus of h5-HT_4(c) displays a high number of putative phosphorylation sites. The h5-HT_4(d) isoform corresponds to an ultrashort form of the receptor, with a truncation two amino acids after the splicing site. Tissue distribution studies revealed some degree of specificity in the pattern of expression of the different isoforms within the human body. The four splice variants transiently expressed in COS-7 cells displayed an identical 5-HT₄ pharmacological profile and showed a similar ability to stimulate adenylyl cyclase activity in the presence of 5-HT. The stimulatory pattern of cyclic AMP formation in response to the 5-HT₄ agonist renzapride was found to be significantly different between h5-HT_4(a) and the other h5-HT₄ isoforms, indicating that the splice variants may differ in the way they trigger the signal transduction cascade following receptor activation.     

Studies on a Capillary-Rich Fraction Isolated from Brain: Histaminic Components and Characterization of the Histamine Receptors Linked to Adenylate Cyclase

Abstract: A fraction enriched in capillaries has been prepared from the guinea pig cerebral cortex. The purity of this fraction was checked by light- and electron-microscopic examination and by its high enrichment in alkaline phosphatase and γ-glutamyl transpeptidase. In the capillary-rich fraction, the endogenous level of histamine was 1.9%'of that measured in the initial hornogenate. The histamine-synthesizing enzyme, I-histidine decarboxylase, and the metabolizing enzyme, histamine-N-methyltransferase, were barely detectable. In addition, histamine elicits a twofold stimulation in the accumulation of cyclic AMP in this capillary fraction with an EC₅₀ of 5 γM. Agonists and antagonists of the two types of histamine receptors (H₁ and H₂) were used for the characterization of the receptors mediating this action: H₂-receptor agonists were able to activate the adenylate cyclase with "relative potencies" similar to that found on typical H₂-receptors, and cimetidine, a specific H₂-receptor antagonist, competitively inhibited the response to histamine with a K₁ value reflecting its interaction with a single population of H₂-receptors. On the contrary, data obtained with H₁-receptor agonists and antagonists reflect their interaction with H₂-receptors rather than H₁-receptors. Thus H₂-receptors are involved in the activation of adenylate cyclase of the capillary fraction.     

HISTAMINE-SENSITIVE ADENYLATE CYCLASE IN HYPOTHALAMUS OF RAT BRAIN: H1 AND H2 RECEPTORS

Abstract— In in vitro experiments on rat hypothalamic homogenates the effects of biogenic amines such as histamine (HA), noradrenaline (NA), dopamine (DA), serotonin (5-HT) and drugs such as isoprenaline (ISP), 2-(2-pyridyl)ethylamine (H₂ stimulant—H_ls), 4-methyl-histamine (H₂ stimulant H_2s), mepyramine (H₁ antagonistp H_la), cimetidine (H₂ antagonist—H_2a) were tested on adenylate cyclase activity. HA possessed a powerful stimulating effect on hypothalamic adenylate cyclase activity, higher than that shown by the other substances.
The stimulating effect of HA was greatest in hypothalamic tissue from male rats, while tissue from females showed only a modest stimulation. H_2s, induced a greater stimulation of adenylate cyclase than H_ls. On the other hand, the H_2a inhibited HA stimulation to a greater extent than the H_la, H_la and H_2a, when used together, completely inhibited the HA stimulation. HA may have a neurotrans-mitter role in the hypothalamus, and in this area there appears to be a mixed population of H₁ and H₂ receptors, with a majority of H₂ receptors.     

Two Intracellular Signaling Pathways for the Dopamine D3 Receptor: Opposite and Synergistic Interactions with Cyclic AMP

Abstract: As cerebral neurons express the dopamine D₁ receptor positively coupled with adenylyl cyclase, together with the D₃ receptor, we have investigated in a heterologous cell expression system the relationships of cyclic AMP with D₃ receptor signaling pathways. In NG108-15 cells transfected with the human D₃ receptor cDNA, dopamine, quinpirole, and other dopamine receptor agonists inhibited cyclic AMP accumulation induced by forskolin. Quinpirole also increased mitogenesis, assessed by measuring [³H]thymidine incorporation. This effect was blocked partially by genistein, a tyrosine kinase inhibitor. Forskolin enhanced by 50–75% the quinpirole-induced [³H]thymidine incorporation. This effect was maximal with 100 n M forskolin, occurred after 6–16 h, was reproduced by cyclic AMP-permeable analogues, and was blocked by a protein kinase A inhibitor. Forskolin increased D₃ receptor expression up to 135%, but only after 16 h and at concentrations of >1 µ M . Thus, in this cell line, the D₃ receptor uses two distinct signaling pathways: it efficiently inhibits adenylyl cyclase and induces mitogenesis, an effect possibly involving tyrosine phosphorylation. Activation of the cyclic AMP cascade potentiates the D₃ receptor-mediated mitogenic response, through phosphorylation by a cyclic AMP-dependent kinase of a yet unidentified component. Hence, transduction of the D₃ receptor can involve both opposite and synergistic interactions with cyclic AMP.     

Glucocorticoids Enhance Histamine-Evoked Catecholamine Secretion from Bovine Chromaffin Cells

Abstract: The synthetic glucocorticoid dexamethasone enhanced histamine-evoked catecholamine secretion from cultured bovine chromaffin cells. Dexamethasone enhanced the effects of histamine on both adrenergic (epinephrine-rich) and noradrenergic (norepinephrine-rich) chromaffin cells but had a more dramatic effect on noradrenergic cells. Histamine-evoked secretion in noradrenergic cells appeared to become rapidly inactivated, whereas the rate of secretion in adrenergic cells was nearly constant for up to 2 h; dexamethasone treatment attenuated the inactivation seen in noradrenergic cells. The effect of dexamethasone appeared after a lag of several hours and was maximal by 24 h. The EC₅₀ for dexamethasone was ∼1 n M . The effect of dexamethasone was mimicked by the glucocorticoid agonist RU 28362 and was blocked by the antagonist RU 38486, indicating that the effects of these steroids were mediated by the glucocorticoid or type II corticosteroid receptor. Histamine-evoked catecholamine secretion in both dexamethasone-treated and untreated cells was blocked by the H₁ histamine receptor antagonist mepyramine but was not affected by the H₂ antagonist cimetidine; thus, dexamethasone appeared to enhance an H₁ receptor-mediated process. In the absence of glucocorticoids, H₁ receptor mRNA levels were higher in adrenergic than in noradrenergic cells. Dexamethasone increased H₁ receptor mRNA levels in both cell types. The increased expression of H₁ receptors presumably contributes to the enhancement of histamine-evoked catecholamine secretion by glucocorticoids. Glucocorticoids may play a physiological role in modulating the responsiveness of chromaffin cells to histamine and other stimuli.     

Ca2+/Calmodulin-Mediated Regulation of Agonist-Induced Sequestration of Gq Protein-Coupled Histamine H1 Receptors in Human U373 MG Astrocytoma Cells

Abstract: We investigated the regulation by intracellular Ca²⁺ of agonist-induced sequestration of G_q protein-coupled histamine H₁ receptors in human U373 MG astrocytoma cells. Histamine-induced sequestration of H₁ receptors from the cell surface membrane was detected as the loss of [³H]mepyramine binding sites on intact cells accessible to the hydrophilic H₁-receptor antagonist pirdonium. The changes in the pirdonium-sensitive binding of [³H]mepyramine were mirrored by changes in the subcellular distribution of H₁ receptors detected by sucrose density gradient centrifugation. The histamine-induced sequestration of H₁ receptors did not occur in hypertonic medium, in which clathrin-mediated endocytosis is known to be inhibited, but was significantly accelerated in the absence of extracellular Ca²⁺ or in the presence of the calmodulin antagonists W-7 and calmidazolium. Inhibitors of protein kinase C (H-7 and GF109203X), Ca²⁺/calmodulin-dependent protein kinase II (KN-62), or protein phosphatase 2B (FK506) did not alter the time course of H₁-receptor sequestration. These results provide the first evidence that agonist-induced, clathrin-mediated sequestration of G_q protein-coupled receptors is transiently inhibited by Ca²⁺/calmodulin, with the result that receptors remain on the cell surface membrane during the early stage of agonist stimulation.     

Interactions Between Neuronal Histamine and Halothane Anesthesia in Rats

Abstract: Using an in vivo microdialysis method, we measured the release of histamine in the anterior hypothalamic area (AHy) of rats under several concentrations of halothane anesthesia (1, 0.5, and 0.2%). The release of histamine increased to 341 and 325% at halothane concentrations of 0.5 and 0.2%, compared with the basal level at anesthesia induced by 1% halothane. α-Fluoromethylhistidine (100 mg/kg i.v.), a specific and irreversible inhibitor of histidine decarboxylase, reduced the histamine release to <35% of the basal value at 1% halothane anesthesia in the AHy, and also decreased the anesthetic requirement for halothane, evaluated as the minimum alveolar concentration (MAC), by 26%. Furthermore, pyrilamine (20 mg/kg i.v.), a brain-penetrating H₁ antagonist, and zolantidine (20 mg/kg i.v.), a brain-penetrating H₂ antagonist, reduced the MAC for halothane by 28.5 and 16%, respectively. Although thioperamide (5 mg/kg i.v.), an antagonist of presynaptic H₃ autoreceptor, induced an approximate twofold increase in the level of histamine release in conscious freely moving rats, the same dose of thioperamide had little effect on the release of histamine under 1% halothane anesthesia in the AHy. Furthermore, thioperamide did not change the anesthetic requirement (MAC) for halothane. The present findings indicate that halothane anesthesia inhibits the release of neuronal histamine and that histaminergic neuron activities change the anesthetic requirement (MAC) for halothane through H₁ as well as H₂ receptors.     

Cloning and Expression of a Human Serotonin 5-HT4 Receptor cDNA

Abstract: Using a combination of library screening and nested PCR based on a partial human serotonin 5-HT₄ receptor sequence, we have cloned the complete coding region for a human 5-HT₄ receptor. The sequence shows extensive similarity to the published porcine 5-HT_4A and rat 5-HT_4L receptor cDNA; however, in comparison with the latter, we find an open reading frame corresponding to only 388 amino acids instead of 406 amino acids. This difference is due to a frame shift caused by an additional cytosine found in the human sequence after position 1,154. Moreover, we also found the same additional cytosine in the rat 5-HT₄ sequence. We confirmed the occurrence of the sequence by examining this part of the sequence in genomic DNA of 10 human volunteers and in rat genomic DNA. Based on a part of the genomic 5-HT₄ receptor sequence that was identified in the cloning process, there seem to be at least two possible splice sites in the coding region of the gene. The human 5-HT₄ receptor, transiently expressed in COS-7 cells, showed radioligand binding properties similar to 5-HT₄ receptors in guinea pig striatal tissue. [³H]GR 113808 revealed K _D values of 0.15 ± 0.01 n M for the human receptor and 0.3 ± 0.1 n M in the guinea pig tissue. Binding constants were determined for four investigated 5-HT₄ antagonists and three agonists, and appropriate binding inhibition constants were found in each case. Stimulation of transfected COS-7 cells with 5-HT₄-specific agonists caused an increase in cyclic AMP levels.     

Differences in the Cyclic AMP-Dependent Phosphorylation of Plasma Membrane Proteins of Differentiated and Undifferentiated L6 Myogenic Cells

Differences in the cyclic AMP-dependent plasma membrane phosphorylation system of undifferentiated and differentiated L₆ myogenic cells have been detected. Endogenous plasma membrane protein phosphorylation in undifferentiated L₆ myoblasts was stimulated more than three fold by 5 × 10⁻⁵ M cyclic AMP, whereas no statistically significant cyclic AMP-dependent phosphorylation of endogenous plasma membrane proteins was observed in differentiated L₆ cells. In undifferentiated cells cyclic AMP promoted the phosphorylation of several proteins, the most prominent of which had a molecular weight of 110,000. In differentiated cells cyclic AMP did not selectively promote the phosphorylation of specific plasma membrane proteins. Both differentiated and undifferentiated L₆ cells, however, contain a cyclic AMP-dependent protein kinase capable of catalyzing the phosphorylation of exogenous substrates, such as histone f₂b. Therefore, the data show that differentiation in L₆ cells is associated with a selective change in the activity of a plasma membrane cyclic AMP-dependent protein kinase which employs endogenous membrane proteins as substrate.     

Receptor-Mediated Desensitisation of Histamine H1 Receptor-Stimulated Inositol Phosphate Production and Calcium Mobilisation in GT1-7 Neuronal Cells Is Independent of Protein Kinase C

Abstract: GT1-7 cells, a clonal line derived from specific tumours of gonadotropin-releasing hormone-secreting neurons from mouse hypothalamus, were used as a model system to investigate the cellular mechanisms underlying the histamine H₁ receptor-mediated desensitisation. GT1-7 cells contain H₁ receptors, acute stimulation of which leads to the desensitisation of histamine-mediated calcium mobilisation and is manifest as a concurrent reduction in both the magnitude of the calcium transient and of the sustained phase. Acute pretreatment of the cells with the phorbol ester, phorbol 12-myristate 13-acetate, can also ablate the histamine-stimulated calcium mobilisation. In addition, acute H₁-receptor stimulation and acute phorbol ester treatment result in the attenuation of histamine-mediated inositol phosphate production. Receptor desensitisation resulting from acute stimulation with histamine is not affected by inhibiting protein kinase C (PKC) activity with Ro 31-7549 or staurosporine. In contrast, the desensitisation of H₁-receptor responses induced by direct activation of protein kinase C is preventable by PKC inhibitors. Thus, these results imply that a PKC-dependent mechanism and PKC-independent mechanism are involved in the H₁-receptor desensitisation cascade in GT1-7 cells and do not support the involvement of PKC in the receptor-mediated desensitisation of H₁ receptor-stimulated calcium and inositol phosphate responses.     

Histamine H1-Receptor Binding Sites in Guinea Pig Brain Membranes: Regulation of Agonist Interactions by Guanine Nucleotides and Cations

Abstract: Agonist, but not antagonist, interactions with histamine H₂-receptors labeled by [³H]mepyramine are regulated selectively by sodium, divalent cations, and guanine nucleotides. Sodium decreases the affinity of histamine and the agonist 2-amino-ethylpyridine for [³H]mepyramine sites in guinea pig brain membranes up to 10-fold. The effect of sodium is exerted to a lesser extent by lithium, while potassium and rubidium are much weaker. Guanine nucleotides also decrease the affinity of histamine for H₁ binding sites about twofold. GTP and its nonmetabolized analogue GMP-PNP as well as GDP exert similar effects, while GMP, ATP, ADP, and AMP are inactive. The effects of GTP and sodium on histamine interactions with H₁-receptors are additive. By contrast, certain divalent cations enhance the potency of histamine at H₁-receptors. Manganese is most potent, while magnesium is almost as active as manganese and calcium is essentially inactive. Sodium, divalent cations, and guanine nucleotides have negligible effects on the interactions of antihistamines with H₁-receptors.     

Mianserin-Induced Down-Regulation of Human 5-Hydroxytryptamine2A and 5-Hydroxytryptamine2C Receptors Stably Expressed in the Human Neuroblastoma Cell Line SH-SY5Y

Abstract: We have assessed the ability of the serotonergic antagonist mianserin to modulate the number and functional activity of human 5-hydroxytryptamine_2A (5-HT_2A) and 5-HT_2C receptors stably expressed in the human neuroblastoma cell line SH-SY5Y. Incubation of cells expressing the 5-HT_2A receptor with mianserin (100 n M ) for 24 h caused a significant decrease (48%) in the binding capacity of [³H]ketanserin. This receptor down-regulation was associated with a corresponding decrease in the maximal production of inositol phosphates induced by 5-HT but not by carbachol. Exposure of cells expressing the 5-HT_2C receptor to mianserin (100 n M ) for 72 h but not for 24 h similarly resulted in a significant reduction (44%) in [³H]mesulergine binding. Corresponding analysis of inositol phosphate production by 5-HT at the 5-HT_2C receptor after incubation with mianserin showed no change in maximal response after 24 h. No change in the binding capacity of either radioligand was seen after incubation with mianserin for 1 h. A decrease in the binding affinity of both radioligands was also observed after mianserin treatment, but this decrease was similar after 1 h of incubation to that seen after 24 or 72 h, and was probably due to the retention of mianserin within the tissue. We conclude that antagonist down-regulation is evident at human 5-HT_2A and 5-HT_2C receptors stably expressed in a human neuroblastoma cell line and is probably mediated by a direct action of mianserin at the receptor.     

Histaminergic H2 Action Protects Hippocampal CA1 Neurons by Prolonging the Onset of the Anoxic Depolarization in Gerbils

Abstract: The central histaminergic action on ischemia-induced neuronal damage was examined by evaluating the histological outcome and the direct current (DC) potential shift in the hippocampal CA1 region in gerbils. An intracerebroventricular administration of histamine (10–100 nmol) improved the delayed ischemic damage in hippocampal CA1 pyramidal cells produced by 3 min of transient forebrain ischemia. A high dose (75 nmol) of mepyramine, an H₁ antagonist, aggravated ischemia-induced neuronal damage, but not a low dose (0.75 nmol). Administration of cimetidine (4 nmol) and ranitidine (3 nmol), H₂ antagonists, aggravated the neuronal damage. An injection of histamine (100 nmol) prolonged the onset time of the ischemia-induced sudden shift in the extracellular DC potential (anoxic depolarization; AD) to 133% of that in control animals. Administration of mepyramine (75 nmol) did not markedly change the AD, whereas injections of cimetidine (40 nmol) and ranitidine (3 nmol) reduced the onset latency to 47 and 45%, respectively. These findings suggest that the central H₂ action serves to protect neurons by delaying the onset of AD in gerbils.