Neuronal differentiation modifies the effect of ethanol exposure on voltage-dependent calcium channels in NG 108-15 cells

The effect of prolonged (72 h) ethanol (200 mM) exposure on the labeling of L-type (using tritiated PN 200-110) and N-type (using iodinated omega-conotoxin) voltage-dependent calcium channels was investigated in cultured NG 108-15 cells. In undifferentiated cells ethanol produced an 80% increase in PN 200-110 Bmax and no changes in omega-conotoxin binding. Differentiation had a profound effect on the response of cells to ethanol, which in differentiated neuron-like cells decreased omega-conotoxin binding (-53.5%) leaving PN 200-110 labeling of L-type channels unaffected. The effect was time dependent and reversible upon ethanol withdrawal. The decreased omega-conotoxin binding was accompanied by a reduced ability of omega-conotoxin to inhibit K+ -stimulated calcium uptake. The results demonstrate that in cultured NG 108-15 cells ethanol differentially affects DHP and omega-conotoxin-sensitive, voltage-dependent calcium channels and that the effect is also modulated by differentiation of the cell to a neuronal phenotype.

The role of palmitoylation of the guanine nucleotide binding protein G11 alpha in defining interaction with the plasma membrane

Mutations of Cys-9 to serine, Cys-10 to serine and a combination of both alterations were produced in a cDNA encoding murine G11 alpha to potentially interfere with the ability of the expressed polypeptides to act as substrates for post-translational palmitoylation. Each of these mutants and the wild-type protein were expressed in simian COS-1 cells. Mutation of either cysteine-9 or cysteine-10 decreased the degree of palmitoylation of the protein by some 80% compared with the wild-type, while the double mutant totally failed to incorporate [3H]palmitate. By contrast, in all transfections the endogenously expressed simian G11 alpha incorporated [3H]palmitate to similar levels. Particulate and cytoplasmic fractions from these cells were subjected to SDS/PAGE under conditions which allow resolution of primate and rodent forms of G11 alpha. Immunoblotting of these fractions demonstrated that in all cases the endogenously expressed simian G11 alpha was exclusively associated with the particulate fraction, as was the transfected and expressed wild-type murine G11 alpha. By contrast, each of the mutated forms of murine G11 alpha displayed a distribution in which approx. 70% of the expressed protein was present in the particulate fraction and 30% in the supernatant. To examine the conformation of the particulate expressed forms of murine G11 alpha, these fractions were treated with various concentrations of sodium cholate and immunoblots were subsequently performed on the solubilized and remaining particulate proteins. Whereas essentially all of the endogenous simian G11 alpha was solubilized by treatment with 1% (w/v) sodium cholate and some 50% with 0.32% cholate, expressed wild-type murine G11 alpha was more recalcitrant to solubilization. However, that fraction of wild-type murine G11 alpha which was solubilized behaved identically to the endogenous simian G11 alpha on Superose-12 gel-exclusion chromatography. The particulate fraction of the C9S/C10S double mutant of murine G11 alpha was highly resistant to solubilization by sodium cholate, whereas the particulate fractions of the two single cysteine to serine mutants were intermediate to the wild-type and double mutant in their ability to be solubilized by this detergent. These data demonstrate that the palmitoylation status of the cysteine residues at positions 9 and 10 in murine G11 alpha plays a central role in defining membrane association of this G-protein and indicate that much of the particulate fraction of the expressed palmitoylation-resistant mutants is likely to represent non-functional rather than correctly folded protein.(ABSTRACT TRUNCATED AT 400 WORDS)

BIMT 17, a 5-HT2A receptor antagonist and 5-HT1A receptor full agonist in rat cerebral cortex

In the search for antidepressant agents with a rapid onset of action, we have found that compound BIMT 17 (1-[2-[4-(3-trifluoromethylphenyl)piperazin-1- yl]ethyl]benzimidazol-[1H]-2-one) shows a good affinity for cerebral cortical 5-HT1A (pKi = 7.72) and 5-HT2A (pKi = 6.90) receptors, with no appreciable affinity for the other 5-HT receptor subtypes, including 5-HT2C. BIMT 17 reduced forskolin-stimulated cAMP accumulation in the cerebral cortex (pEC50 = 6.09) and in the hippocampus (pEC50 = 6.50), and antagonized 5-HT-induced phosphatidylinositol turnover (pKi = 6.96) in the cerebral cortex. The effect on cAMP accumulation was blocked by the 5-HT1A receptor antagonist tertatolol. Buspirone, 8-OH-DPAT and S 14671 (1-[2-(2-thenoylamino)ethyl]- 4[1-(7-methoxynaphtyl)]-piperazine), claimed to be 5-HT1A receptor agonists, did not reduce forskolin-stimulated cAMP formation in the cerebral cortex. On the basis of these data, it was concluded that BIMT 17 was the only compound that behaved as a full agonist with respect to the cAMP response in the cortex, while exerting concurrent agonism at 5-HT1A receptors and antagonism at 5-HT2A receptors. These characteristics might explain the peculiar behavior of BIMT 17 in mimicking the inhibitory action of 5-HT on the basal firing rate of the cortical neurons (see accompanying paper).

Behavioral and biochemical evidence of opioidergic involvement in cocaine sensitization

Chronic administration of cocaine produces sensitization to its behavioral effects in humans and experimental animals. In the present study, rats treated with cocaine (10 mg/kg, i.p.) once daily for 10 days showed an enhancement in the acute drug stimulation of locomotor activity and stereotypy. Biochemical analysis in the nucleus accumbens of chronic cocaine-treated animals indicated that sensitization of D1 dopamine (DA) receptors had also developed. In fact, stimulation of adenylyl cyclase activity by DA was increased in nucleus accumbens membranes from sensitized rats. Our findings suggest that a novel postsynaptic mechanism, i.e., an increased DA-D1 receptor function, may play a role in the sensitization. A causal relationship between the two events is supported by the observation that neither motor behavioral sensitization nor DA-dependent adenylyl cyclase hyperactivity developed when the opiate antagonist naltrexone (2 mg/kg, s.c.) was given daily for 10 days before cocaine. When given alone, naltrexone was inactive in all respects, which rules out any unspecific action and suggests that its effects may be due to competition at receptors with endogenous opioids mobilized by cocaine. This was indirectly supported by the finding that desensitization of opioid inhibition of adenylyl cyclase developed in nucleus accumbens membranes of cocaine-sensitized rats. Chronic blockade of opioid receptors by naltrexone also counteracted the reinforcing properties of cocaine; conditioned place preference, clearly displayed by cocaine-treated animals, was antagonized in a dose-related manner. Overall, these results confirm that endogenous opioid peptides play an important role in cocaine addiction.

The dynamic role of palmitoylation in signal transduction

Guanine nucleotide binding protein (G protein)-linked receptors, the alpha-subunits of heterotrimeric G proteins and members of the Src family of nonreceptor tyrosine kinases are among many polypeptides that are posttranslationally modified by the addition of palmitate, a long-chain fatty acid. Attachment of palmitate to these proteins is dynamic and may be regulated by their activation. The presence of palmitate appears to play a key role in the membrane localization of either the entire polypeptide or parts of it, and may regulate the interactions of these polypeptides with other proteins.

Somatotropic dysregulation in old mammals

In old mammals, including humans, the spontaneous growth hormone (GH) secretory pattern is markedly reduced resulting in lower amounts of GH released over 24 h, and the GH response to administration of GH-releasing hormone (GHRH) is reduced. In agreement with these in vivo findings, an impaired responsiveness to GHRH is evident in the pituitary of old male and female rats in vitro, and this is linked with a diminished stimulation of adenylate cyclase by GHRH. The poor GH responsiveness to GHRH in old mammals, which in the rat is coupled to a defective number of GHRH receptors in the somatotrophs, is likely due to a primary deficiency of GHRH availability, as implied by the diminished GHRH immunoreactivity and gene expression in and GHRH release from the hypothalamus of old rats. Attempts have been made to stimulate the sluggish somatotrophic function in elderly humans and dogs using GHRH; in either species positive results were obtained though, overall, it would seem that the GHRH hypofunction does not entirely account for the GH hyposecretory state during ageing. Concerning somatostatin, although the expression of this peptide decreases with age in the rat hypothalamus, secretion and activity of this hormone is increased, resulting in an altered relationship between GHRH and somatostatin gene expression and secretion. It is likely that defects, especially in catecholaminergic and cholinergic neurons, are instrumental in altering specific peptidergic neurons. Reportedly, catecholamines induce GH release by stimulating GHRH neurons and inhibiting somatostatin-releasing neurons; acetylcholine stimulates GH release via muscarinic receptors, in this way inhibiting the action of somatostatin neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

N-terminal fatty acylation of the alpha-subunit of the G-protein Gi1: only the myristoylated protein is a substrate for palmitoylation

The alpha-subunit of the G-protein Gi1 carries two fatty acyl moieties covalently bound to its N-terminal region: myristic acid is linked to glycine-2 and palmitic acid is linked to cysteine-3. Using site-directed mutagenesis on a cDNA construct of alpha i1 we have generated an alpha i1-G2A mutant, carrying alanine instead of glycine at position 2, and alpha i1-C3S mutant, in which serine replaced cysteine-3 and a double mutant with both substitutions (alpha i1-G2A/C3S). These constructs were individually expressed by transfection in Cos-7 cells, and incorporation of fatty acids into the various mutants was compared with wild-type alpha i1 monitoring metabolic labelling with [3H]palmitate or [3H]myristate. The disruption of the palmitoylation site in alpha i1-C3S did not influence myristoylation, whereas prevention of myristoylation in alpha i1-G2A also abolished palmitoylation. Co-translational myristoylation is thus an absolute requirement for alpha i1 to be post-translationally palmitoylated. The non-palmitoylated alpha i1-C3S showed reduced membrane binding to the same extent as the non-myristoylated/non-palmitoylated alpha i1-G2A and alpha i1-G2A/C3S mutants, indicating that the attachment of palmitic acid is necessary for proper interaction with the membrane.

Expression and regulation of calcium-independent protein kinase C in NG 108-15 cell differentiation

Protein, mRNA and activity levels of the calcium-independent protein kinase C (nPKC) isoenzymes were examined in NG108-15 cells. Western blot analyses reveal that proliferating NG 108-15 cells express the delta, epsilon, and eta, but not the theta species. The atypical species PKC zeta was also detected. Differentiation of these cells with dibutyryl cAMP was associated with increase in the levels of PKC epsilon, with no significant changes in its steady-state mRNA levels. The levels of the other isoforms were not altered by the differentiated state. Similarly, no changes in nPKC activity were discerned in either the soluble or particulate fractions when histone or other proteins were used as substrates. These data suggest that the PKC epsilon isoform may be important for the production and maintenance of the differentiated state in NG 108-15 cells.

The palmitoylation status of the G-protein G(o)1 alpha regulates its activity of interaction with the plasma membrane

Plasmids containing cDNAs encoding either the wild-type guanine-nucleotide-binding protein G(o)1 alpha or the palmitoylation-negative cysteine-3-to-serine (C3S) mutant of G(o)1 alpha were transfected into Rat 1 cells, and clones stably expressing immunoreactivity corresponding to these polypeptides were isolated. Clones C5B (expressing wild-type G(o)1 alpha) and D3 (expressing the mutant form) were selected for detailed study. Immunoprecipitation of whole cell lysates of each clone labelled with either [3H]palmitate or [3H]myristate demonstrated incorporation of [3H]myristate into both wild-type and the C3S mutant of G(o)1 alpha, but that incorporation of hydroxylamine-sensitive [3H]palmitate was restricted to the wild type. When membrane and cytoplasmic fractions were prepared from cells of either the C5B or D3 clones, although immunodetection of wild-type G(o)1 alpha was observed only in the membrane fraction, the C3S mutant was present in both membrane and cytoplasmic fractions. Furthermore, a significant proportion of the C3S G(o)1 alpha immunoreactivity was also detected in the cytoplasmic fraction if immunoprecipitation of recently synthesized G(o)1 alpha was performed from fractions derived from cells pulse-labelled with [35S]Trans label. Pretreatment of cells of both clones C5B and D3 with pertussis toxin led to complete ADP-ribosylation of the cellular population of G(o)1 alpha in both cell types, irrespective of whether the polypeptide was subsequently found in the membrane or cytoplasmic fraction following cellular disruption. By contrast, separation of membrane and cytoplasmic fractions before pertussis-toxin-catalysed [32P]ADP-ribosylation allowed modification only of the membrane-associated G(o)1 alpha (whether wild-type or the C3S mutant). This labelling was decreased substantially by incubation of the membranes with guanosine 5'-[beta gamma-imido]triphosphate. No cytoplasmic G-protein beta subunit was detected immunologically, and the non-membrane-associated C3S G(o)1 alpha from D3 cells migrated as an apparently monomeric 40 kDa protein on a Superose 12 gel-filtration column. Membrane-associated wild-type and C3S G(o)1 alpha appeared to interact with guanine nucleotides with similar affinity, as no alteration in the dose-response curves for guanine-nucleotide-induced maintenance of a stable 37 kDa tryptic fragment was noted for the two forms of G(o)1 alpha. Chemical depalmitoylation of membranes of clone C5B with neutral 1 M hydroxylamine caused a release of some 25-30% of each of G(o)1 alpha, Gi2 alpha and Gq alpha/G11 alpha from the membranes. Equivalent treatment of D3 cells caused an equivalent release of Gi2 alpha and Gq alpha/G11 alpha, but was unable to cause any appreciable release of the CS3 form of G(o)1 alpha, which was membrane-bound.

Prolonged very low calorie diet in highly obese subjects reduces plasma viscosity and red cell aggregation but not fibrinogen

Epidemiological studies have shown that obesity, as well as haemorheological changes are risk factors for cardiovascular disease. The aim of this study performed in grossly obese subjects was to investigate: (a) the effects on haemorheological parameters of a 3 month period of very low calorie diet (VLCD, 514 and 470 Kcal/day in women and men respectively), and (b) the relationship between haemorheological test results at baseline and the different types of body fat distribution. Fifty-two obese healthy subjects (31 women), with BMI > 30, were examined at baseline; 34 of these (19 women), compliant with the diet, were also examined after 3 months VLCD. At baseline, the results of haemorheological variables were not significantly different for patients in the highest waist-to-hip ratio (WHR) tertile vs those in the other two tertiles. After VLCD, body weight and BMI decreased markedly. The values of Ht, plasma viscosity (PV), erythrocyte aggregation index (EAI) values (P < 0.001) and white blood cell (WBC) counts (P < 0.01) significantly dropped. Globulin levels decreased, while albumin levels increased leading to significantly (P < 0.001) higher A/G ratios. No significant changes in fibrinogen (Fgn) levels were recorded after diet. In conclusion, the present study demonstrates that prolonged VLCD associated with slimming in grossly obese subjects is effective in improving related haemorheological disorders, mainly of plasmatic type, except Fgn. Second, we found that, at least in these grossly obese subjects, there is no clear evidence of a relationship between the degree of haemorheological changes and WHR values.

Neuroendocrine aging: its impact on somatotrophic function

In this paper, two different aspects of growth hormone neuroregulation during aging were considered. Twenty-month-old male rats had decreased growth hormone-releasing hormone mRNA levels and a slight reduction of somatostatin mRNA levels in the hypothalamus when compared to 8-month-old counterparts. Short-term administration of biosynthetic human growth hormone (125 micrograms rat twice daily, i.p.) to 8-month-old rats reduced hypothalamic growth hormone-releasing hormone mRNA and increased somatostatin mRNA levels. In old rats, growth hormone administration did not significantly change growth hormone-releasing hormone and somatostatin gene expression. Six old beagle dogs received short-term administration of growth hormone-releasing hormone alone or co-administered with clonidine, an alpha 2-adrenoceptor agonist, and the growth hormone secretory pattern was evaluated during a 6 h period by cluster analysis. In dogs given growth hormone-releasing hormone alone twice daily for 10 days, none of the GH secretory indices were modified except for the increase in the mean GH peak amplitude. By contrast, simultaneous administration of growth hormone-releasing hormone and clonidine, both given twice daily, significantly increased GH peak frequency and total peak area. Administration of clonidine (once daily) associated with growth hormone-releasing hormone (twice daily) further increased the GH secretory indices.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of protein kinase C in NG108-15 cell differentiation

The involvement of PKC in NG108-15 cell differentiation was investigated. Differentiation with dBcAMP was associated with a decrease in total cellular phorbol ester binding. The histone-directed PKC activity was decreased in the soluble fraction. Northern and Western blotting revealed the presence of only PKC alpha but not PKC beta and PKC gamma among the calcium-dependent isoforms. Differentiation induced a decrease of cytosolic PKC alpha immunoreactivity, with no changes of mRNA content or appearance of PKC beta and PKC gamma isoforms. The low levels of PKC alpha in the soluble fraction suggest that the mRNA for this species is less efficiently translated in differentiated NG108-15 cells. The data suggest that down-regulation of PKC alpha protein and kinase activity are associated with induction of neuronal morphology in NG108-15 cells.

Alterations of adenylyl cyclase-coupled growth hormone-releasing hormone (GHRH) pituitary receptors in different conditions of GHRH deprivation

Previous studies have clearly shown that the progressive decrease of growth hormone (GH) secretion occurring during ageing is coupled with a reduced responsiveness of pituitary GHRH receptors both in terms of GH secretion and activation of the adenylyl cyclase (AC), in the presence of increased basal values of the enzyme. The mechanism(s) subserving the age-associated alterations of GHRH-sensitive AC is likely related to the progressive decrease of hypothalamic GHRH function occurring with ageing. In this context, in old male rats, short-term administration of GHRH decreased the high basal AC activity and enhanced the GHRH-stimulated AC activity. Along this line, we decided to investigate whether experimentally induced abrogation of GHRH function in adult rats would induce the same alterations of AC-coupled GHRH receptors present in aged rats. Passive immunization of male young-adult rats with supra-maximal doses of GHRH antiserum (Ab-GHRH) significantly reduced the AC responsiveness to GHRH, an effect already evident 5 days post-injection and still present at 10 days. At this time interval, the treatment also evoked a significant increase of basal AC levels and of Gs alpha protein in the pituitary and completely blocked the GH-releasing effect of a bolus injection of GHRH. Furthermore, mechanical disruption of brain-pituitary links by complete stereotaxical ablation of the mediobasal hypothalamus induced a significant increase of basal AC levels and Gs alpha protein in the pituitary and a strikingly lower AC responsiveness to GHRH.(ABSTRACT TRUNCATED AT 250 WORDS)

[Measurement of C14 beta-radioactivity of stable natural origin taurine (2-aminoethanesulfonic acid) in bovine bile]

Taurine from natural sources has gained great importance as essential nutrient in milk for formula-fed infants. There is a strong request for a method capable of determining the natural origin of taurine. The measure of beta-radioactivity of 14C of taurine by means of liquid scintillation counting proved the most reliable. A simple method is reported.

Cholera toxin antagonizes morphine-induced catalepsy through a cyclic AMP-independent mechanism

We studied the effect of intracerebroventricular pretreatment with pertussis toxin and cholera toxin on morphine catalepsy in rats. Pertussis toxin (1 micrograms/rat, two, three and six days before) did not affect catalepsy evoked by central morphine. Cholera toxin (1 micrograms/rat) did not affect morphine catalepsy after 24 h and 48 h, but significantly reduced it (about 60%) after three and five days. Ten days later the morphine response had totally recovered. This effect was selective, since morphine analgesia was not modified. The reduction of catalepsy appeared unrelated to the ability of cholera toxin to raise cAMP levels, as demonstrated by the different time course of changes in striatal cholera toxin-stimulated adenylate cyclase activity. The effect required an intact cholera toxin molecule and did not occur with a similar dose of cholera toxin-B subunit. These findings demonstrate that catalepsy is an opioid effect not linked to pertussis toxin-sensitive G proteins and suggest that the Gs protein might be involved.

Growth hormone secretion is differently affected in genetically obese male and female rats

Growth hormone (GH) secretion is markedly blunted in obesity. Reportedly, genetically obese Zucker rats show a reduced GH secretion due to an impaired function of hypothalamic neurons producing the GH-releasing hormone (GHRH). The aim of this work was: (1) to compare the in vitro GH responsiveness to GHRH in genetically obese female versus male Zucker rats and, (2) to evaluate the function of hypothalamic GHRH and somatostatin and of pituitary receptors for these neurohormones as assessed by the effectiveness of GHRH and somatostatin on adenylate cyclase (AC) activity. Baseline GH secretion of pituitaries obtained from male and female obese rats was not different and similar to that present in lean counterparts. Stimulation with 10(-7) M GHRH elicited a significantly lower GH secretion from the pituitaries of obese male rats but induced a similar GH secretion from the pituitaries of lean and obese female rats. In these pituitaries, GH concentration was similar in obese versus lean male and female rats [corrected]. A sex-related difference was also evidenced when plasma concentrations of somatomedin C (IGF-I) were evaluated. Obese male rats had lower IGF-I concentrations than lean counterparts, while this was not the case for obese versus lean female rats. Evaluation of AC activity following GHRH disclosed a lower activation in obese than in lean male rats, whereas in the females the enzyme activation was higher in obese than in lean animals. Conversely, the inhibitory effect of somatostatin on forskolin-stimulated AC was similar in pituitary membranes of obese and lean rats of both sexes.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel N-terminal motif for palmitoylation of G-protein alpha subunits

We have examined the post-translational processing of G alpha subunits expressed endogenously in rat PC12 and NG108-15 rat/mouse hybrid cells, and after transfection of cDNA expression constructs into COS cells. Thioester-linked palmitoylation of alpha o, alpha s, alpha q/alpha 11 and alpha 12 has been detected by metabolic labelling with [3H]palmitate and immunoprecipitation. Palmitoylation of alpha o occurs post-translationally in cells treated with protein-synthesis inhibitors, suggesting possible dynamic acylation. Palmitoylation of the C-terminal CAAX motif has been excluded. Site-directed mutagenesis of alpha o has been used to implicate the site of modification as a cysteine residue next to the N-terminal myristoylated glycine, in a novel protein-lipid modification motif Met-Gly-Cys. The non-palmitoylated alpha o mutant is still myristoylated but shows reduced membrane binding, suggesting that reversible palmitoylation may regulate G alpha localization and function.

In situ hybridization reveals specific increases in G alpha s and G alpha o mRNA in discrete brain regions of morphine-tolerant rats

In situ hybridization histochemistry has been used to detect the basal distribution of mRNA encoding the alpha subunit of Gs, Go and Gi2 proteins throughout the rat brain. Based on these data we investigated the effect of chronic morphine on the content of these G protein alpha subunits mRNA. We observed an increase in the expression of alpha s and alpha o messages of chronically morphine-treated animals, while no changes were seen in alpha i2 mRNA. Specifically a 30% increase in expression for alpha s was seen only in the paraventricular nucleus of hypothalamus and a 20% elevation for alpha o was detected in the claustrum and endopiriform nucleus. Immunoblotting analysis was used to correlate the changes in alpha s and alpha o messages with equivalent changes in protein levels. Chronic morphine significantly increased alpha s amounts in the hypothalamus (70%), and produced a minor elevation (30%) in G alpha o levels in the olfactory area. Our results indicate that in discrete brain regions altered G protein expression is part of the adaptive changes underlying opiate tolerance.

Pharmacological characterization of the 5-hydroxytryptamine receptor coupled to adenylyl cyclase stimulation in human brain

Recently, a 5-hydroxytryptamine (5-HT) receptor has been described, whose pharmacology was distinct from that of the already known serotonergic receptors, so that it has been called 5-HT4. Because the lack of a high affinity radioligand, the identification of this receptor depends entirely on functional pharmacological analysis. Its stimulation leads to an increase in cyclic AMP accumulation in mouse embryo colliculi neurons, in guinea pig hippocampus and in human heart. We studied the effect of two indoleamines, 5-HT and 5-methoxytryptamine (5-MeO-T), and a benzimidazolone derivative, BIMU 8, in stimulating basal adenylyl cyclase activity in human frontal cortex, and characterized the receptor subtype involved. In membranes prepared from this tissue, 5-HT, 5-MeO-T and BIMU 8 dose-dependently stimulated (13-25%) the basal enzyme activity (220 pmoles cyclic AMP/min/mg protein). 5-MeO-T behaved as a full agonist, BIMU 8 elicited about 60% of the maximal 5-HT effect. The selective 5-HT1A agonist 8-OH-DPAT, was devoid of any stimulating activity. ICS 205-930, a low affinity 5-HT4 receptor antagonist, completely reversed the effect of all three agonists at high concentrations. Therefore, the present data are consistent with the 5-HT-mediated stimulation of adenylyl cyclase in human frontal cortex resulting by the activation of a 5-HT4 receptor subtype.