The effect of 5-HT receptor ligands on the uptake of [3H]5-hydroxytryptamine into rat cortical synaptosomes

Structure-activity relationship of serotonin derived tocopherol lipids

Tocopherol-based lipids are widely used for nucleic acid delivery. Using tocopherol molecules, we designed and synthesized 5-HT functionalized lipids by tethering 5-hydroxytryptamine (5-HT), a small molecule ligand as the head group to a natural amphiphilic molecule namely α-tocopherol (Vitamin E). This is with the aim of delivering nucleic acids specifically into cells expressing the serotonin receptors (5-hydroxytryptamine[5-HT]) which are abundant in the central nervous system. In order to achieve target recognition, we adopted an approach wherein two structurally different lipid molecules having serotonin as the head group was conjugated to tocopherol via different linkers thus generating lipids with either free -NH₂ or -OH moiety. The corresponding lipids designated as Lipid A (Tocopheryl carbonate serotonin-NH₂) and Lipid B (Tocopheryl 2-hydroxy propyl ammonium serotonin-OH), were formulated with co-lipids 1,2-dioleoyl-sn-glycero-3-phosphatidyl-ethanolamine (DOPE) and 1,2-dioleoyl-sn-glycero-sn-3-phosphatidylcholine (DOPC) and evaluated for their ability to deliver plasmid DNA through reporter gene expression assays in vitro. Furthermore, the physicochemical characteristics and cellular interactions of the formulations were examined using serotonin-receptor enriched cells in order to distinguish the structural and functional attributes of both lipids. Cell-based gene expression studies reveal that in comparison to Lipid A, a formulation of Lipid B prepared with DOPE as the co-lipid, contributes to efficient uptake leading to significant enhancement in transfection. Specific interactions explored by molecular docking studies suggests the role of the hydroxyl moiety and the enantiospecific significance of serotonin- conjugated tocopherol lipids in recognizing these receptors thus signifying a promising lipid-based approach to target the serotonin receptors in the central nervous system.

Antidepressant-like activity of YL-0919: a novel combined selective serotonin reuptake inhibitor and 5-HT1A receptor agonist

It has been suggested that drugs combining activities of selective serotonin reuptake inhibitor and 5-HT1A receptor agonist may form a novel strategy for higher therapeutic efficacy of antidepressant. The present study aimed to examine the pharmacology of YL-0919, a novel synthetic compound with combined high affinity and selectivity for serotonin transporter and 5-HT1A receptors. We performed in vitro binding and function assays and in vivo behavioral tests to assess the pharmacological properties and antidepressant-like efficacy of YL-0919. YL-0919 displayed high affinity in vitro to both 5-HT1A receptor and 5-HT transporter prepared from rat cortical tissue. It exerted an inhibitory effect on forskolin-stimulated cAMP formation and potently inhibited 5-HT uptake in both rat cortical synaptosomes and recombinant cells. After acute p.o. administration, very low doses of YL-0919 reduced the immobility time in tail suspension test and forced swimming test in mice and rats, with no significant effect on locomotor activity in open field test. Furthermore, WAY-100635 (a selective 5-HT1A receptor antagonist, 0.3 mg/kg) significantly blocked the effect of YL-0919 in tail suspension test and forced swimming test. In addition, chronic YL-0919 treatment significantly reversed the depressive-like behaviors in chronically stressed rats. These findings suggest that YL-0919, a novel structure compound, exerts dual effect on the serotonergic system, as both 5-HT1A receptor agonist and 5-HT uptake blocker, showing remarkable antidepressant effects in animal models. Therefore, YL-0919 may be used as a new option for the treatment of major depressive disorder.

Serotonin modulates glutamatergic transmission in the rat olfactory tubercle

The olfactory tubercle (OT) is found in the brains of mammals that are highly dependent on their sense of smell. Its human analogue is the poorly understood anterior perforated substance. Previous work on rat brain slices identified two types of field potential responses from the OT. The association fibre (AF) pathway was sensitive to muscarinic modulation, whereas the lateral olfactory tract (LOT) fibre pathway was not. Here, we establish that serotonin (5-hydroxytryptamine; 5-HT) also inhibits field potential excitatory postsynaptic potentials (EPSPs) in the AF, but not in the LOT fibre, pathway. Parallel experiments with adenosine (ADO) excluded ADO mediation of the 5-HT effect. Exogenous 5-HT at 30 microm caused a long-lasting approximately 40% reduction in the amplitude of AF postsynaptic responses, without affecting the time-course of EPSP decline, indicating a fairly restricted disposition of the 5-HT receptors responsible. The 5-HT(1)-preferring, 5-HT(5)-preferring and 5-HT(7)-preferring agonist 5-carboxamidotryptamine caused similar inhibition at approximately 100 nm. The 5-HT(1A)-preferring ligand 8-hydroxy-di-n-propylamino-tetralin at 10 microm, and the 5-HT uptake inhibitor citalopram at 3 microm, caused inhibition of AF-stimulated field potential responses in the 5-10% range. Order-of-potency information suggested a receptor of the 5-HT(1B) or 5-HT(1D) subtype. The 5-HT(1D) agonist L-694,247 (1 microm) suppressed the AF response by approximately 10% when used on its own. After washing out of L-694,427, inhibition by 30 microm 5-HT was reduced to negligible levels. Allowing for a partial agonist action of L-694,427 and complex interactions of 5-HT receptors within the OT, these results support the presence of active 5-HT(1D)-type receptors in the principal cell layer of the OT.

Pharmacology and metabolism of renzapride : a novel therapeutic agent for the potential treatment of irritable bowel syndrome

BACKGROUND AND OBJECTIVE

Renzapride (ATL-1251), a novel benzamide, is currently under clinical development for the treatment of irritable bowel syndrome (IBS). Previous in vitro and in vivo experimental studies have characterized renzapride as a full serotonin 5-HT(4) receptor agonist on the gut and a 5-HT(3) receptor antagonist. Clinical studies have confirmed the therapeutic efficacy, tolerability and safety of renzapride in patients with constipation-predominant IBS. This study set out to characterize the pharmacological profile of renzapride and its potential metabolic products at both 5-HT and other monoamine receptors in the gut.

METHODS

The affinity of renzapride, its (+) and (-) enantiomers, and its primary metabolite, renzapride N-oxide and its enantiomers, for serotonin receptors was assessed by means of in vitro radioligand binding inhibition studies. After membranes prepared from animal tissue or membranes of cell lines transfected with cloned human receptors had been incubated with radiolabelled ligand with high affinity for a specific receptor, renzapride was added to competitively inhibit this binding. Levels of bound radioligand were measured by filtration and counting of the bound radioactivity. In instances where >50% inhibition of radioligand binding had occurred, the inhibition constant (K(i)) was calculated. Metabolism of renzapride by liver microsomes was assessed by incubating 10 micromol/L renzapride with human liver microsome samples for 60 minutes at 37 degrees C. After the reaction was stopped, the samples were centrifuged and the supernatant analysed for metabolites by high-pressure liquid chromatography (HPLC). The potential inhibitory effects of renzapride on cytochrome P450 (CYP) enzymes were assessed by incubating renzapride at various concentrations over a 1-500 micromol/L concentration range with microsomes genetically engineered to express a single CYP.

RESULTS

Renzapride was selective for serotonergic receptors and, in particular, had high affinity for human 5-HT(3) and guinea-pig 5-HT(4) receptors (K(i) 17 and 477 nm, respectively). Inhibitory properties at 5-HT(2B) receptors were also identified for renzapride, as well as some affinity for 5-HT(2A) and 5-HT(2C) receptors. Renzapride N-oxide and its enantiomers demonstrated much lower affinity for all 5-HT receptors compared with renzapride. Renzapride was metabolized by liver microsomes to a limited extent and there was no significant non-microsomal metabolism of renzapride. Renzapride did not inhibit the major CYP drug-metabolizing enzymes CYP2C9, CYP2D6, CYP1A2, CYP2A6, CYP2C19, CYP2E1 or CYP3A4 at concentrations consistent with use in a clinical setting.

CONCLUSIONS

These results confirm and extend earlier studies in animal and human receptors that show renzapride is a potent and generally full 5-HT(4) receptor agonist and 5-HT(3) receptor antagonist. The results reported in the present study indicate that the metabolites of renzapride are minor and are unlikely to contribute to its therapeutic profile or lead to interaction of renzapride with other drugs that inhibit the major drug-metabolizing enzymes in the liver at therapeutic doses. These data contribute to the understanding of the pharmacological actions and metabolic fate of renzapride in vivo.

The acute and chronic administration of (±)-8-hydroxy-2-(di-n-propylamino)tetralin significantly alters the activity of spontaneously active midbrain dopamine neurons in rats: An in vivo electrophysiological study

This study examined the effect of the acute and chronic systemic administration of (+/-)-8-Hydroxy-2-(Di-n-propylamino)Tetralin(8-OH-DPAT) on the number and firing pattern of spontaneously active dopamine (DA) neurons in the ventral tegmental area (VTA or A10) and substantia nigra pars compacta (SNC or A9) in anesthetized male rats. These parameters were measured using extracellular in vivo electrophysiology. A single s.c. injection of 0.01, 0.1, or 1 mg/kg of 8-OH-DPAT did not significantly alter the number of spontaneously active SNC DA neurons compared to vehicle-treated animals (controls). The acute administration of 0.01 or 0.1 mg/kg of 8-OH-DPAT did not significantly alter, whereas the 1 mg/kg dose significantly decreased the number of spontaneously active VTA DA neurons compared to controls. The acute administration of 8-OH-DPAT significantly increased the percentage of VTA DA neurons firing in a bursting pattern. In contrast, there was a significant decrease in the percentage of SNC DA neurons firing in a bursting pattern following the acute administration of 8-OH-DPAT. The number of spontaneously active SNC DA neurons was not significantly altered by the chronic s.c. administration of 8-OH-DPAT (0.01, 0.1, or 1 mg/kg s.c.) as compared to controls. However, the chronic s.c. administration of all doses of 8-OH-DPAT significantly decreased the number of spontaneously active VTA DA neurons compared to controls. The i.v. administration of (+)-apomorphine (50 microg/kg) did not reverse the 8-OH-DPAT-induced decrease in the number of spontaneously active VTA DA neurons, suggesting that this effect is unlikely due to depolarization blockade. The percentage of VTA DA neurons exhibiting burst firing was significantly increased by 0.01 and 0.1 mg/kg, but significantly decreased by 1 mg/kg of 8-OH-DPAT. Overall, the systemic administration of 8-OH-DPAT preferentially affects the activity of spontaneously active A10 DA neurons in rats.

Uptake of serotonin by adult rat corpus callosum is partially reduced by common antidepressants

The corpus callosum (CC) is the main white matter tract involved in interhemispheric brain communication. We establish that uptake of [3H]5-hydroxytryptamine (5-HT) in CC is partially inhibited by some antidepressants. Slices of the adult rat CC had a high-affinity uptake of 5-HT. About 80% of this uptake was Na+ dependent, with a Michaelis-Menten constant, Km, of 420 +/- 80 nM and a rate of 5-HT uptake, Vmax, of 9.5 +/- 0.8 pmol/mg protein/min. The 5-HT uptake was reduced approximately 60% at pH 5 compared with that at pH 7. Fluoxetine (Prozac) inhibited only 43% of 5-HT uptake in a concentration-dependent manner, with an affinity constant, Ki, of 44.7 +/- 10.0 nM. We also studied the effects of other monoamine uptake inhibitors, all at 10 microM, and found that zimelidine, imipramine, and clomipramine inhibited 5-HT uptake in the CC by approximately 30-40%. The fluoxetine-insensitive 5-HT uptake was not altered by high concentrations of dopamine plus norepinephrine. The present data show that Na(+)-dependent 5-HT uptake occurs in the CC and optic nerve and that this uptake is partially sensitive to antidepressants and probably mediated by the serotonin transporter, which may be relevant during depression.

Antidepressant-like activity of VN2222, a serotonin reuptake inhibitor with high affinity at 5-HT1A receptors

It has been suggested that drugs combining serotonin (5-hydroxytryptamine, 5-HT) transporter blockade and 5-HT1A autoreceptor antagonism could be a novel strategy for a shorter onset of action and higher therapeutic efficacy of antidepressants. The present study was aimed at characterizing the pharmacology of 1-(3-benzo[b]tiophenyl)-3-[4-(2-methoxyphenyl)-1-piperazinyl]-1-propanol (VN2222) a new synthetic compound with high affinity at both the 5-HT transporter and 5-HT1A receptors and devoid of high affinity at other receptors studied, with the only exception of alpha1-adrenoceptors. In keeping with the binding affinity at the 5-HT transporter, VN2222 inhibited 5-HT uptake in vitro both in rat cortical synaptosomes and in mesencephalic cultures and also in vivo when administered locally into the rat ventral hippocampus. After systemic administration, VN2222 exhibited an inverted U-shape effect so the inhibition of [3H]5-HT uptake ex vivo and the increase in 5-HT extracellular levels in microdialysis experiments was observed at low doses of 0.01-0.1 mg/kg whereas higher doses were ineffective. In studies related to 5-HT1A receptor function, 0.01-0.1 microM VN2222 produced a partial inhibition of forskolin-stimulated cAMP formation behaving as a weak agonist of 5-HT1A receptors. In body temperature studies, 5 mg/kg VN2222 produced a mild hypothermic effect in mice, suggesting a weak agonist activity at presynaptic 5-HT1A receptors; much lower doses (0.01-0.5 mg/kg) partially antagonized the hypothermia induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) possibly through 5-HT transporter blockade. In the learned helplessness test in rats, an animal model for antidepressants, 1-5 mg/kg VN2222 reduced significantly the number of escape failures. Consequently, VN2222 is a new compound with a dual effect on the serotonergic system, as 5-HT uptake blocker and 5-HT1A receptor partial agonist, and with a remarkable activity in an animal model of depression with high predictive validity.

The effects of ibogaine on dopamine and serotonin transport in rat brain synaptosomes

Ibogaine has been shown to affect biogenic amine levels in selected brain regions. Because of the involvement of these neurotransmitters in drug addiction, the effects of ibogaine on biogenic amine transport may contribute to the potential anti-addictive properties of ibogaine in vivo. With rat brain synaptosomes as our experimental system, we measured the effects of ibogaine on the uptake and release of dopamine (DA) and serotonin (5-HT). Ibogaine competitively blocked both DA and 5-HT uptake with IC50 values of 20 microM at 75 nM 3H-DA and 2.6 microM at 10 nM 3H-5-HT. Ibogaine had no effect on K+-induced release of 3H-DA from preloaded synaptosomes, but 20 microM and 50 microM ibogaine inhibited roughly 40% and 60%, respectively, of the K(+)-induced release of 3H-5-HT from preloaded synaptosomes. In the absence of a depolarizing stimulus, ibogaine evoked a small release of 3H-DA but not 3H-5-HT. These relatively low-potency effects of ibogaine on DA and 5-HT uptake in synaptosomes are consistent with the low binding affinity of ibogaine that has been previously reported for DA and 5-HT transporters. Our results show that if ibogaine modulates DA and 5-HT levels in the brain by directly blocking their uptake, then a concentration of ibogaine in the micromolar range is required. Furthermore, if the anti-addictive effects of ibogaine require this concentration, then ibogaine likely exerts these effects through a combination of neurotransmitter pathways, because binding affinities and functional potencies of ibogaine in the micromolar range have been reported for a variety of neuronal receptors and transporters.

A microdialysis study of the in vivo release of 5-HT in the median raphe nucleus of the rat

The present study has examined several characteristics of the release of 5-HT in the median raphe nucleus in terms of its dependence of nerve impulse, provenance of a vesicular storage fraction as well as the regulatory role played by 5-HT1A receptors. Tetrodotoxin (1 microM) and reserpine (5 mg kg(-1), i.p.) virtually suppressed the output of 5-HT. The administration of EEDQ (10 mg kg(-1), i.p.) did not alter the basal release of 5-HT but abolished the reduction of 5-HT release induced by 8-OH-DPAT (0.1 mg kg(-1), s.c.). The perfusion of 1-100 microM of 8-OH-DPAT or the novel 5-HT1A agonist BAY x 3702 decreased the efflux of 5-HT, whereas the perfusion of the 5-HT1A antagonist WAY-100635 failed to alter 5-HT release. The decrease in dialysate 5-HT induced by 100 microM 8-OH-DPAT was reversed by the concurrent perfusion of 100 microM WAY-100635. Also, the perfusion of 100 microM WAY-100635 for 2 h inhibited partly the reduction of 5-HT release evoked by the systemic administration of 8-OH-DPAT (0.1 mg kg(-1)). These results indicate that extracellular 5-HT in the median raphe nucleus is stored in vesicles and released in an impulse-dependent manner. Also, the basal release of 5-HT in the median raphe nucleus does not appear to be under the tonic control of somatodendritic 5-HT1A receptors by endogenous 5-HT. Instead, this feedback mechanism seems to be triggered when an excess of the transmitter or a 5-HT1A agonist is present in the extracellular space of the median raphe nucleus.

Serotonin transporters in adult rat brain astrocytes revealed by [3H]5-HT uptake into glial plasmalemmal vesicles

Cultured astrocytes derived from neonatal rat brain exhibited high affinity, Na+-dependent, paroxetine and fluoxetine sensitive [3H]5-HT uptake. Reverse transcriptase-PCR demonstrated that astrocytes in culture expressed messenger RNA for the cloned serotonin transporter protein which has been characterised as the neuronal serotonin transporter. Although the serotonin transporter in cultured astrocytes displayed a Km value approximately 10 times greater than found in adult brain synaptosomes, these observations indicated that astrocytes in vitro may express the same serotonin transporter as neurons. Reverse transcriptase-PCR demonstrated the presence of serotonin transporter mRNA in the adult rat cerebral cortex, suggesting that astrocytes in vivo may express low levels of this mRNA. To investigate whether astrocytes in the adult CNS express functional serotonin transporters, glial plasmalemmal vesicles were prepared from cerebral cortex, representing a subcellular fraction composed primarily of vesicles derived from astrocytes. These vesicles were characterised by [3H]-glutamate and [3H]-dopamine uptake and by immunoblot analysis, using glial and synaptic markers: glutamate synthase, SNAP-25 and synaptobrevin. [3H]5-HT was taken up into glial plasmalemmal vesicles in a high affinity (Km approximately 40 nM), Na+ dependent, paroxetine-sensitive manner. The [3H]5-HT uptake capacity (Vmax) in these vesicles was approximately one quarter of that observed in synaptosomes. These data indicate that astrocytes in culture and in vivo are capable of 5-HT uptake via the previously characterised 'neuronal' serotonin transporter.

Further characterization of forebrain serotonin receptors mediating tachycardia in conscious rats

It has been shown recently that activation of forebrain serotonin1A (5-HT1A) receptors, likely within the preoptic area, elicits a slight increase in blood pressure and a substantial tachycardia. The present studies were designed to characterize: (1) the requirement of the 5-HT1A receptor agonist R(+)-8-hydroxy-2-(di-n-propylamino) tetralin [R(+)8-OH-DPAT]-induced tachycardia on the integrity of serotonergic innervation of the preoptic area, (2) the ability of the 5-HT1A receptor partial agonist buspirone to elicit cardiovascular responses when microinjected into the preoptic area, (3) the role of 5-HT2 and 5-HT3 receptors in the preoptic area in cardiovascular regulation, and (4) the site specificity of the tachycardia produced by R(+)8-OH-DPAT. The data suggest that activation of 5-HT1A receptors, but not 5-HT2 or 5-HT3 receptors, within or very near the preoptic area increases blood pressure and heart rate in conscious rats. Furthermore, the full response is dependent on afferent serotonergic innervation, suggesting a presynaptic modulatory role for 5-HT in the preoptic area.

Possible in vivo 5-HT reuptake blocking properties of 8-OH-DPAT assessed by measuring hippocampal extracellular 5-HT using microdialysis in rats

1. The 5-hydroxytryptamine (5-HT)1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), has been shown to label 5-HT reuptake sites. 2. To study the functional consequences of this property, the effects of 8-OH-DPAT were compared with those of the 5-HT reuptake inhibitors, paroxetine and clomipramine, and of the 5-HT1A receptor agonist flesinoxan, in vitro on 5-HT reuptake, and in vivo on the extracellular concentration of 5-HT by use of microdialysis, in rat hippocampus. Because 5-HT reuptake inhibitors reportedly attenuate the ability of (+)-fenfluramine to increase the extracellular concentration of 5-HT, the possible reversal of these effects of 8-OH-DPAT and by paroxetine were examined. 3. 8-OH-DPAT, paroxetine and clomipramine inhibited [3H]-5-HT reuptake in rat hippocampal synaptosomes (pIC50: 6.00, 8.41 and 7.00, respectively). In contrast, flesinoxan did not alter 5-HT reuptake (pIC50 < 5). 4. 8-OH-DPAT (10 and 100 microM), paroxetine (0.1 microM) and clomipramine (1 microM), administered through the dialysis probe, significantly increased the hippocampal extracellular concentration of 5-HT. In contrast, flesinoxan (100 microM) did not alter extracellular 5-HT. Moreover, the effects of 100 microM 8-OH-DPAT were not blocked by the 5-HT1A receptor antagonist, WAY-100635 (0.16 mg kg-1, s.c.). 5. The increase in extracellular 5-HT induced by 10 mg kg-1, i.p., (+)-fenfluramine was prevented not only by 0.1 microM paroxetine, but also by 100 microM 8-OH-DPAT. In addition, systemic administration of 10 mg kg-1, but not 2.5 mg kg-1, i.p. 8-OH-DPAT attenuated the increase in extracellular 5-HT induced by 2.5 mg kg-1, i.p., (+)-fenfluramine. 6. These findings suggest that the increase in extracellular 5-HT produced by local administration of 8-OH-DPAT does not involve its 5-HT1A receptor agonist properties, but may result, at least in part, from its 5-HT reuptake blocking properties.

Quantitative microdialysis of serotonin and norepinephrine: pharmacological influences on in vivo extraction fraction

The present study was designed to investigate the potential influence of various neuronal processes including uptake, release and metabolism, on the in vivo microdialysis extraction fraction (Ed) of serotonin (5HT) and norepinephrine (NE). Paroxetine administration decreased the Ed of 5HT in the nucleus accumbens from 24 +/- 3 to 18 +/- 0.2% (p < 0.05). Similarly, desipramine infusion reduced the NE Ed from 35 +/- 2 to 26 +/- 1% (p < 0.05). However, perfusion with pargyline or tetrodotoxin had no effect on the Ed of either 5HT or NE. Perfusion with agonists for the 5HT, alpha-adrenergic, D2 and histamine receptors had no effect on the Ed of 5HT. In the same manner, perfusion with the alpha-adrenergic agonists, methoxamine or clonidine, did not affect the Ed of NE. These data are in agreement with experimental results obtained for dopamine in the nucleus accumbens and the theory of quantitative microdialysis which predicts that only changes in the rate of clearance will change Ed of monoamines. These results suggest that, like DA, changes in the Ed for 5HT or NE are indicative of changes in the reuptake of these neurotransmitters. The results also indicate that pharmacological agents which do not affect uptake have no effect on the extraction fraction.

Prozac: panacea or puzzle?

Selective 5-HT (serotonin) reuptake inhibitors (SSRIs) lack many of the adverse side-effects of older antidepressants. One of these compounds, fluoxetine (Prozac), has acquired manifest public awareness that is not evident with other SSRIs. Here, Clare Stanford compares the basic pharmacology of SSRIs to determine whether fluoxetine has any atypical features, and thus provide scientific justification for the attention focused on this compound. Ironically, fluoxetine might be distinctive in that it is the least selective SSRI and has marked effects on catecholamine function in the brain.