Homozygote mice deficient in serotonin 5-HT1B receptor and antidepressant effect of selective serotonin reuptake inhibitors

We use the knockout mice strategy to investigate the contribution of the 5-HT1B receptor in mediating the effects of selective serotonin reuptake inhibitors (SSRI). Using microdialysis in awake 129/Sv mice, we show that the absence of the 5-HT1B receptor in mutant mice (KO 1B -/-) potentiated the effect of paroxetine on extracellular 5-HT levels in the ventral hippocampus, but not in the frontal cortex compared to wild-type mice (WT). Furthermore, using the forced swimming test, we demonstrate that SSRIs decreased immobility of WT mice, and this effect is absent in KO 1B -/- mice showing therefore that activation of 5-HT1B receptors mediate the antidepressant-like effects of SSRIs. Taken together these findings suggest that 5-HT1B autoreceptors limit the effects of SSRI particularly in the hippocampus while postsynaptic 5-HT1B receptors are required for the antidepressant activity of SSRIs.     

5-HT1B Autoreceptors limit the effects of selective serotonin re-uptake inhibitors in mouse hippocampus and frontal cortex

We used knockout mice and receptor antagonist strategies to investigate the contribution of the serotonin (5-hydroxytryptamine, 5-HT) 1B receptor subtype in mediating the effects of selective serotonin re-uptake inhibitors (SSRIs). Using in vivo intracerebral microdialysis in awake mice, we show that a single systemic administration of paroxetine (1 or 5 mg/kg, i.p.) increased extracellular serotonin levels [5-HT]ext in the ventral hippocampus and frontal cortex of wild-type and mutant mice. However, in the ventral hippocampus, paroxetine at the two doses studied induced a larger increase in [5-HT]ext in knockout than in wild-type mice. In the frontal cortex, the effect of paroxetine was larger in mutants than in wild-type mice at the 1 mg/kg, but not at 5 mg/kg. In addition, either the absence of the 5-HT1B receptor or its blockade with the mixed 5-HT1B/1D receptor antagonist, GR 127935, potentiated the effect of a single administration of paroxetine on extracellular 5-HT levels more in the ventral hippocampus than in the frontal cortex. These data suggest that 5-HT1B autoreceptors limit the effects of SSRIs on dialysate 5-HT levels at serotonergic nerve terminals.     

3,4-Dihydro-2H-benzoxazinones as dual-acting 5-HT1A receptor antagonists and serotonin reuptake inhibitors

Investigation of halogen substitution in lead compound 1 has led to the identification of analogues which combine high affinity for 5-HT(1A) receptors and potent serotonin reuptake inhibitory activity. Several compounds show an improved selectivity over 5-HT(1B) and 5-HT(1D) receptors and a superior pharmacokinetic profile in the rat.     

Pulmonary vascular effects of serotonin and selective serotonin reuptake inhibitors in the late-gestation ovine fetus

Maternal use of selective serotonin (5-HT) reuptake inhibitors (SSRIs) is associated with an increased risk for persistent pulmonary hypertension of the newborn (PPHN), but little is known about 5-HT signaling in the developing lung. We hypothesize that 5-HT plays a key role in maintaining high pulmonary vascular resistance (PVR) in the fetus and that fetal exposure to SSRIs increases 5-HT activity and causes pulmonary hypertension. We studied the hemodynamic effects of 5-HT, 5-HT receptor antagonists, and SSRIs in chronically prepared fetal sheep. Brief infusions of 5-HT (3-20 μg) increased PVR in a dose-related fashion. Ketanserin, a 5-HT 2A receptor antagonist, caused pulmonary vasodilation and inhibited 5-HT-induced pulmonary vasoconstriction. In contrast, intrapulmonary infusions of GR127945 and SB206553, 5-HT 1B and 5-HT 2B receptor antagonists, respectively, had no effect on basal PVR or 5-HT-induced vasoconstriction. Pretreatment with fasudil, a Rho kinase inhibitor, blunted the effects of 5-HT infusion. Brief infusions of the SSRIs, sertraline and fluoxetine, caused potent and sustained elevations of PVR, which was sustained for over 60 min after the infusion. SSRI-induced pulmonary vasoconstriction was reversed by infusion of ketanserin and did not affect the acute vasodilator effects of acetylcholine. We conclude that 5-HT causes pulmonary vasoconstriction, contributes to maintenance of high PVR in the normal fetus through stimulation of 5-HT 2A receptors and Rho kinase activation, and mediates the hypertensive effects of SSRIs. We speculate that prolonged exposure to SSRIs can induce PPHN through direct effects on the fetal pulmonary circulation.     

Novel 5-HT(1A/1B/1D) receptors antagonists with potent 5-HT reuptake inhibitory activity

Novel 2-methyl-5-quinolinyl-1-piperazinylalkyl-3,4-dihydro-2H-1,4-benzoxazin-3-ones showing high affinities for the 5-HT(1A/1B/1D) receptors coupled with potent 5-HT reuptake inhibitory activity have been discovered. This is the first report describing docking of the lead compound 6-{2-[4-(2-methyl-5-quinolinyl)-1-piperazinyl]ethyl}-2H-1,4-benzoxazin-3(4H)-one 1, into a model of the 5-HT transporter and the 5-HT(1A) receptor model.     

Naphthyl piperazines with dual activity as 5-HT1D antagonists and 5-HT reuptake inhibitors

SAR around a known molecule with dual 5-HT(1D) antagonist and 5-HT(transporter) inhibitory activity has led to the discovery of molecules with improved dual activity and reduced cross-reactivity toward other aminergic receptors (5-HT(1B), alpha(1), and D(2)).     

Inhibitory mechanisms and binding site location for serotonin selective reuptake inhibitors on nicotinic acetylcholine receptors

Functional and structural approaches were used to examine the inhibitory mechanisms and binding site location for fluoxetine and paroxetine, two serotonin selective reuptake inhibitors, on nicotinic acetylcholine receptors (AChRs) in different conformational states. The results establish that: (a) fluoxetine and paroxetine inhibit hα1β1γδ AChR-induced Ca²⁺ influx with higher potencies than dizocilpine. The potency of fluoxetine is increased ～10-fold after longer pre-incubation periods, which is in agreement with the enhancement of [³H]cytisine binding to resting but activatable Torpedo AChRs elicited by these antidepressants, (b) fluoxetine and paroxetine inhibit the binding of the phencyclidine analog piperidyl-3,4-³H(N)]-(N-(1-(2 thienyl)cyclohexyl)-3,4-piperidine to the desensitized Torpedo AChR with higher affinities compared to the resting AChR, and (c) fluoxetine inhibits [³H]dizocilpine binding to the desensitized AChR, suggesting a mutually exclusive interaction. This is supported by our molecular docking results where neutral dizocilpine and fluoxetine and the conformer of protonated fluoxetine with the highest LUDI score interact with the domain between the valine (position 13′) and leucine (position 9′) rings. Molecular mechanics calculations also evidence electrostatic interactions of protonated fluoxetine at positions 20′, 21′, and 24′. Protonated dizocilpine bridges these two binding domains by interacting with the valine and outer (position 20′) rings. The high proportion of protonated fluoxetine and dizocilpine calculated at physiological pH suggests that the protonated drugs can be attracted to the channel mouth before binding deeper within the AChR ion channel between the leucine and valine rings, a domain shared with phencyclidine, finally blocking ion flux and inducing AChR desensitization.     

Adverse effects associated with selective serotonin reuptake inhibitors and tricyclic antidepressants: a meta-analysis

BACKGROUND: The use of antidepressant medications and the resulting costs have increased dramatically in recent years, partly because of the introduction of selective serotonin reuptake inhibitors (SSRIs). An assessment of the clinical and economic aspects of SSRIs compared with the older tricyclic antidepressants (TCAs) was initiated to generate information for purchasers of these drugs as well as clinicians. One component of this study was an examination of the adverse effects associated with the use of these drugs. METHODS: Searches of bibliographic databases (for January 1980 through May 1996) and manual scanning of both peer-reviewed publications and other documents were used to identify double-blind, randomized controlled trials involving at least one SSRI and one TCA. For the study of adverse effects, only trials that had at least 20 patients in each trial arm and that reported rates of adverse effects in both arms were retained. In total 84 trials reporting on 18 adverse effects were available. Meta-analyses were undertaken to calculate pooled differences in rates of adverse effects. The question of whether the method of eliciting information from patients about adverse effects made a difference in the findings was also examined. Finally, differences in drop-out rates due to adverse effects were calculated. RESULTS: The crude rates of occurrence of adverse effects ranged from 4% (palpitations) to 26% (nausea) for SSRIs and from 4% (diarrhea) to 27% (dry mouth) for TCAs. The differences in the rates of adverse effects between the 2 types of drugs ranged from 14% more with SSRIs (for nausea) to 11% more with TCAs (for constipation). The results did not depend on the method of eliciting information from patients. There were no statistically significant differences between drug classes in terms of drop-outs due to adverse effects. INTERPRETATION: SSRIs and TCAs are both associated with adverse effects, although the key effects differ between the drug classes. Further explanation of the adverse effects and their relation to discontinuation of medication will require better studies involving prospective collection of quality-of-life data.     

SAR development of a selective 5-HT1D antagonist/serotonin reuptake inhibitor lead using rapid parallel synthesis

Incorporation of an SRI (serotonin reuptake inhibitor) pharmacophore into a selective 5-HT(1D) agonist has led to the discovery of a molecule having both 5-HT(1D) antagonist and SRI activity. RPS methodology was used to develop the SAR and identify potential approaches to reduce unwanted adrenergic alpha 1 and dopamine D(2) cross-reactivities.     

Novel selective and potent 5-HT reuptake inhibitors with 5-HT1D antagonist activity: chemistry and pharmacological evaluation of a series of thienopyran derivatives

A series of compounds combining the naphthylpiperazine and thienopyran scaffolds has been prepared and evaluated for 5-HT reuptake inhibition with 5-HT1D antagonist activity. The design of these compounds has been based on the 'overlapping type' strategy where two pharmacophores are linked in a single molecule. The resultant dual pharmacological profile has the potential to deliver a more efficient treatment for depression.     

Neuroadaptive changes in brain during selective serotonin reuptake inhibitors action

Selective serotonin reuptake inhibitors such as fluoxetine that are widely used for the treatment of depression and anxiety disorders produce neuroadaptive change not only in the serotoninergic system but also in other neuromediator systems. These changes may be involved in the therapeutic as well as in side effects of the drugs.     

Homotryptamines as potent and selective serotonin reuptake inhibitors (SSRIs)

A series of N,N-dimethylhomotryptamines was prepared and their binding affinities at the serotonin transporter (SERT) were determined. Compounds possessing an electron withdrawing substituent at the C5-position of the indole nucleus were found to be potent SSRIs. Initial attempts at conformational restriction of the propylamine sidechain by incorporation of a quinuclidine bicyclic structure did not improve binding affinity at SERT.     

Target size analysis of serotonin 5-HT1 and 5-HT2 receptors in bovine brain membranes

Freeze-dried crude synaptic membranes prepared from bovine cerebral cortex and striatum were exposed to high energy gamma ray from the source of 60Co. The size of serotonin 5-HT1 receptors labeled by [3H]serotonin and that of 5-HT2 receptors labeled by [3H]spiperone or [3H]ketanserin was determined by target size analyses. The values were 57,000 daltons, 145,000 daltons and 152,000 daltons for the cerebral cortex and 56,000 daltons, 141,000 daltons and 150,000 daltons for the striatum, respectively. The estimated sizes were deduced by reference to enzyme standards with known molecular masses and which were irradiated in parallel. Our results demonstrate that the molecular entities in situ for 5-HT1 receptors are distinct from those for 5-HT2 receptors, thus supporting data on the existence of two distinct populations of serotonin receptors, hitherto evidenced physiopharmacologically.     

Identification and role of serotonin 5-HT1A and 5-HT1B receptors in primary cultures of rat embryonic rostral raphe nucleus neurons

Autoregulatory mechanisms affecting serotonin [5-hydroxytryptamine (5-HT)] release and synthesis during the early period of development were investigated in dissociated cell cultures raised from embryonic rostral rat rhombencephalon. The presence of 5-HT1A and 5-HT1B receptors in serotoninergic neurons was assessed using binding assays. The involvement of 5-HT1A and 5-HT1B receptors in the control of the synthesis and release of [3H]5-HT was studied using biochemical approaches with several serotoninergic receptor ligands. A mean decrease of 30% in [3H]5-HT synthesis and release was observed in the presence of 5-HT (10(-8) M), the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), the 5HT1B/1A agonist 5-methoxy-3-(1,2,5,6-tetrahydro-4-pyridinyl)-1H-indole (RU 24969), the 5-HT1B agonist 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP-93,129), and the 5-HT(1D/1B) agonist sumatriptan. Inhibition of 5-HT synthesis and release induced by 8-OH-DPAT was blocked by chiral N-tert-butyl-3-[1-[1-(2-methoxy)phenyl]piperazinyl]-1-phenylpropionam ide dihydrochloride quaternary-hydrate (WAY 100135) (10(7) M) or methyl 4-[4-[4-(1,1,3-trioxo-2H-1,2-benzoisothiazol-2-yl)butyl]-1-p iperazinyl]-1Hindole-2-carboxylate (SDZ 216-525) (10(-7)M), and that of CP-93,129 was blocked by methiothepin (10(-7) M). Paradoxically, extracellular levels of [3H]5-HT increased in the presence of 8-OH-DPAT and RU 24969 at 10(-6) M. 5-HT uptake experiments showed that these two agonists interacted with the 5-HT transporter. 5-HT1 binding sites (620 fmol/mg of protein) and 5-HT1A (482 fmol/mg of protein) and 5-HT1B (127 fmol/mg of protein) receptors were detected in 12-day in vitro cell cultures. Experiments carried out with tetrodotoxin suggested that 5-HT1A receptors are located on nerve cell bodies, whereas 5-HT1B receptors are located on the nerve terminals. We concluded that autoregulatory mechanisms involving 5-HT1A and 5-HT1B autoreceptors are functionally mature in cells from rostral raphe nuclei during the early period of development.     

Modulation of selective serotonin reuptake inhibitor and 5-HT1A antagonist activity in 8-aza-bicyclo[3.2.1]octane derivatives of 2,3-dihydro-1,4-benzodioxane

2,3-Dihydro-1,4-benzodioxanes with aryl 8-aza-bicyclo[3.2.1]oct-3-ene attachments 2 produce compounds with potent 5-HT-T affinity, and weak 5-HT(1A) affinity and alpha(1) affinity. This compares with 2,3-dihydro-1,4-benzodioxanes containing 8-aza-bicyclo[3.2.1] octan-3-ol attachments 4 which possess potent 5-HT(1A) affinity, moderate to good selectivity over alpha(1) and little 5-HT-T affinity. A 3-benzothiophene analogue of 4 (30) was synthesized which possesses potent 5-HT(1A) affinity and especially good selectivity over both alpha(1) and 5-HT-T.     

Ligand conformation has a definitive effect on 5-HT1A and serotonin reuptake affinity

Conformationally constrained aryl cyclohexanes and cyclohexenes based on aryl cyclohexanols 1 were prepared. Locking the aryl ring in plane with the cyclohexane moiety provided potent SSRIs 3. Conversely, fixing the aryl ring perpendicular to the cyclohexane ring via a spiro lactone provided balanced 5-HT1A antagonists with mid-nanomolar range SSRI potency (compounds 2).     

Regional changes in density of serotonin transporter in the brain of 5-HT1A and 5-HT1B knockout mice, and of serotonin innervation in the 5-HT1B knockout

5-HT1A knockout (KO) mice display an anxious-like phenotype, whereas 5-HT1B KOs are over-aggressive. To identify serotoninergic correlates of these altered behaviors, autoradiographic measurements of 5-HT1A and 5-HT1B serotonin (5-HT) receptors and transporter (5-HTT) were obtained using the radioligands [3H]8-OH-DPAT, [125I]cyanopindolol and [3H]citalopram, respectively. By comparison to wild-type, density of 5-HT1B receptors was unchanged throughout brain in 5-HT1A KOs, and that of 5-HT1A receptors in 5-HT1B KOs. In contrast, decreases in density of 5-HTT binding were measured in several brain regions of both genotypes. Moreover, 5-HTT binding density was significantly increased in the amygdalo-hippocampal nucleus and ventral hippocampus of the 5-HT1B KOs. Measurements of 5-HT axon length and number of axon varicosities by quantitative 5-HT immunocytochemistry revealed proportional increases in the density of 5-HT innervation in these two regions of 5-HT1B KOs, whereas none of the decreases in 5-HTT binding sites were associated with any such changes. Several conclusions could be drawn from these results: (i) 5-HT1B receptors do not adapt in 5-HT1A KOs, nor do 5-HT1A receptors in 5-HT1B KOs. (ii) 5-HTT is down-regulated in several brain regions of 5-HT1A and 5-HT1B KO mice. (iii) This down-regulation could contribute to the anxious-like phenotype of the 5-HT1A KOs, by reducing 5-HT clearance in several territories of 5-HT innervation. (iv) The 5-HT hyperinnervation in the amygdalo-hippocampal nucleus and ventral hippocampus of 5-HT1B KOs could play a role in their increased aggressiveness, and might also explain their better performance in some cognitive tests. (v) These increases in density of 5-HT innervation provide the first evidence for a negative control of 5-HT neuron growth mediated by 5-HT1B receptors.     

General practitioners' perceptions of the tolerability of antidepressant drugs: a comparison of selective serotonin reuptake inhibitors and tricyclic antidepressants.

OBJECTIVE: To examine inceptions and discontinuations of antidepressants in general practice. DESIGN: An observational study analysing data from an ongoing cross sectional postal survey. Every three months a representative sample of 250 doctors recorded prescribing activity for four weeks. This provided 4000 general practitioner weeks of recording per year. SETTING: A representative panel of general practitioners in England, Wales, and Scotland. SUBJECTS: Patients who began a new course of an antidepressant or had their treatment stopped or changed by the general practitioner between 1 July 1990 and 30 June 1995. MAIN OUTCOME MEASURES: Numbers of patients prescribed a new course of antidepressant; numbers discontinuing treatment; the ratio of antidepressant discontinuations to antidepressant inceptions; reasons for discontinuation; proportion of switches to another antidepressant. RESULTS: There were 13,619 inceptions and 3934 discontinuations of selective serotonin reuptake inhibitors and tricyclic antidepressants during the study. The number of newly prescribed courses of antidepressants increased by 116%, mostly due to an increase in prescribing of serotonin reuptake inhibitors. The ratio of total discontinuations to inceptions was significantly lower for serotonin reuptake inhibitors (22%) than for tricyclic antidepressants (33%). Differences persisted when controlled for age and sex of patients and severity of depression. However, there was more switching away from selective serotonin reuptake inhibitors when they failed (72%) than from tricyclic antidepressants (58%). CONCLUSIONS: Selective serotonin reuptake inhibitors are less likely than tricyclic antidepressants to be discontinued. A prospective study is needed in general practice to assess the implications of differences in discontinuation rates and switches on clinical and economic outcomes.     

Comparison of the effects of serotonin selective,norepinephrine selective,and dual serotonin and norepinephrine reuptake inhibitors on lower urinary tract function in cats

Previous studies showed that the dual serotonin (5-hydroxytryptamine, 5-HT) and norepinephrine (NE) reuptake inhibitor, duloxetine, increases bladder capacity and urethral sphincter electromyographic (EMG) activity in a cat model of acetic acid-induced bladder irritation. The present study aimed to determine the relative importance of 5-HT versus NE reuptake inhibition for mediating these effects by examining drugs that are selective for either the 5-HT or NE system or both. Similar to duloxetine, venlafaxine (0.1 to 10 mg/kg), also a dual serotonin and norepinephrine reuptake inhibitor, produced marked increases in bladder capacity and EMG activity that were reversed by methiothepin (0.3 mg/kg). S-norfluoxetine (0.01 to 10 mg/kg), a serotonin selective reuptake inhibitor, produced small but significant increases in bladder capacity and EMG activity at doses of 3 and 10 mg/kg. Thionisoxetine (0.01 to 3.0 mg/kg), a NE selective reuptake inhibitor, produced no effects on bladder capacity or sphincter EMG activity. Surprisingly, co-administration of thionisoxetine and s-norfluoxetine up to doses of 1 mg/kg of each compound produced no effect on lower urinary tract function. These doses were the maximum that could be administered in combination due to drug-induced emergence of skeletal muscle activity in chloralose-anesthetized animals. These results indicate that there are unexplained pharmacological differences between the effects of single compounds that exhibit dual NE and 5-HT reuptake inhibition and a combination of compounds that exhibit selective NE and 5-HT reuptake inhibition on lower urinary tract function.