Inhibition of neuronal uptake of 3H-biogenic amines into rat cerebral cortex by partially and fully saturated derivatives of imipramine and desipramine. The importance of the aromatic ring in adrenergic amines--part 3.

In order to assess the importance of the aromatic rings in inhibition of neuronal amine uptake produced by tricyclic antidepressant drugs, derivatives of imipramine (IMI) and dcsipramine (DMI) were prepared in which either one (IMIH, DMIH) or both (IMIH2, DMIH2) of the aromatic rings were fully reduced to cyclohexane rings. The relative abilities of these compounds to inhibit the uptake of $\text{l-[}{}^3\text{H}\text{]-}\text{norepinephrine}$ (NE), [³H]-dopamine (DA) and [³H]-5-hydroxytryptamine (5-HT) into chopped tissue of rat cerebral cortex were determined. Reduction of one or both aromatic rings did not alter significantly the inhibition of uptake of [³H]-DA or [³H]5-HT produced by either IMI or DMI (IC₅₀ values 25–80 μM). However, saturation of one or both rings abolished the selectivitv of DMI for inhibition of NE uptake (IC₅₀ 0.12 μM). decreasing potency 150-fold (IC₅₀ 18.3 μM) and 250-fold (IC₅₀ 29.4 μM) respectively. The effect of aromatic ring reduction on the IMI-induced inhibition of NE uptake was much less pronounced. The results suggest that hydrophobic rather than π-electron attractive forces are involved in the interaction of DMI or IMI with DA or 5-HT uptake sites. However, the loss in selectivity for inhibition of NE uptake upon reduction of DMI may reflect loss of π-electron interactions in the binding of DMI to the NE uptake site, or may reflect increased sensitivity to spatial disposition of the hydrophobic binding areas of the drug relative to that found at the DA or 5-HT amine uptake sites.     

Platelets and biogenic amines. 2. Indications for a discrete low affinity uptake mechanism shared by norepinephrine and 5-hydroxytryptamine in human platelets

The uptake of norepinephrine (NE) by human platelets at 10^–-9–5×10^–-4 M of labelled amine concentration was investigated. At physiological concentrations of NE the uptake was unsaturable and could not be inhibited by imipramine or ouabain. At NE concentrations between 25 and 485 M the uptake also comprised a saturable component that could be completely blocked by imipramine and partly by ouabain. The saturable uptake of NE had an apparent K_m of 273±50 M and a V_max of 0.19±0.05 pmole/10⁶ platelets/min. The affinity of NE (IC₅₀) for the 5-HT transporting carrier was 2.3 mM, 8.4 times higher than the apparent K_m for saturable NE uptake. The affinity of 5-HT (IC₅₀) for the NE-transporting carrier was 5.8 M, 5.8 times higher than the apparent K_m for saturable 5-HT transport. Imipramine and norzimeldin were equipotent inhibitors of saturable NE uptake, the potency being of the same degree as that for saturable 5-HT uptake. The tertiary amine amitriptyline was 6 times more effective in inhibiting saturable NE uptake than its demethylated product nortriptyline. Nortriptyline and its hydroxylated E- and Z-isomers had a stronger inhibitory effect on saturable NE uptake than on uptake of 5-HT. The results suggest that human platelets possess two separate amine-transporting carriers, both having their highest affinity for 5-HT. The one with the lowest affinity for 5-HT can also accept NE as a substrate. The human platelet does not possess a high-affinity uptake system for NE comparable to that in adrenergic tissue.     

DU 24565, a quipazine derivative,a potent selective serotonin uptake inhibitor

DU 24565, 6-nitro,2-(1-piperazinyl)quinoline, is a potent and selective inhibitor of the synaptosomal uptake of serotonin (5-HT). At concentrations at least 10³-fold higher it affects the uptake of norepinephrine (NA) and dopamine (DA). The IC₅₀ values are: 5-HT: 4 × 10^?8 M; NA: 6 × 10^?5 M and DA: 4 × 10^?5 M. Uptake of 5-HT by rat blood platelets is also strongly inhibited (K_i ～ 5 × 10^?8 M); the inhibition is probably noncompetitive. In vivo, DU 24565 is active at low oral doses: the 5-HT depletion in rat brain caused by p-chloroamphetamine is antagonized by DU 24565 (oral ED₅₀ 0.7 mg/kg). The decrease in the 5-HT content caused by 4,α-dimethyl-m-tyramine (H 77/77) is antagonized by DU 24565 at 1 mg/kg orally, without any effect on the depletion of catecholamines. 5-HT turnover, measured by the probenecid method, is reduced by the same dose of DU 24565. Other tests confirmed the activity and selectivity of DU 24565: it potentiated the behavioural effects of the 5-HT precursor 5-hydroxytryptophan (5-HTP) in mice (ED₅₀ 1.5 mg/kg orally); it potentiated the temperature increases caused by 5-HTP in the rabbit; it had low activity or no effect at all in NA potentiation tests. This new compound is more potent and selective than the known 5-HT uptake inhibitors. It is a potential antidepressant and can be useful as a pharmacological tool to study the role of 5-HT in the central nervous system.     

Further studies on the mechanism of serotonin-dependent anorexia in rats

4-(3-Indolyl-2-ethyl)piperidine (LM 5008), 2-(1-piperazinyl) quinoline (quipazine), and metachlorophenylpiperazine (mCPP) were studied for their ability to affect serotonergic mechanisms in vitro. Their relative potency in inhibiting serotonin (5-HT) uptake in vivo and reducing food intake in rats was also examined. mCPP was very potent in displacing ³H-5-HT bound to brain membranes (IC₅₀, 6.2×10^-7 M), followed by quipazine, which showed an IC₅₀ of 3.8×10^-6 M. LM 5008 was the least effective with an IC₅₀ of 3.6×10^-5 M. mCPP and quipazine were less potent than d-fenfluramine in releasing ¹⁴C-5-HT from brain synaptosomes, while LM 5008 caused no significant effects at a concentration of 10^-5 M. Conversely, both in vitro and in vivo studies on 5-HT uptake showed that LM 5008 was the most potent compound in inhibiting 5-HT uptake and mCPP the least potent. Since a 50% reduction of food intake was not reached even with a dose of LM 5008 27-times higher than the ED₅₀ for inhibiting 5-HT uptake in vivo, it is suggested that even marked inhibition of 5-HT uptake at central synapses is not sufficient per se to trigger serotonin-dependent anorexia in the rat. Increased release and/or direct stimulation of post-synaptic receptors may be necessary to obtain this effect. This could be of interest for developing new agents which can cause anorexia by interacting with brain serotonin.     

Inhibition of 5-HT cell firing in the DRN by non-selective 5-HT reuptake inhibitors: studies on the role of 5-HT1A autoreceptors and noradrenergic mechanisms

Improved clinical antidepressant efficacy may result if the acute inhibition of 5-HT cell firing induced by antidepressants is prevented. Here we examined whether inhibition of 5-HT cell firing by non-selective 5-HT uptake inhibiting antidepressant drugs is reversed by a selective 5-HT_1A receptor antagonist. In addition, we examined whether concomitant blockade of NA uptake offsets the inhibition of 5-HT cell firing resulting from 5-HT uptake blockade. Antidepressants which block 5-HT uptake (paroxetine, clomipramine, amitriptyline, venlafaxine), all caused dose-dependent and complete inhibition of 5-HT cell firing. Desipramine, a selective NA uptake blocker, caused a slight reduction in firing. The selective 5-HT_1A receptor antagonist, WAY 100635, reversed the inhibition of 5-HT cell firing induced by clomipramine, amitriptyline, venlafaxine, and paroxetine, but not that induced by the α₁ adrenoceptor antagonist, prazosin. Desipramine, at a dose which increased extracellular NA in the DRN, reversed the effect of prazosin but did not alter the ability of paroxetine to inhibit 5-HT cell firing. Our data indicate that antidepressant drugs with 5-HT uptake blocking properties inhibit 5-HT cell firing via activation of 5-HT_1A autoreceptors, and do so irrespective of their effects on NA uptake. These data are discussed in relation to the application of 5-HT_1A receptor antagonists to enhance the clinical efficacy of antidepressant drugs. Received: 15 July 1996 / Final version: 11 November 1996     

Effect of antidepressant and neuroleptic drugs on the electrically evoked release of serotonin from rat cerebral cortex

The effect of antidepressant and neuroleptic drugs on the electrically evoked release of serotonin (5-HT) was investigated in rat brain cortical slices preincubated with 0.1 mol/l ³H-5-HT. Zimelidine, trazodone, clomipramine, doxepin, and viloxazine (1 mol/l each) enhanced the electrically-induced ³H overflow by 20–44%. Six other antidepressants and five neuroleptics did not increase the evoked transmitter release. Only trazodone and viloxazine also increased the ³H overflow in experiments in which neuronal 5-HT reuptake was already blocked by 6-nitroquipazine. 5-HT and clonidine inhibited the electrically-induced ³H-5-HT release by stimulation of presynaptic 5-HT autoreceptors and ₂-adrenoceptors, respectively; trazodone and viloxazine had no effect on the concentration-response curves of 5-HT and clonidine. Other psychotropic agents with well known antiserotonergic activities also failed to block presynaptic 5-HT autoreceptors. It is concluded that zimelidine, clomipramine, and doxepin enhanced the ³H-5-HT overflow by inhibition of neuronal 5-HT uptake, whereas the increase produced by trazodone and viloxazine cannot be explained by reuptake inhibition or interaction with presynaptic receptors.     

Electrophysiological impact of trazodone on the dopamine and norepinephrine systems in the rat brain

Previous study has documented the long-term effects of the antidepressant trazodone on the serotonin (5-HT) system. The present work examined the impact of sustained trazodone on ventral tegmental area (VTA) dopamine (DA) and locus ceruleus (LC) norepinephrine (NE) neurons firing activity, and characterized its effects at 5-HT_2C, 5-HT_2A receptors and α₁- and α₂-adrenoceptors. Electrophysiological recordings were carried out in anesthetized rats. Subcutaneously implanted minipumps delivered vehicle or trazodone (10 mg/kg/day) for 2 or 14 days. Administration of trazodone for 2 and 14 days did not alter the firing activity of DA neurons. Systemic injection of trazodone, however, reversed the inhibitory effect of the 5-HT_2C receptor agonist Ro 60,0175 on the DA neuronal firing, suggesting an antagonistic action of trazodone at this receptor. Administration of trazodone for 2 days significantly enhanced the NE neurons firing. Despite a return of the NE neurons firing rate to the baseline following 14-day trazodone, the percentage of neurons discharging in burst was increased by this regimen. Administration of trazodone for 14 days enhanced the tonic activation of postsynaptic α₂-adrenoceptors, as indicated by the disinhibitory effect of the α₂-adrenoceptor antagonist idazoxan on hippocampus pyramidal neurons firing. The inhibitory effect of acute trazodone on dorsal raphe (DR) 5-HT neurons firing was shown to be through the 5-HT_1A receptor. Systemic injection of trazodone reversed the inhibitory action of 5-HT_2A agonist DOI on the NE neurons firing rate, indicating its antagonistic action at 5-HT_2A receptors. The enhancement in α₂-adrenergic transmission by trazodone, and its 5-HT_2A and 5-HT_2C receptor antagonism may contribute to its therapeutic action in major depression.     

Tricyclic Antidepressant Agents. I. Comparison of the Inhibition of the Uptake of 3H-Noradrenaline and 14C-5-Hydroxytryptamine in Slices and Crude Synaptosome Preparations of the Midbrain-Hypothalamus Region of the Rat Brain

Abstract: The simultaneous uptake of ³H-I-noradrenaline (NA) and ¹⁴C-5-hydroxytryptamine (5-HT) in slices from the midbrain-hypothalamus region of the rat brain was compared with the corresponding uptake in crude synaptosome preparations of the same brain region. In both preparations the uptake of the two amines was selective at the concentration used (1 x 10^-7 M or lower). The K_M values for the amines (NA: 2 x 10^-7 M in synaptosomes and 5 x 10^-7 M in slices; 5-HT: 8 x 10^-8 M in synaptosomes and 6 x 10^-7 M in slices) and the inhibitory concentrations (IC₅₀) of the antidepressant agents were lower in the synaptosome experiments than in the slices experiments. Moreover the order of the inhibitory activities differed between the two preparations. In the slices experiments the NA uptake was inhibited most markedly by desipramine followed by imipramine > chlorimipramine = nortriptyline ≥ amitriptyline ≥ chlordesipramine whereas in the synaptosome experiments the order was desipramine > nortriptyline ≥ chlordesipramine ≥ imipramine > amitriptyline ≥ chlorimipramine. For the 5-HT uptake in slices the order of activity was: chlorimipramine > imipramine ≥ amitriptyline ≥ chlordesipramine = desipramine ≥ nortriptyline whereas in the synaptosome preparations the order was: chlorimipramine > imipramine ≥ amitriptyline ≥ chlordesipramine > nortriptyline = desipramine. The role of protein binding and diffusion barriers in the causation of the difference in the results obtained with the two preparations is discussed.     

Effect of two non tricyclic antidepressant drugs on [14C]5-hydroxytryptamine uptake by rat platelets

The uptake of ¹⁴C-5-HT by rat blood platelets was examined in vitro in experimental conditions which allowed measurement of the initial velocity and excluded other passive processes across the cell membrane. In these conditions, the effect of two non tricyclic antidepressant drugs (Lilly 110140 and trazodone) was investigated. Lilly 110140 was as active as chlorimipramine and several times more active than imipramine as an inhibitor of ¹⁴C-5-HT uptake. Like chlorimipramine, Lilly 110140 appeared to be either a non-competitive or an uncompetitive inhibitor, according to the concentration of drug used. Trazodone also inhibited ¹⁴C-5-HT uptake by platelets but to a lesser extent than chlorimipramine, imipramine or Lilly 110140. m-Chlorophenylpiperazine, a possible metabolite of trazodone, was about 3 times more potent an inhibitor than the parent molecule. Both compounds acted non-competitively. Compared with published data on the effect of Lilly 110140 and trazodone on brain 5-HT, the present results support the suggestion that rat platelets are a useful pharmacological model of serotoninergic nerve endings.     

A comparison of trazodone and fluoxetine: implications for a serotonergic mechanis of antidepressant action

Trazodone is an atypical antidepressant drug that is commonly referred to as a serotonin (5-hydroxytryptamine; 5-HT) uptake inhibitor. However, the most potent pharmacological effect of trazodone appears to be antagonist action at 5-HT_2/1C receptors. This is in contrast to fluoxetine, for which inhibition of 5-HT uptake is the most potent pharmacological action. The effects of trazodone and fluoxetine on several antidepressant drug screens are mediated by antagonist action at 5-HT₂ receptors and inhibition of 5-HT uptake, respectively. While fluoxetine is an effective agent for the treatment of major depression, obsessive-compulsive disorder (OCD) and panic disorder, trazodone does not appear to be effective in the treatment of OCD and panic disorder. In addition, trazodone and fluoxetine differ in humans with respect to their effects on sleep and weight. Taken together, the preclinical and clinical data suggest that trazodone acts as an antidepressant via antagonist action at 5-HT_2/1C receptors, while fluoxetine likely acts as an antidepressant via inhibition of 5-HT uptake.     

Extracellular dopamine, norepinephrine, and serotonin in the ventral tegmental area and nucleus accumbens of freely moving rats during intracerebral dialysis following systemic administration of cocaine and other uptake blockers

Extracellular levels of dopamine (DA), serotonin (5-HT), and norepinephrine (NE) were measured by microdialysis in conscious rats equipped with dual probes, one in the ventral tegmental area (VTA) and another one in the contralateral nucleus accumbens (NACC). Dialysate content of all amines in both regions was essentially abolished by local infusion of tetrodotoxin (1 μM) or Ca²⁺-free buffer. Injection of the selective DA uptake blocker GBR 12935 (15 mg/kg IP) increased DA, as well as NE and, to a lesser extent, 5-HT in the VTA; it increased DA more than 5-HT in the NACC. The selective NE uptake blocker desipramine (10 mg/kg IP) increased NE but also 5-HT in the VTA and NACC; the DA level was persistently enhanced in the VTA, whereas in the NACC it initially rose and then fell below baseline value. The selective 5-HT uptake blocker citalopram (15 mg/kg IP) was generally more effective in elevating dialysate level of 5-HT than that of other amines in both regions. Cocaine (20 mg/kg IP) was non-selective in enhancing all three amines in both regions. There is considerable crosstalk between monoamine systems occurring upon systemic administration of uptake blockers, and the VTA and NACC are notably different in the time course of the DA effect (long-lasting versus transient). Received: 25 February 1997 /Final version: 2 June 1997     

Effect of trazodone (KB-831) and its metabolites on brain monoamines in rat

The effects of trazodone (KB-831) and its metabolites on the uptake, turnover and contents of monoamines in rats were studied in comparison with those of imipramine and mianserin. Trazodone exhibited a more potent inhibitory effect on the uptake of [3H] 5-hydroxytryptamine (5-HT) into brain synaptosomes than on the uptake of [3H]norepinephrine (NE). Trazodone at 10-30 mg/kg, p.o., also inhibited the p-chloramphetamine-induced depletion of 5-HT in rat brain, but not the 6-hydroxydopamine-induced NE depletion in rat heart. Trazodone was the most selective 5-HT uptake inhibitor among the drugs tested in vitro. Its metabolite, m-chlorophenyl-piperazine (m-CPP), inhibited 5-HT and NE uptake in vitro, but not in vivo. Trazodone (100 mg/kg) and imipramine (30-100 mg/kg) inhibited the depletion of NE induced by alpha-methyl-p-tyrosine, whereas mianserin (100 mg/kg) facilitated it. At 1 hr after a single administration of each drug, an increase in 5-HT content and a decrease in 5-hydroxyindole-3-acetic acid (5-HIAA) content were observed when 30 mg/kg trazodone was used. At 100 mg/kg, trazodone increased the levels of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) and decreased the NE content. m-CPP (10-30 mg/kg) produced similar effects on monoamine contents to those of trazodone. Imipramine and mianserin had no effect on monoamine contents even at a dose of 100 mg/kg. After 3 weeks of successive administration, an increase in 5-HT and a decrease in 5-HIAA were induced by trazodone and m-CPP at 1 hr, but not at 17 hr, after the final administration. Imipramine decreased the contents of NE and 5-HIAA, and its effects lasted for 17 hr. These results suggest that trazodone is a selective 5-HT uptake inhibitor and that its neurochemical profile is different from those of imipramine and mianserin.     

A comparison of the inhibitory effects of new non-tricyclic amine uptake inhibitors on the uptake of norepinephrine and 5-hydroxytryptamine into synaptosomes of the rat brain

The effects of new non-tricyclic amine uptake inhibitors, FS32 and FS97, on the uptake of [³H]-norepinephrine (NE) into the hypothalamic synaptosomes and [³H]-5-hydroxytryptamine (5-HT) into whole brain synaptosomes were studied. Their effects were compared with those of tricyclic antidepressants. The uptake of [³H]-NE was inhibited competitively by FS32 and FS97 with a respective K_i value of 6.5 × 10^?7 M and 3.8 × 10^?7 M. The potency of FS32 and FS97 to inhibit this uptake was almost comparable to that of clomipramine and imipramine, respectively. In the case of [³H]-5-HT uptake, FS32 and FS97 also showed competitive inhibition with a respective K_i value of 2.9 × 10^?6 M and 5.9 × 10^?6M. The ability of FS32 to inhibit [³H]-5-HT uptake was almost equal to that of nortriptyline, while FS97 was two times more potent than iprindole in inhibiting this uptake.     

The effects of protriptyline and clomipramine in vitro on the uptake of 5-hydroxytryptamine and dopamine in human platelet-rich plasma

The effects of protriptyline and clomipramine, at concentrations of 10^–7 M to 10^–4 M, were studied in vitro on the uptake of 5-hydroxytryptamine and dopamine uptake in human platelet-rich plasma. It was found that the tertiary amine, clomipramine, was a more potent inhibitor of 5-hydroxytryptamine uptake than the secondary amine, protriptyline. The activity of both compounds was competitive but it was thought unlikely that they acted through tryptamine receptor sites as methysergide 2.5×10^–8 M had very little effect on 5-hydroxytryptamine uptake. Neither tricyclic antidepressant had any marked effect on dopamine uptake.A. T. was supported by a grant from the Mental Health Research Fund.     

Binding and uptake studies with [3H]CP-93, 129, a radiolabeled selective 5-HT1B receptor ligand

3-(1,2,5,6-Tetrahydro-4-pyridyl)-5-pyrrolo[3,2-b]pyridone, CP-93, 129, is a selective agonist ligand for 5-HT_1B receptors. High affinity binding sites of [³H]CP-93, 129 were found in rat whole brain membranes, which showed K_D and B_max values similar to those for 5-HT_1B sites labeled by [³H]5-HT. Uptake of [³H]CP-93, 129 in crude rat synaptosomes was also observed, which was potently inhibited by 5-HT uptake blockers and 5-HT but not by desipramine (NE uptake blocker) or tametraline (NE and DA uptake blocker). Because of this sensitivity to 5-HT uptake inhibitors and the structural similarity of CP-93, 129 to serotonin, [³H]CP-93, 129 uptake probably occurred in 5-HT neurons.     

Duloxetine (LY 248686): an inhibitor of serotonin and noradrenaline uptake and an antidepressant drug candidate

Duloxetine is a potent inhibitor of serotonin (5-hydroxytryptamine, 5-HT) and noradrenaline (NE) uptake in vitro and in vivo and is 3- to 5-times more effective at inhibiting 5-HT uptake. Duloxetine is a weak inhibitor of dopamine (DA) uptake and the binding of radioligands to neurotransmitter receptors. Upon administration of duloxetine in vivo, the inhibitory effects on uptake of 5-HT and NE persist for up to 8 h. Desmethylduloxetine, a potential metabolite, is also an inhibitor of 5-HT and NE uptake. Consistent with the ability to inhibit the uptake of 5-HT, duloxetine blocks p-chloroamphetamine induced depletion of mouse and rat brain 5-HT. Duloxetine also blocks the 6-hydroxydopamine induced depletion of mouse heart NE and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induced depletion of NE in frontal cortex but does not block the MPTP induced depletion of DA in rat striatum. Electrophysiological studies show that duloxetine decreases the activity of 5-HT neurones in dorsal raphe and at a 5-times higher dose also decreases the activity of NE neurones in the locus coeruleus. Microdialysis techniques have demonstrated that duloxetine effectively elevates extracellular 5-HT and NE levels in rat frontal cortex and hypothalamus. Antagonists at somatodendritic 5-HT_1A autoreceptors or at presynaptic alpha₂-adrenergic receptors could augment the duloxetine induced elevation of extracellular 5-HT, NE and DA levels. Duloxetine produces behavioural responses consistent with the enhancement of 5-HT and NE neurotransmission. Pharmacokinetic studies in healthy human volunteers show that duloxetine has a half-life of 10 - 15 h without the influence of food. In preliminary clinical trials, duloxetine has shown antidepressive effects in patients with major depression. Duloxetine offers an opportunity to utilise combined central 5-HT and NE neuronal pathways to improve the treatment of patients with major depression.     

Effects of narcotic analgesics on the uptake and release of 5-hydroxytryptamine in rat synaptosomal preparations

1 The effects of various narcotic analgesics on the uptake and release of labelled 5-hydroxytryptamine (5-HT) in brain and spinal cord synaptosomes were investigated.

2 Methadone was most active in inhibiting 5-HT uptake (IC₅₀ 2.5 × 10^-7 M). Levorphanol also inhibited 5-HT uptake to a large extent (IC₅₀ 8.8 × 10^-7 M) while dextrophan, pethidine and pentazocine showed much less activity. Etorphine and morphine had virtually no such activity, with IC₅₀S higher than 10^-4 and 10^-3 M respectively.

3 The same order of potency as `5-HT releasers' was found when radioactivity was measured in [³H]-5-HT preloaded synaptosomal pellets incubated for 20 min with the various narcotics. Methadone, like chlorimipramine, showed a significant effect at a concentration of 10^-7 M while morphine, at a concentration of 10^-4 M, had no effect.

4 When 5-HT release was studied by a perfusion technique, which largely prevents reuptake of the released amine, only fenfluramine, an anorectic agent proposed as a 5-HT releaser, significantly increased spontaneous 5-HT release. These data suggest that the apparent 5-HT release induced by various narcotics in traditional incubation techniques may largely depend on their ability to interfere with neurotransmitter reuptake mechanisms.

5 The effects of the various narcotics on 5-HT uptake have no relationship to their relative potency as analgesics in the rat. In the light of their poor effectiveness as 5-HT releasers, it can be concluded that mechanisms other than 5-HT uptake inhibition and release are probably involved in the analgesic effects of these compounds in intact animals.

     

An inhibitory prejunctional 5-HT1-like receptor in the isolated perfused rat kidney

Summary The present study has identified a receptor for 5-hydroxytryptamine (5-HT) which functions to inhibit the stimulus-induced release of [³H] noradrenaline following sympathetic periarterial nerve stimulation to the isolated perfused rat kidney. In addition to 5-HT (IC₃₀=4.5×10^–8 mol/l), both 5-carboxamidotryptamine (IC₃₀=8×10^–9 mol/l) and 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl) indole (RU-24969, IC₃₀=2.5×10^–7 mol/l) acted as agonists whereas 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) was inactive. The inhibitory effect of 5-HT on the electrically-evoked release of tritium was antagonized in a concentration-dependent manner by methiothepin (IC₅₀=4×10^–9 mol/l), metergoline (IC₅₀=4×10^–8 mol/l) and methysergide (IC₅₀=1.3×10^–7 mol/l) but not by cyproheptadine, ketanserin, mesulergine, (–)-propranolol, (±)-pindolol, (±)-cyanopindolol, metoclopramide or phentolamine. It is concluded that the receptor to 5-HT conforms to general criteria defining 5-HT₁-like receptors but at the present time the receptor site cannot be fitted to the designated 5-HT_1A, 5-HT_1B or 5-HT_1C subtypes.Preliminary accounts of this work were presented to the British Pharmacological Society in London (December, 1984) and Southampton (July, 1985)     

Platelets and biogenic amines

The uptake of ¹⁴C-dopamine (DA) by human platelets at 10^-7–2.5×10^-4 M of labelled amine concentration was studied in human platelet-rich plasma. The total uptake could be resolved into two components, one of which was saturable and completely inhibited by 10^-5 imipramine and another which was unsaturable but temperature-dependent. The saturable uptake of DA had an apparent K_m of 75×10^-6 M and V _max of 1.34 pmol/10⁶ platelets/min. The uptake of unsaturable DA was 1.33 pmol/10⁶ platelets/min at 10^-4 M DA.Dopamine exerted a mixed non-competitive inhibition of the saturable 5-HT transport and vice versa. Thus the increase in K_m was paralleled by a decrease in V _max.The low-affinity transport of DA by the human platelets does not share any of the dopamine uptake characteristics found in neuronal tissue. The platelet therefore seems to be a poor model for the presynaptic function of the dopamine neurons.     

Preclinical pharmacology of F-98214-TA, a novel potent serotonin and norepinephrine uptake inhibitor with antidepressant and anxiolytic properties

Rationale Serotonin (5-HT) and norepinephrine (NE) re-uptake inhibitors (SNRIs) have been proposed to have a higher efficacy and/or faster onset of action than previously available antidepressants. Objectives We examined in biochemical, electrophysiological and behavioural assays the antidepressant properties of (S)-(−)-4-[(3-fluorophenoxy)-phenyl]methyl-piperidine (F-98214-TA), a compound that displays very high affinity for 5-HT and NE transporters. Results F-98214-TA potently inhibited the uptake of both 5-HT and NE into rat brain synaptosomes (IC₅₀=1.9 and 11.2 nM, respectively) and decreased the electrical activity of dorsal raphe serotonergic neurones (ED₅₀=530.3 μg/kg i.v.), an effect completely abolished by the 5-HT_1A antagonist WAY100,635. In acute behavioural assays in mice, the orally administered compound potentiated the 5-hydroxy-tryptophan (5-HTP)-induced syndrome [minimal effective dose (MED)=10 mg/kg], antagonized the hypothermia induced by a high dose of apomorphine (ED₅₀=2 mg/kg) and reduced the immobility in the tail suspension test (MED=10 mg/kg). Moreover, it also decreased the immobility in the forced swimming test in mice and rats (30 mg/kg, p.o.). Chronic administration of F-98214-TA (14 days, 30 mg kg⁻¹ day⁻¹, p.o.) attenuated the hyperactivity induced by olfactory bulbectomy in rats, confirming its antidepressant-like properties. Interestingly, the same dosage regimen significantly increased the social interaction time in rats, suggesting an additional potential anxiolytic activity. In most assays the compound was more potent than fluoxetine, venlafaxine and desipramine. Conclusions F-98214-TA is a novel SNRI that displays greater potency than other reference antidepressants in animal models predictive of antidepressant and anxiolytic activities. A preliminary report of this work was presented at the 30th Annual Meeting of the Society for Neuroscience, New Orleans, LA, 2000.