Effects of acute and long-term administration of escitalopram and citalopram on serotonin neurotransmission: an in vivo electrophysiological study in rat brain.

The present study was undertaken to compare the acute and long-term effects of escitalopram and citalopram on rat brain 5-HT neurotransmission, using electrophysiological techniques. In hippocampus, after 2 weeks of treatment with escitalopram (10 mg/kg/day, s.c.) or citalopram (20 mg/kg/day, s.c.), the administration of the selective 5-HT(1A) receptor antagonist WAY-100,635 (20-100 microg/kg, i.v.) dose-dependently induced a similar increase in the firing activity of dorsal hippocampus CA(3) pyramidal neurons, thus revealing direct functional evidence of an enhanced tonic activation of postsynaptic 5-HT(1A) receptors. In dorsal raphe nucleus, escitalopram was four times more potent than citalopram in suppressing the firing activity of presumed 5-HT neurons (ED(50)=58 and 254 mug/kg, i.v., respectively). Interestingly, the suppressant effect of escitalopram (100 microg/kg, i.v.) was significantly prevented, but not reversed by R-citalopram (250 microg/kg, i.v.). Sustained administration of escitalopram and citalopram significantly decreased the spontaneous firing activity of presumed 5-HT neurons. This firing activity returned to control rate after 2 weeks in rats treated with escitalopram, but only after 3 weeks using citalopram, and was associated with a desensitization of somatodendritic 5-HT(1A) autoreceptors. These results suggest that the time course of the gradual return of presumed 5-HT neuronal firing activity, which was reported to account for the delayed effect of SSRI on 5-HT transmission, is congruent with the earlier onset of action of escitalopram vs citalopram in validated animal models of depression and anxiety.     

The R-enantiomer of citalopram counteracts escitalopram-induced increase in extracellular 5-HT in the frontal cortex of freely moving rats

The selective serotonin (5-HT) reuptake inhibitor, citalopram, is a racemic mixture of an S(+)- and R(-)-enantiomer, escitalopram and R-citalopram, respectively. The present study compares the effects of escitalopram, R-citalopram and citalopram on extracellular levels of 5-HT in the frontal cortex of freely moving rats. In addition, co-injection of escitalopram and R-citalopram (ratios 1:2 and 1:4) were assessed. In some experiments escitalopram and R-citalopram were infused into the frontal cortex by reverse microdialysis. Finally, the extracellular level of escitalopram in the frontal cortex was studied after administration of escitalopram alone or in combination with R-citalopram. Escitalopram (1.0-3.9 mg/kg, s.c.) produced a greater maximal increase in extracellular 5-HT than citalopram (2.0-8.0 mg/kg, s.c.). R-citalopram (15.6 mg/kg s.c.) did not affect the 5-HT levels. When co-injected, R-citalopram counteracted the escitalopram-induced increase in extracellular 5-HT levels. Local infusion of the two enantiomers into the frontal cortex produced a similar inhibitory response. R-citalopram did not influence the extracellular levels of escitalopram and therefore does not exert its effect via a pharmacokinetic interaction with escitalopram. In conclusion, the 5-HT-reuptake inhibitory activity of citalopram resides in escitalopram, and the R-enantiomer counteracts this effect. This observation would predict an improved clinical profile of escitalopram compared to citalopram.     

In-vivo modulation of central 5-hydroxytryptamine (5-HT1A) receptor-mediated responses by the cholinergic system

The aim of the present study was to investigate a putative modulation of rat 5-HT system by the muscarinic receptor antagonist atropine using in-vivo electrophysiological and behavioural techniques. In the dorsal raphe nucleus, administration of atropine (1 mg/kg i.v.) prevented the suppressant effect of the selective serotonin reuptake inhibitor paroxetine (0.5 mg/kg i.v.) on the spontaneous firing activity of 5-HT neurons, suggesting that atropine could induce an attenuation of somatodendritic 5-HT1A autoreceptors responsiveness. The 5-HT1A receptor agonist 8-OH-DPAT decreased both immobility in the forced swim test and the body core temperature. Pre-treatment with atropine (5 and 10 mg/kg i.p.) enhanced antidepressant-like effect of 8-OH-DPAT (1 mg/kg s.c.) and reduced 8-OH-DPAT (0.1 mg/kg s.c.)-induced hypothermia. In conclusion, the present study reports a functional role of muscarinic receptors in the modulation of pre- and post-synaptic 5-HT1A receptors mediated responses.     

The 5-HT(1A) receptor antagonist robalzotan completely reverses citalopram-induced inhibition of serotonergic cell firing.

5-HT(1A) receptor antagonists have been suggested to increase the efficacy of selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitors in the treatment of depression by enhancing the increase in brain 5-HT induced by 5-HT reuptake blockade. Here, the novel 5-HT(1A) receptor antagonist robalzotan [(R)-3-N, N-dicyclobutylamino-8-fluoro-3, 4-dihydro-2H-1-benzopyran-5-carboxamide hydrogen (2R, 3R) tartrate monohydrate] (12.5, 25, 50, 100 microg/kg, i.v.) was found to completely reverse the acute inhibitory effect of citalopram (300 microg/kg i.v.) or paroxetine (100 microg/kg, i.v.) on the activity of 5-HT neurons in the dorsal raphe nucleus in rats. Robalzotan (5, 50 microg/kg, i.v.) by itself increased the firing rate of the majority of 5-HT cells studied. The present results suggest that robalzotan may indeed augment the increases in 5-HT output induced by selective 5-HT reuptake inhibitors by antagonizing the feedback inhibition of 5-HT cell firing produced by such drugs. Thus, robalzotan may be effective by enhancing the action of selective 5-HT reuptake inhibitors or as monotherapy in the treatment of depression.     

Allosteric effects of R- and S-citalopram on the human 5-HT transporter: evidence for distinct high- and low-affinity binding sites

The human 5-HT transporter (hSERT) has two binding sites for 5-HT and 5-HT uptake inhibitors: the orthosteric high-affinity site and a low-affinity allosteric site. Activation of the allosteric site increases the dissociation half-life for some uptake inhibitors. The objectives of this study were 1) to identify hSERT mutations that inactivate the high-affinity site without affecting the allosteric site and 2) to observe allosteric effects in which hSERT binds R-citalopram with higher affinity than S-citalopram. Wild-type and mutant (Y95F, I172M, and Y95F/I172M) hSERTs were expressed in COS-7 cells, and their 5-HT uptake and uptake inhibitor-binding abilities were studied. The hSERT mutations did not alter affinities for 5-HT or paroxetine, but high-affinity binding of S-citalopram was severely affected, particularly by the I172M, and Y95F/I172M mutations - K(i) respectively 4 nM (wild-type), 35 nM, 1000 nM, and 17.100 nM (mutants). The allosteric site however, in wild-type hSERT and the three mutants was unaffected by the mutations as attenuation of the dissociation rate of the [(3)H]-paroxetine:hSERT complex in the presence of S-citalopram or paroxetine was the same for wild-type hSERT and the three mutants. Further, R-citalopram previously thought of as an inactive enantiomer strongly attenuated dissociation of the wild-type [(3)H]-imipramine:hSERT complex, whereas S-citalopram had almost no effect on this complex. These results suggest that 1: The allosteric site on hSERT is distinct from the site to which S-citalopram binds with high affinity. 2: The allosteric effects of R-citalopram on the dissociation of [(3)H]-imipramine from hSERT indicate that R-citalopram introduces a conformational change in hSERT.     

The S-enantiomer of R,S-citalopram, increases inhibitor binding to the human serotonin transporter by an allosteric mechanism. Comparison with other serotonin transporter inhibitors.

The interaction of the S- and R-enantiomers (escitalopram and R-citalopram) of citalopram, with high- and low-affinity binding sites in COS-1 cell membranes expressing human SERT (hSERT) were investigated. Escitalopram affinity for hSERT and its 5-HT uptake inhibitory potency was in the nanomolar range and approximately 40-fold more potent than R-citalopram. Escitalopram considerably stabilised the [3H]-escitalopram/SERT complex via an allosteric effect at a low-affinity binding site. The stereoselectivity between escitalopram and R-citalopram was approximately 3:1 for the [3H]-escitalopram/hSERT complex. The combined effect of escitalopram and R-citalopram was additive. Paroxetine and sertraline mainly stabilised the [3H]-paroxetine/hSERT complex. Fluoxetine, duloxetine and venlafaxine have only minor effects. 5-HT stabilised the [125I]-RTI-55, [3H]-MADAM, [3H]-paroxetine, [3H]-fluoxetine and [3H]-venlafaxine/SERT complex to some extent. Thus, escitalopram shows a unique interaction with the hSERT compared with other 5-HT reuptake inhibitors (SSRIs) and, in addition to its 5-HT reuptake inhibitory properties, displays a pronounced effect via an affinity-modulating allosteric site.     

Differential regulation of serotonin-1A receptor-stimulated [35S]GTP gamma S binding in the dorsal raphe nucleus by citalopram and escitalopram

The effect of chronic citalopram or escitalopram administration on 5-HT1A receptor function in the dorsal raphe nucleus was determined by measuring [35S]GTP gamma S binding stimulated by the 5-HT1A receptor agonist (R)-(+)-8-OH-DPAT (1nM-10 microM). Although chronic administration of citalopram or escitalopram has been shown to desensitize somatodendritic 5-HT1A autoreceptors, we found that escitalopram treatment decreased the efficacy of 5-HT1A receptors to activate G proteins, whereas citalopram treatment did not. The binding of [3H]8-OH-DPAT to the coupled, high affinity agonist state of the receptor was not altered by either treatment. Interestingly, escitalopram administration resulted in greater occupancy of serotonin transporter sites as measured by the inhibition of [3H]cyanoimipramine binding. As the binding and action of escitalopram is limited by the inactive enantiomer R-citalopram present in racemic citalopram, we propose that the regulation of 5-HT1A receptor function in the dorsal raphe nucleus at the level of receptor-G protein interaction may be a result of greater inhibition of the serotonin transporter by escitalopram.     

Chronic fluoxetine induces opposite changes in G protein coupling at pre and postsynaptic 5-HT1A receptors in rat brain

Chronic treatment with the antidepressant fluoxetine may lead to changes in the properties of pre- and postsynaptic 5-HT(1A) receptors due to modifications in the receptor-G protein coupling process. We have evaluated, in rats, the effect of chronic fluoxetine (10 mg/kg/day) at brain 5-HT(1A) receptors using different techniques. The density of 5-HT(1A) receptors was unchanged in fluoxetine-treated rats vs. vehicle group. Stimulation of [(35)S]GTPgammaS binding induced by (+/-)8-OH-DPAT was significantly attenuated in dorsal raphe nucleus after fluoxetine (+3.7 vs. +31.2% in vehicle). The inhibition of dorsal raphe firing by (+/-)8-OH-DPAT (ED(50) in vehicle = 2.1 microg/kg, i.v.) was also attenuated in rats treated with fluoxetine (ED(50)=4.7 microg/kg). In contrast, a significant increase on (+/-)8-OH-DPAT-induced stimulation of [(35)S]GTPgammaS binding was observed in CA(1) (+53.4 vs.+20.2% in vehicle) and dentate gyrus (+105.7 vs. +52.6% in vehicle) but not in entorhinal cortex. Our data demonstrate that fluoxetine-induced desensitization of 5-HT(1A) autoreceptors occurs at G protein level. Moreover, a relevant finding is the region-specific hypersensitivity of postsynaptic 5-HT(1A) receptors, in the hippocampus but not in entorhinal cortex, following chronic fluoxetine. These differential adaptive changes in brain 5-HT(1A) receptors could underlie the mechanism of action of antidepressants and also contribute to their clinical effects.     

Role of the medial prefrontal cortex in 5-HT1A receptor-induced inhibition of 5-HT neuronal activity in the rat

1. We examined the involvement of the frontal cortex in the 5-HT2A receptor-induced inhibition of 5-HT neurones in the dorsal raphe nucleus (DRN) of the anaesthetized rat using single-unit recordings complemented by Fos-immunocytochemistry. 2. Both transection of the frontal cortex as well as ablation of the medial region of the prefrontal cortex (mPFC) significantly attenuated the inhibition of 5-HT neurones induced by systemic administration of the 5-HT1A receptor agonist, 8-OH-DPAT (0.5-16 microg kg(-1), i.v.). In comparison, the response to 8-OH-DPAT was not altered by ablation of the parietal cortex. The inhibitory effect of 8-OH-DPAT was reversed by the 5-HT1A receptor antagonist, WAY 100635 (0.1 mg kg(-1), i.v.) in all neurones tested. 3. In contrast, cortical transection did not alter the sensitivity of 5-HT neurones to iontophoretic application of 8-OH-DPAT into the DRN. Similarly, cortical transection did not alter the sensitivity of 5-HT neurones to systemic administration of the selective 5-HT reuptake inhibitor, paroxetine (0.1-0.8 mg kg(-1) , i.v.). 4. 8-OH-DPAT evoked excitation of mPFC neurones at doses (0.5-32 microg kg(-1), i.v.) in the range of those which inhibited 5-HT cell firing. At higher doses (32-512 microg kg(-1), i.v.) 8-OH-DPAT inhibited mPFC neurones. 8-OH-DPAT (0.1 mg kg(-1), s.c.) also induced Fos expression in the mPFC. The neuronal excitation and inhibition, as well as the Fos expression, were antagonized by WAY 100635. 5. These data add further support to the view that the inhibitory effect of 5-HT1A receptor agonists on the firing activity of DRN 5-HT neurones involves, in part, activation of a 5-HT1A receptor-mediated postsynaptic feedback loop centred on the mPFC.     

R-citalopram counteracts the antidepressant-like effect of escitalopram in a rat chronic mild stress model

The selective serotonin (5-HT) reuptake inhibitor, citalopram, is a racemic mixture of the stereoisomers, S-(+)-citalopram (escitalopram) and R-(-)-citalopram (R-citalopram). R-citalopram has been shown to counteract the 5-HT enhancing properties of escitalopram in acute studies in animals. In the present study we report, for the first time, on an interaction between R-citalopram and escitalopram after repeated dosing in a rat chronic mild stress (CMS) model of depression. The effect of escitalopram (2.0, 3.9 and 7.8 mg/kg per day), R-citalopram (7.8 mg/kg per day) and escitalopram 3.9 mg/kg per day plus R-citalopram 7.8 mg/kg per day were studied and compared to the effect of citalopram (8.0 mg/kg per day), imipramine and R-fluoxetine (8.9 mg/kg per day). Significant effects relative to a vehicle-treated group were achieved from week 1 for escitalopram (3.9 and 7.8 mg/kg per day), from week 2 for citalopram (8.0 mg/kg per day), from week 3 for R-fluoxetine (8.9 mg/kg per day) and from week 4 for escitalopram (2.0 mg/kg per day) and imipramine (8.9 mg/kg per day). R-citalopram (7.8 mg/kg per day) and escitalopram (3.9 mg/kg per day) plus R-citalopram (7.8 mg/kg per day) did not differ significantly from vehicle. There were no drug-induced effects in non-stressed control groups. In conclusion, escitalopram showed a shorter time to response in the rat CMS model of depression than citalopram, which was faster acting than R-fluoxetine and imipramine. R-citalopram counteracted the effect of escitalopram. The mechanism of action of R-citalopram is, at the moment unclear, but may be relevant to the improved clinical antidepressant activity seen with escitalopram in comparison with citalopram, and may also indicate an earlier response to escitalopram compared to other selective serotonin reuptake inhibitors (SSRIs).     

Comparison of the effects of antidepressants on norepinephrine and serotonin concentrations in the rat frontal cortex: an in-vivo microdialysis study

The present study employed in-vivo microdialysis techniques in the freely moving rat to systematically compare the neurochemical effects of various antidepressant agents on extracellular concentrations of norepinephrine (NE) and serotonin (5-HT) in the frontal cortex. We found that acute administration of the tricyclic antidepressant, desipramine (3-30 mg/kg, s.c.) and the dual serotonin/norepinephrine reuptake inhibitor, venlafaxine (3-30 mg/kg, s.c.), produced dose-dependent and robust increases in cortical NE concentrations (498% and 403%, respectively). Conversely, acute injection of the selective serotonin reuptake inhibitors, fluoxetine (30 mg/kg, s.c.) and paroxetine (1-10 mg/kg, s.c.), did not alter forebrain NE concentrations. However, paroxetine did produce a significant increase in cortical NE concentrations (164%) when administered at 30 mg/kg. These changes in NE were not paralleled by 5-HT, which showed no increase following administration of desipramine, venlafaxine, paroxetine or fluoxetine. Combination treatment with the 5-HT1A receptor antagonist, WAY-100635 (0.3 mg/kg, s.c.), significantly enhanced extracellular 5-HT concentrations following venlafaxine (10 and 30 mg/kg), fluoxetine (30 mg/kg) and paroxetine (3-30 mg/kg). Alternatively, WAY-100635 produced no augmentation of the antidepressant-induced changes in extracellular NE. Collectively, these studies show that paroxetine, at low to intermediate doses, and fluoxetine are selective for 5-HT versus NE systems, whereas venlafaxine produces similar effects on both 5-HT and NE levels at the effective doses tested.     

Effect of chronic and acute administration of fluoxetine and its additive effect with morphine on the behavioural response in the formalin test in rats

Serotonergic systems are involved in the central regulation of nociceptive sensitivity. Fluoxetine, a selective inhibitor of the reuptake of serotonin (5-hydroxytryptamine, 5-HT), was administered orally (0.16, 0.32, 0.8 mg kg(-1) daily for 7 days), intraperitoneally (0.04, 0.08, 0.16 mg kg(-1) day(-1) for 7 days and a single dose of 0.32 mg kg(-1)) and intracerebroventricularly (10 microg/rat) to rats and nociceptive sensitivity was evaluated using the formalin test (50 microL of 2.5% formalin injected subcutaneously). The effect of fluoxetine was also studied in the presence of 5,7-dihydroxytryptamine creatinine sulfate (5,7-DHT) and after co-administration with morphine. Oral (0.8 mg kg(-1)), intraperitoneal (0.16 and 0.32 mg kg(-1)) and intracerebroventricular (10 microg/rat) fluoxetine induced antinociception in the late phase of the formalin test. Furthermore, intrathecal administration of 5-HT (100 microg/rat) induced an analgesic effect. The analgesic effect of fluoxetine (0.16 and 0.32 mg kg(-1), i.p.) and 5-HT (100 microg/rat, i.t.) was abolished by pre-treatment with 5,7-DHT (100 microg/rat, i.t.). In addition, the analgesic effect of 5-HT (100 microg/rat, i.t.) was decreased by pre-treatment with naloxone (2 mg kg(-1), i.p.). Morphine (5 mg kg(-1), i.p.) induced analgesia that was increased by fluoxetine (0.32 mg kg(-1), i.p.). These results suggest that fluoxetine has an antinociceptive effect in tonic inflammatory pain through functional alteration of the serotonergic system and also potentiates the analgesic effect of morphine.     

Potentiation by (-)Pindolol of the activation of postsynaptic 5-HT(1A) receptors induced by venlafaxine.

The increase of extracellular 5-HT in brain terminal regions produced by the acute administration of 5-HT reuptake inhibitors (SSRI's) is hampered by the activation of somatodendritic 5-HT(1A) autoreceptors in the raphe nuclei. The present in vivo electrophysiological studies were undertaken, in the rat, to assess the effects of the coadministration of venlafaxine, a dual 5-HT/NE reuptake inhibitor, and (-)pindolol on pre- and postsynaptic 5-HT(1A) receptor function. The acute administration of venlafaxine and of the SSRI paroxetine (5 mg/kg, i.v.) induced a suppression of the firing activity of dorsal hippocampus CA(3) pyramidal neurons. This effect of venlafaxine was markedly potentiated by a pretreatment with (-)pindolol (15 mg/kg, i.p.) but not by the selective beta-adrenoceptor antagonist metoprolol (15 mg/kg, i.p.). That this effect of venlafaxine was mediated by an activation of postsynaptic 5-HT(1A) receptors was suggested by its complete reversal by the 5-HT(1A) antagonist WAY 100635 (100 microg/kg, i.v.). A short-term treatment with VLX (20 mg/kg/day x 2 days) resulted in a ca. 90% suppression of the firing activity of 5-HT neurons in the dorsal raphe nucleus. This was prevented by the coadministration of (-)pindolol (15 mg/kg/day x 2 days). Taken together, these results indicate that (-)pindolol potentiated the activation of postsynaptic 5-HT(1A) receptors resulting from 5-HT reuptake inhibition probably by blocking the somatodendritic 5-HT(1A) autoreceptor, but not its postsynaptic congener. These results support and extend previous findings providing a biological substratum for the efficacy of pindolol as an accelerating strategy in major depression.     

Effects of acute and chronic tianeptine administration on serotonin outflow in rats: comparison with paroxetine by using in vivo microdialysis

Using in vivo microdialysis, we compared the effects of tianeptine (an antidepressant drug which, in marked contrast with other antidepressants, is thought to increase the uptake of serotonin (5-hydroxytryptamine, 5-HT) on extracellular 5-HT concentrations ([5-HT](ext)) in the frontal cortex and raphe nuclei of freely moving rats with those of paroxetine, a potent selective serotonin reuptake inhibitor. A single paroxetine dose (1 mg/kg, i.p.) increased [5-HT](ext) over baseline in the frontal cortex and raphe nuclei, respectively. A single administration of tianeptine (10 mg/kg, i.p.) did not change [5-HT(ext)] in the two brain regions studied. Repeated exposure to paroxetine (0.5 mg/kg) b.i.d. for 14 days induced a sixfold significant increase in basal [5-HT](ext) in the raphe nuclei. Administration of tianeptine (5 mg/kg) b.i.d. for 14 days did not affect 5-HT baseline concentrations. In rats chronically treated with either paroxetine or tianeptine, drug challenge did not alter area under the curve values. Thus, our in vivo data indicate that tianeptine and paroxetine do not exert a similar in vivo effect on the serotonergic system in rat brain.     

Nicotine inhibits firing activity of dorsal raphe 5-HT neurones in vivo

It is established that the brain monoaminergic systems are among the main targets of several dependence-inducing drugs, including nicotine. In the present study extracellular electrophysiological recordings were performed to investigate the effects of nicotine on dorsal raphe 5-HT neurones. Nicotine, administered systemically (50-400 microg/kg, i.v.) in chloral hydrate-anaesthetised rats, induced a transient inhibition of the majority of 5-HT neurones recorded (38 of 45). The inhibition was rapid in onset (about 30 s) and the firing rate returned to baseline within 1-3 min. No apparent tachyphylaxis was observed to this inhibitory effect. The centrally acting nicotine antagonist mecamylamine (4 mg/kg, i.v.), but not the peripherally acting nicotine antagonist chlorisondamine (0.3 mg/kg, i.v.) antagonised the nicotine-induced inhibition of 5-HT neurones. The inhibition of 5-HT neurones was also blocked with a selective 5-HT1A receptor antagonist (WAY 100635; 0.1 mg/kg, i.v.), indicating a possible involvement of somato-dendritic 5-HT1A receptors in the effect of nicotine. Interestingly, microiontophoretic application of nicotine into the dorsal raphe failed to inhibit 5-HT neurones, suggesting an indirect effect of nicotine on 5-HT neurones, possibly involving afferent pathways.     

The pharmacology of citalopram enantiomers: the antagonism by R-citalopram on the effect of S-citalopram

Recent results on the in vivo and in vitro pharmacology of escitalopram are summarised. The exact molecular mechanism by which R-citalopram inhibits the effect of S-citalopram on the serotonin transporter remains to be elucidated. Preliminary evidence indicates an effect of R-citalopram on the association of escitalopram with the high affinity primary site, and on its dissociation from the serotonin transporter, via an allosteric mechanism. Escitalopram can be considered as an allosteric serotonin reuptake inhibitor. This serotonin dual action in binding to two sites on the serotonin transporter (both the primary site and the allosteric site) is hypothesised to be responsible for a longer binding to, and therefore greater inhibition of the serotonin transporter by escitalopram.     

Effects of the co-administration of mirtazapine and paroxetine on serotonergic neurotransmission in the rat brain.

The alpha(2)-adrenoreceptor antagonist mirtazapine, which is also a 5-HT(2), 5-HT(3) and H(1) receptors antagonist and the selective serotonin (5-HT) reuptake inhibitor paroxetine are effective antidepressant drugs which enhance 5-HT neurotransmission via different mechanisms. The present studies were undertaken to determine whether the mirtazapine-paroxetine combination could induce an earlier and/or a greater effect on the 5-HT system than either drug alone. Using in vivo electrophysiological paradigms, the firing activity of dorsal raphe 5-HT neurons was decreased by 70% in rats treated with paroxetine (10 mg/kg/day, s.c.) for 2 days and was back to normal after 21 days. In contrast, a 2-day treatment with mirtazapine (5 mg/kg/day, s.c.) did not alter the firing of 5-HT neurons whereas it was increased by 60% after 21 days of treatment. A low dose of mirtazapine (5 mg/kg/day, s.c.x2 days) failed to offset the decremental effect of paroxetine on the 5-HT neuron firing activity, but a higher dose (10 mg/kg/day, s.c.x2 days) did attenuate the decremental effect of paroxetine. In the dorsal hippocampus, neither mirtazapine (5 mg/kg/day, s.c.) nor a paroxetine (10 mg/kg/day, s.c.) treatment altered the responsiveness of 5-HT(1A) receptors to microiontophoretically-applied 5-HT. Both in controls and in rats treated for 2 days with paroxetine alone, the administration of the 5-HT(1A) antagonist WAY 100635 (25-100 microg/kg, i.v.) did not change the firing activity of dorsal hippocampus CA(3) pyramidal neurons. However, WAY 100635 increased significantly the firing activity of these neurons in rats treated with mirtazapine alone but to a greater extent with both mirtazapine and paroxetine for 2 days. After 21 days of treatment, WAY 100635 increased to a greater degree the firing rate of CA(3) pyramidal neurons in rats which received the combination over rats given either drug alone. It is concluded that the mirtazapine-paroxetine combination shortened the delay in enhancing the tonic activation of postsynaptic 5-HT(1A) receptors and produced a greater activation of the postsynaptic 5-HT(1A) receptors than either drug given alone. The present results suggested that mirtazapine may have a faster onset of action than a SSRI, and that the co-administration of mirtazapine and paroxetine may accelerate the antidepressant response and as well as being more effective than either drug alone.     

Effects of (-)-tertatolol, (-)-penbutolol and (+/-)-pindolol in combination with paroxetine on presynaptic 5-HT function: an in vivo microdialysis and electrophysiological study.

The antidepressant efficacy of selective serotonin reuptake inhibitors (SSRIs) might be enhanced by co-administration of 5-HT1A receptor antagonists. Thus, we have recently shown that the selective 5-HT1A receptor antagonist, WAY 100635, blocks the inhibitory effect of an SSRI on 5-HT cell firing, and enhances its ability to elevate extracellular 5-HT in the forebrain. Here we determined whether the beta-adrenoceptor/5-HT1A receptor ligands (+/-)-pindolol, (-)-tertatolol and (-)-penbutolol, interact with paroxetine in a similar manner. Both (-)-tertatolol (2.4 mg kg(-1) i.v.) and (-)-penbutolol (2.4 mg kg(-1) i.v.) enhanced the effect of paroxetine (0.8 mg kg(-1) i.v.) on extracellular 5-HT in the frontal cortex, whilst (+/-)-pindolol (4 mg kg(-1) i.v.) did not. (-)-Tertatolol (2.4 mg kg(-1) i.v.) alone caused a slight increase in 5-HT however, (-)-penbutolol (2.4 mg kg(-1) i.v.) alone had no effect. In electrophysiological studies (-)-tertatolol (2.4 mg kg(-1) i.v.) alone had no effect on 5-HT cell firing but blocked the inhibitory effect of paroxetine. In contrast, (-)-penbutolol (0.1-0.8 mg kg(-1) i.v.) itself inhibited 5-HT cell firing, and this effect was reversed by WAY 100635 (0.1 mg kg(-1) i.v.). We have recently shown that (+/-)-pindolol inhibits 5-HT cell firing via a WAY 100635-sensitive mechanism. Our data suggest that (-)-tertatolol enhances the effect of paroxetine on forebrain 5-HT via blockade of 5-HT1A autoreceptors which mediate paroxetine-induced inhibition of 5-HT cell firing. In comparison, the mechanisms by which (-)-penbutolol enhances the effect of paroxetine on extracellular 5-HT is unclear, since (-)-penbutolol itself appears to have agonist properties at the 5-HT1A autoreceptor. Indeed, the agonist action of (+/-)-pindolol at 5-HT1A autoreceptors probably explains its inability to enhance the effect of paroxetine on 5-HT in the frontal cortex. Overall, our data suggest that both (-)-tertatolol and (-)-penbutolol are superior to (+/-)-pindolol in terms of enhancing the effect of an SSRI on extracellular 5-HT. Both (-)-tertatolol and (-)-penbutolol are worthy of investigation for use as adjuncts to SSRIs in the treatment of major depression.     

In vivo actions of the selective 5-HT1A receptor agonist BAY x 3702 on serotonergic cell firing and release

We investigated the effects of the novel 5-HT1A receptor agonist BAY x 3702 on the serotonergic function in rat brain using single unit recordings in the dorsal raphe nucleus (DR) of anesthetized rats and in vivo microdialysis in freely moving rats. The administration of BAY x 3702 (0.25-4 microg/kg i.v.) suppressed the firing activity of 5-HT neurones. This effect was antagonized by a low dose of the selective 5-HT1A receptor antagonist WAY 100635 (5 microg/kg i.v.). In microdialysis experiments, BAY x 3702 (10-100 microg/ kg s.c.) reduced dose-dependently the 5-HT output in the dorsal and median raphe (MnR) nucleus, dorsal hippocampus (DHPC) and medial prefrontal cortex (mPFC) in a regionally selective manner. Maximal effects were observed in the MnR and mPFC, with reductions to approximately 15% of baseline at a dose of 0.1 mg/kg s.c. The decrease in 5-HT output produced in the DR and DHPC was more moderate, to 45% of baseline at 0.1 mg/kg s.c. BAY x 3702. WAY 100635 (0.3 mg/kg s.c.) completely antagonized the effect of BAY x 3702 (30 microg/kg s.c.). The application of BAY x 3702 in the DR (1-100 microM) reduced the local 5-HT output to 25% of baseline. In rats implanted with two dialysis probes (in DR and mPFC) the application of BAY x 3702 (30 microM) in the DR reduced the 5-HT output in the DR and that in mPFC. These effects were significantly antagonized by the co-perfusion of WAY 100635 (100 microM) in the DR. Overall, these results indicate that the systemic administration of BAY x 3702 reduces the 5-HT release with high potency through the activation of midbrain 5-HT1A receptors.     

Diabetes attenuates the antidepressant-like effect mediated by the activation of 5-HT1A receptor in the mouse tail suspension test.

Several lines of evidence have indicated that the prevalence of depression in diabetic subjects is higher than that in the general population, however, little information is available on the effects of antidepressants in diabetes. In the present study, the antidepressant-like effect mediated by the activation of 5-HT(1A) receptors was examined using the tail suspension test in streptozotocin-induced diabetic mice. Long-lasting increases in 5-HT turnover rates were observed in the diabetic mouse midbrain and frontal cortex, but not in the hippocampus. Duration of immobility was significantly longer in diabetic than in nondiabetic mice in the tail suspension test. The 5-HT(1A) receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (3-30 microg/kg, i.p.) reduced the duration of immobility in nondiabetic mice, and this effect was completely antagonized by pretreatment with N-[2-[4-(2-methoxyphenil)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY-100635) (30 microg/kg, s.c.), a selective 5-HT(1A) receptor antagonist. In contrast, 8-OH-DPAT (3 microg/kg-3 mg/kg, i.p.) was ineffective in diabetic mice. The selective 5-HT reuptake inhibitor fluoxetine (3-56 mg/kg, i.p.) reduced the duration of immobility in both nondiabetic and diabetic mice. However, fluoxetine was less effective in diabetic mice than in nondiabetic mice. WAY-100635 (30 microg/kg, s.c.) reversed the suppression of the duration of immobility by fluoxetine (30 mg/kg, i.p.) in nondiabetic mice. On the other hand, the anti-immobility effect of fluoxetine (56 mg/kg, i.p.) was not antagonized by WAY-100635 (30 microg/kg, s.c.) in diabetic mice. The selective 5-HT(2) receptor antagonist 6-methyl-1-(1-methylethyl)-ergoline-8beta-carboxylic acid 2-hydroxy-1-methylpropyl ester (LY53,857) (30 microg/kg, s.c.) reversed the anti-immobility effect of fluoxetine in both nondiabetic and diabetic mice. Spontaneous locomotor activity in diabetic mice was not different from that in nondiabetic mice. 8-OH-DPAT (30 microg/kg, i.p.), but not fluoxetine, increased the spontaneous locomotor activity in both nondiabetic and diabetic mice. The number of 5-HT(1A) receptors in the mouse frontal cortex was unaffected by diabetes. Plasma corticosterone levels in diabetic mice were significantly higher than that in nondiabetic mice. These results suggest that the antidepressant-like effect mediated by 5-HT(1A) receptors may be attenuated by diabetes.