Firing Activity Of Dorsal Raphe Research Articles

Pharmacological Blockade of 5-HT7 Receptors as a Putative Fast Acting Antidepressant Strategy

Current antidepressants still display unsatisfactory efficacy and a delayed onset of therapeutic action. Here we show that the pharmacological blockade of serotonin 7 (5-HT(7)) receptors produced a faster antidepressant-like response than the commonly prescribed antidepressant fluoxetine. In the rat, the selective 5-HT(7) receptor antagonist SB-269970 counteracted the anxiogenic-like effect of fluoxetine in the open field and exerted an antidepressant-like effect in the forced swim test. In vivo, 5-HT(7) receptors negatively regulate the firing activity of dorsal raphe 5-HT neurons and become desensitized after long-term administration of fluoxetine. In contrast with fluoxetine, a 1-week treatment with SB-269970 did not alter 5-HT firing activity but desensitized cell body 5-HT autoreceptors, enhanced the hippocampal cell proliferation, and counteracted the depressive-like behavior in olfactory bulbectomized rats. Finally, unlike fluoxetine, early-life administration of SB-269970, did not induce anxious/depressive-like behaviors in adulthood. Together, these findings indicate that the 5-HT(7) receptor antagonists may represent a new class of antidepressants with faster therapeutic action.

S01-02 - Faster and safer antidepressant-like response induced by pharmacological blockade of 5-HT7 receptors

ObjectivesIn term of efficacy and onset of action, current antidepressant drugs still suffer from important drawbacks. Hence, we recently assessed whether the pharmacological blockade of serotonin 7 receptors [5-hydroxytryptamine (5-HT7) receptors] can produce a faster and safer antidepressant response than fluoxetine (Prozac®), the most prescribed antidepressant.Methods and resultsIn rats, we showed that SB-269970, a selective 5-HT7 receptor antagonist, exerts an effective antidepressant-like effect as assessed in the forced swim test. Moreover, we revealed in vivo that 5-HT7 receptors negatively regulate the firing activity of dorsal raphe 5-HT neurons and become desensitised after a chronic treatment with fluoxetine. In contrast to fluoxetine, a one-week treatment with SB-269970 did not decrease 5-HT neuronal firing activity but desensitized 5-HT autoreceptors, enhanced the hippocampal cellular proliferation rate and counteracted the anxious/depressive-like behaviour that present the olfactory bulbectomized rat model. Finally, early life treatment with fluoxetine, but not with SB-269970, induced anxious/depressive-like behaviours in adulthood.ConclusionsThese results indicated that the 5-HT7 receptor participates in both mood regulation and the antidepressant effect of SSRIs, and that 5-HT7 receptor antagonists may represent a new class of antidepressant drugs with safer and faster therapeutic action.

Functional interactions between dopamine, serotonin and norepinephrine neurons: an in-vivo electrophysiological study in rats with monoaminergic lesions

Anatomical studies have established the existence of reciprocal relationships between the main population of monoamine, serotonin (5-HT), norepinephrine (NE) and dopamine (DA) neurons in the brain. The present study was thus conducted to examine the firing activity of 5-HT and NE neurons in DA-depleted rats, as well as the firing activity of DA neurons in 5-HT- or NE-depleted rats. The selective lesion of DA neurons elicited by 6-hydroxydopamine (6-OHDA) decreased the spontaneous firing activity of dorsal raphe (DR) nucleus 5-HT neurons by 60%, thus revealing the excitatory effect of the DA input on these 5-HT neurons. In contrast, the selective lesion of 5-HT neurons produced by 5,7-dihydroxytryptamine (5,7-DHT) enhanced by 36% the firing activity of VTA DA neurons, thereby indicating an inhibitory effect of the 5-HT input on these DA neurons. With regard to the reciprocal interaction between DA and NE neurons, it was observed that the selective loss of DA neurons achieved by the intra-ventral tegmental area (VTA) injection of 6-OHDA increased the firing activity of a subset of locus coeruleus (LC) NE neurons by 47%. The selective loss of NE neurons in response to the intra-LC injection of 6-OHDA enhanced the firing activity of VTA DA neurons by 70%, demonstrating a net inhibitory role of the NE input on VTA DA neurons. These findings have important consequences for antidepressant treatments aimed at enhancing simultaneously 5-HT, NE and DA transmission. Indeed, based on the understanding of such interactions, it may be possible to develop strategies to improve the effectiveness of antidepressant drugs by preventing counter-productive negative feedback actions.

Allosteric modulation of the effects of the 5-HT reuptake inhibitor escitalopram on the rat hippocampal synaptic plasticity

The present in vivo electrophysiological studies in anesthetized rat were undertaken to assess the effects of the selective serotonin (5-HT) reuptake inhibitor (SSRI) escitalopram alone or in combination with the R-citalopram (the S- and R-enantiomers of citalopram), on both long-term potentiation (LTP) in the CA 1 region of dorsal hippocampus and spontaneous firing activity of dorsal raphe (DR) 5-HT neurons. At the postsynaptic level, neither escitalopram (10 mg/kg, i.p.) nor R-citalopram (20 mg/kg, i.p.) modified basal synaptic transmission but only escitalopram impaired LTP expression. Importantly, R-citalopram counteracted significantly the escitalopram-induced decrease of LTP. At the pre-synaptic level, escitalopram (25–75 μg/kg, i.v.) dose-dependently suppressed the spontaneous firing activity of DR 5-HT neurons and this suppressant effect was significantly prevented by a prior injection of R-citalopram (10 mg/kg, i.p.). These results support a role of allosteric binding sites of 5-HT transporter in the regulation of long-lasting CA 1 synaptic plasticity and DR 5-HT neuronal firing activity.

In vivo modulation of the activity of pyramidal neurons in the rat medial prefrontal cortex by 5-HT2A receptors: relationship to thalamocortical afferents.

The activation of 5-HT(2A) receptors in medial prefrontal cortex (mPFC) by the hallucinogen DOI increases the firing activity of dorsal raphe (DR) 5-HT neurons and prefrontal 5-HT release. Here we show that the i.v. administration of DOI markedly affected the firing rate of identified pyramidal neurons recorded extracellularly. DOI excited (481%) 21/56 neurons, inhibited (11%) 17/56 neurons and left the rest unaffected (overall 2.4-fold increase in firing rate). Both effects were antagonized by 5-HT(2A) receptor blockade. 5-HT(2A)-mediated orthodromic excitations were recorded in pyramidal neurons projecting to DR after electrical stimulation of this nucleus. We also examined whether the effects of DOI in mPFC involve thalamic excitatory inputs. The disinhibition of the mediodorsal and centromedial nuclei of the thalamus by local bicuculline resembled the effects of DOI as it increased pyramidal cell firing and 5-HT release in mPFC. However, the selective activation of prefrontal micro -opioid and mGlu II receptors counteracted the effects of the thalamic disinhibition but not those of DOI. Moreover, extensive thalamic lesions did not alter the effect of DOI on pyramidal cell firing and 5-HT release. We conclude that DOI increases the activity of the mPFC-DR circuit by an action on postsynaptic 5-HT(2A) receptors unrelated to thalamocortical afferents.

Sustained blockade of neurokinin-1 receptors enhances serotonin neurotransmission

Background: Antagonists of neurokinin-1 (NK 1) receptors, through which substance P acts, have been proposed to belong to a new class of antidepressants with a unique mode of action. It was postulated that they exert this putative therapeutic effect independently of the serotonin (5-HT) neurons. Methods: The aim of the present study was to assess, using in vivo electrophysiological paradigms, the effects of sustained administration of the nonpeptidic NK 1 antagonist CP-96,345 on the firing activity of rat dorsal raphe 5-HT neurons, the responsiveness of pre- and postsynaptic 5-HT 1A receptors, and overall 5-HT neurotransmission in the hippocampus. Results: Both short- and long-term treatments with CP-96,345 significantly increased the spontaneous firing activity of dorsal raphe 5-HT neurons, and this increase was associated with an attenuation of somatodendritic 5-HT 1A autoreceptor responsiveness. In contrast, the inactive enantiomer of CP-96,345 at NK 1 receptors, CP-96,344, did not alter these parameters after short-term administration. Because 5-HT 1A receptor activation inhibits the firing activity of dorsal hippocampus CA 3 pyramidal neurons, the degree of disinhibition produced by the selective 5-HT 1A receptor antagonist WAY 100635 was determined to assess the net change in 5-HT neurotransmission. Intravenous injection of WAY 100635 did not disinhibit CA 3 pyramidal neuron firing in rats given saline, CP-96,345 for 2 days, or CP-96,344 for 14 days, but produced a significant enhancement of firing in rats treated with CP-96,345 for 2 weeks. Therefore, only long-term treatment with CP-96,345 enhanced the tonic activation of postsynaptic 5-HT 1A receptors. Conclusions: Similar to all other major types of antidepressant treatments, these data indicate that substance P antagonists might alleviate anxiety and major depression, at least in part, by enhancing the degree of activation of some 5-HT receptors in the forebrain.

Outcome predictors in bipolar disorder: a catamnestic study

The aim of the present study was to assess, using in vivo electrophysiological paradigms, the effect of sustained administration of the selective and reversible monoamine oxidase-A inhibitor beflotaxone on serotonin (5-hydroxytryptamine, 5-HT) neurotransmission. In male Sprague–Dawley rats with the osmotic minipumps in place, a treatment with befloxatone (0.75 mg/kg per day, s.c.) for 2 days decreased the spontaneous firing activity of dorsal raphe 5-HT neurons. The combination of befloxatone and the 5-HT1A/1B receptor antagonist (−)-pindolol (15 mg/kg per day, s.c.) for 2 days slightly increased the firing activity of 5-HT neurons, whereas a treatment with (−)-pindolol alone for 2 days did not modify this parameter. The suppressant effects on the firing activity of 5-HT neurons of the 5-HT autoreceptor agonist lysergic acid diethylamide (LSD), injected intravenously, and of both 5-HT and the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), applied by microiontophoresis, were attenuated in rats treated with befloxatone for 2 days, suggesting an early desensitization of the somatodendritic 5-HT1A receptors. The firing activity of 5-HT neurons was back to normal after a treatment for 21 days with befloxatone but the suppressant effects of LSD, 5-HT or 8-OH-DPAT was the same as in controls. In contrast, the suppressant effect of the α2-adrenoceptor agonist clonidine on the firing activity of 5-HT neurons was significantly attenuated after the treatment with befloxatone for 21 days. At the postsynaptic level, the administration of the selective 5-HT1A receptor antagonist (N-{2-[4(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl)cyclohexanecarboxamide trihydroxychloride (WAY 100635, 100 μg/kg, i.v.) did not modify the firing activity of quisqualate-activated dorsal hippocampus CA3 pyramidal neurons in control rats. In contrast, in rats treated with befloxatone in combination with (−)-pindolol for 2 days as well as with befloxatone alone for 21 days, WAY 100635 significantly increased the firing of CA3 pyramidal neurons. In conclusion, these data suggest that when the firing activity of 5-HT neurons is normal in the presence of befloxatone, either after a two-day treatment together with (−)-pindolol or alone for 21 days, the tonic activation of postsynaptic 5-HT1A receptors is enhanced.

Effects of sustained administration of the serotonin and norepinephrine reuptake inhibitor venlafaxine: I. In vivo electrophysiological studies in the rat

The effect of a 21-day treatment with the dual 5-HT and NE reuptake blocker venlafaxine (delivered s.c. by osmotic minipumps) was assessed on the time required for a 50% recovery (RT 50) of the firing activity of dorsal hippocampus CA 3 pyramidal neurons from the suppression induced by microiontophoretic applications of 5-HT and NE. The RT 50 values for 5-HT were increased by both 10 and 40 mg/kg/day regimens of venlafaxine, whereas those for NE were increased only by the 40 mg/kg/day regimen, indicative of a greater potency of venlafaxine in blocking 5-HT reuptake. The sensitivity of the postsynaptic 5-HT 1A and α 2-adrenergic receptors was altered by neither regimen of venlafaxine. Using a paradigm by which the 5-HT 1A antagonist WAY 100635 can induce a dishinbition of firing activity of CA 3 pyramidal neurons, it was demonstrated that the high, but not the low, dose of venlafaxine led to an enhanced tonic activation of postsynaptic 5-HT 1A receptors in the dorsal hippocampus. The duration of the suppressant effect of the firing activity of CA 3 hippocampus pyramidal neurons produced by the electrical stimulation of the ascending 5-HT pathway was significantly reduced when the frequency of the stimulation was enhanced from 1 Hz to 5 Hz in control rats and in rats treated with 10 mg/kg/day, but not with 40 mg/kg/day of venlafaxine. Hence, venlafaxine induced a desensitization of the terminal 5-HT 1B autoreceptor only at the high dose. A 2-day treatment with 10 mg/kg/day of venlafaxine induced a suppression of the firing activity of 5-HT neurons of the dorsal raphe. The firing activity of these neurons was back to control level in rats that had been treated for 21 days with the same dose of venlafaxine. The suppressant effect of the i.v. administration of the 5-HT autoreceptor agonist LSD on the firing activity of dorsal raphe 5-HT neurons was reduced in rats that had been treated for 21 days with 10 mg/kg/day of venlafaxine. A 2-day treatment with 40 mg/kg/day of venlafaxine, unlike the 10 mg/kg/day regimen, induced a marked suppression of the firing activity of locus coeruleus NE neurons. However, in contrast to 5-HT neurons, NE neurons did not recover their firing activity after a 21-day treatment. Taken together, the results from this study indicate that the low dose of venlafaxine blocked selectively the reuptake of 5-HT, whereas the high dose blocked the reuptake of both 5-HT and NE. Moreover, an enhancement of serotonergic neurotransmission by venlafaxine was only achieved under conditions whereby the desensitization of the terminal 5-HT 1B autoreceptor is appended to that of the somatodendritic 5-HT 1A receptor.

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

The α 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.×2 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.×2 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 μg/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.

Modulation of the firing activity of rat serotonin and noradrenaline neurons by (±)pindolol

Background: (±)Pindolol is a β-adrenergic/5-HT 1A receptor antagonist used in combination with certain antidepressant drugs to accelerate the onset of the antidepressive response. Methods: The aim of the present study was to assess, using an in vivo electrophysiologic paradigm, the effect of (±)pindolol on the spontaneous firing activity of rat dorsal raphe serotonin (5-HT) and locus coeruleus noradrenaline (NA) neurons. Results: (±)Pindolol did not modify the firing activity of dorsal raphe 5-HT neurons at low doses (10 and 200 μg/kg, IV), but it prevented the suppressant effect of the 5-HT autoreceptor agonist lysergic acid diethylamide (LSD, 10 μg/kg, IV) but not that of the 5-HT 1A receptor 8-hydroxy- N,N-dipropyl-aminotetralin (8-OHDPAT, 5 μg/kg, IV). At a higher dose (500 μg/kg, IV), (±)pindolol decreased 5-HT neuronal firing and this effect was reversed by the selective 5-HT 1A receptor antagonist WAY 100635 (100 μg/kg, IV), suggesting that it could act as a partial 5-HT 1A autoreceptor agonist. In the locus coeruleus, the high dose of (±)pindolol decreased the firing activity of NA neurons and this effect was reversed by the 5-HT 2A receptor antagonist MDL 100907 (200 μg/kg, IV). Finally, both a lesion of NA neurons and the administration of MDL 100907 prevented the suppressant effect of (±)pindolol on the firing of 5-HT neurons. Conclusions: It is suggested that, at low doses, (±)pindolol acts as a somatodendritic 5-HT 1A autoreceptor antagonist whereas at a higher dose, it decreases the tonic excitatory input from NA neurons to 5-HT neurons.

Venlafaxine: discrepancy between in vivo 5-HT and NE reuptake blockade and affinity for reuptake sites.

Using an in vivo electrophysiological paradigm, venlafaxine and paroxetine displayed similar potency for suppressing the firing activity of dorsal raphe 5-HT neurons (ED50: 233 and 211 microg/kg i.v., respectively), while venlafaxine was three times less potent than desipramine (ED50: 727 and 241 microg/kg i.v., respectively) to suppress the firing activity of locus coeruleus NE neurons. The selective 5-HT1A receptor antagonist WAY 100635 (100 microg/kg, i.v.) reversed the suppressant effect of venlafaxine and paroxetine on the firing activity of 5-HT neurons and the alpha2-adrenoceptor antagonist piperoxane (1 mg/kg, i.v.) reversed those of venlafaxine and desipramine on the firing activity of NE neurons. The ED50 of venlafaxine on the firing activity of 5-HT neurons was not altered (ED50: 264 microg/kg) in noradrenergic-lesioned rats, while the suppressant effect of venlafaxine on the firing activity of NE neurons was greater in serotonergic-lesioned rats (ED50: 285 microg/kg). Taken together, these results suggest that, in vivo, venlafaxine blocks both reuptake processes, its potency to block the 5-HT reuptake process being greater than that for NE. Since the affinities of venlafaxine for the 5-HT and NE reuptake carriers are not in keeping with its potencies for suppressing the firing activity of 5-HT and NE neurons, the suppressant effect of venlafaxine on the firing activity of 5-HT and NE neurons observed in vivo may not be mediated solely by its action on the [3H]cyanoimipramine and [3H]nisoxetine binding sites. In an attempt to unravel the mechanism responsible for this peculiarity, in vitro superfusion experiments were carried out in rat brain slices to assess a putative monoamine releasing property for venlafaxine. (+/-)Fenfluramine and tyramine substantially increased the spontaneous outflow of [3H]5-HT and [3H]NE, respectively, while venlafaxine was devoid of such releasing properties.

Electrophysiological examination of the effects of sustained flibanserin administration on serotonin receptors in rat brain

5-HT1A receptor agonists have proven to be effective antidepressant medications, however they suffer from a significant therapeutic lag before depressive symptoms abate. Flibanserin is a 5-HT1A receptor agonist and 5-HT2A receptor antagonist developed to possibly induce a more rapid onset of antidepressant action through its preferential postsynaptic 5-HT1A receptor agonism. Flibanserin antagonized the effect of microiontophoretically-applied DOI in the medial prefrontal cortex (mPFC) following 2 days of administration, indicating antagonism of postsynaptic 5-HT2A receptors. This reduction in the effect of locally-applied DOI was no longer present following 7-day flibanserin administration. Two-day flibanserin administration only marginally reduced the firing activity of dorsal raphe (DRN) 5-HT neurons. Following 7 days of administration, 5-HT neuronal firing activity had returned to normal and the somatodendritic 5-HT1A autoreceptors were desensitized. The responsiveness of postsynaptic 5-HT1A receptors located on CA3 hippocampus pyramidal neurons and mPFC neurons, examined using microiontophoretically-applied 5-HT and gepirone, was unchanged following a 7-day flibanserin treatment. As demonstrated by the ability of the 5-HT1A receptor antagonist WAY 100635 to selectively increase the firing of hippocampal neurons in 2- and 7-day treated rats, flibanserin enhanced the tonic activation of postsynaptic 5-HT1A receptors in this brain region. The results suggest that flibanserin could be a therapeutically useful compound putatively endowed with a more rapid onset of antidepressant action.

Acute and long-term actions of the antidepressant drug mirtazapine on central 5-HT neurotransmission

Mirtazapine (ORG 3770, Remeron ®) is a new α 2-adrenoceptor antagonist which has been shown to be an effective antidepressant drug. The aims of the studies were to assess, using an in vivo electrophysiological paradigm in the rat, the effects of acute and long-term treatment with mirtazapine on pre- and postsynaptic α 2-adrenoceptors and to determine whether this drug could modulate serotonin (5-HT) neurotransmission. Acute administration of mirtazapine produced a transient increase of the firing activity of dorsal raphe 5-HT neurons. This effect was mediated via norepinephrine (NE) neurons because it was abolished in NE-lesioned rats. In fact, this increased firing rate of 5-HT neurons was due to their activation by the enhanced release of NE resulting from the blockade of α 2-adrenergic autoreceptors of locus coeruleus neurons. Furthermore, acute mirtazapine injection transiently enhanced the firing activity of locus coeruleus NE neurons and attenuated the suppressant effect of the α 2-adrenoceptor agonist clonidine on these NE neurons. Sustained administration of mirtazapine for 21 days (5 mg/kg/day, s.c., using minipumps) lead to a marked increase in the firing rate of 5-HT neurons (75%) but a more modest increase in the firing rate of NE neurons (30%), as well as to a desensitization of α 2-adrenergic heteroreceptors on 5-HT terminals in the hippocampus. The desensitization of these heteroreceptors, resulting from an increased synaptic availability of NE induced by mirtazapine would free 5-HT terminals from the inhibitory influence of NE on 5-HT release. These modifications of 5-HT neurons lead to an increased tonic activation of postsynaptic 5-HT 1A receptors. The latter conclusion was based on the capacity of the selective 5-HT 1A receptor antagonist WAY 100635 to enhance the firing activity of dorsal hippocampus CA 3 pyramidal neurons in mirtazapine-treated rats but not in controls. This enhanced 5-HT neurotransmission may underlie to the antidepressant effect of mirtazapine.

Effect of the reversible monoamine oxidase-A inhibitor befloxatone on the rat 5-hydroxytryptamine neurotransmission.

The aim of the present study was to assess, using in vivo electrophysiological paradigms, the effect of sustained administration of the selective and reversible monoamine oxidase-A inhibitor beflotaxone on serotonin (5-hydroxytryptamine, 5-HT) neurotransmission. In male Sprague-Dawley rats with the osmotic minipumps in place, a treatment with befloxatone (0.75 mg/kg per day, s.c.) for 2 days decreased the spontaneous firing activity of dorsal raphe 5-HT neurons. The combination of befloxatone and the 5-HT1A/1B receptor antagonist (-)-pindolol (15 mg/kg per day, s.c.) for 2 days slightly increased the firing activity of 5-HT neurons, whereas a treatment with (-)-pindolol alone for 2 days did not modify this parameter. The suppressant effects on the firing activity of 5-HT neurons of the 5-HT autoreceptor agonist lysergic acid diethylamide (LSD), injected intravenously, and of both 5-HT and the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), applied by microiontophoresis, were attenuated in rats treated with befloxatone for 2 days, suggesting an early desensitization of the somatodendritic 5-HT1A receptors. The firing activity of 5-HT neurons was back to normal after a treatment for 21 days with befloxatone but the suppressant effects of LSD, 5-HT or 8-OH-DPAT was the same as in controls. In contrast, the suppressant effect of the alpha2-adrenoceptor agonist clonidine on the firing activity of 5-HT neurons was significantly attenuated after the treatment with befloxatone for 21 days. At the postsynaptic level, the administration of the selective 5-HT1A receptor antagonist (N-[2-[4(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xanecarboxamide trihydroxychloride (WAY 100635, 100 microg/kg, i.v.) did not modify the firing activity of quisqualate-activated dorsal hippocampus CA3 pyramidal neurons in control rats. In contrast, in rats treated with befloxatone in combination with (-)-pindolol for 2 days as well as with befloxatone alone for 21 days, WAY 100635 significantly increased the firing of CA3 pyramidal neurons. In conclusion, these data suggest that when the firing activity of 5-HT neurons is normal in the presence of befloxatone, either after a two-day treatment together with (-)-pindolol or alone for 21 days, the tonic activation of postsynaptic 5-HT1A receptors is enhanced.

Effects of long-term treatment with the alpha 2-adrenoceptor antagonist mirtazapine on 5-HT neurotransmission.

Mirtazapine (ORG 3770, Remeron) is a nonselective alpha 2-adrenoceptor antagonist with antidepressant activity in major depression. The aim of the present study was to assess, using an in vivo electrophysiological paradigm, the effect of long-term treatment with mirtazapine on pre- and postsynaptic alpha 2-adrenoceptors and on 5-HT neurotransmission in male Sprague-Dawley rats. A 21-day treatment with mirtazapine (5 mg/kg/day, s.c., using osmotic minipumps) increased the spontaneous firing activity of locus coeruleus noradrenaline (NA) neurons. Their firing activity was back to normal 48 h after removing the minipump. However, this treatment did not modify the dose-response curve of the suppressant effect of the alpha 2-adrenoceptor agonist clonidine on the firing activity of NA neurons. The spontaneous firing activity of dorsal raphe 5-HT neurons was also markedly increased in mirtazapine-treated rats, and was back to normal 48 h after removing of the minipump. The dose-response curve of the suppressant effect of clonidine on the firing activity of 5-HT neurons was altered in mirtazapine-treated rats. Furthermore, it was further shifted to the left after a 48-h washout. Long-term mirtazapine treatment did not modify the suppressant effects of microiontophoretically-applied NA and 5-HT on the firing activity of CA3 dorsal hippocampus pyramidal neurons. However, this mirtazapine treatment antagonized both the enhancing effect of a low dose (10 micrograms/kg, i.v.) and the reducing effect of a high dose (100 micrograms/kg, i.v.) of the alpha 2-adrenoceptor agonist clonidine on the effectiveness of the electrical stimulation of the ascending 5-HT pathway in suppressing the firing activity of dorsal hippocampus CA3 pyramidal neurons. After a 48-h washout, only the effect of the high dose of clonidine was attenuated, suggesting a desensitization of the terminal alpha 2-adrenergic heteroreceptor, but not of the terminal alpha 2-adrenergic autoreceptor. The decrease in the effectiveness of the stimulation upon increasing its frequency from 1 to 5 Hz (due to the activation of terminal 5-HT autoreceptors) was unaltered after the long-term mirtazapine treatment. In conclusion, the tonic activation of postsynaptic 5-HT receptor is enhanced by a 21-day treatment with mirtazapine, as a result of a sustained increase in 5-HT neuron firing activity in the presence of decreased function of alpha 2-adrenergic heteroreceptors located on 5-HT terminals in the dorsal hippocampus.

Autoregulatory properties of dorsal raphe 5-HT neurons: possible role of electrotonic coupling and 5-HT1D receptors in the rat brain.

In the present study, the hypothesis that somatodendritic availability of 5-hydroxytryptamine (5-HT) could be regulated independently of the firing activity of dorsal raphe 5-HT neurons was tested. The 5-HT pathway was electrically stimulated at the level of the ventromedial tegmentum and the ensuing action potentials, recorded in the dorsal raphe, met all criteria for antidromic invasion of 5-HT neurons. The latency of antidromic spikes was current-dependent and the changes in latency were of quantal nature. This observation suggests an electrotonic coupling between 5-HT neurons. Stimulation of the ventromedial tegmentum also induced a decrease in the probability of firing of 5-HT neurons. This reduction in 5-HT neuron firing activity is a 5-HT-mediated response, due to an increased bioavailability of the neurotransmitter in the biophase of somatodendritic 5-HT1A autoreceptors. The intravenous administration of the 5-HT1 agonists TFMPP and RU 24969 reduced the duration of suppression of firing induced by the 5-HT-pathway stimulation, without altering the spontaneous firing rate of 5-HT neurons. The effect of TFMPP and RU 24969 on duration of suppression was blocked by (+-)mianserin, a drug with high affinity for the rat 5-HT1D, but not 5-HT1B, receptors. On the other hand, (-)propranolol, a mixed 5-HT antagonist also blocked the effect of TFMPP. However, the selective 5-HT1A antagonist (+)WAY 100135 did not alter the effect of TFMPP. These results, in keeping with previous anatomical studies, suggest the existence of electrotonic coupling of 5-HT neurons and indicate that 5-HT release in the rat dorsal raphe nucleus may be controlled independently of firing-regulating 5-HT1A autoreceptors. They also suggest that 5-HT1D receptors may play a role in this regulatory function of 5-HT neurons.

Pre- and post-synaptic effects of the 5-HT3 agonist 2-methyl-5-HT on the 5-HT system in the rat brain.

Microiontophoretic applications of 5-HT and of the 5-HT3 agonist 2-methyl-5-HT produced a current-dependent suppression of firing activity of both hippocampal (CA1 and CA3) and cortical neurons in anesthetized rats. Concomitant microiontophoretic applications of the 5-HT3 antagonists BRL 46470A and S-zacopride, as well as their intravenous injection, did not antagonize the inhibitory effect of 5-HT and 2-methyl-5-HT. In contrast, the 5-HT1A antagonist BMY 7378, applied by microiontophoresis or administered intravenously, significantly reduced the inhibitory action of 5-HT and 2-methyl-5-HT. The firing activity of dorsal raphe 5-HT neurons was also reduced by 5-HT, 2-methyl-5-HT and the 5-HT1A agonist 8-OH-DPAT applied by microiontophoresis. While BRL 46470A (0.1 and 1 mg/kg, i.v.) did not antagonize the inhibitory effect of the three 5-HT agonists on 5-HT neuronal firing activity, only that of 8-OH-DPAT was attenuated by the 5-HT1A antagonist (+) WAY 100135. R-zacopride significantly reduced the duration of suppression of firing activity of CA3 pyramidal neurons induced by the electrical stimulation of the ascending 5-HT pathway, and this reducing effect was prevented by the three 5-HT3/5-HT4 antagonists renzapride, S-zacopride and tropisetron, but not by BRL 46470A. Finally, in in vitro superfusion experiments, both BRL 46470A and S-zacopride antagonized the enhancing action of 2-methyl-5-HT on the electrically-evoked release of [3H]-5-HT in both rat frontal cortex and hippocampus slices. These findings suggest that, in vivo, the suppressant effect of 2-methyl-5-HT on the firing activity of dorsal hippocampus pyramidal, somatosensory cortical, and dorsal raphe 5-HT neurons is not mediated by 5-HT3 receptors, but rather by 5-HT1A receptors. The attenuating effect of R-zacopride on the effectiveness of the stimulation of the ascending 5-HT pathway is not mediated by 5-HT3 receptors. In contrast, in vitro, the enhancing action of 2-methyl-5-HT on the electrically-evoked release of [3H]5-HT in both frontal cortex and hippocampus slices is mediated by 5-HT3 receptors.

Effect of repeated electroconvulsive shocks on serotonergic neurons

The hypothermia induced by the serotonin (5-HT) 1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was attenuated in rats that had received a cource of six electroconvulsive shocks (ECS) over a two-week period. The firing activity of dorsal raphe 5-HT neurons, as well as their responsiveness to microintophoretic applications of 5-HT and 8-OH-DPAT, was unaltered in ECS-treated rats. The electrically evoked overflow of [ 3H]5-HT from preloaded slices of guinea pig hypothalamus was unchanged after the same ECS treatment. The concentration-effect curves of the 5-HT autoreceptor agonist 5-carboxyamindotryptamine (0.1–100 nM) were similar in slices prepared from control and ECS-treated guinea pigs. In addition, the reduction in the evoked [ 3H]5-HT overflow obtained by increasing the stimulation frequency from 1 to 5 Hz, which is due to a greater activation of terminal 5-HT autoreceptors at the higher frequency, was not altered by the ECS treatment. The enhancing effects of the 5-HT autoreceptor antagonist methiothepin (0.1–1 μM) and of the 5-HT 3 (0.1–1 μM) on the evoked [ 3H]5-HT overflow were unaltered by the ECS treatment. These results thus indicate that repeated ECS attenuates the 8-OH-DPAT- induced hypothermia in rats, as previously reported, but does not affect the firing activity of 5-HT neurons and the sensitivity of their somatodendritic 5-HT 1A autoreceptors in the dorsal raphe. The function of 5-HT terminals in the guinea pig hypothalamus was also unaffected by repeated ECS. In conclusion, repeated ECS does not affect the function of 5-HT neurons at the cell body and nerve terminal.

Differential Effect of Gepirone on Presynaptic and Postsynaptic Serotonin Receptors

The sustained administration of the serotonin (5-hydroxytryptamine1A, 5-HT1A) agonist gepirone (15 mg/kg/day subcutaneously) in the rat produced an initial decrease of the firing activity of dorsal raphe 5-HT neurons, which was followed by a progressive recovery to normal after 14 days of treatment. At this point, the somatodendritic 5-HT1A autoreceptor had desensitized, as indicated by the reduced effectiveness of intravenous lysergic acid diethylamide (LSD) and of microiontophoretic applications of 5-HT, LSD, 8-hydroxy-2-(N,N-propylamino) tetralin (8-OH-DPAT), and gepirone, but not of gamma-aminobutyric acid in depressing the firing activity of 5-HT neurons. In contrast, the responsiveness of postsynaptic dorsal hippocampus pyramidal neurons to 5-HT, 8-OH-DPAT, and gepirone was not altered by the 14-day gepirone treatment. In an attempt to unravel the differential effect of sustained gepirone administration on presynaptic and postsynaptic 5-HT1A receptors, the properties of gepirone at these two receptors were assessed. The concurrent microiontophoretic application of gepirone readily blocked the effect of 5-HT on dorsal hippocampus pyramidal neurons, but not on dorsal raphe 5-HT neurons, thus indicating that gepirone is a partial agonist at postsynaptic 5-HT1A receptors and a full agonist at somatodendritic 5-HT1A receptors. It is proposed that gepirone, being a partial agonist at postsynaptic 5-HT1A receptors, fails to desensitize them; whereas, because of its full agonistic activity at the somatodendritic 5-HT1A receptor, it desensitizes this autoreceptor with long-term administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological assessment of putative antagonists of 5-hydroxytryptamine receptors: a single-cell study in the rat dorsal raphe nucleus

The intravenous administration of low doses of lysergic acid diethylamide (LSD) or of the selective 5-hydroxytryptamine1A (5-HT1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) depresses the firing activity of dorsal raphe 5-HT-containing neurons, presumably via the activation of 5-HT1A receptors. The present studies were undertaken to determine the effect of different types of 5-HT receptor antagonists on this effect of LSD and 8-OH-DPAT. (-)-Propranolol (2 mg/kg i.v.), methiothepin (2 mg/kg i.p., twice daily for 4 days followed by an additional dose of 2 mg/kg i.p., prior to the experiment), pelanserine (0.5 mg/kg i.v.), and indorenate (125 micrograms/kg i.v.) failed to block the effects of either LSD or 8-OH-DPAT on the firing activity of 5-HT neurons of the dorsal raphe nucleus. However, spiperone (1 mg/kg i.v.) significantly reduced the effect of both LSD and 8-OH-DPAT. These results indicate that, among the five putative 5-HT receptor antagonists tested, only spiperone can antagonize the suppressant effect of 5-HT receptor agonists on the firing of dorsal raphe 5-HT neurons.