Francesc Artigas

Acceleration of the effect of selected antidepressant drugs in major depression by 5-HT1A antagonists

At clinically relevant doses, selective serotonin (5-HT) reuptake inhibitors (SSRIs) and MAO inhibitors (MAOIs) increase the extracellular concentration of 5-HT in the midbrain raphé nuclei, thereby activating inhibitory somatodendritic 5-HT1A autoreceptors. Consequently, the firing activity of 5-HT neurons is reduced and the enhancement of extracellular 5-HT concentration in forebrain is dampened. Overriding this feedback by using antagonists of 5-HT1A autoreceptors permits SSRIs to produce a marked increase of extracellular 5-HT in the forebrain. Hence, combined treatment with an SSRI and a 5-HT1A antagonist increases the extracellular concentration of 5-HT more so than the former drug alone. The treatment of patients with major depression using an SSRI and pindolol, a 5-HT1A/ beta-adrenoceptor antagonist, markedly reduced the latency of the antidepressant response in previously untreated patients and induced a rapid improvement in treatment-resistant patients.

Randomised, double-blind, placebo-controlled trial of pindolol in combination with fluoxetine antidepressant treatment

BACKGROUND Major depression affects more than 5% of the population and is a serious health and economic problem. Antidepressants have a slow onset of action and are effective in less than two-thirds of patients. The biochemical effects of selective serotonin reuptake inhibitors may be limited by the negative feedback from serotonin autoreceptors. Pindolol is an antagonist of both serotonin autoreceptors and beta-adrenoceptors. We studied the effect of the addition of pindolol to fluoxetine antidepressant treatment. METHODS Of 132 eligible patients with major depression, 111 were randomly assigned to treatment with fluoxetine (20 mg daily) and either placebo or pindolol (7.5 mg daily). Patients were assessed twice a week for the first 3 weeks of active treatment and then once a week until the end of the study (42 days). Hamilton and Montgomery-Asberg depression-rating scales were used to assess depression severity. FINDINGS The proportion of patients who responded to treatment with fluoxetine and pindolol was greater than that with fluoxetine and placebo (41/55 [75%] vs 33/56 [59%], [90% CI 1.1-30.1], p = 0.04). The proportion of patients who achieved a sustained response was also greater in the fluoxetine and pindolol group than in the fluoxetine and placebo group (38/55 [69%] vs 27/56 [48%] [5.9-35.9], p = 0.03). The number of days to reach a sustained response was lower in the fluoxetine and pindolol group than in the fluoxetine and placebo group (median 19 vs 29 days, p = 0.01), however, there was no difference in the time-to-onset of clinical improvement when stringent conditions were used (15 vs 18 days, p = 0.20). INTERPRETATION The addition of pindolol to fluoxetine antidepressant treatment increases the effectiveness of fluoxetine therapy. Further work is needed to resolve whether the time to clinical improvement benefits from this combination and whether the increase in efficacy occurs with other antidepressants.

Fluvoxamine preferentially increases extracellular 5-hydroxytryptamine in the raphe nuclei: an in vivo microdialysis study

The effects of systemic administration of fluvoxamine on extracellular serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in the frontal cortex and raphe nuclei of freely moving rats were examined. Fluvoxamine significantly increased extracellular 5-HT concentrations in both regions at the two doses used (1 and 10 mg/kg i.p.). However, the increase in the raphe nuclei was several-fold that in the frontal cortex. Dialysate 5-HIAA concentrations decreased after treatment with fluvoxamine. These results confirm that 5-HT uptake inhibitors preferentially increase extracellular concentrations of 5-HT in the vicinity of cell bodies and dendrites of serotonergic neurones.

Involvement of 5-HT1A receptors in prefrontal cortex in the modulation of dopaminergic activity : Role in atypical antipsychotic action

Atypical antipsychotics increase dopamine (DA) release in the medial prefrontal cortex (mPFC), an effect possibly involved in the superior effects of atypical versus classical antipsychotics on cognitive/negative symptoms. We examined the role of 5-HT1A receptors in the mPFC on the modulation of dopaminergic activity and the mesocortical DA release in vivo. The highly selective 5-HT1A agonist BAY x 3702 (BAY; 10-40 μg/kg, i.v.) increased the firing rate and burst firing of DA neurons in the ventral tegmental area (VTA) and DA release in the VTA and mPFC. The increase in DA release in both areas was potentiated by nomifensine coperfusion. The selective 5-HT1A antagonist WAY-100635 reversed the effects of BAY in both areas, and the changes in the VTA were prevented by frontocortical transection. The application of BAY in rat and mouse mPFC by reverse dialysis increased local extracellular DA at a low concentration (3 μm) and reduced it at a higher concentration (30 μm). Both effects disappeared in 5-HT1A knock-out mice. In the presence of bicuculline, BAY reduced DA release at all concentrations. The atypical antipsychotics clozapine, olanzapine, and ziprasidone (but not haloperidol) enhanced DA release in the mPFC of wild-type but not 5-HT1A knock-out mice after systemic and local (clozapine and olanzapine) administration in the mPFC. Likewise, bicuculline coperfusion prevented the elevation of DA release produced by local clozapine or olanzapine application. These results suggest that the activation of mPFC 5-HT1A receptors enhances the activity of VTA DA neurons and mesocortical DA release. This mechanism may be involved in the elevation of extracellular DA produced by atypical antipsychotics.

Origin and functional role of the extracellular serotonin in the midbrain raphe nuclei.

There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.

Differential effects of clomipramine given locally or systemically on extracellular 5-hydroxytryptamine in raphe nuclei and frontal cortex. An in vivo brain microdialysis study

SummaryThe antidepressant drug clomipramine (CIM) blocks 5-hydroxytryptamine (5-HT) uptake in vitro. Electrophysiological studies have shown that CIM also reduces the firing of serotonergic neurons in the dorsal raphe nucleus. In order to assess the effects of CIM on serotonergic transmission in vivo, the technique of intracerebral microdialysis was used. CIM was administered either through the dialysis probe or i. p., and dialysate 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) were determined in frontal cortex and/or raphe nuclei. In addition, the action of extracellular 5-HT in raphe nuclei on the release of 5-HT in frontal cortex was studied.The administration of CIM through the dialysis probe increased dialysate 5-HT in frontal cortex in a dose-dependent fashion. An actual ED50 of 3.15 μM CIM for the in vivo inhibition of 5-HT uptake can be calculated in this brain area. When given systemically (10 or 20 mg/kg i. p.), CIM did not increase dialysate 5-HT in the frontal cortex. The occurrence of extracellular 5-HT in the raphe area was demonstrated. This pool of 5-HT increased markedly after local (10 or 40 μM) or systemic (20 mg/kg i. p.) administration of CIM. We also examined the effect of CIM applied locally in the raphe nuclei on extracellular 5-HT in the frontal cortex. The increased dialysate 5-HT in raphe after 10 or 40 μM CIM paralleled a decrease of dialysate 5-HT in the frontal cortex. Values of dialysate 5-HT in the two areas correlated negatively. The administration of CIM through the dialysis probe slightly decreased dialysate 5-HIAA in the frontal cortex. When given systemically, CIM also decreased dialysate 5-HIAA in the frontal cortex, but significantly only after the highest dose tested (20 mg/kg i.p.). Furthermore, the local application of CIM into the raphe nuclei produced a decrease of dialysate 5-HIAA in the frontal cortex.These results prove the in vivo inhibitory activity of CIM on 5-HT uptake. This effect is much more pronounced in the raphe nuclei than in the frontal cortex when CIM is given systemically. However, both areas are equally sensitive to the local application of the drug. The effect of CIM on raphe nuclei partly antagonizes that on the frontal cortex, providing evidence for a functional link between these two brain regions.

Comparative study in the rat of the actions of different types of stress on the release of 5-HT in raphe nuclei and forebrain areas.

The effects of several stress procedures on the release of 5-HT in the dorsal and median raphe nuclei (DRN and MRN, respectively) and in forebrain structures of the rat brain innervated by both nuclei have been studied using intracerebral microdialysis. Handling for 30 sec, a saline injection and forced swimming for 5 min elevated significantly the 5-HT output in the MRN. The 5-HT output in the DRN was also enhanced by a saline injection. With regard to the forebrain structure examined, handling and forced swimming increased dialysate 5-HT in the amygdala. The injection of saline induced a slight, but significant, elevation of 5-HT in the medial prefrontal cortex. In contrast, the outflow of 5-HT was significantly reduced in the ventral hippocampus and medial prefrontal cortex following forced swimming and this effect persisted well beyond the cessation of the swim session. These results indicate that the efflux of 5-HT in the MRN appears to respond to different forms of stress, whereas that in the DRN only increases after the injection of saline. The release of 5-HT in the forebrain structures is also dependent on the type of stress procedure and the region studied.

Effect of pindolol on the function of pre- and postsynaptic 5-HT1A receptors : In vivo microdialysis and electrophysiological studies in the rat brain

In microdialysis studies, somatodendritic 5-HT1A receptors in the dorsal raphe nucleus (DRN) were activated by the local infusion of 50 μM citalopram, a selective 5-HT reuptake inhibitor (SSRI). This reduced extracellular 5-HT by about 50% in dorsal striatum, an area receiving 5-HT afferents exclusively from the DRN. (−)Pindolol dose-dependently attenuated this citalopram-induced reduction of striatal extracellular 5-HT. Consistent with its 5-HT reuptake blocking properties, single doses of the SSRI paroxetine (1 and 3 mg/kg IP) and citalopram (1 mg/kg IP) significantly elevated extracellular 5-HT in the dorsal striatum. Pretreatment with (−)pindolol (15 mg/kg IP) potentiated the effect of 3 mg/kg paroxetine and 1 mg/kg citalopram on striatal extracellular 5-HT. A 2-day treatment with 10 mg/kg/day (SC) of paroxetine reduced by 60% the spontaneous activity of 5-HT neurons of the DRN. However, 5-HT neurons displayed normal activity in rats treated with paroxetine and (−)pindolol for 2 days. The inhibitory effect of LSD on 5-HT neuronal firing activity was also markedly attenuated in (−)pindolol-treated rats, indicating that somatodendritic 5-HT1A receptors were blocked by (−)pindolol. To determine whether (−)pindolol also blocked postsynaptic 5-HT1A receptors in hippocampus, 5-HT and the prototypical 5-HT1A agonist 8-OH-DPAT were applied by microiontophoresis onto CA3 pyramidal neurons following the same treatment. (−)Pindolol did not modify the responsiveness of these neurons to 5-HT and 8-OH-DPAT. Taken together, these results indicate that (−)pindolol can potentiate the effects of an SSRI on extracellular 5-HT concentration by preventing the activation of somatodendritic 5-HT1A autoreceptors resulting from the blockade of the 5-HT transporter in the raphe. This presumably leads to enhanced 5-HT neurotransmission because (−)pindolol would not alter the responsiveness of certain postsynaptic 5-HT1A receptors, such as those located on hippocampal CA3 pyramidal neurons. These results provide a neurobiological basis for the reported potentiation of certain antidepressant drugs by pindolol in major depression.

Role of 5-HT1A autoreceptors in the mechanism of action of serotoninergic antidepressant drugs : recent findings from in vivo microdialysis studies

Summary— Although a new generation of selective serotonin reuptake inhibitors (SSRIs) has been introduced in therapeutics as antidepressant drugs, a two to four week lag period still occurs between starting treatment with SSRIs and the onset of therapeutic effects in man. In vivo cerebral microdialysis can be used to measure extracellular concentrations of serotonin (5‐hydroxytryptamine, 5‐HT), which reflect intrasynaptic events. With the coupling of this new experimental method to very sensitive analytical assays such as liquid chromatography with electrochemical detection, it has recently been possible to obtain two major arguments supporting the hypothesis that somatodendritic 5‐HT1A autoreceptors situated in the raphe nuclei play an important role in the mechanism of action of SSRIs. First, in the rat, single administration of SSRIs at low doses comparable to those used therapeutically increases extracellular 5‐HT concentrations in the vicinity of the cell body and the dendrites of serotoninergic neurones of the raphe nuclei. This effect is more marked than that observed in regions rich in nerve endings (frontal cortex). The magnitude of the activation of the serotoninergic neurotransmission depends on the brain area studied and the dose of the SSRIs administered to rats. This could be explained by simultaneous activation of somatodendritic 5‐HT1A autoreceptors by endogenous 5‐HT in the raphe nuclei, thereby limiting the corticofrontal effects of the antidepressant. Second, SSRIs cause a larger increase in extracellular 5‐HT concentrations in the nerve endings when administered chronically: 5‐HT autoreceptors may have gradually desensitized during the 2–4 weeks of treatment with SSRIs. Preliminary studies of patients with depression appear to confirm these experimental results, as co‐administration of a 5‐HT1A autoreceptor antagonist and a SSRI accelerated the onset of the antidepressant effect (< 1 week).

The somatodendritic release of dopamine in the ventral tegmental area and its regulation by afferent transmitter systems

The release of dopamine in the ventral tegmental area (VTA) plays an important role in the autoinhibition of the dopamine neurons of the mesocorticolimbic system through the activation of somatodendritic dopamine D2 autoreceptors. Accordingly, the intra-VTA application of dopamine D2 receptor agonists reduces the firing rate and release of dopamine in the VTA, and this control appears to possess a tonic nature because the corresponding antagonists enhance the somatodendritic release of the transmitter. In addition, the release of dopamine in the VTA is increased by potassium or veratridine depolarization and abolished by tetrodotoxin and calcium omission. Overall, it appears that the somatodendritic release of dopamine is consistently lower than that in nerve endings. Apart from intrinsic dopaminergic mechanisms, other transmitter systems such as serotonin, noradrenaline, acetylcholine, GABA and glutamate play a role in the control of the activity of dopaminergic neurons of the VTA, although the final action depends on the particular receptor involved as well as the neuronal type where it is localized. Given the involvement of the mesocorticolimbic dopaminergic systems in the pathogenesis of severe neuropsychiatric disorders such as schizophrenia, the knowledge of the factors that regulate the release of dopamine in the VTA could provide new insight into the ethiogenesis of the disease as well as its implication on the mechanisms of action of therapeutic drugs.