Analía Bortolozzi

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.

The activation of 5-HT2A receptors in prefrontal cortex enhances dopaminergic activity

Atypical antipsychotics show preferential 5‐HT2A versus dopamine (DA) D2 receptor affinity. At clinical doses, they fully occupy cortical 5‐HT2 receptors, which suggests a strong relationship with their therapeutic action. Half of the pyramidal neurones in the medial prefrontal cortex (mPFC) express 5‐HT2A receptors. Also, neurones excited through 5‐HT2A receptors project to the ventral tegmental area (VTA). We therefore hypothesized that prefrontal 5‐HT2A receptors can modulate DA transmission through excitatory mPFC–VTA inputs. In this study we used single unit recordings to examine the responses of DA neurones to local (in the mPFC) and systemic administration of the 5‐HT2A/2C agonist 1‐[2,5‐dimethoxy‐4‐iodophenyl‐2‐aminopropane] (DOI). Likewise, using microdialysis, we examined DA release in the mPFC and VTA (single/dual probe) in response to prefrontal and systemic drug administration. The local (in the mPFC) and systemic administration of DOI increased the firing rate and burst firing of DA neurones and DA release in the VTA and mPFC. The increase in VTA DA release was mimicked by the electrical stimulation of the mPFC. The effects of DOI were reversed by M100907 and ritanserin. These results indicate that the activity of VTA DA neurones is under the excitatory control of 5‐HT2A receptors in the mPFC. These observations may help in the understanding of the therapeutic action of atypical antipsychotics.

Serotonin 5-HT1A Receptors as Targets for Agents to Treat Psychiatric Disorders: Rationale and Current Status of Research

Psychiatric disorders represent a large economic burden in modern societies. However, pharmacological treatments are still far from optimal. Drugs used in the treatment of major depressive disorder (MDD) and anxiety disorders (selective serotonin [5-HT] reuptake inhibitors [SSRIs] and serotonin-noradrenaline reuptake inhibitors [SNRIs]) are pharmacological refinements of first-generation tricyclic drugs, discovered by serendipity, and show low efficacy and slowness of onset. Moreover, antipsychotic drugs are partly effective in positive symptoms of schizophrenia, yet they poorly treat negative symptoms and cognitive deficits. The present article reviews the neurobiological basis of 5-HT1A receptor (5-HT1A-R) function and the role of pre- and postsynaptic 5-HT1A-Rs in the treatment of MDD, anxiety and psychotic disorders. The activation of postsynaptic 5-HT1A-Rs in corticolimbic areas appears beneficial for the therapeutic action of antidepressant drugs. However, presynaptic 5-HT1A-Rs play a detrimental role in MDD, since individuals with high density or function of presynaptic 5-HT1A-Rs are more susceptible to mood disorders and suicide, and respond poorly to antidepressant drugs. Moreover, the indirect activation of presynaptic 5-HT1A-Rs by SSRIs/SNRIs reduces 5-HT neuron activity and terminal 5-HT release, thus opposing the elevation of extracellular 5-HT produced by blockade of the serotonin transporter (SERT) in the forebrain. Chronic antidepressant treatment desensitizes presynaptic 5-HT1A-Rs, thus reducing the effectiveness of the 5-HT1A autoreceptor-mediated negative feedback. The prevention of this process by the non-selective partial agonist pindolol accelerates clinical antidepressant effects. Two new antidepressant drugs, vilazodone (marketed in the USA) and vortioxetine (in development) incorporate partial 5-HT1A-R agonist properties with SERT blockade. Several studies with transgenic mice have also established the respective role of pre- and postsynaptic 5-HT1A-Rs in MDD and anxiety. In agreement with pharmacological studies, presynaptic and postsynaptic 5-HT1A-R activation appears necessary for anxiolytic and antidepressant effects, respectively, yet, neurodevelopmental roles for 5-HT1A-Rs are also involved. Likewise, the use of small interference RNA has enabled the showing of robust antidepressant-like effects in mice after selective knock-down of 5-HT1A autoreceptors. Postsynaptic 5-HT1A-Rs in the prefrontal cortex (PFC) also appear important for the superior clinical effects of clozapine and other second-generation (atypical) antipsychotic drugs in the treatment of schizophrenia and related psychotic disorders. Despite showing a moderate in vitro affinity for 5-HT1A-Rs in binding assays, clozapine displays functional agonist properties at this receptor type in vivo. The stimulation of 5-HT1A-Rs in the PFC leads to the distal activation of the mesocortical pathway and to an increased dopamine release in PFC, an effect likely involved in the clinical actions of clozapine in negative symptoms and cognitive deficits in schizophrenia. The anxiolytic/antidepressant properties of 5-HT1A-R agonists in preclinical tests raised expectations enormously. However, these agents have achieved little clinical success, possibly due to their partial agonist character at postsynaptic 5-HT1A-Rs, together with full agonist properties at presynaptic 5-HT1A autoreceptors, as well as their gastrointestinal side effects. The partial 5-HT1A-R agonists buspirone, gepirone, and tandospirone are marketed as anxiolytic drugs, and buspirone is also used as an augmentation strategy in MDD. The development of new 5-HT1A-R agonists with selectivity for postsynaptic 5-HT1A-Rs may open new perspectives in the field.

In vivo actions of aripiprazole on serotonergic and dopaminergic systems in rodent brain

RationaleAripiprazole is an atypical antipsychotic drug with high in vitro affinity for 5-HT1A, 5-HT2A and dopamine (DA) D2 receptors. However, its in vivo actions in the brain are still poorly characterized.ObjectiveThe aim was to study the in vivo actions of aripiprazole in the rat and mouse brain.MethodsBrain microdialysis and single-unit extracellular recordings were performed.ResultsThe systemic administration of aripiprazole reduced 5-HT output in the medial prefrontal cortex (mPFC) and dorsal raphe nucleus of the rat. Aripiprazole also reduced extracellular 5-HT in the mPFC of wild-type (WT) but not of 5-HT1A (−/−) knockout (KO) mice. Aripiprazole reversed the elevation in extracellular 5-HT output produced by the local application of the 5-HT2A/2C receptor agonist DOI in mPFC. Aripiprazole also increased the DA output in mPFC of WT but not of 5-HT1A KO mice, as observed for atypical antipsychotic drugs, in contrast to haloperidol. Contrary to haloperidol, which increases the firing rate of DA neurons in the ventral tegmental area (VTA), aripiprazole induced a very moderate reduction in dopaminergic activity. Haloperidol fully reversed the inhibition in dopaminergic firing rate induced by apomorphine, whereas aripiprazole evoked a partial reversal that was significantly different from that evoked by haloperidol and from the spontaneous reversal of dopaminergic activity in rats treated with apomorphine.ConclusionsThese results indicate that aripiprazole modulates the in vivo 5-HT and DA release in mPFC through the activation of 5-HT1A receptors. Moreover, aripiprazole behaves as a partial agonist at DA D2 autoreceptors in vivo, an action which clearly distinguishes it from haloperidol.

In vivo efflux of serotonin in the dorsal raphe nucleus of 5-HT1A receptor knockout mice

In the dorsal raphe nucleus (DR), extracellular serotonin (5‐HT) regulates serotonergic transmission through 5‐HT1A autoreceptors. In this work we used in vivo microdialysis to examine the effects of stressful and pharmacological challenges on DR 5‐HT efflux in 5‐HT1A receptor knockout (5‐HT1A–/–) mice and their wild‐type counterparts (5‐HT1A+/+). Baseline 5‐HT concentrations did not differ between both lines of mice, which is consistent with a lack of tonic control of 5‐HT1A autoreceptors on DR 5‐HT release. (R)‐(+)‐8‐Hydroxy‐2‐(di‐n‐propylamino)tetralin hydrobromide (8‐OH‐DPAT, 0.5 mg/kg) reduced 5‐HT levels to 30% of basal values in 5‐HT1A+/+ mice, but not in 5‐HT1A–/– mice. The selective 5‐HT1B receptor agonist 1,4‐dihydro‐3‐(1,2,3,6‐tetrahydro‐4‐pyridinyl)‐5H‐pyrrolo[3,2‐b]pyridin‐5‐one dihydrochloride (CP 93129, 300 µm) reduced dialysate 5‐HT to the same extent (30–40% of baseline) in the two genotypes, which suggests a lack of compensatory changes in 5‐HT1B receptors in the DR of such mutant mice. Both a saline injection and handling for 3 min increased DR dialysate 5‐HT in mutants, but not in 5‐HT1A+/+ mice. Fluoxetine (5 and 20 mg/kg) elevated 5‐HT in a dose‐dependent manner in both genotypes. However, this effect was markedly more pronounced in the 5‐HT1A–/– mice. The increased responsiveness of the extracellular 5‐HT in the DR of 5‐HT1A receptor knockout mice reflects a lack of the autoinhibitory control exerted by 5‐HT1A autoreceptors.

Control of 5-Hydroxytryptamine Release in the Dorsal Raphe Nucleus by the Noradrenergic System in Rat Brain. Role of α -Adrenoceptors

The interactions between the brainstem serotonergic (5-hydroxytryptamine, 5-HT) and noradrenergic (NA) systems are important for the pathophysiology and treatment of affective disorders. We examined the influence of α-adrenoceptors on 5-HT and NA release in the rat dorsal raphe nucleus (DR) using microdialysis. 5-HT and NA concentrations in DR dialysates were virtually suppressed by TTX and increased by veratridine. The local and systemic administration of the α1-adrenoceptor antagonist prazosin reduced the DR 5-HT output but not that of NA. The maximal 5-HT reduction induced by local prazosin administration (−78% at 100 μM) was more marked than by its systemic administration (−43% at 0.3 mg/kg). The local application of NA and desipramine, to increase the tone on DR α1-adrenoceptors, did not enhance 5-HT release. The local (100 μM) or systemic (0.1–1 mg/kg s.c.) administration of clonidine reduced 5-HT and NA release (−48 and −79%, respectively, at 1 mg/kg), an effect reversed by RX-821002, which by itself increased both amines when given systemically. DSP-4 pretreatment prevented the effects of clonidine on 5-HT, suggesting the participation of α2-adrenoceptors on NA elements. Moreover, the systemic effect of clonidine on 5-HT (but not NA) was cancelled by lesion of the lateral habenula and by anesthesia, and was slightly enhanced by cortical transection. These data support the view that α1-adrenoceptors in the DR tonically stimulate 5-HT release, possibly at nearly maximal tone. Likewise, the 5-HT release is modulated by α2-adrenoceptors in NA neurons and in forebrain areas involved in the distal control of 5-HT neurons.

Selective siRNA-mediated suppression of 5-HT1A autoreceptors evokes strong anti-depressant-like effects.

Depression is a major health problem worldwide. Most prescribed anti-depressants, the selective serotonin reuptake inhibitors (SSRI) show limited efficacy and delayed onset of action, partly due to the activation of somatodendritic 5-HT1A-autoreceptors by the excess extracellular serotonin (5-HT) produced by SSRI in the raphe nuclei. Likewise, 5-HT1A receptor (5-HT1AR) gene polymorphisms leading to high 5-HT1A-autoreceptor expression increase depression susceptibility and decrease treatment response. In this study, we report on a new treatment strategy based on the administration of small-interfering RNA (siRNA) to acutely suppress 5-HT1A-autoreceptor-mediated negative feedback mechanisms. We developed a conjugated siRNA (C-1A-siRNA) by covalently binding siRNA targeting 5-HT1A receptor mRNA with the SSRI sertraline in order to concentrate it in serotonin axons, rich in serotonin transporter (SERT) sites. The intracerebroventricular (i.c.v.) infusion of C-1A-siRNA to mice resulted in its selective accumulation in serotonin neurons. This evoked marked anti-depressant-like effects in the forced swim and tail suspension tests, but did not affect anxiety-like behaviors in the elevated plus-maze. In parallel, C-1A-siRNA administration markedly decreased 5-HT1A-autoreceptor expression and suppressed 8-OH-DPAT-induced hypothermia (a pre-synaptic 5-HT1AR effect in mice) without affecting post-synaptic 5-HT1AR expression in hippocampus and prefrontal cortex. Moreover, i.c.v. C-1A-siRNA infusion augmented the increase in extracellular serotonin evoked by fluoxetine in prefrontal cortex to the level seen in 5-HT1AR knockout mice. Interestingly, intranasal C-1A-siRNA administration produced the same effects, thus opening the way to the therapeutic use of C-1A-siRNA. Hence, C-1A-siRNA represents a new approach to treat mood disorders as monotherapy or in combination with SSRI.

Dopamine release induced by atypical antipsychotics in prefrontal cortex requires 5-HT1A receptors but not 5-HT2A receptors

Atypical antipsychotic drugs (APDs) increase dopamine (DA) release in prefrontal cortex (PFC), an effect probably mediated by the direct or indirect activation of the 5-HT(1A) receptor (5-HT(1A)R). Given the very low in-vitro affinity of most APDs for 5-HT(1A)Rs and the large co-expression of 5-HT(1A)Rs and 5-HT(2A) receptors (5-HT(2A)Rs) in the PFC, this effect might result from the imbalance of 5-HT(1A)R and 5-HT(2A)R activation after blockade of these receptors by APDs, for which they show high affinity. Here we tested this hypothesis by examining the dependence of the APD-induced DA release in medial PFC (mPFC) on each receptor by using in-vivo microdialysis in wild-type (WT) and 5-HT(1A)R and 5-HT(2A)R knockout (KO) mice. Local APDs (clozapine, olanzapine, risperidone) administered by reverse dialysis induced a dose-dependent increase in mPFC DA output equally in WT and 5-HT(2A)R KO mice whereas the DA increase was absent in 5-HT(1A)R KO mice. To examine the relative contribution of both receptors to the clozapine-induced DA release in rat mPFC, we silenced G-protein-coupled receptors (GPCRs) in vivo with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) while 5-HT(1A)Rs or 5-HT(2A)/2CRs in the mPFC were selectively protected with the respective antagonists WAY-100635 or ritanserin. The inactivation of GPCRs while preserving ∼70% of 5-HT(2A)/(2C)Rs prevented the clozapine-induced DA rise in mPFC. In contrast, clozapine increased DA in mPFC of EEDQ-treated rats whose 5-HT(1A)Rs were protected (∼50% of control rats). These results indicate that (1) 5-HT(1A)Rs are necessary for the APDs-induced elevation in cortical DA transmission, and (2) this effect does not require 5-HT(2A)R blockade by APDs.

In vivo modulation of 5-hydroxytryptamine release in mouse prefrontal cortex by local 5-HT2A receptors: effect of antipsychotic drugs

In the rat, postsynaptic 5‐hydroxytryptamine2A receptors medial prefrontal cortex control the activity of the serotonergic system through changes in the activity of pyramidal neurons projecting to the dorsal raphe nucleus. Here we extend these observations to mouse brain. The prefrontal cortex expresses abundant 5‐ hydroxytryptamine2A receptors, as assessed by immunohistochemistry, Western blots and in situ hybridization procedures. The application of the 5‐hydroxytryptamine2A/2C agonist DOI (100 µm) by reverse dialysis in the medial prefrontal cortex doubled the local release of 5‐hydroxytryptamine. This effect was reversed by coperfusion of tetrodotoxin, and by the selective 5‐hydroxytryptamine2A receptor antagonist M100907, but not by the 5‐hydroxytryptamine2C antagonist SB‐242084. The effect of DOI was also reversed by prazosin (α1‐adrenoceptor antagonist), BAY × 3702 (5‐hydroxytryptamine1A receptor agonist), NBQX (α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole‐4‐propionate/kainic acid antagonist) and 1S,3S‐ACPD (mGluR II/III agonist), but not by dizocilpine (N‐methyl‐d‐aspartate antagonist). α‐Amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole‐4‐propionate mimicked the 5‐hydroxytryptamine elevation produced by DOI, an effect also reversed by BAY × 3702. Likewise, the coperfusion of classical (chlorpromazine, haloperidol) and atypical antipsychotic drugs (clozapine, olanzapine) fully reversed the 5‐hydroxytryptamine elevation induced by DOI. These observations suggest that DOI increases 5‐hydroxytryptamine release in the mouse medial prefrontal cortex through the activation of local 5‐hydroxytryptamine2A receptors by an impulse‐dependent mechanism that involves/requires the activation of local α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole‐4‐propionate receptors. This effect is reversed by ligands of receptors present in the medial prefrontal cortex, possibly in pyramidal neurons, which are involved in the action of antipsychotic drugs. In particular, the reversal by classical antipsychotics may involve blockade of α1‐adrenoceptors, whereas that of atypical antipsychotics may involve 5‐hydroxytryptamine2A receptors and α1‐adrenoceptors.

Stimulation of α1-adrenoceptors in the rat medial prefrontal cortex increases the local in vivo 5-hydroxytryptamine release: reversal by antipsychotic drugs

Pyramidal neurons of the medial prefrontal cortex (mPFC) project to midbrain serotonergic neurons and control their activity. The stimulation of prefrontal 5‐HT2A and AMPA receptors increases pyramidal and serotonergic cell firing, and 5‐hydroxytryptamine (5‐HT) release in mPFC. As the mPFC contains abundant α1‐adrenoceptors whose activation increases the excitability of pyramidal neurons, we examined the effects of their stimulation on local 5‐HT release, using microdialysis. The application of the α1‐adrenoceptor agonist cirazoline by reverse dialysis increased the prefrontal 5‐HT release in a concentration‐dependent manner, an effect antagonized by coperfusion of TTX, prazosin (α1‐adrenoceptor antagonist), BAY × 3702 (5‐HT1A agonist), NBQX (AMPA/KA antagonist) and 1S,3S‐ACPD (mGluR II/III agonist), but not by MK‐801 (NMDA antagonist). Cirazoline also enhanced the increase in 5‐HT release induced by DOI (5‐HT2A/2C agonist) and AMPA. In addition, M100907 (5‐HT2A antagonist) but not SB‐242084 (5‐HT2C antagonist) reversed the cirazoline‐ and AMPA‐induced 5‐HT release. These results suggest that the stimulation of prefrontal α1‐adrenoceptors activates pyramidal afferents to ascending serotonergic neurons. The effect of cirazoline was also reversed by coperfusion of classical (chlorpromazine, haloperidol) and atypical (clozapine, olanzapine) antipsychotics, which suggests that a functional antagonism of the α1‐adrenoceptor‐mediated activation of prefrontal neurons may partly underlie their therapeutic action.