Elena Castro

Strategies for producing faster acting antidepressants

Existing antidepressant treatments exhibit limited efficacy and a slow onset of action. Several neurobiological adaptive mechanisms might delay the clinical effects of antidepressants, whose therapeutic action is primarily triggered by an increase of serotonergic and noradrenergic neurotransmission. Here, we review several potential mechanisms that could be useful to increase the speed of current antidepressant drugs, such as additional blockade of aminergic autoreceptors or antagonism of certain postsynaptic (5-HT2A, 5-HT2C) receptors. The potential use of strategies not based on monoaminergic transmission, such as CRF and NK1 receptor antagonists, or more novel strategies, based on glutamatergic or GABAergic transmission or on intracellular messengers, are also reviewed.

CB1 knockout mice display impaired functionality of 5‐HT1A and 5‐HT2A/C receptors

Interaction between brain endocannabinoid (EC) and serotonin (5‐HT) systems was investigated by examining 5‐HT‐dependent behavioral and biochemical responses in CB1 receptor knockout mice. CB1 knockout animals exhibited a significant reduction in the induction of head twitches and paw tremor by the 5‐HT2A/C receptor selective agonist (±) DOI, as well as a reduced hypothermic response following administration of the 5‐HT1A receptor agonist (±)‐8‐OH‐DPAT. Additionally, exposure to the tail suspension test induced enhanced despair responses in CB1 knockout mice. However, the tricyclic antidepressant imipramine and the 5‐HT selective reuptake inhibitor fluoxetine induced similar decreases in the time of immobility in the tail suspension test in CB1 receptor knockout and wild‐type mice. No differences were found between both genotypes with regard to 5‐HT2A receptor and 5‐HT1A receptors levels, measured by autoradiography in different brain areas. However, a significant decrease in the ability of both, the 5‐HT1A receptor agonist (±)‐8‐OH‐DPAT and the 5‐HT2A/C receptor agonist (−)DOI, to stimulate [35S]GTPγS binding was detected in the hippocampal CA1 area and fronto‐parietal cortex of CB1 receptor knockout mice, respectively. This study provides evidence that CB1 receptors are involved in the regulation of serotonergic responses mediated by 5‐HT2A/C and 5‐HT1A receptors, and suggests that a reduced coupling of 5‐HT1A and 5‐HT2A receptors to G proteins might be involved in these effects.

Long-term fluoxetine treatment modulates cannabinoid type 1 receptor-mediated inhibition of adenylyl cyclase in the rat prefrontal cortex through 5-hydroxytryptamine 1A receptor-dependent mechanisms.

Increasing data indicate that brain endocannabinoid system plays a role in the effects of antidepressant medications. Here we examined the effect of in vivo exposure to the selective serotonin uptake inhibitor fluoxetine on cannabinoid type 1 (CB1) receptor density and functionality in the rat prefrontal cortex (PFC) and cerebellum. Long-term treatment with fluoxetine (10 mg/kg/day) enhanced CB1 receptor inhibition of adenylyl cyclase (AC) in the PFC and reduced it in the cerebellum without altering receptor density and agonist stimulation of guanosine 5′-O-(3-[35S]thio) triphosphate ([35S]GTPγS) in either area. Analysis of [35S]GTPγS-labeled Gα subunits allowed for the detection of up-regulated CB1 receptor coupling to Gαi2, Gαi3 in the PFC, and reduced coupling to Gαi3 in the cerebellum of fluoxetine-treated rats. Concomitant administration of the 5-HT1A receptor antagonist N-[2-[4- (2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate (WAY100635; 0.1 mg/kg/day) reduced fluoxetine-induced modulation of CB1 receptor coupling to Gα subunits and AC in the PFC but not in the cerebellum. These results indicate that increased CB1 receptor signaling at the Gαi-AC transduction level is a long-term adaptation induced by fluoxetine in the PFC and point to a role for 5-HT1A receptors in this effect. Basal AC activity, protein kinase A (PKA) catalytic subunit expression, and phospho-cAMP response element-binding protein (pCREB)/CREB ratio were also up-regulated in the PFC of fluoxetine-treated animals, whereas no differences were detected in the cerebellum. It is interesting that long-term Δ9-tetrahydrocannabinol treatment did not elicit antidepressant-like effects or modulated behavioral responses of fluoxetine in an animal model of depression (olfactory bulbectomy). These data suggest that altered signal transduction through CB1 receptors in the PFC may participate in the regulation of the AC-PKA-CREB cascade induced by fluoxetine in this brain area.

Neural Plasticity and Proliferation in the Generation of Antidepressant Effects: Hippocampal Implication

It is widely accepted that changes underlying depression and antidepressant-like effects involve not only alterations in the levels of neurotransmitters as monoamines and their receptors in the brain, but also structural and functional changes far beyond. During the last two decades, emerging theories are providing new explanations about the neurobiology of depression and the mechanism of action of antidepressant strategies based on cellular changes at the CNS level. The neurotrophic/plasticity hypothesis of depression, proposed more than a decade ago, is now supported by multiple basic and clinical studies focused on the role of intracellular-signalling cascades that govern neural proliferation and plasticity. Herein, we review the state-of-the-art of the changes in these signalling pathways which appear to underlie both depressive disorders and antidepressant actions. We will especially focus on the hippocampal cellularity and plasticity modulation by serotonin, trophic factors as brain-derived neurotrophic factor (BDNF), and vascular endothelial growth factor (VEGF) through intracellular signalling pathways—cAMP, Wnt/β-catenin, and mTOR. Connecting the classic monoaminergic hypothesis with proliferation/neuroplasticity-related evidence is an appealing and comprehensive attempt for improving our knowledge about the neurobiological events leading to depression and associated to antidepressant therapies.

Long-term treatment with fluoxetine induces desensitization of 5-HT4 receptor―dependent signalling and functionality in rat brain

The mode of action of antidepressant drugs may be related to mechanisms of monoamines receptor adaptation, including serotonin 5‐HT4 receptor subtypes. Here we investigated the effects of repeated treatment with the selective serotonin reuptake inhibitor fluoxetine for 21u2003days (5 and 10u2003mg/kg, p.o., once daily) on the sensitivity of 5‐HT4 receptors by using receptor autoradiography, adenylate cyclase assays and extracellular recording techniques in rat brain. Fluoxetine treatment decreased the density of 5‐HT4 receptor binding in the CA1 field of hippocampus as well as in several areas of the striatum over the doses of 5–10u2003mg/kg. In a similar way, we found a significant lower response to zacopride‐stimulated adenylate cyclase activity in the fluoxetine 10u2003mg/kg/day treated group. Furthermore, post‐synaptic 5‐HT4 receptor activity in hippocampus‐measured as the excitatory action of zacopride in the pyramidal cells of CA1 evoked by Schaffer collateral stimulation was attenuated in rats treated with both doses of fluoxetine. Taken together, these results support the concept that a net decrease in the signalization pathway of 5‐HT4 receptors occurs after chronic selective serotonin reuptake inhibitor treatment: this effect may underlie the therapeutic efficacy of these drugs.

Agonist-dependent modulation of G-protein coupling and transduction of 5-HT1A receptors in rat dorsal raphe nucleus

5-HT1A receptors couple to different Go/Gi proteins in order to mediate a wide range of physiological actions. While activation of post-synaptic 5-HT1A receptors is mainly related to inhibition of adenylyl cyclase activity, functionality of autoreceptors located in raphe nuclei has been classically ascribed to modifications of the activity of potassium and calcium channels. In order to evaluate the possible existence of agonist-directed trafficking for 5-HT1A autoreceptors in the rat dorsal raphe nucleus, we studied their activation by two agonists with a different profile of efficacy [(+)8-OH-DPAT and buspirone], addressing simultaneously the identification of the specific Galpha subtypes ([35S]GTPgammaS labelling and immunoprecipitation) involved and the subsequent changes in cAMP formation. A significant increase (32%, p<0.05) in (+)8-OH-DPAT-induced [35S]GTPgammaS labelling of immunoprecipitates was obtained with anti-Galphai3 antibodies but not with anti-Galphao, anti-Galphai1, anti-Galphai2, anti-Galphaz or anti-Galphas antibodies. In contrast, in the presence of buspirone, significant [35S]GTPgammaS labelling of immunoprecipitates was obtained with anti-Galphai3 (50%, p<0.01), anti-Galphao (32%, p<0.01) and anti-Galphai2 (29%, p<0.05) antibodies, without any labelling with anti-Galphai1, anti-Galphaz or anti-Galphas. The selective 5-HT1A antagonist WAY 100635 blocked the labelling induced by both agonists. Furthermore, (+)8-OH-DPAT failed to modify forskolin-stimulated cAMP accumulation, while buspirone induced a dose-dependent, WAY 100635-sensitive, inhibition of this response (Imax 30.8+/-4.9, pIC50 5.95+/-0.46). These results demonstrate the existence of an agonist-dependency pattern of G-protein coupling and transduction for 5-HT1A autoreceptors in native brain tissue. These data also open new perspectives for the understanding of the differential profiles of agonist efficacy in pre- vs. post-synaptic 5-HT1A receptor-associated responses.

Use of Arc expression as a molecular marker of increased postsynaptic 5-HT function after SSRI/5-HT1A receptor antagonist co-administration.

An increase in central postsynaptic 5‐hydroxytryptamine (5‐HT) function activates expression of activity‐related cytoskeletal protein (Arc). Here, Arc expression was used to test whether, in rats, co‐administration of a 5‐HT re‐uptake inhibitor (paroxetine) and a 5‐HT1A receptor antagonist (WAY 100635) increases postsynaptic 5‐HT function. After pre‐treatment with WAY 100635 (0.3u2003mg/kg s.c.), paroxetine (5u2003mg/kg s.c.) caused a threefold increase in 5‐HT in prefrontal cortex microdialysates. In situ hybridization studies found that neither paroxetine (5u2003mg/kg s.c.) nor WAY 1000635 (0.3u2003mg/kg s.c.) altered Arc mRNA abundance in any region examined. In contrast, paroxetine (5u2003mg/kg s.c.) increased Arc mRNA after pre‐treatment with WAY 100635 (0.3u2003mg/kg s.c.). This increase was apparent in cortical regions (frontal, parietal and cingulate) and caudate nucleus but was absent in hippocampus (CA1). Increases in Arc mRNA were accompanied by an increase in c‐fos mRNA. The increase in Arc expression induced by paroxetine/WAY 100635 was abolished by the 5‐HT synthesis inhibitor, p‐chlorophenylalanine (300u2003mg/kg i.p., daily for two days). In conclusion, paroxetine and WAY 100635 injected in combination (but not alone) caused a region‐specific, 5‐HT‐mediated increase in Arc expression. These data provide molecular evidence that co‐administration of a 5‐HT re‐uptake inhibitor and 5‐HT1A receptor antagonist increases 5‐HT function at the postsynaptic level.

Modulation of neuroplasticity pathways and antidepressant-like behavioural responses following the short-term (3 and 7 days) administration of the 5-HT4 receptor agonist RS67333

It has been recently suggested that activation of 5-HT₄ receptors might exert antidepressant-like effects in rats after 3 d treatment, suggesting a new strategy for developing faster-acting antidepressants. We studied the effects of 3 d and 7 d treatment with the 5-HT₄ receptor partial agonist RS67333 (1.5 mg/kg.d) in behavioural tests of chronic efficacy and on neuroplastic-associated changes, such as adult hippocampal neurogenesis, expression of CREB, BDNF, β-catenin, AKT and 5-HT₄ receptor functionality. RS67333 treatment up-regulated hippocampal cell proliferation, β-catenin expression and pCREB/CREB ratio after 3 d treatment. This short-term treatment also reduced immobility time in the forced swim test (FST), together with a partial reversion of the anhedonic-like state (sucrose consumption after chronic corticosterone). Administration of RS67333 for 7 d resulted in a higher increase in the rate of hippocampal cell proliferation, a significant desensitization of 5-HT₄ receptor-coupled adenylate cyclase activity and a more marked increase in the expression of neuroplasticity-related proteins (BDNF, CREB, AKT): these changes reached the same magnitude as those observed after 3 wk administration of classical antidepressants. Consistently, a positive behavioural response in the novelty suppressed feeding (NSF) test and a complete reversion of the anhedonic-like state (sucrose consumption) were also observed after 7 d treatment. These results support the antidepressant-like profile of RS67333 with a shorter onset of action and suggest that this time period of administration (3-7 d) could be a good approximation to experimentally predict the onset of action of this promising strategy.

New strategies in the development of antidepressants: towards the modulation of neuroplasticity pathways.

Over the past five decades, the pharmacological treatment of depression has been based on the pathophysiological hypothesis of a deficiency in monoamines, mainly serotonin and noradrenaline. Antidepressant prescribed today, all of them designed to enhance central monoaminergic tone, present several important limitations, including a 2-5 weeks response lag and also a limited clinical efficacy. As it is increasingly evident that the abnormalities associated to depression go beyond monoamines, the development of better antidepressants will depend on the identification and understanding of new cellular targets. In this regard, much evidence supports a role for cellular and molecular mechanisms of neuroplasticity, including neurotrophic inputs, in mood disorders, in parallel with the biological features associated to stress conditions. In order to illustrate the possible relevance of neuroplasticity-related pathways for the therapy of depressive states, we here review the biological evidence supporting some therapeutic strategies in a very initial phase of development (modulation of the Wnt/GSK-3β/β-catenin pathway, potentiation of endocannabinoid activity, agonism of 5-HT(4) receptors), which involve modulation of downstream mechanisms and neuroplasticity circuits. These strategies also show the existence of mixed mechanisms of action, constituting a nexus between the classic aminergic theory and the new neuroplasticity hypothesis.

In vitro and in vivo characterization of F-97013-GD, a partial 5-HT1A agonist with antipsychotic- and antiparkinsonian-like properties

In order to better define the role of 5-HT(1A) receptors in the modulation of extrapyramidal motor functions, we investigated the effect of 5-HT(1A) agonists on tacrine-induced tremulous jaw movements (TJM) in rats, a putative model of parkinsonian tremor. Acute injection of 5-HT(1A) agonists 8-OH-DPAT and buspirone dose-dependently counteracted the tacrine-induced oral movements (ED(50)=0.04 and 1.0mg/kg, respectively), an effect reversed by the selective 5-HT(1A) antagonist WAY 100,635. In contrast to classical antipsychotics, the atypical antipsychotics risperidone (ED(50)=0.3mg/kg) and clozapine (ED(50)=1.5mg/kg) blocked the oral movements induced by the cholinomimetic agent at or below the doses required for suppression of conditioned avoidance response. The compound F-97013-GD (6-methyl-2-[4-(naphtylpiperazin-1-yl)butyl]-3-(2H)-pyridazinone), a putative antipsychotic drug that in functional in vitro and in vivo assays behaved as a mixed dopamine D(2)-antagonist and 5-HT(1A)-partial agonist, also displayed a potent antitremorgenic effect in this paradigm (ED(50)=0.5mg/kg). Interestingly, pretreatment with WAY 100,635 blocked the inhibitory effect of F-97013-GD but not that of clozapine. The 5-HT depleting agent para-chlorophenylalanine (PCPA) partially attenuated tacrine-induced TJM but did not block the suppressive effect of 5-HT(1A) agonists. In addition, only high doses of F-97013-GD induced catalepsy in rodents and, like 8-OH-DPAT and clozapine, the compound reversed the haloperidol-induced catalepsy in rats. These results show that 5-HT(1A) receptors play a role in the regulation of tacrine-induced TJM and suggest that their activation by novel antipsychotics may not only reduce the extrapyramidal side effects EPS liability, but also be effective in the treatment of parkinsonian tremor.