A cortical GABA–5HT interaction in the mechanism of action of the antidepressant trazodone

doi:10.1016/j.pnpbp.2004.05.046

Progress in Neuro-Psychopharmacology and Biological Psychiatry

Volume 28, Issue 7, November 2004, Pages 1117-1127

https://doi.org/10.1016/j.pnpbp.2004.05.046 Get rights and content

Abstract

The aim of the study was to investigate whether the antidepressant trazodone (TRZ), a serotonin-2 receptor antagonist/reuptake inhibitor, modifies γ-amino-butyric acid (GABA) extracellular levels in the cerebral cortex, by acting on 5-HT_2A receptors, and through this mechanism increases 5-HT levels. For this purpose the effect of TRZ on the release of GABA was studied in adult male rats in synaptosomes, cortical slices, and “in vivo” by microdialysis. In cortical slices, the release of both GABA and 5-HT was determined. GABA and 5-HT were identified and their levels quantified by HPLC. The inhibition of 5-HT uptake by TRZ was also measured. In synaptosomes, TRZ antagonized dose-dependently, at concentrations from 10⁻¹⁰ to 10⁻⁶ M, the increase in GABA release induced by (±)DOI, a 5-HT_2A/2C agonist, and the α receptor agonist phenylephrine, both 10⁻⁶ M. The pIC₅₀ values were 8.31±0.24, and 5.99±0.52, respectively. In the same preparation, [³H]5-HT accumulation was inhibited by citalopram and TRZ with pIC₅₀ of 7.8±0.44 and 5.9±0.09, respectively, a finding confirming the weak activity of TRZ in comparison with a SSRI.

In cortical slices, TRZ exerted a biphasic effect on GABA release. At concentrations from 10⁻¹⁰ to 10⁻⁷ M it inhibited and from 10⁻⁶ to 10⁻⁴ M increased GABA release. 5-HT release was enhanced by TRZ throughout the entire range of concentrations tested. However, the increase was delayed after low and rapid after high concentrations. AMI-193, a 5-HT_2A antagonist (10⁻¹⁰ to 10⁻⁵ M), reduced GABA release in a dose–response manner, while it induced an increase of 5-HT outflow. On the contrary, (±)DOI (10⁻¹⁰ to 10⁻⁵ M) increased GABA release and inhibited 5-HT levels. Perfusion with the GABA_A receptor antagonist bicuculline was also followed by an increase in 5-HT release.

In microdialysis experiments, TRZ 1.25 mg kg⁻¹ s.c. brought about a decrease in GABA extracellular levels, while an increase was found after the dose of 2.5 mg kg⁻¹.

These findings demonstrate that TRZ, at concentrations which do not inhibit 5-HT uptake, reduces the cortical GABAergic tone by decreasing GABA extracellular levels, through the blockade of 5-HT_2A receptors. The attenuation of GABAergic tone is responsible for an increase in 5-HT levels. A further increase also results from 5-HT uptake inhibition caused by higher doses of TRZ. The ensuing high 5-HT levels enhance GABA release, which in turn inhibits 5-HT release.

Introduction

Trazodone (TRZ) was introduced in the 1960s as an antidepressant drug and is still widely used (Baldessarini, 2001). Since it is a weak inhibitor of serotonin (5-HT) reuptake (Owens et al., 1997), it has been considered different from tricyclic antidepressant (TCAs) and selective serotonin reuptake inhibitors (SSRIs). Pazzagli et al. (1999) demonstrated that TRZ administration brings about a large increase in 5-HT extracellular level in rat's cerebral cortex. However, the increase in cortical 5HT levels following TRZ administration shows a different profile and time course when compared with the increase brought about by TCAs (Maione et al., 1997) and SSRIs (Bel and Artigas, 1992). Fuller et al. (1984) suggested that the predominant action of TRZ on 5-HT neuronal systems is a 5-HT receptor antagonism rather than the inhibition of 5-HT reuptake. Marek et al. (1992), comparing TRZ and fluoxetine, concluded that the preclinical and clinical data suggest that TRZ exerts its antidepressant action through an antagonism for 5-HT_2/1C receptors, while fluoxetine likely acts as an antidepressant via inhibition of 5-HT uptake. According to Frazer (1997), TRZ is characterized by high affinity and antagonistic activity for 5-HT_2A receptors subtype, and a weak inhibition of the 5-HT transporter. These data led Stahl (1996) and DeVane (2000) to define TRZ as a SARI compound, i.e. a serotonin-2 receptor antagonist/reuptake inhibitor. However, it has not yet been defined by which mechanism the blockade of 5-HT_2A subtype receptors results in the large increase in 5-HT extracellular levels observed by Pazzagli et al. (1999), which is presumably responsible for the antidepressant effect since, according to Blier and de Montigny (1994), the therapeutic effect of SSRIs and TCAs correlates with the increase in 5-HT extracellular levels in cortical areas brought about by these drugs.

Cozzi and Nichols (1996) demonstrated that selective 5-HT_2A antagonists, such as ketanserin, spiperone, MDL100907 and ritanserin, inhibit K⁺-stimulated γ-amino-butyric acid (GABA) release from cortical slices. Conversely, Abi-Saab et al. (1999) found that local infusion of the 5-HT_2A/2C agonist (±)DOI [(±)-2,5-dimethoxy-4-iodophenyl-2-aminopropane], dose-dependently increases cortical extracellular GABA levels. They also observed the activation of cortical GABAergic interneurons by demonstrating an increase in Fos-like immunoreactivity, following (±)DOI infusion, co-localized with glutamic acid decarboxylase immunoreactivity, a marker of the GABAergic neurons.

Therefore, since cortical GABAergic interneurons can be modulated by 5-HT, acting on 5-HT_2A/2C receptors subtypes, the aim of this study was to investigate whether TRZ, by acting on 5-HT_2A receptor, modifies GABA extracellular levels in the cerebral cortex and through this mechanism increases 5-HT levels. For this purpose, the effect of TRZ on the release of GABA was studied on rat synaptosomes, cortical slices, and “in vivo” in the frontal cortex by microdialysis. In cortical slices, the release of both GABA and 5HT was determined. Finally, 5-HT uptake inhibition by TRZ was also assessed.

Section snippets

Animals

Adult male Sprague–Dawley rats (Harlan, Italy), weighing 250–300 g, were housed at constant temperature (23±1 °C) and relative humidity (50%). Food and water were freely available. All animal-use procedures conformed to the guidelines of the European Community's Council for Animal Experiments (DL 116/92).

Drugs

The following drugs were used: TRZ (ACRAF, Aprilia, Italy), 5-HT, GABA, tetrodotoxin (TTX) and (+)bicuculline (Sigma, St. Louis, MO), (±)DOI (RBI, Natick, MA), phenylephrine (Aldrich Chemical,

Effect of TRZ on GABA release from synaptosomes

The spontaneous GABA release from superfused synaptosomes was 7.37±0.82 pmol/μl (n=6). The release was enhanced in a concentration-dependent manner by the addition of the 5-HT_2A/C receptor agonist (±)DOI, and phenylephrine, a selective α₁ receptor agonist, at concentrations ranging from 10⁻⁸ to 10⁻⁵ M (Fig. 1, insets in panels A and B, respectively) for 2 min. TRZ added to the superfusion fluid at concentrations ranging between 10⁻¹⁰ and 10⁻⁵ M did not modify the spontaneous GABA release (data

Discussion

TRZ is a weak inhibitor of the 5HT transporter (Frazer 1997), as confirmed in our experiments by the difference between the pIC₅₀ of TRZ and the SSRI citalopram on [³H]5-HT uptake by isolated cortical synaptosomes. In the same preparation, Garrone et al. (2000) demonstrated that TRZ is devoid of 5-HT releasing effect. However, systemic TRZ administration results in a large increase in 5-HT extracellular levels in the rat cerebral cortex (Pazzagli et al., 1999). The present experiments indicate

Conclusion

In the present study we demonstrated that TRZ induces a reciprocal modulation between the cortical serotoninergic and GABAergic systems. At low concentrations, TRZ decreases GABA release by removing a serotoninergic tone acting through 5-HT_2a receptors on GABA neurons. In turn, the decrease in GABA level is accompanied by an increase in 5-HT release. At high concentrations, TRZ increases 5-HT release by inhibiting its transporter. The increase in 5-HT release, resulting from this double

Acknowledgements

The experiments carried out in the Department of Pharmacology, University of Florence were supported by a grant from ACRAF, Angelini Ricerche.

References (37)

W.M. Abi-Saab et al.
5-HT₂ receptor regulation of extracellular GABA levels in the prefrontal cortex
Neuropsychopharmacology
(1999)
G.B. Aghajanian et al.
Serotonin model of schizophrenia: emerging role of glutamate mechanisms
Brain Res. Brain Res. Rev.
(2000)
N. Bel et al.
Fluvoxamine preferentially increases extracellular 5-hydroxytryptamine in the raphe nuclei: an in vivo microdialysis study
Eur. J. Pharmacol.
(1992)
P. Blier et al.
Current advances and trends in the treatment of depression
Trends Pharmacol. Sci.
(1994)
M.M. Bradford
A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein–dye binding
Anal. Biochem.
(1976)
N.V. Cozzi et al.
5-HT₂A receptor antagonists inhibit potassium-stimulated γ-aminobutyric acid release in frontal cortex
Eur. J. Pharmacol.
(1996)
R.W. Fuller et al.
Interactions of trazodone with serotonin neurons and receptors
Neuropharmacology
(1984)
M. Harandi et al.
γ-Aminobutyric acid, 5-hydroxytryptamine interrelationships in the rat nucleus raphe dorsalis: combination of radioautographic and immunocytochmical techniques at light and electronmicroscopy level
Neuroscience
(1987)
C.D. Kilts et al.
Simultaneous quantification of dopamine, 5-hydroxytryptamine and four metabolically related compounds by means of reversed-phase high-performance liquid chromatography with electrochemical detection
J. Chromatogr.
(1981)
R. Liu et al.
Serotonin 5-HT₍₂₎ receptors activate local GABA inhibitory inputs to serotonergic neurons of the dorsal raphe nucleus
Brain Res.
(2000)

M. Pazzagli et al.

Trazodone increases extracellular serotonin levels in the frontal cortex of rats

Eur. J. Pharmacol.

(1999)

M. Raiteri et al.

A simple apparatus for studying the release of neurotransmitters from synaptosomes

Eur. J. Pharmacol.

(1974)

M.N. Subhash et al.

Alterations in 5HT_1A receptors and adenylyl cyclase response by trazodone in regions of rat brain

Life Sci.

(2002)

R. Tao et al.

Influence of inhibitory and excitatory inputs on serotonin efflux differs in the dorsal and median raphe nuclei

Brain Res.

(2003)

N.M. Appel et al.

Autoradiographic characterization of (±)-1-(2,5-dimethoxy-4-[¹²⁵I]iodophenyl)-2-aminopropane ([¹²⁵I]doi) binding to 5HT₂ and 5HT_1c receptors in rat brain

J. Pharmacol. Exp. Ther.

(1990)

R.J. Baldessarini

Drugs and the treatment of psychiatric disorders: depression and anxiety disorders

L. Bianchi et al.

Investigation on acetylcholine, aspartate, glutamate and GABA extracellular levels from ventral hippocampus during repeated exploratory activity in the rat

Neurochem. Res.

(2003)

C.L. DeVane

Pharmacologic characteristics of ideal antidepressants in the 21st century

J. Clin. Psychiatry

(2000)

Cited by (37)

Effect of oral trazodone on the minimum alveolar concentration of isoflurane in dogs
2018, Veterinary Anaesthesia and Analgesia
To determine the effect of oral trazodone on the minimum alveolar concentration (MAC) of isoflurane in dogs.
Prospective blinded, single-observer, randomized crossover experimental study.
Six adult (age 6.8 ± 1.6 months) healthy dogs (three males and three females), weighing 24.8 ± 3.4 kg (mean ± standard deviation).
Each dog was anesthetized twice with a minimum of 7 days between anesthetic episodes. Dogs were randomly assigned to be administered two treatments in a crossover design: premedication with trazodone (8 mg kg⁻¹; TRAZ–ISO) orally 2 hours prior to an anesthetic episode or no (ISO). Dogs were anesthetized with intravenous propofol (6 mg kg⁻¹) and isoflurane in >95% oxygen. Isoflurane MAC was determined using an iterative bracketing technique with electrodes placed in the buccal mucosa. Hemodynamic variables were compared at the lowest end-tidal isoflurane concentration at which each dog did not respond. A paired t test was used to assess the effect of treatment on outcome variables with significance set to a value of p < 0.05.
The MAC concentration (mean ± standard deviation) in dogs administered TRAZ–ISO was 0.85 ± 0.17% compared with 1.02 ± 0.11% in those administered ISO (p = 0.01, 95% confidence interval −0.25 to −0.05), resulting in a mean MAC reduction of 17 ± 12%. There were no differences in hemodynamic variables between treatments.
Premedication of dogs with oral trazodone (8 mg kg⁻¹) 2 hours prior to anesthetic induction has a significant isoflurane MAC sparing effect with no significant observed hemodynamic benefit.
Prolonged cortical silent period among drug-naive subjects at ultra-high risk of psychosis
2014, Schizophrenia Research
Citation Excerpt :
UHR subjects in that study were antipsychotic naive, but about half of them were medicated with an antidepressant or zopiclone when participating in the study. A few studies have reported that antidepressants may affect GABA-mediated cortical inhibition (Luparini et al., 2004; Choi et al., 2010; Radhu et al., 2012). Additionally, one study found only lacked transcallosal inhibition rather than that of other TMS parameters in first-degree relatives of schizophrenia patients, who show psychosis-proneness but are free of confounds related to medication and/or florid psychosis (Saka et al., 2005; Nagai et al., 2013).
Deficits in gamma-aminobutyric acid (GABA) inhibitory neurotransmission have been associated with pathophysiological mechanisms underlying schizophrenia. However, little is known about whether these deficits occur before or after the onset of psychosis.
We recruited 16 drug-naive subjects at ultra-high risk of psychosis (UHR), 17 schizophrenia patients and 28 healthy controls. Cortical inhibition was determined using transcranial magnetic stimulation (TMS) over the left primary motor cortex. TMS markers such as short-interval cortical inhibition (SICI), cortical silent period (CSP) and intracortical facilitation (ICF) were obtained from each subject. While SICI can reflect GABA type A (GABA_A) mediated inhibition, CSP is thought to indicate GABA type B (GABA_B) mediated inhibitory circuits.
As compared with healthy controls, UHR subjects showed a prolonged CSP with no change in SICI, whereas schizophrenia patients demonstrated both a prolonged CSP and a reduced SICI. No group differences were found for ICF. CSP in schizophrenia patients also had a positive correlation with positive symptom score of the positive and negative symptom scale (PANSS).
Cortical inhibitory deficits among UHR subjects were relatively limited compared to those among schizophrenia patients. Alterations might occur in some subgroup of GABA-mediated neurotransmitter systems before the onset of psychosis, while alterations in both GABA_A and GABA_B networks might contribute to full-blown psychosis.
Serotonergic modulation of LTP at excitatory and inhibitory synapses in the developing rat visual cortex
2013, Neuroscience
Citation Excerpt :
Then we asked if an increase in the cortical 5-HT tone has opposite effects on the LTP of excitation and inhibition, when compared to 5-HT depleted slices. To do so, we perfused the selective serotonin reuptake inhibitor (SSRI) citalopram (Luparini et al., 2004; Park et al., 2012) in naive (i.e. non depleted) cortical slices. Since we previously showed that citalopram slightly decreases baseline excitatory and inhibitory synaptic transmissions (Moreau et al., 2010), the control recording (before drug) was followed by a pre-TBS recording performed 10 min after citalopram wash-in.
The stability and efficacy of neuronal circuits are achieved through a detailed balance between pyramidal cell and interneuron activities. Interestingly, the neocortical excitatory–inhibitory (E–I) balance is actively maintained at the soma of Layer 5 pyramidal neurons which receive 20% of excitation and 80% of inhibition after dendritic integration, and this is not affected by changes in synaptic strength. To infer the role of serotonergic neuromodulation on the activity-dependent maintenance of the E–I balance, we performed continuous voltage clamp measurements of stimulation-locked conductance dynamics in Layer 5 pyramidal neurons before and after long-term potentiation (LTP) induction, together with chronic or acute manipulation of serotonin function. When a theta-burst stimulation was applied in Layer 2/3 of 5-HT depleted cortical slices (after in vivo treatment with the tryptophan hydroxylase inhibitor p-chlorophenylalanine (pCPA)), or after in vitro perfusion of the potent 5-HT_1A receptor antagonist WAY-100,635, we observed a persistent shift of the ratio between excitation and inhibition toward more inhibition. This was due to a strong LTP of inhibition co-aligned with a weak LTP of excitation, whereas the same protocol caused a similar potentiation of excitatory and inhibitory inputs when applied in control slices. In contrast, neither excitatory nor inhibitory postsynaptic currents were potentiated when LTP protocols were delivered in the presence of either the selective serotonin reuptake inhibitor citalopram or the 5-HT_1A receptor agonist 8-OH-DPAT. This is the first demonstration that serotonergic neuromodulation is crucial for the maintenance of the neocortical E–I balance during high-frequency regimes.
Synergistic antidepressant-like action of gaboxadol and escitalopram
2012, European Neuropsychopharmacology
Citation Excerpt :
It is noteworthy to mention here that systemic gaboxadol (2.5 mg/kg, s.c.) alone produces a slight (50% from baseline) increase of 5-HT release in the prefrontal cortex but when it is co-administrated with escitalopram, the increased 5-HT level in prefrontal cortex is doubled (data not shown), suggesting synergistic action of escitalopram and gaboxadol on prefrontal cortex 5-HT release. Luparini et al. demonstrated that the 5-HT2A receptor antagonist/5-HT uptake inhibitor trazodone, at concentrations which do not inhibit 5-HT uptake, reduces the cortical GABAergic tone by decreasing GABA extracellular levels, through the blockade of 5-HT2A receptors and leading to an increase in 5-HT levels (Luparini et al., 2004). However, a further increase also results from 5-HT uptake inhibition caused by higher doses of trazodone, but the ensuing high 5-HT levels enhance GABA release, which in turn inhibits 5-HT release.
According to current theories on the etiopathogenesis and pathophysiology of depression, both GABAergic and monoaminergic transmitter systems are perturbed. Consequently, the present study addressed the putative antidepressant action of the sedative-hypnotic GABA_A receptor agonist, gaboxadol, separately and in combination with the selective serotonin reuptake inhibitor (SSRI) escitalopram. The rat chronic mild stress model was used to test the chronic antidepressant properties of gaboxadol in this depression model. Sucrose intake used as a read-out on anhedonic-like behavior indicated that the drug response rate for gaboxadol (5 mg/kg/day, i.p.) was similar to that measured for escitalopram (5 mg/kg/day, i.p.), however, the rate increased when the two drugs were co-administered, suggesting a synergistic action. Using in vivo electrophysiological recordings in dorsal raphe nucleus (DRN) of anesthetised rats, the present results showed that one week treatment with gaboxadol (5 mg/kg/day, i.p.) or with escitalopram (5 mg/kg/day, i.p.), followed by a 24 h washout period, did not affect DRN 5-HT neuronal firing per se, but in rats treated with both drugs for one week, the firing rate of DRN 5-HT neurons was significantly increased. Immunohistochemical estimations of cell proliferation in the hippocampal dentate gyrus did not reveal any effect of gaboxadol on chronic mild stressed rats, indicating that neurogenesis per se is not systematically associated with recovery from anhedonic-like behavior. Taken together, our data reveal for the first time an antidepressant action of gaboxadol and indicate a synergistic mechanism, regarding rapid onset of action and efficacy, when co-administered with escitalopram.
Hypnotic use for insomnia management in chronic obstructive pulmonary disease
2009, Sleep Medicine
Citation Excerpt :
Serotonin neurotransmitters act on these G-protein coupled receptors expressed in neurons of the CNS and smooth muscle cells. Trazodone may indirectly lead to increased GABA release as a result of increased levels of serotonin, but it is still unclear if this is how it mediates its sedative effects [57] and, more importantly, at what dose and with what safety cost. The data on the effectiveness of trazodone in normal subjects is limited and there are virtually no studies done in patients with COPD.
Chronic obstructive pulmonary disease (COPD) is one of the leading causes of mortality and morbidity worldwide. Because of the chronic nature of the disease, optimal care for patients includes successful treatment of comorbidities that accompany COPD, including insomnia. Insomnia symptoms and associated disruption of sleep are prevalent in COPD patients but treatment with traditional benzodiazepines may compromise respiratory function. This review summarizes the efficacy and safety consideration of current drugs available for the treatment of insomnia in COPD patients including benzodiazepines, non-benzodiazepine receptor agonists such as eszopiclone, zolpidem, and zaleplon, sedating antidepressants such as trazodone, and the melatonin receptor agonist ramelteon.
Early response to trazodone once-A-day in major depressive disorder: Review of the clinical data and putative mechanism for faster onset of action
2021, CNS Spectrums

View all citing articles on Scopus

View full text

A cortical GABA–5HT interaction in the mechanism of action of the antidepressant trazodone

Abstract

Introduction

Section snippets

Animals

Drugs

Effect of TRZ on GABA release from synaptosomes

Discussion

Conclusion

Acknowledgements

Neuropsychopharmacology

Brain Res. Brain Res. Rev.

Eur. J. Pharmacol.

Trends Pharmacol. Sci.

Anal. Biochem.

Eur. J. Pharmacol.

Neuropharmacology

Neuroscience

J. Chromatogr.

Brain Res.

Eur. J. Pharmacol.

Eur. J. Pharmacol.

Life Sci.

Brain Res.

Autoradiographic characterization of (±)-1-(2,5-dimethoxy-4-[125I]iodophenyl)-2-aminopropane ([125I]doi) binding to 5HT2 and 5HT1c receptors in rat brain

J. Pharmacol. Exp. Ther.

Drugs and the treatment of psychiatric disorders: depression and anxiety disorders

Investigation on acetylcholine, aspartate, glutamate and GABA extracellular levels from ventral hippocampus during repeated exploratory activity in the rat

Neurochem. Res.

Pharmacologic characteristics of ideal antidepressants in the 21st century

J. Clin. Psychiatry

Autoradiographic characterization of (±)-1-(2,5-dimethoxy-4-[¹²⁵I]iodophenyl)-2-aminopropane ([¹²⁵I]doi) binding to 5HT₂ and 5HT_1c receptors in rat brain