Neuropsychopharmacology (2013) 38, 275–284 & 2013 American College of Neuropsychopharmacology. All rights reserved 0893-133X/13 www.neuropsychopharmacology.org Electrophysiological Effects of Repeated Administration of Agomelatine on the Dopamine, Norepinephrine, and Serotonin Systems in the Rat Brain Franck Chenu1, Mostafa El Mansari1 and Pierre Blier*,1,2 1 2 Institute of Mental Health Research, University of Ottawa, Ottawa, ON, Canada; Department of Cellular and Molecular Medicine, University of Ottawa, Ottawa, ON, Canada Agomelatine is a melatonergic MT1/MT2 agonist and a serotonin (5-HT) 5-HT antagonist. The effects of 2-day and 14-day 2C administration of agomelatine were investigated on the activity of ventral tegmental area (VTA) dopamine (DA), locus coeruleus (LC) norepinephrine (NE), and dorsal raphe nucleus (DRN) 5-HT neurons using in vivo electrophysiology in rats. The 5-HT1A transmission was assessed at hippocampus CA3 pyramidal neurons. After a 2-day regimen of agomelatine (40 mg/kg/day, i.p.), an increase in the number of spontaneously active VTA-DA neurons (po0.001) and in the firing rate of LC-NE neurons (po0.001) was observed. After 14 days, the administration of agomelatine induced an increase in: (1) the number of spontaneously active DA neurons (po0.05), (2) the bursting activity of DA neurons (bursts/min, po0.01 and percentage of spikes occurring in bursts, po0.05), (3) the firing rate of DRN-5-HT neurons (po0.05), and (4) the tonic activation of postsynaptic 5-HT1A receptors located in the hippocampus. The increase in 5-HT firing rate was D2 dependent, as it was antagonized by the D2 receptor antagonist paliperidone. The enhancement of NE firing was restored by the 5-HT2A receptor antagonist MDL-100,907 after the 14-day regimen. All the effects of agomelatine were antagonized by a single administration of the melatonergic antagonist S22153 (except for the increase in the percentage of spikes occurring in burst for DA neurons). The present results suggest that (1) agomelatine exerts direct (2 days) and indirect (14 days) modulations of monoaminergic neuronal activity and (2) the melatonergic agonistic activity of agomelatine contributes to the enhancement of DA and 5-HT neurotransmission. Neuropsychopharmacology (2013) 38, 275–284; doi:10.1038/npp.2012.140; published online 8 August 2012 Keywords: agomelatine; antidepressant; electrophysiology; dopamine; norepinephrine; serotonin INTRODUCTION CMS-induced reduction in sucrose consumption, suggesting a reduction of anhedonia in stressed animals (Papp et al, Agomelatine is an antidepressant with a novel mechanism 2003). This effect is, at least in part, related to the of action. It is a melatonergic agonist for MT1 and MT2 melatonergic component of the drug as it is blocked by a receptors and a serotonin (5-HT) receptor antagonist 2C single administration of the MT1/MT2 receptor antagonist (Millan et al, 2003; Audinot et al, 2003; de Bodinat et al, S22153. In the learned helplessness (LH) model, agomela- 2010; Kasper and Hamon, 2009; Ying et al, 1996). The tine induces an antidepressant-like activity after chronic antidepressant-like effect of agomelatine has been demon- administration, which was abolished by pretreatment with strated in different animal models. In the forced swimming the MT1/MT2 receptor antagonist S22153 (Bertaina-An- test, acute and repeated administration of agomelatine glade et al, 2006), suggesting the involvement of the induces an antidepressant-like effect in rats (Bourin et al, melatonergic receptors. These behavioral data results are 2004), whereas in mice only the long-term administration supported by studies showing that the administration of the induces a significant increase in swimming duration. In the melatonergic receptor antagonist S22153 antagonized the chronic mild stress (CMS), repeated administration of increase in brain-derived neurotrophic factor (BDNF) agomelatine in the evening dose-dependently reverses the mRNA levels in rat prefrontal cortex (Racagni et al, 2011). As both the 5-HT2C receptor antagonist SB-242084 and *Correspondence: Dr P Blier, Institute of Mental Health Research melatonin are devoid of effect by themselves in the CMS and (IMHR), 1145 Carling Avenue, University of Ottawa, Room 6412, LH, these preclinical data suggest that the antidepressant- Ottawa K1Z 7K4, ON, Canada, Tel: + 1 613 722 6521 (ext 6944), like effect of agomelatine requires both the activation of Fax: + 1 613 761 3610, E-mail: [email protected] melatonergic receptors and the blockade of 5-HT2C Received 16 March 2012; revised 26 June 2012; accepted 28 June 2012 receptors. This is in line with clinical studies showing that Effects of agomelatine on monoaminergic systems F Chenu et al 276 melatonin by itself does not have any antidepressant effect 1700 h. 5-HT, quisqualic acid, and WAY-100,635 were in patients suffering from major depressive disorder (MDD; bought from Sigma (Oakville, ON, Canada). Paliperidone Dalton et al, 2000; Dolberg et al, 1998). In contrast, the was provided by Janssen (Titusville, NJ). efficacy of agomelatine in MDD has been demonstrated in clinical trials vs placebo (Kennedy and Emsley, 2006) and In Vivo Electrophysiological Recordings using active comparators such as venlafaxine (Lemoine et al, 2007), sertraline (Kasper et al, 2010), and fluoxetine Rats were anesthetized with chloral hydrate (400 mg/kg, i.p.) (Hale et al, 2010). and placed into a stereotaxic frame. The extracellular Regarding its putative mechanism of action, agomelatine recordings of 5-HT, DA, and NE neurons in the DR, the induces a dose-dependent enhancement of extracellular VTA, and the LC, respectively, were carried out using frontocortical dopamine (DA) and norepinephrine (NE) single-barrelled glass micropipettes (Stoelting, Spencerville, transmission measured by microdialysis (Millan et al, MD) preloaded with a 2 M NaCl solution (impedance 4– 2003). These increases are unaffected by the melatonin 7MO). The extracellular recordings of pyramidal neurons in receptor antagonist S22153 and cannot be mimicked by the CA3 region of the hippocampus were carried out using melatonin. However, previous results showed that 5-HT2C multi-barrelled glass micropipettes (impedances: central receptor antagonists induce an increase in extracellular barrel: 2–5MO, side barrels: 20–30MO). The central barrel levels of NE and DA in the prefrontal cortex of freely used for extracellular unitary recording and one side barrel moving rats (Gobert et al, 2000). Therefore, this effect is (automatic current balancing) were filled with 2 M NaCl likely associated with the 5-HT2C receptor antagonist solution. The three other side barrels were filled with 5-HT property of agomelatine. A dose-dependent increase in (25 mM in 0.2 M NaCl, pH ¼ 4) and quisqualate (1.5 mM in extracellular levels of NE was also found in the dorsal 0.2 M NaCl; pH ¼ 8). 5-HT was ejected as cations and hippocampus of freely moving rats following an acute retained with currents of À10 to À8 nA. Quisqualate was administration of agomelatine (Millan et al, 2005). These ejected as an anion and retained with a current of + 5 nA. findings are supported by electrophysiology results showing a dose-dependent increase in the firing rate of locus Recording of DRN-5-HT Neurons coeruleus (LC) NE neurons (Millan et al, 2003) following acute administration of incremental doses of agomelatine. The single-barrelled glass micropipettes were positioned Unexpectedly, the electrical activity of ventral tegmental using the following coordinates (in mm from l): AP: + 1.0 area (VTA) DA neurons was unchanged. to 1.2; L: 0±0.1; and V: 5 to 7. The presumed 5-HT neurons Based on all these previous preclinical studies, it seems that were then identified using the following criteria: a slow (0.5– there is a discrepancy between the behavioral and the 2.5 Hz) and regular firing rate and long-duration (2–5 ms) neurobiochemical effects of agomelatine. Indeed, the anti- bi- or tri-phasic extracellular waveform (Aghajanian and depressant-like effect of agomelatine cannot be fully ex- Vandermaelen, 1982). It is important to mention that there plained by the increase in extracellular levels of NE and DA are slow firing neurons in the DRN that are not serotonergic because this increase is not antagonized by the melatonergic (Allers and Sharp, 2003). However, these neurons have receptor antagonist S22153, whereas its antidepressant-like shorter-duration spikes and do not fire as regularly as 5-HT effect is. The aim of this study was to investigate the effects of neurons. It was deemed that few, if any, such neurons were short (2 days) and long-term (14 days) administration of accidentally introduced in our tally of 5-HT neurons by agomelatine on the electrical activity of VTA-DA neurons, adhering to the above-mentioned criteria. As previously LC-NE neurons, and dorsal raphe nuclei (DRN) 5-HT demonstrated, some 5-HT neurons display a bursting neurons in order to have a better understanding of its activity (Hajos et al, 1995). This occasional firing pattern mechanism of action. of 5-HT neurons was analyzed by spike interval burst analysis. The onset of a burst was defined as the occurrence of two spikes with an interspike interval (ISI) of o10 ms. MATERIALS AND METHODS Animals Recording of VTA-DA Neurons Male Sprague-Dawley rats (Charles River, St Constant, QC, The single-barrelled glass micropipettes were positioned Canada) weighing 250 to 330 g, were used for the experi- using the following coordinates (in mm from bregma): AP: ments. They were kept under standard laboratory condi- À6toÀ5.4; L: 0.6 to 1; and V: 7 to 9. The presumed DA tions (12 : 12 h light/dark cycle, free access to food and neurons were identified according to well-established water). All animals were handled according to the guide- electrophysiological properties in vivo: a typical triphasic lines of the Canadian Council on Animal Care. Protocols in action potential with a marked negative deflection, a this study were approved by the local Animal Care characteristic long duration (42.5 ms) often with an Committee (IMHR, Ottawa, ON, Canada).