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Involvement of potassium channels in the antidepressant-like effect of venlafaxine in mice
Life Sciences 86 (2010) 372–376
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Involvement of potassium channels in the antidepressant-like effect of venlafaxine in mice Cristiani F. Bortolatto, Cristiano R. Jesse, Ethel A. Wilhelm, Cristina W. Nogueira ⁎ Laboratory of Synthesis, Reactivity, Pharmacological and Toxicological Evaluation of Organochalcogens, Natural Science Institute, Federal University of Santa Maria, Santa Maria, CEP 97105-900, RS, Brazil
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Article history: Received 23 September 2009 Accepted 18 January 2010 Keywords: Venlafaxine Antidepressant-like Potassium channels Forced swim test (FST) Mice
a b s t r a c t Aims: Studies have shown that the acute administration of venlafaxine elicits an antidepressant-like effect in the mouse forced swim test (FST) by a mechanism dependent on the L-arginine–nitric oxide (NO)–cyclic guanosine monophosphate (cGMP) pathway. Because it has been reported that NO activates different types of potassium (K+) channels in the brain, this study investigated the involvement of K+ channels in the antidepressant-like effect of venlafaxine in the mouse FST. Main methods: Male adult Swiss mice were pretreated with different K+ channel inhibitors or openers 15 min before venlafaxine administration. After 30 min, the open-field test (OFT) and FST were carried out. Key findings: Intracerebroventricular (i.c.v.) pretreatment of mice with subeffective doses of tetraethylammonium (TEA, a non-specific inhibitor of K+ channels, 25 pg/site), glibenclamide (an ATP-sensitive K+ channel inhibitor, 0.5 pg/site), charybdotoxin (a large- and intermediate-conductance calcium-activated K+ channel inhibitor, 25 pg/site) or apamin (a small-conductance calcium-activated K+ channel inhibitor, 10 pg/site) was able to potentiate the action of a subeffective dose of venlafaxine (2 mg/kg, i.p.). Moreover, the reduction in the immobility time elicited by an effective dose of venlafaxine (8 mg/kg, i.p.) in the FST was prevented by the pretreatment of mice with the K+ channel openers cromakalim (10 µg/site, i.c.v.) and minoxidil (10 µg/site, i.c.v.). The drugs used in this study did not produce any change in locomotor activity. Significance: The results demonstrate that the neuromodulatory effects of venlafaxine, via the inhibition of K+ channels, are possibly involved in its anti-immobility activity in the mouse FST. © 2010 Elsevier Inc. All rights reserved.
Introduction Studies have reported that nitric oxide (NO) can activate different types of potassium (K+) channels (Bolotina et al. 1994; Armstead 1996; Jeong et al. 2001). Several NO physiological actions are mediated through its interaction with the heme iron of soluble guanylate cyclase (sGC), leading to enzyme activation and subsequent increase in cyclic guanosine monophosphate (cGMP). The K+ channel activation by NO is mediated by cGMP (Taniguchi et al. 1993) or by NO itself (Bolotina et al. 1994). The opening of K+ channels leads to hyperpolarization of cell membranes, which results in a decrease in cell excitability (Mackinnon 2003). Venlafaxine is a non-tricyclic antidepressant that inhibits serotonin (5-HT) and noradrenaline (NA) reuptake (Berrocoso et al. 2004). Additionally, the involvement of the L-arginine–NO–cGMP signaling
⁎ Corresponding author. Laboratório de Síntese, Reatividade e Avaliação Farmacológica e Toxicológica de Organocalcogênios, Centro de Ciências Naturais e Exatas, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil. Tel.: +55 55 32208140; fax: +55 55 32208978. E-mail address: [email protected] (C.W. Nogueira). 0024-3205/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.lfs.2010.01.013
pathway in mediating the antidepressant-like activity of venlafaxine in the mouse forced swim test (FST) has been demonstrated, supporting the notion that the inhibition of NO production in the brain may be critical to the action of antidepressants (Dhir and Kulkarni 2007a). Mantovani et al. (2003) have reported that L-arginine–NO–cGMP is an important signaling pathway involved in depression. Although some mechanisms of action of venlafaxine have been reported, additional mechanisms that might be involved in its antidepressant-like effect, including the involvement of K+ channels, still need further investigation. Taking into account that K+ channels represent one of the major downstream targets regulated by activation of the L-arginine–NO–cGMP pathway, it is possible that the inhibition of NO production elicited by venlafaxine in the FST may reflect an inhibition of K+ channels. Studies have suggested the involvement of K+ channels in the modulation of depression. In fact, different types of K+ channel inhibitors, such as tetraethylammonium (TEA), apamin, charybdotoxin, gliquidone and glibenclamide, were able to produce an antidepressantlike effect in the mouse FST (Galeotti et al. 1999; Kaster et al. 2005), whereas the K+ channel openers, such as minoxidil or cromakalim, increased the immobility time, indicating the induction of a depressantlike effect (Galeotti et al. 1999).
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Moreover, evidence indicates that antidepressants may modulate neuronal excitability via K+ channel inhibition, and this may even be a common pathway of pharmacological action of these drugs (Tytgat et al. 1997; Kobayashi et al. 2003). For instance, the antidepressantlike effect of tramadol in the FST involves the inhibition of the Larginine–NO–cGMP pathway, as well as different types of K+ channels (Jesse et al. 2008, 2009). Interestingly, some similarities are claimed to exist structurally and pharmacologically between venlafaxine and tramadol (Markowitz and Patrick 1998). Therefore, the present study attempts to investigate whether different types of K+ channels are involved in the antidepressant-like activity of venlafaxine in the mouse FST. Materials and methods Animals Behavioral experiments were conducted using male adult Swiss mice (25–35 g) maintained at 22–25 °C, with free access to water and food under a 12:12 hour light/dark cycle, with lights on at 6:00 a.m. All manipulations were carried out between 08.00 a.m. and 04.00 p.m. All experiments were performed on separate groups of animals and each animal was used only once in each test. The animals were used in accordance with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1996) and in accordance with the guidelines of the Committee on Care and Use of Experimental Animal Resources of the Federal University of Santa Maria, Brazil. All efforts were made to minimize animal suffering and to reduce the number of animals used in the experiments. Open-field test (OFT) In order to rule out a confounding effect of locomotor activity in the FST, the ambulatory behavior of mice treated with all drugs employed in the present study was assessed in the OFT. The open-field was made of polywood and surrounded by walls 30 cm in height. The floor of the open-field, 45 cm in length and 45 cm in width, was divided by masking tape markers into 9 squares (3 rows of 3). Each animal was placed individually at the center of the apparatus and the number of squares crossed with all paws (crossing) was counted in a 6 min session. The arena floor was cleaned between the trials and the test was carried out in a temperature and light controlled room (Walsh and Cummins 1976). Forced swim test (FST) The FST was conducted using the method described in the literature (Porsolt et al. 1977a). Briefly, mice were individually forced to swim in open cylinders (25 cm height × 10 cm diameter) containing 19 cm of water at 25 ± 1 °C. The duration of immobility was scored during the 6 min test period (Zomkowski et al. 2006). Each mouse was recorded as immobile when floating motionless or making only those movements necessary to keep its head above water. A decrease in the duration of immobility is indicative of an antidepressant-like effect.
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perpendicularly through the skull and no more than 2 mm into the brain of the mouse. The injection was given over 30 s, and the needle remained in place for another 30 s in order to avoid the reflux of the substances injected. The injection site was 1 mm to the right or left of the mid-point on a line drawn through to the anterior base of the ears. To ascertain that the drugs were administered exactly into the cerebral ventricle, the brains were dissected and examined macroscopically after the test (Jesse et al. 2009). Drugs and treatment Venlafaxine hydrochloride (1-[2-(dimethylamino)-1-(4-methoxyphenyl) ethyl] cyclohexanol) was a gift from Medley (Campinas, São Paulo, Brazil). The following drugs were used: tetraethylammonium (TEA, Sigma Chemical Co, USA), apamin, charybdotoxin, cromakalim, minoxidil and glibenclamide (Tocris Cookson, Ballwin, MO, USA). Cromakalim and minoxidil were dissolved in saline (NaCl 0.9%) with 10% Tween 80, whereas all of the other drugs were dissolved in saline solution immediately before use. Appropriate vehicle-treated groups were simultaneously assessed. Venlafaxine was administered to mice by the intraperitoneal (i.p.) route at a dose of 2 mg/kg (a subeffective dose) or 8 mg/kg (an effective dose) at a fixed volume of 10 ml/kg of body weight (Dhir and Kulkarni 2007a). Other drugs were administered by the i.c.v. route in a volume of 5 μl per site/mouse. To test the hypothesis that the antidepressant-like effect of venlafaxine is mediated through an interaction with different types of K+ channels, distinct groups of animals were treated with different classes of drugs. Doses and administration schedules were chosen on the basis of published studies (Kaster et al. 2005; Dhir and Kulkarni 2007a; Budni et al. 2007). In the experiments designed to test the effect of K+ channel inhibitors in the FST, mice were pretreated with subeffective doses of TEA (a non-specific inhibitor of K+ channels, 25 pg/site, i.c.v.), glibenclamide (an ATP-sensitive K+ channel inhibitor, (K+ ATP) 0.5 pg/site, i.c.v.), apamin (a small conductance calcium-activated K+ channel inhibitor (SKCa), 10 pg/site, i.c.v.), charybdotoxin (a largeand intermediate-(IK) conductance calcium-activated K+ channel inhibitor, 25 pg/site, i.c.v.) or vehicle 15 min before the administration of venlafaxine (2 mg/kg, i.p., a subeffective dose). Thirty minutes after venlafaxine administration, mice were tested in the OFT and FST, respectively. The OFT was used to help exclude possible stimulatory effect on motor activity. In another series of experiments, animals were pretreated with cromakalim or minoxidil (K+ channel openers, 10 μg/site, i.c.v.) or vehicle (saline with 10% Tween 80; control group) 15 min before venlafaxine (8 mg/kg, i.p., an effective dose) or saline administration. Thirty minutes after venlafaxine administration, the OFT and FST were carried out. Statistical analysis All experimental results are given as the mean (s) ± S.E.M. Comparisons between experimental and control groups were performed by two-way ANOVA followed by the Newman–Keuls test for post hoc comparison when appropriate. A value of p < 0.05 was considered to be significant. The main effects are presented only when interactions were not significant.
Intracerebroventricular injection technique Results K+ channel openers and inhibitors were administered to mice in a single injection of 5 μl by the intracerebroventricular (i.c.v.) route, directly into the lateral ventricle, as described previously (Zomkowski et al. 2002), with the bregma fissure as a reference. The i.c.v. administration was performed under light isoflurane anesthesia. Briefly, a 0.4 mm external diameter hypodermic needle attached to a cannula, which was linked to a 25 μl Hamilton syringe, was inserted
Effects of K+ channel inhibitors on the antidepressant-like action of venlafaxine in mice evaluated in the FST and OFT The results depicted in Fig. 1A show the effect of TEA (25 pg/site, i.c.v.) in producing an antidepressant-like effect with a subeffective dose of venlafaxine (2 mg/kg, i.p.) in the FST (F(1,24) = 21.89, p <0.0001). The
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Fig. 1. Effect of pretreatment of mice with a subeffective dose of TEA (25 pg/site, i.c.v., panel A), glibenclamide (0.5 pg/site, i.c.v., panel B), charybdotoxin (25 pg/site, i.c.v., panel C) and apamin (10 pg/site, i.c.v., panel D) on the action of a subeffective dose of venlafaxine (2 mg/kg, i.p.) in the mouse FST. Values are expressed as mean ± S.E.M. (n = 7 mice/group). Data were analyzed by two-way analysis of variance (ANOVA) followed by the Newman–Keuls test. *p < 0.001 compared with the K+ channel inhibitor-pretreated group or venlafaxine-treated group.
administration of TEA and venlafaxine to mice did not alter the number of crossings (F(1,24) =0.33, p <0.5734) in the OFT (Table 1). Fig. 1B shows that glibenclamide (0.5 pg/site, i.c.v.) and venlafaxine (2 mg/kg, i.p., a subeffective dose) (F(1,24) = 7.38, p < 0.012) produced an antidepressant-like action in the mouse FST. The number of crossings (F(1,24) = 0.35, p < 0.5609) was unmodified by the combined administration of glibenclamide and venlafaxine to mice (Table 1). The results demonstrated in Fig. 1C show that a subeffective dose of venlafaxine (2 mg/kg, i.p.) produced an antidepressant-like effect with charybdotoxin (25 pg/site, i.c.v.) in the mouse FST (F(1,24) = 7.71, p < 0.0105). The administration of venlafaxine with charybdotoxin to mice did not modify the number of crossings (F(1,24) = 0.36, p < 0.5543) in the OFT (Table 1).
Table 1 Effect of TEA (25 pg/site, i.c.v.), glibenclamide (0.5 pg/site, i.c.v.), charybdotoxin (25 pg/ site, i.c.v.), apamin (10 pg/site, i.c.v.), venlafaxine (2 mg/kg, i.p.) or combined administration of venlafaxine and K+ channel inhibitors on the number of crossings in the OFT. Treatment
Number of crossings
Vehicle TEA Venlafaxine TEA + venlafaxine Vehicle control Glibenclamide Venlafaxine Glibenclamide + venlafaxine Vehicle control Charybdotoxin Venlafaxine Charybdotoxin + venlafaxine Vehicle control Apamin Venlafaxine Apamin + venlafaxine
79.14 ± 13.97 76.29 ± 6.23 72.57 ± 6.02 79.43 ± 4.38 74.71 ± 7.26 72.43 ± 6.30 65.86 ± 8.39 73.43 ± 10.79 73.29 ± 6.50 74.43 ± 6.16 78.29 ± 8.29 82.86 ± 5.08 68.71 ± 5.30 73.86 ± 8.77 71.86 ± 8.77 66.29 ± 8.31
Values are expressed as mean ± S.E.M. (n = 7 mice/group). Data were analyzed by twoway analysis of variance (ANOVA) followed by Newman–Keuls test.
Fig. 1D shows that apamin (10 pg/site, i.c.v.) was able to produce an antidepressant-like action with a subeffective dose of venlafaxine (2 mg/kg, i.p.) in the mouse FST (F(1,24) = 15.89, p < 0.0005). The administration of venlafaxine with apamin to mice did not produce any effect on the number of crossings (F(1,24) = 0.42, p < 0.5221) in the OFT (Table 1). Effects of K+ channel openers on the antidepressant-like action of venlafaxine in mice evaluated in the FST and OFT Fig. 2A shows the pretreatment of mice with cromakalim (10 μg/site, i.c.v.) in the antidepressant-like effect of venlafaxine (8 mg/kg, i.p., an effective dose) in the FST. Cromakalim was able to reverse the reduction of the immobility time produced by venlafaxine (F(1,24) = 14.21, p < 0.0009). Cromakalim and venlafaxine did not alter the number of crossings (F(1,24) = 0.03, p < 0.8744) in the mouse OFT (Table 2). The results depicted in Fig. 2B show that the pretreatment of mice with minoxidil (a K+ channel opener, 10 μg/site, i.c.v.) reversed the antidepressant-like effect of venlafaxine (8 mg/kg, i.p., an effective dose) in the FST (F(1,24) = 14.18, p < 0.001). The combined administration of venlafaxine and minoxidil to mice did not produce any effect on the number of crossings (F(1,24) = 0.59, p < 0.4503) in the OFT (Table 2). Discussion In the present study, we demonstrated that the antidepressantlike effect of the acute administration of venlafaxine, a dual 5-HT and NA reuptake inhibitor, in the mouse FST is linked to the modulation of K+ channels. Pretreatment of mice with subeffective doses of different K+ channel inhibitors and venlafaxine produced an antidepressantlike effect, while pretreatment with K+ channel openers prevented the antidepressant-like effect of an effective dose of venlafaxine in the mouse FST. Venlafaxine has been shown to be superior in efficacy to selective 5HT reuptake inhibitors (SSRIs) in severe major depressive disorder,
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Fig. 2. Effect of pretreatment of mice with cromakalim (10 μg/site, i.c.v., panel A) and minoxidil (10 μg/site, i.c.v., panel B) on venlafaxine (8 mg/kg, i.p., an effective dose)-induced reduction in the immobility time in the mouse FST. Values are expressed as mean ± S.E.M. (n = 7 mice/group). Data were analyzed by two-way analysis of variance (ANOVA) followed by the Newman–Keuls test. *p < 0.001 compared with the vehicle-treated control; #p < 0.001 compared with the venlafaxine-treated group.
treatment-resistant depression and obsessive compulsive disorder (Gutierrez et al. 2003). This antidepressant drug is known to exhibit six- to seven-fold selectivity for inhibition of 5-HT reuptake as compared to NA reuptake in the synaptosomes of rat brain and a 15- to 30-fold higher affinity for 5-HT transporter binding sites as compared to those of the NA transporter (Gould et al. 2006). Various behavioral, biochemical and molecular studies are being carried out to elucidate the exact mechanism of the antidepressant effect of venlafaxine (Gould et al. 2006; Dhir and Kulkarni 2007a,b; Berrocoso and Mico 2009). However, the role of K+ channels in the antidepressant action of venlafaxine has not been investigated. Therefore, the present study aimed to investigate whether the antidepressant-like effect of venlafaxine is mediated through the inhibition of K+ channels, in addition to the other mechanisms already described in the literature. It has been suggested that nitric oxide synthase (NOS) inhibitors, when administered prior to venlafaxine treatment, enhance its antidepressant-like action in the mouse FST (Dhir and Kulkarni 2007a). Moreover, the L-arginine–NO–cGMP signaling pathway has been implicated in depression (Mantovani et al. 2003). Recent evidence has shown that the reduction of NO levels within the hippocampus can induce antidepressant-like effects, thus implicating endogenous hippocampal NO in the neurobiology of stress and depression (Joca and Guimaraes 2006). Kaster et al. (2005) have reported that NO and cGMP are important modulators of some K+ channels at the central level, and the inhibition of these channels might represent an important role in the mechanisms involved in major depressive disorder. Furthermore, several studies employing different pharmacological models have suggested that NO affects K+ channels (Lazaro-Ibanez et al. 2001; Fernandes et al. 2002). For example, NO may activate K+ channels resulting in changes in immobility time in the mouse FST (Inan et al. 2004). Large-conductance Ca2+-activated K+ channels have been suggested as one of the physiological targets of NO in the brain (Jeong et al. 2001). Therefore,
Table 2 Effect of cromakalim (10 µg/site, i.c.v.), minoxidil (10 µg/site, i.c.v.), venlafaxine (8 mg/ kg, i.p.) or combined administration of venlafaxine and cromakalim or minoxidil on the number of crossings in the OFT. Treatment
Number of crossings
Vehicle Cromakalim Venlafaxine Cromakalim + venlafaxine Vehicle control Minoxidil Venlafaxine Minoxidil + venlafaxine
68.00 ± 7.67 72.29 ± 8.46 70.86 ± 9.07 72.43 ± 8.70 73.43 ± 8.90 75.14 ± 8.84 78.71 ± 6.79 68.29 ± 6.83
Values are expressed as mean ± S.E.M. (n = 7 mice/group).Data were analyzed by twoway analysis of variance (ANOVA) followed by Newman–Keuls test.
K+ channels represent one of the major downstream targets regulated by the L-arginine–NO–cGMP pathway. It has also been reported that the mechanism of action and structure of venlafaxine are very similar to tramadol (Markowitz and Patrick 1998). The oral administration of tramadol, a synthetic opioid, produces an antidepressant-like effect in the rat FST by a mechanism that involves the inhibition of the L-arginine–NO pathway (Jesse et al. 2008). Recently, it was demonstrated that the inhibition of different types of K+ channels augments the antidepressant-like effect of the oral administration of tramadol in the mouse FST (Jesse et al. 2009). These results reinforce the idea that the inhibition of the L-arginine–NO–cGMP pathway prevents the activation of K+ channels. As demonstrated in the present study, the inhibition of K+ channels could be one of the mechanisms of action by which venlafaxine elicits its antidepressant-like activity in the mouse FST. In this study, pretreatment of mice with subeffective doses of pharmacological compounds able to block different types of K+ channels, such as TEA, glibenclamide, apamin and charybdotoxin, elicited an antidepressant-like effect with a subeffective dose of venlafaxine. These results show that the inhibition of K+ channels plays an important role in the antidepressant-like effect of venlafaxine in mice, probably by inhibiting membrane hyperpolarization, leading to an increased excitatory response. Moreover, one cannot exclude the possibility that the K+ channel activity is modified by the drugs used in this study, so the neuronal circuits related to the effects of venlafaxine could be affected, and thus, the immobility time with venlafaxine could indirectly change. Pretreatment of mice with drugs able to active K+ channels, such as cromakalim and minoxidil, prevented the anti-immobility effect of an effective dose of venlafaxine in the mouse FST, without affecting the locomotor activity. These results help to support the involvement of K+ channels in the antidepressant-like effect of venlafaxine in mice. Additionally, studies have shown that the pretreatment of animals with cromakalim is able to antagonize the anti-immobility effect of several antidepressants, such as imipramine, amitriptyline, desipramine and paroxetine (Redrobe et al. 1996). Moreover, the blockade of K+ channels enhances the basal release of 5-HT in rat hippocampal slices (Schechter 1997). Imipramine, fluoxetine and agmatine are some examples of antidepressants that influence K+ channels and the L-arginine–NO–cGMP pathway (Choi et al. 2004; Harkin et al. 2004; Kaster et al. 2007). Other antidepressants, such as desipramine, amitriptyline, nortriptyline, clomipramine, maprotiline, citalopram and paroxetine, also produce an inhibition of K+ currents, which might underlie their therapeutic effects (Nicholson et al. 2002; Kobayashi and Washiyama 2004; Kobayashi et al. 2006). It has been suggested that GIRK channels play an important role in the inhibitory regulation of neuronal excitability in most brain regions through the activation of various G protein coupled receptors (Kobayashi and Washiyama 2004).The inhibition of GIRK channels by fluoxetine may contribute to some of its therapeutic effects observed in clinical practice (Kobayashi et al. 2003). In addition, GIRK channel inhibitors
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might be potential agents for the treatment of users of addictive drugs (Kobayashi and Washiyama 2004). Therefore, a study aiming to investigate the involvement of K+ channels in the antidepressant-like effect of venlafaxine is important from the point of view of clinical trials. The FST is widely used as an animal model of depression to screen new potent antidepressant drugs (Bourin et al. 2005). Although the FST model is accepted because it is sensitive to all major classes of antidepressants, including tricyclics, 5-HT-specific reuptake inhibitors, monoamine oxidase inhibitors and atypicals (Porsolt et al. 1977b), it has some drawbacks in terms of the possibility of obtaining false positives or negatives. Drugs enhancing locomotor activity can evoke a false positive effect in these tests, whereas drugs decreasing locomotion may give a ‘false’ negative result (Borsini and Meli 1988). In the present study, animals were submitted to the OFT in order to rule out the possibility that the interaction of K+ channel inhibitors and venlafaxine produces an antidepressant-like effect in the FST due to a stimulant effect. The experimental data obtained in the OFT excludes the possible stimulatory effects of K+ channel inhibitors and venlafaxine on the motor activity of mice. Conclusion The present study demonstrates that the pretreatment of mice with different K+ channel inhibitors produces an antidepressant-like action with a subeffective dose of venlafaxine, while pretreatment with K+ channel openers prevented the antidepressant-like effect of an effective dose of venlafaxine. However, more studies are necessary to elucidate the molecular mechanisms of venlafaxine action in K+ channels, as well as other mechanisms that may be involved in its antidepressant-like activity. Conflict of interest statement None. Acknowledgements Financial support by UFSM, FAPERGS, CAPES and CNPq (PIBIC) is gratefully acknowledged. References Armstead WM. Role of ATP-sensitive K+ channels in cGMP mediated pial artery vasodilation. American Journal of Physiology—Heart and Circulatory Physiology 39 (2), 423–426, 1996. Berrocoso E, Rojas-Corrales MO, Mico JA. Non-selective opioid receptor antagonism of the antidepressant-like effect of venlafaxine in the forced swimming test in mice. Neuroscience Letters 363 (1), 25–28, 2004. Berrocoso E, Mico JA. Role of serotonin 5-HT1A receptors in the antidepressant-like effect and the antinociceptive effect of venlafaxine in mice. International Journal of Neuropsychopharmacology 12 (1), 61–71, 2009. Bolotina VM, Najibi S, Palacino JJ, Pagano PJ, Cohen RA. Nitric oxide directly activates calcium-dependent potassium channels in vascular smooth muscle. Nature 368 (6474), 850–853, 1994. Borsini F, Meli A. Is the forced swimming test a suitable model for revealing antidepressant activity? Psychopharmacology 94 (2), 147–160, 1988. Bourin M, Chenu F, Ripoll N, David DJ. A proposal of decision tree to screen putative antidepressants using forced swim and tail suspension tests. Behavioural Brain Research 164 (2), 266–269, 2005. Budni J, Gadotti VM, Kaster MP, Santos ARS, Rodrigues ALS. Role of different types of potassium channels in the antidepressant-like effect of agmatine in the mouse forced swimming test. European Journal of Pharmacology 575, 87–93, 2007. Choi JS, Choi BH, Ahn HS, Kim MJ, Han TH, Rhie DJ. Fluoxetine inhibits A-type potassium currents in primary cultured rat hippocampal neurons. Brain Research 1018 (2), 201–207, 2004. Dhir A, Kulkarni SK. Involvement of L-arginine–nitric oxide–cyclic guanosine monophosphate pathway in the antidepressant-like effect of venlafaxine in mice. Progress in Neuro-Psychopharmacology & Biological Psychiatry 31 (4), 921–925, 2007a. Dhir A, Kulkarni SK. Involvement of sigma-1 receptor modulation in the antidepressant action of venlafaxine. Neuroscience Letters 420 (3), 204–208, 2007b. Fernandes D, Da Silva-Santos JE, Assreuy J. Nitric oxide-induced inhibition of mouse paw edema: involvement of soluble guanylate cyclase and potassium channels. Inflammation Research 51 (8), 377–384, 2002.
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Involvement of potassium channels in the antidepressant-like effect of venlafaxine in mice Cristiani F. Bortolatto, Cristiano R. Jesse, Ethel A. Wilhelm, Cristina W. Nogueira ⁎ Laboratory of Synthesis, Reactivity, Pharmacological and Toxicological Evaluation of Organochalcogens, Natural Science Institute, Federal University of Santa Maria, Santa Maria, CEP 97105-900, RS, Brazil
a r t i c l e
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Article history: Received 23 September 2009 Accepted 18 January 2010 Keywords: Venlafaxine Antidepressant-like Potassium channels Forced swim test (FST) Mice
a b s t r a c t Aims: Studies have shown that the acute administration of venlafaxine elicits an antidepressant-like effect in the mouse forced swim test (FST) by a mechanism dependent on the L-arginine–nitric oxide (NO)–cyclic guanosine monophosphate (cGMP) pathway. Because it has been reported that NO activates different types of potassium (K+) channels in the brain, this study investigated the involvement of K+ channels in the antidepressant-like effect of venlafaxine in the mouse FST. Main methods: Male adult Swiss mice were pretreated with different K+ channel inhibitors or openers 15 min before venlafaxine administration. After 30 min, the open-field test (OFT) and FST were carried out. Key findings: Intracerebroventricular (i.c.v.) pretreatment of mice with subeffective doses of tetraethylammonium (TEA, a non-specific inhibitor of K+ channels, 25 pg/site), glibenclamide (an ATP-sensitive K+ channel inhibitor, 0.5 pg/site), charybdotoxin (a large- and intermediate-conductance calcium-activated K+ channel inhibitor, 25 pg/site) or apamin (a small-conductance calcium-activated K+ channel inhibitor, 10 pg/site) was able to potentiate the action of a subeffective dose of venlafaxine (2 mg/kg, i.p.). Moreover, the reduction in the immobility time elicited by an effective dose of venlafaxine (8 mg/kg, i.p.) in the FST was prevented by the pretreatment of mice with the K+ channel openers cromakalim (10 µg/site, i.c.v.) and minoxidil (10 µg/site, i.c.v.). The drugs used in this study did not produce any change in locomotor activity. Significance: The results demonstrate that the neuromodulatory effects of venlafaxine, via the inhibition of K+ channels, are possibly involved in its anti-immobility activity in the mouse FST. © 2010 Elsevier Inc. All rights reserved.
Introduction Studies have reported that nitric oxide (NO) can activate different types of potassium (K+) channels (Bolotina et al. 1994; Armstead 1996; Jeong et al. 2001). Several NO physiological actions are mediated through its interaction with the heme iron of soluble guanylate cyclase (sGC), leading to enzyme activation and subsequent increase in cyclic guanosine monophosphate (cGMP). The K+ channel activation by NO is mediated by cGMP (Taniguchi et al. 1993) or by NO itself (Bolotina et al. 1994). The opening of K+ channels leads to hyperpolarization of cell membranes, which results in a decrease in cell excitability (Mackinnon 2003). Venlafaxine is a non-tricyclic antidepressant that inhibits serotonin (5-HT) and noradrenaline (NA) reuptake (Berrocoso et al. 2004). Additionally, the involvement of the L-arginine–NO–cGMP signaling
⁎ Corresponding author. Laboratório de Síntese, Reatividade e Avaliação Farmacológica e Toxicológica de Organocalcogênios, Centro de Ciências Naturais e Exatas, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil. Tel.: +55 55 32208140; fax: +55 55 32208978. E-mail address: [email protected] (C.W. Nogueira). 0024-3205/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.lfs.2010.01.013
pathway in mediating the antidepressant-like activity of venlafaxine in the mouse forced swim test (FST) has been demonstrated, supporting the notion that the inhibition of NO production in the brain may be critical to the action of antidepressants (Dhir and Kulkarni 2007a). Mantovani et al. (2003) have reported that L-arginine–NO–cGMP is an important signaling pathway involved in depression. Although some mechanisms of action of venlafaxine have been reported, additional mechanisms that might be involved in its antidepressant-like effect, including the involvement of K+ channels, still need further investigation. Taking into account that K+ channels represent one of the major downstream targets regulated by activation of the L-arginine–NO–cGMP pathway, it is possible that the inhibition of NO production elicited by venlafaxine in the FST may reflect an inhibition of K+ channels. Studies have suggested the involvement of K+ channels in the modulation of depression. In fact, different types of K+ channel inhibitors, such as tetraethylammonium (TEA), apamin, charybdotoxin, gliquidone and glibenclamide, were able to produce an antidepressantlike effect in the mouse FST (Galeotti et al. 1999; Kaster et al. 2005), whereas the K+ channel openers, such as minoxidil or cromakalim, increased the immobility time, indicating the induction of a depressantlike effect (Galeotti et al. 1999).
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Moreover, evidence indicates that antidepressants may modulate neuronal excitability via K+ channel inhibition, and this may even be a common pathway of pharmacological action of these drugs (Tytgat et al. 1997; Kobayashi et al. 2003). For instance, the antidepressantlike effect of tramadol in the FST involves the inhibition of the Larginine–NO–cGMP pathway, as well as different types of K+ channels (Jesse et al. 2008, 2009). Interestingly, some similarities are claimed to exist structurally and pharmacologically between venlafaxine and tramadol (Markowitz and Patrick 1998). Therefore, the present study attempts to investigate whether different types of K+ channels are involved in the antidepressant-like activity of venlafaxine in the mouse FST. Materials and methods Animals Behavioral experiments were conducted using male adult Swiss mice (25–35 g) maintained at 22–25 °C, with free access to water and food under a 12:12 hour light/dark cycle, with lights on at 6:00 a.m. All manipulations were carried out between 08.00 a.m. and 04.00 p.m. All experiments were performed on separate groups of animals and each animal was used only once in each test. The animals were used in accordance with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1996) and in accordance with the guidelines of the Committee on Care and Use of Experimental Animal Resources of the Federal University of Santa Maria, Brazil. All efforts were made to minimize animal suffering and to reduce the number of animals used in the experiments. Open-field test (OFT) In order to rule out a confounding effect of locomotor activity in the FST, the ambulatory behavior of mice treated with all drugs employed in the present study was assessed in the OFT. The open-field was made of polywood and surrounded by walls 30 cm in height. The floor of the open-field, 45 cm in length and 45 cm in width, was divided by masking tape markers into 9 squares (3 rows of 3). Each animal was placed individually at the center of the apparatus and the number of squares crossed with all paws (crossing) was counted in a 6 min session. The arena floor was cleaned between the trials and the test was carried out in a temperature and light controlled room (Walsh and Cummins 1976). Forced swim test (FST) The FST was conducted using the method described in the literature (Porsolt et al. 1977a). Briefly, mice were individually forced to swim in open cylinders (25 cm height × 10 cm diameter) containing 19 cm of water at 25 ± 1 °C. The duration of immobility was scored during the 6 min test period (Zomkowski et al. 2006). Each mouse was recorded as immobile when floating motionless or making only those movements necessary to keep its head above water. A decrease in the duration of immobility is indicative of an antidepressant-like effect.
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perpendicularly through the skull and no more than 2 mm into the brain of the mouse. The injection was given over 30 s, and the needle remained in place for another 30 s in order to avoid the reflux of the substances injected. The injection site was 1 mm to the right or left of the mid-point on a line drawn through to the anterior base of the ears. To ascertain that the drugs were administered exactly into the cerebral ventricle, the brains were dissected and examined macroscopically after the test (Jesse et al. 2009). Drugs and treatment Venlafaxine hydrochloride (1-[2-(dimethylamino)-1-(4-methoxyphenyl) ethyl] cyclohexanol) was a gift from Medley (Campinas, São Paulo, Brazil). The following drugs were used: tetraethylammonium (TEA, Sigma Chemical Co, USA), apamin, charybdotoxin, cromakalim, minoxidil and glibenclamide (Tocris Cookson, Ballwin, MO, USA). Cromakalim and minoxidil were dissolved in saline (NaCl 0.9%) with 10% Tween 80, whereas all of the other drugs were dissolved in saline solution immediately before use. Appropriate vehicle-treated groups were simultaneously assessed. Venlafaxine was administered to mice by the intraperitoneal (i.p.) route at a dose of 2 mg/kg (a subeffective dose) or 8 mg/kg (an effective dose) at a fixed volume of 10 ml/kg of body weight (Dhir and Kulkarni 2007a). Other drugs were administered by the i.c.v. route in a volume of 5 μl per site/mouse. To test the hypothesis that the antidepressant-like effect of venlafaxine is mediated through an interaction with different types of K+ channels, distinct groups of animals were treated with different classes of drugs. Doses and administration schedules were chosen on the basis of published studies (Kaster et al. 2005; Dhir and Kulkarni 2007a; Budni et al. 2007). In the experiments designed to test the effect of K+ channel inhibitors in the FST, mice were pretreated with subeffective doses of TEA (a non-specific inhibitor of K+ channels, 25 pg/site, i.c.v.), glibenclamide (an ATP-sensitive K+ channel inhibitor, (K+ ATP) 0.5 pg/site, i.c.v.), apamin (a small conductance calcium-activated K+ channel inhibitor (SKCa), 10 pg/site, i.c.v.), charybdotoxin (a largeand intermediate-(IK) conductance calcium-activated K+ channel inhibitor, 25 pg/site, i.c.v.) or vehicle 15 min before the administration of venlafaxine (2 mg/kg, i.p., a subeffective dose). Thirty minutes after venlafaxine administration, mice were tested in the OFT and FST, respectively. The OFT was used to help exclude possible stimulatory effect on motor activity. In another series of experiments, animals were pretreated with cromakalim or minoxidil (K+ channel openers, 10 μg/site, i.c.v.) or vehicle (saline with 10% Tween 80; control group) 15 min before venlafaxine (8 mg/kg, i.p., an effective dose) or saline administration. Thirty minutes after venlafaxine administration, the OFT and FST were carried out. Statistical analysis All experimental results are given as the mean (s) ± S.E.M. Comparisons between experimental and control groups were performed by two-way ANOVA followed by the Newman–Keuls test for post hoc comparison when appropriate. A value of p < 0.05 was considered to be significant. The main effects are presented only when interactions were not significant.
Intracerebroventricular injection technique Results K+ channel openers and inhibitors were administered to mice in a single injection of 5 μl by the intracerebroventricular (i.c.v.) route, directly into the lateral ventricle, as described previously (Zomkowski et al. 2002), with the bregma fissure as a reference. The i.c.v. administration was performed under light isoflurane anesthesia. Briefly, a 0.4 mm external diameter hypodermic needle attached to a cannula, which was linked to a 25 μl Hamilton syringe, was inserted
Effects of K+ channel inhibitors on the antidepressant-like action of venlafaxine in mice evaluated in the FST and OFT The results depicted in Fig. 1A show the effect of TEA (25 pg/site, i.c.v.) in producing an antidepressant-like effect with a subeffective dose of venlafaxine (2 mg/kg, i.p.) in the FST (F(1,24) = 21.89, p <0.0001). The
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Fig. 1. Effect of pretreatment of mice with a subeffective dose of TEA (25 pg/site, i.c.v., panel A), glibenclamide (0.5 pg/site, i.c.v., panel B), charybdotoxin (25 pg/site, i.c.v., panel C) and apamin (10 pg/site, i.c.v., panel D) on the action of a subeffective dose of venlafaxine (2 mg/kg, i.p.) in the mouse FST. Values are expressed as mean ± S.E.M. (n = 7 mice/group). Data were analyzed by two-way analysis of variance (ANOVA) followed by the Newman–Keuls test. *p < 0.001 compared with the K+ channel inhibitor-pretreated group or venlafaxine-treated group.
administration of TEA and venlafaxine to mice did not alter the number of crossings (F(1,24) =0.33, p <0.5734) in the OFT (Table 1). Fig. 1B shows that glibenclamide (0.5 pg/site, i.c.v.) and venlafaxine (2 mg/kg, i.p., a subeffective dose) (F(1,24) = 7.38, p < 0.012) produced an antidepressant-like action in the mouse FST. The number of crossings (F(1,24) = 0.35, p < 0.5609) was unmodified by the combined administration of glibenclamide and venlafaxine to mice (Table 1). The results demonstrated in Fig. 1C show that a subeffective dose of venlafaxine (2 mg/kg, i.p.) produced an antidepressant-like effect with charybdotoxin (25 pg/site, i.c.v.) in the mouse FST (F(1,24) = 7.71, p < 0.0105). The administration of venlafaxine with charybdotoxin to mice did not modify the number of crossings (F(1,24) = 0.36, p < 0.5543) in the OFT (Table 1).
Table 1 Effect of TEA (25 pg/site, i.c.v.), glibenclamide (0.5 pg/site, i.c.v.), charybdotoxin (25 pg/ site, i.c.v.), apamin (10 pg/site, i.c.v.), venlafaxine (2 mg/kg, i.p.) or combined administration of venlafaxine and K+ channel inhibitors on the number of crossings in the OFT. Treatment
Number of crossings
Vehicle TEA Venlafaxine TEA + venlafaxine Vehicle control Glibenclamide Venlafaxine Glibenclamide + venlafaxine Vehicle control Charybdotoxin Venlafaxine Charybdotoxin + venlafaxine Vehicle control Apamin Venlafaxine Apamin + venlafaxine
79.14 ± 13.97 76.29 ± 6.23 72.57 ± 6.02 79.43 ± 4.38 74.71 ± 7.26 72.43 ± 6.30 65.86 ± 8.39 73.43 ± 10.79 73.29 ± 6.50 74.43 ± 6.16 78.29 ± 8.29 82.86 ± 5.08 68.71 ± 5.30 73.86 ± 8.77 71.86 ± 8.77 66.29 ± 8.31
Values are expressed as mean ± S.E.M. (n = 7 mice/group). Data were analyzed by twoway analysis of variance (ANOVA) followed by Newman–Keuls test.
Fig. 1D shows that apamin (10 pg/site, i.c.v.) was able to produce an antidepressant-like action with a subeffective dose of venlafaxine (2 mg/kg, i.p.) in the mouse FST (F(1,24) = 15.89, p < 0.0005). The administration of venlafaxine with apamin to mice did not produce any effect on the number of crossings (F(1,24) = 0.42, p < 0.5221) in the OFT (Table 1). Effects of K+ channel openers on the antidepressant-like action of venlafaxine in mice evaluated in the FST and OFT Fig. 2A shows the pretreatment of mice with cromakalim (10 μg/site, i.c.v.) in the antidepressant-like effect of venlafaxine (8 mg/kg, i.p., an effective dose) in the FST. Cromakalim was able to reverse the reduction of the immobility time produced by venlafaxine (F(1,24) = 14.21, p < 0.0009). Cromakalim and venlafaxine did not alter the number of crossings (F(1,24) = 0.03, p < 0.8744) in the mouse OFT (Table 2). The results depicted in Fig. 2B show that the pretreatment of mice with minoxidil (a K+ channel opener, 10 μg/site, i.c.v.) reversed the antidepressant-like effect of venlafaxine (8 mg/kg, i.p., an effective dose) in the FST (F(1,24) = 14.18, p < 0.001). The combined administration of venlafaxine and minoxidil to mice did not produce any effect on the number of crossings (F(1,24) = 0.59, p < 0.4503) in the OFT (Table 2). Discussion In the present study, we demonstrated that the antidepressantlike effect of the acute administration of venlafaxine, a dual 5-HT and NA reuptake inhibitor, in the mouse FST is linked to the modulation of K+ channels. Pretreatment of mice with subeffective doses of different K+ channel inhibitors and venlafaxine produced an antidepressantlike effect, while pretreatment with K+ channel openers prevented the antidepressant-like effect of an effective dose of venlafaxine in the mouse FST. Venlafaxine has been shown to be superior in efficacy to selective 5HT reuptake inhibitors (SSRIs) in severe major depressive disorder,
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Fig. 2. Effect of pretreatment of mice with cromakalim (10 μg/site, i.c.v., panel A) and minoxidil (10 μg/site, i.c.v., panel B) on venlafaxine (8 mg/kg, i.p., an effective dose)-induced reduction in the immobility time in the mouse FST. Values are expressed as mean ± S.E.M. (n = 7 mice/group). Data were analyzed by two-way analysis of variance (ANOVA) followed by the Newman–Keuls test. *p < 0.001 compared with the vehicle-treated control; #p < 0.001 compared with the venlafaxine-treated group.
treatment-resistant depression and obsessive compulsive disorder (Gutierrez et al. 2003). This antidepressant drug is known to exhibit six- to seven-fold selectivity for inhibition of 5-HT reuptake as compared to NA reuptake in the synaptosomes of rat brain and a 15- to 30-fold higher affinity for 5-HT transporter binding sites as compared to those of the NA transporter (Gould et al. 2006). Various behavioral, biochemical and molecular studies are being carried out to elucidate the exact mechanism of the antidepressant effect of venlafaxine (Gould et al. 2006; Dhir and Kulkarni 2007a,b; Berrocoso and Mico 2009). However, the role of K+ channels in the antidepressant action of venlafaxine has not been investigated. Therefore, the present study aimed to investigate whether the antidepressant-like effect of venlafaxine is mediated through the inhibition of K+ channels, in addition to the other mechanisms already described in the literature. It has been suggested that nitric oxide synthase (NOS) inhibitors, when administered prior to venlafaxine treatment, enhance its antidepressant-like action in the mouse FST (Dhir and Kulkarni 2007a). Moreover, the L-arginine–NO–cGMP signaling pathway has been implicated in depression (Mantovani et al. 2003). Recent evidence has shown that the reduction of NO levels within the hippocampus can induce antidepressant-like effects, thus implicating endogenous hippocampal NO in the neurobiology of stress and depression (Joca and Guimaraes 2006). Kaster et al. (2005) have reported that NO and cGMP are important modulators of some K+ channels at the central level, and the inhibition of these channels might represent an important role in the mechanisms involved in major depressive disorder. Furthermore, several studies employing different pharmacological models have suggested that NO affects K+ channels (Lazaro-Ibanez et al. 2001; Fernandes et al. 2002). For example, NO may activate K+ channels resulting in changes in immobility time in the mouse FST (Inan et al. 2004). Large-conductance Ca2+-activated K+ channels have been suggested as one of the physiological targets of NO in the brain (Jeong et al. 2001). Therefore,
Table 2 Effect of cromakalim (10 µg/site, i.c.v.), minoxidil (10 µg/site, i.c.v.), venlafaxine (8 mg/ kg, i.p.) or combined administration of venlafaxine and cromakalim or minoxidil on the number of crossings in the OFT. Treatment
Number of crossings
Vehicle Cromakalim Venlafaxine Cromakalim + venlafaxine Vehicle control Minoxidil Venlafaxine Minoxidil + venlafaxine
68.00 ± 7.67 72.29 ± 8.46 70.86 ± 9.07 72.43 ± 8.70 73.43 ± 8.90 75.14 ± 8.84 78.71 ± 6.79 68.29 ± 6.83
Values are expressed as mean ± S.E.M. (n = 7 mice/group).Data were analyzed by twoway analysis of variance (ANOVA) followed by Newman–Keuls test.
K+ channels represent one of the major downstream targets regulated by the L-arginine–NO–cGMP pathway. It has also been reported that the mechanism of action and structure of venlafaxine are very similar to tramadol (Markowitz and Patrick 1998). The oral administration of tramadol, a synthetic opioid, produces an antidepressant-like effect in the rat FST by a mechanism that involves the inhibition of the L-arginine–NO pathway (Jesse et al. 2008). Recently, it was demonstrated that the inhibition of different types of K+ channels augments the antidepressant-like effect of the oral administration of tramadol in the mouse FST (Jesse et al. 2009). These results reinforce the idea that the inhibition of the L-arginine–NO–cGMP pathway prevents the activation of K+ channels. As demonstrated in the present study, the inhibition of K+ channels could be one of the mechanisms of action by which venlafaxine elicits its antidepressant-like activity in the mouse FST. In this study, pretreatment of mice with subeffective doses of pharmacological compounds able to block different types of K+ channels, such as TEA, glibenclamide, apamin and charybdotoxin, elicited an antidepressant-like effect with a subeffective dose of venlafaxine. These results show that the inhibition of K+ channels plays an important role in the antidepressant-like effect of venlafaxine in mice, probably by inhibiting membrane hyperpolarization, leading to an increased excitatory response. Moreover, one cannot exclude the possibility that the K+ channel activity is modified by the drugs used in this study, so the neuronal circuits related to the effects of venlafaxine could be affected, and thus, the immobility time with venlafaxine could indirectly change. Pretreatment of mice with drugs able to active K+ channels, such as cromakalim and minoxidil, prevented the anti-immobility effect of an effective dose of venlafaxine in the mouse FST, without affecting the locomotor activity. These results help to support the involvement of K+ channels in the antidepressant-like effect of venlafaxine in mice. Additionally, studies have shown that the pretreatment of animals with cromakalim is able to antagonize the anti-immobility effect of several antidepressants, such as imipramine, amitriptyline, desipramine and paroxetine (Redrobe et al. 1996). Moreover, the blockade of K+ channels enhances the basal release of 5-HT in rat hippocampal slices (Schechter 1997). Imipramine, fluoxetine and agmatine are some examples of antidepressants that influence K+ channels and the L-arginine–NO–cGMP pathway (Choi et al. 2004; Harkin et al. 2004; Kaster et al. 2007). Other antidepressants, such as desipramine, amitriptyline, nortriptyline, clomipramine, maprotiline, citalopram and paroxetine, also produce an inhibition of K+ currents, which might underlie their therapeutic effects (Nicholson et al. 2002; Kobayashi and Washiyama 2004; Kobayashi et al. 2006). It has been suggested that GIRK channels play an important role in the inhibitory regulation of neuronal excitability in most brain regions through the activation of various G protein coupled receptors (Kobayashi and Washiyama 2004).The inhibition of GIRK channels by fluoxetine may contribute to some of its therapeutic effects observed in clinical practice (Kobayashi et al. 2003). In addition, GIRK channel inhibitors
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might be potential agents for the treatment of users of addictive drugs (Kobayashi and Washiyama 2004). Therefore, a study aiming to investigate the involvement of K+ channels in the antidepressant-like effect of venlafaxine is important from the point of view of clinical trials. The FST is widely used as an animal model of depression to screen new potent antidepressant drugs (Bourin et al. 2005). Although the FST model is accepted because it is sensitive to all major classes of antidepressants, including tricyclics, 5-HT-specific reuptake inhibitors, monoamine oxidase inhibitors and atypicals (Porsolt et al. 1977b), it has some drawbacks in terms of the possibility of obtaining false positives or negatives. Drugs enhancing locomotor activity can evoke a false positive effect in these tests, whereas drugs decreasing locomotion may give a ‘false’ negative result (Borsini and Meli 1988). In the present study, animals were submitted to the OFT in order to rule out the possibility that the interaction of K+ channel inhibitors and venlafaxine produces an antidepressant-like effect in the FST due to a stimulant effect. The experimental data obtained in the OFT excludes the possible stimulatory effects of K+ channel inhibitors and venlafaxine on the motor activity of mice. Conclusion The present study demonstrates that the pretreatment of mice with different K+ channel inhibitors produces an antidepressant-like action with a subeffective dose of venlafaxine, while pretreatment with K+ channel openers prevented the antidepressant-like effect of an effective dose of venlafaxine. However, more studies are necessary to elucidate the molecular mechanisms of venlafaxine action in K+ channels, as well as other mechanisms that may be involved in its antidepressant-like activity. Conflict of interest statement None. Acknowledgements Financial support by UFSM, FAPERGS, CAPES and CNPq (PIBIC) is gratefully acknowledged. References Armstead WM. Role of ATP-sensitive K+ channels in cGMP mediated pial artery vasodilation. American Journal of Physiology—Heart and Circulatory Physiology 39 (2), 423–426, 1996. Berrocoso E, Rojas-Corrales MO, Mico JA. Non-selective opioid receptor antagonism of the antidepressant-like effect of venlafaxine in the forced swimming test in mice. Neuroscience Letters 363 (1), 25–28, 2004. Berrocoso E, Mico JA. Role of serotonin 5-HT1A receptors in the antidepressant-like effect and the antinociceptive effect of venlafaxine in mice. International Journal of Neuropsychopharmacology 12 (1), 61–71, 2009. Bolotina VM, Najibi S, Palacino JJ, Pagano PJ, Cohen RA. Nitric oxide directly activates calcium-dependent potassium channels in vascular smooth muscle. Nature 368 (6474), 850–853, 1994. Borsini F, Meli A. Is the forced swimming test a suitable model for revealing antidepressant activity? Psychopharmacology 94 (2), 147–160, 1988. Bourin M, Chenu F, Ripoll N, David DJ. A proposal of decision tree to screen putative antidepressants using forced swim and tail suspension tests. Behavioural Brain Research 164 (2), 266–269, 2005. Budni J, Gadotti VM, Kaster MP, Santos ARS, Rodrigues ALS. Role of different types of potassium channels in the antidepressant-like effect of agmatine in the mouse forced swimming test. European Journal of Pharmacology 575, 87–93, 2007. Choi JS, Choi BH, Ahn HS, Kim MJ, Han TH, Rhie DJ. Fluoxetine inhibits A-type potassium currents in primary cultured rat hippocampal neurons. Brain Research 1018 (2), 201–207, 2004. Dhir A, Kulkarni SK. Involvement of L-arginine–nitric oxide–cyclic guanosine monophosphate pathway in the antidepressant-like effect of venlafaxine in mice. Progress in Neuro-Psychopharmacology & Biological Psychiatry 31 (4), 921–925, 2007a. Dhir A, Kulkarni SK. Involvement of sigma-1 receptor modulation in the antidepressant action of venlafaxine. Neuroscience Letters 420 (3), 204–208, 2007b. Fernandes D, Da Silva-Santos JE, Assreuy J. Nitric oxide-induced inhibition of mouse paw edema: involvement of soluble guanylate cyclase and potassium channels. Inflammation Research 51 (8), 377–384, 2002.
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