Neurol Sci DOI 10.1007/s10072-014-1845-y
ORIGINAL ARTICLE
Single dose varenicline may trigger epileptic activity Haydar Ali Erken • Gu¨lten Erken • Hasan S¸ ims¸ ek Og˘uzhan Korkut • Emine Rabia Koc¸ • ¨ zlem Yavuz • Osman Genc¸ O
•
Received: 26 March 2014 / Accepted: 23 May 2014 Ó Springer-Verlag Italia 2014
Abstract Varenicline is a new drug for smoking cessation, and its effect on epilepsy is not clear. The aim of this study was to investigate whether different doses of varenicline cause epileptic activity. Forty rats were randomly assigned to the following eight groups: control, saline, and 0.025, 0.04, 0.1, 0.5, 1, and 2 mg kg-1 varenicline (single dose, i.p.). EEGs were recorded before the varenicline injection and during the following 240 min. While epileptic discharges were observed on the EEGs of the rats in all of the varenicline-treated groups, motor findings of epileptic seizure were not observed in some rats in these groups except the 1 and 2 mg kg-1 groups. These findings indicate that different single doses of varenicline cause epileptic activity in rats.
H. A. Erken (&) G. Erken Department of Physiology, Faculty of Medicine, Balikesir University, Balikesir, Turkey e-mail: [email protected] H. S¸ ims¸ ek O. Genc¸ Department of Physiology, Faculty of Medicine, Dumlupinar University, Kutahya, Turkey O. Korkut Department of Pharmacology, Faculty of Medicine, Balikesir University, Balikesir, Turkey E. R. Koc¸ Department of Neurology, Faculty of Medicine, Balikesir University, Balikesir, Turkey ¨ . Yavuz O Department of Biochemistry, Faculty of Medicine, Balikesir University, Balikesir, Turkey
Keywords Adverse effect EEG Epilepsy Seizure Varenicline
Introduction Varenicline, which was approved for smoking cessation in the USA (ChantixÒ) and Europe (ChampixÒ) in 2006 [1], is one of the most widely used effective drugs for smoking cessation [2]. Varenicline is a partial agonist at the a4b2 nicotinic acetylcholine receptor (nAChR) and a full agonist at a7 nAChR [3]. Cholinergic projections are widespread and diffuse in the mammalian brain [4]. Neuronal nAChRs are pentameric and are composed of different combinations of a2–a10 and b2–b4 subunits [5]. a7 and a4b2, the two major nAChR subtypes found in the mammalian brain [6], are expressed throughout the brain and are particularly abundant in cortical [5, 7] and subcortical areas such as the thalamus [7] and hippocampus [5], both asynaptically and at synapses [8]. In addition, nAChRs are found on glutamatergic [9] and GABAergic neurons [10], and mediate facilitation of the release of these neurotransmitters [11, 12]. nAChRs also appear to be involved in numerous important processes such as neuronal excitability [13], pain [14], attention [15], learning, and memory [16]. The most commonly reported adverse effects of varenicline are nausea, vomiting, abnormal dreams, insomnia, headache, excitement, agitation, anxiety, irritability, fatigue, tachycardia, taste perversion, flatulence, dyspepsia, and constipation [17, 18]. Although a few cases of epileptic seizure during treatment with varenicline have been reported in humans [19, 20], no clinical or experimental studies regarding the relationship between varenicline and epilepsy have been conducted. Therefore, the aim of this study was to investigate the
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effects of different single doses of varenicline on epileptic activity in rats.
Methods Animals and study design All experimental protocols conducted on the animals were consistent with the National Institutes of Health Guidelines for the Care and Use of Laboratory Animals (NIH Publication No. 85-23) and approved by the Dumlupinar University Local Ethical Committee (approval number: 2013.03.01; approval date: July 3, 2013). Forty adult male Wistar albino rats weighing 359 ± 38 g (mean ± SD) were used. All of the rats were maintained in a 12-h light/ dark cycle environment (lights on 7:00–19:00 h) at a temperature of 23 ± 1 °C and 50 % humidity, and they had access to food and water ad libitum. The rats were randomly assigned to the following eight groups (n = 5 for each group): control (C), saline (S), and 0.025 (V1), 0.04 (V2), 0.1 (V3), 0.5 (V4), 1 (V5), and 2 (V6) mg kg-1 single doses of varenicline [21].
frontoparietal area and the other on the interparietal area. A ground electrode was placed on the tail of the rat. The EEGs were recorded with the PowerLab 26T data acquisition system and Chart 7 program (AD Instruments Co., Australia). The recording parameters were as follows: 0.3–100 Hz low- and high-frequency filter, 50 Hz notch filter, and recording speed of 25 mm s-1. Additional anesthesia was administered as required. The rats were observed and recorded for 240 min after the varenicline application. Thirty-second artifact-free epochs were chosen from the EEG recordings as samples of cortical activity, before injection and at the 90th, 120th, 180th, and 240th min of varenicline/saline injection. The EEG signals obtained from these samples were analyzed using the Chart 7 software program according to the frequency and amplitude. Statistical analysis Repeated measures analysis of variance (ANOVA) and post hoc Tukey test were used for repeated measures of spike frequency and amplitude values. One-way ANOVA and post hoc Tukey test were used to compare the spike frequency and amplitude values of the different dose groups. p \ 0.05 was considered significant.
Drug and chemicals Varenicline tartrate was purchased from Sigma-Aldrich (Taufkirchen, Germany). Aliquot doses of varenicline were measured for each experiment and dissolved in saline immediately before use. Povidone–iodine was purchased from Adeka (Samsun, Turkey); ketamine was purchased from Ege Vet (Izmir, Turkey); xylazine was purchased from aniMedica (Senden-Boesensell, Germany); and EEG conductive paste was purchased from DO Weaver and Co (Aurora, CO, USA). Anesthesia and experimental procedure The rats were anesthetized with ketamine/xylazine (90 and 10 mg kg-1, respectively, i.p.) and their heads were shaved. Then, the rats were placed on a stereotaxic instrument (Stoelting Co., USA) and their heads were disinfected with 10 % povidone–iodine and incised from mid-frontal to mid-occipital. After baseline EEGs were recorded for 30 min, varenicline was administered intraperitoneally (in 0.3 ml sterile saline). The same volume of saline was injected in the S group; the C group received no drug or saline. EEG recording and analysis Two Ag/AgCl flat electrodes were placed on the scalp for bipolar EEG recording; one of them was placed on the right
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Results The EEGs of the rats in all of the varenicline-treated groups (V1–V6) showed that epileptic discharges began 68.10 ± 13.55 min after receiving varenicline and continued until the end of the experiment (240 min after varenicline administration); repetitive group discharges were observed 81.30 ± 8.52 min after the varenicline injection as well (Fig. 1). Motor findings of seizure, such as whisker movements and tonic–clonic contractions of the lower jaw muscles, anterior and posterior extremities, and tails were also observed in these groups, beginning 162.90 ± 47.83 min after the varenicline injection. While epileptic discharges were observed in all the rats in the varenicline-treated groups, three rats in the V1 group, four rats in the V2 group, three rats in the V3 group, and two rats in the V4 group displayed no motor findings of seizure. The EEG and motor findings of epileptic seizure continued until the end of the experiment in the vareniclinetreated groups. There were no epileptic seizure findings in the C and S groups. Spike frequency The spike frequency values of the V3, V4 and V5 groups were significantly increased with time. Also, there are significant differences in spike frequency values of different dose groups (Table 1).
Neurol Sci Fig. 1 EEG sample 180 min after varenicline injection from a rat in the V1 group (0.025 mg kg-1 body weight). Group discharges can be seen at intervals of *5 s (bottom), one of which is enlarged (top)
Spike amplitude The spike amplitude values of all the varenicline-treated groups increased significantly with time. In addition, there were significant differences in the spike amplitude values of the different dose groups (Table 1).
Discussion The present study showed that single-dose varenicline causes epileptic activity in rats without any epileptic agent. While epileptic discharges were observed in all the rats treated with different doses of varenicline, not all those rats displayed motor findings of epileptic seizure. Consistent with previous studies [21, 22], varenicline had a dosedependent effect in this study. There were no epileptic seizure findings in the control and saline groups. Consistent with our findings, a recent report described the case of a 51-year-old man with no medical history of epilepsy or febrile seizures who had his first generalized tonic–clonic seizure while on treatment with varenicline, during the withdrawal phase at 1 mg day-1 [20]. Similarly, the Australian Adverse Drug Reactions Bulletin [19] reported 15 cases of varenicline-induced seizures, but it is not known how many of those patients had a prior history or risk of seizure disorder. However, to the best of our knowledge, there has been no clinical or experimental study regarding the relationship between varenicline and epilepsy; this relationship was first demonstrated in the present study. Varenicline is a partial agonist at a4b2 nAChR and a full agonist at a7 nAChR [3]. These receptors are expressed throughout the brain and are particularly abundant in the cortical and subcortical areas [5, 7]. Previous electrophysiological studies have shown that nAChRs may
mediate neuroexcitation by presynaptic effects on axon terminals and direct postsynaptic effects on cell bodies and dendrites [13]. These receptors are excitatory ligand-gated cation channels, and their activation causes depolarization [23]. It has also been demonstrated that nAChRs exhibit high Ca2? permeability [24], and varenicline has been shown to induce an increase in intracellular Ca2? levels [25]. The increase in intracellular Ca2? concentration is a well-known mechanism for neuronal excitability and epileptic activity [26]. Therefore, it is probable that varenicline activates a4b2 and/or a7 nAChRs, increasing intracellular Ca2? levels and resulting in epileptic activity. The other possible reason for varenicline-induced epileptic activity is the modulating effect of nAChRs on neurotransmitter release in axon terminals [11, 12], as neocortical excitability control depends on a fine balance between excitatory and inhibitory transmission. This balance between glutamate and GABA release is abolished during epileptic activity [27, 28], and nAChRs are known to regulate both [11, 12]. Previous studies have reported that nAChRs were found on glutamatergic [9] and GABAergic neurons [10], which mediate facilitation of the release of glutamate and GABA [11, 12]. It has also been shown that pyramidal neurons in layer V are excited by nAChRs that enhance glutamatergic inputs through stimulation of presynaptic nAChRs [12]. Moreover, varenicline has been reported to stimulate GABA release and enhance GABAergic synaptic transmission in CA1 pyramidal and medial septum/diagonal band neurons in an action potential-independent manner [29]. However, to the best of our knowledge, no studies regarding the effects of varenicline on glutamate release or glutamatergic synaptic transmission have been conducted. There have been reports of seizures induced by some smoking cessation agents, including nicotine replacement
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Neurol Sci Table 1 Spike frequency and amplitude values of varenicline-treated groups Groups
90th min
120th min
180th min
240th min
V1 Spike frequency (min-1)
61.28 ± 24.46
73.95 ± 19.38
90.82 ± 27.51
84.16 ± 18.43
Spike amplitude (lV)
55.32 ± 13.98
64.59 ± 20.52
78.24 ± 16.09b
69.82 ± 15.38
V2 Spike frequency (min-1)
75.25 ± 17.02
89.72 ± 21.86
116.17 ± 38.11
Spike amplitude (lV)
57.42 ± 9.50
63.25 ± 14.01
82.26 ± 9.72c
Spike frequency (min-1)
82.16 ± 11.94
97.70 ± 22.48
158.73 ± 11.30c,e,f
147.90 ± 37.26c,e
Spike amplitude (lV)
74.90 ± 10.59f
87.55 ± 16.24a
103.35 ± 14.97c,e
104.63 ± 12.79c,e,f
124.17 ± 46.57 112.42 ± 10.69c,h,k
182.17 ± 46.83b,g,i 129.74 ± 31.05c,h,j
166.23 ± 33.59b,g 121.56 ± 22.94c,h,j
109.29 ± 19.04 74.65 ± 15.49b
V3
V4 Spike frequency (min-1) Spike amplitude (lV)
86.34 ± 15.61 78.45 ± 22.39g
V5 Spike frequency (min-1)
120.53 ± 19.72h,j,m,o
161.85 ± 32.24g,i,l
194.63 ± 28.54b,h,j
173.15 ± 36.48g
Spike amplitude (lV)
114.82 ± 36.67h,k,n,q
126.47 ± 18.90h,k,l
172.83 ± 26.39b,d,h,k,n,o
169.81 ± 55.40b,d,h,k,n,p
152.96 ± 31.18h,k,n,q
174.22 ± 58.06h,j,m
228.31 ± 49.75h,k
196.41 ± 64.92h,j
V6 Spike frequency (min-1) Spike amplitude (lV)
h,k,n,q
119.87 ± 17.23
h,k,n,q
181.36 ± 65.19
c,e,h,k,n,q,r
275.32 ± 86.70
247.13 ± 62.14c,h,k,n,q,r
Values are presented as mean ± SD. V1, V2, V3, V4, V5 and V6: 0.025, 0.04, 0.1, 0.5, 1 and 2 mg kg-1 body weight single doses of varenicline, respectively a p \ 0.05, different from 90th min of varenicline b
p \ 0.01, different from 90th min of varenicline
c
p \ 0.001, different from 90th min of varenicline
d
p \ 0.05, different from 120th min of varenicline
e
p \ 0.01, different from 120th min of varenicline
f
p \ 0.05, different from V1
g
p \ 0.01, different from V1
h
p \ 0.001, different from V1
i
p \ 0.05, different from V2
j
p \ 0.01, different from V2
k
p \ 0.001, different from V2
l
p \ 0.05, different from V3
m
p \ 0.01, different from V3
n
p \ 0.001, different from V3
o
p \ 0.05, different from V4 p \ 0.01, different from V4
p q
p \ 0.001, different from V4
r
p \ 0.001, different from V5
therapy and cytisine [30, 31]. These results corroborate the possibility of varenicline-induced epilepsy mediated by nAChRs, as nicotine and cytisine have an effect on nAChRs [32–34]. In addition, there have been studies conducted on the relationship between nAChRs and epilepsy [13, 35]. In smoking cessation therapy, dosing of varenicline starts at 0.5 mg once daily for the first 3 days, then increases to 0.5 mg twice daily for the following 4 days and 1 mg twice daily thereafter, for a total of 12 weeks
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[36]. The reason for including the low-dose groups (0.04 and 0.025 mg kg-1) in this study design is that the doses are equivalent to the daily maximum dose used for humans, 2 mg/50 and 80 kg body weight, respectively. These low doses of varenicline caused epileptic activity in rats in the current study. Moreover, this effect was observed after a single-dose application. In healthy adults, maximum plasma concentration of varenicline occurs within 3–4 h of a single dose of oral administration, and steady-state concentration is typically reached within 4 days following
Neurol Sci
regular doses. Furthermore, renal elimination of varenicline is primarily through glomerular filtration, and the elimination half-life is *24 h [37–39] and this elimination rate can contribute to an increase in plasma levels of varenicline. Because plasma concentration is higher in regular doses of varenicline than in a single-dose treatment [37, 39], the possible adverse effects of varenicline may increase in regular doses. While the pharmacokinetic properties of varenicline in rats and humans are similar [38], there may be some differences between the two, such as the pharmacodynamic properties of varenicline and the neurochemical pathways that mediate its effects. The superiority of the current study is that multiple doses of varenicline were tested and its epileptogenic effect was demonstrated in low single doses. In addition, to eliminate individual differences among rats, the EEG findings of the varenicline-treated groups were compared to the initial values of the same groups in addition to the control and saline groups. However, there are some limitations. The study was performed in rats, and there may be some differences between rats and humans. In addition, at this stage, we did not investigate the possible mechanisms that mediate the effects of varenicline on epilepsy. This mechanism and the relationship between glutamate and varenicline should be clarified. In conclusion, the present study shows that intraperitoneal applications of different single doses of varenicline caused epileptic activity in rats. However, the mechanism of this effect is not clear. Nonetheless, according to our findings, epileptic seizures may be accepted as a potential adverse effect of varenicline.
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Acknowledgments The authors thank to Aydın Akc¸ılar for his technical support. This study was supported by Balikesir University Research Fund (Project number: 2012–121).
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Conflict of interest The authors declare that they have no conflicts of interest.
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References 1. Niaura R, Jones C, Kirkpatrick P (2006) Varenicline. Nat Rev Drug Discov 5:537–538. doi:10.1038/nrd2088 2. Cahill K, Stead LF, Lancaster T (2008) Nicotine receptor partial agonists for smoking cessation. Cochrane Database Syst Rev 16:CD006103. doi:10.1002/14651858.CD006103.pub2 3. Mihalak KB, Carroll FI, Luetje CW (2006) Varenicline is a partial agonist at a4b2 and a full agonist at a7 neuronal nicotinic receptors. Mol Pharmacol 70:801–805. doi:10.1124/mol.106. 025130 4. Sarter M, Parikh V, Howe WM (2009) Phasic acetylcholine release and the volume transmission hypothesis: time to move on. Nat Rev Neurosci 10:383–390. doi:10.1038/nrn2635 5. Alkondon M, Albuquerque EX (2004) The nicotinic acetylcholine receptor subtypes and their function in the hippocampus and
19.
20.
21.
22.
cerebral cortex. Prog Brain Res 145:109–120. doi:10.1016/ S0079-6123(03)45007-3 Gotti C, Clementi F, Fornari A et al (2009) Structural and functional diversity of native brain neuronal nicotinic receptors. Biochem Pharmacol 78:703–711. doi:10.1016/j.bcp.2009.05.024 Bina KG, Guzman P, Broide RS, Leslie FM, Smith MA, O’Dowd DK (1995) Localization of alpha 7 nicotinic receptor subunit mRNA and alpha-bungarotoxin binding sites in developing mouse somatosensory thalamocortical system. J Comp Neurol 363:321–332. doi:10.1002/cne.903630212 Descarries L (1998) The hypothesis of an ambient level of acetylcholine in the central nervous system. J Physiol Paris 92:215–220. doi:10.1016/S0928-4257(98)80013-2 Jones IW, Wonnacott S (2004) Precise localization of alpha7 nicotinic acetylcholine receptors on glutamatergic axon terminals in the rat ventral tegmental area. J Neurosci 24:11244–11252. doi:10.1523/JNEUROSCI.3009-04.2004 Le´na C, Changeux JP, Mulle C (1993) Evidence for ‘‘preterminal’’ nicotinic receptors on GABAergic axons in the rat interpeduncular nucleus. J Neurosci 13:2680–2688 Zhu PJ, Chiappinelli VA (2002) Nicotinic receptors mediate increased GABA release in brain through a tetrodotoxin-insensitive mechanism during prolonged exposure to nicotine. Neuroscience 115:137–144. doi:10.1016/S0306-4522(02)00371-8 Aracri P, Amadeo A, Pasini ME, Fascio U, Becchetti A (2013) Regulation of glutamate release by heteromeric nicotinic receptors in layer V of the secondary motor region (Fr2) in the dorsomedial shoulder of prefrontal cortex in mouse. Synapse 67:338–357. doi:10.1002/syn.21655 Dani JA (2000) Properties underlying the influence of nicotinic receptors on neuronal excitability and epilepsy. Epilepsia 41:1063–1065. doi:10.1111/j.1528-1157.2000.tb00300.x Decker MW, Rueter LE, Bitner RS (2004) Nicotinic acetylcholine receptor agonists: a potential new class of analgesics. Curr Top Med Chem 4:369–384. doi:10.2174/1568026043451447 Dalley JW, Theobald DE, Bouger P, Chudasama Y, Cardinal RN, Robbins TW (2004) Cortical cholinergic function and deficits in visual attentional performance in rats following 192 IgG-saporininduced lesions of the medial prefrontal cortex. Cereb Cortex 14:922–932. doi:10.1093/cercor/bhh052 Braida D, Ponzoni L, Martucci R, Sparatore F, Gotti C, Sala M (2014) Role of neuronal nicotinic acetylcholine receptors (nAChRs) on learning and memory in zebrafish. Psychopharmacology 231:1975–1985. doi:10.1007/s00213-013-3340-1 Fagerstrom K, Gilljam H, Metcalfe M, Tonstad S, Messig M (2010) Stopping smokeless tobacco with varenicline: randomised double blind placebo controlled trial. BMJ 341:c6549. doi:10. 1136/bmj.c6549 Tonstad S, Davies S, Flammer M, Russ C, Hughes J (2010) Psychiatric adverse events in randomized, double-blind, placebocontrolled clinical trials of varenicline: a pooled analysis. Drug Saf 33:289–301. doi:10.2165/11319180-000000000-00000 Topliss D, Gold M, Kotsirilos V et al (2008) Varenicline: the Australian experience so far. Aust Adverse Drug React Bull 27(6):22. http://www.tga.gov.au/hp/aadrb-0812.htm#.U49Yw3Y 4TgF. Accessed 12 Jan 2014 Serafini A, Crespel A, Velizara R, Gelisse P (2010) Vareniclineinduced grand mal seizure. Epileptic Disord 12:338. doi:10.1684/ epd.2010.0339 Spiller K, Xi ZX, Li X et al (2009) Varenicline attenuates nicotine-enhanced brain-stimulation reward by activation of alpha4beta2 nicotinic receptors in rats. Neuropharmacology 57:60–66. doi:10.1016/j.neuropharm.2009.04.006 Levin ME, Weaver MT, Palmatier MI, Caggiula AR, Sved AF, Donny EC (2012) Varenicline dose dependently enhances responding for nonpharmacological reinforcers and attenuates the
123
Neurol Sci
23. 24.
25.
26.
27.
28.
29.
30.
31.
reinforcement-enhancing effects of nicotine. Nicotine Tob Res 14:299–305. doi:10.1093/ntr/ntr213 Karlin A (2002) Emerging structure of the nicotinic acetylcholine receptors. Nat Rev Neurosci 3:102–114. doi:10.1038/nrn731 Rathouz MM, Vijayaraghavan S, Berg DK (1996) Elevation of intracellular calcium levels in neurons by nicotinic acetylcholine receptors. Mol Neurobiol 12:117–131. doi:10.1007/BF02740649 Ween H, Thorin-Hagene K, Andersen E et al (2010) Alpha3* and alpha 7 nAChR-mediated Ca2? transient generation in IMR-32 neuroblastoma cells. Neurochem Int 57:269–277. doi:10.1016/j. neuint.2010.06.005 Speckmann EJ, Straub H, Ko¨hling R (1993) Contribution of calcium ions to the generation of epileptic activity and antiepileptic calcium antagonism. Neuropsychobiology 27:122–126. doi:10.1159/000118966 Andre´ V, Marescaux C, Nehlig A, Fritschy JM (2001) Alterations of hippocampal GABAergic system contribute to development of spontaneous recurrent seizures in the rat lithium–pilocarpine model of temporal lobe epilepsy. Hippocampus 11:452–468. doi:10.1002/hipo.1060 Sitges M, Sanchez-Tafolla BM, Chiu LM, Aldana BI, Guarneros A (2011) Vinpocetine inhibits glutamate release induced by the convulsive agent 4-aminopyridine more potently than several antiepileptic drugs. Epilepsy Res 96:257–266. doi:10.1016/j. eplepsyres.2011.06.006 DuBois DW, Damborsky JC, Fincher AS, Frye GD, WinzerSerhan UH (2013) Varenicline and nicotine enhance GABAergic synaptic transmission in rat CA1 hippocampal and medial septum/diagonal band neurons. Life Sci 92:337–344. doi:10.1016/j. lfs.2012.12.013 Beleslin DB, Krstic´ SK (1986) Nicotine-induced convulsions in cats and central nicotinic receptors. Pharmacol Biochem Behav 24:1509–1511. doi:10.1016/0091-3057(86)90476-4 Tutka P, Mro´z T, Bednarski J et al (2013) Cytisine inhibits the anticonvulsant activity of phenytoin and lamotrigine in mice. Pharmacol Rep 65:195–200. doi:10.1016/S1734-1140(13)709782
123
32. Hussy N, Ballivet M, Bertrand D (1994) Agonist and antagonist effects of nicotine on chick neuronal nicotinic receptors are defined by alpha and beta subunits. J Neurophysiol 72:1317–1326 33. Mineur YS, Somenzi O, Picciotto MR (2007) Cytisine, a partial agonist of high-affinity nicotinic acetylcholine receptors, has antidepressant-like properties in male C57BL/6J mice. Neuropharmacology 52:1256–1262. doi:10.1016/j.neuropharm.2007. 01.006 34. Peng C, Stokes C, Mineur YS et al (2013) Differential modulation of brain nicotinic acetylcholine receptor function by cytisine, varenicline, and two novel bispidine compounds: emergent properties of a hybrid molecule. J Pharmacol Exp Ther 347:424–437. doi:10.1124/jpet.113.206904 35. Feuerbach D, Lingenhoehl K, Olpe HR et al (2009) The selective nicotinic acetylcholine receptor alpha7 agonist JN403 is active in animal models of cognition, sensory gating, epilepsy and pain. Neuropharmacology 56:254–263. doi:10.1016/j.neuropharm. 2008.08.025 36. Wang C, Cho B, Xiao D, Wajsbrot D, Park PW (2013) Effectiveness and safety of varenicline as an aid to smoking cessation: results of an inter-Asian observational study in real-world clinical practice. Int J Clin Pract 67:469–476. doi:10.1111/ijcp.12121 37. Faessel HM, Smith BJ, Gibbs MA, Gobey JS, Clark DJ, Burstein AH (2006) Single-dose pharmacokinetics of varenicline, a selective nicotinic receptor partial agonist, in healthy smokers and nonsmokers. J Clin Pharmacol 46:991–998. doi:10.1177/ 0091270006290669 38. Obach RS, Reed-Hagen AE, Krueger SS et al (2006) Metabolism and disposition of varenicline, a selective alpha4beta2 acetylcholine receptor partial agonist, in vivo and in vitro. Drug Metab Dispos 34:121–130. doi:10.1124/dmd.105.006767 39. Kikkawa H, Maruyama N, Fujimoto Y, Hasunuma T (2011) Single- and multiple-dose pharmacokinetics of the selective nicotinic receptor partial agonist, varenicline, in healthy Japanese adult smokers. J Clin Pharmacol 51:527–537. doi:10.1177/ 0091270010372388
ORIGINAL ARTICLE
Single dose varenicline may trigger epileptic activity Haydar Ali Erken • Gu¨lten Erken • Hasan S¸ ims¸ ek Og˘uzhan Korkut • Emine Rabia Koc¸ • ¨ zlem Yavuz • Osman Genc¸ O
•
Received: 26 March 2014 / Accepted: 23 May 2014 Ó Springer-Verlag Italia 2014
Abstract Varenicline is a new drug for smoking cessation, and its effect on epilepsy is not clear. The aim of this study was to investigate whether different doses of varenicline cause epileptic activity. Forty rats were randomly assigned to the following eight groups: control, saline, and 0.025, 0.04, 0.1, 0.5, 1, and 2 mg kg-1 varenicline (single dose, i.p.). EEGs were recorded before the varenicline injection and during the following 240 min. While epileptic discharges were observed on the EEGs of the rats in all of the varenicline-treated groups, motor findings of epileptic seizure were not observed in some rats in these groups except the 1 and 2 mg kg-1 groups. These findings indicate that different single doses of varenicline cause epileptic activity in rats.
H. A. Erken (&) G. Erken Department of Physiology, Faculty of Medicine, Balikesir University, Balikesir, Turkey e-mail: [email protected] H. S¸ ims¸ ek O. Genc¸ Department of Physiology, Faculty of Medicine, Dumlupinar University, Kutahya, Turkey O. Korkut Department of Pharmacology, Faculty of Medicine, Balikesir University, Balikesir, Turkey E. R. Koc¸ Department of Neurology, Faculty of Medicine, Balikesir University, Balikesir, Turkey ¨ . Yavuz O Department of Biochemistry, Faculty of Medicine, Balikesir University, Balikesir, Turkey
Keywords Adverse effect EEG Epilepsy Seizure Varenicline
Introduction Varenicline, which was approved for smoking cessation in the USA (ChantixÒ) and Europe (ChampixÒ) in 2006 [1], is one of the most widely used effective drugs for smoking cessation [2]. Varenicline is a partial agonist at the a4b2 nicotinic acetylcholine receptor (nAChR) and a full agonist at a7 nAChR [3]. Cholinergic projections are widespread and diffuse in the mammalian brain [4]. Neuronal nAChRs are pentameric and are composed of different combinations of a2–a10 and b2–b4 subunits [5]. a7 and a4b2, the two major nAChR subtypes found in the mammalian brain [6], are expressed throughout the brain and are particularly abundant in cortical [5, 7] and subcortical areas such as the thalamus [7] and hippocampus [5], both asynaptically and at synapses [8]. In addition, nAChRs are found on glutamatergic [9] and GABAergic neurons [10], and mediate facilitation of the release of these neurotransmitters [11, 12]. nAChRs also appear to be involved in numerous important processes such as neuronal excitability [13], pain [14], attention [15], learning, and memory [16]. The most commonly reported adverse effects of varenicline are nausea, vomiting, abnormal dreams, insomnia, headache, excitement, agitation, anxiety, irritability, fatigue, tachycardia, taste perversion, flatulence, dyspepsia, and constipation [17, 18]. Although a few cases of epileptic seizure during treatment with varenicline have been reported in humans [19, 20], no clinical or experimental studies regarding the relationship between varenicline and epilepsy have been conducted. Therefore, the aim of this study was to investigate the
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effects of different single doses of varenicline on epileptic activity in rats.
Methods Animals and study design All experimental protocols conducted on the animals were consistent with the National Institutes of Health Guidelines for the Care and Use of Laboratory Animals (NIH Publication No. 85-23) and approved by the Dumlupinar University Local Ethical Committee (approval number: 2013.03.01; approval date: July 3, 2013). Forty adult male Wistar albino rats weighing 359 ± 38 g (mean ± SD) were used. All of the rats were maintained in a 12-h light/ dark cycle environment (lights on 7:00–19:00 h) at a temperature of 23 ± 1 °C and 50 % humidity, and they had access to food and water ad libitum. The rats were randomly assigned to the following eight groups (n = 5 for each group): control (C), saline (S), and 0.025 (V1), 0.04 (V2), 0.1 (V3), 0.5 (V4), 1 (V5), and 2 (V6) mg kg-1 single doses of varenicline [21].
frontoparietal area and the other on the interparietal area. A ground electrode was placed on the tail of the rat. The EEGs were recorded with the PowerLab 26T data acquisition system and Chart 7 program (AD Instruments Co., Australia). The recording parameters were as follows: 0.3–100 Hz low- and high-frequency filter, 50 Hz notch filter, and recording speed of 25 mm s-1. Additional anesthesia was administered as required. The rats were observed and recorded for 240 min after the varenicline application. Thirty-second artifact-free epochs were chosen from the EEG recordings as samples of cortical activity, before injection and at the 90th, 120th, 180th, and 240th min of varenicline/saline injection. The EEG signals obtained from these samples were analyzed using the Chart 7 software program according to the frequency and amplitude. Statistical analysis Repeated measures analysis of variance (ANOVA) and post hoc Tukey test were used for repeated measures of spike frequency and amplitude values. One-way ANOVA and post hoc Tukey test were used to compare the spike frequency and amplitude values of the different dose groups. p \ 0.05 was considered significant.
Drug and chemicals Varenicline tartrate was purchased from Sigma-Aldrich (Taufkirchen, Germany). Aliquot doses of varenicline were measured for each experiment and dissolved in saline immediately before use. Povidone–iodine was purchased from Adeka (Samsun, Turkey); ketamine was purchased from Ege Vet (Izmir, Turkey); xylazine was purchased from aniMedica (Senden-Boesensell, Germany); and EEG conductive paste was purchased from DO Weaver and Co (Aurora, CO, USA). Anesthesia and experimental procedure The rats were anesthetized with ketamine/xylazine (90 and 10 mg kg-1, respectively, i.p.) and their heads were shaved. Then, the rats were placed on a stereotaxic instrument (Stoelting Co., USA) and their heads were disinfected with 10 % povidone–iodine and incised from mid-frontal to mid-occipital. After baseline EEGs were recorded for 30 min, varenicline was administered intraperitoneally (in 0.3 ml sterile saline). The same volume of saline was injected in the S group; the C group received no drug or saline. EEG recording and analysis Two Ag/AgCl flat electrodes were placed on the scalp for bipolar EEG recording; one of them was placed on the right
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Results The EEGs of the rats in all of the varenicline-treated groups (V1–V6) showed that epileptic discharges began 68.10 ± 13.55 min after receiving varenicline and continued until the end of the experiment (240 min after varenicline administration); repetitive group discharges were observed 81.30 ± 8.52 min after the varenicline injection as well (Fig. 1). Motor findings of seizure, such as whisker movements and tonic–clonic contractions of the lower jaw muscles, anterior and posterior extremities, and tails were also observed in these groups, beginning 162.90 ± 47.83 min after the varenicline injection. While epileptic discharges were observed in all the rats in the varenicline-treated groups, three rats in the V1 group, four rats in the V2 group, three rats in the V3 group, and two rats in the V4 group displayed no motor findings of seizure. The EEG and motor findings of epileptic seizure continued until the end of the experiment in the vareniclinetreated groups. There were no epileptic seizure findings in the C and S groups. Spike frequency The spike frequency values of the V3, V4 and V5 groups were significantly increased with time. Also, there are significant differences in spike frequency values of different dose groups (Table 1).
Neurol Sci Fig. 1 EEG sample 180 min after varenicline injection from a rat in the V1 group (0.025 mg kg-1 body weight). Group discharges can be seen at intervals of *5 s (bottom), one of which is enlarged (top)
Spike amplitude The spike amplitude values of all the varenicline-treated groups increased significantly with time. In addition, there were significant differences in the spike amplitude values of the different dose groups (Table 1).
Discussion The present study showed that single-dose varenicline causes epileptic activity in rats without any epileptic agent. While epileptic discharges were observed in all the rats treated with different doses of varenicline, not all those rats displayed motor findings of epileptic seizure. Consistent with previous studies [21, 22], varenicline had a dosedependent effect in this study. There were no epileptic seizure findings in the control and saline groups. Consistent with our findings, a recent report described the case of a 51-year-old man with no medical history of epilepsy or febrile seizures who had his first generalized tonic–clonic seizure while on treatment with varenicline, during the withdrawal phase at 1 mg day-1 [20]. Similarly, the Australian Adverse Drug Reactions Bulletin [19] reported 15 cases of varenicline-induced seizures, but it is not known how many of those patients had a prior history or risk of seizure disorder. However, to the best of our knowledge, there has been no clinical or experimental study regarding the relationship between varenicline and epilepsy; this relationship was first demonstrated in the present study. Varenicline is a partial agonist at a4b2 nAChR and a full agonist at a7 nAChR [3]. These receptors are expressed throughout the brain and are particularly abundant in the cortical and subcortical areas [5, 7]. Previous electrophysiological studies have shown that nAChRs may
mediate neuroexcitation by presynaptic effects on axon terminals and direct postsynaptic effects on cell bodies and dendrites [13]. These receptors are excitatory ligand-gated cation channels, and their activation causes depolarization [23]. It has also been demonstrated that nAChRs exhibit high Ca2? permeability [24], and varenicline has been shown to induce an increase in intracellular Ca2? levels [25]. The increase in intracellular Ca2? concentration is a well-known mechanism for neuronal excitability and epileptic activity [26]. Therefore, it is probable that varenicline activates a4b2 and/or a7 nAChRs, increasing intracellular Ca2? levels and resulting in epileptic activity. The other possible reason for varenicline-induced epileptic activity is the modulating effect of nAChRs on neurotransmitter release in axon terminals [11, 12], as neocortical excitability control depends on a fine balance between excitatory and inhibitory transmission. This balance between glutamate and GABA release is abolished during epileptic activity [27, 28], and nAChRs are known to regulate both [11, 12]. Previous studies have reported that nAChRs were found on glutamatergic [9] and GABAergic neurons [10], which mediate facilitation of the release of glutamate and GABA [11, 12]. It has also been shown that pyramidal neurons in layer V are excited by nAChRs that enhance glutamatergic inputs through stimulation of presynaptic nAChRs [12]. Moreover, varenicline has been reported to stimulate GABA release and enhance GABAergic synaptic transmission in CA1 pyramidal and medial septum/diagonal band neurons in an action potential-independent manner [29]. However, to the best of our knowledge, no studies regarding the effects of varenicline on glutamate release or glutamatergic synaptic transmission have been conducted. There have been reports of seizures induced by some smoking cessation agents, including nicotine replacement
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Neurol Sci Table 1 Spike frequency and amplitude values of varenicline-treated groups Groups
90th min
120th min
180th min
240th min
V1 Spike frequency (min-1)
61.28 ± 24.46
73.95 ± 19.38
90.82 ± 27.51
84.16 ± 18.43
Spike amplitude (lV)
55.32 ± 13.98
64.59 ± 20.52
78.24 ± 16.09b
69.82 ± 15.38
V2 Spike frequency (min-1)
75.25 ± 17.02
89.72 ± 21.86
116.17 ± 38.11
Spike amplitude (lV)
57.42 ± 9.50
63.25 ± 14.01
82.26 ± 9.72c
Spike frequency (min-1)
82.16 ± 11.94
97.70 ± 22.48
158.73 ± 11.30c,e,f
147.90 ± 37.26c,e
Spike amplitude (lV)
74.90 ± 10.59f
87.55 ± 16.24a
103.35 ± 14.97c,e
104.63 ± 12.79c,e,f
124.17 ± 46.57 112.42 ± 10.69c,h,k
182.17 ± 46.83b,g,i 129.74 ± 31.05c,h,j
166.23 ± 33.59b,g 121.56 ± 22.94c,h,j
109.29 ± 19.04 74.65 ± 15.49b
V3
V4 Spike frequency (min-1) Spike amplitude (lV)
86.34 ± 15.61 78.45 ± 22.39g
V5 Spike frequency (min-1)
120.53 ± 19.72h,j,m,o
161.85 ± 32.24g,i,l
194.63 ± 28.54b,h,j
173.15 ± 36.48g
Spike amplitude (lV)
114.82 ± 36.67h,k,n,q
126.47 ± 18.90h,k,l
172.83 ± 26.39b,d,h,k,n,o
169.81 ± 55.40b,d,h,k,n,p
152.96 ± 31.18h,k,n,q
174.22 ± 58.06h,j,m
228.31 ± 49.75h,k
196.41 ± 64.92h,j
V6 Spike frequency (min-1) Spike amplitude (lV)
h,k,n,q
119.87 ± 17.23
h,k,n,q
181.36 ± 65.19
c,e,h,k,n,q,r
275.32 ± 86.70
247.13 ± 62.14c,h,k,n,q,r
Values are presented as mean ± SD. V1, V2, V3, V4, V5 and V6: 0.025, 0.04, 0.1, 0.5, 1 and 2 mg kg-1 body weight single doses of varenicline, respectively a p \ 0.05, different from 90th min of varenicline b
p \ 0.01, different from 90th min of varenicline
c
p \ 0.001, different from 90th min of varenicline
d
p \ 0.05, different from 120th min of varenicline
e
p \ 0.01, different from 120th min of varenicline
f
p \ 0.05, different from V1
g
p \ 0.01, different from V1
h
p \ 0.001, different from V1
i
p \ 0.05, different from V2
j
p \ 0.01, different from V2
k
p \ 0.001, different from V2
l
p \ 0.05, different from V3
m
p \ 0.01, different from V3
n
p \ 0.001, different from V3
o
p \ 0.05, different from V4 p \ 0.01, different from V4
p q
p \ 0.001, different from V4
r
p \ 0.001, different from V5
therapy and cytisine [30, 31]. These results corroborate the possibility of varenicline-induced epilepsy mediated by nAChRs, as nicotine and cytisine have an effect on nAChRs [32–34]. In addition, there have been studies conducted on the relationship between nAChRs and epilepsy [13, 35]. In smoking cessation therapy, dosing of varenicline starts at 0.5 mg once daily for the first 3 days, then increases to 0.5 mg twice daily for the following 4 days and 1 mg twice daily thereafter, for a total of 12 weeks
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[36]. The reason for including the low-dose groups (0.04 and 0.025 mg kg-1) in this study design is that the doses are equivalent to the daily maximum dose used for humans, 2 mg/50 and 80 kg body weight, respectively. These low doses of varenicline caused epileptic activity in rats in the current study. Moreover, this effect was observed after a single-dose application. In healthy adults, maximum plasma concentration of varenicline occurs within 3–4 h of a single dose of oral administration, and steady-state concentration is typically reached within 4 days following
Neurol Sci
regular doses. Furthermore, renal elimination of varenicline is primarily through glomerular filtration, and the elimination half-life is *24 h [37–39] and this elimination rate can contribute to an increase in plasma levels of varenicline. Because plasma concentration is higher in regular doses of varenicline than in a single-dose treatment [37, 39], the possible adverse effects of varenicline may increase in regular doses. While the pharmacokinetic properties of varenicline in rats and humans are similar [38], there may be some differences between the two, such as the pharmacodynamic properties of varenicline and the neurochemical pathways that mediate its effects. The superiority of the current study is that multiple doses of varenicline were tested and its epileptogenic effect was demonstrated in low single doses. In addition, to eliminate individual differences among rats, the EEG findings of the varenicline-treated groups were compared to the initial values of the same groups in addition to the control and saline groups. However, there are some limitations. The study was performed in rats, and there may be some differences between rats and humans. In addition, at this stage, we did not investigate the possible mechanisms that mediate the effects of varenicline on epilepsy. This mechanism and the relationship between glutamate and varenicline should be clarified. In conclusion, the present study shows that intraperitoneal applications of different single doses of varenicline caused epileptic activity in rats. However, the mechanism of this effect is not clear. Nonetheless, according to our findings, epileptic seizures may be accepted as a potential adverse effect of varenicline.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Acknowledgments The authors thank to Aydın Akc¸ılar for his technical support. This study was supported by Balikesir University Research Fund (Project number: 2012–121).
16.
Conflict of interest The authors declare that they have no conflicts of interest.
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18.
References 1. Niaura R, Jones C, Kirkpatrick P (2006) Varenicline. Nat Rev Drug Discov 5:537–538. doi:10.1038/nrd2088 2. Cahill K, Stead LF, Lancaster T (2008) Nicotine receptor partial agonists for smoking cessation. Cochrane Database Syst Rev 16:CD006103. doi:10.1002/14651858.CD006103.pub2 3. Mihalak KB, Carroll FI, Luetje CW (2006) Varenicline is a partial agonist at a4b2 and a full agonist at a7 neuronal nicotinic receptors. Mol Pharmacol 70:801–805. doi:10.1124/mol.106. 025130 4. Sarter M, Parikh V, Howe WM (2009) Phasic acetylcholine release and the volume transmission hypothesis: time to move on. Nat Rev Neurosci 10:383–390. doi:10.1038/nrn2635 5. Alkondon M, Albuquerque EX (2004) The nicotinic acetylcholine receptor subtypes and their function in the hippocampus and
19.
20.
21.
22.
cerebral cortex. Prog Brain Res 145:109–120. doi:10.1016/ S0079-6123(03)45007-3 Gotti C, Clementi F, Fornari A et al (2009) Structural and functional diversity of native brain neuronal nicotinic receptors. Biochem Pharmacol 78:703–711. doi:10.1016/j.bcp.2009.05.024 Bina KG, Guzman P, Broide RS, Leslie FM, Smith MA, O’Dowd DK (1995) Localization of alpha 7 nicotinic receptor subunit mRNA and alpha-bungarotoxin binding sites in developing mouse somatosensory thalamocortical system. J Comp Neurol 363:321–332. doi:10.1002/cne.903630212 Descarries L (1998) The hypothesis of an ambient level of acetylcholine in the central nervous system. J Physiol Paris 92:215–220. doi:10.1016/S0928-4257(98)80013-2 Jones IW, Wonnacott S (2004) Precise localization of alpha7 nicotinic acetylcholine receptors on glutamatergic axon terminals in the rat ventral tegmental area. J Neurosci 24:11244–11252. doi:10.1523/JNEUROSCI.3009-04.2004 Le´na C, Changeux JP, Mulle C (1993) Evidence for ‘‘preterminal’’ nicotinic receptors on GABAergic axons in the rat interpeduncular nucleus. J Neurosci 13:2680–2688 Zhu PJ, Chiappinelli VA (2002) Nicotinic receptors mediate increased GABA release in brain through a tetrodotoxin-insensitive mechanism during prolonged exposure to nicotine. Neuroscience 115:137–144. doi:10.1016/S0306-4522(02)00371-8 Aracri P, Amadeo A, Pasini ME, Fascio U, Becchetti A (2013) Regulation of glutamate release by heteromeric nicotinic receptors in layer V of the secondary motor region (Fr2) in the dorsomedial shoulder of prefrontal cortex in mouse. Synapse 67:338–357. doi:10.1002/syn.21655 Dani JA (2000) Properties underlying the influence of nicotinic receptors on neuronal excitability and epilepsy. Epilepsia 41:1063–1065. doi:10.1111/j.1528-1157.2000.tb00300.x Decker MW, Rueter LE, Bitner RS (2004) Nicotinic acetylcholine receptor agonists: a potential new class of analgesics. Curr Top Med Chem 4:369–384. doi:10.2174/1568026043451447 Dalley JW, Theobald DE, Bouger P, Chudasama Y, Cardinal RN, Robbins TW (2004) Cortical cholinergic function and deficits in visual attentional performance in rats following 192 IgG-saporininduced lesions of the medial prefrontal cortex. Cereb Cortex 14:922–932. doi:10.1093/cercor/bhh052 Braida D, Ponzoni L, Martucci R, Sparatore F, Gotti C, Sala M (2014) Role of neuronal nicotinic acetylcholine receptors (nAChRs) on learning and memory in zebrafish. Psychopharmacology 231:1975–1985. doi:10.1007/s00213-013-3340-1 Fagerstrom K, Gilljam H, Metcalfe M, Tonstad S, Messig M (2010) Stopping smokeless tobacco with varenicline: randomised double blind placebo controlled trial. BMJ 341:c6549. doi:10. 1136/bmj.c6549 Tonstad S, Davies S, Flammer M, Russ C, Hughes J (2010) Psychiatric adverse events in randomized, double-blind, placebocontrolled clinical trials of varenicline: a pooled analysis. Drug Saf 33:289–301. doi:10.2165/11319180-000000000-00000 Topliss D, Gold M, Kotsirilos V et al (2008) Varenicline: the Australian experience so far. Aust Adverse Drug React Bull 27(6):22. http://www.tga.gov.au/hp/aadrb-0812.htm#.U49Yw3Y 4TgF. Accessed 12 Jan 2014 Serafini A, Crespel A, Velizara R, Gelisse P (2010) Vareniclineinduced grand mal seizure. Epileptic Disord 12:338. doi:10.1684/ epd.2010.0339 Spiller K, Xi ZX, Li X et al (2009) Varenicline attenuates nicotine-enhanced brain-stimulation reward by activation of alpha4beta2 nicotinic receptors in rats. Neuropharmacology 57:60–66. doi:10.1016/j.neuropharm.2009.04.006 Levin ME, Weaver MT, Palmatier MI, Caggiula AR, Sved AF, Donny EC (2012) Varenicline dose dependently enhances responding for nonpharmacological reinforcers and attenuates the
123
Neurol Sci
23. 24.
25.
26.
27.
28.
29.
30.
31.
reinforcement-enhancing effects of nicotine. Nicotine Tob Res 14:299–305. doi:10.1093/ntr/ntr213 Karlin A (2002) Emerging structure of the nicotinic acetylcholine receptors. Nat Rev Neurosci 3:102–114. doi:10.1038/nrn731 Rathouz MM, Vijayaraghavan S, Berg DK (1996) Elevation of intracellular calcium levels in neurons by nicotinic acetylcholine receptors. Mol Neurobiol 12:117–131. doi:10.1007/BF02740649 Ween H, Thorin-Hagene K, Andersen E et al (2010) Alpha3* and alpha 7 nAChR-mediated Ca2? transient generation in IMR-32 neuroblastoma cells. Neurochem Int 57:269–277. doi:10.1016/j. neuint.2010.06.005 Speckmann EJ, Straub H, Ko¨hling R (1993) Contribution of calcium ions to the generation of epileptic activity and antiepileptic calcium antagonism. Neuropsychobiology 27:122–126. doi:10.1159/000118966 Andre´ V, Marescaux C, Nehlig A, Fritschy JM (2001) Alterations of hippocampal GABAergic system contribute to development of spontaneous recurrent seizures in the rat lithium–pilocarpine model of temporal lobe epilepsy. Hippocampus 11:452–468. doi:10.1002/hipo.1060 Sitges M, Sanchez-Tafolla BM, Chiu LM, Aldana BI, Guarneros A (2011) Vinpocetine inhibits glutamate release induced by the convulsive agent 4-aminopyridine more potently than several antiepileptic drugs. Epilepsy Res 96:257–266. doi:10.1016/j. eplepsyres.2011.06.006 DuBois DW, Damborsky JC, Fincher AS, Frye GD, WinzerSerhan UH (2013) Varenicline and nicotine enhance GABAergic synaptic transmission in rat CA1 hippocampal and medial septum/diagonal band neurons. Life Sci 92:337–344. doi:10.1016/j. lfs.2012.12.013 Beleslin DB, Krstic´ SK (1986) Nicotine-induced convulsions in cats and central nicotinic receptors. Pharmacol Biochem Behav 24:1509–1511. doi:10.1016/0091-3057(86)90476-4 Tutka P, Mro´z T, Bednarski J et al (2013) Cytisine inhibits the anticonvulsant activity of phenytoin and lamotrigine in mice. Pharmacol Rep 65:195–200. doi:10.1016/S1734-1140(13)709782
123
32. Hussy N, Ballivet M, Bertrand D (1994) Agonist and antagonist effects of nicotine on chick neuronal nicotinic receptors are defined by alpha and beta subunits. J Neurophysiol 72:1317–1326 33. Mineur YS, Somenzi O, Picciotto MR (2007) Cytisine, a partial agonist of high-affinity nicotinic acetylcholine receptors, has antidepressant-like properties in male C57BL/6J mice. Neuropharmacology 52:1256–1262. doi:10.1016/j.neuropharm.2007. 01.006 34. Peng C, Stokes C, Mineur YS et al (2013) Differential modulation of brain nicotinic acetylcholine receptor function by cytisine, varenicline, and two novel bispidine compounds: emergent properties of a hybrid molecule. J Pharmacol Exp Ther 347:424–437. doi:10.1124/jpet.113.206904 35. Feuerbach D, Lingenhoehl K, Olpe HR et al (2009) The selective nicotinic acetylcholine receptor alpha7 agonist JN403 is active in animal models of cognition, sensory gating, epilepsy and pain. Neuropharmacology 56:254–263. doi:10.1016/j.neuropharm. 2008.08.025 36. Wang C, Cho B, Xiao D, Wajsbrot D, Park PW (2013) Effectiveness and safety of varenicline as an aid to smoking cessation: results of an inter-Asian observational study in real-world clinical practice. Int J Clin Pract 67:469–476. doi:10.1111/ijcp.12121 37. Faessel HM, Smith BJ, Gibbs MA, Gobey JS, Clark DJ, Burstein AH (2006) Single-dose pharmacokinetics of varenicline, a selective nicotinic receptor partial agonist, in healthy smokers and nonsmokers. J Clin Pharmacol 46:991–998. doi:10.1177/ 0091270006290669 38. Obach RS, Reed-Hagen AE, Krueger SS et al (2006) Metabolism and disposition of varenicline, a selective alpha4beta2 acetylcholine receptor partial agonist, in vivo and in vitro. Drug Metab Dispos 34:121–130. doi:10.1124/dmd.105.006767 39. Kikkawa H, Maruyama N, Fujimoto Y, Hasunuma T (2011) Single- and multiple-dose pharmacokinetics of the selective nicotinic receptor partial agonist, varenicline, in healthy Japanese adult smokers. J Clin Pharmacol 51:527–537. doi:10.1177/ 0091270010372388
