Effects of fluoxetine on wild and mutant neuronal α7 nicotinic receptors

Abstract

Fluoxetine is used in the treatment of a variety of clinical disorders including depression and obesity, and of cocaine detoxification or alcoholism. It is generally believed that fluoxetine exerts its clinical effects because it selectively blocks 5-hydroxytryptamine (5HT) reuptake into nerve terminals. In here we describe that fluoxetine antagonized the neuronal homomeric ␣ 7 nicotinic acetylcholine receptors (nAChR) expressed in Xenopus oocytes, with an IC 50 of 43 M, when fluoxetine was coapplied with ACh, and of 1.6 M when the oocytes were pretreated briefly with fluoxetine. A similar block occurred in oocytes expressing L247T ␣ 7 mutant nAChR. Furthermore, blockage of mutant ␣ 7 receptors appeared non-competitive and was stronger with cell membrane hyperpolarization. Cell-attached single channel recordings in oocytes expressing L247T ␣ 7 mutant nAChR showed that the voltage-dependence of the blockage by fluoxetine could be due to a drastic decrease in channel opening frequency accompanied by marked channel flickering and reduced channel conductance. We conclude that fluoxetine behaves as a reversible blocker of both wild and mutant ␣ 7 receptors; and that the Leu-247T mutation in the channel domain renders the blockage of ␣ 7 nAChR by fluoxetine voltagedependent. These effects of fluoxetine on ␣ 7 receptors may be clinically important.A great deal of interest has recently focused on the neuronal nicotinic ␣ 7 receptor due to its special structural and functional characteristics as compared to other members of the nicotinic receptor family. In particular, the ␣ 7 subunit forms functional homomeric receptors; 1,2 and the substitution of the conserved leucine 247 for threonine, in the M2 channel domain, changes drastically their function. [3][4][5] Fluoxetine is a potent inhibitor of 5HT uptake that is used extensively in clinical practice; and it was thought that its therapeutic effects were essentially due to blocking of the 5HT-transporter. 6 However, we have recently shown that fluoxetine is a potent blocker of nicotinic receptors, inhibiting not only heteromeric muscular but also neuronal nicotinic receptors expressed in Xenopus oocytes. 7 The experiments reported here were done in oocytes injected with ␣ 7 subunit cDNA to investigate whether fluoxetine would also block homomeric ␣ 7 nAChR. Fluoxetine was also used as a tool to investigate further the role of the Leu-247 residue as a determinant of the functional and pharmacological profiles of the ␣ 7 receptor.Oocytes injected with the wild-type ␣ 7 subunit cDNA had a membrane potential of −38 ± 4 mV and responded to ACh with an inward current (I ACh ) whose peak amplitude depended on transmitter concentration. In oocytes held at −60 mV, 150 M ACh, a concentration close to the EC 50 for ␣ 7 nAChR, 1,8-10 elicited a mean acetylcholine-current (I ACh ) of −77 ± 13 nA (mean ± sem n = 15; two donors, range −20 nA to −177 nA), that decayed to 90% (T 0.1 ) in 0.81 ± 0.01 s. Fluoxetine alone (100 M) did not elicit obvious current responses in either non-inject...

Antagonism of nicotinic acetylcholine receptors by inhibitors of monoamine uptake

“…For instance, fluoxetine has an inhibitory effect on muscle as well as on neuronal AChRs, 11,13,16,17,22 histamine H 1 , adrenergic ␣ 1 and ␣ 2 and muscarinic receptors, 19 and voltage-activated K + and Na + channels. 38 Finally, it is possible that the actions of imipramine, inhibiting neuronal AChRs, are related with therapeutic-and/or side-effects, because the concentration of imipramine in blood of depressed patients may reach ෂ2 M, 39 and that the concentration in the brain is 11-16 fold higher, 40 compared with the IC 50 for imipramine (3.65 M) on neuronal ␣2␤4 AChRs studied here.…”

“…19 In addition, fluoxetine, an antidepressant that initially was thought to be a very high selective inhibitor of the 5-HT uptake system, 20 inhibits nicotinic and serotonin receptors. 11,13,21,22 Similarly, imipramine, a non-selective inhibitor of serotonin uptake, 23 blocks nicotinic receptors of frog motor endplate and electric organ in a noncompetitive and voltage-dependent way, 24,25 inhibits the current evoked by dimethylphenylpiperazinium in human neuroblastoma cells and blocks the secretion of ATP mediated by AChRs in chromaffin cells. 26,27 In this paper, we studied effects of various inhibitors of monoamine transporters on neuronal ␣2␤4 and muscle AChRs, with emphasis on the mechanisms of action of imipramine.…”

Inhibition of G Protein-Activated Inwardly Rectifying K+ Channels by Various Antidepressant Drugs

“…In contrast, the interaction of antidepressants with muscarinic, a 1 adrenergic, and H 1 histamine receptors is involved in some of the adverse side effects (Baldessarini, 2001). It has also been shown that antidepressants inhibit the functions of several other receptors and ion channels, such as 5-HT 2C (Ni and Miledi, 1997) and 5-HT 3 receptors (Fan, 1994), nicotinic acetylcholine receptors (García-Colunga et al, 1997;Maggi et al, 1998), N-methyl-D-aspartate (NMDA) receptor channels (Sernagor et al, 1989), P2X 2 receptors (Nakazawa et al, 1999), voltage-gated Ca 2 þ , Na þ , and K þ channels (Ogata et al, 1989;Mathie et al, 1998;Pancrazio et al, 1998;Teschemacher et al, 1999;Yeung et al, 1999;Deák et al, 2000;Choi et al, 2001;Cuellar-Quintero et al, 2001), Ca 2 þ -activated K þ channels (Kamatchi and Ticku, 1991;Lee et al, 1997;Dreixler et al, 2000;Terstappen et al, 2001), and Cl À channels (Maertens et al, 1999(Maertens et al, , 2002. The effects might also be involved in the molecular and cellular mechanisms underlying some of the therapeutic effects and side effects of various antidepressants.…”

Combined actions of zinc and fluoxetine on nicotinic acetylcholine receptors

“…For instance, they are modulated by cations, [5][6][7][8] 5-HT, 9-13 various agonists and antagonists of different types of 5-HT receptors, [9][10][11][12]14 as well as inhibitors of monoamine transporters that are used as antidepressants. 9,12,[15][16][17][18][19][20] The most common action of clinically used antidepressants on nAChRs appears to be a noncompetitive inhibitory process, 9,12,[15][16][17][18][19][20] and it is probable that this action contributes to improving depressed mood states. 21 On the other hand, zinc is contained in neurons that are widely distributed throughout the nervous system; 22 and it is released from GABAergic and glutamatergic nerve endings, 23,24 reaching an extracellular concentration of B300 mM.…”