Effects of the antidepressant mirtazapine and zinc on nicotinic acetylcholine receptors

Highlights

• The ACh-current mediated by muscle nAChRs was inhibited by mirtazapine.

• Potentiation of ACh-current by zinc was minor in the presence of mirtazapine.

• Mirtazapine inhibited α7 nAChRs in hippocampal CA1 stratum radiatum interneurons.

• The inhibitory effects of mirtazapine depended on membrane potential.

Abstract

Nicotinic acetylcholine receptors (nAChRs) and zinc are associated with regulation of mood and related disorders. In addition, several antidepressants inhibit muscle and neuronal nAChRs and zinc potentiates inhibitory actions of them. Moreover, mirtazapine (a noradrenergic, serotonergic and histaminergic antidepressant) inhibits muscarinic AChRs and its effects on nAChRs are unknown. Therefore, we studied the modulation of muscle α1β1γd nAChRs expressed in oocytes and native α7-containing nAChRs in hippocampal interneurons by mirtazapine and/or zinc, using voltage-clamp techniques. The currents elicited by ACh in oocytes (at −60 mV) were similarly inhibited by mirtazapine in the absence and presence of 100 μM zinc (IC₅₀ ∼15 μM); however, the ACh-induced currents were stronger inhibited with 20 and 50 μM mirtazapine in the presence of zinc. Furthermore, the potentiation of ACh-induced current by zinc in the presence of 5 μM mirtazapine was 1.48 ± 0.06, and with 50 μM mirtazapine zinc potentiation did not occur. Interestingly, in stratum radiatum interneurons (at −70 mV), 20 μM mirtazapine showed less inhibition of the current elicited by choline (Ch) than at 10 μM (0.81 ± 0.02 and 0.74 ± 0.02 of the Ch-induced current, respectively). Finally, the inhibitory effects of mirtazapine depended on membrane potential: 0.81 ± 0.02 and 0.56 ± 0.05 of the control Ch-induced current at −70 and −20 mV, respectively. These results indicate that mirtazapine interacts with muscle and neuronal nAChRs, possibly into the ion channel; that zinc may increase the sensitivity of nAChRs to mirtazapine; and that mirtazapine decreases the sensitivity of nAChRs to zinc.

Introduction

Nicotinic acetylcholine receptors (nAChRs) are pentameric proteins that belong to the superfamily of Cys-loop ligand-gated ion channels. Muscle nAChRs are formed with α1, β1, γ or ε, and δ subunits; whereas neuronal nAChRs are composed of the combination of α2-α10 and β2-β4, which may be assembled as heteromeric nAChRs, or of only one subunit such as homomeric α7 nAChRs [[1], [2], [3]]. nAChRs are widely distributed in the organism, in muscle endplate mediating fast neurotransmission; whereas in the nervous system they mediate fast neurotransmission as well as modulation of transmitter release [[1], [2]]. They are involved in physiological and pathological roles such as skeletal muscle contraction, learning, memory, cognition, congenital myasthenic syndromes, Alzheimer’s disease, epilepsy, depressive disorders, and drug addiction [[4], [5], [6], [7]].

The functioning of nAChRs is modulated by a wide variety of substances interacting in allosteric sites including cations, local anesthetics, antidepressants, among others [[8], [9], [10], [11]]. Particularly, a common feature of different categories of antidepressants, including tricyclics and inhibitors of monoamine uptake, is the modulation of cholinergic signaling by inhibiting both nAChRs [[8], [12], [13], [14], [15], [16]] and muscarinic AChRs [[17], [18], [19]]. These facts strengthen the idea that cholinergic signaling is intimately associated with major depression disorder [[20], [21]].

In the case of mirtazapine – an antidepressant that inhibits norepinephrine, serotonin and histamine receptors [22] – there is one report on cholinergic activity in guinea pig urinary bladder smooth muscle, in which muscarinic AChRs are competitively inhibited by this antidepressant [19].

Interestingly, the divalent cation zinc is found throughout the brain, especially in the cerebral cortex and the hippocampus [23]. This cation allosterically interacts with nAChRs and can potentiate or inhibit them, depending on zinc concentration and receptor subunit composition [[24], [25], [26], [27]]. On other hand, zinc deficiency has been associated with depressive disorders [[28], [29], [30]]. Accordingly, zinc enhances the effect of several antidepressants both in animal models of depression and in depressive patients, and it also enhances inhibitory effects of fluoxetine and bupropion on nAChRs more strongly than the antidepressant alone [[31], [32], [33], [34], [35], [36]]. In this regard, the aim of this work was to study the pharmacological actions of mirtazapine on muscle and α7 nAChRs hippocampal interneurons, in the absence and presence of zinc.