Trazodone inhibits T-type calcium channels
Introduction
Voltage-gated calcium channels control the rapid entry of calcium ions into a wide variety of cells. Electrophysiological studies have identified two major types of calcium channels, high-voltage-activated and low-voltage-activated calcium channels. The latter type was classified as T-type calcium channels, defined by activation at low membrane potentials, fast gating kinetics, and small single channels conductance (Bean, 1989, Carbone and Lux, 1984, Nowycky et al., 1985). Low-voltage-activated T-type calcium channels play important roles in pacing neuronal firing and producing network oscillations generating characteristic EEG patterns during sleep. Calcium-dependent low-threshold currents mediated by T-type calcium channels underlie low-threshold spikes (LTS) that have been demonstrated as intrinsic firing properties of thalamic and cortical neurons. LTS are predominantly found in the thalamus during non-REM sleep and are generally absent during wakefulness and REM sleep, suggesting a key role of T-type calcium channels in the regulation of sleep (Crunelli et al., 2006, McCormick and Bal, 1997, Steriade et al., 1993).
Three T-type channel transcripts, Cav3.1, Cav3.2 and Cav3.3, have been cloned in mice, rats and humans (Perez-Reyes, 2003). These transcripts are highly enriched in brain regions associated with prominent T-type currents, including thalamic relay neurons (Cav3.1) and thalamic reticular neurons (Cav3.2 and Cav3.3) (Talley et al., 1999). Interestingly, it has recently been reported that mice lacking Cav3.1 channels in the thalamus display differences in sleep structure which further suggests a role of T-type calcium channels in maintenance and regulation of sleep (Anderson et al., 2005, Lee et al., 2004).
Trazodone (Desyrel®) is one of the most commonly prescribed agents for treating insomnia (Mendelson, 2005, Nierenberg et al., 1994). The drug produces robust changes in the sleep EEG in patients with sleep complaints (Sharpley and Cowen, 1995). In contrast to other antidepressants, such as selective serotonin reuptake inhibitors and monoamine oxidase inhibitors, trazodone possesses sedative activity (Becker, 2004), reduces sleep latency and increases sleep continuity and slow wave sleep (Montgomery et al., 1983, Saletu-Zyhlarz et al., 2002, Saletu-Zyhlarz et al., 2003, Sharpley and Cowen, 1995), which is believed to be important to restorative sleep (Akerstedt et al., 1997). The pharmacological mechanism of action of trazodone's neurological effects is unclear. Its therapeutic benefit may be linked to its antagonistic effects on postsynaptic 5-HT2A/C receptors which may play an important role in the regulation of slow wave sleep (Sharpley and Cowen, 1995), histamine1 receptors or α1 adrenoceptors (Owens et al., 1997). However, other target activities, at clinical doses ranging from 50 to 600 mg, may be attributable as well.
In an effort to identify novel mechanisms that could contribute to trazodone's neurological effects we have tested the drug and its major metabolite m-chlorophenylpiperazine (m-CPP) (Caccia et al., 1981) for T-type calcium channel activity. Here we report inhibitory effects of trazodone and m-CPP at therapeutically relevant concentrations on Cav3.1, Cav3.2 and Cav3.3 T-type calcium channels stably expressed in HEK-293 cells. We also demonstrate that trazodone inhibits native T-type currents recorded from the subthalamic nucleus in a rat brain slice assay.
Section snippets
Cell culture
For whole-cell voltage-clamp recordings cell lines expressing human Cav3.1, Cav3.2 or Cav3.3 channels were used. Tetracycline inducible HEK-293 cells which stably express the tetracycline repressor protein were transfected with Cav3.1, Cav3.2 and Cav3.3 in pcDNA 4/TO. Cells were maintained in DMEM (Gibco Life Technologies) supplemented with 10% fetal bovine serum, 2 mM l-glutamine, 100 units/mL penicillin/streptomycin, 150 μg/ml Zeocin, and 5 μg/ml Blastocidin. Cells were plated on poly-d-lysine
Trazodone inhibits recombinant low-voltage-gated T-type calcium channels
The effects of trazodone (Fig. 1) on T-type calcium channel currents mediated by human Cav3.1 subunits stably expressed in HEK-293 cells were studied in whole-cell patch-clamp recordings. Cells were held at a polarized membrane potential of −100 mV. Fig. 2A illustrates Cav3.1 currents evoked by depolarizing 30 ms test pulses to −20 mV given every 10 s. Trazodone caused a concentration-dependent decrease of the Cav3.1 peak current amplitude. Inhibition was reversible in all experiments performed.
Characteristics of trazodone block
The main finding of the present study was that trazodone and its major metabolite m-CPP inhibited human T-type calcium channels expressed in HEK-293 cells at clinically relevant concentrations. Inhibitory effects of trazodone were confirmed on native T-type calcium currents recorded from the subthalamic nucleus in a rat brain slice assay. In contrast to other non-selective T-type channel antagonists such as the antiepileptic drug ethosuximide (Gomora et al., 2001), the selective serotonin
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