Abstract
Although the tricyclic antidepressant amitriptyline (ATL) is widely used in the clinic, the mechanism underlying its high therapeutic efficacy against neuropathic pain remains unclear. NMDA receptors (NMDARs) represent a target for ATL and are involved in sensitization of neuropathic pain. Here we describe two actions of ATL on NMDARs: 1) enhancement of Ca2+-dependent desensitization and 2) trapping channel block. Inhibition of NMDARs by ATL was found to be dependent upon external Ca2+ concentration ([Ca2+]) in a voltage-independent manner, with an IC50 of 0.72 μM in 4 mM [Ca2+]. The ATL IC50 value increased exponentially with decreasing [Ca2+], with an e-fold change observed per 0.69 mM decrease in [Ca2+]. Loading neurons with BAPTA abolished Ca2+-dependent inhibition, suggesting that Ca2+ affects NMDARs from the cytosol. Since there is one known Ca2+-dependent process in gating of NMDARs, we conclude that ATL most likely promotes Ca2+-dependent desensitization. We also found ATL to be a trapping open-channel blocker of NMDARs with an IC50 of 220 µM at 0 mV. An e-fold change in ATL IC50 was observed to occur with a voltage shift of 50 mV in 0.25 mM [Ca2+]. Thus, we disclose here a robust dependence of ATL potency on extracellular [Ca2+], and demonstrate that ATL bound in the NMDAR pore can be trapped by closure of the channel.
Highlights
More than half of patients with chronic neuropathic pain suffer from depression[1]
We recently described that NMDARs are upregulated by NCX in physiological conditions and that inhibition of Ca2+ export from neurons by the NCX inhibitor (2-[4-[(4-nitrophenyl)methoxy]phenyl]ethyl ester, methanesulfonate, KB-R7943) or lithium enhances Ca2+-dependent desensitization of NMDARs13,14
We first examined general properties of ATL inhibition of whole-cell currents activated by NMDA in cortical neurons that suggest ATL has multiple mechanisms of action on NMDARs. 100 μM ATL, with 1 mM Ca2+ in the external solution, caused block of NMDAR currents that depended on membrane voltage
Summary
More than half of patients with chronic neuropathic pain suffer from depression[1]. Injury of the dorsal root ganglion and spinal superficial dorsal horn neurons increases expression of α2δ-111, a voltage-activated Ca2+ channel subunit that interacts with NMDARs, promoting synaptic and surface expression of α2δ-1-NMDAR complexes and sensitization of neuropathic pain[12]. The histogram (right) depicts the fraction of blocked current (1 − Ib/Ic) obtained at −70 mV in the presence of 0.25 mM and 2 mM Ca2+ in the bathing solution in each of experiments (circles) as well as mean values ± S.E.M. Histogram (right) depicts fractions of blocked current (1 − Ib/Ic) obtained at −70 mV in the presence of 0.25 mM and 2 mM Ca2+ in the bathing solution in each of experiments (circles) as well as mean values ± S.E.M. Data are not significantly different (p = 0.82, Student’s two-tailed t-tests). Other drugs acting directly on NMDARs (e.g., ketamine[15] and memantine16) or indirectly modulating NMDAR activity (e.g., gabapentinoids, which interact with α2δ-1 and prevent synaptic expression of α2δ-1-NMDAR complexes9) possess analgesic potential[17]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
