Background: The tricyclic antidepressant amitriptyline, the serotonin and noradrenaline reuptake inhibitor duloxetine, and gabapentinoids are first-line drugs for treatment of neuropathic pain. The analgesic effect of these drugs relates to brainstem-spinal descending noradrenergic systems. However, amitriptyline utilizes a variety of mechanisms for analgesia in neuropathic pain, and it is unclear which mechanism is most important. In the present study, we investigated the role of descending noradrenergic systems in the analgesic effect of these drugs for treatment of neuropathic pain. We also examined whether amitriptyline modifies the descending noradrenergic systems.
Methods: Seven days after L5 spinal nerve ligation (SNL), rats received N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, 50 mg/kg) to degenerate noradrenergic fibers. The rats then received 5 daily intraperitoneal injections of amitriptyline (10 mg/kg), duloxetine (10 mg/kg), pregabalin (10 mg/kg), or gabapentin (50 mg/kg) from 21 days after SNL surgery. Paw withdrawal thresholds were determined to assess the effect of the drugs on hyperalgesia after SNL. To determine whether 5 daily injections of amitriptyline activated noradrenergic neurons in the locus coeruleus (LC) and spinal cord with or without DSP-4 treatment, we performed immunohistochemistry using antibodies for c-Fos and dopamine beta-hydroxylase (DβH).
Results: Five daily injections of amitriptyline, duloxetine, pregabalin, and gabapentin exerted antihyperalgesic effects in SNL rats (P < .001; estimated treatment effect of amitriptyline [99% confidence interval]: 59.9 [35.1-84.7] g). The antihyperalgesic effects of duloxetine, pregabalin, and gabapentin were reversed by pretreatment with DSP-4 (P < .001, respectively). However, antihyperalgesia was still observed after treatment of amitriptyline in SNL rats with DSP-4 pretreatment (P < .001, 59.7 [30.0-89.3] g), and this analgesic effect was not reversed by the α2-adrenoceptor antagonist idazoxan (30 μg). Additionally, 5 daily injections of amitriptyline increased the ratio of c-Fos-immunoreactive (IR) cells in noradrenergic LC neurons in SNL rats with or without DSP-4 pretreatment (P < .001, respectively). Five daily injections of amitriptyline increased DβH-IR in the LC and the spinal dorsal horn of SNL rats (P < .001, respectively). With DSP-4 pretreatment, DβH-IR was dramatically decreased with or without 5 daily injections of amitriptyline (P < .001).
Conclusions: Five daily injections of amitriptyline produced antihyperalgesic effects against neuropathic pain despite suppression of noradrenergic descending inhibitory systems. Amitriptyline activated LC neurons and increased noradrenergic fibers density in SNL rats. These results suggest that amitriptyline could still produce analgesia under pathological dysfunction of the descending noradrenergic system. Amitriptyline may enhance the analgesic effect of drugs for neuropathic pain that require normal descending noradrenergic inhibition to produce analgesia, such as serotonin and noradrenaline reuptake inhibitors and gabapentinoids.