Venlafaxine-induced depression of wind-up activity in mononeuropathic rats is potentiated by inhibition of brain 5-HT1A receptor expression in vivo

Monoamines as Drug Targets in Chronic Pain: Focusing on Neuropathic Pain

Monoamines are involved in regulating the endogenous pain system and indeed, peripheral and central monoaminergic dysfunction has been demonstrated in certain types of pain, particularly in neuropathic pain. Accordingly, drugs that modulate the monaminergic system and that were originally designed to treat depression are now considered to be first line treatments for certain types of neuropathic pain (e.g., serotonin and noradrenaline (and also dopamine) reuptake inhibitors). The analgesia induced by these drugs seems to be mediated by inhibiting the reuptake of these monoamines, thereby reinforcing the descending inhibitory pain pathways. Hence, it is of particular interest to study the monoaminergic mechanisms involved in the development and maintenance of chronic pain. Other analgesic drugs may also be used in combination with monoamines to facilitate descending pain inhibition (e.g., gabapentinoids and opioids) and such combinations are often also used to alleviate certain types of chronic pain. By contrast, while NSAIDs are thought to influence the monoaminergic system, they just produce consistent analgesia in inflammatory pain. Thus, in this review we will provide preclinical and clinical evidence of the role of monoamines in the modulation of chronic pain, reviewing how this system is implicated in the analgesic mechanism of action of antidepressants, gabapentinoids, atypical opioids, NSAIDs and histaminergic drugs.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

Increased thermal and mechanical nociceptive thresholds in rats with depressive-like behaviors

Clinical observations suggest that depressed patients were less sensitive to experimental pain than healthy subjects. However, few animal studies are reported concerning the association of depression and pain. The purpose of this study was to investigate the effects of unpredictable chronic mild stress (UCMS) induced depression on the perceived intensity of painful stimulation in rats. We measured the thermal and mechanical paw withdrawal thresholds (PWT) of normal and spinal nerve ligated (SNL) rats using hot plate test and von Frey test, respectively. The results showed that rats exposed to UCMS exhibited significantly higher thermal and mechanical pain thresholds in comparison to the non-depressed controls. In particular, the PWT of the SNL group was restored to nearly normal level after three weeks of UCMS, and even comparable to that of the control group. These results strongly suggest that the depressed subjects have decreased sensitivity to externally applied noxious stimulation, which is consistent with our previous findings.

Role of 5-HT1A and 5-HT1B receptors in the antinociceptive effect of tramadol

Tramadol, (1RS,2RS)-2-[(dimethylamine)-methyl]-1-(3-methoxyphenyl)-cyclohexanol hydrochloride, is an atypical centrally acting analgesic agent with relatively weak opioid receptor affinity and which, like some antidepressants, is able to inhibit the reuptake of serotonin (5-hydroxytryptamine, 5-HT) in the raphe nucleus. We have previously demonstrated that pindolol, a beta-adrenoceptor blocker/5-hydroxytryptamine(1A/1B) receptor antagonist, enhanced tramadol antinociception and that the selective 5-HT1A agonist 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) reduced it. These effects were related to the negative feedback control that regulates raphe region neurones. The current study examines the ability of the selective antagonist at somatodendritic 5-HT1A receptors, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl) cyclohexane carboxamide (WAY100635, 0.8 mg/kg), the selective antagonist at terminal 5-HT1B receptors, N-[3-(2-dimethylamino) ethoxy-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide (SB216641, 0.1-0.8 mg/kg) and the selective agonist at 5-HT1B receptors, 1,4-tDihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b] pyridin-5-one (CP93129, 0.2-0.4 mg/kg), to modify the antinociceptive effect of 4-64 mg/kg of tramadol in the hot plate test in mice. The results show that 0.8 mg/kg of WAY100635 enhanced antinociceptive effect of tramadol while neither agonism nor antagonism at the 5-HT1B receptor modifies it significantly at the doses tested. These results account for involvement of the somatodendritic 5-HT1A receptors in the analgesic effect of tramadol and support the supraspinal interaction of serotonin and the opioid system in the regulation of pain.

The novel analgesic and high-efficacy 5-HT1A receptor agonist F 13640 inhibits nociceptive responses, wind-up, and after-discharges in spinal neurons and withdrawal reflexes

Evidence shows that serotonin (5-HT) is involved in the transmission of nociception in the central nervous system. Using a new electrophysiological method of simultaneous recordings in rats we examined the actions of the novel analgesic and high-efficacy 5-HT1A receptor agonist F 13640 as well as those of the opioid receptor agonist fentanyl on simultaneously evoked responses of spinal dorsal horn (DH) wide-dynamic range (WDR) neurons and spinal withdrawal reflexes. Spinal withdrawal reflexes were studied by assessing the activity of single motor units (SMUs) electromyographically (EMG). Like that of 0.02 mg/kg fentanyl, intraperitoneal injection of 0.31 mg/kg of F 13640 markedly inhibited nociceptive pinch-evoked responses as well as C-fiber-mediated late responses including wind-up of both DH WDR neurons and SMUs to suprathreshold (1.5 x T) repeated (3 Hz) electrical stimulation. Specifically, in contrast to no significant depressive effects by fentanyl on 20 Hz electrically evoked after-discharge of DH WDR neurons, the after-discharges of DH WDR neurons and SMUs were significantly inhibited by F 13640 (P < 0.05 and P < 0.001, respectively). The inhibitory effects of F 13640 and fentanyl on responses of DH WDR neurons and SMUs were reversed by the specific antagonists WAY 100635 and naloxone, respectively, further indicating that this 5-HT1A receptor-modulated anti-nociception is mu-opioid receptor independent. For the first time, 5-HT1A receptors are clearly proved to be involved in the progressive wind-up to 3-Hz frequency of electrical stimulation as well as after-discharges of sensory input of DH WDR neurons, and simultaneously recorded motor output of spinal reflexes to 20-Hz frequency of electrical stimulation; this suggests that serotonin, through 5-HT1A receptors, exerts an inhibitory role in the control of obstinate pathological pain.