Effect of citalopram, amineptine, imipramine and nortriptyline on stress-induced (footshock) analgesia in rats

Antidepressant treatment of neuropathic pain: looking for the mechanism

Neuropathic pain arises as a direct consequence of a lesion or disease affecting the somatosensory system. Among the recommended first-line treatments are antidepressant drugs – that is, molecules that were initially developed to treat other disorders of the nervous system. While their clinical efficacy against neuropathic pain was established more than 30 years ago, there is little information on the mechanism underlying their antidepressant action. However, understanding the therapeutic mechanism of these treatments could help to improve them, or even lead to new therapeutic approaches. In this article, we discuss the difficulties in conducting relevant preclinical research on neuropathic pain treatment with antidepressant drugs and we present the most recent findings on the putative mechanism, which highlight the role of β2-adrenoceptors and δ-opioid receptors.

Effects of Analgesic or Antidepressant Drugs on Pain- or Stress-Evoked Hippocampal and Spinal Neurokinin-1 Receptor and Brain-Derived Neurotrophic Factor Gene Expression in the Rat

Clinical studies show that people suffering from chronic pain are often also burdened by depression. Antidepressants are used to treat some types of chronic pain; however, little is known about their mechanisms of action. This study addressed the effects of a nonsteroidal anti-inflammatory drug and a tricyclic antidepressant drug on pain- and stress-evoked gene expression in the rat spinal cord dorsal horn and hippocampus. Rats were pretreated with either indomethacin or imipramine and then challenged with either intraplantar complete Freund's adjuvant or a bout of immobilization stress. Results showed that indomethacin significantly reduced nociception-related peripheral edema, hyperalgesia, and reversed the pain-evoked up-regulation of neurokinin (NK)-1 receptor and brain-derived neurotrophic factor (BDNF) gene expression in the spinal cord to levels not statistically different from controls. However, indomethacin did not protect against significant pain-induced down-regulation of these genes in the hippocampus by approximately 50%, suggesting that although analgesic drug treatment reduces nociceptive sensory activation in the spinal cord, it is insufficient to prevent the impact of pain on the hippocampus. Conversely, although imipramine did not provide significant behavioral analgesia, it significantly blocked both pain- and stress-evoked alterations in hippocampal and spinal NK-1 and BDNF gene expression. Thus, these results show that application of either analgesic or antidepressant drugs alone does not fully protect against both the behavioral and molecular effects of persistent pain on both "sensory" and "affective" processing within the central nervous system.

Evidence-based data from animal and human experimental studies on pain relief with antidepressants: a structured review

OBJECTIVE

It has been hypothesized that serotonin reuptake inhibitor antidepressants (ADs) are only weakly antinociceptive but augment noradrenergic (NA) antinociception. Thus, ADs with combined serotonergic (SN) and NA activity, (i.e., the serotonergic/noradrenergic (SN/NA) ADs) should have greater antinociceptive activity versus the NA ADs, which in turn should have more antinociceptive activity than the SN ADs. The objective of this structured review was to test this hypothesis by reviewing relevant basic science literature on the treatment of experimental pain with the above different types of ADs. DESIGN, SETTING, PARTICIPANTS, OUTCOME, MEASURES: Animal or human experimental AD pain treatment studies were located by the usual search methods. For animal studies only placebo-controlled studies were included for review. For human studies only double blind placebo-controlled studies were selected for review. The animal and human studies were then sorted according to the pain model represented, e.g., neuropathic pain model. Studies were then characterized according to the type of AD utilized, and the antinociceptive outcome of the AD trial.

RESULTS

Twenty-two animal studies and 5 human studies fulfilled the inclusion criteria of this structured review. Within the animal nonspecific pain model there were 10 SN/NA AD trials, 9 NA AD trials and 7 SN AD trials. Of these trials 100%, 88.9%, and 14.3% respectfully demonstrated a positive AD antinociceptive effect. Overall, for all the animal models there were 25 SN/NA, 9 NA, and 8 SN trials. Of these trials 92%, 88.9%, and 25% respectfully demonstrated a positive AD antinociceptive effect. For the human pain models, only the SN/NA ADs had been utilized in 7 trials. Here in 42.8% of the trials there was a reported antinociceptive effect.

CONCLUSIONS

Overall, the results of this structured review support the above hypothesis.

Imipramine-induced antinociception in the formalin test. Receptor mechanisms involved and effect of swim stress

This study concerned the effect of swim stress on imipramine-induced antinociception in mice. The data showed that intraperitoneal (i.p.) administration of different doses of imipramine (10-40 mg/kg) and 0.5-3 min of swim stress (17 degrees C) induced antinociception in the first and second phases of the formalin test. Low period of swim stress (10 s) with low doses of imipramine (2.5, 5 and 10 mg/kg i.p.), which did not have any effect by themselves, in combination showed antinociception in the second phase of the test. Either yohimbine (0.5 mg/kg i.p.) or naloxone (1 mg/kg i.p.) reversed the response induced by the combination of low doses of imipramine plus swim stress. Yohimbine (1 mg/kg i.p.) decreased the response of imipramine (20 mg/kg i.p.) but not that of 30 s swim stress in the second phase. However, naloxone (1 mg/kg i.p.) reduced the antinociception induced by imipramine (20 mg/kg i.p.) or 30 s swim stress in the second phase of the test, the combination of imipramine with swim stress was not altered by yohimbine or naloxone. Prazosin induced antinociception by itself in the first phase of the test and increased swim-stress-induced antinociception with no interaction. It is concluded that antinociception induced by imipramine in the second phase of formalin test may be mediated through alpha(2)-adrenoceptor antagonists. The results indicate that the responses of swim stress and imipramine may be mediated by an opioid mechanism, but the combination of both drugs induced higher antinociceptive effects.

Inhibition of spinal nociceptive responses after intramuscular injection of capsaicin involves activation of noradrenergic and opioid systems

Extracellular recordings of wide dynamic range neurones in the dorsal horn driven by electrical stimulation of the sciatic nerve were performed in intact urethane-anaesthetized Sprague-Dawley rats. The electrically evoked neuronal responses were defined as A- and C-fibres responses according to latencies, and the effect of a deep nociceptive conditioning stimulus induced by 200 microg capsaicin (8-methyl-N-vanillyl-6-noneamide) injected into the contralateral gastrocnemius-soleus muscle was studied for at least 30 min. Independent of the size and location of the receptive field of the neurone under study, a clear inhibition of the neuronal responses was observed. The electrically evoked C-fibre responses were inhibited to 53% of baseline 15-30 min after injection of capsaicin. This inhibition was only slightly attenuated by 125 nmol of the alpha-adrenoceptor antagonist phentolamine or 250 nmol of the opioid receptor antagonist naloxone applied directly onto the spinal cord when the two compounds were administered separately 5 min before capsaicin. In contrast, when a mixture of the two compounds was given 5 min before capsaicin, the effect of capsaicin was completely abolished. These results indicate that activation of the capsaicin-sensitive afferents in the gastrocnemius-soleus muscle inhibits the electrically evoked C-fibre responses in the dorsal horn by activating noradrenergic and opioidergic inhibitory systems. Moreover, our data indicate that the activation of these two systems following injection of capsaicin has a sub-additive inhibitory effect on the wide dynamic range neurones in the spinal cord. We conclude that only one of these systems is sufficient for the inhibition to occur.

Pharmacological validation of an automated method of pain scoring in the formalin test in rats

In 1997, we described a new automated method of scoring the pain behaviors in the formalin test. The algic behavior was automatically measured with the help of a video-analysis system. The time during which the animal grooms, licks, or bites itself was used as the parameter of pain. In the present study, we tested various analgesics to realize a pharmacological validation of the system. The effect of opiate analgesic (morphine, i.v.), nonsteroidal anti-inflammatory drugs (paracetamol, i.v., piroxicam, i.v., indomethacin, i.v.), antidepressant drugs (clomipramine, desipramine, nortryptyline, and paroxetine, i.p.), and serotonin (i.t.) were analyzed. A dose of 1.25 mg/kg of morphine induced a decrease in the scores of phases 1 and 2. Naloxone (0.25 mg/kg) reversed the effect of morphine (2.5 mg/kg). A 20-mg/kg dose of indomethacin induced a decrease in the second phase, and paracetamol induced a decrease in both phases (analgesic doses were 400 mg/kg and 200 mg/kg for first and second phases, respectively). Piroxicam had no effect on the pain scores. Clomipramine, desipramine, and paroxetine at a dose of 5 mg/kg induced a significant decrease in the second phase. Nortriptyline had no effect on the pain scores. A dose of 75 microg of serotonin induced a decrease in both phases 1 and 2. This study demonstrated that this system shows a good pharmacological sensitivity, although it is lower than that of manual assessment.

Comparative effects of different uptake inhibitor antidepressants in two pain tests in mice

The purpose of this study was to compare the analgesic effect of acute injections (1.25 and 20 mg/kg, ip) of several antidepressants with different effects on monoamine reuptake, on two pain tests in mice (hot-plate and phenylbenzoquinone-induced abdominal writhes). Serotonergic inhibitors (citalopram, fluvoxamine and clomipramine) were more effective in the hot-plate test whereas noradrenaline reuptake inhibitors (desipramine and maprotilline) were more effective in the writhing test. The mixed antidepressants (amitriptyline and to a lesser degree trimipramine) were more effective in the two tests than the other antidepressant drugs. Changes in motor activity of clomipramine and amitriptyline could not account for the modifications of pain threshold. Amineptine (a dopamine reuptake inhibitor) failed to induce any antinociceptive effect in the hot-plate test and was hyperalgesic in the writhing test, which could be explained by an increased motor activity. These findings indicate that the antinociceptive potency of reuptake inhibitors varies according to their monoamine specificity and the nature of stimuli. They would suggest that the preferential choice of serotonergic antidepressants in the management of chronic pain is arguable.

Citalopram. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential in depressive illness

Citalopram is an antidepressant belonging to a new class of drugs which enhance serotoninergic neurotransmission through potent and selective inhibition of serotonin reuptake. Preliminary trials suggest that its short term therapeutic efficacy is significantly greater than that of placebo and mianserin, and comparable to that of amitriptyline, maprotiline and imipramine. It appears to be a weaker antidepressant agent than clomipramine, but better tolerated. Its elimination half-life of 33 hours permits once daily oral administration. Symptomatic improvement obtained with short term treatment has been maintained when therapy has been extended for up to 1 year; in the few patients studied for this extended period, the relapse rate was lower than with fluvoxamine, fluoxetine or imipramine. Compared to standard antidepressant agents, citalopram is well tolerated. It does not appear to be cardiotoxic, has not been associated with seizures in humans, and is relatively nonsedating. Unlike the tricyclic antidepressants, citalopram has minimal anticholinergic effects. Mild and transient nausea, with or without vomiting, is the most frequent adverse effect--occurring in 20% of patients--and increased perspiration, headache, dry mouth, tremor and insomnia are experienced by 15 to 18% of patients. Citalopram thus offers similar therapeutic efficacy and a more favourable tolerability profile than the tricyclic antidepressants. Preliminary data suggest that it may be particularly useful in patients who cannot tolerate the anticholinergic or cardiovascular side effects of tricyclic antidepressants and in those for whom sedation is not indicated.

Chronic administration of clomipramine prevents the increase in serotonin and noradrenaline induced by chronic stress

The effects of chronic clomipramine administration (15 mg/kg daily for 23 days) on changes in serotonin (5-hydroxytryptamine, 5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and noradrenaline (NA) induced by chronic stress have been studied in the rat brain. Chronic stress increased 5-HT in midbrain, pons and hippocampus, 5-HIAA in frontal cortex, midbrain, pons and hippocampus, and NA in midbrain and striatum. Chronic clomipramine significantly decreased the levels of 5-HT in most regions. In hypothalamus, hippocampus and perhaps in frontal cortex this effect possibly reflects decreased synthesis caused by an action on presynaptic 5-HT receptors. However, in midbrain, pons and striatum decreased 5-HT could not be attributed to a decrease in its synthesis since 5-HIAA also increased. This drug treatment also reduced NA in all regions except the striatum. Nevertheless, conclusions on NA synthesis or turnover cannot be drawn since only NA levels were measured. When administered concurrently, chronic clomipramine prevented the increases in 5-HT, 5-HIAA and NA produced by chronic stress. These results are in good accordance with previous findings showing that chronic antidepressant treatment also prevented behavioural disturbances induced by chronic stress.