|Year : 2012 | Volume
| Issue : 3 | Page : 223-228
Evaluation of antinociceptive/analgesic activity of SSRIs (fluoxetine and escitalopram) and atypical antidepressants (venlafaxine and mirtazapine): An experimental study
Pranav Sikka1, Sadhna Kaushik2, Seema Kapoor3, Manish Saini1, KK Saxena1
1 Department of Pharmacology, LLRM Medical College, Meerut, India
2 Department of Pharmacology, MLB Medical College, Jhansi, India
3 Department of Oral Pathology and Microbiology, ITS-CDSR, Muradnagar, Ghaziabad, Uttar Pradesh, India
|Date of Web Publication||8-Aug-2012|
Department of Pharmacology, LLRM Medical College, Meerut, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Objective: To evaluate the antinociceptive/analgesic action of selective serotonin reuptake inhibitors (SSRIs; fluoxetine, escitalopram) and atypical antidepressants (mirtazapine, venlafaxine) and to delineate their probable mechanism. Materials and Methods: The study was conducted on albino mice (25-35 g) and Wistar rats (80-100 g) of either sex. Different doses of morphine (0.5 and 1 mg/kg), fluoxetine (2, 5, and 10 mg/kg), venlafaxine (30, 40, and 50 mg/kg), mirtazapine (3, 5, and 7 mg/kg), and escitalopram (2.5, 5, and 10 mg/kg) were administered subcutaneously in order to ascertain their subanalgesic doses using tail flick analgesiometer and writhing method. Tail flick latencies were obtained at 15, 30, 60, and 120 min after drug administration. In another set of experiments, abdominal writhing was produced by employing 4% saline (1 ml/kg; i.p.) as irritant. Pretested sensitive rats were given test drugs 10 min before saline injection and abdominal writhing was noted between 30 seconds and 3 min of administering saline. Naloxone (1 mg/kg) was administered 10 min prior to test drug for testing antagonism in both sets of experiments. Results: In both sets of experiments, fluoxetine (5 and 10 mg/kg), mirtazapine (5 and 7 mg/kg), and venlafaxine (40 and 50 mg/kg) were found to have antinociceptive activity, but not at lower doses. Escitalopram failed to show antinociceptive activity at any of the doses used. The antinociceptive effect of all the drugs was antagonized by naloxone. Further, subanalgesic doses of fluoxetine, mirtazapine, and venlafaxine showed analgesic activity with suboptimal dose of morphine (0.5 mg/kg). Conclusion: Fluoxetine, mirtazapine, and venlafaxine have antinociceptive activity, whereas escitalopram does not have; their site of action seems to be the same as that of opioid analgesics ("mu" receptors). However, involvement of other pathways (cholinergic, histaminic, noradrenergic, GABAergic) cannot be excluded in mediation of their analgesic activity, which requires further elucidation.
Keywords: Analgesia, antidepressants, selective serotonin reuptake inhibitors
|How to cite this article:|
Sikka P, Kaushik S, Kapoor S, Saini M, Saxena K K. Evaluation of antinociceptive/analgesic activity of SSRIs (fluoxetine and escitalopram) and atypical antidepressants (venlafaxine and mirtazapine): An experimental study. Int J Nutr Pharmacol Neurol Dis 2012;2:223-8
|How to cite this URL:|
Sikka P, Kaushik S, Kapoor S, Saini M, Saxena K K. Evaluation of antinociceptive/analgesic activity of SSRIs (fluoxetine and escitalopram) and atypical antidepressants (venlafaxine and mirtazapine): An experimental study. Int J Nutr Pharmacol Neurol Dis [serial online] 2012 [cited 2020 Feb 22];2:223-8. Available from: http://www.ijnpnd.com/text.asp?2012/2/3/223/99474
| Introduction|| |
Pain is one of the most common problems attended by the physicians. Acute pain usually results from an obvious injury (e.g. a fracture or laceration) or infection. Chronic pain is often characterized by functional loss and precipitated by psychosocial problems and economic stressors and represents a more complex picture in terms of pathology and treatment.  The clinical management of chronic pain remains a challenge. Despite advances in pain research and clinical treatment, magnitude of disability due to chronic pain continues to assume alarming proportions worldwide. Although chronic pain is treated with many medications, such as tricyclic antidepressants, nonsteroidal anti-inflammatory drugs, anticonvulsants, and opioids, none has shown outstanding efficacy. Narcotics are usually avoided because of the risk of developing tolerance, dependence, and functional deterioration.  The association between chronic pain and depression is complex as each can occur as separate primary entity or may jointly coexist. Also, patients with chronic pain are at risk of developing depression. 
It has also been proved in about 75% of studies that some antidepressants are superior to placebo in alleviating pain as they have some intrinsic analgesic activity.  Tricyclic antidepressants have proven efficacy in the treatment of chronic pain conditions such as diabetic neuropathy, fibromyalgia, chronic headaches, and post-herpetic neuralgia. Unfortunately, side effects including dry mouth, constipation, orthostatic hypotension, and urinary retention often limit their use.  Despite the growing popularity of selective serotonin reuptake inhibitors (SSRIs) since 1980s, there are only a few controlled studies of their efficacy in managing chronic pain syndromes. Among the studies available, the data are conflicting. The incidence of treatment-limiting adverse effects with SSRIs is low. Whether they provide analgesia independent of their effect on mood is unclear. Hence, further studies are needed to better elucidate the efficacy of SSRIs for chronic pain.
Another intriguing aspect is meager understanding regarding their mechanism of action. The underlying mechanisms for antinociceptive activity of these agents probably involve a complex interaction between several neurotransmitter systems and neuroreceptors.  There is ample evidence to suggest that pain inhibitory pathway involves monoamines such as noradrenalin (NA) and 5-hydroxy tryptamine (5-HT),  and these monoamines have also been claimed to influence depressive mood.  Antidepressants, especially SSRI, by increasing serotonin level, may inhibit the release of neurotransmitter carrying the pain sensation from nerve endings. 
Despite so much enormity of literature, still it is not clear whether these substances can be used as analgesics and if so, what could be the underlying mechanism. Therefore, the present study was planned with the aim 1) to confirm the analgesic/antinociceptive activity of SSRIs (fluoxetine and escitalopram) and atypical antidepressants (venlafaxine and mirtazapine), 2) to study the interaction of these drugs with morphine, and 3) to delineate their probable site of action.
| Materials and Methods|| |
The study was conducted on healthy albino mice (25-35 g) and Wistar rats (80-100 g) of either sex, maintained at an ambient temperature of 25-35°C with food and water ad libitum. The experimental protocol was approved by the institutional animal ethics committee and was executed according to the guidelines of Committee for the Purpose of Control and Supervision on Experiments on Animals (CPCSEA), India.
Tail flick method
Mice were divided into 20 groups of six each. Individual test drug was given to find its subanalgesic dose. All the drugs were given subcutaneously. The animals in groups 1, 2, and 3 received fluoxetine (2, 5, and 10 mg/kg, respectively), groups 4, 5, and 6 received venlafaxine (30, 40, and 50 mg/kg, respectively), groups 7, 8, and 9 received mirtazapine (3, 5, and 7 mg/kg, respectively), groups 10, 11, and 12 received escitalopram (2.5, 5, and 10 mg/kg, respectively), and groups 13 and 14 received morphine (0.5 and 1 mg/kg, respectively). Groups 15, 16, and 17 received combined treatment consisting of subanalgesic doses of test drugs with subanalgesic dose of morphine, i.e. fluoxetine (2 mg/kg), venlafaxine (30 mg/kg) and mirtazapine (3 mg/kg) with morphine (0.5 mg/kg), respectively. Groups 18, 19, and 20 received fluoxetine (5 mg/kg), venlafaxine (40 mg/kg), and mirtazapine (5 mg/kg) 10 min before naloxone (1 mg/kg). Naloxone was given 10 min after the test drug. In the groups which received combined treatment, both the drugs were administered simultaneously at different sites.
Abdominal writhing method
The rats were divided into 20 groups of six each. Subanalgesic dose of all the test drugs was obtained using 4% saline (1 ml/kg) given intraperitoneally as irritant, 10 min after test drug administration. Only pretested sensitive rats were further taken for drugs studies. All the groups received test drugs in the same manner as in tail flick method, i.e. groups 1, 2, and 3 received fluoxetine (2, 5, and 10 mg/kg, respectively), groups 4, 5, and 6 received venlafaxine (30, 40, and 50 mg/kg, respectively), groups 7, 8, and 9 received mirtazapine (3, 5, and 7 mg/kg, respectively), groups 10, 11, and 12 received escitalopram (2.5, 5, and 10 mg/kg, respectively), and groups 13 and 14 received morphine (0.5 and 1 mg/kg, respectively). Groups 15, 16, and 17 received combined treatment consisting of subanalgesic doses of test drugs with subanalgesic dose of morphine, i.e. fluoxetine (2 mg/kg), venlafaxine (30 mg/kg), and mirtazapine (3 mg/kg) with morphine (0.5 mg/kg), respectively. Groups 18, 19, and 20 received fluoxetine (5 mg/kg), venlafaxine (40 mg/kg), and mirtazapine (5 mg/kg) 10 min before naloxone (1 mg/kg). Naloxone was given 10 min after the test drug. All the drugs were given subcutaneously. In the groups which received combined treatment, both the drugs were administered simultaneously at different sites.
Determination of antinociceptive activity
- Effect of test drugs was obtained in terms of tail flick latency period (the time required for flicking of tail, i.e. reaction time) using analgesiometer at 0, 15, 30, 60, and 120 min. Radiant heat was directed to the proximal third of the tail through hot wire of analgesiometer and reaction time was noted when the mouse tried to pull the tail away.  A mean of two pre-drug readings was taken as basal value (0 min). Mice with a reaction time of more than 6 seconds were not used in the test. In order to prevent tissue injury, a cut-off time of 10 seconds was maintained. The cut-off time was considered as the latency period for the animals not responding up to 10 seconds.
- The abdominal writhing phenomenon (characterized by a wave of contraction of the abdominal musculature followed by extension of hind limbs) was observed in rats using intraperitoneal injection of 4% sodium chloride solution (saline 1 ml/kg; i.p.) as irritant.  Saline was injected 10 min after drug administration and abdominal writhing response was noted 30 seconds to 3 min after saline injection. A reduction in the number of writhes as compared to the control group was considered as evidence for the presence of analgesia, expressed as percent inhibition of writhing,  which is calculated according to the following formula:
The results of tail flick method were expressed as mean ± SD; paired Student's "t" test was employed for comparison between the two means as a measure of significance. P value of <0.05 was regarded as a statistically significant value.
The results of abdominal writhing method were expressed as percent protection of animals showing inhibition of abdominal writhes.
| Results|| |
Effect of test drugs on tail flick latency
Fluoxetine (2 mg/kg), venlafaxine (30 mg/kg), and mirtazapine (3 mg/kg) showed no significant increase in tail flick latency period as compared to the corresponding 0 min values in mice on tail flick analgesiometer. Fluoxetine (5 and 10 mg/kg), venlafaxine (40 and 50 mg/kg), and mirtazapine (5 and 7 mg/kg) produced significant increase in tail flick latency at all time intervals (P < 0.05). These drugs produced a dose-dependent antinociception, but escitalopram did not show any antinociceptive effect at 2.5, 5, and 10 mg/kg doses [Table 1].
|Table 1: Effects of different test drugs on the tail flick latency period (seconds; mean ± SD) at different time intervals|
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Effect of test drugs on abdominal writhing
Pretested sensitive rats that were given fluoxetine (2 mg/kg), venlafaxine (30 mg/kg), and mirtazapine (3 mg/kg) showed no significant inhibition of abdominal writhing. Observations show that fluoxetine (5 and 10 mg/kg), venlafaxine (40 and 50 mg/kg), and mirtazapine (5 and 7 mg/kg) produced significant inhibition of writhes in a dose-dependent manner, but escitalopram did not show any inhibition at all the tested doses, i.e. 2.5, 5, and 10 mg/kg.
Effect of morphine treatment
Morphine (0.5 mg/kg) showed no significant effect in tail flick latency period and abdominal writhing at any time interval (P > 0.1). However, at a dose of 1 mg/kg, morphine produced a significant increase in tail flick latency at every time interval when tested from 0 to 120 min (P < 0.001) and significantly inhibited abdominal writhes (percent protection = 100%) [Table 1].
Effects of combined treatment
The combination of subanalgesic doses of fluoxetine (2 mg/kg), venlafaxine (30 mg/kg), and mirtazapine (3 mg/kg) with morphine (0.5 mg/kg), respectively, produced a significant (P < 0.01 to P < 0.001) increase in tail flick latency period throughout the study [Table 2] and also inhibited abdominal writhing significantly (percent protection from 83.33 to 100%).
|Table 2: Effects on the tail flick latency period (seconds; mean ± SD) after combined treatment of test drugs with morphine at different time intervals|
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The combination of fluoxetine (5 and 10 mg/kg), venlafaxine (40 and 50 mg/kg), and mirtazapine (5 and 7 mg/kg) with naloxone (1 mg/kg) did not produce significant (P > 0.1) increase in tail flick latency during the entire test period [Table 3]. Also, percent protection of abdominal writhing by the above combinations was insignificant. This shows that pretreatment with naloxone (1 mg/kg) completely antagonized the antinociceptive effect of fluoxetine, venlafaxine, and mirtazapine at their analgesic doses.
|Table 3: Effects on the tail flick latency period (seconds; mean ± SD) after combined treatment of test drugs with naloxone|
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| Discussion|| |
Pain is a symptom of many diseases requiring treatment with analgesics. Severe pain due to chronic conditions needs the use of strong analgesics such as opioid drugs. The addiction liability of opioids led to intensive search for compounds without this hazard. Many approaches have been used to differentiate the various actions of analgesics. So, this study has been designed to evaluate not only the action but also the site of action of antidepressants as analgesics. For central activity, the test drugs have been tested through tail flick analgesiometer, and for peripheral analgesic activity, writhing method has been employed. 
Out of the four above-mentioned drugs, escitalopram did not show any analgesic/antinociceptive effect. Our observations are in accordance with the results of various clinical and animal studies. ,
Fluoxetine showed antinociceptive activity at 5 and 10 mg/kg [Table 1] and [Table 4]. These findings are in conjunction with the finding of Max et al. who, in a double-blind cross-over study, observed similar results. Goldenberg et al. compared placebo with fluoxetine in fibromyalgia patients and found significant pain-relieving activity in fluoxetine. Similarly, Rani et al.  compared fluoxetine with amitriptyline and placebo in patients with chronic rheumatic pain and found significant reduction in pain intensity scores and pain relief scores. They suggested fluoxetine to be an effective analgesic with fewer side effects.
|Table 4: Effects of different test drugs on the abdominal writhing (% protection)|
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The antinociceptive effect shown by mirtazapine (5 and 7 mg/kg) [Table 1] and [Table 4] is supported by the findings of Bomholt et al. and Muth-Selbach et al. Brannon and Stone  also reported analgesic effect of mirtazapine for chronic back pain with major depression where amitryptaline, fluoxetine, yohimbine, and bupropione alone and in combinations failed.
In our study, venlafaxine showed antinociceptive effect at 40 and 50 mg/kg [Table 1] and [Table 4]. Various other studies, viz. Lang et al.,  also suggested the antinociceptive effect of venlafaxine in mitigating thermal hyperalgesia in animals. Songer and Schulte  showed the analgesic effect of venlafaxine in radicular back pain associated with depression; also, Bradley et al. showed its effect in migraine, chronic back pain, and chronic regional pain syndrome (CRPS), and Dwight et al. in fibromyalgia with axis I psychiatric disorders.
Further, morphine at a subanalgesic dose (0.5 mg/kg) potentiated all the three above-mentioned antidepressants [Table 2] (supported by the work of Suh and Tseng and Erjavec et al.) ,
Naloxone (1 mg/kg) abolished the analgesic effect of fluoxetine, mirtazapine, and venlafaxine [Table 3]. These results suggest that the antinociceptive activity of these three antidepressant drugs could involve opioid mechanisms. These observations are in agreement with the findings of Singh et al. and Anjaneyulu and Chopra. ,
As the analgesic activity of morphine is mediated through mu (μ) receptors, it is likely that fluoxetine, mirtazapine, and venlafaxine act through opioid pathways involving the μ opioid receptors. However, these drugs interact with other receptor systems also such as cholinergic, muscarinic, histaminergic, noradrenergic, and even the GABAergic system. ,, Hence, it would not be unreasonable to suggest that antidepressant drugs would involve at least some of these systems for their analgesic effect.
In the present study, a combination of subanalgesic doses of morphine and fluoxetine, mirtazapine, and venlafaxine produced a significant additive antinociceptive/analgesic effect. Thus, these combinations of antidepressants with morphine would appear theoretically to minimize the dose requirements, and thus the potential adverse effects of morphine. Above all, further clinical studies are needed to prove the same.
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[Table 1], [Table 2], [Table 3], [Table 4]