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Year : 2012  |  Volume : 44  |  Issue : 2  |  Page : 215--218

Zimelidine attenuates the development of tolerance to morphine-induced antinociception

Ercan Ozdemir1, Sinan Gursoy2, Ihsan Bagcivan3, Nedim Durmus3, Ahmet Altun3,  
1 Department of Physiology, Cumhuriyet University School of Medicine, 58140 Sivas, Turkey
2 Department of Anesthesiology and Reanimation, Cumhuriyet University School of Medicine, 58140 Sivas, Turkey
3 Department of Pharmacology, Cumhuriyet University School of Medicine, 58140 Sivas, Turkey

Correspondence Address:
Ercan Ozdemir
Department of Physiology, Cumhuriyet University School of Medicine, 58140 Sivas


Objectives: The aim of this study was to investigate effect of zimelidine (a serotonin reuptake inhibitor) on morphine-induced tolerance in rats. Materials and Methods: Male Wistar albino rats weighing 160-180 g were used in these experiments (n=72). A 3-day cumulative dosing regimen was used for the induction of morphine tolerance. To constitute of morphine tolerance, animals received morphine twice daily for 3 days. After the last dose morphine was injected on the fourth day, morphine tolerance was evaluated. The analgesic effects of zimelidine (15 mg/kg; i.p.) and morphine (5 mg/kg) were considered at 30-min time intervals (0, 30, 60, 90 and 120 min) by tail-flick and hot-plate analgesiometer (n=6 in each experimental group). Results: The results showed that zimelidine significantly attenuated the development and expression of morphine tolerance. The maximal antinociceptive effect of zimelidine was obtained at the 60 minutes measurements in the zimelidine group and at the 30 minutes measurements in the morphine tolerant group by the tail-flick and hot-plate tests. Administration of zimelidine with morphine showed additive analgesic effect. Conclusion: In conclusion, our results show that zimelidine reduces the development of tolerance to morphine-induced antinociception in rats.

How to cite this article:
Ozdemir E, Gursoy S, Bagcivan I, Durmus N, Altun A. Zimelidine attenuates the development of tolerance to morphine-induced antinociception.Indian J Pharmacol 2012;44:215-218

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Ozdemir E, Gursoy S, Bagcivan I, Durmus N, Altun A. Zimelidine attenuates the development of tolerance to morphine-induced antinociception. Indian J Pharmacol [serial online] 2012 [cited 2022 Jul 3 ];44:215-218
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Opioids such as morphine remain the most efficacious and widely used analgesics for moderate to severe pains. However, long-term administration of opioids can alter the central pain-related systems and lead to the development of tolerance. [1],[2] Tolerance is defined as the phenomenon whereby exposure to opioids results in attenuation of the effect or requirement of a larger dose to produce the same effect. [3] Whereas the conditions required for the development of human opioid tolerance are unclear, this phenomenon is particularly robust in experimental models of acute nociception. [4] The identification of adjuvant drugs that can inhibit the development of tolerance to opioids may lead to the improved management of pain. Neurotransmitter systems that interact with the opioidergic system offer a target for clinically useful strategies to block or delay opioid tolerance. According to recent reports, N-methyl-D-aspartic acid (NMDA)-antagonists [5],[6] and nitric oxide synthase inhibitors [7],[8] attenuate the development of tolerance to morphine in rodents. There is concern over the potential adverse effects of these new pharmacological agents that may limit their clinical applicability as adjuvants in pain management. [9] Also, NMDA-antagonists and nitric oxide synthase inhibitors attenuate rather than completely block the development of morphine tolerance, which suggest that other systems also play essential roles in the tolerance process.

5-Hydroxytryptamine (5-HT) is widely accepted as an important neurotransmitter participating in the central and spinal inhibition of nociceptive transmission. [10],[11] Behavioural studies have demonstrated that 5-HT is implicated in the control exerted by the brain on nociception either by afferent fiber hyperpolarization or through a presynaptic action. Serotonergic deficiency is a common factor in both mental depression and chronic pain. [12],[13] It has been reported that destruction of serotonergic projections greatly affects nociception. In contrast, increasing the availability of 5-HT at the synapse is reported to inhibit nociception by acting at spinal cord, brainstem or thalamic levels. [13]

Several recent lines of physiological, pharmacological and behavioral evidence suggest that a change in serotonergic neurotransmission is involved in mediating the analgesic action of morphine. [14],[15] However, the exact biochemical and physiological mechanisms underlying this effect is not fully understood. It is accepted that opioids establish part of their analgesic effect through stimulation of the serotonergic system. [16] Acute morphine administration enhances 5-HT turnover as evidenced by an increase in its synthesis, release and metabolism. [16] After chronic morphine administration, a decrease in the release of 5-HT from the nerve terminals is observed. [17] Fenfluramine attenuates the development of tolerance to morphine by modulating the process of pain transmission. [16] According to a recent study, 5-HT 1A receptors of the dorsal raphe nucleus are involved in tolerance to the antinociceptive effect of morphine. [18] Zimelidine (ZIM), as a selective serotonin re-uptake inhibitor (SSRIs), changes the neurotransmission in serotonergic system. The mechanism of this antidepressant drug is a strong reuptake inhibition of 5-HT the synaptic cleft and a much less inhibiton of noradrenaline uptake. [19]

Based on these findings, the objective of this study was to investigate the effect of ZIM on the development of tolerance to the analgesic effect of morphine in rats.

 Materials and Methods


The experiments were performed on adult male Wistar albino rats weighing 160-180 g (n=72). The animals were fed a standard laboratory diet and water ad libitum, kept at 22 ± 2°C with a 12-h light/dark cycle. Animals were acclimatized to laboratory conditions before the test. All experiments were carried out blindly between 09:00 and 17:00 h (n=6 in each experimental group in the study). The experimental protocols were approved by the Cumhuriyet University Animal Ethics Committee.

Drug Administration

Morphine sulphate (Cumhuriyet University Hospital, Turkey) and zimelidine (Sigma-Aldrich, USA) were dissolved in saline. The ZIM and morphine were prepared immediately just before use and injected intraperitoneally (i.p.) and subcutaneously (s.c.) in a volume of 10 ml/kg, respectively.

Induction of Morphine Tolerance

A 3-day cumulative dosing regimen was used for the induction of morphine tolerance. The treatment schedule consisted of twice daily s.c. doses of morphine given at 30 mg/kg (a.m.) and 45 mg/kg (p.m.) on day 1; 60 and 90 mg/kg on day 2; and 120 mg/kg twice on day 3. Animals were assessed for tolerance on the fourth day, as described by Way et al. [20]

Assessment of Antinociception

Tail-flick test

We used a standardised tail flick apparatus (May TF 0703 Tail-flick Unit, Commat, Turkey) to evaluate thermal nociception. The radiant heat source was focused on the distal portion of the tail at 3 cm after administration of the vehicle or study drugs. Following vehicle or compound administration, tail-flick latencies (TFL) were obtained. The infrared intensity was adjusted so that basal TFL occurred at 2.8 ± 0.4. Animals with a baseline TFL below 2.4 or above 3.2 s were excluded from further testing. The cutoff latency was set at 15 s to avoid tissue damage. Any animal not responding after 15 s was excluded from the study. The algesic response in the tail-flick test is generally attributed to central mechanisms. [21],[22]

Hot-plate test

The antinociceptive response on the hot-plate is considered to result from a combination of central and peripheral mechanisms. [22] In this test, animals were individually placed on a hot-plate (Eddy's Hot-Plate) with the temperature adjusted to 55 ± 1°C. The latency to the first sign of paw licking or jump response to avoid the heat was taken as an index of the pain threshold; the cut-off time was 30 s in order to avoid damage to the paw.

Experimental Protocols

The analgesic effects of ZIM (15 mg/kg; i.p.) and morphine (5 mg/kg challenge dose; s.c.) were observed at 30-min intervals (0, 30, 60, 90 and 120 min) by tail-flick and hot-plate test as a model of acute pain in rats (n=6 in each group). In the morphine-treated rats after induction of morphine tolerance, analgesic response to the challenge dose was determined again on day 4 at 30-min intervals after the same morphine (5 mg/kg) injection on the first day. To evaluate the ZIM effects on development or expression of morphine tolerance, morphine tolerant animals received zimelidine (15 mg/kg; i.p.). In the saline-treated group, animals received saline (10 ml/kg) instead of morphine during the induction session.

Data Analysis

The percentage of maximum possible effect (% MPE) was calculated for each rat at each dose and time point according to the following formula:


Statistical Analysis

The results obtained are expressed as mean±SEM (standard error of mean). The effect of antinociception was measured and the mean of % MPEs in all groups was calculated. The data were analysed by analysis of variance followed by Tukey test. P<0.05 was considered as significant.


The Antinociceptive Effects of Different Doses of Morphine

The antinociceptive response were measured for the three different doses of morphine (2.5, 5 and 7.5 mg/kg; i.p.) at 30-min intervals by tail-flick and hot-plate test. The % MPE produced by morphine (5 mg/kg) was significantly higher than in the other groups (2.5 mg/kg morphine and saline group) in rats in both tail-flick (F 2,15 =25.51, P<001; [Figure 1]a) and hot-plate (F 2,15 =36.62, P<0001; [Figure 1]b) test. The maximum % MPE was observed at 30 min after administration 5 mg/kg dose of morphine (20.86 ± 1.41 for tail-flick test and 51.33 ± 2.13 for hot-plate test).{Figure 1}

The Expression of Morphine Tolerance

After a 3-day cumulative dosing regimen was used for the induction of morphine tolerance, animals were assessed for tolerance on the 4 th day. The antinociceptive effect of morphine tolerant group was significantly lesser than morphine (5 mg/kg challenge dose) group in both tail-flick (F 2,15 =31.66, P<0.001; [Figure 2]a) and hot-plate test (F 2,15 =380.59, P=0.000; [Figure 2]b). But, there was no significant change when compared to normal saline group. The maximum effect of challenge dose of morphine ocurred at 30 min by tail-flick test (20.86 ± 1.41) and at 60 min by hot-plate test (58.33 ± 2.37) after administration of morphine.{Figure 2}

Effects of ZIM on the Development of Morphine Tolerance

Pretreatment of animals with ZIM (15 mg/kg; i.p.) significantly reduced the development of tolerance to morphine antinociceptive effect, as indicated by increment of %MPE in the ZIM treatment groups in both tail-flick (F 4,25 =64.17, P<0.001; [Figure 3]a) and hot-plate test (F 4,25 =522.31, P<0.001; [Figure 3]b). The peak value of this group was observed at 30 min after administration of morphine and ZIM in both tail-flick and hot-plate test (51.66 ± 4.20 and 54.00 ± 3.89, respectively). The maximum antinociceptive effect was determined in ZIM-morphine group by the tail-flick and hot-plate test. These findings demonstrated that the combinations of ZIM-morphine resulted in an additive interraction. The antinociceptive effect of ZIM group (43.50 ± 6.25) was significantly higher than saline group (0.53 ± 0.11) at 60 min measurements in the hot-plate test (P<0.01).{Figure 3}


One of the major problems associated with the chronic use of morphine is tolerance. Repeated uses of morphine to relieve pain often cause patients to develop increasing resistance to the effects of the drugs, so that progressively higher doses are require to achieve the same analgesic effects. [23],[24] In the present study, we observed that (a) ZIM decreased the development of morphine tolerance, (b) ZIM exhibited additive antinociceptive effect with morphine and (c) ZIM alone has antinociceptive effect in the tail-flick and hot-plate test.

We have found that the % MPE produced by morphine (5 mg/kg) was significantly higher than in the other groups (2.5 mg/kg morphine and saline group) in the analgesia tests to determine the dose of morphine. Interestingly, Joharchi and Jorjani [25] reported that the maximum analgesic effect was observed after administration 7 mg/kg dose of morphine in rats. On the other hand, another study suggested that the maximum analgesic effect was observed 5 mg/kg dose of morphine. [26] These data are consistent with our results.

The link between opioid analgesia and 5-HT has been suggested for many years and it has been reported that serotonergic pathways play an important role in the opioid analgesia. In a number of studies, a direct role of the opioidergic system in antidepressants-induced antinociception is reported. [13],[27] Similarly, in the present study ZIM-induced antinociception may also involve an interaction with opioid receptors to reduce tolerance of morphine in rats. Previous studies have suggested that serotonin plays an important role in opioid-mediated analgesia. [28],[29] One of the studies reported that fluoxetine (a selective serotonin reuptake inhibitor) suppressed the dependence and development of tolerance to the antinociceptive effect of morphine. [30] In another study, it has been stated that systemically administered morphine activates the serotonergic pathways and 5-HT 7 receptors in the spinal cord play an important role in the systemic morphine antinociception. [30] In accordance with our study, Gebhart and Lorens reported that zimelidine and fluoxetine enhanced morphine-induced antinociception on the hot plate. However, the effect of zimelidine pretreatment on pethidine-induced antinociception revealed a significant attenuation of antinociception by pethidine in the hot plate test. [31] These data support a role for 5-HT in the expression of morphine-induced antinociception, and a different mode of antinociceptive action of morphine and pethidine.

It has been shown that chronic morphine administration leads to an increase in GABA tone and subsequently to a decrease in serotonergic activity in the dorsal raphe nucleus. [17] It can be hypothesized that the dorsal raphe serotonergic system has an important role in the manifestation of morphine tolerance. Nayebi et al., [18] reported that direct stimulation of 5-HT 1A receptors in the dorsal raphe nucleus of the rat prolongs the development of tolerance to the analgesic effect of morphine and fluoxetine delays the development of tolerance to morphine analgesia by preventing the decrease of 5-HT in raphe nucleus, which occurs during chronic morphine administration. We observed that coinjection of morphine with ZIM increased the analgesic effects of morphine and reduced development of tolerance to morphine analgesia. These results showed that ZIM enhances the analgesic effect of morphine in rats.


In conclusion, our data suggest that ZIM, as a selective serotonin reuptake inhibitor, attenuates the development of tolerance to morphine in rats. In addition, we suggest that investigation of a possible clinical application for ZIM should be carried out to test its usefulness in diminishing tolerance to morphine. Further studies are needed to elucidate the exact mechanism of ZIM on the neuronal systems, which are responsible for the development of tolerance to morphine.


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