The management of pain is probably one of the most common and yet most difficult aspects in medical practice. Many improved analgesics and anti-inflammatory agents have been developed, but there is considerable opportunity for conceptual innovation.
We used heat induced and formalin induced pain model for evaluating antinociceptive and formalin induced paw edema model for anti-inflammatory effect of Nepeta pogonosperma in experimental rats. Our data demonstrated that the essential oil of this plant elicited potent antinociceptive effects in rats subjected to both the acute thermal (tail-flick) and chronic or persistent formalin pain stimuli and strong anti-inflammatory effect to formalin induced paw edema model.
The Tail flick test is one of the most appropriate techniques to assess the acute somatosensory pain transmission by stimulating thermoreceptors in experimental animal model [25
]. King et al
(1997) showed that this test is sensitive to centrally acting analgesics and supraspinal systems facilitated this tail flicking response which was inhibited by a low dose of morphine [26
]. Since our results mentioned potent analgesia in higher two doses of NP in tail flick test, it may be commented that the effective component(s) of this essential oil exerts its antinociceptive effect by modulating the pain transmission in the central nervous system.
Formalin test is one of the appropriate methods for producing and quantifying the chemical pain in the rat model. Pain intensity in this test is dependent on some objective behavioral categories and the observations are converted to numerical values [22
]. Subcutaneous injection of formalin induces hindpaw inflammation, which leads to a response characterized by jerking, flexing followed by licking of the affected hindlimb. This characteristic response is considered to be a central nociceptive model, and has been associated with increased levels of chemical mediators in tissue fluids. The level of pain in this model is sensitive to both centrally and peripherally acting analgesics. In the present study, as the i.p. administration of different doses of the essential oil of NP
inhibited both phases of pain response relative to controls, it may be suggested that it has both the central and peripheral antinociceptive effects. We checked for anti-inflammatory effect of the essential oil and observed reduced formalin induced edema. This finding proved the peripheral antinociceptive effects of the oil bye ameliorating the formalin induced inflammation. Comparing the antinociceptive and anti-inflammatory effects of the lower dose (50
mg/kg), again makes the central analgesic effect plausible. In the other word, the dose which did not produce anti-inflammatory effect exerted significant analgesic effect.
Similar to the previous report [9
], NP essential oil contained 1, 8-cineole and 4aα, 7α, 7aβ-Nepetalactone as two major constituent.
It has been suggested by many investigators that 1, 8-cineole, the most important constituent of NP
essential oil has potent antinociceptive [14
] and anti-inflammatory activity [11
]. In addition, Aydin and colleagues (1998) suggested that 4aα, 7α, 7aβ-Nepetalactone, might be responsible for the significant analgesic activity and marked sedation in a rat model. They also recommended this nepetalactone might have specific opioid receptor subtype agonist activity [15
]. Again in a behavioral study on rats a significant decrease in performance was observed following i.p. administration of nepetalactone enriched fraction [16
]. Furthermore the nepetalactone isomers were suggested to be the responsible component of the anti-nociceptive and anti-inflammatory actions of Nepeta cataria
L. var. citriodora (Becker) Balb. [17
]. Moreover, the essential oil of Nepeta crispa
Willd. which contained 20.3% 4aα, 7α, 7aβ-Nepetalactone and 47.9% 1,8-cineole, showed strong antibacterial, antifungal, antinociceptive and antiinflammatory activity [19
]. This finding may support the potent anti-inflammatory activity observed in our present experiment. In a previous study, the analgesic activity of the essential oil of Nepeta italica
L. was showed to be correlated with the amount of 1, 8-cineole [27
]. In a recent animal study, cineole has been recommended to reveal an antinociceptive activity comparable to that of morphine in thermal analgesic stimuli [28
]. Hence, both of the components of NP
essential oil, the 4aα, 7α, 7aβ-Nepetalactone and 1, 8-cineole, may be responsible for our experimental findings.