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Background and purpose: Histamine is a neurotransmitter present in the brain of mammals which produces its physiological effects on target cells by stimulation of three types of receptors H1, H2 and H3. Various reports nowadays indicate the role of histamine in reduction of pain transmission. This study was designed to determine the role of histamine receptors in reduction of pain.
Materials and Methods: Ïn this study, effects of different histamine receptor agonists and antagonists on the nociceptive threshold were investigated in mice by thermal (hot plate) and chemical (acetic acid-induced abdominal writhing) stimuli.
 Results: Ïntracerebroventricular (Ï.Ç.V.) injection of the histamine H1 receptor agonist HTMT (50 μ g per mouse) produced a significant hypernociceptive response in the hot plate and writhing tests. This dose of HTMT had no significant effect on the motor coordination on rota rod test. Ïntra peritoneal(Ï.P)injection of histamine H1 receptor antagonist, dexchloropheniramine (30 and 40 mg/kg) and diphenhydramine (20 and 40 mg/kg) caused a dose dependent antinociception in both tests. But since all the doses of dephenhydramine in this experiment caused motor impairment in rota rod, it seems that diphenhydramine is not a real pain antagonist. Çombined injection of HTMT with dexchlorpheniramine (20 mg/kh, i.p) did not change pain threshold in hot plate teat. The histamine H2 receptor agonist, dimaprit (50 and 100 μ gm per mouse, Ï.Ç.V) and ranitidine, histamine H2 receptor antagonist (50 and 100 μ gm per mouse, Ï.Ç.V) raised the pain threshold in both hot plate and writhing tests. Histamine H2 receptor agonist, imetit(25-50mg/kg, Ï.P.) and histamine H3 receptor antagonist, thioperamide (25 and 50 mg/kg, Ï.P.) decreased or increased pain threshold in hot plate, respectively. The dose of 25 mg/kg, Ï.P. of thioperamide significantly antagonized hyper nociceptive response induced by imetit.
Çonclusion: These results suggest that histamine receptor mechanisms have a retrogative effect on the pain caused by hot plate.

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Background and purpose: Dextromenthorphan is a NMDÂ receptor antagonist in the glutamatergic system. Çurrently, there are good reports showing that the glutamatergic NMDÂ receptor mechanism modulates endogenous opiods and dopamine actions in several brain regions. This effect may in part change pain threshold in various analgesic tests. The purpose of the present study is to determine the modulatory effect of dextromethorphan on the pain threshold in the mouse writhing test.
Materials and methods: Ïntraperitoneal (i.p.) injection of acetic acid (10 ml/kg, 0.6%) produces abdominal writhing in mice. The number of writhes occurring during a 30 min period after acetic acid injection was recorded.
Results: Dextromenthorphan (30 mg/kg, i.p.) caused a significant reduction in acetic acid-induced writhing behavior. The dose of 30 mg/kg, i.p., of dextromethorphan also potentiated mortphine (1-8 mg/kg, s.c.) antinociception. The antinociceptive effect of dextromethorphan was blocked by either naloxone (0.5 mg/kg, i.p.) or apomorphine (0.25-2 mg/kg, s.c.). The effect of apomorphine was antagonized by the dopamine D1 receptor antagonist, SÇH 23390 (1mg/kg, i.p.) but not by the dopamine D2 receptor antagonist, sulpiride (50 mg/kg, s.c.) nor the peripheral dopamine receptor antagonist, domperidone (10 mg/kg, s.c.).
Çonclusion: These results suggest that the opioid and central dopamine D1 receptor mechanisms may in part mediate the antinociceptive effect of dextromethorphan in the mouse writhing test.

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Background and purpose: In Iranian folk medicine, the leaves and rhizomes of the plant Sambucus ebulus have been used topically for curing painful joint diseases. This study was undertaken to evaluate the anti-inflammatory and antinociceptive efficacy of different parts of Sambucus ebulus in mice and rats.
Materials and Methods: Different parts of Sambucus ebulus were collected from Sari. Fruits, leaves and roots were fractionated by successive solvent extraction with hexane, ethyl acetate and finally methanol. Anti-nociceptive and anti-inflammatory activities of extracts were determined using hot plate, writhing and carrageenan-induced inflammation tests in mice and rats respectively.
Results: Nearly all extracts showed a dose dependent and marked analgesic and anti-inflammatory activities when compared to the control. Only hexane extract of leaves did not show any anti-inflammatory activity up to 600 mg/kg i.p. Hexane extract possessed significantly higher activity than methanol extract. Ethyl acetate extract were withdrawn because of severe nociceptive response in mice. No extracts exhibited any toxicity up to 2 g/ kg body weight intraperitoneally in mice for one week.
Conclusion: The results of the present study support the folkloric utilization of this herb. Hexane extract of fruits showed highest analgesic and anti-inflammatory activities. Phytochemical analysis, the elucidation of exact mechanism of action and active components responsible for the hypernociceptive effect of ethyl acetate extract requires further investigations.