Further study on the effects of histamine H2 receptor agonist and antagonists on restraint-induced antinociception in mice

Role of the thalamic submedius nucleus histamine H1 and H 2 and opioid receptors in modulation of formalin-induced orofacial pain in rats

Histamine and opioid systems are involved in supraspinal modulation of pain. In this study, we investigated the effects of separate and combined microinjections of agonists and antagonists of histamine H1 and H2 and opioid receptors into the thalamic submedius (Sm) nucleus on the formalin-induced orofacial pain. Two guide cannulas were implanted into the right and left sides of the Sm in ketamine- and xylazine-anesthetized rats. Orofacial formalin pain was induced by subcutaneous injection of a diluted formalin solution (50 μl, 1.5%) into the vibrissa pad. Face rubbing durations were recorded at 3-min blocks for 45 min. Formalin produced a biphasic pain response (first phase: 0-3 min and second phase: 15-33 min). Separate and combined microinjections of histamine H1 and H2 receptor agonists, 2-pyridylethylamine (2-PEA) and dimaprit, respectively, and opioid receptor agonist, morphine, attenuated the second phase of pain. The analgesic effects induced by 2-PEA, dimaprit, and morphine were blocked by prior microinjections of fexofenadine (a histamine H1 receptor antagonist), famotidine (a histamine H2 receptor antagonist), and naloxone (an opioid receptor antagonist), respectively. Naloxone also prevented 2-PEA- and dimaprit-induced antinociception, and the analgesic effect induced by morphine was inhibited by fexofenadine and famotidine. These results showed the involvement of histamine H1 and H2 and opioid receptors in the Sm modulation of orofacial pain. Opioid receptor might be involved in analgesia induced by activation of histamine H1 and H2 receptors and vice versa.

Study on the antinociceptive action of Tyr-K-MIF-1, a peptide from the MIF family

1. Tyr-K-MIF-1 is a melanocyte inhibiting factor (MIF) neuropeptide, isolated from the brain. Opposite to other MIFs (Tyr-MIF-1, Tyr-W-MIF-1), it has a very low affinity for opiate mu-receptors, but interacts with Tyr-MIF-1 specific binding sites. Tyr-MIF-1 and Tyr-W-MIF-1 evoke antinociception mainly by activating opioid receptors. We investigated the possible antinociceptive effect of Tyr-K-MIF-1 and the involvement of histaminergic system in its mechanism of action. 2. Tested on rats by paw-pressure test, Tyr-K-MIF-1 (0.5, 1 and 2 mg kg(-1)) was associated with short-lasting analgesia, which was abolished by naloxone (1 mg kg(-1)). 3. Injected intraperitoneally (i.p.) 15 min before Tyr-K-MIF-1, antagonists of H(1) (diphenhydramine, 100 mg kg(-1)) or H(2) (famotidine, 0.3 and 0.6 mg kg(-1)) histamine receptors diminished peptide antinociceptive effect. Simultaneous H(1)- and H(2) blockade, as well as pretreatment with 5 mg kg(-1) dimaprit (H(2) agonist) abolished Tyr-K-MIF-1-induced analgesia. Tyr-K-MIF-1-induced analgesia was also abolished by treatment with R-(alpha)-methylhistamine (10 mg kg(-1), i.p.), an H(3) histamine receptor agonist that acts to inhibit histamine release. 4. Our results together with data reported in the literature support the conclusion that activation of the histaminergic system is involved in the mechanism of Tyr-K-MIF-1-induced antinociception.

Enhanced antinociception by intrathecally-administered morphine in histamine H1 receptor gene knockout mice

We previously reported that histamine H(1) receptor gene knockout mice (H1KO) showed lower spontaneous nociceptive threshold to pain stimuli when compared to wild-type mice. The objective of the present study was to examine the antinociceptive effect of intrathecally-administered morphine in H1KO mice. The antinociceptive effects of morphine were examined using assays for thermal (tail-flick, hot-plate, paw-withdrawal), mechanical (tail-pressure) and chemical nociception (formalin and capsaicin tests) using H1KO and wild-type mice. In these nociceptive assays, intrathecally-administered morphine produced significant antinociceptive effects in wild-type mice. The antinociceptive effect produced by intrathecally administered morphine was enhanced in the knockout mice. We also examined the effect of an histamine H(1) receptor antagonist, an active (d-) isomer of chlorpheniramine, on morphine-induced antinociception in ICR mice. The intrathecal co-administration of d-chlorpheniramine enhanced the effect of morphine in all nociceptive assays examined. The pharmacological experiments using d-chlorpheniramine further substantiate the evidence for the histamine H(1) receptor-mediated suppression of morphine-induced antinociception. These results suggest that existing H(1) receptors play an inhibitory role in morphine-induced antinociception at the spinal cord level.