Evidence for an inhibitory role of central histamine on carrageenin-induced hyperalgesia

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.

Ghrelin inhibits inflammatory pain in rats: Involvement of the opioid system

This study examined the effects of intracerebroventricular (i.c.v.), intraperitoneal (i.p.) or intraplantar (i.pl.) administration of ghrelin, the endogenous ligand of the growth hormone secretagogue receptor, in the development of hyperalgesia and edema induced by intraplantar injection of carrageenan in rats. Central ghrelin (4 ng to 4 microg/rat) given 5 min before carrageenan produced a dose-related reversal of carrageenan-induced mechanical hyperalgesia measured by Randall-Selitto test with an ED50 of 81.7 ng/rat. Ghrelin at the dose of 4 microg/rat i.c.v. was also effective in inhibiting edema. When ghrelin (4 microg/rat i.c.v.) was administered 150 min after carrageenan, it failed to modify either hyperalgesia or the paw volume. Given i.p., 30 min before carrageenan, ghrelin (20-160 microg/kg) induced a significant dose-dependent inhibition of hyperalgesia with an ED50 of 77 microg/kg and a slight reduction of edema. Intraplantar ghrelin (40 ng to 12 microg/rat) did not significantly modify both the hyperalgesic and edematous activities of carrageenan. The anti-hyperalgesic and anti-edematous effects of ghrelin (4 microg/rat i.c.v.) were reversed by naloxone (10 microg/rat i.c.v.). Systemic administration of the peripheral selective opioid antagonist, naloxone methiodide (3 mg/kg s.c.), did not antagonize antinociception elicited by i.p. ghrelin. Overall these data indicate that ghrelin exerts an inhibitory role on inflammatory pain through an interaction with the central opioid system.

CENTRAL EFFECT OF HISTAMINE AND PERIPHERAL EFFECT OF HISTIDINE ON THE FORMALIN-INDUCED PAIN RESPONSE IN MICE

1. The present study was designed to investigate the role of brain histamine in modulating pain transmission in mice. 2. In conscious mice implanted with an intracerebroventricular (i.c.v.) cannula, the effects of i.v.c. injections of normal saline (control) and low and high doses histamine (2 and 40 microg/mouse, respectively) were investigated on the duration of paw licking and biting induced by subcutaneous (s.c.) injection of formalin (20 microL; 5%) into the plantar surface of the left hindpaw. 3. To clarify the involvement of histidine in the pain response, the effects of intraperitoneal (i.p.) injections of low and high doses of histidine (50 and 1000 mg/kg, respectively) alone or before i.c.v. injection of histamine were also examined. 4. Intraplantar injection of formalin induced a biphasic pain response (first phase: 0-5 min after injection; second phase: 20-40 min after injection). 5. Histamine (2 microg/mouse, i.c.v.) had no effect on the first phase of the pain response, but suppressed the second phase. The higher dose of histamine (40 microg/mouse, i.c.v.) suppressed both phases of the pain response. 6. Histidine, at 50 mg/kg, i.p., had no effect on the pain response, but the higher dose (1000 mg/kg, i.p.) suppressed the both phases of the pain response. 7. Pretreatment with the low dose of histidine (50 mg/kg, i.p.) prior to administration of 2 microg/mouse, i.c.v., histamine did not change the antinociception induced by low-dose histamine. However, pretreatment with the high dose of histidine (1000 mg/kg, i.p.) prior to 2 microg/mouse, i.c.v., histamine produced antinociception that resembled that seen following administration of the high dose of either histidine or histamine. Pretreatment with the low dose of histidine (50 mg/kg, i.p.) prior to administration of 40 microg/mouse, i.c.v., histamine has no effect on the pain response following high-dose histamine. Pretreatment with 1000 mg/kg, i.p., histidine prior to administration of 40 microg/mouse, i.c.v., histamine strongly suppressed both phases of the formalin-induced pain response, particularly the second phase. 8. The results of the present study indicate that: (i) activation of brain histamine produces antinociception in the mouse formalin test; (ii) peripheral loading with a high dose of histidine (1000 mg/kg, i.p.) alone exerts the same effect as that seen following 40 microg/mouse, i.c.v., histamine; and (iii) pretreatment with a high dose of histidine potentiates central histamine-induced antinociception.

Involvement of calmodulin inhibition in analgesia induced with low doses of intrathecal trifluoperazine

We examined which of the known properties of trifluoperazine, including calmodulin inhibition, are involved in its analgesic effect. Furthermore, we tried to find any possible interaction between opioidergic system and calmodulin inhibition-induced analgesia. Intrathecal trifluoperazine (1, 10, 100 microg) showed a biphasic effect in the formalin test; i.e., analgesia at relatively low doses (1, 10 microg) and hyperalgesia at a high dose (100 microg). No analgesic effects were observed after intrathecal injection of sulpiride (1, 10, 100 microg), atropine (0.1, 1, 10 microg), phentolamine (0.1, 1, 10 microg) and brompheniramine (0.1, 1, 10 microg). Meanwhile, intrathecal calmidazolium (10, 50, 250 microg) induced a dose-dependent analgesia. Histamine (1 microg), physostigmine (1 microg), bromocriptine (1 microg) and norepinephrine (1 microg) did not affect trifluoperazine-induced analgesia. Calcium (20 microg) attenuated the antinociceptive effect of trifluoperazine and inhibited the analgesic effect of calmidazolium. Finally, naloxone (2 mg/kg) decreased trifluoperazine-induced antinociception but did not have any effects on calmidazolium-induced analgesia. We concluded that calmodulin inhibition may be involved in the analgesia produced by trifluoperazine. With increasing doses of trifluoperazine, the algesic effect seems to overcome the analgesic effect. It is also suggested that the opioidergic system does not interact with calmodulin inhibition-induced analgesia even though this system has a possible role in trifluoperazine-induced analgesia.

Evidence for hypernociception induction following histamine H1 receptor activation in rodents

To characterize the mechanism of the analgesic action of H1 antihistaminics the effects of a new, highly selective agonist, 2-(3-trifluoromethylphenyl)histamine dihydrogenmaleate (FMPH), and of the better known H1 agonist, 2-thiazolylethylamine (2-TEA), were studied on pain threshold by means of three different kinds of tests for nociception (mouse hot plate and abdominal constriction, and rat paw pressure tests). Low doses of both substances (2.65 and 6.5 microg/animal i.c.v. for FMPH in the hot plate and paw pressure tests, and 0.3 microg/rat i.c.v. for 2-TEA in the paw pressure test) were slightly but significantly hypernociceptive. The selective H1 receptor antagonist, pyrilamine maleate (10-30 mg/kg s.c.), induced a dose-dependent antinociception in all three tests, and both FMPH and 2-TEA prevented its effect, but not that of morphine, thus indicating action on H1 receptors. The same low doses of FMPH were also able to enhance animals' spontaneous motility and curiosity. High doses of FMPH (13.23-132.3 microg/mouse i.c.v.) raised the pain threshold, but due to the severe motor impairment evidenced by the rota rod test, this cannot be considered as real antinociception. An increase in the pain threshold lacking any motor impairment was observed for tenfold higher doses of 2-TEA (3 and 10 microg/mouse i.c.v.). This may be due to action on H2 receptors, with the reported relative potency of 2-TEA for H1 and H2 receptors being about 12:1. It is therefore suggested that H1 receptor activation increases sensitivity to noxious stimuli.

The effect of somatostatin on experimental inflammation in rats

The aim of the present study was to investigate the effect of somatostatin administration on experimentally induced inflammation in rats. Inflammation was induced by the intraplantar injection of carrageenan (50 microL) into the hind paw of the rat. Animals were treated intraplantarly with somatostatin in a volume of 50 microL at different doses (2.5, 25, and 250 ng, 10 microg). The inflammatory response was studied 120, 180, and 240 min after drug administration. The antinociceptive effect of somatostatin was determined by using the Randall and Selitto test and by local production of beta-endorphin from lymphocytes obtained from popliteal lymph nodes. Data show that small doses of somatostatin were the most effective in reducing hyperalgesia. Moreover, our results show that somatostatin treatment significantly increased beta-endorphin in lymphocytes from popliteal lymph nodes. The secretion of opioid peptides, which enhance analgesia, could be stimulated by locally administered somatostatin.

IMPLICATIONS

Acute pain because of intraplantar inflammation induced in rats by carrageenan injection was significantly reduced by small-dose, local administration of somatostatin, which possibly favors beta-endorphin release as a mechanism. These results may have implications regarding treatment of pain conditions associated with an inflammatory response.

Effects of naltrexone and histamine antagonists on the antinociceptive activity of the cimetidine analog SKF92374 in rats

A recent study showed that SKF92374, a structural analog of the histamine H2 receptor antagonist cimetidine, induces antinociception after intraventricular (i.v.t.) administration in the rat. SKF92374 lacked significant activity on H1 or H2 receptors, but had weak activity on H3 receptors. To test the hypothesis that SKF92374-induced antinociception is mediated by an action on H3 receptors, the effects of the H3 agonist R-alpha-methylhistamine (RAMH) and the H3 antagonist thioperamide (both by i.v.t. administration) were investigated on SKF92374 antinociception. SKF92374-induced antinociception was slightly enhanced by thioperamide (30 microg), but unaffected by a range of doses of RAMH (up to 2 microg). Furthermore, SKF92374-induced antinociception was not reduced by large doses of systemically-administered antagonists of H1 (pyrilamine), H2 (zolantidine), H3 (GT-2016), or opioid (naltrexone) receptors. These findings show that the novel compound SKF92374 induces antinociception by a non-opioid mechanism that does not utilize brain H1, H2 or H3 receptors.

Involvement of H1 receptors in the central antinociceptive effect of histamine: pharmacological dissection by electrophysiological analysis

Intracerebroventricular (i.c.v.) administration of histamine (HA, 0.025-0.1 microM/rat) to arthritic rats induces a dose-related inhibition of the neuronal thalamic firing evoked by peripheral noxious stimuli. To characterize the type(s) of HA receptors involved in this depressing activity of the amine we used electrophysiological techniques to examine the effects of i.c.v. administration of H1 and H2 agonists and antagonists on the spontaneous and evoked nociceptive firing of the thalamic neurons in rats rendered arthritic by Freund's adjuvant. The H1 agonist 2-pyridylethylamine (0.4-1.0 microM/rat, i.c.v.) displayed a dose-dependent antinociceptive effect very similar to that of HA, while the H2 agonist dimaprit (0.05-0.2 microM/rat, i.c.v.) did not modify thalamic firing. Neither mepyramine (H1 antagonist, 0.1 microM/rat, i.c.v.) nor zolantidine (H2 antagonist, 0.01 microM/rat, i.c.v.) modified the evoked firing of rat thalamic neurons. When administered before HA (0.1 microM/rat, i.c.v.) mepyramine but not zolantidine was able to inhibit the antinociceptive effect of HA. On the basis of the present electrophysiological results, we suggest that a specific interaction of histamine with H1 receptors may be important for its antinociceptive effect on afferent peripheral inputs to the thalamus.