Activation of peripheral and spinal histamine H3 receptors inhibits formalin-induced inflammation and nociception, respectively

Abstract

Pharmacological activation of histamine H3 receptors is known to reduce the release of inflammatory peptides, thereby reducing pain and inflammation, but the site(s) and mechanism(s) of these effects are currently unknown. The present study addressed these questions by examining the effects of the H3 agonist immepip and the H3 antagonist thioperamide on nociceptive behaviors and swelling produced during the rat formalin test. Systemic administration of immepip (5 and 30 mg/kg, s.c.) significantly attenuated formalin-induced flinching but not licking responses during both phases. This attenuation was reversed by either systemic (15 mg/kg, i.p.) or intrathecal (20 or 50 microg) administration of thioperamide. Furthermore, immepip (30 mg/kg, s.c.) significantly inhibited formalin-induced swelling, an action which was completely reversed by systemic (15 mg/kg, i.p.), but not intrathecal (50 microg) thioperamide. Also consistent with this pattern, intrathecal immepip (50 microg) reduced flinching responses, but had no effect on formalin-induced paw swelling. The present findings suggest that activation of H3 receptors located on peripheral and spinal terminals of deep dermal fibers attenuates formalin-induced swelling and flinching, respectively. Pharmacological stimulation of H3 receptors could be an important therapeutic approach for many disorders related to deep dermal or inflammatory pain.

Figure 1. Effects of the H₃ agonist immepip on formalin-induced flinching responses, licking responses, and paw swelling

Rats received immepip (1, 5, or 30 mg/kg, s.c.) or saline vehicle, followed 30 min later by intraplantar formalin. A: Number of flinches per min was recorded and grouped into 5 min periods (ordinate, mean ± S.E.M.) for the times after formalin shown (abscissa, min). B: The total number of flinches per min was summed for each phase (ordinate, mean ± S.E.M.). Phase 1 was established as 0-10 min, whereas Phase 2 was established as 11-60 min. C: Time spent licking (ordinate, s, mean ± S.E.M.) is shown in 5 min intervals (abscissa, min). D: Paw diameter (ordinate, mm, mean ± S.E.M.) was recorded at the times shown (abscissa, min) after formalin injection. Data are pooled from two separate experiments: a) saline and immepip (5 mg/kg) treatments (n=6) in which paw volume was not measured (data in Figs 1A-1C), and b) all four treatment groups in which behavior and paw volume was measured (data in all 4 figures, n=5). *, ** P< 0.05, P< 0.01 vs. saline at same time interval, respectively.

Figure 2. Effects of the H₃ antagonist thioperamide on immepip-induced attenuation of flinching responses (A, C) and paw swelling (B, D) after formalin treatment

Rats received immepip (imm, 30 mg/kg, s.c.) or saline vehicle (sal, s.c.). A, B: Effects of systemic thioperamide were studied. Ten min after immepip, thioperamide (thio, 15 mg/kg, i.p.) or saline (sal) were injected, followed by intraplantar formalin 20 min later. C, D: Effects of intrathecal thioperamide were studied. Twenty min after immepip, intrathecal thioperamide (50 μg) or saline (sal) were given, followed by formalin 10 min later. Numbers of flinches per min, grouped in 5 min periods (A, C, ordinate, mean ± S.E.M.), and paw diameters (B, D, ordinate, mm, mean ± S.E.M.) were recorded at the times shown after formalin (abscissa, min). No significant differences were found in flinching responses or paw diameters between the following groups, thus permitting the groups to be pooled and identified as in parentheses: 1) s.c. saline/i.p. saline vs. s.c. saline/i.t. saline (s.c. sal/sal, open circles), 2) s.c. saline/i.p. thioperamide vs. s.c. saline/i.t. thioperamide (s.c. sal/thio, open squares), and 3) s.c. immepip/i.p. saline vs. s.c. immepip/i.t. saline (s.c. imm/sal, closed circles). * ,** P< 0.05, P<0.01 vs. sal/sal at same time interval, respectively. ^{+, ++} P< 0.05, P< 0.01 vs. imm/sal at same time interval, respectively.

Figure 3. Effects of intrathecal immepip on formalin-induced flinching responses (A) and paw swelling (B)

Rats received saline (sal, s.c.), followed 20 min later by an intrathecal injection of saline (sal) or immepip (imm, 50 μg). Ten min later, subjects received intraplantar formalin. The experiment proceeded and data are shown as in Fig. 2. * ,** P< 0.05, P<0.01 vs. sal/sal at same time interval, respectively.