Brain Activation by H1 Antihistamines Challenges Conventional View of Their Mechanism of Action in Motion Sickness: A Behavioral, c-Fos and Physiological Study in Suncus murinus (House Musk Shrew)

Abstract

Motion sickness occurs under a variety of circumstances and is common in the general population. It is usually associated with changes in gastric motility, and hypothermia, which are argued to be surrogate markers for nausea; there are also reports that respiratory function is affected. As laboratory rodents are incapable of vomiting, Suncus murinus was used to model motion sickness and to investigate changes in gastric myoelectric activity (GMA) and temperature homeostasis using radiotelemetry, whilst also simultaneously investigating changes in respiratory function using whole body plethysmography. The anti-emetic potential of the highly selective histamine H₁ receptor antagonists, mepyramine (brain penetrant), and cetirizine (non-brain penetrant), along with the muscarinic receptor antagonist, scopolamine, were investigated in the present study. On isolated ileal segments from Suncus murinus, both mepyramine and cetirizine non-competitively antagonized the contractile action of histamine with pK _b values of 7.5 and 8.4, respectively; scopolamine competitively antagonized the contractile action of acetylcholine with pA₂ of 9.5. In responding animals, motion (1 Hz, 4 cm horizontal displacement, 10 min) increased the percentage of the power of bradygastria, and decreased the percentage power of normogastria whilst also causing hypothermia. Animals also exhibited an increase in respiratory rate and a reduction in tidal volume. Mepyramine (50 mg/kg, i.p.) and scopolamine (10 mg/kg, i.p.), but not cetirizine (10 mg/kg, i.p.), significantly antagonized motion-induced emesis but did not reverse the motion-induced disruptions of GMA, or hypothermia, or effects on respiration. Burst analysis of plethysmographic-derived waveforms showed mepyramine also had increased the inter-retch+vomit frequency, and emetic episode duration. Immunohistochemistry demonstrated that motion alone did not induce c-fos expression in the brain. Paradoxically, mepyramine increased c-fos in brain areas regulating emesis control, and caused hypothermia; it also appeared to cause sedation and reduced the dominant frequency of slow waves. In conclusion, motion-induced emesis was associated with a disruption of GMA, respiration, and hypothermia. Mepyramine was a more efficacious anti-emetic than cetirizine, suggesting an important role of centrally-located H₁ receptors. The ability of mepyramine to elevate c-fos provides a new perspective on how H₁ receptors are involved in mechanisms of emesis control.

Keywords: Suncus murinus; gastric myoelectric activity; histamine H1 receptors; hypothermia; motion sickness; muscarinic receptors; respiration pattern.

Figure 1

Illustration of the respiratory pattern of Suncus murinus and analysis of emetic data using burst analysis. (A) Illustration showing elements of the respiratory cycle (inspiration downwards). Mean respiratory rate, tidal volume, inspiration time and inspiration flow were used to characterize the respiratory pattern; (B) Illustration of the raw recording and analysis of emetic data using burst analysis. “Events” are large-amplitude single oscillatory cycles that coincided with visually observed contraction of abdominal muscles. Events per episode, mean inter-event duration, mean retch/vomit frequency, episode duration, interval between episodes (the duration from the end of last episode to the start of the next episode) and cycles between episodes (the duration from the onset of last episode to the start of the next episode) were defined to enable automated analysis of emetic episode data.

Figure 2

Effect of mepyramine and cetirizine on histamine-induced, and scopolamine on acetylcholine-induced contractions of Suncus murinus isolated ileal sections. (A) Concentration-response curve of histamine against ileal sections; (B) Effect of TTX, HEX, and atropine on histamine-induced contraction of isolated ileum; (C,E) Effect of mepyramine and cetirizine on histamine-induced contraction of isolated ileum; (D,F) Double reciprocal plot for histamine in the presence of 30 nM mepyramine and cetirizine, respectively; (G) Effect of scopolamine on acetylcholine-induced contraction of isolated ileum; (H) Schild analysis of scopolamine on acetylcholine-induced contractions. Data represents mean ± s.e.m. of 6–12 determinations. Significant differences compared to the control group are indicated as ^**p < 0.01 (One-way ANOVA followed by Bonferroni test).

Figure 3

Effect of mepyramine (50 mg/kg), cetirizine (10 mg/kg), and scopolamine (10 mg/kg) on motion-induced emesis in Suncus murinus. (A) Number of episodes of emesis; (B) Number of vomits; (C) Number of retches; (D) % change from pre-screening episodes. Drug or vehicle was administered intraperitoneally as a 60 min pretreatment. Results represent the mean ± s.e.m. of 6 animals. Significant differences compared to vehicle group are shown as *p < 0.05, **p < 0.01 (One-way ANOVA followed by Bonferroni tests). Veh, vehicle; Mep, mepyramine; Cet, cetirizine; Sco, scopolamine.

Figure 4

Analysis of emetic data using burst analysis. (A) Events per episode/burst; (B) Mean inter-event duration; (C) Mean retch/vomit frequency; (D) Episodes duration; (E) Interval between episodes; (F) Cycle between episodes. Results represent the mean ± s.e.m. of all animals which vomited (n = 3–6). Significant differences compared to vehicle group are shown as **p < 0.01 (One-way ANOVA followed by Bonferroni tests). Veh, vehicle (saline, 2 ml/kg); Mep, mepyramine (50 mg/kg); Cet, cetirizine (10 mg/kg); Sco, scopolamine (10 mg/kg).

Figure 5

Effect of mepyramine (50 mg/kg), cetirizine (10 mg/kg), and scopolamine (10 mg/kg) on gastric myoelectric activity and core body temperature. (A) Dominant frequency (DF); (B) Dominant power (DP); (C) Bradygastria %; (D) Normogastria %; (E) Tachygastria %; (F) Body temperature. Data represents the mean ± s.e.m. of 6 animals. For inter-group comparison, significant differences compared to vehicle group are shown as *p < 0.05, **p < 0.01 (repeated measures two-way ANOVA followed by Bonferroni tests), ^#p < 0.05, ^##p < 0.01 (repeated measures two-way ANOVA followed by Bonferroni tests) was applied when referring to intra-group comparison. Baseline refers to a 10 min period immediately before provocation motion; recovery indicates 10 min immediately after provocation motion.

Figure 6

Effect of mepyramine (50 mg/kg), cetirizine (10 mg/kg), and scopolamine (10 mg/kg) on respiratory pattern. (A) Respiratory rate; (B) Tidal volume; (C) Inspiration time; (D) Inspiration flow. Data represents the mean ± s.e.m. of 6 animals. For inter-group comparison, significant differences compared to vehicle group are shown as *p < 0.05 (repeated measures two-way ANOVA followed by Bonferroni tests), significant differences compared to baseline are shown as ^#p < 0.05, ^##p < 0.01 (repeated measures two-way ANOVA followed by Bonferroni tests) when referring to intra-group comparison. Baseline refers to a 10 min period immediately before provocation motion; recovery indicates 10 min immediately after provocation motion.

Figure 7

Representative photomicrographs illustrating c-fos expression (violet nuclear label) in the caudal brainstem after administration of saline (2 ml/kg), mepyramine (50 mg/kg), cetirizine (10 mg/kg), and scopolamine (10 mg/kg). Veh/MS, vehicle and motion stimulus; Mep/MS, mepyramine and motion stimulus; Cet/MS, cetirizine and motion stimulus; Sco/MS, scopolamine and motion stimulus; Veh, saline without provocative motion; Mep, mepyramine without provocative motion; Cet, cetirizine without provocative motion; Sco, scopolamine without provocative motion. Arrows show some of the activated c-fos positive cells. AP, area postrema; NTS, nucleus tractus solitarius. Scale bar: 100 μm.

Figure 8

Effect of mepyramine (50 mg/kg), cetirizine (10 mg/kg), and scopolamine (10 mg/kg) on motion-induced c-fos expression in the brain of Suncus murinus. Data represents the mean ± s.e.m. of 6 animals. (A) Area postrema; (B) Nucleus tractus solitarius; (C) Medial vestibular nucleus; (D) Ventromedial hypothalamic nucleus; (E) Dorsomedial hypothalamic nucleus; (F) Bednucleus part of lateral hypothalamus; (G) Paraventricular hypothalamic nucleus; (H) Arcuate hypothalamic nucleus. NC, vehicle control (saline without provocative motion); MC, mepyramine control (mepyramine without provocative motion); CC, cetirizine control (cetirizine without provocative motion); SC, scopolamine control (scopolamine without provocative motion). Significant differences compared to vehicle group or the negative control group are shown as **p < 0.01, ***p < 0.001 (One-way ANOVA followed by Bonferroni tests).