The brain-penetrating, orally bioavailable, ghrelin receptor agonist HM01 ameliorates motion-induced emesis in Suncus murinus (house musk shrew)

Abstract

Background and purpose: HM01, a novel, orally bioavailable, brain-penetrating agonist of ghrelin receptors, ameliorates emesis in Suncus murinus. This study compared HM01's activity against motion sickness with that of the less brain-penetrating ghrelin receptor agonist, HM02.

Experimental approach: The potential of HM01 and HM02 to relax isolated mesenteric arteries and to increase feeding was investigated. Radio telemetry was used to record gastric slow waves and body temperature. Plethysmography was used to measure respiratory function. HM01 and HM02 were administered p.o. 1 hr prior to provocative motion, and c-Fos expression in brain sections was assessed.

Key results: HM01 and HM02 both relaxed precontracted arteries, yielding EC₅₀ values of 2.5 ± 0.5 and 3.5 ± 0.4 nM respectively. HM01 increased feeding, but HM02 did not. Both compounds caused hypothermia and bradygastria. Motion induced 123 ± 24 emetic events. HM01, but not HM02, reduced motion-induced emesis by 67.6%. Motion increased c-Fos expression in the nucleus tractus solitarius (NTS), dorsal motor nucleus of the vagus (DMNV), medial vestibular nucleus (MVe), central nucleus of the amygdala, and paraventricular hypothalamic nucleus (PVH). HM01 alone increased c-Fos expression in the area postrema, NTS, DMNV, PVH, and arcuate hypothalamic nucleus; HM02 had a similar pattern except it did not increase c-Fos in the PVH. Both compounds antagonized the motion-induced increases in c-Fos expression in the MVe.

Conclusions and implications: HM01 is more effective than HM02 in preventing motion-induced emesis. The difference in potency may relate to activation of ghrelin receptors in the PVH.

Figure 1

Chemical structure and potency of ghrelin agonist HM01 and HM02. (a and b) Chemical structure of HM01 and HM02; (c) effect of ghrelin and ghrelin agonists on phenylephrine‐induced contraction of the mesenteric artery; and (d) representative raw traces of ghrelin‐, HM01‐, and HM02‐induced relaxation. Data represent the mean ± SEM of six animals

Figure 2

Effect of HM01 and HM02 on food and water intake in Suncus murinus. (a) Cumulative daily food intake; (b) mean daily food intake (average of Days 1–6); (c) cumulative water intake; and (d) mean daily water intake (average of Days 1–6). Animals were treated daily with vehicle (saline, 10 ml·kg⁻¹, p.o.), HM01 (10 mg·kg⁻¹, p.o.), or HM02 (30 mg·kg⁻¹, p.o.) for six consecutive days, and food and water intake were measured daily shortly before treatments. Data represent the mean ± SEM of eight animals. ^* P < .05, significantly different from vehicle‐treated animals; one‐way ANOVA followed by Bonferroni tests

Figure 3

Effect of HM01 (10 mg·kg⁻¹) and HM02 (30 mg·kg⁻¹) on motion‐induced emesis in Suncus murinus. (a) Latency to the first episode; (b) number of episodes of emesis; (c) number of vomits; (d) number of retches; (e) number of vomits + retches; and (f) % change from prescreening episodes. Drug or vehicle was administered p.o. as a 60‐min pretreatment. Results represent the mean ± SEM of six animals. ^* P < .05, significantly different from vehicle‐treated animals; one‐way ANOVA followed by Bonferroni tests. Latency data to the first episode were assessed by a Kruskal–Wallis test followed by a Bonferroni tests. Individual animal latencies to the first episode of retching and/or vomiting and lines indicating medians with interquartile ranges are shown as are the number of animals exhibiting retching and/or vomiting out of the number of animals tested

Figure 4

Effect of HM01 (10 mg·kg⁻¹) and HM02 (30 mg·kg⁻¹) on gastric myoelectric activity (GMA) and body temperature. (a) Body temperature; (b) dominant frequency (DF); (c) dominant power (DP); (d) bradygastria %; (e) normogastria %; and (f) tachygastria %. Data represent the mean ± SEM of six animals. ^* P < .05, significantly different from vehicle‐treated animals; two‐way ANOVA followed by Bonferroni tests. Vertical lines indicate moments when drugs/saline was administered, and the motion was turned on and off. All data collected by radio telemetry were calculated by taking average of the data per 10 min

Figure 5

Effect of HM01 (10 mg·kg⁻¹) and HM02 (30 mg·kg⁻¹) on respiratory activity. (a) Respiratory rate; (b) adjusted tidal volume; (c) inspiration time; and (d) inspiration flow. Data represent the mean ± SEM of six animals. ^* P < .05, significantly different from vehicle‐treated animals; two‐way ANOVA followed by Bonferroni tests. Vertical lines indicate moments when drugs/saline was administered, and the motion was turned on and off. All data collected by whole body plethysmography were calculated by taking average of the data per 2 min and eventually averaged into 10‐min segments for statistical analysis. Respiratory data, excepting respiration rate, could not be collected accurately during motion due to disruption of recordings caused by the shaking of the recording equipment, therefore were not analysed statistically

Figure 6

Representative photomicrographs illustrating c‐Fos expression (violet nuclear label) in the caudal brainstem (top panel) and hypothalamus (bottom panel) after administration of vehicle (saline, 2 ml·kg⁻¹), HM01 (10 mg·kg⁻¹), and HM02 (30 mg·kg⁻¹). Vehicle + Motion: vehicle and motion stimulus; HM01 + Motion: HM01 and motion stimulus; HM02 + Motion: HM02 and motion stimulus; Vehicle: saline without provocative motion; HM01: HM01 without provocative motion; HM02: HM02 without provocative motion; 3V, the 3rd ventricle; AP, area postrema; Arc, arcuate hypothalamic nucleus; DMNV, dorsal motor nucleus of vagus; NTS, nucleus tractus solitarius. Scale bar: 100 μm

Figure 7

Effect of HM01 (10 mg·kg⁻¹) and HM02 (30 mg·kg⁻¹) on motion‐induced c‐Fos expression in the brain of Suncus murinus. Data represent the mean ± SEM of six animals. (a) area postrema (AP); (b) nucleus tractus solitarius (NTS); (c) dorsal motor nucleus of vagus (DMNV); (d) medial vestibular nucleus (MVe); (e) paraventricular hypothalamic nucleus (PVH); (f) central amygdaloid nucleus (CeA); and (g) arcuate hypothalamic nucleus (Arc). ^* P < .05, significantly different from vehicle‐treated animals; ^# P < .05, significantly different from Vehicle + M group; one‐way ANOVA followed by Bonferroni tests