Meclizine

Vomiting is a centrally regulated reflex mechanism that initiates from the vomiting center and the chemoreceptor trigger zone (CTZ) located in the medulla. Motion sickness is also regulated by CTZ. The blood-brain barrier near the CTZ is relatively permeable to circulating mediators and CTZ can transmit impulses to vomiting center located in the brainstem. Different receptors responding to different factors, including histamine, 5-HT, enkephalins, substance P, and dopamine, are expressed along the brainstem to activate respective pathways and contribute to the control of vomiting. Histamine H1 receptors are expressed on the vestibular nuclei and nucleus of the solitary tract (NTS) that are activated by motion sickness and stimuli from the pharynx and stomach. When activated, H1 receptor signaling from these nuclei is transmitted to the CTZ and vomiting centre.

Through its antagonistic action on the H1 receptors, meclizine primarily works by inhibiting signaling pathway transduction through histaminergic neurotransmission from the vestibular nuclei and NTS to the CTZ and medullary vomiting center. Meclizine may also decrease the labyrinth excitability and vestibular stimulation.