Mechanisms of Nausea and Vomiting: Current Knowledge and Recent Advances in Intracellular Emetic Signaling Systems

Abstract

Nausea and vomiting are common gastrointestinal complaints that can be triggered by diverse emetic stimuli through central and/or peripheral nervous systems. Both nausea and vomiting are considered as defense mechanisms when threatening toxins/drugs/bacteria/viruses/fungi enter the body either via the enteral (e.g., the gastrointestinal tract) or parenteral routes, including the blood, skin, and respiratory systems. While vomiting is the act of forceful removal of gastrointestinal contents, nausea is believed to be a subjective sensation that is more difficult to study in nonhuman species. In this review, the authors discuss the anatomical structures, neurotransmitters/mediators, and corresponding receptors, as well as intracellular emetic signaling pathways involved in the processes of nausea and vomiting in diverse animal models as well as humans. While blockade of emetic receptors in the prevention of vomiting is fairly well understood, the potential of new classes of antiemetics altering postreceptor signal transduction mechanisms is currently evolving, which is also reviewed. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide potential answers.

Keywords: Ca2+; G-protein coupled receptors; brainstem; emesis; nausea; signaling pathway; the gastrointestinal tract; vomiting.

Figure 1

Central and peripheral anatomical sites involved in nausea and vomiting induced by various stimuli. Nausea and vomiting can be generated by diverse stimuli and are mediated by the bidirectional interaction between brain and gut. In brief: (1) The brainstem area postrema in the floor of the fourth ventricle lacks blood brain barrier and thus serves as direct central receptor sites for circulating and systemic emetic stimuli in the cerebrospinal fluid and the blood [11]. (2) Systemically administered drugs can activate corresponding receptors present on vagal afferents, which project sensory signals to the nucleus of the solitary tract [11,12]. (3) Peripheral stimuli such as toxic drugs and microbials (e.g., bacteria, viruses, fungi) that enter the lumen of the gastrointestinal tract (GIT) and pathologies in the GIT cause release of local emetic neurotransmitters/modulators, which subsequently act on the corresponding receptors present on vagal afferents and/or stimulate the brainstem area postrema via circulating blood [9,10]. Besides the area postrema and the sensory vagal afferents, the nucleus of the solitary tract is also the recipient of: (i) direct neural inputs from the splanchnic nerves carrying sensation caused by diseases of visceral organs (e.g., cardiac, kidney); (ii) brainstem vestibular nuclei collecting signals from vestibular apparatus in inner ear and/or cerebellum, caused by stimuli related to motion sickness and opioid analgesics [13,14]; and (iii) the cerebral cortex and limbic system, which accept and process emotional and cognitive stimuli [3,4,5,6,7,8]. The nucleus of the solitary tract has output pathways to the dorsal motor nucleus of the vagus, which further project to the upper gastrointestinal tract to produce the vomiting reflex [11]. In addition, the nucleus of the solitary tract has projections to the mid- and forebrain for the perception of nausea [15].

Figure 2

A schematic representation of emetic ligands or agonists, acting on the corresponding cell membrane-located receptors, including calcium-permeable ion channels and G-protein coupled receptors.