<unk> s is a family of endogenous molecules of gases or gaseous signaling molecules, including NO, CO, H2S, and others.  These particular gases share many common features in their production and function but carry on their tasks in unique ways, which differ from classical signaling molecules, in the human body. The first suggestion that a gas, , had a direct action at  pharmacological receptors and thereby acting as a neurotransmitter was first suggested in 1981 from clinical work with nitrous oxide (1,2,3).  In vitro experiments confirmed these observations (4) which were replicated at NIDA later(5).
The terminology and characterization criteria of<unk> ” were firstly introduced in 2002 (see ref. 14).  For one gas molecule to be categorized as a<unk> s, all of the following criteria should be met (see ref. 13 &14).  (i) It is a small molecule of gas; (ii) It is freely permeable to membranes.  As such, its effects do not rely on the cognate membrane receptors. It can have endocrine, paracrine, and autocrine effects.  In their endocrine mode of action, for example,<unk> s can enter the blood stream; be carried to remote targets by scavengers and released there, and modulate functions of remote target cells; (iii) It is endogenously and enzymatically generated and its production is regulated; (iv) It has well defined and specific functions at physiologically relevant concentrations.  Thus, manipulating the endogenous levels of this gas evokes specific physiological changes; (v) Functions of this endogenous gas can be mimicked by its exogenously applied counterpart; (vi) Its cellular effects may or may not be mediated by second messengers, but should have specific cellular and molecular targets.
Gasotransmitter
Sympathomimetic amine
Stimulant
Eicosanoid
Carotenoid
Growth factor
Bryostatin
Steroid hormone