Figure 2: Schematic illustration of major nonneuronal neuroactive mediators and their corresponding receptors involved in the modulation of NAU afferent impulses. Molecules in red, green, and violet color represent stimulatory, inhibitory, and both effects on afferent fiber excitability, respectively. Autoreceptors to be identified are indicated with question symbols (?). A1, adenosine A1 receptor; ACh, acetylcholine; Auto-R, autoreceptor; B1/2, bradykinin receptors 1 and 2; BK, bradykinin; CGRP, calcitonin-gene-related peptide; β-END, β-endorphin; EP, prostaglandin E receptor; GABA, γ-aminobutyric acid; Glu, glutamate; H1/H2, histamine H1/H2 receptors; His, histamine; 5-HT, 5-hydroxytryptamine; IL, interleukin; M2, muscarinic M2 receptor; NA, noradrenaline; NO, nitric oxide; PG, prostaglandins; P2X/P2Y, purinergic receptors P2X and P2Y; -R, adrenoceptor; SP, substance P; SS, somatostatin; SSR, somatostatin receptor; TNF-α, tumor necrosis factor-α.