The concept of inverse agonism, whereby "antagonists" exert actions opposite to those of agonists at constitutively active receptors, has been documented both at receptor-modulated ion channels as well as at G-protein-coupled receptors (GPCR) in recombinant expression systems. However, it remains unclear whether physiologically or therapeutically relevant inverse agonists actions at GPCRs occur in the CNS in vivo. The present overview discusses our recent observations concerning 5-HT1B receptors, and focuses on the relationship between actions at heterologous Chinese hamster ovary (CHO) expression systems compared with native CNS populations of receptors. To this end, we have exploited several novel and selective ligands, notably the inverse agonist and neutral antagonist at 5-HT1B receptors, SB224,289 and S18127, respectively. Like 5-HT itself, the agonist, GR46611, markedly increases the binding of [35S]-GTP gamma S binding to h5-HT1B receptors expressed in CHO cells, while the "antagonist", GR127,935, modestly stimulates binding suggesting partial agonist properties. However, SB224,289 markedly suppresses binding at these sites. S18127, which does not alter [35S]GTP gamma S binding alone, abolishes the actions of both GR46611 and SB224,289. Nevertheless, in quantitative autoradiographical studies, S18127 and SB224,289 cannot be distinguished as concerns modulation of [35S]-GTP gamma S binding at substantia nigra and caudate nucleus-localized 5-HT1B receptors, inasmuch as they each block the action of the 5-HT1B agonist, CP93129, yet fail to modify binding alone. Further, S18217 and SB224,289, as well as GR127,935, all abolish the inhibitory influence of GR46611 upon dialysis levels of 5-HT in the frontal cortex of freely moving rats without themselves modifying release. Moreover, they all block the hypothermic actions of GR46611 without themselves modifying core temperature. Thus, differences in intrinsic activity of S18127, SB224,289 and GR127,935 seen at cloned, h5-HT1B receptors cannot be detected in vivo. Most notably, no evidence for opposite actions of the inverse agonist, SB224,289, as compared to 5-HT1B agonists is apparent. These data suggest that in vitro observations of inverse agonist actions cannot necessarily be extrapolated to intact systems in vivo.