The basic secretagogue compound 48/80 activates G proteins indirectly via stimulation of phospholipase D–lysophosphatidic acid receptor axis and 5‐HT 1A receptors in rat brain sections

The basic secretagogues, such as compound 48/80 (c48/80) and mastoparans, are widely used histamine‐releasing agents and their mechanism of action is commonly attributed to a direct, receptor‐bypassing property to activate the G i/o class of G proteins. We tested here whether c48/80 could directly stimulate [ 35 S]guanosine‐5′‐[ γ ‐thio]triphosphate ([ 35 S]GTP γ S) binding to rat brain sections in an attempt to visualize the entire signaling pool of G i/o in its native neuroanatomical context. Instead of direct G i/o activation, c48/80 (100 μ g ml −1 ) from various suppliers stimulated brain phospholipase D (PLD) activity, leading to the generation of endogenous phospholipids capable of activating brain white matter‐enriched, G i/o ‐coupled lysophosphatidic acid (LPA) receptors. This response was sensitive to 1‐butanol and was potently reversed by the LPA 1 /LPA 3 receptor‐selective antagonist Ki16425 (IC 50 59±13 n M , mean±s.e.m.), and showed age‐dependent decline, closely reflecting known developmental regulation of the PLD–LPA 1 receptor axis in the CNS. In addition, c48/80 was found to modestly activate hippocampal 5‐HT 1A receptors in a pH‐dependent and antagonist‐sensitive manner. Consistent with the lack of direct G i/o ‐activating properties in brain sections, c48/80 showed no activity in classical membrane [ 35 S]GTP γ S binding assays. Instead, c48/80 from one particular manufacturer elicited non‐specific effect in these assays, therefore challenging the previous interpretations regarding the compound's ability to activate G proteins directly. We conclude that c48/80 is not a receptor‐bypassing general G protein activator but rather activates PLD, leading to generation of endogenous LPA receptor‐activating phospholipids. This property may also contribute to the compound's ability to release histamine from mast cells. British Journal of Pharmacology (2006) 147 , 596–606. doi: 10.1038/sj.bjp.0706671