Platelet-activating factor (PAF) may be a neuromodulator involved in neural cell differentiation, cerebral inflammation, and ischemia. The PAF receptor is a member of the G protein-coupled receptor superfamily. In the present study, we sought to define the specific G protein(s) that mediate PAF-stimulated phosphoinositide (PI) metabolism in an immortalized hippocampal cell line, HN33.11. PAF increased the production of 3H-labeled inositol phosphates (IPs) with EC50 values of 1.2-1.5 nM. The effect of PAF on 3H-IPs formation was completely blocked by the PAF antagonist BN 50739 at a concentration of 300 nM. Pertussis toxin pretreatment attenuated PAF-stimulated 3H-IPs production by 20-30% (p < 0.05). Consistent with a role for Gi1/2 in this response, antiserum against G alpha i1/2 blocked the response to a similar degree. Pretreatment of permeabilized cells with G alpha q/11 antiserum attenuated the response by 70% (p < 0.05), suggesting a role for Gq/11 in mediating the PAF response in this cell line. Stimulation with PAF increased [alpha-32P]-GTP binding to both G alpha q and G alpha i1/2 proteins. Moreover, specific [3H]PAF binding sites coprecipitated with G alpha q and G alpha i1/2 proteins. The results suggest that PAF-stimulated PI metabolism in HN33.11 cells is mediated by both Gq and Gi1/2 proteins.