Phosphoinositide 3-Kinase g (PI3Kg) is the only member of the class Ib PI3Ks and can be activated by G-protein coupled receptors (GPCRs), producing the second messenger phosphatidylinositol-3,4,5-trisphosphate. In addition, PI3Kg possesses an intrinsic protein kinase activity and might exert a scaffold function in the signalling network of different cells. We show that PI3Kg is expressed in small diameter neurons of dorsal root ganglia (DRG), i.e. neurons that are involved in pain perception (nociception). PI3Kg-knockout mice (KO) show an altered response to morphine treatment in their nociceptive behaviour. Analysis of animal nociceptive behaviour and in vitro studies on primary neuronal cell culture indicates an involvement of PI3Ks in mu-opioid receptor pathways (antinociception) (Narita et al., 2004, Neurosci. 124:515; 2002, Neurosci. 113:647). Voltage-gated calcium channels are inhibited upon stimulation of mu-opioid receptor GPCR activation and the PI3K cascade was shown to interfere with G-protein / calcium channel interactions (Tan et al., 2003, J. Neurosci. 23:10292–10301).

We utilized PI3Kg-/- mice (KO) and those expressing a PI3Kg variant without kinase activity (KD). DRG neurons were isolated from wild type (WT) and transgenic mice and Ca²⁺ currents were measured using the whole-cell patch-clamp technique after one day in culture. Acute application of the mu-opioid agonists [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) or morphine inhibited voltage-dependent calcium currents in DRG neurons to about 50%. For 2 mM DAMGO: WT 45.81.8%, n=32, KO 44.62.5%, n=15, KD 50.15.9%, n=6; for 2 mM morphine: WT 55.82.7%, n=24, KO 46.32.3%, n=24. After long-term incubation (6 hrs) with 2 mM DAMGO desensitisation was observed in WT resulting on only 17.92.4% block (n=9). mu-opioid inhibition of voltage-gated calcium channels was desensitized to a smaller extent after 6 hrs incubation with 2 mM morphine (36.13.2% block, n=12). DRGs from KO and KD mice did not show a desensitisation after long-term incubation with DAMGO. Furthermore, DAMGO-induced desensitisation of WT DRGs could be abolished when cells were incubated for 30 minutes with the PKC blocker bisindolylmaleimide I. Our experiments on mu-opioid inhibition of calcium channels suggest that after long-term incubation with mu-opioid agonist PI3Kg controls the amount of Gßg subunits capable of interfering with the calcium channels, via a PKC-dependent pathway. For this task the kinase activity of PI3Kg is required because DRG neurons from KO and KD mice showed the same lack of mm-opioid desensitisation.