Phosphatidylinositol 4,5-bisphosphate (PIP ) modulates afterhyperpolarizations in oxytocin neurons of the supraoptic nucleus.

Key Points: Afterhyperpolarizations (AHPs) generated by repetitive action potentials in supraoptic magnocellular neurons regulate repetitive firing and spike frequency adaptation but relatively little is known about PIP 's control of these AHPs. We examined how changes in PIP levels affected AHPs, somatic [Ca ] , and whole cell Ca currents. Manipulations of PIP levels affected both medium and slow AHP currents in oxytocin (OT) neurons of the supraoptic nucleus. Manipulations of PIP levels did not modulate AHPs by influencing Ca release from IP -triggered Ca stores, suggesting more direct modulation of channels by PIP . PIP depletion reduced spike-evoked Ca entry and voltage-gated Ca currents. PIP appears to influence AHPs in OT neurons by reducing Ca influx during spiking.

Abstract: Oxytocin (OT)- and vasopressin (VP)-secreting magnocellular neurons of the supraoptic nucleus (SON) display calcium-dependent afterhyperpolarizations (AHPs) following a train of action potentials that are critical to shaping the firing patterns of these cells. Previous work demonstrated that the lipid phosphatidylinositol 4,5-bisphosphate (PIP ) enabled the slow AHP component (sAHP) in cortical pyramidal neurons. We investigated whether this phenomenon occurred in OT and VP neurons of the SON. Using whole cell recordings in coronal hypothalamic slices from adult female rats, we demonstrated that inhibition of PIP synthesis with wortmannin robustly blocked both the medium and slow AHP currents (I and I ) of OT, but not VP neurons with high affinity. We further tested this by introducing a water-soluble PIP analogue (diC -PIP ) into neurons, which in OT neurons not only prevented wortmannin's inhibitory effect, but slowed rundown of the I and I . Inhibition of phospholipase C (PLC) with U73122 did not inhibit either I or I in OT neurons, consistent with wortmannin's effects not being due to reducing diacylglycerol (DAG) or IP availability, i.e. PIP modulation of AHPs is not likely to involve downstream Ca release from inositol 1,4,5-trisphosphate (IP )-triggered Ca -store release, or channel modulation via DAG and protein kinase C (PKC). We found that wortmannin reduced [Ca ] increase induced by spike trains in OT neurons, but had no effect on AHPs evoked by uncaging intracellular Ca . Finally, wortmannin selectively reduced whole cell Ca currents in OT neurons while leaving VP neurons unaffected. The results indicate that PIP modulates both the I and I in OT neurons, most likely by controlling Ca entry through voltage-gated Ca channels opened during spike trains.