Modulation of hypothalamic NMDA receptor function by cyclic AMP-dependent protein kinase and phosphatases

In the present study we investigated the modulation of hypothalamic NMDA receptor-mediated currents by cyclic AMP-dependent protein kinase (PKA) using the two-electrode voltage-clamp technique in Xenopus oocytes injected with rat hypothalamic mRNA. Application of forskolin, which activates PKA by means of cyclic AMP stimulation, caused a transient increase of NMDA-induced currents, whereas the inactive forskolin analogue 1,9-dideoxyforskolin had no effect. Incubation of oocytes with a membrane-permeable analogue of cyclic AMP, 8-bromoadenosine 3',5' -cyclic monophosphate, potentiated NMDA responses even more prominently than with forskolin. NMDA-induced currents recorded from Xenopus oocytes injected with cRNA encoding the NMDA receptor subunits NR1, NR2A, and/or NR2B, mainly found in rat hypothalamus, were not affected by PKA activation but were increased by protein kinase C (PKC) stimulation. It is interesting that inhibition of endogenous protein phosphatase 1 and/or 2A by calyculin A resulted in a similar enhancement of hypothalamic NMDA-induced currents. Preinjection of oocytes with calyculin A impeded the PKA- but not the PKC-mediated potentiation of hypothalamic NMDA-induced currents. We propose the involvement of an additional third messenger in the PKA effect, which acts most likely via the inhibition of tonically active protein phosphatase 1 and/or 2A.