AbstractOur laboratory has reported previously the characteristics of specific AVP binding to rat hippocampal synaptic membranes (SPM) in the presence of Ni2+[Costantini MG, Pearlmutter AF:J Biol Chem259:11739–11745, 1984]. We extended our investigation to determine the effects of Ni2+, (AVP), and AVP analogs on SPM protein phosphorylation. Ni2+(5 mM) caused a dramatic reduction in phosphorylation of most SPM phosphoproteins. The most prominent protein which is phosphorylated in SPM has a molecular weight of 48 kilodaltons (KDa) and has been named B50 or F1; this protein shows altered phosphorylation in vitro in response to long‐term potentiation in vivo as well as changes induced by exposure of SPM to ACTH (1–24), dopamine, and somatostatin. AVP and related peptides reduced phosphorylation of this presynaptic phosphoprotein in the following order of potency: AVP = oxytocin>DG‐AVP>dDAVP>d(CH2)5Tyr(Me) AVP = [pGlu4, Cyt6]AVP‐(4–9). Except for the pressor antagonist d(CH2)5Tyr(Me)AVP, this corresponds to their relative efficacy in displacing3H‐AVP from high‐affinity specific binding sites on rat hippocampal synaptic membranes. Ni2+did not alter the degree of inhibition caused by the peptides. When SPM were treated with AVP after the attainment of maximum32P incorporation, AVP inhibited dephosphorylation over a 30‐min period. Our results show that AVP can alter both phosphorylation and dephosphorylation of hippocampal SPM phosphoproteins in vitro; the direction of these effects depends upon experimental conditions. Since B50/F1 is known to be a substrate for protein kinase C, AVP may act by inhibition of protein kinase C activity, either direc