We have sought to elucidate the biochemical mechanisms that underlie the memory 
enhancing properties of the neural peptide vasopressin. Toward that goal we have 
investigated vasopressin induction of calcium signaling cascades, long held to 
be involved in long-term memory function, in neurons derived from the cerebral 
cortex, a brain region associated with long-term memory. Our previous studies 
demonstrated that in cultured cortical neurons, V1a vasopressin receptor (V1aR) 
activation resulted in a sustained rise in intracellular calcium concentration 
that was dependent on calcium influx (Son & Brinton, 1998). To investigate the 
mechanism of V1aR-induced calcium influx, we investigated V1aR activation of the 
calcium channel subtype(s) in cortical neurons cultured from Sprague-Dawley rat 
embryonic day 18 fetuses. The results of these analyses demonstrated that the 
L-type calcium channel blocker nifedipine blocked 250 nM V1 vasopressin receptor 
agonist (V1 agonist)-induced calcium influx. Intracellular calcium imaging 
analyses using fura-2AM demonstrated that blockade of L-type calcium channels 
prevented the 250 nM V1 agonist-induced rise in intracellular calcium 
concentration. These results indicate that the influx of extracellular calcium 
via L-type calcium channels is an essential step in the initiation of the V1 
agonist-induced rise in intracellular calcium concentration. To determine the 
mechanism of V1aR activation of L-type calcium channels, regulatory components 
of the phosphatidylinositol signaling pathway were investigated. The results of 
these analyses demonstrated that V1 agonist-induced calcium influx was blocked 
by both a phospholipase C inhibitor (U-73122) and a protein kinase C inhibitor 
(bisindolylmaleimide I). Further analysis of V1aR activation of protein kinase C 
(PKC) demonstrated that V1 agonist induced PKC activity within 1 min of exposure 
in cultured cortical neurons. These data indicate that in cultured cortical 
neurons, V1aR activation regulates the influx of extracellular calcium via 
L-type calcium channel activation through a protein kinase-C-dependent 
mechanism. The results of these studies provide biochemical mechanisms by which 
vasopressin could enhance memory function. Those mechanisms include a complex 
cascade that is initiated by activation of the phosphatidylinositol pathway, 
activation of protein kinase C, followed by phosphorylation of L-type calcium 
channels to initiate the influx of extracellular calcium to activate a cascade 
of calcium-dependent release of intracellular calcium.