Five minutes after the induction of long-term potentiation (LTP) in the intact hippocampal formation of chloralose/urethane-anesthetized rats there was a selective increase in the in vitro phosphorylation state of a 47-kDa band (designated Protein F1). When low frequency, nonpotentiating stimulation was used, no change in Protein F1 was observed. LTP had no significant effect on other phosphoproteins measured at the 5-min time point. In vitro phosphorylation of Protein F1 5 min after LTP was directly related to the change in synaptic efficiency (r = +0.86, p < .01). The calcium-dependent, synaptically localized Protein F1 may be the same as the brain-specific Protein B-50. The regulation of the plastic LTP response may involve the novel multifunctional phospholipid-dependent enzyme. Protein Kinase C, which has been shown to phosphorylate B50.
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