Modulation of hippocampal calcium signalling and plasticity by serine/threonine protein phosphatases

Kinases and phosphatases act antagonistically to maintain physiological phosphorylation/dephosphorylation at numerous intracellular sites critical for neuronal signalling. In this study, it was found that inhibition of serine/threonine phosphatases by exposure of hippocampal slices to okadaic acid (OA) or cantharidin (CA; 100 nmol/L) for 2 h resulted in reduced basal synaptic transmission and blocked the induction of synaptic plasticity in the form of long-term potentiation as determined by electrophysiological analysis. Fura-2 Ca²⁺ imaging revealed a bidirectional modulation of N-methyl-d-aspartate (NMDA) -mediated Ca²⁺ responses and reduced KCl-mediated Ca²⁺ responses in neonatal cultured hippocampal neurons after phosphatase inhibition. While OA inhibited NMDA-induced Ca²⁺ influx both acutely and after incubation, CA-enhanced receptor-mediated Ca ²⁺ signalling at low concentrations (1 nmol/L) but reduced NMDA and KCl-mediated Ca²⁺ responses at higher concentrations (100 nmol/L). Changes in Ca²⁺ signalling were accompanied by increased phosphorylation of cytoskeletal proteins tau and neurofilament and the NMDA receptor subunit NR1 in selective treatments. Incubation with OA (100 nmol/L) also led to the disruption of the microtubule network. This study highlights novel signalling effects of prolonged inhibition of protein phosphatases and suggests reduced post-synaptic signalling as a major mechanism for basal synaptic transmission and long-term potentiation impairments.