An experimental test of the role of postsynaptic calcium levels in determining synaptic strength using perirhinal cortex of rat.

1. We have investigated the prediction of a relationship between the magnitude of activity-dependent increases in postsynaptic calcium and both the magnitude and direction of synaptic plastic change in the central nervous system. 2. Activity-dependent increases in calcium were buffered to differing degrees using a range of concentrations of EGTA and the effects on synaptic plasticity were assessed. Activity-dependent synaptic plasticity was induced during whole-cell recording in rat perirhinal cortex in vitro. In control conditions (0.5 mM EGTA) low frequency stimulation (LFS; 200 stimuli) delivered to neurones held at -40 or -70 mV induced long-term depression (LTD) or, at -10 mV, induced long-term potentiation (LTP). 3. The relationship between EGTA concentration (0.2 to 10 mM) and the magnitude of LTD was examined. This relationship described a U-shaped curve, as predicted by models of synaptic plasticity. This provides strong evidence that the magnitude of LTD is determined by the magnitude of the increase in intracellular calcium concentration. 4. LFS paired with depolarisation to -10 mV induced LTD, no change or LTP as activity-dependent postsynaptic calcium levels were allowed to increase progressively by the use of progressively lower concentrations of buffer (10 to 0.2 mM EGTA). 5. We investigated if the lack of plasticity that occurs at the transition between LTD and LTP is due to induction of both of these processes with zero net change, or is due to neither LTD nor LTP being induced. These experiments were possible as LTP but not LTD was blocked by the protein kinase inhibitor staurosporine while LTD but not LTP was blocked by the mGlu receptor antagonist MCPG. At the transition between LTD and LTP, blocking LTP mechanisms did not uncover LTD whilst blocking LTD mechanisms did not uncover LTP. This suggests that the transition between LTD and LTP is due to the lack of induction of both of these processes and also suggests that these two processes are induced independently of one another.