DIFFERENTIAL MODULATION OF CORTICAL SYNAPTIC ACTIVITY BY CALCINEURIN (PHOSPHATASE-2B) VERSUS PHOSPHATASE-1 AND PHOSPHATASE-2A

Reversible protein phosphorylation is thought to play an important reg ulatory role in synaptic neurotransmission. We recently have shown in cultured rat cortical neurons that inhibition of the Ca2+/calmodulin-d ependent phosphatase calcineurin (phosphatase 2B) increases the freque ncy, but not the amplitude, of postsynaptic glutamatergic currents, im plicating a presynaptic site of action for calcineurin. The specific p resynaptic phosphoprotein substrates for calcineurin are unknown, howe ver, calcineurin has been implicated in the control of the Ca2+-indepe ndent phosphatases, phosphatases I and 2A. To determine whether calcin eurin's effects on synaptic transmission are direct or are mediated by changes in phosphatase 1 and/or 2A activities, we used whole-cell vol tage clamp to record spontaneous and miniature excitatory postsynaptic currents in the presence of calyculin A (I mu M in bath solution), a membrane permeant inhibitor of phosphatases 1 and 2A which has no effe ct on calcineurin. Calyculin increased postsynaptic current amplitude without changing current frequency. In these same neurons, subsequent inhibition of calcineurin with cyclosporine A or FK506 had no further effect on current amplitude, but increased current frequency. The incr eased current amplitude seen with calyculin involved a postsynaptic me chanism, since the effect was reproduced by microcystin (10 mu M in pi pette solution), which is a membrane-impermeant inhibitor of phosphata ses 1 and 2A. Thus, in rat cortical neurons, glutamatergic neurotransm ission appears to be frequency-modulated through a presynaptic mechani sm by calcineurin, and amplitude-modulated through a postsynaptic mech anism by phosphatases 1 and 2A.