Activation of presynaptic cAMP-dependent protein kinase is required for induction of cerebellar long-term potentiation

Cerebellar long-term potentiation (LTP) is a persistent increase in the str ength of the granule cell-Purkinje neuron synapse that occurs after brief s timulation of granule cell axons at 2-8 Hz. Previous work has indicated tha t cerebellar LTP induction requires presynaptic Ca influx, stimulation of C a-sensitive adenylyl cyclase, and activation of PKA. The evidence implicati ng PKA has come from bath application of drugs during LTP induction, an app roach that does not distinguish between PKA activation in the presynaptic o r postsynaptic cell. Although bath application of PKA inhibitor drugs (KT57 20, Rp-8CPT-cAMP-S) blocked LTP induction in granule cell-Purkinje neuron p airs in culture, selective application to granule cell or Purkinje neuron s omata via patch pipettes did not. We hypothesized that presynaptic PKA acti vation is required for LTP induction but that drugs applied to the granule cell soma cannot diffuse to the terminal within this timescale. To test thi s hypothesis, we transfected cerebellar cultures with an expression vector encoding a peptide inhibitor of PKA [Rous sarcoma virus (RSV)-protein kinas e A inhibitor (PKI)]. Transfection of RSV-PKI into presynaptic granule cell s, but not postsynaptic Purkinje neurons or glial cells, blocked LTP induct ion produced by either synaptic stimulation or an exogenous cAMP analog. An expression vector encoding a control peptide with no PKA inhibitory activi ty was ineffective. These results show that induction of cerebellar LTP req uires a presynaptic signaling cascade, including Ca influx, stimulation of Ca-sensitive adenylyl cyclase, and activation of PKA, and argue against a r equirement for postsynaptic Ca signals or their sequelae.