PTP and UP at a hippocampal mossy fiber-interneuron synapse

The mossy fiber-CA3 pyramidal neuron synapse is a main component of the hip pocampal trisynaptic circuitry. Recent studies, however, suggested that inh ibitory interneurons are the major targets of the mossy fiber system. To st udy the regulation of mossy fiber-interneuron excitation, we examined unita ry and compound excitatory postsynaptic currents in dentate gyrus basket ce lls, evoked by paired recording between granule and basket cells or extrace llular stimulation of mossy fiber collaterals. The application of an associ ative high-frequency stimulation paradigm induced posttetanic potentiation (PTP) followed by homosynaptic longterm potentiation (LTP). Analysis of num bers of failures, coefficient of variation, and paired-pulse modulation ind icated that both PTP and LTP were expressed presynaptically. The Ca2+ chela tor 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) did no t affect PTP or LTP at a concentration of 10 mM but attenuated LTP at a con centration of 30 mM. Both forskolin, an adenylyl cyclase activator, and pho rbolester diacetate, a protein kinase C stimulator, lead to along-lasting i ncrease in excitatory postsynaptic current amplitude. H-89, a protein kinas e A inhibitor, and bisindolylmaleimide, a protein kinase C antagonist, redu ced PTP, whereas only bisindolylmaleimide reduced LTP. These results may su ggest a differential contribution of protein kinase A and C pathways to mos sy fiber-interneuron plasticity. Interneuron PTP and LTP may provide mechan isms to maintain the balance between synaptic excitation of interneurons an d that of principal neurons in the dentate gyrus-CA3 network.