Induction Properties of Synaptic Plasticity in Rat Prefrontal Neurons

In rat prelimbic (prefrontal) slices, layer I–II to layer V pyramidal neuron glutamatergic synapses show long-term depression (LTD) and potentiation (LTP) of synaptic strength. First, LTD is induced by high-frequency synaptic stimuli (100 pulses at 50 Hz, 4 times) in the presence of dopamine. Our analyses show that the synaptic responses and postsynaptic depolarization during high-frequency stimuli are larger in the presence of dopamine than in its absence. These dopamine effects are N-methyl-d-aspartate (NMDA) receptor-dependent. Second, LTP is induced by the same stimuli, in the presence of dopamine, if the synapses are previously exposed to dopamine. Interestingly, the synaptic responses and depolarization during the LTP-inducing high-frequency stimuli are smaller than those in control and LTD conditions. Third, LTP can also be induced by short burst-type stimuli without dopamine (5–10 pulses at 50 Hz, 5–6 times). With the short burst stimuli, prelimbic neurons show little inter-burst synaptic fatigue and fire to each burst episode, a property not seen in any of the groups in which the long trains of stimuli were applied. These data suggest that there are separate induction mechanisms for synaptic plasticity in rat prefrontal neurons. We will then discuss on functional implications of these LTP and LTD.