Noradrenergic Enhancement of Long-Term Potentiation at Mossy Fiber Synapses in the Hippocampus

摘要

The study shows that norepinephrine (NE) enhances the magnitude, duration, and probability of induction of long-term potentiation (LTP) at mossy fiber synapses. This modulatory effect of NE on mossy fiber synaptic transmission appears to be dependent on the frequency of the stimulation, that is, NE exerts little or no effect at low rates of stimulation but enhances transmission following high-frequency trains of stimulation. This frequency- or activity-dependent action of NE stands in contrast to NE's reported effects at other excitatory synapses in the hippocampus. Our experiments support the conclusion that the modulation of LTP by NE is mediated by the activation of Beta-adrenoceptors, leading to a stimulation of cyclic AMP in the postsynaptic neuron. We also found that NE, Beta-adrenoceptor agonists, and 8-bromo-cyclic AMP enhanced voltage-dependent calcium conductance mechanisms. A working hypothesis is presented that the enhancement of LTP by NE is through increased calcium channel activity, leading to greater calcium influx in the postsynaptic neuron.