Y.Murayama, I.Fujita, and M.Kato

  Long-term changes in synaptic efficacy known as long-term potentiation (LTP) and depression (LTD) are possible cellular bases for learning and memory.  To determine whether LTP and LTD are elicited in primates, the animal group having the most developed learning abilities, we examined three Japanese macaque monkeys anesthetized with a mixture of N2O and O2.
  Extracellular field potentials were recorded in layer 2?3 of the inferior temporal cortex (TE) and of the striate cortex (V1) which are the final and first stages of the visual cortical pathway for object recognition.  Stimulating electrodes were placed 0.5`1mm lateral to the recording site to activate horizontal axons in layer 2/3.  After a high-frequency conditioning stimulus, the amplitude of field potentials evoked by a single shock in the TE gradually increased over 50-70 min to a maximum, and this potentiation lasted for more than 3 hours.  An identical stimulus protocol did not potentiate field potentials in V1, but instead caused a depression which developed over 5-10 min and remained stable until the end of the recording.  Steady state depression with a slow initial development suggests that this depression is a physiological phenomenon not caused by an experimental artifact such as a damage to the stimulating site.  In both areas, field potentials evoked from an unconditioned pathway were not changed in their amplitude and wave  form.  These findings demonstrate that homosynaptic LTP and LTD are elicited in the adult primate neocortex, and suggest that the TE and V1 differed from each other in their synaptic plasticity.  Supported by grants from the Uehara Memorial Foundation and the Inamori Foundation.