LEARNING-RELATED CHANGES IN HIPPOCAMPAL FIELD POTENTIALS

It is commonly believed that learning is based on modifications of syn aptic strength. Much of the evidence for this comes from the observati on that blockade of processes necessary for induction of long-term pot entiation in the hippocampus also blocks certain forms of learning. As such correlations may have many causes, an understanding of the mecha nisms for memory formation might also profit from direct recording of cellular activity in learning tasks. Field potential recording represe nts one such approach. Although changes in field potentials are unlike ly to uncover modifications in synaptic strength related to the storag e of memory, any general facilitation (or reduction) of synaptic trans mission taking place in populations of neurons during the acquisition stage might be picked up by a field measure. One problem related to th e approach is that field potentials are heavily affected by nonlearnin g factors. It is shown that field potentials in the hippocampus are hi ghly sensitive to changes in brain temperature and that a significant part of the increase in field excitatory postsynaptic potentials (f-EP SPs) during learning reflects warming of the brain. Temperature-relate d changes in synaptic transmission do not affect the efficiency of spa tial learning, as the acquisition of a water-maze task is equally effi cient at low (30-32 degrees C) and high (37-39 degrees C) brain temper atures. Subtraction of the temperature component of the held potential alterations during learning in an exploration task shows that explora tion is accompanied by a temperature-independent synaptic potentiation as well. Both the f-EPSP and the population spike are increased, and both decay gradually within 15-20 min. It is important to find out whe ther this potentiation reflects learning-related processes and whether such a potentiation is useful to the brain given the apparent 'noise' caused by temperature-related physiological changes.