Potentiation of dentate synapses initiated by exploratory learning in rats: dissociation from brain temperature, motor activity, and arousal.

Abstract

Certain kinds of learning may be related to potentiation of transmission at specific hippocampal synapses. We investigated whether transmission across the perforant-path/granule-cell synapses of the dentate gyrus is facilitated when rats are learning about novel objects in an open field during exploration. Such studies are complicated by the sensitivity of hippocampal field potentials to brain temperature change. To control for this, we have recorded both brain temperature and field potentials and compared potentials sampled during exploration with potentials taken at corresponding brain temperature in a passive warming situation, with the animals at rest. Relative to these reference potentials, both the f-EPSP slope and the population spike were elevated while the rats explored. The potentiation reached its maximum within < 5 sec after the exploration began. During the first 2 min, the f-EPSP slope was enhanced by 6.5% relative to the control values. The potentiation then decayed, reaching the reference values after 20-30 min of exploration. Significant potentiation required exploration above a certain minimum intensity. Control experiments showed that the changes were neither mimicked by arousal in response to aversive stimuli nor by motor activity. It is suggested that the facilitated transmission across the perforant-path/dentate synapses may be involved in learning during exploration.