The action of norepinephrine in the dentate gyrus: beta-mediated facilitation of evoked potentials in vitro.

The effects of superfusion of norepinephrine (NE) on perforant path (PP) evoked potentials in the dentate gyrus were evaluated in the rat hippocampal slice preparation. Superfusion of NE (10 microM) produced a facilitation of the PP evoked responses. Facilitation of the synaptically-evoked responses was expressed in the field potential as an increase in extracellular excitatory postsynaptic potential (EPSP) (117% of control), a decrease in population spike onset latency (94% of control) and an increase in population spike amplitude (131% of control). In 24% of the slices the facilitation of the population spike amplitude lasted longer than 30 min. Isoproterenol, a beta-agonist, mimicked NE effects while timolol, a beta-antagonist, blocked them. Facilitation of the population spike amplitude by NE could not be accounted for solely by the increase in EPSP slope also produced by NE. Superfusion of NE did not produce facilitation of the antidromically evoked field potentials, but in 4 of 8 slices produced a small decrease. NE effects were activity-independent, since the subsequently evoked PP responses were facilitated even when the PP was not concurrently stimulated during superfusion with NE.