Mechanisms of norepinephrine actions on hippocampal pyramidal cells in vitro

Responses of pyramidal cells to topical application of norepinephrine (NE) were studied by intracellular recording in hippocampal slices in vitro. Norepinephrine hyperpolarized CA1 cells. Simultaneously, there was a decreased response to constant hyperpolarizing and depolarizing current pulses. The number of spikes evoked by constant depolarizing pulses was reduced. Spontaneous activity, when present, was reduced or abolished. The response to depolarizing current pulses was reduced more than the response to hyperpolarizing current pulses. The reduction of the depolarizing response was minimal for the first 6-8 msec of the pulse, whereafter it increased. The effects persisted after blocking synaptic transmission with low calcium-high magnesium concentrations in the incubation fluid. We conclude that the hyperpolarization is most likely due to a conductance increase. The mechanism behind the reduced response to depolarizing current pulses is discussed.