Effects of norepinephrine and serotonin upon spontaneous activity and responses to mossy fiber stimulation of CA3 neurons in hippocampal slices.

Effects of norepinephrine (NE) and 5-hydroxytryptamine (5-HT) upon spontaneous activity and responses to mossy fiber stimulation (mfs) were tested in 192 units of the field CA3 in the guinea pig and rat hippocampal slices. The drugs were added to the incubating medium or ejected by pressure from a micropipette. After NE superfusion firing rate increased in 52% of the reactive units, while activity of 48% was suppressed. The direction of the effect strongly correlated with pattern of spontaneous activity: only the cells with 'complex discharges' (short bursts of 2-4 spikes with attenuation of amplitude) were suppressed by NE; the cells with single spikes increased the level of activity. Similar excitatory effects of NE were observed in all units (n = 14) with single spike activity recorded in the field CA1. 5-HT increased activity in 30% of the reactive units and suppressed it in 70% of them. Some of the cells which were suppressed by 5-HT, were excited by NE. In more than a half of the units tested, 5-HT led to prolonged (up to 30-40 min) increase of the level of background activity irrespective of the initial excitatory or suppressive action; periodic grouped discharges appeared in some units under the influence of 5-HT. The response to mfs usually changed in the same direction as the level of background activity during application of NE and 5-HT, though some exceptions were observed in both cases. Prolonged (up to 30-40 min) facilitation of responses to mfs was present after application of 5-HT.