Spontaneous activity and sensory responses of hippocampal neurons during persistent theta-rhythm evoked by median raphe nucleus blockade in rabbit

Spontaneous activity and responses to sensory stimuli were analysed in the hippocampal CA1 neurons of chronic unanesthetized rabbits before and after reversible functional blockade of the median raphe nucleus and medial septa l area by local microinjections of anesthetic lidocaine. This evoked, corre spondingly, persistent theta rhythm and its complete blockade for about 30 min. The results were compared to the neuronal data obtained earlier in the experiments with cholinergic drugs modulating expression of theta rhythm. Intra-median raphe nucleus injection of lidocain evoked uniform increase of discharge rate in the hippocampal neurons with low and high spontaneous ac tivity. Theta modulation of neuronal activity had increased regularity and frequency (by 0.5-2.0 Hz) and appeared in additional group of the neurons s imultaneously with expression of persistent theta in the hippocampal electr oencephalogram. Sensory responsiveness of the hippocampal neurons was drast ically decreased (45% of the responses preserved). Reactions of all types w ere blocked, diminished, or inverted, but inhibitory responses were the mos t severely affected. Injection of lidocaine into medial septal area also blocked all brain stem afferents ascending to the hippocampus via medial septal area, thus, totall y depriving hippocampus of brainstem-septal input. However, besides the tot al absence of theta modulation, spontaneous activity in majority of neurons was not significantly changed. Responsiveness to sensory stimuli also rema ined relatively high (77% of the responses preserved); on-effects were espe cially resistant to medial septal area blockade. Comparison of spontaneous and evoked activity in two theta states (physostigmine and median raphe nuc leus blockade) revealed striking similarity of all characteristics, which s uggested that theta-suppressing influences of median raphe nucleus (presuma bly serotonergic) are realized primarily through the control of cholinergic septo-hippocampal theta-generating mechanism. However, as the frequency of theta rhythm does not depend on it, an additional effect of disinhibition of activating reticular formation by the median raphe nucleus suppression i s suggested. The data confirm that theta rhythm may be regarded as active Filter in the information processing by the hippocampal neurons. (C) 1999 IBRO. Published by Elsevier Science Ltd.