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Electrical activity in the hippocampal formation of the rat : role of ascending monoamine-containing systems Assaf, Souhile Y.


The effects of electrical stimulation of the monoamine-containing nuclei, the median raphe (MR) and the locus coeruleus (LC), on extracellularly recorded hippocampal electrical activty were studied in urethane anaesthetized rats. The following observations suggested that serotonergic and noradrenergic systems originating in MR and LC, respectively, modulate rhythmical slow activity (RSA) recorded in the dentate gyrus and/or the population response of granule cells to stimulation of the perforant path (PP) input. 1. RSA (3-7 Hz,) recorded in the dentate gyrus (DG) was related to the bursting discharge pattern of medial septal (MS) neurones. Electrolytic or kainate lesions of the MS abolished RSA thereby supporting the conclusion that rhythmical hippocampal activity was initiated by MS neurones. 2. Stimulation of MR resulted in disruption of the bursting discharge of MS neurones and desynchronization of RSA. Depletion of forebrain serotonin following pretreatment with para-chlorophenylalanine (p-CPA) eliminated these responses while quipazine (1 mg/kg), a serotonin agonist, mimicked the effects of MR stimulation. These data confirmed that a serotonin-containing system mediates the disruption of bursting discharge in MS and desynchronization of hippocampal electrical activity. 3. LC stimulation evoked RSA. Intracerebral injections of 6-hydroxydopamine (6-OHDA) which depleted hippocampal noradrenaline (NA) did not eliminate this response. In addition, electrolytic lesions of the LC did not eliminate RSA. These data suggested that NA-containing afferents to the hippocampal formation are not essential for the generation of rhythmical activity. 4. Stimulation of the PP resulted in synaptic potentials in the dendritic layer of DG and population spikes recorded in the granule cell body layer. Conditioning stimulation of ME or LC increased the amplitude of the population spike evoked by a test PP pulse without altering the amplitude or rate of rise of the synaptic potential. The effects of MR and LC stimulation were blocked by p-CPA and 6-OHDA, respectively, suggesting that these monoamine-containing nuclei specifically influenced the responsiveness of dentate granule cells. 5. The observations that changes in the amplitude of the population spike, such as those observed following stimulation of MR and LC, could occur in the absence of changes in the synaptic potential were confirmed using an additional afferent to the dentate via the commissural projection. These data indicated that extrinsic afferents to DG may potentiate the discharge of a population of granule cells without enhancing synaptic potentials. On the basis of the above data, it was concluded that pharmacologically distinct neuronal systems originating in MR and LC modulate electrical activity in the hippocampal formation.

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