Decrease in both choline acetyltransferase activity and EEG patterns in the hippocampal formation of the rat following septal macroelectrode implantation

Effects of RU 52583, an alpha 2-antagonist, on memory in rats with excitotoxic damage to the septal area

The anti-amnesic action of RU 52583, an alpha 2-adrenergic receptor antagonist, was evaluated through performance of spatial tasks in a radial maze by rats with N-methyl-D-aspartic acid (NMDA) lesion of the medial septal (MS) nuclei. Memory performance of lesioned or sham-operated rats was evaluated by measuring reference memory as long-term maintenance of an acquired performance and working memory or memory for recent events. The lesion: a produced significant impairments of the animals' memory performance, b) significantly reduced the sodium-dependent high-affinity choline uptake in the hippocampal formation, and c) deeply disrupted cholinergic hippocampal theta waves. Oral administration of RU 52583 at 1 and 2 mg/kg (tested doses: 1-5 mg/kg) prior to performance of the task markedly reduced memory impairments, whereas idazoxan, another alpha 2-adrenergic receptor antagonist, had no effect at tested doses (2-5 mg/kg). Cholinergic drugs--arecoline at 0.1 and 1 mg/kg (tested doses: 0.05-1 mg/kg) and physostigmine at 0.02 and 0.1 mg/kg (tested doses: 1, 2, and 5 mg/kg)-administered intraperitoneally showed a tendency to alleviate memory deficits. The present results show that the alpha 2-adrenergic antagonist RU 52583 possesses cognition-enhancing properties in rats with damage to the septohippocampal system.

Effects of medial and lateral septal lesions on acquisition of a place and cue radial maze task

Rats with lesions limited to either the medial septal (MS) or dorsolateral septal (LS) nuclei were trained on a place and cue version of the radial maze using a procedure that permits determination of both reference memory (RM) and working memory (WM). Following training the presence or absence of hippocampal theta for MS and LS groups was determined. The results showed that both MS- and LS-lesioned rats were impaired in acquisition. Specifically, the pattern of results indicated a general impairment in working memory that was found on both the place and cue tasks together with impaired acquisition of the place (but not the cue) task. While both septal lesion groups evidenced a similar impairment, rats in the MS group made more errors than LS rats on several components of the tasks. Electroencephalographic recordings revealed that hippocampal theta was not affected in the LS group but was abolished in rats with MS lesions. Thus, it appears that the functional role of the septo-hippocampal system cannot be fully described by a single mechanism or process but rather several processes seem to be affected when the septal area is damaged.

Changes in hippocampal acetylcholine and glutamate extracellular levels during soman-induced seizures: Influence of septal cholinoceptive cells

The changes in extracellular acetylcholine and glutamate levels were determined, during the course of seizures induced by soman, an irreversible inhibitor of acetylcholinesterase, in the CA1 hippocampal area of rats previously injected with atropine or normal saline into septum. The marked increases observed in soman-treated animals were abolished in rats receiving atropine. These data strongly suggest that, during soman intoxication, septal cholinoceptive cells play a key role in controlling the release of acetylcholine and glutamate in hippocampus. The mechanisms underlying this phenomenon are discussed.

Effects of transient cerebral ischemia on the hippocampal dentate theta (theta) profile in the acute rat: a study 4-5 months following recirculation

This study mainly describes the long-term effects of 20 min of cerebral ischemia on the profile of the presumed cholinergic theta rhythm in the rat dorsal hippocampal formation during ether anesthesia and injection of the muscarinic agonist agent arecoline. The experimental data were collected 4-5 months after ischemia. They show that ischemia results in a statistically significant reduction in both superficial and deep theta recorded from the CA1 area of the hippocampus and the dentate gyrus, respectively. Amplitude reduction is similar for both rhythms and co-varies positively with the extent of CA1 stratum pyramidale damage which, from light microscope observation, appeared to be the major neuroanatomical consequence of ischemic insult in the dorsal hippocampal formation. The medial septal nucleus-diagonal band of Broca complex involved in theta generation did not suffer visible anatomical damage. Moreover, no significant alteration in the spatial distribution and the density of hippocampal dentate acetylcholinesterase reaction product was seen in ischemic animals. These histological data were statistically confirmed by computerized image analysis. Finally, this is the first investigation to show that transient interruption of cerebral blood flow results in a long-lasting alteration of theta rhythm which is probably the major aspect of the basic activity of the hippocampal formation. Thus, the present findings obtained in the acute rat at 4-5 months postischemia confirm and extend, in most respects, our previous results collected in the chronic animal 2-29 days following 4-vessel occlusion. Possible significance of these findings for the hypothesis of the dependent generation sites of superficial and deep thetas in the hippocampus assumed to be crucial in learning and memory, is discussed.

Modulation of the afferent input to the septal neurons by cholinergic drugs

The effects of cholinergic drugs upon the evoked activity of extracellularly recorded neurons of the medial septal nucleus-nucleus of the diagonal band (MS-DB complex) were tested in unanesthetized rabbits. Electrical stimulation of MFB resulted in entrainment of the background theta-cycles in the neurons with strong rhythmic discharge (types I and II). Phase-locking of the background theta-cycles to the stimulus occurred 'by the burst', or 'by the pause' within the theta-range of frequencies (3-12 Hz). Single-spike responses, following up to 30 Hz and more, were also evoked by MFB stimulation, especially in the cells with weak theta-modulation (type III) or without it (type IV). Injection of physostigmine increased background theta-modulation of neuronal activity and simultaneously blocked or diminished responses to repetitive MFB stimulation in 82% of the MS-DB units, independent of their type of response. Driving of theta-cycles both 'by the burst' and 'by the pause' was ineffective or drastically reduced. Single spike responses disappeared or became unstable, though their minimal latencies did not change. Initial inhibitory responses were blocked or became significantly shorter. Antimuscarinic drugs, scopolamine and atropine, which abolished theta-modulation in many MS-DB units, restored responses and sometimes enhanced them. Repetitive stimulation of the MFB in this condition was effective up to the high frequencies, well beyond the theta-range. Thus, the majority of the MS-DB units did not respond to the afferent stimuli during prominent theta-activity evoked by physostigmine. The role of the septal cholinergic system in gating of afferent input during the theta-state and its importance for learning and memory is suggested.

Control of the neuronal rhythmic bursts in the septal pacemaker of theta-rhythm: effects of anaesthetic and anticholinergic drugs

The drugs, described as blocking the high-frequency (pentobarbital) or low-frequency (scopolamine, atropine) theta rhythm of the hippocampal electroencephalogram, were tested upon the rhythmically bursting septal cells. Three groups of chronic, unanaesthetized rabbits were used for the experiments: with intact septum; with septohippocampal disconnection; with complete basal undercutting of the septum, depriving it of ascending brainstem influences (MFB lesion). While the frequency and other parameters of theta bursts did not differ in the first two groups (5.2-5.5 Hz), in MFB-lesioned septum their frequency was significantly lower (3.5 Hz). Intravenous injection of pentobarbital suppressed theta bursts in some cells with unstable, periodic rhythmic activity and lowered the frequency of the bursts in continuously bursting cells. The parameters of bursts in intact and hippocampectomized septum under pentobarbital did not differ from those of undercut septum in undrugged state. Acetylcholine-blocking drugs suppressed theta modulation in some intermittently bursting cells, but only slightly decreased regularity of the bursts in some cells with continuous theta bursting even in sublethal doses; physostigmine has the opposite effect. Neither scopolamine and atropine, nor physostigmine influenced frequency of theta bursts in any way. Sensory or reticular stimulation could temporarily restore both the theta rhythm of hippocampal EEG and the rhythmic bursting of some septal cells under pentobarbital or anticholinergic drugs. On the basis of the experiments a unitary concept of theta rhythm origin is proposed. Pentobarbital influences ascending excitatory input to the septum, which results in a decrease of the burst frequency in the limited group of septal cells, regarded as endogenous bursting pacemakers, and in restriction of the population of high-threshold secondary rhythmic cells, synaptically involved in the rhythmic process. Anticholinergic drugs do not influence the pacemaker cells, but block intraseptal and septohippocampal cholinergic transmission. Both cholinergic and non-cholinergic neurons projecting to the hippocampus exist among septal cells synaptically involved in the rhythmic activity.