Accepted in revised form 18 December 2001; recieved 15 August 2001.
In cats with chronically implanted electrodes in different brain structures study was made of the effect of postconvulsive depression of the hippocampal theta rhythm before and after learning sessions on the elaboration and extinction of instrumental alimentary reflexes based on sound discrimination; the effect of paradoxical sleep deprivation (PSD) by the method of non-emotional awakening, which prevents the development of the hippocampal theta rhythm on the elaboration and extinction of instrumental alimentary reflexes based on sound discrimination; the effect of lesion of various septal areas on (a) general behavior, (b) the sleep-wakefulness cycle, (c) the elaboration and extinction of these reflexes and (d) the conditioned delayed responses; the effect of functional elimination of the hippocampus by way of induction of local epileptiform discharges in it on the elaboration and extinction of instrumental conditioned alimentary reflexes and on conditioned delayed responses. The data were statistically processed.
Analysis of our experimental results allows to come to the following conclusion: 1. Depression of the hippocampal theta rhythm by way of repeated epileptiform discharges both prior to and following learning sessions leads to a considerable retardation of elaboration of instrumental alimentary reflexes in the initial sessions but does not prevent subsequent elaboration of discrimination of conditioned signals. 2. Depression of the hippocampal theta rhythm by way of inducing in it repeated epileptiform discharges before and after learning sessions has no considerable effect on the extinction of conditioned instrumental alimentary reflexes. 3. PSD in the intersession period and thus prevention of the development of the hippocampal theta rhythm without any accessory changes in the functional state of the brain has no significant effect on the elaboration of instrumental alimentary reflexes based on sound discrimination. 4. Lesion of the medial septum, causing a complete abolition of the hippocampal theta rhythm, has no tangible effect either on the structure of the sleep-wakefulness cycle or on the cat's general behavior: the signs of the so-called "septal syndrome" are lacking. The rate of elaboration of instrumental alimentary reflexes based on sound discrimination and extinction of these reflexes remains unchanged. Yet, lesion of the medial septum results in the disturbance of delayed responses to conditioned signals. 5. Massive lesion of the septum, when, apart from the medial area, the lateral one is also involved, considerably alters the structure of the sleep-wakefulness cycle (wakefulness becoming longer and PS shorter) and the character of general behavior. The cats are rendered more agitated, aggressive, diverted and hyperphagic. Motor activity also augments significantly. All these changes are the components of the "septal syndrome". Elaboration of instrumental alimentary reflexes in such animals is hindered, especially affected is the elaboration of discrimination of conditioned sounds because of the development of perseverative movements. The process of extinction of instrumental alimentary reflexes is sharply retarded and the delayed responses to conditioned signals are completely disturbed. 6. Functional elimination of the hippocampus by way of inducing in it epileptiform discharges in various intervals (1 min, 25 sec, immediately) after each pairing of a conditioned signal with food reinforcement facilitates the elaboration of instrumental alimentary reflexes based on sound discrimination. Such epileptiform discharges accelerate also the extinction of conditioned alimentary reflexes when they are induced immediately after the animal approaches an empty feeder, and has no effect if they are induced l min after approach. 7. Epileptiform discharges in the hippocampus induced immediately or 2-5 min after the onset of this or another conditioned signal disturb the delayed responses and have no effect if they are induced at long intervals.
The role of the septo-hippocampal system in the regulation of learning and memory is discussed on the basis of the foregoing. The view is advanced and substantiated according to which the descending influence of the hippocampus via the lateral septum on the activating and motivating structures of the mesodiencephalon is more important in the regulation of these processes than is the ascending influence of the mesodiencephalon via the medial septum, as a result of which the theta rhythm is formed at the hippocampal level.
Moreover, it has been revealed that the septo-hippocampal system plays a crucial role in the regulation of short-term operative memory responsible for delayed conditioned reactions. What is more, in the regulation of memory of this type both the input (the medial septum) and the output (the lateral septum) of the hippocampus are equally important.
Key Words: Hippocampus, Septum, Theta rhythm, Learning, Memory
Correspondence: Oniani Tengiz, Prof.,
Department of Neurobiology of Sleep-Wakefulness Cycle,
I.S. Beritashvili Institute of Physiology, Georgian Academy of Sciences,
14, L. Gotua str., Tbilisi, 380060, Georgia.