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The firing pattern of septal neurons and the form of the hippocampal theta wave
BRAIN RESEARCH
201
T H E F I R I N G P A T T E R N OF SEPTAL N E U R O N S A N D T H E F O R M OF T H E H I P P O C A M P A L T H E T A WAVE
G. GOGOL/~K, CH. STUMPF, H. PETSCHE ANDJ. ~TERC * Pharmacological and Neurological Institute of the University of Vienna, Vienna (Austria)
(Accepted July 30th, 1967)
INTRODUCTION Several studies carried out previously3,S,10,11 have shown that the hippocampal theta rhythm depends upon the activity of neurons of the dorsal part of the diagonal band of Broca (referred to as BD by Andy and Stephan2). Each unit discharges in bursts synchronous with a particular phase of the theta rhythm which, however, varies from unit to unit. It has been shown histologically 7 that the axons of these neurons project predominantly to the rostral part of the dorsal hippocampus. Moreover, the theta rhythm has been found to spread symmetrically from this part along the hippocampus as a travelling wave 9. It may be asked how this highly synchronous EEG activity can arise in spite of the fact that the hippocampus is impinged upon by burst discharges from different cells which, as a whole, are not synchronous. This apparent contradiction may perhaps be explained on the basis that the essential factor might be the total number of action potentials arriving at the hippocampus from the BD at any instant, rather than the phase relationship of the individual burst to the theta wave. The present series of experiments was undertaken to test this hypothesis. METHODS Extracellular records were taken by steel microelectrodes from a total of 108 BD cells in 8 curarized rabbits. Eserine was repeatedly given intravenously in doses of 0.1-0.3 mg/kg, to maintain a continuous theta rhythm. Details of the experimental procedure have been previously described 10. The hippocampal theta rhythm was recorded bipolarly across the CA1 pyramidal layer. Two techniques of recording were used. (1) Measurement of phase relationship between the first action potential of each burst and the theta wave (Fig. 1): a CAT 1000 (TMC) was triggered externally over a period of 5 min by the first action po* Visiting scientist. Supported by an IBRO/UNESCO Interdisciplinary Temporary Research Team Grant. Present address: Institute of Physiology, Czechoslovak Academy of Sciences, Prague. Brain Research. 7 (1968) 201-207
202
G. GOGOL,~Ket al.
Fig. 1. Measurement of phase relationship between burst and theta wave.
tential of each burst, the hippocampal theta rhythm being fed simultaneously into the analog input of the CAT. 512 addresses, with 1 msec per address were used. The CAT was programmed for signal averaging. Because theta waves are phasecorrelated to the bursts and, hence, to the trigger pulses, the summation of theta waves gives an average value for the theta wave which permits the measurement of the average phase relationship. (2) Determination of frequency distribution of firing within the bursts (sequential histogram) (Fig. 2) : tape recordings of the unit activity were used. Again, over a period of 5 min, each first discharge of the bursts was used to trigger the CAT. After conversion into pulses of constant parameters by an amplitude discriminator, the action potentials were fed into the direct input of the CAT, programmed for sequential histogram recording. 256 addresses, with 0.5 msec per address, were used. RESULTS
Each experiment was plotted as shown in Fig. 3. The histograms of all cells were arranged according to their time relation to the theta wave, and their ordinates were summated at l0 msec intervals. By this procedure, a sinusoidal curve with one minimum and one maximum was obtained. There was a positive correlation between the number of cells recorded per experiment and the amplitude of this sinusoidal curve. The result of a typical experiment in which recordings were made from 22 cells is shown in Fig. 3. The curves obtained from all experiments are asymmetrical, with shapes similar to that of the theta wave, the similarity being more pronounced Brain Research, 7 (1968) 201-207
SEPTUM NEURONS AND THETA WAVE FORM
203
Fig. 2. Determination of histograms (1000 bursts summed up). in experiments in which it was possible to record from a higher number of cells. Because there is some variation in theta frequency between the experiments, the individual curves first had to be standardized before comparing them. For this purpose, the duration of the average theta wave of each experiment was taken as 100 %. Fig. 4 shows the standardized curves of those experiments in which more than l0 units were recorded, arranged in order according to decreasing number of units per experiment. The peaks of the plotted curves are more or less at the same position. The summation curve of Fig. 4 represents the average of all 8 experiments (henceforth referred to as the 'average curve'). This average curve approaches more than any of the individual curves the shape of the theta wave recorded across the pyramidal layer (Fig. 5). As previously 5 observed, the theta wave deviates significantly from a true sine wave; a similar asymmetry is exhibited by the average curve, as well as by the individual curves of each experiment. The peak of the average curve is not exactly synchronous with the corresponding peak of the theta wave but precedes it by about 12 msec. As to the polarity of the theta wave, the more pointed peak which is uppermost in Fig. 5 corresponds to the phase which goes from positive to negative in the basal dendritic layer. Brain Research, 7 (1968) 201-207
G. GOGOL~,K et al.
204
Fig. 3. Presentation of a typical experiment. Histograms of all 22 units recorded were arranged according to their phase relationship to the theta wave, the distance between the ordinate and starting points being equal to their latencies. Arrangement in vertical position is meaningless. Summation of the histograms of all 22 cells gives an asymmetrical curve which represents the firing distribution of BD cells with respect to the theta wave. Portions of the histograms extending beyond the average duration of the theta wave were transferred to the initial part of the diagram as if they were plotted on a cylinder with theta duration as the circumference. Because the moment when each unit starts firing remains constant in relation to the time course of the theta wave, the curve is cyclically recurring and may therefore be extended (dotted). Abscissa: Time in msec. The average duration of the theta wave is marked (247 msec). Ordinate: Discharges/5 min/0.5 msec (CAT). The ratio between the maximal and minimal number of discharges is 4 : 1. DISCUSSION The similarity between the shape of the average curve and that of the theta wave may be interpreted in the following manner: since the average curve represents t h e f r e q u e n c y d i s t r i b u t i o n o f B D u n i t d i s c h a r g e s , it s h o u l d a l s o r e p r e s e n t t h e frequency distribution of PSPs generated at the hippocampal
p y r a m i d a l cells. It m a y
be concluded from the similar asymmetrical form of both curves that at the time of the highest pulse density in the septum (peak of the average curve), the basal dendritic
Brain Research, 7 (1968) 201-207
205
SEPTUM NEURONS A N D THETA WAVE FORM
2 2 u n i t s IL
17 u n i t s Ii
•
i
13 u n i t s I
i ~.
0.2 n~
f
f 12 u n i t s , Average )
theta
period
247 msec 4
Summation
Fig. 4. In each of these 5 experiments, more than 10 BD units were recorded. The 5 curves were obtained in a similar manner to the curves of Fig. 3. The lower right curve represents the average of all BD cells from which recordings were made.
BASAL DENDRITIC LAYER +
~o TOTAL OUTPUT OF A POPULATION OF 108 SEPIUM BO CELLS
l
,
100 MSEC
Fig. 5. Upper curve: A bipolar recording across the pyramidal layer of the hippocampus. Lower curve: The average curve as presented in Fig. 4 drawn repetitively to the same scale as the upper curve, in order to show the correspondence between the total output of the septum BD cells and the frequency and shape of the theta wave.
Brain Research, 7 (1968) 201-207
206
G. GOGOL~,Ke t al.
layer is negative and the apical dendritic layer positive. Moreover, it is obvious that the periodicity of pulse density arriving at the hippocampus from the septum is a determinant factor for the rhythmical hippocampal activity, i.e. the theta rhythml,S,1°,11. Consequently, the theta rhythm may be assumed to be generated in the hippocampus, either by summation of EPSPs at the basal dendritic layer or by summation of IPSPs at the apical dendritic layer. Which of these two possibilities is correct cannot be decided solely on the basis of this study; but the finding that the BD axons projecting towards the hippocampus mainly terminate in the basal dendritic layer7 supports the assumption that EPSPs are the essential source of the theta rhythm. Fujita and Sato 4 also emphasize the significance of rhythmically occurring EPSPs in'the genesis of the hippocampal theta rhythm. The time difference of about 12 msec between the peak of the average curve and the peak of the average theta wave can be accounted for both by the conduction along the septo-hippocampal fibres and by the spreading time of the theta rhythm within the hippocampus. The time required for the projection of pulses from the septum to the hippocampus may be assumed to be of the order of 5 msec and that for the spreading of theta waves from the rostral part of the hippocampus to the site of the recording electrode of the order of l0 msecL This explanation of the genesis of the theta rhythm is offered as an alternative to the existing ones suggested by other authors1, 6. In contrast to other explanations, it takes into account the finding that the rhythmicity of theta activity is already formed in the septum. SUMMARY
Septum units in the rabbit discharge in regular bursts synchronous with the hippocampus theta rhythm but with different phase relationships. Superimposition of time histograms of these bursts with regard to their phase relationships to the theta wave, results in a curve similar to the theta wave in shape. Therefore, the theta rhythm can be interpreted as being determined by the frequency distribution of the firing of septum units.
REFERENCES I ANDERSEN,P., ECCLES, J. C., AND LOYNING, Y., Pathway of postsynaptic inhibition in the hippocampus, J. NeurophysioL, 27 (1964) 608-619. 2 ANDY, O. J., AND STEPHAN, H., The Septum of the Cat, Thomas, Springfield, Ill., 1964. 3 BRI3CKE,F., PETSCHE,H., PILLAT, B., UND DEISENHAMMER,E., Ein Schrittmacher in der medialen Septumregion des Kaninchengehirnes, Pfliigers Arch. ges. Physiol., 269 (1959) 135-140. 4 FUJITA, Y., AND SATO, T., Intracellular records from hippocampal pyramidal cells in rabbit during theta rhythm activity, J. NeurophysioL, 27 (1964) 1011-1025. 5 GREEN, J. D., MAXWELL, D. S., AND PETSCHE, H., Hippocampal electrical activity. III. Unitary events and genesis of slow waves, Electroenceph. clin.rNeurophysiol., 13 (1961) 854-867. 6 KANDEL,E. R., SPENCER,W. A., AND BRINLEY,F. J., Electrophysiology of hippocampal neurons. I. Sequential invasion and synaptic organization, J. Neurophysiol., 24 (1961) 225-242. 7 PETSCHE, H., GOGOL.~K, G., UND STUMPF, CH., Die Projektion der Zellen des Schrittmachers ffir den Thetarhythmus auf den Kaninchenhippocampus, J. Hirnforsch., 8 (1966) 129-136.
Brain Research, 7 (1968) 201-207
SEPTUM NEURONS AND THETA WAVE FORM
207
8 PETSCHE,H., GOGOLAK, A., AND VAN ZWIETEN, P. A., Rhythmicity of septal cell discharges at various levels of reticular excitation, Electroenceph. clin. Neurophysiol., 19 (1965) 25-33. 9 PETSCHE,H., AND STUMPF, CI-t., Topographic and toposcopic study of origin and spread of the regular synchronised arousal pattern in the rabbit, Electroenceph. clin. Neurophysiol., 12 (1960) 589-600. 10 PETSCHE, H., STUMPF,CH., AND GOGOLAK, G., The significance of the rabbit's septum as a relay station between the midbrain and the hippocampus. I. The control of hippocampus arousal activity by the septum cells, Electroenceph. clin. Neurophysiol., 14 (1962) 202-211. l 1 STUMPF,CH., PETSCHE, H., AND GOGOL.~K, G., The significance of the rabbit's septum as a relay station between the midbrain and the hippocampus, lI. The differential influence of drugs upon both the septal cell firing pattern and the hippocampus theta activity, Electroenceph. clin. Neurophysiol., 14 (1962) 212-219.
Brain Research, 7 (1968) 201-207
201
T H E F I R I N G P A T T E R N OF SEPTAL N E U R O N S A N D T H E F O R M OF T H E H I P P O C A M P A L T H E T A WAVE
G. GOGOL/~K, CH. STUMPF, H. PETSCHE ANDJ. ~TERC * Pharmacological and Neurological Institute of the University of Vienna, Vienna (Austria)
(Accepted July 30th, 1967)
INTRODUCTION Several studies carried out previously3,S,10,11 have shown that the hippocampal theta rhythm depends upon the activity of neurons of the dorsal part of the diagonal band of Broca (referred to as BD by Andy and Stephan2). Each unit discharges in bursts synchronous with a particular phase of the theta rhythm which, however, varies from unit to unit. It has been shown histologically 7 that the axons of these neurons project predominantly to the rostral part of the dorsal hippocampus. Moreover, the theta rhythm has been found to spread symmetrically from this part along the hippocampus as a travelling wave 9. It may be asked how this highly synchronous EEG activity can arise in spite of the fact that the hippocampus is impinged upon by burst discharges from different cells which, as a whole, are not synchronous. This apparent contradiction may perhaps be explained on the basis that the essential factor might be the total number of action potentials arriving at the hippocampus from the BD at any instant, rather than the phase relationship of the individual burst to the theta wave. The present series of experiments was undertaken to test this hypothesis. METHODS Extracellular records were taken by steel microelectrodes from a total of 108 BD cells in 8 curarized rabbits. Eserine was repeatedly given intravenously in doses of 0.1-0.3 mg/kg, to maintain a continuous theta rhythm. Details of the experimental procedure have been previously described 10. The hippocampal theta rhythm was recorded bipolarly across the CA1 pyramidal layer. Two techniques of recording were used. (1) Measurement of phase relationship between the first action potential of each burst and the theta wave (Fig. 1): a CAT 1000 (TMC) was triggered externally over a period of 5 min by the first action po* Visiting scientist. Supported by an IBRO/UNESCO Interdisciplinary Temporary Research Team Grant. Present address: Institute of Physiology, Czechoslovak Academy of Sciences, Prague. Brain Research. 7 (1968) 201-207
202
G. GOGOL,~Ket al.
Fig. 1. Measurement of phase relationship between burst and theta wave.
tential of each burst, the hippocampal theta rhythm being fed simultaneously into the analog input of the CAT. 512 addresses, with 1 msec per address were used. The CAT was programmed for signal averaging. Because theta waves are phasecorrelated to the bursts and, hence, to the trigger pulses, the summation of theta waves gives an average value for the theta wave which permits the measurement of the average phase relationship. (2) Determination of frequency distribution of firing within the bursts (sequential histogram) (Fig. 2) : tape recordings of the unit activity were used. Again, over a period of 5 min, each first discharge of the bursts was used to trigger the CAT. After conversion into pulses of constant parameters by an amplitude discriminator, the action potentials were fed into the direct input of the CAT, programmed for sequential histogram recording. 256 addresses, with 0.5 msec per address, were used. RESULTS
Each experiment was plotted as shown in Fig. 3. The histograms of all cells were arranged according to their time relation to the theta wave, and their ordinates were summated at l0 msec intervals. By this procedure, a sinusoidal curve with one minimum and one maximum was obtained. There was a positive correlation between the number of cells recorded per experiment and the amplitude of this sinusoidal curve. The result of a typical experiment in which recordings were made from 22 cells is shown in Fig. 3. The curves obtained from all experiments are asymmetrical, with shapes similar to that of the theta wave, the similarity being more pronounced Brain Research, 7 (1968) 201-207
SEPTUM NEURONS AND THETA WAVE FORM
203
Fig. 2. Determination of histograms (1000 bursts summed up). in experiments in which it was possible to record from a higher number of cells. Because there is some variation in theta frequency between the experiments, the individual curves first had to be standardized before comparing them. For this purpose, the duration of the average theta wave of each experiment was taken as 100 %. Fig. 4 shows the standardized curves of those experiments in which more than l0 units were recorded, arranged in order according to decreasing number of units per experiment. The peaks of the plotted curves are more or less at the same position. The summation curve of Fig. 4 represents the average of all 8 experiments (henceforth referred to as the 'average curve'). This average curve approaches more than any of the individual curves the shape of the theta wave recorded across the pyramidal layer (Fig. 5). As previously 5 observed, the theta wave deviates significantly from a true sine wave; a similar asymmetry is exhibited by the average curve, as well as by the individual curves of each experiment. The peak of the average curve is not exactly synchronous with the corresponding peak of the theta wave but precedes it by about 12 msec. As to the polarity of the theta wave, the more pointed peak which is uppermost in Fig. 5 corresponds to the phase which goes from positive to negative in the basal dendritic layer. Brain Research, 7 (1968) 201-207
G. GOGOL~,K et al.
204
Fig. 3. Presentation of a typical experiment. Histograms of all 22 units recorded were arranged according to their phase relationship to the theta wave, the distance between the ordinate and starting points being equal to their latencies. Arrangement in vertical position is meaningless. Summation of the histograms of all 22 cells gives an asymmetrical curve which represents the firing distribution of BD cells with respect to the theta wave. Portions of the histograms extending beyond the average duration of the theta wave were transferred to the initial part of the diagram as if they were plotted on a cylinder with theta duration as the circumference. Because the moment when each unit starts firing remains constant in relation to the time course of the theta wave, the curve is cyclically recurring and may therefore be extended (dotted). Abscissa: Time in msec. The average duration of the theta wave is marked (247 msec). Ordinate: Discharges/5 min/0.5 msec (CAT). The ratio between the maximal and minimal number of discharges is 4 : 1. DISCUSSION The similarity between the shape of the average curve and that of the theta wave may be interpreted in the following manner: since the average curve represents t h e f r e q u e n c y d i s t r i b u t i o n o f B D u n i t d i s c h a r g e s , it s h o u l d a l s o r e p r e s e n t t h e frequency distribution of PSPs generated at the hippocampal
p y r a m i d a l cells. It m a y
be concluded from the similar asymmetrical form of both curves that at the time of the highest pulse density in the septum (peak of the average curve), the basal dendritic
Brain Research, 7 (1968) 201-207
205
SEPTUM NEURONS A N D THETA WAVE FORM
2 2 u n i t s IL
17 u n i t s Ii
•
i
13 u n i t s I
i ~.
0.2 n~
f
f 12 u n i t s , Average )
theta
period
247 msec 4
Summation
Fig. 4. In each of these 5 experiments, more than 10 BD units were recorded. The 5 curves were obtained in a similar manner to the curves of Fig. 3. The lower right curve represents the average of all BD cells from which recordings were made.
BASAL DENDRITIC LAYER +
~o TOTAL OUTPUT OF A POPULATION OF 108 SEPIUM BO CELLS
l
,
100 MSEC
Fig. 5. Upper curve: A bipolar recording across the pyramidal layer of the hippocampus. Lower curve: The average curve as presented in Fig. 4 drawn repetitively to the same scale as the upper curve, in order to show the correspondence between the total output of the septum BD cells and the frequency and shape of the theta wave.
Brain Research, 7 (1968) 201-207
206
G. GOGOL~,Ke t al.
layer is negative and the apical dendritic layer positive. Moreover, it is obvious that the periodicity of pulse density arriving at the hippocampus from the septum is a determinant factor for the rhythmical hippocampal activity, i.e. the theta rhythml,S,1°,11. Consequently, the theta rhythm may be assumed to be generated in the hippocampus, either by summation of EPSPs at the basal dendritic layer or by summation of IPSPs at the apical dendritic layer. Which of these two possibilities is correct cannot be decided solely on the basis of this study; but the finding that the BD axons projecting towards the hippocampus mainly terminate in the basal dendritic layer7 supports the assumption that EPSPs are the essential source of the theta rhythm. Fujita and Sato 4 also emphasize the significance of rhythmically occurring EPSPs in'the genesis of the hippocampal theta rhythm. The time difference of about 12 msec between the peak of the average curve and the peak of the average theta wave can be accounted for both by the conduction along the septo-hippocampal fibres and by the spreading time of the theta rhythm within the hippocampus. The time required for the projection of pulses from the septum to the hippocampus may be assumed to be of the order of 5 msec and that for the spreading of theta waves from the rostral part of the hippocampus to the site of the recording electrode of the order of l0 msecL This explanation of the genesis of the theta rhythm is offered as an alternative to the existing ones suggested by other authors1, 6. In contrast to other explanations, it takes into account the finding that the rhythmicity of theta activity is already formed in the septum. SUMMARY
Septum units in the rabbit discharge in regular bursts synchronous with the hippocampus theta rhythm but with different phase relationships. Superimposition of time histograms of these bursts with regard to their phase relationships to the theta wave, results in a curve similar to the theta wave in shape. Therefore, the theta rhythm can be interpreted as being determined by the frequency distribution of the firing of septum units.
REFERENCES I ANDERSEN,P., ECCLES, J. C., AND LOYNING, Y., Pathway of postsynaptic inhibition in the hippocampus, J. NeurophysioL, 27 (1964) 608-619. 2 ANDY, O. J., AND STEPHAN, H., The Septum of the Cat, Thomas, Springfield, Ill., 1964. 3 BRI3CKE,F., PETSCHE,H., PILLAT, B., UND DEISENHAMMER,E., Ein Schrittmacher in der medialen Septumregion des Kaninchengehirnes, Pfliigers Arch. ges. Physiol., 269 (1959) 135-140. 4 FUJITA, Y., AND SATO, T., Intracellular records from hippocampal pyramidal cells in rabbit during theta rhythm activity, J. NeurophysioL, 27 (1964) 1011-1025. 5 GREEN, J. D., MAXWELL, D. S., AND PETSCHE, H., Hippocampal electrical activity. III. Unitary events and genesis of slow waves, Electroenceph. clin.rNeurophysiol., 13 (1961) 854-867. 6 KANDEL,E. R., SPENCER,W. A., AND BRINLEY,F. J., Electrophysiology of hippocampal neurons. I. Sequential invasion and synaptic organization, J. Neurophysiol., 24 (1961) 225-242. 7 PETSCHE, H., GOGOL.~K, G., UND STUMPF, CH., Die Projektion der Zellen des Schrittmachers ffir den Thetarhythmus auf den Kaninchenhippocampus, J. Hirnforsch., 8 (1966) 129-136.
Brain Research, 7 (1968) 201-207
SEPTUM NEURONS AND THETA WAVE FORM
207
8 PETSCHE,H., GOGOLAK, A., AND VAN ZWIETEN, P. A., Rhythmicity of septal cell discharges at various levels of reticular excitation, Electroenceph. clin. Neurophysiol., 19 (1965) 25-33. 9 PETSCHE,H., AND STUMPF, CI-t., Topographic and toposcopic study of origin and spread of the regular synchronised arousal pattern in the rabbit, Electroenceph. clin. Neurophysiol., 12 (1960) 589-600. 10 PETSCHE, H., STUMPF,CH., AND GOGOLAK, G., The significance of the rabbit's septum as a relay station between the midbrain and the hippocampus. I. The control of hippocampus arousal activity by the septum cells, Electroenceph. clin. Neurophysiol., 14 (1962) 202-211. l 1 STUMPF,CH., PETSCHE, H., AND GOGOL.~K, G., The significance of the rabbit's septum as a relay station between the midbrain and the hippocampus, lI. The differential influence of drugs upon both the septal cell firing pattern and the hippocampus theta activity, Electroenceph. clin. Neurophysiol., 14 (1962) 212-219.
Brain Research, 7 (1968) 201-207