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Temperature change-induced potentiation: A comparative study of facilitatory mechanisms in aged and young rat hippocampal slices
Neuroscience Vol. 68, No. 2, pp. 395 397, 1995 Pergamon
0306-4522(95)00135-2
Elsevier ScienceLtd Copyright © 1995 IBRO Printed in Great Britain. All rights reserved 0306-4522/95 $9.50 + 0.00
TEMPERATURE CHANGE-INDUCED POTENTIATION: A C O M P A R A T I V E S T U D Y OF F A C I L I T A T O R Y M E C H A N I S M S IN A G E D A N D Y O U N G R A T H I P P O C A M P A L SLICES S. B U L D A K O V A , E. D U T O V A , S. I V L E V and M. WEISS* I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, 44 M. Torez Prospect, 194223 St Petersburg, Russia Abstract--The effect of temperature changes in the medium on the evoked potentials of pyramidal neurons in response to the electrical stimulation of Schaffer collaterals was investigated in both young (one to two months) and aged (24 28 months) rat hippocampal slices. Temperature increase was shown to reduce, and subsequent temperature decrease to enhance, the population spike amplitude in both young and aged rats. Temperature decrease produced a long-lasting (> 30 min) and important potentiation (180%) of population spike amplitude in young but not in aged rats. Long-term post-tetanic potentiation was maintained in young but suppressed in aged rats by temperature changes. The impairment of temperature-induced potentiation in aged rats is suggested as a tool for studying promnesic drugs.
T e m p e r a t u r e changes were d e m o n s t r a t e d as m o d u l a t ing the responsiveness o f rat dorsal h i p p o c a m p u s n e u r o n s to the electrical stimulation o f the Schaffer collateral pathway. T e m p e r a t u r e increase was observed to reduce and t e m p e r a t u r e decrease to enhance p o p u l a t i o n spike (PS) amplitude. 4'6 Moreover, further experiments on young rat slices d e m o n s t r a t e d the occurrence o f a low temperature-induced long-term potentiation o f the responsiveness o f h i p p o c a m p a l neurons. 2 The present report describes a comparative study o f this low t e m p e r a t u r e potentiated effect o f stimulation which was carried out on y o u n g and aged rat slices. It was aimed to determine whether the t e m p e r a t u r e - i n d u c e d p o t e n t i a t i o n in h i p p o c a m p a l slices from aged rats was impaired, as reported for the other synaptic potentiation processes. 5
EXPERIMENTAL PROCEDURES
Experiments were carried out on 15 completely submerged hippocampal slices prepared from six 24 28-monthold and nine one- to two-month-old Wistar rats. Slices were cut at 400-500/~m and immediately placed for 1 h at 28°C in an artificial cerebrospinal fluid of composition (mmol/l); NaC1 124; KC1 5.0; CaC12 2.5; MgSO4 2.4; KH2PO4 26; glucose 10; pH 7.4-7.5. The medium was saturated with carbogen (95% 02 + 5% CO 0. For recording, slices were transferred to a recording chamber with a dead space of 0.8 ml. A wire coil was used to control the temperature of perfusion, which could be varied from 24 to 40°C. Perfusion *To whom correspondence should be addressed. Present address: Laboratoire de Pharmacodynamie, Facult6 de Pharmacie, 27, Bd Jean-Moulin, 13385 Marseille Cedex 5, France. Abbreviations: PS, population spike. NS¢68/2--E
395
with artificial cerebrospinal fluid was carried out at a rate of 2 ml/min. Concentric bipolar glass stimulating electrodes were inserted into the Schaffer collaterals for orthodromic stimulation (0.1-0.3 Hz, 30 50V, 0.2ms duration) and tetanic activation (100s -I for 1 s). Field evoked responses were recorded in the CA1 pyramidal layer by single glass microelectrodes filled with 2 M sodium citrate (tip resistance 10-25 Mf~). The negative potential (PS) of each field response is thought to reflect the summed discharge of synchronous pyramidal cell activities? In all experiments, evoked responses were amplified, stored and averaged (I0 sweeps) before being plotted. Response amplitude was taken to be the difference between the spike peak negativity and the preceding and following positivities in each averaged field potential. The mean effect of temperature changes on the magnitude of PS is expressed as a percentage (+ S.E.M.) of the control amplitude at the initial temperature value. As a measure of tetanic potentiation, PS amplitude was expressed as a percentage of the control value before tetanization. The Mann Whitney test was used to assess the significance of results.
RESULTS AND DISCUSSION
A rapid increase o f temperature from 25 to 32°C in 5 rain was p e r f o r m e d on b o t h young and aged rat slices in I°C steps with 0 . 7 m i n intervals between successive steps (Fig. 1A). In the whole range o f temperatures, lower PS amplitudes were generally observed for similar stimulus intensities in aged rat brains than in young ones (respective m e a n PS amplitudes at 25°C: 0.5___0.24mV, n = 6 and 1.1 _+ 0.47mV, n = 9). After each I°C temperature increment, in b o t h y o u n g and aged rats PS amplitude was observed to decrease relative to the control value
396
S. Buldakova et al.
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Fig. 1. Time course of temperature-induced PS amplitude changes in hippocampus slices. (A) Comparative effects of temperature changes on PS amplitude in young ( 0 ) and aged (A) rat hippocampus slices. Plots of temperature changes (top) and PS amplitude (bottom) over the course of the experiment are shown. Ordinate, temperature (T, in cC; upper curve) and PS amplitude (A, in mV; lower curves). Abscissa, time (min). Note the successive changes (-//-) on the time scale. In this experiment the temperature curves were similar for both the young and the aged rats. (B) Comparative effects of temperature changes on the post-tetanic potentiation in young and aged rat hippocampal slices. Notation as in A. The vertical dotted lines indicate the period of tetanization (100 s - ], 1 s).
at 25°C, practically disappearing at 32°C. Subsequent cooling of the solution to the initial value reversed the high temperature-induced depressing effect in both young and aged rats. While PS amplitudes returned progressively to their initial amplitudes in aged rats (106 4- 11%, n = 6), a long ( > 3 0 min) and considerable increase o f PS amplitude relative to the control at 25°C (184 4 - 4 6 % , n = 9) was observed in young rat slices, mimicking the effects o f tetanization. To ascertain whether the two mechanisms are similar, we
examined the effects o f temperature changes on tetanic potentiation (Fig. 1B). Potentiation was considered to occur for PS amplitudes greater than 125% o f the control value, 30 min after tetanization. As c o m p a r e d to their control value before tetanization, a significant e n h a n c e m e n t o f PS amplitude could be noted in eight out o f nine young rat slices tested (mean value; 1 6 3 + 4 4 % , n = 9 ) . However, subsequent temperature changes were performed on only five o f them (see Table 1). Although
Table I. Effect of temperature changes on population spike amplitude A Control, % (mV) Young Aged
100 (1.1±0.47) 100 (0.50 ± 0.24)
B
After temperature changer (% of control)
n++
184 ± 46*
9
106 ± 11
6
Control, % (mV) 100 (1.3±0.23) 100 (0.43 ± 0.32)
After tetanization (% of control)
After temperature changer (% of control)
n~
183 + 49*
154 ± 30*
5
112 ± 15
92 ± 11
6
A, Effect of temperature changes on PS amplitude; B, effect of temperature changes on PS amplitude after tetanization. "I'PS amplitude after return to the initial temperature value. In each experiment temperature was increased to suppress the evoked PS, prior to subsequent cooling. :~The same slices were tested before (A) and after (B) tetanization. One slice per rat was studied (n). Tetanization was carried out on five of nine young rat slices tested. Mean time interval between A and B was 2 h. *P < 0.05 compared with corresponding control values.
Temperature-induced long-term potentiation more difficult to obtain in aged rat slices, two of the six aged rats presented a significant increment of their PS amplitude (133% and 128 %, respectively). We did not obtain tetanic potentiation in the other four rats. In good agreement with these results, Landfield et al. 5 reported that potentiation was observed in only 28% of aged rats and never exceeded 130-140%. In our experiment, temperature increase induced a loss of the PS potentiation when present, with a return to the initial amplitude at about 27-28°C. Subsequent temperature increase produced, in all cases, a progressive decrease of PS amplitude, which practically disappeared at temperatures above 32°C, as observed previously. Return to the initial temperature produced a recovery of the post-tetanic PS amplitude in young rats, while PS values of all aged rats returned to their control amplitude before tetanization. CONCLUSION We have sought in this study to determine whether temperature effects on pyramidal neurons are different in young and aged rat brains, as suggested by multiple reports on the effects of ageing in the hippocampus, e.g. morphological alterations, ~ decline of synaptic neurotransmission processes v and correlative age-related electrophysiological changes. ~ The
397
present results show that the magnitude of PS is temperature dependent in both young and aged rat hippocampal slices. Although, as reported previously, 2 a low-temperature potentiation similar to that obtained by tetanic stimulation 9 was observed in young rat slices, the same temperature decrease failed to enhance PS amplitude in hippocampal slices from aged rats. Moreover, temperature changes did not affect the maintenance of long-term post-tetanic potentiation in young rat brains, but suppressed it in aged animals. In all cases, temperature changes failed to produce potentiation in aged rat brains, even though a posttetanic potentiation could be obtained. This fact suggests that low temperature and tetanization might produce their potentiating effects via different mechanisms. Finally, temperature-induced long-term potentiation might be useful in the study of the decrement of mechanisms of facilitation in aged brains. This experimental paradigm is suggested as a tool for the electrophysiological screening of promnesic drugs. Acknowledgements--We thank Mme M. Galante for sec-
retarial assistance, Dr M. Ptito for improving the English and Prof. M. Jalfre for his helpful comments on the manuscript. This work was supported by the Sechenov Institute and a special grant from the French Embassy in Moscow.
REFERENCES
1. Barnes C. (1993) Electrophysiological changes in hippocampus of aged rodents: predictions for functional changes in aged primate. Neurobiol. Aging 14, 645-647. 2. Buldakova S. L., Dutova E. A. and Ivlev S. V. (1993) Temperature-induced long term potentiation in hippocampal slices. J. evol. Biochem. Physiol. 29, 365-373. 3. Creager R., Dunwiddie T. and Lynch G. (1980) Paired pulse and frequency facilitation in the CA1 region of the in vitro rat hippocampus. J. Physiol. 299, 409-424. 4. Fujii T. and Yoshizaki K. (1976) Temperature influence on the development of electrical activities in mammalian brain slice during incubation. Jap. J. Physiol. 26, 355 365. 5. Landfield P. W., McGaugh I. L. and Lynch G. (1978) Impaired synaptic potentiation process in the hippocampus of aged, memory deficient rats. Brain Res. 150, 85 102. 6. Schiff S. J. and Somjen G. G. (1985) The effect of temperature on synaptic transmission in hippocampal tissue slices. Brain Res. 345, 279 284. 7. Taylor L. and Griffith W. H. (1993) Age-related decline in cholinergic synaptic transmission in hippocampus. Neurobiol. Aging 14, 509-512. 8. West M. J. (1993) Regionally specific loss of neurons in the aging human hippocampus. Neurobiol. Aging 14, 287-294. 9. Yamamoto C. and Chujo T. (1978) Long term potentiation in thin hippocampal sections studied by intracellular and extracellular recording. Expl Neurol. 58, 242-250. (Accepted 7 March 1995)
0306-4522(95)00135-2
Elsevier ScienceLtd Copyright © 1995 IBRO Printed in Great Britain. All rights reserved 0306-4522/95 $9.50 + 0.00
TEMPERATURE CHANGE-INDUCED POTENTIATION: A C O M P A R A T I V E S T U D Y OF F A C I L I T A T O R Y M E C H A N I S M S IN A G E D A N D Y O U N G R A T H I P P O C A M P A L SLICES S. B U L D A K O V A , E. D U T O V A , S. I V L E V and M. WEISS* I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, 44 M. Torez Prospect, 194223 St Petersburg, Russia Abstract--The effect of temperature changes in the medium on the evoked potentials of pyramidal neurons in response to the electrical stimulation of Schaffer collaterals was investigated in both young (one to two months) and aged (24 28 months) rat hippocampal slices. Temperature increase was shown to reduce, and subsequent temperature decrease to enhance, the population spike amplitude in both young and aged rats. Temperature decrease produced a long-lasting (> 30 min) and important potentiation (180%) of population spike amplitude in young but not in aged rats. Long-term post-tetanic potentiation was maintained in young but suppressed in aged rats by temperature changes. The impairment of temperature-induced potentiation in aged rats is suggested as a tool for studying promnesic drugs.
T e m p e r a t u r e changes were d e m o n s t r a t e d as m o d u l a t ing the responsiveness o f rat dorsal h i p p o c a m p u s n e u r o n s to the electrical stimulation o f the Schaffer collateral pathway. T e m p e r a t u r e increase was observed to reduce and t e m p e r a t u r e decrease to enhance p o p u l a t i o n spike (PS) amplitude. 4'6 Moreover, further experiments on young rat slices d e m o n s t r a t e d the occurrence o f a low temperature-induced long-term potentiation o f the responsiveness o f h i p p o c a m p a l neurons. 2 The present report describes a comparative study o f this low t e m p e r a t u r e potentiated effect o f stimulation which was carried out on y o u n g and aged rat slices. It was aimed to determine whether the t e m p e r a t u r e - i n d u c e d p o t e n t i a t i o n in h i p p o c a m p a l slices from aged rats was impaired, as reported for the other synaptic potentiation processes. 5
EXPERIMENTAL PROCEDURES
Experiments were carried out on 15 completely submerged hippocampal slices prepared from six 24 28-monthold and nine one- to two-month-old Wistar rats. Slices were cut at 400-500/~m and immediately placed for 1 h at 28°C in an artificial cerebrospinal fluid of composition (mmol/l); NaC1 124; KC1 5.0; CaC12 2.5; MgSO4 2.4; KH2PO4 26; glucose 10; pH 7.4-7.5. The medium was saturated with carbogen (95% 02 + 5% CO 0. For recording, slices were transferred to a recording chamber with a dead space of 0.8 ml. A wire coil was used to control the temperature of perfusion, which could be varied from 24 to 40°C. Perfusion *To whom correspondence should be addressed. Present address: Laboratoire de Pharmacodynamie, Facult6 de Pharmacie, 27, Bd Jean-Moulin, 13385 Marseille Cedex 5, France. Abbreviations: PS, population spike. NS¢68/2--E
395
with artificial cerebrospinal fluid was carried out at a rate of 2 ml/min. Concentric bipolar glass stimulating electrodes were inserted into the Schaffer collaterals for orthodromic stimulation (0.1-0.3 Hz, 30 50V, 0.2ms duration) and tetanic activation (100s -I for 1 s). Field evoked responses were recorded in the CA1 pyramidal layer by single glass microelectrodes filled with 2 M sodium citrate (tip resistance 10-25 Mf~). The negative potential (PS) of each field response is thought to reflect the summed discharge of synchronous pyramidal cell activities? In all experiments, evoked responses were amplified, stored and averaged (I0 sweeps) before being plotted. Response amplitude was taken to be the difference between the spike peak negativity and the preceding and following positivities in each averaged field potential. The mean effect of temperature changes on the magnitude of PS is expressed as a percentage (+ S.E.M.) of the control amplitude at the initial temperature value. As a measure of tetanic potentiation, PS amplitude was expressed as a percentage of the control value before tetanization. The Mann Whitney test was used to assess the significance of results.
RESULTS AND DISCUSSION
A rapid increase o f temperature from 25 to 32°C in 5 rain was p e r f o r m e d on b o t h young and aged rat slices in I°C steps with 0 . 7 m i n intervals between successive steps (Fig. 1A). In the whole range o f temperatures, lower PS amplitudes were generally observed for similar stimulus intensities in aged rat brains than in young ones (respective m e a n PS amplitudes at 25°C: 0.5___0.24mV, n = 6 and 1.1 _+ 0.47mV, n = 9). After each I°C temperature increment, in b o t h y o u n g and aged rats PS amplitude was observed to decrease relative to the control value
396
S. Buldakova et al.
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B
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Fig. 1. Time course of temperature-induced PS amplitude changes in hippocampus slices. (A) Comparative effects of temperature changes on PS amplitude in young ( 0 ) and aged (A) rat hippocampus slices. Plots of temperature changes (top) and PS amplitude (bottom) over the course of the experiment are shown. Ordinate, temperature (T, in cC; upper curve) and PS amplitude (A, in mV; lower curves). Abscissa, time (min). Note the successive changes (-//-) on the time scale. In this experiment the temperature curves were similar for both the young and the aged rats. (B) Comparative effects of temperature changes on the post-tetanic potentiation in young and aged rat hippocampal slices. Notation as in A. The vertical dotted lines indicate the period of tetanization (100 s - ], 1 s).
at 25°C, practically disappearing at 32°C. Subsequent cooling of the solution to the initial value reversed the high temperature-induced depressing effect in both young and aged rats. While PS amplitudes returned progressively to their initial amplitudes in aged rats (106 4- 11%, n = 6), a long ( > 3 0 min) and considerable increase o f PS amplitude relative to the control at 25°C (184 4 - 4 6 % , n = 9) was observed in young rat slices, mimicking the effects o f tetanization. To ascertain whether the two mechanisms are similar, we
examined the effects o f temperature changes on tetanic potentiation (Fig. 1B). Potentiation was considered to occur for PS amplitudes greater than 125% o f the control value, 30 min after tetanization. As c o m p a r e d to their control value before tetanization, a significant e n h a n c e m e n t o f PS amplitude could be noted in eight out o f nine young rat slices tested (mean value; 1 6 3 + 4 4 % , n = 9 ) . However, subsequent temperature changes were performed on only five o f them (see Table 1). Although
Table I. Effect of temperature changes on population spike amplitude A Control, % (mV) Young Aged
100 (1.1±0.47) 100 (0.50 ± 0.24)
B
After temperature changer (% of control)
n++
184 ± 46*
9
106 ± 11
6
Control, % (mV) 100 (1.3±0.23) 100 (0.43 ± 0.32)
After tetanization (% of control)
After temperature changer (% of control)
n~
183 + 49*
154 ± 30*
5
112 ± 15
92 ± 11
6
A, Effect of temperature changes on PS amplitude; B, effect of temperature changes on PS amplitude after tetanization. "I'PS amplitude after return to the initial temperature value. In each experiment temperature was increased to suppress the evoked PS, prior to subsequent cooling. :~The same slices were tested before (A) and after (B) tetanization. One slice per rat was studied (n). Tetanization was carried out on five of nine young rat slices tested. Mean time interval between A and B was 2 h. *P < 0.05 compared with corresponding control values.
Temperature-induced long-term potentiation more difficult to obtain in aged rat slices, two of the six aged rats presented a significant increment of their PS amplitude (133% and 128 %, respectively). We did not obtain tetanic potentiation in the other four rats. In good agreement with these results, Landfield et al. 5 reported that potentiation was observed in only 28% of aged rats and never exceeded 130-140%. In our experiment, temperature increase induced a loss of the PS potentiation when present, with a return to the initial amplitude at about 27-28°C. Subsequent temperature increase produced, in all cases, a progressive decrease of PS amplitude, which practically disappeared at temperatures above 32°C, as observed previously. Return to the initial temperature produced a recovery of the post-tetanic PS amplitude in young rats, while PS values of all aged rats returned to their control amplitude before tetanization. CONCLUSION We have sought in this study to determine whether temperature effects on pyramidal neurons are different in young and aged rat brains, as suggested by multiple reports on the effects of ageing in the hippocampus, e.g. morphological alterations, ~ decline of synaptic neurotransmission processes v and correlative age-related electrophysiological changes. ~ The
397
present results show that the magnitude of PS is temperature dependent in both young and aged rat hippocampal slices. Although, as reported previously, 2 a low-temperature potentiation similar to that obtained by tetanic stimulation 9 was observed in young rat slices, the same temperature decrease failed to enhance PS amplitude in hippocampal slices from aged rats. Moreover, temperature changes did not affect the maintenance of long-term post-tetanic potentiation in young rat brains, but suppressed it in aged animals. In all cases, temperature changes failed to produce potentiation in aged rat brains, even though a posttetanic potentiation could be obtained. This fact suggests that low temperature and tetanization might produce their potentiating effects via different mechanisms. Finally, temperature-induced long-term potentiation might be useful in the study of the decrement of mechanisms of facilitation in aged brains. This experimental paradigm is suggested as a tool for the electrophysiological screening of promnesic drugs. Acknowledgements--We thank Mme M. Galante for sec-
retarial assistance, Dr M. Ptito for improving the English and Prof. M. Jalfre for his helpful comments on the manuscript. This work was supported by the Sechenov Institute and a special grant from the French Embassy in Moscow.
REFERENCES
1. Barnes C. (1993) Electrophysiological changes in hippocampus of aged rodents: predictions for functional changes in aged primate. Neurobiol. Aging 14, 645-647. 2. Buldakova S. L., Dutova E. A. and Ivlev S. V. (1993) Temperature-induced long term potentiation in hippocampal slices. J. evol. Biochem. Physiol. 29, 365-373. 3. Creager R., Dunwiddie T. and Lynch G. (1980) Paired pulse and frequency facilitation in the CA1 region of the in vitro rat hippocampus. J. Physiol. 299, 409-424. 4. Fujii T. and Yoshizaki K. (1976) Temperature influence on the development of electrical activities in mammalian brain slice during incubation. Jap. J. Physiol. 26, 355 365. 5. Landfield P. W., McGaugh I. L. and Lynch G. (1978) Impaired synaptic potentiation process in the hippocampus of aged, memory deficient rats. Brain Res. 150, 85 102. 6. Schiff S. J. and Somjen G. G. (1985) The effect of temperature on synaptic transmission in hippocampal tissue slices. Brain Res. 345, 279 284. 7. Taylor L. and Griffith W. H. (1993) Age-related decline in cholinergic synaptic transmission in hippocampus. Neurobiol. Aging 14, 509-512. 8. West M. J. (1993) Regionally specific loss of neurons in the aging human hippocampus. Neurobiol. Aging 14, 287-294. 9. Yamamoto C. and Chujo T. (1978) Long term potentiation in thin hippocampal sections studied by intracellular and extracellular recording. Expl Neurol. 58, 242-250. (Accepted 7 March 1995)