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Potassium and short-term response plasticity in the hippocampal slice
Brain Research, 159 (1978) 239-242 © Elsevier/North-Holland Biomedical Press
239
Potassium and short-term response plasticity in the hippocampal slice
BRADLEY E. ALGER* and TIMOTHY J. TEYLER Departments of Pharmacology and Physiology, School of Medicine, University of California, San Francisco, Calif. 94143 and (T.J.T.) Neurobiology Division, Northeastern Ohio University, College of Medicine, Rootstown, Ohio 44272 (U.S.A.)
(Accepted August 24th, 1978)
The idea that extracellular potassium ([K+]0) accumulation resulting from neuronal activity might directly modify neuronal responses is an old oneT,s. The expected action of [K+]0 accumulation is not clear and both enhancing6,11 and depressive 15 effects have been found. Usually, however, the potassium hypothesis 5,s, 9,15 appears to predict a close temporal correlation between [K+]0 and the neuronal responses modified. This prediction has been tested in the hippocampal slice. We have found that stimulation-related build-up of [K+]0 does not persist as long as either: (1) post-tetanic facilitation of CA 1 EPSPs or (2) post-tetanic depression of dentate gyrus EPSPs, and hence, may not play a direct role in mediating these shortterm effects. However, the time course of [K+]0 lecovery following a tetanus is similar to CA1 population spike facilitation. Furthermore, stimulation of two separate inputs to CA1 can result in a 'heterosynaptic facilitation '9 of this response, which is similar in duration to that of the associated [K+]0 increase. We suggest that stimulation-related increases in [K+]0 may play a role in post-tetanic facilitation of the CA1 population spike response. Our slice preparation and recording conditions have been describedX,2. Extracellular potassium was measured simultaneously with field potentials using commercially available K+-sensitive microelectrodes (ISMs) and conventional techniques 5. The ISM was placed within 100 # m of the field pipette. ISM sensitivit.y was approximately 45 mV for changes in [K +] of 5-50 m M in otherwise normal Earle solution. Tissue [K +] was within 0.1 m M of saline [K ÷] (5.37 mM) measured on withdrawal, and a factor of 0.32 m M [K+]/mV was used for converting small ( < 2 mM) changes in [K+]0 into mV. Response plasticity ((Rn-R1)/(Rmax-R1) × 100; where: Rn = the response to a stimulus either during or post-tetanus; Rmax greatest deviation from the first tetanic response (R1)) was measured in a slice by giving 10 'tetani' (2 Hz, 5 sec) and following each by a single pulse at intervals of 2-30 sec. Such normalized data are interpreted in arbitrary units of facilitation or
* The results reported in this paper formed part of a Doctoral Dissertation submitted by B.E.A. to Harvard University, Cambridge, Mass. 02138 (1977).
240 depression. Statistical tests were performed on recovery rate constants using two-tailed t-tests with a 'significant' level o f P < 0.01 and a 'not-significant' level o f P > 0.05. Population EPSPs of dentate molecular layer and CAI stratum radiatum and population spikes of CAI stratum pyramidale were studied. Experiments were performed on 33 slices from 23 male Sprague-Dawley rats. Post-tetanic potassium clearance in the hippocampus proceeds exponentially 10. For small changes [K+]0 recovery can be described by" VK+ -- ae-kt; (VK+ = ISM output, a -- constant, e = base of natural logarithms, t ~ time post-tetanus and k = rate constant). All data were fitted by negative exponential equations. Correlation coefficients of the least squares regression lines were high and statistically significant (0.916-0.989 ([K+]0); 0.867-0.970 (neuronal responses). Fig. 1 depicts moderate response plasticity (facilitation in CAI" Fig. IA, C: depression in dentate gyrus: Fig. 1 B). In the lower row post-tetanic neuronal responses are replotted (open circles) on semilogarithmic coordinates together with associated [K~]0 values. Potassium recovers significantly faster in CAI stratum radiatum (n ~ 7; Fig. 1A2) and dentate molecular layers (n -- 8; Fig. 1B2) than EPSPs measured in these areas. Half-times for EPSP recovery is 6.4 sec, for [K+]0 2.5 and 2.2 sec in CAI and dentate, respectively. However, the duration of CAI population spike post-tetanic facilitation is not significantly different from that of [K+]0 recovery in stratum pyramidale (n ~ 9; Fig. 1C2). These correlational data suggest that, while increased [K ~]0 does not persist long enough to play a direct role in post-tetanic EPSP plasticity, [K+]0 could be involved in post-tetanic spike facilitation. AI '°° l ~P~
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Fig. 1. Short-term response plasticities and associated potassium recovery values. Top: response facilitation (A1 and C]) and depression (BD during and post-tetanus. Bottom : post-tetanic responses (open circles, replotted from top row) together with associated potassium values. (In C1 the final 3 points are depressed with respect to control and a constant was added for replotting in C2. The slope of the line is not changed.) Ordinates read in arbitrary units. [K+]~ always increased during a tetanus. Typical field potentials, traced from the film by hand, are shown in the top row.
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Fig. 2. Heterosynapticfacilitation of C A ] population spike. Test stimuli were delivered to one afferent system (either stratum oriens or stratum radiatum) before and at various times after a brief tetanus to the other system. M e a n p o s t - t e t a n i c values are plotted for field p o t e n t i a l s (n = 7; o p e n circles) a n d
[K+10(n = 3) after similartetani.
If this interpretation were correct, then activity which increases [K÷]0 should increase the ability of inputs to the pyramidal cells to activate them, as reported in vivo a. To test this, stimulating electrodes were located simultaneously on pathways in stratum oriens 3 (anterior) and in stratum radiatum (posterior) with a recording electrode in stratum pyramidale. A test pulse ($1) via one electrode preceded a brief tetanus (4 Hz, 5 sec) via the other, followed at 2-30 sec by a se6ond test pulse ($2) via the first. Enhanced post-tetanic responsiveness (R2/R1 × 100) of the non-tetanized pathway was produced in 7 of 9 slices. In 3 slices (3 determinations each) [K+]0 responses in stratum pyramidale following an identical tetanus were analyzed. The recovery rate constants for [K]0 and 'heterosynaptic facilitation' (Fig. 2) do not differ significantly. These preliminary experiments support the hypothesis that [K+]0 influences pyramidal cell excitability. They do not themselves address the possibility of alternative mechanisms (e.g. excitatory interneurons). However: (1) stimuli presented in a 'double-shock' paradigm between stratum oriens and stratum radiatum do not produce 'heterosynaptic facilitation'13; (2) heterosynaptic long-term potentiation is not seen1,4,12; and (3) during prolonged high frequency (200 Hz) stimulation which blocks synaptic responses of one pathway, the single shock response of the nontetanized pathway is greatly increased over control values (Alger, 1976; unpublished observations; see also ref. 12). These findings suggest that heterosynaptic interactions in this system are not mediated solely by excitatory interneurons. EPSP plasticity was not closely correlated with [K+]0 recovery and we conclude that [K+]0 probably does not play a roletn mediating post-tetanic EPSP plasticity. The possibility of [K+]0 involvement in dentate depression is further reduced by the observation that degree of depression is inversely 14, and [K+]0 build-up directly, related to stimulus intensity. There is a correspondence between the duration of posttetanic facilitation of the CA1 population spike and [K÷]0 recovery. Experiments demonstrating a 'heterosynaptic facilitation' of the CA1 population spike are consistent with the suggestion that stimulation-dependent extracellular [K+]o may have a direct effect on CA1 pyramidal cell excitability. Intracellular recordings will be necessary to clarify these issues.
242 W e t h a n k Dr. R. A. Nicoll for r e a d i n g a n d c o m m e n t i n g on the m a n u s c r i p t . The w o r k was s u p p o r t e d in p a r t by N S F G r a n t BMS75-02802 to T . J . T . B . E . A . held N I M H P r e d o c t o r a l F e l l o w s h i p 1F31 MH05628-01.
1 Alger, B. E., Megela, A. L. and Teyler, T. J., Transient heterosynaptic depression in the hippocampal slice, Brain Res. Bull., 3 (1978) 181-184. 2 Alger, B. E. and Teyler, T. J., Longrterm and short-term plasticity in the CAI, CA3 and dentate regions of the rat hippocampal slice, Brain Research, 110 (1976) 463-480. 3 Alger, B. E. and Teyler, T. J., A monosynaptic fiber track studied in vitro: evidence ofa hippocampal CA1 associational system.'?, Brain Res. Bull., 2 (1977) 355-365. 4 Andersen, P., Sundberg, S. H., Sveen, O. and Wigstr6m, H., Specific long-lasting potentiation of synaptic transmission in hippocampal slices, Nature (Lond.), 266 (1977) 736-737. 5 Fritz, L. C. and Gardner-Medwin, A. R., The effect of synaptic activity on extracellular potassium concentration in the hippocampal dentate region in vitro, Brain Research, 112 (1976) 183-187. 6 Gage, P. W. and Quastel, D. M. J., Dual effect of potassium on transmitter release, Nature (Lond.), 206 (1965) 625-626. 7 Green, J. D., The hippocampus, Physiol. Rev., 44 (1961) 561-608. 8 Hughes, J. R., Post-tetanic potentiation, Physiol. Rev., 38 (1958) 91-113. 9 Izquierdo, I. and Vasquez, B., Field potentials in rat hippocampus: monosynaptic nature and heterosynaptic post-tetanic potentiation, Exp. Neurol., 21 (1968) 133-146. 10 Lewis, D. V. andSchuette, W. H.,NADHfluorescenceand potassium concentration changes during hippocampal electrical stimulation, J. Neurophysiol., 38 (1975) 405-4 17. 11 Liley, A. W., Effects of presynaptic polarization on spontaneous activity at the mammalian neuromuscular junction, J. Physiol. (Lond.), 134 (1956) 427-443. 12 Lynch, G. S., Dunwiddie, T. and Gribkoff, V., Heterosynaptic depression: a postsynaptic correlate of long-term potentiation, Nature (Lond.), 266 (1977) 737-739. 13 Teyler, T. J. and Alger, B. E., Candidate mechanisms of plasticity in the hippocampus, Neurosci. Abstr., 2 (1976) 1212. 14 Teyler, T. J. and Alger, B. E., Monosynaptic habituation in the vertebrate forebrain: the dentate gyrus examined in vitro, Brain Research, i 15 (1976) 413-425. 15 Weight, F. F. and Erulkar, S. D., Modulation of synaptic transmitter release by repetitive postsynaptic action potentials, Science, 193 (1976) 1023-1025.
239
Potassium and short-term response plasticity in the hippocampal slice
BRADLEY E. ALGER* and TIMOTHY J. TEYLER Departments of Pharmacology and Physiology, School of Medicine, University of California, San Francisco, Calif. 94143 and (T.J.T.) Neurobiology Division, Northeastern Ohio University, College of Medicine, Rootstown, Ohio 44272 (U.S.A.)
(Accepted August 24th, 1978)
The idea that extracellular potassium ([K+]0) accumulation resulting from neuronal activity might directly modify neuronal responses is an old oneT,s. The expected action of [K+]0 accumulation is not clear and both enhancing6,11 and depressive 15 effects have been found. Usually, however, the potassium hypothesis 5,s, 9,15 appears to predict a close temporal correlation between [K+]0 and the neuronal responses modified. This prediction has been tested in the hippocampal slice. We have found that stimulation-related build-up of [K+]0 does not persist as long as either: (1) post-tetanic facilitation of CA 1 EPSPs or (2) post-tetanic depression of dentate gyrus EPSPs, and hence, may not play a direct role in mediating these shortterm effects. However, the time course of [K+]0 lecovery following a tetanus is similar to CA1 population spike facilitation. Furthermore, stimulation of two separate inputs to CA1 can result in a 'heterosynaptic facilitation '9 of this response, which is similar in duration to that of the associated [K+]0 increase. We suggest that stimulation-related increases in [K+]0 may play a role in post-tetanic facilitation of the CA1 population spike response. Our slice preparation and recording conditions have been describedX,2. Extracellular potassium was measured simultaneously with field potentials using commercially available K+-sensitive microelectrodes (ISMs) and conventional techniques 5. The ISM was placed within 100 # m of the field pipette. ISM sensitivit.y was approximately 45 mV for changes in [K +] of 5-50 m M in otherwise normal Earle solution. Tissue [K +] was within 0.1 m M of saline [K ÷] (5.37 mM) measured on withdrawal, and a factor of 0.32 m M [K+]/mV was used for converting small ( < 2 mM) changes in [K+]0 into mV. Response plasticity ((Rn-R1)/(Rmax-R1) × 100; where: Rn = the response to a stimulus either during or post-tetanus; Rmax greatest deviation from the first tetanic response (R1)) was measured in a slice by giving 10 'tetani' (2 Hz, 5 sec) and following each by a single pulse at intervals of 2-30 sec. Such normalized data are interpreted in arbitrary units of facilitation or
* The results reported in this paper formed part of a Doctoral Dissertation submitted by B.E.A. to Harvard University, Cambridge, Mass. 02138 (1977).
240 depression. Statistical tests were performed on recovery rate constants using two-tailed t-tests with a 'significant' level o f P < 0.01 and a 'not-significant' level o f P > 0.05. Population EPSPs of dentate molecular layer and CAI stratum radiatum and population spikes of CAI stratum pyramidale were studied. Experiments were performed on 33 slices from 23 male Sprague-Dawley rats. Post-tetanic potassium clearance in the hippocampus proceeds exponentially 10. For small changes [K+]0 recovery can be described by" VK+ -- ae-kt; (VK+ = ISM output, a -- constant, e = base of natural logarithms, t ~ time post-tetanus and k = rate constant). All data were fitted by negative exponential equations. Correlation coefficients of the least squares regression lines were high and statistically significant (0.916-0.989 ([K+]0); 0.867-0.970 (neuronal responses). Fig. 1 depicts moderate response plasticity (facilitation in CAI" Fig. IA, C: depression in dentate gyrus: Fig. 1 B). In the lower row post-tetanic neuronal responses are replotted (open circles) on semilogarithmic coordinates together with associated [K~]0 values. Potassium recovers significantly faster in CAI stratum radiatum (n ~ 7; Fig. 1A2) and dentate molecular layers (n -- 8; Fig. 1B2) than EPSPs measured in these areas. Half-times for EPSP recovery is 6.4 sec, for [K+]0 2.5 and 2.2 sec in CAI and dentate, respectively. However, the duration of CAI population spike post-tetanic facilitation is not significantly different from that of [K+]0 recovery in stratum pyramidale (n ~ 9; Fig. 1C2). These correlational data suggest that, while increased [K ~]0 does not persist long enough to play a direct role in post-tetanic EPSP plasticity, [K+]0 could be involved in post-tetanic spike facilitation. AI '°° l ~P~
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Fig. 1. Short-term response plasticities and associated potassium recovery values. Top: response facilitation (A1 and C]) and depression (BD during and post-tetanus. Bottom : post-tetanic responses (open circles, replotted from top row) together with associated potassium values. (In C1 the final 3 points are depressed with respect to control and a constant was added for replotting in C2. The slope of the line is not changed.) Ordinates read in arbitrary units. [K+]~ always increased during a tetanus. Typical field potentials, traced from the film by hand, are shown in the top row.
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Fig. 2. Heterosynapticfacilitation of C A ] population spike. Test stimuli were delivered to one afferent system (either stratum oriens or stratum radiatum) before and at various times after a brief tetanus to the other system. M e a n p o s t - t e t a n i c values are plotted for field p o t e n t i a l s (n = 7; o p e n circles) a n d
[K+10(n = 3) after similartetani.
If this interpretation were correct, then activity which increases [K÷]0 should increase the ability of inputs to the pyramidal cells to activate them, as reported in vivo a. To test this, stimulating electrodes were located simultaneously on pathways in stratum oriens 3 (anterior) and in stratum radiatum (posterior) with a recording electrode in stratum pyramidale. A test pulse ($1) via one electrode preceded a brief tetanus (4 Hz, 5 sec) via the other, followed at 2-30 sec by a se6ond test pulse ($2) via the first. Enhanced post-tetanic responsiveness (R2/R1 × 100) of the non-tetanized pathway was produced in 7 of 9 slices. In 3 slices (3 determinations each) [K+]0 responses in stratum pyramidale following an identical tetanus were analyzed. The recovery rate constants for [K]0 and 'heterosynaptic facilitation' (Fig. 2) do not differ significantly. These preliminary experiments support the hypothesis that [K+]0 influences pyramidal cell excitability. They do not themselves address the possibility of alternative mechanisms (e.g. excitatory interneurons). However: (1) stimuli presented in a 'double-shock' paradigm between stratum oriens and stratum radiatum do not produce 'heterosynaptic facilitation'13; (2) heterosynaptic long-term potentiation is not seen1,4,12; and (3) during prolonged high frequency (200 Hz) stimulation which blocks synaptic responses of one pathway, the single shock response of the nontetanized pathway is greatly increased over control values (Alger, 1976; unpublished observations; see also ref. 12). These findings suggest that heterosynaptic interactions in this system are not mediated solely by excitatory interneurons. EPSP plasticity was not closely correlated with [K+]0 recovery and we conclude that [K+]0 probably does not play a roletn mediating post-tetanic EPSP plasticity. The possibility of [K+]0 involvement in dentate depression is further reduced by the observation that degree of depression is inversely 14, and [K+]0 build-up directly, related to stimulus intensity. There is a correspondence between the duration of posttetanic facilitation of the CA1 population spike and [K÷]0 recovery. Experiments demonstrating a 'heterosynaptic facilitation' of the CA1 population spike are consistent with the suggestion that stimulation-dependent extracellular [K+]o may have a direct effect on CA1 pyramidal cell excitability. Intracellular recordings will be necessary to clarify these issues.
242 W e t h a n k Dr. R. A. Nicoll for r e a d i n g a n d c o m m e n t i n g on the m a n u s c r i p t . The w o r k was s u p p o r t e d in p a r t by N S F G r a n t BMS75-02802 to T . J . T . B . E . A . held N I M H P r e d o c t o r a l F e l l o w s h i p 1F31 MH05628-01.
1 Alger, B. E., Megela, A. L. and Teyler, T. J., Transient heterosynaptic depression in the hippocampal slice, Brain Res. Bull., 3 (1978) 181-184. 2 Alger, B. E. and Teyler, T. J., Longrterm and short-term plasticity in the CAI, CA3 and dentate regions of the rat hippocampal slice, Brain Research, 110 (1976) 463-480. 3 Alger, B. E. and Teyler, T. J., A monosynaptic fiber track studied in vitro: evidence ofa hippocampal CA1 associational system.'?, Brain Res. Bull., 2 (1977) 355-365. 4 Andersen, P., Sundberg, S. H., Sveen, O. and Wigstr6m, H., Specific long-lasting potentiation of synaptic transmission in hippocampal slices, Nature (Lond.), 266 (1977) 736-737. 5 Fritz, L. C. and Gardner-Medwin, A. R., The effect of synaptic activity on extracellular potassium concentration in the hippocampal dentate region in vitro, Brain Research, 112 (1976) 183-187. 6 Gage, P. W. and Quastel, D. M. J., Dual effect of potassium on transmitter release, Nature (Lond.), 206 (1965) 625-626. 7 Green, J. D., The hippocampus, Physiol. Rev., 44 (1961) 561-608. 8 Hughes, J. R., Post-tetanic potentiation, Physiol. Rev., 38 (1958) 91-113. 9 Izquierdo, I. and Vasquez, B., Field potentials in rat hippocampus: monosynaptic nature and heterosynaptic post-tetanic potentiation, Exp. Neurol., 21 (1968) 133-146. 10 Lewis, D. V. andSchuette, W. H.,NADHfluorescenceand potassium concentration changes during hippocampal electrical stimulation, J. Neurophysiol., 38 (1975) 405-4 17. 11 Liley, A. W., Effects of presynaptic polarization on spontaneous activity at the mammalian neuromuscular junction, J. Physiol. (Lond.), 134 (1956) 427-443. 12 Lynch, G. S., Dunwiddie, T. and Gribkoff, V., Heterosynaptic depression: a postsynaptic correlate of long-term potentiation, Nature (Lond.), 266 (1977) 737-739. 13 Teyler, T. J. and Alger, B. E., Candidate mechanisms of plasticity in the hippocampus, Neurosci. Abstr., 2 (1976) 1212. 14 Teyler, T. J. and Alger, B. E., Monosynaptic habituation in the vertebrate forebrain: the dentate gyrus examined in vitro, Brain Research, i 15 (1976) 413-425. 15 Weight, F. F. and Erulkar, S. D., Modulation of synaptic transmitter release by repetitive postsynaptic action potentials, Science, 193 (1976) 1023-1025.