- Email: [email protected]
Transient heterosynaptic depression in the hippocampal slice
Brain ~eseareft &lletin.
Vol.
3,
pp. 181-184.
Printed in the U.S.A
BRIEF COMMUNICATION Transient Heterosynaptic
Depression in the Hippocampal Slice
BRADLEY E. ALCER,’ ANDREA L. MEGELA’ AND TIMOTHY J. TEYLER3 Deyartmeilt
ofFs~~cholo~v arzd Social Relations, Harvard Universit_v Cambridge, MA 02138 (Received
14 October
1977)
ALCI’R, B. E., A. L. Mt-GfiLA AND T. J. TEYLER. Transient hetemsynaptic depression in thehippocampalslice. BRAIN RES. BULL. 3(2) 181-184, 1978. -- Two independent, excitatory, monosynaptic afferent fiber systems projecting onto basal and apical dendritic layers of the CA1 sub-field of the hippocampus were tested for interactive effects. Long-term potentiation of either of the pathways was accompanied by a transient depression in the other pathway. Similar transient depression could be elicited by antidromic stimulation of the CA1 cells. No long-term heterosynaptic effects were observed. Hippocampus
Hippocampal slice
Neuronal response plasticity
Heterosynaptic
depression
be significant; guinea pigs have been used by some investigators (31 and rats [5,6] by others. In addition it has not been shown that the reported changes are dependent on synaptic activation of the cells and are not present following non-synaptic activation. Finally, some measure of response variability is essential in order to evaluate any changes. Although usually quite stable for hours, hippocampal slices do deteriorate [ 1,7] and hippocampal responses are variable over time [ 1,4] . Using laminar analysis, microsurgical techniques and control experiments to assess possible current spread, we have recently shown that stimulation delivered to stratum oriens (s. oriens) activates fibers traveling only in s. oriens [2]. Measurements of the frequency following characteristics and synaptic delay of the extracellular EPSP indicates that the s. oriens fibers make monosynaptic intracellular with CA1 cells. Furthermore, contact recordings from individual CA1 cells indicated action potentials could be triggered equally well from either s. oriens or s. radiatum stimulation. Thus a single population of cells appears to be involved in both responses. From these results arose the question of possible interactions between effects of stimulation to each fiber system [ 81. This communication confirms the presence of a short-term heterosynaptic depression following low-frequency tetanic stimulation. The depression was not restricted to instances in which long-term potentiation was produced and indeed was seen following antidromic tetani-
LONG-TERM potentiation (LTP) is a dramatic increase in neuronal responsiveness produced by a brief train, or series of trains, of stimuli [ 1, 2, 4, 5, 6, 7j which in the hippocampus lasts for at least 30 minutes. The ease with which this form of neuronal plasticity is instituted and its durability make it an important model for the study of how the nervous system registers events of behavioral significance. A fundamental question in understanding the basis of LTP concerns how this phenomenon is mediated, whether pre- or postsynaptically. Evidence consonant with a general postsynaptic effect is the demonstration in the hippocampal slice preparation that glutamate sensitivity of CA1 cells is depressed when long-term potentiation is produced but not otherwise 161. In other experiments the establishment of LTP by stimulation delivered to one pathway depressed the responsiveness of a given cell group to stimulation by another input pathway (heterosynaptic depression [ 5 j ; but see [ 81). However, when the size of the input volley is monitored a lasting general effect is not found [31. None of these experiments demonstrated that stimulation delivered to two different areas of the hippocampal slice in fact activated two separate afferent pathways. Differential current spread to nearby areas could conceivably account for the differences in results. Alternatively, even if two distinct inputs exist, the same cell population may not be influenced by both. Species differences might
1This work was supported by NSF Grant BMS75 -02802 (TJT); BEA and ALM held NIMH Predoctoral Fellowships. ‘Present address: Department of Pharmacology and Physiology, School of Medicine, University of California, San Francisco, CA 94143. Send reprint requests to B.E.A. ‘Present address: Section of Neurobiology and Behavior, Langmuir Laboratory, Cornell University, Ithaca, NY 14853. ‘Present address: Neurobiology Program, Northeastern Ohio Universities College of Medicine, Rootstown, OH 44272. 181
0361-9230/78/0302-0181$00.60
Copyright Q 1978 ANKHO
International Inc.
ALGER, MEGELA AND TEYLER
182
zation of CA1 cells. Long-term not evident.
heterosynaptic
effects were
METHOD
The rat in vitro hippocampal slice was used in these experiments. Details of the slice preparation as used in our laboratory are available [see 11 Experiments were conducted on 43 slices maintained at 33-35°C from 2.5 male Sprague-Dawley rats. Stimulation ( I -I 0 V) was delivered to s. oriens, s. radiatum or the alveus with small concentric bipolar electrodes (o.d. 300 microns). Stimulating electrodes were placed in s. oriens anterior (i.e., towards the fimbria) with respect to recording electrodes and in s. radiatum (or the alveus in certain experiments) posterior (i.e., towards the subiculum) with respect to recording electrodes. The evoked population spikes were recorded at the CA1 cell body iayer with micropipettes (1M NaCl) and their amplitude measured from the photographic film [see 11. Procedure In the basic experiment a stable baseline was first obtained by recording and averaging the amplitudes of
1 ct
I I I I TET
I 5
I IQ
three population spike responses to 0.1 Hz stimulation every five minutes. Following a control period of at least IO min during which these responses remained stahle (wrthin approximately 10% of the maximum response) a sin& tetanus of 33 Hz for 3 set was administered to either s. oriens, s. radiatum or the alveus. Responses to stimulation of both pathways were then tested at one minute intervals for the first five minutes and thereafter at five mrnutc intervals up to 30 min posttetanus. .4t each interval three responses were obtained at 0.1 HL and the means of these used in analyzing the data. Because a transient short-term heterosynaptic depression was apparent (as has hcen prcviously reported [3,5] ) responses were evaluated throughout the 30 min interval in order to ;ISSCSSlasting effects.
The means of 16 such experiments, 8 in which s. oriens was tetanized and 8 in which s. radiatum was tetanized is illustrated in Fig. 1. Responses to stimulation of the tetanized pathway were subsequently potentiated while responses to stimulation of the nontetanized path were depressed below control level for approximately five minutes posttetanus. Potentiation and depression as measured at the two minute interval were 309% and 80.6% of
I
15
I
I
I
20
25
30
o--o
MiNUTES FIG. 1. Basic test for heterosynaptic effects. Points are population spike amplitudes expressed as percent of pretetanus control values (a). Each data point is the mean of amplitudes obtained during 16 experiments (8 in which s. radiatum was tetanized and 8 in which s. oriens was tetanized). Responses to stimulation of the tetanized pathway (0) showed a potentiation which lasted throughout the 30 minute posttetanus period. Responses to stimulation of the nontetanized pathway (*I were transiently depressed before regaining control levels at about five minutes posttetanus.
HIPPOCAMPAL
HETEROSYNAPTIC
c r
TET
183
DEPRESSION
5
IO
15
20
25
-
3or
5
TET
10
15
20
25
30
e-----o
MINUTES FIG. 2. Extension of the basic experiment. Thirty minutes following establishment of LTP in response to stimulation of one pathway (o), the originally nontetanized pathway (0) was potentiated by a tetanus (at 2nd arrow). Notice values for the initially potentiated pathway are only slightly affected by this treatment. Points represent means of four experiments, two in which s. oriens tetanization preceded s. radiatum stimulation and two in which the reverse was true.
control levels, respectively. These values compare well with previously published data [5]. The depression of the nontetanized pathway lasted from less than one minute to 12 minutes, averaging 4.84 f 3.96 minutes (mean and standard error). Figure 2 presents results of four experiments in which, at 30 minutes following potentiation of one pathway, the originally nontetanized pathway was tetanically stimulated (at second arrow). Responses to stimulation of this second pathway were subsequently potentiated while the responses to the first were only slightly affected. That no substantial change in responsivity to the originally stimulated pathway (open circles) has occurred may be seen by comparing the mean of the potentiated values (309%) from 5-30 minutes after the first tetanus with mean of these same points (291%) after the second tetanus. A f-test (N = 6) reveals no difference for p>O.2. Since the responses often decrease slightly with time posttetanus (e.g., Fig. 3; 1.1, 5, 61) the small decrease seen here (5%) is probably unrelated to tetanization of the second pathway. Although population responses appear relatively stable in the in vitro preparation, some long-term variability may occur. In 15 control experiments (12 in which no tetani were given and 3 in which tetani were delivered but no LTP was produced) the mean response amplitude at 30 minutes postcontrol relative to basal levels was 97,1%, standard
error + 26.3%. Such variability affects the interpretation of subtle long lasting alterations. In order to distinguish the effects of simply activating the cells from effects due to synaptic activation and long-term potentiation, the basic experiment was repeated with stimulation given antidromically and paired in different experiments with either s. oriens or s. radiatum stimulation. Figure 3 shows, as previously reported [ 71, that antidromic stimulation does not produce long-term potentiation. Antidromic stimulation depressed responses to orthrodromic synaptic activation for a short period (Fig. 3Bl). The antidromic response itself appeared relatively insensitive to the effects of tetanization (Fig. 3A or 3B2).
DISCUSSION
These experiments confirm the existence of a heterosynaptic depression of short duration. However, the depression occurred after non-successful tetanization (i.e., when LTP was not produced) as well as after antidromic tetani. Therefore, it seems to represent a nonspecific effect of neuronal activation as distinct from synaptic activation or LTP. This short-term influence may have been missed in a previous study [S] because of an overly coarse time
ALGER, MEGELA AND TEYLER
184
I~IG.3. Effects of antidromic stimulation. In A stimulation was delivered orthodromically to CA1 (1~) (in two experiments s. oriens was tetanized and in two s. radiatum was tetanized). Note the relative lack of effect on antidromic responses (0) while orthodromic responses are potentiated. In B stimulation was delivered antidromically. The orthodromic response Bl was transiently depressed, while again the antidromic response B2 was unaffected. The results of four experiments are represented in B (different experiments than in A); two in which the s. oriens response was paired with the antidromic response, and two in which the s. radiatumevoked response was paired with the antidromic response.
analysis. The present experiments confirm our earlier results showing no long-term interactive effects. Hence, LTP appears to be an input-specific alteration, possibly due to presynaptic mechanisms such as an augmentation of transmitter release [ 31 or to postsynaptic mechanisms such as local excitatory alterations at the dendritic synapses [ 41. It may be that the short-term heterosynaptic depression
seen in this tissue is of some functional importance in the registration of new information in the nervous system. It could, for example, represent a contrast enhancing property of these cells somewhat analagous to lateral inhibition. Such a property might serve to accentuate a recently encoded change in synaptic efficacy and to increase its signal-to-noise ratio.
REFERENCES Alger, B. E. and T. .I. Teyler. Long-term and short-term plasticity in the CAl, CA3 and dentate regions of the rat hippocampal slice. Bruin Res. 110: 463-480, 1976. Alger, B. E. and T. J. Teyler. A monosynaptic excitatory fiber system studied in vitro: Evidence for an ipsilateral associational pathway in CA1 stratum oriens. Bruin Res. Bull. 2: 355-365, 1977. Andersen, P., S. H. Sundberg, 0. Sveen and H. Wigstrbm. Specific long-lasting potentiation of synaptic transmission in hippocampal slices. Nature 266: 736-737, 1977. Bliss, T. V. P. and T. L$mo. Long-lasting potentiation ot synaptic transmission in the dentate area of the anesthetized rabbit following stimulation of the perforant path. J. Physiol 232: 331-356, 1973.
Lynch, G. S., T. Dunwiddie and V. Gribkoff. Heterosynaptic depression: A postsynaptic correlate of long-term potentiation. h’ature 266: 737-739, 1977. 6. Lynch, G. S., V. K. Gribkoff and S. A. Deadwyler. Long-term potentiation is accompanied by a reduction in dendritic responsiveness to glutamic acid. Nature 263: 151-153, 1976. 7. Schwartzkroin, P. A. and K. Wester. Long-lasting facilitation of a synaptic potential following tetanization in the in vitro hippocampal slice. Bruin Rex 89: 107-119, 1975. of plasticity 8. Teyler, T. J. and B. E. Alger. Candidate mechanisms in the hippocampus. Neuroscience Abs., Vol. 2, 1976, No. 1212. 5.
Vol.
3,
pp. 181-184.
Printed in the U.S.A
BRIEF COMMUNICATION Transient Heterosynaptic
Depression in the Hippocampal Slice
BRADLEY E. ALCER,’ ANDREA L. MEGELA’ AND TIMOTHY J. TEYLER3 Deyartmeilt
ofFs~~cholo~v arzd Social Relations, Harvard Universit_v Cambridge, MA 02138 (Received
14 October
1977)
ALCI’R, B. E., A. L. Mt-GfiLA AND T. J. TEYLER. Transient hetemsynaptic depression in thehippocampalslice. BRAIN RES. BULL. 3(2) 181-184, 1978. -- Two independent, excitatory, monosynaptic afferent fiber systems projecting onto basal and apical dendritic layers of the CA1 sub-field of the hippocampus were tested for interactive effects. Long-term potentiation of either of the pathways was accompanied by a transient depression in the other pathway. Similar transient depression could be elicited by antidromic stimulation of the CA1 cells. No long-term heterosynaptic effects were observed. Hippocampus
Hippocampal slice
Neuronal response plasticity
Heterosynaptic
depression
be significant; guinea pigs have been used by some investigators (31 and rats [5,6] by others. In addition it has not been shown that the reported changes are dependent on synaptic activation of the cells and are not present following non-synaptic activation. Finally, some measure of response variability is essential in order to evaluate any changes. Although usually quite stable for hours, hippocampal slices do deteriorate [ 1,7] and hippocampal responses are variable over time [ 1,4] . Using laminar analysis, microsurgical techniques and control experiments to assess possible current spread, we have recently shown that stimulation delivered to stratum oriens (s. oriens) activates fibers traveling only in s. oriens [2]. Measurements of the frequency following characteristics and synaptic delay of the extracellular EPSP indicates that the s. oriens fibers make monosynaptic intracellular with CA1 cells. Furthermore, contact recordings from individual CA1 cells indicated action potentials could be triggered equally well from either s. oriens or s. radiatum stimulation. Thus a single population of cells appears to be involved in both responses. From these results arose the question of possible interactions between effects of stimulation to each fiber system [ 81. This communication confirms the presence of a short-term heterosynaptic depression following low-frequency tetanic stimulation. The depression was not restricted to instances in which long-term potentiation was produced and indeed was seen following antidromic tetani-
LONG-TERM potentiation (LTP) is a dramatic increase in neuronal responsiveness produced by a brief train, or series of trains, of stimuli [ 1, 2, 4, 5, 6, 7j which in the hippocampus lasts for at least 30 minutes. The ease with which this form of neuronal plasticity is instituted and its durability make it an important model for the study of how the nervous system registers events of behavioral significance. A fundamental question in understanding the basis of LTP concerns how this phenomenon is mediated, whether pre- or postsynaptically. Evidence consonant with a general postsynaptic effect is the demonstration in the hippocampal slice preparation that glutamate sensitivity of CA1 cells is depressed when long-term potentiation is produced but not otherwise 161. In other experiments the establishment of LTP by stimulation delivered to one pathway depressed the responsiveness of a given cell group to stimulation by another input pathway (heterosynaptic depression [ 5 j ; but see [ 81). However, when the size of the input volley is monitored a lasting general effect is not found [31. None of these experiments demonstrated that stimulation delivered to two different areas of the hippocampal slice in fact activated two separate afferent pathways. Differential current spread to nearby areas could conceivably account for the differences in results. Alternatively, even if two distinct inputs exist, the same cell population may not be influenced by both. Species differences might
1This work was supported by NSF Grant BMS75 -02802 (TJT); BEA and ALM held NIMH Predoctoral Fellowships. ‘Present address: Department of Pharmacology and Physiology, School of Medicine, University of California, San Francisco, CA 94143. Send reprint requests to B.E.A. ‘Present address: Section of Neurobiology and Behavior, Langmuir Laboratory, Cornell University, Ithaca, NY 14853. ‘Present address: Neurobiology Program, Northeastern Ohio Universities College of Medicine, Rootstown, OH 44272. 181
0361-9230/78/0302-0181$00.60
Copyright Q 1978 ANKHO
International Inc.
ALGER, MEGELA AND TEYLER
182
zation of CA1 cells. Long-term not evident.
heterosynaptic
effects were
METHOD
The rat in vitro hippocampal slice was used in these experiments. Details of the slice preparation as used in our laboratory are available [see 11 Experiments were conducted on 43 slices maintained at 33-35°C from 2.5 male Sprague-Dawley rats. Stimulation ( I -I 0 V) was delivered to s. oriens, s. radiatum or the alveus with small concentric bipolar electrodes (o.d. 300 microns). Stimulating electrodes were placed in s. oriens anterior (i.e., towards the fimbria) with respect to recording electrodes and in s. radiatum (or the alveus in certain experiments) posterior (i.e., towards the subiculum) with respect to recording electrodes. The evoked population spikes were recorded at the CA1 cell body iayer with micropipettes (1M NaCl) and their amplitude measured from the photographic film [see 11. Procedure In the basic experiment a stable baseline was first obtained by recording and averaging the amplitudes of
1 ct
I I I I TET
I 5
I IQ
three population spike responses to 0.1 Hz stimulation every five minutes. Following a control period of at least IO min during which these responses remained stahle (wrthin approximately 10% of the maximum response) a sin& tetanus of 33 Hz for 3 set was administered to either s. oriens, s. radiatum or the alveus. Responses to stimulation of both pathways were then tested at one minute intervals for the first five minutes and thereafter at five mrnutc intervals up to 30 min posttetanus. .4t each interval three responses were obtained at 0.1 HL and the means of these used in analyzing the data. Because a transient short-term heterosynaptic depression was apparent (as has hcen prcviously reported [3,5] ) responses were evaluated throughout the 30 min interval in order to ;ISSCSSlasting effects.
The means of 16 such experiments, 8 in which s. oriens was tetanized and 8 in which s. radiatum was tetanized is illustrated in Fig. 1. Responses to stimulation of the tetanized pathway were subsequently potentiated while responses to stimulation of the nontetanized path were depressed below control level for approximately five minutes posttetanus. Potentiation and depression as measured at the two minute interval were 309% and 80.6% of
I
15
I
I
I
20
25
30
o--o
MiNUTES FIG. 1. Basic test for heterosynaptic effects. Points are population spike amplitudes expressed as percent of pretetanus control values (a). Each data point is the mean of amplitudes obtained during 16 experiments (8 in which s. radiatum was tetanized and 8 in which s. oriens was tetanized). Responses to stimulation of the tetanized pathway (0) showed a potentiation which lasted throughout the 30 minute posttetanus period. Responses to stimulation of the nontetanized pathway (*I were transiently depressed before regaining control levels at about five minutes posttetanus.
HIPPOCAMPAL
HETEROSYNAPTIC
c r
TET
183
DEPRESSION
5
IO
15
20
25
-
3or
5
TET
10
15
20
25
30
e-----o
MINUTES FIG. 2. Extension of the basic experiment. Thirty minutes following establishment of LTP in response to stimulation of one pathway (o), the originally nontetanized pathway (0) was potentiated by a tetanus (at 2nd arrow). Notice values for the initially potentiated pathway are only slightly affected by this treatment. Points represent means of four experiments, two in which s. oriens tetanization preceded s. radiatum stimulation and two in which the reverse was true.
control levels, respectively. These values compare well with previously published data [5]. The depression of the nontetanized pathway lasted from less than one minute to 12 minutes, averaging 4.84 f 3.96 minutes (mean and standard error). Figure 2 presents results of four experiments in which, at 30 minutes following potentiation of one pathway, the originally nontetanized pathway was tetanically stimulated (at second arrow). Responses to stimulation of this second pathway were subsequently potentiated while the responses to the first were only slightly affected. That no substantial change in responsivity to the originally stimulated pathway (open circles) has occurred may be seen by comparing the mean of the potentiated values (309%) from 5-30 minutes after the first tetanus with mean of these same points (291%) after the second tetanus. A f-test (N = 6) reveals no difference for p>O.2. Since the responses often decrease slightly with time posttetanus (e.g., Fig. 3; 1.1, 5, 61) the small decrease seen here (5%) is probably unrelated to tetanization of the second pathway. Although population responses appear relatively stable in the in vitro preparation, some long-term variability may occur. In 15 control experiments (12 in which no tetani were given and 3 in which tetani were delivered but no LTP was produced) the mean response amplitude at 30 minutes postcontrol relative to basal levels was 97,1%, standard
error + 26.3%. Such variability affects the interpretation of subtle long lasting alterations. In order to distinguish the effects of simply activating the cells from effects due to synaptic activation and long-term potentiation, the basic experiment was repeated with stimulation given antidromically and paired in different experiments with either s. oriens or s. radiatum stimulation. Figure 3 shows, as previously reported [ 71, that antidromic stimulation does not produce long-term potentiation. Antidromic stimulation depressed responses to orthrodromic synaptic activation for a short period (Fig. 3Bl). The antidromic response itself appeared relatively insensitive to the effects of tetanization (Fig. 3A or 3B2).
DISCUSSION
These experiments confirm the existence of a heterosynaptic depression of short duration. However, the depression occurred after non-successful tetanization (i.e., when LTP was not produced) as well as after antidromic tetani. Therefore, it seems to represent a nonspecific effect of neuronal activation as distinct from synaptic activation or LTP. This short-term influence may have been missed in a previous study [S] because of an overly coarse time
ALGER, MEGELA AND TEYLER
184
I~IG.3. Effects of antidromic stimulation. In A stimulation was delivered orthodromically to CA1 (1~) (in two experiments s. oriens was tetanized and in two s. radiatum was tetanized). Note the relative lack of effect on antidromic responses (0) while orthodromic responses are potentiated. In B stimulation was delivered antidromically. The orthodromic response Bl was transiently depressed, while again the antidromic response B2 was unaffected. The results of four experiments are represented in B (different experiments than in A); two in which the s. oriens response was paired with the antidromic response, and two in which the s. radiatumevoked response was paired with the antidromic response.
analysis. The present experiments confirm our earlier results showing no long-term interactive effects. Hence, LTP appears to be an input-specific alteration, possibly due to presynaptic mechanisms such as an augmentation of transmitter release [ 31 or to postsynaptic mechanisms such as local excitatory alterations at the dendritic synapses [ 41. It may be that the short-term heterosynaptic depression
seen in this tissue is of some functional importance in the registration of new information in the nervous system. It could, for example, represent a contrast enhancing property of these cells somewhat analagous to lateral inhibition. Such a property might serve to accentuate a recently encoded change in synaptic efficacy and to increase its signal-to-noise ratio.
REFERENCES Alger, B. E. and T. .I. Teyler. Long-term and short-term plasticity in the CAl, CA3 and dentate regions of the rat hippocampal slice. Bruin Res. 110: 463-480, 1976. Alger, B. E. and T. J. Teyler. A monosynaptic excitatory fiber system studied in vitro: Evidence for an ipsilateral associational pathway in CA1 stratum oriens. Bruin Res. Bull. 2: 355-365, 1977. Andersen, P., S. H. Sundberg, 0. Sveen and H. Wigstrbm. Specific long-lasting potentiation of synaptic transmission in hippocampal slices. Nature 266: 736-737, 1977. Bliss, T. V. P. and T. L$mo. Long-lasting potentiation ot synaptic transmission in the dentate area of the anesthetized rabbit following stimulation of the perforant path. J. Physiol 232: 331-356, 1973.
Lynch, G. S., T. Dunwiddie and V. Gribkoff. Heterosynaptic depression: A postsynaptic correlate of long-term potentiation. h’ature 266: 737-739, 1977. 6. Lynch, G. S., V. K. Gribkoff and S. A. Deadwyler. Long-term potentiation is accompanied by a reduction in dendritic responsiveness to glutamic acid. Nature 263: 151-153, 1976. 7. Schwartzkroin, P. A. and K. Wester. Long-lasting facilitation of a synaptic potential following tetanization in the in vitro hippocampal slice. Bruin Rex 89: 107-119, 1975. of plasticity 8. Teyler, T. J. and B. E. Alger. Candidate mechanisms in the hippocampus. Neuroscience Abs., Vol. 2, 1976, No. 1212. 5.