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Interactions between septal and entorhinal inputs to the rat dentate gyrus: Facilitation effects
Brain Research, 379 (1986) 63-67 Elsevier
63
BRE 11875
Interactions Between Septal and Entorhinal Inputs to the Rat Dentate Gyrus: Facilitation Effects G.B. ROBINSON* and R.J. RACINE Department of Psychology, McMaster University, Hamilton, Ont. (Canada)
(Accepted December 10th, 1985) Key words: facilitation - - depression - - paired-pulse - - entorhinal cortex - - septum - - dentate gyrus
We examined facilitation effects between the medial septum and perforant path inputs to the dentate gyrus for the four possible combinations of paired-pulse activation. Facilitation effects occurred in all cases. The largest facilitation effects occurred when the septal pulse served as the conditioning pulse for the population spike subsequently evoked by a perforant path pulse. Using 3 pulses, we also examined the influence of septal activation on paired-pulse facilitation of the perforant path-granule cell population spike. A septal stimulation pulse, applied 6-10 ms prior to the onset of the population spike evoked by a perforant path conditioning pulse, did not affect the perforant path-dentate test response at any interpulse interval. If the septal pulse occurred immediately prior to population spike onset, however, there was a significantly greater depression of the test response from 70-3000 ms, but no effect at early intervals (20-50 ms). The effect of the septal pulse appears more consistent with a direct action of the septal terminals on granule cells than with an indirect action via the recurrent inhibitory interneurons.
INTRODUCTION In the previous report we d e m o n s t r a t e d a cooperativity effect between the septal and entorhinal inputs to dentate gyrus granule cells. Activation of both pathways resulted in significantly greater longterm potentiation (LTP) of the perforant path-granule cell (PP-GC) population spike than occurred following trains applied to the perforant path alone. This cooperativity effect persisted when the septal trains p r e c e d e d the perforant path trains by up to 2000 ms, with the largest effects occurring at intertrain intervals of 100 ms or less 21. There are at least two possible mechanisms for this effect. First, the septal input may directly activate the granule cells, increasing the level of afferent excitatory input and thereby granule cell discharge, in support of this hypothesis there is both anatomical and physiological evidence of an input from the septum to granule cell somata and dendrites 4"9'16. Second, the
septal input may be inhibitory to hilar interneurons 17, thereby suppressing the recurrent inhibitory circuits 7'14 and increasing the postsynaptic effect of each perforant path train. Blocking recurrent inhibition in the h i p p o c a m p a l slice p r e p a r a t i o n has been shown to increase the magnitude of P P - G C LTP 26. To distinguish between these two possibilities we examined the effect of septal stimulation on P P - G C paired-pulse depression and facilitation. Pairedpulse tests provide a measure of the strength of the recurrent inhibitory circuits 15'2°. If the conditioning pulse triggers cell discharge (as indicated by the presence of the population spike3), then a test response falling within the period of active inhibition will be depressed. If the septal input blocks or reduces recurrent inhibition then it should also block or reduce paired-pulse depression in the P P - G C circuit. Activation of the septal input a few ms prior to the perforant path conditioning pulse (PP1), for example, should block the interneurons before they are activated by
* Current address: University of Pittsburgh, Psychobiology Program, 459 Crawford Hall, Pittsburgh, PA 15260, U.S.A. Correspondence: G.B. Robinson, University of Pittsburgh, Psychobiology Program, 459 Crawford Hall, Pittsburgh, PA 15260, U.S.A. 0006-8993/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)
64 granule cell collaterals. A subsequent perforant path pulse (PP2; the test pulse) would, therefore, be expected to generate a population spike of relatively greater amplitude. MATERIALS AND METHODS
Double-pulse tests Twelve male Long-Evans Hooded rats were prepared for surgery and acute electrophysioiogical recording of PP-GC and SD-GC field potentials as previously described 21"22. To study paired-pulse effects within a pathway (PP1-PP2; SD1-SD2), two stimuli of equal amplitude were delivered through the same electrode. The initial pulse (PP1) served as a conditioning stimulus and evoked the first response, and the second pulse (PP2) evoked the test response. To examine heterosynaptic effects (PP1-SD2; SD1-PP2 ) either the septal or perforant path pulse served as the conditioning pulse, while the test pulses were applied to the perforant path or septum, respectively. Perforant path stimulation was set at an intensity sufficient to consistently evoke a small population spike in response to a single pulse. The septal intensity was 1.2 mA, which was the same intensity used for testing train effects in the previous experiments 21,22. The interpulse intervals (IPIs) for the pulse-pairs were 20. 30, 50.70. 100. 150, 200. 300. 500 and 1000 ms. There was a 10 s interval between each pulse-pair.
Triple-pulse tests Eleven rats were prepared for these experiments. The anesthetic used in this experiment was urethane (1.5 g/kg), as sodium pentobarbitol was known to increase recurrent inhibition in the hippocampal formation t,6,19. Urethane leaves an apparently normal early inhibition and does not interfere with the magnitude of the heterosynaptic interactions observed during double pulse tests (Robinson and Racine. unpublished observations). Two conditions were examined: (1) SD-PP1-PP2 (septal pulse 6-10 ms prior to the onset of the population spike onset); and (2) PP1-SD-PP2 (septal twice the latency between the PP1 pulse and the population spike onset): and (2) PP1-SD-PP2 (septal pulse immediately prior to population spike and consequently following the PP1 pulse by 3-5 ms). The
effect of the septal pulse on PPI-PP2 depression and/or facilitation was compared against the effect of pulse-pairs applied to the perforant path alone (PPtPP2). The IPIs were the same as those used in the double pulse experiment, except that IPIs of 2, 3, 5 and 10 s were also tested. This was done to determine the duration of the late depression. The interpair interval was increased from 10 to 15 s. For the SD-PPI-PP2 condition, the intensities of the perforant path pulses were adjusted to correct for facilitation induced by the septal pulse. This correction equated the PP-GC population spike amplitude with that evoked by the conditioning pulse (PPl) in the other conditions.
Data analysis Population spike amplitude was determined as previously described 22. Briefly, the population spike amplitude was measured from the tangent, joining spike onset and offset, to the spike peak height ~. For the granule cell response to activation of the septum. the initial positive component was measured from onset to peak. The magnitude of the paired-pulse effects were calculated as the percent change in the amplitude of the test response over the conditioning response. If PP1 had no effect on the test response, the percent change was 0%. so the test response amplitude was recorded as 100% of baseline. The data were then averaged for each interval, across all animals. RESULTS
Double-pulse effects Fig. I illustrates the effects of perforant path and septal conditioning pulses on the PP-GC population spike (the test response m these measures). Pulsepairs applied only to the perforant path resulted in the typical triphasic depression/facilitation/ depression response curve 6A5"2°. The period of early depression was from 20 to 100 ms. and at the two shortest intervals (20 and 30 ms), the test population spike was completely eliminated. Peak facilitation occurred at an IPI of 150 ms (152 -~ 12%). The delay to peak facilitation may, in part. be due to activation of the recurrent inhibitory circuits 2°. The late depression phase occurred from 500 to 1000 ms (the largest IPI examined in this group).
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Fig. I. Effect of conditioning pulse on test PP-GC population spike as a function of the interpulse interval. Both depression and facilitation were observed when the conditioning pulse was applied to the perforant path (solid line; n = 50). When the conditioning pulse was applied to the septal input, only heterosynaptic facilitation was observed (dashed line; n = 70).
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Fig. 2. E f f e c t o f c o n d i t i o n i n g p u l s e o n test S D - G C E P S P as a f u n c t i o n o f the i n t e r p u l s e i n t e r v a l . C o n d i t i o n i n g p u l s e s w e r e a p p l i e d to e i t h e r the s e p t a l a f f e r e n t s (solid line; n = 85) o r the p e r f o r a n t p a t h a f f e r e n t s ( d a s h e d line; n = 6 0 ) .
Triple-pulse effects In experiment 1, the paired-pulse curves produced by the SD1-PP2 and PP1-PP2 pairings (see Fig. 1)
In contrast, there was no early depression observed with the SD1-PP2 pairings, which showed a peak facilitation of 408 + 29%, at the 20 ms IPI. In addition to its greater magnitude, the heterosynaptic facilitation was also longer-lasting than homosynaptic facilitation. At an IPI of 500 ms, for example, the magnitude of heterosynaptic facilitation was 138% compared to a 9% depression with the homosynaptic pairings. There was no late depression with the SD1PP2 pairings. These results are similar to those previously reported for septal-perforant path pair-
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ings 2,1o,22. Fig. 2 illustrates the results of the paired-pulse tests when the S D - G C field potential was the test response. Peak facilitation following the SD1-SD2 pairings (175 + 5%) was greater than that observed following the PP1-SD2 pairings (151 + 2%). Peak facilitation effects occurred at 70 ms and 30 ms for the homosynaptic and heterosynaptic pairings, respectively. The initial delay to peak facilitation may have been due to transmitter depletion or to activation of the recurrent inhibitory circuits 2°. Whatever the mechanism, the initial depression (delay to peak facilitation) was never as large as observed when testing the population spike evoked in the dentate by perforant path stimulation. Homosynaptic facilitation effects were also longer lasting than those observed for the heterosynaptic pairing.
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Fig. 3. A: comparison of PPI-PP2 (solid line) and SD-PP1-PP2
(dashed line) effects (n = 60). B: PPI-PP2 (solid line) and PP1SD-PP2 (dashed line) effects (n = 70) at interpulse intervals up to 10 s.
66 were mirror images of each other up to an IPI of 150 ms. The perforant path pairs resulted in depression of the test response, whereas only facilitation was observed with the SD1-PP2 pairs. This is consistent with a possible septal suppression or blockade of granule cell recurrent inhibition. We tested this possibility further with a triple-pulse experiment. Fig. 3A compares the SD-PP1-PP2 and PP1-PP2 conditions when the amplitudes of the perforant path pulses were corrected for the effects of the septal pulse. Application of the septal pulse, prior to PP1, appeared to have no effect on the magnitude of paired-pulse depression or facilitation. For example, peak facilitation and depression observed in the PP1PP2 condition was 238 ( + 8 % , n = 9) and 59 (+2%), respectively. For the SD-PP1-PP2 condition the respective values were 222 (+14%) and 59 (_+5%). In contrast, when the septal pulse immediately preceded the population spike evoked by PP1 (PPISD-PP2) the magnitude of the late depression was significantly increased from 70 to 3000 ms (Fig. 3B). The average test response at the peak of this depression was 48% (+1%) of normal amplitude in the PP1-SD-PP2 group and 63% (___3%) in the PP1-PP2 group (P < 0.05; t-test for correlated means). There was no effect on the magnitude of either the early depression (72 + 10% vs 71 +_ 9%) or peak facilitation (234 + 8% vs 226 + 7%). DISCUSSION Short-term interaction effects were found for all four pulse-pair combinations. The largest and longest-lasting effects occurred when the septal pulse served as the conditioning pulse for the PP-GC population spike. These results are similar to those previously reported 2"6"1°A5"16"22. In the triple-pulse experiments, it appeared that single pulse activation of the septal area did not interfere with recurrent inhibition. If the septal input were mainly to basket cell interneurons, then application of the septal pulse prior to PP1 should have blocked the interneurons before they were activated by collateral discharge from the granule cells. The septal pulse, however, had no effect on the magnitude of the early depression. G. Goddard (personal communication) has recently found that a septal pulse applied within a few ms of the onset of the popula-
tion spike (presumably falling between the "immediate' and 6 ms intervals used in our experiment) did interfere with subsequent paired pulse depression effects. This temporal window is far too narrow to account for either the facilitation effects of septal pulses reported in this paper, or the cooperativity effects of septal trains reported in the previous paper zl. Our results, therefore, appear to be more consistent with a direct action of the septal terminals on granule cells rather than an indirect action via inhibitory interneurons. This interpretation is also consistent with previous results showing that the SD-GC pathway is itself capable of LTP 16'22. and that septal terminals contact granule cell somata and may have an additional direct projection to the molecular laver 9A°. The putative transmitter in the septal pathway is acetvlcholine (ACh), and it has been suggested that ACh may exert its excitatory influence by decreasing recurrent inhibition 7'14. Our triple-pulse results do not appear consistent with an ACh mechanism, however. as the recurrent inhibition did not appear to be blocked. Furthermore. there is evidence that ACh may decrease K* conductance in the hippocampus 7. Our PP1-SD-PP2 group showed an increase in the magnitude of the late depression which would seem to be more consistent with an increase in K* conductance ls'25 Fantie and Goddard 1° found that various cholinergic agonists and antagonists had no effect on the magnitude of SD-PP facilitation. It is possible that we were acuvatmg fibres of passage. Noradrenergic fibres run through this region. and Hopkins and Johnson 12 have shown that noradrenerglc activation can facilitate LTP in the mossy fiber-CA3 pathway. On the other hand. noradrenaline depletion does not reduce either SD-PP facilitation 1° or PP-GC LTP z3. Although the effects of serotonin depletion on SD-PP facilitation have not been tested, paired stimulation of the raphe nucleus, a source of hippocampal serotonin H. and the perforant path has been shown to increase the magnitude of the PP-GC population spike 5. Also. serotonin depletion appears to decrease the synaptic component o f LTP s. and Sega124 has suggested that serotonin, applied to CA I pyramidal cells, increases K ~ conductance (consistent with the triple-pulse resultsl. A serotonergic modulation of neural plasticity could utilize a second messenger system, as reported for Aplysia L~ which might account for the duration over which sep-
67 tal trains can i n f l u e n c e P P - G C L T P 21. I n t r a c e l l u l a r
helpful c o m m e n t s on e a r l i e r drafts of this m a n u -
r e c o r d i n g and n e u r o c h e m i c a l t e c h n i q u e s m a y be re-
script. A p r e l i m i n a r y r e p o r t of these results was pre-
q u i r e d to resolve the q u e s t i o n s raised by t h e s e data.
s e n t e d at the 14th A n n u a l M e e t i n g of the Society for N e u r o s c i e n c e , O c t o b e r , 1984. This r e s e a r c h was sup-
ACKNOWLEDGEMENTS
p o r t e d by an o p e r a t i n g grant to R . J . R . and a p o s t g r a d u a t e scholarship to G . B . R . , f r o m the N a t u r a l Sci-
W e w o u l d like to t h a n k Dr. T . W . B e r g e r for his
REFERENCES 1 Adamec, R.E., McNaughton, B., Racine, R. and Livingston, K.E., Effects of diazepam on hippocampal excitability in the rat: action in the dentate area, Epilepsia, 22 (1981) 205-215. 2 Alvarez-Leefmans, F.J. and Gardner-Medwin, A.R., Influences of the septum on the hippocampal dentate area which are unaccompanied by field potentials, J. Physiol. (London), 249 (1975) 14P-16P. 3 Andersen, P., Bliss, T.V.P. and Skrede, K.K., Unit analysis of hippocampal population spikes, Exp. Brain Res., 13 (1971) 208-221. 4 Andersen, P., Bruland, H. and Kaada, B.R., Activation of the dentate area by septal stimulation, Acta Physiol. Scand., 51 (1961) 17-28. 5 Assaf, S.Y. and Miller, J.J., Neuronal transmission in the dentate gyrus: role of inhibitory mechanisms, Brain Research, 151 (1978) 587-592. 6 Barnes, C.A., Memory deficits associated with senescence: a neurophysiological and behavioral study in the rat, J. Comp. Physiol. Psychol., 93 (1979) 74-104. 7 Ben-Ari, Y., Krnjevic, K., Reinhardt, W. and Ropert, N., Intracellular observations on the disinhibitory action of acetylcholine in the hippocampus, Neuroscience, 6 (1981) 2475-2484. 8 Bliss, T.V.P., Goddard, G.V. and Riives, M., Reduction of long-term potentiation in the dentate gyrus of the rat following selective depletion of monoamines, J. Physiol. (London), 334 (1983) 475-491. 9 Chandler, J.P. and Crutcher, K.A., The septohippocampal projection in the rat: an electron microscopic horseradish peroxidase study, Neuroscience, 10 (1983) 685-696. 10 Fantie, B.D. and Goddard, G.V., Septal modulation of the population spike in the fascia dentata produced by perforant path stimulation in the rat, Brain Research, 252 (1982) 227-237. 11 Halaris, A.E., Jones, B.E. and Moore, R.Y., Axonal transport in serotonin neurons of the midbrain raphe, Brain Research, 107 (1976) 555-575. 12 Hopkins, W.F. and Johnston, D., Frequency-dependent noradrenergic modulation of long-term potentiation in the hippocampus, Science, 226 (1984) 350-352. 13 Kandel, E.R. and Schwartz, J.H., Molecular biology of learning: modulation of transmitter release, Science, 218
ences and E n g i n e e r i n g R e s e a r c h C o u n c i l of C a n a d a .
(1982) 433-443. 14 Krnjevic, K., Reiffenstein, R.J. and Ropert, N., Disinhibitory action of acetylcholine in the rat's hippocampus: extracellular observations, Neuroscience, 6 (1981) 2465-2474. 15 L0mo, T., Potentiation of monosynaptic EPSPs in the perforant path-dentate granule cell synapse, Exp. Brain Res., 12 (1971) 46-63. 16 McNaughton, N. and Miller, J.J., Medial septal projections to the dentate gyrus of the rat: electrophysiological analysis of distribution and plasticity, Exp. Brain Res., 56 (1984) 243-256. 17 Mosko, S., Lynch, G. and Cotman, C.W., The distribution of septal projections to the hippocampus of the rat, J. Comp. Neurol., 52 (1973) 163-174. 18 Nicoll, R.A. and Alger, B.E., Synaptic excitation may activate a calcium dependent potassium conductance in hippocampal pyramidal cells, Science, 212 (1981) 957-959. 19 Nicoll, R.A., Eccles, J.C., Oshima, T. and Rubia, F., Prolongation of hippocampal inhibitory post-synaptic potentials by barbiturates, Nature (London), 258 (1975) 625-627. 20 Racine, R.J. and Milgram, N.W., Short-term potentiation phenomena in the rat limbic forebrain, Brain Research, 260 (1983) 201-216. 21 Robinson, G.B., Enhanced long-term potentiation induced in rat dentate gyrus by co-activation of septal and entorhihal inputs: temporal constraints, Brain Research, 379 (1986) 56-62. 22 Robinson, G.B. and Racine, R.J., Heterosynaptic interactions between septal and entorhinal inputs to the dentate gyrus: long-term potentiation effects, Brain Research, 249 (1982) 162-166. 23 Robinson, G.B. and Racine, R.J., Long-term potentiation in the dentate gyrus: effects of noradrenaline depletion in the awake rat, Brain Research, 325 (1985) 71-78. 24 Segal, M., The action of serotonin in the rat hippocampal slice preparation, J. Physiol. (London), 303 (1980) 423-439. 25 Thalmann, R.H. and Ayala, G.F., A late increase in potassium conductance follows synaptic stimulation of granule neurons of the dentate gyrus, Neurosci. Lett., 29 (1982) 243-248. 26 Wigstrom, H. and Gustafsson, B., Large long-lasting potentiation in the dentate gyrus in vitro during blockade of inhibition, Brain Research, 275 (1983) 153-158.
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Interactions Between Septal and Entorhinal Inputs to the Rat Dentate Gyrus: Facilitation Effects G.B. ROBINSON* and R.J. RACINE Department of Psychology, McMaster University, Hamilton, Ont. (Canada)
(Accepted December 10th, 1985) Key words: facilitation - - depression - - paired-pulse - - entorhinal cortex - - septum - - dentate gyrus
We examined facilitation effects between the medial septum and perforant path inputs to the dentate gyrus for the four possible combinations of paired-pulse activation. Facilitation effects occurred in all cases. The largest facilitation effects occurred when the septal pulse served as the conditioning pulse for the population spike subsequently evoked by a perforant path pulse. Using 3 pulses, we also examined the influence of septal activation on paired-pulse facilitation of the perforant path-granule cell population spike. A septal stimulation pulse, applied 6-10 ms prior to the onset of the population spike evoked by a perforant path conditioning pulse, did not affect the perforant path-dentate test response at any interpulse interval. If the septal pulse occurred immediately prior to population spike onset, however, there was a significantly greater depression of the test response from 70-3000 ms, but no effect at early intervals (20-50 ms). The effect of the septal pulse appears more consistent with a direct action of the septal terminals on granule cells than with an indirect action via the recurrent inhibitory interneurons.
INTRODUCTION In the previous report we d e m o n s t r a t e d a cooperativity effect between the septal and entorhinal inputs to dentate gyrus granule cells. Activation of both pathways resulted in significantly greater longterm potentiation (LTP) of the perforant path-granule cell (PP-GC) population spike than occurred following trains applied to the perforant path alone. This cooperativity effect persisted when the septal trains p r e c e d e d the perforant path trains by up to 2000 ms, with the largest effects occurring at intertrain intervals of 100 ms or less 21. There are at least two possible mechanisms for this effect. First, the septal input may directly activate the granule cells, increasing the level of afferent excitatory input and thereby granule cell discharge, in support of this hypothesis there is both anatomical and physiological evidence of an input from the septum to granule cell somata and dendrites 4"9'16. Second, the
septal input may be inhibitory to hilar interneurons 17, thereby suppressing the recurrent inhibitory circuits 7'14 and increasing the postsynaptic effect of each perforant path train. Blocking recurrent inhibition in the h i p p o c a m p a l slice p r e p a r a t i o n has been shown to increase the magnitude of P P - G C LTP 26. To distinguish between these two possibilities we examined the effect of septal stimulation on P P - G C paired-pulse depression and facilitation. Pairedpulse tests provide a measure of the strength of the recurrent inhibitory circuits 15'2°. If the conditioning pulse triggers cell discharge (as indicated by the presence of the population spike3), then a test response falling within the period of active inhibition will be depressed. If the septal input blocks or reduces recurrent inhibition then it should also block or reduce paired-pulse depression in the P P - G C circuit. Activation of the septal input a few ms prior to the perforant path conditioning pulse (PP1), for example, should block the interneurons before they are activated by
* Current address: University of Pittsburgh, Psychobiology Program, 459 Crawford Hall, Pittsburgh, PA 15260, U.S.A. Correspondence: G.B. Robinson, University of Pittsburgh, Psychobiology Program, 459 Crawford Hall, Pittsburgh, PA 15260, U.S.A. 0006-8993/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)
64 granule cell collaterals. A subsequent perforant path pulse (PP2; the test pulse) would, therefore, be expected to generate a population spike of relatively greater amplitude. MATERIALS AND METHODS
Double-pulse tests Twelve male Long-Evans Hooded rats were prepared for surgery and acute electrophysioiogical recording of PP-GC and SD-GC field potentials as previously described 21"22. To study paired-pulse effects within a pathway (PP1-PP2; SD1-SD2), two stimuli of equal amplitude were delivered through the same electrode. The initial pulse (PP1) served as a conditioning stimulus and evoked the first response, and the second pulse (PP2) evoked the test response. To examine heterosynaptic effects (PP1-SD2; SD1-PP2 ) either the septal or perforant path pulse served as the conditioning pulse, while the test pulses were applied to the perforant path or septum, respectively. Perforant path stimulation was set at an intensity sufficient to consistently evoke a small population spike in response to a single pulse. The septal intensity was 1.2 mA, which was the same intensity used for testing train effects in the previous experiments 21,22. The interpulse intervals (IPIs) for the pulse-pairs were 20. 30, 50.70. 100. 150, 200. 300. 500 and 1000 ms. There was a 10 s interval between each pulse-pair.
Triple-pulse tests Eleven rats were prepared for these experiments. The anesthetic used in this experiment was urethane (1.5 g/kg), as sodium pentobarbitol was known to increase recurrent inhibition in the hippocampal formation t,6,19. Urethane leaves an apparently normal early inhibition and does not interfere with the magnitude of the heterosynaptic interactions observed during double pulse tests (Robinson and Racine. unpublished observations). Two conditions were examined: (1) SD-PP1-PP2 (septal pulse 6-10 ms prior to the onset of the population spike onset); and (2) PP1-SD-PP2 (septal twice the latency between the PP1 pulse and the population spike onset): and (2) PP1-SD-PP2 (septal pulse immediately prior to population spike and consequently following the PP1 pulse by 3-5 ms). The
effect of the septal pulse on PPI-PP2 depression and/or facilitation was compared against the effect of pulse-pairs applied to the perforant path alone (PPtPP2). The IPIs were the same as those used in the double pulse experiment, except that IPIs of 2, 3, 5 and 10 s were also tested. This was done to determine the duration of the late depression. The interpair interval was increased from 10 to 15 s. For the SD-PPI-PP2 condition, the intensities of the perforant path pulses were adjusted to correct for facilitation induced by the septal pulse. This correction equated the PP-GC population spike amplitude with that evoked by the conditioning pulse (PPl) in the other conditions.
Data analysis Population spike amplitude was determined as previously described 22. Briefly, the population spike amplitude was measured from the tangent, joining spike onset and offset, to the spike peak height ~. For the granule cell response to activation of the septum. the initial positive component was measured from onset to peak. The magnitude of the paired-pulse effects were calculated as the percent change in the amplitude of the test response over the conditioning response. If PP1 had no effect on the test response, the percent change was 0%. so the test response amplitude was recorded as 100% of baseline. The data were then averaged for each interval, across all animals. RESULTS
Double-pulse effects Fig. I illustrates the effects of perforant path and septal conditioning pulses on the PP-GC population spike (the test response m these measures). Pulsepairs applied only to the perforant path resulted in the typical triphasic depression/facilitation/ depression response curve 6A5"2°. The period of early depression was from 20 to 100 ms. and at the two shortest intervals (20 and 30 ms), the test population spike was completely eliminated. Peak facilitation occurred at an IPI of 150 ms (152 -~ 12%). The delay to peak facilitation may, in part. be due to activation of the recurrent inhibitory circuits 2°. The late depression phase occurred from 500 to 1000 ms (the largest IPI examined in this group).
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Fig. I. Effect of conditioning pulse on test PP-GC population spike as a function of the interpulse interval. Both depression and facilitation were observed when the conditioning pulse was applied to the perforant path (solid line; n = 50). When the conditioning pulse was applied to the septal input, only heterosynaptic facilitation was observed (dashed line; n = 70).
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Fig. 2. E f f e c t o f c o n d i t i o n i n g p u l s e o n test S D - G C E P S P as a f u n c t i o n o f the i n t e r p u l s e i n t e r v a l . C o n d i t i o n i n g p u l s e s w e r e a p p l i e d to e i t h e r the s e p t a l a f f e r e n t s (solid line; n = 85) o r the p e r f o r a n t p a t h a f f e r e n t s ( d a s h e d line; n = 6 0 ) .
Triple-pulse effects In experiment 1, the paired-pulse curves produced by the SD1-PP2 and PP1-PP2 pairings (see Fig. 1)
In contrast, there was no early depression observed with the SD1-PP2 pairings, which showed a peak facilitation of 408 + 29%, at the 20 ms IPI. In addition to its greater magnitude, the heterosynaptic facilitation was also longer-lasting than homosynaptic facilitation. At an IPI of 500 ms, for example, the magnitude of heterosynaptic facilitation was 138% compared to a 9% depression with the homosynaptic pairings. There was no late depression with the SD1PP2 pairings. These results are similar to those previously reported for septal-perforant path pair-
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ings 2,1o,22. Fig. 2 illustrates the results of the paired-pulse tests when the S D - G C field potential was the test response. Peak facilitation following the SD1-SD2 pairings (175 + 5%) was greater than that observed following the PP1-SD2 pairings (151 + 2%). Peak facilitation effects occurred at 70 ms and 30 ms for the homosynaptic and heterosynaptic pairings, respectively. The initial delay to peak facilitation may have been due to transmitter depletion or to activation of the recurrent inhibitory circuits 2°. Whatever the mechanism, the initial depression (delay to peak facilitation) was never as large as observed when testing the population spike evoked in the dentate by perforant path stimulation. Homosynaptic facilitation effects were also longer lasting than those observed for the heterosynaptic pairing.
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Fig. 3. A: comparison of PPI-PP2 (solid line) and SD-PP1-PP2
(dashed line) effects (n = 60). B: PPI-PP2 (solid line) and PP1SD-PP2 (dashed line) effects (n = 70) at interpulse intervals up to 10 s.
66 were mirror images of each other up to an IPI of 150 ms. The perforant path pairs resulted in depression of the test response, whereas only facilitation was observed with the SD1-PP2 pairs. This is consistent with a possible septal suppression or blockade of granule cell recurrent inhibition. We tested this possibility further with a triple-pulse experiment. Fig. 3A compares the SD-PP1-PP2 and PP1-PP2 conditions when the amplitudes of the perforant path pulses were corrected for the effects of the septal pulse. Application of the septal pulse, prior to PP1, appeared to have no effect on the magnitude of paired-pulse depression or facilitation. For example, peak facilitation and depression observed in the PP1PP2 condition was 238 ( + 8 % , n = 9) and 59 (+2%), respectively. For the SD-PP1-PP2 condition the respective values were 222 (+14%) and 59 (_+5%). In contrast, when the septal pulse immediately preceded the population spike evoked by PP1 (PPISD-PP2) the magnitude of the late depression was significantly increased from 70 to 3000 ms (Fig. 3B). The average test response at the peak of this depression was 48% (+1%) of normal amplitude in the PP1-SD-PP2 group and 63% (___3%) in the PP1-PP2 group (P < 0.05; t-test for correlated means). There was no effect on the magnitude of either the early depression (72 + 10% vs 71 +_ 9%) or peak facilitation (234 + 8% vs 226 + 7%). DISCUSSION Short-term interaction effects were found for all four pulse-pair combinations. The largest and longest-lasting effects occurred when the septal pulse served as the conditioning pulse for the PP-GC population spike. These results are similar to those previously reported 2"6"1°A5"16"22. In the triple-pulse experiments, it appeared that single pulse activation of the septal area did not interfere with recurrent inhibition. If the septal input were mainly to basket cell interneurons, then application of the septal pulse prior to PP1 should have blocked the interneurons before they were activated by collateral discharge from the granule cells. The septal pulse, however, had no effect on the magnitude of the early depression. G. Goddard (personal communication) has recently found that a septal pulse applied within a few ms of the onset of the popula-
tion spike (presumably falling between the "immediate' and 6 ms intervals used in our experiment) did interfere with subsequent paired pulse depression effects. This temporal window is far too narrow to account for either the facilitation effects of septal pulses reported in this paper, or the cooperativity effects of septal trains reported in the previous paper zl. Our results, therefore, appear to be more consistent with a direct action of the septal terminals on granule cells rather than an indirect action via inhibitory interneurons. This interpretation is also consistent with previous results showing that the SD-GC pathway is itself capable of LTP 16'22. and that septal terminals contact granule cell somata and may have an additional direct projection to the molecular laver 9A°. The putative transmitter in the septal pathway is acetvlcholine (ACh), and it has been suggested that ACh may exert its excitatory influence by decreasing recurrent inhibition 7'14. Our triple-pulse results do not appear consistent with an ACh mechanism, however. as the recurrent inhibition did not appear to be blocked. Furthermore. there is evidence that ACh may decrease K* conductance in the hippocampus 7. Our PP1-SD-PP2 group showed an increase in the magnitude of the late depression which would seem to be more consistent with an increase in K* conductance ls'25 Fantie and Goddard 1° found that various cholinergic agonists and antagonists had no effect on the magnitude of SD-PP facilitation. It is possible that we were acuvatmg fibres of passage. Noradrenergic fibres run through this region. and Hopkins and Johnson 12 have shown that noradrenerglc activation can facilitate LTP in the mossy fiber-CA3 pathway. On the other hand. noradrenaline depletion does not reduce either SD-PP facilitation 1° or PP-GC LTP z3. Although the effects of serotonin depletion on SD-PP facilitation have not been tested, paired stimulation of the raphe nucleus, a source of hippocampal serotonin H. and the perforant path has been shown to increase the magnitude of the PP-GC population spike 5. Also. serotonin depletion appears to decrease the synaptic component o f LTP s. and Sega124 has suggested that serotonin, applied to CA I pyramidal cells, increases K ~ conductance (consistent with the triple-pulse resultsl. A serotonergic modulation of neural plasticity could utilize a second messenger system, as reported for Aplysia L~ which might account for the duration over which sep-
67 tal trains can i n f l u e n c e P P - G C L T P 21. I n t r a c e l l u l a r
helpful c o m m e n t s on e a r l i e r drafts of this m a n u -
r e c o r d i n g and n e u r o c h e m i c a l t e c h n i q u e s m a y be re-
script. A p r e l i m i n a r y r e p o r t of these results was pre-
q u i r e d to resolve the q u e s t i o n s raised by t h e s e data.
s e n t e d at the 14th A n n u a l M e e t i n g of the Society for N e u r o s c i e n c e , O c t o b e r , 1984. This r e s e a r c h was sup-
ACKNOWLEDGEMENTS
p o r t e d by an o p e r a t i n g grant to R . J . R . and a p o s t g r a d u a t e scholarship to G . B . R . , f r o m the N a t u r a l Sci-
W e w o u l d like to t h a n k Dr. T . W . B e r g e r for his
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