Single granule cells reliably discharge targets in the hippocampal CA3 network in vivo

Transcription

1 Single grnule cells relily dischrge trgets in the hippocmpl CA3 network in vivo Drrell A. Henze, Luci Wittner nd György Buzsáki Center for Moleculr nd Behviorl Neuroscience, Rutgers University, The Stte University of New Jersey, 197 University Avenue, Newrk, New Jersey 07102, USA Correspondence should e ddressed to G.B. (uzski@xon.rutgers.edu) Pulished online: 15 July 2002, doi: /nn887 Processing of neuronl informtion depends on interctions etween the ntomicl connectivity nd cellulr properties of single cells. We exmined how these computtionl uilding locks work together in the intct rt hippocmpus. Single spikes in dentte grnule cells, controlled intrcellulrly, generlly filed to dischrge either interneurons or CA3 pyrmidl cells. In contrst, trins of spikes effectively dischrged oth CA3 cell types. Incresing the dischrge rte of the grnule cell incresed the dischrge proility of its trget neuron nd decresed the dely etween the onset of grnule cell trin nd evoked firing in postsynptic trgets. Thus, we conclude tht the grnule cell to CA3 synpses re conditionl detontors, dependent on grnule cell firing pttern. In ddition, we suggest tht informtion in single grnule cells is converted into temporl dely code in trget CA3 pyrmidl cells nd interneurons. These dt demonstrte how neurl circuit of the CNS my process informtion. Input from the dentte gyrus to the hippocmpus is implicted in vrious physiologicl functions, such s memory nd sptil representtion 1 3, nd in pthologicl sttes such s epilepsy 4 6. Neocorticl informtion is conveyed to hippocmpl CA3 pyrmidl neurons oth directly vi the perfornt pth from the entorhinl cortex nd indirectly y wy of the mossy fiers of grnule cells. A lrge numer of grnule cells (1 million in the rt 7 ) innervte the 300,000 CA3 pyrmidl cells vi the mossy fiers with very limited convergence ( 50 grnule cells per pyrmidl cell) nd divergence ( 14 pyrmidl cells per grnule cell 8 ) vi specilized presynptic gint mossy fier outon 9. In ddition, ech grnule cell innervtes 40 to 50 CA3 interneurons vi filopodil extensions tht extend from the gint outon nd en pssnt outons long the min mossy fier xon 10,11. The ntomicl differentition of mossy fier synpses onto pyrmidl cells nd interneurons is ssocited with functionl segregtion with respect to short-term nd long-term plsticity 12,13. Despite the welth of knowledge out the ntomy nd synptic physiology of this system, nothing is known how grnule cell ctivity influences the ctivity of CA3 trgets in the intct network in vivo. This knowledge is importnt ecuse models of hippocmpl function ssume tht the mossy fier synpse is detontor or techer synpse criticl for directing the storge of informtion in the uto-ssocitive CA3 network 3,14. A detontor or techer synpse strongly controls the ctivity of the postsynptic trget without need of coordinted input from other synpses onto the postsynptic trget 14. The hypothesized deterministic firing of CA3 pyrmidl cells y grnule cells is ssumed to provide the Hein postsynptic depolriztion 15,16 required to strengthen the commissurl/ssocitionl nd/or perfornt pth inputs to the ctivted CA3 pyrmidl neurons. Although previous in vitro investigtions hve illuminted some importnt properties of the mossy fier synpses (for review, see ref. 17), the conditions required to dischrge CA3 neurons y grnule cells in the intct rin hve remined unexplored. Our dt demonstrte tht spike trnsmission ( precise temporl reltionship etween single pre- nd postsynptic spikes) occurs etween grnule cells nd their pyrmidl cell nd interneuron trgets in re CA3. Spike trnsmission from grnule cell to CA3 ws not oserved for single spikes ut insted required trins of spikes with the mximum spike trnsmission proility depending on the frequency of grnule cell firing. Finlly, physiologicl ptterns of grnule cell firing contin periods of high-frequency firing tht effectively drove CA3 pyrmidl cells nd interneurons. RESULTS We directly ddressed the influence of grnule cells on the ctivity of CA3 trgets y recording in vivo from popultion of potentil trgets in the CA3c region while controlling grnule cell ctivity vi intrcellulr current injection using shrp microelectrode (ref. 18; Methods). The current injections llowed the ritrry control of grnule cell spiking, thus permitting us to dissocite the influence of single grnule cell ctivity from ckground network drive. Multiple single units in re CA3c were recorded extrcellulrly nd isolted using multichnnel silicon proes nd clustering nlysis techniques (Methods). Individul units were identified s pyrmidl cells or interneurons sed on pulished criteri A plot of unit spike symmetry (durtion of the first hlf of the spike divided y the durtion of the lst 790 nture neuroscience volume 5 no 8 ugust 2002

2 hlf) versus hlf-width resulted in digonl seprtion of units (Fig. 1) tht llowed unit identifiction when comined with the unit utocorrelogrms (Fig. 1). Puttive pyrmidl cells (n = 22) hd low men firing rtes outside the evoked ctivity (0.91 ±0.18 Hz) nd hd wider, symmetricl widend wveforms (0.32 ± 0.01 ms hlf-width; 0.52 ± 0.05 symmetry rtio). Four puttive pyrmidl cell units hd enough spontneous spikes for us to construct spike utocorrelogrms, typicl of pyrmidl neurons 19 (Figs. 1 nd 2d). Puttive interneurons (n = 26), on verge, dischrged fster (3.38 ± 0.75 Hz) nd hd nrrower spikes (0.18 ± 0.01 ms hlf-width; 1.46 ± 0.09 symmetry rtio), nd the utocorrelogrms did not show pek t less thn 10 ms (Figs. 1 nd 2f). Ten neurons could not e clssified into either ctegory using these criteri. Intrcellulrly evoked grnule cell ction potentils resulted in time-locked unit ctivity in suset of trget CA3c neurons (Fig. 2). The fixed short ltency of the unit ctivity indicted monosynptic connection etween the grnule cell nd its CA3c trgets 19,21. Spike trnsmission, the occurrence of precise timelocked spike y postsynptic cell following spike in presy- Fig. 1. Clssifiction of extrcellulr units. () Extrcellulr unit wveform symmetry (time from seline to negtive pek/time from negtive pek to seline, see horizontl rrows in 1) versus unit spike hlf-mplitude width. Closed circles nd open tringles re puttive pyrmidl cells nd interneurons, respectively. () Exmple widend (1 Hz 8 khz) unit wveforms nd utocorrelogrms tht were used to confirm unit clssifiction. The symmetry mesurement is illustrted with verticl dotted lines nd horizontl rrows in 1. Pyrmidl cells hve wider nd more symmetric wveforms nd pek t short interspike intervls in the utocorrelogrm. Interneurons hve nrrower nd more symmetric wveforms nd lck pek t short interspike intervls in the utocorrelogrms. The lettered rrows in () correspond to the supnels in (). Scle rs: horizontl, 1 ms; verticl, 100 µv, 50 µv, 50 µv, 25 µv for 1, 2, 3 nd 4, respectively. nptic cell, occurred etween grnule cells nd CA3 trgets in 37 of 69 rts. A connection ws considered for detiled nlysis when there ws significnt pek in the cross-correltion in the 6 ms following the grnule cell ction potentil (defined s over 3 stndrd devitions ove the men of the overll cross-correltion). Of the 37 rts with qulittively oserved spike trnsmission, 13 nimls yielded 58 pirs with recordings of sufficient qulity nd length to generte unit clusters nd quntify spike trnsmission proility (1 11 driven units per successful experiment). Twenty-two of the driven units were pyrmidl cells (Fig. 2c nd d), nd 26 were interneurons (Fig. 2e nd f). The verge distnce etween the closest en pssnt or gint mossy fier outon nd the proe (Fig. 2, inset) ws 51.4 ± 10.1 µm (± stndrd error of the men, s.e.m.) for the 7 experiments in which mossy fiers were recovered ntomiclly. On verge, puttive pyrmidl cells hd shorter nd more precise spike trnsmission ltencies thn puttive interneurons (Tle 1). The spike trnsmission proility depended on the comined effect of grnule cell dischrge frequency nd the numer of spikes within the trin (Fig. 3). Single grnule cell spikes Fig. 2. Spike trnsmission etween grnule cell nd its interneuron nd pyrmidl cell trgets in CA3c. () Cmer lucid reconstruction of iocytin-leled grnule cell nd the extrcellulr electrode trck where spike trnsmission ws oserved. Scle r, 50 µm. m, moleculr lyer; g, grnule cell lyer; h, hilus; IC, intrcellulr electrode trck; EC, extrcellulr electrode trck. Inset, higher-power view of the mossy fier xon ner the proe trck. Arrowheds, mossy fier outons; scle r, 20 µm. () Superimposed (n = 60) intrcellulrly evoked ction potentils in grnule cell (ottom trces) nd simultneously recorded extrcellulr units (filtered khz). Note the time-locked response of puttive pyrmidl cell to the grnule cell ction potentils. The grnule cell ws driven y 350 ms depolrizing steps. Scle r, 1 ms, 25 mv, 75 µv. (c) Cross-correlogrm (shuffle corrected) expressed s proility (numer of unit spikes per in/totl numer of grnule cell spikes) etween the evoked grnule cell ction potentils nd the ctivity of puttive CA3c pyrmidl cell. The rrowhed indictes the pek time of the grnule cell ction potentil. (d) Autocorrelogrm of the pyrmidl cell unit during nonevoked periods. Left inset, the verge wide-nd wveform of the pyrmidl cell (high-pss filtered t 1 Hz). Scle r, 1 ms, 20 µv. Right inset, spontneous complex urst of the pyrmidl cell. Scle r, 5 ms, 20 µv. As in (c), ut for n isolted CA3 interneuron. (e, f) As in (c, d), ut the extrcellulr unit is puttive CA3c interneuron. c d e f nture neuroscience volume 5 no 8 ugust

3 were generlly ineffective, filing to show significnt peks in the cross-correltions in 54 of the 58 pirs (including 10 unidentified trgets). However, high-frequency grnule cell spike trins could result in spike trnsmission with proility greter thn 0.8 (pyrmidl cells, to 0.842; interneurons, to 0.57). In the pir shown in Fig. 3, the first ction potentil in the trin filed to evoke extrcellulr spikes, wheres mximum spike trnsmission proility occurred following the fifth spike (puttive pyrmidl cell unit; mximum proility, 0.52). For 45 of the 48 Tle 1. Grnule cell CA3 spike trnsmission dynmics. Numer of units Men ltency Men precision Men spike numer to (ms) (ms) mximum proility Overll (including 10 unclssified units) ± ± ± 0.51 Pyrmidl cell ± * 0.81 ± 0.046** 3.94 ± 0.44 Interneuron ± * 1.24 ± 0.049** 4.62 ± 0.40 *P < 0.01, **P < , two-tiled t-tests (pyrmidl cells versus interneurons). Ltency ws clculted etween the pek of the grnule cell ction potentil nd the negtive pek of the extrcellulr spike. Men precision ws clculted s the men stndrd devition of the ltency from ech driven unit. identified pirs, spike trnsmission ( significnt pek t 2 6 ms in the cross-correltion) required t lest two presynptic spikes. Susequently, the mximum spike trnsmission proility occurred lter in the trin (Tle 1; Fig. 3), followed y lte trin reduction in spike trnsmission proility. In the three remining identified pirs (one pyrmidl cell, two interneurons), the first spike proility ws greter thn or equl to lter spikes (first spike proility, 0.26, 0.02, 0.30, respectively). The overll men spike trnsmission proility ws higher for the puttive pyrmidl cell trgets thn for puttive interneurons, regrdless of the chnges over the course of the trin (0.157 ± versus ± 0.004; P < ; two-wy ANOVA). The dischrge frequency of the grnule cell ffected oth dischrge proility nd the ltency etween the onset of the presynptic trin nd postsynptic firing. At 10 Hz, spike trnsmission proility ws low, wheres t 100 Hz, it ws high. In the pir shown in Fig. 4, mximum proility occurred fter the third to sixth spike. The comined effect of spike frequency nd spike numer in the grnule cell resulted in considerle time shift etween the onset of the grnule cell trin nd the mximum spike trnsmission proility (Fig. 4, rrows). Figure 4 summrizes the effect of grnule cell firing frequency on the time of mximum spike trnsmission proility (n = 5) from the eginning of the trin. For ll pirs, the time etween the onset of the presynptic trin nd the time when highest spike trnsmission proility ws reched decresed severl-fold s function of c Fig. 3. Spike trnsmission dynmics. () Top, filtered (0.8 8 khz) extrcellulr trce of puttive pyrmidl cell. Middle, grnule cell ction potentils evoked y rief current injections t 100 Hz (rtifcts digitlly removed). Bottom, current injection pttern. Scle r, top, 100 µv; middle, 25 mv, 10 ms. () Spike trnsmission proility (shuffle-corrected proility of spike in 6 ms following intrcellulr spike) s function of spike numer in evoked 100 Hz trin (± s.e.m). Solid line, puttive interneurons (n = 24); dotted line, puttive pyrmidl cells (n = 21). Fig. 4. Frequency dependence of grnule cell CA3 spike trnsmission. () Representtive results of the effect of intrtrin frequency on spike trnsmission proility for puttive pyrmidl cell. () Time of mximum proility following onset of the grnule cell spike trin s function of frequency for two pyrmidl cells (dotted lines) nd three interneurons (solid lines). As grnule cell spiking frequency increses, the ltency from the grnule cell trin onset of the mximum dischrge proility of the postsynptic unit decreses. (c) The mximum proility of spike trnsmission normlized cross grnule cell spike trin frequencies for two pyrmidl cells (dotted lines) nd three interneurons (solid lines). The mximum proility of spike trnsmission ws oserved for the highest frequency (100 Hz) in 4 of 5 cells. The different symols represent the different cells in () nd (c). 792 nture neuroscience volume 5 no 8 ugust 2002

4 DISCUSSION These experiments directly ddress long-stnding question in hippocmpl physiology: is the connection etween single grn c d e f the frequency of grnule cell spiking. The mximum spike trnsmission proility incresed with the frequency of grnule cell dischrge (Fig. 4 nd c). In 4 of 5 pirs tested, 100 Hz trins (the highest frequency tested) generted the highest spike trnsmission proility (Fig. 4c). The frequency sensitivity nd in-trin dynmics of grnule cell CA3 spike trnsmission in Figs. 3 nd 4 indicte tht the oserved monosynptic effects do not result merely from the coincidence of spontneous ckground ctivity nd current-induced spiking of the single grnule cell. Grnule cells of the dentte gyrus hve not een reported to fire for extended periods of times t high fixed frequencies 2. Therefore, we next exmined whether the nturl physiologicl pttern of ctivity of single grnule cell is cple of dischrging CA3 trgets. To mimic the sitution in the ehving rt, the presynptic grnule cell firing pttern ws ssigned y injecting spike trin sed upon templte otined from firing of grnule cell in ehving niml trversing the unit s plce field (Methods). As expected, high-frequency periods of the spike trin were more effective in driving oth pyrmidl cells nd interneurons thn isolted spikes or low-frequency periods (Fig. 5). Longer interspike intervls were ssocited with lower spike trnsmission proilities, in ccordnce with the findings using regulr spike trins. Fig. 5. Spike trnsmission from grnule cell to CA3c during physiologicl grnule cell firing ptterns. () Rstergrm of 36 trils of grnule cell ction potentils evoked using templte recorded during explortion while the niml trversed the plce field of the grnule cell (Methods). () Instntneous frequency (1/previous interspike intervl) of the evoked grnule cell ctivity. (c) Rstergrm of the ctivity of puttive CA3c pyrmidl neuron. (d) Men percent of mximum spike trnsmission proility (± s.e.m.) onto interneurons s function of ech spike in the grnule cell spike trin (n = 9). The proility of ech unit spiking ws normlized to the mximum proility for tht unit nd then verged cross units. (e) As in (d), ut for pyrmidl cells (n = 6). (f) Men percent of mximum spike trnsmission proility (± s.e.m.) onto interneurons (n = 9) s function of instntneous frequency (1/ previous interspike intervl) for ech spike in the grnule cell trin shows tht higher proility of spike trnsmission is ssocited with higher frequencies. The est liner fit regression line is shown (R = 0.49; P < ). (g) As in (f), ut for pyrmidl cells (n = 6; R = 0.45; P < 0.002). ule cells nd CA3 pyrmidl cells strong enough to e high-fidelity detontor or techer synpse for the storge of informtion in the CA3 uto-ssocitive network 3,14? According to computtionl models, the sprse convergence of grnule cell CA3 pyrmidl cell connections 11 nd the low men firing rtes of grnule cells 2,22 provide n idel mechnism for incresing the storge cpcity of the hippocmpus 23,24. Our dt indicte tht the mossy fier synpses onto CA3 trgets re conditionl detontors dependent on fst repetitive firing of the presynptic grnule cell. Therefore, single grnule cell cn e considered techer during periods of medium to high ctivity y precisely timing the ctivity of CA3 pyrmidl cells with high reliility. The temporlly precise ctivtion of ction potentils in CA3 pyrmidl cells would provide the necessry postsynptic depolriztion for the induction of Hein forms of NMDA receptor dependent plsticity t ssocitionl/commissurl nd very distl perfornt pth synpses on CA3 pyrmidl cells 25. Thus grnule cell ctivity would direct or tech the CA3 network during the storge of informtion 14,26,27. Previous in vitro experiments reveled strong excittory postsynptic potentil fcilittion y mossy fiers onto CA3 pyrmidl cells 28 nd oth fcilittion nd depression onto interneurons 13,29,30. Although single grnule cells dischrge mossy cells of the hilus in vitro 31, ctivting single mossy fiers projecting to CA3 nd mimicking the physiologicl phrmcology of the system in vivo is difficult to control in the slice preprtion. In our in vivo experiments, grnule cells rrely drove their trgets with single spikes, wheres high-frequency trins of spikes nd ptterns with short interspike intervls in trins mimicking dischrge ptterns in the ehving niml roustly incresed spike trnsmission proility to oth pyrmidl cells nd interneurons. These findings mplify the functionl importnce of the roust frequency fcilittion oserved for the mossy fier synpses onto CA3 trgets 13,28,32. Furthermore, the physiologicl stimultion pttern dt illustrtes how the mossy fier synpse, the postsynptic cellulr properties nd the locl CA3 network con- nture neuroscience volume 5 no 8 ugust

5 nections ll work together to generte specific pttern of output in CA3 pyrmidl cells tht is dependent on, yet different from, the grnule cell input. The requirement for trins of spikes to evoke spike trnsmission etween grnule cells nd CA3 is distinctly different from spike trnsmission etween pyrmidl cells nd interneurons in CA1. In contrst to the grnule cell CA3 connection, single CA1 pyrmidl cell spikes or low frequency trins (less thn 20 Hz) re most effective in driving nery interneurons, nd the men proility of spike trnsmission decreses during high-frequency trins 33. Our findings indicte tht the spike numer nd frequencydependent recruitment of the postsynptic dischrge is n effective mechnism to vry the time dely etween the onset of grnule cell ctivity nd trget neuron ctivity. Vrile spike trnsmission dely lines re suggested to e criticl for encoding informtion in computtionl models 34,35. In the cse of the mossy fier input to CA3, this property cn e thought of s progrmmle dely line, in which the timing of spiking onset in CA3 vries y n order of mgnitude s function of the short-term firing rte of grnule cells. For exmple, if entorhinl input evoked period of 50 Hz firing in grnule cell, then the trgets of tht grnule cell would hve the highest proility of firing 200 ms from the onset of the grnule cell ctivity. However, if entorhinl input evoked 100 Hz firing in grnule cell, then the highest proility of CA3 ctivtion would occur 40 ms lter. Such mechnism could convert the spike frequency of grnule cells to precise timing of CA3 trget neurons during the thet cycle 22,24. In ddition, grnule cells with similr mgnitude of ctivtion will dischrge CA3 trgets together in time nd therey my increse their mutul connectivity 15,16,36. Similrly, the delyed ctivtion of CA3 pyrmidl cells would e ssocited with dendritic ckpropgting ction potentils tht my then fcilite potentition of direct entorhinl inputs to CA3. This conversion of frequency to spike timing could e used to encode sequences such s n niml s trjectory through n environment 22,37,38. first moment of the utocorrelogrm (Fig. 1; ref. 19). The existence of monosynptic connection ws determined from the cross-correltion etween the grnule cell ction potentil peks nd the spikes from ech unit. Spike trnsmission proility ws defined s the proility of finding unit spike in the 6 ms following the grnule cell ction potentil. The ckground proility (clculted vi shuffling) ws sutrcted from ll reported proility vlues 19. An ovious cvet of this method is tht it fils to detect wek monosynptic connections due to the sence of driven spikes. Thus, our findings my e ised for estimting strong connections. Physiologicl firing ptterns. The physiologicl grnule cell firing pttern used in Fig. 5 ws otined from mouse freely exploring cm open field. The selected unit is considered plce cell 43 ecuse it ws ctivted selectively in limited portion of the experimentl pprtus. The unit hd low overll men rte of 1.7 Hz nd high (26 Hz) in-field firing rte. A single representtive 1.8-s period of ctivity when the niml trversed the plce field ws selected s templte of physiologicl ctivity. This sequence of spike times ws converted into n nlog wveform for controlling the injection of intrcellulr current into the grnule cells. Ech unit spike ws converted to sine wve pulse 5 ms in durtion nd 2 3 na in mplitude. The mplitude of the pulse ws djusted t the strt of the experiment so tht single ction potentils were relily elicited in the grnule cell for every sine pulse (Fig. 5). The entire 1.8-s spike trin ws repetedly injected every 5 s. We used sine wve pulses to reduce the lrge stimulus rtifcts ssocited with injecting lrge-mplitude squre wve steps. The sence of the stimulus rtifcts simplified detecting nd mesuring the intrcellulr ction potentils evoked y the spike trin. Acknowledgments This work ws supported y grnts from the Ntionl Institutes of Helth (NS34994, MH54671, MH12403, NS43157, 5P41RR009754). Competing interests sttement The uthors declre tht they hve no competing finncil interests. RECEIVED 13 MAY; ACCEPTED 7 JUNE 2002 METHODS Sujects nd surgery. Sixty-nine Sprgue Dwley rts (Hilltop Lortories, Scottdle, Pennsylvni, or Zivic Miller lortories, Zellenople, Pennsylvni), g, were nesthetized with urethne (1.5 g/kg) nd plced in stereotxic frme. Urethne nesthesi ws chosen for oth the stility of the plne of nesthesi nd ecuse ll known hippocmpl network ptterns re preserved under this nesthetic 39,40. However, ecuse urethne potentites GABA A currents nd depresses glutmte currents 41, the results presented here must eventully e confirmed in the drug-free niml. All nimls were treted in ccordnce with experimentl protocols pproved y the Rutgers University Animl Cre nd Use Committee. Electrophysiology. Extrcellulr units were recorded using three-shnk silicon tetrodes plced in the CA3c pyrmidl lyer. Extrcellulr signls were high-pss filtered (1 Hz) nd mplified (1000 ) using multichnnel mplifier (Sensorium, Inc. Chrlotte, Vermont). Intrcellulr recordings nd current injections were chieved using shrp glss micropipettes (finl resistnce MΩ), filled with 1 M potssium cette nd 2% iocytin, nd DC mplifier (Axoclmp-2A, Axon Instruments, Union City, Cliforni). Current injections were either squre or positively offset sine wves ( π/2 3π/2). Biocytin ws injected intrcellulrly using positive current steps fter ech experiment nd susequently developed using diminoenzidine. All dt were digitized t 20 khz nd stored on computer disk for lter nlysis. Unit clustering ws done using n in-house softwre pckge s descried sed upon principl component mesurements of the extrcellulr units 19,42. Three criteri were used to seprte pyrmidl cells nd interneurons: spike wveform symmetry, hlf-width of the spike nd, when ville, the 1. Collier, T. J., Miller, J. S., Trvis, J. & Routtenerg, A. Dentte gyrus grnule cells nd memory: electricl stimultion disrupts memory for plces rewrded. Behv. Neurl Biol. 34, (1982). 2. Jung, M. W. & McNughton, B. L. Sptil selectivity of unit ctivity in the hippocmpl grnulr lyer. Hippocmpus 3, (1993). 3. Rolls, E. T. in Neurl Models of Plsticity (eds. Byrne, J. H. & Berry, W. O.) (Acdemic, Sn Diego, 1989). 4. Sloviter, R. S. The functionl orgniztion of the hippocmpl dentte gyrus nd its relevnce to the pthogenesis of temporl loe epilepsy. Ann. Neurol. 35, (1994). 5. Mody, I., Lmert, J. D. & Heinemnn, U. Low extrcellulr mgnesium induces epileptiform ctivity nd spreding depression in rt hippocmpl slices. J. Neurophysiol. 57, (1987). 6. Lothmn, E. W., Stringer, J. L. & Bertrm, E. H. in The Dentte Gyrus nd its Role in Seizures (eds. Rik, C. E., Gll, C. M. & Mody, I.) (Elsevier, Amsterdm, 1992). 7. Boss, B. D., Peterson, G. M. & Cown, W. M. On the numer of neurons in the dentte gyrus of the rt. Brin Res. 338, (1985). 8. Amrl, D. G., Ishizuk, N. & Cliorne, B. Neurons, numers nd the hippocmpl network. Prog. Brin Res. 83, 1 11 (1990). 9. Amrl, D. G. & Dent, J. A. Development of the mossy fiers of the dentte gyrus: I. A light nd electron microscopic study of the mossy fiers nd their expnsions. J. Comp. Neurol. 195, (1981). 10. Frotscher, M., Sorino, E. & Misgeld, U. Divergence of hippocmpl mossy fiers. Synpse 16, (1994). 11. Acsdy, L., Kmondi, A., Sik, A., Freund, T. & Buzsáki, G. GABAergic cells re the mjor postsynptic trgets of mossy fiers in the rt hippocmpus. J. Neurosci. 18, (1998). 12. Mccferri, G., Toth, K. & McBin, C. J. Trget-specific expression of presynptic mossy fier plsticity. Science 279, (1998). 13. Toth, K., Sures, G., Lwrence, J. J., Philips-Tnsey, E. & McBin, C. J. Differentil mechnisms of trnsmission t three types of mossy fier synpse. J. Neurosci. 20, (2000). 14. McNughton, B. L. & Morris, R. G. M. Hippocmpl synptic enhncement nd informtion storge within distriuted memory system. Trends Neurosci. 10, (1987). 794 nture neuroscience volume 5 no 8 ugust 2002

6 15. Mrkrm, H., Luke, J., Frotscher, M. & Skmnn, B. Regultion of synptic efficcy y coincidence of postsynptic APs nd EPSPs. Science 275, (1997). 16. Mgee, J. C. & Johnston, D. A synpticlly controlled, ssocitive signl for Hein plsticity in hippocmpl neurons. Science 275, (1997). 17. Henze, D. A., Urn, N. N. & Brrionuevo, G. The multifrious hippocmpl mossy fier pthwy: review. Neuroscience 98, (2000). 18. Henze, D. A. et l. Intrcellulr fetures predicted y extrcellulr recordings in the hippocmpus in vivo. J. Neurophysiol. 84, (2000). 19. Csicsvri, J., Hirse, H., Czurko, A. & Buzsáki, G. Reliility nd stte dependence of pyrmidl cell-interneuron synpses in the hippocmpus: n ensemle pproch in the ehving rt. Neuron 21, (1998). 20. Rnck, J. B. Jr. Studies on single neurons in dorsl hippocmpl formtion nd septum in unrestrined rts. I. Behviorl correltes nd firing repertoires. Exp. Neurol. 41, (1973). 21. Glrret, M. & Hestrin, S. Spike trnsmission nd synchrony detection in networks of GABAergic interneurons. Science 292, (2001). 22. Skggs, W. E., McNughton, B. L., Wilson, M. A. & Brnes, C. A. Thet phse precession in hippocmpl neuronl popultions nd the compression of temporl sequences. Hippocmpus 6, (1996). 23. Rolls, E. T. in The Computing Neuron (eds. Durin, R., Mill, C. & Mitchison, G.) (Addison-Wesley, Wokinghm, 1989). 24. Lismn, J. E. Relting hippocmpl circuitry to function: recll of memory sequences y reciprocl dentte-ca3 interctions. Neuron 22, (1999). 25. Pulsen, O. & Sejnowski, T. J. Nturl ptterns of ctivity nd long-term synptic plsticity. Curr. Opin. Neuroiol. 10, (2000). 26. Buzsáki, G. Two-stge model of memory trce formtion: role for noisy rin sttes. Neurosci. 31, (1989). 27. Chttrji, S., Stnton, P. K. & Sejnowski, T. J. Commissurl synpses, ut not mossy fier synpses, in hippocmpl field CA3 exhiit ssocitive long-term potentition nd depression. Brin Res. 495, (1989). 28. Slin, P. A., Scnzini, M., Mlenk, R. C. & Nicoll, R. A. Distinct short-term plsticity t two excittory synpses in the hippocmpus. Proc. Ntl. Acd. Sci. USA 93, (1996). 29. Alle, H., Jons, P. & Geiger, J. R. PTP nd LTP t hippocmpl mossy fierinterneuron synpse. Proc. Ntl. Acd. Sci. USA 98, (2001). 30. Doherty, J. & Dingledine, R. Reduced excittory drive onto interneurons in the dentte gyrus fter sttus epilepticus. J. Neurosci. 21, (2001). 31. Schrfmn, H. E., Kunkel, D. D. & Schwrtzkroin, P. A. Synptic connections of dentte grnule cells nd hilr neurons: results of pired intrcellulr recordings nd intrcellulr horserdish peroxidse injections. Neuroscience 37, (1990). 32. Urn, N. N., Henze, D. A. & Brrionuevo, G. Revisiting the role of the hippocmpl mossy fier synpse. Hippocmpus 11, (2001). 33. Mrshll, L. et l. Hippocmpl pyrmidl cell-interneuron spike trnsmission is frequency dependent nd responsile for plce modultion of interneuron dischrge. J. Neurosci. 22, RC197 (2002). 34. Lorincz, A. & Buzsáki, G. Two-phse computtionl model trining longterm memories in the entorhinl-hippocmpl region. Ann. NY Acd. Sci. 911, (2000). 35. Tnk, D. W. & Hopfield, J. J. Neurl computtion y concentrting informtion in time. Proc. Ntl. Acd. Sci. USA 84, (1987). 36. Bi, G. Q. & Poo, M. M. Synptic modifictions in cultured hippocmpl neurons: dependence on spike timing, synptic strength, nd postsynptic cell type. J. Neurosci. 18, (1998). 37. O Keefe, J. & Recce, M. L. Phse reltionship etween hippocmpl plce units nd the EEG thet rhythm. Hippocmpus 3, (1993). 38. Hrris, K. D. et l. Spike trin dynmics predicts thet-relted phse precession in hippocmpl pyrmidl cells. Nture (in press). 39. Vnderwolf, C. H. Cererl ctivity nd ehvior: control y centrl cholinergic nd serotonergic systems. Int. Rev. Neuroiol. 30, (1988). 40. Ylinen, A. et l. Shrp wve-ssocited high-frequency oscilltion (200 Hz) in the intct hippocmpus: network nd intrcellulr mechnisms. J. Neurosci. 15, (1995). 41. Hr, K. & Hrris, R. A. The nesthetic mechnism of urethne: the effects on neurotrnsmitter-gted ion chnnels. Anesth. Anlg. 94, (2002). 42. Hrris, K. D., Henze, D. A., Csicsvri, J., Hirse, H. & Buzsáki, G. Accurcy of tetrode spike seprtion s determined y simultneous intrcellulr nd extrcellulr mesurements. J. Neurophysiol. 84, (2000). 43. O Keefe, J. & Dostrovsky, J. The hippocmpus s sptil mp. Preliminry evidence from unit ctivity in the freely-moving rt. Brin Res. 34, (1971). nture neuroscience volume 5 no 8 ugust