The impact of bursting thalamic impulses at a neocortical synapse

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1 The impct of bursting thlmic impulses t neocorticl synpse Hrvey A. Swdlow 1 nd Alexnder G. Gusev 1,2 1 Deprtment of Psychology, The University of Connecticut, 406 Bbbidge Rod, Storrs, Connecticut 06269, USA 2 Moscow Brin Reserch Institute, Russin Acdemy of Medicl Sciences, Moscow , Russi Correspondence should be ddressed to H.A.S. (swdlow@psych.psy.uconn.edu) Considerble effort hs gone into understnding the mechnisms underlying high-frequency bursting of thlmocorticl impulses, their sensory informtion content nd their involvement in perception. However, little is known bout the influence of such impulses on their corticl trgets. Here we follow bursting thlmic impulses to their terminus t the thlmocorticl synpse of the wke rbbit, nd exmine their influence on clss of somtosensory corticl neurons. We show tht thlmic bursts potently ctivte corticl circuits. Initil impulses of ech burst hve gretly enhnced bility to elicit corticl ction potentils, nd lter impulses in the burst further rise the probbility of eliciting spikes. In some cses, multiple corticl spikes result from single burst. Moreover, we show tht the intervl preceding ech burst is crucil for generting the enhnced corticl response. The powerful ctivtion of neocortex by thlmocorticl bursts is fully consistent with n involvement of these impulses in perceptul/ttentionl processes. Neurons of the mmmlin thlmus provide gtewy through which ll sensory informtion other thn olfction must pss before reching the neocortex. Thlmic neurons operte in two distinct modes tht regulte the trnsfer of sensory informtion to the neocortex 1 3. When subjects re wke nd vigilnt, thlmic neurons re generlly in rely mode: impulses occur t high nd regulr rtes, nd synptic trnsmission through the thlmus is fithful. However, when subjects re inttentive, drowsy or nesthetized, thlmic cells re generlly in burst mode. Here, trnsmission through the thlmus is less relible, nd impulses occur t low nd irregulr rtes punctuted by high-frequency bursts. The ssocition of burst mode with sleep nd nesthetic sttes hs led to suggestions tht thlmic nuclei my be functionlly disconnected from neocortex during these periods. However, bursts my occur in the fully wke stte 4 7, nd decoupling of thlmic neurons from their sensory inputs is selective 8. Indeed, dorsl lterl geniculte nucleus (LGND) neurons re highly sensitive to some types of visul stimultion during these periods 9, nd the informtion content of ech burst is high 10. These nd other properties of thlmic burst mode hve led to suggestions tht bursts could serve s ttentionl serchlights 11 or s wke-up clls 12 to sensory neocortex for novel nd potentilly interesting or dngerous stimuli. However, these hypotheses require thlmocorticl synpses to be effective during burst mode. Although synptic trnsmission between sensory fferents nd dorsl thlmic rely nuclei hs been extensively studied during these periods, we hve no comprble informtion bout wht occurs t the thlmocorticl synpse. On the one hnd, thlmocorticl synptic trnsmission my be wekened considerbly during burst mode becuse of the reduction in cholinergic inputs to both thlmus nd cortex during periods of drowsiness nd sleep Indeed, reduced gin or trnsfer rtio is precisely wht occurs t the retinogeniculte synpse during sttes of drowsiness, sleep nd light nesthesi tht re ssocited with burst mode 3,16. Alterntively, thlmocorticl efficcy could be fcilitted during burst mode becuse of presynptic 17,18 (see below) or postsynptic 19,20 mechnisms. The present work directly investigtes this question in the somtosensory thlmocorticl system of the wke rbbit. To do this, we exploited potent synptic connection between ventrobsl (VB) thlmocorticl neurons nd puttive fst-spike GABAergic interneurons (suspected inhibitory interneurons, SINs) of the topogrphiclly ligned somtosensory corticl brrel column We recorded extrcellulr ction potentils from these popultions nd used cross-correltion methods to exmine the efficcy with which the first nd subsequent impulses generted by bursting thlmocorticl neuron elicit ction potentils in corticl SINs. Our strtegy ws to compre the efficcy vlues generted from bursting thlmic spikes with those generted from the entire spike trin of the thlmic neuron (the control efficcy 25 ). Our results show tht bursting thlmic impulses powerfully ctivte neocorticl circuits, nd re consistent with n involvement of thlmocorticl bursts in ttentionl mechnisms of sensory neocortex. RESULTS Rely mode nd burst mode in VB thlmus Rbbits generlly st quietly during recording sessions. The eyes were kept open, nd we observed only occsionl smll body movements. Under these conditions, the overll rte of spontneous VB impulses vried considerbly. Some seconds or minutes would pss when the spontneous ctivity of ll recorded thlmic neurons ws high nd few bursts occurred. This would be followed by vrible periods during which the spontneous 402 nture neuroscience volume 4 no 4 pril 2001

2 rticles b d impulse ctivity of VB neurons decresed, nd burst responses were common. The irregulr nd bursting responses redily converted to high, regulr firing either spontneously, or fter uditory or tctile stimultion. EEG recordings from the hippocmpus confirmed tht the periods of regulr firing t high rtes were ssocited with thet ctivity (in the 4 8 Hz rnge, sign of rousl 26 ). In contrst, bursts nd low spontneous firing rtes were ssocited with high-voltge, irregulr ctivity (HVIR), sign of drowsiness, inttention nd sleep. In one such recording session (Fig. 1), two thlmocorticl neurons (VB1 nd VB2) were recorded simultneously from the sme VB brreloid using two microelectrodes seprted by 160 microns. Hippocmpl EEG ctivity (Fig. 1, lower trce) shows tht the initil severl seconds of this record re dominted by thet ctivity. During this period, spontneous VB ctivity ws high in both neurons, nd single burst response occurred (circles bove verticl lines; individul spikes within the bursts cnnot be resolved t this time scle). Within bout one second, however, the EEG converted to HVIR ctivity, when spontneous ctivity of VB neurons ws much lower, nd bursts were common. To quntify this reltionship, we segmented dt files into periods dominted by either hippocmpl thet ctivity or by HVIR ctivity. Files were segmented by visul inspection (without knowledge of VB neuronl ctivity), ided by fst c e Fig. 1. Rely mode nd burst mode in VB thlmus () Trnsition from rely mode to burst mode. Upper trces, 10 s of records from the spike trins of two thlmocorticl neurons (VB1 nd VB2) recorded vi two microelectrodes seprted by 160 microns. Verticl lines, individul ction potentils; verticl line with blck circle bove, bursts of 2 5 ction potentils. (The seprte ction potentils within the bursts cnnot be resolved t this time scle.) Left, neurons in rely mode, showing high rtes of firing nd only single burst of ction potentils. After bout 4 s, the firing rte decreses, nd the number of bursts increse. Lower trce shows tht hippocmpl EEG ctivity is correlted with these different firing ptterns. During rely mode, thet ctivity is seen in the hippocmpl EEG. This chnges to high-voltge, irregulr ctivity (HVIR) during burst mode. Summed FFT power spectr (946-ms smples) of ll hippocmpl EEG segments clssified s being dominted by thet (b) nd HVIR (c). The mximl vlue in ech distribution ws normlized to vlue of 1. (d) Burst cross-correlogrm, the correltion between the initil spike in ech burst of neuron VB1 nd VB2 (). (e) Autocorrelogrms of the spike trins of neurons VB1 (top) nd VB2 (bottom). Fourier trnsforms (FFT) of ech segment. In this mnner, 20 55% of ech dt file ws clssified s thet, nd 30 40% ws clssified s HVIR. The remining portions of the files could not be clssified. For ech of six VB neurons studied in this mnner, bseline spontneous firing rte ws pproximtely two times higher during the periods of thet ctivity (pired t-test, p ; Tble 1), but bursts were 8 20 times more prevlent during HVIR ctivity (pired t-test, p = 0.001). The summed FFT power spectr for thet nd HVIR segments obtined from the entire dt file tht yielded the two neurons in Fig. 1 (Fig. 1b nd c) hd thet segments dominted by frequencies of 4 7 Hz, but HVIR segments with considerbly more power in both lower nd higher frequencies. A burst cross-correlogrm (for the initil spike in ech burst) ws clculted for these two VB neurons (Fig. 1d), nd no shrp synchrony ws seen. However, bursts showed some brod synchrony, with hlf- mplitude response of somewht less thn one second. Very similr burst cross-correlogrms were seen for ech of 14 pirs of VB neurons. Ech member of ech pir of VB neurons emitted more thn 100 bursts, nd ech pir ws locted within the sme VB brreloid nd recorded on seprte electrodes tht were spced t pproximtely 160 microns. No cler rhythmicity, other thn preferred inter-burst intervl of ms, occurred in the burst utocorrelogrms (utocorrelogrm Tble 1. Frequency for different clsses of spike ctivity in ech of six VB neurons tht were studied during recording of hippocmpl EEG ctivity. VB neuron Spikes/s Bursts/s Isolted spikes/s Pseudo-bursts/s Thet HVIR Thet HVIR Thet HVIR Thet HVIR Men ± s.d ± ± ± ± ± ± ± ± 0.07 The EEG ws segmented into periods tht were dominted by thet ctivity or by high-voltge irregulr ctivity (HVIR). nture neuroscience volume 4 no 4 pril

3 Fig. 2. Methods nd results from one experiment. () An extrcellulr microelectrode recorded the spontneous ction potentils of single VB thlmocorticl neuron, while two electrodes recorded the spikes of two corticl SINs (SIN x nd y). All correlogrms were constructed using bin width of 0.1 ms nd were clculted in the bsence of ny stimultion. (b) Cross-correlogrms were initilly clculted using the entire spike trins of the VB neuron (verticl rrows, 68,345 spikes), SINx (128,096 spikes) nd SINy (184,006 spikes). An index of the potency of this functionl connectivity is given by the efficcy, which ws computed for period of 0.6 ms on either side of the pek in the cross-correlogrm (indicted by horizontl lines bove the peks in the crosscorrelogrms). The computed efficcies of nd indicte n extremely potent synptic contct between the VB neuron nd these two SINs. The rtio of the numbers of SIN spikes in the pek of ech correlogrm (minus the bseline) nd the number of VB spikes used in the clcultions is given beside ech pek. (c) Method for selecting only initil b c d spikes in thlmocorticl burst nd computing the cross-correlogrms e bsed on these spikes. We selected only the first spike in ech burst (verticl rrow) nd computed the cross-correlogrm with ech SIN. This resulted in n enhnced efficcy for these selected VB spikes. (d) Method for selecting only isolted VB spikes nd computed crosscorrelogrms bsed on these spikes. The durtion of the intervl preceding such spikes ws chosen to mtch those seen in the burst condition (c, see Methods). These isolted VB spikes were highly potent in ctivting the corticl SINs. (e) For these sme neurons, the durtion of the required silent intervl tht preceded VB spike ws vried from 0 to 750 ms. Efficcy vlues were clerly relted to the durtion of this intervl. All cross-correlogrms were normlized for spike rte. The verticl scle br in (b) pplies to ll correlogrms, represents 500 nd 100 spikes per second in the VB SINx nd VB SINy correlogrms, respectively. of the initil spike in ech burst) for the 2 VB neurons shown in Fig. 1 (Fig. 1e), or in 15 other such burst utocorrelogrms. The impct of the initil spike in burst Of more thn 50 VB SIN pirs studied, we selected subset for detiled nlysis tht showed evidence of very potent synptic connectivity (efficcy vlues of greter thn 5%). In one cse (Fig. 2), n extrcellulr microelectrode recorded the spontneous ction potentils of single VB thlmocorticl neuron, nd two dditionl electrodes, locted in the topogrphiclly ligned S1 corticl brrel, recorded the spikes of two SINs (SINx nd SINy; Fig. 2). Our initil cross-correltion nlyses (Fig. 2b) were done using the entire spike trins of the VB neuron (more thn 68,000 spikes) nd the SINs (more thn 120,000 spikes ech). This nlysis reveled potent functionl connectivity between the VB neuron nd both of the SINs. In these correlogrms, the VB spike occurs t 0 dely, nd the corticl spike follows fter brief dely. A very brief ( 1 ms) nd potent increse in spike rte occurs t intervls of ms following the VB ction potentil 23. An index 404 of the potency of this functionl connectivity is given by the efficcy. This vlue compres the number of SIN spikes in the pek (± 0.6 ms) of ech cross correlogrm with the totl number of VB spikes 25 (see Methods). Computed efficcies were nd 0.128, between the VB neuron nd SINs x nd y, respectively, which indictes n extremely potent synptic contct between the VB neuron nd these two SINs. Next, we identified 1216 bursts of ction potentils tht occurred within the spike trin of the VB neuron during the two hours of this recording session (Fig. 2c). We then selected the first spike in ech of these bursts (verticl rrow) nd computed the cross-correlogrm of these selected spikes with the spike trins of ech SIN. The efficcy of the initil VB spike in ech burst ws clculted in mnner identicl to tht of the control efficcy, round the pek in the cross-correlogrm (± 0.6 ms). Burst efficcy incresed to nd for SINx nd SINy, respectively. Similr increses in the efficcy for the initil spike in the thlmocorticl burst were seen in ech of the seven VB SIN pirs studied, with men efficcy vlues more thn doubling (men, 221% of the control vlue; p = 0.005, pired t-test). nture neuroscience volume 4 no 4 pril 2001

4 rticles b c d e The intervl preceding the initil spike in burst The enhnced efficcy of initil spikes in thlmic burst could be due to vriety of postsynptic or presynptic mechnisms. One simple explntion is relted to the long interspike intervl tht precedes ech thlmic burst. In the slice preprtion, thlmocorticl synpses demonstrte pronounced piredpulse depression tht lsts for severl hundred milliseconds 17,18. In the wke stte, most thlmic neurons hve spontneous ctivity levels of more thn 10 spikes per second (for exmple, see Tble 1), nd thlmocorticl synpses, therefore, could be in chronic stte of depression. The puse in firing tht necessrily precedes thlmic burst could relieve this depression nd produce lrger-mplitude EPSP on the postsynptic membrne. If this is the cse, single, isolted thlmic spikes tht re preceded by silent periods comprble to those seen before thlmic bursts should lso show n enhnced efficcy in eliciting spikes in corticl SINs. We found tht this is indeed the cse (Fig. 2d). The 1216 bursts recorded in this VB neuron followed the preceding spike t medin intervl of 278 ms. Therefore, isolted VB spikes were selected tht occurred t similr intervl following the preceding spike (medin vlue, 280 ms, mtched intervl condition; Fig. 2d, verticl rrow; Methods). These isolted VB spikes were highly potent in ctivting the corticl neurons, showing efficcies of nd in SINs x nd y, respectively (Fig. 2d). These vlues re very similr to those seen under the burst condition (Fig. 2c). The mgnitude of the enhnced thlmocorticl efficcy for these neurons ws clerly relted to the durtion of the silent period tht preceded the VB spike (Fig. 2e). Similr increses in the efficcy for such isolted VB spikes (with mtched preceding intervl) were seen in ech of the seven VB SIN pirs studied, with men efficcy vlues nerly doubling (men, 174% of the control vlue; p = 0.013, pired t-test). Fig. 3. Thlmocorticl efficcy of first nd second impulses of bursts nd pseudo-bursts. ( d) Anlysis for one of the VB SIN pirs shown in Fig. 2 (VB SINx) of thlmocorticl efficcy of the first nd second spikes in bursts (n = 1216) nd pseudo-bursts (n = 2453). (, b) Crosscorrelogrm generted by the first nd second spike, respectively, in ech burst. (c, d) Correlogrms generted by the first nd second spike of pseudo-bursts. For nlyses generted by the second VB spike in bursts/pseudo-bursts (b, d), rrows indicte the increse in SIN spike probbility tht ws due to the initil VB spike. Efficcy vlues for these nlyses re clculted identiclly to tht described in Fig. 2, with the exception tht the chnce levels of responding re bsed on the intervl of from 50 ms to 10 ms preceding ech spike. All cross-correlogrms re normlized for spike rte (verticl scle br, 500 spikes/s; bin width, 0.1 ms). (e) The summed results for the six VB SIN pirs studied in this mnner. The efficcies of the first nd second spike in ech burst nd pseudo-burst re presented, normlized with respect to the control efficcy for ech VB SIN pir (the efficcy s clculted for the entire dt file). Error brs indicte the stndrd devition vlues for the combined normlized efficcies generted by the first nd second spike of ech burst/pseudo-burst. To further exmine the involvement of the intervl tht precedes ech burst, we compred the thlmocorticl efficcy of bursts with those of pseudo-bursts. Like norml bursts (see Methods), pseudo-bursts were required to consist of t lest two spikes, t interspike intervls of less thn 4 ms. Unlike norml bursts, however, pseudo-bursts hd no lengthy preceding intervl, nd were required to hve t lest one spike in the intervl of ms preceding the initil spike in the pseudo-burst. Our nlysis compred the thlmocorticl efficcy of the initil two spikes of regulr bursts with those of such pseudo-bursts (Fig. 3), nd showed tht the peks in the cross-correlogrms in response to both the first nd second VB spikes re considerbly stronger in bursts thn in pseudo-bursts (Fig. 3 d). In ech of the VB SIN pirs studied, the combined efficcy of the first two spikes ws greter in norml bursts thn in pseudo-bursts (p < 0.001, pired t-test). The men efficcy vlues of the first nd second thlmic spike in ech burst nd pseudo-burst were normlized with respect to the control efficcy for ech pir (the efficcy s clculted for the entire dt file; Fig. 3e). The impct of lter spikes in burst The bove nlysis shows tht the second spike in thlmocorticl bursts (nd pseudo-bursts) contributed to the corticl response. Moreover, inspection of the cross-correlogrms generted by the initil spike of burst (Fig. 2c) show prolonged secondry peks (rrows) following the shrp initil peks, which re not seen in Fig. 2b or d. For the seven VB SIN pirs studied, we nlyzed the thlmocorticl efficcy of lter spikes in burst in which the preceding spikes ll filed to generte corticl ction potentils (Fig. 4). We compred the thlmocorticl efficcy for initil spikes in burst (Fig. 4, left br) with those of the second spike (Fig. 4, middle br) or third spike (Fig. 4, right br). Thlmocorticl synptic efficcy remins slightly bove control vlues under these conditions for both the second nd third spike in the burst. We lso sked whether the multiple spikes of thlmocorticl burst cn elicit multiple impulses in their corticl trgets. For exmple, it is possible tht corticl spikes in the pek of the crosscorrelogrm in Fig. 3b were elicited by the second spike of the VB burst only when the first VB spike filed to elicit corticl spike. We showed for this sme VB SIN pir tht this is not the cse (Fig. 4b). A cross-correlogrm ws generted by selecting for nture neuroscience volume 4 no 4 pril

5 Fig. 4. The impct of lter spikes in burst. () Thlmocorticl efficcy of lter spikes in burst when the initil spike(s) fil to elicit corticl response. For the seven VB SIN pirs under study, efficcy of the initil spike in the burst (left) is compred with the efficcy of the second (middle) nd third (right) spike in the burst under these conditions. Error brs, s.d. (b) For the sme VB SIN pir shown in Fig. 3, we selected for nlysis only the second spike in ech VB burst, under conditions in which the first VB spike ws successful (rrow) in eliciting corticl spike. The resulting efficcy of the second spike remined high (0.176), showing tht bursts in single VB neurons cn elicit multiple spikes in their trgets. (c) We nlyzed only the third spike in ech VB burst, under conditions in which the first VB spike ws successful (rrow), but the second ws not. (d) Only the fourth spike in ech VB burst is selected for nlysis, under conditions when the first VB spike ws successful (rrow), nd the second nd third spike in the burst were not successful. All cross-correlogrms re normlized for spike rte (verticl scle br, 500 spikes/s; bin width, 0.1 ms). nlysis only the second spike in ech VB burst, under conditions in which the first VB spike ws successful in eliciting corticl spike (Fig. 4b, rrow). Even under these conditions, the efficcy of the second spike ws very high (0.176), showing tht bursts in single VB neurons cn elicit multiple spikes in their trgets. We then selected for nlysis only the third spike in ech VB burst (Fig. 4c), under conditions in which the first VB spike ws successful (rrow), but the second VB spike ws not successful in eliciting corticl spike. Finlly, we selected for nlysis only the fourth spike in ech VB burst (Fig. 4d), under conditions in which the first VB spike ws successful (rrow), nd the second nd third spikes in the burst were not successful. Efficcies remin t very high level under ll conditions for this very strongly connected VB SIN pir. These dt show tht the sequentil impulses of single bursts tht occur in some potently connected VB neurons my elicit multiple ction potentils in corticl trget neurons. Thlmocorticl efficcy nd EEG stte Bursts were reltively rre during hippocmpl thet, nd pseudo-bursts were rre during HVIR (Tble 1). Becuse of this, we could not compre thlmocorticl efficcy of these events between the two EEG sttes. However, we could compre, cross EEG sttes, the thlmocorticl efficcy of isolted VB spikes with long preceding interspike intervls (see bove). These spikes re of specil interest becuse they my be generted by the sme underlying mechnism (low-threshold clcium spike) tht genertes thlmic burst responses 6. In ech of the five VB SIN pirs tht were dequtely studied under both sttes, thlmocorticl efficcy of such isolted spikes ws similrly enhnced over control vlues (men ± s.d., 183 ± 21% of control vlues during thet ctivity; 177 ± 27% of control vlues during HVIR ctivity). DISCUSSION High-frequency bursts of ction potentils re chrcteristic of mny neuronl popultions, nd their involvement in neurl informtion processing hs generted considerble interest 19. In some systems tht demonstrte pired-pulse fcilittion, the synptic efficcy of lter impulses in burst is much higher thn for erlier impulses, nd bursting my serve s kind of high-pss filter 18. In other systems, ech spike in the burst hs synptic efficcy tht is similr to tht of n verge spike, but the overll probbility of generting t lest one postsynptic spike is higher for bursts thn for single spikes 27. The present results show tht bursts of thlmocorticl impulses generte yet nother type of enhnced postsynptic response. Here, the enhnced ctivtion of corticl neurons 406 b c d occurs in two wys: first, the initil impulse of ech burst hs synptic efficcy tht is much higher thn tht of n verge thlmocorticl impulse, nd second, the efficcy of subsequent impulses within the burst re very ner the verge vlue nd further rise the overll probbility of successfully eliciting neocorticl spikes. In some strongly connected VB SIN pirs, multiple corticl spikes re elicited by the consecutive impulses of single burst. Our dt suggest tht the intervl preceding the initil spike in the burst is key fctor in generting the enhnced synptic efficcy to this spike. Thus, we first found cler reltionship between the durtion of this preceding intervl nd thlmocorticl synptic efficcy. Second, we found tht similr enhnced thlmocorticl efficcy is generted by single isolted spikes (with mtched preceding intervls) tht re not prt of burst. Third, we found tht the enhnced thlmocorticl efficcy generted by such isolted VB spikes is independent of the EEG stte (hippocmpl thet or HVIR) during which they occur. Fourth, we found tht the initil spike of pseudo-bursts (which hve no preceding spike-free intervl) does not show n enhnced thlmocorticl efficcy. Nevertheless, it is possible tht hidden postsynptic fctors re correlted with long interspike intervls in VB neurons, nd tht these contribute to the enhnced thlmocorticl efficcy of VB bursts nd isolted spikes. Studies showing ctivity-dependent depression of thlmocorticl synpses provide plusible explntion for the potent influence of interspike intervl on thlmocorticl synptic efficcy. Thlmocorticl synpses onto both spiny neurons nd fst-spike GABAergic interneurons show considerble pired-pulse depression 17,18 tht grdully recovers over hundreds of milliseconds. If such ctivity-dependent depression lso occurs t the thlmocorticl synpse of dult, intct subjects, the high levels of spontneous ctivity generted by thlmocorticl neurons (for exmple, see Tble 1) would be expected to result in chronic stte of synptic depression. In light of this, it hs been suggested 6 tht the long intervl tht necessrily precedes the initil spike of thlmocorticl burst would llow thlmic neurons to recover from such ctivity- nture neuroscience volume 4 no 4 pril 2001

6 rticles dependent depression, nd tht initil spikes in burst should evoke mximum EPSP in postsynptic trgets. Our results re in full greement with this suggestion. Bursting of thlmocorticl neurons hs often been ssocited with sleep nd nesthetic sttes 1. The present observtions in wke rbbits gree with previous work showing tht thlmic bursting lso occurs in the wke stte 4 7. Thlmocorticl bursts were pproximtely 12 times more frequent during hippocmpl HVIR ctivity thn during thet ctivity. HVIR hs been ssocited with relxed immobility, grooming, inttention nd slow-wve sleep in the rbbit, nd thet ctivity hs been ssocited with rousl nd lertness, both during movement nd during immobility 26,28. We believe tht our rbbits were wke but drowsy nd inttentive during the HVIR seen in our experiments, for the following resons. First, lthough the eyelids sometimes drooped, they were lwys open. Second, rbbits were responsive to sensory stimultion during these periods. An pproching visul stimulus would rpidly convert the HVIR into thet ctivity, nd thet ctivity ws lso generted by low-intensity uditory or tctile stimultion. Third, hippocmpl EEG chnged spontneously between thet ctivity nd HVIR, sometimes within period of less thn one second (for exmple, Fig. 1). We found tht spontneous bursts were only wekly synchronous, even mong VB neurons of the sme brreloid. However, work in LGNd shows tht bursts re generted t very constnt ltency to very low intensity visul stimultion 9. This suggests tht, when in burst mode, mny thlmocorticl neurons with similr sptil receptive fields would generte synchronous burst responses to even wek sensory stimultion. Although the present results showing enhnced corticl responses to thlmic bursts were bsed on spontneous thlmic nd corticl impulses, we would expect bursts triggered by sensory events to lso show enhnced efficcy t the thlmocorticl synpse. If so, synchronous bursts generted in multiple thlmocorticl neurons by even wek sensory stimulus should, in turn, generte powerful postsynptic corticl response. It is importnt to emphsize tht our findings re bsed on thlmocorticl synptic input to specil clss of corticl neuron: puttive fst-spike GABAergic interneurons. These neurons hve membrne properties tht differ considerbly from those of most corticl neurons 18,29. It is possible, therefore, tht other corticl popultions my respond differently to thlmocorticl bursts. Our results, however, suggest tht recovery from ctivity-dependent depression underlies the enhnced synptic efficcy of the initil spike of thlmocorticl burst. If this is the cse, one would expect similr enhnced efficcy t the thlmocorticl synpse onto spiny stellte neurons of lyer 4. This popultion shows considerble ctivity-dependent depression t the thlmocorticl synpse 17,18 nd receives potent input from subset of thlmic fferents 30. An enhnced, burst-induced response in both corticl excittory (spiny stellte nd locl pyrmidl neurons) nd inhibitory neurons would result in n enhnced erly response in the excittory popultion tht is swiftly terminted (t disynptic dely of 1 ms) by powerful feed-forwrd inhibition 21,31. Such potent nd shrply synchronous corticl excittory popultion response would be well suited to ctivte trget neurons lying downstrem nd is consistent with n involvement of thlmocorticl bursts in ttentionl mechnisms of the neocortex 4 7,9 11. METHODS Extrcellulr recordings were obtined from somtosensory (S1) corticl brrel columns nd from VB thlmic brreloids of dult, Dutch-belted rbbits. Initil surgery ws done under nesthesi using septic procedures. Subsequent recordings were obtined in the wke, unnesthetized stte using procedures pproved by the Institutionl Animl Cre nd Use Committee t the University of Connecticut in ccordnce with NIH guidelines. Methods used to ensure the comfort of our subjects hve been described in detil 23,24. Rbbits were held snugly within stocking nd were plced on fom rubber pd. The steel br on the hed ws fstened to restrining devise in mnner tht minimized stress on the neck. Rbbits generlly st quietly for severl hours of ech recording session nd then were returned to their home cge. Microelectrodes were constructed of qurtz-pltinum filments (40 microns, mximum dimeter 32 ) tht were pulled under high temperture nd shrpened to fine tip. A concentric rry of 7 19 such electrodes (inter-electrode spcing of 160 microns) ws chroniclly implnted within VB thlmus, with ech electrode independently controlled by miniture microdrive. Corticl recordings were obtined from topogrphiclly ligned S1 brrels, following mpping procedures. Corticl recordings were obtined cutely, using 5 7 of the sme type of electrodes described bove (electrode spcing < 160 microns), but positioned using 7-chnnel microdrive system 33. VB thlmocorticl neurons were identified by spike-triggered verges of field potentils elicited in the ligned S1 brrel 31. Corticl SINs were identified by high-frequency (> 600 Hz) burst of three or more spikes elicited by electricl stimultion of VB thlmus 23,24 nd by their short-durtion ction potentil 22,23,29. All dt were collected in the bsence of peripherl stimultion, under conditions of spontneous ctivity. We selected brief window (± 0.6 ms on ech side of the pek of the cross-correlogrm) for clcultion of efficcy vlues. We restricted our nlysis to this brief window becuse peks in the thlmocorticl cross-correlogrm in this system re comprbly short 23 nd becuse we wnted to limit the nlysis to the effects of single presynptic impulse. Efficcy vlues were clculted by counting the number of ction potentils tht occurred in the SIN during this brief temporl window, subtrcting bseline number of spikes expected by chnce during this period, nd dividing this vlue by the number of triggering VB spikes. The number of SIN spikes expected by chnce ws bsed on the men number of spikes per bin tht occurred between 4 ms nd +1 ms of the VB spike time. We limited our nlysis to those VB SIN pirs tht showed control efficcy greter thn In ddition, we studied only those pirs in which greter thn 100 burst responses of the VB neuron occurred. Thlmocorticl bursts were identified ccording to previously described criteri 34. Thus, the initil impulse in burst ws required to be preceded by n intervl of t lest 100 ms during which no ction potentils occurred nd to be followed by nother ction potentil t n intervl of less thn 4 ms. Subsequent impulses tht occurred t intervls of less thn 4 ms were identified s being prt of the burst. Isolted spikes (mtched intervl condition, Fig. 2d) were preceded by defined period during which no ction potentils occurred, nd were followed by period greter thn 10 ms during which no spikes were permitted. Initilly, we selected vlue of 100 ms for the period preceding the isolted spikes. When using this vlue, however, mesured interspike intervls preceding the isolted spike were considerbly less thn those preceding VB bursts. Therefore, for ech VB cell, we lengthened the required intervl preceding the isolted spike until interspike intervls mtched (medin vlues, ± 5 ms) those preceding VB bursts. This required period vried between 145 nd 195 ms for the VB neurons studied in this mnner. Pseudo-bursts consisted of n initil spike tht ws followed by nother spike t n intervl of less thn 4 ms nd ws preceded by t lest one spike t intervls of ms. Wheres most bursts consisted of three or more spikes, most pseudo-bursts consisted of two spikes. For this reson, we compred only the initil two spikes of bursts nd pseudo-bursts. The procedure for clculting efficcy vlues to the second spike in burst/pseudo-burst ws complicted by the fct tht the time period usully used for clculting chnce spike vlues (between 4 ms nd +1 ms of the VB spike time, bove) showed elevted spike counts due to the corticl spikes tht were elicited by the first spike in the burst (rrows, Fig. 3b nd d). For this nlysis, therefore, we clculted the chnce level of responding bsed on the time period from 10 to 50 ms preceding the VB spike. As in the nlyses presented in nture neuroscience volume 4 no 4 pril

7 Fig. 2, efficcy vlues were bsed on brief window (± 0.6 ms on ech side of the pek of the cross-correlogrm). Hippocmpl EEG ws obtined vi two pltinum-iridium microwires plced bove nd below (1-mm verticl seprtion) the CA1 lyer of the hippocmpus. ACKNOWLEDGEMENTS Supported by grnts from NSF (IBN ), NIMH (MH-59322) nd the Russin Foundtion of Bsic Reserch ( ). We thnk J.-M. Alonso nd K. Luktel for discussions nd for comments on this mnuscript. RECEIVED 22 NOVEMBER 2000; ACCEPTED 16 FEBRUARY Steride, M. & Llins, R. R. The functionl sttes of the thlmus nd the ssocited neuronl interply. Physiol. Rev. 68, (1988). 2. McCormick, D. A. & Feeser, H. R. Functionl implictions of burst firing nd single spike ctivity in lterl geniculte rely neurons. Neuroscience 39, (1990). 3. Shermn, S. M. & Koch, C. The control of retinogeniculte trnsmission in the mmmlin lterl geniculte nucleus. Exp. Brin Res. 63, 1 20 (1986). 4. Guido, W. & Weynd, T. Burst responses in thlmic rely cells of the wke behving ct. J. 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Shrp, locl synchrony mong puttive feed-forwrd inhibitory interneurons of rbbit somtosensory cortex. J. Neurophysiol. 79, (1998). 25. Levick, W. R., Clelnd, G. G. & Dubin, M. W. Lterl geniculte neurons of ct: retinl inputs nd physiology. Invest. Opththlmol. 11, (1972). 26. Green, J. D. & Arduini, A. A. Hippocmpl electricl ctivity nd rousl. J. Neurophysiol. 57, (1954). 27. Csicsvri, J., Hirse, H., Czurko, A. & Buzski, G. Relibility nd stte dependence of pyrmidl cell-interneuron synpses in the hippocmpus: n ensemble pproch in the behving rt. Neuron 21, (1998). 28. Krmis, R., Vnderwolf, C. H. & Blnd, B. H. Two types of hippocmpl rhythmicl slow ctivity in both the rbbit nd the rt: reltions to behvior nd effects of tropine, diethyl ether, urethne, nd pentobrbitl. Exp. Neurol. 49, (1975). 29. McCormick, D. A., Connors, B. W., Lighthll, J. W. & Prince, D. A Comprtive electrophysiology of pyrmidl nd sprsely spiny stellte neurons of the neocortex. J. Neurophysiol. 54, (1985). 30. Reid, R. C. & Alonso, J.-M. Specificity of monosynptic connections from thlmus to visul cortex. Nture 378, (1995). 31. Swdlow, H. A. & Gusev, A. G. The influence of single VB thlmocorticl impulses on brrel columns of rbbit somtosensory cortex. J. Neurophysiol. 83, (2000). 32. Reitboeck, H. J. Fiber microelectrodes for electrophysiologicl recordings. J. Neurosci. Methods 8, (1983). 33. Eckhorn, R. & Thoms, U. A new method for the insertion of multiple microprobes into neurl nd musculr tissue, including fiber electrodes, fine wires, needles nd microsensors. J. Neurosci. Methods 49, (1993). 34. Lu, S.-M., Guido, W. & Shermn, S. M. Effects of membrne voltge on receptive field properties of lterl geniculte neurons in the ct: contributions of the low threshold C 2+ conductnce. J. Neurophysiol. 68, (1992). 408 nture neuroscience volume 4 no 4 pril 2001