C. Wyart, J.-F. Léger, C. Herr, S. Cocco, D. Chatenay, L. Bourdieu

1. Dynamics of excitatory synaptic components in sustained firing at low rates C. Wyart, J.-F. Léger, C. Herr, S. Cocco, D. Chatenay, L. Bourdieu Laboratoire de Dynamique des Fluides Complexes UMR 7506, ULP, Strasbourg Laboratoire de Neurobiologie Cellulaire et Moléculaire UMR 8544, ENS, Paris

2. Low frequency f persistent activity of neuronal networks : working memory in the prefrontal cortex 1) Standard model : the role of propagation of reverberating excitation ? High efficiency of recurrent synapses 2) Role of the temporal synaptic dynamics (Wang, 1999) ? Fast negative feedback mechanisms tneg to prevent high frequency firing SLOW positive feedback mechanisms tpos To allow refiring after a long delay • synchronous networks tpos >1/f • asynchronous networks tpos > tneg R-NMDA

3. ~10Hz ~2Hz The case of highly synchronous glutamatergic netwoks AUTAPSES • model of synchronous networks • silences and self maintained active states • no propagation through the network • low and very low frequency regimes Cell attached recordings

4. Anti-GAD Anti-GABA 2-5µm Neuronal networks of controlled architecture: towards neuron-electronic interface • Spatial adhesion contrats Fluorosilane vs. Polylysine • The physiological activity is preserved. Électrophysiology Type of neurons Synapses 50µm 50µm

5. 4 3 1 5 2 6 11 8 EPSC autaptique = -86pA+/-29pA 7 EPSC (pA) 10 9 voisin • Orientation of the connectivity • « known » architecture • interfacing with MEA 1ers voisins 90% 2nds voisins 40% Direct connexion • fixed position of neurons • nature of neurons • connexion efficiency Optimization of neuron to electronic interface Connexion efficiency

6. MK801/ APV CONTROLE CONTROLE Persistence at low frequency (~10Hz) II weeks MK801, APV • Remove : - the slow component of the EPSP • - the persistent activity - Timescale of the EPSP’s slowest component > ~ 1/frequency - Persistence at ~ 10 Hz requires R-NMDA

7. Persistence at very low frequency (~2Hz) III-IV weeks Irregular very low frequency firing MK801, APV • - slightly modifies the slow component • - Do not prevent the persistence of activity

8. Persistence at very low frequency (~2Hz) III-IV weeks Asynchronous miniatures Very slow synaptic component • removes: • the slow component • asynchronous miniatures • persistence of activity at very low frequency EGTA-AM

9. Persistence at very low frequency (~2Hz) glutamate mgluR group I Ca2+ out Intracellular Calcium stores Ca2+ in Slow depolarizing conductance (ICAN: cationic non selective calcium activated conductance ?) L-type Ca channels Congar, Ben-ari 1997

10. Role of the late release of glutamate • Experiments with EGTA • Irregular bursts with APV/MK801 • Experiments with strontium Strontium : diminish the fast release, increase the late release Strontium : Increases persistence [Strontium] 3mM [Ca++ ] 3mM

11. Persistence at very low frequency (~2Hz) EGTA-AM, MCPG, Nifedipin • Persistence at very low frequency: • intracellular calcium dependant conductance • presumably ICAN • role of asynchronous release • Timescale of the slowest depolarizing component of the EPSP >1/frequency

12. Model : rate control • Fast negative feedback mechanism : • Drop of the cell resistance • Slow positive feedback: • The slowest depolarizing component of the EPSP

13. What about hetero-synapses ? paires isolées

14. Conclusion • Direct demonstration of the role of the temporal synaptic dynamics in the persistence of activity at low firing rate • Identification of two conductances NMDA et ICAN • - in vivo evidences of R-NMDA implications in persistence of activity • - NMDAR antagonists in the prefrontal cortex impair working memory in rat performing a spatial delayed alternation task (Verma 1996 and Romanides 1999) • NMDA antagonists abolish epileptiform discharges induced by picrotoxin in hippocampal slices (Lee, Hablitz, 1990, 1991) • induction by NMDA of "theta-like" rhythmic oscillations and bursts in hippocampal slices (Bonansco et al., 2002) • - in vivo role of mGluRs in persistence of activity • Evidence for a role of a mGluRs synaptic activation during prolonged epileptiform discharges induced by bicuculline and 4-AP (Lee et al., 2002) • when glutamate ionotropic receptors were blocked, a long lasting post synaptic component sensitive to MCPG allowed the signal propagation elicited by stimulation of association fibers in the hippocampus (Sugitani et al., 2002).