Fast Oscillatory Activity in the Hippocampus can Back-Propagate Electrotonically to the Dentate Gyrus

F. ORTIZ1,2, R. GUTIÉRREZ1

1 CINVESTAV, Mexico City, Mexico; 2 Instituto de Fisiología Celular, UNAM, Mexico City, Mexico

INTRODUCTION

Fast ripples (FRs; intermittent high frequency oscillations of >250 Hz) are the hallmark of epileptic foci in the limbic system. While electrotonic coupling is a mechanism by which high frequency network activity (150-200 Hz) becomes synchronized, FRs depend mainly on chemical synaptic transmission. Although a synaptic back-projection from CA3 to the DG has been described, the mechanisms by which FRs emerge in the DG remain elusive.

METHODS

Entorhinal cortex (EC)-hippocampal horizontal slices were obtained from rats with EC lesions made prior to slicing. This procedure provokes FRs in vitro1. We recorded multiunitary and field activity with a 4096 microelectrode matrix (3Brain). We used Granger causality to determine the directionality of FRs propagation. Recordings were conducted under normal neurotransmision during perfusion of low Ca++ medium (0.2 mM) and during perfusion of the connexin 36 gap junction blocker, mefluoquine (50 µM). We assessed functional connections between neurons of CA3 and DG (2-10 ms window) with a Montecarlo algorithm.

Field events are rarely seen in control slices; however they display multi-unitary activity (4 sec of activity in loop).














Spontaneous FRs in slices from in situ cortical lesioned rats (duration: 40 ms).






FRs were detected in the hippocampus and DG


FRs emerge mainly in medial and distal CA3














FRs propagate in a disorganized manner




Site-specific back-propagation of FRr from CA3 to the DG determined by Granger causality test. Each frame represents a FR event. Connecting lines represent significant Granger causality (5 min of activity in loop).

















Total number of Granger Causality connections










Activity after blockage of chemical synaptic transmission in FRs (4 sec of activity in loop).







CA3 to DG functional connections after blockage of chemical synaptic transmission in FRs (5 min of activity in loop).













Mefluoquine reduced the number of active sites after chemical synaptic transmission was blocked.

CONCLUSION

FRs emerge in CA3 but can appear simultaneously in discontinuous sites and with different oscillatory frequencies along the CA region. Importantly, FRs emerging in CA3 could propagate to the DG, were suppressed by low Ca++ medium, and mefluoquine significantly reduced the number of remaining active sites in the DG and distal CA3. Back propagation of FRs from CA3 to the DG may be due to electrotonic coupling mediated by gap junctions2,3, and may play a significant role in DG activation by CA3 activity.

References:

1. Ortiz, F., & Gutiérrez, R. (2015). Entorhinal cortex lesions result in adenosine-sensitive high frequency oscillations in the hippocampus. Experimental Neurology, 271, 319–328.

2. Dudek, F. E., Yasumura, T., & Rash, J. E. (1998). “Non-synaptic” mechanisms in seizures and epileptogenesis. Cell Biology International, 22(11–12), 793–805.

3. Vivar, C., Traub, R. D., & Gutiérrez, R. (2012). Mixed electrical-chemical transmission between hippocampal mossy fibers and pyramidal cells. The European Journal of Neuroscience, 35(1), 76–82.