Excitation–inhibition balance in the CA3 network – neuronal specificity and activity-dependent plasticity Mario Trevin˜o,* Carmen Vivar   and Rafael Gutie ´ rrez Departamento de Fisiologı ´a, Biofı ´sica y Neurociencias, Centro de Investigacio ´ n y de Estudios Avanzados del Instituto Polite ´ cnico Nacional Apartado Postal 14-740, Me ´ xico D.F. 07000, Mexico Keywords: CA3, epilepsy, excitation–inhibition balance, GABA, mossy fibers, rat Abstract Activation of the axons of the granule cells, the mossy fibers, excites pyramidal cells and interneurons in the CA3 area, which, in turn, inhibit pyramidal cells. The integration of the various inputs that converge onto CA3 cells has been studied by pharmacological dissection of either the excitatory or inhibitory components. This strategy has the disadvantage of partially isolating the recorded cell from the network, ignoring the sources and the impact of concurrent inputs. To overcome this limitation, we dissociated excitatory and inhibitory synaptic conductances by mathematical extraction techniques, and analysed the dynamics of the integration of excitatory and inhibitory inputs in pyramidal cells and stratum lucidum interneurons (Sl-Ints) of CA3. We have uncovered a shunting mechanism that decreases the responsiveness of CA3 output cells to mossy fiber input after a period of enhanced excitability. The activation of the dentate gyrus (DG) after applying a kindling-like protocol in vitro, or after producing one or several seizures in vivo, results in a graded and reversible increase of inhibitory conductances in pyramidal cells, while in Sl-Ints, an increase of excitatory conductances occurs. Thus, interneurons reach more depolarized membrane potentials on DG activation yielding a high excitatory postsynaptic potential–spike coupling, while the contrary occurs in pyramidal cells. This effective activation of feedforward inhibition is synergized by the emergence of direct DG-mediated inhibition on pyramidal cells. These factors force the synaptic conductance to peak at a potential value close to resting membrane potential, thus producing shunt inhibition and decreasing the responsiveness of CA3 output cells to mossy fiber input. Introduction It has been proposed that the dentate gyrus (DG) is mainly an inhibitory structure controlling CA3 excitability (Henze et al., 2000), and that it serves as a gate of information transfer from the entorhinal cortex to the hippocampus proper (Heinemann et al., 1992). Because each granule cell of the DG makes more contacts with interneurons than with pyramidal cells of CA3 (Acsa ´dy et al., 1998), DG activation drives a large number of interneurons, which, in turn, inhibit pyramidal cells, making feedforward inhibition a mechanism through which DG modulates CA3 excitability (Henze et al., 2000; Lawrence & McBain, 2003). Intuitively, it is clear that the loss of the excitation ⁄ inhibition (E ⁄ I) balance in favor of excitation would result in epileptiform activity in the CA3 area. The ongoing excitatory and inhibitory inputs are integrated by the neuron, setting its membrane potential to a depolarized or hyperpolarized value, and determining its ulterior activity. This integration depends on the membrane potential at rest, the time constant of the membrane, the leak conductance, as well as on the amplitude, kinetics, reversal potentials and the degree of temporal overlap between excitation and inhibition. The various synaptic inputs that CA3 cells receive have been traditionally studied by pharmaco- logical dissection of the excitatory and inhibitory responses. This strategy, besides modifying the resting membrane potential (RMP), partially isolates the recorded cell from the network, and ignores the sources and the impact of the concurrent excitatory and inhibitory inputs. Therefore, this approach does not allow the direct determination of a realistic excitation–inhibition ratio. However, a conductance-based analysis (Borg-Graham et al., 1998; Borg-Graham, 2001) allows the estimation of the time course of excitatory and inhibitory conductances and the composite reversal potential from complex synaptic events intracellularly recorded at several different membrane potentials. Using this approach, we here studied long-lasting, activity-depen- dent changes in synaptic integration in CA3 pyramidal cells and stratum lucidum interneurons (Sl-Ints) in control conditions, after providing a kindling-like stimulation protocol in vitro, and after producing one or several seizures in vivo. Specifically, we addressed the possibility that these long-lasting synaptic changes are differen- tially expressed in pyramidal cells and Sl-Int of CA3, probably modifying the way they integrate further complex synaptic activity driven by the DG. We show that after the kindling-like stimulation in vitro and after an acute or several kindled seizures in vivo, DG stimulation produces a graded and reversible increase in its inhibitory Correspondence: Dr R. Gutie ´rrez, as above. E-mail: grafael@fisio.cinvestav.mx *Present address: Max-Planck Institute for Medical Research Heidelberg 69120, Germany.   Present address: NIH, National Institute on Aging, Biomedical Research Center, Laboratory of Neuroscience, Baltimore, MD, 21224, USA. Received 30 November 2010, revised 1 March 2011, accepted 4 March 2011 European Journal of Neuroscience, pp. 1–15, 2011 doi:10.1111/j.1460-9568.2011.07670.x ª 2011 The Authors. European Journal of Neuroscience ª 2011 Federation of European Neuroscience Societies and Blackwell Publishing Ltd European Journal of Neuroscience