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1 Fisiologia, Biofisica y Neurociencias, Centro de Investigacion y Estudios Avanzados, Mexico D.F., Mexico
2 Instituto de Investigaciones Biomedicas, Universidad Nacional Autonoma de Mexico, Mexico D.F., Mexico
* To whom correspondence should be addressed. E-mail: grafael{at}fisio.cinvestav.mx.
The granule cells of the dentate gyrus (DG) are considered to be glutamatergic but they contain glutamic acid decarboxylase, 
-amino butyric acid (GABA), and the vesicular GABA transporter mRNA. Their expression is regulated in an activity dependent manner and coincides with the appearance of GABAergic transmission from the mossy fibers (MF) to pyramidal cells in area CA3. These data support the hypothesis that MF are able to release glutamate and GABA. Following the principle that a given neuron releases the same neurotransmitter(s) onto all its targets, we here demonstrate the emergence, after a generalized convulsive seizure, of MF GABAergic signaling sensitive to activation mGluR-III onto pyramidal cells and interneurons of CA3. Despite this, excitation overrides inhibition in interneurons, preventing disinhibition. Furthermore, upon blockade of GABA and glutamate ionotropic receptors, an M1-cholinergic depolarizing signal is also revealed in both targets, which postsynaptically modulates the glutamatergic and GABAergic fast neurotransmission. The emergence of these non-glutamatergic signals depends on protein synthesis. In contrast to cholinergic responses evoked by associational/commissural fibers activation, cholinergic transmission evoked by DG stimulation is only observed after seizures and is strongly depressed by the activation of mGluR-II, whereas both are depressed by M2-AChR activation. With immunohistological experiments, we show that this cholinergic pathway runs parallel to the MF. Thus, seizures compromise a delicate balance of excitation and inhibition, upon which a complex interaction of different neurotransmitters emerges to counteract excitation at pre- and post-synaptic sites. Particularly, MF GABAergic inhibition emerges to exert an overall inhibitory action on CA3.
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