Co-release of glutamate and GABA from single, identified mossy fibre giant boutons

Key points The granule cells and their mossy fibres (MFs) can express, besides glutamate, all the markers of the GABAergic phenotype during development, suggesting that they can co-release glutamate and GABA. Several groups have presented substantial electrophysiological evidence, albeit indirect, supporting this hypothesis. We investigated the co-release of these amino acids by recording synaptic responses in mechanically dissociated pyramidal cells to stimulation of single, identified MF boutons attached to their apical dendrite. In pyramidal cells from developing rats, MF bouton stimulation evoked responses that were mediated by either glutamate receptors (R) only or by both glutamate-R and GABAA-R; in adult rats stimulation of MF boutons produced exclusively glutamate-R-mediated responses. By contrast, responses to stimulation of interneuronal boutons on the same cells were exclusively GABAA-R mediated. We demonstrate that the pharmacologically isolated GABAergic responses evoked by MF stimulation in CA3 cells in slice preparations may indeed be of MF origin. Abstract Several laboratories have provided immunohistochemical, molecular biological and electrophysiological evidence that the glutamatergic granule cells of the dentate gyrus can transiently express a GABAergic phenotype during development. Electrophysiological recordings on hippocampal slices obtained during this period have shown that stimulation of the mossy fibres (MFs) provokes simultaneous monosynaptic GABAA and glutamate receptor-mediated responses in their target cells, which have the pharmacological and physiological characteristics of MF neurotransmission. This evidence, although strongly supporting the hypothesis that MFs co-release glutamate and GABA, is indirect, as the extracellular stimulation used in slice experiments could activate fibres other than MFs. In this study, we show that selective stimulation of single, identified MF boutons (MFBs) attached to the apical dendrites of dissociated pyramidal cells of developing rats produced synaptic currents mediated by either glutamate receptors only or by both glutamate and GABAA receptors. By contrast, stimulation of MFBs of adult rats produced exclusively glutamate receptor-mediated responses. All responses evoked by stimulation of MFBs underwent strong frequency-dependent potentiation and were depressed by the activation of presynaptic metabotropic glutamate receptors. On the other hand, synaptic responses evoked by stimulation of interneuronal boutons located on the soma or on the basal dendrites of the same pyramidal cells were exclusively mediated by GABAA receptors, underwent frequency-dependent depression and were unaffected by mGluR agonists. We here demonstrate that the simultaneous glutamatergic and GABAergic responses evoked by MF stimulation in pyramidal cells of CA3 during development have a common origin in the giant MFBs.