Synaptically-silent immature neurons show GABA and glutamate receptor-mediated currents in adult rat dentate gyrus

The fate of adult-generated neurons in dentate gyrus is mainly determined early, before they receive synapses. In developing brain, classical neurotransmitters such as GABA and glutamate exert trophic effects before synaptogenesis. In order for this to occur in adult brain as well, immature non-contacted cells must express functional receptors to GABA and glutamate. In this investigation, patch-clamp recordings were used in adult rat dentate gyrus slices to assess the presence and analyze the characteristics of GABA- and glutamate-evoked currents in highly immature, synaptically-silent granule cells. Whole-cell patch-clamp recordings showed that all the analyzed cells responded to puff application of GABA and most of them responded to glutamate. Currents evoked by GABA were mediated exclusively by GABAA receptors and those elicited by glutamate were mediated by NMDA and AMPA/Kainate receptors. GABAA receptor-mediated currents were reduced by furosemide, which suggests that synaptically-silent immature neurons express high-affinity, alpha4-subunit-containing GABAA receptors. Gramicidin-perforated-patch recordings showed that GABAA receptor-mediated currents exerted a depolarizing effect due to high intracellular chloride concentration. Synaptically-silent immature cells shared morphological and electrophysiological properties with GFP-expressing, 7-day-old adult-generated granule layer cells, indicating that they could be in the first week of life, the period of maximal newborn cell death. Moreover, the presence of functional GABA and glutamate receptors was confirmed in these GFP-expressing cells. Present findings are mostly consistent with previous data obtained in female mice undergoing spontaneous activity and in transgenic mice, except for some inconsistencies about the presence of functional glutamatergic receptors. We speculate that adult-generated, non-contacted granule cells may be able to sense activity-related variations of GABA and glutamate extracellular levels. This condition is necessary, even if not sufficient, for these neurotransmitters to have a direct role in addressing cell survival