Ruolo del cross-talking GABA-glutammato nello sviluppo morfo-funzionale dei neuroni ippocampali di Mesocricetus auratus

Abstract

The hippocampus (HIP) is recognized as a major telencephalic area processing learning and episodic memory events through the accumulation of neuronal signals deriving from the different subregions. In particular, hippocampal neurons have received particular attention due to some of their functional properties being regulated by GABAergic and Glutamat- (Glu)-ergic neuronal signals during both postnatal and adult stages. For the present study, growth of hamster hippocampal neurons on biohybrid membrane substrates allowed us to show for the first time that the two major GABAA α receptor subunits (α2,5) promote early neuronal shaping plus expression differences of the main neuronal cytoskeletal factors (GAP-43, MAP2, Syn, the neurotrophin BDNF) along with Gluergic subtypes. In a first case, the selective GABAAR agonist diazepam (DZP; α2,5) caused very great (p<0.001) increases of dendritic sprouting and branching processes mainly at DIV3, while its effects continued to account even for great (p<0.01) axonal length during the entire culture period. In addition, DZP also accounted for great (p<0.01) up-regulation of neuritic NR1, GluR2 and MAP2 while it moderately (p<0.05) increased synaptophysin (Syn) at DIV7. Such effects were abolished by its highly selective antagonist flumazenil. The application of the inverse α5 agonist (RY-080) tended to strongly suggest its specific role on the dendritic component via the inhibition of BDNF, as shown by a reduction of dendritic length at DIV7 concomitantly to very low levels of the neurotrophin. Conversely, the effects of the preferentially specific agonist for hippocampal α2 subunit (flunitrazepam) were, instead, directed at the formation of growth cones at DIV3 in the presence of greatly (p<0.01) diminished GAP-43 levels as displayed by strongly reduced axonal sprouting. It is interesting to note that contextually to these morphological variations, the transcription of some Gluergic receptor subtypes resulted to be altered. Indeed, the α2 agonist was responsible for a distinctly rising expression pattern of axonal NR1 mRNA levels from DIV3 (p<0.01) until DIV7 (p<0.001), along with a very great up-regulation of Syn at DIV7. In the case of RY-080, it evoked a very great (p<0.001) downregulation of dendritic GluR2 at only DIV7. Together, our results demonstrate that GABAA α2,5 receptor-containing subunits are considered key modulating neuronal elements of HIP morphological features by regulating the precise synchronization of cytoskeletal factors. Moreover, the notable NR1 and GluR2 transcription differences promoted by these GABAA α subunits tend to favorably corroborate the early role of α2 + α5 on HIP neuronal networks in hibernating rodents through the recruitment and activation of silent neurons. We are still at the beginning but further studies are required to establish the nature of molecular signals controlling responses of the different hippocampal subregions, which may have therapeutic bearings for neurological disorders.