Distinct α subunits of the GABA_A receptor are responsible for early hippocampal silent neuron‐related activities

Abstract

The modulatory actions of GABA(A) receptor subunits are crucial for morphological and transcriptional neuronal activities. In this study, growth of hamster hippocampal neurons on biohybrid membrane substrates allowed us to show for the first time that the two major GABA(A) alpha receptor subunits (alpha(2,5)) are capable of early neuronal shaping plus expression differences of some of the main neuronal cytoskeletal factors (GAP-43, the neurotrophin--BDNF) and of Gluergic subtypes. In a first case the inverse alpha(5) agonist (RY-080) seemed to account for the reduction of dendritic length at DIV7, very likely via lower BDNF levels. Conversely, the effects of the preferentially specific agonist for hippocampal alpha(2) 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 concomitantly to these morphological variations, the transcription of some Gluergic receptor subtypes resulted to be altered. In particular, flunitrazepam was responsible for a distinctly rising expression of axonal NR1 mRNA levels from DIV3 (P < 0.01) until DIV7 (P < 0.001), whereas RY-080 evoked a very great (P < 0.001) downregulation of dendritic GluR2 at only DIV7. Together, our results demonstrate that GABA(A) alpha(2,5) receptor-containing subunits by regulating the precise synchronization of cytoskeletal factors are considered key modulating neuronal elements of hippocampal morphological growth features. Moreover, the notable NR1 and GluR2 transcription differences promoted by these GABA(A) alpha subunits tend to favorably corroborate the early role of alpha(2) + alpha(5) on hippocampal neuronal networks in hibernating rodents through the recruitment and activation of silent neurons, and this may provide useful insights regarding molecular neurodegenerative events.

GABAergic control of neurite outgrowth and remodeling during development and adult neurogenesis: general rules and differences in diverse systems

“…This is based on two pieces of evidence. (1) BDNF expression and release from target neurons depends on GABAergic depolarization in the first 2 weeks in vitro , whereas GABAergic activity reduces the synthesis of BDNF mRNA in mature neurons (Berninger et al, 1995; Marty et al, 1996; Vicario-Abejon et al, 1998; Giusi et al, 2009). (2) BDNF signaling deprivation leads to dendrite growth impairment in newborn neurons during development and in the adult (McAllister et al, 1996; Berghuis et al, 2006; Chen et al, 2006; Takemoto-Kimura et al, 2007; Bergami et al, 2008; Chan et al, 2008; Ageta-Ishihara et al, 2009; Suh et al, 2009).…”

“…Notably, application of muscimol reduced the number of primary dendrites when applied 2 weeks after plating (after the excitatory to inhibitory switch of GABA responses takes place), again suggesting the requirement of depolarizing GABA. Interestingly, in a recent paper, a possible involvement of the α5 subunit (but not β2) of the GABA A receptor was suggested for the GABA-induced effect on neurite outgrowth via lowering of brain-derived neurotrophic factor (BDNF) levels in hamster hippocampal neurons treated with a specific α5 inverse agonist (Giusi et al, 2009). The latter results, again, point to a possible involvement of tonic GABA signaling in neuritogenesis.…”

“…The effect of GABA on axonal growth may be specifically mediated by the α 2 subunit of the GABA A receptor, as a α 2 (but not α5) selective agonist strongly reduced axonal sprouting in cell cultures of hamster hippocampal neurons (Giusi et al, 2009). Furthermore, a new study has recently addressed specifically whether GABA signaling may affect axonal morphological maturation both in vitro and in vivo (Ageta-Ishihara et al, 2009).…”

Neuroprotective Effect of Didymin on Hydrogen Peroxide-Induced Injury in the Neuronal Membrane System

“…The present study investigates the protective effect of didymin against H 2 O 2 -induced damage to the neuronal cells in a biohybrid membrane system model. Previous studies have demonstrated that semipermeable polymeric membranes in flat and hollow fiber configurations, thanks to their highly selective structural, physicochemical and transport properties, allow the successful in vitro reconstruction of neuronal tissue, reproducing a tissue model for studying metabolic diseases and drug effects [Woerly et al, 1996;Schmidt and Leach, 2003;Zhang et al, 2005;De Bartolo et al, 2008;Giusi et al, 2009;He et al, 2009;Morelli et al, 2010;Di Vito et al, 2011;Morelli et al, 2012b, c]. It was recently reported that polycaprolactone (PCL)-based membranes successfully supported outgrowth and differentiation of human neuronal cells [Morelli et al, 2012a].…”

Distinct α subunit variations of the hypothalamic GABAA receptor triplets (αβγ) are linked to hibernating state in hamsters

“…Indeed during the arousal state, the switching ON of α 1 may lead to a structurally well-assembled GABA A R complex [29] and consequently the activation of motor-controlling neurogenic programs in order to face new functional plasticity states [30]. Moreover, the predominance of a α 1 -dependent pharmacological organizational and functional features [8] have already been reflected during the early neuronal developmental stages of another major limbic region in hamsters and precisely the hippocampus [31] as well as on the induction of visual functions in other adult rodents [32]. As a consequence, it might very well be that the high levels of hypothalamic α 1 -containing neurons may assure a pharmacological protective role against ischemic insults during the awakening phase [19,33] especially since an increased gene expression of this subunit has been correlated to the new functional plasticity states during the arousal phase [34].…”