6-Containing GABAA Receptors Are the Principal Mediators of Inhibitory Synapse Strengthening by Insulin in Cerebellar Granule Cells

Abstract

Activity-dependent strengthening of central synapses is a key factor driving neuronal circuit behavior in the vertebrate CNS. At fast inhibitory synapses, strengthening is thought to occur by increasing the number of GABA A receptors (GABARs) of the same subunit composition to preexisting synapses. Here, we show that strengthening of mouse cerebellar granule cell GABAergic synapses occurs by a different mechanism. Specifically, we show that the neuropeptide hormone, insulin, strengthens inhibitory synapses by recruiting ␣6-containing GABARs rather than accumulating more ␣1-containing receptors that are resident to the synapse. Because ␣6-receptors are targeted to functionally distinct postsynaptic sites from ␣1-receptors, we conclude that only a subset of all inhibitory synapses are strengthened. Together with our recent findings on stellate cells, we propose a general mechanism by which mature inhibitory synapses are strengthened. In this scenario, ␣1-GABARs resident to inhibitory synapses form the hardwiring of neuronal circuits with receptors of a different composition fulfilling a fundamental, but unappreciated, role in synapse strengthening.

Dynamic Regulation of the GABAA Receptor Function by Redox Mechanisms

“…Endogenous ROS production was also shown to be a key factor that can strengthen inhibitory neurotransmission mediated by GABA A Rs in cerebellar neurons (Accardi et al, 2014(Accardi et al, , 2015. Specifically, synaptic GABA A responses were recorded in stellate and granule cells, whereas ROS levels were enhanced, for example, by intracellular perfusion of the mitochondrial uncoupler antimycin A (Fig.…”

“…Thiol reagents elicited opposite effects on glycine-evoked responses evoked in these cells, indicating that other Cys-loop receptors could also be sensitive to redox modulation (Pan et al, 1995). Redox susceptibility of GABA A Rs to diverse endogenous agents (e.g., ascorbic acid, GSH, ROS, RNS) was later corroborated in several neuronal types, including cerebellar granule cells (Amato et al, 1999;Accardi et al, 2015), superior cervical ganglionic neurons (Amato et al, 1999), retinal bipolar neurons (Calero et al, 2011), hippocampal CA1 pyramidal neurons (Penna et al, 2014), and cerebellar stellate cells (Accardi et al, Fig. 2.…”

“…The damaging effects of ROS can be counteracted by the specific detoxifying enzymes and high levels of antioxidant agents in a sophisticated way such that their local concentration and distribution can be rapidly and precisely controlled to allow dynamic cell signaling (Rhee, 2006;Bao et al, 2009;Woo et al, 2010;Rice, 2011;Finkel, 2011). ROS effects in the nervous system can include transient changes in neuronal activity and synaptic plasticity (Klann, 1998;Bao et al, 2009), as well as diverse actions on both excitatory and inhibitory neurotransmission (Frantseva et al, 1998;Sah and Schwartz-Bloom, 1999;Kamsler and Segal, 2003), including regulatory changes on neurotransmitter receptors (Aizenman et al, 1990;Chu et al, 2006;Coddou et al, 2009;Accardi et al, 2014Accardi et al, , 2015Beltrán González et al, 2014;Penna et al, 2014).…”

“…2) (Pan et al, 1995;Amato et al, 1999;Sah et al, 2002;Calero et al, 2011;Penna et al, 2014;Accardi et al, 2014Accardi et al, , 2015. Previous receptor-binding studies indicated that sulfhydryl and disulfide groups may play a role in the responses to GABA and glycine (referenced in Pan et al, 1995); however, the functional modulation of GABA A Rs by redox agents was revealed by studying the effects of endogenous and exogenous thiol reagents on inhibitory responses in retinal neurons (Pan et al, 1995) (Fig.…”

“…Cerebellum: (A) GABA A responses (GABA 10 mM) in acutely dissociated granule cells showed very low sensitivity to DTT (5 mM) and DTNB (500 mM) (Amato et al, 1999). (B) After an increase in ROS in granule cells, induced by mitochondrial uncoupling (bottom trace) (Accardi et al, 2015), a higher frequency of low-amplitude postsynaptic responses was detected. Expression in oocytes: (A) Effects of DTT (2 mM) on responses mediated by human GABA A R variants (GABA 10 mM) (Pan et al, 2000).…”