GABA _A transmission is a critical step in the process of triggering homeostatic increases in quantal amplitude

Abstract

When activity levels are altered over days, a network of cells is capable of recognizing this perturbation and triggering several distinct compensatory changes that should help to recover and maintain the original activity levels homeostatically. One feature commonly observed after activity blockade has been a compensatory increase in excitatory quantal amplitude. The sensing machinery that detects altered activity levels is a central focus of the field currently, but thus far it has been elusive. The vast majority of studies that reduce network activity also reduce neurotransmission. We address the possibility that reduced neurotransmission can trigger increases in quantal amplitude. In this work, we blocked glutamatergic or GABAA transmission in ovo for 2 days while maintaining relatively normal network activity. We found that reducing GABAA transmission triggered compensatory increases in both GABA and AMPA quantal amplitude in embryonic spinal motoneurons. Glutamatergic blockade had no effect on quantal amplitude. Therefore, GABA binding to the GABAA receptor appears to be a critical step in the sensing machinery for homeostatic synaptic plasticity. The findings suggest that homeostatic increases in quantal amplitude may normally be triggered by reduced levels of activity, which are sensed in the developing spinal cord by GABA, via the GABA A receptor. Therefore, GABA appears to be serving as a proxy for activity levels.activity ͉ neurotransmitter receptor ͉ postsynaptic ͉ synaptic scaling ͉ synaptic plasticity S pontaneous network activity (SNA) is a prominent feature of developing neural networks that consists of episodic bursts of activity separated by periods of quiescence (1-3). SNA is produced by hyperexcitable, recurrently connected circuits in which glutamate and GABA are both excitatory early in development. In the spinal cord, SNA drives embryonic limb movements and is known to be important for various aspects of limb (4) and motoneuron development (5, 6). Reducing spontaneous network activity in the embryonic chick in vivo leads to compensatory increases in the strength of excitatory GABAergic and AMPAergic synapses (7). These compensatory increases in synaptic strength have been described in several systems where they appear to act in a manner to recover and maintain network activity levels homeostatically (homeostatic synaptic plasticity) (8, 9). Activity perturbations lead to several different forms of homeostatic synaptic plasticity, including changes in quantal amplitude, probability of release, and frequency of miniature postsynaptic currents (mPSCs). In the present work, we focus on compensatory changes in quantal (mPSC) amplitude. Changes in mPSC amplitude have been studied intensely in cultured networks where prolonged activity reduction results in an increase in the amplitude of excitatory mPSCs and a decrease in the amplitude of inhibitory mPSCs (10-12). Changes in mPSC amplitude are achieved through alterations in the number of postsynaptic receptors and/or the amount of transmitter re...

Compensatory changes in cellular excitability, not synaptic scaling, contribute to homeostatic recovery of embryonic network activity

“…This recovery of activity occurred before compensatory increases in quantal amplitude were observed, as described previously (Fig. 1A) (17). To determine whether cellular excitability contributed to the recovery of embryonic activity after GABA A receptor blockade, we treated embryos in ovo for either 12 h (E9.5-E10) or 48 h (E8-E10) with saline, the GABA A receptor antagonist gabazine, or the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX) and DL-2-amino-5-phosphonopentanoic acid (APV).…”

“…We have further shown that the compensatory changes in quantal amplitude were triggered through a reduction in GABA A receptor activation (17). In this way, reductions in GABA binding to its receptor signal that network activity levels have been reduced.…”

“…Blocking GABA A transmission at embryonic day 8 (E8) reduced embryonic movements, which then recovered within 12 h. Movements (our measure of SNA) were counted during a 5-min observation period (17). This recovery of activity occurred before compensatory increases in quantal amplitude were observed, as described previously (Fig.…”

Synaptic Scaling and the Development of a Motor Network

“…We may have underestimated the extent of synaptic scaling up as we observed partial reversal of inhibition by CNQX. Nonetheless, Wilhelm and Wenner (2008) used a similar technique of a bolus injection of activity-reducing drugs in the in ovo chick spinal cord and measured the recovery of spontaneous network activity by mechanisms that include synaptic scaling. They showed that injection of a single receptor antagonist such as CNQX resulted in a temporary reduction in spontaneous network activity (as measured by immature limb movements) that recovered to control levels within hours.…”

Control of lung ventilation following overwintering conditions in bullfrogs,Lithobates catesbeianus

“…From the neural perspective, a wealth of information exists regarding how neurons and circuits maintain target levels of activity despite removing or enhancing activity inputs (termed homeostatic plasticity). However, models to study these processes use pharmacological or pathological paradigms to manipulate neuronal activity in vivo and in vitro primarily at early developmental stages (Hengen et al, 2013;Knogler et al, 2010;Ngodup et al, 2015;Schacher and Hu, 2014;Turrigiano, 2012;Wilhelm and Wenner, 2008). Thus, the bullfrog respiratory control system following overwintering offers a powerful ability to uncover mechanisms leading to and resulting in the preservation (respiratory motor output and hypoxia sensitivity) and deterioration (CO 2 sensitivity) of sensorimotor function in the same species, individual and neural control system after ecologically relevant inactivity.…”