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GABAA Transmission Is a Critical Step in the Process of Triggering Homeostatic Increases in Quantal Amplitude

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GABAA Transmission Is a Critical Step in the Process of Triggering Homeostatic Increases in Quantal Amplitude GABAA transmission is a critical step in the process of triggering homeostatic increases in quantal amplitude Jennifer C. Wilhelm and Peter Wenner* Department of Physiology, Emory University School of Medicine, Atlanta, GA 30322 Communicated by Lynn T. Landmesser, Case Western Reserve University, Cleveland, OH, June 23, 2008 (received for review March 1, 2008) When activity levels are altered over days, a network of cells is models suggests that reducing network activity produces a corre- capable of recognizing this perturbation and triggering several dis- sponding reduction in cellular spiking activity, thereby reducing tinct compensatory changes that should help to recover and maintain intracellular calcium levels in a postsynaptic cell. In this model, the original activity levels homeostatically. One feature commonly which we will refer to as the cell activity model, the postsynaptic cell observed after activity blockade has been a compensatory increase in senses changes in intracellular calcium levels as a measure of altered excitatory quantal amplitude. The sensing machinery that detects activity and triggers compensatory changes in mPSC amplitude. altered activity levels is a central focus of the field currently, but thus However, the studies that have inspired this model not only block far it has been elusive. The vast majority of studies that reduce spiking activity, they also block or reduce neurotransmitter binding network activity also reduce neurotransmission. We address the to its receptor and any associated downstream signaling cascades. possibility that reduced neurotransmission can trigger increases in Thus, it remains possible that neurotransmission is a critical step in quantal amplitude. In this work, we blocked glutamatergic or GABAA the sensing process that triggers changes in quantal amplitude. In an transmission in ovo for 2 days while maintaining relatively normal alternative model, the neurotransmitter model, changes in activity network activity. We found that reducing GABAA transmission trig- levels would be sensed by alterations in neurotransmitter receptor gered compensatory increases in both GABA and AMPA quantal binding. To address such a possibility, one would need to block amplitude in embryonic spinal motoneurons. Glutamatergic blockade either activity or neurotransmission, but not both. Recent studies had no effect on quantal amplitude. Therefore, GABA binding to the have demonstrated that neither hyperpolarization nor reducing GABAA receptor appears to be a critical step in the sensing machinery spike rate in the postsynaptic cell leads to compensatory changes in for homeostatic synaptic plasticity. The findings suggest that homeo- mPSC amplitude as would be predicted by the cell activity model static increases in quantal amplitude may normally be triggered by (8, 16–19). In those studies, the release and binding of neurotrans- reduced levels of activity, which are sensed in the developing spinal mitters to their receptors was not altered, and thus the results are cord by GABA, via the GABAA receptor. Therefore, GABA appears to consistent with the neurotransmitter model. be serving as a proxy for activity levels. To test the neurotransmitter model directly, one would like to block neurotransmission without affecting activity levels. In the activity ͉ neurotransmitter receptor ͉ postsynaptic ͉ synaptic scaling ͉ present work, we block GABAergic or glutamatergic neurotrans- synaptic plasticity mission in the developing spinal cord in vivo while maintaining significant levels of SNA caused by the fast compensatory nature of the highly excitable developing cord (20, 21). We have found a pontaneous network activity (SNA) is a prominent feature of particular importance of GABA transmission in triggering com- developing neural networks that consists of episodic bursts of A S pensatory changes in quantal amplitude. Our findings suggest that activity separated by periods of quiescence (1–3). SNA is produced the GABA receptor is part of the sensing machinery for homeo- by hyperexcitable, recurrently connected circuits in which gluta- A static synaptic plasticity in the developing spinal cord. mate and GABA are both excitatory early in development. In the spinal cord, SNA drives embryonic limb movements and is known Results to be important for various aspects of limb (4) and motoneuron Neurotransmitter Block in Ovo. Previously, we reduced SNA in ovo development (5, 6). Reducing spontaneous network activity in the from embryonic day 8–10 (E8–E10) through infusion of the sodium embryonic chick in vivo leads to compensatory increases in the channel blocker lidocaine and found compensatory increases in strength of excitatory GABAergic and AMPAergic synapses (7). both GABAergic and AMPAergic mPSC amplitude at E10 (7). To These compensatory increases in synaptic strength have been test the possibility that reduced ionotropic neurotransmitter signal- described in several systems where they appear to act in a manner ing triggered compensatory changes in quantal amplitude, we to recover and maintain network activity levels homeostatically injected various neurotransmitter antagonists in ovo.Wefirst (homeostatic synaptic plasticity) (8, 9). determined the concentration of receptor antagonists necessary to Activity perturbations lead to several different forms of homeo- reduce neurotransmission significantly for 48 h in ovo. Embryonic static synaptic plasticity, including changes in quantal amplitude, limb movements were assessed to determine the effects of neuro- probability of release, and frequency of miniature postsynaptic transmitter receptor block on spontaneous network activity in ovo. currents (mPSCs). In the present work, we focus on compensatory Spinally generated embryonic limb movements were observed changes in quantal (mPSC) amplitude. Changes in mPSC ampli- through windows in the eggshell. SNA was quantified in ovo by tude have been studied intensely in cultured networks where counting the total time the chicken embryo moved during a prolonged activity reduction results in an increase in the amplitude 5-min period of observation (Fig. 1A). We also monitored the of excitatory mPSCs and a decrease in the amplitude of inhibitory number of bouts of movement (Fig. 1B) and the duration of each mPSCs (10–12). Changes in mPSC amplitude are achieved through bout (Fig. 1C). alterations in the number of postsynaptic receptors and/or the amount of transmitter released per vesicle (8, 9). However, far less is known about the machinery that senses perturbations in the level Author contributions: J.C.W. and P.W. designed research; J.C.W. performed research; of activity, which then trigger compensatory changes in mPSC J.C.W. and P.W. analyzed data; and J.C.W. and P.W. wrote the paper. amplitude. The authors declare no conflict of interest. Several signals have been suggested as possible sensors for *To whom correspondence should be addressed at: 615 Michael Street, Department of Phys- tracking changes in activity, including spike rate, depolarization, iology, Emory University, Atlanta, GA 30322. E-mail: [email protected]. and intracellular calcium levels (8, 9, 13–15). One of the favored © 2008 by The National Academy of Sciences of the USA 11412–11417 ͉ PNAS ͉ August 12, 2008 ͉ vol. 105 ͉ no. 32 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0806037105 Downloaded by guest on October 6, 2021 A 1.5 B 5 4 d) ize l 3 control rma 2 o 1.0 (N 1 ts Number of bouts 0 men 0 1020304050 e v C Time (Hours) 1.5 mo control 0.5 b cocktail m li bicuculline f ormalized) 1.0 o gabazine N n CNQX o ti CNQX + APV ra 0.5 u D 0 01020304050 Time (Hours) 0 01020304050 Bout duration ( E8 E10 Time (Hours) Fig. 1. GABAergic or glutamatergic receptors can be chronically blocked without a prolonged reduction of spontaneous network activity. (A) Graph of the average amount of time chick embryos moved during a 5-min period of observation, obtained once every 0.5–12 h. Data are normalized to control values at each observation time point/stage, thus producing the dotted line for comparison. Treatment with a single receptor antagonist (bicuculline, n ϭ 4 embryos; gabazine, n ϭ 5; CNQX, n ϭ 3; CNQX ϩ APV, n ϭ 6) transiently reduces embryonic movements; however, movements return to control (n ϭ 13) levels within 12 h. A mixture of gabazine, CNQX, strychnine, and APV (n ϭ 9) nearly abolished limb movements for 48 h. (B and C) Graph of the median number of bouts or episodes (B) or average bout duration (C) during a 5-min period of observation. Error bars, SE. To determine the effective concentrations of receptor antago- Table 1). Further, average mPSC frequency, mPSC rise and decay nists needed to block activity significantly in ovo, a mixture of kinetics, and cellular passive membrane properties were unaltered ionotropic receptor antagonists was infused into the egg, including after glutamate receptor blockade from E8 to E10 (Tables 1 and 2). aGABAA receptor antagonist (bicuculline or gabazine, concen- We did see a reduction in the variability of mPSC amplitude when trations shown in Experimental Procedures; assume a 50 ml egg volume), a glycine receptor antagonist (strychnine), an AMPA/KA (referred to as AMPA from this point) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX), and an A Control NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic CNQX acid (APV). The injection of the mixture of these four antagonists at E8 resulted in a near cessation of all limb movements from E8 CNQX + APV to E10 (Fig. 1, red). 20 pA The exclusion of any one of these drugs from the mixture reduced 2s NEUROSCIENCE limb movements but did not abolish them for the entire 2 days (data B CNQX + C CNQX + not shown). The injection of a single receptor antagonist or a Control CNQX APV Control CNQX APV combination of CNQX and APV caused a temporary reduction in limb movements that recovered to control levels within hours (Fig. 10 pA 1). Doubling the concentration of any drug or the addition of a 100 ms second bolus at E9 produced no additional effect on limb move- ments (data not shown).
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