Spontaneous Network Activity in the Embryonic Spinal Cord Regulates AMPAergic and GABAergic Synaptic Strength

Abstract

Spontaneous network activity (SNA) has been described in most developing circuits, including the spinal cord, retina, and hippocampus. Despite the widespread nature of this developmental phenomenon, its role in network maturation is poorly understood. We reduced SNA in the intact embryo and found compensatory increases in synaptic strength of spinal motoneuron inputs. AMPAergic miniature postsynaptic current (mPSC) amplitude and frequency increased following the reduction of activity. Interestingly, excitatory GABAergic mPSCs also increase in amplitude through a process of synaptic scaling. Finally, the normal modulation of GABAergic mPSC amplitude was accelerated. Together, these compensatory responses appear to increase the excitability of the cord and could act to maintain appropriate SNA levels, thus demonstrating a distinct functional role for synaptic homeostasis. Because spontaneous network activity can regulate AMPAergic and GABAergic synaptic strength during development, SNA is likely to play an important role in a coordinated maturation of excitatory and inhibitory synaptic strength.

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

“…At E10, the two dominant spinal motoneuron inputs are GABAergic and glutamatergic (7). Blocking AMPA and NMDA receptors failed to trigger changes in the amplitude of mPSCs.…”

“…Previously, we reduced SNA in ovo from embryonic day 8-10 (E8-E10) through infusion of the sodium channel blocker lidocaine and found compensatory increases in both GABAergic and AMPAergic mPSC amplitude at E10 (7). To test the possibility that reduced ionotropic neurotransmitter signaling triggered compensatory changes in quantal amplitude, we injected various neurotransmitter antagonists in ovo.…”

“…Further, we did not observe differences in several features of the in ovo embryonic movements in treated and control embryos, including total movement duration, bout duration, bout frequency, or recovery of the movements. We remain open to the possibility that subtle distinctions between the embryonic movements and SNA may exist; however, it is unlikely that increases in quantal amplitude are triggered by low activity levels in GABA A -blocked embryos because they exhibit larger increases in mPSC amplitude than activityblocked lidocaine-treated embryos (7). For this reason it is also highly unlikely that a transient reduction in SNA, after antagonist injection, could account for the observed increase in quantal amplitude.…”

“…Whole-cell voltage clamp recordings of mPSCs were obtained from antidromically identified femorotibialis or adductor motoneurons as described in ref. 7. Muscle nerve recordings were obtained from suction electrodes connected to high-gain differential amplifiers (A-M Systems).…”

“…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).…”