Co-release of GABA does not occur at glycinergic synapses onto lumbar motoneurons in juvenile mice

G Bhumbra

doi:10.3389/fncel.2012.00008

Abstract

The fast inhibitory neurotransmitters glycine and GABA are co-localized in synaptic terminals of inhibitory interneurons in the spinal cord and co-released onto lumbar motoneurons in neonatal rats. We performed whole-cell voltage-clamp experiments on spinal cord preparations obtained from juvenile (P8–14) mice to determine whether inhibitory currents exhibited GABAergic components in motoneurons of animals of weight-bearing age. Subsequently we established whether or not GABA is co-released at glycinergic synapses onto motoneurons by determining if it conferred modulatory effects on the kinetics of glycinergic currents. Exponential fitting analysis showed that evoked and miniature inhibitory post-synaptic currents (IPSCs) were best-fitted with a single decay time constant. Responses recorded from connected interneuron-motoneuron pairs showed no effect of a benzodiazepine or a GABAA receptor antagonist. Similarly IPSCs evoked by extracellular stimulation and miniature IPSCs were not affected by either agent, indicating the absence of co-detection. Experimental manipulation of the relative content of pre-synaptic GABA and glycine conferred no effect on post-synaptic responses. It is thus unlikely that GABA is co-released in biologically relevant amounts at glycinergic synapses onto lumbar motoneurons in mice of this age.

Highlights

Fast inhibition in the nervous system is mediated by γaminobutyric acid (GABA) and glycine
Recordings of miniature inhibitory post-synaptic currents from motoneurons (Jonas et al, 1998) have demonstrated that a high proportion of currents are mediated by both glycine and GABA
Since each miniature inhibitory post-synaptic currents (mIPSCs) constitutes the post-synaptic response to an individual quantum of transmitter, the observation suggests that vesicles are loaded with both neurotransmitters which are released together; the notion of co-release was supported by similar observations in recordings from connected interneuron-motoneuron pairs

Summary

Introduction

Fast inhibition in the nervous system is mediated by γaminobutyric acid (GABA) and glycine. Recordings of miniature inhibitory post-synaptic currents (mIPSCs) from motoneurons (Jonas et al, 1998) have demonstrated that a high proportion of currents are mediated by both glycine and GABA. GABA even modulates the glycinergic current and shortens the time constant of the decay phase (Lu et al, 2008) through its action as a partial agonist. Vesicular filling of both neurotransmitters is possible since the vesicular inhibitory amino acid transporter (VIAAT) is shared between GABA and glycine albeit with higher affinity for GABA (Wojcik et al, 2006)

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