Proceedings of The Physiological Society
Cardiff University (2009) Proc Physiol Soc 17, C05
The development of glycinergic inhibition in the spinal dorsal horn
S. Koch1, G. Hathway2,1, M. Fitzgerald1
1. Department of Neuroscience, Physiology and Pharmacology, UCL, London, United Kingdom. 2. School of Biomedical Sciences, University of Nottingham, Nottingham, United Kingdom.
Newborn spinal sensory circuits are poorly organised: reflex thresholds are lower and dorsal horn cutaneous receptive are poorly tuned. Spinal inhibitory systems appear to mature slowly over the first postnatal weeks. The two major inhibitory neurotransmitters in the spinal cord are GABA and glycine. We have previously shown that GABA signalling is fully functional in the newborn cord (1), but the role of glycine in development has not been fully investigated. In the adult spinal dorsal horn cells in lamina (L) III-VI are under tonic glycinergic control and intrathecal (i.t.) strychnine, a specific glycine receptor antagonist, induces spontaneous pain and sensitization to mechanical stimuli (2). Glycinergic neurons receive monosynaptic input from both low threshold myelinated and nociceptive afferents acting as a negative feedback to excitatory neurones in the dorsal horn. Whole cell patching of LII neurons in newborn spinal cord slices revealed an absence of glycinergic mIPSCs and little or no afferent evoked glycinergic activity, despite the presence of functional glycine receptors from birth (3). The aim of this study was to undertake an in vivo analysis of the functional maturation of glycinergic synaptic transmission in the spinal dorsal horn and establish the role of glycinergic signalling in the postnatal organisation of dorsal horn circuits. Spontaneous and brush-evoked responses of individual dorsal horn neurons were recorded extracellularly in isoflurane gas-anaesthetised rats (1.8% in O2), and strychnine applied to the surface of the exposed cord. Glycinergic activity was also examined by quantifying the change in fos expression in the dorsal horn 20 minutes following i.t. strychnine with and without brush stimulation of the hind paw. Immunostaining for GlyT2 (1:40,000, kind gift from Professor F Zafra) was used to map the distribution of glycinergic terminals in the developing dorsal horn. Spontaneous activity of spinal dorsal horn neurons increased in postnatal day (P)21 rats following strychnine (mean increase 0.26±0.16 spikes/sec (SEM) from baseline, P= 0.01, non-parametric ttest) but had no effect on that of P3 dorsal horn neurons. Brush-evoked activity increased markedly in mature neurons 20 minutes post strychnine (mean increase 6.61±1.81 spikes/sec, P<0.05) but was inhibited in P3 neurons (mean decrease 2.17±0.73 spikes/sec, P<0.05). Fos expression mirrored in vivo findings, with no apparent increase in activated neurons following strychnine in the neonatal dorsal horn. GlyT2 staining revealed an absence of glycinergic terminals in the superficial dorsal horn at P3, with gradual appearance in LIII by the P21. From these results, we can conclude that specific glycinergic inhibition of low threshold afferent sensory evoked activity is absent in neonatal dorsal horn circuits and the primary role of glycinergic transmission in the early postnatal period is to facilitate transmission of sensory input.
Where applicable, experiments conform with Society ethical requirements