The development of inhibitory mechanisms in spinal pain pathways

• Main Author: Harrop, Jane Emily
• Published: University College London (University of London) 2006
• Subjects: 617.56407
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.431667

The aim of this thesis was to investigate the postnatal development of inhibitory signalling in spinal pain pathways by the inhibitory transmitters GABA and glycine. Immunohistochemical studies and western blot analysis have shown that two key molecules, the scaffolding protein gephyrin and the potassium cation co-transporter KCC2, are postnatally upregulated in the rat superficial dorsal horn. Low levels of gephyrin limit postsynaptic clustering of GABAa and glycine receptors reducing the efficiency of synaptic inhibition, whereas the relative paucity of the cation-chloride co-transporter KCC2 in the neonatal period reduces the cell's ability to actively extrude chloride, producing a reversal of the electrochemical gradient. The upregulation of KCC2 was accelerated in vivo by peripheral inflammation and in cultured neurons by potassium induced depolarisation, while spike blockade with TTX produced the opposite effect. Gephyrin expression on the other hand, was not activity dependent. The functional consequences of these low levels of KCC2 in the neonatal period upon GABA and glycine signalling were tested using intrathecal injections of GABA and glycine antagonists. In adults these antagonists are known to produce sensitisation of mechanical withdrawal thresholds but in neonates they had the opposite effect, causing reduced sensitivity. Correspondingly, GABA and glycine, whilst having no effect in adult rats produced sensitisation in neonatal rat pups. This difference was abolished on spinalisation, demonstrating the importance of supra spinal connections in controlling segmental inhibition. Benzodiazepines, are GABA agonists commonly used in neonatal intensive care. In order to test whether the results of this thesis have practical implications for clinical care, a questionnaire survey of neonatologists was carried out. Myoclonus and seizures were reported following the use of midazolam in young patients suggesting that low levels of KCC2 may also be a feature of the human neonatal CNS.