Study of the dorsal root reflex activity in an isolated mammalian spinal cord preparation

Chen, Ying (1991) Study of the dorsal root reflex activity in an isolated mammalian spinal cord preparation. University of Southampton, Doctoral Thesis.

Abstract

Dorsal root reflex activity has been demonstrated as action potentials antidromically conducting along the primary afferent fibres generated synaptically inside the spinal cord. Primary afferent depolarization (PADF) is a presynaptic inhibitory mechanism to sensory inputs to the spinal cord, by depolarizing the primary afferent terminals. Dorsal root reflex activity, therefore, has been suggested to be generated by suprathreshold PAD in the primary afferents. The aim of the present project was to study the spinal mechanisms underlying the generation of PAD by examining the characteristics of the dorsal root activity in the lumbar and lower thoracic regions of an isolated whole spinal cord preparation of the adult hamster. Spontaneous and evoked action potentials in dorsal roots and dorsal horn neurons were recorded extracellularly. The study revealed an extensive neural network related to the dorsal root reflex activity within the spinal cord. Evoked dorsal root reflex activity was recorded propagating for ten segments rostrocaudally and also across the cord. Cross-correlation of spontaneous activity was also observed between these ipsilateral and contralateral dorsal roots. Each individual segment was demonstrated able to generate spontaneous dorsal root activity, and the activity was intersegmentally coupled via an excitatory transmission. The intersegmental transmission was performed principally by nerve tracts on the dorsal surface and the dorsal horn, and affected by strychnine. Further investigation showed that action potentials reocrded from spontaneously active neurons in the dorsal four laminae of the lumbar cord correlated with the dorsal root activity, suggesting that the four laminae are the probable site for the generation of the dorsal root reflex activity. An inhibition following excitation was observed in both the evoked and spontaneous dorsal root activity, and also in the spontaneously active dorsal horn neurons, suggesting an inhibitory neuronal circuit involved in the dorsal root reflex pathway. From pharmacological observations, we found that the GABAergic system (GABA_A and GABA_B receptors) is involved in the generation of the dorsal root activity, and so are the glycine receptors. The spontaneous activity was increased on the removal of 1mM Mg⁺⁺ but not NMDA-mediated. The similarity of the pharmacological responses shown in both the spontaneous and the evoked dorsal root activities also suggested that the two forms of the dorsal root activity may share a common pathway. This study revealed a wide spread neural network associated with the generation mechanisms of the dorsal root activity, and providedadditional information concerning the synaptic mechanisms of the dorsalroot activity and hence PAD in the mammalian spinal cord.

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