GLYCINERGIC NEURONS IN NEURONAL CIRCUITS PROCESSING PAIN IN THE SPINAL DORSAL HORN
Evidences indicate that glycinergic neurotransmission in the spinal dorsal horn plays a critical role in the development of hyperalgesia and allodynia. It is poorly understood how glycinergic neurons form neural circuits underlying spinal pain processing. Thus, our present experiments are focused on the glycinergic neurons morphological and functional properties in laminae I-IV of the spinal dorsal horn. We label glycinergic neurons with transgenic technology, crossing GlyT2::Cre mice (kind gift of H.U. Zeilhofer, Zurich, Switzerland) with tdTomato reporter mice. To be sure that we can visualize glycinergic neurons with this technology, we performed some control experiments. Spinal cord tissue sections of GlyT2::Cre-tdTomato mice were immunostained for PAX2, a nuclear transcription factor for inhibitory neurons, and we found that 95.6 ± 3.4 % of tdTomato expressing neurons in the spinal dorsal horn were PAX2 positive. To explore whether the tdTomato labeled axon terminals are axon terminals of local spinal glycinergic neurons, we performed hemisections at the level of Th12 of the spinal cord. The hemisection decreased the numbers of tdTomato labelled profiles and GlyT2-IR axon terminals, indicating that approximately one-third and a bit more than 10 % of the GlyT2-IR axon terminals in laminae I-II and lamina III, respectively, are terminals of axons descending from higher brain centers; but the rest can be regarded as axon terminals of local spinal neurons. The results show that the tdTomato labeled cells in the GlyT2::Cre-tdTomato mice are indeed glycinergic, and they may have a wide axonal arbor in the superficial spinal dorsal horn.