Spinal neuronal thermosensitivity in vivo and in vitro in relation to hypothalamic neuronal thermosensitivity

In the spinal cord, temperature signals are generated which serve as specific inputs in the central nervous control of body temperature. Because of the spatially distinct organization of afferent and efferent neuronal systems at the spinal level, the afferent pathway for temperature signal transmission could be identified in vivo in the ascending, anterior and lateral tracts with a relationship of about 75:25% between warm and cold sensitive neuraxons. Analysis of spinal neuronal thermosensitivity in vitro on spinal cord tissue slices has been concerned, so far, with the superficial laminae of the dorsal horn as the site of origin of ascending nerve fibers conveying mostly temperature and pain signals, and with lamina X as a site of origin of afferent as well as efferent neurons. A relationship of about 95:5% between warm and cold sensitive neurons was found at the segmental level, indicating that warm sensitivity is the prevailing, primary property of spinal neurons, whereas cold sensitivity seems to be mainly generated by synaptic interaction as a secondary modality. Dynamic responses to temperature changes were frequently displayed in vitro at the spinal segmental level in lamina I + II but not in lamina X, even by neurons whose static activity was little influenced by local temperature. Dynamic thermosensitivity was found less frequently in ascending tract neuraxons and was not observed in hypothalamic neurons receiving temperature signal inputs from the spinal cord, and thus, does not seem to be relevant for the thermosensory function of spinal cord neurons, unlike peripheral warm and cold receptors. A majority of spinal warm sensitive neurons displayed both static and dynamic warm sensitivity as an inherent property after synaptic blockade. In the further analysis of spinal cord thermosensitivity, the in vitro approach permits application of the same electrophysiological and neuropharmacological methods as were established for the analysis of hypothalamic thermosensitivity. In addition, the topography of the spinal cord will provide additional structural and possibly histochemical information to characterize the functions of neurons independently of their thermal properties.