| Cold and heat stresses are the common environmental stimulators, which evoke the adaptive and protective responses of the organisms, namely, stress responses. Signal transduction pathway of Mitogen- Activated Protein Kinase (MAPK) is part of the intracellular signaling systems and closely related to the extracellular stress. However, the effect of cold and heat stresses on signaling pathway of MAPKs in the central nervous system is not clear. We reported that the acute cold stress (4℃waterbath of 20 min) and heat stresses (44℃waterbath of 20 min) had different degrees of influence on the activation of ERK and p38, two major components of MAPK family, in the spinal cord and spinal ganglia of model rats by using immunohistochemistry and Western blotting analysis.1. The light microscopy showed that the phosphorylated ERK1/2 (p-ERK1/2)-positive cells in the spinal cord were mainly located in all Rexed layers, and evenly scattered in the grey matter, occasionally in the white matter known as glial cells. The immunoreactive substances displayed brown granular and were homogeneously located in cytoplasm, especially around nucleus, sometimes within nucleus. In spinal ganglia, the p-ERK1/2 immunoreaction was distributed in the majority of large-, medium-, and small-sized neurons, and mainly located in the nucleus with different degrees of immunostaining, rarely seen in cytoplasm and nucleoli. After 20 minutes of acute heat stress in animals, the positive cells in the Rexed II layer significantly increased, especially in cytoplasm, without clear alteration in other Rexed layers. In the Rexed V-X layers, the positive cytonuclei were stained more unevenly, with a larger area around nucleus and a clearer broken nuclear membraine; The positive cells in spinal ganglia had no significant alteration, but the staining in nucleus slightly increased; After 10 minutes of acute heat stress, both the positive cell number and the nucleus staining appearance were not greatly changed. After 20 minutes of acute cold stress, the positive cells in the spinal Rexed II layers were significantly decreased, with a smaller area around nucleus in V-X layers and the rare broken nucleus membrane; The positive cells in spinal ganglia was not of greatly changed number, but displayed the reduced degree of nucleus staining; After 10 minutes of acute cold stress, the positive cell number was slightly decreased, with the rare broken nucleus membrane. The Western immunoblot analysis for the thoracic spinal cord showed that the activation level of ERK was gradually enhanced following the time of heat stress increased by comparison to the normal control groups and normal body temperature (38℃)-bathed groups, with a increasing downregulation of ERK at the different time-points of cold stress.2. The light microscopy showed that in spinal cord, the phosphorylated p38 (p-p38)-positive cells were mainly distributed in the Rexed II layer of dorsal horn, without immunostaining in the Rexed III-X layers. The immunoreactive substances were lacated in cytonuclei, displaying brown granular and homogeneous, without immunostaining in cytoplasm. In spinal ganglia, the pp38 immunoreaction was mainly located in the minority of small-sized neurons, with rare immunostaining in cytoplasm and nucleoli. After 10 and 20 minutes of acute heat stress, the positive cells in the Rexed II layer of spinal cord were gradually decreased, with a weaker immunostaining in cytonuclei; After 10 and 20 minutes of acute cold stress, the positive cells in the Rexed II layer of spinal cord were slightly decreased or unchanged, with a similar immunostaining in cytonuclei. In spinal ganglia, the p-p38 immunoreactions were not affected by cold and heat stresses. The Western immunoblot analysis for the thoracic spinal cord showed that activation level of p-p38 was gradually downregulated following the time of heat stress increased by comparison to the normal control groups and normal body temperature (38℃)-bathed groups, with a gentle down-regulation of p-p38 at the different timepoints of cold stress.In summary, cold and heat stresses altered the phosphorylation levels of ERK and p-p38 in spinal cord and spinal ganglia, with the acute heat stress enhancing the ERK phosphorylation and reducing the p-p38 phosphorylation, and the cold stress mainly reducing ERK phosphory- lation and not altering the p-p38 phosphorylation. It's suggested that the ERK and p-p38 signaling pathways played the opposite-regulating roles in the thermostress responses of the CNS in the rat, displaying the distinct specificities for different thermostresses and participating in the different functional activities of sensory afferent neurons. The ERK signaling pathway may be related to the sensory afferent including noxious and un-noxious signals, whereas the p-p38 signaling pathway may only be related to the noxious sensory afferents.
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