| Objective: To investigate the change of pain threshold in SD rats before and after exercise fatigue, and the relationship between exercise fatigue and pain threshold. Methods:The mechanical threshold and the thermal pain threshold were measured and compared before and after exercise fatigue induced by forced exhaustive swimming (one-time and repeated exhaustive models). The change of pain threshold could be obtained. The expression of nNOS in spinal cord of rats (the number of positive neuron) was detected by immunohistochemical technology.Results: The paw withdrawal thermal latency (PWTL) was increased apparently during two different exercise-induced fatigue. The PWTL average value is 16.55±5.81s after one-time exhaustive fatigue. Compared with the PWTL average value (11.21±1.15s) before fatigue, there was significant difference (P<0.01); The PWTL average value is 16.6±7.37s after one-week exhaustive fatigue. Compared with the PWTL average value (11.14±1.14s) before fatigue, there was significant difference (P<0.01); There were significant differences between different exhaustive groups (P<0.05), or the thermal pain threshold was increased. The PWMT average value is 5.41±0.01g after one-time exhaustive fatigue. Compared with the PWMT average value (5.19±0.004g) before fatigue, there was significant difference (P<0.01); The PWMT average value is 5.74±0.001g after one-week exhaustive fatigue. Compared with the PWMT average value (5.169±0.07g) before fatigue, there was significant difference (P<0.01); There were significant differences between different exhaustive groups (P<0.05), or the mechanical pain threshold was increased.The one-way analysis of variance of the number, area, average gray, light density and equivalent circle diameter of nNOS positive neuron in front of spinal cord, posterior horn and central canal spinal cord of rats in three groups was conducted. The result shows that compared with control group, the number [F(2,42)=109.758,P<0.001], area [F(2,42)=281.804,P<0.001], average gray [F(2,42)=17.366,P<0.001] , light density [F(2,42)=3.949,P<0.05] and equivalent circle diameter [F(2,42)=4.730,P<0.05] of nNOS positive neuron in front of spinal cord in different-extent exhaustive fatigue groups have significant differences. After the comparison between two groups tested by LSD method, compared with one-time exhaustive fatigue group, the number of nNOS positive neuron in spinal cord in one-week exhaustive fatigue group has significant differences(P<0.05);The average gray and light density also have significant difference(sP<0.01, P<0.001);The area and equivalent circle diameter have no significant differences(P>0.05). Compared with control group, the number [F(2,42)=411.42,P<0.001], area [F(2,42)=132.61, P<0.001] and equivalent circle diameter [F(2,42)=3.581 , P < 0.05] of nNOS positive neuron in posterior horn in different-extent exhaustive fatigue groups have significant differences. The average gray [F(2,42)=2.128,P>0.05] and light density [F(2,42)=0.321,P>0.05] have no significant differences. After the comparison between two groups tested by LSD method, compared with one-time exhaustive fatigue group, the number of nNOS positive neuron in spinal cord in one-week exhaustive fatigue group has great significant difference(sP<0.001);The average gray, light density, area and equivalent circle diameter have no significant differences(P>0.05). Compared with control group, the number [F(2,42)=121.151,P<0.001] of nNOS positive neuron in central canal in different-extent exhaustive fatigue groups have great significant differences. The area [F(2,42)=2.179,P>0.05] ,average gray [F(2,42)=1.027,P>0.05], equivalent circle diameter [F(2,42)=0.544,P<0.05]and light density [F(2,42)=0.999,P>0.05] have no significant differences. After the comparison between two groups tested by LSD method, compared with one-time exhaustive fatigue group, the number of nNOS positive neuron in spinal cord in one-week exhaustive fatigue group has great significant differences(P<0.001);The area , average gray, light densityand equivalent circle diameter have no significant differences(P>0.05).Conclusions: The change of PWMT was increased before and after exercise fatigue, in accordance with PWLT. The expression of nNOS in spinal cord of rats was significantly decreased after two types of exhaustive exercise. The change of nNOS in spinal cord of rats indicated that it may be involved in the appearance of central exercise fatigue. The increase of PWMT showed that the reaction of pain perception was slowly and the decrease of NO in fatigue inhibited normal transmission of pain perception.
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