Thyrotropin-releasing hormone (TRH) and serotonin (5-HT) are well known as neurotransmitters of descending bulbo-spinal tracts. 5-HT uptake caudal to (5-HT decreased) graded spinal lesions has been measured in rats and dogs and significantly correlated with the degree of cord injury. We studied 5-HT content via high-pressure liquid chromatography and radioimmunoassay measurements of TRH in dog spinal cord, both rostral and caudal, to 6 week T6 spinal transection (T) (n = 7) or T6 hemisections (H) (n = 7). Sham controls (n = 7) were used. Mean 5-HT content values were (pmole/mg): rostral sham 1.25 ± 0.02, caudal sham 1.35 ± 0.17, rostral T 2.65 ± 0.36, caudal T 0.19 ± 0.06, rostral H 2.1 ± 0.22, and caudal H 1.0 ± 0.31. A significant decrease in 5-HT caudal to transection versus control (P < 0.001) was seen. A trend for decreased 5-HT caudal to hemisection versus control (P < 0.1) was also seen. Mean TRH levels (pg/mg protein) were: rostral sham 40.02 ± 18.47, caudal sham 30.61 ± 10.03, brainstem sham 18.9 ± 5.13, rostral T 52.4 ± 21.34, caudal T 3.52 ± 1.67, brainstem T 19.25 ± 4.11, rostral H 43.45 ± 18.61, caudal H 14.24 ± 5.7, and brainstem H 21.69 ± 1.23. Significant decreases for caudal TRH transection versus controls (P < 0.001) and for caudal TRH hemisection versus controls (P < 0.02) were seen. A significant difference between caudal hemisection and caudal transection TRH levels (P < 0.001) was seen. The ratio of the percentage of drop between H and T was 60%, which is near the predicted ideal of 50%. Significant increases for rostral 5-HT transection versus control (P < 0.02) and rostral 5-HT hemisection versus control (P < 0.02) were seen. The increase in TRH above a transection was not statistically significant. The increase in 5-HT rostral to the level of injury may result from blockade of transport. The decrease in measured TRH below the level of injury correlated with the severity of injury. These results suggest that TRH measurements are superior to 5-HT content for correlating the degree of long tract injury.
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