DYNORPHIN A CONTENT IN THE RAT-BRAIN AND SPINAL-CORD AFTER A LOCALIZED TRAUMA TO THE SPINAL-CORD AND ITS MODIFICATION WITH PARA-CHLOROPHENYLALANINE - AN EXPERIMENTAL-STUDY USING RADIOIMMUNOASSAY TECHNIQUE

The distribution of dynorphin A in the spinal cord and brain of normal rats and of rats subjected to a focal injury of the spinal cord was examined in a rat model using a radioimmunoassay (RIA) technique. The validity of RIA was checked by high performance liquid chromatography (HPLC). Furthermore, the possibility that the peptide is somehow functionally related with endogenous 5-hydroxytryptamine (5-HT, serotonin), was also evaluated using a pharmacological approach. In normal animals, the peptide content was very similar in the spinal cord segments (T9, T10-11, and T12) examined whereas, the dynorphin content of the whole brain was about two-fold higher compared with that in the spinal cord. A focal injury to the spinal cord in the right dorsal horn (about 1.5 mm deep, 2.5 mm long and 1.5 mm to the right of the midline) of the lower thoracic cord (T10-11) in urethane anaesthetised animals significantly altered the peptide content in the whole brain as well as in the spinal cord. Thus, a decrease in the peptide level in whole brain, T10-11 and in the T12 segments of the spinal cord was observed 1 and 2 h after trauma. At 5 h, the peptide had accumulated markedly in the T9 segment (about a two-fold increase) as compared with the controls. At this time, the peptide content had been restored in the T10-11 and T12 segments. On the other hand, the whole brain dynorphin level continued to remain low (by 55%) as compared to the control group. Pretreatment with p-chlorophenylalanine (p-CPA, a 5-HT synthesis inhibitor) prevented the increase of the peptide in the T9 segment of the cord seen after 5 h injury. The peptide level of the other two spinal cord samples and the whole brain was similar to that of untreated traumatised rats. Furthermore, p-CPA treatment alone (in untraumatised animals) significantly reduced the whole brain dynorphin content (45% decrease) without having any effect on the spinal cord peptide level. Our results, for the first time, provide biochemical evidence that (i) dynorphin actively participates in the tissue response to a spinal cord trauma, (ii) its concentration is widely altered in the spinal cord and brain after a focal trauma to the cord, and (iii) suggest that a functional interaction between serotonin and dynorphin exists in the central nervous system.