Towards a Translational Pain Model - Techniques and developments

Pain is a major health issue for each affected individual and has a large impact on health costs. Most of our knowledge about this defensive system rely on data from different animal experiments. There is an obvious need for a translational pain model allowing comparisons between experimental animals and humans. UVB irradiation of skin has been introduced as a valid translational pain model and this model have resulted in comparable behavioral and electrophysiological data obtained from animals and humans. This thesis partly focus on detecting alterations for two neuropeptides (galanin and substance P), known to be involved in pain modulation, in the rat sensory nervous system after UVB irradiation of the rat hind paw on one side. UVB irradiation induced increased immunoreactivity for galanin in the lateral spinal nucleus (LSN) and in the dorsal horn of the rat spinal cord and also a reduction of the proportion of galanin positive neurons in corresponding DRGs. The changes observed for galanin in the LSN area have previously not been reported. The peak of the changes occurred 24 - 48 h after UVB irradiation which correlates with the time frame when primary and secondary hyperalgesia can be observed in rats and humans after UVB irradiation. UVB irradiation also induced increased c-fos activity in the dorsal part of the spinal cord and in the area around the central canal. For the other neuropeptide, substance P, an increase was observed 48 h in the LSN area. The changes observed on the ipsilateral side could also be observed on the contralateral side. This observation implies that the contralateral side should not be used as the control after induced inflammation on one side. Another goal of this thesis was to develop a mechanically flexible electrode system that precisely could target neurons deep in the brain. An electrode system containing 28 individual recording electrodes was constructed and it could be implanted into the brain after gelatine embedding. This electrode system made it possible to obtain stable neurophysiological recordings during eight weeks in awake, freely moving rats. Histological examination verified that the individual electrodes had reached their target, the subthalamic nucleus, and that they also spread out within the target. This electrode system can be used as a tool for further experimental pain studies. The fourth article focus on visualising neurons in histological sections from cortex, spinal cord and DRG using two pan-neuronal markers, HuC/HuD and NeuN. Different staining procedures were used, glass mounted sections or free floating ones with or without an antigen retrieval protocol. The anti-HuC/HuD antibody together with a standard immunohistochemical protocol worked well for cortical sections stained on glass or in a free floating mode. For the anti-NeuN antibody the best results were obtained after an antigen retrieval protocol. For spinal cord sections the use of the anti-HuC/HuD antibody gave the most favourable results. This antibody was used when the spinal cord sections were stained for evaluating the changes induced in the dorsal part of the spinal cord after UVB irradiation. Both antibodies worked well for DRG sections. In summary, this thesis demonstrates that UVB induced skin inflammation will cause rapid biochemical alterations in the parts of the nervous system related to pain signalling. A novel electrode system has been developed that can be used for further experimental pain studies. Furthermore, different pan-neuronal markers differ in their ability to stain neurons from different parts of the nervous system.