Antinociceptive effects of sensory stimulation involve dynorphin B supraspinally in rats

The aim was to investigate the mechanisms behind sensory stimulation which can be used to desensitize CNS in patients with atypical orofacial pain. Earlier studies have shown that the kappa-receptor in the periaqueductal gray (PAG) is involved in sensory stimulation induced antinociception. A possible antinociceptive role for dynorphin B (DynB) in supraspinal regions was tested. The behavioral effect of sensory stimulation in conscious rats, by stroking the fur, was tested using the nociceptive test hotplate and the hindpaw withdrawal latency (HWL) was measured. In anesthetized rats sensory stimulation during different modalities, stroking or pinching was performed and the microdialysis technique was used to determine the extra cellular level of DynB in the ventrolateral PAG. To evaluate the antinociception after sensory stimulation DynB was microinjected into the PAG and the effect was measured with the HWL to heat. The results showed that sensory stimulation in conscious rats significantly increased the HWL as an antinociceptive effect. Innocuous sensory stimulation such as stroking the fore paw significantly elevated the DynB level in the PAG compared to internal control. After pinching a tendency to delayed release of DynB was seen and a possible discharge of the nerve terminals could be speculated upon. The blood pressure did significantly increase after pinching but not after stroking. An intra-PAG injection of DynB into the PAG increased the HWL to heat after 24 h compared to basal level of HWL and to saline treated animals. In conclusion, DynB is involved in the antinociception that is triggered by sensory stimulation.