Corticothalamic modulation on formalin-induced changes of spontaneous thalamic activities

Objectives: The inflammatory pain-induced responses of central somatosensory nervous system are unclear, except the spinal cord. In this study, we observed the changes of spontaneous activities in ventral posterior medial (VPM) thalamus by formalin injected to vibrissae area of anesthetized rats, and the alteration of thalamic inflammatory responses during inactivation of primary somatosensory (SI) cortex. Methods: For this issue, we have performed the in-vivo extracellular single unit recording in VPM thalamic nucleus. Results: After injection of formalin (2.5%, 5%, 30ul) to the peripheral receptive field (RF), spontaneous firing rates of thalamus were rapidly increased during initial 10 minutes, and then gradually restored the control values. The second phase that have been frequently observed in spinal cord by formalin, was not observed in VPM thalamus. However, this thalamic responsiveness was altered by change of cortical activity. When SI cortical activities were inactivated by microinjection of musimol, GABA agonist, to layer IV of SI cortex, both first and second phase induced by formalin were observed in VPM. This second phase was generated after 60 to 90 minutes post-drug. First phase and second phase in thalamus during inactivation of cortex, were different from those of spinal cord reported previously. In this study, the inhibitory period between first and second phase in thalamus were not clear. Bicuculline microinjected to SI cortex did not alter the pattern of formalin-induced changes of spontaneous thalamic activities, while amplitude of changes by formalin were decreased. Conclusion: These results suggest that VPM thalamus may be modulated by corticofugal projection from cortex during inflammatory pain, and that corticothalamic pathway may normally play a role to inhibit the pain response of subcortical nuclei. This work was supported by grant of Kyungpook National Univ. Fund 2003.