Objectives: 

Stress activates multiple neuronal and endocrine systems that modify pain sensation. In particular, the cingulate cortex is an area in the limbic system strongly involved in the stress-pain interaction. In the present study, we investigated changes in nociceptive responses in the cingulate cortex after stress loading.

Materials: 

Twenty Wistar rats (male, body weight: about 160g) were used. Six hours/day of stress was applied using metal mesh-restraint for 3, 7 and 21 days. Neuronal activities evoked by pinch stimuli were recorded from single neurons in the cingulate cortex. Changes in spike numbers before and after 5s-pinch stimuli were compared between control (without stress conditioning) and stress -conditioned rats.

Results: 

After pinch stimuli, both excitatory and inhibitory responses were observed in the cingulate cortex. Excitatory responses evoked by tail-pinch stimuli were significantly enhanced after 7-day stress-conditioning (t-test, p<0.05), by contrast, inhibitory responses were not changed. Three or 21 day-stress conditioning did not affect neuronal activities in the cingulate cortex. Face and neck-evoked nociceptive responses were not changed by stress conditioning.

Conclusions: 

It was shown that stress-conditioning enhanced excitatory nociceptive responses following tail stimuli at an early stage. By contrast, long-term stress did not affect these responses, suggesting that adaptation against stress conditioning was induced in the limbic system. Nociceptive responses evoked by face and neck stimulation were not changed by stress conditioning. This suggests that the emotional influence in nociception is weaker in the ascending pathways from face and neck as compared to that from tail in rats.