Effect of local anaesthesia on neuronal c-fos expression in the spinal dorsal horn and hypothalamic paraventricular nucleus after surgery in rats

The surgical stress response is the neurophysiologic reflex response to surgery, which involves activation of the hypothalamic-pituitary-adrenal axis and is regulated by the hypothalamic paraventricular nucleus. The effect of preoperative use of local anaesthetics on activation of neurones in the paraventricular nucleus during surgery was studied by quantification of the neuronal expression of the c-fos-gene after a standardized plantar incision in rats. Furthermore, c-fos expression in the spinal dorsal horn was used as a measure of spinal nociception. Six halothane-anaesthetized animals underwent surgery following infiltration with lidocaine and bupivacaine, six animals were operated without local anaesthetics, and six control animals were subjected to the anaesthetic procedures. After two hours, the animals were perfused with 4% formaldehyde and the spinal cords and brains were collected and processed by immunohistochemistry for stereological quantification of the number of neurones with Fos-like immunoreactivity. Furthermore, brain and spinal cord were sampled from nine control animals right after induction of halothane anaesthesia. Surgery without local anaesthetics caused a significant increased number of neurones with Fos-like immunoreactivity in the spinal cord (4258 +/- 1710; mean +/- S.D.; P < 0.01) compared to the anaesthesia control group (1204 +/- 436). Local anaesthetics reduced this number to 2029 +/- 919 (P < 0.05), which was not significantly different from the anaesthesia control group. After surgery, the number of neurones with Fos-like immunoreactivity in paraventricular nucleus increased from 2948 +/- 1365 in the anaesthetized control group to 5550 +/- 3875 and 5191 +/- 1558 in the surgery and local anaesthetics plus surgery group, respectively, although significance was only reached for the group receiving local anaesthetics (P < 0.05). In conclusion, preoperative local anaesthetic infiltration did not reduce the surgery-induced c-fos expression in paraventricular nucleus after paw surgery in rats, although spinal nociception was reduced.