Functional activity mapping of the rat spinal cord during formalin-induced noxious stimulation

The functional activity pattern in the cervical enlargement of the spinal cord (as expressed by changes in local glucose utilization) was investigated by the semi-quantitative [14C]2-deoxyglucose technique 2 min ("early" group) or 60 min ("late" group) after injection of a small amount of dilute formalin (0.06-0.08 ml, 5%) in a forepaw of unanesthetized, freely-moving rats. Control animals were either injected with an equivalent volume of saline or simply handled. In both formalin groups a tonic flexion of the injected limb was present during the experiments, while supraspinal-integrated behavior (such as licking the affected paw) was sharply reduced in the late group. A bilateral increase of metabolic activity indexes, more pronounced on the ipsilateral side, was found in the "early" formalin-injected animals. The highest increase over control values was found in the medial part of the superficial (laminae I-II) region of the ipsilateral dorsal horn. However, the [14C]2-deoxyglucose uptake was found to be elevated over the whole extent of the dorsal horns, as well as in the gray matter surrounding the central canal, anterior horns and ipsilateral dorsolateral funiculus. In a parallel group of experiments performed in pentobarbital-anesthetized rats metabolic increases in the early period after formalin injection were less pronounced; they were only found in the ipsilateral side of the cord. In the "late" formalin group the overall metabolic changes were less conspicuous. They were mainly observed in the side ipsilateral to the injection, the highest increase being found in the deep portion (laminae V-VI) of the dorsal horn. Therefore, the spatial distribution of functional activation elicited during prolonged noxious stimulation in the spinal cord gray matter of unanesthetized rats varies according to time and changes in animal behavior.