Effect of lesions of A5 or A7 noradrenergic cell group or surgical transection of brainstem catecholamine pathways on plasma catecholamine levels in rats injected subcutaneously by formalin.

The pain-induced activation of the sympatho-adrenal system is modulated by several brain areas, including brainstem catecholamine cell groups. In the present study, we evaluated the effect of bilateral lesions of the A5 or A7 cell groups or bilateral transections of brainstem catecholaminergic pathways on plasma catecholamine levels in Sprague-Dawley rats injected subcutaneously by formalin or saline. Plasma levels of both epinephrine and norepinephrine were slightly elevated after formalin injections within 15-30 min in rats with or without lesions of the A7 catecholamine cell group. However, saline but not formalin elicited a significant increase in plasma epinephrine level in both sham-operated and A5-lesioned groups. It is more likely, that formalin blocks the effect of the handling and the painful injection procedure. In rats with bilateral partial transections of the lower brainstem, formalin was more effective than saline in the elevation of plasma epinephrine and norepinephrine levels at several time-points through the investigation period. Our data indicate the involvement of A5 and A7 norepinephrine neurons and brainstem catecholaminergic pathways in the regulation of the activity of the sympatho-adrenal system during acute painful situations. Their modulatory effect, however, seems to be a very rapid one, short and moderate.