Noxious somatic stimulation-induced expression of Fos-like immunoreactivity in catecholaminergic neurons with habenular nucleus projection in the medullary Visceral zone of rat

In order to study the expression of Fos protein in catecholaminergic neurons in the medullary visceral zone (MVZ), which project to the habenular nucleus (HE), a triple-labeling method combining wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) retrograde tracing with anti-Fos and anti-tyrosine hydroxylase (TH) immunohistochemical staining was used in the rat. WGA-HRP was stereotaxically injected into unilateral HE. Forty-eight hours later, 50 mu l of 8% formalin was injected into the foot pad of the right front paw. Two hours after formalin injection, animals were anesthetized and perfused transaortically. Coronal sections (40 mu m) were cut from the cervical segment of spinal cord, the medulla oblongata and WGA-HRP injected area with a cryostat. First, sections of the injected area and the medulla oblongata were histochemically processed to demonstrate the presence of retrogradely transported WGA-HRP using the chromogen tetramethylbenzidine (TMB). Then sections of the spinal cord and the medulla oblongata were immunostained with anti-Fos and anti-TH antibodies using the ABC method. Under the light microscope, seven types of variously labeled neurons could be identified in MVZ, namely Fos and TH-immunoreactive (Fos-or TH-IL) neurons, WGA-HRP labeled ones, Fos/HRP, Fos/TH and HRP/TH double-labeled and Fos/HRP/TH triple-labeled cells. The results suggest that some catecholaminergic neurons in MVZ could send projections to HE and this pathway may be involved to relay nociceptive information from spinal cord to brainstem and on to the forebrain. (C) 1998 Elsevier Science B.V.