INCREASED NERVE GROWTH-FACTOR INDUCIBLE-A GENE AND C-FOS MESSENGER-RNA LEVELS IN THE RAT MIDBRAIN AND HINDBRAIN ASSOCIATED WITH THE CARDIOVASCULAR-RESPONSE TO ELECTRICAL-STIMULATION OF THE MESENCEPHALIC CUNEIFORM NUCLEUS

Functional neuronal connections associated with the cardiovascular res ponse to unilateral low-intensity electrical stimulation of the mesenc ephalic cuneiform nucleus were examined in the halothane-anaesthetized and paralysed rat by in situ hybridization histochemistry using speci fic S-35-labelled oligonucleotides for detection of nerve growth facto r inducible-A gene (NGFI-A) and c-fos messenger RNAs. Stimulation of t he cuneiform nucleus increased mean arterial pressure and heart rate b y 20 +/- 0.5 mmHg and 35 +/- 3 b.p.m., respectively, while no signific ant cardiovascular response was observed in animals stimulated in the inferior colliculus or in sham-operated animals. Cuneiform nucleus sti mulation produced increased NGFI-A and c-fos messenger RNA levels in t he Kolliker-Fuse and parabrachial nuclei ipsilaterally, and the cuneif orm nucleus, dorsal periaqueductal gray and caudal ventrolateral medul la bilaterally at levels significantly greater than those in inferior colliculus-stimulated, sham-operated and naive, unoperated animals. NG FI-A, but not c-fos, messenger RNA expression was increased bilaterall y in the caudal portion of the nucleus of the solitary tract and infer ior olive. These results are consistent with previous neuroanatomical tract-tracing studies of afferent and efferent pathways from the cunei form nucleus and indicate that these midbrain and hindbrain structures may be involved in the presser and tachycardic responses associated w ith stimulation of the cuneiform nucleus. The ipsilateral nature of re sponses in certain brain areas may be explained by the absence of decu ssating pathways and/or the presence of multisynaptic connections whic h attenuate bilateral signal transmission. Characterization of these a ctivated neuronal structures using other compatible labelling techniqu es should further elucidate the mechanisms by which these central nerv ous system structures are integrated in the cardiovascular responses t o stimulation of the cuneiform nucleus.