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 response to unilateral low-intensity electrical stimulation of the mesencephalic cuneiform nucleus were examined in the halothane-anaesthetized and paralysed rat by in situ hybridization histochemistry using specific 35S-labelled oligonucleotides for detection of nerve growth factor inducible-A gene (NGFI-A) and c-fos messenger RNAs. Stimulation of the cuneiform nucleus increased mean arterial pressure and heart rate by 20 +/- 0.5 mmHg and 35 +/- 3 b.p.m., respectively, while no significant cardiovascular response was observed in animals stimulated in the inferior colliculus or in sham-operated animals. Cuneiform nucleus stimulation produced increased NGFI-A and c-fos messenger RNA levels in the Kölliker-Fuse and parabrachial nuclei ipsilaterally, and the cuneiform nucleus, dorsal periaqueductal gray and caudal ventrolateral medulla bilaterally at levels significantly greater than those in inferior colliculus-stimulated, sham-operated and naive, unoperated animals. NGFI-A, but not c-fos, messenger RNA expression was increased bilaterally in the caudal portion of the nucleus of the solitary tract and inferior olive. These results are consistent with previous neuroanatomical tract-tracing studies of afferent and efferent pathways from the cuneiform nucleus and indicate that these midbrain and hindbrain structures may be involved in the pressor and tachycardic responses associated with stimulation of the cuneiform nucleus. The ipsilateral nature of responses in certain brain areas may be explained by the absence of decussating pathways and/or the presence of multisynaptic connections which attenuate bilateral signal transmission. Characterization of these activated neuronal structures using other compatible labelling techniques should further elucidate the mechanisms by which these central nervous system structures are integrated in the cardiovascular responses to stimulation of the cuneiform nucleus.