NEURONAL ACTIVATION IN THE FOREBRAIN FOLLOWING ELECTRICAL-STIMULATIONOF THE CUNEIFORM NUCLEUS IN THE RAT - HYPOTHALAMIC EXPRESSION OF C-FOS AND NGFI-A MESSENGER-RNA

Forebrain neuronal connections associated with the cardiovascular resp onse to unilateral, low-intensity, electrical stimulation of the mesen cephalic cuneiform nucleus were examined in the halothane-anaesthetize d and paralysed rat by in situ hybridization histochemistry using spec ific S-35-labelled oligonucleotides for detection of c-fos and nerve g rowth factor inducible-A gene (NGFI-A) messenger RNAs. Stimulation of the cuneiform nucleus led to increases in mean arterial pressure and h eart rate, whereas no cardiovascular response was observed in animals stimulated in the inferior colliculus or in sham-operated animals [see concurrent mid- and hindbrain study [Lam W. et ni. (1996) Neuroscienc e 71, 193-211. Cuneiform nucleus stimulation was associated with incre ased c-fos and NGFI-A messenger RNA levels bilaterally in the ventrome dial, dorsomedial and lateroanterior hypothalamic nuclei, lateral and anterior hypothalamic areas, and ipsilaterally in the medial amygdaloi d nucleus, at levels significantly greater than those in inferior coll iculus-stimulated, sham-operated and naive, unoperated animals. C-fos, but not NGFI-A, messenger RNA expression was increased bilaterally in the piriform cortex and subparafascicular thalamic nucleus. These res ults are consistent with the existence of direct and indirect projecti ons between the cuneiform nucleus and the aforementioned activated are as, the functions of which may include the control of reproduction and metabolism, as well as cardiovascular regulation. The ipsilateral nat ure of responses in certain brain areas may be explained by the absenc e of decussating pathways and/or the presence of multisynaptic connect ions which attenuate bilateral signal transmission. The existence of s tructures that are known to receive afferent projections from the cune iform nucleus, but that were not activated, may be explained by synapt ic depolarization not reaching the threshold for immediate early gene expression or by a net inhibitory effect on innervated neurons. Charac terization of these activated forebrain regions using other compatible labelling techniques should further elucidate the mechanisms by which these central nervous system structures are integrated in the respons e to stimulation of the cuneiform nucleus. (C) 1997 IBRO. Published by Elsevier Science Ltd.