Functional mapping of the brainstem during centrally evoked bradycardia: a 2-deoxyglucose study

Autoradiographic 2-deoxy-[14C] glucose (2-DG) procedures were used to map the functional activity of the brainstem during bradycardia elicited in awake rats by stimulation of the deep mesencephalic nucleus of the midbrain reticular formation (MRF). Quantitative determinations of 2-DG uptake in 46 brainstem structures of MRF-stimulated rats were compared to those of control rats without stimulation. This paper is the first 2-DG study to map the brainstem structures involved in a heart rate response evoked by central stimulation. The structures activated in the midbrain, caudal to the stimulation site, are part of the reticular formation and the central gray. The greater focuses of labeling were concentrated on the lateral aspects of the deep mesencephalic nucleus and on the lateral divisions of the midbrain central gray. The remaining structures activated during bradycardia were all located in the caudal medulla. The largest increase was observed in the caudal nucleus ambiguus. Significant increases were also found in the dorsal motor nucleus of vagus and in the nucleus of the solitary tract. The region of the caudal inferior olive showed a small increase in 2-DG uptake, whereas structures like the raphe magnus and parvocellular reticular nucleus showed a tendency to reduce 2-DG uptake levels in the stimulated rat. It was concluded that bradycardia induced centrally by MRF stimulation may be mediated by well-defined brainstem descending pathways, direct and indirect, between the activated regions of the midbrain and the various medullary nuclei known to induce bradycardia upon electrical stimulation. The results suggest that the midbrain central gray and reticular formation may play a role as intermediates in an indirect hypothalamus-medullary circuitry for bradycardia. In addition, descending MRF information and afferent baroreceptor inputs appear to exert their inhibitory influences on heart rate via a common set of neuroanatomical substrates in the medulla.