Potent inhibitory input to locus coeruleus from the nucleus prepositus hypoglossi

Previous studies reported afferents to LC from the nucleus tractus solitarius (NTS), while more recent anatomic experiments indicate that the area of the nucleus prepositus hypoglossi (PrH), but not the NTS, provides robust innervation of LC. In the present experiments, the contribution of these two dorsomedial medullary areas to LC innervation was assessed with electrophysiologic methods in anesthetized rats. Focal electrical stimulation of LC antidromically activated a substantial number of PrH neurons; such stimulation failed to antidromically activate NTS neurons. Electrical activation of PrH evoked potent, uniform inhibition of LC discharge. In contrast, NTS activation produced only weak, long latency responses in only a few LC neurons. In agreement with these results, WGA-HRP injections into NTS did not yield consistent anterograde labeling in LC. These results confirm our previous anatomic findings that PrH, but not NTS, provides major innervation of LC. Furthermore, the input from PrH potently inhibits LC activity.