Bulbar influences on the periaqueductal gray. Potential role in nociception

Unit activity was recorded from 96 neurons in the periaqueductal gray (PAG) in response to focal electrical stimulation of the lateral reticular nucleus (LRN) and the nucleus gigantocellularis (NGC), in anesthetized, curarized rats. Consistent with anatomical studies showing direct projections from these nuclei to PAG, short latency excitatory responses (less than 5 ms) were found in 63 neurons and inhibition of the activity in 28 neurons. Peripheral noxious heat stimulation was effective in altering the activity in 36 of these neurons (38%). The LRN and NGC stimuli inhibited the noxious evoked responses in 26 of these neurons. The inhibition could also be induced when the dorsolateral funiculi (DLFs) were cut. These results indicate that LRN and NGC can have a direct influence on PAG neurons, which are probably involved in the processing of noxious information.