Differential effects of noxious and non-noxious input on neurones according to location in ventral periaqueductal grey or dorsal raphe nucleus.
Nociceptive and non-nociceptive input to the dorsal raphe nucleus (DR) and to the surrounding periaqueductal grey (PAG) was studied in chloralose-anaesthetized rats. Single units in the midbrain responding to electrical stimulation of a coccygeal nerve were recorded with glass micropipettes. A fluorescence histochemical technique was applied to identify recording sites in the DR and PAG. 109 DR-units, 141 PAG-units and 95 units from surrounding structures were tested for responsiveness to electrical nerve stimulation. In 53% of the DR-units, but in only 20% of the PAG- and SN-units, ongoing activity was inhibited by electrical stimulation (I-units) while 42% of the PAG- and SN-units but only 24% of the DR-units were electrically excited (E-units). 40 E-units and 24 I-units were tested with repeated noxious radiant heat stimuli applied to the tail or hindpaws. 70% of the E-units were excited by heating, and in 54% of the I-units ongoing activity was inhibited by heating. The majority of the former units were located in the PAG, and most of the latter were proven to be DR-neurones. In 75% of the E-units and in 12.5% of the I-units the heat effect was in the opposite direction. The findings are discussed in terms of the now well-established role of the PAG-region in the descending control of pain. The properties of the PAG-E-units suggest that this system is involved in a negative feedback circuit by which pain transmission to the CNS limits itself. DR-I-units may be involved via an additional small loop with the PAG to disinhibit the activation of the PAG pain control system.
D000818:Animals; D001933:Brain Stem / Q000502:physiology*; D002535:Cerebral Aqueduct / Q000502:physiology*; D004558:Electric Stimulation; D005071:Evoked Potentials; D005727:Ganglia, Spinal / Q000502:physiology; D008297:Male; D008465:Mechanoreceptors / Q000502:physiology; D009412:Nerve Fibers / Q000502:physiology; D009433:Neural Inhibition; D009434:Neural Pathways / Q000502:physiology; D009474:Neurons / Q000502:physiology; D009619:Nociceptors / Q000502:physiology*; D011903:Raphe Nuclei / Q000502:physiology*; D051381:Rats; D012154:Reticular Formation / Q000502:physiology; D012701:Serotonin / Q000378:metabolism; D012867:Skin / Q000294:innervation; D013623:Tail / Q000294:innervation
D012701:Serotonin