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.