FORMALIN AS A PERIPHERAL NOXIOUS STIMULUS CAUSES A BIPHASIC RESPONSE IN NUCLEUS PARAGIGANTOCELLULARIS NEURONS

The effects of formalin as a peripheral noxious stimulus on the activity of lateral paragigantocellularis nucleus (LPGi) neurons were examined. Spontaneous activity ofLPGi neurons was recorded after confirmation of their responsiveness to acute pain, and thereafter formalin (50 µL, 2.5%) was injected in the contralateral hindpaw. The response of the LPGi neurons was monitored for 60 min. A biphasic response with a peak lasting 3 to 5 min post-injection, and a second more prolonged tonic excitatory response were obtained which corresponds to the nature and time course of behavioral studies. It is concluded that LPGi neurons may be involved in the processing of nociceptive information related to formalin as a noxious stimulus.