The effect of subcutaneous formalin (5%, 0.05 ml) on responses to nociceptive neurons in the medial bulboreticular formation was studied in barbiturate-anesthetized rats. In general, the activity in the studied neurons was not increased following the application of formalin, except for the first few minutes. Concomitantly, no formalin-induced prolonged increase in spontaneous impulse discharge was seen in nociceptive spinal cord neurons with antidromically identified projections to the bulboreticular formation. Also, following formalin application no hypersensitivity was seen in responses of bulboreticular neurons to high-intensity electric stimuli applied to the paw contralateral to the formalin-treated paw. The results suggest that it is not a general property of nociceptive neurons in the medial bulboreticular formation that they are activated in a prolonged fashion by subcutaneous formalin. It is possible that the medial bulboreticular formation is involved in the first rapid phase but not the second prolonged phase of formalin-induced pain.
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