Behavioral responses and Fos activation following orofacial formalin injection in a rodent model of Parkinson's disease

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We investigated the modulation of the nociceptive stimuli from the orofacial region in Parkinson's disease (PD) model rats, which were produced by the unilateral or bilateral injection of 6-OHDA into the medial forebrain bundle. Behavioral responses to formalin injection into the upper lip and the expression of c-Fos and pERK in trigeminal spinal subnucleus caudalis (Vc) were explored.Unilateral PD rats showed increased nociceptive responses when formalin was injected ipsilaterally to the lesion. This hyperalgesic response was associated with increased c-Fos expression in Vc. When formalin was injected into contralateral side to 6-OHDA lesion, behavioral responses were reduced in 6-OHDA rats. However, the expression of pERK in Vc remained unchanged. Although the hyperalgesic response was not observed in bilateral PD rats, c-Fos expression in Vc increased.These results suggest that 6-OHDA resion causes hyperalgesic response to nociceptive stimulation from the orofacial region.