Trigeminal neuropathic pain causes changes in affective processing of pain in rats

Trigeminal neuropathic pain has been modeled in rodents through the constriction of the infraorbital nerve (CCI-ION). Sensory alterations, including spontaneous pain, and thermal and mechanical hyperalgesia are well characterized, but there is a notable lack of evidence about the affective pain component in this model. Evaluation of the emotional component of pain in rats has been proposed as a way to optimize potential translational value of non-clinical studies. In rats, 22 and 50 kHz ultrasonic vocalizations (USVs) are considered well-established measures of negative and positive emotional states, respectively. Thus, this study tested the hypothesis that trigeminal neuropathic pain would result, in addition to the sensory alterations, in a decrease of 50 kHz USV, which may be related to altered function of brain areas involved in emotional pain processing. CCI-ION surgery was performed on 60-day-old male Wistar rats. 15 days after surgery, von Frey filaments were applied to detect mechanical hyperalgesia, and USV was recorded. At the same timepoint, systemic treatment with d,l-amphetamine (1 mg/kg) allowed investigation of the involvement of the dopaminergic system in USV emission. Finally, brain tissue was collected to assess the change in tyrosine hydroxylase (TH) expression in the nucleus accumbens (NAc) and c-Fos expression in brain areas involved in emotional pain processing, including the prefrontal cortex (PFC), amygdala, and NAc. The results showed that CCI-ION rats presented mechanical hyperalgesia and a significant reduction of environmental-induced 50 kHz USV. Amphetamine caused a marked increase in 50 kHz USV emission in CCI-ION rats. In addition, TH expression was lower in constricted animals and c-Fos analysis revealed an increase in neuronal activation. Taken together, these data indicate that CCI-ION causes a reduction in the emission of environmental-induced appetitive calls concomitantly with facial mechanical hyperalgesia and that both changes may be related to a reduction in the mesolimbic dopaminergic activity.