Abdominal surgery increases activity in several phoenixin immunoreactive nuclei

Research on the peptide phoenixin has increased in recent years and greatly widened the known scope of its functions since its discovery in 2013. Involvement of phoenixin has since been shown in anxiety, food intake, reproduction as well as emotional and immunological stress. To further evaluate its involvement in stress reactions, this study aims to investigate the effects of abdominal surgery, a well-established physical stressor, on the activity of phoenixin-immunoreactive brain nuclei. Male Sprague-Dawley rats (n = 6/group) were subjected to either an abdominal surgery stress protocol or a sham operation. Animals in the verum group were anesthetized, the abdominal cavity opened and the cecum palpated, followed by closing of the abdomen and recovery. Sham operated animals only received inhalation anesthesia and time for recovery. All animals were subsequently sacrificed and brains processed and evaluated for c-Fos activity as well as phoenixin density. Compared to control, abdominal surgery significantly increased c-Fos activity in the paraventricular nucleus (PVN, 6.4-fold, p < 0.001), the medial part of the nucleus of the solitary tract (mNTS, 3.8-fold, p < 0.001), raphe pallidus (RPa, 3.6-fold, p < 0.001), supraoptic nucleus (SON, 3.2-fold, p < 0.001), ventrolateral medulla (VLM, also called A1C1, 3.0-fold, p < 0.001), dorsal motor nucleus of vagus (DMN, 2.9-fold, p < 0.001), locus coeruleus (LC, 1.8-fold, p < 0.01) and Edinger-Westphal nucleus (EW, 1.6-fold, p < 0.05), while not significantly altering c-Fos activity in the amygdala (CeM, 1.3-fold, p > 0.05). Phoenixin immunoreactivity was not significantly affected by abdominal surgery (p > 0.05). The observed abdominal surgery-related increase in activity in phoenixin immunoreactive nuclei compared to sham surgery controls supports the hypothesis of an involvement of phoenixin in stress reactions. Interestingly, various psychological and physical stressors lead to specific changes in activity and immunoreactivity in phoenixin-containing nuclei, giving rise to a stressor-specific involvement of phoenixin.