Akt‐PI3K signaling pathway is a crucial player in dopamine D2 receptor‐mediated emesis in least shrews

Abstract

Dopamine is a member of the catecholamine family involved in multiple physiological functions. Together with its five dopaminergic receptors (DRs), DA is associated with several neurological diseases including restless leg syndrome, schizophrenia, Parkinson’s disease, and depression. Unfortunately, most DR‐based agonists used to treat some of these diseases evoke nausea and vomiting as impending side‐effects. Coincidently, activation of the kinase phosphoinositide 3‐kinase (PI3K)/protein kinase B (Akt) pathway not only plays a significant role in the onset and the progression of neurodegenerative disorders but may also evoke emesis caused by stimulation of dopamine D2 receptor (DR2). Thus, the aim of this study was to decipher the role of Akt in DR2‐induced emesis using the least shrew model of vomiting (Cryptotis parva). Shrews were injected intraperitoneally (i.p.) with a fully effective emetic dose of the DR2 preferring agonist quinpirole (2 mg/kg) in the presence and the absence of either: i) a corresponding DR2 antagonist sulpiride (subcutaneous (s.c.), 8 mg/kg), or ii) incremental doses of the PI3K inhibitor LY294002 (0, 5, 10 mg/kg, i.p.). The treated shrews were observed for 30 mins for vomiting behavior following quinpirole injection. Another set of shrews were treated with 2 mg/kg quinpirole alone (N = 4 per group) and sacrificed at 5, 15, and 30 mins to determine the time‐dependent profile of Akt phosphorylation. The animals were then perfused, and brains collected for immunohistochemistry studies or brainstems harvested for Western blot analysis on protein extracts. For antagonist studies, different groups of shrews were given sulpiride (8 mg/kg, s.c) or LY294002 (10 mg/kg, i.p.) (N = 4–6 per group) for 30 mins prior to injection of quinpirole (2 mg/kg) and brainstems were collected after 15 mins for Western blot analysis. Behavioral results showed that quinpirole induced vomiting was significantly blocked by sulpiride (8 mg/kg) and LY296004 (10 mg/kg). Akt phosphorylation at Ser473 was significantly increased at 15 and 30 mins in the brainstem with quinpirole treatment alone. The increase of phospho‐Akt protein in brainstem emetic nuclei [area postrema (AP), nucleus of the solitary tract (NTS), and dorsal motor nucleus of the vagus (DMNX)] was further confirmed through immunostaining brainstem sections of quinpirole‐treated least shrews. Sulpiride on the other hand significantly inhibited quinpirole‐induced Akt and PI3K protein phosphorylation at 15 mins. Likewise, LY29004 prevented Akt activation at 15 mins. Our findings uncovered a crucial role for DR2‐mediated Akt activation in emesis and suggest targeting signals up upstream of Akt enzyme may offer new avenues for potential potent antiemetics.Support or Funding InformationThis work was supported in part by NIH‐NCI grant RO1CA207287 and the WesternU startup fund (1395) to ND.