Assessing impacts of two forms of PACAP-38 (pituitary adenylate cyclase-activating polypeptide) on infectious Nile tilapia (Oreochromis niloticus) immunophysiology

  • Fajei E, Department of Pathology and Microbiology, Atlantic Veterinary College, University of Prince Edward Island
  • Cai WC, Department of Infectious Diseases and Public Health, Jockey Club College of Veterinary Medicine and Life Sciences, City University of Hong Kong
  • Whyte SK, Department of Pathology and Microbiology, Atlantic Veterinary College, University of Prince Edward Island
  • Despres B, Department of Pathology and Microbiology, Atlantic Veterinary College, University of Prince Edward Island
  • Fast MD, Department of Pathology and Microbiology, Atlantic Veterinary College, University of Prince Edward Island

In teleosts, pituitary adenylate cyclase-activating polypeptide (PACAP) has been demonstrated to have direct antimicrobial activity against several aquatic pathogens, including those from the genus Flavobacterium.

Our goal was to assess the impacts of PACAP and possible favourable modification in immunophysiology of infectious Nile tilapia (Oreochromis niloticus) and determine whether this was impacted by the route of administration. Over the course of four studies, tilapia (416.1±116.7 g) randomly assigned to replicate tanks were administered with either PACAP-38 or a modified form of PACAP-38 via intraperitoneal injection, bath, nares flush, or gill flush, and compared to PBS controls. In the first two studies, following PACAP individual treatments, tilapia underwent a bath exposure (40 L tank for 45 min) to F. columnare (isolate ALG-00-530; at 2.1 x 108 CFU/ml) or sham exposure without the addition of the bacterial culture. Fish were sampled before exposure, 48 h after stimulation, at 1 day after the onset of mortality in exposed tanks, and resolution of mortality. Tilapia that received i.p. injection of PACAP-38 unlike modified form showed significantly lower mortality from F. columnare (10%) than those receiving PBS i.p. (25%).

Furthermore, PACAP-38 also induced inflammatory gene expression in the spleen, and eosinophilic granule cell aggregation in the nares, following flushing. These data suggest that PACAP-38 induces protection against infection and stress mainly with a reduction of IL1-B expression. Eventually, exposing tilapia to low temperature (15-17°C) for 30 minutes (cold shock), confirmed PACAP-38 capacity to reduce secondary impacts of stress, and we are currently examining these mechanisms.