Assessing the impact of two forms of PACAP-38 (pituitary adenylate cyclase-activating polypeptide) on Nile tilapia (Oreochromis niloticus) immune response following challenge with Flavobacterium columnare

Abstract

Antimicrobial peptides have been isolated from various organisms and play an essential role in defense against infections. They are small peptides (less than 60 amino acids) with broad-spectrum antibacterial activity. 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 examine the impacts of PACAP and route of administration on the immune response of Nile tilapia (Oreochromis niloticus). 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 (i.p.) injection, bath, nares flush, or gill flush, and compared to PBS controls. In the first two studies, following individual treatments, tilapia underwent a bath exposure (40 L tank for 45 min) to F. columnare (isolate ALG-00-530; at 2.1 × 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 showed significantly lower mortality from F. columnare (10 %) than those receiving PBS i.p. (25 %). However, bath immersion of fish, both with and without F. columnare, resulted in significant mortality due to secondary infections with Edwardsiella tarda. Administration of modified PACAP-38 via nares/gill flush, or i.p. injection did not, however, result in significantly fewer mortalities compared to sham/PBS. Furthermore, the same modified form of PACAP-38 also induced inflammatory gene expression in the spleen, and eosinophilic granule cell aggregation in the nares, following flushing. The data from the first study suggest that PACAP-38 induces protection against infection and stress mainly through a reduction of inflammatory-il1β expression. Moreover, data analysis of spleen samples from these two studies also indicates that PACAP-38 might have more anti-inflammatory impacts than its modified form. In the fourth trial, after exposing tilapia to low temperature (15–17 °C) for 30 min (cold stress), the PACAP-38 treated group had significantly lower cortisol levels compared to both PBS injected group and the negative control. Due to the observed impacts of PACAP-38 treatment on the immunophysiology of Nile tilapia, its potential for therapeutic use should be further investigated.