Pain sensitivity of fishes and analgesia induced by opioid and nonopioid agents

Experiments were performed on carp Cyprinus carpio, rainbow trout Oncorhynchus mykiss, cod Gadus morhua and sturgeon Acipenser ruthenus. An originally designed optico-mechanical system was used to record the response to painful electrical stimulation. before and after administration of analgetic agents. Drugs used were mu agonists tramadol, dermorphine and beta-casomorphine, kappa agonist U-50488, delta agonist DADLE, and nonopioid agents sydnophenum, analginum, novocainum. Drugs were administrated by different ways peritoneally, subcutaneously, intranasally. Administration of drugs produced dose-dependent and lasting for at least 1 h increase of NT in 1.5-3 times. In rainbow trout, intranasal administration of dermorphine 0.20-0.75 mg/kg caused a concentration-dependent decrease in the pain sensitivity by 12-55%. The analgesic effect was usually observed within 10 min after administration and it lasted for at least 1 h (up to 2-3 h in some fish). In cod, intranasal administration of beta-casomorphine 2.5-12.5 mg/kg and peritoneal one 10-30 mg/kg decreased the pain sensitivity by 15-37% and 14-35%, respectively. In carp, nociceptive thresholds significantly increasd following the intramuscular injection of agonists mu, delta, and kappa opioid receptors, tramadol 10-100 nmol/g, DADLE 10-50 nmol/g, and U-50488 30-80 nmol/g, respectively. Antinociceptive effects of opioid agents were blocked or significantly reduced by pretreatment with naloxone. In cod, injected peritoneally sydnophenum 15-100mg/kg decreased the pain sensititivity by 15-89%. Intraperitoneal injection of 50% solution of analginum 0.5-2.5 ml/100g and subcutaneous one 0.25-1 ml/100g decrease the pain sensitivity by 16-21% and 29-45%, respectively. Local subcutaneous injections of 2% solution of novocainum blocked the nociceptive reactions. Stress significantly reduced nociceptive responses.