Vasoactive intestinal polypeptide- and pituitary adenylate cyclase activating polypeptide-mediated control of catecholamine release from chromaffin tissue in the rainbow trout, Oncorhynchus mykiss.

The aim of the present investigation was to assess the relative contributions of cholinergic (acetylcholine) and non-cholinergic vasoactive intestinal polypeptide (VIP), and pituitary adenylate cyclase activating polypeptide (PACAP) neurotransmitters in the neuronal control of catecholamine secretion from the chromaffin tissue lining the posterior cardinal vein of the rainbow trout (Oncorhynchus mykiss). Using an in situ saline-perfused posterior cardinal vein preparation, it was demonstrated that exogenous administration of chicken VIP or human PACAP-27 caused a dose-dependent increase in adrenaline secretion; noradrenaline secretion was unaffected. Analysis of dose-response curves indicated that VIP and PACAP stimulated the secretion of adrenaline with a similar degree of potency (ED(50) for VIP=1.90x10(-11) mol/kg; ED(50) for PACAP=1.03x10(-11) mol/kg). The VIP/PACAP-elicited secretion was diminished in the presence of the VIP receptor antagonist, VIP 6-28, but was unaffected by the PACAP receptor antagonist, PACAP 6-27, or the cholinergic antagonists, hexamethonium and atropine. Thus, this is the first study to demonstrate a direct stimulatory role for VIP or PACAP in catecholamine secretion from piscine chromaffin cells. The relative contribution of cholinergic and non-cholinergic neurotransmitters in the neuronal control of catecholamine secretion from the chromaffin tissue was evaluated using an in situ nerve-stimulating technique previously validated by us in the rainbow trout. This was accomplished by comparing catecholamine secretion in the presence or absence of cholinergic and the VIP and PACAP receptor antagonists during different levels of electrical stimulation. The results demonstrated that cholinergic stimulation predominated during high frequency of electrical stimulation (20 Hz) while the non-cholinergic component prevailed at low frequency (1 Hz). Overall, the results of the present investigation demonstrate that VIP and/or PACAP may directly stimulate adrenaline secretion from trout chromaffin cells at low levels of neuronal activity. Therefore, the neuronal control of catecholamine secretion in teleosts may not be confined to cholinergic-evoked events.