Neuronal control of catecholamine release in the rainbow trout (Oncorhynchus mykiss)

Abstract

The aim of this thesis was to study aspects of the neuronal control of catecholamine secretion in a teleost, the rainbow trout. The development and validation of a nerve stimulation technique made it possible to determine that a portion of the neuronal control of catecholamine release, which prevailed at low frequency stimulation, could be attributed to vasoactive intestinal polypeptide (VIP) and/or pituitary adenylate cyclase activating polypeptide (PACAP). On the other hand, cholinergic stimulation predominated during higher levels of neuronal activity. Fluorescent histochemical techniques in combination with pharmacological approaches provided direct evidence that VIP and PACAP can elicit the secretion of adrenaline, only, from the chromaffin tissue via specific VIP binding sites that exhibited properties of VPAC receptors. Using in situ perfused posterior cardinal vein preparations, evidence was provided that while the nicotinic receptor appears to be the predominant pathway mediating the effects of acetylcholine on catecholamine secretion, muscarinic receptor stimulation may augment the cellular response to nicotinic receptor activation. Under extreme conditions, muscarinic receptors may directly elicit the secretion of catecholamines. The impact of extracellular catecholamines on catecholamine secretion from chromaffin cells was also investigated. Results revealed that the mechanisms of adrenergic inhibition of catecholamine secretion in response to cholinergic stimulation include activation of chromaffin cell membrane beta2-receptors and presynaptic alpha2-adrenergic receptors. However, catecholamine release in response to VIP appears to be insensitive to the adrenergic negative feedback mechanisms. Finally, despite the rapid progress in cDNA cloning, molecular information on the receptors mediating the effects of VIP and PACAP in fish is scant. In this thesis, I report preliminary findings of the cloning of the trout PAC1, VPAC1, and VPAC2 receptors from brain cDNA. In summary, VIP and PACAP appear to function as neurotransmitters in the neuronal regulation of catecholamine release in this species. The apparent complexity of the mechanisms regulating the secretion of catecholamines from trout chromaffin cells may reflect the precise control required for these hormones to play their role in physiological and biochemical homeostasis in these organisms.