Neuro-endocrine pathways in the control of stress in fish
The increasing intensification and subsequent contamination of many bodies of natural water bodies around the world by several anthropogenic substances is creating the major category of environmental stressor. Various stressors are necessary components of modern intensive fish culture (e.g., grading, transportation). The initial response represents the perception of an altered state and initiates a neuroendocrine/endocrine response that forms part of the generalized stress response in fish. This response includes the rapid release of stress hormones, such as catecholamines and cortisol into the circulation. Catecholamines are released from the chromaffin tissue situated in the head kidney of teleost’s and also from the endings of adrenergic nerves. Cortisol is released from the interrenal tissue, located in the head (anterior) kidney, in response to several pituitary hormones, but most potently to adrenocorticotrophic hormone (ACTH). A recent study showed that ACTH may also stimulate adrenaline release and that chronic cortisol treatment may affect catecholamine storage and release in trout. As both the chromaffin tissue and the interrenal tissue lie in close proximity in fish, it is suggested that there is a possibility that a paracrine control for stress hormone regulation exists in fish. The response of the fish to such stressors involves all levels of organization, from the cell to the individual organism to the structure of the population. Stress response in fish is characterized by the activation of hypothalamo-pituitary- interrenal axis or HPA, activation of which starts in the pre-optic nucleus of hypothalamus (NPO) otherwise known as nucleus preopticus, which release corticotrophin releasing hormone (CPH). CPH, in turn, stimulates pituitary to release ACTH, which stimulates head kidney to release cortisol. Other pathways such as hypothalamo – pituitary – interenal (HPI) & pituitary – thyroid (PT) axis play an important role in stress control.