The response of brain serotonergic and dopaminergic systems to an acute stressor in rainbow trout: a time course study

The brain monoaminergic neurotransmitter systems are known to be involved in the integrated response to stress in vertebrates. However, present knowledge about the timing of their actions as well as their specific roles in the regulation of the endocrine axes that drive the stress response is incomplete. This is partly because of the complexity of the reciprocal interactions among the monoaminergic systems and other biochemical effectors of the stress response such as corticotropin-releasing factor (CRF), arginine vasotocin (AVT), adrenocorticotropic hormone (ACTH) and corticosteroids. In this study, we show for the first time in teleost fish (rainbow trout) the short-and mid-term time course of the response of the forebrain serotonergic and dopaminergic activities after exposure to an acute stressor. Other stress markers like the plasma levels of cortisol, glucose and lactate were also monitored, providing a context in which to precisely locate the monoaminergic activation within the fish acute stress response. Our results show that acute stress induced a rapid increase in forebrain serotonergic activity, which became elevated after only 15 s of chasing. Several hours after stress, serotonergic activity recovered its basal levels, in parallel with the recovery of other stress markers such as plasma catecholamines and cortisol. Dopaminergic activity was also increased after stress, but only in the telencephalon and only after 20 min. The increase in serotonergic activity happened before the elevation of plasma catecholamines, suggesting that this monoamine system could have a key role in triggering the initial steps of the activation of not only the hypothalamus-pituitary-inter-renal axis but also the brain-sympathetic-chromaffin axis in fish.