Central control of locomotor activity in juvenile salmonids: The role of corticotropin-releasing hormone in the brain

The present study investigated the neurocrine and neuroendocrine control of locomotor activity, habitat choice, social behavior, and migratory behavior in juvenile chinook salmon (Oncorhynchus tshawytscha). This was achieved by the manipulation of three neurotransmitter systems; serotonin (5-HT), dopamine (DA), γ-amino-n-butyric acid (GABA) and the neuropeptide corticotropin-releasing hormone (CRH). Chemicals were administered into the third ventricle of the brain and both behavioral and physiological assays were used to evaluate the effects.; These studies established that CRH causes a dose dependent increase in locomotor activity. Interactions between CRH and the serotonergic system were also investigated. Activation of the serotonergic system potentiated the effect of CRH on locomotor activity and the location of fish in the tanks. In addition, the role of GABA in the downregulation locomotor activity that was stimulated by CRH or by the combination of CRH and serotonin was investigated. The results suggest that endogenous GABA inhibits locomotor activity due to the interaction between CRH and serotonin but not CRH alone. By itself however, muscimol stimulated activity, an effect appears to be mediated by an indirect action on the dopaminergic system.; The present study also evaluated the effect of CRH on downstream swimming behavior in wild and hatchery juvenile chinook salmon. Fish were given an injection of CRH or saline and released into an artificial stream system. CRH increased the tendancy of nonmigratory fish to move downstream. However, in fish that were already migrating CRH significantly altered their behavior reminiscent of the changes observed in field studies of stressed fish.; These results provide evidence that all four systems are involved in the regulation of locomotor activity. We hypothesize that CRH alters locomotor activity by stimulation of the serotonergic system and that this is inhibited by the action of GABA. In addition, we speculate that the dopaminergic mechanisms involved in the control of locomotor activity are indirectly mediated by GABA. This study also provides support for the hypothesis that CRH is involved in mediating some aspects of migratory behavior.