Induction of bradycardia in trout by centrally administered corticotropin-releasing-hormone (CRH)

Central hyperventilatory action of the stress-related neurohormonal peptides, corticotropin-releasing factor and urotensin-I in the trout Oncorhynchus mykiss

The stress-related neurohormonal peptides corticotropin-releasing factor (CRF) and urotensin-I (U-I), an ortholog of mammalian urocortin 1, are widely distributed in the central nervous systems of teleost fish but little is known about their possible central neurotropic actions. In the present study, we investigated the effect of intracerebroventricular (ICV) injection of CRF and U-I (1-10pmol) on ventilatory and cardiovascular variables in our established unanaesthetized trout model. CRF and U-I produced a significant dose-dependent and long-lasting increase in the ventilatory frequency (VF) and the ventilatory amplitude (VA). Consequently the net effect of these peptides was a hyperventilatory response since the total ventilation (VTOT) was significantly elevated. However, CRF evoked a significant hyperventilatory response 5-10min sooner than that observed after ICV administration of U-I and the hyperventilatory effect of 10pmol CRF was twofold higher than that of equimolar dose of U-I. Pre-treatment of the trout with the antagonist, alpha-helical CRF(9-41), significantly reduced by about threefold the CRF-induced increase in VF, VA and VTOT. The most significant cardiovascular action of central CRF and U-I was to evoke a hypertensive response without changing the heart rate. Peripheral injection of CRF and U-I at doses of 5 and 50pmol produced no change in VF, VA or VTOT. Only a transient hypertensive response without change in heart rate was observed after the injection of the highest dose of U-I. Our results demonstrate that in a teleost fish, CRF and U-I produce a potent hyperventilatory response only when injected centrally. The two endogenous stress-related neuropeptides may play an important stimulatory role acting as neurotransmitters and/or neuromodulators in the central control of ventilatory apparatus during stress.

The corticotropin-releasing factor system as a mediator of the appetite-suppressing effects of stress in fish

A characteristic feature of the behavioural response to intensely acute or chronic stressors is a reduction in appetite. In fish, as in other vertebrates, the corticotropin-releasing factor (CRF) system plays a key role in coordinating the neuroendocrine, autonomic, and behavioural responses to stress. The following review documents the evidence implicating the CRF system as a mediator of the appetite-suppressing effects of stress in fish. Central injections of CRF or the related peptide, urotensin I (UI), or pharmacological treatments or stressors that result in an increase in forebrain CRF and UI gene expression, can elicit dose-dependent reductions in food intake that are at least partially reversed by pre-treatment with a CRF receptor antagonist. In addition, the appetite suppressing effects of various environmental, pathological, physical, and social stressors are associated with elevated levels of forebrain CRF and UI gene expression and with an activation of the hypothalamic-pituitary-interrenal (HPI) stress axis. In contrast, although stressors can also be associated with an increase in caudal neurosecretory system CRF and UI gene expression and an endocrine role for CRF-related peptides has been suggested, the physiological effects of peripheral CRF-related peptides on the gastrointestinal system and in the regulation of appetite have not been investigated. Overall, while CRF and UI appear to participate in the stress-induced changes in feeding behaviour in fish, the role of other know components of the CRF system is not known. Moreover, the extent to which the anorexigenic effects of CRF-related peptides are mediated through the hypothalamic feeding center, the HPI axis and cortisol, or via actions on descending autonomic pathways remains to be investigated.

Cardiovascular actions of the stress-related neurohormonal peptides, corticotropin-releasing factor and urotensin-I in the trout Oncorhynchus mykiss

In this review, we summarize the most significant data concerning the cardiovascular effects of centrally and peripherally administered synthetic trout corticotropin-releasing factor (CRF) and urotensin-I (U-I) in our animal model, the unanesthetized trout Oncorhynchus mykiss. Although there is more than 60% sequence identity between these two stress-related neurohormonal peptides, CRF and U-I-induced differential actions upon the mean dorsal aortic blood pressure (Pda) and the heart rate (HR) in trout maintained under similar experimental situations. After intracerebroventricular injections, only U-I induced an increase in Pda while in non-cannulated trout, CRF only decreased the HR and elevated the heart rate variability by a presumed activation of the parasympathetic nervous system activity to the heart. The CRF antagonist, the alpha-helical CRF(9-41) blocked these central actions of CRF. After intra-arterial (IA) injections, U-I induced a direct hypotensive action and an elevation in HR. This hypotensive phase was reversed to hypertension by the release of catecholamines. IA injection of CRF caused no change in Pda or HR. These cardiovascular effects are compared with the much better established actions of CRF and the orthologous urocortins in mammals.

Time-Course Effects of Centrally Administered Native Urotensin-II on Motor and Cardioventilatory Activity in Trout

Although in most vertebrate species urotensin-II (UII) is synthesized in neurons of the central nervous system, little is known regarding the physiological actions of UII in the brain. We have investigated the effects of intracerebroventricular (ICV) administration of synthetic trout UII (1, 5, and 50 pmol) on total motor activity (ACT), ventilatory frequency (VF), ventilatory amplitude (VA), and heart rate (HR) in the unanesthetized trout. ICV injection of UII increased ACT in a dose-dependent manner, and the maximal effect was observed at a dose of 5 pmol. At doses of 1 and 5 pmol, UII did not affect VF, VA, or HR. At the highest dose tested (50 pmol), UII not only increased ACT, but also significantly activated VF, VA, and HR. In contrast, ICV injection of synthetic trout angiotensin-II (5 pmol) did not produce any effect on ACT, VF, or VA, but sharply increased HR. These data provide the first evidence that UII can act centrally to induce motor activity in a nonmammalian vertebrate species.

Corticotropin-Releasing Factor Receptor 1 and Central Heart Rate Regulation in Mice during Expression of Conditioned Fear

The present study was performed to 1) determine heart rate (HR) effects mediated through central corticotropin-releasing factor receptor subtypes 1 (CRF(1)) investigate and 2 (CRF(2)) and 2) to the contribution of endogenous CRF to baseline HR and its fear-induced adjustment in freely moving mice. CRF ligands were injected into both lateral ventricles (i.c.v.) 15 min before the presentation of a conditioned auditory fear stimulus (CS). Initial behavioral results suggest an ovine CRF (oCRF)-mediated enhanced baseline fear and mildly enhanced conditioned auditory fear. In contrast, i.c.v. injection of oCRF (35-210 ng/mouse) dose-dependently decreased baseline HR, increased HR variability, and attenuated the CS-induced tachycardia. This effect is suggested to depend on a combined activation of sympathetic and parasympathetic activity referred to as enhanced sympathovagal antagonism. An extreme bradycardia was elicited by oCRF injection into the lower brainstem. All HR effects were probably mediated by CRF(1) because injection of the CRF(2)-selective agonist mouse urocortin II was ineffective, and the baseline bradycardia by i.c.v. CRF was preserved in CRF(2)-deficient mice. Injection of various CRF receptor antagonists including the CRF(2)-selective antisauvagine-30 did not affect the conditioned HR response. This finding suggests that endogenous CRF does not contribute to the fear-mediated tachycardia. Thus, the hypothesis of an involvement of CRF in HR responses of mice to acute aversive stimulation is rejected. Pharmacological evidence points at the involvement of CRF(1) in enhanced sympathovagal antagonism, a pathological state contributing to elevated cardiac risk, whereas the physiological role of the brain CRF system in cardiovascular regulation remains to be determined.

Central effects of native urotensin II on motor activity, ventilatory movements, and heart rate in the trout Oncorhynchus mykiss

Urotensin II (UII) has been originally isolated from fish urophysis. However, in fish as in mammals, UII is also produced in brain neurons. Although UII binding sites are widely distributed in the fish central nervous system (CNS), little is known regarding its central activities. In the present study, we have investigated the effects of intracerebroventricular (ICV) administration of synthetic trout UII on the duration of motor activity (ACT; evidenced by bursts of activity on the trace of the ventilatory signal), ventilatory frequency (VF), ventilatory amplitude (VA), and heart rate (HR) in unanesthesized trout, Oncorhynchus mykiss. ICV injection of very low doses of UII (1 and 5 pmol) produced a dose-dependent increase of ACT without affecting VF, VA, or HR. At a higher dose (50 pmol), UII stimulated ACT as well as VF, VA, and HR. ICV injection of trout angiotensin II (5 pmol) did not affect ACT, VF, and VA, but provoked a robust increase in HR. These data provide the first evidence that central administration of UII stimulates motor activity in a nonmammalian vertebrate.

Captopril blocks the cardiac actions of centrally administered angiotensin I in the trout Oncorhynchus mykiss

The present study was performed in order to gain new insights into the existence of a brain renin-angiotensin system (RAS) in teleost fish. For this purpose, we investigated the effects of centrally administered angiotensin (ANG) I ([Asn(1),Val(5),Asn(9)]ANG I) and ANG II ([Asn(1),Val(5)]ANG II) on heart rate (HR) and heart rate variability (HRV) in the unanesthetized trout. The animals were studied before and after treatment with captopril, an angiotensin-converting enzyme (ACE) inhibitor. Trout were equipped with two subcutaneous electrocardiographic electrodes and with an intracerebroventricular (i.c.v.) cannula inserted within the third ventricle of the brain. The i.c.v. injection of vehicle had no effect on the recorded parameters. The i.c.v. injections of ANG I and ANG II at doses of 5 and 50 pmol had a marked effect on HR and HRV. At a dose of 50 pmol, ANG I and ANG II produced a progressive and significant increase in HR (+36% and+45%, respectively) but elicited a profound decrease in HRV (-88% and-92%, respectively). I.c.v. injection of captopril (10 microg) had no effect on HR or HRV. However, this ACE inhibitor prevented the tachycardia and abolished the decrease in HRV mediated by 50 pmol of ANG I. In contrast, captopril had no effect upon the cardiac actions of 50 pmol of ANG II. These results give the first support for the existence of functional important ACE-like activity in the brain of a teleost fish and suggest that the brain RAS in this class of vertebrate may be involved in the control of cardiac chronotropic activity.