2047 1 We acknowledge contributions of representatives of the Dutch veal industry to Exp. 1, and we are grateful to SODIAVIT for providing experimental facilities for Exp. 2. This work was supported by INRA funds (AIP Bien-e ˆtre et Sciences du Comporte- ment); by the Dutch Board Animal Feed; by the Dutch Ministry of Agriculture, Nature Management and Fisheries; and by European funds (Shared cost action: Chain Management of Veal Calves’ Welfare). We thank G. Coulaud and F. Decuq for their help in taking blood samples, B. Engel for statistical advice, P. Morme `de for reviewing this paper, and G. Littlejohn for improving the English. Received October 2, 1998. Accepted February 22, 1999. Adrenocorticotropic Hormone and Cortisol in Calves After Corticotropin-Releasing Hormone 1 I. Veissier*, C. G. van Reenen † , S. Andanson*, and I. E. Leushuis † *INRA, Centre de Clermont-Ferrand-Theix, Unite ´ de Recherche sur les Herbivores, 63122 Saint-Gene `s Champanelle, France and † Institute for Animal Science and Health (ID-DLO), 8200 AB Lelystad, The Netherlands ABSTRACT: The aim for this study was to analyze responsiveness of the hypothalamo-pituitary- adrenocortical axis to exogenous bovine corticotropin- releasing hormone (bCRH) in calves. Two dose- response studies were carried out, using either bCRH alone (dose rates of 0, .01, .03, and .1 mg bCRH/kg live weight) or in combination with arginine-vasopressin (bCRH:AVP, 0:0, .1:.05, .5:.25, and 1:.5 mg kg live weight). The bCRH was administered i.v. to calves (n = 5 to 7 per dose) housed individually or in groups. Serial blood samples were obtained from before to 300 min after injection and analyzed for plasma ACTH and cortisol concentrations. The lowest bCRH dose that produced a response in all calves was .1 mg/kg. In the experiment using bCRH with AVP, increasing the bCRH dose from .1 to 1 mg/kg resulted in an increase in peak ACTH concentration (321 vs 2,003 pg/mL) but did not significantly affect the peak cortisol concentration (37 vs 40 ng/mL). The time to reach the peak cortisol concentration increased with the dose of bCRH with AVP (from 38 to 111 min). The ACTH and cortisol concentrations determined at any time be- tween 20 and 90 min after bCRH injection were correlated to the integrated responses calculated as areas under the ACTH and the cortisol curves ( r between .61 and .99, P < .05). In comparison with results from studies in humans, pigs, and sheep, our data showed that the pituitary of calves seems less sensitive to CRH than that of other mammals, despite a greater capacity to produce ACTH. Moreover, the calf’s adrenals seem to have a lower capacity to produce cortisol than adrenals of other mammals. As in other species, it seems that AVP enhances the release of ACTH and cortisol. For CRH challenge to be used in calves, we suggest injecting at least .1 mg of bCRH/kg live weight either with or without AVP and taking several blood samples before injection and between 20 and 90 min after injection. Key Words: Calves, Cortocoliberin, Corticotropin, Hydrocortisone, Stress 1999 American Society of Animal Science. All rights reserved. J. Anim. Sci. 1999. 77:2047–2053 Introduction The hypothalamo-pituitary-adrenocortical ( HPA) axis is a major component of the neuroendocrine responses to stressful events (Selye, 1973). Altered HPA axis function has been observed in depressive patients using reactivity tests measuring, for example, suppression of corticoid release after dexamethasone, ACTH release in response to corticotropin-releasing hormone ( CRH) , or corticoid release in response to ACTH (Carroll et al., 1981; Modell et al., 1998; O’Toole et al., 1998). The ACTH challenge has been extended to domestic animals to detect states of chronic activation of the HPA axis due to external stressors (cattle: Friend et al., 1979; Dantzer et al., 1983; pigs: Janssens et al., 1994; Klemcke, 1994). Recently, stimulation of the HPA axis by CRH has also been used (rats: Garcia-Belenguer et al., 1993; pigs: Janssens et al., 1995). Few studies investigated the CRH challenge in cattle. Abebe et al. (1993) monitored HPA axis reactivity to CRH in African Boran cattle in the context of infectious disease. To our knowledge, the effect of CRH on the HPA axis has not been studied in calves, except by Veissier et al. (1998). However, in this latter study only a limited number of blood samples were obtained for a relatively short time after administration of one dose of CRH.