Adrenalectomy has been shown to reverse most facets of the syndrome of the genetically obese fa/fa rat. However, a detailed analysis of the hypothalamo-pituitary-adrenal (HPA) axis in these animals is lacking. In the present study, morning corticosteronemia was higher in obese rats of both sexes than in lean ones, whereas evening corticosteronemia was higher only in obese male rats. The HPA axis was further investigated using stressful stimuli. Immobilization, ether, and cold stresses resulted in greater corticosterone levels in obese than in lean animals. These abnormalities consisted in upward shifts of the corticosterone response in obese females and absolute increases in that of obese males, indicating that such alterations were more pronounced in obese male than obese female rats. Due to this, the putative origin of the increased corticosterone output of obese rats was studied in males. Greater levels of ACTH were reached in obese than in lean rats when submitted to a cold stress (6 C). Dexamethasone produced a complete suppression of corticosterone output in both lean and obese rats. During the recovery from such suppression, corticosterone levels rose to higher values in obese than in lean rats. This observation together with the greater cold-induced ACTH output in obese rats suggest that the increased activity of the HPA axis of these animals is of central origin. Whatever its precise etiology within the central nervous system, it is proposed that the increased HPA axis activity in obese rats and its resultant hypercorticism play a role in the establishment and maintenance of their syndrome.

The mechanism by which glucocorticoids induce insulin resistance was studied in normal rats administered for 2 days with corticosterone then tested by euglycaemic hyperinsulinaemic clamps. Corticosterone administration induced a slight hyperglycaemia, hyperinsulinaemia and increased non-esterified fatty acid levels. It impaired insulin-stimulated total glucose utilization (corticosterone 15.7 +/- 0.7; controls 24.6 +/- 0.8 mg.kg-1 x min-1), as well as residual hepatic glucose production (corticosterone 4.9 +/- 1.0; controls 2.0 +/- 0.7 mg.kg-1 x min-1). During the clamps, insulin did not decrease the elevated non-esterified fatty acid levels in corticosterone-administered rats (corticosterone 1.38 +/- 0.15, controls 0.22 +/- 0.04 mmol/l). Corticosterone administration decreased the in vivo insulin-stimulated glucose utilization index by individual muscles by 62 +/- 6%, and the de novo glycogen synthesis by 78 +/- 2% (n = 8-9 muscles). GLUT4 protein and mRNA levels were either unchanged or slightly increased by corticosterone administration. Inhibition of lipid oxidation by etomoxir prevented corticosterone-induced muscle but not hepatic insulin resistance. In conclusion, glucocorticoid-induced muscle insulin resistance is due to excessive non-esterified fatty acid oxidation, possibly via increased glucose fatty-acid cycle ultimately inhibiting glucose transport, or via decreased glycogen synthesis, or by a direct effect on glucose transporter translocation or activity or both.

Most metabolic disorders of genetically obese Zucker rats are reversed by adrenalectomy and are restored by corticosterone treatment, thus suggesting that a functional hypercorticosteronemic state is involved in the pathogenesis of the obesity syndrome in fa/fa rats. However, the hormone content and morphology of the hypothalamo-pituitary-adrenal axis of this animal model have to our knowledge not yet been described. We, thus, investigated morphologically and morphometrically the hypothalamic regions involved in CRF synthesis and secretion in male fa/fa rats. To ascertain if the brain is selectively or uniformly affected, we studied the main nuclei of the lateral and mediobasal hypothalamus, i.e. arcuate, lateral hypothalamic, and ventromedial nucleus and the parvicellular portion of the paraventricular nucleus. Moreover, after immunocytochemical labeling, we analyzed densitometrically the CRF-bearing axons of the median eminence and the ACTH-containing cells of the anterior and intermediate lobe of the pituitary gland. Finally, we investigated the adrenal glands by qualitative light microscopy. In fa/fa rats most hypothalamic nuclei were structurally changed. Furthermore, hypothalamic CRF and anterior pituitary ACTH contents as well as adrenal weight were increased, the zona fasciculata of the adrenal cortex was hypertrophic, and the ACTH content of the intermediate lobe was reduced. In conclusion, our results demonstrate that the obesity syndrome in genetically obese fa/fa rats is associated with lesions of the hypothalamo-pituitary-adrenal axis consistent with hyperadrenocorticism due to hyperactivity of the whole adrenal axis. Alterations also occur in the hypothalamic nuclei controlling glycemia, insulinemia, and circadian corticosterone secretion.