105 Horm. metabol. Res. 19 (1987) 105-109 © Georg Thieme Verlag Stuttgart • New York Glucocorticoid Negative Feedback and Glucocorticoid Receptors After Hippocampectomy in Rats A.M. Magarinos, G. Somoza and A.F. De Nicola Laboratorios de Esteroides and Neuroendocrinologia, Instituto de Biologia y Medicina Experimental, Buenos Aires, Argentina Summary In rats with dorsal hippocampectomy, glucocorticoid receptors in the hypothalamus and anterior pituitary, as well as the pituitary transcortin-like compound, are preserved, in spite of a 60% depletion of glucocorticoid receptors in the hippocampus. In the hippocampec- tomized group, basal levels of serum corticosterone (CORT) were increased, although there was a normal response to ether stress. Inhibition of the response to ether with dexamethasone (DEX) was dose-dependent: whereas 100Mg/kg completely suppressed serum CORT, 10 Mg/kg were ineffective. However, we observed a re- duced sensitivity to DEX inhibition with 25 fig/kg in hippocampectomized animals. These results indicate that the hippocampus is involved in negative feedback mech- anisms, although different doses of DEX are needed for this demonstration. The inhibition of serum CORT due to 100 Mg/kg DEX suggests that negative feedback at sites other than the hippocampus was still operative, in agreement with normal levels of glucocorticoid receptors in the anterior pituitary and hypothalamus of hippo- campectomized rats. Key-Words: Glucocorticoid Receptors - Dexamethasone — Corticosterone Hippocampus • Introduction It is recognized that the feedback action of glucocorticoids on ACTH secretion is composed of a fast, rate-sensitive, and a delayed, level sensitive, mechanism (Jones, Tiptaft, Brush, Ferguson and Neame 1974; Keller-Wood and Dallman 1984). Delayed feedback is probably dependent on the genomic interaction of the hormone-receptor complex (McEwen 1982; de Kloet and Veldhuis 1984). The struc- tures mediating delayed feedback have been explored, and the anterior pituitary, the hypothalamus and the extra- hypothalamic limbic system have been implicated to vary- ing degrees (Knigge and Hay 1963;De Wied 1964; Casady and Taylor 1976;Feldman and Conforti 1980;McEwen 1982; Keller-Wood and Dallman 1984). The role of the anterior pituitary and hypothalamus stems from observations on the direct effects of glucocorticoids upon secretion and or synthesis of ACTH and CRF (De Wied 1964; Jones et al. 1974). However, demonstrations of the inhibitory role of the hippocampus on hypothalamic- pituitary-adrenal secretion (KniggeandHay 1963; Casady Received: 7 Jan. 1986 Accepted: 12 May 1986 and Taylor 1976; Feldman and Conforti 1980; Wilson, Greer, Greer and Roberts 1980) and of the regulatory effect of glucocorticoids on hippocampal function (McEwen 1982) suggest its participation in steroid nega- tive feedback. Data employing radioactive corticosterone (CORT) demonstrated a preferential nuclear uptake, cyto- solic binding and autoradiographic localization in the hippocampus (Warembourg 1975; Stumpf and Sar 1979; McEwen 1982) and suggestions for the involvement of hippo- campal glucocorticoid receptors in feedback mechanisms have been given (Tornello, Orti, De Nicola, Rainbow and McEwen 1982;Sapolsky and McEwen 1985). In the present report, we show that upon ablation of the hippocampus, rats present normal binding of ( 3 H)-dexamethasone (DEX) in the pituitary and hypothalamus. In spite of the con- servation of receptors in areas implicated in feedback mechanisms, our animals also showed some abnormalities of the function of the brain-pituitary-adrenal axis. Materials and Methods Surgery Male Wistar rats (250-300 g) were maintained on a 14-h light, 10-h dark schedule (lights on: 05.00-19.00 h) with controlled temperature (24°C) and rat chow and water ad libitum. After anesthesia with sodium Embutal (4 mg/100 g BW in distilled water), hippocampectomy was carried out by aspiration as described by Murphy and Brown (1970) and Schmajuk, Spear and Isaacson (1983). The rats were used 7 days after operation. COR T determination A volume of 0.4 ml blood was obtained from control and hippo- campectomized rats at 9.00-10.00 a.m. by cutting the tip of the tail in less than 2 min after cage removal (basal levels). Ether stress was studied after placing the animals for 1 min in an ether-saturated jar and then returned to their cages; a second blood sample was ob- tained 15 min after stress. For glucocorticoid suppression, rats were injected at 9.00 a.m. with either 10, 25 or 100 Mg/kg of DEX- phosphate (Decadron, Merck, Sharp &Dohme) s.c. and 4 h later subjected to ether stress. Bleeding was performed 15 min after stress as described above. CORT was determined in serum by a competitive protein binding assay (Murphy 1967). Results were ex- pressed in Mg/dl and were obtained after logit-log transformation of displacement curves. Steroid binding assay Control and hippocampectomized rats were bilaterally adrenalec- tomized under ether anesthesia and maintained on 0.9% NaCl in drinking water for 2-3 days. After this period the animals were Downloaded by: University of Ottawa. Copyrighted material.