Hypothalamic CRH neurons that control ACTH secretion from the pituitary gland have secretory terminals in the external zone of the median eminence (ZEME). These neurons can coproduce vasopressin (AVP), a neuropeptide that potentiates the ACTH releasing effects of CRH. Recently, we found increased AVP production in adult rats weeks after single exposure to a stressor, which may play a role in event-induced stress disorders. Here, we describe the long-term changes in the HPA axis of adult male rats following a single exposure to a stressor, the cytokine interleukin-1 beta (IL-1 beta). The effects on storage and release of AVP and CRH were established by quantitative immunocytochemistry, the effects on ACTH and corticosterone responses by radioimmunoassay. Single administration of IL-1 beta (5 micrograms/kg i.p.) induces a delayed (at least 4 d) and a long-lasting (at least 3 weeks) increase of vasopressin (AVP) stores in CRH terminals of the ZEME without affecting the CRH stores, and a marked increase of the fraction of CRH terminals that costore AVP. Eleven days after IL-1 beta administration, a second IL-1 beta challenge causes a marked depletion of the AVP stores in the ZEME within 2 hr, which is not seen in rats treated with vehicle 11 d earlier. This is accompanied by twofold higher ACTH and corticosterone responses, as compared to those in vehicle pretreated rats. IL-1 beta-pretreated rats also showed increased ACTH and corticosterone responses to electric footshocks. We conclude that transient activation of the HPA axis by a single administration of IL-1 beta induces a delayed and long-lasting hyperproduction, hyperstorage, and hypersecretion of AVP from hypothalamic CRH neurons that results in hyperresponsiveness of the HPA axis to subsequent stimuli.

Inflammation following an infection induces a range of nonspecific symptoms of sickness in animals and humane. The cytokine interleukin‐1 (IL‐1) mediates many of the brain‐mediated symptoms of sickness. Binding sites for IL‐1 have been found in mouse brain, but not in the brains of rats. This raises questions as to the involvement of these neuronally localized IL‐1 binding sites in the induction of sickness symptoms. Based on observations of IL‐1 receptor mRNA in close vicinity to the vasculature in the mouse and rat brain, we studied the possibility that endothelial cells in the rat brain exhibit IL‐1 receptors to transduce information to the brain. Ligand binding studies reveal that cultured endothelial cells of adult rat brain exhibit specific binding sites for rat IL‐ 1β. Polymerase chain reaction experiments demonstrated that mRNA of the type I but not that of the type II IL‐1 receptor is present in rat brain endothelial cells. Incubation of these endothelial cells with recombinant rat IL‐1β showed a dose‐dependent increase in interleukin‐6, prostaglandin E2, and prostacyclin secretion. Intravenous administration of rat IL‐1β to adult rats enhanced prostaglandin E2 immunoreactivity in en‐dothelial cells of the brain microvasculature. These results indicate that functional type I IL‐1 receptors are present on endothelial cells of adult rat brain. We postulate that circulating IL‐1 can be translated by brain endothelial cells into other signals such as interleukin‐6 or prostaglandins that have access to the brain and induce sickness symptoms.—Van Dam, A.‐M., de Vries, H. E., Kuiper, J., Zijlstra, F. J., de Boer, A. G., Tilders, F. J. H., Berkenbosch, F. Interleukui‐1 receptors on rat brain endothelial cells: a role in neuroimmune interaction? FASEB J. 10, 351‐356 (1996)

In this study changes in serum FSH concentration, hypothalamic LHRH content and in vitro release of LHRH from hypothalami of female Wistar rats during prepubertal development are described. The concentration of FSH in serum was determined by radioimmunoassay. Extremely high levels (1.800 ng/ml) were found between 12 and 16 days of age. Thereafter, FSH levels rapidly fell to low levels (135 ng/ml) on day 32. LHRH was measured by radioimmunoassay in extracts of mediobasal hypothalami (MBH) and showed a gradual increase during the first 10 days of life. Between 10 and 16 days of age, hypothalamic LHRH content did not change. After day 16, LHRH content sharply increased to a maximum found on day 32. The release of LHRH from MBH of developing rats was studied during in vitro incubation in Krebs-Ringer bicarbonate buffer. LHRH in the medium was measured by radioimmunoassay. The spontaneous release rate of LHRH was maximal from MBH of 12-day-old rats and minimal from those of 32 days. Depolarization of neural membranes, by medium containing 45 mM K⁺ induced a stimulation of LHRH release. This K⁺-induced release of LHRH was also most prominent on day 12. These observations are consistent with the hypothesis that the high circulating FSH levels found in female rats between 12 and 16 days of age are at least in part caused by a high secretion rate of LHRH during this period.

In the present study the inhibitory effects of a panel of 21 monoclonal antibodies (moabs) to rat interleukin 1 beta (rIL-1 beta) on the binding of 125I-labeled rIL-1 beta to murine type I IL-1 receptors on EL4 cells were investigated. Furthermore, the epitopes of these moabs were determined by the use of the pepscan technique, and these epitopes were visualized on a three-dimensional model of rIL-1 beta. Some moabs (SILK 3, 4, 5, 6, and 22) inhibited receptor binding of radioiodinated rIL-1 beta at concentrations that are similar to the dissociation constant values of antibody-rIL-1 beta binding. Another group of moabs (SILK 7, 11, 20, 21, and 23) also inhibited receptor binding but only at concentrations that are 10-150 times higher than their dissociation constants. A large group of moabs did not affect receptor binding in the concentration range tested, and two moabs enhanced the binding of rIL-1 beta to type I receptors. The result of pepscan analysis shows that the moabs bound to one or more of the amino acid sequences 35-49, 66-85, 78-97, 106-124, and 123-143 of mature rIL-1 beta. Modeling of rIL-1 beta shows that the binding domains of SILK 4, 5, 6, and 22 (sequence 123-143) is located at the closed end of the molecule, indicating that this part of rIL-1 beta harbors domains that are crucial for type I receptor binding. The binding domain of SILK 3 (sequence 66-85) is also located at this end of the molecule. In contrast, the binding domains of SILK 7, 11, 20, 21, and 23 (sequence 78-97) are located at the open end of the molecule, which is at the same face as the amino- and carboxy-terminals. The binding domain of SILK 16 (sequence 106-124) is positioned at the center of the molecule. It is concluded that the closed end of rIL-1 beta contains sequences that are crucial for its binding to type I receptors on murine EL4 cells. Because of the high concentrations of antibodies to residues 78-97 of rIL-1 beta that are needed to interfere with receptor binding, the importance of these domains in binding to type I receptors remains uncertain.

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