Central Nervous System IL-1β System and Neuropeptide Y mRNAs During IL-1β-induced Anorexia in Rats

doi:10.1016/S0361-9230(97)00159-7

Brain Research Bulletin

Volume 44, Issue 3, 1997, Pages 311-317

https://doi.org/10.1016/S0361-9230(97)00159-7 Get rights and content

Abstract

Interleukin-1β (IL-1β) induces anorexia and neuropeptide Y (NPY) increases feeding by direct action in the central nervous system (CNS). IL-1β, depending on the dose, attenuates or blocks NPY-induced feeding. This suggests that IL-1β-NPY interactions may be involved in IL-1β-induced anorexia. Here, RNase protection assays were used to investigate the effects of the chronic intracerebroventricular (ICV) administration of IL-1β (at a dose that yields estimated pathophysiological concentrations in the cerebrospinal fluid) on mRNA levels of IL-1β system components and NPY in the cerebellum, parietofrontal cortex, hippocampus, hypothalamus, and midbrain. The results show that the chronic ICV administration of IL-1β (8.0 ng/24 h for 72 h) differentially induced IL-1β system components across brain regions in anorectic rats. IL-1β mRNA and IL-1 receptor antagonist (IL-1Ra) mRNA were induced similarly, exhibiting highest and lowest expression levels in the hypothalamus and hippocampus, respectively. IL-1 receptor type I (IL-1RI) mRNA and the soluble form of IL-1 receptor accessory protein (IL-1R AcP II) mRNA were also induced in the hypothalamus and cerebellum. NPY mRNA expression showed a small, but significant decrease in the hypothalamus. Heat-inactivated IL-1β (8.0 ng/24 h for 72 h) had no effect on the behavioral or molecular profiles. The results suggest that endogenous upregulation of IL-1β contributes to IL-1β-induced anorexia, and that modification of NPY mechanisms also may be involved.

Introduction

Interleukin-1β (IL-1β) acts directly in the central nervous system (CNS) to induce a variety of neurological manifestations including anorexia 14, 17, 20. The IL-1β system includes: IL-1β (the predominant released form of IL-1 [8]), IL-1 receptor type I (IL-1RI; responsible for IL-1β signaling [23] and induction of neurological manifestations [16]), IL-1 receptor accessory protein (IL-1R AcP; a protein that increases the binding affinity of IL-1RI for IL-1 9, 15), and IL-1 receptor antagonist (IL-1Ra; an endogenous inhibitor that antagonizes, by competitive inhibition, IL-1β-induced CNS actions 18, 20).

Neuropeptide Y (NPY) also acts directly in the CNS to induce a robust feeding response 6, 25, and NPY expression is modulated by the feeding status 7, 21. Previous work showed that the intracerebroventricular (ICV) administration of IL-1β antagonizes NPY-induced feeding in rats [25]; NPY also inhibits IL-1β-induced anorexia [25]. The mechanism of this cytokine-neuropeptide interaction, however, has not been determined.

The present study investigated whether transcriptional mechanisms related to the IL-1β system and NPY participate in IL-1β-induced anorexia. Specifically, the mRNA levels of IL-1β system components and NPY were examined in anorectic rats responding to the chronic ICV administration of IL-1β, at a dose that yields estimated pathophysiological concentrations in the cerebrospinal fluid (CSF). Anorexia induced by the chronic ICV microinfusion of IL-1β is observed at doses that yield estimated pathophysiological concentrations in the CSF [19]. Briefly, the delivery rate of the minipumps used is 1.0 μl/h, and therefore, the estimated concentration of IL-1β released into the CSF will be 333 pg/h with an 8.0 ng/24 h dose. Considering the rat’s normal CSF volume (300 to 400 μl), it is estimated that the concentration of IL-1β in the CSF will be 8.3 to 11.1 pg/10 μl. Also taking into account the rat’s normal rate of CSF turnover and secretion (about 0.7% of the total volume/min (based on a CSF production rate of 1.99 ± 0.16 μl/min, mean ± SD from three studies: see [24]), the concentration of IL-1β would decrease by an additional 42%, that is, to 4.8 and 6.4 pg IL-1β/10 μl for a volume of 400 μl and 300 μl, respectively. Thus, the dose of IL-1β administered in the present study is estimated to yield the pathophysiological range observed in the CSF, a range that extends from >1.0 to 13 pg IL-1/10 μl CSF (summarized in [19]).

Section snippets

Subjects and Maintenance

Male Wistar (VAF) rats weighing between 250 and 275 g at the beginning of the experiments were used. Rats were maintained ad lib as previously described [19]. Rats were allowed several days of adaptation before being cannulated into the third ventricle. Test solutions were administered at least 10 days postsurgery.

Implantation of Brain Cannulas

Under intraperitoneal (IP) ketamine (100 mg/kg) plus xylazine (5 mg/kg) anesthesia, a guide cannula was implanted into the third ventricle at stereotaxic coordinates: −2.1

Effects of Chronic ICV Administration of IL-1β on Food and Water Intakes and Body Weight

The ICV administration of IL-1β induced significant anorexia. Nighttime food intake (during the period corresponding to 60–72 h after initiating the chronic ICV infusion, that is, just before sacrifice) was 18.8 ± 0.5 g (n = 9) and 12.3 ± 0.4 g (n = 9) for the heat-inactivated and active IL-1β-treated groups, respectively (Fig. 1A); t-test showed that treatments differed significantly in food intake [t = 9.92, p < 0.0001, power of performed test (ppt) = 1.0]. Water intake, on the other hand,

Discussion

The chronic ICV microinfusion of IL-1β, at a dose that yields estimated pathophysiological concentrations in the CSF, differentially induced IL-1β system mRNAs in the cerebellum, parietofrontal cortex, hippocampus, hypothalamus, and midbrain from anorectic rats. IL-1β and IL-1Ra had a similar mRNA induction profile with highest and lowest levels being exhibited by the hypothalamus and hippocampus, respectively. IL-1β-treated animals also displayed high levels of IL-1RI mRNA and IL-1R AcP II

IL-1β System mRNAs

The similar mRNA profile of IL-1β and IL-1Ra within a brain region (Fig. 3 and Fig. 4) indicates an equivalent induction of the stimulatory ligand and endogenous inhibitor in anorectic rats responding to IL-1β. The high correlation between IL-1β and IL-1Ra mRNA levels suggests a critical balance for the fine regulation of IL-1β signaling. An endogenous cytoprotective mechanism to prevent deleterious effects of uncontrolled IL-1β action has been proposed [13]. That is, the antagonist (IL-1Ra)

NPY mRNA

IL-1β significantly decreased NPY mRNA levels in the hypothalamus, an important feeding-regulatory brain site. NPY mRNA level changes in all other brain regions examined were minimal; this suggests that IL-1β-induced decrease in hypothalamic NPY mRNA levels was specific and not associated with a generalized reduction. Studies have proposed that NPY is a mediator of normal feeding 1, 10, 11, 26, and that fluctuations in hypothalamic NPY mRNA levels are associated with changes in feeding 7, 21.

Acknowledgements

We thank Dr. Ronald P. Hart (Department of Biological Sciences, Rutgers University) for providing the rat IL-1β, IL-1Ra, IL-1RI and IL-1R AcP cDNAs, and Dr. Steven L. Sabol (Laboratory of Biochemical Genetics, National Heart, Lung, and Blood Institute, NIH) for providing the rat NPY cDNA. This research was supported by funds from the University of Delaware and the National Institutes of Health to C.R.P.S.

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