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Endocrinology Vol. 139, No. 2 466-473
Copyright © 1998 by The Endocrine Society


ARTICLES

Anorectic Effects of the Cytokine, Ciliary Neurotropic Factor, Are Mediated by Hypothalamic Neuropeptide Y: Comparison with Leptin1

B. Xu, M. G. Dube, P. S. Kalra, W. G. Farmerie, A. Kaibara, L. L. Moldawer, D. Martin and S. P. Kalra

Departments of Neuroscience (B.X., S.P.K.), Physiology (M.G.D., P.S.K.), and Surgery (A.K., L.L.M.) and ICBR (W.G.F.), University of Florida College of Medicine, Gainesville, Florida 32610; and Amgen, Inc. (D.M.), Boulder, Colorado 80301

Address all correspondence and requests for reprints to: Satya P. Kalra, Ph.D., Department of Neuroscience, University of Florida College of Medicine, P.O. Box 100244, Gainesville, Florida 32610-0244. E-mail: skalra{at}neocortex.health.ufl.edu


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Although ciliary neurotropic factor (CNTF) is a tropic factor in nervous system development and maintenance, peripheral administration of this cytokine also causes severe anorexia and weight loss. The neural mechanism(s) mediating the loss of appetite is not known. As hypothalamic neuropeptide Y (NPY) is a potent orexigenic signal, we tested the hypothesis that CNTF may adversely affect NPYergic signaling in the hypothalamus. Intraperitoneal administration of CNTF (250 µg/kg) daily for 4 days significantly suppressed 24-h food intake in a time-dependent manner and decreased body weight. The loss in body weight was similar to that which occurred in pair-fed (PF) rats. As expected, hypothalamic NPY gene expression, determined by measurement of steady state prepro-NPY messenger RNA by ribonuclease protection assay, significantly increased in PF rats in response to energy imbalance. However, despite a similar loss in body weight, there was no increase in NPY gene expression in CNTF-treated rats. Daily administration of CNTF intracerebroventricularly (0.5 or 5.0 µg/rat) also produced anorexia and body weight loss. In this experiment, negative energy balance produced by both PF and food deprivation augmented hypothalamic NPY gene expression. However, despite reduced intake and loss of body weight, no similar increment in hypothalamic NPY gene expression was observed in CNTF-treated rats. In fact, in rats treated with higher doses of CNTF (5.0 µg/rat), NPY gene expression was reduced below the levels seen in control, freely fed rats. Furthermore, CNTF treatment also markedly decreased NPY-induced feeding. These results suggested that anorexia in CNTF-treated rats may be due to a deficit in NPY supply and possibly in the release and suppression of NPY-induced feeding. The possibility that CNTF-induced anorexia may be caused by increased leptin was next examined. Daily intracerebroventricular injections of leptin (7 µg/rat) decreased food intake, body weight, and hypothalamic NPY gene expression in a manner similar to that seen after CNTF treatment. Leptin administration also suppressed NPY-induced feeding. However, peripheral and central CNTF injections markedly decreased leptin messenger RNA in lipocytes, indicating a deficiency of leptin in these rats; thus, leptin was unlikely to be involved in appetite suppression. Thus, these results show that a two-pronged central action of CNTF, causing diminution in both NPY availability and the NPY-induced feeding response, may underlie the severe anorexia. Further, unlike other members of the cytokine family, suppression of NPYergic signaling in the hypothalamus by CNTF does not involve up-regulation of leptin, but may involve a direct action on hypothalamic NPY neurons or on neural circuits that regulate NPY signaling in the hypothalamus.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
CILIARY neurotropic factor (CNTF) has been studied extensively as a tropic factor in nervous system development and maintenance and has been shown to promote the survival of a variety of neuronal cell types in vitro and in vivo (1, 2, 3, 4, 5, 6). CNTF is a member of the cytokine family structurally related to leukemia inhibitory factor, interleukin-6, and other cytokines (7). It exerts a spectrum of biological effects similar to those attributed to other proinflammatory cytokines, such as induction of fever (8), hepatic acute phase protein responses (9), anorexia, weight loss, and cachexia (9, 10, 11, 12). Anorexia accompanied by weight loss is a common neurological manifestation in acute and chronic diseases and after injury and damage in the central nervous system. A large body of evidence indicates that anorexia produced by a number of cytokines is mediated by central neural mechanisms, and the primary targets of action may reside in the hypothalamus (13, 14), a site previously implicated in the regulation of energy homeostasis. Stimulation of appetite, the basic urge to eat, is one of the hypothalamic mechanisms employed to replenish weight loss and sustain the body’s energy balance. Recently, an interconnected neural pathway has been identified in the hypothalamus that generates and transmits stimuli for stimulation of appetite (15, 16). Within this pathway, neuropeptide Y (NPY) is an important messenger molecule for the relay of appetite-stimulating signals (15, 16). Neuroanatomical mapping studies showed that NPY-producing neurons located in the brain stem and in the arcuate nucleus (ARC) of the basal hypothalamus innervate various hypothalamic sites including the paraventricular nucleus (PVN) (17, 18). Experimental evidence indicates that NPY synthesis in the ARC and release in the PVN are up-regulated when feeding occurs either normally during the night or after fasting and dietary restriction (19, 20, 21, 22, 23). Blockade of NPY action by either immunoneutralization (23) or the administration of an NPY Y1 receptor antagonist (24) resulted in inhibition of feeding under these physiological and semiphysiological paradigms. These observations imply that NPY released from the nerve terminals of axons emanating mainly from neurons in the ARC is a physiological appetite transducer (16). On the basis of these findings, we reasoned that the hypothalamic NPY system may be one of the neural pathways disrupted in CNTF-induced anorexia. Therefore, in the first series of experiments we evaluated components of the hypothalamic NPY signaling system in association with CNTF-induced anorexia and weight loss (10, 11, 12). Leptin produced by lipocytes is another cytokine shown to inhibit food intake and to rapidly induce a loss in body weight in genetically obese and normal rodents (25, 26, 27, 28). As there is evidence to suggest that leptin down-regulates the hypothalamic NPY system in conjunction with inhibition of food intake (29), we have compared the action and efficacy of leptin with those of CNTF in modifying food consumption and the hypothalamic NPYergic system.


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Adult male Sprague-Dawley (225–250 g), purchased from Zivic Miller Laboratories, (Zelienople, PA) were housed individually in a light- and temperature-controlled room (lights on, 0500–1900 h; 22 C); food and water were available ad libitum unless otherwise stated. During the experiments some groups of rats were either food deprived (FD) or pair fed (PF; to CNTF-treated rats), as described below. These studies were approved by the University of Florida institutional animal care and use committee.

Exp 1
In this experiment the effects of recombinant rat CNTF (Amgen, Boulder, CO) administration on hypothalamic prepro-NPY messenger RNA (mRNA) levels were evaluated. The selection of CNTF dose was based on previous studies (8, 9, 10, 11, 12). Two groups of freely fed (FF) rats received either saline alone or CNTF in saline (250 µg/kg) ip at 1000 h daily for 4 days. Another group of PF rats that received the average of amount of food consumed by CNTF-treated rats was injected with saline ip daily at 1000 h. Food intake and body weight were monitored daily. Twenty-four hours after the fourth injection, rats were killed by decapitation. The brains were rapidly removed, and the medial basal hypothalamus (MBH) was dissected, frozen on dry ice, and stored at -80 C for NPY mRNA analysis with a ribonuclease (RNase) protection assay (30).

Exp 2
In this experiment the effects of intracerebroventricular (icv) administration of CNTF on hypothalamic NPY gene expression were examined. Rats were implanted with a permanent cannula in the lateral cerebroventricle under ketamine/xylazine (30) anesthesia. After a recovery period of 10 days, rats were randomly divided into four groups, designated FF, FD, PF, and CNTF. FF, FD, and PF rats were injected icv daily at 1000 h with phosphate buffer (PBS; 3 µl), and the CNTF group received CNTF (0.5 µg in 3 µl PBS) for 4 days. Body weight and food intake were monitored. Rats were killed 24 h after the fourth icv injection. The MBH was processed for NPY mRNA analysis as described above.

Two additional groups of FF rats implanted with permanent lateral ventricle cannulas were injected icv with PBS or CNTF (5 µg/3 µl PBS) daily for 4 days as described above. These rats were killed 24 h after the last icv injection, and the MBH was processed for NPY mRNA.

Exp 3
To investigate the effects of CNTF on NPY-induced food intake, FF rats were preimplanted with permanent lateral ventricle cannulas as described above. CNTF (0.5 or 5 µg/rat) or PBS (control) was injected icv daily at 0900 h. On days 1 and 4 of treatment these rats received NPY (1 nmol/3 µl PBS·rat) icv 1 h after the injection of CNTF or PBS, and 2-h food intake was measured. This dose of NPY has been shown to produce a near-maximal increase in food intake in satiated rats (31).

Exp 4
To analyze the effects of CNTF on leptin mRNA levels in lipocytes, rats were injected ip with either PBS or CNTF (250 µg/kg) and killed 5 h later. Epididymal fat was collected and rapidly frozen at -80 C for subsequent analysis of leptin mRNA. In the second series of experiments, groups of FF, FD, and PF rats received daily injections of PBS, and the fourth group received CNTF (250 µg/kg) ip for 4 days. Rats were killed 24 h after the fourth injection. Epididymal fat from these rats was processed for leptin mRNA analysis.

In the third series of experiments, the effects of daily icv injections of CNTF on leptin mRNA were analyzed. As described for Exp 2, groups of rats received daily either PBS (3 µl/rat) or CNTF (5 µg in 3 µl PBS/rat, icv) for 4 days. Epididymal fat samples were collected 24 h after the fourth injection for leptin mRNA analysis.

Exp 5
The effects of daily icv recombinant leptin (rMuleptin, Amgen, Thousand Oaks, CA) injection on hypothalamic prepro-NPY mRNA levels were examined. Rats with permanent lateral ventricle cannulas received daily either PBS (7 µl) or leptin (7 µg/7 µl PBS) for 4 days and were killed 24 h after the last injection. The MBH was dissected out for analysis of NPY mRNA.

Exp 6
The effects of daily icv injection of leptin (3.5 or 7 µg) or PBS on NPY-induced food intake were studied. The experimental design was the same as that described for Exp 3, except that instead of CNTF, leptin was injected daily. On days 1 and 4, leptin injection was followed 1 h later by the injection of 1 nmol NPY icv. Two-hour food intake after the NPY injection was monitored.

RNase protection assay for NPY and leptin
The leptin probe was constructed using the rat leptin plasmid in pGEM-T vector (Promega), which was ligated with a 318-bp complementary DNA (cDNA) fragment obtained from reverse transcription-PCR using a 19-mer upper primer (5'-CCC ATT CTG AGT TTG TCC A-3'; 5' position 262) and an 18-mer lower primer (5'-GCA TTC AGG GCT AAG GTC-3'; 3' position 561) based on rat leptin cDNA (GenBank accession no. D45862) (32). Total RNA isolated from rat fat tissue with RNA STAT-60 (Tel-Test, Friendswood, TX) was used for reverse transcription. The plasmid DNA was linearized by digestion with Spe I, and an antisense probe of 364 bp was produced by in vitro transcription using T7 RNA polymerase. Total RNA (5 µg) was hybridized overnight at 45 C with 32P-labeled leptin antisense probe and 152-bp ß-actin antisense riboprobe (cDNA template purchased from Ambion, Austin, TX). After hybridization, RNase A/T1 digestion was performed for 1 h at 37 C. Protected hybrids were isolated by ethanol precipitation and separated on a 6% polyacrylamide denaturing sequencing gel. The dried gel was quantitated by a Molecular Dynamics PhosphorImager (Sunnyvale, CA). The expected protected leptin fragments and ß-actin fragments were 318 and 126 bp, respectively. Leptin mRNA levels were standardized relative to the ß-actin value to minimize gel loading variations.

Complementary DNA to NPY was obtained from Dr. S. L. Sabol (NIH, Bethesda, MD). The standard RNase protection assay protocol, followed for hypothalamic prepro-NPY mRNA was described previously (30). The hybrid signal was analyzed by PhosphorImager, and the NPY mRNA values were normalized to cyclophilin mRNA.

Statistical analysis
Data are presented as the mean ± SE. Leptin and prepro-NPY mRNA in various experiments were measured in several RNase protection assays at different times. Although the leptin and NPY mRNA values were calculated relative to ß-actin and cyclophilin in each sample, due to variations in sensitivities of the ß-actin and cyclophilin probes in different assays, the mRNA values for controls varied between experiments. To make comparisons between experiments, the mRNA values for controls were assigned an arbitrary value of 1.0, and the levels in experimental groups were calculated relative to this. Data were statistically analyzed by one-way ANOVA followed by Tukey’s multiple comparison test post-hoc. Daily body weight changes are presented as a percentage of the initial body weight. Body weight changes and food intake within a group were analyzed by repeated measures ANOVA and Dunnett’s multiple test to compare with initial values. Comparisons between two treatment groups at any single time point or dose level were made by the unpaired t test for two groups and by one-way ANOVA followed by Tukey’s multiple comparison test post-hoc for more than two groups. The level of significance was set at P < 0.05.


    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Effects of ip CNTF on food intake, body weight, and hypothalamic NPY gene expression (Fig. 1Go)
Daily ip injections of CNTF (Fig. 1AGo) significantly reduced 24-h food intake in a time-related fashion, with the lowest intake observed after the fourth CNTF injection. Whereas FF rats injected with saline maintained their body weight, the anorexia in CNTF-treated rats resulted in a significant loss in body weight beginning on day 2 (P < 0.05) and reaching the lowest level on day 4 (Fig. 1BGo). PF rats that were fed amounts equivalent to CNTF rats showed a pattern of body weight loss similar to that of the CNTF-treated rats. The effects of these treatments on hypothalamic NPY gene expression are shown in Fig. 1CGo. In association with reduced intake and loss in body weight, the steady state prepro-NPY mRNA levels in the hypothalami of PF rats increased significantly compared with that in FF control rats (P < 0.05). However, in CNTF-treated rats, despite a similar loss in body weight, there was no increase in NPY gene expression; prepro-NPY mRNA levels in the hypothalami of CNTF-treated rats were similar to those in saline-injected FF rats.



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Figure 1. The effects of daily CNTF (250 µg, ip) or saline (Cont) injections on 24-h food intake (A), body weight change (B), and hypothalamic NPY mRNA (C). *, P < 0.05 vs. initial value of the treatment. Daily food intake in CNTF and PF rats averaged 20.2 ± 1.6 and 20.5 ± 1.2 g, respectively, for the 4-day experimental period. a and b, Statistically significant differences between treatment groups at the same time point (P < 0.05; n = 6 rats/group).

 
Effects of icv CNTF on food intake, body weight, and hypothalamic NPY gene expression (Figs. 2Go and 3Go)
Daily icv injection of 0.5 µg CNTF (Fig. 2Go) produced a significant decrease in food intake and loss of body weight compared with those in FF rats (P < 0.05). A decrease in food intake to about 50% of the control range was seen between days 2 and 4 in CNTF-treated rats (Fig. 2AGo); the loss in body weight in these rats was comparable to that observed in PF rats, but was significantly less than that seen in FD rats (P < 0.05; Fig. 2BGo).



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Figure 2. A, Effects of daily icv injections (arrowheads) of PBS (control) or CNTF (0.5 µg/rat) on food intake. B, Patterns of body weight loss in control (cont), PF, and FD rats injected daily with PBS or CNTF. C, Hypothalamic NPY mRNA levels in these four groups of rats killed 24 h after the fourth injection (n = 7 rats/group). The average 4-day daily food intakes in CNTF and PF rats were 6.6 ± 0.7 and 6.8 ± 0.4, respectively. The letters a, b, and c represent statistically significant differences between treatment groups at the same time point (P < 0.05).

 


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Figure 3. Effect of four daily icv injection of PBS or CNTF (5 µg/rat) on food intake (A), body weight change (B), and hypothalamic NPY mRNA levels 24 h after the fourth injection. *, P < 0.05 vs. controls (n = 7 rats/group).

 
As shown in Fig. 2CGo, hypothalamic NPY gene expression was augmented in PF rats compared with that in FF control rats (P < 0.05) and was even further increased in FD rats (P < 0.05 vs. PF rats). Despite the anorexia and loss in body weight equivalent to those observed in PF rats, NPY gene expression failed to increase in CNTF-treated rats. In fact, prepro-NPY mRNA levels in these rats were similar to those in FF rats (Fig. 2CGo).

Further, as shown in Fig. 3Go, a 10-fold higher dose of icv CNTF (5 µg/rat) also reduced food intake, and the loss in body weight was slightly greater (15% of initial body weight) than that in response to 0.5 µg CNTF (12% of initial body weight; Fig. 2Go). However, in contrast to the lack of effect of 0.5 µg CNTF treatment (Fig. 2CGo) on NPY mRNA levels, the higher dose of CNTF significantly suppressed the steady state levels of prepro-NPY mRNA below those in FF control rats (P < 0.05; Fig. 3CGo).

Effects of icv CNTF on NPY-induced feeding (Fig. 4Go)
To investigate the effects of daily 0.5- and 5-µg CNTF treatment on NPY-induced feeding, we analyzed 2-h food intake in response to NPY (1 nmol) on days 1 and 4 of treatment. As expected on both days 1 and 4, NPY evoked robust feeding responses in control rats injected with PBS daily. Rats injected with CNTF alone (0.5 or 5 µg) ate a little, but this response was not different from that in the controls or that seen normally in untreated rats (31). In rats pretreated with CNTF, decreases in NPY-induced food intake were observed on day 1; however, the decrease was statistically significant only in the group receiving 5 µg CNTF (P < 0.05). On day 4, NPY-induced food intake was suppressed in rats receiving either dose of CNTF, and the magnitude of suppression was related to the dose of CNTF (P < 0.05).



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Figure 4. The effects of icv CNTF on NPY-induced food intake on day 1 (A) and day 4 (B) of treatment. CNTF (0, 0.5, and 5 µg) was injected daily. On days 1 and 4, NPY (1 nmol) was injected icv 1 h after CNTF, and 2-h food intake was monitored. Different letters represent statistically significant differences from other treatment groups (n = 7 rats/group).

 
Effects of CNTF on leptin gene expression in lipocytes
The data presented above show that CNTF administration produces anorexia and a marked loss in body weight. As cytokines have been shown to up-regulate leptin in fat cells (33, 34), it appeared likely that the anorectic effects of CNTF may be mediated through leptin action in the hypothalamus. Therefore, we studied the effects of CNTF treatment on leptin mRNA levels in lipocytes.

Effects of ip CNTF. As shown in Fig. 5AGo, one injection of CNTF failed to alter leptin mRNA levels in the lipocytes when analyzed 5 h later. However, daily injections of CNTF produced a profound suppression of leptin mRNA in lipocytes in association with a loss in body weight. In accord with the results of Exp 1 presented in Fig. 1Go, in this experiment FF rats increased body weight by 15% during the 4-day treatment with PBS compared with their initial body weight, whereas CNTF-treated rats lost 5% of their body weight, equivalent to that seen in PF rats, but significantly less than that in FD rats (22%; data not shown). As shown in Fig. 5BGo, this CNTF treatment decreased lipocyte leptin mRNA (P < 0.05 vs. FF control rats); the reduction in leptin gene expression was similar to those in FD and PF rats.



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Figure 5. A, Relative levels of leptin mRNA in lipocytes of FF rats 5 h after an ip injection of PBS or CNTF (250 µg/kg BW). B, Relative levels of leptin mRNA in lipocyte of rats 24 h after four daily ip injections of PBS or CNTF (250 µg/kg BW). Leptin mRNA levels in FD and PF rats given daily PBS injections are included for comparison. Different letters represent statistically significant differences from other treatment groups (n = 7 rats/group).

 
Effects of icv CNTF. As noted in the earlier experiments and Fig. 3Go, daily icv injections of 5 µg CNTF for 4 days reduced body weight by 15% (P < 0.05; data not shown). This loss in body weight was accompanied by a significant decrease in leptin mRNA in lipocytes of CNTF-treated compared with control PBS-injected rats (Fig. 6Go).



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Figure 6. Relative levels of lipocyte leptin mRNA 1 day after four daily icv injections of PBS or CNTF (5 µg). *, P < 0.05 vs. PBS (control) group (n = 7 rats/group).

 
Effects of icv leptin injection on food intake and NPY gene expression in the hypothalamus
Daily injections of leptin (7 µg/rat) decreased body weight by 14% compared with that in control rats receiving PBS alone. The 24-h food intake monitored between days 2 and 3 showed that leptin-treated rats ate 7.8 ± 1.6 g, significantly less than the PBS-treated controls (15 ± 0.7 g; P < 0.05). Analysis of hypothalamic prepro-NPY mRNA showed that leptin treatment for 4 days significantly suppressed NPY gene expression compared with that in control PBS-treated rats (P < 0.05; Fig. 7Go).



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Figure 7. Relative levels of NPY mRNA in the MBH of rats 24 h after the fourth daily PBS (n = 5 rats) or leptin (7 µg/rat; n = 6 rats) icv injection. *, P < 0.05 vs. PBS (control) group.

 
Effects of icv leptin injection on NPY-induced feeding
As shown in Fig. 8Go, icv NPY (1 nmol) induced a robust feeding response on both days 1 and 4 in control rats injected daily with PBS. Injection of leptin (3.5 or 7 µg) 1 h before NPY on day 1 reduced food intake, but the response was not statistically significant. However, on day 4 of leptin treatment (3.5 or 7 µg), NPY-induced feeding was significantly reduced; feeding was reduced by 88% in 3.5 µg leptin-treated rats and by 90% in rats treated with 7 µg leptin (P < 0.05).



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Figure 8. The effects of daily PBS or leptin (7 µg/rat) injections on the 2-h NPY-induced food intake on days 1 and day 4. Different letters represent statistically significant differences from other treatment groups (n = 6 rats/group).

 

    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
In this study we investigated the effects of peripheral and central administration of the cytokine CNTF on food intake and body weight, and correlated these with the impact on hypothalamic NPY, the endogenous orexigenic signaling modality (15, 16, 19). Additionally, the CNTF-induced responses were compared with those produced by the endogenous cytokine-like hormone, leptin, produced by lipocytes. Several new findings have emerged from these investigations. In agreement with the previous reports (11, 14), we observed that peripheral administration of CNTF produced anorexia. We have extended these results to show that central administration of CNTF (in much smaller doses) is just as effective as peripheral injections in reducing food intake. Consequently, it is likely that the targets for CNTF action involved in producing anorexia may reside within the brain (15, 16). Our results also showed that the anorectic effects of central CNTF persist for a considerable period of time, because CNTF injection 9 h before the normal nighttime feeding consistently reduced intake for 24 h, and daily injections sustained anorexia for at least 5 days; compared with the effects of other cytokines, such as interleukin-1 (15, 16, 35), the anorectic effects of CNTF apparently are more profound and longer lasting.

NPY is a potent endogenous orexigenic signal (15, 16). Our findings show that CNTF may act on hypothalamic NPYergic signaling in three ways to produce anorexia. First, it is likely that the supply of NPY for release at the appetite-stimulating targets located in the PVN and surrounding sites (15, 16, 19, 20) may be drastically curtailed. This is evidenced by our findings that CNTF inhibited the basal steady state levels of prepro-NPY mRNA in the ARC by high doses of CNTF and suppressed the augmentation in NPY gene expression normally produced by restricted feeding or fasting. In fact, the degree of suppression of NPY gene expression was positively correlated with the dose of CNTF and the reduction in food intake. Second, CNTF injected ip or icv in small doses produced anorexia, with no change in hypothalamic prepro-NPY mRNA compared with that in freely fed controls. Seemingly, the supply of NPY for release may not be disrupted under these conditions. Although the current studies do not provide any supportive evidence, it remains possible that the amount of NPY release responsible for normal food intake may be attenuated in CNTF-treated rats, thereby leading to decreased food intake. Indeed, immunoneutralization of endogenously released NPY has been shown to suppress food intake and induce a loss in body weight in a fashion similar to that seen in CNTF-treated rats (23). Third, CNTF suppressed the feeding induced by NPY. Suppression of intake in response to NPY was evident on day 1 of CNTF treatment, and the magnitude of suppression was highest on day 4. These findings lead us to suspect that an additional site of CNTF action in counteracting NPY-induced appetite most likely resides in the PVN and surrounding areas, where NPY microinjection readily stimulated feeding (36). Overall, these revelations strongly imply that CNTF-induced body weight loss is the consequence of anorexia caused by a reduction in the NPY supply for release, possibly release itself, and suppression of NPY action at hypothalamic target sites engaged in enhancing appetite.

These conclusions are supported by another line of evidence. It is known that energy depletion produced by either restricted availability or complete absence of food up-regulates NPY synthesis and release (19, 20, 21), which, in turn, is apparently responsible for enhancing appetite to replenish the loss in energy (19, 23). These energy-depleted rats feed avidly when allowed access to food and quickly regain the loss in body weight. In our study, food and water were available ad libitum to CNTF-treated rats, and despite the disruption in energy homeostasis similar to that in PF and FD rats, as indicated by weight loss, these rats apparently made little attempt to replenish the energy loss. Also, we observed that daily icv CNTF administration to fasted rats prevented the fasting-induced up-regulation of NPY expression in the hypothalamus (37). Taken together, these findings clearly show that CNTF is a powerful appetite suppressant and curtailment of NPYergic signaling in the hypothalamus may in part underlie the anorexia. The precise mechanism by which CNTF reduces both NPY gene expression and action, presumably at two different sites in the hypothalamus, remains to be ascertained. Although the topography of the neuronal network producing CNTF in the hypothalamus is currently unknown, both CNTF receptor-{alpha} immunoreactivity and CNTF receptor-{alpha} mRNA are expressed in various hypothalamic sites, including the PVN (38, 39). Thus, the potential to influence the NPYergic system by CNTF exists in the adult rat hypothalamus.

Concurrent with the loss in body weight due to anorexia, we observed that the steady state leptin mRNA levels were reduced in the lipocytes of CNTF-treated rats. These findings complement several reports showing that leptin gene expression in fat cells is directly related to body mass index (reviewed in Refs. 28 and 40). However, we suspect that the act of eating itself may serve as a trigger to up-regulate leptin gene expression, and the anorexia in CNTF-treated rats may be responsible for diminished leptin gene expression. This inference is in harmony with a number of observations that the daily increases in leptin gene expression in lipocytes and the circulating levels of leptin closely follow the normal feeding pattern during the night (28, 39).

Our consistent findings of a reduction in leptin gene expression in lipocytes by CNTF administered either systemically or centrally differ from those reported for several other peripherally administered cytokines, such as tumor necrosis factor, interleukin-1, and leukemia inhibitor factors as well as the endotoxin, lipopolysaccharide (33, 34). These cytokines were shown to augment leptin secretion and lipocyte leptin gene expression, both acutely and on a long term basis in fasted mice and free-fed hamsters. Based on these results, it was suggested that the cytokine-induced increase in leptin may be responsible for the anorexia seen in these rodents (34, 35). In contrast, our results show that ip CNTF failed to change leptin mRNA acutely in satiated rats, an observation similar to that reported in fasted mice (33). In addition, severe anorexia in conjunction with reduced leptin mRNA was observed in rats receiving CNTF either systemically or centrally. Consequently, our findings demonstrate that anorexia produced by CNTF involves a direct central action mediated through diminution in NPYergic signaling and not indirectly via up-regulation of leptin.

Additionally, a comparison of our results of CNTF and leptin studies clearly show that these two members of the cytokine family engage a similar central modality, i.e. suppression of NPY synthesis and action in the hypothalamus, to cause anorexia. Evidently, these cytokines suppress the basal, as shown in this study, and fasting-induced NPY gene expression (37), and in this respect, although additional studies are needed, CNTF appears slightly more effective than leptin on a molar basis. However, in each of these two cases anorexia at the doses tested was quite severe, and injection of NPY only marginally stimulated feeding in these rats. Therefore, we are tempted to propose that CNTF and leptin may involve similar intracellular JAK-STAT signal transduction modalities in target cells in the hypothalamus to produce anorexia leading to body weight loss (41, 42, 43).

In summary, the results of these studies show that CNTF is a potent anorectic cytokine. One of the central CNTF actions in producing anorexia and body weight loss involves suppression of NPYergic signaling in the hypothalamus in a manner similar to that effected by leptin. Identification of the NPY neuronal system as one of the target neural pathways for CNTF action is important for further understanding the cellular and molecular events underlying the anorexia accompanying infection and brain injury.


    Acknowledgments
 
We thank Ms. Sally McDonell for secretarial assistance, and Amgen (Boulder, CO) and Amgen (Thousand Oaks, CA) for the supply of recombinant rat CNTF and recombinant leptin, respectively.


    Footnotes
 
1 Presented in part at the 26th Annual Meeting of the Society for Neuroscience, Washington DC, 1996. This work was supported by NIH Grants DK-37273 (to S.P.K.), NS-32727 (to P.S.K.), and GM-40586 and GM-53252 (to L.M.). Back

Received July 9, 1997.


    References
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Ip Y, Yancopoulus GD 1992 Ciliary neurotropic factor and its receptor complex. Progr Growth Factors 4:139–155
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