Main

There is some evidence that leptin's actions in the brain may involve CRF or a related neuropeptide. Expression of c-fos (a measure of neuronal activation) increases in the paraventricular nucleus of the hypothalamus in response to intracerebroventricular (icv) injection of leptin5. Leptin also induces expression of CRF mRNA in the paraventricular nucleus6, and leptin receptors are present on CRF-containing neurons7. Although CRF is best known for its involvement in stress responses, it also potently inhibits food intake and mediates several actions of cytokines in the brain8,9. The regulation of energy balance probably involves CRF because it inhibits food intake and stimulates energy expenditure and sympathetic nervous system activity9. In addition, the CRF-like neuropeptide urocortin, which also acts at CRF receptors, is a more potent suppressor of appetite than CRF10.

The experiments described here test the hypothesis that CRF or urocortin mediates the actions of leptin on food intake. The CRF-receptor antagonist D-Phe CRF12-41 has similar affinity for type I and type II CRF receptors11. Intracerebroventricular administration of leptin (Insight Biotechnology, UK; 4 μg per rat at 18:00 h) to individually housed, free-feeding male Sprague-Dawley rats (250–300 g) inhibited food intake, measured 1 h prior to the injection and again 14 h afterward, by 50% ( p<0.001) compared to vehicle-treated animals (Fig. 1a). Administration of the CRF-receptor antagonist D-Phe CRF12-41 (Bachem, UK; 1–5 μg per rat) alone did not significantly affect food intake. Co-administration of the lowest dose (1 μg per rat) of D-Phe CRF12-41 with leptin had no significant effect on food intake, but higher doses (2.5 and 5 μg per rat) attenuated effects of leptin on food intake by 58% (p<0.001), such that food intake of these animals was restored to 80% of the intake of vehicle-treated controls. Leptin injection also caused marked loss of body weight (12 ± 3 g) over 14 h, which was attenuated by cotreatment with all doses of CRF receptor antagonist used (Fig. 1b).

Figure 1: Leptin-induced reduction in food intake and body weight is attenuated by D-Phe CRF.
figure 1

(a) Injection of leptin (4 μg, icv; n = 10) reduced food intake by 50% over 14 h (p<0.001) compared with vehicle-treated animals. Co-injection of D-Phe CRF12-41 (2.5 or 5 μg/rat , icv; n = 6,7) significantly attenuated this effect (p<0.001) to 80% of control food intake. (b) Injection of leptin (4 μg, icv; n = 10) reversed the gain in body weight seen in vehicle-treated rats over 14 h (p<0.001). Co-injection of D-Phe CRF12-41 (1, 2.5 or 5 μg/rat, icv; n = 6,7) significantly attenuated this effect in a dose-dependent manner (p<0.001). Animals were housed under a 12 h light-dark cycle (08:00 to 20:00) and had free access to food (pelleted rat chow, Beekay International, UK) and water. All drugs were made up in sterile water for injection. Data were analyzed using ANOVA followed by Newman-Keuls post test.

Full size image

The antagonist D-Phe CRF12-41 has been used widely at doses similar to those used here to inhibit actions of CRF12. This antagonist (5 μg, icv; n = 5) failed to influence food intake or body weight gain in vehicle-treated rats (n = 10) and did not influence the hypophagia or reduced body-weight gain induced by icv injection of prostaglandin E2 (500 ng per rat, icv; n = 6), suggesting that its effects are specific to the CRF receptor. (Food intake: vehicle, 28 ± 1 g; D-Phe CRF12-41, 28 ± 1 g; PGE2, 20 ± 1 g; PGE2 and D-Phe CRF12-41, 21 ± 1 g. Body weight gain: vehicle, 19 ± 1 g; D-Phe CRF12-41, 18 ± 2 g; PGE2, 11 ± 1 g; PGE2 and D-Phe CRF12-41, 11 ± 1 g).

Because effects of leptin were attenuated but not abolished by the CRF receptor antagonist, CRF or urocortin may act in conjunction with other mediators, such as melanocortin, which have been implicated in the regulation of appetite13,14. Nevertheless, these results indicate that CRF or another CRF-like neuropeptide, such as urocortin, is important for the actions of leptin on food intake.