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Brain-derived neurotrophic factor (BDNF), corticotropin-releasing factor (CRF), and hypothalamic neuronal histamine are anorexigenic substances within the hypothalamus. This study examined the interactions among BDNF, CRF, and histamine during the regulation of feeding behavior in rodents. Food intake was measured after treatment with BDNF, α-fluoromethyl histidine (FMH; a specific suicide inhibitor of histidine decarboxylase that depletes hypothalamic neuronal histamine), or CRF antagonist. We measured food intake in wild-type mice and mice with targeted disruption of the histamine H1 receptor (H1KO mice) after central BDNF infusion. Furthermore, we investigated CRF content and histamine turnover in the hypothalamus after BDNF treatment, and conversely, BDNF content in the hypothalamus after histamine treatment. We used immunohistochemical staining for histamine H1 receptors (H1-R) in BDNF neurons. BDNF-induced feeding suppression was partially attenuated in rats pre-treated with FMH or a CRF antagonist, and in H1KO mice. BDNF treatment increased CRF content and histamine turnover in the hypothalamus. Histamine increased BDNF content in the hypothalamus. Immunohistochemical analysis revealed that H1-Rs were expressed on BDNF neurons in the ventromedial nucleus of the hypothalamus. These results indicate that CRF and hypothalamic neuronal histamine mediate the suppressive effects of BDNF on feeding behavior and body weight.
Brain-derived neurotrophic factor (BDNF) plays an important role in the central regulation of energy metabolism. Intracerebroventricular (i.v.t.) BDNF administration results in decreased food intake and body weight (Pelleymounter et al. 1995). In animals with conditional BDNF deletion, mutation, or in BDNF (+/−) heterozygous mice, hyperphagia and obesity are accompanied by significantly reduced BDNF gene expression in the hypothalamus, including the ventromedial nuclei (VMH; Lyons et al. 1999; Rios et al. 2001). Further, exogenous BDNF reverses the phenotype of these animals, suggesting that endogenous BDNF reduces feeding and body weight gain (Kernie et al. 2000). Tropomyosin-related kinase receptor type B (TrkB) is a high-affinity BDNF receptor that is widely expressed in the adult central nervous system, notably in several nuclei involved in energy balance, including the VMH (Yan et al. 1997). Mutant mice that express TrkB in the brain at approximately one-quarter the normal level exhibit hyperphagia and excessive weight gain on high-fat diets (Xu et al. 2003).
Neuronal histamine is one of anorexigenic substances produced mainly in the tuberomammillary nucleus (TMN) of the posterior hypothalamus, which has diffuse projections throughout the brain, including projections to the VMH and paraventricular nucleus (PVN), where the histamine H1-receptors (H1-R) are localized (Fukagawa et al. 1989). In addition, corticotropin-releasing factor (CRF), which is mainly synthesized in the PVN, directly activates histamine neurons (Gotoh et al. 2005). Local BDNF infusion into the PVN and VMH markedly decreased food intake and body weight (Wang et al. 2007a, b). These effects of locally infused BDNF were attenuated by local infusion of α-helical CRF into the PVN, suggesting that the PVN is targeted by the BDNF-evoked CRF pathway (Toriya et al. 2010). From these behavioral and neuroanatomical studies, we hypothesized that BDNF, CRF, and hypothalamic neuronal histamine constitute a neuronal network within the hypothalamus that regulates food intake.
We examined this hypothesis from several different perspectives. First, we investigated whether the anorectic effect of BDNF was modulated in rats pre-treated with FMH, a suicide inhibitor of histidine decarboxylase (HDC), or α-helical CRF9-41 (α-helical CRF), a competitive CRF receptor antagonist that blocks both CRF type 1 receptors (CRF1-R) and type 2 receptors (CRF2-R). We also examined the effect of BDNF treatment on food intake in H1-R knockout (H1KO) mice, which lack the H1-R for neuronal histamine. Then, we determined whether the BDNF treatment affected CRF content and histamine turnover in the hypothalamus. Finally, we investigated whether the central infusion of histamine activated BDNF neurons and whether the blockade of endogenous BDNF biological activity modulated the suppressive effect of histamine on food intake.
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We investigated that the relationship among BDNF, CRF, and neuronal histamine on feeding behavior. Our results showed that the FMH-induced depletion of neuronal histamine partially weakened the reduction of food intake by BDNF. We suggest that HDC activity is important to regulate energy metabolism because our previous studies revealed the decrease of HDC activity in Zucker fatty (fa/fa) rats which have a missense mutation in the leptin receptor gene (Yoshimatsu et al. 1992; Sakata and Yoshimatsu 1997). In this study, we used FMH to investigate whether BDNF-induced anorexia is related to the activity of HDC. In addition, this reduced BDNF-induced response was also mimicked in H1KO mice, indicating that neuronal histamine and H1-R partially mediate the BDNF-induced hypophagia. We demonstrated that BDNF treatment elevated t-MH levels in the PVN, VMH, and TMN of hypothalamus. However, TrkB was not localized in histamine neurons, indicating that BDNF might stimulate histamine neurons indirectly. In addition, BDNF also increased CRF content in the PVN, which is compatible with the observation that ectopic expression of hypothalamic BDNF increases CRF mRNA expression (Jeanneteau et al. 2012). Therefore, we speculate that BDNF activates histamine neurons through CRF neurons.
First, we demonstrated that central BDNF infusion increased the CRF content in the PVN of the hypothalamus. The central administration of α-helical CRF prevented the reduction in food intake induced by BDNF treatment, suggesting that CRF mediates the anorectic effect of BDNF in the hypothalamus. Moreover, it has been reported that CRF suppresses feeding when it is injected into the PVN, but not the LH, of rats (Krahn et al. 1988). There is the report that few VMH neurons project to the PVN, suggesting that it does not make much sense to propose that BDNF-expressing neurons in the VMH project to the PVN to affect CRF-expressing neurons (Canteras et al. 2004; Lin and York 2004). However, we identified that microinfusion of BDNF into the PVN also increased t-MH contents in the PVN, VMH, and TMN in the hypothalamus. Considering other studies that VMH neurons project to the PVN using retrograde tracer fluorogold and that VMH-selective knockdown of BDNF induces hyperphagia and obesity, BDNF neurons in the VMH may innervate and stimulate CRF in the PVN (Buijs et al. 2001; Unger et al. 2007).
Our previous study indicated that the CRF activated histamine neuron in the TMN through CRF1-R (Gotoh et al. 2005). Furthermore, we hypothesized that BDNF regulates neuronal histamine through CRF neurons. Our results demonstrated that BDNF induces the reduction of feeding by activating histamine neurons in the TMN through CRF neurons in the PVN, supporting our hypothesis. A previous study indicated that the suppressive effect of the CRF2-R antagonist on BDNF-induced hypophagia was less than that of α-helical CRF, the dual antagonist of both CRF1-R and CRF2-R, suggesting that the CRF1-R pathway can cooperate with the CRF2-R pathway to fully mediate the BDNF action to regulate feeding (Toriya et al. 2010).
Finally, we examined whether neuronal histamine activated BDNF neurons because neuronal histamine stimulates CRF neurons through H1-R (Kjaer et al. 1998). We showed that histamine also elevates BDNF expression in the VMH, suggesting that it is possible that some BDNF neurons may be stimulated with activated histamine neurons by CRF.
It seems reasonable to speculate that BDNF administered peripherally enters the hypothalamus by crossing the blood–brain barrier (BBB) and regulates energy metabolism and appetite because it has been reported that blood BDNF stored in platelets crosses the BBB and reaches the central nervous system (Pan et al. 1998). Furthermore, it is always possible that BDNF infused centrally might leak into the periphery via the venous system. This study showed that a single central administration of BDNF at a dose too low to have an effect when delivered peripherally was able to immediately suppress feeding behavior and there was no significant difference in plasma BDNF levels between ivt and ip administrations of BDNF, supporting the findings that BDNF has a short serum half-life of just minutes and the dose of peripherally injected BDNF that required to show any effect on feeding is 500 times greater than the dose that induced appetite suppression centrally (Kishino et al. 2001; Suwa et al. 2010).
A previous study demonstrated that selective microinfusion of FMH to the VMH and PVN induces feeding (Sakata et al. 1990). Here, essential questions can be raised as to why hyperphagia was not observed in histamine-depleted rats by FMH and food intake did not differ between wild-type and H1KO mice. There is no definite answer to the query to date. However, these results are compatible with previous findings that histamine depletion by the treatment of FMH did not affect daily food intake and basal food intake in H1KO mice tended to be higher than in wild-type mice, but this effect was not significant (Yoshimatsu et al. 1999; Mollet et al. 2001). There might be some possible and reasonable explanations. One is that FMH infusion to the third ventricle is weaker to deplete histamine than the infusion into specific nuclei. The other is that the differences in mean daily food intake between PBS treatment and FMH treatment as well as between wild-type and H1KO mice is relatively small compared with the deviation of daily food intake in both groups. In addition, the increase of food intake with the treatment of α-helical CRF alone was not observed. This finding is also compatible with previous reports that α-helical CRF attenuates leptin-induced reduction of food intake although a single administration of α-helical CRF does not significantly affect food consumption, and α-helical CRF locally infused into the PVN had no significant effect on food intake (Uehara et al. 1998; Toriya et al. 2010). There may be a possibility that a higher dose of α-helical CRF would have resulted in a significant reduction of food intake compared to control group as well as a more sustained attenuation of the anorexigenic effect of BDNF. Considering that BDNF stimulates histamine neurons through CRF neurons and neuronal histamine activates BDNF neurons directly, there might be a close relationship between histamine and BDNF in circadian rhythm regulation.
BDNF is synthesized in several areas of the hypothalamus, including the PVN, VMH, and LH (Conner et al. 1997). Hypothalamic BDNF expression is highest in the VMH (Unger et al. 2007). This region is important in the regulation of energy metabolism. The VMH is connected to several hypothalamic areas and contains receptors for nutritional status signals, including leptin (King 2006). However, far less is known regarding the role of BNDF and TrkB in the LH. The LH has often been referred to as a feeding center, as lesions to this region result in hypophagia and weight loss (Anand and Brobeck 1951). It contains cells that synthesize melanin concentrating hormone (MCH) and hypocretin, two orexigenic factors (Date et al. 1999). We found that central administration of histamine increased BDNF content in the PVN and VMH, but not in the LH, supporting a previous finding that BDNF injected in the LH did not reduce feeding and body weight, although BDNF and its receptors are expressed in the LH (Wang et al. 2007a, b).
This study supports the possibility that the anorectic effects of BDNF are because of the proposed VMH-PVN-TMN network (Fig. 6), providing novel insight into the action of BDNF in the hypothalamic regulation of energy metabolism.
Figure 6. Proposed feeding model for the interactions among brain-derived neurotrophic factor (BDNF), corticotropin-releasing factor (CRF), and histamine neurons. BDNF regulates histamine neurons in the tuberomammillary nucleus (TMN) through CRF neurons, whose receptors are expressed on the histamine neurons. Neuronal histamine suppresses food intake directly via H1-R, which is expressed on feeding-related neurons, including BDNF neurons, in the ventromedial nucleus (VMH) and paraventricular nucleus (PVN). ‘Feeding-related neurons’ indicates neurons that produce peptides such as BDNF and neuropeptide Y (NPY) or are co-localized with receptors such as H1-R and influence feeding behavior.
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