You may be familiar with Leptin and Ghrelin, and even know them as appetite-regulating hormones. But how do they work to make us raid the fridge at inappropriate times? Contrary to common beliefs, hunger does not always stem from our stomachs, and could all very well be in your head !
This study explains the behavior of such metabolic hormones causing rewiring within the brain’s appetite centre (synaptic plasticity); and hence, the brain's involvement with the physiology of energy regulation.
Understanding informational signaling within the brain, and how such signaling might affect energy balance and metabolism in our bodies.
Neurons are brain cells. They interact by synapsing (information transmitted across the gap between neurons), from the axon (one end) of one neuron to the dendrite (the other end) of an adjacent neuron. Information transmission can occur chemically via neurotransmitters and neuromodulators (molecules which act like messengers between neurons); or electrically (direct contact between two neurons); or by the release of gaseous substances. The specific type of signaling is determined by respective neuronal connections, the availability of receptors for different neurotransmitters, modulators and the various metabolic hormones.
Our body's changing metabolic state affects the wiring and rewiring of metabolic circuits in the hypothalamus (located at the base of the brain, above the brain stem). Previous studies conducted on non human subjects revealed that short term fasting resulted in strong and quick remodeling of synaptic inputs : first, in key orexigenic (appetite inducing) neurons on the hypothalamus; and second, in the respective interneuronal modulators.
As such, it is a foregone conclusion that the hypothalamus is not hardwired but indeed, is capable of rewiring itself in response to different stimulus and inputs. In the same way, this synaptic plasticity (remodeling of synapses) within the hypothalamus, provides the pathway through which orexigenic (appetite inducing) and anorexigenic (appetite reducing) hormones such as Leptin, Ghrelin as well as Estradial (a form of estrogen), act to mediate energy homeostasis i.e. balance, and metabolism.
How is Leptin linked to metabolism?
There is a strong relationship between Leptin and Metabolism. Leptin is an anorexigenic (appetite reducing) hormone and modulates energy balance via its regulatory effects on neurons Neuropeptide Y (NPY), Proopiomelacortin (POMC), and Orexin. Here is what happens.
First, it was observed that lowered levels of Leptin during periods of fasting coincided with rapid rearrangements of synapses within the appetite centre of the brain. Second, Leptin has a suppressive effect on the functions of neurons Neuropeptide Y (NPY) and Orexin, thereby diminishing their ability to secrete Leptin receptors. Since receptors are imperative to the function of hormones, a change in the levels of Leptin receptors will in turn have adverse effects on the proper mechanism of the Leptin hormone.
Third, Leptin modulates feeding patterns via its effects on NPY and POMC. First, it is important to know that increased activity in NPY, and reduced activity in POMC, encourages feeding and fat conservation. Conversely, the reverse in activity of such neurons discourages eating and promotes lean body mass. Studies have shown lowered levels of Leptin to cause an increase in NPY and a corresponding decrease of POMC, consequently heightening hunger and weight gain. Thus, when Leptin was replaced in Leptin-deficient mice, their food intake decreased dramatically and triggered weight loss.
How does Ghrelin factor in?
Ghrelin is an orexigenic (appetite inducing) hormone; and hence, has the opposite effect that Leptin has on our appetite centres. While overnight food deprivation suppresses Leptin secretion, the same would induce an increase in levels of Ghrelin, triggering more synaptic current and rewiring of the appetite-regulating neurons of the brain.
It is also interesting to note that Ghrelin has been found to have the ability to bind to neurons which govern spatial learning and memory function. In this way, Ghrelin might be the linking pin between metabolism and the higher learning capabilities of the brain.
One Final Word.
Certainly, it is important to acknowledge that daily fluctuations of levels of these hunger- regulating hormones are intrinsic to us and are very vital to our survival. However, comprehension of the physiology of hunger signaling and energy metabolism underscores the importance of maintaining our body’s hormonal balance in areas where managing eating behaviours, and on a larger scale, weight management are concerned.
Furthermore, it is my opinion that a well modulated hormonal state can bring us benefits that extend beyond eating patterns, such as emotional equilibrium; and that such a state might be be achieved through the influences of lifestyle choices, sufficient sleep, diet, exercise etc.
Source:
Horvath, Tamas L (2006). Synaptic Plasticity in Energy Balance Regulation. Obesity. Aug 2006. Vol 14. pp 228S-232S.
