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Leptin Hormone, Produced in Fat, Inhibits Running; New Studies

2015-09-27

It is already established that running increases one’s blood levels of endorphins.  The same is true of anandamide, the body’s analogue to cannabinoids; running for 30 minutes increases blood levels of anandamide.  More recently, blood levels of leptin have been found to change in relation to exercise.

In humans and most mammals, leptin, a hormone produced in fat and circulated in the bloodstream, reduces the appetite and indicates to the body that there is sufficient energy available.  Fasting and running are associated with reduced levels of leptin, and faster running is associated with even lower levels.

Most recent, though, is an experimental setup in which mice had their STAT3 receptors deleted genetically.  These receptors, in dopamine neurons, are associated with the mediation of the the leptin response in these neurons, which are important for their association with rewarding stimulation.  In the absence of these internal mediating receptors, the neurons are continually stimulated and the mice respond by running twice as much as normal.

Mice do normally run several kilometers a day voluntarily on treadmills.  But in the absence of the STAT3 receptors, they ran as much as 11 kilometers a day.  The researchers say that they gave leptin to normal mice (intra-tegmentally) and they stopped running.  The mice with the genetic deletion of the leptin mediator didn’t respond to leptin injections and kept running.  The odd thing was that the leptin deficient mice didn’t have any changes in eating behavior.  This receptor deletion study was published this month, here.

The trend of research recently has been towards multiplying the known endohormonal systems that are associated with changes in the individual’s metabolism and activities.  There has been, over the last fifty years, a plethora of discoveries of new hormones and receptors.  First, there are the brain and gut receptors that correspond to specific drugs: opioids, known as endorphins, that are subdivided into mu1, mu2, mu3, k1, k2, k3, d1, d2, and ORL1; GABA-A and GABA-B; nicotinic receptors, subtypes composed of twelve different subunits, and muscarinic, five subtypes; serotonin 1A-1F, 2A-2C, 3-7; endocannabinoid 1, 2, and possibly 3-5; testosterone, estradiol, and progesterone, using four types of nuclear receptors, luteinizing hormone, chorionic gonadotropin and follicle stimulating hormone, using membrane receptors ; oxytocin;  finally there are the metabolism-related hormones insulin (probably the first) and glucagon,  leptin, and ghrelin.  There may be numerous others.  In addition, the five known types of endovascular nitric oxide should be mentioned.  Some of these function as both neurotransmitters and hormones, such as oxytocin.

 

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