I want to make my experimentatal rat very fat and I found out that
reducing Leptin will make my rat eat and eat so eventually make it
fat. But I don't know the name of the thing that reduces Leptin so I'm
asking you. By the way can you tell me the name of the thing that
makes the body produce more Leptin.
(DO NOT WANT TO KNOW THE NAME OF THE MEDICINE!!! WANT THE NAME OF THE CHEMICAL)
*Question: What is the name of the thing that reduces Leptin?
           What is the name of the thing that make body produce more Leptin?
           How to measure the amount of Leptin produced?
           What is the unit used to measure the amount of Leptin?

Howdy;

First, Leptin is produces by adipocytes (fat cells).  Following
ingestion, leptin (which is encoded by the gene ob), is produced.
Leptin acts on the hypothalamus.
Lets review the neurological systems that control feeding behavior. 
There are leptin-sensative cells int he arcuate nucleus.  These cells
project to the lateral hypothalamus.  There are neurons in the lateral
hypothalamus which are sensative to inputs from the arcuate. A rise in
leptin results in arcuate neurons releasing alpha-melanin
concentrating hormone. The Mc4 receptors in the lateral hypothalamic
area are sensative to MCH. MCH activates these receptors, which
produces an inhibition of feeding behavior.  In the case of low levels
of leptin When leptin levels drop, a second population of neurons in
the arcuate which produce AgRP (agouti reactive protien) release this
NT to the same population of neurons with MC4 receptors in the Lateral
hypothalamic area. AgRP inhibits the MC4 receptor action, which causes
an INHIBITION of the INHIBITION of feeding behavior.  (This theme of 
disinhibition is common in the neurosciences). So, the long and short
of the above is, when leptin is high, feeding is inhibited.  So, with
regards to your questions....

1) The "thing" that reduces leptin - To the best of my knowledge, no
companies are researching anti-leptin drugs.  It is counterproductive
to their goal of money making.  Leptin has been considered for its
utility as a weight loss drug, but most people now discount the idea. 
The body is really good at maintaing weight.  If you wanted to have
the same effect, you could think about the neurophysioilogy above -
Rather than reducing leptin, think about infusing AgRP.  This would
promote feeding behavior.  The signaling pathway of leptin production
is not well understood yet. There are plenty of investigators working
on this though

2) The body produces more leptin in response to depleated fat stores.

3) You can measure leptin a few ways, if you are looking at brain
tissue, get an anti-leptin antibody, and look at sections of the
arcuate nucleus of the hypothalamus.  You can also look at serum
leptin levels.

4) As with most protein, you can measure it in mg/ml.

You can also measure leptin levels in the blood, keeping the animal
alive, using a technique called microdialysis