continue of the book "Fat Chance"

FYI: I forgot to mention what the (VTA) means.  The hedonic pathway comprises a neural conduit between two brain areas: the ventral tegmental area (VTA) and the nucleus accumbens (NA, also known as the reward center), both of which are deep-brain structures. Pleasure occurs when the VTA signals the NA to release dopamine, a neurotransmitter.  It's a signal from one brain center to another. When the released dopamine binds to its specific dopamine D2 receptor in the NA, the sense of pleasure is experienced. So what are neurotransmitters and receptors? Think of keys and locks. Each neuron is a cell body, and at its end is an axon (special fiber of the neuron that sends information).  This axon has a synapse, or pathway, that connects to the dendrites (specialized fibers of the nerve cell that receive information) of the next neuron.  When a neural impulse is generated in the first cell, it pulses down to the end of the axon, which contains little packets of neurotransmitters that are then released. These are the keys. They travel across the synapse to the receptors (locks), located in the dendrites of the next cell.  There are many keys that take the path along the synapse, and not all of them make it to their destination. Along their way via the synapse, some are metabolized and some are "re-uptaken." Dopamine is one of these types of keys traveling to fit into the locks of the D2 receptors in the next cell, thus determining the triggering and firing of the next cells down the chain.

Food intake is just one readout of the hedonic pathway.  It appears to mediate feeding on the basis of palatability rather than energy need: I'm stuffed, but that chocolate cake looks so good.  When functional, the hedonic pathway helps to curtail food intake in situations where energy stores are replete: I don't need to finish that macaroni and cheese.  However, when dysfunctional, this pathway can increase food intake, leading to obesity.
1. Anything that increases the dopamine transmission to the NA increases  the feeling of reward.
2. Anything that clears dopamine from the NA will extinguish the feeling of reward.
3. Anything that reduces the number of D2 receptors in the NA, or binding of dopamine to those receptors (such as chronic overuse of a substance), will shortchange reward. You then need more dopamine, and hence more of the substance, to get the same feeling of pleasure.
These precepts are as true for food as they are for addictive drugs. And food and drugs cross over. With time we can become sensitized to a substance and need more of it to get the same effect. Once sensitized, animals and humans may become hyperresponsive to a new substance; this is known as cross-sensitization.  In other words, if the brain has been wired for addition, it's easy to switch from one substance to another. Ask recovering alcoholics about their incessant need for coffee, tobacco, and/or sugar. If you have a decreased dopamine binding capacity, there is a perceived need  for compulsive food intake to provide excess stimulation of these depressed circuits, thereby driving continued weight gain.

P.S. This is so interesting, I hope you all read this book!  I will talk about why I'm onto this subject after while.  But let me read a little more and I can tell you what happens. TO BE CONTINUED