http://www.nida.nih.gov/PDF/NicotineRR.pdf
Of primary
importance to its addictive nature
are findings that nicotine activates
the brain circuitry that regulates
feelings of pleasure, the so-called
reward pathways. A key brain
chemical involved in mediating
the desire to consume drugs is
the neurotransmitter dopamine,
and research has shown that
nicotine increases the levels of
dopamine in the reward circuits.
Nicotine’s pharmacokinetic
properties have been found also
to enhance its abuse potential.
Cigarette smoking produces a
rapid distribution of nicotine to
the brain, with drug levels peaking
within 10 seconds of inhalation.
The acute effects of nicotine
dissipate in a few minutes, causing
the smoker to continue dosing
frequently throughout the day to
maintain the drug’s pleasurable
effects and prevent withdrawal.
What people frequently do
not realize is that the cigarette
is a very efficient and highly
engineered drug-delivery system.
By inhaling, the smoker can get
nicotine to the brain very rapidly
with every puff. A typical smoker
will take 10 puffs on a cigarette
over a period of 5 minutes that
the cigarette is lit. Thus, a person
who smokes about 1-1/2 packs
(30 cigarettes) daily, gets 300
“hits” of nicotine to the brain each
day. These factors contribute
considerably to nicotine’s highly
addictive nature.