Only some smokers develop an instant liking to cigarettes after their first puff; now researchers want to know why.

Experts at the University of Ontario used rats to study the pathways involved in nicotine addiction.

They were able to identify a receptor that controlled the brain's initial reaction to the rewarding and addictive properties of nicotine.

They hope to study this pathway more so they can develop drugs to prevent nicotine vulnerability, or treat nicotine withdrawal.

Studies up until now have looked at brain pathways involved in nicotine dependence; few have looked at the initial addictive mechanism.

In other medical news…

While flu season is still months away, preparations are happening now.

On Tuesday, FDA officials announced the approval of this year's seasonal flu vaccine.

The vaccine is completely reformulated, including three new strains of the virus suspected to cause the most illness this year.

Normally only one to two strains are changed between years. Two of the strains are currently in use in the southern hemisphere, where flu season is underway.

Experts estimate 5-20% of Americans gets the flu each year. They are especially encouraging health care workers to get a flu shot because they are in constant contact with vulnerable patients.