Biology 202
2000 First Web Report
On Serendip

For the majority of this class I have been a fence-sitter with 'mystical' leanings. However when I decided to do my paper on smoking, I found in the phenomina of addiction a very strong argument for our brain=behavior hypothesis. In the following essay, I will describe the biological effects of nicotine on the Central Nervous System and some of the resulting behavioral effects. Afterwards I will present thoughts as to why I believe addiction is a very important point to be considered in the question we've been asking since the first day of class.

When an individual smokes a cigarette, the nicotine travels to the brain where it binds to nicotinic acetylcholine receptors, causing them to fire action potentials (2). This in turn causes the release of dopamine in the nucleus accumbens (2). Professor Gaylord Ellison in the department of psychology at UCLA, while testing the effects of drugs on rats found that nicotine is destructive to the part of the brain (called the fasciculus retroflexus) that determines the levels of dopamine and seretonin in an organism. "Dopamine controls movement, emotional response, and the ability to experience pleasure and pain, while seretonin regulates a person'a mood" (4). When nicotine binds with nicotinic acetylcholine receptors, the resulting release of dopamine increases its concentration from what would normally be found in the brain, thus producing a pleasurable effect on the smoker. When nicotine binds with these acetylcholine receptors it slows down their natural rate of deterioration (being proteins, they are constantly breaking down as new ones are being made), but the rate at which new ones are produced remains unchanged. This leads to an overall increase in the number of receptors.(3).

Over time and with constant exposure, the nicotine will begin to act as an antagonist, desensitizing the receptors it once activated. Thus a smoker will have to smoke more and more to try and achieve the same pleasurable effects.(3).

The half-life of nicotine in humans is two hours. Thus, a person who smokes regularly will be constantly exposed to the substance untill he goes to sleep at night and his neurons have a chance to recover, or if he for some other reason abstains from smoking for long periods of time. Tests were conducted at Baylor College Of Medicine that compaired (among other things) the effects on relevant neurons of a pressure injection of nicotine comparable to the amount received when a cigarette is smoked, with the effects seen on those neurons when the nicotine was applied constantly over time. In the first instance, the application of nicotine, ". . .caused a depolarization of the resting membrane potential. After 1 min in 0.5 M nicotine, the neuron fired continuous action potentials that were no longer influenced by the pressure injections of ACh, as would be expected if most of the nAChRs were desensitized" (2). However, when the nicotine was introduced for a period of nineteen minutes, after the first five minutes, ". . .the currents activated by pressure injection of ACh were more strongly desensitized, and the baseline current decreased nearly to its original pre-nicotine position" (2). This shows that the long exposure to nicotine that most smokers experience can severely desensitize nicotonic acetylcholine receptors.

People who are addicted to cigarettes show behavioral patterns that are often contrary to those they would chose to exhibit. A big part of the discussion on whether brain=behavior has been the issue of free will. We all must eat, drink, sleep, and breathe; these are behaviors dictated by the brain as a result of the body's needs. If we want to survive, we have relatively little choice in these matters. However, no one needs to smoke to survive. If we define free will as the ability, when given more than one option or course, do choose and enact the option you want, then the choice to smoke or not to smoke should be a matter of free will. For a smoker however, this is not the case. I live in a smoking dormitory, and have heard numerous times the exclamations of friends swearing that they are going to quit smoking - I usualy see them lighting up before the end of the day.

Typically, the first cigarette smoked after waking is the most enjoyable because, as was stated earlier, neurons have had a chance to recover somewhat from their usual onsaught of nicotine. It is estimated that at least eight hours of sleep are required to allow for this recovery (1). ' "A person trying to quit has to get out of bed without that cigarette, and we have been getting a lot of anecdotal material telling us that people just can't" Dain said' . Their altered nuerons that have resulted from regular smoking have become used to functioning with constant nicotine bound to their receptors. Without the nicotine they cannot function smoothly. Of the 30% of smokers who try to quit each year, only about 3% succeed (3). But even a smoker does not need nicotine to survive. I found no evidence of people having died from nicotine withdrawl. So if neuron functions can win out over free will - if matter can win out over "mind" - then this leads me to believe that the brain really does dictate behavior. Though 30% of smokers may desire to quit, the functioning of their neurons will not allow them to.

WWW Sources

2) , Dani, John A., DeBiasi, Mariella, Pidoplichko, Volodymyr I., Williams, John T. (1997) Nicotine activates and desensitizes midbrain dopamine neurons. Nature. 390(6657): 401-404

3) , Ember, L.R. (1994) The Nicotine Connection. Chem. Eng. News 72(48):8-18

4)The Daily Bruin, Article on nicotine brain targeting

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