When it comes to substance abuse like smoking or abuse of intoxicating drugs, it is very difficult to determine what a “safe” limit of exposure is.  Quite often, the initial exposure to mood altering substances like nicotine occurs during the teenage years. The period of adolescence is marked by a tendency towards risk-taking behavior which often results in ‘experimental’ exposure to psychedelic substances. Adolescents who tend to flirt with danger in this fashion are often convinced that a small trial will not actually have lasting damaging effects. However, research on the development of prefrontal cortex in similarly age-matched animals says otherwise.

The prefrontal cortex in teenagers is in a state of growth and development. Contrary to established notions, brain development continues well into the teenage years and changes in synapses (connections between brain cells that facilitate the transmission of chemical messengers between cells) occur well into adolescence.  Research on adolescent mice and rats shows that exposure to nicotine during this period has long-lasting effects. For starters, nicotine is known to be able to excite neurons bearing nicotinic acetylcholine receptors. In the prefrontal cortex, nicotine has been shown to induce greater expression of a specific subset of nicotinic acetylcholine receptors, by 34%.  In the normal course of development, the number of acetylcholine receptors declines in these cells. This phenomenon is specific to the period of adolescence since a similar increase in the number of receptors is not seen when the initial exposure to nicotine occurs in adulthood, in these animals. Research has also shown that exposure to nicotine in early adolescence enhances the nicotinic ‘reward’ feeling during adulthood. It is therefore surmised that early exposure to smoking is likely to set the stage for long-term addiction and perhaps it also explains why addiction to nicotine is so prevalent, worldwide.

One might argue that since the teenage years are a short period in the life-span of a person, occasional exposure to nicotine is not likely to leave lasting damage. Here’s the catch. Exposure to nicotine also changes the pattern of synaptic connectivity between neurons in the prefrontal cortex.  The ability of neurons to establish new synaptic connections and develop new firing patterns in response to different stimuli is also known as “synaptic plasticity”. Neuroscientists have shown that all “learning” as well as  information analysis and assimilation in the brain is a net result of the pattern of exchange of neurotransmitter molecules (also referred to as pattern of ‘firing’) between neurons which respond to training stimuli. So, the more you learn, the better you get at learning by stimulating your neurons to make new synaptic connections. However, exposure to nicotine in early adolescence, changes the pattern of firing of neurons in the prefrontal cortex. Now this change reduces the capability of neurons in the prefrontal cortex to make new synaptic connections. Therefore exposure to psychedelic and addictive substances like nicotine results in reduced synaptic plasticity and has a negative impact on cognitive processes in adult life.

All these significant changes take place in early adolescence and perhaps parental guidance may play a huge role in preventing nicotine addiction and associated cognitive deficits.

References

Adriani W, Macrì S, Pacifici R, & Laviola G (2002). Peculiar vulnerability to nicotine oral self-administration in mice during early adolescence. Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 27 (2), 212-24 PMID: 12093595

Counotte DS, Goriounova NA, Moretti M, Smoluch MT, Irth H, Clementi F, Schoffelmeer AN, Mansvelder HD, Smit AB, Gotti C, & Spijker S (2012). Adolescent nicotine exposure transiently increases high-affinity nicotinic receptors and modulates inhibitory synaptic transmission in rat medial prefrontal cortex. FASEB journal : official publication of the Federation of American Societies for Experimental Biology, 26 (5), 1810-20 PMID: 22308197

Goriounova NA, & Mansvelder HD (2012). Nicotine exposure during adolescence alters the rules for prefrontal cortical synaptic plasticity during adulthood. Frontiers in synaptic neuroscience, 4 PMID: 22876231

Goriounova NA, & Mansvelder HD (2012). Nicotine exposure during adolescence leads to short- and long-term changes in spike timing-dependent plasticity in rat prefrontal cortex. The Journal of neuroscience : the official journal of the Society for Neuroscience, 32 (31), 10484-93 PMID: 22855798

Kawai HD, Kang HA, & Metherate R (2011). Heightened nicotinic regulation of auditory cortex during adolescence. The Journal of neuroscience : the official journal of the Society for Neuroscience, 31 (40), 14367-77 PMID: 21976522

Kota D, Robinson SE, & Imad Damaj M (2009). Enhanced nicotine reward in adulthood after exposure to nicotine during early adolescence in mice. Biochemical pharmacology, 78 (7), 873-9 PMID: 19576867

Image via william langeveld / Shutterstock.