The first step in processing nicotine's effects on the brain is the drug's
interaction with neuronal nicotinic receptors (nAChR). The diversity of nAC
hR subtypes, their various modes of response (activation, desensitisation,
prolonged inactivation), and the complex pharmacokinetics of nicotine deliv
ery conspire to make this a complex issue that is difficult to unravel. The
alpha4beta2 nAChR subtype has the highest affinity for nicotine and is the
primary candidate for mediating nicotine's central effects. Chronic nicoti
ne exposure (in both humans, animals and cell culture systems) leads to an
increase in numbers of alpha4beta2 nAChR (upregulation), with functional im
plications for withdrawal. However, there is little evidence presently that
nAChR upregulation is pertinent to the induction or maintenance of depende
nce. However, the particular characteristics of the alpha7 subtype of nAChR
suggest that it may participate in long term changes in synaptic efficacy
that could be relevant to nicotine dependence.