Nicotine inside neurons

Tobacco smoking is a major source of preventable morbidity and mortality worldwide particularly due to lung tumours and/or cardiovascular pathologies. Continued tobacco smoking is primarily driven by dependence on nicotine, one of the main psychoactive ingredients of tobacco smoke, which initiates a series of adaptive changes in brain neurons and circuits by binding with high affinity to neuronal nicotinic acetylcholine receptors (nAChRs) [1]. Nicotine is a highly lipophilic compound and, once in the blood stream, rapidly crosses the blood brain barrier and accumulates in the brain. nAChRs are a family of pentameric ion channels, whose variety is mainly due to the diversity of the possible homomeric or heteromeric combinations of nine α (α2−α10) and three β (β2−β4) subunits, which respond to endogenous neurotransmitter acetylcholine (ACh) and exogenous nicotine. Interestingly, the half-life of nicotine in brain tissue, is much longer than that of ACh because acetylcholinesterase does not hydrolyse nicotine, which is only metabolised by liver enzyme [2]. nAChRs are widely and unevenly distributed in the central nervous system (CNS). The majority of them have a presynaptic and/or preterminal localisation, where they modulate the release of many neurotransmitters, whereas those in the peripheral nervous system have a postsynaptic localisation and mediate fast synaptic transmission. Chronic exposure to cigarette smoke or nicotine gives rise to neural adaptations. One prominent neuroadaptation in the brain of humans and animals chronically exposed to nicotine [3] is the cell and regionspecific up-regulation of nAChRs [4] that leads to change in downstream plasticity at molecular, cellular and circuit levels [1]. The α4β2 nAChR subtype is the most highly expressed subtype in the CNS and when is expressed in heterologous systems can exist in alternate (α4β2)2α4 and (α4β2)2β2 stoichiometries that have different calcium permeability and agonist or antagonist sensitivity [5]. In both stoichiometries two ACh binding site located at α4/ β2 subunit interfaces are present, but in the (α4β2)2α4 stoichiometry an additional ACh binding site, with low affinity for ACh, is also present at the α4/α4 interface [5]. Prolonged exposure to nicotine in vivo, increases the number of α4β2 nAChR binding sites as a result of an increase in α4 and β2 subunit protein levels without any change in mRNA levels (reviewed in [6]). This upregulation in the brain is not uniform and so we analyzed the effect of chronic (14 days) nicotine on the expression of α4β2 nAChRs in the cortex and thalamus, two brain regions of C57Bl/6 mice that respond differently to nicotine [7]. We found that the α4 and β2 subunits under control conditions, are only present in assembled α4β2 receptors in the cortex and thalamus and in this latter region a significantly higher proportion of receptors have the (α4β2)2β2 stoichiometry. The chronic in vivo administration of nicotine at a concentration that is in the range of that found in the blood of heavy smokers, more markedly up-regulated α4β2 nAChRs in the cortex than those in the thalamus. This up-regulation was paralleled by an increased proportion of receptors with (α4β2)2β2 stoichiometry in the cortex but not in the thalamus. The up-regulation and altered stoichiometry are transient because they returned to control levels with an average half-time of 2.7 days after nicotine withdrawal. This change in stoichiometry is very important because it can greatly influence the physiological response of neurons to nicotine and ACh.