Interoception refers to the representation of the internal world, and includes the processes by which an organism senses, interprets, integrates, and regulates signals from within itself.The brain communicates with internal organs via the peripheral nervous system and non-neuronal systems.

Nicotine addiction is initiated by its binding to high-affinity nicotinic receptors in brain composed primarily of ␣4 and ␤2 subunits. For nicotinic receptors expressed in vivo or heterologously, nicotine exposure over hours to days increases or "upregulates" high-affinity nicotine binding to receptors through a posttranslational mechanism thought to increase receptor numbers. Using heterologous expression, we find nicotine exposure causes a fourfold to sixfold higher binding to ␣4␤2 receptors that does not correspond with any significant change in the number of surface receptors or a change in the assembly, trafficking, or cell-surface turnover of the receptors. However, upregulation does alter the functional state of the receptor, slowing desensitization and enhancing sensitivity to acetylcholine. Based on these findings, we propose an alternative mechanism to explain nicotine-induced upregulation in which nicotine exposure slowly stabilizes ␣4␤2 receptors in a high-affinity state that is more easily activated, thereby providing a memory for nicotine exposure.

Recent evidence suggests that in addition to ␣4␤2 and ␣3-containing nicotinic receptors, ␣6-containing receptors are present in midbrain dopaminergic neurons and involved in the nicotine reward pathway. Using heterologous expression, we found that ␣6␤2, like ␣3␤2 and ␣4␤2 receptors, formed high affinity epibatidine binding complexes that are pentameric, trafficked to the cell surface, and produced acetylcholineevoked currents. Chronic nicotine exposure up-regulated ␣6␤2 receptors with differences in up-regulation time course and concentration dependence compared with ␣4␤2 receptors, the predominant high affinity nicotine binding site in brain. The ␣6␤2 receptor up-regulation required higher nicotine concentrations than for ␣4␤2 but lower than for ␣3␤2 receptors. The ␣6␤2 up-regulation occurred 10-fold faster than for ␣4␤2 and slightly faster than for ␣3␤2. Our data suggest that nicotinic receptor up-regulation is subtype-specific such that ␣6-containing receptors up-regulate in response to transient, high nicotine exposures, whereas sustained, low nicotine exposures upregulate ␣4␤2 receptors.The addictive actions of nicotine are initiated by its binding to nicotinic acetylcholine receptors (nAChRs) 2 (1, 2). nAChRs are ligand-gated ion channels (3). Muscle nAChRs are pentamers containing four different subunits, ␣, ␤, ␥ (or ⑀), and ␦. Neuronal nAChRs are homologous to muscle nAChRs and fall into two different pharmacological classes. One class binds ␣-bungarotoxin with high affinity and is predominantly pentamers composed of only ␣7 subunits (4 -6). The other class of neuronal nAChRs bind agonists with high affinity and is composed of ␣ (␣2-␣6) and ␤ (␤2-␤4) subunits, homologous to the muscle subunits (7, 8).After chronic nicotine exposure, high-affinity agonist binding to nicotinic receptors is increased in murine (9, 11, 62) and human brains (12). This process, termed nicotine-induced upregulation, is linked to nicotine addiction. The sites up-regulated appear to be predominantly ␣4␤2 receptors (13). Yet other nAChR subtypes that display high affinity binding in the presence of cytisine (10, 14) are up-regulated with nicotine exposure (15). Whether these nAChRs are ␣3-, ␣6-, or even ␣4-containing receptors is unknown (16,17). Recent evidence using ␣-conotoxin MII, an antagonist specific for ␣6-and ␣3-containing receptors (18 -20) and ␣6 null mice (16, 21), suggest that these additional nAChRs are ␣6-containing receptors.The identification of different neuronal nAChR subtypes has been hampered by a lack of subtype-specific ligands and antibodies, receptor heterogeneity, and low levels of expression (2). Because of these problems heterologous expression of nAChR subunits has been used to help identify nAChR subtypes and to study up-regulation (22). Several features of the up-regulation allow it to be studied in heterologous systems. First, the mechanisms underlying up-regulation are predominantly, if not exclusively, posttranslational (62, 45) and, thus, independent of elements regulating nAChR transcription and...

Neuronal nicotinic α7 subunits assemble into cell-surface complexes that neither function nor bind α-bungarotoxin when expressed in tsA201 cells. Functional α-bungarotoxin receptors are expressed if the membrane-spanning and cytoplasmic domains of the α7 subunit are replaced by the homologous regions of the serotonin-3 receptor subunit. Bgt-binding surface receptors assembled from chimeric α7/serotonin-3 subunits contain subunits in two different conformations as shown by differences in redox state and other features of the subunits. In contrast, α7 subunit complexes in the same cell line contain subunits in a single conformation. The appearance of a second α7/serotonin-3 subunit conformation coincides with the formation of α-bungarotoxin–binding sites and intrasubunit disulfide bonding, apparently within the α7 domain of the α7/serotonin-3 chimera. In cell lines of neuronal origin that produce functional α7 receptors, α7 subunits undergo a conformational change similar to α7/serotonin-3 subunits. α7 subunits, thus, can fold and assemble by two different pathways. Subunits in a single conformation assemble into nonfunctional receptors, or subunits expressed in specialized cells undergo additional processing to produce functional, α-bungarotoxin–binding receptors with two α7 conformations. Our results suggest that α7 subunit diversity can be achieved postranslationally and is required for functional homomeric receptors.

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