We used immunoprecipitation with subunit‐specific antibodies to examine the distribution of heteromeric neuronal nicotinic acetylcholine receptors (nAChRs) that contain the α5 subunit in the adult rat brain. Among the regions of brain we surveyed, the α5 subunit is associated in ∼37% of the nAChRs in the hippocampus, ∼24% of the nAChRs in striatum, and 11–16% of the receptors in the cerebral cortex, thalamus, and superior colliculus. Sequential immunoprecipitation assays demonstrate that the α5 subunit is associated with α4β2* nAChRs exclusively. Importantly, in contrast to α4β2 nAChRs, which are increased by 37–85% after chronic administration of nicotine, the α4β2α5 receptors are not increased by nicotine treatment. These data thus indicate that the α4β2α5 nAChRs in rat brain are resistant to up‐regulation by nicotine in vivo, which suggests an important regulatory role for the α5 subunit. To the extent that nicotine‐induced up‐regulation of α4β2 nAChRs is involved in nicotine addiction, the resistance of the α4β2α5 subtype to up‐regulation may have important implications for nicotine addiction.

We investigated the effects of chronic nicotine on ␣6-and ␤3-containing nicotinic acetylcholine receptors (nAChRs) in two rat brain regions using three methodological approaches: radioligand binding, immunoprecipitation, and nicotine-stimulated synaptosomal release of dopamine. Nicotine was administered by osmotic minipumps for 2 weeks. Quantitative autoradiography with [125 I]␣-conotoxin MII to selectively label ␣6* nAChRs showed a 28% decrease in binding in the striatum but no change in the superior colliculus. Immunoprecipitation of nAChRs labeled by [ 3 H]epibatidine in these two regions showed that chronic nicotine increased ␣4-and ␤2-containing nAChRs by 39 to 67%. In contrast, chronic nicotine caused a 39% decrease in ␣6-containing nAChRs in striatum but no change in superior colliculus. No changes in ␤3-containing nAChRs were seen in either region after chronic nicotine. The decreased expression of ␣6-containing nAChRs persisted for at least 3 days, recovering to baseline by 7 days after removal of the pumps. There was a small but significant decrease in total nicotine-stimulated dopamine release in striatal synaptosomes after nicotine exposure. However, the component of dopamine release that was resistant to ␣-conotoxin MII blockade was unaffected, whereas dopamine release that was sensitive to blockade by ␣-conotoxin MII was decreased by 56%. These findings indicate that the ␣6* nAChR is regulated differently from other nAChR subtypes, and they suggest that the inclusion of a ␤3 subunit with ␣6 may serve to inhibit nicotineinduced down-regulation of these receptors.

Nicotinic cholinergic receptors (nAChRs) are present in ganglia in the peripheral nervous system. In autonomic ganglia, they are responsible for fast synaptic transmission, whereas in the sensory ganglia and sensory neurons, they may be involved in modulation of neurotransmission. The present study measured nAChRs in several rat autonomic ganglia: the superior cervical ganglia (SCG), sensory nodose ganglia, stellate ganglia, and pelvic ganglia. The densities of the heteromeric nAChRs determined by receptor binding assay in those four ganglia are 481, 45, 9, and 11 fmol/mg protein, respectively. Immunoprecipitation studies with subunit-specific antibodies showed that a majority of the nAChRs in the SCG and nodose ganglia contain the ␣3 and ␤4 subunits, but a significant percentage of the nAChRs in these ganglia also contain ␣5 and ␤2 subunits. A small percentage of the nAChRs in nodose ganglia also contain ␣2 and ␣4 subunits. Sequential immunoprecipitation assays indicated that in the SCG, all ␣5 subunits are associated with ␣3 and ␤4 subunits, forming the mixed heteromeric ␣3␤4␣5 subtype. A receptor composed of ␣3, ␤2, and ␤4 subunits in the SCG was also detected. In rat SCG, we found the following distribution of nAChRs subtypes: 55 to 60% simple ␣3␤4 subtype, 25 to 30% ␣3␤4␣5 subtype, and 10 to 15% ␣3␤4␤2 subtype. These findings indicate that the nAChRs in SCG and nodose ganglia are heterogeneous, which suggests that different receptor subtypes may play different roles in these ganglia or may be activated under different conditions. Neuronal nicotinic cholinergic receptors (nAChRs) are ligand-gated ion channels present throughout the central and peripheral nervous systems. In the autonomic nervous system, they mediate the fast synaptic transmission crucial to the functions of both sympathetic and parasympathetic ganglia. In addition, they are found in sensory ganglia and sensory neurons (Mandelzys et al., 1990(Mandelzys et al., , 1994Flores et al., 1996), in which they may modulate neurotransmission (Cooper, 2001). These receptors are pentameric structures composed of ␣ and ␤ subunits. Nine ␣ (␣2-␣10) and three ␤ (␤2-␤4) subunits have been identified in the vertebrate nervous system. The heteromeric nAChRs are composed of at least one type of ␣ subunit and one type of ␤ subunit. For example, the ␣3␤4* and ␣4␤2* are the predominant heteromeric subtypes in autonomic ganglia and the central nervous system, respectively. By convention, the asterisk denotes the possibility that one or more unidentified subunits are present in those receptors. For example, although expression of ␣3 and ␤4 subunits or ␣3 and ␤2 subunits results in functional nAChRs in heterologous expression systems (Covernton et al

Drug-induced changes in synaptic strength are hypothesized to contribute to appetitive behavior and addiction. Nicotine, the major addictive substance in tobacco, activates nicotinic receptors (nAChRs) to initiate a series of adaptive changes at the cellular and circuit levels in brain, particularly the ventral tegmental area (VTA). Our lab previously reported that nicotine facilitates induction of long-term potentiation (LTP) in VTA dopamine (DA) neurons by increasing glutamate release via activation of α7 nAChRs on the glutamate terminals, suggesting a critical presynaptic contribution of nicotine in LTP induction. In the present study, we used an in vitro exposure paradigm to study nicotine’s effect on excitatory synaptic strength. Brief exposure of nicotine to brain slices from drug naïve adult rats followed by a period of recovery resulted in an NMDA receptor (NMDAR)-dependent increase of AMPAR/NMDAR ratio in VTA DA neurons, which is consistent with the induction of LTP. These effects are similar to that induced by a single in vivo nicotine injection i.p. The induction of synaptic potentiation required excitation of DA neurons mediated by somatodendritic α4β2 nAChRs, as well as enhancement of NMDAR function via D5 dopamine receptors, also on DA neurons. Nicotine-induced increase of presynaptic glutamate release also contributed to the induction of synaptic plasticity, likely through increased activation of NMDAR. These results identified important receptor systems involved in nicotine-induced long-term changes in excitatory synaptic input to VTA DA neurons. The data also revealed remarkable similarity in the mechanisms underlying synaptic plasticity induced by nicotine and cocaine in the VTA.

Use of tobacco products contributes to hundreds of thousands of premature deaths and untold millions of dollars in health care costs in this country each year. Nicotine is the principal neuroactive component in tobacco, but, despite ongoing research efforts, the cellular basis of its effects on behavior remains unclear. Efforts to resolve this conundrum have focused on the mesoaccumbens dopamine system, which contributes to the rewarding effects of many addictive drugs, including nicotine. The goal of this review is to outline recent advances and highlight some of the important unanswered questions regarding nicotine's effects on neuronal excitability and synaptic plasticity within the brain reward pathways.