Intermittent nicotine exposure upregulates nAChRs in VTA dopamine neurons and sensitises locomotor responding to the drug.

Dopaminergic projections from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) mediate the behavioral and motivational effects of many drugs of abuse, including nicotine. Repeated intermittent administration of these drugs, a pattern often associated with initial drug exposure, sensitises the reactivity of dopamine (DA) neurons in this pathway, enhances the locomotor behaviors the drugs emit, and promotes their pursuit and self-administration. Here we show that activation of nicotinic acetylcholine receptors (nAChRs) in the VTA, but not the NAcc, is essential for the induction of locomotor sensitisation by nicotine. Repeated intermittent nicotine exposure (4 × 0.4 mg/kg, base, i.p., administered over 7 days), a regimen leading to long-lasting locomotor sensitisation, also produced upregulation of nAChRs in the VTA, but not the NAcc, in the hours following the last exposure injection. Functional nAChR upregulation was observed selectively in DA but not GABA neurons in the VTA. These effects were followed by long-term potentiation of excitatory inputs to these cells and increased nicotine-evoked DA overflow in the NAcc. Withdrawal symptoms were not observed following this exposure regimen. Thus, intermittent activation and upregulation by nicotine of nAChRs in DA neurons in the VTA may contribute to the development of behavioral sensitisation and increased liability for nicotine addiction.

Nicotine potentiation of excitatory inputs to ventral tegmental area dopamine neurons.

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 laboratory 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 the effect of nicotine on excitatory synaptic strength. Brief exposure of nicotine to brain slices from drug-naive adult rats followed by a period of recovery resulted in an NMDA receptor (NMDAR)-dependent increase of AMPA receptor/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 intraperitoneally. The induction of synaptic potentiation required excitation of DA neurons mediated by somatodendritic α4ß2 nAChRs, as well as enhancement of NMDAR function via D(5) 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.

The alpha4beta2alpha5 nicotinic cholinergic receptor in rat brain is resistant to up-regulation by nicotine in vivo.

We used immunoprecipitation with subunit-specific antibodies to examine the distribution of heteromeric neuronal nicotinic acetylcholine receptors (nAChRs) that contain the alpha5 subunit in the adult rat brain. Among the regions of brain we surveyed, the alpha5 subunit is associated in approximately 37% of the nAChRs in the hippocampus, approximately 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 alpha5 subunit is associated with alpha4beta2* nAChRs exclusively. Importantly, in contrast to alpha4beta2 nAChRs, which are increased by 37-85% after chronic administration of nicotine, the alpha4beta2alpha5 receptors are not increased by nicotine treatment. These data thus indicate that the alpha4beta2alpha5 nAChRs in rat brain are resistant to up-regulation by nicotine in vivo, which suggests an important regulatory role for the alpha5 subunit. To the extent that nicotine-induced up-regulation of alpha4beta2 nAChRs is involved in nicotine addiction, the resistance of the alpha4beta2alpha5 subtype to up-regulation may have important implications for nicotine addiction.

Nicotine and behavioral sensitization.

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.

Chronic nicotine differentially regulates alpha6- and beta3-containing nicotinic cholinergic receptors in rat brain.

We investigated the effects of chronic nicotine on alpha6- and beta3-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]alpha-conotoxin MII to selectively label alpha6(*) 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 alpha4- and beta2-containing nAChRs by 39 to 67%. In contrast, chronic nicotine caused a 39% decrease in alpha6-containing nAChRs in striatum but no change in superior colliculus. No changes in beta3-containing nAChRs were seen in either region after chronic nicotine. The decreased expression of alpha6-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 alpha-conotoxin MII blockade was unaffected, whereas dopamine release that was sensitive to blockade by alpha-conotoxin MII was decreased by 56%. These findings indicate that the alpha6(*) nAChR is regulated differently from other nAChR subtypes, and they suggest that the inclusion of a beta3 subunit with alpha6 may serve to inhibit nicotine-induced down-regulation of these receptors.

Heterogeneity of nicotinic cholinergic receptors in rat superior cervical and nodose Ganglia.

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 alpha3 and beta4 subunits, but a significant percentage of the nAChRs in these ganglia also contain alpha5 and beta2 subunits. A small percentage of the nAChRs in nodose ganglia also contain alpha2 and alpha4 subunits. Sequential immunoprecipitation assays indicated that in the SCG, all alpha5 subunits are associated with alpha3 and beta4 subunits, forming the mixed heteromeric alpha3beta4alpha5 subtype. A receptor composed of alpha3, beta2, and beta4 subunits in the SCG was also detected. In rat SCG, we found the following distribution of nAChRs subtypes: 55 to 60% simple alpha3beta4 subtype, 25 to 30% alpha3beta4alpha5 subtype, and 10 to 15% alpha3beta4beta2 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.