Chronic Nicotine Cell Specifically Upregulates Functional α4* Nicotinic Receptors: Basis for Both Tolerance in Midbrain and Enhanced Long-Term Potentiation in Perforant Path

Abstract

Understanding effects of chronic nicotine requires identifying the neurons and synapses whose responses to nicotine itself, and to endogenous acetylcholine, are altered by continued exposure to the drug. To address this problem, we developed mice whose ␣4 nicotinic receptor subunits are replaced by normally functioning fluorescently tagged subunits, providing quantitative studies of receptor regulation at micrometer resolution. Chronic nicotine increased ␣4 fluorescence in several regions; among these, midbrain and hippocampus were assessed functionally. Although the midbrain dopaminergic system dominates reward pathways, chronic nicotine does not change ␣4* receptor levels in dopaminergic neurons of ventral tegmental area (VTA) or substantia nigra pars compacta. Instead, upregulated, functional ␣4* receptors localize to the GABAergic neurons of the VTA and substantia nigra pars reticulata. In consequence, GABAergic neurons from chronically nicotine-treated mice have a higher basal firing rate and respond more strongly to nicotine; because of the resulting increased inhibition, dopaminergic neurons have lower basal firing and decreased response to nicotine. In hippocampus, chronic exposure to nicotine also increases ␣4* fluorescence on glutamatergic axons of the medial perforant path. In hippocampal slices from chronically treated animals, acute exposure to nicotine during tetanic stimuli enhances induction of long-term potentiation in the medial perforant path, showing that the upregulated ␣4* receptors in this pathway are also functional. The pattern of cell-specific upregulation of functional ␣4* receptors therefore provides a possible explanation for two effects of chronic nicotine: sensitization of synaptic transmission in forebrain and tolerance of dopaminergic neuron firing in midbrain.

Alpha4 containing nicotinic receptors are positioned to mediate postsynaptic effects on 5-HT neurons in the rat dorsal raphe nucleus

“…Consistent with previous descriptions that report that more then 99% of TH cells contain alpha4 (Arroyo-Jimenez, et al, 1999;Nashmi et al, 2007), a high level of co-incident labeling was observed. However there was also labeling for alpha4 present in non-TH cells, consistent with the presence of alpha4 within some interneurons in the substantia nigra (Nashmi et al, 2007). Subsequently, the distribution in the DR was examined.…”

α4* Nicotinic Acetylcholine Receptors Modulate Experience-Based Cortical Depression in the Adult Mouse Somatosensory Cortex

“…Adult (2.5-4 months old) male wild-type or ␣4-YFP knock-in mice (Nashmi et al, 2007) with C57BL/6J background were used. The ␣4-YFP knock-in mice also co-expressed a hemagglutinin (HA) epitope tag.…”

“…Immunostaining for HA on YFP-tagged ␣4* nAChRs (Nashmi et al, 2007) was achieved by immersing fixed brain sections in 0.3% H 2 O 2 for 15 min followed by overnight incubation in PBS containing rabbit anti-HA antibodies (1:1000, Sigma). Sections were then incubated for 2 h in PBS containing biotin-conjugated anti-rabbit antibodies (1:1000, Vectastain Elite kit, Vector Labs), followed by incu- Figure 1.…”

“…To test this idea, we used knock-in mice that express YFP on the ␣4* subunit of the nicotine receptor (Nashmi et al, 2007). Previous data has shown that inclusion of the YFP construct on the ␣4* subunit does not alter the normal expression of these receptors nor affect their function or sensitivity to agonists such as acetylcholine (Nashmi et al, 2007). Similar to VSD imaging experiments mentioned above, we trimmed C row whiskers on the left side of the face and then killed mice 3 h or 1, 3, 7, and 21 d after trimming.…”

“…Neuronal nAChRs are ligand-gated, cation-permeable channels that are assembled in pentamers composed in a variety of subunit combinations (␣2-␣10 and ␤2-␤4; (Dani, 2001;Barik and Wonnacott, 2009), with the ␣4* bearing nAChR being the most prevalent (* indicates that there are other subunits in the pentameric nAChR). Studies have shown that nAChRs can regulate synaptic plasticity in the hippocampus and reward pathways (McKay et al, 2007;Nashmi et al, 2007;Changeux, 2010). In the visual cortex, these receptors are highly expressed during the critical period of ocular dominance plasticity (Prusky et al, 1988), which can be extended by removing a molecular brake on nAChR function (Morishita et al, 2010).…”