Precise Localization of α7 Nicotinic Acetylcholine Receptors on Glutamatergic Axon Terminals in the Rat Ventral Tegmental Area

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Abstract
α7 neuronal nicotinic acetylcholine receptors (nAChRs) constitute one of the predominant nAChR subtypes in the mammalian brain. Within the ventral tegmental area (VTA), nicotine application, paired with postsynaptic stimulation, contributes to a form of long-term potentiation, an effect attributed to presynaptic α7 nAChRs on glutamatergic afferents (Mansvelder and McGehee, 2000). The aim of this study was to examine the precise subcellular distribution of α7 nAChRs in the adult rat VTA to establish whether these receptors are indeed present on glutamatergic axon terminals and to determine their relationship with cholinergic afferents. The spatial relationship between α7 nAChRs, labeled using the α7 nAChR-specific antagonist α-bungarotoxin, and the local neurochemical environment was investigated by the application of multiple labeling strategies with antibodies against tyrosine hydroxylase, vesicular glutamate transporters (VGluTs), vesicular acetylcholine transporter, and glial fibrillary acidic protein. α7 nAChRs were localized at both somatodendritic and presynaptic loci within the VTA: on subpopulations of dopaminergic and nondopaminergic neurons and glutamatergic and nonglutamatergic terminals. There was no detectable α7 nAChR expression within astrocytes in the VTA. Most α7 nAChRs were cytoplasmic (82%), and the remainder were associated with the plasma membrane. Most presynaptic receptors (75%) were on glutamatergic axon terminals, with similar levels of α-bungarotoxin binding present on both VGluT1- and VGluT2-immunoreactive boutons. Both preembedding and postembedding electron microscopy revealed that presynaptic α7 nAChRs are often located at extrasynaptic (27%) and perisynaptic (61%) loci. α7 nAChRs were not associated with cholinergic synapses, consistent with their activation by a paracrine mode of acetylcholine or choline delivery.