Presynaptic localisation of the nicotinic acetylcholine receptor β2 subunit immunoreactivity in rat nigrostriatal dopaminergic neurones

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Abstract
Nicotinic acetylcholine receptors (nAChR) are widely distributed in the central nervous system, where they exert a modulatory influence on synaptic transmission. For the striatum, pharmacological evidence supports the presence of presynaptic α3β2* and α4β2* nAChR that modulate dopamine release from nigrostriatal terminals. The objective of this study was to examine the precise subcellular distribution of the nAChR β2 subunit in these neurones and its localisation at presynaptic sites. Double immunolabelling with tyrosine hydroxylase (TH) at the confocal level revealed that the cell bodies and axon terminals (synaptosomes) of nigrostriatal neurones were also immunoreactive for the nAChR β2 subunit. Double‐preembedding electron microscopy confirmed that β2‐immunogold labelling was enriched in TH‐positive terminals in the dorsal striatum. Quantitative analysis of doubly immunogold‐labelled sections in postembedding electron microscopy showed that 86% of TH‐positive axonal boutons are also labelled for the nAChR β2 subunit, whereas 45% of β2 subunit‐immunolabelled boutons do not contain TH. Thus the β2 subunit is localised within at least two populations of axon terminals in the dorsal striatum. In these structures, 15% of β2 subunit immunoreactvity was at the plasma membrane but was rarely associated with synapses. These findings are compatible with functional presynaptic β2‐containing nAChR that may be stimulated physiologically by acetylcholine that diffuses from synaptic or nonsynaptic sites of acetylcholine release. These results demonstrate the presynaptic localisation of an nAChR subunit in nigrostriatal dopaminergic neurones, providing morphological evidence for the presynaptic nicotinic modulation of dopamine release. J. Comp. Neurol. 439:235–247, 2001.

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