Presynaptic localisation of the nicotinic acetylcholine receptor beta 2 subunit immunoreactivity in rat nigrostriatal dopaminergic neurones

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 alpha3 beta2* and alpha4 beta2* nAChR that modulate dopamine release from nigrostriatal terminals. The objective of this study was to examine the precise subcellular distribution of the nAChR beta2 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 beta2 subunit. Double-preembedding electron microscopy confirmed that beta2-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 beta2 subunit, whereas 45% of beta2 subunit-immunolabelled boutons do not contain TH. Thus the beta2 subunit is localised within at least two populations of axon terminals in the dorsal striatum. In these structures, 15% of beta2 subunit immunoreactvity was at the plasma membrane but was rarely associated with synapses. These findings are compatible with functional presynaptic beta2-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.