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 cen tral nervous system, where they exert a modulatory influence on synaptic tr ansmission. For the striatum, pharmacological evidence supports the presenc e of presynaptic alpha3 beta2* and alpha4 beta2* nAChR that modulate dopami ne 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 immunolab elling with tyrosine hydroxylase (TH) at the confocal level revealed that t he cell bodies and axon terminals (synaptosomes) of nigrostriatal neurones were also immunoreactive for the nAChR beta2 subunit. Double-preembedding e lectron microscopy confirmed that beta2-immunogold labelling was enriched i n TH-positive terminals in the dorsal striatum. Quantitative analysis of do ubly immunogold-labelled sections in postembedding electron microscopy show ed that 86% of TH-positive axonal boutons are also labelled for the nAChR b eta2 subunit, whereas 45% of beta2 subunit-immunolabelled boutons do not co ntain TH. Thus the beta2 subunit is localised within at least two populatio ns of axon terminals in the dorsal striatum. In these structures, 15% of be ta2 subunit immunoreactvity was at the plasma membrane but was rarely assoc iated with synapses. These findings are compatible with functional presynap tic beta2-containing nAChR that may be stimulated physiologically by acetyl choline that diffuses from synaptic or nonsynaptic sites of acetylcholine r elease. These results demonstrate the presynaptic localisation of an nAChR subunit in nigrostriatal dopaminergic neurones, providing morphological evi dence for the presynaptic nicotinic modulation of dopamine release. (C) 200 1 Wiley-Liss, Inc.