Immunohistochemical localization of voltage-gated calcium channels in substantia nigra dopamine neurons

The rhythmic firing of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) is thought to be mediated by nifedipine-sensitive Ca2+ channels, although an involvement of omega -conotoxin-sensitive Ca2+ channels is also suggested. In an attempt to localize such Ca2+ channels at both the regional and cellular levels, their expression and distribution patterns were immunohistochemically investigated in the rat SNc. The three distinct subtypes of voltage-gated Ca2+ channels were tested: the class B N-type alpha1 subunit (CNB1), the class C L-type alpha1 subunit (CNC1) and the class D L-type alpha1 subunit (CND1). A large number of SNc neurons showed intense immunoreactivity against CND1 and they were distributed throughout the entire extent. By contrast, many fewer neurons displayed less intense CNC1 immunoreactivity and many of them were located in the lateral aspect of the SNc. No immunoreactivity against CNB1 was detected in the SNc. Moreover, double immunofluorescence analysis in combination with tyrosine hydroxylase staining revealed that virtually all DA neurons were CND1-immunoreactive whereas many DA neurons especially in the medial SNc exhibited only faint or no immunoreactivity against CNC1. Both CNC1 and CND1 were expressed in cell bodies and proximal dendrites of SNc DA neurons, whilst their distal dendrites that penetrated into the substantia nigra pars reticulata expressed CND1 alone. Thus, the ubiquitously and intensely expressed class D alpha1 subunit of L-type Ca2+ channels that is sensitive to both nifedipine and omega -conotoxin may be responsible for the pacemaker activity of SNc DA neurons.