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Low-Threshold L-type Calcium Channels in Rat Dopamine Neurons

¹Department of Anatomy and Neurobiology, University of Tennessee Health Science Center, Memphis, Tennessee 38163; and ²Department of Anatomy and Neurobiology, Morehouse School of Medicine, Atlanta, Georgia 30310

Submitted 21 October 2003; accepted in final form 18 November 2003

Ca²⁺ channel subtypes expressed by dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) were studied using whole cell patch-clamp recordings and blockers selective for different channel types (L, N, and P/Q). Nimodipine (Nim, 2 µM), -conotoxin GVIA (Ctx, 1 µM), or -agatoxin IVA (Atx, 50 nM) blocked 27, 36, and 37% of peak whole cell Ca²⁺ channel current, respectively, indicating the presence of L-, N-, and P-type channels. Nim blocked approximately twice as much Ca²⁺ channel current near activation threshold compared with Ctx or Atx, suggesting that small depolarizations preferentially opened L-type versus N- or P-type Ca²⁺ channels. N- and L-channels in DA neurons opened over a significantly more negative voltage range than those in rat dorsal root ganglion cells, recorded from using identical conditions. These data provide an explanation as to why Ca²⁺-dependent spontaneous oscillatory potentials and rhythmic firing in DA neurons are blocked by L-channel but not N-channel antagonists and suggest that pharmacologically similar Ca²⁺ channels may exhibit different thresholds for activation in different types of neurons.

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