Single-Cell RT-PCR and Functional Characterization of Ca2+ Channels in Motoneurons of the Rat Facial Nucleus

Voltage-dependent Ca 2+ channels are a major pathway for Ca 2+ entry in neurons. We have studied the electrophysiological, pharmacological, and molecular properties of voltage-gated Ca 2+ channels in motoneurons of the rat facial nucleus in slices of the brainstem. Most facial motoneurons express both low voltage-activated (LVA) and high voltage-activated (HVA) Ca 2+ channel currents. The HVA current is composed of a number of pharmacologically separable components, including 30% of N-type and ∼5% of L-type. Despite the dominating role of P-type Ca 2+ channels in transmitter release at facial motoneuron terminals described in previous studies, these channels were not present in the cell body. Remarkably, most of the HVA current was carried through a new type of Ca 2+ channel that is resistant to toxin and dihydropyridine block but distinct from the R-type currents described in other neurons. Using reverse transcription followed by PCR amplification (RT-PCR) with a powerful set of primers designed to amplify all HVA subtypes of the α 1 -subunit, we identified a highly heterogeneous expression pattern of Ca 2+ channel α 1 -subunit mRNA in individual neurons consistent with the Ca 2+ current components found in the cell bodies and axon terminals. We detected mRNA for α 1A in 86% of neurons, α 1B in 59%, α 1C in 18%, α 1D in 18%, and α 1E in 59%. Either α 1A or α 1B mRNAs (or both) were present in all neurons, together with various other α 1 -subunit mRNAs. The most frequently occurring combination was α 1A with α 1B and α 1E . Taken together, these results demonstrate that the Ca 2+ channel pattern found in facial motoneurons is highly distinct from that found in other brainstem motoneurons.