Inhibition of N- and L-type Ca2+ currents by dopamine in lamprey spinal motoneurons.

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Dopamine is co-localized with 5-hydroxytryptamine in a ventromedially located plexus in the lamprey spinal cord and reduces Ca2+ currents in motoneurons that express high-voltage activated Ca2+ currents of the N-, L- and P/Q-types. Blockade of L- and P/Q- type channels leaving N-type channels intact reduced but did not prevent the inhibition of the Ca2+ current by dopamine. Dopamine also reduced the L-type current potentiated by BAY K 8644. During simultaneous blockade of N-type and L-type currents, dopamine was unable to affect the remaining Ca2+ current. In addition, blockade of G-proteins abolished the dopaminergic modulation. The inhibition was unaffected by depolarizing prepulses. Thus, dopamine mediates inhibition of N- and L-type currents through a G-protein-dependent, voltage-independent pathway in lamprey spinal motoneurons.