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The Journal of Neuroscience, September 1, 1998, 18(17):6693-6703

L-Type Calcium Channels Mediate a Slow Excitatory Synaptic Transmission in Rat Midbrain Dopaminergic Neurons

Antonello Bonci¹, Pernilla Grillner¹, Nicola B. Mercuri^{1, 2}, and Giorgio Bernardi^{1, 2}

¹ Ístituto Ricovero e Cura a Carattere Scientifico Santa Lucia, 00179 Rome, Italy, and ² Clinica Neurologica, University of Tor Vergata, 00173 Rome, Italy

Patch pipettes were used to record whole-cell synaptic currents under voltage-clamp in dopaminergic neurons in slices of rat substantia nigra pars compacta and ventral tegmental area. We report that dihydropyridines (DHPs), L-type Ca²⁺ channel antagonists, depressed a slow EPSC (EPSC_slow) evoked by a train of focally delivered electrical stimuli. In fact, the amplitude of the EPSC_slow was reduced by the DHP antagonists nifedipine (1-100 µM), nimodipine (1-100 µM), and isradipine (30 nM-100 µM) in a concentration-dependent and reversible manner. On the other hand, Bay-K 8644 (1 µM), an L-type Ca²⁺ channel agonist, increased the EPSC_slow. The DHPs depressed the EPSC_slow only when the high-frequency stimulation that was used to evoke this synaptic current lasted >70 msec. On the other hand, Bay-K 8644 increased the amplitude of the EPSC_slow only when it was evoked by a train <70 msec. Moreover, the DHPs did not affect the EPSC_fast, the IPSC_fast, and the IPSC_slow. The inhibition of the EPSC_slow caused by the DHPs is attributed to presynaptic mechanisms because (1) the inward current generated by exogenously administered glutamate was not affected and (2) the EPSC_slow was reduced to a similar degree even when the activation state of postsynaptic L-type Ca²⁺ channels was changed by holding the neurons at 100, 60, and +30 mV. Finally, a DHP-sensitive component of the EPSC_slow could even be detected after the blockade of N-, Q-, and P-type Ca²⁺ channels by the combination of -conotoxin GVIA, -agatoxin IVA, and -conotoxin MVIIC. Taken together, these results indicate that under certain patterns of synaptic activity, L-type Ca²⁺ channels regulate the synaptic release of excitatory amino acids on the dopaminergic neurons of the ventral mesencephalon.

Key words: dopamine neurons; L-type calcium channels; dihydropyridines; excitatory postsynaptic currents; midbrain; electrophysiology

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Copyright © 1998 Society for Neuroscience  0270-6474/98/18176693-11$05.00/0

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