L-type calcium channels mediate a slow excitatory synaptic transmission in rat midbrain dopaminergic neurons

Abstract

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 ω-conotoxin GVIA, ω-agatoxin IVA, and ω- 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.