Extrasynaptic Vesicular Transmitter Release from the Somata of Substantia Nigra Neurons in Rat Midbrain Slices

Abstract

Substantia nigra neurons release dopamine from their somatodendritic regions. A long-unresolved question is whether this release occurs by exocytosis or by a nonvesicular mechanism. We used carbon fiber microelectrodes in a brainstem slice to assay secretion from single cell bodies that had been cleared of connective tissue. Amperometry at the carbon fiber microelectrodes revealed unitary events in ∼90% of cells in resting conditions. These events had charge integrals ranging from a few femtocoulombs to several hundred femtocoulombs (fC). Local glutamate application enhanced the event frequency by 3.5-fold on average and up to 10-fold in highly responsive cells, although the mean charge integral was not modified. Local application of a high K⁺-containing saline had effects similar to those of glutamate. The frequency of resting and stimulated amperometric events was much lower at 21–22°C than at 32–35°C. The addition of Cd²⁺ (50 μm), a blocker of voltage-dependent Ca²⁺ channels, to the bath solution blocked the stimulatory effects of glutamate. These results suggest that dopamine is released from the somata of substantia nigra neurons by exocytosis and that this mechanism is regulated by neuronal electrical activity. More generally, this study demonstrates the applicability of carbon fiber microelectrodes to the measurement of quantal monoamine secretion in brain slices.