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 approximately 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 degreesC than at 32-35 degreesC. The addition of Cd2+ (50 microM), 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.

Fig. 1.

Cleaning procedure for amperometric recording in substantia nigra. A, Appearance of substantia nigra neuron during amperometric recording. A substantia nigra neuron from the pars compacta was partly freed from overlying connective tissue with the help of a saline-filled glass pipette. A carbon fiber electrode (EDP-insulated type) then was positioned to attain contact with the exposed part of the neuronal soma. Scale bar, 10 μm. B, Cyclic voltammograms with a scan rate of 100 mV/sec were performed in potassium ferricyanide solution (see Materials and Methods). The vertical excursion gives the limiting current, an indication of the sensitivity of the electrode. The limiting current is reduced by a factor of ∼5 after contact with brain tissue.

Fig. 2.

Spontaneous amperometric events recorded from a substantia nigra neuron. A, Examples are shown of spontaneous events recorded from a single cell. Notice the large variability in size and time course of these events. B, Shown is the distribution of the mean frequencies of spontaneous amperometric events in the different cells in which events could be recorded.

Fig. 3.

Enhancement of the frequency of amperometric events by glutamate applications. A, Sample recordings at rest (top) and a few seconds after local application of 0.3 mm glutamate (bottom).B, Timing of amperometric events after three successive applications of glutamate to the same cell. The three plots are consecutive. The events illustrated in A are taken before and during the first glutamate application to this cell. Each event is represented by a vertical box. There are 17 events during the first application, which lasted 30 sec, as compared with three events during the preceding 30 sec period. Note that the effects of glutamate become weaker for the second and third applications than for the first. C, Effect of glutamate on event frequency. Results are cumulative from 10 applications obtained in five cells. The results have been aligned with respect to the start of the applications. Applications ranged between 27 sec (solid line) and 47 sec (the end of thedotted line) in duration.

Fig. 4.

Summary histograms of amperometric event characteristics (498 events). A, Charge integral.B, Half-widths (with 200 Hz of low-pass filtering).C, Comparison of cumulative charge histograms for spontaneous and for stimulated events. Both histograms have been normalized. There was no significant difference between the two curves.