Relationship Between Dopamine Release in the Rat Nucleus Accumbens and the Discharge Activity of Dopaminergic Neurons During Local in Vivo Application of Amino Acids in the Ventral Tegmental Area

Amino acids were pressure-ejected in the ventral tegmental area of rats which were anesthetized with chloral hydrate and treated with pargyline. The extracellular dopamine concentration was recorded from the nucleus accumbens with an electrochemically treated carbon fiber electrode combined either with differential normal pulse voltammetry or with differential pulse amperometry. In distinct rats the discharge activity of single dopaminergic neurons was monitored in the ventral tegmental area while amino acids were pressure-injected at a distance of 200-300 microns from the recorded cell. GABA (24 and 50 nl, 1 M) induced a complete and reversible inhibition of the firing rate lasting for 3-6 min and a decrease in the basal extracellular dopamine level (-54% and -66%, respectively). Glutamate (32 nl, 10 mM), N-methyl-D-aspartate and quisqualate (100 microM) stimulated the firing rate and enhanced the dopamine extracellular concentration up to 10-times the basal one (18 nM). These increases subsided within 1-5 min. Their amplitude depended on the ejected volume (from 16 to 65 nl). At the time-resolution of the method (some seconds) all these variations in the dopamine release appeared closely time-correlated with those of the firing rate. When the mean discharge rate is considered, N-methyl-D-aspartate was as potent as quisqualate but the former promoted burst firing while the latter induced a sustained activity. As regards dopamine release, N-methyl-D-aspartate was twice as potent as quisqualate. This further shows that dopaminergic terminals convert physiological impulse flow into dopamine release as a high pass filter which favors bursts of action potentials.