Species differences in somatodendritic dopamine transmission determine D2-autoreceptor-mediated inhibition of ventral tegmental area neuron firing

Abstract

The somatodendritic release of dopamine within the ventral tegmental area (VTA) and substantia nigra pars compacta activates inhibitory postsynaptic D2-receptors on dopaminergic neurons. The proposed mechanisms that regulate this form of transmission differ between electrochemical studies using rats and guinea pigs and electrophysiological studies using mice. This study examines the release and resulting dopamine D2-autoreceptor-mediated IPSCs (D2-IPSCs) in the VTA of mouse, rat, and guinea pig. Robust D2-IPSCs were observed in all recordings from neurons in slices taken from mouse, whereas D2-IPSCs in rat and guinea pig were observed less frequently and were significantly smaller in amplitude. In slices taken from guinea pig, dopamine release was more persistent under conditions of reduced extracellular calcium. The decline in the concentration of dopamine was also prolonged and not as sensitive to inhibition of reuptake by cocaine. This resulted in an increased duration of D2-IPSCs in the guinea pig. Therefore, unlike the mouse or the rat, the time course of dopamine in the extracellular space of the guinea pig determined the duration the D2-IPSC. Functionally, differences in D2-IPSCs resulted in inhibition of dopamine neuron firing only in slices from mouse. The results suggest that the mechanisms and functional consequences of somatodendritic dopamine transmission in the VTA vary among species. This highlights the complexity that underlies dopamine-dependent transmission in one brain area. Differences in somatodendritic transmission would be expected in vivo to affect the downstream activity of the mesocorticolimbic dopamine system and subsequent terminal release.

Figure 1.

Interspecies characterization of dopamine release and D2-IPSCs in the VTA. A, FSCV measurements of the average evoked [DA]_o by a train of stimuli (5 pulses, 0.5 ms, 40 Hz) in the mouse (black, n = 11), rat (blue, n = 10), and guinea pig (red, n = 11). Stimulation evoked less dopamine release in the rat than in the guinea pig or mouse. B, Summary data illustrating that the decay time constants for the [DA]_o was longer in the guinea pig VTA than in the rat or mouse VTA. C, Representative traces of D2-IPSCs evoked by a train of stimuli (5 pulses, 0.5 ms, 40 Hz) in the mouse (black), rat (blue), and guinea pig (red). Synaptic currents were larger in amplitude in the mouse VTA than in the guinea pig or rat VTA. D, Summary distribution of D2-IPSC amplitude of all neurons recorded in the VTA of the mouse, rat, and guinea pig. Numbers in parenthesis refer to the proportion of neurons recorded in the VTA of each species that expressed a measurable D2-IPSC. Large filled circle illustrates the average synaptic currents of responding neurons. E, Normalized D2-IPSCs illustrated in D. D2-IPSCs have been scaled to the peak of their amplitude. D2-IPSCs from the mouse and rat exhibited similar kinetics while D2-IPSCs in the guinea pig had a slower rise time, time to peak, and time of decay. F, Summary data illustrating the slower time constant of decay of D2-IPSCs from the guinea pig (n = 23) than from the mouse (n = 37) or rat (n = 26). **p < 0.01; ***p < 0.001.

Figure 2.

Dopamine release in the VTA shows a weaker dependence on extracellular calcium in guinea pigs than in rats or mice. A–C, Averaged [DA]_o (n = 7–10) and representative traces of D2-IPSCs from the mouse, rat, and guinea pig under control conditions (2.5 mm [Ca²⁺]_o; black) and low calcium (0.5 mm [Ca²⁺]_o; gray). D, Summary data illustrating the [DA]_o remaining in 0.5 mm [Ca²⁺]_o in the VTA and striatum and the amplitude of D2-IPSCs in the VTA in 0.5 mm Ca²⁺compared with control (2.5 mm [Ca²⁺]_o). **p < 0.01; ***p < 0.001.

Figure 3.

Dopamine-mediated D2-autoreceptor currents are smallest in guinea pig VTA dopamine neurons. A, Representative traces of outward currents evoked by the exogenous iontophoretic application of dopamine (160 nA, 1 s). B, Current density (pA/pF) showing that a maximal application of dopamine evoked the largest currents in the mouse and the smallest currents in the guinea pig. *p < 0.05; ***p < 0.001.

Figure 4.

Dopamine uptake in the VTA regulates D2-IPSCs to a greater extent in mice and rats than in guinea pigs. A, Representative traces illustrating evoked D2-IPSCs under control conditions (black) and in the presence of cocaine (1 μm, gray) from VTA dopamine neurons in mice, rats, and guinea pigs. Note the greater increase in amplitude of D2-IPSCs recorded from rats and mice than from guinea pigs in the presence of cocaine. D2-IPSCs under control conditions have been sized to the same initial amplitude to illustrate the relative increase induced by blocking reuptake. B, Summarized data illustrating the increase in amplitude of D2-IPSCs in the presence of cocaine (1 μm). *p < 0.05; ***p < 0.001.

Figure 5.

Evoking D2-IPSCs does not induce a pause in the firing of VTA dopamine neurons from the rat or guinea pig. A, Voltage-clamp recording of a D2-IPSC (top trace) from a mouse VTA dopamine neuron and current-clamp recordings of tonic pacemaker firing of the same neuron (lower traces). The time course of the evoked D2-IPSCs was similar to that of the evoked hyperpolarization and the pause in firing when recorded in current clamp. The D2-receptor antagonist, sulpiride (200 nm), blocks the evoked pause in firing, suggesting that D2-receptor activation during phasic release underlies the pause in firing. B, C, Current-clamp recordings of tonic pacemaker firing of rat and guinea pig VTA dopamine neurons under control conditions and in the presence of sulpiride (200 nm). Stimulation does not induce an extended pause in firing. Sulpiride does not change the firing rate following stimulation, suggesting that D2-receptor activation by evoked release in the rat and guinea pig is not sufficient to induce a phasic pause in firing. D, Summary data of the above conditions. Data are illustrated as the time from the start of stimulation to the first action potential following the stimulation as a percentage of the average interspike interval recorded from each cell during 3 s of baseline tonic firing. ***p < 0.001.