Short-acting cocaine and long-acting GBR-12909 both elicit rapid dopamine uptake inhibition following intravenous delivery

Abstract

The rewarding effects of cocaine have been reported to occur within seconds of administration. Extensive evidence suggests that these actions involve the ability of cocaine to inhibit the dopamine (DA) transporter. We recently showed that 1.5 mg/kg i.v. cocaine inhibits DA uptake within 5 s. Despite this evidence, there remains a lack of consensus regarding how quickly i.v. cocaine and other DA uptake inhibitors elicit DA uptake inhibition. The current studies sought to better characterize the onset of cocaine-induced DA uptake inhibition and to compare these effects to those obtained with the high-affinity, long-acting DA transporter inhibitor, GBR-12909 (1-(2-bis(4-fluorphenyl)-methoxy)-ethyl)-4-(3-phenyl-propyl)piperazine). Using in vivo fast scan cyclic voltammetry, we showed that i.v. cocaine (0.75, 1.5, and 3.0 mg/kg) significantly inhibited DA uptake in the nucleus accumbens of anesthetized rats within 5 s. DA uptake inhibition peaked at 30 s and returned to baseline levels in approximately 1 h. The effects of cocaine were dose-dependent, with the 3.0 mg/kg dose producing greater uptake inhibition at the early time points and exhibiting a longer latency to return to baseline. Further, the blood-brain barrier impermeant cocaine-methiodide had no effect on DA uptake or peak height, indicating that the generalized peripheral effects of cocaine do not contribute to the CNS alterations measured here. Finally, we show that GBR-12909 (0.75, 1.5, and 3.0 mg/kg) also significantly inhibited DA uptake within 5 s post-injection, although the peak effect and return to baseline were markedly delayed compared with cocaine, particularly at the highest dose. Combined, these observations indicate that the central effects of dopamine uptake inhibitors occur extremely rapidly following i.v. drug delivery.

Figure 1

Significant DA uptake inhibition within 5 sec of i.v. cocaine. Shown are representative concentration-time plots (top), color plots (bottom) and cyclic voltammograms (insets) of DA responses from representative rats following saline (left), 0.75 mg/kg, (left middle), 1.5 mg/kg, (right middle), and 3.0 mg/kg cocaine (right) injections. Concentration–time plots; electrical stimulation of VTA (60 Hz for 1 sec; red bar) rapidly induced DA release in the NAc. Relative to saline injection, i.v. cocaine elicits robust uptake inhibition, (descending portion of the curve is less steep than prior to cocaine). Color plots; the voltammetric current (displayed in color in the z-axis) is plotted against the applied potential (y-axis) and the acquisition time (x-axis). Relative to saline, cocaine increased electrically evoked release of DA and inhibited DA uptake as shown by an increase in oxidative current (plotted as green on color plots). Inset; background-subtracted cyclic voltammograms depict two current peaks, one at 600 mV (positive deflection) for DA oxidation and one at −200 mV (negative deflection) for reduction of DA-o-quinone. The position of the peaks identifies the substance oxidized as DA.

Figure 2

Cocaine dose-dependently inhibits DA uptake within 5 sec of i.v. injection. Shown are means ± SEMs for DA uptake (apparent K_m). Saline injections did not produce changes in DA uptake. DA uptake was significantly decreased 5 sec after i.v. cocaine injections. *P<0.01 relative to saline injections. ^#P<0.01 relative to 0.75 mg/kg cocaine.

Figure 3

Time-course of cocaine effects on DA uptake. Shown are the mean ± SEM of DA uptake inhibition (apparent K_m) for the hour following saline and cocaine (0.75 mg/kg, 1.5 mg/kg, and 3.0 mg/kg). Saline injections did not produce changes in DA uptake. By comparison, all doses of cocaine significantly inhibited DA uptake within 5 sec and maximal levels of inhibition were observed within 30 sec. These effects were maintained through much of the experimental session. Note that the x-axis is divided into second (5–60 sec) and minute (5–60 min) intervals. *P<0.01 relative to saline injections.

Figure 4

Cocaine-methiodide has no effect on DA uptake. Shown are the mean ± SEM of DA uptake inhibition (apparent K_m) during the hour following saline, cocaine (1.5 mg/kg) and cocaine-methiodide (1.97 mg/kg). Relative to saline injections, cocaine methiodide did not produce changes in DA uptake inhibition. Consistent with this, cocaine produced significantly greater DA uptake inhibition when compared to cocaine methiodide. Note that the x-axis is divided into second (5–60 sec) and minute (5–60 min) intervals. ⁺P<0.05, *P<0.01 relative to cocaine methiodide. Statistical significance for 1.5mg/kg cocaine vs. saline is shown in Figure 3.

Figure 5

Significant DA uptake inhibition within 5 s of i.v. GBR-12909 administration. Shown are representative concentration/time plots (top), color plots (bottom) and cyclic voltammograms (insets) of DA responses from representative rats following saline injection (left) and 5 sec (left middle), 30 sec (right middle), and 20 min (right) after GBR-12909 injection. Concentration–time plots; relative to saline injection, i.v. GBR-12909 elicited modest yet significant uptake inhibition at the 5 sec and 30 sec time points and robust uptake inhibition and increased DA peak height at the 20 min time point. Color plots; the voltammetric current (displayed in color in the z-axis) is plotted against the applied potential (y-axis) and the acquisition time (x-axis). Relative to saline, GBR-12909 increased DA uptake inhibition and increase peak height as shown by an increase in oxidative current (plotted as green on color plots). This effect was the most robust at the 20 min time point. Inset; background-subtracted cyclic voltammograms depict two current peaks, one at 600 mV (positive deflection) for DA oxidation and one at −200 mV (negative deflection) for reduction of DA-o-quinone. The position of the peaks identifies the substance oxidized as DA.

Figure 6

GBR-12909 dose-dependently inhibits DA uptake within 5 sec of i.v. injection. Shown are means ± SEMs for DA uptake (apparent K_m). Saline injections did not produce changes in DA uptake. In contrast, DA uptake was significantly decreased 5 sec after i.v. GBR-12909 injections. ⁺P<0.05, *P<0.01 relative to saline. ^#P<0.01, relative to 0.75 mg/kg GBR-12909. ^†P<0.01 relative to 1.5 mg/kg GBR-12909.

Figure 7

Time-course effects of GBR-12909 on DA uptake. Shown are the mean ± SEM of DA uptake inhibition (apparent K_m) for the hour following saline and GBR-12909 (0.75 mg/kg, 1.5 mg/kg, and 3.0 mg/kg). Relative to saline injections, GBR-12909 produced marked inhibition of DA uptake as early as 5 sec following i.v. administration. Maximal levels were reached in approximately 20 min. Note that the y-axis is divided to show the full range of apparent K_m values and that the x-axis is divided into second (5–60 sec) and minute (5–60 min) intervals. ⁺P<0.05, * P<0.01 relative to saline.