The rate of intravenous cocaine administration alters c-fos mRNA expression and the temporal dynamics of dopamine, but not glutamate, overflow in the striatum

Abstract

The rapid entry of drugs into the brain is thought to increase the propensity for addiction. The mechanisms that underlie this effect are not known, but variation in the rate of intravenous cocaine delivery does influence its ability to induce immediate early gene expression (IEG) in the striatum, and to produce psychomotor sensitization. Both IEG induction and psychomotor sensitization are dependent upon dopamine and glutamate neurotransmission within the striatum. We hypothesized, therefore, that varying the rate of intravenous cocaine delivery might influence dopamine and/or glutamate overflow in the striatum. To test this we used microdialysis coupled to on-line capillary electrophoresis and laser-induced fluorescence, which allows for very rapid sampling, to compare the effects of a rapid (5 s) versus a slow (100 s) intravenous cocaine infusion on extracellular dopamine and glutamate levels in the striatum of freely moving rats. An acute injection of cocaine had no effect on extracellular glutamate, at either rate tested. In contrast, although peak levels of dopamine were unaffected by infusion rate, dopamine levels increased more rapidly when cocaine was administered over 5 versus 100 s. Moreover, c-fos mRNA expression in the region of the striatum sampled was greater when cocaine was administered rapidly than when given slowly. These data suggest that small differences in the temporal dynamics of dopamine neurotransmission may have a large effect on the subsequent induction of intracellular signalling cascades that lead to immediate early gene expression, and in this way influence the ability of cocaine to produce long-lasting changes in brain and behavior.

Figure 1

Intensity of c-fos mRNA expression as a function of rate of i.v. cocaine infusion. The left panel shows representative densitograms of c-fos mRNA expression in the brains of animals given cocaine delivered over 5 or 100 seconds (2.0 mg/kg) or saline delivered over 5 seconds. The black box overlay on the densitograms shows the area in which c-fos mRNA expression was quantified. This region was chosen because it corresponds to the central location of the microdialysis sampling region for dopamine detection. Individual microdialysis probe placements are shown at the bottom right. Each line represents the active area (3 mm) of the microdialysis probe (40 μm inner and 200 μm outer diameter) for one animal and distance from bregma is indicated on each section. Animals in the saline group showed low levels of c-fos mRNA expression in the striatum, possibly due to the mild stress of handling. However, there was no effect of rate of saline infusion (5 versus 100 seconds) on c-fos mRNA expression. In order to account for c-fos induction in the control group, the average response to saline was subtracted from each individual value obtained for rats receiving cocaine. The bar graph in the upper right hand corner shows average (+SEM) c-fos mRNA signal intensity for animals receiving cocaine over 5 or 100 seconds. Consistent with a previous report (Samaha et al., 2004), cocaine-induced c-fos mRNA expression within the mircodialysis sampling region was significantly greater when the drug was administered over 5 versus 100 seconds (unpaired t-test t _{(1, 20)} = 2.36, p < .05). These data were obtained by analysis of autoradiographs obtained in a study by Samaha et al. (2004).

Figure 2

Mean (±SEM) percent change from baseline of extracellular glutamate in response to cocaine (2.0 mg/kg) administered over 5 (top panel) or 100 seconds (bottom panel). The arrowhead indicates the start of cocaine infusion. Microdialysis samples were taken every 12 seconds and baseline was calculated as average extracellular glutamate detected over 15 samples taken prior to cocaine infusion. Cocaine delivered at either rate did not significantly alter extracellular glutamate levels in the dorsal striatum. The inset shows the microdialysis probe placements (guide cannula coordinates: 0.3 mm posterior and 2.5 mm lateral to bregma, and 3 mm ventral to bregma); each line represents the active area (3 mm) of the microdialysis probe for one animal (40 μm inner and 200 μm outer diameter; the distance from bregma is indicated on the section). Similar results were obtained when samples were taken from the nucleus accumbens (Venton et al., 2006a).

Figure 3

Mean (±SEM) percent change from baseline of extracellular dopamine in response to cocaine (2.0 mg/kg) administered over 5 or 100 seconds (guide cannula coordinates: 1mm posterior and 2 mm lateral to bregma, and 4 mm ventral to dura). The arrow indicates the start of cocaine infusion. Microdialysis samples were taken every 90 seconds, and baseline was calculated as average extracellular dopamine detected over 15 samples taken prior to cocaine infusion. Cocaine infusion significantly increased extracellular dopamine above baseline, and this effect varied with the rate of intravenous drug delivery (mixed model ANOVA effect of time F _{(35, 1)} = 18.5, p < 0.001, effect of rate F _{(1, 1)} = 7.5, p < 0.01, time × rate interaction F _{(35, 1)} = 2.13, p = 0.02). The peak level of extracellular dopamine induced by cocaine delivery did not differ with infusion rate (planned paired t-test, p > 0.05). In addition, there was no difference in total dopamine detected between the two rates (area under the curve analysis from start of infusion to 50 min post infusion: p > 0.05). However, the rise of extracellular dopamine in response to cocaine administered over 5 seconds was steeper than that following delivery of cocaine over 100 seconds (mixed model ANOVA from start of infusion to peak: effect of time F _{(3, 1)} = 45.8, p < 0.001, effect of rate F _{(1, 1)} = 16.1, p < 0.001, time × rate interaction F _{(3, 1)} = 6.5, p < 0.01) and the subsequent return to baseline was faster when cocaine was given over 5 versus 100 seconds (mixed model ANOVA from peak through end of data collection: effect of time F _{(31, 1)} = 9.8, p < 0.001, effect of rate F _{(1, 1)} = 19.2, p < 0.001, time × rate interaction F _{(31, 1)} = 0.4, p > 0.05).

Figure 4

Overlaid electropherograms taken before (lower trace) and after (upper trace) a single intravenous cocaine infusion (2.0 mg/kg). Microdialysis samples were taken and analyzed on line every 90 seconds. Each peak in the above traces corresponds to a compound in the sample, and the arrow indicates the peak corresponding to dopamine. The increase in the height of the dopamine peak (increase in intensity of fluorescence) indicates an increase in extracellular levels of the transmitter. Standards of known dopamine concentration were used to determine the average basal dopamine concentration in the sample: 18.1 ± 1.53 nM (n = 12, Mean ± S.E.M). For complete methods see (Bowser and Kennedy, 2001; Shou et al., 2006).