In vivo effects of amphetamine analogs reveal evidence for serotonergic inhibition of mesolimbic dopamine transmission in the rat

Abstract

Evidence suggests that elevations in extracellular serotonin (5-HT) in the brain can diminish stimulant effects of dopamine (DA). To assess this proposal, we evaluated the pharmacology of amphetamine analogs (m-fluoroamphetamine, p-fluoroamphetamine, m-methylamphetamine, p-methylamphetamine), which display similar in vitro potency as DA releasers (EC(50) = 24-52 nM) but differ in potency as 5-HT releasers (EC(50) = 53-1937 nM). In vivo microdialysis was used to assess the effects of drugs on extracellular DA and 5-HT in rat nucleus accumbens, while simultaneously measuring ambulation (i.e., forward locomotion) and stereotypy (i.e., repetitive movements). Rats received two intravenous injections of drug, 1 mg/kg at time 0 followed by 3 mg/kg 60 min later. All analogs produced dose-related increases in dialysate DA and 5-HT, but the effects on DA did not agree with in vitro predictions. Maximal elevation of dialysate DA ranged from 5- to 14-fold above baseline and varied inversely with 5-HT response, which ranged from 6- to 24-fold above baseline. All analogs increased ambulation and stereotypy, but drugs causing greater 5-HT release (e.g., p-methylamphetamine) were associated with significantly less forward locomotion. The magnitude of ambulation was positively correlated with extracellular DA (p < 0.001) and less so with the ratio of DA release to 5-HT release (i.e., percentage DA increase divided by percentage 5-HT increase) (p < 0.029). Collectively, our findings are consistent with the hypothesis that 5-HT release dampens stimulant effects of amphetamine-type drugs, but further studies are required to address the precise mechanisms underlying this phenomenon.

Fig. 1.

Chemical structures of PAL analogs tested in this study. Amphetamine is shown for comparison.

Fig. 2.

Effects of PAL analogs on extracellular DA in nucleus accumbens of rats undergoing in vivo microdialysis. Rats received intravenous injection of 1 mg/kg of drug at time 0, followed by 3 mg/kg at 60 min. Extracellular DA was determined by HPLC-ECD as described under Materials and Methods; mean basal concentration of DA was 1.58 ± 0.12 pg/5 μl for n = 28 rats. Left, for time course data, dialysate DA values are mean ± S.E.M. expressed as a percentage of three preinjection baseline samples (% basal). Arrows indicate times of drug injection. Right, for mean effect data, DA values are mean ± S.E.M. expressed as the average DA response determined from three postinjection samples after each dose. n = 7 rats/group. *, p < 0.05 compared with all other groups; #, p < 0.05 compared with PAL-353.

Fig. 3.

Effects of PAL analogs on extracellular 5-HT in nucleus accumbens of rats undergoing in vivo microdialysis. Rats received intravenous injection of 1 mg/kg of drug at time 0, followed by 3 mg/kg at 60 min. Extracellular 5-HT was determined by HPLC-ECD as described under Materials and Methods; mean basal concentration of 5-HT was 0.19 ± 0.03 pg/5 μl for n = 28 rats. Left, for time course data, dialysate 5-HT values are mean ± S.E.M. expressed as a percentage of three preinjection baseline samples (% basal). Arrows indicate times of drug injection. Right, for mean effect data, 5-HT values are mean ± S.E.M. expressed as the average 5-HT response determined from three postinjection samples after each dose. n = 7 rats/group. *, p < 0.05 compared with all other groups.

Fig. 4.

Effects of PAL analogs on ambulation in rats undergoing in vivo microdialysis. Rats received intravenous injection of 1 mg/kg of drug at time 0, followed by 3 mg/kg at 60 min. Ambulation was determined as described under Materials and Methods; mean basal value for ambulation was 109 ± 7 cm/20 min for n = 28 rats. Left, for time course data, ambulation values are mean ± S.E.M. expressed as a percentage of three preinjection baseline samples (% basal). Arrows indicate times of drug injection. Right, for mean effects data, ambulation values are the mean ± S.E.M. expressed as the average ambulation response determined from three postinjection samples after each dose. n = 7 rats/group. *, p < 0.05 compared with all other groups.

Fig. 5.

Effects of PAL analogs on stereotypy in rats undergoing in vivo microdialysis. Rats received intravenous injection of 1 mg/kg of drug at time 0, followed by 3 mg/kg at 60 min. Stereotypy was determined as described under Materials and Methods; mean basal value for stereotypy was 116 ± 9 episodes/20 min for n = 28 rats. Left, for time course data, stereotypy values are mean ± S.E.M. expressed as a percentage of three preinjection baseline samples (% basal). Arrows indicate times of drug injection. Right, for mean effect data, stereotypy values are the mean ± S.E.M. expressed as the average stereotypy response determined from three postinjection samples after each dose. n = 7 rats/group.

Fig. 6.

Correlation of dialysate DA with ambulation (left) and stereotypy (right). Mean group data for dialysate DA, ambulation, and stereotypy at each time point after drug treatment (i.e., from 20–120 min postinjection for each drug) were used to construct correlation plots. Dialysate DA is positively correlated with ambulation and stereotypy (p < 0.001) based on Pearson's coefficient (r).

Fig. 7.

Correlation of dialysate 5-HT with ambulation (left) and stereotypy (right). Mean group data for dialysate 5-HT, ambulation, and stereotypy at each time point after drug treatment (i.e., from 20–120 min postinjection for each drug) were used to construct correlation plots. Dialysate 5-HT is positively correlated with stereotypy (p < 0.001) based on Pearson's coefficient (r).

Fig. 8.

Correlation of the ratio of DA/5-HT response with ambulation (left) and stereotypy (right). Mean group data for DA/5-HT ratio, ambulation, and stereotypy at each time point after drug treatment (i.e., from 20–120 min postinjection for each drug) were used to construct correlation plots. The ratio of DA/5-HT response was positively correlated with ambulation (p < 0.029) based on Pearson's coefficient (r).