Decreased vesicular somatodendritic dopamine stores in leptin-deficient mice

Abstract

An increasing number of studies indicate that leptin can regulate the activity of the mesolimbic dopamine system. The objective of this study was to examine the regulation of the activity of dopamine neurons by leptin. This was accomplished by examining the dopamine D2 receptor-mediated synaptic current that resulted from somatodendritic release of dopamine in brain slices taken from mice that lacked leptin (Lep(ob/ob) mice). Under control conditions, the amplitude and kinetics of the IPSC in wild-type and Lep(ob/ob) mice were not different. However, in the presence of forskolin or cocaine, the facilitation of the dopamine IPSC was significantly reduced in Lep(ob/ob) mice. The application of L-3,4-dihydroxyphenylalanine (L-DOPA) increased the IPSC in Lep(ob/ob) mice significantly more than in wild-type animals and fully restored the responses to both forskolin and cocaine. Treatment of Lep(ob/ob) mice with leptin in vivo fully restored the cocaine-induced increase in the IPSC to wild-type levels. These results suggest that there is a decrease in the content of somatodendritic vesicular dopamine in the Lep(ob/ob) mice. The release of dopamine from terminals may be less affected in the Lep(ob/ob) mice, because the cocaine-induced rise in dopamine in the ventral striatum was not statistically different between wild-type and Lep(ob/ob) mice. In addition, the relative increase in cocaine-induced locomotion was similar for wild-type and Lep(ob/ob) mice. These results indicate that, although basal release is not altered, the amount of dopamine that can be released is reduced in Lep(ob/ob) mice.

Figure 1.

There is no difference in the basal D2R IPSC between wild-type and Lep^ob/ob mice. A, Average baseline D2R IPSCs from wild-type (n = 28) and Lep^ob/ob (n = 28) mice. Calibration: 30 pA, 0.5 s. B, Mean amplitude of baseline D2R IPSCs from wild-type and Lep^ob/ob mice. C, Mean time to peak of baseline D2R IPSCs from wild-type and Lep^ob/ob mice.

Figure 2.

The increase in the D2R IPSC elicited by forskolin and cocaine is significantly reduced in Lep^ob/ob mice. A, Sample D2R IPSCs from wild-type mice in response to forskolin (10 μm). B, Sample D2R IPSCs from Lep^ob/ob mice in response to forskolin. C, Mean increase in the D2R IPSC after forskolin treatment (WT, n = 15; Lep^ob/ob, n = 13; *p < 0.05). D, Sample D2R IPSCs from wild-type mice in response to cocaine (1 μm). E, Sample D2R IPSCs from Lep^ob/ob mice in response to cocaine. F, Mean increase in the D2R IPSC after cocaine treatment (WT, n = 19; Lep^ob/ob, n = 15; *p < 0.05). Calibration: 50 pA, 1 s.

Figure 3.

l-DOPA has a significantly larger effect on the D2R IPSC in Lep^ob/ob mice and fully restores the response to cocaine. A, Sample D2R IPSCs from wild-type mice in response to l-DOPA (10 μm). B, Sample D2R IPSCs from Lep^ob/ob mice in response to l-DOPA. C, Mean increase in the D2R IPSC in response to l-DOPA (WT, n = 14; Lep^ob/ob, n = 9; *p < 0.05). D–H, Effects of forskolin (10 μm) and cocaine (1 μm) on the D2R IPSC after pretreatment with l-DOPA (10 μm). D, Sample D2R IPSCs from wild-type mice in response to forskolin after pretreatment with l-DOPA. E, Sample D2R IPSCs from Lep^ob/ob mice in response to forskolin after pretreatment with l-DOPA. F, Sample D2R IPSCs from wild-type mice in response to cocaine after pretreatment with l-DOPA. G, Sample D2R IPSCs from Lep^ob/ob mice in response to cocaine after pretreatment with l-DOPA. H, Mean increase in the D2R IPSC in response to each of the treatments described in this figure and in Figure 2 (l-DOPA–forskolin: WT, n = 13, Lep^ob/ob, n = 7; l-DOPA–cocaine: WT, n = 12, Lep^ob/ob, n = 7; *p < 0.05). Calibration: 50 pA, 0.5 s.

Figure 4.

The alterations in the D2R IPSC in Lep^ob/ob mice are not attributable to decreased vesicle pool size or altered probability of release. A, Perfusion of reserpine (1 μm) reduced the D2R IPSC equally in wild-type and Lep^ob/ob mice. The black bar denotes the time of perfusion of reserpine (WT, n = 5; Lep^ob/ob, n = 5). B–F, The D2R IPSCs from wild-type and Lep^ob/ob mice show similar levels of paired-pulse depression. B, Sample traces of five repeated D2R IPSCs from wild-type mice. C, Sample traces of five repeated dopamine iontophoretic currents from wild-type mice. D, Sample traces of five repeated D2R IPSCs from Lep^ob/ob mice. E, Sample traces of five dopamine iontophoretic currents from Lep^ob/ob mice. F, Mean amplitude of the D2R IPSCs and dopamine iontophoretic currents during the repeated pulses (WT, n = 5; Lep^ob/ob, n = 6). Calibration: 20 pA, 1 s.

Figure 5.

Treatment of Lep^ob/ob mice with leptin in vivo restores the full response of the D2R IPSC to cocaine. A, Sample traces of the effects of forskolin on the D2R IPSC from Lep^ob/ob mice treated with saline in vivo. B, Sample traces of the effects of forskolin on the D2R IPSC from Lep^ob/ob mice treated with leptin in vivo. C, Sample traces of the effects of cocaine on the D2R IPSC from Lep^ob/ob mice treated with saline in vivo. D, Sample traces of the effects of cocaine on the D2R IPSC from Lep^ob/ob mice treated with leptin in vivo. E, Mean increase in the D2R IPSC after treatment with forskolin and cocaine in Lep^ob/ob mice treated in vivo with saline or leptin (saline–forskolin, n = 17; leptin–forskolin, n = 14; saline–cocaine, n = 18; leptin–cocaine, n = 14). Calibration: 50 pA, 0.5 s.

Figure 6.

The cocaine-stimulated release of dopamine in the ventral striatum is increased in Lep^ob/ob mice. Microdialysis measurement of dopamine levels in the ventral striatum in wild-type and Lep^ob/ob mice in response to saline or cocaine (20 mg/kg) injection. Each time point represents a 15 min period. A, Mean dopamine levels (expressed as percentage baseline dopamine levels) in wild-type and Lep^ob/ob mice in response to saline or cocaine injection. Saline or cocaine was injected at time 0. B, Fold increase in dopamine in response to cocaine injection for each individual animal (WT, n = 12; Lep^ob/ob, n = 10).

Figure 7.

Cocaine-induced locomotion is altered in Lep^ob/ob mice. A, Total distance traveled in wild-type and Lep^ob/ob mice in response to injections of saline and cocaine (20 mg/kg). Each time point represents a 5 min interval (*p < 0.05 WT saline vs cocaine; **p < 0.05 Lep^ob/ob saline vs cocaine; n = 22 for both WT and Lep^ob/ob mice for all panels). B, Increase in distance traveled in response to cocaine (relative to saline) in wild-type and Lep^ob/ob mice (*p < 0.05). C, Fold increase in distance traveled in response to cocaine in wild-type and Lep^ob/ob mice. D, Scatter plot of the fold increase in the total distance traveled in response to cocaine over the entire 60 min session for each individual mouse.