Evidence for defective mesolimbic dopamine exocytosis in obesity-prone rats

Abstract

The association between dietary obesity and mesolimbic systems that regulate hedonic aspects of feeding is currently unresolved. In the present study, we examined differences in baseline and stimulated central dopamine levels in obesity-prone (OP) and obesity-resistant (OR) rats. OP rats were hyperphagic and showed a 20% weight gain over OR rats at wk 15 of age, when fed a standard chow diet. This phenotype was associated with a 50% reduction in basal extracellular dopamine, as measured by a microdialysis probe in the nucleus accumbens, a projection site of the mesolimbic dopamine system that has been implicated in food reward. Similar defects were also observed in younger animals (4 wk old). In electrophysiology studies, electrically evoked dopamine release in slice preparations was significantly attenuated in OP rats, not only in the nucleus accumbens but also in additional terminal sites of dopamine neurons such as the accumbens shell, dorsal striatum, and medial prefrontal cortex, suggesting that there may be a widespread dysfunction in mechanisms regulating dopamine release in this obesity model. Moreover, dopamine impairment in OP rats was apparent at birth and associated with changes in expression of several factors regulating dopamine synthesis and release: vesicular monoamine transporter-2, tyrosine hydroxylase, dopamine transporter, and dopamine receptor-2 short-form. Taken together, these results suggest that an attenuated central dopamine system would reduce the hedonic response associated with feeding and induce compensatory hyperphagia, leading to obesity.

Figure 1.

Elevated body weight of adult obesity-prone rats is linked to increased chow intake and decreased extracellular dopamine levels in the nucleus accumbens. A) Food intake during the dark cycle of obesity-prone (OP) animals (n=4 in triplicate) is greater than food intake of obesity-resistant (OR) animals (n=6 in triplicate). B) Body weight of the young adult OP rats (n=3) used for microdialysis was significantly higher than the body weight of OR rats (n=4). C) Basal accumbens extracellular dopamine in freely moving OP rats (n=3) was significantly lower than that in OR rats (n=4). *P < 0.01; ^#P < 0.05.

Figure 2.

Stimulated dopamine release is attenuated in the nucleus accumbens, dorsal striatum, and prefrontal cortex of adult obesity-prone rats. A) Representative amperometric traces of stimulated dopamine release from acute coronal slices. B, C) Average peak amplitude (pA) (B) and number of molecules released after stimulation of dopamine release (C) were significantly lower in obesity-prone (OP) rats than in obesity-resistant (OR) rats in the nucleus accumbens (NAc) [OP (n=45) stimulations in 9 slices; OR (n=53) stimulations in 11 slices], dorsal striatum (DS) [OP (n=40) stimulations in 8 slices; OR (n=40) stimulations in 8 slices], and prefrontal cortex (PFC) [OP (n=32) stimulations in 7 slices; OR (n=35) stimulations in 9 slices]. D) In 4- to 5-wk-old rats, the average peak amplitude after stimulation of dopamine release was also significantly lower in OP rats than in OR rats [OP (n=37) stimulations in 10 slices; OR (n=56) stimulations in 13 slices]. *P < 0.01.

Figure 3.

Reduced dopamine quantal size in VTA-derived neurons from P0–P1 obesity-prone pups. A) Representative amperometric traces from VTA cultures of obesity-resistant (OR) (top) and obesity-prone (OP) (bottom) neonates. Individual events are shown at higher resolution below. B) Quantal size distribution in cultures from neonatal OP and OR animals. Note that the OP distribution is skewed to the left due to lack of events in the higher quantal size bins. C) Quantal size (left panel) and amplitude (right panel) of stimulated dopamine release from VTA-derived neuronal cultures from OP pups (gray bars, n=110 events) is lower than that in cultures derived from OR pups (black bars, n=182 events). *P < 0.01.

Figure 4.

Lower mRNA and protein expression regulators of dopamine synthesis and exocytosis VTA cell cultures from obesity-prone neonatal animals. A) mRNA expression of TH was significantly lower in dopamine neuronal cultures derived from obesity-prone (OP) rats (gray bar) than in those from obesity-resistant (OR) rats (black bar, n=5 pooled cultures/group in triplicate). B) TH-immunopositive cells in cultures from OP rats (n=11 cultures) were significantly less than those in cultures from OR rats (n=6 cultures). C) mRNA expression of VMAT2 was significantly lower while mRNA expression of endogenous VMAT2 down-regulator Gα_q was significantly higher in cultures from OP rats than in those from OR rats (n=5 cultures/group). D) VMAT2-immunopositive sites in cultures from OP rats (n=12 cultures) were significantly less than those in sites from OR rats (n=16 cultures). E) Representative VMAT2 immunostaining in VTA cultures from OR and OP pups, 7 days postplating. Arrows point to VMAT2-positive sites. Scale bar = 100 μm; ×20 view. F) Lower mRNA expression of the DAT transporter and the presynaptic dopamine autoreceptor D2S in cultures from OP (gray bars) than OR neonates (black bars, n=5 cultures/group). *P < 0.01.