While both studies are thorough, the Johnson and Kenny paper is more complete. Critically, the authors first prepared rats for brain stimulation reward (BSR) experiments. After recovery, the rats consumed three different diets for 40 days. The authors then examined food intake, weight gain, and most importantly, BSR levels. In a separate experiment, they also examined D2R density in the striatum. The authors then worked backwards. This time, they induced D2R knock down and then determined whether they observed similar behaviors to those noted in the first part of the study. Such confirmation of results definitely supported their conclusions well. Still, one slight discrepancy appeared. Although the Lenti-D2Rsh rats demonstrated increased BSR levels, they did not eat more or gain more weight than their control counterparts (in either the cafeteria or chow situations). Perhaps this is due to the fact that the rats in this portion of the experiment did not have previous access to the fatty food. Also, they only had two weeks of access to the cafeteria diet.
The Kim et al. paper also attempted to work backwards to confirm initial results, though maybe not as successfully. Specifically, before treating WT mice with leptin, they used haloperidol, a D2R antagonist. This scenario allowed the WT mice to mimic the D2R-/- mice, as now the WT mice did not have functional D2Rs. The authors note that the haloperidol treated WT mice were more sensitive to leptin, just like the D2R-/- mice. They ate less food and lost more weight. Despite such observations, one must consider the results with caution, as haloperidol alters locomotor activity and could skew results in terms of energy homeostasis.
It is also relevant to compare the genetic engineering techniques employed in the two papers. Johnson and Kenny used an interfering RNA knockdown method to create transgenic rats, while Kim et al. employed knockout mice as part of their evaluation. Both techniques have certain advantages and disadvantages. Johnson and Kenny utilized RNA interference in the form of lentiviral vectors that delivered shRNA to knock down D2Rs. While the researchers were able to directly target locations in the striatal hemisphere, they did not completely knock out all D2Rs in the brain. Thus, this technique may be more useful than a knockout mice, simply because embryonic development can proceed normally. In addition, they did not disrupt D2R-mediated homeostatic functions in the hypothalamus. Kim et al created knockout mice by destroying the base sequence of the D2R gene on the actual chromosome before inserting the genes into fertilized eggs in a surrogate mother. The resulting pups were heterozygous +/-. The heterozygous mice then mated to produce homozygous knockout mice. Polymerase chain reaction (PCR) verified the genotypes of the mice. While the knockout technique is very specific, it does have certain caveats. For instance, though not in this case, the eliminated gene might be required for survival, in which case the knockout mice do not survive. Still in other cases, the protein product may be important in embryonic development, so destruction of the gene may lead to confounding phenotypes.
Another noteworthy comparison between the two papers involves the brain region of study. The Johnson and Kenny paper looked at D2Rs in the striatum, while the Kim et al. paper examined the D2Rs in the arcuate nucleus of the hypothalamus. The striatum is part of the forebrain and includes the nucleus accumbens. Dopaminergic neurons from the ventral tegmental area (VTA) in the midbrain extend to the nucleus accumbens via the mesocorticolimbic dopaminergic pathway, also known as the reward circuitry (Bear et al., 2006). Dopaminergic neurons in both of these regions have receptors for addictive drugs such as cocaine, nicotine, and heroin. Taking these drugs stimulates and/or enhances dopamine function in the brain reward circuits (Di Chiara and Imperato, 1988). With chronic drug administration, however, there is down regulation of the dopaminergic pathway (Kenny et al., 2006). Similarly, in the Johnson and Kenny paper, the authors note that rats on the cafeteria diet developed a decreased sensitivity of the brain reward pathway (increased BSR levels). This hypofunction may contribute to a vicious cycle such that the rats then eat more fatty foods to avoid a negative state (Wang et al., 2002). Johnson and Kenny maintain that because rats on the cafeteria diet displayed fewer striatal D2Rs, there is a causal relationship such that access to palatable food leads to overeating and decreased D2Rs, resulting in hypofunction of the reward circuitry, which then leads to continued over eating. One final consideration is that the authors targeted the dorsolateral striatum when they knocked down the D2Rs with the lentivirus vector. In contrast, drug addiction studies have generally found down regulation of D2Rs in the ventral striatum and not in the dorsolateral striatum (Dalley et al., 2007). Still, the results in the Johnson and Kenny paper point to the importance of the reward circuitry underlying aspects of obesity.
The Johnson and Kenny paper is certainly convincing, but one must remember that the regulation of feeding behavior is also connected with metabolism. Hedonic mechanisms are not the only factors to consider in eating and obesity. Thus, the Kim et al. paper did not consider the reward circuitry specifically. These authors looked at D2Rs in the arcuate nucleus of the hypothalamus, a region of the brain involved in homeostasis regulation. The authors state that because dopamine inhibits leptin signaling, a lack of D2Rs allows for increased leptin sensitivity and an alteration in food intake and energy expenditure. Still, because they used knockout mice, they could not negate the fact that D2R-/- mice might have had altered reward circuitry anyway. The results would have been more convincing had the authors used either some sort of conditional knock down or a specific knock down of receptors such as with a lentivrius like in the Johnson and Kenny article.
Finally, it is worthwhile to consider the role of leptin. Although the Johnson and Kenny paper did not study the effects of leptin, they briefly referred to this hormone in their conclusion. As mentioned previously, leptin is a homeostatic hormone released by fat cells that signals satiety. The Kim et al. paper concentrated on the effects of leptin and revealed that dopamine reduces leptin-induced behaviors. If one considers these results in the context of the Johnson and Kenny paper, then it could be that overeating leads to a reduction in D2R density, which results in an increase in serum leptin levels, as the dopamine system no longer negatively regulates leptin to the same extent. Increased leptin signals satiety, so perhaps this represents inhibition of the brain’s reward circuitry such that homeostatic signals can then be overridden to allow for continued overeating. Such a model remains to be tested. In other words, it might be interesting to target D2Rs or leptin receptors in the hypothalamus in such a way so as to induce obesity/compulsive eating.
In elucidating the possible role of leptin in the reward circuitry, it is beneficial to consider two 2006 studies cited in the Kim et al. paper. First, one lab used WT and ob/ob mice to analyze pSTAT3 levels in the VTA upon leptin injection (Fulton et al., 2006). Just as in the Kim et al. paper, Fulton et al. noticed that leptin induced phosphorylation of STAT3. The first figure in this article seems to show, though, that pSTAT3 levels were enhanced in the ob/ob mice much more so than in the WT mice, though the authors state that this was not a significant difference (see below). It is not clear whether STAT3 is even present in significant quantities in the VTA as compared to in the hypothalamus. In another article in the same issue of Neuron, Hommel et al. considered the prevalence of leptin receptors in the VTA. These authors were able to regulate feeding behavior by directly injecting leptin into the VTA of mice. They also demonstrated that leptin inhibited dopaminergic neurons from the VTA (see below). This is a rather confusing result, as one would then expect ob/ob mice (that do not have leptin) to have enhanced dopamine levels and signaling. However, when Fulton et al. electrically stimulated the axons of dopaminergic neurons from the VTA to measure dopamine release, they found that dopamine levels were reduced by about 46% in ob/ob mice as compared to WT mice (see below). Such conflicting results lead to the question of whether leptin is relevant to reward circuitry. Hence, the leptin controversy that was sparked in the 1990s continues today. Researchers recognize that leptin is not a miracle cure to obesity, but they still hold contrasting views regarding leptin's effects on brain reward circuitry.
Phosphorylation of STAT3 in the VTA of WT and ob/ob mice as displayed in the Fulton et al. article.
LEFT: Using extracellular, single-unit recordings, Hommel et al. revealed that intravenous leptin administration inhibits VTA dopaminergic neuron firing in rats.
RIGHT: Fulton et al. - decrease in dopamine release in ob/ob mice.
Ultimately, the two papers developed slightly different models for obesity and regulation of feeding behavior. Johnson and Kenny concluded that rats on the cafeteria diet had disrupted reward circuitry and ignored adverse consequences such as foot shocks in order to continue eating the cafeteria food. These two aspects of the study truly parallel human models of obesity. Stice et al. presented research in which women who gained weight over the course of six months had fewer D2Rs and less caudal activation in response to a milkshake (Stice et al., 2010). In addition, many people know that overeating has negative health and even social consequences (a parallel to the foot shock), yet they continue to binge. In contrast, the Kim et al. paper offered support for the role of D2Rs in energy homeostasis. More studies are definitely needed to clarify the complex links among leptin, dopamine, reward circuitry, and overeating.
Bibliography for Synthesis
Bear, M.F., B.W. Connors, and M.A. Paradiso. Neuroscience: Exploring the Brain. Philadelphia, PA: Lippincott Williams & Wilkins, 2006.
Dalley, J.W., Fryer, T.D., Brichard, L. et al. Nucleus Accumbens D2/3 Receptors Predict Trait Impulsivity and Cocaine Reinforcement. Science 315: 1267-1270, 2007.
Di Chiara, G. and Imperato, A. Drugs abused by humans preferentially increase synaptic dopamine concentrations in the mesolimbic system of freely moving rats. Proc Natl Acad Sci USA 85: 5274–5278, 1988.
Fulton, S., Pissios, P., Manchon, R.P. et al. Leptin regulation of the mesoaccumbens dopamine pathway. Neuron 51: 811-822, 2006.
Hommel, J.D., Trinko, R., Sears, R.M. et al. Leptin receptor signaling in midbrain dopamine neurons regulates feeding. Neuron 51: 801-810, 2006.
Kenny, P.J., Chen, S.A., Kitamura, O., et al. Conditioned withdrawal drives heroin consumption and decreases reward sensitivity. J Neurosci 26: 5894-5900, 2006.
Stice, E., Yokum, S., Blum., K. et al. Weight Gain Is Associated with Reduced Striatal Response to Palatable Food. J Neurosci 30(39): 13105-13109, 2010.
Wang, G.J., Volkow, N.D. and Fowler, J.S. The role of dopamine in motivation for food in hmans: implicatison for obesity. Expert Opin Ther Targets 6: 601-609, 2002.
Please note that all other sources are cited on the initial project page.
Return to the project page Neural Reward, Energy Homeostasis, and Addiction-like Compulsive Eating.