Food restriction markedly increases dopamine D2 receptor (D2R) in a rat model of obesity as assessed with in-vivo muPET imaging ([11C] raclopride) and in-vitro ([3H] spiperone) autoradiography.
The genetically obese Zucker rat presents several abnormalities related to insulin and brain monoamines, which may play a role in its impaired regulation of food intake and body weight. In a previous study, the possible insulin-monoamine interplay was investigated by measuring brain monoamine and metabolite levels in the three genotypes of the Zucker strain. In addition to the expected results, insulin had a particular effect on striatal dopamine (DA) release, regardless of ponderal status and genotype. We further investigated this point in the present study, using the brain microdialysis technique in the striatum. Lean homozygous Fa-Fa rats responded as expected to insulin with regard to striatal DA release, with increases in DA and 3-methoxy-tyramine levels and decreases in dihydroxyphenylacetic acid and homovanillic acid. Lean heterozygous Fa-fa rats showed a very specific response profile, with decreases in all dopaminergic parameters, suggestive of an effect on DA synthesis rather than DA release. This further emphasizes the marked differences between homozygous and heterozygous lean rats. The obese fa-fa rats clearly fell into two populations. The first showed a profile of response to insulin similar to that of the lean Fa-fa rats, in keeping with the disturbances related to the "fa" gene. The second showed an increase in all the dopaminergic parameters. This pattern of response was, however, different from that of the Fa-Fa rats. These opposing responses in the two obese populations did not reflect differences in the blood glucose response to insulin. One explanation is that 16 wk may be a critical transition period in the development of genetic obesity, with regard to brain monoamine disturbances and the response to insulin.