Effects of dietary preference and galanin administration in the paraventricular or amygdaloid nucleus on diet self-selection.
Microinjection of galanin into the paraventricular nucleus (PVN) of the hypothalamus has been reported to predominantly increase dietary fat consumption during the light or late dark periods. However, there have been no studies of the macronutrient-specific effects of galanin administered into the amygdala (AMY). We used male Sprague-Dawley rats to test the effect of galanin administered into the PVN or central nucleus of the AMY on diet selection using two different protocols: the two-choice composite diet (10% or 55% energy as fat) (Study 1) or the three-choice macronutrient diet (Study 2). In the first study, total food intake was significantly increased 60 min after galanin injection (0.3 nmol) into either the PVN or AMY, but there was no difference in intake between the low- and high-fat diets. In Study 2, using the three-choice macronutrient diet, the feeding stimulation produced by galanin in the PVN also was not diet-dependent. In the AMY, there was a significant effect of diet on the feeding response to galanin, due primarily to the lack of stimulation of protein intake, whereas the intakes of fat and carbohydrate were not significantly different. In summary, there was no difference in the effect of galanin on fat versus carbohydrate intake for either nuclei, whether animals were injected during the light or the late-dark phase. Because rats often display preferences when allowed to choose among individual macronutrient sources, we also examined the relationship between baseline macronutrient preference and the feeding response to galanin in groups of highly fat-preferring and carbohydrate-preferring rats (Study 3). After PVN galanin injection, fat-preferring rats demonstrated significant increases in their consumption of both carbohydrate and fat, whereas carbohydrate-preferring rats responded by eating carbohydrate almost exclusively. There were significant positive associations between baseline macronutrient intake and galanin-induced intake. The results of these studies combined suggest that the effect of galanin on macronutrient selection is not specific for fat and that underlying macronutrient preferences are important in determining the feeding response to galanin.