The role of glucagon-like peptide-1 (GLP-1) in fluid and food intakes in vasopressin-deficient Brattleboro rats

Abstract

Eating and drinking co-occur and many of the same mechanisms that control one are involved in the control of the other, making it difficult to isolate specific mechanisms for the control of fluid intake. Glucagon-like peptide-1 (GLP-1) is a peptide that seems to be involved in the endogenous control of both ingestive behaviors, but we lack a thorough understanding of how and where GLP-1 is acting to control fluid intake. Vasopressin-deficient Brattleboro rats are a model of hereditary hypothalamic diabetes insipidus that have been used extensively for the study of vasopressin actions in behavior and physiology. Here, we propose that these rats, that eat normally but drink excessively, provide a useful model to dissociate central controls of food and fluid intakes. As an initial step toward establishing this model for these purposes, we focused on GLP-1. Similar to the effect observed after treatment with a GLP-1 receptor (GLP-1R) agonist, the intake difference between wildtype and Brattleboro rats was largely a function in the number of licking bursts, indicating differences in post-ingestive feedback (e.g., satiation). When given central injections of a GLP-1R agonist, the effect on feeding was comparable between wildtype and Brattleboro rats, but the effect of drug on fluid intake was markedly exaggerated in Brattleboro rats. Additionally, Brattleboro rats did not respond to GLP-1R antagonism, whereas wildtype rats did. Taken together, these results suggest that Brattleboro rats exhibit a selective disruption to GLP-1's control of water intake. Overall, these experiments provide foundational studies of the ingestive behavior of Brattleboro rats and demonstrate the potential to use these rats to disentangle the effects of GLP-1 on food and fluid intakes.

Keywords: Drinking; Feeding; Glucagon-like peptide-1; Polydipsia; Thirst.

Figure 1.

Body weight over time in Brattleboro and wildtype rats. Body weight was measured for the duration of all experiments. A) The development of the differences in body weight in the genotypes for rats in Experiments 1–3 (summary for illustration only). B) Average body weight for each group of rats from Experiments 1–3 from PD 57–63. Wildtype rats weighed more than Brattleboro rats and male rats weighed more than female rats, but there was no statistically significant interaction. C) Significant main effect of genotype for PD 57–63 highlighted; wildtype rats weighed more than Brattleboro rats during this time range. D) Significant main effect of sex for PD 57–63; male rats weighed more than female rats during this time range. E) Average body weight for each group of rats from Experiment 4 from PD 113–119. There was a significant sex*genotype interaction: wildtype male rats weighed more than Brattleboro male rats. Male rats weighed more than the female rats of either genotype, however, there were no genotype differences in female rats. Statistical significance (p ≤ 0.05) is indicated by asterisks or by bars with different letters. Abbreviations used: Male, M; Female, F; Wildtype, WT; Brattleboro, BB.

Figure 2.

Food intake by wildtype and Brattleboro rats. Food intake was measured for 5 consecutive 24-h periods. A) When differences in body weight were controlled for, same sex wildtype and Brattleboro rats ate similar amounts of food and male rats of both genotypes ate significantly more than female rats of either genotype. B) Significant main effect of sex highlighted. C) When examining food intake across days of the estrous cycle, there were no differences between the genotypes, but there was a main effect of cycle stage, with no comparisons reaching the threshold for significance in the post hoc test. D) Although ANOVA did not reveal differences between the days of the cycle, an exploratory t-test found a significant difference in food intake in all female rats between diestrus 1 and proestrus. Asterisk indicates significant difference, p ≤ 0.05. Abbreviations used: Body weight, BW; Male, M; Female, F; Wildtype, WT; Brattleboro, BB; Diestrus 1, D1; Diestrus 2, D2; Proestrus, P; Estrus, E.

Figure 3.

Water intake by wildtype and Brattleboro rats. Water intake was measured for 5 consecutive 24-h periods. A) Brattleboro rats drank significantly more than wildtype rats. B) Water intake across the estrous cycle. There was a main effect of genotype without an effect of cycle stage in wildtype or Brattleboro rats. C) An exploratory t-test between diestrus 1 and proestrus found a significant reduction in water intake during proestrus in wildtype female rats (y axis changed to illustrate the effect), but not in Brattleboro rats (data not shown). D) Brattleboro rats had a significantly higher number of licking bursts that underlie the differences in baseline drinking between wildtype and Brattleboro rats. E) The number of licking bursts is a function of estrous cycle stage, but only in wildtype rats. ANOVA detected a main effect of genotype and a lower number of bursts on the day of estrus compared to the day of diestrus 1 in wildtype rats, but the effect of cycle was not observed in Brattleboro rats. F) There were no differences between the genotypes on burst size. G) Female rats had fewer licks per burst. H) There were no differences in burst size across the estrous cycle between genotypes or in either genotype. Asterisks indicates significant differences, p ≤ 0.05. Abbreviations used: Male, M; Female, F; Wildtype, WT; Brattleboro, BB; Diestrus 1, D1; Diestrus 2, D2; Proestrus, P; Estrus, E.

Figure 4.

The GLP-1R agonist Ex4, injected into the LV immediately before dark phase onset. A) All groups had an agonist-induced decrease in food intake (normalized by body weight) without any detected genotype or sex differences. The main effect of drug is illustrated in panel B. C) There was a significant drug*genotype interaction on Ex4 drinking with Brattleboro rats showing a larger magnitude of an effect of Ex4 that was statistically significant in the post hoc testing. Main effects of drug and sex in wildtype rats are illustrated in panels D and E, respectively, with y axes changed to illustrate the effects. F) The effect of Ex4 on fluid intake also was analyzed as percent of baseline, further revealing the exaggerated response to Ex4 in Brattleboro rats. G) When data were analyzed as a percent of baseline across time, suppression was equivalent between the two genotypes for the first 16 h of the test, but Brattleboro rats were significantly more suppressed than wildtype rats were during the later hours of the test. Asterisks indicate significant differences, p ≤ 0.05. Abbreviations used: Male, M; Female, F; Wildtype, WT; Brattleboro, BB.

Figure 5.

Drinking microstructure across the Ex4 intake measure. A) Brattleboro rats had a greater number of licking bursts across the first 12 hours of the test. B) In wildtype rats, there was a main effect of drug in the first 12 hours, with Ex4 causing a decrease in the number of bursts. C) In Brattleboro rats, there was no effect of drug on burst number in the first 12 hours, but there was a main effect of sex (highlighted here). D) In the second 12 hours of the test, there was a significant drug*genotype*sex interaction. Brattleboro rats had a greater number of licking bursts. Brattleboro female rats had a greater number of bursts after Ex4 treatment. E) When the genotypes were analyzed separately, there was a main effect of drug on burst number in wildtype rats, with more licking bursts in the Ex4-treated group in the second half of the test (y-axis changed to illustrate effect). F) When the genotypes were analyzed separately, there was a main effect of drug on burst number in Brattleboro rats, with more licking bursts in the Ex4-treated group in the second half of the test. This effect seemed to be driven by changes in Brattleboro female rats. G) Analyses of burst size in the first half of the test found that Brattleboro rats had more licks per burst than wildtype rats. H) When analyzed separately, Ex4-treatment was associated with a decrease in the number of licks per burst in wildtype rats. I) The same was true for Brattleboro rats—Ex4-treatment was associated with a decrease in the number of licks per burst. J) There was a significant main effect of sex on burst size in Brattleboro rats. K) There was a significant drug*genotype interaction on burst size in the last 12 hours of the test. Post hoc test showed a main effect of drug in Brattleboro rats that was not present in wildtype rats. Asterisks indicate significant differences, p ≤ 0.05. Abbreviations used: Male, M; Female, F; Wildtype, WT; Brattleboro, BB.

Figure 6.

The GLP-1R antagonist, administered during the light phase via an indwelling cannula aimed at the LV. A) There was a main effect of genotype on water intake after Ex9. B) When not otherwise stimulated to drink, an injection of Ex9 caused an increase in drinking, but only in male wildtype rats (y-axis changed to illustrate effect). C) There was a significant effect of genotype on burst number during the Ex9 test. D) There was a significant increase in burst number in wildtype male rats given Ex9 (y-axis changed to illustrate effect). E) There was a significant difference between the size of the licking bursts between genotypes, but there were no changes in burst size in response to Ex9 injection. F) There was a main effect of sex on burst size in Brattleboro rats. Asterisk indicates significant difference, p ≤ 0.05. Abbreviations used: Male, M; Female, F; Wildtype, WT; Brattleboro, BB.

Figure 7.

The GLP-1R antagonist, as well as a subcutaneous injection of hypertonic saline to further stimulate drinking, administered during lights on via an indwelling cannula aimed at the LV. A) There was a main effect of genotype on drinking. B) When the genotypes were analyzed separately, there was a significant difference in the amount of water consumed between male and female wildtype rats (y-axis changed to illustrate effect). C). There was a main effect of Ex9 on drinking in wildtype rats. This effect seemed to be driven by male wildtype rats (y-axis changed to illustrate effect). D) There was a main effect of genotype on burst number, but there were no drug-induced differences in burst number in any group. E) There was a main effect of genotype on burst size, but there were no drug-induced differences in burst size in any group. Asterisks indicate significant differences, p ≤ 0.05. Abbreviations used: Male, M; Female, F; Wildtype, WT; Brattleboro, BB.