This study provides novel information on the expression of GLP-1 receptors in human brains as well as on how liraglutide 1.8 mg may influence short-term appetite regulation among individuals with diabetes. This is the first randomised, placebo-controlled, crossover trial to examine potential neural mechanisms activated by subcutaneous liraglutide therapy. More specifically, we observed changes in activation of the parietal cortex, insula and putamen in response to highly palatable food cues with liraglutide as compared with placebo. Our results suggest that liraglutide may exert central effects by decreasing attention to highly palatable foods, given decreased parietal cortex activation; additional central mechanisms will need to be explored more fully by means of larger trials and/or using higher doses of liraglutide.
Expression of GLP-1 receptors in the human brain
We observed expression of GLP-1 receptors in the hypothalamic nuclei and medulla oblongata of humans, confirming the findings of studies in rodents and primates [10–12]. This suggests that GLP-1 analogues may confer some of their effects on appetite through actions on the hypothalamus. The identification of GLP-1 receptors in the area postrema may also explain why nausea is a relatively common side effect of GLP-1 analogues , considering the area postrema has a large role in nausea and vomiting .
Interestingly, we observed for the first time GLP-1 receptors on neurons in the parietal cortex, suggesting our findings of altered parietal activation with liraglutide could be a direct rather than an indirect effect (e.g. acting through hypothalamic circuitry) of the medication. This needs further study. Our findings are strengthened by the translational finding that activation of this area in fMRI was directly correlated with the desirability of food to patients. Our findings are also distinct from those of other studies, using the same antibody, reporting that there were no GLP-1 receptors in the parietal cortex of non-human primates . Although we did not see changes in the hypothalamus in response to food cues while participants were taking liraglutide, this may be due to the limitations of MRI, due partly to the size of this structure and partly to its proximity to the sinuses, which creates artefacts . As far as identification of GLP-1 in the human brain, Drucker and Asa described in 1988 GLP-1 (also known as GCG) mRNA-positive neurons in the region of dorsal motor nucleus of the vagus nerve in the medulla oblongata . Recently, GLP-1-immunopositive neurons within the human medulla oblongata have been observed in two patients . Immunoreactivity was displayed in the nucleus of the solitary tract, in area postrema and in the dorsal motor nucleus of the vagus nerve. We will expand our findings to determine whether the GLP-1 receptor also exists in these areas and explore its presence in more patients.
Metabolic impact of liraglutide
In our trial GLP-1 immunoreactivity in the circulation of individuals increased in a step-wise manner with increasing doses of liraglutide. Additionally, fasting blood sugar levels improved without weight loss in this short course of 1.8 mg liraglutide therapy. No significant changes were noted in fructosamine, HbA1c, lipid profiles, blood pressure or anthropometric measurements, such as BMI and fat mass, as expected with the short duration of therapy designed to eliminate weight loss as a potential confounder. We did notice decreased energy intake with the 1.8 mg liraglutide dose with no accompanying changes in resting energy expenditure; this would likely lead to weight loss with a study of longer duration.
Central mechanisms of liraglutide
We observed that, in response to liraglutide treatment, parietal activation decreased when participants viewed images of highly desirable food. The inferior parietal cortex is a well-known part of the fronto-parietal attention network, which demonstrates increased activation to salient or important and, in this case, highly desirable stimuli [21–23]. Thus, we suggest that this decreased activation represents the decreased significance or appeal of more desirable (high-energy) foods while participants were taking liraglutide. Parietal activation in response to images of high-energy foods inversely correlated with weight loss in another fMRI study, suggesting that decreased attention to these images leads to greater weight loss . Additionally, our participants rated themselves as feeling fuller in the fasting state while receiving liraglutide and had a lower energy intake with increasing liraglutide doses, altogether suggesting that they would indeed lose weight. Those with the least inferior parietal activation identified in the whole-brain analysis also reported it would be less pleasant to eat while on liraglutide during the fasting state, further supporting this notion. This change in activation may indicate neural changes that could proceed to and predict future weight loss with liraglutide, a notion to be fully evaluated by long-term studies.
Further supporting these findings are our whole-brain regression results, which show that activation of areas related to attention and saliency processing, including the occipital cortex, cuneus, precuneus and parietal cortex, were correlated with ratings of hunger and appetite. Greater activation in these areas in response to highly desirable vs less desirable food cues during the fasting state while on liraglutide were associated with higher levels of hunger, larger appetite and a feeling of how pleasant it would be to eat. These occipital areas (cuneus, precuneus and occipital cortex) are known to respond to food cues and this has been attributed to the emotional salience of food cues [25, 26]. Indeed, increased activation of occipital cortex has also been linked with emotional images [27, 28]. This may indicate individual differences in response to liraglutide, where participants who have decreased activation in these areas show less hunger and appetite. Interestingly, ratings of nausea while on liraglutide correlated inversely with activations in some of the same attention-related brain areas, likely indicating that participants are less interested in food items while feeling nauseous, as would be expected. We also report changes in activation in sensorimotor areas, including the cingulate, superior frontal and somatosensory cortex, which are often linked with feelings of nausea [29, 30]. These may be deactivated in response to highly desirable food cues simply due to the feeling of nausea and lack of desirability of these types of foods when one is feeling nauseous. Additionally, nausea is a side effect of medication, so these changes might be a result of increased medication effects for certain individuals at 17 days.
There are limited other studies examining the effects of other GLP-1 analogues in the human brain. One study has demonstrated changes in resting-state hypothalamic connectivity with a single continuous infusion of exenatide in obese responders (e.g. those who consumed less food at a subsequent ad libitum buffet) . A recent study using exenatide found changes in brain areas of the reward system, including the insula and orbital frontal cortex, with a similar task . Nausea could be considered a confounder in that study which used a single dose, whereas in our study, nausea had subsided for most patients but weight loss had not yet occurred . We did observe similar activation in the insula and putamen with the same small volume correction on our data. It remains to be seen whether statistically significant results would also be observed with a higher dose of liraglutide (3 mg) or more participants. More recently, another study comparing liraglutide with insulin showed similar decreases in activation to food cues after 10 days, but not 12 weeks, in the insula and putamen . It is well known that the insula is involved in saliency processing and satiety [33–35]. Insular activity has been shown to be modulated with ratings of hunger and fasted/fed states [34, 35], indicating a role in the salience of food images similar to the parietal cortex, consistent with its activity during non-food salient stimuli . Some studies have suggested that the putamen may be involved in the reward processing of food [15, 37–39] and have shown links between activation in the putamen and future weight gain . Thus, since these areas decrease in activation on liraglutide, we would assume that participants find highly palatable foods less rewarding or relevant while on liraglutide. We report such data as only preliminary, however, and they need to be confirmed by future studies.
Limitations and future directions
We demonstrate a central role for GLP-1 analogues in mediating weight loss in humans. Results from immunohistochemical studies support the findings of our fMRI study, suggesting that liraglutide likely acts through the parietal cortex directly to decrease attention to highly palatable food cues. Due to the crossover design of our study with randomly assigned sequence, we are confident that potential covariates are well-controlled and data are appropriately analysed, since each individual serves as his or her own control. However, the strength of our findings could be amplified by the addition of more participants and/or with studies of longer duration.
We did not see any changes in fMRI activation of the hypothalamus with liraglutide, in contrast to immunohistochemistry findings in rodents and primates [10, 12]. This may be due to differences in terms of control of eating between rodents and humans; in the latter the homeostatic control of eating is often overridden by more complex cortical systems regulating the rewarding value of food . This may also be a limitation of fMRI in general, as the hypothalamus is difficult to detect with typical fMRI measures due in part to its small size and in part to its proximity to the sinuses, which create artefacts .
The fMRI outcomes of our study need to be studied in different time frames after administration to explore possible sequential effects (e.g. changes at first dose and after longer-term use). Additionally, comparative studies to explore potential differences between types of GLP-1 analogues would be useful. Also, different doses (e.g. 3.0 mg vs 1.8 mg liraglutide) need to be examined further. It is possible that more pronounced changes could have been observed with the newly recommended higher dose of liraglutide (3.0 mg), which has been shown to cause weight loss more dramatically . In fact, despite no difference in gastric emptying or nausea, the 3.0 mg dose of liraglutide caused greater weight loss than the 1.8 mg dose, suggesting greater central/brain changes . Thus, future studies need to examine how this higher dose alters brain activations.
We did not see any significant changes with respect to neurocognition; the variable closest to significance was the stop signal reaction time (i.e. the most appropriate test as per our a priori hypotheses), which can be used for future power calculations with higher doses and/or longer duration. The stop signal reaction time is a measure of inhibitory control, where a lower time indicates greater inhibitory control, which has been shown to be impaired with obesity . This may indicate that with higher doses and/or more participants, liraglutide may increase inhibitory control, but this will need to be confirmed.