Interleukin‐6 in the central amygdala is bioactive and co‐localised with glucagon‐like peptide‐1 receptor

Abstract

Neuronal circuits involving the central amygdala (CeA) are gaining prominence as important centres for regulation of metabolic functions. As a part of the subcortical food motivation circuitry, CeA is associated with food motivation and hunger. We have previously shown that interleukin (IL)‐6 can act as a downstream mediator of the metabolic effects of glucagon‐like peptide‐1 (GLP‐1) receptor (R) stimulation in the brain, although the sites of these effects are largely unknown. In the present study, we used the newly generated and validated RedIL6 reporter mouse strain to investigate the presence of IL‐6 in the CeA, as well as possible interactions between IL‐6 and GLP‐1 in this nucleus. IL‐6 was present in the CeA, mostly in cells in the medial and lateral parts of this structure, and a majority of IL‐6‐containing cells also co‐expressed GLP‐1R. Triple staining showed GLP‐1 containing fibres co‐staining with synaptophysin close to or overlapping with IL‐6 containing cells. GLP‐1R stimulation enhanced IL‐6 mRNA levels. IL‐6 receptor‐alpha (IL‐6Rα) was found to a large part in neuronal CeA cells. Using electrophysiology, we determined that cells with neuronal properties in the CeA could be rapidly stimulated by IL‐6 administration in vitro. Moreover, microinjections of IL‐6 into the CeA could slightly reduce food intake in vivo in overnight fasted rats. In conclusion, IL‐6 containing cells in the CeA express GLP‐1R, are close to GLP‐1‐containing synapses, and demonstrate increased IL‐6 mRNA in response to GLP‐1R agonist treatment. IL‐6, in turn, exerts biological effects in the CeA, possibly via IL‐6Rα present in this nucleus.

Anatomical and Functional Characterization of Central Amygdala Glucagon-Like Peptide 1 Receptor Expressing Neurons

“…Tac2 is a neuropeptidergic gene enriched in medial and some lateral CeA neurons and modulates the expression of conditioned fear (Andero et al, 2014(Andero et al, , 2016. A recent study identified that ∼50% of Glp1r CeA neurons coexpress the cytokine interleukin-6 (IL-6) and that direct administration of IL-6 to the CeA could produce a subtle decrease in feeding, but the functional impact of IL-6 in combination with CeA GLP-1Rs is unclear (Anesten et al, 2019). The limited overlap we observed between Prkcd/Sst/Tac2 and Glp1r in the CeA suggests that the anorexigenic actions of peripherally-applied or intracerebral GLP-1R agonists are likely not due to the direct activation of these neurons, but do not preclude their activation through indirect network effects (Gabery et al, 2020).…”

“…Glp1r CeA neurons are located throughout the rostral-caudal axis with a non-significant increase in the middle third of the CeA and are more densely distributed in the CeM. Coincidently, the CeA also receives innervation from Preproglucagon (PPG, the precursor gene of GLP-1)-expressing neurons in the nucleus of the solitary tract and this innervation is most dense in the CeM (Llewellyn-Smith et al, 2011;Williams et al, 2018;Anesten et al, 2019). The functional and behavioral role of PPG inputs to the CeA are unknown.…”

Overview of the central amygdala role in feeding behaviour

“…A study has shown that direct injection of GABA A receptor agonist in the CeA decreased food consumption (3) . According to Anesten et al (2019), the stimulation of glucagon-like peptide-1 receptors in the CeA nucleus reduces food intake (12) . Food consumption may be decreased by the microinjection of IL-6 into the CeA via the enhancement of glucagon-like peptide-1 receptors (12) .…”

“…Nevertheless, the amygdala is seen as the main brain region with a critical role in feeding regulation (11) . The central amygdala (CeA) is a significant extra-hypothalamic region that has a determining impact on different physiological aspects and nutrition behaviours (12,13) . Several CeA inputs/outputs provide a vast connection surface for this core (14) .…”

“…The CeA acts as the major output nucleus of the amygdala (17) . Finally, the significance of CeA relates to different functions of the neurotransmitters and hormones involving in feeding behaviour (12,21,(30)(31)(32) . Several brain nuclei participate in the regulation of feeding behaviour, and many studies have focused on identifying the neural mechanisms of food intake regulation (2,33) ; however, the CeA is largely understudied.…”