Mapping of g-protein coupling reveals a specialized role for mu-opioid receptors in the parabrachial nucleus in the control eating
Authors:   , Ward Heather G. , Harvey John A , Simansky Kenny J 
Published: July 2024

Numerous studies have implicated the endogenous opioid system in ingestive behavior. Morphine and opioid peptides stimulate food intake, whereas antagonists to opioid receptors reduce intake. Attention has primarily focused on reward-related regions in opioid-mediated feeding, such as the nucleus accumbens and the ventral tegmental area; however, less attention has focused on the parabrachial nucleus (PBN) of the hindbrain. The PBN communicates with brain regions involved in taste, visceral function, reward, basic autonomic activities, and eating. The PBN contains a large population of mu-opioid receptors (MORs), fewer kappa-opioid receptors (KORs), and an extremely small number of delta-opioid receptors (DORs). The PBN communicates with numerous brain regions that have been implicated in opioid-mediated food intake and thus, appears to be ideally located within the neurocircuitry to control feeding. Previous data from our laboratory demonstrated that acute stimulation of MORs in the PBN increased consumption of standard chow in rats and this effect was blocked by a reversible MOR antagonist. However, the study did not address the anatomical regions affected by the drug, whether persistent ingestive changes would occur with long-term impairment of receptor function, or the degree of inhibition of receptors. The studies in this thesis focused on the role of MORs in different subregions of the PBN in the control of food intake through pharmacological, behavioral, and cellular approaches. In particular, these data addressed whether endogenous MORs in the PBN modulate eating and whether this regulation depends on the composition of the food; whether long-term reduction of the coupling of MORs to their G-proteins in the PBN would persistently reduce food intake; and whether the physiological roles of parabrachial MORs are localized to a specific anatomical subregion. In order to address each question, we made a single infusion of the irreversible antagonist, [beta]-funaltrexamine ([beta]-FNA) into different subregions of the PBN and measured the consumption of standard chow and either high fat/high sucrose (HF/HS) chow or HF/HS Ensure®. To selectively verify the inhibition of MOR function at the receptor level, each study used the irreversible MOR antagonist, [beta]-FNA in conjunction with the cellular assay, [³⁵S]GTP[gamma]S autoradiography, to visually map and quantify regional loss of MOR G-protein coupling after microinfusion of irreversible antagonist. The studies in this thesis strongly implicate MORs in the PBN of the pons in the regulation of food intake. Irreversible antagonism of MORs completely prevented receptor function at the G-protein level and thus reduced standard chow intake by approximately 30%. In contrast, inhibition of MORs did not affect consumption of a solid HF/HS diet in either single- or dual-diet studies. In a separate study, [beta]-FNA decreased consumption of standard chow for 18 days after infusion, but did not alter intake of the HF/HS liquid Ensure® diet, which contained the same relative macronutrient composition as the HF/HS solid chow. The long-term decrease and subsequent recovery of standard chow intake on Day 19 corresponded with the complete prevention of MOR coupling in the inferior lateral aspect of the lateral PBN (LPBN) on Days 2 and 8 and recovery of MOR function on Day 20. Smaller infusions of the irreversible antagonist into different subregions of the PBN revealed that MORs in the inferior lateral aspect of the PBN (LPBC and LPBE) appear to modulate the consumption of standard chow but not HF/HS Ensure®. In contrast, complete inhibition of MOR function in the medial subregion of the PBN (MPBN) decreased Ensure® intake for only 2 days but did not alter standard chow consumption. Inhibition of MOR function in the ventral lateral subregion of the PBN (LPBV) did not affect intakes of either diet. These studies strongly suggest that the physiological coupling of MORs in the inferior lateral PBN may serve to regulate the consumption of standard food. Foods that are high in fat and sucrose may serve as a feedforward mechanism via taste receptors in the tongue through the nucleus of the solitary tract and then the PBN to drive food intake and thus override MOR inhibition in the PBN. In summary, the results of this thesis support a potential new target for the treatment of obesity.