Regulation of feeding by the parabrachial nucleus: A model for studying the mu-opioid receptor system

The parabrachial nucleus (PBN) of the brain---especially the lateral PBN (LPBN)---is densely innervated by neurons that synthesize opioid neuropeptides and which express mu opioid receptors (MOPRs). Additionally, the PBN receives afferent input from gustatory and visceral sensory processes (Norgren et al, 1971) and possesses bidirectional communication with other brain regions related to homeostatic and nonhomeostatic feeding. Previous studies from our laboratory showed that parabrachial MOPRs are implicated in diverse roles related to ingestion, making it a useful model system to investigate the function of these receptors (Wilson et al., 2003; Ward and Simansky, 2006).;This thesis defines more precisely the structure and function of MOPRs in the PBN in regulating food intake. The work extended this role by linking events at the receptor level with intracellular signal transduction pathways and associating this cellular physiology with feeding. Thus, these data demonstrated the value of this parabrachial circuit as a model for analyzing in great detail the pharmacological mechanisms and cellular responses associated with MOPR function. These studies revealed several novel findings of methodological and theoretical as well as fundamental significance. First, (+)-naloxone [(+)-NLX] was used both in vitro and in vivo as a pharmacological tool to characterize the stereospecificity of the parabrachial MOPRs. Specifically, we have demonstrated that only (-)-NLX, and not (+)-NLX, blocks parabrachial MOPR-stimulated G-protein coupling further indicating the stereospecificity of the MOPR ligand binding domain.;Furthermore, by using (+)-NLX, we also established a novel mechanism for enhancing G-protein coupling and feeding behavior elicited by the direct agonist DAMGO. In particular, we showed that an ultra-low dose of (-)-NLX (0.001nM) enhanced DAMGO-stimulated G-protein coupling at 15min. The enantiomer (+)-NLX also enhanced DAMGO-stimulated G-protein coupling, but in a wider concentration range (0.01-10 nM). What is more important, (+)-NLX potentiated DAMGO-stimulated feeding behavior using doses that are commonly used with the active (-)-enantiomer for an antagonistic action (1-10nmo1/0.5ul). Based on these results, we suggest that (+)-NLX is a novel pharmacological tool for the study of MOPR function. Moreover, these outcomes ---enhancing agonist action at MOPRs--- would appear to have therapeutic relevance. This thesis further analyzed the MOPR subtype expressed in the PBN. Using the irreversible, RI-antagonist naloxonazine (Nlxz), we revealed that the functional MOPRs located in the parabrachial nucleus are of the micro 1 subtype and that these receptors mediate processes that modulate food intake in rats. Finally, we extended our studies intracellularly and showed that infusion of Nlxz into the LPBN increased the phosphorylation of the transcription factor CREB. With these data, we provided the first evidence implying that the phosphorylation of the cAMP response element binding protein (CREB) can be used as a biological marker that links the downstream cellular cascade of MOPR-associated G-protein coupling to feeding behavior.