Affective anxiety is characterized by behavioral inhibition accompanied by autonomic arousal and vigilance, reflecting a future-oriented emotional state that is generated by perceived threat. Identification of neural circuits that underlie anxiety in animal models is essential for understanding neurobiological mechanisms that contribute to normative and pathological anxiety in humans. Basic and clinical research has emphasized the importance of the central nucleus of the amygdala (CEA) and the anterolateral bed nucleus of stria terminalis (alBST) in regulating the affective and physiological components of anxiety. The CEA and alBST are heavily interconnected via GABAergic and corticotropin releasing-factor expressing neurons, and share many common sources of input, including input from interoceptive (i.e., visceral sensory) brainstem nuclei. Interoceptive feedback from body to brain strongly modulates emotional state, and can elicit anxiety in the absence of environmental threat. The proposed research will test new hypotheses regarding the anxiogenic role of a neurochemically discrete interoceptive pathway that targets the CEA-alBST, arising from hindbrain neurons that express glucagon-like peptide-1 (GLP1). GLP1 neurons occupy the most caudal (visceral) portion of the nucleus of the solitary tract (cNST) and medullary reticular formation, where they receive direct and relayed input from visceral sensory afferents. GLP1 axon terminals target CEA-alBST subregions that express CRF and GLP1-receptor(R). GLP1 administered centrally or directly into the CEA increases anxiety-like behavior in rats, whereas anxiety is attenuated after central GLP1-R antagonism. Our pilot data demonstrate that GLP1 neurons are robustly activated in rats after ethologically relevant anxiogenic threats that also activate CRF-rich regions of the lateral (la)CEA and fusiform nucleus of the alBST (BSTfu), where GLP1 fibers are present. Further, manipulation of interoceptive state by fasting rats overnight blocks the ability of anxiogenic threat to recruit GLP1 neurons, and also significantly attenuates anxiety-like behavior. Based on these and other data, we posit that GLP1 signaling facilitates CEA-mediated inhibition of GABAergic tone within the oval nucleus of the alBST (BSTov), thereby contributing to the disinhibition of BST outflow posited to model many aspects of generalized anxiety disorder. The proposed work will define and manipulate GLP1 inputs to the CEA-alBST in adult male Sprague- Dawley rats to test our hypothesis that anxiety-like behavior is increased by endogenous GLP1 signaling from the cNST to the CEA-alBST. To challenge this central hypothesis and to explore underlying mechanisms, proposed experiments are organized into two Specific Aims.
The first Aim will characterize the synaptic arrangement of GLP1 axonal inputs to anatomically-defined targets within the CEA and alBST, in order to test the hypothesis that GLP1 neurons are synaptically linked to CRF-positive laCEA neurons that innervate the BSTov. We also will test the hypothesis that GLP1 axon terminals form excitatory synaptic inputs to the dendrites/soma and axon terminals of laCEA neurons that innervate CRF neurons in the BSTov, and to the dendrites/soma of CRF neurons within BSTfu. Anatomical data will be supplemented by ex vivo acute slice physiology experiments that will record from laCEA, BSTov, and BSTfu neurons with known axonal projections to investigate their responses to a GLP1 analogue.
The second Aim will probe the anxiogenic importance of endogenous GLP1 signaling to the CEA-alBST, to test the hypothesis that chronic knockdown of GLP1 expression by cNST neurons will attenuate anxiety-like behavior and alter CEA-alBST neural responses to anxiogenic threat. We also will test the hypothesis that anxiety-like behavior will be attenuated in rats after chronic knockdown of GLP1-R expression by CEA neurons and/or BSTfu neurons.
The proposed research will use laboratory rats to test the hypothesis that anxiety-like behavior is increased by endogenous glucagon-like peptide 1 (GLP1) signaling from the caudal, visceral portion of the nucleus of the solitary tract to the central nucleus of the amygdala and the anterior lateral bed nucleus of the stria terminalis. Central administration of GLP1 induces anxiety-like behavior in rats. GLP1-receptor mRNA and protein are expressed in human brain, and we have localized GLP1-positive neurons in human brainstem. Thus, revealing the functional organization of central GLP1 circuits in rats has clinica relevance, and may contribute to the development of new therapeutic approaches for treating anxiety disorders in humans.
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