Role of neurokinin B-expressing neurons in the bed nucleus of the stria terminalis

Anxiety disorders constitute the largest group of brain diseases in European countries and are a major burden for the society. A considerable portion of patients displays a chronic disease course or does not respond to available treatment. Thus, a better understanding of the underling mechanisms may eventually lead to the development of improved therapies. Anxiety disorders are characterized by pathological, dys-regulated anxiety and fear. While anxiety is considered as a general apprehension of future potential harm, fear is a specific emotional reaction to a clearly identified threat and involves selective learning processes. Recent evidence points towards an important role of neuropeptides in the modulation of both, fear and anxiety. For instance tachykinins are peptidergic neurotransmitters in the mammalian brain, particularity enriched in limbic brain areas that are relevant for mediating emotional behavior. Neurokinin B (NKB), a member of the tachykinin family, is highly expressed in the bed nucleus of the stria terminalis (BNST), a brain area that modulates anxiety-related behavior. However, its role in processing of learned fear, in particular sustained fear and fear generalization, is increasingly recognized. The goal of the proposed project is to characterize NKB neurons in the BNST by (1) identifying their axonal projections and their behavioral relevance for fear processing and (2) by investigating functionally-connected neuronal ensembles and their function in other brain areas. Thus, our primary intention is to elucidate projections of NKB neurons, originating from the BNST and targeting the main fear-processing centers in the amygdala, a brain area central for emotional-processing. In preliminary experiments, we illustrate the expression and initial neurochemical characterization of NKB-neurons in different BNST subdivisions. By combining immunohistochemistry with genetic neuronal tract tracing, we mapped an unexpected elaborated axonal network established by NKB- BNST neurons. Furthermore, by using vrial-vector mediated site-specific expression of artificial designer receptors (these are receptors that have no endogenous ligand but can instead be activated by pharmaceutical compounds) demonstrated that NKB-BNST neurons are increasing fear expression while promoting fear extinction learning. A more specific and detailed investigation will now address the role of the individual BNST subdivisions. Interestingly, in a pilot experiment, we identified functionally connected, neuronal ensembles in the amygdala that were activated by exogenous excitation of NKB-BNST neurons. We are now planning to further characterize these neuronal ensembles, in terms of projection targets, neurochemical content as well as electrophysiological and fear-related properties. In summary, we will focus on feedback-projections of NKB-BNST neurons targeting the main fear- processing nuclei in the amygdala, and we will investigate the neuroanatomical and functional properties of the neuronal ensembles that are activated by this feedback loop.

Role of neurokinin B-expressing neurons in fear integration Anxiety disorders constitute worldwide the largest group of brain diseases and are a major burden for the society. A considerable portion of patients displays a chronic disease course or does not respond to available treatment. Thus, a better understanding of the underlying mechanisms may eventually lead to the development of improved therapies. Anxiety disorders are characterized by pathological dysregulation of innate anxiety and learned fear. Recent evidence points towards an important role of neuropeptides in the modulation of both, fear and anxiety. For instance, tachykinins are neuromodulators particularity enriched in limbic brain areas that are relevant for mediating emotional behavior. Neurokinin B (NKB), a member of the tachykinin family, is highly expressed in forebrain areas that modulate emotional affective behavior, such as the bed nucleus of the stria terminalis (BNST). However, the relevance of NKB BNST neurons in the processing of anxiety, sustained fear and fear generalization, all hallmarks of human post-traumatic stress disorder, is unclear. The primary intention of this project was to elucidate, anatomically and functionally, the neuronal network of NKB neurons in the BNST and the projections to other brain areas relevant for emotional-processing. The findings of the project suggest that NKB neurons in specific subdivisions of the BNST promote anxiety and long-duration, sustained fear expression, without affecting short-term, phasic fear responses. Importantly, these changes in sustained fear reactions were particular strong in males, while not seen in females, indicating sex differences in neuronal functioning. Interestingly, a reduction in food intake was discovered during normal feeding, fasting-induced refeeding and a reduction in body weight upon repetitive stimulation of NKB BNST neurons. These results suggest that NKB in the BNST controls the integration of emotional and metabolic stimuli to produce an adapted behavior in a sex-specific manner. They further highlight the neuronal substrate that links increased anxiety and sustained fear expression with reduced food intake. In particular, the integration and mutual interaction of food intake and emotional responses and the potential sex differences in the underlying circuitries and/or neurotransmitter systems, may have important implications when translating experimental findings into clinical treatment. Imbalances and dysfunctions in NKB BNST circuitries may eventually serve as an explanation for the high co-morbidity of anxiety and eating disorders in human subjects.