Functional neuroanatomy of brainstem circuits for the defense reaction

Challenged with threats, mammals exhibit a plethora of adaptive survival responses. The biological machinery underlying the survival responses and their neuronal substrates that support them has been conserved throughout evolution. In order to react to innate or learned threatening stimuli the mammalian brain generates integrated affective states that encompass various defensive responses including behavioral changes, adaptations of the autonomic system, and endocrine fluctuations. Brain areas such as the central amygdala (CeA), periaqueductal gray (PAG), and the dorsal vagal complex (DVC) have been demonstrated to play important roles in mediating behavioral and autonomic aspects of the defense reaction. However, while the integrated nature of defensive reactions has long been known, the underlying circuit mechanisms remain poorly understood. Novel technologies such as the genetic dissection of brain circuits enable us to better understand the neurochemical and molecular identity and the specific connectivity of targeted neuronal populations. This allows for detailed characterization of the anatomical basis for defensive circuit functions. In the present dissertation, genetic and viral approaches were used to characterize the connectivity of the DVC, a key regulator of cardiovascular functions with midbrain and forebrain areas. A retrogradely transported adeno-associated virus (AAV) injected into the DVC allowed visualization of excitatory afferents from vesicular glutamate transporter-2-positive neurons (Vglut2+) of the periaqueductal gray (PAG) and inhibitory afferents from glutamate decarboxylase-2 enzyme-positive (Gad2+) neurons of the CeA. Furthermore, co-infection mediated by the injection of a double-conditional, anterogradely transported AAV injected into the PAG and the CeA allowed labeling of the synaptic terminals of their afferents to the DVC as well as their brain-wide collaterals. The main results suggest that a population of glutamatergic neurons of the ventrolateral column of the PAG (vlPAG) innervates the dorsal motor nucleus of the vagus (DMV) within the DVC along its anterior-posterior extension and sends collaterals to other nuclei in the ventral hindbrain, including the magnocellular medulla (Mc), a brain region essential for freezing. In addition, the obtained data revealed that long-range GABAergic projections from the CeA preferentially target the nucleus of the solitary tract (NTS) in the DVC and send collaterals to the vlPAG. Following up on the identified neuroarchitecture of the CeA to DVC and vlPAG pathway, we performed intersectional optogenetics experiments in the CeA to understand the role of this neuronal population in the defense reaction. The results shed light on the implications of this inhibitory circuit in defensive brain states. In sum, our data builds the basis for future functional studies aiming at how these projection pathways coordinate the integration of different components of the defense reaction.