In lymph nodes, fibroblastic reticular cells (FRCs) form a collagen-based reticular network that supports migratory dendritic cells (DCs) and T cells and transports lymph. A hallmark of FRCs is their propensity to contract collagen, yet this function is poorly understood. Here, we demonstrate that podoplanin (PDPN) regulated actomyosin contractility in FRCs. Under resting conditions, when FRCs are unlikely to encounter mature DCs expressing the PDPN receptor, CLEC-2, PDPN endowed FRCs with contractile function and exerted tension within the reticulum. Upon inflammation, CLEC-2 on mature DCs potently attenuated PDPN-mediated contractility, resulting in FRC relaxation and reduced tissue stiffness. Disrupting PDPN function altered the homeostasis and spacing of FRCs and T cells, resulting in an expanded reticular network and enhanced immunity.

SUMMARY Ventral tegmental area (VTA) dopamine (DA) neurons play a central role in mediating motivated behaviors, but the circuitry through which they signal positive and negative motivational stimuli is incompletely understood. Using in-vivo fiber photometry, we simultaneously recorded activity in DA terminals in different nucleus accumbens (NAc) subnuclei during an aversive and reward conditioning task. We find that DA terminals in the ventral NAc medial shell (vNAcMed) are excited by unexpected aversive outcomes and to cues that predict them, whereas DA terminals in other NAc subregions are persistently depressed. Excitation to reward-predictive cues dominated in the NAc lateral shell and was largely absent in the vNAcMed. Moreover, we demonstrate that glutamatergic (VGLUT2-expressing) neurons in the lateral hypothalamus represent a key afferent input for providing information about aversive outcomes to vNAcMed-projecting DA neurons. Collectively, we reveal the distinct functional contributions of separate mesolimbic DA subsystems and their afferent pathways underlying motivated behaviors.

SUMMARY Mesolimbic dopamine (DA) neurons play a central role in motivation and reward processing. Although the activity of these mesolimbic DA neurons is controlled by afferent inputs, little is known about the circuits in which they are embedded. Using retrograde tracing, electrophysiology, optogenetics and behavioral assays we identify principles of afferent-specific control in the mesolimbic DA system. Neurons in the medial shell subdivision of the nucleus accumbens (NAc) exert direct inhibitory control over two separate populations of mesolimbic DA neurons by activating different GABA receptor subtypes. In contrast, NAc lateral shell neurons mainly synapse onto ventral tegmental area (VTA) GABA neurons, resulting in disinhibition of DA neurons that project back to the NAc lateral shell. Lastly, we establish a critical role for NAc subregion-specific input to the VTA underlying motivated behavior. Collectively, our results suggest a distinction in the incorporation of inhibitory inputs between different subtypes of mesolimbic DA neurons.

CHAPTER 1: INTRODUCTION Heterogeneity of major depressive disorder Depression, or major depressive disorder, is a mental disorder that dramatically affects a person's health and life. It is characterized by persistent low mood, inability to feel pleasure in previously enjoyable activities, feeling of low-esteem, fatigue, sleep disturbances, appetite changes, pain without a clear cause, and thoughts of suicide. It has become the leading cause of disability worldwide with over 300 million people affected. The prevalence of major depressive disorder increased by 18% from 2005 to 2015 (World Health Organization, 2017). Depression is also a chronic disease, as half of a people who experienced a single episode are likely to have recurrent episodes with higher frequency and severity (Akil et al., 2018). Even though major depressive disorder is diagnosed as a single entity, it is a really heterogeneous disorder characterized by patients having widely varied symptoms, with little to even no overlap of symptomatologies in some cases (Akil et al., 2018). This heterogeneity of depression hinders both research and treatment of this highly prevalent disorder (Fried, 2017). Antidepressants available on the market today were developed based on a theory called monoamine hypothesis of depression, which was established on several key observations made in 1950s. The hypothesis states that the underlying biological reason for depression is depletion of dopamine, serotonin, and/or norepinephrine levels in the central nervous system. It has been demonstrated that increasing the levels of the aforementioned monoamine neurotransmitters in the brain, either by blockage of their reuptake or inhibition of their degradation, alleviates the depression symptoms in patients (Delgado, 2000; Hirschfeld, 2000). Despite the relative effectiveness of currently available antidepressant medications, they are still lacking and possess a variety of drawbacks. Less than half of the patients achieve full remission after the first treatment with antidepressants (Rush, 2007). That leads to trial-and-error approach, where multiple trials of different treatment are needed until the patient is matched with optimal medication. Even then, for patients that do respond to treatment, it takes weeks or months until the depressive symptoms are alleviated (Berton and Nestler, 2006). Some patients also exhibit resistance to antidepressants, which can develop spontaneously in patients previously responsive to treatment or as a result of worsening illness over the course of time (Thase and Schwartz, 2015). Moreover, treatment with antidepressants has numerous side effects, such as fatigue, sleep disturbances, weight and appetite, and sexual dysfunction (Fergusson, 2001). Therefore, there is still an unmet need for more effective, faster, and safer treatment for major depressive disorder. A different approach to depression treatment would be to conceptualize this disease as a circuit dysfunction instead of a neurotransmitter dysfunction. It is possible that the heterogene...

The neural representation of directional heading is conveyed by head direction (HD) cells located in an ascending circuit that includes projections from the lateral mammillary nuclei (LMN) to the anterodorsal thalamus (ADN) to the postsubiculum (PoS). The PoS provides return projections to LMN and ADN and is responsible for the landmark control of HD cells in ADN.However, the functional role of the PoS projection to LMN has not been tested. The present study recorded HD cells from LMN after bilateral PoS lesions to determine whether the PoS provides landmark control to LMN HD cells. After the lesion and implantation of electrodes, HD cell activity was recorded while rats navigated within a cylindrical arena containing a single visual landmark or while they navigated between familiar and novel arenas of a dual-chamber apparatus. PoS lesions disrupted the landmark control of HD cells and also disrupted the stability of the preferred firing direction of the cells in darkness. Furthermore, PoS lesions impaired the stable HD cell representation maintained by path integration mechanisms when the rat walked between familiar and novel arenas. These results suggest that visual information first gains control of the HD cell signal in the LMN, presumably via the direct PoS ¡ LMN projection. This visual landmark information then controls HD cells throughout the HD cell circuit.Key words: landmark; mammillary; navigation; rat; spatial orientation; visual IntroductionMost mammals are able to reliably perceive their momentary directional heading relative to the environment. In rodents, this directional perception is thought to be encoded by head direction (HD) cells, which are located throughout the limbic system (for review, see Taube, 2007). This HD cell signal appears to be generated from self-movement information that arrives from the vestibular system, but proprioceptive and/or motor efference cues also play a major role in updating the signal during movement (Taube and Burton, 1995; Blair et al., 1997; Stackman and Taube, 1997; Frohardt et al., 2006;Yoder et al., 2011a). Although these idiothetic cues are important for generating and updating the signal, visual landmarks dominantly control the preferred firing direction of the HD cell when these cues are available (Goodridge and Taube, 1995;Zugaro et al., 2003). The neural circuit responsible for providing this "landmark control" to the HD signal is not fully understood but is particularly important for our understanding of navigation and spatial learning.HD signal generation appears to occur within the reciprocal connections between the dorsal tegmental nuclei and the lateral mammillary nuclei (LMN), in which vestibular, motor efference copy, and optic flow information arrive from the medial vestibular nucleus, nucleus prepositus hypoglossi, supragenual nucleus, and paragigantocellular reticularis nucleus (Taube and Bassett, 2003;Song and Wang, 2005;Biazoli et al., 2006). From the LMN, the HD signal is projected bilaterally to the anterodorsal thalamus (ADN), which pr...