A Neurobiologia Vegetal é um recente ramo das ciências vegetais que objetiva esclarecer os complexos padrões de comportamento vegetal, no que se refere à percepção, processamento, armazenamento e transmissão de sinais na planta e entre plantas. A detecção de vizinhos, é uma capacidade que implica em auto reconhecimento, uma vez que um organismo só terá sucesso em interações competitivas se for capaz de auto/não-auto discriminação. Assim, objetivou-se com este trabalho verificar se raízes de ervilha (Pisum sativum) cv. Mikado apresentam crescimento diferenciado quando na presença de raízes da mesma planta, e de raízes de outras plantas, mas pertencentes ao mesmo genótipo, para que se possa averiguar sua capacidade de auto/não-auto discriminação. Além disso, avaliou-se também o crescimento da parte aérea para observar em que grau a presença de plantas vizinhas pode influenciar o desenvolvimento vegetativo de plantas de ervilha. Quatro dias após a germinação, plântulas de Pisum sativum cv. Mikado tiveram a raiz principal cortada 5 mm abaixo do hipocótilo. Passados sete dias, foram retiradas as raízes secundárias, deixando-se apenas duas raízes, de igual tamanho, por planta (split-root). Plantas com duas raízes iguais foram replantadas, com cada vaso contendo duas raízes da mesma planta (tratamento Auto) ou duas raízes de plantas diferentes (Tratamento Não-auto). Os vasos foram agrupados em tríades. O experimento foi mantido em estufa incubadora sob condições de temperatura e fotoperíodo controladas e após 18 dias foram feitas avaliações do crescimento da parte aérea e das raízes, através das medições de: altura da planta (cm), peso fresco de parte aérea e de raiz (g), peso seco de parte aérea e de raiz (g), área foliar (cm2), área radicular (cm2), comprimento total de raiz (cm) e diâmetro médio de raiz (cm). A análise dos dados considerando os valores médios de cada tríade revelou não haver diferença significativa entre os tratamentos Auto e Não-auto com relação ao crescimento de parte aérea. No que se refere ao crescimento da raiz, com exceção do diâmetro médio, as demais variáveis diferiram significativamente, sendo que as plantas pertencentes ao tratamento Auto apresentaram valores de peso seco, área superficial e comprimento total 36,71%, 27,84% e 23,18%, respectivamente, maiores do que as plantas do tratamento Não-auto. Ou seja, as plantas que não estavam sob competição apresentaram maior crescimento de raiz. No entanto, quando se observou o comportamento das plantas entre si, em cada tríade, verificou-se, no tratamento não-auto, diferenças visíveis de crescimento tanto em parte aérea como na raiz entre as três plantas que constituía cada tríade. Verificou-se também que a raiz de uma mesma planta cresceu diferentemente de acordo com a identidade da raiz vizinha. Enquanto que no tratamento auto as três plantas que constituíam uma tríade tinham aproximadamente o mesmo tamanho de parte aérea e raiz. Assim, podemos afirmar que o crescimento das plantas no tratamento não-auto foi influenciado pelas interações entre as raízes e mais que isto, foi dependente da identidade da raiz vizinha implicando em auto/não-auto discriminação e reconhecimento parental. / The Plant Neurobiology is a recent branch of plant science that aims to clarify the complex patterns of behavior vegetable, with respect to perception, processing, storage and transmission of signals in plant and between plants. The detection of neighbors, is a capacity that involves self-recognition and an individual will only be successful in competitive interactions if it is capable of self/non-self discrimination. Thus, the objective was to determine whether roots of pea (Pisum sativum) cv. Mikado grow differently in the presence of the same plant roots, and roots of other plants, but within the same genotype, so that we can determine its capacity for self/non-self discrimination. In addition, we assessed also the growth of the shoot to see to what degree the presence of neighboring plants can influence the vegetative growth of pea plants. Five days from germination, seedlings of Pisum sativum cv. Mikado had the seminal root severed 5 mm below the hypocotyl. After seven days, all but two of these roots were removed, leaving only two roots of equal size per plant (split-root). Plants with two equal roots were replanted, with each pot containing two roots of the same plant (treatment self) or two roots of different plants (Treatment non-self). Pots were grouped in triplets. The experiment was kept in an incubator camera under controlled conditions of temperature and photoperiod and after 18 days were evaluated for growth of shoots and roots. It was measure plant height (cm), fresh weight of shoot and root (g), dry weight of shoot and root (g), leaf area (cm2), root area (cm2), total length of root (cm) and average root diameter (cm). The analysis of data considering the average values of each triplets showed no significant difference between treatments self and non-self in relation to the growth of shoots. With respect to root growth, except for the diameter, the other variables differed significantly, and plants belonging to treatment self had values of dry weight, surface area and total length of 36.71%, 27.84 % and 23.18%, respectively, higher than the treatment plants non-self. That is, plants that were not under competition had higher root growth. However, when we observe the behavior of plants in each triplet, it was found that the treatment non-self, the plants had sizes of shoot and root differ. It was also found that the root of the same plant grew differently depending on the identity of neighboring roots. While in treatment self, the three plants that constituted a triplet had, approximately, the same size of shoot and root. Thus, we can say that the growth of plants to treatment non-self was influenced by the interactions between roots and more that this was dependent on the identity of neighboring roots implying self/non-self discrimination and kin recognition.

Encontramos no bulbo diversos núcleos, assim como diversos neurotransmissores, relacionados com a manutenção da pressão arterial. Dentre os núcleos, o núcleo do trato solitário se destaca por ser um dos principais moduladores do sistema nervoso autônomo, sendo o primeiro a receber aferências dos barorreceptores e encaminhá-los para diversos outros núcleos. Dentre estes neurotransmissores, encontramos o glutamato e as catecolaminas, sendo ambos essenciais para a manutenção da pressão arterial. É sabido que a atuação de transmissores em células do sistema nervoso pode levar a alterações em outras vias de neurotransmissão, alterando assim a resposta das células a estímulos. Levando em consideração a importância do glutamato e das catecolaminas na modulação da pressão arterial, e que tanto os receptores glutamatérgicos quanto catecolaminérgicos podem interferir no metabolismo celular e gerar mudanças estruturais nos neurônios, cogitamos que a atuação do sistema glutamatérgico poderia modular o sistema catecolaminérgico. Neste trabalho, avaliamos se o sistema glutamatérgico e catecolaminérgico podem interagir em culturas de células do bulbo de ratos neonatos, a partir de tratamentos das culturas com glutamato ou noradrenalina. Observamos que o tratamento destas culturas com glutamato leva a uma redução nos níveis de proteína e de mRNA da enzima tirosina hidroxilase e do receptor _2 adrenérgico. A modulação do sistema glutamatérgico a partir de tratamentos com noradrenalina não mostrou variações significativas. Concluímos que o sistema glutamatérgico pode modular o sistema catecolaminérgico em células do bulbo de ratos neonatos, e que esta modulação pode ser importante na regulação da pressão arterial pelos núcleos bulbares. / It is found in the medulla oblongata several nuclei, as well as several neurotransmitters, related with the maintenance of the arterial pressure. Among these nuclei, the nucleus of the solitary tract stands aside for being one of the main modulators of the autonomic nervous system, being the first to receive afferences from baroreceptors and to send their stimuli to other nuclei. Among these neurotransmitters, glutamate and the catecholamines are both essentials to the maintenance of the arterial pressure. It is known that the stimulation of brain cells by neurotransmitters can result in changes in other neurotransmitter pathways, changing the cell response to certain stimuli. Taking in consideration the importance of glutamate and the catecholamines in the modulation of the arterial pressure, and that both of them can interfere in the cellular metabolism and create structural changes in neurons, we have speculated that the stimulation of the glutamatergic system could modulate the catecholaminergic system. In this work, it was evaluated if the glutamatergic and catecholaminergic systems could interact in cell cultures of the medulla oblongata of newborn rats, from treatments of the cultures with glutamate or noradrenaline. It was found that the treatment of these cultures with glutamate leads to a reduction in the protein and mRNA levels of the enzyme tyrosine hydroxylase and the receptor _2 adrenergic. The modulation of the glutamatergic system from treatments with noradrenaline did not show significative variation. We concluded that the glutamatergic system can modulate the catecholaminergic system in medulla oblongata cell cultures, and that this modulation can be important in the regulation of the arterial pressure by nuclei present in the medulla oblongata.

It has long been known that attending to the right place at the right time can improve performance and reaction time in a wide variety of the tasks that humans engage in. If attention is defined as a general mechanism by which a nervous system’s economy of resources and information are distributed to enable a perceptual state that is well-aligned with the goals of the biological system, then changes in visual attention should emerge as changes in the distribution of resources and information in the visual cortex. A growing body of evidence supports this proposition in non-human primates, and suggests that visual spatial attention affects perceptual state through top-down signaling that refines the neural representation of the attended stimulus (Desimone and Duncan, Chelazzi and Reyonlds, Mayo). This refinement is correlated with, and often believed to cause, the change in behavioral performance accompanied by visual spatial attention. To test whether similar mechanisms of visual spatial attention affect perceptual state in mice (our central hypothesis), we recorded neuronal activity in the primary visual cortex while mice engaged in a contrast detection task. This task was designed to induce endogenous shifts in visual spatial attention by changing the probability that a stimulus would appear at a particular location on the monitor. We found that our subject’s contrast detection threshold did not depend on this manipulation, suggesting that either: our subject was unable to distinguish between different probability conditions, AND/OR there was no advantage in attending to the side of higher probability, AND/OR visual spatial attention does not affect perceptual state in mice in ways similar to that of non-human primates. Analyses of pupil recordings have helped us learn more about the subject’s strategy and the ways in which we can modify the task to encourage sizeable shifts in visual spatial attention. In parallel with this experiment, a process was developed to aggregate neuronal data from the same population of neurons across days. Although difficult and time-consuming, this strategy enables analyses of individual neurons across many days and trial types. Once we are successful in designing a task to induce shifts in visual spatial attention, this routine will allow us to determine whether mice and non-human primates share a common mechanism of visual spatial attention. Doing so will elucidate whether the mouse model should be used to study the physiology and pathophysiology of visual spatial attention.

The neonatal subventricular zone (SVZ) is a site of continued postnatal neurogenesis, and is the source of cortical glial cells. Apoptosis is an endogenous process of cell destruction, and is a key event in the proper development of the SVZ. Despite its importance, there is still a lack of knowledge regarding the temporal and spatial occurrence of neonatal SVZ apoptosis, cell types affected and the underlying intrinsic and extrinsic mechanisms that guide the process. This thesis addresses these issues, and in addition, finds a nontraditional mode of neurotrophic action for cell survival in the neonatal SVZ. We assessed SVZ apoptosis by subregion, employing the cell death markers, pH2ax and cleaved caspase 3. The medial SVZ contained the highest density of dying cells at p0, while at p7 there was no significant difference in the apoptotic cell density distribution in the SVZ subregions. Combining cell type specific markers with the death markers used, revealed immature postmitotic neurons were the primary cell type cleared in the p0 medial SVZ. The majority of dying cells in the p7 dorsolateral SVZ (SVZdl) were unable to be identified. Using stereotactic injection of a GFP expressing lentivirus, we determined the p0 medial SVZ cell population to be migratory cells bound for the olfactory bulb. An investigation into the intrinsic and extrinsic mechanisms mediating cell death in the neonatal SVZ, showed BH3-only protein Bim expression in the p0 and p7 SVZ, as well as significantly decreased p0 medial SVZ apoptosis in Bim knockout mice. Bim knockout mice did not show a significant change in apoptosis in the p7 SVZdl. TrkB knockout mice have shown a survival role for the receptor in the lateral ganglionic eminence of the neonatal SVZ. To test this in the p0 medial SVZ using a more specific method, a TrkB blocking antibody was injected into the p0 medial SVZ. This resulted in a significantly higher number of apoptotic cells in the p1 medial SVZ versus controls. These studies demonstrate the dynamic nature of the SVZ with its changing density and identity of apoptotic cells within the subregions. It has also shown the influence of Bim and TrkB signaling in neonatal SVZ apoptosis and survival. Finally, it has identified a premigratory cell population in the p0 medial SVZ, whose survival is mediated by neurotrophin signaling at their site of origin.

The hippocampus is a brain structure involved in memory as well as anxiety and depression-related behavior. One unique property of the hippocampus is that adult neurogenesis occurs in this region. Rodent studies in which adult hippocampal neurogenesis is ablated have shown a role for this process in the cognitive domain, specifically in pattern separation tasks, as well as in mediating the behavioral effects of antidepressants. These studies have furnished the intriguing hypothesis that increasing adult hippocampal neurogenesis may improve these functions and therefore serve as a target for novel treatments for cognitive impairments as well as depression and anxiety disorders. Here, we use both genetic and pharmacological models to increase adult neurogenesis in mice. Under baseline conditions, we find that increasing adult hippocampal neurogenesis is sufficient to improve performance in a fear-based pattern separation task, but has no effect on exploratory, anxiety or depression-related behavior. In mice exposed to voluntary exercise, increasing adult hippocampal neurogenesis increases exploration, without affecting anxiety or depression-related behavior. Finally, in mice treated with chronic corticosterone, a model of anxiety and depression, increasing adult hippocampal neurogenesis is sufficient to prevent the behavioral effect of CORT on anxiety and depression-related behavior. Here, we therefore describe dissociations between the effects of increasing adult hippocampal neurogenesis under baseline, voluntary exercise and chronic stress conditions. Together, our results suggest that increasing adult hippocampal neurogenesis has therapeutic potential for both cognitive, and anxiety and depression-related disorders.

In the adult brain, localized increases in neural activity almost always result in increases in local blood flow, a relationship essential for normal brain function. This coupling between neural activity and blood flow provides the basis for many neuroimaging techniques including functional magnetic resonance imaging (fMRI) and near-infrared spectroscopy (NIRS). However, functional brain imaging studies in newborns and children have detected a range of responses, including some entirely inverted with respect to those of the adult. Confusion over the properties of functional hemodynamics in the developing brain has made it challenging to interpret functional imaging data in infants and children. Additionally, developmental differences in functional hemodynamics would suggest postnatal neurovascular maturation and a unique metabolic environment in the developing brain. This thesis begins with a series of studies in which I tracked and characterized postnatal changes in functional hemodynamics in rodent models utilizing high-speed, high-resolution multi-spectral optical intrinsic and fluorescent signal imaging. I demonstrated that in early postnatal development increases in cortical blood flow do not occur in response to somatosensory stimulation. In fact, I observed stimulus-linked global vasoconstrictions in the brain. In slightly older age groups, I observed biphasic hemodynamic responses, with initial local hyperemia followed by global vasoconstriction, eventually progressing with age to recognizable adult-like hemodynamic responses. In these studies, I also found that the postnatal development of autoregulation is a potential confound in the study of early functional activation, and may account for some of the variability seen in prior human studies. Charting this progression led to the hypothesis that anomalous functional responses observed in human subjects are due to the postnatal development of neurovascular coupling itself. To directly assess neurovascular development, I performed a further set of studies in Thy1-GCaMP3 mice, permitting simultaneous observation of the development of neural function and connectivity along with functional hemodynamics. My results demonstrate that the spatiotemporal properties of neural development do not predict observed changes in the hemodynamic response, consistent with the parallel development of neural networks and neurovascular coupling. Confirming the presence of vascularly-uncoupled neural activity in the newborn brain led me to question how the brain supports its energy needs in the absence of evoked hyperemia, prompting the exploration of the potential metabolic bases and consequences of developmental changes in neurovascular coupling. Finally, I explore the cellular and vascular morphological and functional correlates of functional neurovascular development. My results confirm that neurovascular development occurs postnatally, which has critical implications for the interpretation of functional imaging studies in infants and children. My work also provides new insights into postnatal neural, metabolic, and vascular maturation and could have important implications for the care of infants and children, and for understanding the role of neurovascular development in the pathophysiology of developmental disorders.

A central question of developmental neurobiology is how the extraordinary variety of cell types in the nervous system is generated. A large body of evidence suggests that transcription factors acting as terminal selectors control cell fate determination by directly activating cell type-specific gene regulatory programs during neurogenesis. Neurons within the same class often further differentiate into subtypes that have distinct cellular morphology, axon projections, synaptic connections, and neuronal functions. The molecular mechanism that controls the subtype diversification of neurons sharing the same general fate is poorly understood, and only a few studies have addressed this question, notably the motor neuron subtype specification in developing vertebrate spinal cord and the segment-specific neuronal subtype specification of the peptidergic neurons in Drosophila embryonic ventral nerve cord. In this dissertation, I investigate the genetic basis of neuronal subtype specification using the Touch Receptor Neurons (TRNs) of Caenorhabditis elegans. The six TRNs are mechanosensory neurons that can be divided into four subtypes, which are located at various positions along the anterior-posterior (A-P) axis. All six neurons share the same TRN fate by expressing the POU-domain transcription factor UNC-86 and the LIM domain transcription factor MEC-3, the terminal selectors that activate a battery of genes (referred as TRN terminal differentiation genes) required for TRN functions. TRNs also have well-defined morphologies and synaptic connections, and therefore serve as a great model to study neuronal differentiation and subtype diversification at a single-cell resolution. This study primarily focuses on the two embryonically derived TRN subtypes, the anterior ALM and the posterior PLM neurons; each contains a pair of bilaterally symmetric cells. Both ALM and PLM neurons have a long anteriorly-directed neurite that branches at the distal end; the PLM, but not the ALM, neurons are bipolar, having also a posteriorly-directed neurite. ALM neurons form excitatory gap junctions with interneurons that control backward movement and inhibitory chemical synapses with interneurons that control forward movement, whereas PLM neurons do the reverse. Therefore, the clear differences between ALM and PLM neurons offer the opportunity to identify the mechanisms controlling subtype specification. Using the TRN subtypes along the A-P axis, I first found that the evolutionarily conserved Hox genes regulate TRN differentiation by both promoting the convergence of ALM and PLM neurons to the common TRN fate (Chapter II) and inducing posterior subtype differentiation that distinguishes PLM from the ALM neurons (Chapter III). First, distinct Hox proteins CEH-13/lab/Hox1 and EGL-5/Abd-B/Hox9-13, acting in ALM and PLM neurons respectively, promote the expression of the common TRN fate by facilitating the transcriptional activation of TRN terminal selector gene mec-3 by UNC-86. Hox proteins regulate mec-3 expression through a binary mechanism, and mutations in ceh-13 and egl-5 resulted in an “all or none” phenotype: ~35% of cells lost the TRN cell fate completely, whereas the rest ~65% of cells express the TRN markers at the wild-type level. Therefore, Hox proteins contribute to cell fate decisions during terminal neuronal differentiation by acting as reinforcing transcription factors to increase the probability of successful transcriptional activation. Second, Hox genes also control TRN subtype diversification through a “posterior induction” mechanism. The posterior Hox gene egl-5 induces morphological and transcriptional specification in the posterior PLM neurons, which distinguish them from the ALM. This subtype diversification requires EGL-5-induced repression of TALE cofactors, which antagonize EGL-5 functions, and the activation of rfip-1, a component of recycling endosomes, which mediates Hox activities by promoting subtype-specific neurite outgrowth. Thus, these results suggest that neuronal subtype diversification along the A-P axis is mainly driven by the posterior Hox genes, which induces the divergence of posterior subtypes away from the common state of the neuron type. I have also performed an RNAi screen to identify novel regulators of the TRN fate and identified the LIM domain-binding protein LDB-1 and the Zinc finger homeodomain transcription factor ZAG-1 as part of the regulatory network that determines TRN fate (Chapter IV). LDB-1 binds to and stabilizes MEC-3 and is also required for the activation of TRN terminal differentiation genes by MEC-3. ZAG-1 promotes TRN fate by preventing the expression other transcription factors EGL-44 and EGL-46, which inhibits the expression of TRN fate by competing for the cis-regulatory elements normally bound by the TRN fate selectors UNC-86/MEC-3. The mutual inhibition between ZAG-1 and EGL-44 establishes a bistable switch that regulates cell fate choice between TRNs and FLP neurons. I also investigated the genetic basis of neuronal morphogenesis using TRNs. By conducting a forward genetic screen searching for mutants with TRN neurite outgrowth defects, I identified a series of genes required for axonal outgrowth and guidance in TRNs. Following a few genes identified from the screen, genetic studies have revealed two novel mechanisms for neuritogenesis. First, Dishevelled protein DSH-1 attenuates the strength of Wnt signaling to allow the PLM posterior neurite to grow against the gradient of repulsive Wnt proteins, which are enriched at the posterior side of PLM cell body and normally repel the axons toward the anterior (Chapter V). Second, guanine nucleotide exchange factor UNC-73 and TIAM-1 promotes anteriorly and posteriorly directed neurite outgrowth, respectively; and outgrowth in different directions can suppress each other by competing for the limited neurite extension capacity (Chapter VI). As side projects, I performed mRNA expression profiling using isolated and separated populations of in vitro cultured ALM and PLM neurons and identified hundreds of genes differentially expressed between the two subtypes (Appendix I). I have also studied subtype differentiation of the VC motor neurons in the ventral nerve cord of C. elegans and discovered a mechanism by which histone modification patterns the expression of subtype-specific genes during terminal neuronal differentiation (Appendix II). In summary, my doctoral research established a framework for the study of neuronal subtype specification using the C. elegans TRNs and uncovered the genetic mechanisms for a variety of aspects of terminal neuronal differentiation. By investigating the generation of neuron type and subtype diversity in a well-defined model organism, my study provides novel insights for understanding the development of the nervous system.

To understand the emergent properties of neural circuits it would be ideal to record the activity of every neuron in a behaving animal and decode how it relates to behavior. We have achieved this with the cnidarian Hydra vulgaris, using calcium imaging of genetically engineered animals to measure the activity of essentially all of its neurons. While the nervous system of Hydra is traditionally described as a simple nerve net, we surprisingly find instead a series of functional networks that are anatomically non-overlapping and are associated with specific behaviors. Three major functional networks extend through the entire animal and are activated selectively during longitudinal contractions, elongations in response to light and radial contractions, while an additional network is located near the hypostome and is active during nodding. Additionally, we show that the behavior of Hydra is made of regularly occurring radial contractions, which expel the content of the gastric cavity about every 45 minutes. These results demonstrate the functional sophistication of apparently simple nerve nets, and the potential of Hydra and other basal metazoans as a model system for neural circuit studies.

The motivation for this thesis is to devise a simple model of transient dynamics in neural networks. Neural circuits are capable of performing many computations without reaching an equilibrium, but instead through transient changes in activity. Thus, having a good model for transient activity is important. In particular, this thesis focuses on a firing-rate description of neural activity. Firing rates offer a convenient simplification of neural activity, and have been shown experimentally to convey information about stimuli and behavior. This work begins by review the philosophy of modeling firing rates, as well as the problems that go with it. It examines traditional approaches to modeling firing rates, and in particular how common assumptions lead to a model that fails to capture transient dynamics. Chapter 2 applies a traditional model of firing rates in order to gain insight into properties of cortical circuitry. In collaboration with the lab of David Ferster at Northwestern University, we found that surround suppression in cat primary visual cortex is mediated by a withdrawal of excitation in the cortical circuit. In theoretical work, we find that this behavior can only arise if excitatory recurrence alone is strong enough to destabilize visual responses but feedback inhibition maintains stability. Chapter 3 reviews concepts and literature related to the dynamics of large networks of spiking neurons. Population density approaches are common for describing the dynamics of networks of spiking neurons. These approaches allow for a rigorous approach to relate the dynamics of individual neurons to the population firing rate. Chapter 4 explores a method for accurately approximating the firing-rate dynamics of a population of spiking neurons. We describe the population by the probability density of membrane potentials, so the dynamics are governed by a Fokker-Planck equation. Using a spiking model with periodic boundary conditions, we write the Fokker-Planck dynamics in a Fourier basis. We find that the lowest Fourier modes dominate the dynamics. Chapter 5 presents a novel rate model that successfully captures synchronous dynamics. As in the previous chapter, we invoke an approximation to the dynamics of a population of spiking neurons in order to develop a firing-rate model. Our approach derives from an eigenfunction expansion of a Fokker-Planck equation, which is a common approach to solving such problems. We find that a very simple approximation turns out to be surprisingly accurate. This approximation allows us to write a closed-form expression for the firing rate that resembles the equations for a damped harmonic oscillator. Finally, chapter 6 uses the formalism derived in the previous chapter to analyze activity in a large randomly-connected network of neurons. Comparing this large spiking network to a network of two coupled rate units, we find that the firing rate network gives a good approximation to the time-varying activity of a spiking network across a wide range of parameters. Perhaps most surprisingly, we also find that the rate network can approximate the phase diagram of the spiking network, predicting the bifurcation line between synchronous and asynchronous states.

Organisms possess internal sensory systems to detect changes in physiological state. Despite the importance of these sensory systems for maintaining homeostasis, their development, sensory mechanisms, and circuitry are relatively poorly understood. To help address these gaps in knowledge, I used the tracheal dendrite (td) sensory neurons of Drosophila larvae as a model to gain insights into the cellular and molecular organization, developmental regulators, sensory functions and mechanisms, and downstream neural circuitry of internal sensory systems. In this thesis, I present data to show that td neurons comprise defined classes with distinct gene expression and axon projections to the CNS. The axons of one class project to the subesophageal zone (SEZ) in the brain, whereas the other terminates in the ventral nerve cord (VNC). This work identifies expression and a developmental role of the transcription factor Pdm3 in regulating the axon projections of SEZ-targeting td neurons. I find that ectopic expression of Pdm3 alone is sufficient to switch VNC-targeting td neurons to SEZ targets, and to induce the formation of putative synapses in these ectopic target regions. These results define distinct classes of td neurons and identity a molecular factor that contributes to diversification of central axon targeting. I present data to show that td neurons express chemosensory receptor genes and have chemosensory functions. Specifically, I show that td neurons express gustatory and ionotropic receptors and that overlapping subsets of td neurons are activated by decrease in O2 or increase in CO2 levels. I show that respiratory gas-sensitive td neurons are also activated when animals are submerged for a prolonged duration, demonstrating a natural-like condition in which td neurons are activated. I assessed the roles of chemosensory receptor genes in mediating the response of td neurons to O2 and CO2. As a result, I identify Gr28b as a mediator of td responses to CO2. Deletion of Gr28 genes or RNAi knockdown of Gr28b transcripts reduce the response of td neurons to CO2. Thus, these data identify two stimuli that are detected by td neurons, and establish a putative role for Gr28b in internal chemosensation in Drosophila larvae. Finally, I present data to elucidate the neural circuitry downstream of td sensory neurons. I show that td neurons synapse directly and via relays onto neurohormone populations in the central nervous system, providing neuroanatomical basis for internal sensory neuron regulation of hormonal physiology in Drosophila. These results pave the way for future work to functionally dissect the td circuitry to understand its function in physiology and behavior.