The functions of serotonin have been assigned through serotonin-receptor-specific drugs and mutants; however, because a constellation of receptors remains when a single receptor subtype is inhibited, the coordinate responses to modulation of serotonin levels may be missed. Here we report the analysis of behavioural and neuroendocrine defects caused by a complete lack of serotonin signalling. Analysis of the C. elegans genome sequence showed that there is a single tryptophan hydroxylase gene (tph-1)-the key enzyme for serotonin biosynthesis. Animals bearing a tph-1 deletion mutation do not synthesize serotonin but are fully viable. The tph-1 mutant shows abnormalities in behaviour and metabolism that are normally coupled with the sensation and ingestion of food: rates of feeding and egg laying are decreased; large amounts of fat are stored; reproductive lifespan is increased; and some animals arrest at the metabolically inactive dauer stage. This metabolic dysregulation is, in part, due to downregulation of transforming growth factor-beta and insulin-like neuroendocrine signals. The action of the C. elegans serotonergic system in metabolic control is similar to mammalian serotonergic input to metabolism and obesity.

Drugs that target the serotonergic system are the most commonly prescribed therapeutic agents and are used for treatment of a wide range of behavioral and neurological disorders. However, the mechanism of the drug action remain a conjecture. Here, we dissect the genetic targets of serotonin (5HT), the selective 5HT reuptake inhibitor (SSRI) fluoxetine (Prozac), the tricyclic antidepressant imipramine, and dopamine. Using the well-established serotonergic response in C. elegans egg-laying behavior as a paradigm, we show that action of fluoxetine and imipramine at the 5HT reuptake transporter (SERT) and at 5HT receptors are separable mechanisms. Even mutants completely lacking 5HT or SERT can partially respond to fluoxetine and imipramine. Furthermore, distinct mechanisms for each drug can be recognized to mediate these responses. Deletion of SER-1, a 5HT1 receptor, abolishes the response to 5HT but has only a minor effect on the response to imipramine and no effect on the response to fluoxetine. In contrast, deletion of SER-4, a 5HT2 receptor, confers significant resistance to imipramine while leaving the responses to 5HT or fluoxetine intact. Further, fluoxetine can stimulate egg laying via the Gq protein EGL-30, independent of SER-1, SER-4, or 5HT. We also show that dopamine antagonizes the 5HT action via the 5HT-gated ion channel MOD-1 signaling, suggesting that this channel activity couples 5HT and dopamine signaling. These results suggest that the actions of these drugs at specific receptor subtypes could determine their therapeutic efficacy. SSRIs and tricyclic antidepressants may regulate 5HT outputs independently of synaptic levels of 5HT.T HE selective serotonin reuptake inhibitor (SSRI) transmission (Di Matteo et al. 2001). On the other fluoxetine and the tricyclic antidepressant imiprahand, antagonists to the ionotropic receptor 5HT3 inmine represent two major classes of therapeutics used hibit dopamine function in the rat mesocorticolimbic for treatment of a wide range of behavioral and neurosystem (Gillies et al. 1996). Therefore, the action of logical disorders from depression to autism and schizoantidepressants at specific 5HT receptor subtypes could phrenia. The classically recognized action of these drugs underlie the unique spectrum of the therapeutic effiis to block uptake of serotonin (5HT) by the membrane cacy of the drugs. However, distinction between the 5HT transporter (SERT), which results in an increase action of a drug on SERT and on 5HT receptors in vivo in synaptic 5HT availability (Baldessarini 1996). In is difficult, and the role of 5HT receptor subtypes in addition, it has been shown in vitro that these drugs,

Serotonin (5HT) is a pivotal signaling molecule that modulates behavioral and endocrine responses to diverse chemical and physical stimuli. We report cell-specific regulation of 5HT biosynthesis by transient receptor potential V(TRPV) ion channels in C. elegans. Mutations in the TRPV genes osm-9 or ocr-2 dramatically downregulate the expression of the gene encoding the 5HT synthesis enzyme tryptophan hydroxylase(tph-1) in the serotonergic chemosensory neurons ADF, but neither the mutation nor the double mutation of both channel genes affects other types of serotonergic neurons. The TRPV genes are expressed in the ADF neurons but not in other serotonergic neurons, and act cell-autonomously to regulate a neuron-specific transcription program. Whereas in olfactory neurons OSM-9 and OCR-2 function is dependent on ODR-3 Gα, the activity of ODR-3 or two other Gα proteins expressed in the ADF neurons is not required for upregulating tph-1 expression, thus the TRPV ion channels in different neurons may be regulated by different mechanisms. A gain-of-function mutation in CaMKII UNC-43 partially suppresses the downregulation of tph-1 in the TRPV mutants, thus CaMKII may be an effector of the TRPV signaling. Mutations in the TRPV genes cause worms developmentally arrest at the Dauer stage. This developmental defect is due in part to reduced 5HT inputs into daf-2/insulin neuroendocrine signaling.

Stress response is a fundamental form of behavioral and physiological plasticity. Here we describe how serotonin (5HT) governs stress behavior by regulating DAF-2 insulin/IGF-1 receptor signaling to the DAF-16/FOXO transcription factor at the nexus of development, metabolism, immunity, and stress responses in C. elegans. Serotonin-deficient tph-1 mutants, like daf-2 mutants, exhibit DAF-16 nuclear accumulation and constitutive physiological stress states. Exogenous 5HT and fluoxetine (Prozac) prevented DAF-16 nuclear accumulation in wild-type animals under stresses. Genetic analyses imply that DAF-2 is a downstream target of 5HT signaling and that distinct serotonergic neurons act through distinct 5HT receptors to influence distinct DAF-16-mediated stress responses. We suggest that modulation of FOXO by 5HT represents an ancient feature of stress physiology and that the C. elegans is a genetically tractable model that can be used to delineate the molecular mechanisms and drug actions linking 5HT, neuroendocrine signaling, immunity, and mitochondrial function.

ULK1 and ULK2 are thought to be essential for initiating autophagy, and Ulk1/2-deficient mice die perinatally of autophagy-related defects. Therefore, we used a conditional-knockout approach to investigate the roles of ULK1/2 in the brain. Although the mice showed neuronal degeneration, the neurons showed no accumulation of P62+/ubiquitin+ inclusions or abnormal membranous structures, which are observed in mice lacking other autophagy genes. Rather, neuronal death was associated with activation of the unfolded protein response (UPR) pathway. An unbiased proteomics approach identified SEC16A as an ULK1/2-interacting partner. ULK-mediated phosphorylation of SEC16A regulated the assembly of endoplasmic reticulum (ER) exit sites and ER-to-Golgi trafficking of specific cargo, which did not require other autophagy proteins (e.g. ATG13). The defect in ER-to-Golgi trafficking activated the UPR pathway in ULK-deficient cells; both processes were reversed upon expression of SEC16A with a phosphomimetic substitution. Thus, the regulation of ER-to-Golgi trafficking by ULK1/2 is essential for cellular homeostasis.

In the absence of the leucine biosynthetic precursor alpha-isopropylmalate (alpha-IPM), the yeast LEU3 protein (Leu3p) binds DNA and acts as a transcriptional repressor in an in vitro extract. Addition of alpha-IPM resulted in a dramatic increase in Leu3p-dependent transcription. The presence of alpha-IPM was also required for Leu3p to compete effectively with another transcriptional activator, GAL4/VP16, for limiting transcription factors. Therefore, the addition of alpha-IPM appears to convert a transcriptional repressor into an activator. This represents an example in eukaryotes of direct transcriptional regulation by a small effector molecule.

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Serotonin (5-HT)-absorbing neurons use serotonin reuptake transporter (SERT) to uptake serotonin (5-HT) from extracellular space but do not synthesize it. While 5-HT-absorbing neurons have been identified in diverse organisms from C. elegans to humans, their function has not been elucidated. Here, we show that SERT in 5-HT-absorbing neurons controls behavioral response to food deprivation in C. elegans. The AIM and RIH interneurons uptake 5-HT released from chemosensory neurons and secretory neurons. Genetic analyses suggest that 5-HT secreted by both synaptic vesicles and dense core vesicles diffuse readily to the extrasynaptic space adjacent to the AIM and RIH neurons. Loss of mod-5/SERT function blocks the 5-HT absorption. mod-5/SERT mutants have been shown to exhibit exaggerated locomotor response to food deprivation. We found that transgenic expression of MOD-5/SERT in the 5-HT-absorbing neurons fully corrected the exaggerated behavior. Experiments of cell-specific inhibition of synaptic transmission suggest that the synaptic release of 5-HT from the 5-HT-absorbing neurons is not required for this behavioral modulation. Our data point to the role of 5-HT-absorbing neurons as temporal-spatial regulators of extrasynaptic 5-HT. Regulation of extrasynaptic 5-HT levels by 5-HT-absorbing neurons may represent a fundamental mechanism of 5-HT homeostasis, integrating the activity of 5-HT-producing neurons with distant targets in the neural circuits, and could be relevant to some actions of SSRIs in human.