Jafari, Gholamali et al. (2011) International Worm Meeting "Stress response to food depravation is modulated by non-synthesizing 5-HT neurons that absorb non-synaptic 5-HT, via 5-HT transporter, MOD-5/SERT, in C. elegans."

Sze, Ji Ying, Kullyev, Andrey, Liang, Bin, Jafari, Gholamali

[International Worm Meeting, 2011]

The significance of Selective Serotonin Reuptake Inhibitors (SSRIs) has been proven clinically in the treatment of depression. Although, it is believed that these drugs increase the level of synaptic serotonin (5-HT), the precise mechanism through which they act remains unclear. In C. elegans, we found that 5-HT is secreted from 5-HT producing neurons, NSMs, ADFs, and HSNs and then absorbed by specific 5-HT absorbing neurons, AIMs and RIH, which do not form any synapse with the 5-HT producing neurons. While AIMs and RIH are not able to synthesize 5-HT, they express the 5-HT transporter, MOD-5/SERT, to absorb 5-HT. Our genetic analyses show that 5-HT producing neurons secrete 5-HT via both synaptic vesicles (SVs), clustered at the synaptic sites, and dense-core vesicles (DCVs), clustered at dendrites, and axons, and somata. By ectopically expressing 5-HT transporter, MOD-5/SERT, we show that non-synaptic 5-HT is available throughout the nervous system and can be absorbed by non-serotonergic neurons as well. Finally, our behavioral study indicates that the 5-HT absorbing neurons represent a reservoir of 5-HT to fine-tune the level of 5-HT during and post stress conditions. The presence and roles of absorbing neurons in higher organism remain to be investigated. Knowing that the therapeutic effects of antidepressant, SSRIs, take place in two to three weeks in human, the immediate rise of synaptic 5-HT cannot fully explain the mechanism of action of those drugs. Our findings suggest that the 5-HT activity in neuronal circuits and consequently behavior may not be limited at the synaptic sites.

Jafari G et al. (2011) J Neurosci "Regulation of extrasynaptic 5-HT by serotonin reuptake transporter function in 5-HT-absorbing neurons underscores adaptation behavior in Caenorhabditis elegans."

Liang B, Jafari G, Sze JY, Kullyev A, Xie Y

[J Neurosci, 2011]

Serotonin [5-hydroxytryptamine (5-HT)]-absorbing neurons use serotonin reuptake transporter (SERT) to uptake 5-HT from extracellular space but do not synthesize it. While 5-HT-absorbing neurons have been identified in diverse organisms from Caenorhabditis 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 selective serotonin reuptake inhibitors in humans.

Michael Y Chao et al. (2002) East Coast Worm Meeting "Serotonin modulation of a chemosensory circuit in C. elegans"

Michael Y Chao, Rhonda Hyde, Hidetoshi Komatsu, Anne C Hart, Heather M Dionne

[East Coast Worm Meeting, 2002]

C. elegans responds to the volatile repellant 1-octanol by moving backwards. To study this chemosensory response we screened for mutants that do not respond to octanol, and found that a subset of the mutants obtained were defective only in the absence of food. Because the presence of food is thought to increase serotonin (5-HT) signaling, we investigated whether 5-HT is involved in this food-dependent regulation of octanol detection. Exogenous 5-HT restores the ability to respond to octanol in the absence of food in most of these mutants. Furthermore, at least two of these mutants were also partially resistant to 5-HT-induced paralysis, suggesting that they may be defective in 5-HT signaling in motor neurons as well. Because eat-4 animals (which are defective for glutamatergic signaling) are mutant for octanol avoidance, but tph-1 animals (which lack 5-HT biosynthesis) and dgk-1 animals (which are defective for 5-HT signaling) are only defective in detecting dilute octanol, our results suggest that 5-HT modulates glutamatergic signaling in this circuit. We are currently further characterizing these mutants in the hope of identifying genes involved in 5-HT modulation of synaptic signaling.

Michael Y Chao et al. (2002) West Coast Worm Meeting "Serotonin modulation of a chemosensory circuit in C. elegans"

Heather M Dionne, Michael Y Chao, Anne C Hart, Hidetoshi Komatsu, Rhonda Hyde

[West Coast Worm Meeting, 2002]

C. elegans responds to the volatile repellant 1-octanol by moving backwards. To study this chemosensory response we screened for mutants that do not respond to octanol, and found that a subset of the mutants obtained were defective only in the absence of food. Because the presence of food is thought to increase serotonin (5-HT) signaling, we investigated whether 5-HT is involved in this food-dependent regulation of octanol detection. Exogenous 5-HT restores the ability to respond to octanol in the absence of food in most of these mutants. Furthermore, at least two of these mutants were also partially resistant to 5-HT-induced paralysis, suggesting that they may be defective in 5-HT signaling in motor neurons as well. Because eat-4 animals (which are defective for glutamatergic signaling) are mutant for octanol avoidance, but tph-1 animals (which lack 5-HT biosynthesis) and dgk-1 animals (which are defective for 5-HT signaling) are only defective in detecting dilute octanol, our results suggest that 5-HT modulates glutamatergic signaling in this circuit. We are currently further characterizing these mutants in the hope of identifying genes involved in 5-HT modulation of synaptic signaling.

Weinshenker DZ et al. (1995) J Neurosci "Genetic and pharmacological analysis of neurotransmitters controlling egg laying in C. elegans."

Thomas JH, Garriga G, Weinshenker DZ

[J Neurosci, 1995]

We have investigated the neurotransmitters used to control egg-laying in C. elegans. Previous studies suggested that 5-HT released by the HSN motor neurons stimulates egg laying, and that tricyclic antidepressants potentiate egg laying by blocking reuptake of 5-HT by the HSN neurons. We report studies of the wild type and a mutant that lacks detectable 5-HT that suggest 5-HT is not required for egg-laying. Furthermore, we find that ACh is required for egg laying in response to 5-HT, suggesting that 5-HT is not sufficient to activate egg laying. The dominant egl-2(n693) mutation, which causes animals to lay eggs in response to tricyclics but not 5-HT, also conflicts with the model for egg laying. Experiments in which the HSN neurons or 5-HT are removed from egl-2 animals indicate that the action of tricyclics cannot be explained by a block of 5-HT reuptake. We find that D-2 family dopamine antagonists can also induce egg laying in egl-2(n693) mutants, and that dopamine inhibits egg laying in the wild type. These results suggest that dominant egl-2 mutations activate an inhibitory dopaminergic pathway that can be blocked by tricyclics and D-2 antagonists. We also find that these drugs stimulate egg laying in mutants lacking 5-HT or the HSN neurons, consistent with a target on the egg-laying muscles. In contrast to tricyclics, fluoxetine and other selective 5-HT reuptake inhibitors appear to be specific for 5-HT reuptake in C. elegans egg laying.

Foss, Eric P. et al. (2009) International Worm Meeting "Measuring Effects of Serotonin on Locomotory Behavior."

Foss, Eric P., Carnell, Lucinda

[International Worm Meeting, 2009]

Serotonin (5-HT) is a neuromodulator that regulates food-associated behaviors in both vertebrates and invertebrates. In C. elegans exposure to exogenous 5-HT slows locomotion. Several 5-HT receptors, mod-1, ser-1, and ser-4 have been implicated in mediating this 5-HT-dependent decrease of locomotion rates. We are interested in studying the acute and long-term effects of 5-HT on locomotory behavior using the automated tracking software WormTracker (Ramot et al. 2008) to determine average velocities of a population of animals over time. From initial studies, examining the effects of increasing 5-HT concentration, we found that without food wild-type (N2) animals were slowed to 16% of control speeds at 4 mg/ml (10.3mM) 5-HT after 30 minutes. Control wild-type animals without 5-HT also display an increase of speed during the first 30 minutes, which is followed by a decrease after 90 minutes. We measured the speed of three 5-HT receptor mutants, mod-1, ser-1, and ser-4 to assess their contribution to this slowing response with increasing concentrations of 5-HT. After 30 minutes at 4 mg/mL 5-HT, these mutant animals slowed to 28%, 32%, and 36% of control rates, respectively. However, these mutants all displayed greater resistance to the effect of 5-HT at lower concentrations. The larger goal of this project is to identify an adaptation response where the animals recover after time from the inhibitory effects of 5-HT, which has been previously observed. Because none of the animals displayed recovery over the 90 minute period, most likely due to the lack of bacteria, we have begun to measure speed over longer time periods in the presence of food. Ramot et al. 2008 PLoS ONE 3(5): e2208. Doi:10.1371/journal.pone.0002208.

Nizar Mohamed et al. (2008) C.elegans Neuronal Development Meeting "Modulation Of Muscle Function By Multiple Serotonin Receptors"

Kathleen Davis, Bruce Bamber, Nizar Mohamed

[C.elegans Neuronal Development Meeting, 2008]

Biogenic amines, including serotonin (5-HT), dopamine, octopamine and tyramine, modulate behaviors in vertebrates and invertebrates. The receptors for biogenic amines are generally G-protein coupled receptors. When exposed to exogenous 5-HT, C. elegans increases pharyngeal pumping and egg laying, and reduces locomotion. This response mimics the behavior of worms in the presence of food, suggesting that 5-HT may play a role in normal behavioral modulation related to feeding. Because muscle contraction is central to all behavior, we hypothesize that 5-HT may modulate muscle excitability. In fact, vulval muscles express three 5-HT receptors: Galpha coupled SER-1, Galpha coupled SER-7, and Galpha coupled F16D3.7 (a putative 5-HT receptor). F16D3.7 expression is also observed in body wall muscles. Our goal is to understand how 5-HT modulates muscle activity, and how multiple 5-HT receptors within a single tissue function together to shape muscle activity patterns using patch-clamp electrophysiology. In particular, we will focus on the regulation of muscle activity in the vulval muscles that are required for egg laying. Eggs are laid as a result of intrinsic vulval muscle activity, with synaptic input regulating the frequency of the contractions. 5-HT is released by the HSN motor neurons onto the vulval muscles, and exogenous 5-HT increases the frequency of calcium transients in vulval muscles. Because mutations that affect egg-laying include several potassium channels and an L-type voltage-gated calcium channel, we will characterize potassium and calcium currents in vulval muscles in the presence and absence of 5-HT, and delineate the modulatory pathways using loss of function mutants and RNAi to inactivate 5-HT receptors, G-protein subunits, and ion channels. We will also determine whether 5-HT affects body wall muscle. In contrast to vulval muscles, the contraction of body wall muscle is synaptically controlled by acetylcholine and GABA inputs. A single putative 5-HT receptor, SER-5, is expressed on body wall muscles, but no direct effect of 5-HT on body wall muscle contraction have been reported. We will examine whether 5-HT acts through SER-5 in body wall muscles to change its responsiveness to ACh or GABA, or to change the activity of voltage gated channels. We will present progress toward measuring multiple muscle voltage- and neurotransmitter-activated currents in the presence and absence of 5-HT.

Marishka K. Brown et al. (2007) International Worm Meeting "Modulation of serotonin neurotransmission by EGb 761 in C.elegans."

Marishka K. Brown, Yuan Luo

[International Worm Meeting, 2007]

Serotonin (5-hydroxytryptamine; 5-HT) is a monoamine that modulates behaviors in both invertebrates and vertebrates. Drugs that alter 5-HT signaling are potent antidepressants and affect metabolic rates. Previously, we observed that the Ginkgo biloba extract EGb 761 modulates the expression of 5-HT2_A receptors in mouse N2A neuroblastoma cells expressing amyloid beta and it enhances adult neurogenesis in aged mice in a mouse model of Alzheimer''s disease. The aim of this present study is to determine the mechanism of action of EGb 761 on 5-HT transmission. In Caenorhabditis elegans, the 5-HT regulates several different behaviors including: egg-laying, pharyngeal pumping and locomotion. It has been shown that 5-HT regulates movement through MOD-1, which is a 5-HT chlorine gated channel found in the postsynaptic receptors in the worms. When exogenous 5-HT is applied to wild type worms, this causes a decrease in locomotion. Mutant mod-1 worms are resistant to levels of exogenous 5-HT. In this study, wild type and mutant mod-1 worms were treated with EGb 761 in the presence or absence of known 5-HT receptor agonist and antagonist, meta chlorophenylpiperazine (mCPP) and mianserin, respectively. Our results show that wild type worms treated with EGb 761 demonstrate a similar resistance to exogenous 5-HT as the mod-1 mutants. However, mod-1 worms that were treated with EGb 761 are less resistant to exogenous 5-HT, suggesting that the effect of EGb 761 depends on the postsynaptic 5-HT receptors in the worms. The question of whether the modifying effects of EGb 761 on 5-HT receptors are direct or indirect is presently under investigation.

McLaughlin CN et al. (2018) Trends Genet "Keeping Neurons Young and Foxy: FoxOs Promote Neuronal Plasticity."

McLaughlin CN, Broihier HT

[Trends Genet, 2018]

Any adult who has tried to take up the piano or learn a new language is faced with the sobering realization that acquiring such skills is more challenging as an adult than as a child. Neuronal plasticity, or the malleability of brain circuits, declines with age. Young neurons tend to be more adaptable and can alter the size and strength of their connections more readily than can old neurons. Myriad circuit- and synapse-level mechanisms that shape plasticity have been identified. Yet, molecular mechanisms setting the overall competence of young neurons for distinct forms of plasticity remain largely obscure. Recent studies indicate evolutionarily conserved roles for FoxO proteins in establishing the capacity for cell-fate, morphological, and synaptic plasticity in neurons.

Chao MY et al. (2004) Proc Natl Acad Sci U S A "Feeding status and serotonin rapidly and reversibly modulate a Caenorhabditis elegans chemosensory circuit."

Fukuto HS, Komatsu H, Chao MY, Hart AC, Dionne HM

[Proc Natl Acad Sci U S A, 2004]

Serotonin (5-HT) modulates synaptic efficacy in the nervous system of vertebrates and invertebrates. In the nematode Caenorhabditis elegans, many behaviors are regulated by 5-HT levels, which are in turn regulated by the presence or absence of food. Here, we show that both food and 5-HT signaling modulate chemosensory avoidance response of octanol in C. elegans, and that this modulation is both rapid and reversible. Sensitivity to octanol is decreased when animals are off food or when 5-HT levels are decreased; conversely, sensitivity is increased when animals are on food or have increased 5-HT signaling. Laser microsurgery and behavioral experiments reveal that sensory input from different subsets of octanol-sensing neurons is selectively used, depending on stimulus strength, feeding status, and 5-HT levels. 5-HT directly targets at least one pair of sensory neurons, and 5-HT signaling requires the Galpha protein GPA-11. Glutamatergic signaling is required for response to octanol, and the GLR-1 glutamate receptor plays an important role in behavioral response off food but not on food. Our results demonstrate that 5-HT modulation of neuronal activity via G protein signaling underlies behavioral plasticity by rapidly altering the functional circuitry of a chemosensory circuit.