Dissecting the Serotonergic Food Signal Stimulating Sensory-Mediated Aversive Behavior in C. elegans

Nutritional state often modulates olfaction and in Caenorhabditis elegans food stimulates aversive responses mediated by the nociceptive ASH sensory neurons. In the present study, we have characterized the role of key serotonergic neurons that differentially modulate aversive behavior in response to changing nutritional status. The serotonergic NSM and ADF neurons play antagonistic roles in food stimulation. NSM 5-HT activates SER-5 on the ASHs and SER-1 on the RIA interneurons and stimulates aversive responses, suggesting that food-dependent serotonergic stimulation involves local changes in 5-HT levels mediated by extrasynaptic 5-HT receptors. In contrast, ADF 5-HT activates SER-1 on the octopaminergic RIC interneurons to inhibit food–stimulation, suggesting neuron-specific stimulatory and inhibitory roles for SER-1 signaling. Both the NSMs and ADFs express INS-1, an insulin-like peptide, that appears to cell autonomously inhibit serotonergic signaling. Food also modulates directional decisions after reversal is complete, through the same serotonergic neurons and receptors involved in the initiation of reversal, and the decision to continue forward or change direction after reversal is dictated entirely by nutritional state. These results highlight the complexity of the “food signal” and serotonergic signaling in the modulation of sensory-mediated aversive behaviors

Monoamines differentially modulate neuropeptide release from distinct sites within a single neuron pair

<div><p>Monoamines and neuropeptides often modulate the same behavior, but monoaminergic-peptidergic crosstalk remains poorly understood. In <i>Caenorhabditis elegans</i>, the adrenergic-like ligands, tyramine (TA) and octopamine (OA) require distinct subsets of neuropeptides in the two ASI sensory neurons to inhibit nociception. TA selectively increases the release of ASI neuropeptides encoded by <i>nlp-14</i> or <i>nlp-18</i> from either synaptic/perisynaptic regions of ASI axons or the ASI soma, respectively, and OA selectively increases the release of ASI neuropeptides encoded by <i>nlp-9</i> asymmetrically, from only the synaptic/perisynaptic region of the right ASI axon. The predicted amino acid preprosequences of genes encoding either TA- or OA-dependent neuropeptides differed markedly. However, these distinct preprosequences were not sufficient to confer monoamine-specificity and additional N-terminal peptide-encoding sequence was required. Collectively, our results demonstrate that TA and OA specifically and differentially modulate the release of distinct subsets of neuropeptides from different subcellular sites within the ASIs, highlighting the complexity of monoaminergic/peptidergic modulation, even in animals with a relatively simple nervous system.</p></div

TA- and OA-dependent neuropeptides co-localize at ASI synapses.

<p>(A) Schematic representation of an ASI neuron highlighting the predicted synaptic/perisynaptic and extra-synaptic sites. (B-D) Translational neuropeptide fusions (ASI::<i>nlp-14</i>::<i>gfp</i>, ASI::<i>nlp-18</i>::<i>gfp</i> or ASI::<i>nlp-9</i>::<i>gfp)</i> expressed in their respective null backgrounds in the left ASI sensory neurons driven by the ASI-specific promoter <i>gpa-4</i>. Anterior is to the left and dorsal is up. (E) Straightened axons of <i>nlp-9</i> null animals co-expressing <i>gpa-4</i>::<i>nlp-9</i>::<i>gfp</i> and <i>srg-47</i>::<i>mCherry</i>::<i>rab-3</i> transgenes. Circles indicate the sites of predicted ASI synapses based on electron microscopy. Arrowheads denote potential additional extra-synaptic sites of release. (F) Straightened axons of an animal co-expressing <i>gpa-4</i>::<i>nlp-14</i>::<i>gfp</i> and <i>srg-47</i>::<i>nlp-9</i>::<i>mCherry</i> transgenes. (G) Extra-synaptic/synaptic ratios of ASI::<i>nlp-14</i>::<i>gfp</i> or ASI::<i>nlp-9</i>::<i>gfp</i> expressed in their respective null backgrounds. * Denotes significantly different to <i>nlp-14</i> (<i>p</i> = 0.0001). Data are presented as a mean ±SE (<i>n</i>) and were analyzed by two-tailed Student’s <i>t</i> test (<i>n</i> = 28–55).</p

Model of monoamine-specific ASI neuropeptide release.

<p>Neuropeptides encoded by <i>nlp-9</i>, <i>-14</i> and <i>-18</i> are expressed in the soma and predicted synaptic/perisynaptic and extra-synaptic sites in both ASI neurons. TA selectively stimulates the release of NLP-18::GFP from ASI soma and NLP-14::GFP from synaptic/presynaptic regions of both the right and left ASI axons. In contrast, OA selectively stimulates the asymmetric release of NLP-9::GFP from only synaptic/perisynaptic regions in the right ASI axon.</p

A Genetic Survey of Fluoxetine Action on Synaptic Transmission in Caenorhabditis elegans

Fluoxetine is one of the most commonly prescribed medications for many behavioral and neurological disorders. Fluoxetine acts primarily as an inhibitor of the serotonin reuptake transporter (SERT) to block the removal of serotonin from the synaptic cleft, thereby enhancing serotonin signals. While the effects of fluoxetine on behavior are firmly established, debate is ongoing whether inhibition of serotonin reuptake is a sufficient explanation for its therapeutic action. Here, we provide evidence of two additional aspects of fluoxetine action through genetic analyses in Caenorhabditis elegans. We show that fluoxetine treatment and null mutation in the sole SERT gene mod-5 eliminate serotonin in specific neurons. These neurons do not synthesize serotonin but import extracellular serotonin via MOD-5/SERT. Furthermore, we show that fluoxetine acts independently of MOD-5/SERT to regulate discrete properties of acetylcholine (Ach), gamma-aminobutyric acid (GABA), and glutamate neurotransmission in the locomotory circuit. We identified that two G-protein–coupled 5-HT receptors, SER-7 and SER-5, antagonistically regulate the effects of fluoxetine and that fluoxetine binds to SER-7. Epistatic analyses suggest that SER-7 and SER-5 act upstream of AMPA receptor GLR-1 signaling. Our work provides genetic evidence that fluoxetine may influence neuronal functions and behavior by directly targeting serotonin receptors