A model for regulation of appetitive state and behavioral choice by satiation via serotonin.

<p>A. The model builds on the relation that the default turn response to somatotopically mapped stimuli from the oral veil is avoidance, and that corollary outputs of the feeding network can change this response to orienting. The resting excitation state of the feeding motor network is set by endogenous 5-HT level, which is an inverse function of satiation state. Thus, network excitation state sums effects of satiation and appetitive sensory inputs (including effects of learned values of different prey odors). At increasing levels of feeding network excitation the turn response is switched from avoidance to approach. B. Increasing levels of 5-HT, through decline of satiation or by exogenous addition, increase feeding CPG excitation and thereby reduce the sensory thresholds for orienting and active feeding.</p

5-HT stimulates general excitation state of the feeding motor network in isolated CNSs.

<p>Similar stimulation of the feeding network by driving command neurons or gastroesophageal stimulation converts fictive avoidance turn responses to orienting <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102240#pone.0102240-Hirayama1" target="_blank">[9]</a>. A. 5 µM 5-HT induces spike activity in nerve root 3 (R3) of the buccal ganglion. B. A separate experiment in which sequential applications of 10 and 50 µM 5-HT show dose-dependence of induced spiking activity recorded intracellularly in a right retraction phase buccal motorneuron (rRMN) and extracellularly in a left cerebrobuccal connective (lCBC), and left nerve roots 2 and 3 (lR2, lR3).</p

5-HT injections reduced feeding thresholds, suppressed avoidance, and promoted orienting turn responses to a noxious stimulus in intact animals.

<p>A. Injection of 23 animals with 5-HT (1–2.5 µM final hemolymph dilution) significantly reduced median feeding thresholds to betaine measured 12 minutes later (Wilcoxon matched-pairs signed-ranks tests, PE: W = −28, *p<0.02; Bite: W = −36, **p<0.01). Control saline injections were ineffective. B. Responses to noxious unilateral taurine application (10⁻² M) to the tentacle/oral veil were compared before and after 5-HT injection to an estimated final hemolymph concentration of 2.5 µM. Among 20 animals tested, 18 initially showed avoidance and 2 animals showed orienting responses to taurine application. 12 minutes after 5-HT injection, avoidance was replaced by null responses in 11 animals and orienting turns were observed in 6 (p<0.005, Fishers' Exact Test); only 3 animals continued to show avoidance.</p

5-HT altered fictive turn preference from avoidance to orienting.

<p>Application of 5 µM 5-HT to isolated CNS from each of 4 donors changed fictive turn preference from avoidance to orienting. The figure shows a fictive avoidance turn (A) changed to fictive orienting (B) within 5 minutes following 5-HT addition. Fictive turn direction is shown in differing relative spike rates of the LBWNs following stimulation of the ipsilateral LOVN (short horizontal bar at bottom) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102240#pone.0102240-Hirayama1" target="_blank">[9]</a>. Fictive turns (solid double arrows) are measured following transient fictive withdrawal responses (dashed double arrows). C. Box and whiskers plot of ratios of spike frequencies for the LBWNs calculated across bins to assess orienting vs. avoidance turns in control vs. 5-HT conditions. Variations across ratio medians were significant (Kruskal–Wallis ANOVA; p<0.0001). Ratios of spike frequencies in 5-HT were significantly different from controls (two-tailed Dunn's Multiple Comparisons Test; *p<0.01; **p<0.001; <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102240#pone-0102240-t001" target="_blank">Table 1</a>).</p

Chemoceptive responses of papillae to betaine perfusion (0.1M trimethylglycine).

A. The record shows the latency from activity induced by perfusion onset to activity induced by betaine stimulation. B. Betaine stimulation (arrowhead) induces spike activity. Shown are 8 recordings of different papillae from 3 different animals. The arrowhead indicates betaine contact with papillae. C. In recordings of B, comparing activity 3 seconds before and after papillar contact, betaine increased spike frequency from an average baseline of 4.04 ± 0.68 (median: 4.00 Hz, standard error: 0.678) to 14.04 ± 1.1 Hz (median: 13.33 Hz, standard error: 1.01) with significant difference (* p < 0.008, Wilcoxon Signed-Rank test).</p

LOVN afferent spikes originate in the subepithelial network.

Orthodromic action potentials originating in papillae were not identifiable in the sensory LOVN. A. A gentle water jet from a Pasteur pipet was applied twice (arrows) at the midline of the isolated oral veil, likely stimulating multiple papillae mechanically. Recordings of a single papilla (upper trace) and the LOVN (lower trace) show typical loosely correlated activities. B. An expanded record of A, showing difficulty in identifying common spikes. Recording sites were separated by ~2 cm of nerve. The record also shows the relatively longer duration spikes of the ciliary clusters relative to nerve. Other parts of the expanded records are available in Supplementary Information. C. Stimulating a single papilla with a suction electrode (0.2 V, 4 msec duration, single stimuli at intervals of 4–10 sec) over 55 trials failed to drive correlated spikes in the LOVN with similar latencies but drove a few larger spikes at long latencies (triangles). Larger trace: Overlays of the 55 trials. Spiking post-stimulus was significant (Wilcoxon Signed Rank test, using right-tailed Z distribution, p = 0.018). Latencies of these large-amplitude spikes were quite variable, as expected if synaptically activated. Smaller dark trace: The average of the 55 trials in which, if present, smaller, orthodromically driven spikes should have additively emerged from the noise but did not.</p

The oral veil (OV) with papillae, tentacle, and their innervation.

A: Pleurobranchaea foraging (La Jolla Shores, CA.; photo by Tracy Clark). B: Papillae (pp) on the oral veil. T, the tentacle portion of the oral veil C: A single papilla. Papillae are multi-lobed and vary in size, with larger papillae near the midline of the oral veil. D: Branching of the two sensory nerves, the large oral veil nerve (LOVN) and the tentacle nerve (TN) innervating both the tentacle (T) and adjacent oral veil. The nerves overlap in their innervation of the OV and bring sensory information to the cerebropleural ganglion (CPlG).</p

Spontaneous spiking activity in a focally recorded cilia cluster and the lack thereof in adjacent skin.

Spontaneous spiking activity in a focally recorded cilia cluster and the lack thereof in adjacent skin.</p

Ratios of relative spike rate responses of contralateral vs. ipsilateral turn nerves to unilateral LOVN stimulation.

<p>Dunn's Multiple Comparison Test; *p<0.01, **p<0.001. Avoidance turns are characterized by ratios <1.0, and orienting turns by ratios >1.0. Fisher's Exact Test supports a significant effect of 5-HT in changing avoidance responses to orienting in the four experiments of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102240#pone-0102240-g003" target="_blank">Figure 3</a>.</p

Anti-tubulin and phalloidin staining of papillae in whole-mounted tissues.

Cilia clusters on the lobules are green (anti-tubulin). Muscle is red (phalloidin). A-C: Antibody and phalloidin penetrated only the superficial level of the tissues, probably through the clusters, revealing the clusters of sensory cilia in the lobules (arrowheads) and the adjacent muscle fibers (arrows). Finer cilia surround the central bundles. Muscle fibers lie immediately beneath the cilia clusters. D: A confocal section of C showing off the glomerular tangles below the cilia (arrowhead) and muscle fibers (arrows). Scale bars A, 200 μm; B-D, 50 μm.</p