Abstract
Serotonin (5-hydroxytryptamine, 5-HT) is involved in the regulation of feeding and digestion in many animals from worms to mammals. In insects, 5-HT functions both as a neurotransmitter and as a systemic hormone. Here we tested its role as a neurotransmitter in feeding and crop contractions and its role as a systemic hormone that affected feeding in adult foraging honeybees. We found 5-HT immunoreactive processes throughout the gut, including on the surface of the oesophagus, crop, proventriculus, and the midgut, as well as in the ventral nerve cord. mRNA transcripts for all four of the known bee 5-HT receptors (Am5-ht1A,2α,2β,7) were expressed in the crop and the midgut suggesting a functional role for 5-HT in these locations. Application of a cocktail of antagonists with activity against these known receptors to the entire gut in vivo reduced the rate of spontaneous contraction in the crop and proventriculus. Although feeding with sucrose caused a small elevation of endogenous 5-HT levels in the haemolymph, injection of exogenous 5-HT directly into the abdomen of the bee to elevate 5-HT in the haemolymph did not alter food intake. However, when 5-HT was injected into directly into the brain there was a reduction in intake of carbohydrate, amino acid, or toxin-laced food solutions. Our data demonstrate that 5-HT inhibits feeding in the brain and excites muscle contractions in the gut, but general elevation of 5-HT in the bee’s haemolymph does not affect food intake.
Highlights
The commencement and cessation of feeding is orchestrated by a diverse set of internal cues that provide the brain with information about nutritional state and satiety
This idea has been supported by a recent study in the blowfly (Phormia regina) demonstrating that 5-HT applied to the crop increases muscle contractions and crop emptying rate (Liscia et al, 2012)
Immunohistochemistry Using bees collected as described above, ventral nerve cords (VNC; N = 4), and digestive tracts (N = 8) were dissected in air and fixed for 1–3 h in 4% paraformaldehyde in 0.1 M phosphate buffered saline (PBS)
Summary
The commencement and cessation of feeding is orchestrated by a diverse set of internal cues that provide the brain with information about nutritional state and satiety. 5-HT neurons innervate the crop and midgut (Budnik et al, 1989; Haselton et al, 2006; Molaei and Lange, 2003; Pietrantonio et al, 2001) indicating that they are likely to play an important role in the movement of food through the digestive tract. This idea has been supported by a recent study in the blowfly (Phormia regina) demonstrating that 5-HT applied to the crop increases muscle contractions and crop emptying rate (Liscia et al, 2012).
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