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5–HT and 5–HT‐SO4, but not tryptophan or 5‐HIAA levels in single feeding neurons track animal hunger state

Serotonin (5‐HT) is an intrinsic modulator of neural network excitation states in gastropod molluscs. 5‐HT and related indole metabolites were measured in single, well‐characterized serotonergic neurons of the feeding motor network of the predatory sea‐slug Pleurobranchaea californica. Indole amounts were compared between paired hungry and satiated animals. Levels of 5‐HT and its metabolite 5‐HT‐SO4 in the metacerebral giant neurons were observed in amounts approximately four‐fold and two‐fold, respectively, below unfed partners 24 h after a satiating meal. Intracellular levels of 5‐hydroxyindole acetic acid and of free tryptophan did not differ significantly with hunger state. These data demonstrate that neurotransmitter levels and their metabolites can vary in goal‐directed neural networks in a manner that follows internal state.