Journal of Comparative Physiology A

, Volume 168, Issue 2, pp 191–200 | Cite as

Serotonin in the leech central nervous system: anatomical correlates and behavioral effects

  • Charles M. Lent
  • David Zundel
  • Edward Freedman
  • James R. Groome
Review Article


  1. 1.

    Serotonin is sequestered by a limited population of identified neurons in the 32 ganglia of the leech nervous system. A major fraction of the serotonin in each ganglion is contained in the paired Retzius cells, colossal effector neurons whose size varies longitudinally. The 5 other classes of identified serotonin-containing neurons, one effector cell and 4 interneurons, are approximately twice as numerous in anterior as in posterior ganglia.

  2. 2.

    We dissected 6 longitudinal samples from the ventral nerve cords of hungry Hirudo medicinalis, and measured their serotonin content using high pressure liquid chromatography with electrochemical detection. A consistent neurochemical pattern emerged in which segmental ganglia 2–4 had the highest quantity of serotonin: 18.51 pmol per ganglion. The anterior cerebral ganglion contained 14.78 pmol, and the content of the 4 posterior samples, segmental ganglia 7–10, 12–15, 17–20 and the caudal ganglion, decreased continuously from 16.35, 15.08, 10.75 to 2.51 pmol per ganglion, respectively. Morphometric analyses indicated that this pattern of ganglionic serotonin correlated primarily with longitudinal variations in the number of serotonin neurons per ganglion and secondarily with volume of the Retzius cells. Retzius cell volume correlated highly with the mass of their innervated body segments both of which are largest in mid-body domains.

  3. 3.

    Serotonin expresses leech feeding, and its ganglionic levels are a potentially useful index of behavioral state. We measured serotonin in the ganglionic samples from hungry and satiated leeches. The samples from recently fed animals contained 28% less serotonin than those from hungry ones. The amounts of serotonin in the cerebral and all the segmental samples from satiated leeches were significantly lower than equivalent samples of hungry animals. A similar pattern of depletion was seen in leeches which fed for a prolonged period (90 to 120 min) rather than the normal period of 30 min.

  4. 4.

    The effects of ingestion on serotonin-containing neurons was examined with the glyoxylic acid-induced histochemical fluorescence. The levels of fluorescence in all serotonin neurons in fed leeches were consistently lower than those in equivalent neurons in hungry animals, corroborating the ganglionic decrease in serotonin in satiated leeches.

  5. 5.

    To examine effects of body wall distension on serotonin levels, hungry leeches were fed to satiation, and half of them were relaxed by removing their distending blood meals. After 6 weeks, ganglionic serotonin in leeches with relaxed bodies was 21% higher than in those with distended bodies.

  6. 6.

    Ingestive behavior depletes serotonin from leech neurons and body wall distension appears to interfere with its synthesis. The behavioral states of leech feeding are implicated in the turnover and the ganglionic levels of a behaviorally important monoamine.


Key words

Leech Retzius cells Feeding Satiation Serotonin Fluorescence 



subesophageal ganglion


supraesophageal ganglion


Retzius cell


segmental ganglion 1–21


Caudal ganglion


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  1. de la Torre JC (1979) Standardization of the SPG histofluorescence method for transmitters. Soc Neurosci Abstr. 5:333Google Scholar
  2. Elliot E (1986) Chemosensory stimuli in feeding behavior of the leech Hirudo medicinalis. J Comp Physiol A 159:391–401Google Scholar
  3. Globus H, Lux HD, Schubert P (1973) Transfer of amino acids between neuroglia cells and neurons in the leech ganglion. Exp Neurol 40:104–113Google Scholar
  4. Glover JG (1987) Serotonin storage and uptake by identified neurons in the leech Haementeria ghilianii. J Comp Neurol 256:117–127Google Scholar
  5. Glover JC, Lent CM (1990) Serotonin is released from isolated leech ganglia by potassium-induced depolarization. Comp Biochem Physiol (in press)Google Scholar
  6. Glover JC, Mason A (1986) Morphogenesis of an identified leech neuron: Segmentai specification of axonal outgrowth. Dev Biol 115:256–260Google Scholar
  7. Leake LD, Woodward SKA (1988) The leech Retzius cell: A multiaction neurone. In: Salanki J, Rozsa KS (eds) Neurobiology of invertebrates. Transmitters, modulators and receptors. Symposia Biologica Hungarica 36:37–50Google Scholar
  8. Lent CM (1982) Fluorescent properties of monoamine neurons following glyoxylic acid treatment of intact leech ganglia. Histochemistry 75:77–89Google Scholar
  9. Lent CM (1984) Quantitative effects of a neurotoxin on serotonin levels within tissue compartments of the medicinal leech. J Neurobiol 15:309–323Google Scholar
  10. Lent CM (1985) Ingestive behavior decreases the serotonin in the leech CNS Soc Neurosci Abstr 11:480Google Scholar
  11. Lent CM, Dickinson MH (1984a) Serotonin integrates the feeding behavior of the medicinal leech. J Comp Physiol A 154:457–471Google Scholar
  12. Lent CM, Dickinson MH (1984b) Retzius cells retain functional membrane properties following ‘ablation’ by the neurotoxin, 5,7-DHT. Brain Res 300:167–171Google Scholar
  13. Lent CM, Dickinson MH (1987) On the termination of ingestive behaviour by the medicinal leech. J Exp Biol 131:1–15Google Scholar
  14. Lent CM, Dickinson MH, Marshall CG (1989) Serotonin and leech feeding behavior: Obligatory neuromodulation. Am Zool 24:1241–1254Google Scholar
  15. Loer CM, Kristan WB Jr (1989) Peripheral target choice by homologous neurons during embryogenesis of the medicinal leech. I. Segment-specific preferences of Retzius cells. J Neurosci 9:513–527Google Scholar
  16. Mason A, Sunderland AJ, Leake LD (1979) Effects of leech Retzius cells on body wall muscles. Comp Biochem Physiol 63C:359–361Google Scholar
  17. McAdoo DJ, Coggeshall RE (1976) Gas chromatographic-mass spectrometric analysis of biogenic amines in identified neurons and tissues of Hirudo medicinalis. J Neurochem 26:163–167Google Scholar
  18. Muller KJ, Nicholls JG, Stent GS (eds) (1981) Neurobiology of the leech. Cold Spring Harbor Laboratory Press, New YorkGoogle Scholar
  19. Orchard I, Lange AB, Barrett FM (1988) Serotonergic supply to the epidermis of Rhodnius prolixus: Evidence for serotonin as the plasticising factor. J Insect Physiol 34:873–879Google Scholar
  20. Ramón y Cajal S (1909) Histologie du Système Nerveux de l'Homme et des Vertébrés. Maloine, ParisGoogle Scholar
  21. Retzius G (1891) Zur Kenntnis des centralen Nervensystems der Würmer. Biol Unters (NF) 2–128Google Scholar
  22. Rude S (1966) Monoamine-containing neurons in the central nervous system and peripheral nerves of the leech, Hirudo medicinalis. J Comp Neurol 136:349–372Google Scholar
  23. Sokal RR, Rohlf FJ (1981) Biometry, 2nd ed. W.H.Freeman, New YorkGoogle Scholar
  24. Stuart AE, Hudspeth AJ, Hall ZW (1974) Vital staining of monoamine-containing cells in the leech nervous system. Cell Tissue Res 153:55–61Google Scholar
  25. Torrence SA, Law MI, Stuart DK (1989) Leech neurogenesis II. Mesodermal control of neuronal patterns. Dev Biol 136:40–60Google Scholar
  26. Willard AL (1981) Effects of serotonin on the generation of the motor program for swimming by the medicinal leech. J Neurosci 1:936–944PubMedGoogle Scholar

Copyright information

© Springer-Verlag 1991

Authors and Affiliations

  • Charles M. Lent
    • 1
  • David Zundel
    • 1
  • Edward Freedman
    • 2
  • James R. Groome
    • 1
  1. 1.Department of BiologyUtah State UniversityLoganUSA
  2. 2.Department of Neurobiology and BehaviorBrown UniversityProvidenceUSA

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