LOCALIZATION AND RELEASE OF 5-HYDROXYTRYPTAMINE IN THE CRAYFISH EYESTALK

The content and regional distributioon of 5-hydroxytryptamine (5-HT) i n the crayfish eyestalk was was determined by high-performance liquid chromatography. Levels of the 5-HT precursors L-tryptophan (L-TRP) and 5-hydroxytryptophan (5-OH-TRP), and of three metabolites, 5-hydroxytr yptophol (5-HTPH), N-acetylserotonin (NA-5-HT) and 5-hydroxy-indole-3- acetic acid (5-HIAA), were also determined. The total content of 5-HT in the eyestalk was 95.4+/-49.3 pg mg(-1) wet mass (mean+/-S.D. N=55) while the specific content was 9.6+/-1.9 fmol mu g(-1) protein (mean /- S.D. N=5). 5-HT was present in all four ganglia of the eyestalk. Th e highest proportion was found in the medulla terminalis (40.2%) and t he lowest in the retina lamina ganglionaris (9.9%), which also had the lowest specific content. Conversely, the highest specific content of L-TRP was in the retina lamina ganglionaris. 5-HT biosynthesis and met abolism were explored in isolated eyestalks The monoamine oxidase bloc ker pargyline, at concentrations between 0.8 and 10 mmol(-1), elicited a dose-dependent increase in 5-HT content, The biosynthesis of 5-HT i n the crayfish eyestalk is suggested by the presence of its immediate precursor (5-OH-TRP) and by the suppression of 5-HT synthesis induced by m-hydroxybenzyl-hydrazine (m-HBH), a blocker of 5-OH-TRP decarboxyl ase. The presence of immunopositive cell bodies and axons was demonstr ated using an anti-5-HT antiserum, 5-HT-like immunopositivity was dete cted in various regions of the eyestalk. Efferent immunopositive axons were also identified in the optic nerve, and these may have originate d in the protocerebral lobe of the suprsoesophageal ganglion, The bran chings of these axons were profusely distributed in the neuropile of t he medulla terminalis. A basal level release of 5-HT was detected in i solated eyestalks. The amount recovered was increased two-to threefold after blocking 5-HT uptake with fluoxetine (1 mu mol l(-1)). Incubati on of eyestalks in solutions containing a high K+ concentration (80 mm ol l(-1)) released 5-HT. Electrical stimulation of the optic nerve rel eased 5-HT as a function of the intensity of stimulation Both the basa l and evoked release were suppressed by lowering the Ca2+ concentratio n in the medium. These observations support a role for 5-HT as a neuro transmitter or neuromodulator in the crayfish eyestalk.