Marine Biomedical Institute, Sapporo Medical College, Higashirishiri, Hokkaido 097-01, Japan
In the sea urchin, Strongylocentrotus intermedins, gonad response to γ-aminobutyric acid (GABA) was examined by indirect measurements of gonad contraction. (1) Mechanical response was noted to increase with gonad growth. Young growing gonads showed small response but for maturing, ripe and post-spawning specimens, the response was large. (2) Two different GABA responses on the part of maturing, ripe and post-spawning gonads, and young and growing gonads were thus in evidence. In the former, there were both phasic and rhythmical contractions but in the latter, only phasic contraction. (3) GABA response was also noted to vary according to sex. In the female, the appearance of rhythm was high, but low in the male. (4) The duration of GABA response was about 20 min in maturing and ripe gonads. (5) The threshold concentration of GABA response was 0.01 mM. The 50% effective and maximal doses were 0.1 mM and 10 mM, respectively. (6) The sites of action of GABA injected at a concentration of 10 mM were examined. GABA acted directly on the gonads. The generator inducing phasic and rhythmical contraction is discussed.