A 5-HT1A-like receptor is involved in the regulation of the embryonic rotation of Lymnaea stagnalis L.

Cilia driven rotation of the pond snail Lymnaea stagnalis embryos is regulated by serotonin (5-HT). In the present study, physiological and biochemical assays were used to identify the 5-HT receptor type involved in rotation. The 5-HTergic agonists applied stimulated the rotation by 180-400% and their rank order potency was as follows: LSD > 5-HT > 8-OH-DPAT > WB4101 ≫ 5-CT. The applied antagonists, spiperone, propranalol and mianserin inhibited the 5-HT or 8-OH-DPAT stimulated rotation of the embryos by 50-70%. 3H-5-HT was bound specifically to the washed pellet of the embryo homogenates. The specific binding of 3H-5-HT was saturable and showed a single, high affinity binding site with Kd 7.36 nM and Bmax 221 fmol/mg pellet values. This is the first report demonstrating the high affinity binding of 3H-5-HT to the native receptor in molluscs. All of the pharmacons that stimulated the rotation or inhibited the 5-HT or 8-OH-DPAT evoked stimulation displaced effectively the binding of 3H-5-HT. 5-HT resulted in the inhibition of forskolin stimulated cAMP accumulation, showing that 5-HT is negatively coupled to adenylate cyclase. Our results suggest that in the 5-HTergic regulation of the embryonic rotation in L. stagnalis a 5-HT1A-like receptor of the vertebrate type is involved.