An octopaminergic system in the CNS of the snails, Lymnaea stagnalis and Helix pomatia

Abstract

Octopamine (OA) levels in each ganglion of the terrestrial snail,Helix pomatia, and the pond snail,Lymnaea stagnalis, were measured by using the HPLC technique. In both species an inhomogeneous distribution of OA was found in the central nervous system. The buccal ganglia contained a concentration of OA (12.6 pmol mg-1and 18.8 pmol mg-1) that was two to three times higher than the pedal (4.93 pmol mg-1and 9.2 pmol mg-1) or cerebral (4.46 pmol mg-1and 4.9 pmol mg-1) ganglia ofHelixandLymnaea, respectively, whereas no detectable amount of OA could be assayed in the visceroparietal complex. InLymnaeaganglia, the OA uptake into the synaptosomal fraction had a high (Km1= 4.07 ± 0.51 νM,Vmax1= 0.56 ±0.11 pmol mg-1per 20 min), and a low (Km2= 47.6 ± 5.2 νM,Vmax2= 4.2 ± 0.27 pmol mg201per 20 min), affinity component. A specific and dissociable3H–OA binding to the membrane pellet prepared from the CNS of bothHelixandLymnaeawas demonstrated. The Scatchard analysis of the ligand binding data showed a one'binding site, representing a single receptor site. TheKdandBmaxvalues were found to be 33.7 ± 5.95 nM and 1678 ± 179 fmol g-1tissue inHelixand 84.9 + 17.4 nM and 3803 ± 515 fmol g-1tissue inLymnaeapreparation. The pharmacological properties of the putative molluscan OA receptor were characterized in both species and it was demonstrated that the receptor resembled the insect OA2rather than to the clonedLymnaeaOA receptor. Immunocytochemical labelling demonstrated the presence of OA–immunoreactive neurons and fibres in the buccal, cerebral and pedal ganglia in the central nervous system of both species investigated. Electrophysiological experiments also suggested that theLymnaeabrain possessed specific receptors for OA. Local application of OA onto the identified buccal B2 neuron evoked a hyperpolarization which could selectively be inhibited by the OAergic agents phentolamine, demethylchlordimeform and 2–chloro–4–methyl–2–(phenylimino)–imidazolidine. Among the dopamine antagonists, ergotamine reversibly inhibited the OA response, whereas sulpiride had no effect. Based on our findings, a neurotransmitter–modulator role of OA is suggested in the gastropod CNS.