Excitatory effects of ergot alkaloids and their derivatives on the excitability of an identifiable giant neuron of the African giant snail (Achatina fulica Férussac).

The effects of various ergot alkalids and related substances on the neuronal excitability were examined using a spontaneously firing neuron (PON, periodically oscillating neuron) identified in the suboesophageal ganglia of the African giant snail (Achatina fulica Férussac). Of the substances examined, ergometrine and methylergometrine had a marked excitatory effect on the neuron (their critical concentration by bath application: 5 x 10(-6) kg/l). D-lysergic acid (the critical concentration: 3 x 10(-5) kg/l), D-lysergic acid amide (3 x 10(-5), D-lysergic acid 3-hydroxypiperidinamide (3 x 10(-5) and D-lysergic acid 4-hydroxypiperidinamide (10(-4) had also an excitatory effect which was weaker than that of the two alkaloids mentioned above. Agroclavine and elymoclavine had a very slight excitatory effect in some cases. The other substances examined, for example, lysergine, 9, 10-dihydro-D-lysergic acid, 9, 10-dihydro-D-lysergic acid amide, nicergoline, metergoline, methysergide, ergotamine, bromocriptine, lergotrile etc., had no effect on the neuron at a high concentration (10(-4) kg/l). When local (microdrop) application of ergometrine and methylergometrine in the physiological state was performed, the depolarizing shift of the PON membrane and exaggeration of the membrane potential oscillations were observed. In the Na+-free (replaced with Li+) state, local application of the two effective alkaloids produced a marked frequency increase of tonic spike discharges with depolarization of the neuronal membrane.