| Abstract: | Membrane potential and voltage clamp measurements of the responses to octopamine, dopamine, O-acetylcholine, and the formamidine derivatives chlordimeform (CDM) and N1-demethylchlordimeform (DCDM) have been made on identified central neurones of Helix aspersa. DCDM and CDM were agonists on both the inhibitory and excitatory octopamine receptors on these neurones but were less potent than octopamine. The direct inhibitory effect of DCDM was antagonised by both phentolamine and cyproheptadine. DCDM was also found to antagonise the effects of octopamine, dopamine and O-acetylcholine on these neurones. The pA2 values for DCDM against dopamine and O-acetylcholine were 5.2 (±0.1) and 5.35 (±0.15), respectively. The pA2 value=logDR-(1/I)], where I is the concentration of the antagonist, and DR=dose ratio =A2/A1; A1 is the dose of agonist required to produce 50% of the maximum response, and A2 is the dose of agonist needed to produce the same degree of response in the presence of the antagonist. It is concluded that DCDM and CDM act specifically on the octopamine receptors of H. aspersa central neurones, both as agonists and antagonists. The antagonist effect of DCDM against dopamine receptors is probably non-specific. |
