An analysis of the inhibitory responses of dopamine and octopamine on Helix central neurons.

1. Electrophysiological recordings were made from identified neurons in the isolated suboesophageal ganglionic mass of Helix aspersa. Cells were voltage clamped at testing membrane potential. 2. Bath addition of 1 microM dibutyryl cAMP caused a time dependent enhancement of an evoked IPSP and the dopamine (DA) and octopamine (OA) induced outward currents obtained in these neurons. Forskolin, 0.1 microM, which enhances and MDL 12,330A, 0.12 microM, which depresses adenylate cyclase activity also modified these responses. 3. The DA and OA inhibitory responses were both shown to be potassium mediated events. They were preferentially antagonised by low micromolar concentrations of 4-aminopyridine. Two other potassium channel antagonists, tetraethylammonium and apamin had little effect on the DA and OA responses. 4. Cell sensitivity to DA and OA was greatly enhanced in calcium free/2 mM cobalt Ringer. The reversal potential of the DA response was shifted to a more negative value in calcium free Ringer. Sodium free Ringer was also found to alter the responses to DA or OA but those results were not consistent.