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MacDonald JF. 
“Pharmacology of dopamine receptors in the central nervous system of Planorbis corneas”. 
J.Physiol.. 1976;263(1):205P-06P.
A discrete dopaminergic system in the CNS of the gastropod mollusc Planorbis corneus consists of a dopamine (DA) containing pedal neurons that makes monosynaptic connections with neurones (follower cells) in the visceral and parietal ganglia. Stimulation of the DA cell evokes either e.p.s.p.s, i.p.s.p s or biphasic potentials (e.i,p.s.p.s) in follower cells. DA is probablythe transmitter mediating these responses. The pharmacology of DA receptors was studied by recording responses of follower cells to iontophoresis of DA, and to stimulation of the identified DA cell. 3 Distinct responses to DA were found: 2 were characterized by depolarization (D) and excitation of follower cells, the 3rd by hyperpolarization (M) and inhibition. D responses were distinguishable on the basis of duration and rate of desensitization. Apomorphine (AP) mimicked one of the D responses, but perfusion with AP antagonized subsequent responses to DA. Neither of the remaining 2 responses were antagonized by AP (1-10 M). AP ( 1-100 mcM) did lot alter synaptically evoked i.p.s.p.s; however, some e.p.s.p.s were antagonized by AP ( 1-10 M). H responses to DA and i.p.s.p.s were greatly reduced by perfusion of haloperidol, fluphenazine and metoclopramicle (50 AM). Reduction of these responses was accompanied by changes in membrane properties which were apparently non-specific, because responses to ACh iontophoresis were also reduced by these drugs. However, reduction of DA responses was greater and recovery was delayed following washing (30 min) whereas ACh responses returned directly after washing. Ergometrine, and LSD-25 demonstrated some capacity to mimic H responses to DA. Perfusion of LSD-25 and ergometrine (both 1-10 M) blocked or reduced inhibitory synaptic and iontophoretic dopaminergic responses. The actions of some of the drugs employed have been previously reported, However, the Planorbis system has the advantage that neurones known to possess a dopaminergic input are examined, both in response to DA and by synaptic activation. Furthermore, evidence suggests that there are probably 3 types of DA receptors in the CNS of Planorbis corneus,
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