Dopaminergic transmission between identified neurons from the mollusc,
Lymnaea stagnalis.
Magoski, Neil S., Lorenzo G. Bauce, Naweed I. Syed, and Andrew G.M. Bulloch.
Department of Medical Physiology and Neuroscience Research Group, Faculty
of Medicine, University of Calgary, 3330 Hospital Drive N.W., Calgary,
Alberta, Canada, T2N 4N1.
APStracts 2:0148N, 1995.
SUMMARY AND CONCLUSIONS
1. Dopaminergic transmission was investigated in the central nervous system
(CNS) of the freshwater snail, Lymnaea stagnalis. 2. The giant pedal neuron,
designated as right pedal dorsal one (RPeD1), makes chemical, monosynaptic
connections with a number of identifed follower cells in the CNS. Previous
work has shown that RPeD1 is an interneuron and a important component of the
Lymnaea respiratory central pattern generator. In this study, the hypothesis
that RPeD1 uses dopamine as its neurotransmitter was tested by
chromatographic, pharmacological, and electrophysiological methods.
Characterization of RPeD1's transmitter pharmacology is essential to clearly
understand its role in Lymnaea. 3. Earlier studies have demonstrated that the
soma of RPeD1 contains dopamine. This was quantitated in the present study by
high performance liquid chromatography (with electrochemical detection) of
isolated RPeD1 somata and growth cones, which yielded 0.8 +/- 0.3 and 0.10 +/-
0.08 pmol of dopamine per soma and growth cone, respectively. 4. Bath or
pressure application of dopamine to follower cells of RPeD1, in situ ,
mimicked the effects of RPeD1 stimulation. Dose response curves were
constructed for: i) the excitatory effect of dopamine on follower cells,
visceral dorsal two and three (ED 50 =39 [mu]M; Hill coefficient=1.03); ii)
the inhibitory effect of dopamine on follower cell, visceral dorsal four (ED
50 =33 [mu]M; Hill coefficient=0.92). 5. The following dopamine agonists (100
[mu]M) were tested by bath application: 2-amino-6,7-dihydroxy-1,2,3,4-
tetrahydronapthlene (ADTN), apopmorphine, 2-bromo-[alpha]-ergocryptine,
deoxyepinephrine (DE), mesulergine, (-) quinpirole, SKF 38393, and tyramine.
Only the general dopamine agonists, ADTN and DE mimicked RPeD1's effects on
its follower cells. 6. When VD2/3 was isolated and plated in vitro , it
maintained a depolarizing response to dopamine. This response was reduced by
intracellular injection of the G-protein blocker, GDP-[beta]-S (2 mM in
electrode). Similarly, incubation of VD2/3, in vitro for _18 hours, with
pertussis toxin (PTX; 5 [mu]g/mL), the G-protein inactivating endotoxin, also
reduced the dopamine response. Injecting GDP or incubating in heat-inactivated
PTX, did not effect the response. 7. Several dopamine antagonists were used in
an attempt to block RPeD1's synapses: chlorpromazine, ergonovine,
fluphenazine, haloperidol, 6-hydroxydopamine, SCH 23390, (+/-) sulpiride, and
tubocurarine. Only the D-2 dopamine receptor antagonist, (+/-) sulpiride,
reversibly blocked synaptic transmission from RPeD1 to its follower cells.
Both the (+) and the (-) enantiomer of sulpiride also antagonized synaptic
transmission. A dose inhibition curve for (+/-) sulpiride was constructed (IC
50 =47 [mu]M). (+/-) Sulpiride also blocked the effects of bath applied
dopamine. 8. Collectively, these data suggest that RPeD1 uses dopamine as a
neurotransmitter and that dopamine may elicit its effects via a G-protein
coupled, D-2-like dopamine receptor.
Received 17 February 1994; accepted in final form 2 May 1995.
APS Manuscript Number J118-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 16 May 1995.