A biphasic dopaminergic modulation of the high voltage-activated Ba2+ current of identified snail neurons
Tóm tắt
We have investigated the intracellular mechanisms by which dopamine induced a biphasic modulation of the Ba2+ current amplitude through the high voltage-activated Ca2+ channel (HVA-IBa) in identifiedHelix aspersa neurons. We used the two electrode voltage clamp technique on a group of identified neurons of the right parietal ganglionin situ, and the whole cell patch clamp technique on these same neurons in primary culture. Brief application of dopamine induced an initial fast reduction of the HVA-IBa followed by a slower enhancement of HVA-IBa. This enhancement was not due to a shift of the current-voltage curve. Repetitive application of dopamine did not attenuate this phase of the response. During longer application, the inhibition began to ‘sag’ and returned towards control levels. These results indicate that the enhancement was not due to a desensitization of the receptor or a relief from tonic G-protein mediated inhibition of the current. Manipulations of the levels of intracellular second messengers such as Ca2+, cGMP, cAMP, and arachidonic acid, as well as inhibition of protein kinases and phosphatases, had no effect on the dopamine induced biphasic effect on HVA-IBa. Pertussis toxin added to the patch pipette had a slow but simultaneous blocking effect on both phases of the dopamine action on HVA-IBa. Since our results show that pertussis toxin affects both phases of the dopamine action on this current, we suggest that both phases of the dopamine action on HVA-IBa are mediated by a pertussis toxin-sensitive G-protein. If a second messenger is implicated, it is none of the ‘classical’ second messenger systems.
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