Effect of pertussis toxin and N-ethylmaleimide on voltage-dependent and -independent calcium current modulation in serotonergic neurons.
Introduction of GTP-gamma-S into a neuronal cell spontaneously results in G-protein activation. A possible contribution to this mechanism is that some receptors have a constitutive activity that stimulates GDP/GTP exchange resulting in increased GTPase activity of G-protein alpha subunits, leading to a facilitation of GTP-gamma-S binding. It follows that partial or complete uncoupling of receptors and G-proteins could inhibit Ca(2+) current modulation by GTP-gamma-S. This possibility was tested in acutely isolated rat dorsal raphe neurons by uncoupling the receptor and G-protein using N-ethylmaleimide and pertussis toxin. Since these compounds have been suggested to differentially block voltage-dependent inhibition, relative to voltage-independent, we investigated whether the apparent voltage-independent component of Ca(2+) channel modulation by 5-hydroxytryptamine (5-HT) shares the same mechanism as the voltage-dependent component. N-ethylmaleimide inhibited the response to 5-HT by about 50% but had no effect on the response to GTP-gamma-S. In dorsal raphe neurons 28.9% of the total response to 5-HT was voltage-independent. N-ethylmaleimide had identical effects on the voltage-dependent and -independent components as measured by tail current inhibition. The response to 5-HT was completely sensitive to pertussis toxin, and completely uncoupling the receptors and G-proteins did not affect the maximal response to GTP-gamma-S. Our results suggest that the apparent voltage-independent component of Ca(2+) channel modulation by 5-HT in dorsal raphe neurons might share the same mechanism as does the voltage-dependent component. In addition, these experiments provided evidence that partial or even complete uncoupling of receptors and G-proteins did not affect Ca(2+) current modulation by direct activators of G-proteins.
Chen
Y
Y
Department of Pharmacology, University of Washington, Seattle 98195, USA.
Yao
Y
Y
Penington
N J
NJ
D000818:Animals; D015220:Calcium Channels / Q000187:drug effects* / Q000502:physiology*; D005033:Ethylmaleimide / Q000494:pharmacology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D008297:Male; D009474:Neurons / Q000378:metabolism*; D037342:Pertussis Toxin; D011903:Raphe Nuclei / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000378:metabolism*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology*
D015220:Calcium Channels; D010566:Virulence Factors, Bordetella; D012701:Serotonin; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D037342:Pertussis Toxin; D005033:Ethylmaleimide