Differential occurrence of reluctant openings in G-protein-inhibited N- and P/Q-type calcium channels.
Voltage-dependent inhibition of N- and P/Q-type calcium channels by G proteins is crucial for presynaptic inhibition of neurotransmitter release, and may contribute importantly to short-term synaptic plasticity. Such calcium-channel modulation could thereby impact significantly the neuro-computational repertoire of neural networks. The differential modulation of N and P/Q channels could even further enrich their impact upon synaptic tuning. Here, we performed in-depth comparison of the G-protein inhibition of recombinant N and P/Q channels, expressed in HEK 293 cells with the m2 muscarinic receptor. While both channel types display classic features of G-protein modulation (kinetic slowing of activation, prepulse facilitation, and voltage dependence of inhibition), we confirmed previously reported quantitative differences, with N channels displaying stronger inhibition and greater relief of inhibition by prepulses. A more fundamental, qualitative difference in the modulation of these two channels was revealed by a modified tail-activation paradigm, as well as by a novel "slope" analysis method comparing time courses of slow activation and prepulse facilitation. The stark contrast in modulatory behavior can be understood within the context of the "willing-reluctant" model, in which binding of G-protein betagamma subunits to channels induces a reluctant mode of gating, where stronger depolarization is required for opening. Our experiments suggest that only N channels could be opened in the reluctant mode, at voltages normally spanned by neuronal action potentials. By contrast, P/Q channels appear to remain closed, especially over these physiological voltages. Further, the differential occurrence of reluctant openings is not explained by differences in the rate of G-protein unbinding from the two channels. These two scenarios predict very different effects of G-protein inhibition on the waveform of Ca(2+) entry during action potentials, with potentially important consequences for the timing and efficacy of synaptic transmission.
Colecraft
H M
HM
Program in Molecular and Cellular Systems Physiology, Departments of Biomedical Engineering and Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
Patil
P G
PG
Yue
D T
DT
D000200:Action Potentials / Q000502:physiology; D020864:Calcium Channels, N-Type / Q000502:physiology*; D020865:Calcium Channels, P-Type / Q000502:physiology*; D020907:Calcium Channels, Q-Type / Q000502:physiology*; D002478:Cells, Cultured; D004594:Electrophysiology; D019204:GTP-Binding Proteins / Q000494:pharmacology*; D006801:Humans; D007668:Kidney / Q000166:cytology; D018377:Neurotransmitter Agents / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011976:Receptors, Muscarinic / Q000502:physiology; D009435:Synaptic Transmission / Q000502:physiology*
D020864:Calcium Channels, N-Type; D020865:Calcium Channels, P-Type; D020907:Calcium Channels, Q-Type; D018377:Neurotransmitter Agents; D011976:Receptors, Muscarinic; D019204:GTP-Binding Proteins