CHANNELS, RECEPTORS, AND ELECTRICAL SIGNALING |
GABAA receptor kinetics in the cerebellar nuclei:
Evidence for detection of transmitter from distant release
sites
Jason R. Pugh 1 and Indira M. Raman 1*
1 Northwestern University
* To whom correspondence should be addressed. E-mail: i-raman{at}northwestern.edu.
Submitted on November 5, 2004
Revised on November 29, 2004
Accepted on 14 December 2004
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Abstract |
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Neurons of the cerebellar nuclei receive GABAergic input from Purkinje cells. Purkinje boutons have several closely spaced presynaptic densities without GABA transporters, raising the possibility that neurotransmitter released by one presynaptic site diffuses to multiple postsynaptic sites. To test whether such "local" spillover may contribute to transmission, we studied gating of GABAA receptors at 31-33°C in cerebellar nuclear neurons acutely dissociated from mice. Currents were evoked by rapid application of long steps, brief pulses, and high-frequency trains of GABA to outside-out patches. Receptors desensitized and deactivated rapidly, and dose-response measurements estimated an EC50 of ~30 µM. From these data, a kinetic scheme was developed that replicated the recorded currents. Next, we simulated diffusion of GABA in the synaptic cleft, constrained by previous electron microscopic data, and drove the kinetic GABAA receptor model with modeled concentration transients. Simulations predicted receptor occupancies of ~100% directly opposite the release site and ~50% at distant postsynaptic densities, such that receptors up to 700 nm from a release site opened on the time scale of the IPSC before desensitizing. Further simulations of probabilistic release from multiple-site boutons suggested that local spillover-mediated transmission slows the onset and limits the extent of depression during high-frequency signaling.
Key Words:
IPSC, Purkinje, deep cerebellar nuclei, desensitization, spillover, synaptic depression
