Tonic inhibition was imaged in cerebellar granule cells of transgenic mice expressing the optogenetic chloride indicator, Clomeleon. Blockade of GABAA receptors substantially reduced chloride concentration in granule cells due to block of tonic inhibition. This indicates that tonic inhibition is a significant contributor to the resting chloride concentration of these cells. Tonic inhibition was observed not only in granule cell bodies, but also in their axons, the parallel fibers (PFs). This presynaptic tonic inhibition could be observed in slices both at room and physiological temperatures, as well as in vivo, and has many of the same properties as tonic inhibition measured in granule cell bodies. GABA application revealed that PFs possess at least two types of GABAA receptor: one high-affinity receptor that is activated by ambient GABA and causes a chloride influx that mediates tonic inhibition, and a second with a low affinity for GABA that causes a chloride efflux that excites PFs. Presynaptic tonic inhibition regulates glutamate release from PFs because GABAA receptor blockade enhanced both the frequency of spontaneous EPSCs and the amplitude of evoked EPSCs at the PF-Purkinje cell synapse. We conclude that tonic inhibition of PFs could play an important role in regulating information flow though cerebellar synaptic circuits. Such cross talk between phasic and tonic signaling could be a general mechanism for fine tuning of synaptic circuits.
Bibliographical noteFunding Information:
This work was supported by National Institute of Mental Health Grants, Alfred P. Sloan Research Fellowship, Klingenstein Fellowship Award in the Neuroscience, Beckman Young Investigator Award, World Class Institute program of the National Research Foundation of Korea funded by Ministry of Education, Science, and Technology WCI 2009-003, National Research Foundation of Singapore CRP Grant, National Science Foundation Grant 1512826, and BRAIN Initiative MH106013. We thank S. Van Hooser and D. Fitzpatrick for teaching us about in vivo imaging; K. Tanaka for electrophysiological advice; C.J. Lee for helpful discussion; and K. Chung, E. Ellison, D. Given, S. Kay, M. Lee, and P. Teo for technical assistance.
© 2016 the authors.
- Parallel fibers
- Tonic inhibition