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¹ Department of Molecular pharmacology and physiology, College of Medicine, University of South Florida, Tampa, FL, 33612-4799, USA

The onset of motor learning in rats coincides with exclusive expression of GABA_A receptors containing ₆ and subunits in the granule neurons of the cerebellum. This development temporally correlates with the presence of a spontaneously active chloride current through ₆-containing GABA_A receptors, known as tonic inhibition. Here we report that the coexpression of ₆, ₂, and subunits produced receptor–channels which possessed two distinct and separable states of agonist affinity, one exhibiting micromolar and the other nanomolar affinities for GABA. The high-affinity state was associated with a significant level of spontaneous channel activity. Increasing the level of expression or the ratio of ₂ to ₆ and subunits increased the prevalence of the high-affinity state. Comparative studies of ₆₂, ₁₂, ₆₂₂, ₁₂₂ and ₄₂ receptors under equivalent levels of expression demonstrated that the significant level of spontaneous channel activity is uniquely attributable to ₆₂ receptors. The pharmacology of spontaneous channel activity arising from ₆₂ receptor expression corresponded to that of tonic inhibition. For example, GABA_A receptor antagonists, including furosemide, blocked the spontaneous current. Further, the neuroactive steroid 5-THDOC and classical glycine receptor agonists -alanine and taurine directly activated ₆₂ receptors with high potency. Specific mutation within the GABA-dependent activation domain (^Y157F) impaired both low- and high-affinity components of GABA agonist activity in ₆^Y157F receptors, but did not attenuate the spontaneous current. In comparison, a mutation located between the second and third transmembrane segments of the subunit (^R287M) significantly diminished the nanomolar component and the spontaneous activity. The possibility that the high affinity state of the ₆₂ receptor modulates the granule neuron activity as well as potential mechanisms affecting its expression are discussed.

(Received 26 March 2007; accepted after revision 28 March 2007; first published online 29 March 2007)
Corresponding author J. Amin: University of South Florida, College of Medicine, Department of Pharmacology and Therapeutics, MDC box 9, 12901 Bruce B. Downs Blvd. Tampa, FL 33612, USA. Email: jamin{at}health.usf.edu

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