Development of Action Potential‐dependent and Independent Spontaneous GABA_A Receptor‐mediated Currents in Granule Cells of Postnatal Rat Cerebellum

Abstract

The postnatal development of spontaneous GABAergic transmission between cerebellar Golgi cells and granule cells was investigated with voltage-clamp recording from rat cerebellar slices, in symmetrical Cl- conditions. Between postnatal days 7 and 14 (P7-14), bicuculline- and TTX (tetrodotoxin)-sensitive spontaneous inhibitory postsynaptic currents (sIPSCs), occurred at high frequency in 56% of granule cells. Between P10 and P14, sIPSCs were superimposed on a tonic current of -12 +/- 1.8 pA at -70 mV, that was accompanied by noise with a variance of 17 +/- 3 pA2. Both the current and noise were inhibited by bicuculline. TTX blocked the bicuculline-sensitive current and noise by approximately 60%. Between P18 and P25, sIPSCs were less frequent; all cells showed tonic, bicuculline-sensitive currents, but these were partially inhibited by TTX (approximately 35%). Between P40 and P53, sIPSCs were rare; tonic, bicuculline-sensitive currents and noise were greater in amplitude, with mean values of -17 pA and 22 pA2 at -70 mV, they were present in all cells but they were not inhibited by TTX. Glycine receptor channels that were expressed in immature, but not adult cells, did not mediate spontaneous currents. Our results indicate that spontaneous transmission onto cerebellar granule cells in immature animals consists primarily of action potential-dependent, phasic release of vesicular GABA. This generates GABAA receptor-mediated sIPSCs. The effects of GABA transporter blockers suggest that it also produces the TTX-sensitive current-noise, as GABA spills out of synapses to activate extrasynaptic receptors or receptors in neighbouring synapses. In older animals, action potential-independent release of transmitter is predominant and results in tonic activation of GABAA receptors. This does not appear to be spontaneous vesicular release of GABA. Neither does it appear to be reversed uptake of GABA, although further work is required to rule out these possibilities.

Morphological development and maturation of the GABAergic synapses in the mouse cerebellar granular layer

“…The cerebellum is deeply involved in learning motor skills [29,40,41] Our study demonstrated that during the same developmental period, functional GABAergic synapses are formed synapse and GABAergic inhibition starts in the cerebellar glomeruli [32,66,70,71]. Overall, these results suggested that the start of GABAergic inhibition is crucial for acquisition of motor learning.…”

GABAAReceptor-Mediated Tonic Inhibition in Thalamic Neurons

“…Tonic GABA A receptor-mediated inhibition has been shown previously in a variety of cell types, including DGGCs, CGCs, and pyramidal cells and interneurons of the hippocampus (Kaneda et al, 1995;Brickley et al, 1996;Wall and Usowicz, 1997;Bai et al, 2001;Brickley et al, 2001;Nusser and Mody, 2002;Stell and Mody, 2002;Semyanov et al, 2003). The receptors that underlie tonic inhibition are exclusively located extrasynaptically, but their subunit composition is cell type dependent.…”

“…Tonic GABA A currents have been observed primarily in cerebellar (CGCs) and dentate gyrus (DGGCs) granule cells (Kaneda et al, 1995;Brickley et al, 1996;Wall and Usowicz, 1997;Nusser and Mody, 2002;Stell and Mody, 2002). In CGCs, the ␦ subunit is intimately associated with the ␣6 subunit (Jones et al, 1997;Nusser et al, 1999), whereas in DGGCs it probably associates with the ␣4 subunit (Sur et al, 1999).…”

Neurosteroid regulation of central nervous system development

“…These receptors are responsible for tonic inhibition and responses to GABA spillover (Brickley et al, 1996;Brickley et al, 2001;Hamann et al, 2002;Nusser and Mody, 2002;Rossi et al, 2003;Semyanov et al, 2003;Stell and Mody, 2002;Wall and Usowicz, 1997). GABA A receptors containing the delta subunit are restricted to these extrasynaptic locations (Nusser et al, 1998;Wei et al, 2003).…”