Author

Yuil Kim

Date

March 2009

Document Type

Dissertation

Degree Name

Ph.D.

Institution

Oregon Health & Science University

Abstract

Cartwheel cells in the dorsal cochlear nucleus (DCN) are glycinergic inhibitory interneurons which form synapses on principal neurons and among themselves. Their activity is characterized by the presence of the complex spike, a brief burst of high-frequency spikes riding on a slow depolarization. The feedforward inhibition mediated by cartwheel cells prominently shapes the response of DCN principal cells to somatosensory input. Therefore, knowledge on how activity of cartwheel cells is regulated is important for input processing in the DCN. I examined two aspects of cartwheel cells, the ionic mechanism of complex spike generation and the glycine reversal potential, using gramicidin perforated-patch recording in a mouse brain slice preparation. Complex spikes require both Ca[superscript 2+] and Na[superscript +] currents. I monitored the change in spike waveforms elicited by subtype-specific blockers Ca[superscript 2+] channels and Ca[superscript 2+]-activated K[superscript +] channels. T/R- and L-type Ca[superscript 2+] channels contributed to the slow depolarization of complex spikes, whereas the P/Q-type Ca[superscript 2+] channels reduced the duration of the slow depolarization by coupling to SK channels. Single action potentials repolarized to more negative potential due to the presence of BK channels, which lowered the tendency to fire complex spikes. Thus, the balance between the depolarizing influence of a variety of Ca[superscript 2+] channels and the repolarizing effect of Ca[superscript 2+]-activated K[superscript +] channels shaped the complex spike and controlled its occurrence. In a second set of experiments, I examined the factors controlling the activity-dependence of glycine responses in cartwheel cells. Glycine reversal potential (E[subscript gly]) determines whether the glycine response of the cell is hyperpolarizing or depolarizing. E[subscript gly] shifted negative, increasing inhibition, after complex spiking or Ca[superscript 2+] spiking, and this effect was prevented by blocking Ca[superscript 2+] influx. I hypothesized that Ca[superscript 2+]-dependent intracellular acidification led to a negative shift in E[subscript gly] as a result of a decrease in intracellular HCO[superscript -][subscript 3] and fall in intracellular Cl[superscript -], the latter via activation of the Na[superscript +]-driven Cl[superscript -]-HCO[superscript -][subscript 3] exchanger (NDCBE). I used simultaneous measurements of intracellular pH and E[subscript gly] to examine the relationship between intracellular pH and E[subscript gly]. Intracellular acidification did occur with spiking activity, and such acidification, as well as the negative E[subscript gly] shift was sensitive to Ca[superscript 2+] channel block. Intracellular acidification induced with a weak acid in the quiescent state could shift the E[subscript gly] negative. The intracellular Cl[superscript -] concentration, monitored with the Cl[superscript -]-sensitive dye MQAE, indeed fell after complex spiking. Blocking NDCBE with H[subscript 2]DIDS or HCO[superscript -][subscript 3]/CO[subscript 2] removal eliminated the negative E[subscript gly] shift and slowed the recovery from intracellular acidification. These results largely supported the hypothesis. The Ca[superscript 2+]-dependent negative shift in E[subscript gly] may be a feedback suppression mechanism for excessive complex spiking.

Identifier

doi:10.6083/M4QN64PS

Division

Neuroscience Graduate Program

School

School of Medicine

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