Date of Award

2015

Degree Type

Thesis

Degree Name

Master of Science (MS)

First Advisor

Harold Komiskey Jr, PhD

Second Advisor

Shu Zhu, MD, PhD

Third Advisor

Richard White, PhD

Fourth Advisor

Valerie E Cadet, PhD

Abstract

Over five million Americans suffer from Alzheimer’s Disease (AD), with an expected 34% increase in the incidence in this disease over the next decade. Unfortunately, there is no cure for AD. Recent studies have indicated that drugs which increase the levels of cyclic guanosine-3’5’-monophosphate (cGMP) may help preserve learning and memory in AD and enhance cognition in the aging brain; however, the mechanism(s) of how cGMP exerts this beneficial effect is unknown. The present findings now suggest that elevation of cGMP in astrocytes depresses inhibitory potassium currents in these cells to stimulate their protective influence on neuronal activity. Cellular currents were measured directly in rat embryonic astrocytes via the perforated-patch whole-cell patch-clamp technique, and these experiments demonstrated robust outward currents due to potassium efflux. Currents exhibited slowly-activating, non-inactivating kinetics, and were sensitive to inhibition by tetraethylammonium. Thus, these currents were carried predominately by the delayed rectifier potassium (KDR) channel, which is highly expressed in astrocytes. Stimulation of cGMP signaling with sodium nitroprusside (a nitric oxide donor; 10mM; 10min) depressed steady-state outward currents by an average of 34% (n=3). In other cells addition of 8Br-cGMP (500mM; a membrane-permeable cGMP derivative) also depressed these currents (18.4%, n=3). In contrast, neither SNP nor 8Br-cGMP altered rapidly-inactivating, A-type potassium currents significantly. Lastly, elevating cyclic adenosine-3’5’-monophosphate (cAMP) levels in astrocytes with forskolin (10mM, 10 min) also depressed outward currents (17%; n=3). Taken together, these findings suggest a novel cellular transduction mechanism that could contribute to the beneficial effect of NO/cGMP signaling on learning and memory: depression of KDR currents, which depolarizes astrocytes, and thereby increases calcium influx via voltage-dependent (L-type) calcium channels. Increased astrocytic calcium levels would in turn excite these cells and enhance release of gliotransmitters to promote normal synaptic neurotransmission.

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