The overall specific aims of this project are to understand the action of ethanol on brain pathways that mediate reward. The dopaminergic neurons of the ventral tegmental area (DA-VTA neurons) are important for the mediation of the reinforcing properties of ethanol, and the firing rate of these neurons is increased by ethanol. We have shown that serotonin potentiates ethanol excitation of DA-VTA neurons. Our subsequent studies have investigated ionic currents in the membrane of DA-VTA neurons to elucidate the ionic mechanism of this potentiation. In the previous project period, we focused on two membrane currents that may underlie the serotonin potentiation, the calcium-dependent potassium current SK, and the mixed cationic current lh. SK can be reduced by drugs like apamin or by depletion of intracellular calcium. We found that serotonin reduces Ih, and produces changes in the spike afterhyperpolarization similar to those produced by apamin. In our preliminary studies, we found that concurrent reduction of both of these currents produces profound potentiation of ethanol excitation. We have developed a model to explain this observation. In the proposed studies, five specific aims will seek to explore the relationship between ethanol excitation and potentiation of that excitation by serotonin.
Specific Aim #1 will establish the concentration-response curve for very low concentrations of ethanol during blockade of SK and Ih.
Specific Aim #2 will ascertain whether serotonin potentiation of ethanol excitation is mediated by concurrent reduction of SK and Ih.
Specific Aims #3 and #4 will determine how blockade of SK and Ih alter specific membrane properties of DA-VTA neurons, and how ethanol effects on specific membrane properties are altered by SK and Ih blockade.
Specific Aim #5 will use imaging of intracellular free calcium to determine whether prolonged administration of serotonin depletes intracellular stores of calcium. These studies directly build on our interesting initial observations of ethanol potentiation by blockade of SK and Ih, and will provide important information on how sensitivity to ethanol excitation may be modulated by neurotransmitter effects on ionic currents in DA-VTA neurons. Ultimately, information generated by this project may help in the development of pharmacotherapies for the amelioration of alcoholism.
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