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FIGURE 5–7. Determining the effects of systemic and direct drug administration on neuronal activity.

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FIGURE 5–7. Determining the effects of systemic and direct drug administration on neuronal activity.There are several means of applying drugs to a neuron to examine their actions. During in vivo recording, drugs may be administered systemically (i.e., intravenously, intraperitoneally, subcutaneously, intraventricularly, intramuscularly) or directly to the neuron by microiontophoresis or pressure ejection. (A) Systemic administration of a drug is useful for determining how a drug affects neurons in the intact organism, regardless of whether the action is direct or indirect. In this case, intravenous administration of the -aminobutyric acid (GABA) agonist muscimol (solid arrows) causes a dose-dependent increase in the firing rate of this dopamine-containing neuron. (B) In contrast, direct administration of a drug to a neuron will provide information about the site of action of the drug, at least as it concerns the discharge of the neuron under study. In this case, GABA is administered directly to a dopamine neuron by microiontophoresis. In this technique, several drug-containing pipettes are attached to the recording electrode. The pH of the drug solutions is adjusted to ensure that the drug molecules are in a charged state (e.g., GABA is used at pH = 4.0 to give it a positive charge), and the drug is ejected from the pipette tip by applying very small currents to the drug-containing pipette. Because the total diameter of the microiontophoretic pipette tip is only about 5 m, the drugs ejected typically affect only the cell being recorded. In this case, GABA is applied to a dopamine neuron by microiontophoresis; the horizontal bars show the time during which the current is applied to the drug-containing pipette, and the amplitude of the current (indicated in nA) is listed above each bar. Note that, unlike the excitatory effects produced by a systemically administered GABA agonist in (A), direct application of GABA will inhibit dopamine neurons. This has been shown to be caused by inhibition of a much more GABA-sensitive inhibitory interneuron by the systemically administered drug and illustrates the need to compare systemic drug administration with direct drug administration to ascertain the site of action of the drug of interest.Source. Adapted from Grace AA, Bunney BS: "Opposing Effects of Striatonigral Feedback Pathways on Midbrain Dopamine Cell Activity." Brain Research 333:271–284, 1985. Copyright 1985, Elsevier. Used with permission.

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