Inhibitory or Excitatory Potential Changes :: Drugs Psychology Psychiatry Medical Essays

Inhibitory or Excitatory Potential Changes Risperidone is a serotonin antagonist which blocks GABA-evoked currents in pyramidal neurons (Feng et al, 2001). Risperidone lengthens action potentials and blocks potassium current in rabbit heart muscle. In human atrial muscle risperidone decreases outward current but has no effect on inward current. It lengthens the repolarization of atrial and ventricular action potentials only at high drug levels. Risperidone reduces the membrane resting potential and prolongs action potential duration. In human heart preparations risperidone, at high drug concentrations, lengthens repolarization of atrial and ventricular myocardium. (Gluais et al, 2004).Risperidone lengthens the QT action potential of rabbit heart. It lengthens the action potential and blocks repolarization potassium flow. (Gluais et al, 2004). To decrease background noise when using a patch electrode a patch of membrane is isolated with a micropipette. This allows current measurement in the pico ampere range. The tube is sealed to the membrane by suction forming a "gigaseal" which refers to the electrical resistance across the membrane. Such techniques were instrumental in finding that ion channels are gated and modulated not only by voltage and external ligands but also by second messengers, regulatory proteins and by phosphorylation. Patch clamp recording can be used with any cell culture. Voltage dependant sodium, calcium, and potassium channels have a huge number of subtypes. Another large category is transmitter-gated channels regulated by nucleotides, intra-cellular sodium, and calcium, and GTP-binding proteins. There are hundreds of channel subtypes regulated by a variety of mechanisms. Activity of ion channels directly or indirectly affects second messenger calcium ionic concentration. Voltage-dependant calcium channels gate the entry of calcium ions and help shape the action potentials. The entry of calcium ions activates kinases, contractile proteins and ion channels. There are voltage-independent calcium channels activated by ligand binding of receptors on the outside of the cell membranes. Both receptor-linked GTP-binding protein and second messengers affect ion channel activity. Modulation of channel activity by applying agonists to the cell culture indicates a second messenger system. Patch clamp electrodes have been used on the calcium transporter channels in organelles such as endoplasmic reticulum and sarcoplasmic reticulum by rupturing the cell, isolating the organelle, and inducing the membranes to form vesicles. To study microbe channels techniques are used to merge several microbes into a giant microbe large enough to attach a patch electrode to (Rudy,B. & Iverson, L, 1991). Viruses contain coding to create ion channels in host membranes.