The Effect of Ambient on Photoluminescence from GaN

The effect of ambient on photoluminescence (PL) from GaN was studied. We found that the PL intensity in vacuum was nearly four times higher than in air. The PL intensity also increased after etching the sample in Aqua Regia and BOE to remove the native oxide layer. After etching, the PL intensity was very stable in vacuum, but substantially degraded in air ambient. In HCl vapor (low pH), the PL intensity increased as compared to air ambient, while in NH3 vapor (high pH) it decreased. The quantum efficiency of the exciton and blue luminescence bands increased significantly with increasing excitation power density. This increase could not be explained by reduction of the depletion region width (field effect mechanism), but could be explained by changes in the nonradiative recombination rate at the surface (recombination mechanism). We therefore assume that in vacuum and acid vapor some surface species are desorbed or passivated, resulting in a decreased nonradiative recombination rate and increased PL intensity

Retigabine holds KV7 channels open and stabilizes the resting potential

The anticonvulsant Retigabine is a KV7 channel agonist used to treat hyperexcitability disorders in humans. Retigabine shifts the voltage dependence for activation of the heteromeric KV7.2/KV7.3 channel to more negative potentials, thus facilitating activation. Although the molecular mechanism underlying Retigabine\u27s action remains unknown, previous studies have identified the pore region of KV7 channels as the drug\u27s target. This suggested that the Retigabine-induced shift in voltage dependence likely derives from the stabilization of the pore domain in an open (conducting) conformation. Testing this idea, we show that the heteromeric KV7.2/KV7.3 channel has at least two open states, which we named O1 and O2, with O2 being more stable. The O1 state was reached after short membrane depolarizations, whereas O2 was reached after prolonged depolarization or during steady state at the typical neuronal resting potentials. We also found that activation and deactivation seem to follow distinct pathways, suggesting that the KV7.2/KV7.3 channel activity displays hysteresis. As for the action of Retigabine, we discovered that this agonist discriminates between open states, preferentially acting on the O2 state and further stabilizing it. Based on these findings, we proposed a novel mechanism for the therapeutic effect of Retigabine whereby this drug reduces excitability by enhancing the resting potential open state stability of KV7.2/KV7.3 channels. To address this hypothesis, we used a model for action potential (AP) in Xenopus laevis oocytes and found that the resting membrane potential became more negative as a function of Retigabine concentration, whereas the threshold potential for AP firing remained unaltered

EXPANDING MONOAMINE TRANSPORTERS PHARMACOLOGY USING CALCIUM CHANNELS

Research in drug development meets many challenges including lengthy, complex and costly procedures to identify novel pharmacotherapies. In our lab, we developed a method for fast screening of small molecules that interact with monoamine transports – dopamine and serotonin (DAT, SERT). These membrane proteins play important roles in brain neurotransmission responsible for cognition, motion and pleasure. Dysfunction in dopaminergic and serotonergic systems result in neurological disorders such as depression, Attention Deficit Hyperactivity Disorder (ADHD), schizophrenia and addiction. DAT and SERT are responsible for uptake of dopamine (DA) or serotonin (5HT) into the synapse and they limit neurotransmitter signaling. Drugs that mimic or antagonize actions of endogenous neurotransmitters (DA and 5HT) increase the concentrations of DA and/or 5HT either by blocking the transporter (blockers) or by competing uptake with neurotransmitter (substrate). The uptake of substrates is associated to an inward current that depolarizes the cell membrane. Voltage-gated calcium channels (CaV) can respond to small changes in membrane potential. In our method, we combined permanent cell line expressing the human dopamine transporter (hDAT) or the human serotonin transporter (hSERT) (FlpIn TREx expression system) with transient transfection of CaV. This system works as a tightly electrically coupled system. Cells challenged with substrate of the transports produce detectable Ca2+ signal while monoamine transporter blockers can inhibit these Ca2+ signals. The novelty of this method relies on the ability to discriminate between substrate and blockers of monoamine transporters. Preliminary experiments measuring our optimized cell system in a Flex Station 3 plate reader suggest that the co-expression of a voltage-gated Ca2+ channel, a monoamine transporter and a genetically encoded Ca2+ sensor constitute a rapid screening biosensor to identify active drugs at monoamine transporters. Our novel methodology can rapidly assess drug-effect profile on monoamine transporters and benefit development of new psychotherapeutics for treatment of mental illnesses. It can also be used to characterize mechanism of action of emerging drug of abuse, as well as to discover small molecules with novel drug-effect profile useful in basic neuroscience research

Retigabine holds KV7 channels open and stabilizes the resting potential

The anticonvulsant Retigabine is a KV7 channel agonist used to treat hyperexcitability disorders in humans. Retigabine shifts the voltage dependence for activation of the heteromeric KV7.2/KV7.3 channel to more negative potentials, thus facilitating activation. Although the molecular mechanism underlying Retigabine\u27s action remains unknown, previous studies have identified the pore region of KV7 channels as the drug\u27s target. This suggested that the Retigabine-induced shift in voltage dependence likely derives from the stabilization of the pore domain in an open (conducting) conformation. Testing this idea, we show that the heteromeric KV7.2/KV7.3 channel has at least two open states, which we named O1 and O2, with O2 being more stable. The O1 state was reached after short membrane depolarizations, whereas O2 was reached after prolonged depolarization or during steady state at the typical neuronal resting potentials. We also found that activation and deactivation seem to follow distinct pathways, suggesting that the KV7.2/KV7.3 channel activity displays hysteresis. As for the action of Retigabine, we discovered that this agonist discriminates between open states, preferentially acting on the O2 state and further stabilizing it. Based on these findings, we proposed a novel mechanism for the therapeutic effect of Retigabine whereby this drug reduces excitability by enhancing the resting potential open state stability of KV7.2/KV7.3 channels. To address this hypothesis, we used a model for action potential (AP) in Xenopus laevis oocytes and found that the resting membrane potential became more negative as a function of Retigabine concentration, whereas the threshold potential for AP firing remained unaltered

Effects of <i>N</i>‑Alkyl-4-Methylamphetamine Optical Isomers on Plasma Membrane Monoamine Transporters and Abuse-Related Behavior

4-Methylamphetamine (4-MA) is an emerging drug of abuse that acts as a substrate at plasma membrane transporters for dopamine (DAT), norepinephrine (NET), and serotonin (SERT), thereby causing nonexocytotic release of monoamine transmitters via reverse transport. Prior studies by us showed that increasing the <i>N</i>-alkyl chain length of N-substituted 4-MA analogues converts 4-MA from a transportable substrate (i.e., releaser) at DAT and NET to a nontransported blocker at these sites. Here, we studied the effects of the individual optical isomers of <i>N</i>-methyl-, <i>N</i>-ethyl-, and <i>N</i>-<i>n</i>-propyl 4-MA on monoamine transporters and abuse-related behavior in rats because action/function might be related to stereochemistry. Uptake inhibition and release assays were conducted in rat brain synaptosomes whereas electrophysiological assessments of drug–transporter interactions were examined using cell-based biosensors. Intracranial-self-stimulation in rats was employed to assess abuse potential in vivo. The experimental evidence demonstrates that <i>S</i>(+)<i>N</i>-methyl 4-MA is a potent and efficacious releaser at DAT, NET, and SERT with the highest abuse potential among the test drugs, whereas <i>R</i>(−)<i>N</i>-methyl 4-MA is a less potent releaser with reduced abuse potential. The <i>S</i>(+)­ethyl analogue has decreased efficacy as a releaser at DAT but retains full release activity at NET and SERT with a reduction in abuse-related effects; the <i>R</i>(−)­ethyl analogue has a similar profile but is less potent. <i>S</i>(+)<i>N</i>-Propyl 4-MA is a nontransported blocker at DAT and NET but an efficacious releaser at SERT, whereas the <i>R</i> enantiomer is almost inactive. In conclusion, the <i>S</i> enantiomers of the <i>N</i>-alkyl 4-MA analogues are most potent. Lengthening the <i>N</i>-alkyl chain converts compounds from potent nonselective releasers showing abuse-related effects to more selective SERT releasers with no apparent abuse potential

Retigabine holds KV7 channels open and stabilizes the resting potential

The anticonvulsant Retigabine is a K(V)7 channel agonist used to treat hyperexcitability disorders in humans. Retigabine shifts the voltage dependence for activation of the heteromeric K(V)7.2/K(V)7.3 channel to more negative potentials, thus facilitating activation. Although the molecular mechanism underlying Retigabine’s action remains unknown, previous studies have identified the pore region of K(V)7 channels as the drug’s target. This suggested that the Retigabine-induced shift in voltage dependence likely derives from the stabilization of the pore domain in an open (conducting) conformation. Testing this idea, we show that the heteromeric K(V)7.2/K(V)7.3 channel has at least two open states, which we named O(1) and O(2), with O(2) being more stable. The O(1) state was reached after short membrane depolarizations, whereas O(2) was reached after prolonged depolarization or during steady state at the typical neuronal resting potentials. We also found that activation and deactivation seem to follow distinct pathways, suggesting that the K(V)7.2/K(V)7.3 channel activity displays hysteresis. As for the action of Retigabine, we discovered that this agonist discriminates between open states, preferentially acting on the O(2) state and further stabilizing it. Based on these findings, we proposed a novel mechanism for the therapeutic effect of Retigabine whereby this drug reduces excitability by enhancing the resting potential open state stability of K(V)7.2/K(V)7.3 channels. To address this hypothesis, we used a model for action potential (AP) in Xenopus laevis oocytes and found that the resting membrane potential became more negative as a function of Retigabine concentration, whereas the threshold potential for AP firing remained unaltered