Neurotransmitter modulation of extracellular H+ fluxes from isolated retinal horizontal cells of the skate

Self-referencing H+-selective microelectrodes were used to measure extracellular H+ fluxes from horizontal cells isolated from the skate retina. A standing H+ flux was detected from quiescent cells, indicating a higher concentration of free hydrogen ions near the extracellular surface of the cell as compared to the surrounding solution. The standing H+ flux was reduced by removal of extracellular sodium or application of 5-(N-ethyl-N-isopropyl) amiloride (EIPA), suggesting activity of a Na+–H+ exchanger. Glutamate decreased H+ flux, lowering the concentration of free hydrogen ions around the cell. AMPA/kainate receptor agonists mimicked the response, and the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) eliminated the effects of glutamate and kainate. Metabotropic glutamate agonists were without effect. Glutamate-induced alterations in H+ flux required extracellular calcium, and were abolished when cells were bathed in an alkaline Ringer solution. Increasing intracellular calcium by photolysis of the caged calcium compound NP-EGTA also altered extracellular H+ flux. Immunocytochemical localization of the plasmalemma Ca2+–H+-ATPase (PMCA pump) revealed intense labelling within the outer plexiform layer and on isolated horizontal cells. Our results suggest that glutamate modulation of H+ flux arises from calcium entry into cells with subsequent activation of the plasmalemma Ca2+–H+-ATPase. These neurotransmitter-induced changes in extracellular pH have the potential to play a modulatory role in synaptic processing in the outer retina. However, our findings argue against the hypothesis that hydrogen ions released by horizontal cells normally act as the inhibitory feedback neurotransmitter onto photoreceptor synaptic terminals to create the surround portion of the centre-surround receptive fields of retinal neuron

Oscillating magnetoresistance in diluted magnetic semiconductor barrier structures

Ballistic spin polarized transport through diluted magnetic semiconductor (DMS) single and double barrier structures is investigated theoretically using a two-component model. The tunneling magnetoresistance (TMR) of the system exhibits oscillating behavior when the magnetic field are varied. An interesting beat pattern in the TMR and spin polarization is found for different NMS/DMS double barrier structures which arises from an interplay between the spin-up and spin-down electron channels which are splitted by the s-d exchange interaction.Comment: 4 pages, 6 figures, submitted to Phys. Rev.

Aharonov-Bohm Physics with Spin II: Spin-Flip Effects in Two-dimensional Ballistic Systems

We study spin effects in the magneto-conductance of ballistic mesoscopic systems subject to inhomogeneous magnetic fields. We present a numerical approach to the spin-dependent Landauer conductance which generalizes recursive Green function techniques to the case with spin. Based on this method we address spin-flip effects in quantum transport of spin-polarized and -unpolarized electrons through quantum wires and various two-dimensional Aharonov-Bohm geometries. In particular, we investigate the range of validity of a spin switch mechanism recently found which allows for controlling spins indirectly via Aharonov-Bohm fluxes. Our numerical results are compared to a transfer-matrix model for one-dimensional ring structures presented in the first paper (Hentschel et al., submitted to Phys. Rev. B) of this series.Comment: 29 pages, 15 figures. Second part of a series of two article