Hyperpolarizing pulses

Up to now, neither this method nor STARTMAS has been used by researchers other than their authors, especially because they are subjected to many imperfections of the pulse sequence. Still, it may be anticipated that they will open up new possibilities in a variety of applications, including studies on unstable systems, in-situ high-temperature experiments, hyperpolarized solids, or measurements on very slowly relaxing spins. 

The immediate short-term effects of dopamine are mediated by voltage and receptor-operated channels. The effects depend on the membrane potential of the postsynaptic cell, and its recent history, because these variables determine the state of the channels modulated by dopamine. At hyperpolarized potentials, rapidly inactivating channels are available, and modulating them can have effects on the transition from hyperpolarized Down states to depolarized Up states. After prolonged periods in the depolarized states, these channels are inactivated. If a dopamine pulse occurs at this time the effects on noninactivating channels will predominate. 

For dye injection, tips are backfilled with a solution of 5 mg/ml carboxyfluorescein that has been dissolved at pH 10 and then adjusted to pH 7.4 in 0.2 M KCl (Goodall and Johnson, 1984), while the shaft is filled with 0.2 M KCl. Dye and current injection are performed using 500 msec hyperpolarizing current pulses, with 500 msec intervals, at a magnitude of 2 nA (Goodall and Maro, 1986). 

Figure 3. Changes in the membrane potential and input conductance of muscle bags during the sequential application of increasing concentrations of a) acetylcholine (Ach) and b) pyrantel The input conductance was determined from the amplitude of the hyperpolarizations resulting from 400 ms pulses of current (50 nA) passed across the bag membrane Ach and pyrantel were applied by microfusion (concentrations shown) to different muscle bags Note the decrease in input conductance and repolarization in the presence of 1 and 10 mM pyrantel. (Adapted with permission from Ref. 1. Copyright 1985, Society of Chemical Industry).

Figure 22 Schematic description of a structure determination of a denatured protein by combining pulse labelling with NOE. In the denatured state U, hyperpolarization (CIDNP) is generated in a tryptophan residue by the reaction with a sensitizer PS, transferred to nearby amino acids by cross-relaxation (NOE), and then transferred to the native protein N by fast refolding, where it is observed. Further explanation, see text. Reproduced from Ref. 232 with permission copyright...

Fig. 9 NMR spectral changes revealed by a 5 mm solution of hyperpolarized choline upon undergoing phosphorylation by 0.5 units of choline kinase, (a) Emergence of the new phosphocholine resonance shown by directly detected single-pulse N NMR spectroscopy experiments, (b) Emergence of the H NMR resonance associated with the methylenes in the C2-position of phosphocholine, (c) Comparison between the expected enzyme kinetics of kinase with results afforded by the N- ( ) and H-detected ( ) hyperpolarized experiments, as derived from the relative peak ratios of the NMR peaks in (a) and (b). The straight line illustrates the best fit of the combined set of data points, and corresponds to an initial phosphorylation rate of 0.3 mM min under these conditions. Reproduced with permission from [55]...

FIGURE 30.5 Action potential initiation by extracellular stimulation of CNS neurons by cathodic and anodic stimuli. Each trace shows transmembrane voltage as a function of time for different sections of the neuron, (a) Stimulation with a monophasic cathodic stimulus pulse from an electrode positioned f mm over a node of Ranvier of the axon. Depolarization occurs in the node directly beneath the elertrode (solid arrowhead) and hyperpolarization occurs in the adjacent nodes of Ranvier (open arrowhead). Action potential initiation occurs in the node of Ranvier directly under the electrode (arrow) and the action potential propagates in both directions, (b) During threshold stimulation with an electrode positioned 1 mm over the cell body, action potential initiation occurs at a node of Ranvier of the axon. With cathodic stimuli (duration 0.1 msec) action potential initiation occurred at the second node of Ranvier from the cell body (arrow), (c) With anodic stimuli (duration 0.1 msec) action potential initiation occurred in the third node of Ranvier from the cell body (arrow). 

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