How could a chirp be more effective than a louder clock--resonant transfer of energy between subthreshold excitation pulses and excitable tissues.

By definition, electrical resonance of excitable biomembranes reflects their remarkable ability to select input signals with preferential frequencies as ones that will pass through the chain of information exchange within a coordinated organism. This feature is known to be caused and regulated by the voltage-dependent kinetics of transmembrane ion channels. Knowledge about the resonant behavior of excitable cells may prove crucial toward better underpinning those essential properties of neurons which make them to function as a highly coordinated network. In this work we present novel experimental evidence which strongly supports the paradigm of selective communication between excitable cells, by no more than efficient transfer of energy which takes place when the frequency of subthreshold presynaptic pulses matches the natural, resonant frequency of a target excitable tissue. As a possible direct application, our novel experimental results prove the functional importance of bursting activity of pre-synaptic neurons as an effective physiological mechanism for discriminatory communication between neurons.

The influence exerted by the beta(3) subunit on MVIIA omega-conotoxin binding to neuronal N-type calcium channels.

In the present study, two-electrode voltage-clamp techniques have been used to assess the interaction between the MVIIA omega-conotoxin and an isoform of the N-type Ca(2+) channel alpha subunit (alpha(1B-d)). Cloned alpha(1B-d) Ca(2+) channels were expressed in Xenopus laevis oocytes in the presence and absence of the beta(3) subunit. Coexpression of the beta(3) subunit significantly shifted the IC(50) value for MVIIA inhibition of central N-type Ca(2+) channel current. Analysis of the peak conductance vs. depolarising voltage dependence suggested that the beta(3) subunit has no apparent effect on the gating charge which accompanies the closed-open transition of the channels. Instead, coexpression of the beta(3) subunit led to an approx. 10 mV shift to more hyperpolarised potentials in the voltage-dependent activation of N-type Ca(2+) channels. We conclude that MVIIA alters the surface charge on the N-type Ca(2+) channels and might induce allosteric changes on the structure of the channel, leading to an increase in the dissociation constant of MVIIA binding.

Novel omega-conotoxins from Conus catus discriminate among neuronal calcium channel subtypes.

omega-Conotoxins selective for N-type calcium channels are useful in the management of severe pain. In an attempt to expand the therapeutic potential of this class, four new omega-conotoxins (CVIA-D) have been discovered in the venom of the piscivorous cone snail, Conus catus, using assay-guided fractionation and gene cloning. Compared with other omega-conotoxins, CVID has a novel loop 4 sequence and the highest selectivity for N-type over P/Q-type calcium channels in radioligand binding assays. CVIA-D also inhibited contractions of electrically stimulated rat vas deferens. In electrophysiological studies, omega-conotoxins CVID and MVIIA had similar potencies to inhibit current through central (alpha(1B-d)) and peripheral (alpha(1B-b)) splice variants of the rat N-type calcium channels when coexpressed with rat beta(3) in Xenopus oocytes. However, the potency of CVID and MVIIA increased when alpha(1B-d) and alpha(1B-b) were expressed in the absence of rat beta(3), an effect most pronounced for CVID at alpha(1B-d) (up to 540-fold) and least pronounced for MVIIA at alpha(1B-d) (3-fold). The novel selectivity of CVID may have therapeutic implications. (1)H NMR studies reveal that CVID possesses a combination of unique structural features, including two hydrogen bonds that stabilize loop 2 and place loop 2 proximal to loop 4, creating a globular surface that is rigid and well defined.

Ion permeation through a G-protein activated (GIRK1/GIRK5) inwardly rectifying potassium channel.

In order to further investigate a G-protein activated inwardly rectifying potassium channel subunit, GIRK1 was expressed in Xenopus oocytes (where it coassembles with the endogenous GIRK5). The mechanism underlying ion permeation and rectification were measured in isolated inside-out patches. Single channel current amplitudes under symmetrical K+ concentrations at different holding potentials were evaluated. Inward-rectification of K+-currents through open GIRK1/GIRK5 channels was removed by washing out polyamines and Mg2+ ions. We developed a simple 'two-sites-three-barrier' (2S3B) Eyring rate theory model of K+ ion permeation for GIRK1/GIRK5 channels. The resulting optimized parameter-set will be used as a working model for subsequent investigation regarding K+ permeation process through the GIRK1/GIRK5 channel.

A C-terminal peptide of the GIRK1 subunit directly blocks the G protein-activated K+ channel (GIRK) expressed in Xenopus oocytes.

1. In order to find out the functional roles of cytosolic regions of a G protein-activated, inwardly rectifying potassium channel subunit we studied block of GIRK channels, expressed in Xenopus laevis oocytes, by synthetic peptides in isolated inside-out membrane patches. 2. A peptide (DS6) derived from the very end of the C-terminus of GIRK1 reversibly blocked GIRK activity with IC50 values of 7.9 +/- 2.0 or 3.5 +/- 0.5 micrograms ml-1 (corresponding to 3.7 +/- 0.9 or 1.7 +/- 0.2 mumol l-1) for GIRK1/GIRK5 or GIRK1/GIRK4 channels, respectively. 3. Dose dependency studies of GIRK activation by purified beta gamma subunits of the G protein (G beta gamma) showed that DS6 block of GIRK channels is not the result of competition of the peptide with functional GIRK channels for the available G beta gamma. 4. Burst duration of GIRK channels was reduced, whereas long closed times between bursts were markedly increased, accounting for the channel block observed. 5. Block by the DS6 peptide was slightly voltage dependent, being stronger at more negative potentials. 6. These data support the hypothesis that the distal part of the carboxy-terminus of GIRK1 is a part of the intrinsic gate that keeps GIRK channels closed in the absence of G beta gamma.

Actinic light density dependence of the O intermediate of the photocycle of bacteriorhodopsin.

The O intermediate of the photocycle of bacteriorhodopsin (BR) was studied by absorption kinetic measurements at different actinic light densities. With increasing exciting flash intensity, the relative yield of O slightly increases, while that of Mf strongly decreases at the expense of Ms. Kinetic calculations and the optical anisotropy of O show that O can be formed only from Mf although Mf and O have different light intensity dependences. In order to resolve the apparent contradiction, a phenomenologically new cooperative regulatory mechanism seems to be necessary.