Adaptation without parameter change: Dynamic gain control in motion detection

Many sensory systems adapt their input-output relationship to changes in the statistics of the ambient stimulus. Such adaptive behavior has been measured in a motion detection sensitive neuron of the fly visual system, H1. The rapid adaptation of the velocity response gain has been interpreted as evidence of optimal matching of the H1 response to the dynamic range of the stimulus, thereby maximizing its information transmission. Here, we show that correlation-type motion detectors, which are commonly thought to underlie fly motion vision, intrinsically possess adaptive properties. Increasing the amplitude of the velocity fluctuations leads to a decrease of the effective gain and the time constant of the velocity response without any change in the parameters of these detectors. The seemingly complex property of this adaptation turns out to be a straightforward consequence of the multidimensionality of the stimulus and the nonlinear nature of the system.     

The mouse Chrna4 A529T polymorphism alters the ratio of high to low affinity alpha 4 beta 2 nAChRs

Previously, a missense polymorphism was identified in the mouse nicotinic receptor alpha4 subunit gene, Chrna4. This polymorphism leads to an Ala/Thr variation at amino acid position 529 of the alpha4 subunit. Chrna4 A529T is associated with several measures of acute sensitivity to nicotine as well as with mouse strain differences in nicotine-stimulated (86)Rb(+) efflux from synaptosomes. Here, we report that the variant forms of the mouse alpha4 subunit confer functional differences when expressed with the beta2 subunit in a heterologous system. alpha4beta2 receptors containing the T529 variant of the alpha4 subunit exhibited a higher EC(50) value for the high affinity receptor population and an apparent reduced sensitivity to blockade by DHbetaE relative to alpha4beta2 receptors containing the A529 variant of the alpha4 subunit. Moreover, the proportion of the total agonist-elicited current contributed by the high affinity alpha4beta2 receptor population was greater for alpha4beta2 receptors containing the alpha4(T529) variant (64%) than the alpha4beta2 receptors containing the alpha4(A529) variant (41%). These data suggest that the polymorphism in the mouse alpha4 subunit is located in a previously unidentified functional domain of the receptor subunit that influences receptor function, including regulation of the affinity population ratio of alpha4beta2 receptors.