The tetrameric kainate receptors can be assembled from a combination of five different subunit subtypes. While GluK1-3 subunits can form homomeric receptors, GluK4 and GluK5 require a heteromeric partner to assemble, traffic to the membrane surface, and produce a functional channel. Previous studies have shown that incorporation of a GluK4 or GluK5 subunit changes both receptor pharmacology and channel kinetics. We directly compared the functional characteristics of recombinant receptors containing either GluK4 or GluK5 in combination with the GluK1 or GluK2 subunit. In addition, we took advantage of mutations within the agonist binding sites of GluK1, GluK2, or GluK5 to isolate the response of the wild-type partner within the heteromeric receptor. Our results suggest that GluK1 and GluK2 differ primarily in their pharmacological properties, but that GluK4 and GluK5 have distinct functional characteristics. In particular, while binding of agonist to only the GluK5 subunit appears to activate the channel to a non-desensitizing state, binding to GluK4 does produce some desensitization. This suggests that GluK4 and GluK5 differ fundamentally in their contribution to receptor desensitization. In addition, mutation of the agonist binding site of GluK5 results in a heteromeric receptor with a glutamate sensitivity similar to homomeric GluK1 or GluK2 receptors, but which requires higher agonist concentrations to produce desensitization. This suggests that onset of desensitization in heteromeric receptors is determined more by the number of subunits bound to agonist than by the identity of those subunits. The distinct, concentration-dependent properties observed with heteromeric receptors in response to glutamate or kainate are consistent with a model in which either subunit can activate the channel, but in which occupancy of both subunits within a dimer is needed to allow desensitization of GluK2/K5 receptors.
Accurate estimates of age and growth of fishes are important in the management and conservation of species and for the development of modeling approaches. Assessments of endangered or rare species typically are limited by poor or inadequate data owing to low abundance, unrepresentative sampling, and/or restrictions on sampling. Atlantic Sturgeon Acipenser oxyrinchus oxyrinchus, which occurs along the east coast of North America, has five distinct population segments (DPSs) listed under the U.S. Endangered Species Act. The New York Bight (NYB) DPS is listed as endangered and represents the largest DPS in the United States. Coastal trawl surveys from 2005 to 2012 were used to evaluate the current age structure of the NYB DPS. A total of 21 year‐classes (mean age = 8.89 years, n = 742 fish) were observed. Age data for the NYB DPS were combined with other available age estimates from multiple research laboratories and sources (n = 2,774) in the Hudson River and Delaware River as well from the coastal regions of New York, New Jersey, and Delaware from 1975 to 2012. Collectively, the combined data set captured much of the age range of the species, minimizing age biases and resulting in improved von Bertalanffy parameter estimates (L∞ = 278.87, K = 0.057, t0 = −1.27) with high overall model fit (r2 = 0.87). We assessed the effects of individual data sets through a series of leave‐one‐out bootstrap routines that evaluated the influence of each data set on growth parameter estimates. The parameter estimates of the von Bertalanffy growth function were influenced by sampling location and/or researcher effects. Despite these differences, the combined data set approach used here represents the most comprehensive study on the age‐and‐growth relationship of Atlantic Sturgeon and provides parameter estimates for the development of population dynamics models and valuable information for future management. Received February 23, 2015; accepted September 24, 2015
The Atlantic Sturgeon Acipenser oxyrinchus oxyrinchus historically supported a significant commercial fishery along the eastern coast of North America. However, overfishing led to substantial population declines with contributions from other anthropogenic impacts, including vessel strikes and contaminants that continue to impede recovery. Our work is the first to estimate the abundance of early juvenile (age 0–1), resident Atlantic Sturgeon in the Delaware River estuary. Using the Schumacher and Eschmeyer mark–recapture estimator for multiple censuses, we estimated 3,656 (95% CI = 1,935–33,041) individuals used the Delaware River estuary as a nursery in 2014. We found no significant change in mean length during the course of our study (November–December), and lengths of age 0–1 Atlantic Sturgeon ranged from 220 to 515 mm TL. Further, using a passive acoustic receiver array, we identified significant habitat areas where age‐0–1 juveniles spend considerable amounts of time; this included the Marcus Hook area and some habitat use downriver and upriver of Marcus Hook at Cherry Island and the Chester Range. Our results support the idea that a spawning population of Atlantic Sturgeon exists in the Delaware River and that some level of early juvenile recruitment is continuing to persist despite current depressed population levels. Understanding trends in abundance, habitat use, and other population metrics for natal river Atlantic Sturgeon will allow for better conservation and management of the species.Received March 24, 2016; accepted July 10, 2016Published online September 26, 2016
The GABAA receptors are ligand-gated chloride channels which are the targets for many clinically used sedatives, including the barbiturates. The barbiturate pentobarbital acts through multiple sites on the GABAA receptor. At low concentrations (μM), it acts as a positive allosteric modulator while at higher concentrations it can directly activate the receptor. This agonist action is influenced by the subunit composition of the receptor, and pentobarbital is a more effective agonist than GABA only at receptors containing an α6 subunit. The conformational change that translates GABA binding into channel opening is known to involve a lysine residue located in an extracellular domain between the 2nd and 3rd transmembrane domains. Mutations of this residue disrupt activation of the channel by GABA and have been linked to inherited epilepsy. Pentobarbital binds to the receptor at a different agonist site than GABA, but could use a common signal transduction mechanism to gate the channel. To address this question, we compared the effect of a mutating the homologous lysine residue in the α1 or α6 subunits (K278 or K277, respectively) to methionine on direct activation of recombinant GABAA receptors by GABA or pentobarbital. We found that this mutation reduced GABA sensitivity for both α1 and α6 subunits, but affected pentobarbital sensitivity only for the α1 subunit. This suggests that pentobarbital acts through a distinct signal transduction pathway at the α6 subunit, which may account for its greater efficacy compared to GABA at receptors containing this subunit.
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