Biomedical subjects

S Rajendra

Publications and source records attributed to S Rajendra.

23 records · Page 2Linked to original sources

PubMed1994

Startle disease mutations reduce the agonist sensitivity of the human inhibitory glycine receptor.

The receptor for the inhibitory neurotransmitter glycine is a member of the ligand-gated ion channel receptor superfamily. Point mutations in the gene encoding the alpha 1 subunit of the glycine receptor-channel complex (GlyR) have recently been identified in pedigrees with the autosomal dominant neurological disorder, startle disease (hyperekplexia). These mutations result in the substitution of leucine or glutamine for arginine 271. This charged residue is located near the ion channel region and is predicted to affect chloride permeation through the GlyR. We found little evidence for this role from the anion/cation selectivity and lack of pronounced rectification of currents flowing through recombinant human alpha 1 subunit GlyRs containing the startle disease mutations. We reveal, however, that the startle disease mutations profoundly disrupt GlyR function by causing 230-410-fold decreases in the sensitivity of receptor currents activated by the agonist glycine. Additionally, we report corresponding 56- and 120-fold reductions in the apparent binding affinity (Ki) of glycine to the mutant GlyRs, but no change in the binding affinity of the competitive antagonist, strychnine. Thus, startle disease reduces the efficacy of glycinergic inhibitory neurotransmission by producing GlyRs with diminished agonist responsiveness. Our results show that startle disease mutations define a novel receptor activation site.

Humans↗

PubMed1993

Modulation of the development of light-initiated asymmetry in chick thalamofugal visual projections by oestradiol.

Thalamofugal visual projections of light-incubated chicks are organised asymmetrically. This asymmetry, which is generated by light stimulation of the embryo during the final days of incubation, is sexually dimorphic, being more pronounced in males than in females. We have shown that the development of the asymmetry can be prevented by elevating circulating levels of 17 beta-oestradiol in the embryo prior to hatching. Light-incubated male chicks were treated 5 days prior to hatching with either one of two doses of 17 beta-oestradiol (1.5 mg or 2.5 mg in a 0.1 ml solution of 10% ethanol in olive oil) or the vehicle only. After hatching the retrogradely labelling fluorescent dyes True Blue and Fluorogold were injected into the left and right side of the hyperstriatal region of the forebrain, consequently labelling the cell bodies of thalamic neurons which project to this region. Although a pronounced asymmetry was present in the control group, it was not present in both of the 17 beta-oestradiol-treated groups. These results suggest that the asymmetrical development of thalamofugal visual projections in response to lateralised light stimulation of the embryo is dependent on circulating levels of steroid hormones, and that compared to the male, the lesser degree of asymmetry found in thalamofugal projections of untreated females may be related to the higher levels of circulating oestradiol present in females prior to hatching.

Animals↗

PubMed1993

Asymmetry is present in the thalamofugal visual projections of female chicks.

Asymmetry of the thalamofugal visual projections of the chick has been previously reported to exist in males but not females. Using a larger sample size and a double-labelling procedure, projections to the visual Wulst were labelled with the retrograde tracers True Blue and Fluoro-gold and the ratio of contralateral projections to ipsilateral projections was calculated. We found that asymmetry does exist in female chicks, but to a lesser degree than in males.

Animals↗

PubMed1993

The extracellular disulfide loop motif of the inhibitory glycine receptor does not form the agonist binding site.

Inhibitory (glycine and gamma-aminobutyric acid type A) and excitatory (nicotinic acetylcholine and serotonin 5-hydroxytryptamine type 3) receptors of the ligand-gated ion channel superfamily are related by both structural similarities and primary sequence identity. One invariant feature of all members of this receptor superfamily is the presence of an extracellular disulfide loop motif. This structural motif has been modeled, and Cockcroft et al. [Proteins 8:386-397 (1990)] have suggested that it forms the agonist binding site of the ligand-gated ion channel receptors. Using site-directed mutagenesis of the inhibitory glycine receptor (GlyR), we have specifically tested this hypothesis. The lysine residue at position 143 is proposed to form the binding site for the negatively charged carboxyl group of the agonist glycine. Differing residues at this position in other ligand-gated receptors are proposed to confer agonist specificity. The aspartic acid residue at position 148 is an invariant residue in every known subunit of the ligand-gated ion channel receptor superfamily. This residue has been proposed as the binding site for the positively charged amino group of the various agonists. Mutation of the lysine at position 143 to alanine resulted in essentially unaltered GlyRs, showing only modest decreases in strychnine affinity (Kd, 8.1 +/- 1.4 nM versus 13.4 +/- 1.3 nM), glycine displacement of strychnine binding (Ki, 25 +/- 5 microM versus 49 +/- 9 microM), and glycine activation of chloride currents (EC50, 27 +/- 6 microM versus 114 +/- 14 microM). Thus, we conclude that Lys-143 does not play a major role in either agonist or antagonist binding or agonist activation of the GlyR. Mutation of Asp-148 to either alanine or asparagine disrupted the expression and/or assembly of the receptor, and no binding sites or ion channels were expressed on the cell surface. The conservative mutation of the aspartic acid at position 148 to glutamic acid (D148E) allowed the expression of receptors, although with reduced efficiency. The D148E GlyRs showed a 1 order of magnitude decrease in strychnine affinity (Kd, 8.1 +/- 1.4 nM versus 82 +/- 21 nM), without any change in the glycine displacement of strychnine binding (Ki, 25 +/- 5 microM versus 29 +/- 8 microM) or glycine activation of chloride currents (EC50, 27 +/- 6 microM versus 20 +/- 1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Aspartic Acid↗

PubMed1992

An analysis of Na+ currents in rat olfactory receptor neurons.

Na+ currents were observed in acutely-dissociated adult rat olfactory receptor neurons using the whole-cell recording techniques. The threshold for current activation was near -70 mV and currents were fully activated by -10 mV (midpoint: -45 mV). Steady-state inactivation was complete at potentials more positive than -70 mV and half complete at -110 mV (+/- less than 1, n = 8). Complete recovery from inactivation required one second at -100 mV (n = 7). The addition of 10 microM tetrodotoxin or 1 mM Zn2+ to the external solution was required to completely block the current. The current differs from those in amphibian and cultured neonatal rat olfactory neurons in its unusually negative voltage-dependence and slow recovery. Since mammalian olfactory neurons have very high input resistances, physiological resting potentials cannot usually be measured using whole-cell recording techniques. However, predominantly-capacitatively-coupled spikes activated by depolarisation were frequently observed in cell-attached patches. This indicates that the cells were excitable and implies that they must have had resting potentials more negative than -90 mV in order for this current to underlie the action potential.

Action Potentials↗