Two distinct modalities of NMDA-receptor inactivation induced by calcium influx in cultured rat hippocampal neurons

Repetitive stimulation of glutamate (glu) receptors elicits increasingly smaller ionic currents in hippocampal neurons. To investigate mechanisms underlying this phenomenon, voltage clamp whole-cell currents evoked by glu (100 microM) were recorded from hippocampal neurons in culture. These currents were primarily carried by N-methyl-D-aspartate-receptor (NMDA-R) channels, as shown by the voltage-dependent sensitivity to extracellular Mg2+ blockade, and inhibition by the specific antagonist MK-801. In the presence of 2.2 mM extracellular Ca2+ ([Ca2+]e), repetitive glu applications (15 episodes of 4 s/min) elicited progressively smaller currents that stabilized at 45% of their initial peak value. Replacement of [Ca2+]e by Ba2+ produced similar effects. This phenomenon, defined as interepisode inactivation, was exacerbated by elevating [Ca2+]e to 11 mM, attenuated by reducing [Ca2+]e to 0.22 mM, and further diminished by shortening the length of the glu pulse to 2 s. Current decay exhibited during individual stimuli, or intraepisode inactivation, was dependent on [Ca2+]e yet remained stable during repetitive stimulation. We conclude that interepisode and intraepisode inactivations of NMDA-R currents are the expression of two distinct processes triggered by Ca2+. These modalities of inactivation may arise from Ca2+ binding either to the receptor or to closely associated regulatory proteins.

Potassium channels from normal and denervated mouse skeletal muscle fibers

The properties of singles K+ channels in normal and denervated muscles were compared using the "patch-clamp" technique. Single channels were recorded from vesicles obtained by stretching bundles of normal and denervated extensor digitorium longus (EDL) muscles. The most frequently observed channel in normal muscles was a high conductance (266 pS) Ca++ activated K+ channel. Although channel density, as estimated by patch recording, showed a significant decrease in denervated muscles, no differences were found in conductance and gating properties. Another voltage-dependent K+ channel (81 pS) was only recorded from normal muscles, but never from denervated ones. In addition, a 35 pS conductance was recorded from both normal and denervated fibers. This channel displayed neither voltage dependence nor sensitivity to tetraethylammonium (TEA). In contrast, another TEA-insensitive (16 pS) channel was recorded only from denervated muscles. We conclude that denervation induces significant changes in the distribution and expression of K+ channels in mammalian skeletal muscles.

Primary structure, chromosomal localization, and functional expression of a voltage-gated sodium channel from human brain

A cDNA library derived from human cerebral cortex was screened for the presence of sodium channel alpha subunit-specific clones. Ligation of three overlapping clones generated a full-length cDNA clone, HBA, that provided the complete nucleotide sequence coding for a protein of 2005 amino acids. The predicted structure suggests four homologous repeats and exhibits greatest homology and structural similarity to the rat brain sodium channel II. A second cDNA clone, HBB, that encodes a different subtype of sodium channel was isolated. Hybridization of DNA fragments from the 3' untranslated region of HBA and PCR with primers derived from HBB with human-hamster somatic cell hybrids localized these clones to human chromosome 2. In situ hybridization to human metaphase chromosomes mapped the structural genes for both HBA and HBB sodium channels to chromosome 2q23-24.3. The sodium channel HBA gene product was expressed by transfection in CHO cells. Expressed HBA currents were voltage-dependent, sodium-selective, and tetrodotoxin-sensitive and, thus, exhibit the biophysical and pharmacological properties characteristic of sodium channels.

Molecular cloning, chromosomal mapping, and functional expression of human brain glutamate receptors

A full-length cDNA clone encoding a glutamate receptor was isolated from a human brain cDNA library, and the gene product was characterized after expression in Xenopus oocytes. Degenerate PCR primers to conserved regions of published rat brain glutamate receptor sequences amplified a 1-kilobase fragment from a human brain cDNA library. This fragment was used as a probe for subsequent hybridization screening. Two clones were isolated that, based on sequence information, code for different receptors: a 3-kilobase clone, HBGR1, contains a full-length glutamate receptor cDNA highly homologous to the rat brain clone GluR1, and a second clone, HBGR2, contains approximately two-thirds of the coding region of a receptor homologous to rat brain clone GluR2. Southern and PCR analysis of a somatic cell-hybrid panel mapped HBGR1 to human chromosome 5q31.3-33.3 and mapped HBGR2 to chromosome 4q25-34.3. Xenopus oocytes injected with in vitro-synthesized HBGR1 cRNA expressed currents activated by glutamate receptor agonists with the following specificity sequence: domoate greater than kainate much greater than quisqualate greater than or equal to alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid greater than or equal to L-glutamate much greater than N-methyl-D-aspartate. The kainate-elicited currents were specifically blocked by 6-cyano-7-nitroquinoxaline-2,3-dione but were insensitive to 2-amino-5-phosphonovalerate and kynurenic acid. These results indicate that clone HBGR1 codes for a glutamate receptor of the kainate subtype cognate to members of the glutamate receptor family from rodent brain.

Positive interaction between alpha-1 adrenergic and dopamine-2 receptors in locomotor activity of normo and supersensitive mice

In normosensitive mice either the D1 antagonist SCH 23390 or the D2 antagonist sulpiride inhibited the reversion of reserpine-induced akinesia elicited by the mixed D1/D2 agonist pergolide. In mice rendered supersensitive by a five days' reserpine treatment, sulpiride did not prevent the pergolide-induced reversal of akinesia while SCH 23390 disclosed two subpopulations of mice. One population responded to pergolide with marked locomotor activity whereas in the other subpopulation this response was absent. However, all mice challenged with pergolide failed to reverse reserpine-akinesia after alpha-methyl-p-tyrosine (AMPT) pretreatment. The alpha 1/alpha 2 agonist clonidine restored the ability of pergolide to overcome reserpine akinesia in supersensitive mice pretreated with SCH 23390. Clonidine reversed the akinesia in supersensitive mice but in normal animals it did not. However, in these last conditions, the combined use of clonidine plus the D2 agonist LY 171555 was effective to induce locomotion. Neither AMPT nor SCH 23390 inhibited this response whereas the alpha-adrenergic antagonists prazosin and yohimbine did prevent it. The alpha 2 agonist B-HT 920 failed to induce locomotor responses when given together with LY 171555. The same occurred with the D1 agonist SKF 38393 when given together with clonidine. The combined use of SCH 23390 plus prazosin in chronic reserpinized mice prevented pergolide-induced locomotion. Adrenergic stimulation, acting on alpha 1 receptors, could be an alternative to D1 stimulation as a necessary factor to obtain D2-induced motor responses under normo and supersensitive conditions.