Numbers of neurones, synapses and axon terminals were quantified in a murine scrapie model with severe hippocampal pyramidal cell loss, in which definite clinical scrapie is evident from 226 days post-infection (dpi) and death occurs around 250 dpi. Disease-specific PrP accumulations were first seen at 70 dpi (28% of the incubation period (IP)) in thalamus and as sparse foci within the stratum pyramidale of CA1. By 98 dpi (39% IP), PrP was seen in the stratum radiatum and was found at later stages throughout all levels of the hippocampus. At the ultrastructural level in the stratum radiatum of CA1, a decrease in the numbers of simple synapses from 84 dpi (34% IP) and in perforated synapses from 98 dpi (42% IP) was found using an unbiased stereological method, the disector analysis. Degeneration of axon terminals was found from 98 dpi (39% IP) onwards. Neuronal loss was detected in CA1 from 180 dpi (72% IP). The results suggest that the fundamental lesion in the hippocampus of ME7-infected mice is associated with PrP release from CA1 pyramidal neurones, which perturbs synaptic function and leads to degeneration of preterminal axons, and that subsequent pathological changes including neurone loss are sequelae to this initial insult.

We have used site-directed mutagenesis of amino acids located within the S1 and S2 ligand binding domains of the NR2A N-methyl-D-aspartate (NMDA) receptor subunit to explore the nature of ligand binding. Wild-type or mutated NR1/NR2A NMDA receptors were expressed in Xenopus laevis oocytes and studied using two electrode voltage clamp. We investigated the effects of mutations in the S1 and S2 regions on the potencies of the agonists L-glutamate, L-aspartate, (R,S)-tetrazol-5yl-glycine, and NMDA. Mutation of each of the corresponding residues found in the NR2A receptor subunit, suggested to be contact residues in the GluR2 ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit, caused a rightward shift in the concentration-response curve for each agonist examined. None of the mutations examined altered the efficacy of glutamate as assessed by methanethiosulfonate ethylammonium potentiation of agonist-evoked currents. In addition, none of the mutations altered the potency of glycine. Homology modeling and molecular dynamics were used to evaluate molecular details of ligand binding of both wild-type and mutant receptors, as well as to explore potential explanations for agonist selectivity between glutamate receptor subtypes. The modeling studies support our interpretation of the mutagenesis data and indicate a similar binding strategy for L-glutamate and NMDA when they occupy the binding site in NMDA receptors, as has been proposed for glutamate binding to the GluR2 AMPA receptor subunit. Furthermore, we offer an explanation as to why "charge conserving" mutations of two residues in the binding pocket result in nonfunctional receptor channels and suggest a contributing molecular determinant for why NMDA is not an agonist at AMPA receptors.The NMDA receptor channel is thought to be formed from the coassembly of two NR1 subunits and two NR2 subunits in a dimer of dimers configuration (Schorge and Colquhoun, 2003). NMDA receptors are unique among ligand-gated ion channels in that the binding of two different ligands is required for the activation of the receptor channel complex. Glycine, a coagonist, binds to residues located in the NR1 subunits, whereas glutamate binds to residues located in NR2 subunits (for review, see Erreger et al., 2004) of which there are four types (termed NR2A-D or ⑀ 1-4) and which are the major determinants of the pharmacological and biophysical properties of these receptors (Monyer et al., 1992(Monyer et al., , 1994Vicini et al., 1998;Wyllie et al., 1998). Ionotropic glutamate receptor subunits are comprised of distinct functional regions-an amino terminal domain, a ligand binding domain, a membrane-associated region, and an intracellular carboxyl-terminal domain (Fig. 1A). The ligand binding domain is thought to form a hinged clamshell-like structure (Armstrong et al., 1998) and is comprised of a region preceding the first membrane spanning domain (termed S1) and a region between the second and third membrane spanning domains (termed S2). In NR2 NMDA receptor s...

1. Intracellular recordings were made from 123 tonically active medial vestibular nucleus (MVN) neurones in a horizontal slice preparation of the dorsal brainstem of the rat. On the basis of their averaged action potential shapes, the cells were classified as either type A, having a single deep after-hyperpolarization (AHP; 40/123 cells, 33%), or type B, having an early fast AHP and a delayed slow AHP (83/123 cells, 67%). The two cell types were distributed throughout the rostrocaudal extent of the MVN. 2. In type A cells TEA reduced the single deep AHP and decreased the rate of spike repolarization. Depolarizing current pulses from a hyperpolarized membrane potential elicited spikes with short plateau potentials in TEA. These persisted in Ca2"-free medium but were abolished along with the spontaneous activity in 1'TX. Ca2+-free medium did not affect the initial rate of repolarization but reduced the deep AHP. Apamin and carbachol had little effect. 4-Aminopyridine (4-AP) slowed spike repolarization and the AHP amplitude by a small amount. Thus, in type A cells spike repolarization and AHP appear to be mediated largely by a TEA-sensitive potassium current (presumably IK) and an apamin-insensitive Ca2+-activated potassium current (presumably Ic).3. The early fast AHP in type B cells was readily abolished in TEA. In seven of ten type B cells tested, the spontaneous spikes developed plateau potentials of 100-120 ms duration in 10 mM TEA, which then became 7-9 s long in Ca2+-free medium. In the remaining three cells, the spontaneous plateaux were 1-75-2 s long in TEA, and were reduced to 30-100 ms in Ca2+-free medium. ITX abolished the spontaneous spikes and plateaux.The delayed AHP was abolished by apamin, which induced irregular firing. 4-AP slowed spike repolarization and abolished the fast AHP, but did not induce plateaux. Thus, in type B cells spike repolarization involves a TEA-sensitive current (presumably IK) as well as Ic and the 4-AP-sensitive potassium current IA, while the apaminsensitive potassium current IAHP is responsible for the delayed AHP. 4. The tonic activity in type B cells appears to be regulated mainly by interactions between a persistent Na+ current, which in most cells is large enough to generate plateaux when repolarization is impeded in TEA, and the hyperpolarization mediated by 'AHP. About 30% of type B cells have an additional inward Ca2+ current. In type A cells the persistent Na+ current is either not large enough to generate plateaux or is more effectively counteracted by TEA-insensitive outward currents. The pacemaker conductances in type A and type B cells thus appear to be distinct.Medial vestibular nucleus (MVN) neurones are the reciprocal inhibitory interactions between the vestibular primary central targets of afferents from the ipsilateral nuclei of the two sides, and by cerebellar Purkinje cell horizontal semi-circular canal, and their projections to the projections onto MVN neurones. Cervical proprioceptive oculomotor nuclei and the cervical spinal cord are amongst and eye p...

The postnatal maturation of medial vestibular nucleus (MVN) neurones was examined in slices of the dorsal brainstem prepared from balb/c mice at specific stages during the first postnatal month. Using spike-shape averaging to analyse the intracellularly recorded action potentials and after-hyperpolarizations (AHPs) in each cell, all the MVN neurones recorded in the young adult (postnatal day 30; P30) mouse were shown to have either a single deep AHP (type A cells), or an early fast and a delayed slow AHP (type B cells). The relative proportions of the two subtypes were similar to those in the young adult rat. At P5, all the MVN cells recorded showed immature forms of either the type A or the type B action potential shape. Immature type A cells had broad spontaneous spikes, and the characteristic single AHP was small in amplitude. Immature type B cells had somewhat narrower spontaneous spikes that were followed by a delayed, apamin-sensitive AHP. The delayed AHP was separated from the repolarisation phase of the spike by a period of isopotentiality. Over the period P10-P15, the mean resting potentials of the MVN cells became more negative, their action potential fall-times became shorter, the single AHP in type A cells became deeper, and the early fast AHP appeared in type B cells. Until P15 cells of varying degrees of electrophysiological maturity were found in the MVN but by P30 all MVN cells recorded were typical adult type A or type B cells. Exposure to the selective blocker of SK-type Ca-activated K channels, apamin (0.3 microM), induced depolarising plateaux and burst firing in immature type B cells at rest. The duration of the apamin-induced bursts and the spike frequency during the bursts were reduced but not abolished after blockade of Ca channels in Ca-free artificial cerebrospinal fluid containing Cd2+. By contrast, in mature type B cells at rest apamin selectively abolished the delayed slow AHP but did not induce bursting activity. Apamin had no effect on the action potential shape of immature type A cells. These data show that the apamin-sensitive I(AHP) is one of the first ionic conductances to appear in type B cells, and that it plays an important role in regulating the intrinsic rhythmicity and excitability of these cells.

We investigated changes in intrinsic excitability and GABA receptor efficacy in rat medial vestibular nucleus (MVN) neurons following 48 h and 7-10 days of behavioral recovery after unilateral labyrinthectomy (UL) in the rat. The mean in vitro discharge rate of rostral ipsilesional MVN cells at both time points was significantly higher than normal, indicating that the intrinsic excitability of the deafferented cells undergoes a sustained up-regulation during vestibular compensation. In slices from animals that had compensated for 7-10 days after UL, the responsiveness of rostral ipsilesional MVN cells to the GABA(A) agonist muscimol was not different from normal, while the responsiveness to the GABA(B) agonist baclofen was significantly down-regulated. This is in contrast to the situation soon after UL, where the efficacy of both GABA(A) and GABA(B) receptors is markedly down-regulated. The recovery of fast GABA(A) mediated neurotransmission by 7-10 days post-UL presumably enables ipsilesional cells to again respond to vestibular stimulation, through commissural inhibitory modulation from the intact side. The permanent loss of excitatory input from the lesioned side may be, in effect, counteracted by the long-term down-regulation of slow GABA(B) receptors in the de-afferented neurons.

The spontaneous discharge of 48 medial vestibular nucleus (MVN) neurones was recorded extracellularly in horizontal slices of the rat brainstem in vitro. The mean tonic rate of discharge was 17.1 +/- 8.2 imp/s, similar to that observed by others in transverse (coronal) slices of the rat and guinea pig MVN. The tonic rate of discharge of individual MVN cells either increased or decreased after synaptic blockade in low Ca2+ media, suggesting that ongoing synaptic activity has an important influence on the spontaneous activity of MVN cells in vitro. However the persistence of tonic activity after synaptic blockade indicates that an intrinsic, pacemaker-like mechanism is involved in the generation of the tonic activity. GABA, muscimol, baclofen and 3-APA inhibited the tonic activity of all MVN cells tested. Bicuculline antagonised, and picrotoxin blocked, the inhibitory responses to muscimol, but the effects of GABA were only partially blocked in 50 microM picrotoxin. The effects of baclofen and 3-APA persisted in low Ca2+ media, and were antagonised by saclofen and phaclofen. Picrotoxin-resistant responses to GABA persisted in low Ca2+ media, and were also antagonised by saclofen. These results suggest that the inhibitory control of MVN neurones by GABA involves both the GABAA and GABAB subtypes of GABA receptor. GABAB receptors appear to be distributed both pre- and post-synaptically in the rat MVN. The possible significance of the intrinsic, tonic activity of MVN cells in normal vestibular function and in vestibular compensation, and the effects of GABA, are discussed.

The use of second-order Jahn-Teller active Mo (VI) centers and chiral organic amines is discussed as an approach to crystallographic noncentrosymmetry. Several series of reactions, conducted under mild hydrothermal conditions, were designed to probe important reaction variables. Correlations between reagent and solvent concentrations and the molybdate structure were investigated using composition space analysis, which allows for the isolation of specific reaction variables. The effects of amine structure variation were probed using multiple series of related amines, which consisted of either linear diamines or ethylenediamine derivatives. The addition of fluoride results in the loss of amine-based structural variations. Chiral organic amines were used to demonstrate the viability of using such components to control the three-dimensional symmetry in new materials. The synthesis, structure, and characterization of eight new organically templated polyoxomolybdates and polyoxofluoromolybdates are reported.

We investigated the role of the cerebellar flocculus in mediating the adaptive changes that occur in the intrinsic properties of brainstem medial vestibular nucleus (MVN) neurons during vestibular compensation. Ipsi-lesional, but not contra-lesional, flocculectomy prevented the compensatory increase in intrinsic excitability (CIE) that normally occurs in the de-afferented MVN neurons within 4 h after unilateral labyrinthectomy (UL). Flocculectomy did not, however, prevent the down-regulation of efficacy of GABA receptors that also occurs in these neurons after UL, indicating that these responses of the MVN neurons to deafferentation are discrete, parallel processes. CIE was also abolished by intra-floccular microinjection of the metabotropic glutamate receptor (mGluR) antagonist AIDA, and the protein kinase C inhibitor bisindolymaleimide I (BIS-I). The serene-threonine kinase inhibitor H-7 had no effect when microinjected at the time of de-afferentation, but abolished CIE if microinjected 2 h later. These cellular effects are in line with the recently reported retardatory effects of BIS-I and H-7 on behavioural recovery after UL. They demonstrate that the increase in intrinsic excitability in MVN neurons during vestibular compensation is cerebellum dependent, and requires mGluR activation and protein phosphorylation in cerebellar cortex. Furthermore, microinjection of the glucocorticoid receptor (GR) antagonist RU38486 into the ipsi-lesional flocculus also abolished CIE in MVN neurons. Thus an important site for glucocorticoids in facilitating vestibular compensation is within the cerebellar cortex. These observations ascribe functional significance to the high levels of GR and 11-beta-HSD Type 1 expression in cerebellum.