Anne-Lise Jaton

Epilepsy, Hyperalgesia, Impaired Memory, and Loss of Pre- and Postsynaptic GABA B Responses in Mice Lacking GABA B(1)

GABA(B) (gamma-aminobutyric acid type B) receptors are important for keeping neuronal excitability under control. Cloned GABA(B) receptors do not show the expected pharmacological diversity of native receptors and it is unknown whether they contribute to pre- as well as postsynaptic functions. Here, we demonstrate that Balb/c mice lacking the GABA(B(1)) subunit are viable, exhibit spontaneous seizures, hyperalgesia, hyperlocomotor activity, and memory impairment. Upon GABA(B) agonist application, null mutant mice show neither the typical muscle relaxation, hypothermia, or delta EEG waves. These behavioral findings are paralleled by a loss of all biochemical and electrophysiological GABA(B) responses in null mutant mice. This demonstrates that GABA(B(1)) is an essential component of pre- and postsynaptic GABA(B) receptors and casts doubt on the existence of proposed receptor subtypes.

In vivo detection of amyloid-β deposits by near-infrared imaging using an oxazine-derivative probe

As Alzheimers disease pathogenesis is associated with the formation of insoluble aggregates of amyloid β-peptide, approaches allowing the direct, noninvasive visualization of plaque growth in vivo would be beneficial for biomedical research. Here we describe the synthesis and characterization of the near-infrared fluorescence oxazine dye AOI987, which readily penetrates the intact blood-brain barrier and binds to amyloid plaques. Using near-infrared fluorescence imaging, we demonstrated specific interaction of AOI987 with amyloid plaques in APP23 transgenic mice in vivo, as confirmed by postmortem analysis of brain slices. Quantitative analysis revealed increasing fluorescence signal intensity with increasing plaque load of the animals, and significant binding of AOI987 was observed for APP23 transgenic mice aged 9 months and older. Thus, AOI987 is an attractive probe to noninvasively monitor disease progression in animal models of Alzheimer disease and to evaluate effects of potential Alzheimer disease drugs on the plaque load.

Redistribution of GABAB(1) Protein and Atypical GABAB Responses in GABAB(2)-Deficient Mice

GABAB receptors mediate slow synaptic inhibition in the nervous system. In transfected cells, functional GABAB receptors are usually only observed after coexpression of GABAB(1) and GABAB(2) subunits, which established the concept of heteromerization for G-protein-coupled receptors. In the heteromeric receptor, GABAB(1) is responsible for binding of GABA, whereas GABAB(2) is necessary for surface trafficking and G-protein coupling. Consistent with these in vitro observations, the GABAB(1) subunit is also essential for all GABAB signaling in vivo. Mice lacking the GABAB(1) subunit do not exhibit detectable electrophysiological, biochemical, or behavioral responses to GABAB agonists. However, GABAB(1) exhibits a broader cellular expression pattern than GABAB(2), suggesting that GABAB(1) could be functional in the absence of GABAB(2). We now generated GABAB(2)-deficient mice to analyze whether GABAB(1) has the potential to signal without GABAB(2) in neurons. We show that GABAB(2)-/- mice suffer from spontaneous seizures, hyperalgesia, hyperlocomotor activity, and severe memory impairment, analogous to GABAB(1)-/- mice. This clearly demonstrates that the lack of heteromeric GABAB(1,2) receptors underlies these phenotypes. To our surprise and in contrast to GABAB(1)-/- mice, we still detect atypical electrophysiological GABAB responses in hippocampal slices of GABAB(2)-/- mice. Furthermore, in the absence of GABAB(2), the GABAB(1) protein relocates from distal neuronal sites to the soma and proximal dendrites. Our data suggest that association of GABAB(2) with GABAB(1) is essential for receptor localization in distal processes but is not absolutely necessary for signaling. It is therefore possible that functional GABAB receptors exist in neurons that naturally lack GABAB(2) subunits.

Dynamics of Aβ Turnover and Deposition in Different β-Amyloid Precursor Protein Transgenic Mouse Models Following γ-Secretase Inhibition

Human β-amyloid precursor protein (APP) transgenic mice are commonly used to test potential therapeutics for Alzheimers disease. We have characterized the dynamics of β-amyloid (Aβ) generation and deposition following γ-secretase inhibition with compound LY-411575 [N2-[(2S)-2-(3,5-difluorophenyl)-2-hydroxyethanoyl]-N1-[(7S)-5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]azepin-7-yl]-l-alaninamide]. Kinetic studies in preplaque mice distinguished a detergent-soluble Aβ pool in brain with rapid turnover (half-lives for Aβ40 and Aβ42 were 0.7 and 1.7 h) and a much more stable, less soluble pool. Aβ in cerebrospinal fluid (CSF) reflected the changes in the soluble brain Aβ pool, whereas plasma Aβ turned over more rapidly. In brain, APP C-terminal fragments (CTF) accumulated differentially. The half-lives for γ-secretase degradation were estimated as 0.4 and 0.1 h for C99 and C83, respectively. Three different APP transgenic lines responded very similarly to γ-secretase inhibition regardless of the familial Alzheimers disease mutations in APP. Amyloid deposition started with Aβ42, whereas Aβ38 and Aβ40 continued to turn over. Chronic γ-secretase inhibition lowered amyloid plaque formation to a different degree in different brain regions of the same mice. The extent was inversely related to the initial amyloid load in the region analyzed. No evidence for plaque removal below baseline was obtained. γ-Secretase inhibition led to a redistribution of intracellular Aβ and an elevation of CTFs in neuronal fibers. In CSF, Aβ showed a similar turnover as in preplaque animals demonstrating its suitability as marker of newly generated, soluble Aβ in plaque-bearing brain. This study supports the use of APP transgenic mice as translational models to characterize Aβ-lowering therapeutics.

Macrocyclic BACE-1 inhibitors acutely reduce Aβ in brain after po application

A series of macrocyclic peptidic BACE-1 inhibitors was designed. While potency on BACE-1 was rather high, the first set of compounds showed poor brain permeation and high efflux in the MDRI-MDCK assay. The replacement of the secondary benzylamino group with a phenylcyclopropylamino group maintained potency on BACE-1, while P-glycoprotein-mediated efflux was significantly reduced and brain permeation improved. Several compounds from this series demonstrated acute reduction of Abeta in human APP-wildtype transgenic (APP51/16) mice after oral administration.

Dopamine D1- and D2-receptor interaction in turning behaviour induced by dopamine agonists in 6-hydroxydopamine-lesioned rats

The selective dopamine D2-antagonist sulpiride potentiated contralateral circling behaviour induced by the D1-agonist CY 208-243 in rats with unilateral lesions of substantia nigra, but reduced the effects of the selective D2-agonist bromocriptine. Similarly, the D1-antagonist SCH 23390 tended to increase the effects of bromocriptine but markedly inhibited CY 208-243 induced turning. The mixed D1/D2-antagonist fluphenazine was effective in reducing circling behaviour induced by either agonist, whereas pimozide (D1/D2) inhibited only the actions of bromocriptine. These results indicate that the actions of CY 208-243 and bromocriptine are mediated via distinct but interacting receptor subtypes.

NEUROPHARMACOLOGY OF ERGOT DERIVATIVES

ABSTRACT A short review of neuropharmacological effects of ergot derivatives is given. Based on in vitro experiments it is shown that ergot derivatives possess high affinities to noradrenaline, dopamine and serotonin receptor sites in the brain. Biochemical and functional studies in vivo indicate that dopamine agonist and serotonin agonist effects may provide the basis for effects on motor function and changes in animal electroencephalogram. Finally, experiments in a maze learning paradigm in rats are reported to illustrate the actions of Hydergine and bromocriptine on cognitive and motor performance.