Ute Neubacher

Dynamic patterns of USPIO enhancement can be observed in macrophages after ischemic brain damage

Cells of the mononuclear phagocytotic system (MPS) are often found near to or within ischemic tissue and can potentially aggravate cellular damage. Hence, visualization of those cells would allow demarcation of putatively affected from intact tissue. Experimental MRI studies have shown that ultrasmall particles of dextran‐coated iron oxide (USPIO) are internalized into cells of the MPS. To test if this cell tagging method may be also applied to cerebral infarction, USPIOs were administered to Fisher rats 5.5 h after permanent occlusion of the middle cerebral artery (pMCAO). During the first 2 days USPIO were preferentially found in patches within the lesion and in surrounding areas. On day 4, USPIOs expanded within the core of the lesion. On day 7 they were found predominantly within the boundary area. Histological analysis showed large populations of macrophages containing iron particles in the infarcted tissue. We conclude, therefore, that it is possible to monitor MPS activity after focal cerebral ischemia using USPIOs. Magn Reson Med 46:1018–1022, 2001.

Dose-dependence of changes in cortical protein expression induced with repeated transcranial magnetic theta-burst stimulation in the rat

BACKGROUND Theta Burst stimulation (TBS) applied via transcranial magnetic stimulation (TMS) effectively modulates human neocortical excitability but repeated applications of the same TBS protocol at short intervals may be not simply accumulative. OBJECTIVE Our aim was to investigate the impact of multiple blocks of either intermittent (iTBS) or continuous TBS (cTBS) on the expression of neuronal activity marker proteins in rat cortex. METHODS Up to four iTBS- or cTBS-blocks of 600 stimuli were applied to urethane-anesthetized rats followed by immunohistochemical and Western blot analyses. RESULTS The effects of iTBS and cTBS were similar but slightly differed with regard to the number of stimuli applied. The expression of the 65-kD isoform of glutamic acid decarboxylase (GAD65) increased with each stimulation block, while that of the 67-kD isoform (GAD67), and that of the calcium-binding proteins (CaBP) Parvalbumin (PV) and Calbindin (CB) and that of the immediate early gene c-Fos progressively decreased. Both TBS protocols increased the expression of the vesicular glutamate transporter 1 (VGLUT1) with 1200-1800 stimuli but then decreased them after the 4th block. CONCLUSION Our findings indicate that repeated TBS elicits no simple accumulative dose-dependent effect for all activity-markers but distinct profiles with threshold characteristics and a waxing-and-waning effect especially for the markers of inhibitory activity CB and GAD67. Interestingly, somatic activity markers, such as c-Fos for mainly excitatory and GAD67, CB and PV for inhibitory neurons, decreased with repeated stimulation while synaptic activity markers mainly increased which could be a result of the artificial stimulation of axons.

Modulation of Inhibitory Activity Markers by Intermittent Theta-burst Stimulation in Rat Cortex is NMDA-receptor Dependent

BACKGROUND Intermittent theta-burst stimulation (iTBS) applied via transcranial magnetic stimulation has been shown to increase cortical excitability in humans. In the rat brain it strongly reduced the number of neurons expressing the 67-kD isoform of the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD67) and those expressing the calcium-binding proteins parvalbumin (PV) and calbindin (CB), specific markers of fast-spiking (FS) and non-FS inhibitory interneurons, respectively, an indication of modified cortical inhibition. OBJECTIVE Since iTBS effects in humans have been shown to be NMDA receptor sensitive, we wondered whether the iTBS-induced changes in the molecular phenotype of interneurons may be also sensitive to glutamatergic synaptic transmission mediated by NMDA receptors. METHODS In a sham-controlled fashion, five iTBS-blocks of 600 stimuli were applied to rats either lightly anesthetized by only urethane or by an additional low (subnarcotic) or high dose of the NMDA receptor antagonist ketamine before immunohistochemical analysis. RESULTS iTBS reduced the number of neurons expressing GAD67, PV and CB. Except for CB, a low dose of ketamine partially prevented these effects while a higher dose almost completely abolished the iTBS effects. CONCLUSIONS Our findings indicate that iTBS modulates the molecular, and likely also the electric, activity of cortical inhibitory interneurons and that the modulation of FS-type but less that of non-FS-type neurons is mediated by NMDA receptors. A combination of iTBS with pharmacological interventions affecting distinct receptor subtypes may thus offer options to enhance its selectivity in modulating the activity of distinct cell types and preventing others from being modulated.

Changes in NMDA-Receptor Function in the First Week Following Laser-Induced Lesions in Rat Visual Cortex

Focal brain injuries are accompanied by processes of functional reorganization that partially compensate the functional loss. In a previous study, extracellular recordings at the border of a laser-induced lesion in the visual cortex of rats showed an enhanced synaptic plasticity, which was mediated by the activity of NR2B-contaning NMDA-receptors (NMDARs) shedding light on the potential cellular mechanisms underlying this reorganization. Given the potentially important contribution of NMDARs in processes of functional reorganization, in the present study, we used the same lesion model to further investigate lesion-induced changes in function and localization of NMDARs in the vicinity of the lesion. The most important finding was a lesion-mediated functional reexpression of nonpostsynaptic, but according to our data, presynaptic or peri-/extrasynaptic NMDARs (preNMDARs), which were undetectable in age-matched (>P21) sham-operated controls. Notably, preNMDARs were able to boost both spontaneous and evoked synaptic glutamatergic transmission. At the postsynaptic site, we also disclosed an increase in the decay time constant of NMDARs mediated currents, which was accompanied by a decreased NR2A/NR2B ratio, as revealed by Western blot analysis. All together these findings provide new insights into the role of NMDARs activity during processes of functional reorganization following a focal lesion in the cerebral cortex.

Recovery of function is not associated with proliferation of retinogeniculate synapses after chronic deafferentation in the dorsal lateral geniculate nucleus of the adult cat.

A restricted recovery of visual excitation occurs in the partially deafferented dorsal lateral geniculate nucleus (dLGN) of the cat after retinal lesions. In the absence of axonal growth an increase of retinogeniculate synapses at the peripheral dendrites of deafferented cells could be a possible underlying mechanism. We labeled optic tract terminals with horseradish peroxidase and [3H]proline. The size and density of labeled boutons in the vicinity of regions deafferented by chronic retinal lesions were not different from those in normal parts of the dLGN. There was no indication for synaptic proliferation as a response to partial visual deafferentiation in the adult cat dLGN.

Shift from phasic to tonic GABAergic transmission following laser-lesions in the rat visual cortex

Reduction in the strength of GABAergic neurotransmission has often been reported following brain lesions. This weakened inhibition is believed to influence neurological deficits, neuronal hyperexcitability and functional recovery after brain injuries. Uncovering the mechanisms underlying the altered inhibition is therefore crucial. In the present study we used an ex vivo–in vitro model of laser lesions in the rat visual cortex to characterize the cellular correlates of changes in GABAergic transmission in the tissue adjacent to the injury. In the first week post-injury the number of VGAT positive GABAergic terminals as well as the expression level of the GABA synthesizing enzymes GAD67 and GAD65 remained unaltered. However, a reduced frequency of miniature inhibitory postsynaptic currents (mIPSCs) together with an increased paired-pulse ratio (PPR) of evoked IPSCs suggested a functional reduction of phasic GABA release. In parallel, we found an enhancement in the GABAA receptor-mediated tonic inhibition. On the basis of these findings, we propose that cortical lesions provoke a shift in GABAergic transmission, decreasing the phasic and reinforcing the tonic component. We therefore suggest that it is not, as traditionally assumed, the overall inhibitory strength to be primarily compromised by a cortical lesion but rather the temporal accuracy of the GABAergic synaptic signaling.

NO regulates the strength of synaptic inputs onto hippocampal CA1 neurons via NO-GC1/cGMP signalling

GABAergic interneurons are the predominant source of inhibition in the brain that coordinate the level of excitation and synchronization in neuronal circuitries. However, the underlying cellular mechanisms are still not fully understood. Here we report nitric oxide (NO)/NO-GC1 signalling as an important regulatory mechanism of GABAergic and glutamatergic synaptic transmission in the hippocampal CA1 region. Deletion of the NO receptor NO-GC1 induced functional alterations, indicated by a strong reduction of spontaneous and evoked inhibitory postsynaptic currents (IPSCs), which could be compensated by application of the missing second messenger cGMP. Moreover, we found a general impairment in the strength of inhibitory and excitatory synaptic inputs onto CA1 pyramidal neurons deriving from NO-GC1KO mice. Finally, we disclosed one subpopulation of GABAergic interneurons, fast-spiking interneurons, that receive less excitatory synaptic input and consequently respond with less spike output after blockage of the NO/cGMP signalling pathway. On the basis of these and previous findings, we propose NO-GC1 as the major NO receptor which transduces the NO signal into cGMP at presynaptic terminals of different neuronal subtypes in the hippocampal CA1 region. Furthermore, we suggest NO-GC1-mediated cGMP signalling as a mechanism which regulates the strength of synaptic transmission, hence being important in gating information processing between hippocampal CA3 and CA1 region.

Neuropeptide Y as a possible homeostatic element for changes in cortical excitability induced by repetitive transcranial magnetic stimulation

BACKGROUND Repetitive transcranial magnetic stimulation (rTMS) is able to modify cortical excitability. Rat rTMS studies revealed a modulation of inhibitory systems, in particular that of the parvalbumin-expressing (PV+) interneurons, when using intermittent theta-burst stimulation (iTBS). OBJECTIVE The potential disinhibitory action of iTBS raises the questions of how neocortical circuits stabilize excitatory-inhibitory balance within a physiological range. Neuropeptide Y (NPY) appears to be one candidate. METHODS Analysis of cortical expression of PV, NPY and vesicular glutamate transporter type 1 (vGluT1) by immunohistochemical means at the level of cell counts, mean neuropil expression and single cell pre-/postsynaptic expression, with and without intraventricular NPY-injection. RESULTS Our results show that iTBS not only reduced the number of neurons with high-PV expression in a dose-dependent fashion, but also increased the cortical expression of NPY, discussed to reduce glutamatergic transmission, and this was further associated with a reduced vGluT1 expression, an indicator of glutamateric presynaptic activity. Interneurons showing a low-PV expression exhibit less presynaptic vGluT1 expression compared to those with a high-PV expression. Intraventricular application of NPY prior to iTBS prevented the iTBS-induced reduction in the number of high-PV neurons, the reduction in tissue vGluT1 level and that presynaptic to high-PV cells. CONCLUSIONS We conclude that NPY, possibly via a global but also slow homeostatic control of glutamatergic transmission, modulates the strength and direction of the iTBS effects, likely preventing pathological imbalance of excitatory and inhibitory cortical activity but still allowing enough disinhibition beneficial for plastic changes as during learning.