Elke Weiler

Theta-Burst Transcranial Magnetic Stimulation Alters Cortical Inhibition

Human cortical excitability can be modified by repetitive transcranial magnetic stimulation (rTMS), but the cellular mechanisms are largely unknown. Here, we show that the pattern of delivery of theta-burst stimulation (TBS) (continuous versus intermittent) differently modifies electric activity and protein expression in the rat neocortex. Intermittent TBS (iTBS), but not continuous TBS (cTBS), enhanced spontaneous neuronal firing and EEG gamma band power. Sensory evoked cortical inhibition increased only after iTBS, although both TBS protocols increased the first sensory response arising from the resting cortical state. Changes in the cortical expression of the calcium-binding proteins parvalbumin (PV) and calbindin D-28k (CB) indicate that changes in spontaneous and evoked cortical activity following rTMS are in part related to altered activity of inhibitory systems. By reducing PV expression in the fast-spiking interneurons, iTBS primarily affected the inhibitory control of pyramidal cell output activity, while cTBS, by reducing CB expression, more likely affected the dendritic integration of synaptic inputs controlled by other classes of inhibitory interneurons. Calretinin, the third major calcium-binding protein expressed by another class of interneurons was not affected at all. We conclude that different patterns of TBS modulate the activity of inhibitory cell classes differently, probably depending on the synaptic connectivity and the preferred discharge pattern of these inhibitory neurons.

Proliferation in the vomeronasal organ of the rat during postnatal development.

We investigated proliferation of sensory cell precursors in the rat vomeronasal organ (VNO) at various postnatal ages from birth (P1) to P666. In the rat, which continues to grow during most of its adult life, proliferation might be related to growth and/or replacement. Proliferating cells were labelled by BrdU injection, and histological sections of the VNO were evaluated after immunohistochemical detection of BrdU. Proliferation density (number of proliferating cells/section) decreased dramatically from 115 at P1 to 27.2 at P21, although the area increased. Adult values were reached at P66–P333 (10.3 cells/section); at P400–P666 the value was 8.6 cells/section. Distribution of labelled cells changed considerably with age: in neonates the cells were nearly equally distributed throughout the sensory epithelium, whereas from P21 onwards most proliferating cells were concentrated in clusters near the boundaries with non‐sensory epithelium. Labelled cells in the sensory neuronal layer were adjacent to the undulating basement membrane‐bordering capillaries that intrude into the sensory epithelium, indicating that they were true basal cells. The volume of the sensory epithelium increased between P1 and P66, and remained constant thereafter, although the length still increased. Length and volume of the sensory epithelium were related to body size, not to sex; males and females of the same body size had the same VNO size. The complex changes in proliferation pattern during postnatal development indicate differential growth and replacement. We suggest that in adults the labelled cell clusters near the boundaries are a pool for growth, whereas proliferation in the central parts represents a replacement pool.

Supporting cell proliferation in the olfactory epithelium decreases postnatally

It is well known that progenitor cells in the basal layer of olfactory epithelium proliferate continuously throughout life; the offspring of these dividing cells produce replacements for receptor neurons. In the rat the number of proliferating basal cells/mm length of epithelium (proliferation density) decreases with postnatal age while the area of the olfactory sheet increases. The supporting cells, which act as the glia of the olfactory epithelium, also divide. We examined in detail some aspects of the dynamics of olfactory supporting cell proliferation to determine whether their rate of proliferation changes with age, and how it compares with the rate in basal progenitor cells. Using BrdU to label dividing cells, we determined the proliferation density of supporting cells and basal cells in 10 μm coronal sections from six different anterior‐posterior regions in rats ranging in age from birth (P1) until P333. We observed a dramatic decrease in supporting cell proliferation density from P1 (80 cells/mm) to P11 (32 cells/mm) to P21 (12 cells/mm); the density decreases continuously to P333 (0.4 cells/mm). This reduction was even more dramatic than that in the basal cell population (Weiler and Farbman, 1997). Analysis of the data for correlation between basal and supporting cell proliferation revealed a weak correlation in neonates but no correlation in older animals. This suggests that the mechanisms that regulate proliferation of the two cell types are different. Our data also indicate that the proliferation of supporting cells is related only to growth in surface area of the epithelium. No turnover seems to occur in the supporting cells as it does in the olfactory neurons, where proliferation of basal cells is necessary for both growth and replacement. GLIA 22:315–328, 1998.

Reduced presynaptic efficiency of excitatory synaptic transmission impairs LTP in the visual cortex of BDNF-heterozygous mice.

The neurotrophin brain‐derived neurotrophic factor (BDNF) plays an important role in neuronal survival, axonal and dendritic growth and synapse formation. BDNF has also been reported to mediate visual cortex plasticity. Here we studied the cellular mechanisms of BDNF‐mediated changes in synaptic plasticity, excitatory synaptic transmission and long‐term potentiation (LTP) in the visual cortex of heterozygous BDNF‐knockout mice (BDNF+/–). Patch‐clamp recordings in slices showed an ∼ 50% reduction in the frequency of miniature excitatory postsynaptic currents (mEPSCs) compared to wild‐type animals, in the absence of changes in mEPSC amplitudes. A presynaptic impairment of excitatory synapses from BDNF+/– mice was further indicated by decreased paired‐pulse ratio and faster synaptic fatigue upon prolonged repetitive stimulation at 40 Hz. In accordance, presynaptic theta‐burst stimulation (TBS) failed to induce LTP at layer IV to layers II‐III synapses during extracellular field‐potential recordings in BDNF+/– animals. Changes in postsynaptic function could not be detected, as no changes were observed in either the amplitudes of evoked EPSCs, the ratios of AMPA : NMDA currents or the kinetics of evoked AMPA and NMDA EPSCs. In line with this observation, an LTP pairing paradigm that relies on direct postsynaptic depolarization under patch‐clamp conditions could be induced successfully in BDNF+/– animals. These data suggest that a chronic reduction in the expression of BDNF to nearly 50% attenuates the efficiency of presynaptic glutamate release in response to repetitive stimulation, thereby impairing presynaptically evoked LTP in the visual cortex.

Excitation and Inhibition Jointly Regulate Cortical Reorganization in Adult Rats

The primary somatosensory cortex (SI) retains its capability for cortical reorganization after injury or differential use into adulthood. The plastic response of SI cells to peripheral stimulation is characterized by extension of cortical representations accompanied by changes of the receptive field size of neurons. We used intracortical microstimulation that is known to enforce local, intracortical synchronous activity, to induce cortical reorganization and applied immunohistochemical methods in the same individual animals to investigate how plasticity in the cortical topographic maps is linked to changes in the spatial layout of the inhibitory and excitatory neurotransmitter systems. The results reveal a differential spatiotemporal pattern of upregulation and downregulation of specific factors for an excitatory (glutamatergic) and an inhibitory (GABAergic) system, associated with changes of receptive field size and reorganization of the somatotopic map in the rat SI. Predominantly local mechanisms are the specific reduction of the calcium-binding protein parvalbumin in inhibitory neurons and the low expression of the activity marker c-Fos. Reorganization in the hindpaw representation and in the adjacent SI cortical areas (motor cortex and parietal cortex) is accompanied by a major increase of the excitatory transmitter glutamate and c-Fos. The spatial extent of the reorganization appears to be limited by an increase of glutamic acid decarboxylase and the inhibitory transmitter GABA. The local and medium-range net effects are excitatory and can facilitate receptive field enlargements and cortical map expansion. The longer-range increase of inhibition appears suited to limit these effects and to prevent neurons from pathological hyperexcitability.

Olfactory deprivation enhances normal spine loss in the olfactory bulb of developing ferrets

Ferrets show a sensitive phase in their postnatal development during which they can become imprinted to food odors. At the same time the number of granule cell spines in the olfactory bulb reaches a maximum, declining significantly thereafter. In ferrets, exposed continuously to saturated levels of geraniol odor in the cage environment, the normal decline in spine number (occurring between day 60 and 90) is significantly enhanced. No such effects were observed during earlier ontogenetic phases. This late postnatal phase is further associated with a marked and significant decrease in total brain weight. The significance of these events to olfactory imprinting and plasticity in the developing brain is discussed.

Mitral cell loss following lateral olfactory tract transection increases proliferation density in rat olfactory epithelium.

Olfactory sensory neurons are replaced throughout the life of vertebrates by proliferation of basal cells and differentiation of the new cells into neurons. Removal of their target, the olfactory bulb, increases proliferation twofold because sensory neurons die prematurely, suggesting that the olfactory bulb provides a trophic substance required for survival. We asked whether mitral cells, a major postsynaptic target of olfactory sensory neurons, are involved in their survival. We report here that depletion of mitral cells increases proliferation and cell death in the olfactory sensory neuron population. Mitral cell loss was induced unilaterally by transection of their axons in the lateral olfactory tract in 18‐day‐old rats. At all time points after surgery (3 weeks, 7 weeks, 3 months, 14 months) there was a 29% mean reduction in the number of mitral cells ipsilateral to the transection. The surviving mitral cells were smaller than controls and had less rough endoplasmic reticulum. In the olfactory epithelium, proliferation density (BrdU‐positive cells/mm epithelial length) in the progenitor basal cells was increased by an average of 20–25% at all time points, as was the number of TUNEL‐positive dying cells. The results are consistent with the notion that mitral cells, or the synaptic sites on them, are a source of trophic factor required for maintenance of the lives of olfactory sensory cells. The target field of postsynaptic neurons remaining after lateral olfactory tract transection is insufficient to maintain normal survival of all existing olfactory neurons. In unperturbed animals the proliferation density declines in an age‐dependent manner and interestingly the decline on the tractotomized side is parallel. This suggests that with age the sensory cells are less dependent on their targets.

Dynamics of endogenous growth regulators during the growth cycle of a hormone-autotrophic plant cell culture

ist das Czochralski-Verfahren als gfinstigstes Ziichtungsverfahren anzusehen. Die Gitterkonstanten der reinen Aluminate passen allerdings nicht zu denen magnetischer Hexaferrite und mtissen durch Substitution des A1 durch gr613ere diamagnetische Kationen geeignet aufgeweitet werden. Unter der Voraussetzung, dal3 solche Substitutionsmischkristalle ebenfalls kongruent schmelzen, bietet sich z.B. Ga als Substituent an. Untersuchungen an Sinterproben zeigen eine vollstfindige Mischbarkeit im System LaMgAl~O19-LaMgGa~O~9 (W. Sch6nwelski, private Mitteilung).

The effects of naris occlusion on mouse nasal turbinate development

Unilateral naris occlusion, a standard method for causing odor deprivation, also alters airflow on both sides of the nasal cavity. We reasoned that manipulating airflow by occlusion could affect nasal turbinate development given the ubiquitous role of environmental stimuli in ontogenesis. To test this hypothesis, newborn mice received unilateral occlusion or sham surgery and were allowed to reach adulthood. Morphological measurements were then made of paraffin sections of the whole nasal cavity. Occlusion significantly affected the size, shape and position of turbinates. In particular, the nasoturbinate, the focus of our quantitative analysis, had a more delicate appearance on the occluded side relative to the open side. Occlusion also caused an increase in the width of the dorsal meatus within the non-occluded and occluded nasal fossae, compared with controls, and the position of most turbinates was altered. These results suggest that a mechanical stimulus from respiratory airflow is necessary for the normal morphological development of turbinates. To explore this idea, we estimated the mechanical forces on turbinates caused by airflow during normal respiration that would be absent as a result of occlusion. Magnetic resonance imaging scans were used to construct a three-dimensional model of the mouse nasal cavity that provided the input for a computational fluid dynamics simulation of nasal airflow. The simulation revealed maximum shear stress values for the walls of turbinates in the 1 Pa range, a magnitude that causes remodeling in other biological tissues. These observations raise the intriguing possibility that nasal turbinates develop partly under the control of respiratory mechanical forces.

Selective and Reversible Reduction of Odor Sensitivity in the Rat by Concanavalin A

Rats were trained using an olfactometer to detect low concentrations of ethyl acetate (EA) or dimethyl disulphide vapor (DMDS). Concanavalin A (ConA) applied to the olfactory mucosa had no effect on ability of rats to detect low concentrations of EA but produced a marked deficit in detection in DMDS. DMDS detection performance returned to control levels 3 days after the ConA treatment. These results provide the first behavioral evidence for odor-specific inhibition by chemical modification of the olfactory epithelium and support the notion that ConA selectively inactivates one or more types (subclasses) of olfactory receptor proteins.