Eva Fifková

Long-lasting morphological changes in dendritic spines of dentate granular cells following stimulation of the entorhinal area.

SummaryStimulation of the perforant path induces a long-lasting increase in the area of dendritic spines, which are sites of termination of the stimulated pathway in the distal third of the dentate molecular layer. No enlarged spines were found in the proximal third of the dentate molecular layer, where the commissural afferents terminate. Following a single tetanic stimulus of 30 sec duration at 30/sec, spines became significantly larger by 15%, 38%, 35% and 23% within poststimulation intervals of 2–6 min, 10–60 min, 4–8 h, and 23 h, respectively. Axon terminals decreased their area by 15% within the 2–6 min interval and the vesicle density was decreased by 19% within the 10–60 min interval. Both changes were reversible and terminals resumed their prestimulation condition at longer intervals (>4 h). The initial enlargement of spines was interpreted as being due to a glutamate-induced increase in the sodium permeability of the spine membrane, whereas for the long-lasting enlargement an increase in protein synthesis was postulated. The long-lasting enlargement of dendritic spines in the dentate molecular layer following a short train of stimuli delivered to the perforant path, supports the postulate which links such a change to the mechanism of long-lasting postactivation potentiation observed in this pathway.

Monoaminergic synapses, including dendro-dendritic synapses in the rat substantia nigra

SummaryIntraventricular administration of 1 or 2 mg of the osmiophilic “false transmitter” 5-hydroxydopamine (5-OHDA) was used to label monoamine storage and release sites in the rat substantia nigra. Vesicles containing unusually dense cores indicative of the presence of the marker were seen forming from the Golgi apparatus in the cell bodies of medium-sized neurons of the substantia nigra, pars compacta, and from smooth endoplasmic reticulum in the dendrites of those neurons and in small unmyelinated axons of unknown origin. In serial sections, both axons and dendrites containing synaptic vesicles marked with 5-OHDA were seen to form synapses “en passage” in pars compacta, and some presynaptic dendrites containing vesicles filled by the marker were also observed to form contacts with dendrites in pars reticulata. The only identified postsynaptic elements engaging in monoaminergic synapses in the substantia nigra were dendrites of medium-sized pars compacta neurons.

Stimulation-induced changes in dimensions of stalks of dendritic spines in the dentate molecular layer

Abstract Tetanic stimulation of the lateral and medial segments of the entorhinal area induces an increase in the width of the spine head and spine stalk in the middle and distal thirds of the dentate molecular layer 4 min after stimulation. Ninety minutes after stimulation, both spine stalk and head were wider in the distal third, whereas, in the middle third, the stalk returned to control values. Whereas the length of the spine head did not change systematically with stimulation, the length of the spine stalk was systematically reduced in the distal third in both poststimulation intervals.

Calcium in the spine apparatus of dendritic spines in the dentate molecular layer

With a pyroantimonate precipitation technique, we have demonstrated Ca2+ in the sacs of the dendritic spine apparatus in the SER of dendrites and axon terminals, in synaptic vesicles, multivesicular bodies, mitochondria, and glial processes of the dentate molecular layer. It is speculated that the spine apparatus may be a Ca2+ sequestering organelle which may regulate levels of intraspinal and intradendritic Ca2+ during synaptic activity.

Actin Matrix of Dendritic Spines, Synaptic Plasticity, and Long-Term Potentiation

Publisher Summary This chapter describes the composition of the spine cytoplasm along with the synaptic activity generated by the spine. Dendritic spines are appendage-like outgrowths of dendrites. They contain an enlarged terminal part (the spine head) and a slender stalk connecting the head with the parent dendrite. A small protrusion emanating from the spine head into the axon terminal contacting the dendritic spine is termed as “spinule.” Dendritic spines are endowed with a considerable degree of plasticity because they are readily modified by stimulation and changes in the environment. Synaptic plasticity is considered one of the most important properties of the brain as it allows for adaptive changes in the brain in response to past events and experiences. The spine apparatus (SA) consists of parallel membranous sacs that alternate with plates of dense material, which are contacted by dendritic microtubules during development. The SA membranous sacs are in continuity with the smooth endoplasmic reticulum (SER) of the parent dendrite. The absence of cytoplasmic organelles from the spine allows for a full use of the actin cytomatrix, which is particularly dense. Out of the entire neuron, the dendritic spines contain the highest density of actin filaments, which are arranged in such a way as to determine the characteristic shape of the spine.

A multisensory zone in rat parietotemporal cortex: intra- and extracellular physiology and thalamocortical connections.

Multisensory integration is essential for the expression of complex behaviors in humans and animals. However, few studies have investigated the neural sites where multisensory integration may occur. Therefore, we used electrophysiology and retrograde labeling to study a region of the rat parietotemporal cortex that responds uniquely to auditory and somatosensory multisensory stimulation. This multisensory responsiveness suggests a functional organization resembling multisensory association cortex in cats and primates. Extracellular multielectrode surface mapping defined a region between auditory and somatosensory cortex where responses to combined auditory/somatosensory stimulation were larger in amplitude and earlier in latency than responses to either stimulus alone. Moreover, multisensory responses were nonlinear and differed from the summed unimodal responses. Intracellular recording found almost exclusively multisensory cells that responded to both unisensory and multisensory stimulation with excitatory postsynaptic potentials (EPSPs) and/or action potentials, conclusively defining a multisensory zone (MZ). In addition, intracellular responses were similar to extracellular recordings, with larger and earlier EPSPs evoked by multisensory stimulation, and interactions suggesting nonlinear postsynaptic summation to combined stimuli. Thalamic input to MZ from unimodal auditory and somatosensory thalamic relay nuclei and from multisensory thalamic regions support the idea that parallel thalamocortical projections may drive multisensory functions as strongly as corticocortical projections. Whereas the MZ integrates uni‐ and multisensory thalamocortical afferent streams, it may ultimately influence brainstem multisensory structures such as the superior colliculus. J. Comp. Neurol. 460:223–237, 2003.

Effect of anisomycin on stimulation-induced changes in dendritic spines of the dentate granule cells

SummaryTetanic stimulation of the entorhinal area induces significant enlargement of the average dendritic spine area and perimeter in the middle and distal thirds of the dentate molecular layer 4 and 90 min following stimulation. Four minutes after stimulation, the differences between the stimulated and control animals were 20% for the dendritic spine area and 9% for the perimeter in the middle third, and in the distal third 32 and 14%, respectively. Ninety minutes after stimulation the differences were 28 and 11% for the area and perimeter in the middle third, and 33 and 18% in the distal third, respectively. Anisomycin at a dose of 25 mg/kg had no significant effect on the average spine area or perimeter in the various thirds of the dentate molecular layer in the 19 and 105 min post-application intervals. This dose of anisomycin given 15 min prior to the stimulation suppresses the stimulation-induced spine changes in the 4 min interval. In the 90 min interval when the effect of anisomycin on protein synthesis is largely terminated, spine enlargement reappears, being 21% higher than the controls in the middle and distal thirds. The differential effect of anisomycin on dendritic spines in the two post-stimulation intervals is discussed in relation to the effect of anisomycin on protein synthesis. The present experiments thus demonstrate that the stimulation-induced spine enlargement in the dentate fascia can be suppressed by a protein synthesis blocking drug.

Distribution of MAP 2 in dendritic spines and its colocalization with actin

The distribution of MAP2 and actin in dendritic spines of the visual and cerebellar cortices, dentate fascia, and hippocampus was determined by using immunogold electron microscopy. By this approach, we have confirmed the presence of MAP2 in dendritic spines and identified substructures within the spine compartment showing MAP2 immunoreactivity. MAP2 immunolabeling was mainly associated associated with filaments which reacted with a monoclonal anti-actin antibody. Also, by immunogold double-labeling we colocalized MAP2 with actin on the endomembranes of the spine apparatus, smooth endoplasmic reticulum, and in the postsynaptic density. Labeling was nearly absent in axons and axonal terminals. These results indicate that MAP2 is an actin-associated protein in dendritic spines. Thus, MAP2 may organize actin filaments in the spine and endow the actin network of the spine with dynamic properties that are necessary for synaptic plasticity.SummaryThe distribution of MAP2 and actin in dendritic spines of the visual and cerebellar cortices, dentate fascia, and hippocampus was determined by using immunogold electron microscopy. By this approach, we have confirmed the presence of MAP2 in dendritic spines and identified substructures within the spine compartment showing MAP2 immunoreactivity. MAP2 immunolabeling was mainly associated with filaments which reacted with a monoclonal anti-actin antibody. Also, by immunogold double-labeling we colocalized MAP2 with actin on the endomembranes of the spine apparatus, smooth endoplasmic reticulum, and in the postsynaptic density. Labeling was nearly absent in axons and axonal terminals. These results indicate that MAP2 is an actin-associated protein in dendritic spines. Thus, MAP2 may organize actin filaments in the spine and endow the actin network of the spine with dynamic properties that are necessary for synaptic plasticity.

Actin filament organization within dendrites and dendritic spines during development

The myosin S-1 subfragment was used to label actin filaments in the developing rat brain. The results show actin filaments present throughout the dendritic region with highest concentrations within growth cones and regions of spine development. Between 6 and 25 days postnatal, spines became more complex and actin filaments within them increased in number and formed a complex network. The observed organization of actin supports the hypothesis that actin has a role in the protrusion of spines from the dendrite during development.

Effect of age on blood vessels and neurovascular appositions in the rat dentate fascia

Rats aged 3, 9, 24 and 30 months were used in this study. We show increased basal lamina thickening and increased mitochondrial presence in walls of capillaries and not in walls of large vessel populations with age. This suggests that age selectively affects capillary structure. Ultrastructural differences between capillaries and two types of large vessels are reported and discussed in terms of their probable functional significance. In particular it was noted that there are more axon terminals, axons and dendrites adjacent to capillaries than to large vessels and that this was unaffected by increasing age. It is not clear whether the proximity of neuronal processes to a vessel wall serves a function, however, the larger number adjacent to capillaries than to large vessels indicates a more significant role for them in capillary rather than in large vessel function. Since increasing age did not alter the number of neuronal processes adjacent to vessels, age-related compromises in vessel function may be unrelated to neuronal regulation. The age-related changes are discussed as possible vascular markers for the aging brain.