Goal-directed whisking increases phase-locking between vibrissa movement and electrical activity in primary sensory cortex in rat

We tested the hypothesis that behavioral context modulates phase-locking between rhythmic motor activity and concomitant electrical activity induced in primary sensory (S1) cortex. We used exploratory whisking by rat as a model system and recorded two measures: (i) the mystacial electromyogram ( nabla EMG) as a surrogate of vibrissa position, and (ii) the field potential ( nabla LFP) in S1 cortex as an indicator of electrical activity. The degree to which the nabla EMG and nabla LFP were phase-locked was compared for three categories of rhythmic whisking: (i) searching for an object with the vibrissae for a food reward, (ii) whisking in air for the goal of returning to the home cage, and (iii) whisking with no reward. We observed that the magnitude of phase-locking was nearly tripled for the two rewarded conditions compared to unrewarded whisking. Critically, increased locking was not accompanied by an increase in the amplitude of the cortical nabla LFP for the rewarded tasks. Additional experiments showed that there was no significant relation between the amplitude of a sensory-evoked response in S1 cortex and the magnitude of the locking between the nabla EMG and the nabla LFP during whisking. We conclude that the behavioral context of a whisking task can increase the modulation of S1 cortical activity by motor output without a concomitant increase in the magnitude of activity.     

Fluctuating and sensory-induced vasodynamics in rodent cortex extend arteriole capacity

Neural activity in the brain is followed by localized changes in blood flow and volume. We address the relative change in volume for arteriole vs. venous blood within primary vibrissa cortex of awake, head-fixed mice. Two-photon laser-scanning microscopy was used to measure spontaneous and sensory evoked changes in flow and volume at the level of single vessels. We find that arterioles exhibit slow (<1 Hz) spontaneous increases in their diameter, as well as pronounced dilation in response to both punctate and prolonged stimulation of the contralateral vibrissae. In contrast, venules dilate only in response to prolonged stimulation. We conclude that stimulation that occurs on the time scale of natural stimuli leads to a net increase in the reservoir of arteriole blood. Thus, a "bagpipe" model that highlights arteriole dilation should augment the current "balloon" model of venous distension in the interpretation of fMRI images.     

Characterization of functional organization within rat barrel cortex using intrinsic signal optical imaging through a thinned skull.

We used optical imaging of intrinsic signals to characterize the functional representations of mystacial vibrissae (whiskers) in rat somatosensory cortex. Stimulation of individual whiskers for 2 s at 5 Hz resulted in a discrete area of functional activity in the cortex. Images of whisker representations were collected both through the dura and through a thinned skull. We characterized the functional representation of a whisker both spatially and temporally with two-dimensional images and three-dimensional surface plots of intrinsic signal development in the cortex in response to whisker stimulation. Single unit recordings verified that the representation of the whisker obtained with optical imaging corresponded with the electrophysiological response area of that whisker in the cortex. Lesions in the center of the functional activity were found to be in the center of the dense cytochrome oxidase patch for the corresponding whisker. In addition, a 3 x 3 matrix of whiskers was stimulated and the distances between the centers of the imaged representations and the distances between the centers of the layer IV cytochrome oxidase staining of the nine whiskers were found to be highly correlated (r = 0.98). This study shows a striking correspondence among imaging, physiology, and anatomy in the rat somatosensory cortex. Furthermore, the ability to use optical imaging through a thinned skull should allow investigations into the long-term changes in a sensory representation within a single animal.     

Histochemical changes in cytochrome oxidase of cortical barrels after vibrissal removal in neonatal and adult mice.

The posteromedial barrel subfield of the somatosensory cortex of mice was examined histochemically for cytochrome oxidase activity (cytochrome c oxidase; ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1). In normal mice a high enzymatic activity was found within the barrel hollows, rather than in the sides and septa. Electron microscopic examination indicated that within the hollows reactive mitochondria reside in many dendrites, in some axonal terminals, and in a few neuronal perikarya. After neonatal cauterization of selected row(s) of vibrissae, the corresponding row(s) of barrels appeared as narrowed fused band(s) and their cytochrome oxidase activity was much reduced. Removal of vibrissae in the adult, by either cauterization or repeated plucking, did not cause size changes of cortical barrels. However, there was a significant decrease in the oxidative enzymatic activity within these barrels. Thus, the deprivation of sensory input through damage to, or removal of, the peripheral sensory organ induces an enzymatic response in neurons that are at least two to three synapses away from the periphery.     

Barrel construction in rodent neocortex: role of thalamic afferents versus extracellular matrix molecules.

The rodent primary somatosensory cortex is characterized by aggregates of cellular and axonal elements that replicate the distribution of mystacial vibrissae on the face. The periphery-related cortical pattern ("barrels") is influenced by an amalgam of elements extrinsic (i.e., afferents) and intrinsic (i.e., neurons, glia, and their substrate) to the developing neocortex. To assign the role of some of these elements in cortical pattern formation, we have examined the temporal correlation between periphery-related patterns formed by thalamocortical axons and by extracellular matrix (ECM) molecules associated with neurons and glia in the cortex. Thalamocortical axons were labeled with the lipophilic tracer 1,1'-dioctydecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) in aldehyde-fixed neonatal rat brains, and the same brains were also prepared for immunohistochemical localization of ECM molecules cytotactin and cytotactin-binding proteoglycan. We present evidence that thalamocortical axons form a periphery-related pattern well before such an organization is detectable in the distribution of ECM molecules. Furthermore, a patterned distribution of ECM molecules results from the down-regulation of these molecules from barrel centers, where thalamic axons have established vibrissa-specific patches. We conclude that thalamic axons convey the blueprint of the sensory periphery onto the neocortex and that ECM molecules do not participate in the initial formation of this pattern.     

Peripheral multidendritic sensory neurons are necessary for rhythmic locomotion behavior in Drosophila larvae

From breathing to walking, rhythmic movements encompass physiological processes important across the entire animal kingdom. It is thought by many that the generation of rhythmic behavior is operated by a central pattern generator (CPG) and does not require peripheral sensory input. Sensory feedback is, however, required to modify or coordinate the motor activity in response to the circumstances of actual movement. In contrast to this notion, we report here that sensory input is necessary for the generation of Drosophila larval locomotion, a form of rhythmic behavior. Blockage of all peripheral sensory inputs resulted in cessation of larval crawling. By conditionally silencing various subsets of larval peripheral sensory neurons, we identified the multiple dendritic (MD) neurons as the neurons essential for the generation of rhythmic peristaltic locomotion. By recording the locomotive motor activities, we further demonstrate that removal of MD neuron input disrupted rhythmic motor firing pattern in a way that prolonged the stereotyped segmental motor firing duration and prevented the propagation of posterior to anterior segmental motor firing. These findings reveal that MD sensory neuron input is a necessary component in the neural circuitry that generates larval locomotion.     

Target-derived influences on axon growth modes in cultures of trigeminal neurons.

Cellular and molecular signals involved in axon elongation versus collateral and arbor formation may be intrinsic to developing neurons, or they may derive from targets. To identify signals regulating axon growth modes, we have developed a culture system in which trigeminal ganglion cells are challenged by various target tissues. Embryonic day 15 (E15) rat trigeminal ganglion explants were placed between peripheral (vibrissa pad) and central nervous system targets. Normally, bipolar trigeminal ganglion cells extend one process to the vibrissa pad and another to the brainstem trigeminal complex. Under coculture conditions, the peripheral processes invade the vibrissa pad explants and form a characteristic circumfollicular pattern. Central processes of E15 ganglion cells invade many, but not all, central nervous system tissues. In isochronic (E15) central nervous system explants such as brainstem, olfactory bulb, or neocortex, these central processes elongate and form a "tract" but have virtually no arbors. However, in more mature targets (e.g., a section from postnatal brainstem or neocortex), they form arbors instead of a tract. We conclude from these observations that whether trigeminal axons elongate to form a tract, or whether they begin to arborize, is dictated by the target tissue and not by an intrinsic developmental program of the ganglion cell body. The explant coculture system is an excellent model for analysis of the molecular basis of neuron-target interactions.     

Development of a Method to Evaluate Glutamate Receptor Function in Rat Barrel Cortex Slices

The rat is a nocturnal animal and uses its vibrissae extensively to navigate its environment. The vibrissae are linked to a highly organized part of the sensory cortex, called the barrel cortex which contains spiny neurons that receive whisker specific thalamic input and distribute their output mainly within the cortical column. The aim of the present study was to develop a method to evaluate glutamate receptor function in the rat barrel cortex. Long Evans rats (90 – 160g) were killed by cervical dislocation and decapitated. The brain was rapidly removed, cooled in a continuously oxygenated, ice-cold Hepes buffer (pH 7.4) and sliced using a vibratome to produce 0.35mm slices. The barrel cortex was dissected from slices corresponding to 8.6 to 4.8mm anterior to the interaural line and divided into rostral, middle and caudal regions. Depolarization-induced uptake of ⁴⁵Ca²⁺ was achieved by incubating test slices in a high K⁺ (62.5mM) buffer for 2 minutes at 35°C. Potassium-stimulated uptake of ⁴⁵Ca²⁺ into the rostral region was significantly lower than into middle and caudal regions of the barrel cortex. Glutamate had no effect. NMDA significantly increased uptake of ⁴⁵Ca²⁺ into all regions of the barrel cortex. The technique is useful in determining NMDA receptor function and will be applied to study differences between spontaneously hypertensive rats (SHR) that are used as a model for attention deficit disorder and their normotensive control rats.     

Slow intrinsic rhythm in the koniocellular visual pathway

Slow rhythmic changes in nerve-cell activity are characteristic of unconscious brain states and also may contribute to waking brain function by coordinating activity between cortical and subcortical structures. Here we show that slow rhythms are exhibited by the koniocellular (K) pathway, one of three visual pathways beginning in the eye and projecting through the lateral geniculate visual relay nucleus to the cerebral cortex. We recorded activity in pairs and ensembles of neurons in the lateral geniculate nucleus of anesthetized marmoset monkeys. We found slow rhythms are common in K cells but are rare in parvocellular and magnocellular cell pairs. The time course of slow K rhythms corresponds to subbeta (<10 Hz) EEG frequencies, and high spike rates in K cells are associated with low power in the theta and delta EEG bands. By contrast, spontaneous activity in the parvocellular and magnocellular pathways is neither synchronized nor strongly linked to EEG state. These observations suggest that parallel visual pathways not only carry different kinds of visual signals but also contribute differentially to brain circuits at the first synapse in the thalamus. Differential contribution of sensory streams to rhythmic brain circuits also raises the possibility that sensory stimuli can be tailored to modify brain rhythms.     

A complexity measure for selective matching of signals by the brain.

We have previously derived a theoretical measure of neural complexity (CN) in an attempt to characterize functional connectivity in the brain. CN measures the amount and heterogeneity of statistical correlations within a neural system in terms of the mutual information between subsets of its units. CN was initially used to characterize the functional connectivity of a neural system isolated from the environment. In the present paper, we introduce a related statistical measure, matching complexity (CM), which reflects the change in CN that occurs after a neural system receives signals from the environment. CM measures how well the ensemble of intrinsic correlations within a neural system fits the statistical structure of the sensory input. We show that CM is low when the intrinsic connectivity of a simulated cortical area is randomly organized. Conversely, CM is high when the intrinsic connectivity is modified so as to differentially amplify those intrinsic correlations that happen to be enhanced by sensory input. When the input is represented by an individual stimulus, a positive value of CM indicates that the limited mutual information between sensory sheets sampling the stimulus and the rest of the brain triggers a large increase in the mutual information between many functionally specialized subsets within the brain. In this way, a complex brain can deal with context and go "beyond the information given."     

Context-dependent perceptual modulation of single neurons in primate visual cortex

Some neurons in the visual cortex alter their spiking rate according to the perceptual interpretation of an observed stimulus, rather than its physical structure alone. Experiments in monkeys have suggested that, although the proportion of neurons showing this effect differs greatly between cortical areas, this proportion remains similar across different stimuli. These findings have raised the intriguing questions of whether the same neurons always participate in the disambiguation of sensory patterns and whether such neurons might represent a special class of cortical cells that relay perceptual signals to higher cortical areas. Here we explore this question by measuring activity in the middle temporal cortex of monkeys and asking to what degree the percept-related responses of individual neurons depend upon the specific sensory input. In contrast to our expectations, we found that even small differences in the stimuli led to significant changes in the signaling of the perceptual state by single neurons. We conclude that nearly all feature-responsive neurons in this area, rather than a select subset, can contribute to the resolution of sensory conflict, and that the role of individual cells in signaling the perceptual outcome is tightly linked to the fine details of the stimuli involved.     

Cardiac extrinsic apoptotic pathway is silent in young but activated in elder mice overexpressing bovine GH: interplay with the intrinsic pathway

Apoptosis may occur through the mitochondrial (intrinsic) pathway and activation of death receptors (extrinsic pathway). Young acromegalic mice have reduced cardiac apoptosis whereas elder animals have increased cardiac apoptosis. Multiple intrinsic apoptotic pathways have been shown to be modulated by GH and other stimuli in the heart of acromegalic mice. However, the role of the extrinsic apoptotic pathways in acromegalic hearts is currently unknown. In young (3-month-old) acromegalic mice, expression of proteins of the extrinsic apoptotic pathway did not differ from that of wild-type animals, suggesting that this mechanism did not participate in the lower cardiac apoptosis levels observed at this age. On the contrary, the extrinsic pathway was active in elder (9-month-old) animals (as shown by increased expression of TRAIL, FADD, TRADD and increased activation of death inducing signaling complex) leading to increased levels of active caspase 8. It is worth noting that changes of some pro-apoptotic proteins were induced by GH, which seemed to have, in this context, pro-apoptotic effects. The extrinsic pathway influenced the intrinsic pathway by modulating t-Bid, the cellular levels of which were reduced in young and increased in elder animals. However, in young animals this effect was due to reduced levels of Bid regulated by the extrinsic pathway, whereas in elder animals the increased levels of t-Bid were due to the increased levels of active caspase 8. In conclusion, the extrinsic pathway participates in the cardiac pro-apoptotic phenotype of elder acromegalic animals either directly, enhancing caspase 8 levels or indirectly, increasing t-Bid levels and conveying death signals to the intrinsic pathway.     

Decoding temporally encoded sensory input by cortical oscillations and thalamic phase comparators

The temporally encoded information obtained by vibrissal touch could be decoded “passively,” involving only input-driven elements, or “actively,” utilizing intrinsically driven oscillators. A previous study suggested that the trigeminal somatosensory system of rats does not obey the bottom-up order of activation predicted by passive decoding. Thus, we have tested whether this system obeys the predictions of active decoding. We have studied cortical single units in the somatosensory cortices of anesthetized rats and guinea pigs and found that about a quarter of them exhibit clear spontaneous oscillations, many of them around whisking frequencies (≈10 Hz). The frequencies of these oscillations could be controlled locally by glutamate. These oscillations could be forced to track the frequency of induced rhythmic whisker movements at a stable, frequency-dependent, phase difference. During these stimulations, the response intensities of multiunits at the thalamic recipient layers of the cortex decreased, and their latencies increased, with increasing input frequency. These observations are consistent with thalamocortical loops implementing phase-locked loops, circuits that are most efficient in decoding temporally encoded information like that obtained by active vibrissal touch. According to this model, and consistent with our results, populations of thalamic “relay” neurons function as phase “comparators” that compare cortical timing expectations with the actual input timing and represent the difference by their population output rate.     

Peroxisome biogenesis disorders: The role of peroxisomes and metabolic dysfunction in developing brain

Summary Peroxisome biogenesis disorders, of which Zellweger syndrome is the most severe, result in severe neurological dysfunction associated with abnormal CNS neuronal migrations due to the lack of functional peroxisomes. The PEX2 ^-/- mouse model for Zellweger syndrome has enabled us to evaluate the role of peroxisomes in the development and functioning of the nervous system. These studies have shown that, in addition to disturbances in neuronal migration in developing cerebral cortex and cerebellum, defects in neuronal differentiation, proliferation and survival may also contribute to the CNS malformations. However, owing to the multiorgan dysfunction in peroxisomal disorders, it has been difficult to clearly define an intrinsic role for the peroxisome in brain cells. The use of several in vitro cell culture assays to evaluate the migration and differentiation of cerebellar neurons demonstrates a persistence of defects in peroxisome-deficient neurons. The absence of potential systemically derived, extrinsic factors in these in vitro systems indicates that CNS intrinsic defects contribute to the pathogenesis of disease in these disorders. However, bile acid treatment also increases the survival and growth of PEX2 ^-/- mice and improves some aspects of cerebellar development, indicating that extrinsic factors also affect the developing peroxisome-deficient brain. Therefore, the final phenotype of nervous system dysfunction in peroxisomal disorders will reflect a combination of both CNS intrinsic and extrinsic factors.     

Immediate and simultaneous sensory reorganization at cortical and subcortical levels of the somatosensory system

The occurrence of cortical plasticity during adulthood has been demonstrated using many experimental paradigms. Whether this phenomenon is generated exclusively by changes in intrinsic cortical circuitry, or whether it involves concomitant cortical and subcortical reorganization, remains controversial. Here, we addressed this issue by simultaneously recording the extracellular activity of up to 135 neurons in the primary somatosensory cortex, ventral posterior medial nucleus of the thalamus, and trigeminal brainstem complex of adult rats, before and after a reversible sensory deactivation was produced by subcutaneous injections of lidocaine. Following the onset of the deactivation, immediate and simultaneous sensory reorganization was observed at all levels of the somatosensory system. No statistical difference was observed when the overall spatial extent of the cortical (9.1 ± 1.2 whiskers, mean ± SE) and the thalamic (6.1 ± 1.6 whiskers) reorganization was compared. Likewise, no significant difference was found in the percentage of cortical (71.1 ± 5.2%) and thalamic (66.4 ± 10.7%) neurons exhibiting unmasked sensory responses. Although unmasked cortical responses occurred at significantly higher latencies (19.6 ± 0.3 ms, mean ± SE) than thalamic responses (13.1 ± 0.6 ms), variations in neuronal latency induced by the sensory deafferentation occurred as often in the thalamus as in the cortex. These data clearly demonstrate that peripheral sensory deafferentation triggers a system-wide reorganization, and strongly suggest that the spatiotemporal attributes of cortical plasticity are paralleled by subcortical reorganization.     

Early visual brain areas reflect the percept of an ambiguous scene

When a visual scene allows multiple interpretations, the percepts may spontaneously alternate despite the stable retinal image and the invariant sensory input transmitted to the brain. To study the brain basis of such multi-stable percepts, we superimposed rapidly changing dynamic noise as regional tags to the Rubin vase-face figure and followed the corresponding tag-related cortical signals with magnetoencephalography. The activity already in the earliest visual cortical areas, the primary visual cortex included, varied with the perceptual states reported by the observers. These percept-related modulations most likely reflect top-down influences that accentuate the neural representation of the perceived object in the early visual cortex and maintain the segregation of objects from the background.     

Rapid reorganization of adult rat motor cortex somatic representation patterns after motor nerve injury.

The potential for peripheral nerve injury to reorganize motor cortical representations was investigated in adult rats. Maps reflecting functional connections between the motor cortex and somatic musculature were generated with intracortical electrical stimulation techniques. Comparison of cortical somatotopic maps obtained in normal rats with maps generated from rats with a facial nerve lesion indicated that the forelimb and eye/eyelid representations expanded into the normal vibrissa area. Repeated testing from an electrode placed chronically in the motor cortex showed a shift from vibrissa to forelimb within hours after facial nerve transection. These comparatively quick changes in motor cortex representation pattern suggest that synaptic relations between motor cortex and somatic musculature are continually reshaped in adult mammals.     

Evolution of rhythms during periodic stimulation of embryonic chick heart cell aggregates

During periodic stimulation of spontaneously beating chick heart cell aggregates, there is often an evolution of coupling patterns between the stimulator and the aggregate action potential. For example, at rapid stimulation frequencies, a rhythm that is initially 1:1 (stimulus frequency:aggregate frequency) can evolve to other rhythms such as 5:4 and 4:3. Time-dependent effects generated during periodic stimulation are characterized by three types of experiments to determine 1) the effect of periodic stimulation on the intrinsic cardiac beat rate (overdrive suppression), 2) the effect of periodic stimulation on the phase resetting properties of the aggregate, and 3) the time-dependent changes in the coupling patterns between the stimulator and the aggregate during periodic stimulation. The protocols involved variations in the duration and rate of periodic stimulation. A mathematical model is developed in the form of a two-dimensional finite difference equation based on the data from experiments 1 and 2. The model is used to predict the data generated by experiment 3. There is good correspondence with the experiments in that the theory reproduces complex transitions between various rhythms and displays irregular rhythms similar to those observed experimentally. These results have implications for the evolution of cardiac arrhythmias such as atrioventricular heart block and modulated parasystole.     

Alcohol, but not lipopolysaccharide-induced liver apoptosis involves changes in intracellular compartmentalization of apoptotic regulators

BACKGROUND: While alcohol-induced augmentation of liver apoptosis has been demonstrated in humans and laboratory animals, the underlying mechanisms are not fully elucidated. This study addresses the question whether alcohol and bacterial lipopolysaccharide (LPS), a putative mediator of alcohol effects on the liver, induce augmentation of liver apoptosis by intrinsic or extrinsic signaling pathways. This information may prove important for future design of therapies for alcoholic liver disease. METHODS: Male rats were fed either an alcohol-containing liquid diet or an isocaloric, control diet for 15-16 weeks. At the end of feeding period, the rats were treated with LPS (0.8 mg.kg-1 body weight) or sterile saline and killed 3 and 24 hr later. The liver and blood were sampled for histology and biochemical assays. Hepatocytes were isolated by collagenase perfusion and fractionated to yield mitochondria and cytoplasm. The propensity of mitochondria to undergo permeability transition in the presence of a Ca2+ overload was determined along with distribution of various apoptotic regulators (AIF, Smac2, Bax, cytochrome c, Bcl-XL, Bfl-1, and caspase-2) between mitochondria and cytoplasmic fractions. RESULTS: Increased liver apoptosis in alcohol-treated rats was associated with translocation of several apoptotic regulators between mitochondria and cytoplasm in a manner suggesting that alcohol induces augmentation of apoptosis by recruiting intrinsic apoptotic signals. LPS treatment of rats counteracted alcohol-induced changes in intracellular compartmentalization of apoptotic regulators despite an increased rate of apoptosis. LPS may, therefore, recruit extrinsic apoptotic signals, such as proinflammatory cytokines. CONCLUSIONS: Hepatocytes are to be able to mount an apoptotic response to both intrinsic and extrinsic signals. Alcohol increases liver apoptosis predominantly through an intrinsic signaling pathway while LPS recruits extrinsic signaling pathways.