Changes in endogenous enzymatic reactivity to DAB induced by neuronal inactivity

Micropopulation mapping of the mouse parafascicular nucleus connections reveals diverse input-output motifs

Introduction

In primates, including humans, the centromedian/parafascicular (CM-Pf) complex is a key thalamic node of the basal ganglia system. Deep brain stimulation in CM-Pf has been applied for the treatment of motor disorders such as Parkinson's disease or Tourette syndrome. Rodents have become widely used models for the study of the cellular and genetic mechanisms of these and other motor disorders. However, the equivalence between the primate CM-Pf and the nucleus regarded as analogous in rodents (Parafascicular, Pf) remains unclear.

Methods

Here, we analyzed the neurochemical architecture and carried out a brain-wide mapping of the input-output motifs in the mouse Pf at micropopulation level using anterograde and retrograde labeling methods. Specifically, we mapped and quantified the sources of cortical and subcortical input to different Pf subregions, and mapped and compared the distribution and terminal structure of their axons.

Results

We found that projections to Pf arise predominantly (>75%) from the cerebral cortex, with an unusually strong (>45%) Layer 5b component, which is, in part, contralateral. The intermediate layers of the superior colliculus are the main subcortical input source to Pf. On its output side, Pf neuron axons predominantly innervate the striatum. In a sparser fashion, they innervate other basal ganglia nuclei, including the subthalamic nucleus (STN), and the cerebral cortex. Differences are evident between the lateral and medial portions of Pf, both in chemoarchitecture and in connectivity. Lateral Pf axons innervate territories of the striatum, STN and cortex involved in the sensorimotor control of different parts of the contralateral hemibody. In contrast, the mediodorsal portion of Pf innervates oculomotor-limbic territories in the above three structures.

Discussion

Our data thus indicate that the mouse Pf consists of several neurochemically and connectively distinct domains whose global organization bears a marked similarity to that described in the primate CM-Pf complex.

Analyzing Thalamocortical Tract-Tracing Experiments in a Common Reference Space

Current mesoscale connectivity atlases provide limited information about the organization of thalamocortical projections in the mouse brain. Labeling the projections of spatially restricted neuron populations in thalamus can provide a functionally relevant level of connectomic analysis, but these need to be integrated within the same common reference space. Here, we present a pipeline for the segmentation, registration, integration and analysis of multiple tract-tracing experiments. The key difference with other workflows is that the data is transformed to fit the reference template. As a test-case, we investigated the axonal projections and intranuclear arrangement of seven neuronal populations of the ventral posteromedial nucleus of the thalamus (VPM), which we labeled with an anterograde tracer. Their soma positions corresponded, from dorsal to ventral, to cortical representations of the whiskers, nose and mouth. They strongly targeted layer 4, with the majority exclusively targeting one cortical area and the ones in ventrolateral VPM branching to multiple somatosensory areas. We found that our experiments were more topographically precise than similar experiments from the Allen Institute and projections to the primary somatosensory area were in agreement with single-neuron morphological reconstructions from publicly available databases. This pilot study sets the basis for a shared virtual connectivity atlas that could be enriched with additional data for studying the topographical organization of different thalamic nuclei. The pipeline is accessible with only minimal programming skills via a Jupyter Notebook, and offers multiple visualization tools such as cortical flatmaps, subcortical plots and 3D renderings and can be used with custom anatomical delineations.

Cerebellar and basal ganglia inputs define three main nuclei in the mouse ventral motor thalamus

The thalamus is a central link between cortical and subcortical brain motor systems. Axons from the deep nuclei of the cerebellum (DCN), or the output nuclei of the basal ganglia system (substantia nigra reticulata, SNr; and internal pallidum GPi/ENT) monosynaptically innervate the thalamus, prominently some nuclei of the ventral nuclear group. In turn, axons from these ventral nuclei innervate the motor and premotor areas of the cortex, where their input is critical for planning, execution and learning of rapid and precise movements. Mice have in recent years become a widely used model in motor system research. However, information on the distribution of cerebellar and basal ganglia inputs in the rodent thalamus remains poorly defined. Here, we mapped the distribution of inputs from DCN, SNr, and GPi/ENT to the ventral nuclei of the mouse thalamus. Immunolabeling for glutamatergic and GABAergic neurotransmission markers delineated two distinct main territories, characterized each by the presence of large vesicular glutamate transporter type 2 (vGLUT2) puncta or vesicular GABA transporter (vGAT) puncta. Anterograde labeling of axons from DCN revealed that they reach virtually all parts of the ventral nuclei, albeit its axonal varicosities (putative boutons) in the vGAT-rich sector are consistently smaller than those in the vGLUT2-rich sector. In contrast, the SNr axons innervate the whole vGAT-rich sector, but not the vGLUT2-rich sector. The GPi/ENT axons were found to innervate only a small zone of the vGAT-rich sector which is also targeted by the other two input systems. Because inputs fundamentally define thalamic cell functioning, we propose a new delineation of the mouse ventral motor nuclei that is consistent with the distribution of DCN, SNr and GPi/ENT inputs and resembles the general layout of the ventral motor nuclei in primates.

How the Barrel Cortex Became a Working Model for Developmental Plasticity: A Historical Perspective

For half a century now, the barrel cortex of common laboratory rodents has been an exceptionally useful model for studying the formation of topographically organized maps, neural patterning, and plasticity, both in development and in maturity. We present a historical perspective on how barrels were discovered, and how thereafter, they became a workhorse for developmental neuroscientists and for studies on brain plasticity and activity-dependent modeling of brain circuits. What is particularly remarkable about this sensory system is a cellular patterning that is induced by signals derived from the sensory receptors surrounding the snout whiskers and transmitted centrally to the brainstem (barrelettes), the thalamus (barreloids), and the neocortex (barrels). Injury to the sensory receptors shortly after birth leads to predictable pattern alterations at all levels of the system. Mouse genetics have increased our understanding of how barrels are constructed and revealed the interplay of the molecular programs that direct axon growth and cell specification, with activity-dependent mechanisms. There is an ever-rising interest in this sensory system as a neurobiological model to study development of somatotopy, patterning, and plasticity at both the morphologic and physiological levels. This article is part of a group of articles commemorating the 50th anniversary of the Society for Neuroscience.

Subset of Cortical Layer 6b Neurons Selectively Innervates Higher Order Thalamic Nuclei in Mice

The thalamus receives input from 3 distinct cortical layers, but input from only 2 of these has been well characterized. We therefore investigated whether the third input, derived from layer 6b, is more similar to the projections from layer 6a or layer 5. We studied the projections of a restricted population of deep layer 6 cells ("layer 6b cells") taking advantage of the transgenic mouse Tg(Drd1a-cre)FK164Gsat/Mmucd (Drd1a-Cre), that selectively expresses Cre-recombinase in a subpopulation of layer 6b neurons across the entire cortical mantle. At P8, 18% of layer 6b neurons are labeled with Drd1a-Cre::tdTomato in somatosensory cortex (SS), and some co-express known layer 6b markers. Using Cre-dependent viral tracing, we identified topographical projections to higher order thalamic nuclei. VGluT1+ synapses formed by labeled layer 6b projections were found in posterior thalamic nucleus (Po) but not in the (pre)thalamic reticular nucleus (TRN). The lack of TRN collaterals was confirmed with single-cell tracing from SS. Transmission electron microscopy comparison of terminal varicosities from layer 5 and layer 6b axons in Po showed that L6b varicosities are markedly smaller and simpler than the majority from L5. Our results suggest that L6b projections to the thalamus are distinct from both L5 and L6a projections.

Long-Term Deficits in Cortical Circuit Function after Asphyxial Cardiac Arrest and Resuscitation in Developing Rats

Cardiac arrest is a common cause of global hypoxic-ischemic brain injury. Poor neurologic outcome among cardiac arrest survivors results not only from direct cellular injury but also from subsequent long-term dysfunction of neuronal circuits. Here, we investigated the long-term impact of cardiac arrest during development on the function of cortical layer IV (L4) barrel circuits in the rat primary somatosensory cortex. We used multielectrode single-neuron recordings to examine responses of presumed excitatory L4 barrel neurons to controlled whisker stimuli in adult (8 ± 2-mo-old) rats that had undergone 9 min of asphyxial cardiac arrest and resuscitation during the third postnatal week. Results indicate that responses to deflections of the topographically appropriate principal whisker (PW) are smaller in magnitude in cardiac arrest survivors than in control rats. Responses to adjacent whisker (AW) deflections are similar in magnitude between the two groups. Because of a disproportionate decrease in PW-evoked responses, receptive fields of L4 barrel neurons are less spatially focused in cardiac arrest survivors than in control rats. In addition, spiking activity among L4 barrel neurons is more correlated in cardiac arrest survivors than in controls. Computational modeling demonstrates that experimentally observed disruptions in barrel circuit function after cardiac arrest can emerge from a balanced increase in background excitatory and inhibitory conductances in L4 neurons. Experimental and modeling data together suggest that after a hypoxic-ischemic insult, cortical sensory circuits are less responsive and less spatially tuned. Modulation of these deficits may represent a therapeutic approach to improving neurologic outcome after cardiac arrest.

Comparable reduction in Zif268 levels and cytochrome oxidase activity in the retrosplenial cortex following mammillothalamic tract lesions

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Mammillothalamic tract lesions impaired T-maze alternation performance.

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Mammillothalamic tract lesions reduced Zif268 levels in retrosplenial cortex.

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Mammillothalamic tract lesions reduced cytochrome oxidase in retrosplenial cortex.

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No changes were found in the dorsal hippocampus.

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These distal changes may contribute to the memory impairments.

Damage to the mammillothalamic tract (MTT) produces memory impairments in both humans and rats, yet it is still not clear why this diencephalic pathway is vital for memory. One suggestion is that it is an important route for midbrain inputs to reach a wider cortical and subcortical network that supports memory. Consistent with this idea, MTT lesions produce widespread hypoactivity in distal brain regions as measured by the immediate-early gene, c-fos. To determine whether these findings were selective to c-fos or reflected more general changes in neuronal function, we assessed the effects of MTT lesions on the expression of the immediate-early gene protein, Zif268 and the metabolic marker, cytochrome oxidase, in the retrosplenial cortex and hippocampus. The lesions decreased levels of both activity markers in the superficial and deep layers of the retrosplenial cortex in both its granular and dysgranular subregions. In contrast, no significant changes were observed in the hippocampus, despite the MTT-lesioned animals showing marked impairments on T-maze alternation. These findings are consistent with MTT lesions providing important, indirect inputs for normal retrosplenial cortex functioning. These distal functional changes may contribute to the memory impairments observed after MTT lesions.

Metabolic Maturation of Auditory Neurones in the Superior Olivary Complex

Neuronal activity is energetically costly, but despite its importance, energy production and consumption have been studied in only a few neurone types. Neuroenergetics is of special importance in auditory brainstem nuclei, where neurones exhibit various biophysical adaptations for extraordinary temporal precision and show particularly high firing rates. We have studied the development of energy metabolism in three principal nuclei of the superior olivary complex (SOC) involved in precise binaural processing in the Mongolian gerbil (Meriones unguiculatus). We used immunohistochemistry to quantify metabolic markers for energy consumption (Na⁺/K⁺-ATPase) and production (mitochondria, cytochrome c oxidase activity and glucose transporter 3 (GLUT3)). In addition, we calculated neuronal ATP consumption for different postnatal ages (P0–90) based upon published electrophysiological and morphological data. Our calculations relate neuronal processes to the regeneration of Na⁺ gradients perturbed by neuronal firing, and thus to ATP consumption by Na⁺/K⁺-ATPase. The developmental changes of calculated energy consumption closely resemble those of metabolic markers. Both increase before and after hearing onset occurring at P12–13 and reach a plateau thereafter. The increase in Na⁺/K⁺-ATPase and mitochondria precedes the rise in GLUT3 levels and is already substantial before hearing onset, whilst GLUT3 levels are scarcely detectable at this age. Based on these findings we assume that auditory inputs crucially contribute to metabolic maturation. In one nucleus, the medial nucleus of the trapezoid body (MNTB), the initial rise in marker levels and calculated ATP consumption occurs distinctly earlier than in the other nuclei investigated, and is almost completed by hearing onset. Our study shows that the mathematical model used is applicable to brainstem neurones. Energy consumption varies markedly between SOC nuclei with their different neuronal properties. Especially for the medial superior olive (MSO), we propose that temporally precise input integration is energetically more costly than the high firing frequencies typical for all SOC nuclei.

Thalamocortical dysfunction and thalamic injury after asphyxial cardiac arrest in developing rats

Global hypoxia-ischemia interrupts oxygen delivery and blood flow to the entire brain. Previous studies of global brain hypoxia-ischemia have primarily focused on injury to the cerebral cortex and to the hippocampus. Susceptible neuronal populations also include inhibitory neurons in the thalamic reticular nucleus. We therefore investigated the impact of global brain hypoxia-ischemia on the thalamic circuit function in the somatosensory system of young rats. We used single neuron recordings and controlled whisker deflections to examine responses of thalamocortical neurons to sensory stimulation in rat survivors of 9 min of asphyxial cardiac arrest incurred on postnatal day 17. We found that 48-72 h after cardiac arrest, thalamocortical neurons demonstrate significantly elevated firing rates both during spontaneous activity and in response to whisker deflections. The elevated evoked firing rates persist for at least 6-8 weeks after injury. Despite the overall increase in firing, by 6 weeks, thalamocortical neurons display degraded receptive fields, with decreased responses to adjacent whiskers. Nine minutes of asphyxial cardiac arrest was associated with extensive degeneration of neurites in the somatosensory nucleus as well as activation of microglia in the reticular nucleus. Global brain hypoxia-ischemia during cardiac arrest has a long-term impact on processing and transfer of sensory information by thalamic circuitry. Thalamic circuitry and normalization of its function may represent a distinct therapeutic target after cardiac arrest.

Spine formation and maturation in the developing rat auditory cortex

The rat auditory cortex is organized as a tonotopic map of sound frequency. This map is broadly tuned at birth and is refined during the first 3 weeks postnatal. The structural correlates underlying tonotopic map maturation and reorganization during development are poorly understood. We employed fluorescent dye ballistic labeling ("DiOlistics") alone, or in conjunction with immunohistochemistry, to quantify synaptogenesis in the auditory cortex of normal hearing rats. We show that the developmental appearance of dendritic protrusions, which include both immature filopodia and mature spines, on layers 2/3, 4, and 5 pyramidal and layer 4 spiny nonpyramidal neurons occurs in three phases: slow addition of dendritic protrusions from postnatal day 4 (P4) to P9, rapid addition of dendritic protrusions from P9 to P19, and a final phase where mature protrusion density is achieved (>P21). Next, we combined DiOlistics with immunohistochemical labeling of bassoon, a presynaptic scaffolding protein, as a novel method to categorize dendritic protrusions as either filopodia or mature spines in cortex fixed in vivo. Using this method we observed an increase in the spine-to-filopodium ratio from P9-P16, indicating a period of rapid spine maturation. Previous studies report mature spines as being shorter in length compared to filopodia. We similarly observed a reduction in protrusion length between P9 and P16, corroborating our immunohistochemical spine maturation data. These studies show that dendritic protrusion formation and spine maturation occur rapidly at a time previously shown to correspond to auditory cortical tonotopic map refinement (P11-P14), providing a structural correlate of physiological maturation.

Architectonic subdivisions of neocortex in the Galago (Otolemur garnetti)

In the present study, galago brains were sectioned in the coronal, sagittal, or horizontal planes, and sections were processed with several different histochemical and immunohistochemical procedures to reveal the architectonic characteristics of the various cortical areas. The histochemical methods used included the traditional Nissl, cytochrome oxidase, and myelin stains, as well as a zinc stain, which reveals free ionic zinc in the axon terminals of neurons. Immunohistochemical methods include parvalbumin (PV) and calbindin (CB), both calcium-binding proteins, and the vesicle glutamate transporter 2 (VGluT2). These different procedures revealed similar boundaries between areas, which suggests that functionally relevant borders were being detected. These results allowed a more precise demarcation of previously identified areas. As thalamocortical terminations lack free ionic zinc, primary cortical areas were most clearly revealed by the zinc stain, because of the poor zinc staining of layer 4. Area 17 was especially prominent, as the broad layer 4 was nearly free of zinc stain. However, this feature was less pronounced in the primary auditory and somatosensory cortex. As VGluT2 is expressed in thalamocortical terminations, layer 4 of primary sensory areas was darkly stained for VGluT2. Primary motor cortex had reduced VGluT2 staining, and increased zinc-enriched terminations in the poorly developed granular layer 4 compared to the adjacent primary somatosensory area. The middle temporal visual (MT) showed increased PV and VGluT2 staining compared to the surrounding cortical areas. The resulting architectonic maps of cortical areas in galagos can usefully guide future studies of cortical organizations and functions.

Passive vs. active touch-induced activity in the developing whisker pathway

The mouse trigeminal (V) system undergoes significant postnatal structural and functional developmental changes. Histological modules (barrelettes, barreloids and barrels) in the brainstem, thalamus and cortex related to actively moved (whisking) tactile hairs (vibrissae) on the face allow detailed studies of development. High-resolution [(3) H]2-deoxyglucose (2DG) emulsion autoradiography with cytochrome oxidase histochemistry was used to analyze neuronal activity changes related to specific whisker modules in the developing and mature mouse V system provoked by passive (experimenter-induced) and active (animal-induced) displacements of a single whisker (D4). We tested the hypothesis that neuronal activity patterns change in relation to the onset of active touch (whisking) on postnatal day (P)14. Quantitative image analyses revealed: (i) on P7, when whisker-like patterns of modules are clear, heightened 2DG activity in all appropriate modules in the brainstem, thalamus and cortex; (ii) on P14, a transitory activity pattern coincident with the emergence of whisking behavior that presages (iii) strong labeling of the spinal V subnucleus interpolaris and barrel cortex produced by single-whisker-mediated active touch in adults and (iv) at all above-listed ages and structures, significant suppression of baseline activity in some modules surrounding those representing the stimulated whisker. Differences in activity patterns before and after the onset of whisking behavior may be caused by neuronal activity induced by whisking, and by strengthening of modulatory projections that alter the activity of subcortical inputs produced by whisking behavior during active touch.

Innervation and activity dependent dynamics of postsynaptic oxidative metabolism

Despite extensive investigations into the mechanisms of aerobic respiration in mitochondria, the spontaneous metabolic activity of individual cells within a whole animal has not been observed in real time. Consequently, little is known about whether and how the level of mitochondrial energy metabolism is regulated in a cell during development of intact systems. Here we studied the dynamics of postsynaptic oxidative metabolism by monitoring the redox state of mitochondrial flavoproteins, an established indicator of energy metabolism, at the developing Drosophila neuromuscular junction. We detected transient and spatially synchronized flavoprotein autofluorescence signals in postsynaptic muscle cells. These signals were dependent on the energy substrates and coupled to changes in mitochondrial membrane potential and Ca2+ concentration. Notably, the rate of autofluorescence signals increased during synapse formation through contact with the motoneuronal axon. This rate was also influenced by the magnitude of synaptic inputs. Thus, presynaptic cells tightly regulate postsynaptic energy metabolism presumably to maintain an energetic balance during neuromuscular synaptogenesis. Our results suggest that flavoprotein autofluorescence imaging should allow us to begin assessing the progress of synapse formation from a metabolic perspective.

Cortical adenylyl cyclase 1 is required for thalamocortical synapse maturation and aspects of layer IV barrel development

Experimental evidence from mutant or genetically altered mice indicates that the formation of barrels and the proper maturation of thalamocortical (TC) synapses in the primary somatosensory (barrel) cortex depend on mechanisms mediated by neural activity. Type 1 adenylyl cyclase (AC1), which catalyzes the formation of cAMP, is stimulated by increases in intracellular Ca(2+) levels in an activity-dependent manner. The AC1 mutant mouse, barrelless (brl), lacks typical barrel cytoarchitecture, and displays presynaptic and postsynaptic functional defects at TC synapses. However, because AC1 is expressed throughout the trigeminal pathway, the barrel cortex phenotype of brl mice may be a consequence of AC1 disruption in cortical or subcortical regions. To examine the role of cortical AC1 in the development of morphological barrels and TC synapses, we generated cortex-specific AC1 knock-out (CxAC1KO) mice. We found that neurons in layer IV form grossly normal barrels and TC axons fill barrel hollows in CxAC1KO mice. In addition, whisker lesion-induced critical period plasticity was not impaired in these mice. However, we found quantitative reductions in the quality of cortical barrel cytoarchitecture and dendritic asymmetry of layer IV barrel neurons in CxAC1KO mice. Electrophysiologically, CxAC1KO mice have deficits in the postsynaptic but not in the presynaptic maturation of TC synapses. These results suggest that activity-dependent postsynaptic AC1-cAMP signaling is required for functional maturation of TC synapses and the development of normal barrel cortex cytoarchitecture. They also suggest that the formation of the gross morphological features of barrels is independent of postsynaptic AC1 in the barrel cortex.

Development of thalamocortical response transformations in the rat whisker-barrel system

Extracellular single-unit recordings were used to characterize responses of thalamic barreloid and cortical barrel neurons to controlled whisker deflections in 2, 3-, and 4-wk-old and adult rats in vivo under fentanyl analgesia. Results indicate that response properties of thalamic and cortical neurons diverge during development. Responses to deflection onsets and offsets among thalamic neurons mature in parallel, whereas among cortical neurons responses to deflection offsets become disproportionately smaller with age. Thalamic neuron receptive fields become more multiwhisker, whereas those of cortical neurons become more single-whisker. Thalamic neurons develop a higher degree of angular selectivity, whereas that of cortical neurons remains constant. In the temporal domain, response latencies decrease both in thalamic and cortical neurons, but the maturation time-course differs between the two populations. Response latencies of thalamic cells decrease primarily between 2 and 3 wk of life, whereas response latencies of cortical neurons decrease in two distinct steps--the first between 2 and 3 wk of life and the second between the fourth postnatal week and adulthood. Although the first step likely reflects similar subcortical changes, the second phase likely corresponds to developmental myelination of thalamocortical fibers. Divergent development of thalamic and cortical response properties indicates that thalamocortical circuits in the whisker-to-barrel pathway undergo protracted maturation after 2 wk of life and provides a potential substrate for experience-dependent plasticity during this time.

Total sleep deprivation inhibits the neuronal nitric oxide synthase and cytochrome oxidase reactivities in the nodose ganglion of adult rats

Sleep disorders are a form of stress associated with increased sympathetic activity, and they are a risk factor for the occurrence of cardiovascular disease. Given that nitric oxide (NO) may play an inhibitory role in the regulation of sympathetic tone, this study set out to determine the NO synthase (NOS) reactivity in the primary cardiovascular afferent neurons (i.e. nodose neurons) following total sleep deprivation (TSD). TSD was performed by the disc-on-water method. Following 5 days of TSD, all experimental animals were investigated for quantitative nicotinamine adenine dinucleotide phosphate-diaphorase (NADPH-d, a co-factor of NOS) histochemistry, neuronal NOS immunohistochemistry and neuronal NOS activity assay. In order to evaluate the endogenous metabolic activity of nodose neurons, cytochrome oxidase (COX) reactivity was further tested. All the above-mentioned reactivities were objectively assessed by computerized image analysis. The clinical significance of the reported changes was demonstrated by alterations of mean arterial blood pressure (MAP). The results indicated that in normal untreated rats, numerous NADPH-d/NOS- and COX-reactive neurons were found in the nodose ganglion (NG). Following TSD, however, both the labelling and staining intensity of NADPH-d/NOS as well as COX reactivity were drastically reduced in the NG compared with normal untreated ganglions. MAP was significantly higher in TSD rats (136+/-4 mmHg) than in normal untreated rats (123+/-2 mmHg). NO may serve as an important sympathoinhibition messenger released by the NG neurons, and decrease of NOS immunoexpression following TSD may account for the decrease in NOS content. In association with the reduction of NOS activity, a defect in NOS expression in the primary cardiovascular afferent neurons would enhance clinical hypertension, which might serve as a potential risk factor in the development of TSD-relevant cardiovascular disturbances.

Barrel map development relies on protein kinase A regulatory subunit II beta-mediated cAMP signaling

The cellular and molecular mechanisms mediating the activity-dependent development of brain circuitry are still incompletely understood. Here, we examine the role of cAMP-dependent protein kinase [protein kinase A (PKA)] signaling in cortical development and plasticity, focusing on its role in thalamocortical synapse and barrel map development. We provide direct evidence that PKA activity mediates barrel map formation using knock-out mice that lack type IIbeta regulatory subunits of PKA (PKARIIbeta). We show that PKARIIbeta-mediated PKA function is required for proper dendritogenesis and the organization of cortical layer IV neurons into barrels, but not for the development and plasticity of thalamocortical afferent clustering into a barrel pattern. We localize PKARIIbeta function to postsynaptic processes in barrel cortex and show that postsynaptic PKA targets, but not presynaptic PKA targets, have decreased phosphorylation in pkar2b knock-out (PKARIIbeta(-/-)) mice. We also show that long-term potentiation at TC synapses and the associated developmental increase in AMPA receptor function at these synapses, which normally occurs as barrels form, is absent in PKARIIbeta(-/-) mice. Together, these experiments support an activity-dependent model for barrel map development in which the selective addition and elimination of thalamocortical synapses based on Hebbian mechanisms for synapse formation is mediated by a cAMP/PKA-dependent pathway that relies on PKARIIbeta function.

Organization of somatosensory cortical areas in the naked mole-rat (Heterocephalus glaber)

Multiunit electrophysiology was combined with histological analysis of cortical sections to investigate the organization of somatosensory areas in the naked mole-rat. We provide new details for the organization of primary somatosensory cortex (S1) and identify cortical modules and barrels that correspond to the representations of different body parts. In addition, details of the location and organization of secondary somatosensory cortex (S2) are reported, and evidence for a third somatosensory representation, likely the parietal ventral area (PV), is provided and discussed. S1 contained a complete and systematic representation of the contralateral body surface and oral structures. The orientation of S1 was inverted, with the lower body represented medially and the face and oral structures located rostrolaterally. The S2 representation was found in caudolateral cortex forming a mirror image of S1. The two areas were joined at the representation of the vibrissae and snout, so that the orientation of S2 formed an upright representation of the body in cortex. Receptive fields for S2 were consistently larger than those in S1. Evidence for the presumptive parietal ventral area, lateral to S2, suggests that this area may be an inverted mirror image of S2. By aligning the electrophysiological maps of body representations with cytochrome oxidase-reacted cortical sections we were able to identify modules related to the buccal pad, chin, vibrissae, forelimb, hindlimb, trunk, tongue, lower incisor, and upper incisors. The orofacial modules in lateral cortex resemble similar modules reported to relate to oral structures previously described in the laboratory rat, owl monkey, and squirrel monkey.

Subbarrel domains in rat somatosensory (S1) cortex

We used cytochrome oxidase (CO) histochemistry in conjunction with other histological methods to investigate the histochemoarchitecture of barrel hollows in rat somatosensory cortex. We found that individual large barrels in the posteromedial barrel subfield encompass two or three discrete subbarrel domains. Detailed analysis revealed, further, that subbarrel domains are relatively consistent in size, each having average dimensions that approximate those of large barrels in mouse S1. Unexpectedly, subbarrel domains are organized into a few distinct, repeated patterns. The small barrels in rat anterolateral barrel subfield and all barrel hollows in mouse S1 appear to consist of single CO domains. Subbarrel domains revealed here by CO are columnar entities that correspond with cyto- and myeloarchitectonic inhomogeneities within the barrels and are enriched in thalamocortical axon terminals. The present findings together with existing data indicate that barrels in rat posteromedial barrel subfield are structurally and functionally heterogeneous.

Melatonin restores the cytochrome oxidase reactivity in the nodose ganglia of acute hypoxic rats

This study aimed to elucidate whether melatonin would exert beneficial effects on the neuronal functions of the nodose ganglion (NG) following acute hypoxic insult. The cytochrome oxidase (COX) and the nicotinamine adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry along with the nitric oxide synthase (NOS) immunofluorescence were used to examine the metabolic stage and nitric oxide production in nodose neurons respectively. Adult rats were injected intraperitoneally with melatonin at 5 or 100 mg/kg. Hypoxia was achieved by placing the rats into an altitude chamber (PO2 = 43 torr) for 4 hr. The results show that in normal untreated rats, nearly all and about 43% of the NG neurons displayed COX and NOS/NADPH-d reactivities with various staining intensities respectively. However, COX reactivity was drastically decreased while NOS/NADPH-d reactivity was significantly upregulated following hypoxia treatment. In melatonin pretreated rats, the hypoxia-induced reduction of COX reactivity was obviously prevented and the augmentation of NOS/NADPH-d reactivity was successfully suppressed. The deficit in the metabolic stage and the over-activation of NOS would contribute to the generation of oxidative stress. By effectively preventing the metabolic disruption, melatonin may have potential utility in therapeutic treatment of neuronal dysfunctions where oxidative stress is a participant.

Effects of prenatal alcohol exposure on the development of the vibrissal somatosensory cortical barrel network

We have previously shown that the serotonin (5-HT) and its thalamocortical afferents are compromised by prenatal alcohol exposure (PAE). The development of the sensory cortical barrels is regulated by 5-HT-rich thalamocortical afferents. Therefore, it is hypothesized that PAE will deleteriously affect the postnatal development of the cortical barrel formations. On embryonic day (E)7, C57BL/6 mice were grouped into: Alcohol (Alc), Pair-fed (PF), or Chow, and maintained on diet until E18. On postnatal day 7, cortices were stained with 5-HT for thalamocortical fibers, and a NeuN for identification of mature neurons. The area of the posterior medial barrel subfield (PMBSF), was measured as well as the number of NeuN+ neurons within the barrel patches. Though brain weight and brain volume were similar among the three groups, a significant reduction was seen in total area of the PMBSF, and in the average individual barrel area in the Alc group as compared to Chow. Furthermore, the volumes of the B, but not C row barrels were significantly reduced. Barrels were found missing in layer IV, specifically in the posterior aspects of the A, B, and straddler row in the Alc group. Cell counts demonstrated a nearly 50% reduction in NeuN+ neuron number in both rows. This reduction in size of the PMBSF and fewer neurons within these sensory barreloids may underlie a change in the development of the discriminatory sensitivity of the whiskers and serves as an excellent model for the study of a compromised sensory modality following PAE.

Changes in the metabolic activity of neurons in the anterior hypothalamic nuclei in rats during hyperthermia, fever, and hypothermia

Histochemical methods were used to detect differently directed changes in the metabolic activity of neurons in the anterior hypothalamic nuclei in rats during hyperthermia, fever, and hypothermia. Hyperthermia induced by high temperatures was associated with increases in the activities of enzymes involved in energy metabolism, with increases in RNA contents in neurons in the supraoptic, paraventricular, and median preoptic nuclei of the anterior hypothalamus of the rat, which is evidence for increases in metabolic activity in the neurons of these nuclei. Endotoxin-induced fever was accompanied by decreases in the metabolic activity of neurons in the median preoptic nucleus, while activity in neurons of the supraoptic and paraventricular nuclei showed no significant change. The development of hypothermia induced by low temperatures was characterized by decreases in the metabolic activity of neurons in the supraoptic, paraventricular, and median preoptic nuclei of the anterior hypothalamus. It is suggested that the differently directed changes in metabolic activity in the neurons of the anterior hypothalamus in hyperthermia, fever, and hypothermia are associated with their roles in the central mechanisms of thermoregulation (median preoptic nucleus) and neurosecretory processes (supraoptic and paraventricular nuclei).

The chemo- and somatotopic architecture of the Galago cuneate and gracile nuclei

The pattern of peripheral nerve inputs into the dorsal column nuclei, cuneate and gracile, was investigated in the prosimian Galago garnetti. The major findings were, that there is a greater segregation of the inputs from the fingers/hand within the cuneate compared with input form the toes/foot within the gracile. In both nuclei, cell clusters can be identified as cytochrome oxidase dense blotches, reactive also for the activity-dependent enzyme nitric oxide synthase. In the cuneate, cell clusters were apparent as six main cytochrome oxidase/nitric oxide synthase-reactive ovals arranged in a medial to lateral sequence. In contrast in the gracile, a higher degree of parcellation was noted and several cytochrome oxidase/nitric oxide synthase blotches were distributed along the rostrocaudal axis of the nucleus. This different architecture parallels differences in the organization of the inputs from the hand and from the foot. In the cuneate, cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals from the glabrous and hairy skin of digits d1 to d5 segregated in each of the five most lateral cytochrome oxidase/nitric oxide synthase blotches. Afferents from the thenar, palmar pads and hypothenar overlapped with those from digit 1, digit 2 to digit 4 and digit 5, respectively. Inputs from wrist arm and shoulder were segregated in the most medial blotch. In the gracile, multiple foci of cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals were observed upon injections of single sites in the toes or plantar pads. Although in multiple foci, inputs from different toes segregated from one another as well. Terminals from the plantar pads appeared to converge on the same cytochrome oxidase/nitric oxide synthase blotches targeted by inputs from the toes. In both the cuneate and the gracile, cytochrome oxidase/nitric oxide synthase blotches also presented intense immunoreactivity for GABA, calbindin, parvalbumin, and brain derived neurotrophic factor. Finally, in the cuneate the cell cluster region presented similarities in prosimian galagos and four species of New World monkeys, whereas it appeared more differentiated and complex in the Old Word macaque monkeys. In conclusion, the different pattern of segregation of the inputs from the hand and from the foot can be related to the different metabolic organization of the cuneate and of the gracile, respectively.

Conductive hearing loss results in changes in cytochrome oxidase activity in gerbil central auditory system

Conductive hearing loss (CHL) restricts auditory input to an intact peripheral auditory system. Effects of deprivation on the central auditory system (CAS) have been debated, although a number of studies support the hypothesis that CHL can cause modification of CAS structure and function. The present study was designed to test the hypothesis that unilateral CHL results in a decrease in cytochrome oxidase (CO) activity in CAS nuclei that receive major afferent input from the affected ear. Gerbils at postnatal day 12 (P21) or 6-8 weeks underwent left unilateral CHL (malleus removal), cochlear ablation, or a sham surgical procedure. After a survival time of 48 hours or 3 weeks, animals were sacrificed and tissue was processed for cytochrome oxidase histochemistry. Optical density (OD) measurements were made from individual neurons in the anteroventral cochlear nucleus (AVCN) and from medial and lateral dendritic fields in the medial superior olivary nucleus (MSO), the lateral superior olivary nucleus, and the inferior colliculus. The width of the CO-stained neuropil in MSO was also measured as an estimate of dendritic length. OD measures were corrected to neutral areas of the brain. Cochlear ablation caused significant decreases in CO activity in left lower brainstem nuclei, particularly in adult animals. Following CHL, a significant decrease in CO activity was observed in the ipsilateral AVCN and a significant increase was observed in the contralateral AVCN. Cochlear ablation resulted in decreased width of MSO neuropil containing dendrites that receive primary input from the ablated ear. CHL resulted in a significant increase in the width of MSO neuropil on both sides of the brain in the P21 animals that survived 3 weeks but not in P21 animals that survived only 48 hours or in the adult animals. Unilateral CHL is associated with changes in CO activity in the AVCN and may affect MSO dendritic length in younger animals.

Deafness-induced changes in the auditory pathway: implications for cochlear implants

A profound sensorineural hearing loss induces significant pathological and atrophic changes within the cochlea and central auditory pathway. We describe these deafness-induced morphological and functional changes following controlled lesions of the cochlea in experimental animals. Such changes are generally consistent with the limited number of reports describing deafness-induced changes observed in human material. The implications of these pathophysiological changes within the auditory pathway on cochlear implant function are discussed. Finally, the plastic response of the deafened auditory system to electrical stimulation of the auditory nerve is reviewed in light of the clinical implications for cochlear implant recipients.

Neonatal sensorineural hearing loss affects neurone size in cat auditory midbrain

We examined the effect of a neonatal sensorineural hearing loss on the soma area of neurones in the central nucleus of the inferior colliculus (ICC) in adult cats to evaluate the role of auditory experience on neuronal atrophy within the auditory midbrain. Three groups of animals were used: bilaterally deafened, unilaterally deafened and normal hearing controls. Soma area measurements were made from the laminated central and medial divisions of the ICC of eight deafened and two normal hearing cats. A small but significant reduction in soma area was evident for bilaterally deafened animals compared with normal hearing controls (P<0.05, Dunnett's test). In contrast, there was no significant difference in mean soma area between normal hearing and unilaterally deafened animals (P0.05) irrespective of whether the ICC examined was ipsi- or contralateral to the deafened ear. These results demonstrate that the reduction in soma area of auditory brainstem neurones reported following a sensorineural hearing loss is also evident at the level of the auditory midbrain.

Effects of a restricted unilateral neocortical lesion upon cerebral glucose and oxidative metabolisms in fetal and neonatal cats

The present study was designed to measure cerebral glucose and oxidative metabolisms and to assess relationships with previously identified morphological changes in adult cats with a unilateral, restricted neocortical lesion sustained either during fetal life or neonatally. Local cerebral metabolic rates for glucose (LCMR(glc)) were measured using the [14C]2-deoxy-D-glucose (2 DG) autoradiography method and oxidative capacity was determined using cytochrome oxidase histochemistry (C.O.). Only glucose metabolism in the fetal-lesioned cats was affected substantially. There was a global decrease (31.0% relative to controls) of the LCMR(glc) for both cerebral hemispheres while focal decreases were seen mainly in thalamic and neostriatal nuclei (and reaching declines of over 50%). Cats with a neonatal lesion showed only a tendency to such declines (19.5% and 22.0% for the right and left hemispheres, respectively). C.O. values were not affected significantly either globally or locally in any of the age-at-lesion groups. In previous work using fetal animals with similar lesions, morphological evidence of subcortical neuropile degeneration was not observed; instead, a marked decrease in size of the ipsilateral remaining neocortex and a pronounced distortion of gyri and sulci patterns bilaterally were found. In this context, we propose that in the fetal-lesioned cats, there was a widespread lesion-induced decrease in corticofugal (and transcortical) synaptic inputs which was responsible for a decline in functional (synaptic) activities, and that this, in turn, caused a downturn in glucose utilization. In the neonatal cats minor degeneration, coupled with lack of reinnervation, would account for the tendency to 2 DG declines. These results indicate that the long-term metabolic response of the fetal brain to injury is also less adaptive than that of the neonatal brain. Since standard methods are available to measure cerebral metabolism in humans, our studies in animal models may help understanding the long term physiological consequences of developmental focal brain damage in patients as well as to predict the relationships between cerebral metabolism and the underlying long-term morphological effects of such lesions.

Metabolic and morphological stability of motoneurons in response to chronically elevated neuromuscular activity

The purpose of this study was to determine the plasticity of spinal motoneuron size and succinate dehydrogenase activity in response to increased levels of neuromuscular activation and/or increased target size. The plantaris muscles of adult rats were functionally overloaded for one or 10 weeks via the removal of the soleus and gastrocnemius muscles bilaterally. In addition, one group of functionally overloaded rats at each time period was trained daily (1 h/day) on a treadmill. The plantaris muscle on one side in each rat was injected with the fluorescent tracer Nuclear Yellow two days prior to the end of the study to retrogradely label the associated motor pool. At one week, the plantaris weight was increased compared to control, whereas there was no change in motoneuron size. Succinate dehydrogenase activity was unaffected in either the muscle or motoneurons. At 10 weeks, the plantaris muscle weight was larger and the succinate dehydrogenase activity lower in the functionally overloaded rats compared to age-matched controls. Training further increased the hypertrophic response, whereas the succinate dehydrogenase activity returned to control levels. In contrast, mean motoneuron size and succinate dehydrogenase activity were similar among the three groups. These data indicate that overload of a specific motor pool, involving both an increase in activation and an increase in target size, had a minimal effect on the size or the oxidative potential of the associated motoneurons. Thus, it appears that the spinal motoneurons, unlike the muscle fibers, are highly stable over a wide range of levels of chronic neuromuscular activity.

Sensorineural hearing loss during development: morphological and physiological response of the cochlea and auditory brainstem

We have investigated the effects of sensorineural hearing loss on the cochlea and central auditory system of profoundly deafened cats. Seventeen adult cats were used: four had normal hearing; 12 were deafened neonatally for periods of < 2.5 years (five bilaterally, seven unilaterally); and one animal had a long-term (approximately 8 years) profound bilateral hearing loss. Bipolar scala tympani stimulating electrodes were bilaterally implanted in each animal, and electrically evoked auditory brainstem responses (EABRs) were recorded in an acute study to evaluate the basic physiologic response properties of the deafened auditory pathway. The cochleae and cochlear nuclei (CN) of each animal were examined with light microscopy. Spiral ganglion cell density in neonatally deafened cochleae was 17% of normal, and only 1.5% of normal in the long-term deaf animal. There was a 46% reduction in total CN volume in neonatally deafened animals compared to normal, and a 60% reduction in the long-term deaf animal. Neural density in the anteroventral CN of bilaterally deafened animals was 37% higher than normal; 44% higher in the long-term deaf animal. Significantly, however, we saw no evidence of a loss of neurones within the anteroventral CN in any deafened animal. There was a significant increase in EABR threshold and wave IV latency in the deafened animals, and a significant decrease in response amplitude and input/output function gradient. Again, these changes were more extensive in the long-term deaf animal. These data show that a sensorineural hearing loss can evoke significant morphological and physiological changes within the cochlea and auditory brainstem, and these changes become greater with duration of deafness. It remains to be seen whether these changes can be reversed following the introduction of afferent activity via chronic electrical stimulation of the auditory nerve.

Cerebral metabolism following neonatal or adult hemineodecortication in cats: effect on oxidative capacity using cytochrome oxidase histochemistry

In order to determine the degree and extent of changes in cerebral oxidative capacity following cerebral hemineodecortication, adult cats which had undergone surgery early postnatally (mean age: 11.4 days) or during adulthood were studied using cytochrome oxidase histochemistry. A total of 18 animals were employed and 50 brain regions were quantified bilaterally using optical densitometry. Although many subcortical regions exhibiting extensive degenerative features revealed lower levels of cytochrome oxidase (C.O.) activity, this reduction was relatively unremarkable compared to intact controls. Nevertheless, it was interesting that this decrease (down to 66-89%) of normal was more pronounced in neonatal-lesioned cats, reaching significance in a number of ipsilateral thalamic nuclei, compared to adult-lesioned animals (91-100% of normal), suggesting a contribution of glial cells to the density of C.O. staining in the latter cats. Regions of the brain spared from degeneration exhibited a bilateral increase in C.O. activity which may reflect the demands for energy to support the anatomical reorganization which is prevalent in these animals. Surprisingly, such increases were more robust within spared regions of the adult-lesioned brain, reaching significance in four ipsilateral and nine contralateral areas with the density of the reaction attaining levels over 125% of control. This may indicate different demands for oxidative metabolism in the adult-lesioned cats. These results enhance our understanding of the mechanism(s) underlying the greater extent of functional sparing or recovery in cats sustaining injury to the cerebral cortex early vs. late in life. In addition, the findings complement our previous companion report on glucose metabolism supporting the concept of energy compartmentalization, which reflects the dynamic interaction between anatomical and functional changes in this age-at-lesion model of recovery.

Downregulation of oxidative phosphorylation in Alzheimer disease: loss of cytochrome oxidase subunit mRNA in the hippocampus and entorhinal cortex

Messenger RNA (mRNA) for cytochrome oxidase subunit II (COX II) was localized by in situ hybridization in the entorhinal cortex and hippocampal formation of postmortem brain tissue from normal human subjects and from patients with Alzheimer disease (AD). In the control entorhinal cortex, COX II mRNA was detected mainly in neuronal cell bodies of layers II and IV. In control hippocampal formation, highest levels were localized in neuronal cell bodies of the dentate gyrus and the CA3 and CA1 regions, neurons that are involved in the major input and output pathways of the hippocampal formation. In AD brain, COX II mRNA was markedly reduced in the entorhinal cortex and the hippocampal formation compared with control brain. In the AD hippocampal formation, reductions were in regions severely affected by AD pathology as well as in regions that were relatively spared. These results are consistent with the hypothesis that reduced mitochondrial energy metabolism reflects loss of neuronal connections in AD.

The central auditory system and auditory deprivation: experience with cochlear implants in the congenitally deaf

In the present paper we briefly review the response of the central auditory system to auditory deprivation and describe recent experimental and clinical experience with cochlear implants. While the central auditory system undergoes marked changes in response to auditory deprivation, it would appear that at least a rudimentary cochleotopic organisation is maintained at the level of the brainstem and auditory cortex in animals deafened from birth. Moreover, recent studies have demonstrated the ability of the central auditory system to undergo functional reorganisation in response to changes in the pattern of afferent activity. Clinical experience has shown that deaf children with little or no prior auditory experience can obtain significant benefit from cochlear implants, provided the device is fitted at a young age. Furthermore, factors predicting successful clinical outcomes with these devices reflect the importance of auditory experience, either prior to an acquired loss or with the use of a cochlear implant. These findings suggest that functional reorganisation within the central auditory pathway can at least partially account for improvements in clinical performance over time.

Consequences of nigrostriatal denervation on the functioning of the basal ganglia in human and nonhuman primates: an in situ hybridization study of cytochrome oxidase subunit I mRNA

To examine the consequences of nigrostriatal denervation and chronic levodopa (L-DOPA) treatment on functional activity of the basal ganglia, we analyzed, using in situ hybridization, the cellular expression of the mRNA encoding for cytochrome oxidase subunit I (COI mRNA), a molecular marker for functional neuronal activity, in the basal ganglia. This analysis was performed in monkeys rendered parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) Intoxication, some of which had been receiving L-DOPA, and in patients with Parkinson's disease (PD). In MPTP-intoxicated monkeys compared with control animals, COI mRNA expression was increased in the subthalamic nucleus (STN) and in the output nuclei of the basal ganglia, i.e., the internal segment of the globus pallidus and the substantia nigra pars reticulata. This increase was partially reversed by L-DOPA treatment. COI mRNA expression remained unchanged in the external segment of the globus pallidus (GPe). In PD patients, all of whom had been treated chronically by L-DOPA, COI mRNA expression in the analyzed basal ganglia structures was similar to that in control subjects. These results are in agreement with the accepted model of basal ganglia organization, to the extent that the output nuclei of the basal ganglia are considered to be overactive after nigrostriatal denervation, partly because of increased activity of excitatory afferents from the STN. Yet, our results would also seem to contradict this model, because the overactivity of the STN does not seem to be attributable to a hypoactivation of the GPe.

Effects of 14 days of spaceflight and nine days of recovery on cell body size and succinate dehydrogenase activity of rat dorsal root ganglion neurons

The cross-sectional areas and succinate dehydrogenase activities of L5 dorsal root ganglion neurons in rats were determined after 14 days of spaceflight and after nine days of recovery. The mean and distribution of the cross-sectional areas were similar to age-matched, ground-based controls for both the spaceflight and for the spaceflight plus recovery groups. The mean succinate dehydrogenase activity was significantly lower in spaceflight compared to aged-matched control rats, whereas the mean succinate dehydrogenase activity was similar in age-matched control and spaceflight plus recovery rats. The mean succinate dehydrogenase activity of neurons with cross-sectional areas between 1000 and 2000 microns2 was lower (between 7 and 10%) in both the spaceflight and the spaceflight plus recovery groups compared to the appropriate control groups. The reduction in the oxidative capacity of a subpopulation of sensory neurons having relatively large cross-sectional areas immediately following spaceflight and the sustained depression for nine days after returning to 1 g suggest that the 0 g environment induced significant alterations in proprioceptive function.

Cytochrome oxidase staining in the major pelvic ganglion of the male rat

Cytochrome oxidase staining was used as a marker of metabolic activity in neural elements in the rat major pelvic ganglion. Many neurons in the ventral pole of the ganglion have little cytochrome oxidase activity, while neurons in other locations show gradations in staining intensity. Punctate staining around principal neurons may represent preganglionic terminals, since it was greatly reduced after denervation of the ganglion. Image analysis was used to compare neuronal size to staining intensity. There was a negative correlation between cell size and staining intensity; the largest neurons were only lightly stained for cytochrome oxidase, while the medium and the small neurons showed a full range of metabolic activity. To study metabolic activity of an identified neuronal population, the seminal vesicles were injected with a retrograde tracer. The largest seminal vesicles neurons (1500 to 3200 microns2) had low enzyme activity, whereas the majority of neurons to this organ were smaller with gradations in staining. These results are indicative of the metabolic activity of the autonomic innervation to various pelvic tissues. Cytochrome oxidase histochemistry should prove valuable in assessing the demands placed on autonomic ganglia in differing functional and dysfunctional states.

Distribution of calcium-binding protein immunoreactivities in the guinea pig auditory brainstem

This study was intended to provide an overview of the distribution of calcium-binding proteins in the rodent auditory brainstem. We based our observations on immunohistochemical material obtained in the guinea pig, a species widely used in auditory research in which a mapping of calcium-binding proteins in the auditory brainstem is still missing. Differences in the amounts of these proteins throughout the auditory brainstem were further analyzed semiquantitatively. Parvalbumin was present in most neurons and their axon terminals throughout the ascending auditory brainstem. Nuclei that surround the main relay nuclei of the ascending auditory pathway lacked labeling. Calretinin staining was prominent in spherical and globular cells of the cochlear nucleus, in their axon terminals in the superior olivary complex, and in principal cells of the medial superior olive. Measures of optical densities showed that auditory neurons involved in sound localization had the highest calretinin labeling levels. Calbindin D-28k was present in cartwheel cells of the dorsal cochlear nucleus, in almost all neurons of the medial nucleus of the trapezoid body, and in globular cells in the ventral nucleus of the lateral lemniscus. The labeling patterns for calretinin and calbindin D-28k were non-overlapping throughout the auditory brainstem. This was also evident in the ventral nucleus of the lateral lemniscus where calbindin D-28k-immunoreactive terminals were found in the medial portion, while the calretinin-immunoreactive terminals were observed in the lateral portion. This study presents the first direct and comprehensive comparison of these three calcium-binding proteins in the auditory brainstem of a rodent. Each antibody yields a unique staining pattern that provides a basis for further defining neuronal populations. In addition, since their axons are also selectively stained, auditory nuclei can further be compartmentalized based on different terminal fields. These immunoreactivities have provided clues to the complex structure of the auditory brainstem.

Cytochrome oxidase activity during acute focal ischaemia in rat brain. A pathophysiology of acute focal ischaemia: Part 2

An enzyme-histochemical technique was used to examine the changes in cytochrome oxidase activity during acute focal ischaemia in the rat. In the somatosensory cortex, the enzyme activity began to increase significantly (p < 0.01) 1 hour after middle cerebral artery occlusion (MCAO) and continued to increase up to 3 hours, during which ischaemic cell damage was not detected. In the striatum, the enzyme activity increased significantly (p < 0.01) 1 hour after MCAO in the absence of morphological evidence of ischaemic cell damage; a peak activity was reached at 2 hours, and began to decline 3 hours after MCAO when moderate ischaemic change was detected. In both cortical and subcortical areas, the enzyme activity tended to decrease from 4 hours after MCAO, and was reduced to a level similar to or below that of the non-ischaemic hemisphere 5 hours after MCAO, when severe ischaemic damage was demonstrated. The relation of this transient increase of cytochrome oxidase activity in the early stage of acute ischaemia and the hypermetabolism of neuronal cells during ischaemic insult was discussed.

Does age at cochlear implantation affect the distribution of 2-deoxyglucose label in cat inferior colliculus?

Cochlear implants are one treatment for children who are born deaf or become deaf before acquiring language. The question of optimum age for implantation arises. Using an animal model, we have studied the response of the auditory brainstem to implantation at various ages. Neonatally, pharmacologically deafened cats were implanted with a 4-electrode array in the left cochlea at ages from 100 to over 180 days. Eleven were chronically stimulated (1000 h if possible) with charge-balanced, biphasic current pulses; eight were unstimulated controls. In a terminal experiment, each animal received [14C]2-deoxyglucose i.v. preceding a 45-min stimulation program. The fraction of the right inferior colliculus (IC) with a significant accumulation of label was calculated. If age at implantation were a significant factor in determining the size of the responding region, the fraction would depend on the age; this was not observed. However, there was considerable variation in the IC fraction sizes within both stimulated and unstimulated groups, leading to the conclusion that there are factors other than age which determine the size of the responding region. Thus, for deaf children of corresponding ages, age at implantation may not be of critical importance.

On the specificity of neurons and visual areas

The dominant view during the past 40 years has been that the visual system analyzes the visual scene by breaking it down into basic attributes such as color, form, motion, depth and texture. Individual dedicated neurons and specific visual areas were believed to be devoted to the analysis of each of these attributes. Current research has challenged these views by emphasizing that neurons, especially in the cortex, have multifunctional properties and therefore serve as general-purpose analyzers rather than feature detectors. Consequently, it appears that most extrastriate visual areas, rather than each being devoted to the analysis of a specific basic visual attribute, perform several different tasks and thereby engage in more advanced and complex analyses than had been realized.

Metabolic activity of the basal ganglia in parkinsonian syndromes in human and non-human primates: a cytochrome oxidase histochemistry study

In order to examine the consequences of nigrostriatal denervation on metabolic and functional activity of the basal ganglia, we analysed the distribution of cytochrome oxidase, a metabolic marker for neuronal functional activity, throughout the different basal ganglia structures in parkinsonian syndromes. The study was performed using enzyme histochemistry and densitometric measurements in patients with Parkinson's disease and in monkeys rendered parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrydine (MPTP) intoxication. In MPTP-intoxicated monkeys compared to control animals, enzyme activity was significantly increased in the subthalamic nucleus and in the output nuclei of the basal ganglia, e.g. the internal segment of the globus pallidus and the substantia nigra pars reticulata, but remained unchanged in the external segment of the globus pallidus and the striatum. L-DOPA treatment reversed the increased enzyme activity in all of the affected structures studied. In contrast, in parkinsonian patients, who had all been chronically treated with L-DOPA, no changes in enzyme activity were detected compared to control subjects. The results in MPTP-intoxicated monkeys are in agreement with the accepted model of basal ganglia organization, in which the output nuclei of the basal ganglia are considered to be overactive after nigrostriatal denervation, partly due to increased activity of excitatory afferents from the subthalamic nucleus. Since the increased enzyme activity in MPTP-intoxicated monkeys was reversed by L-DOPA therapy, the unchanged cytochrome oxidase activity observed in parkinsonian patients might result from L-DOPA treatment, combined with the chronicity of nigrostriatal denervation.

Regional brain effects of sodium azide treatment on cytochrome oxidase activity: a quantitative histochemical study

The objective of the present study was to determine if regional variation in brain cytochrome oxidase activity was observed following systemic administration of sodium azide. An image analysis system calibrated with internal standards of known cytochrome oxidase activity was used to quantify cytochrome oxidase in histochemically stained brain sections. Rats receiving chronic infusion of sodium azide (400 micrograms/hr), which were sacrificed after two weeks, showed a substantial decrease in brain cytochrome oxidase activity over those infused with saline. All of the 22 regions sampled from telencephalic, diencephalic, and mesencephalic levels, showed a significant activity reduction which ranged between 26% and 37%. The regions that appeared significantly more vulnerable to the sodium azide effects were the mesencephalic reticular formation and the central amygdala, which displayed the largest decrease in activity. In addition, interregional correlations of activity showed a deeply modified pattern of correlative metabolic activity between hippocampal, amygdaloid and cortical areas after azide treatment. The regional effects found were consistent with azide-induced learning and memory dysfunctions.

Development of parvalbumin immunoreactivity in the chick Edinger Westphal nucleus

To determine when the calcium-binding protein parvalbumin appears during development, neurons in the chick Edinger Westphal nucleus were examined for parvalbumin immunoreactivity at a variety of embryonic stages. Parvalbumin immunoreactivity appeared on embryonic day 14 (E14, Hamburger and Hamilton stage 40) in predominantly lateral Edinger Westphal neurons. Cytochrome oxidase activity within the nucleus was examined throughout development, as an indicator of physiological activity, and expression of cytochrome oxidase was compared with that of parvalbumin. Cytochrome oxidase activity was found to be uniformly high in all parts of the Edinger Westphal nucleus throughout development. Either the Edinger Westphal nucleus in physiologically active quite early in its development or other energy demands mask the correlation of cytochrome oxidase with electrical activity. Cytochrome oxidase was expressed well before parvalbumin immunoreactivity appeared. Voltage-activated calcium currents were characterized in E12 Edinger Westphal neurons. In both amplitude and composition, E12 calcium currents resemble those of E16 neurons, excluding the possibility that calcium currents appear de novo during or just prior to the appearance of parvalbumin. Both cytochrome oxidase activity and calcium currents are observed in Edinger Westphal neurons well before the appearance of parvalbumin during development. These findings do not exclude the possibility that physiological activity affects the expression of parvalbumin since other factors such as changing patterns of synaptic activity or the appearance of calcium conducting NMDA receptors have yet to be examined. However, they raise the possibility that additional factors such as an intrinsic developmental program or a change in the neuron's basal intracellular calcium requirements may also be involved.

Disproportionate regulation of nuclear- and mitochondrial-encoded cytochrome oxidase subunit proteins by functional activity in neurons

Cytochrome oxidase is the terminal enzyme in the mitochondrial respiratory chain engaged in oxidative metabolism and energy production. In mammals, the holoenzyme is composed of 13 subunits encoded by both nuclear and mitochondrial genomes. The goal of the present study was to compare the effect of afferent impulse blockade on the expression of these two genomes at the subunit protein level. It also aimed to determine the correlation between the level of cytochrome oxidase activity and the relative amount of subunit proteins. Relative enzyme activity was analysed histochemically, and relative amounts of subunits IV (nuclear-encoded) and II/III (mitochondrial-derived) proteins were obtained immunohistochemically by anti-subunit IV and anti-subunit II/III antibodies in the lateral geniculate nucleus and the primary visual cortex of adult monkeys. In the normal visual centers, similar staining patterns were found for all three markers. After three and seven days of tetrodotoxin treatment, levels of enzyme activity and subunit proteins declined disproportionately in the deprived laminae of the visual center. Densitometric analysis indicates that changes in enzyme activity and subunit IV proteins were significantly greater than those of subunit II/III proteins (P < 0.01). The finding that nuclear and mitochondrial genomes are disproportionately regulated at subunit protein levels by neuronal activity implies that the two genomes operate under different regulatory mechanisms. Changes in subunit IV paralleled most closely those of cytochrome oxidase activity (coefficient of determination r2 = 0.95). This suggests that nuclear-derived subunit IV protein may play a pivotal role in controlling cytochrome oxidase holoenzyme activity.

Changes in regional cytochrome oxidase activity in the fetal, newborn and adult ovine brainstem

Metabolic activity of specific brain regions (e.g. brainstem respiratory centers) may increase during the physiologic adaptations at birth. Since regional activity of cytochrome oxidase is correlated with the level of oxidative metabolism, cytochrome oxidase histochemistry was used to investigate whether there are sustained changes in metabolic activity within specific nuclei of the ovine brainstem during the perinatal period and whether further changes occur in the adult. Histochemistry was performed on 10-microns-thick frozen sections of the perinatal (130 d fetus, 140 d fetus, 8 h newborn and 10 d newborn) and adult ovine brainstem (n = 3 at each age). Computer-assisted image analysis was performed on 20 brainstem regions. A general decreasing trend, interrupted by a tendency for a transient increase at 8 h after birth was observed in most regions analyzed. Statistically significant decreases (P < 0.05) in cytochrome oxidase levels between the perinatal age groups and the adult were found in 7 brainstem nuclei studied: ambiguus, cuneate, inferior olivary, reticularis lateralis, spinal trigeminal, parabrachial and superior olivary nuclei. Within the perinatal period, the nucleus gracilis was the only region to show statistically significant decreases in 140 d fetus and 8 h newborns in all nuclei analyzed, but this change was not statistically significant (P > 0.1). These results indicate that the dramatic changes in physiology and environment at birth do not result in a significant change in the metabolic capacity of brainstem nuclei in the immediate perinatal period. However, more gradual developmental changes are observed in specific brainstem nuclei suggesting a decrease in neuronal activity occurs in these areas during development in the sheep.

Functional alterations in neural circuits in Alzheimer's disease

While a correlation exists at the regional level between the distribution of neurofibrillary tangles and the predicted sites of brain dysfunction based on clinical and functional neuroimaging studies, the relationship between neurofibrillary tangles and neuronal dysfunction is poorly understood. Using cytochrome oxidase activity as a marker of neuronal functional activity, we found reductions in metabolic activity both in a hippocampal subfield with a high density of neurofibrillary tangles (CA1) as well as in subfields relatively spared (CA3, dentate granule cells). In contrast, we found no reduction in activity in primary visual cortex. Using in situ hybridization, we found a selective reduction in a mitochondrial-encoded cytochrome oxidase mRNA transcript with sparing of a nuclear-encoded transcript. These results suggest that the reduction in cytochrome oxidase activity in Alzheimer's disease brain may be related to an alteration in mitochondrial gene expression. The absence of a direct correlation between structural pathology and cytochrome oxidase activity suggests that neurons that remain structurally intact in Alzheimer's disease may nonetheless undergo substantial changes in metabolic activity.

Regional distributions of hippocampal Na+,K(+)-ATPase, cytochrome oxidase, and total protein in temporal lobe epilepsy

Na+,K(+)-ATPase (the sodium pump) is a ubiquitous enzyme that consumes ATP to maintain an adequate neuronal transmembrane electrical potential necessary for brain function and to dissipate ionic transients. Reductions in sodium pump function augment the sensitivity of neurons to glutamate, increasing excitability and neuronal damage in vitro. Temporal lobe epilepsy (TLE) is one disease characterized by hyperexcitability and marked hippocampal neuronal losses that could depend in part, on impaired sodium pump capacity secondary to changes in sodium pump levels and/or insufficient ATP supply. To assess whether abnormalities in the sodium pump occur in this disease, we used [3H]ouabain to determine the density of Na+,K(+)-ATPase for each anatomic region of hippocampus by in vitro autoradiography. Tissues were surgically obtained from epileptic patients with hippocampal sclerosis and compared with specimens from patients with seizures originating from temporal lobe tumors and autopsy controls. Changes in cellular population arising from neuronal losses or gliosis were assessed by protein densities derived from quantitative computerized densitometry of Coomassie-stained tissue sections. We estimated regional differences in capacity for ATP generation by determining cytochrome c oxidase (CO) activity. Principal neurons of hippocampus exhibit high levels of sodium pump enzyme. Both epilepsy groups exhibited slight but significant increases in sodium pump density/unit mass of protein in the dentate molecular layer, CA2, and subiculum as compared with autopsy controls. Greater hilar sodium pump density was also observed in sclerotic hippocampi. In contrast, CO activity was reduced in both epilepsy types throughout hippocampus. Results suggest that although sodium pump protein in surviving neurons appears to be upregulated in epilepsy, sodium pump capacity may be limited by the reduced levels of CO activity. Functional reduction in sodium pump capacity may be an important factor in hyperexcitability and neuronal death.