Short exposure to an enriched environment accelerates plasticity in the barrel cortex of adult rats.

Cortical sensory neurons adapt their response properties to use and disuse of peripheral receptors in their receptive field. Changes in synaptic strength can be generated in cortex by simply altering the balance of input activity, so that a persistent bias in activity levels modifies cortical receptive field properties. Such activity-dependent plasticity in cortical cell responses occurs in rat cortex when all but two whiskers are trimmed for a period of time at any age. The up-regulation of evoked responses to the intact whiskers is first seen within 24 h in the supragranular layers [Laminar comparison of somatosensory cortical plasticity. Science 265(5180):1885-1888] and continues until a new stable state is achieved [Experience-dependent plasticity in adult rat barrel cortex. Proc Natl Acad Sci U S A 90(5):2082-2086; Armstrong-James M, Diamond ME, Ebner FF (1994) An innocuous bias in whisker use in adult rat modifies receptive fields of barrel cortex neurons. J Neurosci 14:6978-6991]. These and many other results suggest that activity-dependent changes in cortical cell responses have an accumulation threshold that can be achieved more quickly by increasing the spike rate arising from the active region of the receptive field. Here we test the hypothesis that the rate of neuronal response change can be accelerated by placing the animals in a high activity environment after whisker trimming. Test stimuli reveal an highly significant receptive field bias in response to intact and trimmed whiskers in layer IV as well as in layers II-III neurons in only 15 h after whisker trimming. Layer IV barrel cells fail to show plasticity after 15-24 h in a standard cage environment, but produce a response bias when activity is elevated by the enriched environment. We conclude that elevated activity achieves the threshold for response modification more quickly, and this, in turn, accelerates the rate of receptive field plasticity.

Cortical activation during Braille reading is influenced by early visual experience in subjects with severe visual disability: a correlational fMRI study.

Functional magnetic resonance imaging was performed on blind adults resting and reading Braille. The strongest activation was found in primary somatic sensory/motor cortex on both cortical hemispheres. Additional foci of activation were situated in the parietal, temporal, and occipital lobes where visual information is processed in sighted persons. The regions were differentiated most in the correlation of their time courses of activation with resting and reading. Differences in magnitude and expanse of activation were substantially less significant. Among the traditionally visual areas, the strength of correlation was greatest in posterior parietal cortex and moderate in occipitotemporal, lateral occipital, and primary visual cortex. It was low in secondary visual cortex as well as in dorsal and ventral inferior temporal cortex and posterior middle temporal cortex. Visual experience increased the strength of correlation in all regions except dorsal inferior temporal and posterior parietal cortex. The greatest statistically significant increase, i.e., approximately 30%, was in ventral inferior temporal and posterior middle temporal cortex. In these regions, words are analyzed semantically, which may be facilitated by visual experience. In contrast, visual experience resulted in a slight, insignificant diminution of the strength of correlation in dorsal inferior temporal cortex where language is analyzed phonetically. These findings affirm that posterior temporal regions are engaged in the processing of written language. Moreover, they suggest that this function is modified by early visual experience. Furthermore, visual experience significantly strengthened the correlation of activation and Braille reading in occipital regions traditionally involved in the processing of visual features and object recognition suggesting a role for visual imagery.

Theory for normal and impaired experience-dependent plasticity in neocortex of adult rats.

We model experience-dependent plasticity in the cortical representation of whiskers (the barrel cortex) in normal adult rats, and in adult rats that were prenatally exposed to alcohol. Prenatal exposure to alcohol (PAE) caused marked deficits in experience-dependent plasticity in a cortical barrel-column. Cortical plasticity was induced by trimming all whiskers on one side of the face except two. This manipulation produces high activity from the intact whiskers that contrasts with low activity from the cut whiskers while avoiding any nerve damage. By a computational model, we show that the evolution of neuronal responses in a single barrel-column after this sensory bias is consistent with the synaptic modifications that follow the rules of the Bienenstock, Cooper, and Munro (BCM) theory. The BCM theory postulates that a neuron possesses a moving synaptic modification threshold, theta(M), that dictates whether the neuron's activity at any given instant will lead to strengthening or weakening of its input synapses. The current value of theta(M) changes proportionally to the square of the neuron's activity averaged over some recent past. In the model of alcohol impaired cortex, the effective theta(M) has been set to a level unattainable by the depressed levels of cortical activity leading to "impaired" synaptic plasticity that is consistent with experimental findings. Based on experimental and computational results, we discuss how elevated theta(M) may be related to (i) reduced levels of neurotransmitters modulating plasticity, (ii) abnormally low expression of N-methyl-d-aspartate receptors (NMDARs), and (iii) the membrane translocation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in adult rat cortex subjected to prenatal alcohol exposure.

Enhancement of cortical plasticity by behavioral training in acetylcholine-depleted adult rats.

Trimming all whiskers except two on one side of an adult rat's face results in cortical plasticity in which the spared whiskers, D2 and one D-row surround whisker (either D1 or D3), evoked responses containing more spikes than the response evoked by the cut whisker (called whisker pairing plasticity). Previously we have reported that acetylcholine (ACh) depletion in cortex prevents surround D-row whisker plasticity from developing within the barrel cortex. In this study we examined whether the animal's active use of its two intact whiskers can restore some aspects of plasticity in the ACh-depleted cortex. To achieve this goal, ACh was depleted from barrel field cortex, and 14 days after the depletion surgery, whiskers were trimmed and animals were trained on a whisker-dependent gap crossing task. After 7 days of training, animals were anesthetized with urethan and prepared for single-unit recording. Training the ACh-depleted, whisker-paired animals resulted in a significant enhancement of responses to paired surround whiskers: the D-paired whisker-evoked response contained more spikes than the D-cut evoked response. We conclude that training whisker paired rats has a positive impact on response properties of neurons in S1 cortex, even in ACh-depleted animals.

Direct inhibition evoked by whisker stimulation in somatic sensory (SI) barrel field cortex of the awake rat.

Whisker deflection typically evokes a transient volley of action potentials in rat somatic sensory (SI) barrel cortex. Postexcitatory inhibition is thought to quickly terminate the cortical cell response to whisker deflection. Using dual electrode extracellular recording in awake rats, we describe an infrequent type of cell response in which stimulation of single hairs consistently blocks the ongoing discharge of neurons without prior excitation (I-only inhibition). Reconstruction of the recording sites indicates that I-only inhibition occurs most frequently when the recording site is clearly in the septum or at the barrel-septum junction. The same cells that respond with I-only inhibition to one whisker can show an excitatory discharge to other whiskers, usually followed by inhibition. Stimulation of either nose hairs or the large mystacial vibrissa can evoke I-only inhibition in SI cortex. I-only inhibition is most commonly observed at low stimulus frequencies ( approximately 1 Hz). At stimulus frequencies of >6 Hz, I-only inhibition typically converts to excitation. We conclude that single whisker low-frequency stimulation can selectively block the spontaneous discharge of neurons in SI barrel field septa. The observation that this cell response is found most often in or at the edge of septa and at relatively long latencies supports the idea that I-only inhibition is mediated through cortical circuits. We propose that in these cells inhibition alone or a combination of inhibition and disfacilitation play a role in suppressing neuronal discharge occasioned by low frequency contact of the whiskers with the environment.

Effect of enriched environment rearing on impairments in cortical excitability and plasticity after prenatal alcohol exposure.

The daily ingestion of alcohol by pregnant mammals exposes the fetal brain to varying levels of alcohol through the placental circulation. Here we focus on the lingering impact on cortical function of 6.5% alcohol administered in a liquid diet to pregnant rats throughout gestation, followed by 3 alcohol-free months before brain function was analyzed in the offspring. Both spontaneous activity of the neurons in the barrel cortex and the level of response to test stimuli applied to the whiskers remained reduced by >75% after alcohol exposure. Whisker pairing, a type of cortical plasticity induced by trimming all but two whiskers in adult rats, occurred in <1 d in controls, but required 14 d to reach significance after alcohol exposure. These long-term neuronal deficits are present in all layers of cortex and affect neurons with both fast and slow action potentials. Plasticity is first seen in the total sample of neurons at 14 d; however, by 7 d, neurons in layer II/III already show plasticity, with no change in layer IV neurons, and a reverse shift occurs toward the inactive whisker in layer V neurons. Analysis of NMDA receptor subunits shows a persistent, approximately 30-50% reduction of NR1, NR2A, and NR2B subunits at postnatal day 90 in the barrel field cortex. Exposing the prenatal alcohol-exposed rats to enriched rearing conditions significantly improves all measured cortical functions but does not restore normal values. The results predict that combinations of interventions will be necessary to completely restore cortical function after exposure of the fetal brain to alcohol.

Differential cellular and subcellular localization of protein phosphatase 1 isoforms in brain.

Protein phosphatase 1 (PP1) is a gene family with a number of important functions in brain. Association with a wide variety of regulatory/targeting subunits is thought to be instrumental in directing the phosphatase to specific subcellular locations and substrates. By using antibodies directed against specific PP1 isoforms, we asked whether PP1 isoforms are differentially distributed in brain. Immunoblotting detects in brain the PP1gamma2 isoform, which had previously been thought to be testis specific, in addition to alpha, beta, and gamma1 isoforms. PP1 isoform expression varies modestly in extracts from different subdissected brain regions and is relatively constant during postnatal development, except for an about twofold increase in PP1gamma2. By immunohistochemical analyses of rat brain, PP1beta and PP1gamma1 cellular expression is widespread but quite distinct from one another. Subcellular fractionation studies demonstrate that PP1beta and PP1gamma1 are selectively associated with different cytoskeletal elements: PP1beta with microtubules, PP1gamma1 with the actin cytoskeleton. Double-immunofluorescence labeling of cultured cortical neurons further reveals a strikingly different and nonoverlapping localization of PP1beta and PP1gamma1: whereas PP1beta localizes to a discrete area of the soma, PP1gamma1 is highly enriched in dendritic spines and presynaptic terminals of cultured neurons. These results show that PP1 isoforms are targeted to different neuronal cytoskeletal compartments with a high degree of specificity, presumably by isoform-specific association with regulatory/targeting proteins. Furthermore, the synaptic localization of PP1gamma1 indicates that it is this isoform that is involved in the regulation of synaptic phosphoproteins such as neurotransmitter receptors and ion channels implicated in synaptic plasticity.

Barrels and septa: separate circuits in rat barrels field cortex.

The neural circuitry within sensory cortex determines its functional properties, and different solutions have evolved for integrating the activity that arises from an array of sensory inputs to cortex. In rodent, circumscribed receptors, such as whiskers, are represented in somatic sensory (S-I) cortex in islands of cells in layer IV called "barrels" surrounded by narrow channels that separate barrels called "septa." These two cortical domains were previously shown to receive sensory inputs through parallel subcortical pathways. Here, by using small biocytin injections, we demonstrate that distinct intrinsic and corticocortical circuitries arise from barrel and septal columns. The intracortical S-I projections originating from barrel columns are rather short-ranged, terminating for the most part within the far boundaries of the most immediate neighboring barrel columns, whereas corticocortical projections reach the second somatosensory (S-II) cortex. In contrast, the intrinsic projections arising from septal columns extend two to three barrels' distance along the row of whisker representation, producing terminals preferentially in other septal columns. Septal corticocortical projections terminate in the dysgranular cortex anterior to E-row barrels and in the posteromedial parietal cortex in addition to S-II. Whereas layer IV barrels are largely isolated from lateral connections, septa are the main conduits of intracortical projections arising from neighboring barrel and septal columns. These results indicate that the two subcortical pathways from whiskers to cortex continue as two distinct partially segregated pathways in cortex.

Modulation of receptive field properties of thalamic somatosensory neurons by the depth of anesthesia.

Modulation of receptive field properties of thalamic somatosensory neurons by the depth of anesthesia. The dominant frequency of electrocorticographic (ECoG) recordings was used to determine the depth of halothane or urethan anesthesia while recording extracellular single-unit responses from thalamic ventral posterior medial (VPM) neurons. A piezoelectric stimulator was used to deflect individual whiskers to assess the peak onset latency, magnitude, probability of response, and receptive field (RF) size. There was a predictable increase in the dominant ECoG frequency from deep stage IV to light stage III-1 anesthetic levels. There was no detectable frequency at stage IV, a 1- to 2-Hz dominant frequency at stage III-4, 3-4 Hz at stage III-3, 5-7 Hz at stage III-2, and a dual 6- and 10- to 13-Hz pattern at stage III-1. Reflexes and other physical signs showed a correlation with depth of anesthesia but exhibited too much overlap between stages to be used as a criterion for any single stage. RF size and peak onset latency of VPM neurons to whisker stimulations increased between stage III-4 and III-1. A dramatic increase in RF size and response latency occurred at the transition from stage III-3 (RF size approximately 2 whiskers, latency approximately 7 ms) to stage III-2 (RF size approximately 6 whiskers, latency approximately 11 ms). Response probability and magnitude decreased from stage III-4 to stage III-3 and III-2. No responses were ever evoked in VPM cells by vibrissa movement at stage IV. These changes in VPM responses as a function of anesthetic depth were seen only when the nucleus principalis (PrV) and nucleus interpolaris (SpVi) trigeminothalamic pathways were both intact. Eliminating SpVi inputs to VPM, either by cutting the primary trigeminal afferent fibers to SpVi or cutting axons projecting from SpVi to VPM, immediately reduced the RF size to fewer than three whiskers. In addition, the predictable changes in VPM response probability, response magnitude, and peak onset latency at different anesthetic depths were all absent after SpVi pathway interruption. We conclude that 1) the PrV input mediates the near "one-to-one" correspondence between a neuronal response in VPM and a single mystacial whisker, 2) in contrast, the SpVi input to VPM is primarily responsible for the RF properties of VPM neurons at light levels of anesthesia and presumably in the awake animal, and 3) alterations in VPM responses produced by changing the depth of anesthesia are due to its selective influence on the properties mediated by SpVi inputs at the level of the thalamus.

Computational study of experience-dependent plasticity in adult rat cortical barrel-column.

We model experience-dependent plasticity in the adult rat S1 cortical representation of the whiskers (the barrel cortex) which has been produced by trimming all whiskers on one side of the snout except two. This manipulation alters the pattern of afferent sensory activity while avoiding any direct nerve damage. Our simplified model circuitry represents multiple cortical layers and inhibitory neurons within each layer of a barrel-column. Utilizing a computational model we show that the evolution of the response bias in the barrel-column towards spared whiskers is consistent with synaptic modifications that follow the rules of the Bienenstock, Cooper and Munro (BCM) theory. The BCM theory postulates that a neuron possesses a dynamic synaptic modification threshold, thetaM, which dictates whether the neuron's activity at any given instant will lead to strengthening or weakening of the synapses impinging on it. However, the major prediction of our model is the explanation of the delay in response potentiation in the layer-IV neurons through a masking effect produced by the thresholded monotonically increasing inhibition expressed by either the logarithmic function, h(x) = mu log(1 + x), or by the power function, h(x) = mu x(0.8-0.9), where mu is a constant. Furthermore, simulated removal of the supragranular layers (layers II/III) reduces plasticity of neurons in the remaining layers (IV-VI) and points to the role of noise in synaptic plasticity.

Contribution of supragranular layers to sensory processing and plasticity in adult rat barrel cortex.

Contribution of supragranular layers to sensory processing and plasticity in adult rat barrel cortex. J. Neurophysiol. 80: 3261-3271, 1998. In mature rat primary somatic sensory cortical area (SI) barrel field cortex, the thalamic-recipient granular layer IV neurons project especially densely to layers I, II, III, and IV. A prior study showed that cells in the supragranular layers are the fastest to change their response properties to novel changes in sensory inputs. Here we examine the effect of removing supragranular circuitry on the responsiveness and synaptic plasticity of cells in the remaining layers. To remove the layer II + III (supragranular) neurons from the circuitry of barrel field cortex, N-methyl--aspartate (NMDA) was applied to the exposed dura over the barrel cortex, which destroys those neurons by excitotoxicity without detectable damage to blood vessels or axons of passage. Fifteen days after NMDA treatment, the first responsive cells encountered were 400-430 micrometers below the pial surface. In separate cases triphenyltetrazolium chloride (TTC), a vital dye taken up by living cells, was absent from the lesion area. Cytochrome oxidase (CO) activity was absent in the first few tangential sections through the barrel field in all cases before arriving at the CO-dense barrel domains. These findings indicate that the lesions were quite consistent from animal to animal. Controls consisted of applying vehicle without NMDA under similar conditions. Responses of D2 barrel cells were assessed for spontaneous activity and level of response to stimulation of the principal D2 whisker and four surround whiskers D1, D3, C2, and E2. In two additional groups of animals treated in the same way, sensory plasticity was assessed by trimming all whiskers except D2 and either D1 or D3 (called Dpaired) for 7 days before recording cortical responses. Such whisker pairing normally potentiates D2 barrel cell responses to stimulation of the two intact whiskers (D2 + Dpaired). After NMDA lesions, cortical cells still responded to all whiskers tested. Cells in lesioned cortex showed reduced response amplitude compared with sham-operated controls to all D-row whiskers. In-arc surround whisker (C2 or E2) responses were normal. Spontaneous activity did not change significantly in any remaining layer at the time tested. Modal latencies to stimulation of principal D2 or surround D1 or D3 whiskers showed no significant change after lesioning. These findings indicate that there is a reasonable preservation of the response properties of layer IV, V, VI neurons after removal of layer II-III neurons in this way. Whisker pairing plasticity in layer IV-VI D2 barrel column neurons occurred in both lesioned and sham animals but was reduced significantly in lesioned animals compared with controls. The response bias generated by whisker trimming (Dpaired/Dcut + Dpaired ratio) was less pronounced in NMDA-lesioned than sham-lesioned animals. Proportionately fewer neurons in layer IV (52 vs. 64%) and in the infragranular layers (55 vs. 68%) exhibited a clear response bias to paired whiskers. We conclude that receptive-field plasticity can occur in layers IV-VI of barrel cortex in the absence of the supragranular layer circuitry. However, layer I-III circuitry does play a role in normal receptive-field generation and is required for the full expression of whisker pairing plasticity in granular and infragranular layer cells.

Experience-dependent plasticity of adult rat S1 cortex requires local NMDA receptor activation.

The effect of blocking NMDA glutamate receptors in adult rat cortex on experience-dependent synaptic plasticity of barrel cortex neurons was studied by infusing D-AP5 with an osmotic minipump over barrel cortex for 5 d of novel sensory experience. In acute pilot studies, 500 microM D-AP5 was shown to specifically suppress NMDA receptor (NMDAR)-dependent responses of single cells in cortical layers I-IV. To induce plasticity, all whiskers except D2 and D1 were cut close to the face 1 d after pump insertion. The animals were housed with 2 cage mates before recording 4 d later. This pairing of two whiskers for several days in awake animals generates highly significant biases in responses from D2 layer IV (barrel) cells to the intact D1 whisker as opposed to the cut D3 whisker. D-AP5 completely prevented the D1/D3 surround whisker bias from occurring in the D2 barrel cells (p > 0.6 for D1 > D3, Wilcoxon). Fast-spike and slow-spike barrel cells were affected equally, suggesting parity for inhibitory and excitatory cell plasticity. D-AP5 only partially suppressed the D1/D3 bias in supragranular layers (layers II-III) in the same penetrations (p < 0.042 for D1 > D3). In control animals, the inactive L-AP5 isomer allowed the bias to develop normally toward the intact surround whisker (p < 0.001 for D1 > D3) for cells in all layers. We conclude that experience-dependent synaptic plasticity of mature barrel cortex is cortically dependent and that modification of local cortical NMDARs is necessary for its expression.

Role of the basal forebrain cholinergic projection in somatosensory cortical plasticity.

Trimming all but two whiskers in adult rats produces a predictable change in cortical cell-evoked responses characterized by increased responsiveness to the two intact whiskers and decreased responsiveness to the trimmed whiskers. This type of synaptic plasticity in rat somatic sensory cortex, called "whisker pairing plasticity," first appears in cells above and below the layer IV barrels. These are also the cortical layers that receive the densest cholinergic inputs from the nucleus basalis. The present study assesses whether the cholinergic inputs to cortex have a role in regulating whisker pairing plasticity. To do this, cholinergic basal forebrain fibers were eliminated using an immunotoxin specific for these fibers. A monoclonal antibody to the low-affinity nerve growth factor receptor 192 IgG, conjugated to the cytotoxin saporin, was injected into cortex to eliminate cholinergic fibers in the barrel field. The immunotoxin reduces acetylcholine esterase (AChE)-positive fibers in S1 cortex by >90% by 3 wk after injection. Sham-depleted animals in which either saporin alone or saporin unconjugated to 192 IgG is injected into the cortex produces no decrease in AChE-positive fibers in cortex. Sham-depleted animals show the expected plasticity in barrel column neurons. In contrast, no plasticity develops in the ACh-depleted, 7-day whisker-paired animals. These results support the conclusion that the basal forebrain cholinergic projection to cortex is an important facilitator of synaptic plasticity in mature cortex.

Brain protein phosphatase 2A: developmental regulation and distinct cellular and subcellular localization by B subunits.

Protein phosphatase 2A (PP2A) is a heterotrimeric enzyme consisting of a catalytic subunit (C), a structural subunit (A), and a variable regulatory subunit (B). We have investigated the spatial and temporal expression patterns of three members of the B subunit family, Balpha, Bbeta, and Bgamma, both at the message level by using ribonuclease protection analysis and at the protein level by using specific antibodies. Although A, Balpha, and C protein are expressed in many tissues, Bbeta and Bgamma were detectable only in brain. Balpha, Bbeta, and Bgamma are components of the brain PP2A heterotrimer, because they copurified with A and C subunits on immobilized microcystin. Whereas Balpha and Bbeta are mainly cytosolic, Bgamma is enriched in the cytoskeletal fraction. In contrast to A, C, and Balpha, which are expressed at constant levels, Bbeta and Bgamma RNA and protein are developmentally regulated, with Bbeta levels decreasing and Bgamma levels increasing sharply after birth. RNA and immunoblot analyses of subdissected brain regions as well as immunohistochemistry demonstrated that B subunits are expressed in distinct but overlapping neuronal populations and cellular domains. These data indicate that B subunits confer tissue and cell specificity, subcellular localization, and developmental regulation to the PP2A holoenzyme. The Balpha-containing heterotrimer may be important in general neuronal functions that involve its partially nuclear localization. Holoenzymes containing B likely function in early brain development as well as in somata and processes of subsets of mature neurons. Bgamma may target PP2A to cytoskeletal substrates that are important in the establishment and maintenance of neuronal connections.

Protein serine/threonine phosphatase 1 and 2A associate with and dephosphorylate neurofilaments.

The phosphorylation state of neurofilaments plays an important role in the control of cytoskeletal integrity, axonal transport, and axon diameter. Immunocytochemical analyses of spinal cord revealed axonal localization of all protein phosphatase subunits. To determine whether protein phosphatases associate with axonal neurofilaments, neurofilament proteins were isolated from bovine spinal cord white matter by gel filtration. approximately 15% of the total phosphorylase a phosphatase activity was present in the neurofilament fraction. The catalytic subunits of PP1 and PP2A, as well as the A and B alpha regulatory subunits of PP2A, were detected in the neurofilament fraction by immunoblotting, whereas PP2B and PP2C were found exclusively in the low molecular weight soluble fractions. PP1 and PP2A subunits could be partially dissociated from neurofilaments by high salt but not by phosphatase inhibitors, indicating that the interaction does not involve the catalytic site. In both neurofilament and soluble fractions, 75% of the phosphatase activity towards exogenous phosphorylase a could be attributed to PP2A, and the remainder to PP1 as shown with specific inhibitors. Neurofilament proteins were phosphorylated in vitro by associated protein kinases which appeared to include protein kinase A, calcium/calmodulin-dependent protein kinase, and heparin-sensitive and -insensitive cofactor-independent kinases. Dephosphorylation of phosphorylated neurofilament subunits was mainly (60%) catalyzed by associated PP2A, with PP1 contributing minor activity (10-20%). These studies suggest that neurofilament-associated PP1 and PP2A play an important role in the regulation of neurofilament phosphorylation.

Selenoprotein P associates with endothelial cells in rat tissues.

Selenoprotein P is an extracellular heparin-binding protein that has been implicated in protecting the liver against oxidant injury. Its location in liver, kidney, and brain was determined by conventional immunohistochemistry and confocal microscopy using a polyclonal antiserum. Selenoprotein P is associated with endothelial cells in the liver and is more abundant in central regions than in portal regions. It is also present in kidney glomeruli associated with capillary endothelial cells. Staining of selenoprotein P in the brain is also confined to vascular endothelial cells. The heparin-binding properties of selenoprotein P could be the basis for its binding to tissue. Its localization to the vicinity of endothelial cells is potentially relevant to its oxidant defense function.

Localization of the calcium/calmodulin-dependent protein phosphatase, calcineurin, in the hindbrain and spinal cord of the rat.

The calcium/calmodulin-dependent protein phosphatase calcineurin was localized at the light microscopic level in the rat hindbrain and spinal cord by using an antibody against the alpha-isoform of the catalytic subunit. Calcineurin was highly concentrated in axons, dendrites, and cell bodies of a subpopulation of alpha-motoneurons in hindbrain motor nuclei and the lateral motor column along the length of the spinal cord. These calcineurin-positive alpha-motoneurons appeared to be randomly distributed and represented approximately 25% of the total alpha-motoneuron pool in the motor trigeminal nucleus and the spinal cord lateral motor column. Within the facial nucleus, calcineurin-containing motoneurons were present in the medial and dorsal subdivision but not in the lateral and intermediate subdivision. In addition to the enrichment in motoneurons, calcineurin was enriched in cells of the superficial laminae of the spinal cord dorsal horn and its extension into the medulla, the caudal spinal trigeminal nucleus. Axonal staining in the white matter of the spinal cord was generally weak, except in the dorsolateral funiculus, where strongly calcineurin-positive axons formed a putative ascending tract that appeared to terminate uncrossed in the caudal lateral reticular nucleus of the medulla. This tract may originate from calcineurin-positive cells in the dorsolateral funiculus. We also compared the distribution of calcineurin with calcium/calmodulin-dependent kinase II in the spinal cord and found that the kinase is more widely expressed. Thus, calcineurin is highly restricted to a few locations in the hindbrain and spinal cord. Selective staining in facial subnuclei that innervate phasically active muscles suggests that calcineurin-positive motoneurons represent a subset of alpha-motoneurons innervating a metabolic subtype of muscle fibers, possibly fast-twitch fibers.

Postnatal changes in NMDAR1 subunit expression in the rat trigeminal pathway to barrel field cortex.

N-methyl-D-aspartate (NMDA) type glutamate receptors are constituted of one obligatory subunit (NR1), expressed as eight splice variants, combined with one or more of four NMDAR2 subunits. Polyclonal antibodies were produced to an N-terminal domain of the NR1 subunit that recognize all eight splice variants. The antibody was used to localize NR1 in the trigeminal pathway to barrel field cortex in rats. The distribution and density of NR1 changes between birth (postnatal day 0 = P-0) and P-360. The trigeminal nuclei already contain a high level of NR1 immunoreactivity on the day of birth. The ventral posterior lateral, ventral posterior medial, and posterior nucleus, medial division, thalamic nuclei show fluctuations in NR1 immunoreactivity levels, starting at birth with moderate densities in neuropil which decrease at P-7, and peak again in neuronal cell bodies as well as the neuropil at P-21. In the cortex, the density of NR1 in layer VI fluctuates with low points at P-7 and P-40. Superficial cortical layers I, II, and III reach adult levels at P-14 and remain high. NR1 levels decrease sharply in layer IV just prior to P-40 and then slowly recover over the next 3 months to stabilize at moderate levels in the adult. In addition to neuronal expression there is a transient high level of labeling in glial cells with a peak density of staining at P-21. The results emphasize that NR1 subunit expression is finely regulated in rat somatic sensory pathways for periods as long as 7-8 weeks after birth in the barrel field cortex.

An innocuous bias in whisker use in adult rats modifies receptive fields of barrel cortex neurons.

The effect of innocuously biasing the flow of sensory activity from the whiskers for periods of 3-30 d in awake, behaving adult rats on the receptive field organization of rat SI barrel cortex neurons was studied. One pair of adjacent whiskers, D2 and either D1 or D3, remained intact unilaterally (whisker pairing), all others being trimmed throughout the period of altered sensation. Receptive fields of single cells in the contralateral D2 barrel were analyzed under urethane anesthesia by peristimulus time histogram (PSTH) and latency histogram analysis after 3, 7-10, and 30 d of pairing and compared with controls, testing all whiskers cut to the same length. Response magnitudes to surround receptive field in-row whiskers D1 and D3 were not significantly different for control animals. The same was found for surround in-arc whiskers C2 and E2. However, after 3 d of whisker pairing a profound bias occurred in response to the paired D-row surround whisker relative to the opposite trimmed surround D-row whisker and to the C2 and E2 whiskers. This bias increased with the duration of pairing, regardless of which surround whisker (D1 or D3) was paired with D2. For all three periods of pairing the mean response to the paired surround whisker was increased relative to controls, but peaked at 7-10 d. Response to the principal center-receptive (D2) whisker was increased for the 3 and 7-10 d groups and then decreased at 30 d. Responses to trimmed arc surround whiskers (C2 and E2) were decreased in proportion to the duration of changed experience. Analysis of PSTH data showed that earliest discharges (5-10 msec poststimulus) to the D2 whisker increased progressively in magnitude with duration of pairing. For the paired surround whisker similar early discharges newly appeared after 30 d of pairing. At 3 and 7-10 d of pairing, increases in response to paired whiskers and decreases to cut surround whiskers were confined to late portions of the PSTH (10-100 msec poststimulus). Changes at 3-10 d can be attributed to alterations in intracortical synaptic relay between barrels. Longer-term changes in response to both paired whisker inputs (30 d) largely appear to reflect increases in thalamocortical synaptic efficacy. Our findings suggest that novel innocuous somatosensory experiences produce changes in the receptive field configuration of cortical cells that are consistent with Hebbian theories of experience-dependent potentiation and weakening of synaptic efficacy within SI neocortical circuitry, for correlated and uncorrelated sensory inputs, respectively.

Laminar comparison of somatosensory cortical plasticity.

During tactile learning there is a transformation in the way the primary somatosensory cortex integrates, represents, and distributes information from the skin. To define this transformation, the site of earliest modification has been identified in rat somatosensory cortex after a change in sensory experience. Afferent activity was manipulated by clipping all except two whiskers on one side of the snout ("whisker pairing"), and the receptive fields of neurons at different cortical depths were mapped 24 hours later. Neurons in layer IV, the target of the primary thalamic pathway, were unaltered, whereas neurons located above and below layer IV showed significant changes. These changes were similar to those that occur in layer IV after longer periods of whisker pairing. The findings support the hypothesis that the layers of cortex contribute differently to plasticity. Neurons in the supragranular and infragranular layers respond rapidly to changes in sensory experience and may contribute to subsequent modification in layer IV.