Synaptic proliferation in the auditory cortex of the young adult rat following callosal lesions

Callosal bisection and transcallosal secondary antiepileptogenesis

More than 28,000 neuroscientists and 3,000 epileptologists gathered at their respective 2001 meetings of the Society for Neuroscience and the American Epilepsy Society. Yet only six articles, one directly and five indirectly, discussed the corpus callosum (CC). Is not this in itself a remarkable finding? Are there no mysteries left? The reality is that considerable uncertainties exist regarding the rationale for callosal bisection (CCB) that causes contrasting effects (i.e., amelioration of generalized seizure, at times leading to freedom from seizure, and intensification of postoperatively fragmented seizure, at times leading to status epilepticus). Similarly, the clinical relevance of EEG mirror focus formation, an experimentally well-established transcallosal consequence of partial cortical epileptogenesis, continues to be debated. This presentation revisits these unresolved issues (a) to gain insight into the dynamic role played by the CC in medically refractory epilepsy, and (b) to promote the development of antiepileptogenic tools that are currently unavailable.

Thalamocortical patches in auditory neocortex

Thalamocortical afferents to the primary auditory cortex of the rabbit were labeled by the iontophoretic injection of the anterograde tracers PHA-L or biocytin into the ventral division of the medial geniculate body (vMGB). Single injections of either tracer into the vMGB labeled multiple "patches" of afferent axons in lamina III/IV of the ipsilateral auditory cortex. Serial section analysis revealed that single patches were elongated in the rostral-caudal axis forming bands of approximately 2 mm in length. The orientation of the bands was similar to the isofrequency contours of the tonotopic maps derived from prior electrophysiological experiments. Within the coronal plane, the topography of the patches is remarkably similar to the intermittent distribution of binaural interaction subclasses described in physiological studies. Our results are consistent with a model of vMGB organization containing functionally distinct, parallel anatomical pathways to AI.

Nucleus basalis stimulation facilitates thalamocortical synaptic transmission in the rat auditory cortex

Nucleus basalis (NB) neurons are a primary source of neocortical acetylcholine (ACh) and likely contribute to mechanisms of neocortical activation. However, the functions of neocortical activation and its cholinergic component remain unclear. To identify functional consequences of NB activity, we have studied the effects of NB stimulation on thalamocortical transmission. Here we report that tetanic NB stimulation facilitated field potentials, single neuron discharges, and monosynaptic excitatory postsynaptic potentials (EPSPs) elicited in middle to deep cortical layers of the rat auditory cortex following stimulation of the auditory thalamus (medial geniculate, MG). NB stimulation produced a twofold increase in the slope and amplitude of the evoked short-latency (onset 3.0 +/- 0.13 ms, peak 6.3 +/- 0.21 ms), negative-polarity cortical field potential and increased the probability and synchrony of MG-evoked unit discharge, without altering the preceding fiber volley. Intracortical application of atropine blocked the NB-mediated facilitation of field potentials, indicating action of ACh at cortical muscarinic receptors. Intracellular recordings revealed that the short-latency cortical field potential coincided with a short-latency EPSP (onset 3.3 +/- 0.20 ms, peak 5.6 +/- 0.47 ms). NB stimulation decreased the onset and peak latencies of the EPSP by about 20% and increased its amplitude by 26%. NB stimulation also produced slow membrane depolarization and sometimes reduced a long-lasting IPSP that followed the EPSP. The combined effects of NB stimulation served to increase cortical excitability and facilitate the ability of the EPSP to elicit action potentials. Taken together, these data indicate that NB cholinergic neurons can modify neocortical functions by facilitating thalamocortical synaptic transmission.

Long-term callosal lesions and learning of a black-white discrimination by one-eyed rats

We know from our previous studies that mature rats with monocular enucleation at birth (OEBs), as well as animals enucleated at maturity (OETs), were unable to learn a black-white discrimination when they were trained after lesions of the visual cortex contralateral to the remaining eye. Since it is well known that synaptic reorganization takes place in the adult rat brain through reactive synaptogenesis following deafferentation, we wondered if long-term callosal lesions in OEBs and OETs would bring out such synaptic reorganization in the visual cortex and, consequently, affect the outcome of the discrimination mentioned above. In the present study, two experiments were carried out: in Experiment 1 the previous experiment was replicated in that OEBs and OETs of 3 months of age were trained on the discrimination 10 days following unilateral visual cortex lesions; in Experiment 2, effects of callosal lesions made 10 weeks earlier either at 3 weeks of age or 13 weeks of age were investigated. The results were: 1) the findings of the previous experiment were confirmed; 2) the long-term callosal lesions facilitated the acquisition of discrimination in OEBs but not in OETs; 3) the facilitative effects were more prominent in OEBs with callosal lesions at 3 weeks of age than in those at 13 weeks of age. The findings were discussed in relation to possible synaptic reorganization produced in the visual cortex ipsilateral to the remaining eye following callosal lesions made 10 weeks earlier and also in relation to reorganization of the uncrossed visual pathways resulting from monocular enucleation at birth.

Cortical and striatal structure and connectivity are altered by neonatal hemidecortication in rats

The cortical cytoarchitecture, cortical thickness, corticostriatal connections, cortical dendritic arborization, and striatal patch-matrix compartmentalization were compared in rats with neonatal (1 day of age) or adult hemidecortication. Neonatal hemidecortication produced few changes in cytoarchitecture of the remaining hemisphere and did not preclude the development of a patch-matrix compartmentalization in either striatum. There was a significant modification of contralateral cortical-striatal connections, however, as there were extensive crossed connections from layer II/III of the prefrontal cortex in the neonatal hemidecorticates, which contrasts with connections from layer V in the normal brain. Adult hemidecorticates had no crossed corticostriatal connections. Neonatal hemidecortication also led to an increase in cortical thickness relative to adult operates or controls and the neonatal hemidecorticates, and led to an increase in dendritic arborization in layer II/III pyramidal cells of the somatosensory and motor cortex but not in the visual or temporal cortex. The results suggest that the behavioral sparing of sensorimotor and some prefrontal functions after neonatal hemidecortication could be supported, in part, by the anatomical changes in the prefrontal and sensorimotor connectivity and dendritic arborization.

Primary auditory cortex in the rat: transient expression of acetylcholinesterase activity in developing geniculocortical projections

A characteristic pattern of acetylcholinesterase (AChE) activity is expressed transiently in primary auditory cortex (cortical area 41) of developing laboratory rats during early postnatal life. This AChE activity occurs as a dense plexus in cortical layer IV and the deep part of layer III. This transient band of AChE activity is first detected by histochemical techniques on postnatal day (P) 3, reaches peak intensity at approximately P8-10, and declines to form the adult pattern by P23. The ventral nucleus of the medial geniculate body of the thalamus also displays prominent, and transient, staining for AChE. This intense staining for AChE, found within neuronal somata and neuropil, is detected at the time of birth, reaches peak intensity around P8, and declines to adult levels by P16. The areal and laminar patterns of the transient band of AChE activity in temporal cortex correspond to the patterns of anterograde transneuronal labeling of geniculocortical terminals following injection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the inferior colliculus. Placement of lesions that include the medial geniculate nucleus or the geniculocortical axons results in a marked decrease in AChE staining in thalamorecipient layers of auditory cortex. Placement of lesions that include the medial globus pallidus reduce AChE staining of some axons in temporal cortex of developing rats, but the dense band of AChE in layers III and IV remains. Placement of lesions in the inferior colliculus in newborn animals results in marked decrease in AChE staining in cells of the ipsilateral ventral medial geniculate nucleus and in ipsilateral auditory cortex of developing pups. These data indicate that transiently expressed AChE activity is characteristic of geniculocortical neurons, including their somata in the medial geniculate body and their terminal axons in primary auditory cortex. This AChE activity is expressed early in postnatal development, probably during the time when thalamocortical axons are proliferating in cortical layer IV and forming synaptic contacts with cortical neurons.

Synapses made by axons of callosal projection neurons in mouse somatosensory cortex: emphasis on intrinsic connections

This is one of a series of papers aimed at identifying the synaptic output patterns of the local and distant projections of subgroups of pyramidal neurons. The subgroups are defined by the target site to which their main axon projects. Pyramidal neurons in areas 1 and 40 of mouse cerebral cortex were labeled by the retrograde transport of horseradish peroxidase (HRP) transported from severed callosal axons in the contralateral hemisphere. Terminals of the local axon collaterals of these neurons ("intrinsic" terminals) were identified in somatosensory areas 1 and 40, and their distribution and synaptic connectivity were examined. Also examined were the synaptic connections of "extrinsic" callosal axon terminals labeled by lesion induced degeneration consequent to the severing of callosal fibers. A post-lesion survival time of 3 days was chosen because by this time the extrinsic terminals were all degenerating, whereas the intrinsic terminals were labeled by HRP. Both intrinsic and extrinsic callosal axon terminals occurred in all layers of the cortex where they formed only asymmetrical synapses. Layers II and III contained the highest concentrations of both types of callosal axon terminal. Analyses of serial thin sections through layers II and III in both areas 1 and 40 yielded similar results: 97% of the extrinsic (277 total sample) and of the intrinsic (1215 total sample) callosal axon terminals synapsed onto dendritic spines, likely those of pyramidal neurons; the remainder synapsed onto dendritic shafts of both spiny and nonspiny neurons. Thus the synaptic output patterns of intrinsic vs. extrinsic callosal axon terminals are strikingly similar. Moreover, the high proportion of axospinous synapses formed by both types of terminal contrasts with the proportion of asymmetrical, axospinous synapses that occur in the surrounding neuropil where only about 80% of the asymmetrical synapses are onto spines. This result is in accord with previous quantitative studies of the synaptic connectivities of both extrinsic and intrinsic axonal pathways in the cortex (White and Keller, 1989: Cortical Circuits; Boston: Birkhauser): in all instances, axonal pathways are highly selective for the types of elements with which they synapse.

Topographic organization of the auditory thalamocortical system in the albino rat

The organization of the auditory thalamocortical connections was studied in rats. Retrograde transport of horseradish peroxidase conjugated to wheat germ agglutinin following injections into parietal, occipital and temporal cortex was used. The medial geniculate body, the suprageniculate, the lateral part of the nucleus posterior thalami, the posterior part of the nucleus lateralis thalami, and the nucleus ventroposterior project to the investigated part of the neocortex. Corresponding to different patterns of labeling, five areas of auditory neocortex were distinguished: 1. The rostral area is innervated by neurons of the nucleus ventroposterior, the lateral part of the nucleus posterior thalami, and the medial division of the medial geniculate body. 2. The dorsal area is innervated by neurons of the suprageniculate, the posterior part of the nucleus lateralis thalami and the rostral region of the dorsal division of the medial geniculate body. 3. The caudal area is innervated by neurons of the posterior part of the nucleus lateralis thalami, the suprageniculate, the medial division, the caudal region of the dorsal division and the ventrolateral nucleus of the medial geniculate body. 4. The ventral area is innervated by neurons of the suprageniculate, the medial division, the caudal region of the dorsal division, and the ventrolateral nucleus of the medial geniculate body. 5. The core area of the temporal cortex is exclusively connected to the caudal region of the medial division and the ventral division of the medial geniculate body. The findings of the present study indicate topographic organizations of the ventral division of the medial geniculate body and of the corea area. Four segments (a-d) of the ventral division each show a different set of topographic axes. They correspond to sets of topographic axes in the core area of the auditory cortex. These topographies characterize the segments which are each exclusively connected to one of the four fields of the core area.

The cholinergic nuclei of the basal forebrain of the rat: hypertrophy following contralateral cortical damage or section of the corpus callosum

After damage of the neocortex of one hemisphere by removal of the pia-arachnoid mater, the cholinergic cells of the basal nucleus of the unoperated hemisphere show a marked increase in their cross-sectional areas. This hypertrophy reaches a maximum of 25% by 3 weeks after operation and persists indefinitely. The cell bodies appear normal in shape, are often paler-staining and the hypertrophy includes the proximal dendrites. The hypertrophy is confined to the part of the basal nucleus corresponding to that which shows shrinkage on the operated side. The enlargement is greatest in animals operated upon on the first day after birth (+31%), is less in adult animals (mean +22%) but occurs at all ages up to 496 days, the oldest animal used. After unilateral removal of the hippocampus or section of the fimbria there is hypertrophy of the cholinergic neurones of the contralateral medial septal nucleus (+24%) and vertical nucleus of the diagonal band (+18%). Removal of the olfactory bulb on one side had no apparent effect upon the cholinergic neurones in the contralateral horizontal nucleus of the diagonal band. Damage of the neocortex by exitotoxic amino acids did not result in hypertrophy of the cholinergic cells of the contralateral basal nucleus despite marked shrinkage of the neurones in the basal nucleus on the operated side. After section of the corpus callosum the neurones throughout the basal nucleus of both sides are significantly larger than in the normal animal; the hypertrophy has occurred by 20 days after operation and persists indefinitely.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensory interactive zones in the rat cerebral cortex

The acoustic, somatosensory and visual areas of the rat cerebral cortex were mapped in view of overlapping regions, giving polysensory evoked responses. In case of each pair of modalities overlapping areas were found between the cortical representations. The evoked potentials of two different modalities show occlusive and facilitatory interactions in these overlapping areas. In the "core" region of these areas, points with especially high values of occlusion are concentrated while at the periphery weak occlusion or even facilitation is characteristic. It is supposed that the polysensory areas interposed between primary sensory fields represent not only areas with mixed responsiveness but also stage for real physiological interactions between different sensory systems.

Proliferation of thalamic afferents in cerebral cortex altered by callosal deafferentation

In area 41, the auditory region of rat neocortex, callosal afferents project to layers I through III and thalamic afferents project to deep layer III through IV. Thus, these two extrinsic systems of afferents project simultaneously to only a narrow lamina in mid to low layer III. For this study, this narrow region of overlap is quantitatively examined to determine the distribution of callosal and thalamic afferents by observing degenerating terminals produced by separate callosal and thalamic lesions. The results show that of all asymmetric synapses observed in the neuropil of this narrow zone, 84% are dendritic spines and the balance are dendritic shafts. Although both callosal and thalamic afferents prefer to synapse with dendritic spines in the neuropil, 78% of the thalamic afferents synapse with dendritic spines while 93% of the callosal afferents synapse with dendritic spines. Vaughan & Foundas (1982) have shown that 3 months after callosal lesions in 1-month-old animals, additional thalamic axons have grown into, and proliferated in, this part of mid to low layer III. Quantitative analysis of the distribution of the degenerating thalamic axon terminals in these long-term callosally lesioned animals has been used to determine whether the proliferating thalamic afferents demonstrate any specificity in the pattern of synapses they make or whether the callosally deafferented neurons determine the pattern of synapses. The results indicate that thalamic axons do exhibit axon specificity, for after they have proliferated into the callosal domain, 80% of the degenerating terminals synapse with dendritic spines and 20% synapse with shafts. This distribution is most comparable to the normal distribution of thalamic axons in this region.

Hypertrophy of cholinergic neurones of the rat basal nucleus following section of the corpus callosum

The effect of division of the corpus callosum on immunohistochemically identified cholinergic neurones of the basal nucleus has been examined in rats. Following callosal section the cholinergic cell bodies on both sides are significantly larger (25%) than those in normal animals. This hypertrophy persists for at least 62 days after operation, the longest survival time examined. It is greatest when the animal is operated on in infancy, but it occurs at all ages examined. The enlargement is similar to that seen in the cells of the same nucleus on one side following contralateral cortical damage.

Long term effects of callosal lesions in the auditory cortex of rats of different ages

The corpus callosum was sectioned in groups of rats 3, 12, and 24 months of age, and the auditory cortex was examined three months later to determine whether there were age-related differences in the morphological response to the partial deafferentation. Material from the three groups of long-term callosally-lesioned rats were compared with three groups of age-matched control animals. Analysis focused on those cortical layers known to receive the heaviest callosal projection (layers II and III) and those neurons known to be postsynaptic to callosal afferents (layer V pyramidal neurons). There were no age-related changes in cortical thickness or in the relative thickness of the cortical layers in the control groups. However, the apical dendrites of layer V pyramidal neurons did lose dendritic spines and became thinner with age. In all three lesion groups, the cortex became thinner without altering the relative thickness of cortical layers; there was a decrease in the relative density of apical dendrite spines in layer III, but an increase in the density of these spines in layer IV. Both effects varied with age. Spine decreases in layer III were greatest in older animals and spine increases in layer IV were greatest in younger animals. The mean diameters of apical dendrites decreased in the youngest group of lesioned animals but increased in the oldest group. The results indicate that the effects of callosal deafferentation are age dependent.

The termination of callosal fibres in the auditory cortex of the rat. A combined Golgi--electron microscope and degeneration study

When the corpus callosum of the rat is sectioned, the callosal fibres in the cerebral cortex undergo degeneration. In the auditory cortex (area 41) the degenerating axon terminals form asymmetric synapses, and the vast majority of them synapse with dendritic spines. Some other synapse with the shafts of both spiny and smooth dendrites, and a few with the perikarya of non-pyramidal cells. The degenerating axon terminals are contained principally within layer II/III, in which they aggregate in patches. Using a technique in which neurons within the cortex are Golgi-impregnated, then gold-toned and examined in the electron microscope, it has been shown that the dendritic spines of pyramidal neurons with cell bodies in different layers receive the degenerating callosal afferents. The spines arise from the main apical dendritic shafts and their branches, from the dendrites of the apical tufts, and in some cases from the basal dendrites of the pyramidal neurons. The shafts of some pyramidal cell apical dendrites also form asymmetric synapses with callosal afferents. Since we have encountered no spiny non-pyramidal neurons in Golgi preparations of rat auditory cortex, and because other types of non-pyramidal cells have few dendritic spines, it is concluded that practically all of the dendritic spines synapsing with callosal afferents originate from pyramidal neurons.

Thalamic and callosal connections of the rat auditory cortex

This study was designed to assess the relative distributions of two extrinsic afferent fiber systems in the rat auditory cortex as indicated by the patterns of specific lesion-induced degeneration evident in Fink-Heimer preparations. The auditory cortex consists of cytoarchitectural areas 41, 20 and 36. Lesions were made in the medial geniculate body (MGB) or the corpus callosum in some rats, while in other rats, lesions were made in both the MGB and the corpus callosum. Following the thalamic lesions, degenerating terminals occur throughout the auditory region of cortex, principally in layer IV and deep layer III, but also in layer VI and in the superficial part of layer I. With the exception of the band of degeneration in layer I, the density of the thalamic degeneration is uneven, such that patches of increased density of degeneration are separated by regions with few degenerating terminals. Following lesions of the corpus callosum, degenerating callosal terminals are also evident throughout the auditory region of cortex and they occur in deep layer I through layer III, superficial layer V and in layer VI. The density of the degenerating callosal terminals is not uniform throughout most of area 41, to the extent that there are radially-oriented bands of increased density which appear within the continuous callosal projection. Following the double lesions, degenerating terminals throughout the auditory region are distributed homogeneously within all cortical layers with the exception of deep layer V which is relatively free of degeneration. The results indicate that all regions within the rat auditory cortex are subject to both thalamic and callosal influence, although the input is not completely uniform, for the zones in layers IV and VI which have decreased thalamic input appear to have increased callosal input.