Pathoanatomy of cerebellar degeneration in spinocerebellar ataxia type 2 (SCA2) and type 3 (SCA3)

The cerebellum is one of the well-known targets of the pathological processes underlying spinocerebellar ataxia type 2 (SCA2) and type 3 (SCA3). Despite its pivotal role for the clinical pictures of these polyglutamine ataxias, no pathoanatomical studies of serial tissue sections through the cerebellum have been performed in SCA2 and SCA3 so far. Detailed pathoanatomical data are an important prerequisite for the identification of the initial events of the underlying disease processes of SCA2 and SCA3 and the reconstruction of its spread through the brain. In the present study, we performed a pathoanatomical investigation of serial thick tissue sections through the cerebellum of clinically diagnosed and genetically confirmed SCA2 and SCA3 patients. This study demonstrates that the cerebellar Purkinje cell layer and all four deep cerebellar nuclei consistently undergo considerable neuronal loss in SCA2 and SCA3. These cerebellar findings contribute substantially to the pathogenesis of clinical symptoms (i.e., dysarthria, intention tremor, oculomotor dysfunctions) of SCA2 and SCA3 patients and may facilitate the identification of the initial pathological alterations of the pathological processes of SCA2 and SCA3 and reconstruction of its spread through the brain.

The human premotor oculomotor brainstem system - can it help to understand oculomotor symptoms in Huntington's disease?

Recent progress in oculomotor research has enabled new insights into the functional neuroanatomy of the human premotor oculomotor brainstem network. In the present review, we provide an overview of its functional neuroanatomy and summarize the broad range of oculomotor dysfunctions that may occur in Huntington's disease (HD) patients. Although some of these oculomotor symptoms point to an involvement of the premotor oculomotor brainstem network in HD, no systematic analysis of this functional system has yet been performed in brains of HD patients. Therefore, its exact contribution to oculomotor symptoms in HD remains unclear. A possible strategy to clarify this issue is the use of unconventional 100-microm-thick serial tissue sections stained for Nissl substance and lipofuscin pigment (Nissl-pigment stain according to Braak). This technique makes it possible to identify the known nuclei of the premotor oculomotor brainstem network and to study their possible involvement in the neurodegenerative process. Studies applying this morphological approach and using the current knowledge regarding the functional neuroanatomy of this human premotor oculomotor brainstem network will help to elucidate the anatomical basis of the large spectrum of oculomotor dysfunctions that are observed in HD patients. This knowledge may aid clinicians in the diagnosis and monitoring of the disease.

Widespread thalamic and cerebellar degeneration in a patient with a complicated hereditary spastic paraplegia (HSP)

The hereditary spastic paraplegias (HSP) are a heterogeneous group of familial movement disorders sharing progressive spastic paraplegia as a common disease sign. In the present study, we performed the first pathoanatomical investigation of the central nervous degeneration of a female patient with a complicated HSP form who suffered from progressive spastic paraplegia, dysarthria, emotional symptoms, cognitive decline and a variety of additional neuropsychological deficits. This pathoanatomical investigation revealed in addition to loss of layer V Betz pyramidal cells in the primary motor cortex, widespread cerebellar neurodegeneration (i.e., loss of Purkinje cells and neuronal loss in the deep cerebellar nuclei), extensive and severe neuronal loss in a large number of thalamic nuclei, involvement of some brainstem nuclei, as well as damage to descending (i.e., lateral and ventral corticospinal tracts) and ascending (i.e., dorsal and ventral spinocerebellar tracts, gracile fascicle) fiber tracts. In view of their known functional role, damage to these central nervous gray and white matter components offers explanations for the patient's pyramidal signs, her cerebellar, psychiatric and neuropsychological disease symptoms.

Accumulation of phosphorylated I kappaB alpha and activated IKK in nodes of Ranvier

AIMS

Nuclear factor-kappaB (NF-kappaB) is an ubiquitously expressed transcription factor that modulates inducible gene transcription crucial for the regulation of immunity, inflammatory processes, and cell survival. In the mammalian nervous system, constitutive NF-kappaB activation is considered to promote neuronal cell survival by preventing apoptosis. Increasing evidence suggests a critical role for NF-kappaB activation in acute and chronic neurodegenerative diseases. Recently, a striking enrichment of phosphorylated I kappaB alpha (pI kappaB alpha) and activated I KappaB Kinase (IKK), two key components of the NF-kappaB activation pathway, was demonstrated in the axon initial segment (AIS) of neurons. As the AIS shares fundamental features with nodes of Ranvier (NR), we examined whether pI kappaB alpha and activated IKK are also enriched in NR.

METHODS

Double-immunofluorescence labelling was performed with vibratome sections of the rodent central and peripheral nervous system. Sections were analysed using confocal laser scanning microscopy and preembedding electron microscopy.

RESULTS

Here we report a remarkable accumulation of pI kappaB alpha and activated IKK in NR in the central and peripheral nervous system. Immunolabelling for both proteins extended from NR into the adjacent paranode. pI kappaB alpha predominantly accumulated within the cytoplasm and was associated with fasciculated microtubules. This association was confirmed by electron microscopy. By comparison, activated IKK preferentially clustered beneath the cytoplasmic membrane.

CONCLUSION

In conclusion, the coincident accumulation of pI kappaB alpha and activated IKK in AIS and NR suggests that these specific axonal compartments contribute to neuronal NF-kappaB activation.

Degeneration of ingestion-related brainstem nuclei in spinocerebellar ataxia type 2, 3, 6 and 7

Dysphagia, which can lead to nutritional deficiencies, weight loss and dehydration, represents a risk factor for aspiration pneumonia. Although clinical studies have reported the occurrence of dysphagia in patients with spinocerebellar ataxia type 2 (SCA2), type 3 (SCA3), type 6 (SCA6) and type 7 (SCA7), there are neither detailed clinical records concerning the kind of ingestive malfunctions which contribute to dysphagia nor systematic pathoanatomical studies of brainstem regions involved in the ingestive process. In the present study we performed a systematic post mortem study on thick serial tissue sections through the ingestion-related brainstem nuclei of 12 dysphagic patients who suffered from clinically diagnosed and genetically confirmed spinocerebellar ataxias assigned to the CAG-repeat or polyglutamine diseases (two SCA2, seven SCA3, one SCA6 and two SCA7 patients) and evaluated their medical records. Upon pathoanatomical examination in all of the SCA2, SCA3, SCA6 and SCA7 patients, a widespread neurodegeneration of the brainstem nuclei involved in the ingestive process was found. The clinical records revealed that all of the SCA patients were diagnosed with progressive dysphagia and showed dysfunctions detrimental to the preparatory phase of the ingestive process, as well as the lingual, pharyngeal and oesophageal phases of swallowing. The vast majority of the SCA patients suffered from aspiration pneumonia, which was the most frequent cause of death in our sample. The findings of the present study suggest (i) that dysphagia in SCA2, SCA3, SCA6 and SCA7 patients may be associated with widespread neurodegeneration of ingestion-related brainstem nuclei; (ii) that dysphagic SCA2, SCA3, SCA6 and SCA7 patients may suffer from dysfunctions detrimental to all phases of the ingestive process; and (iii) that rehabilitative swallow therapy which takes specific functional consequences of the underlying brainstem lesions into account might be helpful in preventing aspiration pneumonia, weight loss and dehydration in SCA2, SCA3, SCA6 and SCA7 patients.

Diffusion tensor MRI shows abnormal brainstem crossing fibers associated with ROBO3 mutations

Horizontal gaze palsy with progressive scoliosis (HGPPS) is caused by mutations in the ROBO3 gene, critical for the crossing of long ascending medial lemniscal and descending corticospinal tracts in the medulla. Diffusion tensor imaging in a patient with HGGPS revealed the absence of major pontine crossing fiber tracts and no decussation of the superior cerebellar peduncles. Mutations in the ROBO3 gene lead to a widespread lack of crossing fibers throughout the brainstem.

Spinocerebellar ataxia type 7 (SCA7): first report of a systematic neuropathological study of the brain of a patient with a very short expanded CAG-repeat

Spinocerebellar ataxia type 7 (SCA7) represents a very rare and severe autosomal dominantly inherited cerebellar ataxia (ADCA). It belongs to the group of CAG-repeat or polyglutamine diseases with its underlying molecular genetical defect on chromosome 3p12-p21.1. Here, we performed a systematic study of the neuropathology on unconventional thick serial sections of the first available brain tissue of a genetically confirmed late-onset SCA7 patient with a very short CAG-repeat expansion. Along with myelin pallor of a variety of central nervous fiber tracts, we observed i) neurodegeneration in select areas of the cerebral cortex, and ii) widespread nerve cell loss in the cerebellum, thalamus, nuclei of the basal ganglia, and brainstem. In addition, upon immunocytochemical analysis using the anti-polyglutamine antibody 1C2, immunopositive neuronal intranuclear inclusions bodies (NI) were observed in all cerebellar regions, in all parts of the cerebral cortex, and in telencephalic and brainstem nuclei, irrespective of whether they underwent neurodegeneration. These novel findings provide explanations for a variety of clinical symptoms and paraclinical findings of both our and other SCA7 patients. Finally, our immunocytochemical analysis confirms previous studies which described the presence of NI in obviously degenerated brain and retinal regions as well as in apparently well-preserved brain regions and retina of SCA7 patients.

Spinocerebellar ataxia type 4 (SCA4): Initial pathoanatomical study reveals widespread cerebellar and brainstem degeneration

Spinocerebellar ataxia type 4 (SCA4), also known as 'hereditary ataxia with sensory neuropathy', represents a very rare, progressive and untreatable form of an autosomal dominant inherited cerebellar ataxia (ADCA). Due to a lack of autopsy cases, no neuropathological or clinicopathological studies had yet been performed in SCA4. In the present study, the first available cerebellar and brainstem tissue of a clinically diagnosed and genetically-confirmed German SCA4 patient was pathoanatomically studied using serial thick sections. During this systematic postmortem investigation, along with an obvious demyelinization of cerebellar and brainstem fiber tracts we observed widespread cerebellar and brainstem neurodegeneration with marked neuronal loss in the substantia nigra and ventral tegmental area, central raphe and pontine nuclei, all auditory brainstem nuclei, in the abducens, principal trigeminal, spinal trigeminal, facial, superior vestibular, medial vestibular, interstitial vestibular, dorsal motor vagal, hypoglossal, and prepositus hypoglossal nuclei, as well as in the nucleus raphe interpositus, all dorsal column nuclei, and in the principal and medial subnuclei of the inferior olive. Severe neuronal loss was seen in the Purkinje cell layer of the cerebellum, in the cerebellar fastigial nucleus, in the red, trochlear, lateral vestibular, and lateral reticular nuclei, the reticulotegmental nucleus of the pons, and the nucleus of Roller. In addition, immunocytochemical analysis using the anti-polyglutamine antibody 1C2 failed to detect any polyglutamine-related immunoreactivity in the central nervous regions of this SCA4 patient studied. In view of the known functional role of affected nuclei and related fiber tracts, the present findings not only offer explanations for the well-known disease symptoms of SCA4 patients (i.e. ataxic symptoms, dysarthria and somatosensory deficits), but for the first time help to explain why diplopia, gaze-evoked nystagmus, auditory impairments and pathologically altered brainstem auditory evoked potentials, saccadic smooth pursuits, impaired somatosensory functions in the face, and dysphagia may occur during the course of SCA4. Finally, the results of our immunocytochemical studies support the concept that SCA4 is not a member of the CAG-repeat or polyglutamine diseases.

Involvement of the cranial nerves and their nuclei in spinocerebellar ataxia type 2 (SCA2)

Although the cranial nerves, their nuclei and related fiber tracts are crucial for a variety of oculomotor, somatomotor, somatosensory, auditory, vestibular-related, autonomic and ingestion-related functions, knowledge regarding the extent of their involvement in spinocerebellar ataxia type 2 (SCA2) patients is incomplete. Accordingly, we performed a pathoanatomical analysis of these structures in six clinically diagnosed SCA2 patients. Unconventionally thick serial sections through the brainstem stained for lipofuscin pigment (aldehyde-fuchsin) and Nissl material (Darrow red) showed that all oculomotor, somatomotor, somatosensory, auditory, vestibular and autonomic cranial nerve nuclei may undergo neurodegeneration during SCA2. Similarly, examination of myelin-stained thick serial sections revealed that nearly all cranial nerves and associated fiber tracts may sustain atrophy and myelin loss in SCA2 patients. In view of the known functional role of the affected cranial nerves, their nuclei and associated fiber tracts, the present findings provide appropriate pathoanatomical explanations for some of the disease-related and unexplained symptoms seen in SCA2 patients: double vision, gaze palsy, slowing of saccades, ptosis, ingestion-related malfunctions, impairments of the optokinetic nystagmus and the vestibulo-ocular reaction, facial and tongue fasciculation-like movements, impaired centripetal transmission of temperature-related information from the face, dystonic posture of the neck, as well as abnormalities of the brainstem auditory evoked potentials.

Extended pathoanatomical studies point to a consistent affection of the thalamus in spinocerebellar ataxia type 2

The involvement of the thalamus during the course of the currently known polyglutamine diseases is still a matter of debate. While it is well-known that this diencephalic nuclear complex undergoes neurodegeneration in some polyglutamine diseases such as Huntington's disease (HD), it has remained unclear whether and to what extent the thalamus is also involved in spinocerebellar ataxia type 2 (SCA2) patients. Encouraged by our recent post-mortem findings in one German SCA2 patient and the results of a recent nuclear magnetic resonance (NMR) study, we extended our pathoanatomical analysis to serial thick sections stained for lipofuscin granules and Nissl substance through the thalami of four additional German and Cuban SCA2 patients. According to this analysis the thalamus is consistently affected by the destructive process of SCA2. In particular, during our study we observed a consistent involvement of the lateral geniculate body, the lateral posterior, ventral anterior, ventral lateral, ventral posterior lateral, and ventral posterior medial thalamic nuclei as well as the extraterritorial reticular nucleus. In four of the SCA2 cases studied additional damage was seen in the inferior and lateral nuclei of the pulvinar, whereas in the minority of the patients a subset of the limbic nuclei of the thalamus (i.e. anterodorsal, anteroprincipal, laterodorsal, fasciculosus, mediodorsal, central lateral, central medial, cucullar, and paracentral nuclei, medial nucleus of the pulvinar) underwent neurodegeneration. These interindividual differences in the distribution pattern of thalamic neurodegeneration indicate that the thalamic nuclei differ in their proclivities to degenerate in SCA2 and may suggest that they become involved at different phases in the evolution of the underlying degenerative process.

Spinocerebellar ataxias types 2 and 3: degeneration of the pre-cerebellar nuclei isolates the three phylogenetically defined regions of the cerebellum

The pre-cerebellar nuclei act as a gate for the entire neocortical, brainstem and spinal cord afferent input destined for the cerebellum. Since no pathoanatomical studies of these nuclei had yet been performed in spinocerebellar ataxia type 2 (SCA2) or type 3 (SCA3), we carried out a detailed postmortem study of the pre-cerebellar nuclei in six SCA2 and seven SCA3 patients in order to further characterize the extent of brainstem degeneration in these ataxic disorders. By means of unconventionally thick serial sections through the brainstem stained for lipofuscin pigment and Nissl material, we could show that all of the pre-cerebellar nuclei (red, pontine, arcuate, prepositus hypoglossal, superior vestibular, lateral vestibular, medial vestibular, interstitial vestibular, spinal vestibular, vermiform, lateral reticular, external cuneate, subventricular, paramedian reticular, intercalate, interfascicular hypoglossal, and conterminal nuclei, pontobulbar body, reticulotegmental nucleus of the pons, inferior olive, and nucleus of Roller) are among the targets of both of the degenerative processes underlying SCA2 and SCA3. These novel findings are in contrast to the current neuropathological literature, which assumes that only a subset of pre-cerebellar nuclei in SCA2 and SCA3 may undergo neurodegeneration. Widespread damage to the pre-cerebellar nuclei separates all three phylogenetically and functionally defined regions of the cerebellum, impairs their physiological functions and thus explains the occurrence of gait, stance, limb and truncal ataxia, dysarthria, truncal and postural instability with disequilibrium, impairments of the vestibulo-ocular reaction and optokinetic nystagmus, slowed and saccadic smooth pursuits, dysmetrical horizontal saccades, and gaze-evoked nystagmus during SCA2 and SCA3.

Damage to the reticulotegmental nucleus of the pons in spinocerebellar ataxia type 1, 2, and 3

BACKGROUND

The reticulotegmental nucleus of the pons (RTTG) is among the precerebellar nuclei of the human brainstem. Although it represents an important component of the oculomotor circuits crucial for the accuracy of horizontal saccades and the generation of horizontal smooth pursuits, the RTTG has never been considered in CAG repeat or polyglutamine diseases.

METHODS

Thick serial sections through the RTTG of 10 patients with spinocerebellar ataxias (SCAs) assigned to the CAG repeat or polyglutamine diseases (2 SCA-1 patients, 4 SCA-2 patients, and 4 SCA-3 patients) were stained for neuronal lipofuscin pigment and Nissl material.

RESULTS

The unconventionally thick tissue sections revealed the hitherto overlooked involvement of the RTTG in the degenerative processes underlying SCA-1, SCA-2, and SCA-3, whereby in one of the SCA-1 patients, in two of the SCA-2 patients, and in all of the SCA-3 patients, the RTTG underwent a conspicuous loss of its nerve cells.

CONCLUSIONS

Neurodegeneration may not only affect the cranial nerve nuclei (i.e., oculomotor and abducens nuclei) of SCA-1, SCA-2 and SCA-3 patients integrated into the circuits, subserving accuracy of horizontal saccades and the generation of horizontal smooth pursuits, but likewise involves the premotor networks of these circuits. This may explain why the SCA-1, SCA-2, and SCA-3 patients in this study with a heavily damaged reticulotegmental nucleus of the pons developed dysmetric horizontal saccades and impaired smooth pursuits during the course of the disease.

Stereological analysis of the reorganization of the dentate gyrus following entorhinal cortex lesion in mice

Denervation of the dentate gyrus by entorhinal cortex lesion has been widely used to study the reorganization of neuronal circuits following central nervous system lesion. Expansion of the non-denervated inner molecular layer (commissural/associational zone) of the dentate gyrus and increased acetylcholinesterase-positive fibre density in the denervated outer molecular layer have commonly been regarded as markers for sprouting following entorhinal cortex lesion. However, because this lesion extensively denervates the outer molecular layer and causes tissue shrinkage, stereological analysis is required for an accurate evaluation of sprouting. To this end we have performed unilateral entorhinal cortex lesions in adult C57BL/6J mice and have assessed atrophy and sprouting in the dentate gyrus using modern unbiased stereological techniques. Results revealed the expected increases in commissural/associational zone width and density of acetylcholinesterase-positive fibres on single brain sections. Yet, stereological analysis failed to demonstrate concomitant increases in layer volume or total acetylcholinesterase-positive fibre length. Interestingly, calretinin-positive fibres did grow beyond the border of the commissural/associational zone into the denervated layer and were regarded as sprouting axons. Thus, our data suggest that in C57BL/6J mice shrinkage of the hippocampus rather than growth of fibres underlies the two morphological phenomena most often cited as evidence of regenerative sprouting following entorhinal cortex lesion. Moreover, our data suggest that regenerative axonal sprouting in the mouse dentate gyrus following entorhinal cortex lesion may be best assessed at the single-fibre level.

Sprouting in the hippocampus after entorhinal cortex lesion is layer- specific but not translaminar: which molecules may be involved?

Entorhinal cortex lesion partially denervates the rat fascia dentata. This is said to induce sprouting of intact fibers from neighboring layers that invade the zone of the degenerating axons. However, recent in vivo and in vitro studies failed to demonstrate sprouting across laminar boundaries. Sprouting does occur, but it mainly involves unlesioned fiber systems terminating within the layer of fiber degeneration. These findings point to laminar cues that promote sprouting of fibers within the denervated zone while repelling other, adjacent fiber systems that try to grow into the denervated zone. A group of molecules that are likely to guide the sprouting process and the formation of borders are extracellular matrix molecules synthesized by reactive astrocytes. These molecules provide boundaries for growing axons during development. Some extracellular matrix molecules (tenascin-C, DSD- 1 -proteoglycan, neurocan, and brevican) were upregulated within the denervated outer molecular layer after lesion of the entorhinal cortex, suggesting a similar role after lesion. These extracellular matrix components forin a sharp molecular border towards the adjacent nondenervated inner molecular layer, and their pattern of distribution correlates precisely with the laminar termination pattern of the sprouting fiber populations. Thus, extracellular matrix molecules could delineate boundaries of axonal growth after entorhinal cortex lesion and could thus contribute to the molecular processes underlying the postlesional re-patterning of the fascia dentata.

Stereological estimates of total neuron numbers in the hippocampus of adult reeler mutant mice: Evidence for an increased survival of Cajal-Retzius cells

The cytoarchitecture of the brain is disrupted severely in reeler mice. This is caused by a deficiency in the protein, Reelin, which is essential for the normal migration and positioning of neurons during development. Although cell migration is clearly affected by the reeler mutation, it is believed that the total number of neurons is not. Thus, we were surprised to find an unusually large number of calretinin-immunopositive cells, presumably Cajal-Retzius cells, in the molecular layer of the adult reeler hippocampus (Deller et al. [1999]; Exp. Neurol. 156:239-253). This suggested that the reeler mutation affects the number of neurons in the hippocampus. In order to verify this hypothesis, unbiased stereological methods were employed. Calretinin immunostaining was used as a marker for Cajal-Retzius cells in control as well as reeler mice and Nissl staining was used to identify hippocampal principal neurons. Total numbers of calretinin-immunopositive cells, calretinin-immunoreactive Cajal-Retzius cells, and Nissl-stained neurons were estimated in different subfields of the reeler and the control hippocampus. Stereological estimates (P < 0.05) revealed that the total number of calretinin-immunopositive and Cajal-Retzius cells in reeler mice are 1.5 and 2.1 times that of controls, respectively. No significant difference in total neuron number was found in any hippocampal subfield. These data demonstrate that the reeler mutation affects the number of calretinin-immunoreactive Cajal-Retzius cells in the adult hippocampus, probably due to a reduced excitatory innervation by entorhinal terminals in the absence of reelin. However, the reeler mutation does not affect mechanisms that determine total hippocampal neuron number.

3D-Reconstruction of microglia and amyloid in APP23 transgenic mice: no evidence of intracellular amyloid

Microglia cells are closely associated with compact amyloid plaques in Alzheimer's disease (AD) brains. Although activated microglia seem to play a central role in the pathogenesis of AD, mechanisms of microglial activation by beta-amyloid as well as the nature of interaction between amyloid and microglia remain poorly understood. We previously reported a close morphological association between activated microglia and congophilic amyloid plaques in the brains of APP23 transgenic mice at both the light and electron microscopic levels [25]. In the present study, we have further examined the structural relationship between microglia and amyloid deposits by using postembedding immunogold labeling, serial ultrathin sectioning, and 3-dimensional reconstruction. Although bundles of immunogold-labeled amyloid fibrils were completely engulfed by microglial cytoplasm on single sections, serial ultrathin sectioning and three-dimensional reconstruction revealed that these amyloid fibrils are connected to extracellular amyloid deposits. These data demonstrate that extracellular amyloid fibrils form a myriad of finger-like channels with the widely branched microglial cytoplasm. We conclude that in APP23 mice a role of microglia in amyloid phagocytosis and intracellular production of amyloid is unlikely.

Potential role of synaptopodin in spine motility by coupling actin to the spine apparatus

Dendritic spines are dynamic structures that rapidly remodel their shape and size. These morphological adaptations are regulated by changes in synaptic activity, and result from rearrangements of the postsynaptic cytoskeleton. A cytoskeletal molecule preferentially found in mature spines is the actin-associated protein synaptopodin. It is strongly expressed by spine-bearing neurons in the olfactory bulb, striatum, cerebral cortex, and hippocampus. In the hippocampus, principal cells express synaptopodin mRNA and sort the protein to the spine compartment. Within the spine microdomain, synaptopodin is preferentially located in the spine neck and is closely associated with the spine apparatus. On the basis of these data we hypothesize that synaptopodin could affect spine motility by bundling actin filaments in the spine neck. In addition, it could link the actin cytoskeleton of spines to intracellular calcium stores, i.e., the spine apparatus and the smooth endoplasmic reticulum.

Up-regulation of growth-associated protein 43 mRNA in rat medial septum neurons axotomized by fimbria-fornix transection

Transection of septohippocampal fibres is widely used to study the response of CNS neurons to axotomy. Septohippocampal projection neurons survive axotomy and selectively up-regulate the transcription factor c-Jun. In the present study we investigated whether these cells concomitantly up-regulate the growth-associated protein-43 (GAP-43), a potential target gene of c-Jun implicated in axonal growth and regeneration. Using in situ hybridization histochemistry (ISHH) it was demonstrated that postlesional c-jun mRNA expression is accompanied by an increased expression of GAP-43 mRNA in the medial septum 3 days following fimbria-fornix transection (FFT). The increase reached a maximum at 7 days and gradually declined thereafter (17 days, 3 weeks). Retrograde prelabeling with Fluoro-Gold followed by axotomy and ISHH revealed that GAP-43 mRNA was up-regulated in septohippocampal projection neurons. Colocalization of GAP-43 mRNA and choline acetyltransferase protein showed that GAP-43 mRNA was expressed by cholinergic medial septal neurons after axotomy. Selective immunolesioning of the cholinergic component of the septohippocampal projection with 192 IgG-saporin followed by FFT demonstrated that GAP-43 mRNA was also synthesized by axotomized GABAergic neurons. These results demonstrate an up-regulation of GAP-43 mRNA in axotomized septohippocampal projection neurons independent of their transmitter phenotype which is closely correlated with c-Jun expression. Because the GAP-43 gene contains an AP-1 site, we hypothesize a c-Jun-driven up-regulation of GAP-43 in lesioned medial septal neurons that may contribute to their survival and regenerative potential following axotomy.

Retrograde and anterograde tracing combined with transmitter identification and electron microscopy

Fiber tracts in the brain are formed by neurochemically heterogeneous neuron populations. To distinguish between the different neurons that contribute to a fiber tract it is necessary to combine anterograde and retrograde tracing techniques with immunocytochemistry. In this article, we describe two techniques which allow for the neurochemical identification of retrogradely labeled neurons and anterogradely labeled axons on the ultrastructural level. The identification of the neurotransmitter identity of retrogradely labeled neurons is achieved by combining retrograde Fluoro-Gold tracing with preembedding immunocytochemistry, while the neurotransmitter identity of anterogradely labeled axons can be revealed by combining anterograde Phaseolus vulgaris-leucoagglutinin (PHAL) tracing and postembedding immunostaining.

Entorhinal cortex lesion does not alter reelin messenger RNA expression in the dentate gyrus of young and adult rats

The extracellular matrix protein reelin plays an important role in neuronal pattern formation and axonal collateralization during the development of the central nervous system. With the concept that reelin might also be important for axonal growth in the injured nervous system we investigated whether reelin is re-expressed in areas of collateral sprouting after brain injury. The expression of reelin messenger RNA was studied in the denervated fascia dentata of adult rats one, four, seven and 14 days following entorhinal cortex lesion. In adult control animals, in situ hybridization histochemistry with digoxigenin-labeled reelin riboprobes revealed reelin messenger RNA expression in neurons located in the outer molecular layer and beneath the granule cell layer of the dentate gyrus. After entorhinal cortex lesion, this expression pattern did not change during the whole post-lesional time period investigated despite a strong glial activation and reactive sprouting in the outer molecular layer of the dentate gyrus as visualized by immunohistochemistry for glial fibrillary acidic protein and acetylcholinesterase histochemistry, respectively. The expression of reelin messenger RNA was also unaffected by entorhinal cortex lesion in the dentate gyrus of young animals (postnatal day seven), where an even stronger sprouting response occurs.

The chondroitin sulphate proteoglycan brevican is upregulated by astrocytes after entorhinal cortex lesions in adult rats

The chondroitin sulphate proteoglycan brevican is one of the most abundant extracellular matrix molecules in the adult rat brain. It is primarily synthesized by astrocytes and is believed to influence astroglial motility during development and under certain pathological conditions. In order to study a potential role of brevican in the glial reaction after brain injury, its expression was analysed following entorhinal cortex lesion in rats (12 h, 1, 2, 4, 10, 14 and 28 days and 6 months post lesion). In situ hybridization and immunohistochemistry were employed to study brevican mRNA and protein, respectively, in the denervated outer molecular layer of the fascia dentata and at the lesion site. In both regions brevican mRNA was upregulated between 1 and 4 days post lesion. The combination of in situ hybridization with immunohistochemistry for glial fibrillary acidic protein demonstrated that many brevican mRNA-expressing cells are astrocytes. In the denervated zone of the fascia dentata, immunostaining for brevican was increased by 4 days, reached a maximum by 4 weeks and remained detectable up to 6 months post lesion. Electron microscopic immunocytochemistry showed that brevican is a component of the extracellular matrix compartment. At the lesion site a similar time course of brevican upregulation was observed. These data demonstrate that brevican is upregulated in areas of brain damage as well as in areas denervated by a lesion. They suggest a role of brevican in reactive gliosis and are compatible with the hypothesis that brevican is involved in the synaptic reorganization of denervated brain areas.

Reorganization of the rat fascia dentata after a unilateral entorhinal cortex lesion. Role of the extracellular matrix

Entorhinal cortex lesion (ECL) partially denervates the fascia dentata of the hippocampus. This is said to induce the sprouting of intact fibers from neighboring layers that invade the zone of the degenerating axons. However, recent studies using anterograde tracing failed to demonstrate sprouting across laminar boundaries. Sprouting does occur, but it mainly involves unlesioned fiber systems terminating within the layer of fiber degeneration. It is now of interest to identify the cues that could underlie this layer-specific sprouting response. Since extracellular matrix (ECM) molecules delineate boundaries of axonal growth during development, it was tested whether these molecules play a similar role during the sprouting process following ECL. After ECL, reactive astrocytes rapidly synthesize and secrete growth-inhibiting ECM molecules, such as tenascin-C and the chondroitin sulfate proteoglycan neurocan, into the ECM of the outer molecular layer. These molecules form a sharp border against the nondenervated inner molecular layer. This pattern of ECM molecule expression may contribute to the layer-specific sprouting response of surviving afferents after ECL: axons trying to grow into the denervated outer molecular layer, for example, from the inner molecular layer, would be deflected by a growth-inhibiting ECM barrier.

Ultrastructural localization of activated NF-kappaB in granule cells of the rat fascia dentata

The distribution of activated NF-kappaB (p65) was studied in granule cells of the rat fascia dentata using confocal laser scanning microscopy and preembedding immunogold electron microscopy. Activated NF-kappaB, detected with a monoclonal antibody specific for the nuclear localization signal-epitope, was found in proximal dendrites, somata, and nuclei but not in axon terminals of granule cells. Within the nuclei of granule cells, clusters of NF-kappaB immunoreactivity were detected. These data are in line with the proposed function of NF-kappaB as a retrograde signal transducer which transports information from synaptic sites to the nucleus to initiate gene expression.

Actin-associated protein synaptopodin in the rat hippocampal formation: localization in the spine neck and close association with the spine apparatus of principal neurons

Dendritic spines are sites of synaptic plasticity in the brain and are capable of remodeling their shape and size. However, little is known about the cellular mechanisms that regulate spine morphology and motility. Synaptopodin is a recently described actin-associated protein found in renal podocytes and dendritic spines (Mundel et al. J Cell Biol. [1997] 139:193-204), which is believed to play a role in spine plasticity. The present study analyzed the distribution of synaptopodin in the hippocampal formation. In situ hybridization histochemistry revealed a high constitutive expression of synaptopodin mRNA in the principal cell layers. Light microscopic immunohistochemistry showed that the protein is distributed throughout the hippocampal formation in a region- and lamina-specific manner. Postembedding immunogold histochemistry demonstrated that synaptopodin is exclusively present in dendrites and spines, specifically in the spine neck in close association with the spine apparatus. Spines lacking a spine apparatus are not immunoreactive for synaptopodin. These data suggest that synaptopodin links the spine apparatus to actin and may thus be involved in the actin-based plasticity of spines.

Entorhinal cortex lesion in adult rats induces the expression of the neuronal chondroitin sulfate proteoglycan neurocan in reactive astrocytes

The chondroitin sulfate proteoglycan neurocan is a major component of brain extracellular matrix during development. Neurocan is primarily synthesized by neurons and has the ability to interact with cell adhesion molecules involved in the regulation of cell migration and axonal growth. Within the first weeks postnatally, neurocan expression is strongly downregulated. To test whether neurocan is reexpressed in areas of axonal growth (sprouting) after brain injury, the time course of neurocan expression was analyzed in the denervated fascia dentata of the rat after entorhinal cortex lesion (12 hr; 1, 2, 4, and 10 d; 2 and 4 weeks; and 6 months after lesion). In the denervated zone, immunohistochemistry revealed neurocan-positive astrocytes by 2 d after lesion and a diffuse labeling of the extracellular matrix at all later time points. Electron microscopy confirmed the deposition of neurocan in the extracellular matrix compartment. In situ hybridization demonstrated a strong upregulation of neurocan mRNA within the denervated outer molecular layer 1 and 4 d after lesion. The combination of in situ hybridization with immunohistochemistry for glial fibrillary acidic protein demonstrated that the neurocan mRNA-expressing cells are astrocytes. These data demonstrate that neurocan is reexpressed in the injured brain. In contrast to the situation during development, astrocytes, but not neurons, express neurocan and enrich the extracellular matrix with this molecule. Similar to the situation during development, neurocan is expressed in an area of active axon growth, and it is suggested that neurocan acts to maintain the boundaries of the denervated fascia dentata after entorhinal cortex lesion.

Cerebral amyloid induces aberrant axonal sprouting and ectopic terminal formation in amyloid precursor protein transgenic mice

A characteristic feature of Alzheimer's disease (AD) is the formation of amyloid plaques in the brain. Although this hallmark pathology has been well described, the biological effects of plaques are poorly understood. To study the effect of amyloid plaques on axons and neuronal connectivity, we have examined the axonal projections from the entorhinal cortex in aged amyloid precursor protein (APP) transgenic mice that exhibit cerebral amyloid deposition in plaques and vessels (APP23 mice). Here we report that entorhinal axons form dystrophic boutons around amyloid plaques in the entorhinal termination zone of the hippocampus. More importantly, entorhinal boutons were found associated with amyloid in ectopic locations within the hippocampus, the thalamus, white matter tracts, as well as surrounding vascular amyloid. Many of these ectopic entorhinal boutons were immunopositive for the growth-associated protein GAP-43 and showed light and electron microscopic characteristics of axonal terminals. Our findings suggest that (1) cerebral amyloid deposition has neurotropic effects and is the main cause of aberrant sprouting in AD brain; (2) the magnitude and significance of sprouting in AD have been underestimated; and (3) cerebral amyloid leads to the disruption of neuronal connectivity which, in turn, may significantly contribute to AD dementia.

Cholinergic innervation of mossy cells in the rat fascia dentata

Hilar mossy cells are the main cells of origin of the commissural/associational projection to the inner molecular layer of the rat fascia dentata. In order to analyze the cholinergic innervation of hilar mossy cells, a light and electron microscopic double-labeling technique was used. Immunolabeling for calcitonin gene-related peptide (CGRP) was employed to identify mossy cells and immunocytochemistry for choline acetyltransferase (ChAT) was used to label cholinergic septohippocampal fibers. Cholinergic boutons were abundant around mossy cell somata and on their proximal dendrites. Electron microscopy confirmed that many of these boutons formed synapses with the CGRP-positive mossy cells. These data demonstrate a direct innervation of hilar mossy cells by cholinergic septohippocampal afferents. This connectivity could contribute to the electrophysiological behavior of mossy cells during theta oscillations.

Different primary target cells are important for fiber lamination in the fascia dentata: a lesson from reeler mutant mice

The factors determining the lamina-specific termination of entorhinal and commissural afferents to the fascia dentata are poorly understood. Recently it was shown that early generated Cajal-Retzius (CR) cells in the outer molecular layer and reelin, synthesized by CR cells, play a role in the lamina-specific termination of entorhinal fibers which form transient synapses with CR cells before establishing their definite contacts with granule cell dendrites (J. A. del Rio et al., 1997, Nature 385, 70-74). By using anterograde tracing with Phaseolus vulgaris leukoagglutinin we show that the normal, sharply delineated entorhinal projection to the outer molecular layer is retained in reeler mutant mice lacking reelin. This coincides with the regular presence of CR cells, the primary, transient target cells of entorhinal fibers. In contrast, the commissural fibers were found to terminate in an abnormal broad, not clearly defined area. This widespread projection coincides with the distribution of granule cells which in the mutant do not form a dense cell layer but are scattered all over the hilus due to a migration defect. Unlike the entorhinal fibers, the commissural fibers arrive in their target layer late in development, when granule cell dendrites are already there. We hypothesize from these results that the presence of the adequate postsynaptic element at the time of fiber ingrowth, CR cells for the early ingrowing entorhinal fibers and granule cells for the late-arriving commissural fibers, is crucial for the normal formation of these layer-specific projections.

Differential induction of c-Fos, c-Jun and Jun B in the rat central nervous system following unilateral entorhinal cortex lesion

In order to identify some of the molecular mechanisms that occur after a central nervous system trauma, the immediate early gene encoded proteins c-Fos, c-Jun and Jun B were analysed by immunocytochemistry following unilateral entorhinal cortex lesion (controls, 30 min, 2, 5, 12 and 24 h, two, six, 10 and 14 days, four weeks and six months postlesion). In the dentate gyrus, c-Fos was induced in some supragranular neurons (30 min), massively expressed in granule cells ipsilaterally to the lesion (2 h), expressed in hilar neurons (5 h and two days) and was absent at all later stages. A basal expression of c-Jun was found in dentate granule cells of controls, which was strongly increased on the lesion side (2 h) and on the side contralateral to the lesion (12 h). c-Jun expression returned to control levels by 24 h. Jun B was induced in granule cells ipsilateral to the lesion within 2 h and was back to control levels by 5 h. In the lateral septal area, c-Fos and c-Jun were induced 30 min postlesion and decreased rapidly thereafter. In the cerebral cortex, a widespread induction of c-Fos and c-Jun occurred within 30 min after entorhinal cortex lesion and this up-regulation lasted until two days postlesion. These data indicate that electrolytic lesion of the entorhinal cortex leads to a rapid and widespread induction of c-Fos, c-Jun and Jun B. Within the denervated fascia dentata, some of these changes may be linked to the reorganization processes following the lesion. Alternatively, the alterations in immediate early gene expression reported here may be due to changes in synaptic activity or postlesional seizures which occur in this lesioning paradigm.

The hippocampus of the reeler mutant mouse: fiber segregation in area CA1 depends on the position of the postsynaptic target cells

Area CA1 of the rodent hippocampus is characterized by a highly lamina-specific and nonoverlapping termination of afferent fiber tracts. Entorhinal fibers terminate in stratum lacunosum-moleculare and commissural/associational fibers terminate in strata radiatum and oriens. It has been hypothesized that this fiber lamination depends on specific signals for the different afferent fiber tracts that are located on distinct dendritic segments of the postsynaptic neuron. In order to test this hypothesis, entorhinal and commissural/associational afferents to Ammon's horn were investigated in the adult reeler mutant mouse, in which developmental cell migration defects have disrupted the normal array of cells. Golgi staining revealed a deep and a superficial principal cell layer in the mutant. The morphology of the cells located in the deep pyramidal cell layer was considerably abnormal, whereas most cells located in the superficial pyramidal cell layer showed an almost normal cellular and dendritic morphology. Anterograde tracing with Phaseolus vulgaris leukoagglutinin revealed that the duplication and disorganization of the pyramidal cell layer in area CA1 are mirrored by the duplication and disruption of afferent fiber termination zones. In the zone above the abnormal deep pyramidal cell layer, i.e., between the two cell layers, entorhinal fibers as well as commissural/associational fibers terminate and intermingle. In contrast, in the zone above the fairly normal superficial pyramidal cell layer, entorhinal and commissural/associational fibers retain their normal fiber segregation. These data suggest that the normal laminar organization of the murine hippocampus depends on positional cues presented by their target cells.

Lamina-specific cell adhesion on living slices of hippocampus

Laminar distribution of fiber systems is a characteristic feature of hippocampal organization. Ingrowing afferents, e.g. the fibers from the entorhinal cortex, terminate in specific layers, which implies the existence of laminar recognition cues. To identify cues that are involved in the laminar segregation of fiber systems in the hippocampus, we used an in vitro assay to study the adhesion of dissociated entorhinal cells on living hippocampal slices. Here we demonstrate that dissociated entorhinal cells adhere to living hippocampal slices with a lamina-specific distribution that reflects the innervation pattern of the entorhino-hippocampal projection. In contrast, laminae which are not invaded by entorhinal fibers are a poor substrate for cell adhesion. Lamina-specific cell adhesion does not require the neural cell adhesion molecule or the extracellular matrix glycoprotein reelin, as revealed in studies with mutants. However, the pattern of adhesive cues in the reeler mouse hippocampus mimics characteristic alterations of the entorhinal projection in this mutant, suggesting a role of layer-specific adhesive cues in the pathfinding of entorhinal fibers. Lamina-specific cell adhesion is independent of divalent cations, is abolished after cryofixation or paraformaldehyde fixation and is recognized across species. By using a novel membrane adhesion assay, we show that lamina-specific cell adhesion can be mimicked by membrane-coated fluorescent microspheres. Recognition of the adhesive properties of different hippocampal laminae by growing axons, as either a growth permissive or a non-permissive substrate, may provide a developmental mechanism underlying the segregation of lamina-specific fiber projections.

Effects of repeated administration of amphetamine on behavioral vigilance: evidence for "sensitized" attentional impairments

The effects of repeated intermittent administration of amphetamine (1, 2, 3 mg/kg, i.p.) on the performance of rats in a task designed to assess sustained attention were tested. A substantial increase in the number of false alarms (i.e., "claims" for hits in nonsignal trials) was observed following subsequent administrations of amphetamine. This effect could not be accounted for by drug-induced side or position biases, switching behavior or stereotypy. The effects of repeated amphetamine may model some of the cognitive processes which mediate the attribution of incentive salience to stimuli associated with repeated psychostimulant administration and the development of psychostimulant-induced psychotic symptoms.

The anatomical organization of the rat fascia dentata: new aspects of laminar organization as revealed by anterograde tracing with Phaseolus vulgaris-Luecoagglutinin (PHAL)

The rat fascia dentata is characterized by a simple cytoarchitecture and characteristic lamination of afferents. Entorhinal afferents are believed to terminate exclusively in the outer two thirds of the molecular layer, whereas commissural fibers are believed to terminate exclusively in the inner molecular layer of the fascia dentata. A sharp border divides these two major afferent fiber systems and is regarded as the main boundary of the fascia dentata. This concept of a highly laminated brain structure has made the fascia dentata attractive for studies analyzing normal or pathological processes of the brain. Recently, entorhinal as well as commissural fibers have been identified which do not follow the classical lamination of the fascia dentata. Using anterograde tracing with Phaseolus vulgaris-Leucoagglutinin, an entorhino-dentate projection to the molecular layer, granule cell layer, and hilus of the fascia dentata was described. With the same technique, GABAergic commissural fibers to the outer molecular layer of the fascia dentata were revealed and a previously unknown heterogeneity of the commissural projection was demonstrated. These previously unknown fiber systems complicate the interpretation of lesion effects in this brain region and have to be taken into account as possible sources of sprouting fibers following the partial denervation of the fascia dentata.

The anatomy of the rat fascia dentata--new vistas

The rat fascia dentata is frequently used as a model system to analyze normal as well as pathological processes of the brain. The normal anatomy of the fascia dentata is the basis for a meaningful interpretation of experimentally induced changes in this brain region. Using anterograde tracing with Phaseolus vulgaris-leucoagglutinin (PHAL) previously unknown commissural as well as entorhinal fiber systems to the fascia dentata were described. These fiber systems need to be incorporated into current concepts of the hippocampal network since they have profound implications for studies of lesion effects in this brain region.

Up-regulation of astrocyte-derived tenascin-C correlates with neurite outgrowth in the rat dentate gyrus after unilateral entorhinal cortex lesion

The extracellular matrix protein tenascin-C has been implicated in the regulation of axonal growth. Using unilateral entorhinal cortex lesions, which induce a massive sprouting response in the denervated outer molecular layer of the rat fascia dentata, the role of tenascin-C for axonal growth was investigated in vivo. Monoclonal antibodies against the neurite outgrowth and anti-adhesive domains of the molecule were employed. Immunostaining was increased throughout the denervated outer molecular layer by day 2, reached a maximum around day 10, and was back to control levels by four weeks post lesion. Growth cone deflecting as well as neurite outgrowth promoting isoforms of tenascin-C were up-regulated after the lesion. Using electron microscopy, single intensely tenascin-C immunoreactive cells were identified as reactive astrocytes that phagocytose degenerated terminals. In situ hybridization histochemistry for tenascin-C messenger RNA revealed numerous cellular profiles in the denervated outer molecular layer of the ipsilateral and contralateral dentate gyrus two days post lesion. Tenascin-C messenger RNA-positive cells in the outer molecular layer were identified as astrocytes using double-labelling for tenascin-C messenger RNA and glial fibrillary acidic protein immunohistochemistry. Thus, a tenascin-C-rich substrate is present in the outer molecular layer during the time of sprouting and a sharp boundary is formed against the inner molecular layer. This pattern may contribute to the layer-specific sprouting response of surviving afferents after entorhinal lesion. Neurite outgrowth may be promoted within the denervated zone, whereas axons trying to grow into the denervated outer molecular layer, for example from the inner molecular layer, would be deflected by a tenascin-C-rich barrier.

Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion

In response to a central nervous system trauma surviving neurons reorganize their connections and form new synapses that replace those lost by the lesion. A well established in vivo system for the analysis of this lesion-induced plasticity is the reorganization of the fascia dentata following unilateral entorhinal cortex lesions in rats. After general considerations of neuronal reorganization following a central nervous system trauma, this review focuses on the sprouting of single fibres in the rat hippocampus after entorhinal lesion and the molecular factors which may regulate this process. First, the connectivity of the fascia dentata in control animals is reviewed and previously unknown commissural fibers to the outer molecular layer and entorhinal fibres to the inner molecular layer are characterized. Second, sprouting of commissural and crossed entorhinal fibres after entorhinal cortex lesion is described. Single fibres sprout by forming additional collaterals, axonal extensions, boutons, and tangle-like axon formations. It is pointed out that the sprouting after entorhinal lesion mainly involves unlesioned fibre systems terminating within the layer of fibre degeneration and is therefore layer-specific. Third, molecular changes associated with axonal growth and synapse formation are considered. In this context, the role of adhesion molecules, glial cells, and neurotrophic factors for the sprouting process are discussed. Finally, an involvement of sprouting processes in the formation of neuritic plaques in Alzheimer's disease is reviewed and discussed with regard to the axonal tangle-like formations observed after entorhinal cortex lesion.

Developmental distribution of a reeler gene-related antigen in the rat hippocampal formation visualized by CR-50 immunocytochemistry

During histogenesis of the neocortex, Cajal Retzius cells in the marginal zone express the glycoprotein reelin which is developmentally regulated and involved in the formation of the inside out mode of cortical layering. Cajal Retzius cells are also present in the developing hippocampus. There, inhibition of reelin by blocking with CR-50, an antibody which recognizes the N-terminus of this protein, leads to abnormal development of layer-specific connections. Here we report the developmental distribution pattern of reelin expressing neurons in the rat hippocampal formation using CR-50 immunocytochemistry. Labelled Cajal Retzius cells were located near the hippocampal fissure in neonate rats. Many of these cells were still present in the adult. From postnatal day 4 on, neurons in other layers were stained with the CR-50 antibody. In adult rats immunopositive neurons were found in all hippocampal subfields and in the entorhinal cortex. These observations indicate that in the rat hippocampal formation reelin is expressed in different neuronal types during development and in adulthood. Moreover, Cajal Retzius cells in the marginal zone near the hippocampal fissure are still found in adult animals.

Basal expression, subcellular distribution, and up-regulation of the proto-oncogene c-JUN in the rat dentate gyrus after unilateral entorhinal cortex lesion

The expression of the transcription factor c-JUN was investigated in the rat fascia dentata under normal conditions and after entorhinal cortex lesion. As shown by immunocytochemistry and in situ hybridization histochemistry c-JUN and its messenger RNA are present in the principal cell layers of the dentate gyrus and Ammon's horn (except hippocampal region CA2). Pre-embedding immunogold electron microscopy revealed an almost exclusive nuclear localization of c-JUN, where it is associated with chromatin. In addition, double immunolabelling for c-JUN and parvalbumin demonstrated that c-JUN immunoreactivity is primarily found in principal neurons since GABAergic parvalbumin-positive interneurons did not express c-JUN. After unilateral electrolytic lesion of the entorhinal cortex c-JUN was strongly up-regulated in the ipsilateral dentate gyrus within 2 h postlesion. This up-regulation was also present in the contralateral fascia dentata 12 h after entorhinal cortex lesion and returned to control levels on both sides 24 h postlesion. The cellular distribution of c-JUN did not change after entorhinal cortex lesion: parvalbumin-positive interneurons never contained c-JUN. These results point to a specific role of c-JUN in the granule cells of the fascia dentata in the normal animal and in rats with entorhinal cortex lesions. The selective induction of c-JUN after entorhinal lesion could be one of the first molecular steps that regulate transneuronal changes within granule cells after their denervation. A different mechanism has to be assumed for GABAergic interneurons known to receive an entorhinal innervation as well.

Cholinergic sprouting in the rat fascia dentata after entorhinal lesion is not linked to early changes in neurotrophin messenger RNA expression

After unilateral entorhinal cortex lesion cholinergic septohippocampal fibres sprout in the denervated fascia dentata. This process is dependent on neurotrophin changes following the lesion. Thus, there is an up-regulation of nerve growth factor and brain-derived neurotrophic factor messenger RNA expression in the denervated granule cells which is detectable 4 h postlesion and returns to control levels by 24 h. Here, using a competitive polymerase chain reaction and in situ hybridization, a transient neurotropin messenger RNA increase could be demonstrated bilaterally following unilateral electrolytic entorhinal cortex lesion. Treatment of the animals with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate blocked this messenger RNA increase, suggesting an involvement of this receptor type in the neurotrophin changes. However, in spite of this blockade, the typical cholinergic sprouting response as visualized with acetylcholinesterase histochemistry was present in animals four weeks after entorhinal cortex lesion. These data suggest that brief initial changes in neurotrophin messenger RNA expression in dentate granule cells are not responsible for the induction of the cholinergic sprouting. Changes in neurotrophin messenger RNA expression occurring immediately postlesion may be linked to glutamate release from entorhinal terminals resulting from the electrolytic lesion of the projection cells in the entorhinal cortex. We hypothesize that later changes in neurotrophin expression, for example in glial cells, are more likely to be related to the cholinergic sprouting process.