Induced acetylcholinesterase-rich layer in rat dentate gyrus following entorhinal lesions

Glutamate producing aspartate aminotransferase in glutamatergic perforant path terminals of the rat hippocampus. Cytochemical and lesion studies

The enzyme aspartate aminotransferase was demonstrated cytochemically in the rat hippocampus 4, 7, and 14 days after unilateral entorhinal cortex lesion. At the light microscopic level the enzyme showed a significant activity decrease in the ipsilateral entorhinal terminal field which was similar at all postlesion times investigated. Non-denervated areas, i.e. the inner one-third of the dentate gyrus molecular layer and the radiatum layer of CA2/3, showed an increase of aminotransferase activities. At the electron microscopic level in the entorhinal terminal field of the control (unoperated) side aspartate aminotransferase was localized preferentially in a great number of boutons, containing the cytoplasmic and mitochondrial isoenzymes. Following entorhinal lesion a significant loss of these positively reacting boutons was seen. Most of the degenerating boutons contained reaction product but a small number was negative for aspartate aminotransferase. From 4 to 14 postlesion days the positively reacting boutons of the non-denervated supragranular zone expanded outward into the denervated area according to the known terminal proliferation of the commissural and associational systems. The remaining denervated entorhinal terminal field was reinnervated predominantly by negatively reacting boutons (probably terminal proliferations of septal afferents) and by a small number of positively reacting boutons (probably terminal proliferations of the crossed temporo-dentate pathway). The presence of cytoplasmic aspartate aminotransferase in the terminals of a well-known glutamatergic system is discussed in relation to the possible importance of this enzyme for the production of releasable glutamate.

The pattern of GAP-43 immunostaining changes in the rat hippocampal formation during reactive synaptogenesis

The reactive synaptogenesis that takes place in the rat hippocampal formation after certain experimental manipulations affords an opportunity to investigate the molecular events that underlie structural remodeling in the adult CNS. Between 2 and 4 days after lesioning the perforant pathway, levels of the synaptic phosphoprotein, GAP-43 (B50, F1, pp46, neuromodulin), were found to increase markedly in the inner molecular layer (iml) of the dentate gyrus, coincident with the time at which commissural-associational (CA) fibers begin to sprout axon collaterals into dendritic portions denervated by the lesion. GAP-43 immunostaining in the iml began to decline by 8 days but continued to define an expanded CA projection for at least one month. In the outer molecular layer (oml), GAP-43 levels decreased after the loss of perforant pathway terminals and did not return for 2-3 weeks, the time at which sprouting of septal inputs into this layer can be visualized by cholinesterase histochemistry. These results demonstrate that GAP-43 levels change during reactive synaptogenesis, and point to differences among neural systems in their expression of this protein.

Long-term survival of neural transplants to senescence in rats

A critical issue for clinical and research applications of transplant techniques is the long-term survival of transplanted tissue and its effect on the host brain. In this study, entorhinal cortices from donor embryos were transplanted into the lesioned angular bundle of juvenile male Sprague-Dawley rats. Animals were maintained for 2 years and then sacrificed for histological and histochemical examinations. The results indicate that entorhinal transplants survive to old age and that both the host and transplant tissues maintain morphological features consistent with those of short-term neural grafts. An unexpected finding of this experiment was the persistence in the transplanted tissue and adjacent host cortex of a pattern of AChE staining which is typical of early postnatal development.

The nature and causes of hippocampal long-term potentiation

One of the most fascinating features of the hippocampus is its capacity for plasticity. Long-term potentiation (LTP), a stable facilitation of synaptic potentials after high-frequency synaptic activity, is very prominent in hippocampus and is a leading candidate memory storage mechanism. Here, we discuss the nature and causes of LTP and relate them to endogenous rhythmic neuronal activity patterns and their potential roles in memory. Anatomical studies indicate that LTP is accompanied by postsynaptic structural modifications while pharmacological studies strongly suggest that LTP is not due to an increase in presynaptic transmitter release. In field CA1, LTP induction appears to be triggered by a postsynaptic influx of calcium through NMDA receptor-linked channels. Possible roles of several calcium-sensitive enzyme systems in LTP are discussed and it is argued that activation of a calcium-dependent protease (calpain) could produce the structural changes linked to LTP. Rhythmic bursting activity is highly effective in inducing LTP and it is argued that the endogenous hippocampal theta rhythm plays a role in LTP induction in vivo. Finally, studies indicate that LTP and certain types of memory share a common pharmacology and the use of electrical brain stimulation as a sensory cue suggests that LTP develops when the significance of that cue is learned.

NGF-dependent sprouting and regeneration in the hippocampus

While a variety of sprouting and regenerative responses have been investigated in the hippocampus, the cellular and molecular events responsible for these plastic responses have not been determined. One transmitter system, the cholinergic system, shows several distinct responses to damage in the septohippocampal circuit. Present evidence strongly supports a role for nerve growth factor (NGF) in these responses. NGF is not only important for the survival of the adult cholinergic neurons, but can also induce regrowth of the damaged fibers given an appropriate substratum for growth. These reparative effects of NGF can manifest themselves in functional recovery in the aged rat and the young rat with fimbria-fornix lesions. Finally, a role for glia cells is proposed to clarify how NGF availability may be regulated during the degenerative and regenerative events. While all plasticity events certainly cannot be explained by the coincidence of NGF and the cholinergic system, their interaction may provide a template for other transmitter/trophic factor interactions.

Axon sprouting in the rodent and Alzheimer's disease brain: a reactivation of developmental mechanisms?

Research over the past 15 years has led to a comprehensive description of the processes of axonal sprouting and synaptic reorganization in the hippocampus. Previous studies on axonal sprouting have now been supplemented with recent studies on excitatory amino acid receptor plasticity. These and related studies pave the way to research strategies which detail the molecular mechanisms of the sprouting response. The re-expression of the fetal form of alpha-tubulin mRNA in rat after entorhinal lesions was found to be similar to the re-expression of the human fetal form of alpha-tubulin in Alzheimer's brain. This result suggests that the sprouting process may involve a reactivation of certain developmental mechanisms and that this may possibly contribute to the etiology of Alzheimer's disease.

Exposure to trimethyltin significantly enhances acetylcholinesterase staining in the rat dentate gyrus

Trimethyltin (TMT) is known to produce substantial damage to the hippocampal formation. It also destroys neurons within the entorhinal cortex, thereby causing degeneration of perforant path afferents that terminate in the outer molecular layer (OML) of the dentate gyrus. Surgical destruction of the entorhinal cortex also causes the perforant path to degenerate. This leads to reactive synaptogenesis (axonal sprouting) of septal afferents to the dentate gyrus. The purpose of the present study was to determine whether administration of 6 mg/kg of TMT by gavage to rats would cause axonal sprouting within the septo-dentate projection. A histochemical stain for acetylcholinesterase (AChE) was used. Compared to control subjects rats given TMT exhibited significantly denser AChE staining in the dentate OML. This is putative indication of reactive synaptogenesis within the cholinergic projection to this layer of the dentate and is somewhat surprising because other neurotoxins, such as lead and ethanol, that affect neurons within the hippocampal formation reduce the capacity for reactive synaptogenesis in response to lesions of the entorhinal cortex.

Morphological correlates of long-term potentiation imply the modification of existing synapses, not synaptogenesis, in the hippocampal dentate gyrus

This report evaluates two morphological markers of synaptogenesis following the induction of long-term potentiation (LTP) in the dentate gyrus of the anesthetized rat. These two morphological features, polyribosomes and multiple synaptic contacts, are known to increase in number with synaptogenesis in the mature hippocampus. The analysis focused on the middle third of the dentate molecular layer. As shown previously, this is the region of primary synaptic activation in our electrophysiological protocol and the region of localized morphological changes with LTP. Here the incidence of a polyribosome at the base of a dendritic spine declined 57% with LTP. In addition, the number of multiple synaptic contacts decreased 18% there with LTP. Both decreases were more pronounced immediately following conditioning stimulation than at later intervals. Because both morphological features decrease with LTP but increase with synaptogenesis, the data do not support the hypothesis that new synapses form with LTP. Instead, the data add further support to the view that the strengthening of existing excitatory synapses underlies LTP.

Hippocampal plasticity in normal aging and decreased plasticity in Alzheimer's disease

Different patterns of age-related dendritic change have been reported in different zones of the human hippocampal region in the normal and Alzheimer's disease (AD) brain. In normal aging there is an increase in average (net) dendritic extent (which we interpret as plasticity) in the parahippocampal gyrus and dentate gyrus. There is net stability of dendritic extent in CA2-3, CA1, and subiculum. In regions that show plasticity in normal aging, dendrites in AD show reduced or aberrant plasticity. In regions that show stability in normal aging, dendrites either are stable or regress in AD, depending upon how severely involved the region is with the pathology of AD.

Selective alterations of RNA in rat hippocampus after entorhinal cortex lesioning

In vitro translation products from RNA of rat hippocampus after deafferentation by entorhinal cortex lesions were analyzed by two-dimensional gel electrophoresis. Although hippocampal total RNA yield was not affected 14 days after the lesion, analysis of the gels showed reproducible changes in the steady-state level of several transcripts. Glial fibrillary acidic protein RNA increased 2-fold over control hippocampi RNA. Moreover, seven other transcripts of unknown identity had increased prevalence in the denervated hippocampus. The changes, which ranged from 2- to 20-fold, involved mRNA encoding small slightly acidic polypeptides: 12 kDa (pI 5.6), 13 kDa (pI 6.1), 20 kDa (pI 5.8), 31 kDa (pI 5.7), 33 kDa (pI 5.7), 35 kDa (pI 5.6), and 53 kDa (pI 5.4). These results suggest new molecular markers for analyzing the complex mechanisms of synaptic reorganization in the dentate gyrus after deafferentation.

Acetylcholinesterase and Nissl staining in the same histological section

Acetylcholinesterase (AChE) enzyme histochemistry and Nissl staining are commonly utilized in neural architectonic studies. However, the opaque reaction deposit produced by the most commonly used AChE histochemical methods is not compatible with satisfactory Nissl staining. As a result, precise correlation of AChE and Nissl staining necessitates time-consuming comparisons of adjacent sections which may have differential shrinkage. Here, we have modified the Koelle-Friedenwald histochemical reaction for AChE by omitting the final intensification steps. The modified reaction yields a non-opaque reaction product that is selectively visualized by darkfield illumination. This non-intensified darkfield AChE (NIDA) reaction allows clear visualization of Nissl staining in the same histological section. This combined AChE-Nissl method greatly facilitates detailed correlation of enzyme and cytoarchitectonic organization.

Increase in acetylcholinesterase in the molecular layer of the dentate gyrus in the absence of septal inputs following selective granule cell lesions

Granule cell lesions cause an increase in acetylcholinesterase (AChE) staining in the molecular layer of the dentate gyrus. The source of this response was examined by combining granule cell lesions with lesions of the fornix-fimbria, thereby removing the cholinergic input from the septum to the hippocampus. The increased AChE staining was present in animals with granule cell lesions regardless of whether the fornix was lesioned or intact. The increase in AChE staining occurred without a corresponding increase in choline acetyltransferase staining. These findings suggest that an AChE-positive, but non-cholinergic, sprouting response occurred within the dentate gyrus following selective lesions of the granule cells. The source of this sprouting may be from AChE-positive hilar interneurons.

Confirmation of the cholinergic specificity of the Chol-1 gangliosides in mammalian brain using affinity-purified antisera and lesions affecting the cholinergic input to the hippocampus

An antiserum raised to Torpedo electromotor synaptosomal membranes (anti-TSM antiserum) induces a cholinergic-specific immune lysis of mammalian brain synaptosomes and recognizes a group of minor gangliosides appeared, therefore, to be specific to the cholinergic neuron and were designated Chol-1. To confirm the cholinergic specificity of the Chol-1 gangliosidic antigens, we have shown that not only does a mammalian ganglioside fraction that is enriched with respect to the Chol-1 gangliosides inhibit the cholinergic-specific immune lysis induced by the anti-TSM antiserum, but also it can be used to affinity-purify a subpopulation of immunoglobulins from the anti-TSM antiserum that also induce a cholinergic-specific lysis. Furthermore, we have demonstrated that fimbrial lesions, which cause a massive degeneration of cholinergic terminals in the ipsilateral hippocampus, lead to a loss of the Chol-1 gangliosides concomitant with that shown by choline acetyltransferase activity and that lesions to the entorhinal cortex, which cause a loss of mainly glutamergic synapses in the ipsilateral dentate gyrus leading to cholinergic sprouting from adjacent hippocampal areas and an increase in cholinergic markers in the dentate gyrus, produce concomitant increases in choline acetyltransferase activity and Chol-1 content. These results provide strong evidence in favour of the cholinergic specificity of the Chol-1 gangliosides.

Synaptic reorganization within the human central nervous system following injury

Behavioral recovery following brain injury in humans is well recognized; however, the anatomical basis for such recovery has not been demonstrated. Two cases are presented that show reorganization of synaptic connections (plasticity) in the dentate gyrus of human brain following uncal herniation. The neurohistological appearance of these cases is very similar to a well-described animal model of anatomical, physiological, and behavioral recovery following experimental surgical injury.

Long-term deficits in water maze spatial conditional alternation performance following retrohippocampal lesions in rats

The effects of large bilateral retrohippocampal lesions on long-term performance of conditional spatial alternation, incorporating a strong working memory component, were examined using a T-maze task motivated by swim-escape. The lesions, which included entorhinal cortex, subiculum, pre- and parasubiculum and invaded the molecular layer of the dentate gyrus, completely eliminated the previously acquired conditional alternation learning, and performance failed to recover with 40 days of testing. These findings support the contention that retrohippocampal structures are an important and necessary component of the neural circuitry mediating working memory.

Neural plasticity in vivo: opioid sensitivity of memory develops gradually after a septal lesion

Neuronal plasticity can manifest itself in alterations in the sensitivity of memory to the effects of drugs. After the production of a brain lesion, the memory processing of a passive-avoidance task in mice gradually becomes sensitive to the effect of morphine, i.e., an improvement in retention performance is seen after 6 weeks, but not after 1 or 2 weeks. The results presented demonstrate that, even if they lead to no discernible changes in behaviour, plastic processes can still be detected by means of behavioural tests.

Reorganization of neuronal connections following CNS trauma: principles and experimental paradigms

The present review summarizes how the nervous system responds to trauma. The goal is to provide an introduction to the problems, techniques, experimental paradigms, current issues, and future promise. The review is especially designed for basic scientists and clinicians who are not currently involved in research on CNS reorganization, and for students just entering the field. The review characterizes the secondary degenerative events that occur after trauma, and the types of growth that commonly occur. A standard terminology is set forth with criteria for differentiating between related phenomena. Experimental methods are described that can be used documenting reorganization of circuitry. The principles that determine whether a given process will or will not occur are summarized, and some of the factors that may regulate the nature and extent of growth are considered. Research strategies are outlined that have been used to evaluate whether reorganization of circuitry is functionally significant. Finally, future directions in research and clinical application are discussed, focusing especially on the efforts to facilitate regeneration, and the work on transplants of CNS tissue to facilitate growth of surviving connections, and to replace tissue destroyed by trauma.

Lesions of excitatory pathways reduce hippocampal cell death after transient forebrain ischemia in the gerbil

Transient forebrain ischemia produces a spatially and temporally selective pattern of neuronal degeneration in the hippocampal formation of the Mongolian gerbil. Ischemic neuronal death has been suggested to depend on the activation of excitatory hippocampal pathways that project to the vulnerable neurons. This idea was tested by examining the effect of a unilateral entorhinal cortical lesion or a unilateral knife cut lesion of intrahippocampal pathways on the neuropathology produced by 5 min of complete forebrain ischemia. A prior lesion of either the ipsilateral entorhinal cortex or the mossy fiber and Schaffer collateral-commissural pathways partially prevented the destruction of CA1b pyramidal cells in most animals. It did not, however, reduce the extent of ischemic neuronal death in any other hippocampal subfield. Within area CA1b, an entorhinal lesion protected an average of 23% of the pyramidal cells and a transection of both mossy and Schaffer collateral-commissural fibers protected an average of 36.5%. CA1b pyramidal cells saved from ischemia-induced degeneration appeared clearly abnormal when stained with cresyl violet or by silver impregnation. It is suggested that lesions of excitatory pathways attenuate ischemic damage to area CA1b by directly or indirectly reducing the level of synaptic excitation onto the vulnerable neurons. However, only a relatively small percentage of hippocampal neurons can be protected by these lesions in the gerbil ischemia model and there is reason to believe that the neurons protected in this manner may not be electrophysiologically competent. Synaptic excitation therefore appears to play an important, but not an essential, role in this model of ischemic brain damage.

Histochemical changes in enzymes of energy metabolism in the dentate gyrus accompany deafferentation and synaptic reorganization

The dentate gyrus of adult rats was examined histochemically for cytochrome oxidase and lactate dehydrogenase activity after unilateral lesions of the entorhinal cortex. In normal animals, synaptic terminal fields of the perforant pathway from the entorhinal cortex show high levels of cytochrome oxidase activity (the other two-thirds dentate molecular layer), whereas terminal zones of the commissural and associational fibers show high levels of lactate dehydrogenase activity (the inner one-third dentate molecular layer). Lesions of the entorhinal cortex result in a significant reduction in staining for cytochrome oxidase in the deafferented outer molecular layer of the dentate gyrus. The changes become prominent at 16-24 h after the lesion and persist until 90 days, the longest post-lesion survival time studied. In the non-deafferented inner zones ipsilateral to the lesion, there is an increase in staining for cytochrome oxidase and lactate dehydrogenase at 24 h post-lesion that disappears by days 2-4. From 8 to 90 days post-lesion, the band of high reactivity for lactate dehydrogenase in the inner molecular layer spreads approximately 40 microns into the overlying deafferented zone. This expansion parallels the expansion of the commissural and associational terminal fields into the adjacent deafferented molecular layer. Thus, lesion-induced synaptogenesis in the dentate gyrus is accompanied by a corresponding change in enzyme activity. The results indicate that the pattern of activity of enzymes involved in energy metabolism in the dentate gyrus depends on the distribution of pathway-specific synaptic input.

Chol-1: a cholinergic-specific ganglioside of possible significance in central nervous system neurochemistry and neuropathology

By use of an antiserum raised against presynaptic plasma membrane purified from the purely cholinergic electromotor system of Torpedo marmorata we have been able to identify a group of antigenically-related minor gangliosides (collectively designated Chol-1) that appear to be exclusively localized on cholinergic neurons. The cholinergic-specificity of these antigens has been shown by the following findings: a) The anti-Chol-1 antiserum induces a selective complement-mediated lysis of the cholinergic subpopulation of mammalian brain synaptosomes; b) Section of the fimbria, which causes a massive degeneration of cholinergic terminals in the hippocampus, leads to a concomitant depletion of the level of the Chol-1 gangliosides in the hippocampus; c) The anti-Chol-1 serum can be used to immunostain cholinergic elements in the central and peripheral nervous systems of the rat. The discovery of a cell surface cholinergic-specific antigen has provided a new and effective tool with which to study the cholinergic neuron. For instance, we have immuno-isolated cholinergic synaptosomes from rat cortex and used this preparation to study transmitter coexistence. Our results indicate that approximately 75% of the cortical cholinergic neurons also express the neuropeptide VIP. Furthermore, we are investigating the expression of Chol-1 in patients affected by diseases such as ALS which primarily involve central cholinergic neurons.

Embryonic entorhinal transplants project selectively to the deafferented entorhinal zone of adult mouse hippocampi, as demonstrated by the use of Thy-1 allelic immunohistochemistry. Effect of timing of transplantation in relation to deafferentation

The mouse Thy-1.1/Thy-1.2 allelic marking system is used to show that transplanted embryonic entorhinal cortex can reinnervate adult host hippocampi. The projection is limited to the appropriate terminal zones--viz. the outer two-thirds of the stratum moleculare of the dentate gyrus, and the stratum lacunosum-moleculare of the hippocampus--and extends for up to about 2 mm into the denervated host terminal field. The reconstruction of the entorhinal projections to the host requires direct contact between the embryonic donor tissue and the denervated adult host terminal field, and is dependent upon removal of the ipsilateral host entorhinal area. In the absence of an overall deafferenting host entorhinal lesion the transplanted entorhinal area forms only small local projections which are confined to areas which would have been locally deafferented as a result of direct damage to the host entorhinal afferents (i.e. during their intrahippocampal course) by the hippocampal lesion caused at the time when the transplant was inserted. The correct relative timing of deafferentation and transplantation is vital for the formation of the transplant-to-host projection. The host dendrites can be made receptive to entorhinal transplant projections by removal of the host entorhinal area at the time of transplantation. When deafferentation is performed first and transplantation is delayed, it is found that the deafferented host dendrites retain this receptivity even when deafferentation has been performed as much as two months before transplantation. Reversing the order of transplantation and deafferentation, however, shows that the transplants have only a transient ability to project to the deafferented host territory. Thus, transplants inserted and allowed to become established for one week before host deafferentation make very much reduced projections to the host, and from two weeks onwards are incapable of any detectable response to subsequent removal of the host entorhinal area. Coextensive with the formation of transplant-to-host entorhinodentate projections, the host entorhinal lesion also induces an intensification of the acetylcholinesterase staining of the host septodentate afferents in the denervated outer dentate stratum moleculare. The findings demonstrate the accurate reconstruction of a lost projection in adult brain by transplanting the appropriate type of embryonic tissue, but the results of altering the relative timing of deafferentation and transplantation raise currently unsolved questions about the nature of the competitive interactions between transplant and host axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Astrocytes are important for sprouting in the septohippocampal circuit

Damage to the fimbria-fornix, and separately to the perforant path, leads to distinct and dramatic time-dependent increases in glial fibrillary acidic protein immunoreactivity (GFAP-IR) in specific areas of the hippocampal formation. Specifically, fimbria-fornix lesions resulted in an increase in the GFAP-IR in the pyramidal and oriens area of the CA3 as well as the inner molecular layer of the dentate gyrus. In addition, in the septum ipsilateral to the lesion, there was a rapid and robust increase in GFAP-IR in the dorsal lateral quadrant of the septum, but not in the medial region. Only after 30 days did the GFAP-IR reach the medial septum. Following perforant path lesions, there was a selective increase in GFAP-IR in the outer molecular layer of the dentate gyrus. Most of these changes were transient and had disappeared by 30 days postlesion. We speculate that the increase in GFAP-IR in these target areas is a necessary requirement for the sprouting responses that are observed. This hypothesis is supported by the fact that astrocytes secrete NGF in vitro and that NGF activity increases in these target areas following these same lesions. A mechanism for the selective activation of the astrocytes through the initial activation of microglia and secretion of interleukin-1 is postulated.

Changes in septo-hippocampal projections after lateral entorhinal or combined entorhinal-raphé lesions as studied by anterograde tracing methods

Septal and entorhinal projections to the hippocampus show a considerable overlap in their target structures in the molecular layer of the dentate gyrus (DG) and stratum lacunosum-moleculare of the cornu ammonis (CA). Employing anterograde tracing methods, it was investigated in which way the morphological pattern of the septohippocampal projections were influenced by lateral entorhinal cortex (LEA) lesions. Anterograde filling of neurons from soma to axonal terminals with Phaseolus vulgaris leucoagglutinin (PHA-L) revealed lesion-induced changes in innervation patterns in the DG but not in CA fields. LEA lesions provoke an impressive shift of septo-dentate projections from a predominant middle molecular layer innervation to the outer molecular layer, whereas septal projections to the CA remain unchanged. Comparison with concurrent acetylcholinesterase (AChE) staining and immunocytochemical demonstration of choline acetyltransferase (ChAT) confirm the cholinergic nature of this plasticity response. This response was equally strong in unilateral or bilateral damage to the LEA and was neither enhanced nor inhibited by simultaneous injury to the median raphé nuclei.

Electron microscopy of plasticity in rat olfactory cortex

Electron microscopy (EM) is being used to study the ultrastructural basis for the age-dependent reorganization of afferents in the olfactory cortex (OC) of rat after deafferentation of the area by removal of the ipsilateral olfactory bulb (OB). The double-lesion technique was used with a primary lesion of the OB at various postnatal (PN) ages between PN 0 and 30 and in the adult (PN 100). After appropriate survival times to remove initial lesion-degenerated terminals from the OB lesion, a second lesion was placed in the ipsilateral OC. One to 3 days later the tissue is prepared for EM with emphasis on a study of changes in the superficial and deep dendritic layer (Ia and Ib respectively) rostral to the lesion. In control litter mates with both OBs intact, but with a single OC lesion only, degenerating synaptic terminals occur onto dendritic spines and branches only in deeper Ib. However, in adults with OB lesions at PN 0-9, OC lesions produce degenerating terminals throughout Ia and Ib including immediately subjacent to the pia. In Ia degenerating terminals are greatly reduced in the PN 13 group and rare to absent in experiments with OB lesions at older ages (PN 30-100). Electron-dense debris within glia occurs throughout layer I in each double-lesion group but is greatest in experiments with OB lesions at older ages. Some transsynaptic alterations are seen throughout, especially in the PN 30-100 group even at a distance from the OC lesion. The results support earlier light microscopic (LM) findings, suggesting PN 9-13 as critical ages for developmental plasticity and prove that at least in the younger ages, synapses are involved in the phenomenon. This may be explained by either reinnervation of deafferented sites or persistence of synapses that would otherwise have been eliminated by afferents from the OB. In addition, some of the LM degeneration particles probably are engulfed masses of debris and not synaptic structures, especially in cases which were operated at older ages and survived for 3 days. The various afferent pathways involved in the events as well as factors that limit the phenomenon in older ages are discussed.

Neuroplasticity and the progression of Alzheimer's disease

A neurobiological hypothesis is proposed to explain the relation between the percentage cell loss in the cholinergic basal forebrain and the density of neuritic plaques in cortex, as found by Arendt et al. (1985) in Alzheimer's disease: When cells in the cholinergic basal forebrain die, their cortical synaptic target sites can be reoccupied by axonal sprouting of other neurons from the basal forebrain. This neuroplasticity hypothesis leads to equations that are consistent with the quantitative data of Arendt et al. (1985), and it makes specific predictions that can be tested experimentally. Moreover, this hypothesis suggests that the more rapid course of the presenile form of Alzheimer's disease and its more extensive pathology can be understood as a consequence of the decline in neuroplasticity with age.

Time course and reversibility of ethanol's suppressive effects on axon sprouting in the dentate gyrus of the adult rat

Ethanol was administered chronically to adult rats in a liquid diet for 14 days preceding and for 5, 7, 8, 9, or 10 days following the unilateral destruction of the entorhinal cortex. Control groups received a diet of lab chow and water and were sacrificed at comparable survival times. An additional experimental group was given ethanol until 9 days after the lesion, then switched to lab chow and water and sacrificed 1 day later. Coronal sections through the dorsal hippocampal formation were stained and analyzed histochemically for the localization of acetylcholinesterase (AChE). Quantitative measurements of the histochemical patterns in the molecular layer of the dentate gyrus were obtained. Ethanol exposure inhibited the withdrawal of the acetylcholinesterase-stained septohippocampal fibers and limited the typical lesion-induced expansion of the pale-staining commissural/associational zone in the molecular layer of the denervated dentate gyrus. However, abstinence from ethanol for just 24 h released the inhibitory effect on the acetylcholinesterase-staining fibers, resulting in a significant expansion of the commissural/associational zone.

Denervation induces long-lasting changes in the distribution of microtubule proteins in hippocampal neurons

The cellular distributions of tubulin and microtubule-associated protein 2 (MAP2) were examined in the dentate gyrus of the rat hippocampus after unilateral lesion of the entorhinal cortex, which destroys the major afferent pathway to dentate granule cells. Changes were observed in distribution of both tubulin and MAP2 in granule cell dendrites on the denervated side. After 24 h there was a noticeable increase in both anti-tubulin and anti-MAP2 staining in the outer two-thirds of the dentate molecular layer, corresponding to the area of denervation. This increased staining reached a maximum 1 week after the lesion. There was no change on the unlesioned side. During a subsequent second phase the region of increased anti-tubulin and anti-MAP2 staining became restricted, by 35 days after lesioning, to a narrow band mid-way through the molecular layer. This pattern remained the same until 6 months after the lesion, the longest time point examined. The results indicate that there is considerable plasticity in the microtubular cytoskeleton of dendrites in the adult brain and that rearrangements induced in it by axotomy can persist long after the immediate effects of denervation and subsequent re-innervation have subsided.

Differential regulation of amyloid-beta-protein mRNA expression within hippocampal neuronal subpopulations in Alzheimer disease

We have mapped the neuroanatomical distribution of amyloid-beta-protein mRNA within neuronal subpopulations of the hippocampal formation in the cynomolgus monkey (Macaca fascicularis), normal aged human, and patients with Alzheimer disease. Amyloid-beta-protein mRNA appears to be expressed in all hippocampal neurons, but at different levels of abundance. In the central nervous system of monkey and normal aged human, image analysis shows that neurons of the dentate gyrus and cornu Ammonis fields contain a 2.5-times-greater hybridization signal than is present in neurons of the subiculum and entorhinal cortex. In contrast, in the Alzheimer disease hippocampal formation, the levels of amyloid-beta-protein mRNA in the cornu Ammonis field 3 and parasubiculum are equivalent. These findings suggest that within certain neuronal subpopulations cell type-specific regulation of amyloid-beta-protein gene expression may be altered in Alzheimer disease.

Ultrastructural changes in acetylcholinesterase activity in the deafferented spinal trigeminal nucleus

Acetylcholinesterase (AChE) activity was investigated in synaptic areas of the cat spinal trigeminal nucleus (pars interpolaris and pars caudalis) ipsilateral and contralateral to complete retrogasserian rhizotomy. Vibratome sections of tissue taken from animals of 1, 3, 6, 14, and 21 days survival were examined by electron microscopy following a histochemical reaction for AChE activity employing a method based on the Karnovsky-Roots technique for demonstrating reaction product. As degeneration progressed with survival time, enzymatic activity was initially reduced in synaptic clefts of injured afferent terminals and subsequently was enhanced throughout the extracellular space, including within synaptic clefts of possibly reinnervated sites. These changes in enzymatic activity with primary deafferentation are discussed in relation to the process of reinnervation, the development of neuronal hyperactivity, and possible noncholinergic functions of AChE.

Cholinomimetics induce theta rhythm and reduce hippocampal pyramidal cell excitability

The actions of cholinomimetics and of physostigmine were tested on two parameters reflecting hippocampal activity, namely theta activity and pyramidal cell excitability. In rats pretreated with methylscopolamine and anaesthetized with urethane i.v. administration of the cholinomimetics oxotremorine and arecoline and the cholinesterase blocker physostigmine evoked theta wave activity in the hippocampus, which was blocked by scopolamine. Spectral analysis demonstrated that the frequency of the theta waves induced was dose-related, ranging from about 3 Hz to between 5 and 6 Hz. theta Activity could not be induced by arecoline in animals with large septal lesions. Pyramidal cell excitability is known to be increased by endogenous acetylcholine released from cholinergic fibres. In the present study, however, i.v. injections of oxotremorine, arecoline and physostigmine in doses that induce theta activity diminished the excitability of CA1 pyramidal cells in a dose-dependent manner, as judged by the reduction in the amplitude of the population spike and the dendritic epsp. These depressant effects were attenuated by scopolamine but not by methylscopolamine. The depressant effect of arecoline was attenuated in rats with extensive lesions in the medial septal area. The present findings demonstrate that exogenously administered cholinomimetics only partly mimic the action of endogenous acetylcholine in the hippocampus. The central sites of action of exogenously administered cholinomimetics for mediation of theta activity and alteration of pyramidal cell excitability remain to be elucidated.

Ethanol intoxication fails to affect sprouting induced by entorhinal cortex lesions

After unilateral entorhinal cortex lesions, acute ethanol exposure (mean daily intake = 16.3 +/- 0.3 g/kg for 15 days) of juvenile rats failed to alter lesion-induced axonal sprouting in the dentate gyrus. The distribution of acetylcholinesterase in the dentate gyrus was identified histochemically as an indicator of axonal sprouting. Comparisons between operated and intact sides were based on qualitative observations and quantitative morphometry techniques using a computerized image analyser to evaluate the widths of the bands of the molecular layer. Whether ethanol-exposed or not, rats with unilateral entorhinal cortex lesions exhibited substantial qualitative and quantitative evidence of axonal sprouting. These results indicate that a 15-day post-operative ethanol exposure had no effect on axonal sprouting in juvenile rats and thus qualify previous findings about ethanol-mediated effects on axonal sprouting.

The response of the associational afferents to the dentate gyrus to simultaneous or sequential elimination of the commissural and entorhinal afferents

Previous studies have shown that following the removal of the commissural afferents to the dentate gyrus, the ipsilateral association afferents, which are normally distributed within the same region of the molecular layer, sprout new collateral branches and in time occupy essentially all the vacated synaptic sites. It is also known that when the entorhinal afferents to the dentate gyrus are interrupted the associational and commissural fibers can both undergo a similar phase of reactive synaptogenesis and give rise to new collaterals which extend for some distance into the denervated zone. Since the associational fibers can sprout after the removal of either the commissural or entorhinal afferents experiments were designed to determine their capacity for sprouting in newborn and young adult rats when both groups of afferents were eliminated either simultaneously or sequentially (i.e., after an interval of 8 weeks). The resulting changes in the terminal field of the associational afferents were assessed, two months after the last deafferentation, by measuring in autoradiographs the width of the zone occupied by the associational afferents labeled with [3H]proline, and by estimating the volume of this region in Timm-stained sections. The results indicate that under these conditions the associational afferents are capable of expanding their terminal field not only to occupy essentially all of the synaptic sites made available by the elimination of commissural fibers, but also to occupy a significant proportion of the space vacated by the removal of the entorhinal afferents. This suggests that the capacity of the associational afferents for reactive synaptogenesis is greater than that expressed after either commissural of entorhinal lesions alone.

Fetal brain transplants induce recuperation of taste aversion learning

Rats showing disrupted taste aversion due to gustatory neocortex or amygdala lesions were transplanted into the lesioned area with homologous brain tissue obtained from 17-day-old fetuses. Comparisons of taste aversions scores before and after the graft, revealed that the grafted animals significantly recuperated taste aversions, whereas cortical lesioned animals without grafts did not. Surprisingly, however, amygdala-lesioned animals without graft presented spontaneous recovery. These results not only support the hypothesis that fetal brain transplants can restore cognitive functions, but also that there are some fundamental functional differences between the gustatory neocortex and the amygdala in the regulation of the processes involved in the acquisition and retention of taste aversion.

Sympathetic sprouting in the central nervous system: a model for studies of axonal growth in the mature mammalian brain

Sympathetic fibers innervate many peripheral tissues but are normally confined to extracerebral structures within the cranial cavity, e.g. blood vessels. The invasion of the central nervous system by vascular sympathetic axons is a unique example of neuronal plasticity which provides new information concerning the regulation and mechanisms of neuronal sprouting in both the peripheral and central nervous systems. In this paper, the principal findings concerning the conditions under which such sprouting occurs, the mechanisms which may be involved, and the question of its possible function are reviewed. Of special interest is the fact that a nerve growth factor-like brain factor may be involved in this growth response. The principles gleaned from studies of this sprouting phenomenon may be applicable to other models of neuronal plasticity and may have clinical relevance.

Reinnervation of the hippocampal perforant pathway zone in Alzheimer's disease

The perforant pathway originates from the entorhinal cortex of the anterior parahippocampal gyrus and terminates on the outer dendritic branches of the granule cells of the dentate gyrus and pyramidal cells of the subiculum and hippocampus. It carries the principal cortical input to the hippocampal formation. Destruction of the perforant pathway in experimental animals leads to a partial deafferentation of its target neurons, followed by a robust sprouting of acetylcholinesterase (AChE) terminals in the deafferented perforant pathway zone. In Alzheimer's disease, the cells of origin of the perforant pathway are laden with neurofibrillary tangles. AChE staining in the terminal zone of the perforant pathway in Alzheimer's disease shows several distinct patterns that are not found in control brains. These changes are consistent with the results of experimental studies demonstrating reinnervation in laboratory mammals, including nonhuman primates. The results suggest that in Alzheimer's disease sprouting of AChE-containing systems occurs in the hippocampal formation in response to disease-related cellular damage in the entorhinal cortex.

Entorhinal lesions result in increased nerve growth factor-like growth-promoting activity in medium conditioned by hippocampal slices

Nerve growth factor (NGF) is present in high concentrations in the rat hippocampal formation where it may be involved in sympathetic sprouting following septohippocampal denervation. In addition, recent evidence suggests that some forebrain cholinergic neurons, including septohippocampal neurons, are responsive to exogenous NGF. Since septohippocampal neurons have been shown to sprout in response to entorhinal lesions both in rats and, recently, in humans, we sought to determine whether endogenous NGF-like activity increases in the rat hippocampal formation following injury to the entorhinal cortex. We found that entorhinal lesions which result in extensive denervation of the dentate granule cells, and subsequent sprouting of septohippocampal axons, do result in greater NGF-like growth-promoting activity in medium conditioned by slices of the denervated tissue when compared to medium conditioned by control tissue. These results suggest that brain NGF may be involved in injury-induced sprouting of forebrain cholinergic neurons.

Studies on retrograde and anterograde amnesia of olfactory memory after denervation of the hippocampus by entorhinal cortex lesions

The effect of hippocampal denervation on olfactory memory in rats was tested after interrupting the lateral olfactory tract projections at the level of the entorhinal cortex. When lesioned animals were trained to learn new odors, they showed no evidence of retention 3 h after acquisition. These results confirm earlier data on rapid forgetting in rats after hippocampal deafferentation and are in parallel to the anterograde amnesia typically found in humans with hippocampal damage. On the other hand, preoperatively learned information was minimally impaired after hippocampal deafferentation even if it was acquired within less than 1 h before the lesion. This finding differs from reports on humans as well as monkeys with hippocampal damage where memories formed during a critical time span of months or even years before the lesion are found to be impaired. This may suggest that the consolidation process in humans and rodents has different time scales or that the roles of the human and the rat hippocampal structure in memory formation are somewhat different.

Thalamic control of dopaminergic functions in the caudate-putamen of the rat--III. The effects of lesions in the parafascicular-intralaminar nuclei on D2 dopamine receptors and high affinity dopamine uptake

Dopamine receptor binding in the caudate-putamen was studied following bilateral lesions of the thalamostriatal pathway. Receptor binding was assayed using [3H]spiperone and defined with both (+)-butaclamol and S(-)-sulpiride. Radiofrequency lesions resulted in an increase in the Bmax of [3H]spiperone binding defined with both (+)-butaclamol and S(-)-sulpiride between 7 and 14 days following surgery. At longer survival times a fluctuating response was seen in which a decrease in receptor binding was observed at 28 days following lesion and a further rise again at 70 days. At no time point was significant change in Kd recorded. Further experiments were carried out to control for the possible effects of damage to fibres of passage and for inadvertent damage to habenula, as well as to define the receptor subtype involved. Ibotenic acid lesions resulted in similar effects to those reported with the radiofrequency method. Thus, 7 days following lesion, Bmax for (+)-butaclamol-defined [3H]spiperone binding increased by approximately 14-20% over that recorded in sham-lesioned animals. Using S(-)-sulpiride to define binding, Bmax was found to increase 13-17% in the same membrane preparations. Similar results were obtained in experiments at 14 days following ibotenic acid induced lesions. Again, no change in Kd was recorded. When radiofrequency lesions were made, which were largely restricted to habenula and associated fibres of passage, only small [(+)-butaclamol defined] or insignificant [S(-)-sulpiride defined] changes in Bmax were recorded. Combined radiofrequency lesions of habenula and ibotenic acid lesions of the thalamus resulted in a larger increase in Bmax for (+)-butaclamol defined binding than with S(-)-sulpiride defined binding. Our interpretation of these findings, in the light of the histology of the lesions, is that the predominant effect of removing thalamic input to the caudate-putamen is an increase in the number of D2 receptors, but without any change of affinity. A small component of the change in Bmax defined with (+)-butaclamol found with radiofrequency lesions may be due to a response at non-dopamine sites (possibly a 5-hydroxytryptamine receptor subtype) following damage to other caudate-putamen afferents which pass near the habenula or fasciculus retroflexus. Following unilateral ibotenic acid lesions of the thalamus, the number of high affinity uptake sites for dopamine was increased at long survival times.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholinergic neurons in the hippocampus. A combined light- and electron-microscopic immunocytochemical study in the rat

We report here on cholinergic neurons in the rat hippocampal formation that were identified by immunocytochemistry employing a monoclonal antibody against choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme. In general, ChAT-immunoreactive cells were rare, but were observed in all layers of the hippocampus proper and fascia dentata with a preponderance in zones adjacent to the hippocampal fissure and in the part of CA1 bordering the subiculum. All immunoreactive cells found were non-pyramidal neurons. They were relatively small with round or ovoid perikarya, which gave rise to thin spine-free dendrites. These hippocampal neurons were very similar to ChAT-immunoreactive cells in the neocortex of the same animals but were quite different from cholinergic neurons in the basal forebrain, medial septal nucleus, and neostriatum, which were larger and more intensely immunostained. Electron-microscopic analysis of ChAT-immunoreactive cells in the hippocampus and fascia dentata revealed synaptic contacts, mainly of the asymmetric type, on cell bodies and smooth proximal dendrites. The nuclei of the immunoreactive cells exhibited deep indentations, which are characteristic for non-pyramidal neurons. Our results provide evidence for an intrinsic source of the hippocampal cholinergic innervation in addition to the well-established septo-hippocampal cholinergic projection.

A comparison of the effects of climbing fiber deafferentation in adult and weanling rats

The climbing fiber input to the cerebellar cortex was destroyed using both electrolytic and chemical (3-acetylpyridine) lesions. The long-term effects of climbing fiber deafferentation on the ansiform lobule of weanling and adult rats were examined at both the light and electron microscopic levels. Image analysis of Golgi-impregnated Purkinje cells indicated a significantly lower number of smooth branches and spiny branchlets following climbing fiber deafferentation of both adult and weanling rats. The results suggest that the lower number of smooth branches and spiny branchlets following climbing fiber deafferentation of the weanling rat is the result of a loss of postnatal growth rather than transneuronal degeneration. Ultrastructural evidence is provided in confirmation of these quantitative findings. Formation of ectopic dendritic spines was found following climbing fiber deafferentation of the weanling rat, but not the adult. It is shown that ectopic spines and the denervated dendritic thorns of these animals were synaptically innervated by the parallel fiber system and basket axons. The formation of ectopic spines on climbing fiber deafferentated Purkinje cells may represent a form of dendritic plasticity. Ultrastructurally, the dendritic arborizations of weanling deafferentated Purkinje cells showed no signs of transneuronal degeneration. However, the primary response to climbing fiber deafferentation in the adult rat was marked transneuronal degeneration of the Purkinje cell dendrites. It is suggested that the inability of the adult Purkinje cell to form ectopic spines and to replace the excitatory postsynaptic potential of the climbing fiber varicosity is directly related to the Purkinje cell's subsequent transneuronal degeneration.

Learning sets, discrimination reversal, and hippocampal function

Impairments of discrimination reversal are commonly found following lesions of the hippocampal system. In a recent experiment, however, acquisition of a reversed discrimination was actually facilitated, rather than impaired, by partial lesions of the fimbria-fornix (FFX), an extrinsic fiber connection to the hippocampus. That experiment differed from most previous reversal experiments in that the discriminative stimuli were olfactory rather than spatial or visual, and 3 discriminations were given prior to the reversal of the third discrimination, rather than a single discrimination. The present experiment was designed to determine the generality of the results obtained in the olfactory experiment. Rats with partial lesions of the FFX and operated controls were tested on a series of 3 Go, No-go spatial discriminations, a reversal of the third discrimination, and 4 subsequent discriminations, in that order. Control rats acquired a learning set in the first 3 discriminations, reaching criterion on the third discrimination in fewer trials than they took on the first discrimination. Their choice accuracy was not significantly affected by the reversal and discrimination performance reached an asymptotic level after the reversal. Rats with FFX lesions also acquired a learning set in the first 3 discriminations, but they consistently took longer than control rats to learn each of the 7 discriminations. These rats were especially impaired on the reversal. These results contrast markedly with those obtained when a similar procedure was used with olfactory discriminations. Identification of the variables responsible for these differences should help distinguish those behaviors that require hippocampal function from those that do not.

Induced homotypic sprouting of serotonergic fibers in hippocampus. II. An immunocytochemistry study

The dorsal hippocampus of the rat normally receives its 5-HT innervation from two homologous groups of cells in the median raphe nucleus via the cingulum bundle-induseum griseum (CB-IG) and the fornix-fimbria (FF) (J. Comp. Neurol., 179 (1978) 641-667 and Brain Res. Bull., 10 (1983) 445-451). 5-HT immunoreactive (IR) fibers are distributed in a laminar pattern in the hippocampus. These fibers have large varicosities and are densely distributed in the infragranular layer of dentate gyrus, in the stratum lacunosum-moleculare of the cornu Ammonis and in the area fasciola cinerea (FC). The present study provides evidence that the density of the 5-HT-IR fibers in the dorsal hippocampus is greatly decreased but maintained a similar laminar pattern 3 days after lesioning by microinjection of 4 micrograms of 5,7-dihydroxytryptamine (5,7-DHT) into the CB-IG. An apparently normal density and distribution pattern of the 5-HT-IR fiber is seen by 42 days postlesion. The FC in the hippocampus is among the first regions reinnervated by 5-HT-IR fibers with very dense and large varicosities. The restitution of 5-HT-IR fibers in the dorsal hippocampus after the 5,7-DHT lesion in the CB-IG is accompanied by a marked increase in the number and intensity of 5-HT-IR fibers in the FF. No evidence of a regeneration of 5-HT-IR fibers is seen distal to the injection site in the CB-IG. These observations provide direct evidence for homotypic collateral sprouting in the CNS induced by removal of a single fiber type.

Plasticity of hippocampal circuitry in Alzheimer's disease

Two markers of neuronal plasticity were used to compare the response of the human central nervous system to neuronal loss resulting from Alzheimer's disease with the response of rats to a similar neuronal loss induced by lesions. In rats that had received lesions of the entorhinal cortex, axon sprouting of commissural and associational fibers into the denervated molecular layer of the dentate gyrus was paralleled by a spread in the distribution of tritiated kainic acid-binding sites. A similar expansion of kainic acid receptor distribution was observed in hippocampal samples obtained postmortem from patients with Alzheimer's disease. An enhancement of acetylcholinesterase activity in the dentate gyrus molecular layer, indicative of septal afferent sprouting, was also observed in those patients with a minimal loss of cholinergic neurons. These results are evidence that the central nervous system is capable of a plastic response in Alzheimer's disease. Adaptive growth responses occur along with the degenerative events.

Replacement of damaged cortical projections by homotypic transplants of entorhinal cortex

The extent to which transplants of embryonic cortical tissue can be used to replace damaged cortical projections has been examined. Embryonic entorhinal cortex was implanted into the entorhinal region of young adult rats that had previously received a lesion through the angular bundle. Projections between transplant and host were examined by using WGA-HRP and the fluorescent dye Fast Blue. Implants selectively innervated areas of the host hippocampus and amygdala which normally receive entorhinal afferents. Implants were innervated by cells in the host diagonal band and, in one case, by cells in the contralateral entorhinal and/or presubicular cortex. In most cases, host fibers were differentially distributed within transplants, possibly reflecting an ability of host fibers to recognize and selectively innervate their appropriate targets even though the cellular organization of the implant is different from that present during normal development. These data suggest that homotypic implants of embryonic entorhinal cortex can, in some ways, replace severed cortical projections and may eventually be able to reconstitute normal cortical circuitry.