An electron microscopic study of lesion-induced synaptogenesis in the dentate gyrus of the adult rat. II. Reappearance of morphologically normal synaptic contacts

Cellular and connective organization of slice cultures of the rat hippocampus and fascia dentata

This study examined the cellular and connective organization of hippocampal tissue taken from 6-8-day-old rats and cultured by the roller tube technique for 3-6 weeks. In the cultures containing the fascia dentata and the hippocampus proper (CA1, CA3, CA4) the main cell and neuropil layers were organotypically organized when observed in ordinary cell stains. The normal distribution of smaller cell populations of AChE-positive neurons and somatostatin-reactive neurons was demonstrated by histochemical and immunohistochemical methods. Both cell types were mainly confined to str. oriens of CA3 and CA1 and the dentate hilus (CA4). Individual dentate granule cells and hippocampal pyramidal cells were injected with lucifer yellow and HRP, revealing great stability of the dendritic patterns of these cells in the culture condition. The same was found for the axonal branching and termination of HRP-filled mossy fibers arising from an HRP-injected granule cell. The preservation of organotypic afferent patterns in the cultures was also shown by Timm staining of the terminal distribution of the mossy fiber system. Mossy fiber terminals, with characteristic ultrastructural features verified in the electron microscope, were thus found in the hilus (CA4) and along the CA3 pyramidal cell layer onto the CA3-CA1 transition. Depending on the amount of dentate tissue relative to CA3 the terminals could stop before reaching CA1 (small fascia dentata) or take up additional intra and infrapyramidal locations along CA3 (small CA3). In cultures with a gap in the CA3 pyramidal cell layer some mossy fiber terminals were found in contact with the CA3 pyramidal cells beyond the gap. In all cultures there was an aberrant projection of supragranular mossy fibers. This projection is analogous to the one known from lesion and transplant studies to form in the absence of the entorhinal perforant path input to the dentate molecular layer. Also, in accordance with these studies the Timm staining pattern of the outer parts of the dentate molecular layer and the entire molecular layer of the hippocampus was altered corresponding to the spread of afferents normally confined to the inner zone of the dentate and str. radiatum of CA3 and CA1. Possibly as a consequence of the lack of normal targets for projections from CA1, this subfield contained an unusually dense Timm staining suggestive of autoinnervation.(ABSTRACT TRUNCATED AT 400 WORDS)

Remodeling of the cerebellar glomeruli after long-term alcohol consumption in the adult rat

The morphological effects of the prolonged alcohol consumption on the cerebellar glomeruli of adult rats were studied in groups of controls and animals fed alcohol for 1, 3, 6, 12 and 18 months, by applying qualitative and quantitative ultrastructural methods. Following 6 months of alcohol consumption degenerated granule cell dendritic profiles were randomly dispersed over the granular layer surrounding mossy fiber terminals. In 12- and 18-month alcohol-fed groups, some glomeruli appeared with an atrophic design owing to the lack of their post-synaptic targets while others presented a remodeled organization, with Golgi cell dendrites replacing the missing granule cell digits. An extensive glial reaction was seen investing both types of glomeruli. The total number of synapses per glomerulus remained unchanged in all groups. This is due to an increase of the mossy fiber terminal--Golgi cell dendrite synapses, which compensate the decrease of the mossy fiber terminal--granule cell dendrite synapses. It is suggested that the morphological remodeling of the cerebellar glomeruli after long-term alcohol consumption can lead to changes in the balance of the excitatory-inhibitory activities of the cerebellar circuitry. This could be related to the important functional changes observed in the cerebellum under these circumstances.

Reciprocal relationship between size of postsynaptic densities and their number: constancy in contact area

Plasticity in the size of postsynaptic membrane specializations (postsynaptic densities) was analyzed by quantitation following lesioning of parallel fiber afferent axons to Purkinje cells in the cerebellum. Double sectioning of parallel fibers in the same folium as well as longitudinal undercutting of the molecular layer to destroy granule cells and their parallel fibers were used to produce various levels of afferent reduction to Purkinje cells. The length of profiles of membrane densities was measured utilizing semi-automated, computer-electron microscopy and the number of synapses was determined from their volume density and changes in volume estimated from cortical thickness. Correlation between the number of synapses on Purkinje cells and their average contacts area revealed a reciprocal relationship throughout a range of 0-67% reductions in parallel fiber synapses. Larger reduction levels had a progressive decrease in average size of contacted postsynaptic densities and an accompanying increase in the number of vacant postsynaptic specializations. Total absence of parallel fibers resulted in nearly all vacant sites on spines (except for a few connections with boutons having irregular shaped vesicles). These vacant sites were, on the average, only half the size of controls but their number was approximately double the control amount. This study confirms that location and size of synapses are not permanent and that plasticity in size of contacts allows reorganization in circuitry to compensate alterations in the number of inputs following a number of perturbations. The finding of a reciprocal relationship indicates that total contact area on Purkinje cells remained relatively constant throughout the entire range of reductions in the number of afferents. A 'constancy principle for total postsynaptic contact area' is envisioned to stabilize functional aspects of circuitry during developmental organization and to direct compensation following reduction in pre- or postsynaptic neurons from environmental effects or attrition in aging. Constancy in target area provides a realm under which the size of individual synapses can be modified as functional adaptations in circuitry even without changes in the number of connections.

Synaptic junctions isolated from cerebellum and forebrain: comparisons of morphological and molecular properties

Synaptic junction (SJ) fractions have been isolated from rat cerebellum which are similar to forebrain SJs on the basis of morphology and enrichment of synaptic structures. The polypeptide compositions of SJ fractions were analyzed by one- and two-dimensional SDS-gel electrophoresis and stained by the silver technique. Equivalent numbers of proteins that possess similar relative mobilities (Mr), isoelectric points and staining intensities were present in cerebellum and forebrain synaptic fractions. A few prominent differences were observed between cerebellum and forebrain synaptic fractions; cerebellum SJs contained a 240 K protein that was not detected in the forebrain and the 52,000 K, major PSDp protein was present in forebrain SJs in amounts that are approximately 5-fold greater than in cerebellum SJ fractions. The identity of the cerebellum mPSDp was verified by electrophoretic mobility, peptide fingerprinting and [125I]calmodulin binding. Differences between various synaptic fractions in mannose containing glycoproteins were examined by the binding of [125I]concanavalin A (Con A) to gels. On the basis of apparent molecular weights, the glycoproteins in forebrain and cerebellum SPMs were very similar. In contrast, however, the prominent glycoproteins that reside in the postsynaptic junctional membrane of forebrain SJs were undetectable in SJ fractions isolated from cerebellum. SJ fractions from cerebellum contained their own distinct group of Con A binding glycoproteins. SPM and SJ fractions from forebrain and cerebellum were examined for receptors for excitatory (aspartate, glutamate and kainic acid) and inhibitory (GABA) neurotransmitters and the benzodiazepine analog flunitrazepam. On the basis of relative receptor contents, SJ fractions isolated from either brain region were qualitatively similar and bound significant amounts of excitatory and inhibitory transmitters. These findings support the notion that SJs from cerebellum contain a distinct class of synaptic elements that are in large part derived from asymmetric, type I synapses.

Learning deficits after lesions of dentate gyrus granule cells

The effect of destroying granule cells in the dentate gyrus on learning was examined with a behavioral testing protocol. These neurons were destroyed by microinjections of the selective neurotoxin colchicine in the hippocampal formation of rats. After a 30-day recovery period, the animals were trained in an operant chamber with an appetitive conditioning paradigm. The learning abilities of the animals with lesions were compared with two control groups--naive, unoperated rats and those with control injections of saline. The basic task required the animal to discriminate between two spatially separate visual stimuli which represented positive and negative cues. Testing and training was separated into four progressively more difficult phases with various stimulus schedules, contingencies of reinforcement, and stimulus positions. Colchicine-treated animals demonstrated significantly poorer performance than naive animals and those receiving saline control injections. None of the colchicine-treated animals achieved criterion performance in the stimulus position reversal paradigm, and half had difficulty with variable ratio schedules of reinforcement. Our experiments suggested that granule cells in the dentate gyrus played a pivotal role in certain learning tasks.

Entorhinal lesions result in shrinkage of the outer molecular layer of rat dentate gyrus leading subsequently to an apparent increase of glutamate decarboxylase and cytochrome oxidase activities

In intact dentate gyrus, glutamate decarboxylase immunoreactivity (GAD) and cytochrome oxidase activity (CyO) showed different distributions patterns. Entorhinal lesions caused increases of GAD and CyO in the outer molecular layer (OML) of the ipsilateral side. Submicroscopical localization of these enzymes did not change, except for CyO labeling more astrocytic mitochondria. The increase in numerical density of GAD puncta correlated quantitatively with shrinkage of OML, whereas in the whole molecular layer the number of GAD puncta remained unchanged. Hence, the localized increase of enzyme activities and lysosomes is apparently related to shrinkage of OML, but does not indicate plasticity of GABAergic neurons.

Enhanced acetylcholinesterase staining in the hippocampal perforant pathway zone after combined lesions of the septum and entorhinal cortex

A lesion of the septum or a transection of the fimbria-fornix diminishes most, but not all, acetylcholinesterase (AChE) staining in the hippocampal formation. The residual AChE is located in the outer part of the molecular layer of the hippocampal CA1 zone and adjacent subicular field (zone 31). We report that following combined lesions of the septum and entorhinal cortex, the residual hippocampal AChE staining pattern expands and occupies the zone innervated normally by perforant pathway terminals from the entorhinal cortex.

Rate of synaptic replacement in denervated rat hippocampus declines precipitously from the juvenile period to adulthood

Synaptic contacts per unit area in the rat dentate gyrus reach adult numbers by the end of the first month after birth and remain constant thereafter. This experiment demonstrated that the rate at which synapses were replaced by sprouting after a lesion declined dramatically from 35 to 90 days of age. Thus, the juvenile period of the rat's life is marked by a considerable change in neuronal plasticity. This may be related to age-dependent effects in recovery from brain damage.

Ganglioside treatment improves recovery of alternation behavior after unilateral entorhinal cortex lesion

Exogenous gangliosides have been reported to enhance neurite formation in vitro and in vivo after damage to peripheral nerves. We report here the effects of ganglioside treatment on the course of recovery of alternation behavior that follows a unilateral lesion of the rat entorhinal cortex. The recovery of this function is known to parallel rapid synaptic reinnervation (collateral sprouting) into the partially denervated dentate gyrus of the hippocampus which previously received afferent input from the entorhinal region. Rats trained on an alternation behavior were subjected to a unilateral entorhinal lesion and subsequently given daily injections of total ganglioside (50 mg/kg, i.m.). Testing of the behavior continued for 2 weeks to assess the extent of behavioral impairment and the rate of recovery. Rats treated with gangliosides showed reduced behavioral impairment, accelerated recovery of the learned behavior, and final performance levels greater than controls. We hypothesize that the gangliosides may be interacting with regenerating neuronal membranes, either acting as receptors for trophic growth factors or altering membrane structure itself.

Relative slowness of heterotypic synaptogenesis in the septal nuclei

The dorsolateral quadrant of the lateral septal nucleus receives a bilateral projection from the fimbria. When the fimbria of one side is cut, the axons of the remaining fimbria take over its synaptic sites preferentially, but when both fimbrias are cut the sites are reinnervated by non-fimbrial axons. To explore the basis of this preference, the present study plots the time courses of the appearance and disappearance of degenerating synapses, and the loss and recovery of non-degenerating synapses after ipsi-, contra- and bi-lateral fimbrial lesions. A preliminary investigation showed that at any time after these three lesions there was no change in the numerical density per unit area of 'control' structures such as shaft synapses (which do not degenerate) and neuronal perikarya (which neither shrink nor degenerate). This indicates that the changes in the numerical density of fimbrial (spine) synapses can be used as a measure of the processes of deafferentation and reinnervation without the danger of the numerical data being distorted by shrinkage. In the sampled area, the ipsilateral fimbrial axons account for about 45% of the synapses and the contralateral fimbrial axons for 25%. The number of degenerating synapses appearing at any one time underestimates the loss of non-degenerating synapses by about one-third, and a photographic simulation of degeneration suggests that a major factor in this discrepancy is the difficulty in recognizing degenerating synapses. Our main finding is that there is a major delay in the rate of removal of degeneration, and in the rate of reinnervation, after bilateral as opposed to unilateral lesions. This delay cannot be accounted for in any simple way by the greater amounts of degeneration. Thus after unilateral lesions, which cause the turnover of 25% (contralateral) or 45% (ipsilateral) of the synapses, 50% of the degeneration is removed in 1-2 days after the peak, whereas after bilateral lesions, which affect 70% of the synapses, it takes 20 days for 50% of the degeneration to be removed. That the synaptic changes after bilateral lesions involve a qualitatively different mechanism is also suggested by the observations of a much greater proportional increase in the multiple synapse index, and a decreased astroglial response.

Plasticity in the central nervous system: do synapses divide?

Changes in the proportion of synapses containing postsynaptic densities with perforations during periods of increased synapse formation have led us to propose a hypothesis describing a possible division of preexisting synapses. Relevant features of this model are that various types of stimulation result in the following sequence of events: (i) the synaptic junction increases in area; (ii) a perforation forms in the enlarging synaptic junction; (iii) a synaptic spinule appears apposed to the perforation in the postsynaptic density; (iv) the perforation in the synaptic junction increases in size until the synaptic junction splits into two separate synaptic junctions within the same synaptic terminal; and (v) the dendritic spine divides into two, each containing a synaptic junction. Physiological responses in which synapse division may possibly play a role include hormone-induced neuronal changes, reinnervation of dendrites after lesions, and learning and memory.

Heart conditioned medium elicits post-lesion muscarinic receptor recovery in vivo

Intraseptal administration of heart conditioned medium (HCM) stimulates the growth of injured cholinergic fibers into iris implants placed in the anterodorsal hippocampus. The aims of the present report were to monitor muscarinic cholinergic receptor concentrations during denervation and central innervation of the peripheral tissue targets, and to evaluate the effect of HCM on these changes. For 0, 4, 8, 16 and 28 days after mechanical injury to septohippocampal axons, animals received either intraseptal injections of HCM or control vehicle. Binding of [3H]quinuclidinylbenzilate ([3H]QNB) within iris implants was used as an index of cholinergic muscarinic receptor concentration. The results indicate that within the iris implants: (1) a dramatic drop in the number of muscarinic receptors is observed 4 days after denervation; (2) under control conditions, central cholinergic innervation is not associated with muscarinic receptor recovery; and (3) after administration of HCM, muscarinic receptor levels begin to increase within two weeks and approach the pre-lesion endogenous concentration following 28 days of treatment. These results support the hypothesis that trophic factors may facilitate the restoration of effective, appropriate connections between nerve fibers and their targets.

Lesion-induced mossy fibers to the molecular layer of the rat fascia dentata: identification of postsynaptic granule cells by the Golgi-EM technique

The axons of the dentate granule cells, the hippocampal mossy fibers, sprout "backward" into the dentate molecular layer when this is heavily denervated. Using the combined Golgi-electron microscopy (EM) technique we now demonstrate that these aberrant supragranular mossy fibers at least in part terminate on granule cell dendrites. Sprouting of mossy fibers into the dentate molecular layer was induced in adult rats by simultaneous surgical removal of the commissural and entorhinal afferents to the fascia dentata. After at least 7 weeks survival, the presence of mossy fiber terminals in the inner part of the dentate molecular layer was demonstrated by light microscopy. In the electron microscope the mossy fiber terminals were identified by their unique structural characteristics, namely, the unusually large size of the terminals, the dense packing of clear synaptic vesicles with a few dense core vesicles intermingled, the presence of asymmetric synaptic contacts with spines and desmosome-like contacts with dendritic shafts, and the continuity with a thin unmyelinated preterminal axon. Golgi-stained granule cells were first identified in the light microscope, and then, after deimpregnation, the same cells were examined in the electron microscope. In ultrathin, serial sections lesion-induced mossy fiber terminals were found in synaptic contact with spines on proximal dendritic segments of such identified Golgi-impregnated granule cells. From this we conclude that the aberrant, supragranular mossy fibers can innervate dendrites of the parent cell group, the dentate granule cells. The results, moreover, provide an example of reactive synaptogenesis where both the sprouted afferents and its postsynaptic element have been identified.

Lysosomal enzyme changes in young and aged control and entorhinal-lesioned rats

In the hippocampus of young and aged rats, lysosomal acid phosphatase and beta-glucuronidase were localized by enzyme histochemistry to the pyramidal and granule cells and to the mossy fiber zone (acid phosphatase only). Following a unilateral entorhinal lesion, enhancement in enzyme staining was observed within GFA (glial fibrillary acidic) protein-positive astrocytes and in cells resembling microglia and oligodendroglia in the ipsilateral dentate outer molecular layer and the hippocampal stratum lacunosum-moleculare. In young animals, these changes, first detected on day 2 post-lesion, were maximum on days 4 and 10 post-lesion and declined subsequently. The aged animals exhibited a delayed time-course of enzyme changes with minimal increases on days 2 and 4 post-lesion and greater increases on days 10 and 30 and gradual diminution thereafter. The delayed lysosomal response in the aged animal paralleled a reduction in the induction of glial cells following the lesion. Our findings suggest that the previously reported impairment in the clearing of degeneration debris in the aged animal is mediated by reduced glial induction and delayed glial lysosomal activation.

'Free' postsynaptic-like densities in normal adult brain: their occurrence, distribution, structure and association with subsurface cisterns

Samples of cerebral cortex (parietal and occipital) and thalamic nuclei (ventrobasal, posterolateral, dorsal lateral geniculate) from normal, adult, aldehyde perfusion fixed mice and rats were examined by electron microscopy for the presence of free postsynaptic-like densities (FPSDs). FPSDs are plaques of intracellular paramembranous electron-dense material, ultrastructurally indistinguishable from postsynaptic densities, but not aligned with a presynaptic specialization. In a systematic survey of the neuropil around 6000 neuronal perikarya, 250 FPSDs were encountered. Almost all of these were within dendritic spines and shafts and about 90% of them were apposed by a neuronal perikaryon, the remainder by a dendritic shaft. In every case a subsurface cistern (SSC) was present in the cell body or dendrite apposed to the FPSD, and was flattened along the extent of the FPSD. In none of the material were the FPSDs associated, even remotely, with degenerating elements, suggesting that they are not formed by degeneration of presynaptic boutons. The incidence of FPSD-SSC complexes was higher in thalamus than in cerebral cortex which, together with previous observations indicating their absence from normal cerebellar cortex, suggests significant regional variations in distribution. It is suggested that FPSDs might represent synaptic precursors perhaps induced to form as a response to loss (possibly age-dependent loss) of synaptic contacts on a neuron and that the SSCs are somehow involved in maintaining the FPSDs and/or preparing them for innervation by adjacent axon terminals to form new synaptic contacts.

Synaptic remodelling and astrocytic hypertrophy in rat cerebral cortex from early to late adulthood

Ultrastructural observation of the molecular layer of the parietal cortex of rats, aged 3, 6, 10 and 17 months, revealed various atypical synaptic profiles besides typical synapses. The atypical synapses were frequently in the vicinity of hypertrophied astrocytic profiles, and were sometimes completely surrounded by astrocytic processes. The presynaptic terminal contained either no vesicles or a few small distorted vesicles. Vacant postsynaptic terminals were occasionally seen. The total surface area of astrocytic profiles and the numbers of atypical synapses increased significantly between 3 and 10 months. The astrocytic acquisition of degenerating terminals was repeatedly observed over this period. Since there was no decrease in total synaptic number at this age, the astrocytic phenomenon may represent a stage in a continuous cycle of synaptic loss and replacement in the normal brain. By 17 months, when total synapse numbers decrease, synaptic replacement may be less than optimal.

Plasticity of the parallel fiber-Purkinje cell synapse by spine takeover and new synapse formation in the adult rat

Alteration in synaptic connectivity between Purkinje cell spines and parallel fibers of the cerebellum were studied following partial deafferentation of Purkinje cells in the the adult rat. Transection of parallel fibers by two lesions placed at a 1 mm interval on the folial crest were used to produce degeneration of these afferents. Ultrastructural analysis of synapses on Purkinje cell spines revealed degeneration with vacating of postsynaptic sites within 6 h. Reactive synaptogenesis as takeover of Purkinje cell spines by formation of new synapses from remaining parallel fibers occurred even before degenerating parallel fibers had vacated postsynaptic sites. This was accompanied by a marked increase in the number of dual innervations by reactive parallel fibers within one day. Some vacated postsynaptic sites were lost as indicated by a reduction in the number of synapses and others may have been taken over by newly formed synapses on spines. In addition, new synapses formed between the shafts of Purkinje cell branchlets and parallel fibers. Sprouting of parallel fibers occurred as small extensions without tubules while Purkinje cell spines reacted by forming elongated and multiple heads which contacted different parallel fibers. After 5 days degenerating boutons were rarely found. Enlarged spine heads were each capped by a proportionally enlarged parallel fiber bouton and joined by an elongated synaptic junction to parallel fibers. Some parallel fiber boutons were greatly enlarged and capped numerous profiles of spines. This study shows that formation of new pre- and postsynaptic sites takes precedence over reoccupation of original contacts and that multiple synapses on individual spines are being eliminated to give rise to single contacts with boutons. This elimination resulted in enlargement of synaptic contact areas between Purkinje cell spines and parallel fibers by taking over postsynaptic sites from some vacated and eliminated boutons.

Neurons in the rat dentate gyrus granular layer substantially increase during juvenile and adult life

Volumetric estimates of the total number of granule cells in rats 30, 120, 200, and 365 days old increase linearly by approximately 35 to 43 percent between 1 month and 1 year. Total volume of the granular layer also grows linearly during that time. These results demonstrate a numerical increase in a neuronal population during adulthood in the mammalian brain.

Lesion-induced synaptogenesis in the dentate gyrus of aged rats: I. Loss and reacquisition of normal synaptic density

Quantitative electron microscopy was used to examine the ability of aged (2-year-old) and young adult (90-day-old) rats to replace those synapses lost (85-90%) in the outer two-thirds of the molecular layer of the dentate gyrus after a complete unilateral lesion of the entorhinal cortex. In aged rats the synaptic density is significantly lower than that of young adults at 10 days postlesion. Synaptic replacement begins between 2 and 4 days postlesion in young adults, whereas there is a delay until after 10 days postlesion in aged rats. Once synapse replacement begins in aged rats, the rate of synapse reappearance is about equal that of young adults. Thus the initial 10 days postlesion appears critical to growth of responding afferents and reformation of synaptic contacts. Analysis of synapses in terms of noncomplex and complex synaptic types shows that the noncomplex type accounts for the significant synaptic density difference between the two age groups. Replacement of complex synapses is nearly indistinguishable between age groups and is complete by 60 days postlesion. In contrast the initial replacement rate of noncomplex synapses in aged rats is much slower than young adults, though the control synaptic density is achieved by the end of the time course.

The effect of entorhinal cortical ablation on the distribution of muscarinic cholinergic receptors in the rat hippocampus

Removal of the entorhinal cortical projection to the hippocampus in adult rats decreased the density of muscarinic cholinergic receptors in the denervated dentate gyrus outer molecular layer at two days postlesion. Thirty days following the lesion (in adults and neonates) there is a small receptor density increase in the outer molecular layer (may be due to tissue shrinkage), and a larger increase in the lacunosum-moleculare. The receptor density decrease seen two days postlesion suggests the presence of presynaptic muscarinic receptors on the lost entorhinal cortical fibers. The distribution and extent of the receptor changes seen at 30 days postlesion are inconsistent with the cholinergic fiber reorganization which follows an entorhinal cortical lesion, but are consistent with a proposed model of non-cholinergic afferent mediated control of muscarinic receptor density in the rat hippocampus.

The sprouting of septal afferents to the dentate gyrus after lesions of the entorhinal cortex in adult rats

The projection of the septum to the dentate gyrus has been demonstrated autoradiographically and the pattern of acetylcholinesterase (AChE) staining in the dentate gyrus has been mapped histochemically, in a series of normal young adult rats and in a group of animals in which the entorhinal cortex had been ablated or its efferents to the dentate gyrus interrupted, some weeks earlier. It is clear from this material that the normal disposition of the septal projection to the dentate gyrus differs significantly from the pattern of AChE staining; however, in the denervated region of the molecular layer in the experimental animals there is a marked increase in the density of the septal projection which precisely coincides with the zone of intensification of AChE staining. It follows from this that although the distribution AChE does not accurately reflect the organization of the septo-dentate projection in normal animals, the intensification of AChE staining provides a good indication of the reorganization which occurs in this pathway following entorhinal deafferentation.

Sprouting in the avian brainstem auditory pathway: dependence on dendritic integrity

The brainstem auditory pathway of the chicken were used to examine the relationship between the maintenance of dendrites following denervation and the successful reinnervation (sprouting) by surviving afferents. In the system the third-order cells in n. laminaris receive spatially segregated binaural innervation from n. magnocellularis. Afferents from the ipsilateral n. magnocellularis innervate the dendrites on the dorsal aspect of n. laminaris cells, while afferents from contralateral magnocellular neurons innervate ventral dendrites via the crossed dorsal cochlear tract. Denervation of the ventral dendrites of n. laminaris cells by transection at the midline results in rapid and severe atrophy of the denervated dendrite. Unilateral cochlea removal induces transneuronal degeneration of 30-45% of the ipsilateral magnocellular cells, thereby partially denervating one dendrite of the n. laminaris cells on each side of the brain. In animals with long-standing transections of the crossed dorsal cochlear tract there is no evidence of sprouting the fibers from the ipsilateral n. magnocellularis when the projections of the surviving magnocellular neurons are traced with degeneration methods after a secondary cochlea removal. However, when dendrites of n. laminaris are partially denervated dendrites do not disappear. Furthermore, secondary lesions of the crossed dorsal cochlear tract or secondary cochlea removal reveal that these denervated dendrites are reinnervated by the afferents from the opposite n. magnocellularis which are normally restricted to the opposite dendrite of the n. laminaris cells.

A new type of lesion-induced synaptogenesis: II. The effect of aging on synaptic turnover in non-denervated zones

Partial denervation of the dentate molecular layer causes sprouting and reinnervation by undamaged afferents within the denervated zones of young adult animals and to a lesser extent in aged animals. We have previously reported a non-degenerative remodeling of the dentate molecular layer in areas outside the primary denervated zone of young adult rats after a unilateral entorhinal lesion. In this study, we evaluate the response of aged rats under the same conditions, to see if aged animals also respond to injury in non-denervated zones. After a unilateral entorhinal lesion, the outer two-thirds of the ipsilateral dentate molecular layer loses about 85% of its input, while the outer two-thirds of the contralateral molecular layer loses less than 5% of its input (crossed temporo-dentate path). Denervation does not occur in the inner one-third of the molecular layer on either side. Within the ipsilateral inner molecular layer, the synaptic density rapidly drops 21% in the absence of degeneration and then recovers by 10 days post-lesion, as is the case in young adult animals. On the contralateral side, young adult animals show synapse turnover similar to the ipsilateral inner molecular layer. In contrast, no significant response in the total synaptic density was observed in the non-denervated contralateral inner molecular layer or the partially denervated outer two-thirds of the contralateral molecular layer. Thus, in aged animals, synaptic turnover is restricted to the massively denervated ipsilateral side. The small loss of input to the contralateral side apparently is not sufficient to initiate quantifiable turnover of synaptic contacts. This steady-state situation may be the result of an on-going stabilization of neuronal circuitry, which may limit restoration of function after injury in aged animals.

A new type of lesion-induced synaptogenesis: I. Synaptic turnover in non-denervated zones of the dentate gyrus in young adult rats

It is well established that partial denervation causes the formation of new synapses within denervated areas. It is also possible that synapse formation and remodeling occurs outside denervated zones. In this study we evaluate this possibility by examining the effect of a unilateral entorhinal lesion on the number and characteristics of synapses in non-denervated zones of the dentate gyrus within the hippocampal formation. A unilateral entorhinal lesion massively denervates the outer two-thirds of the ipsilateral dentate molecular layer and also causes a minor loss of synapses in the outer two-thirds of the contralateral dentate molecular layer. The inner one-third of the molecular layer is not denervated on either side. In the ipsilateral inner molecular layer the number of synapses rapidly decreases by about 20% and recovers by 10 days post-lesion. Similarly, in the contralateral inner molecular layer, synapses are lost and replaced, but the time course is slower. Loss is maximal at 60 days post-lesion and this recovers by 180 days post-lesion. Thus, a complete cycle of turnover occurs in both of the inner molecular layers. No degenerating terminals of any type were seen throughout the time course in these layers. Small synapses with non-complex synaptic junctions appear to account for most of the changes. Also the outer two-thirds of the contralateral molecular layer, which has lost less than 5% of its input, loses about 37% of its synapses and replaces the majority of them over time. However, the total number of synapses in the contralateral molecular layer never fully attains the value of unoperated animals. The total synaptic population reaches a value such that the ipsilateral and contralateral molecular layers are nearly equivalent. These changes, achieved through synaptic turnover, may represent a homeostatic response to nearby denervation which may facilitate restoration of bilateral function in the dentate gyrus.

Perforant path lesion depletes glutamate content of fascia dentata synaptosomes

Destruction of the hippocampal perforant path fibers reduces the glutamate content of a crude synaptosomal fraction of the rat fascia dentata by about 40%, but does not reduce the aspartate content. This result supports the hypothesis that the perforant path fibers use glutamate as their transmitter. Since a perforant path lesion reduces the glutamate content of dentate homogenates to a much lesser degree, the reduction in synaptosomal glutamate may be offset by an increase in extraterminal glutamate stores. Thus, when evaluating the possible transmitter role of glutamate or aspartate, one can probably gain more definitive information by measuring the glutamate and aspartate content of a synaptosomal preparation than of the target region as a whole.

The normal organization of the lateral posterior nucleus in the golden hamster and its reorganization after neonatal superior colliculus lesions

We have studied the normal organization of the hamster lateral posterior nucleus and its reorganization after neonatal superior colliculus lesions. First, we divided the lateral posterior nucleus into rostrolateral, rostromedial and caudal subdivisions and determined the normal distributions of terminals contributed to each division by the ipsilateral and contralateral superior colliculi, the ipsilateral posterior neocortex and the contralateral retina. Since the rostrolateral subdivision receives most of the projections from the ipsilateral superior colliculus, our studies concentrated on this region. The rostrolateral subdivision contains synaptic clusters formed primarily by medium-sized M-terminals synapsing around a central dendrite. Electron microscopic observations showed that the majority of M-terminals are from the ipsilateral colliculus, although a few are contributed by the contralateral colliculus and retina. Therefore, after an ipsilateral neonatal colliculus lesion the synaptic clusters must develop in the absence of their major input. The next step was to examine the distributions of the remaining afferents to the rostrolateral subdivision in adult animals which had received ipsilateral neonatal colliculus lesions. In the cases, the normally restricted projection fields of the contralateral colliculus and the retina expand until they share a border in the rostrolateral subdivision. In contrast the cortical projection, which normally extends throughout the lateral posterior nucleus, is reduced in the region containing retinal terminals. At the ultrastructural level, we found morphologically normal synaptic clusters and showed the M-terminals now occupying the clusters are contributed by the remaining colliculus and the retina. The results suggested that the afferents to the lateral posterior nucleus normally compete for synaptic space and that this competition continues after a neonatal colliculus lesion. In our final experiments, we performed various combinations of neonatal lesions (bilateral superior colliculus, superior colliculus and retina, superior colliculus and cortex) and found that the remaining afferent expand their terminal fields still further in the absence of two inputs.

Regeneration of the septohippocampal pathways in adult rats is promoted by utilizing embryonic hippocampal implants as bridges

The ability of embryonic hippocampal tissue to promote regeneration of cholinergic axons in the septohippocampal system has been studied in adult rats. Strips of embryonic hippocampus, taken from 7-40 mm rat fetuses, were implanted into a 2-3 mm wide cavity which completely transected the septal cholinergic axons innervating the intrinsic hippocampus. The ingrowth of cholinergic fibres into the denervated host hippocampal formation was monitored by measuring the activity of the enzyme, choline acetyltransferase (ChAT), and by acetylcholine esterase (AChE) histochemistry. The results demonstrated a gradual, partial return of both ChAT enzyme activity and AChE-positive fibres in the initially denervated hippocampal formation of the adult recipient. Time-course studies indicated that this ingrowth progressed from the implant into the rostral tip of the host hippocampus, and continued caudally to cover the entire dorsal hippocampus by 3-6 months postoperative. Although the regenerating AChE-positive fibres reached the hippocampal target in the recipient along abnormal routes, they reinnervated selectively the appropriate terminal areas within the host hippocampus and dentate gyrus, suggesting the presence of quite specific mechanisms to guide the regenerating axons back to their original targets. Lesions of the medial septum-diagonal band area of the host and horseradish peroxidase (HRP) injections into the host hippocampus, caudal to the implant, indicated that the origin of the regenerating axons was predominately from the ipsilateral ventral medial septum and diagonal band area of the host. The results provide evidence that axonal regeneration and reinnervation of a denervated target zone can be promoted by utilizing implants of embryonic CNS tissue to bridge a tissue defect between the target and the lesioned axonal stumps.

Neuronal plasticity in the deafferented hypothalamic arcuate nucleus of adult female rats and its enhancement by treatment with estrogen

The hypothalamic arcuate nucleus (ARCN) was examined ultrastructurally 3 or 21 days after complete deafferentation of the medial basal hypothalamus (MBH) in ovariectomized adult female rats. Axodendritic shaft (SHS) and spine synapses (SPS) were counted in a field of 18,000 mu m2 in the middle part of the ARCN in each brain. The mean numbers of SHS and SPS at 3 or 21 days after deafferentation were reduced to about half of those in intact control animals. When the ARCNs of the ovariectomized MBH-island rats were examined 3 days after treatment with estradiol benzoate (EB, 2 mu g/day), the numbers of SHS and SPS did not differ from those in ovariectomized MBH-island rats without EB treatment. However, EB treatment for 21 days produced a marked increase in the number of both SHS and SPS in the ovariectomized MBH-island females, with the number of SHS in these females being restored to almost 75% of the intact level; the incidence of SPS was also significantly greater than that in the intact control animals. In these EB-treated, ovariectomized MBH-island rats, double synapses (spine-spine and spine-shaft double synapses) were frequently observed. In ovariectomized females without MBH deafferentation, however, estrogen failed to increase the numbers of SHS, SPS, and double synapses, which were almost comparable to those in intact and ovariectomized controls. These results suggest that estrogen has a facilitatory effect on SHS and SPS formation in the deafferented ARCN, presumably stimulating not only axonal sprouting but also dendritic spine formation by intact arcuate neurons in the MBH island.

Is aspartic acid the neurotransmitter of the perforant pathway?

In order to determine whether an amino acid may act as a neurotransmitter in the perforant pathway we examined the effect of lesion of rat entorhinal cortex on the concentrations of various amino acids in the hippocampus proper and fascia dentata. Only the aspartic acid content was found significantly decreased after the lesion. This decreases is not due to a loss from target cells of the perforant pathway, but rather to a loss from its degenerating terminals.

Decline in reactive fiber growth in the dentate gyrus of aged rats compared to young adult rats following entorhinal cortex removal

The reaction of septal and commissural-associational afferents in the dentate gyrus was examined at various times following a unilateral entorhinal lesion in 2- and 3-month-old, 12- to 18-month-old and 25--30-month-old rats. The response of septo-hippocampal fibers was examined histochemically by staining for acetylcholinesterase (AChE) activity; and that of commissural-associational fibers by the Holmes' fiber stain. In 2- and 3-month-old rats, AChE staining fibers, which project to the outer three-fourths of the molecular layer of the dentate gyrus, increased their staining intensity within 5--6 days following lesion of the entorhinal cortex. The rate of the response and the eventual magnitude declined progressively with the age of the subject. In 2- and 3-month-old rats, the commissural-associational fiber plexus appeared to expand partially into the entorhinal zone within 6 days following the lesion. This response also decreased progressively in rate and magnitude with age. Animals in the oldest age group showed at 12 days after the lesion a greater variability in the expansion of the commissural-associational fiber plexus than all younger groups. Astrocytes in the dentate molecular layer appeared to become more abundant and more hypertrophied in unoperated animals with age. The appearance of astrocytes in 25- to 30-month-old rats was similar to that seen in 2- and 3-month-old animals following an entorhinal lesion. An entorhinal lesion in the aged animals did not appear to cause a marked change in the appearance of astrocytes.

A delayed sprouting response to partial hippocampal deafferentation: time course of sympathetic ingrowth following fimbrial lesions

Sympathetic, noradrenaline-containing fibers grow into the hippocampal formation following lesions of the medial septum or fimbria/fornix. Fluorescent histochemical analysis reveals that these fibers begin to arise as collateral sprouts of the normal sympathetic innervation of the internal and external transverse hippocampal arteries at 9 days post-lesion. These initial fibers are oriented orthogonally to the septo-temporal axis of the hippocampal formation. They grow towards the granule cells of the fascia dentata and the CA3 pyramidal cells, where they begin to proliferate at 14 days post-lesion. This process continues until 29 days, resulting in a final distribution of fibers in areas of septal deafferentation: stratum lucidum, the inner one-third of stratum oriens and stratum pyramidale of CA3; and the hilus, the inner one-third of stratum moleculare and stratum granulosum of the fascia dentata. The time course of this sprouting response is relatively late in onset and slow in completion when compared to sprouting responses of intrinsic afferent systems of the hippocampal formation following entorhinal cortical or commissural deafferentation.

The organization of the lateral posterior nucleus in neonatal golden hamsters

The present series of experiments was designeneonatal golden hamsters.

Alterations in axons and synapses of olfactory cortex following olfactory bulb lesions in newborn rats

The olfactory cortex of rats is being studied at various survival times following deafferentating olfactory bulb ablation on the day of birth. The neonatal axons and synaptic terminals undergo rapid, flocculent degeneration and fragmentation. Most are not electron-dense and therefore probably not argyrophilic at this particular age of the lesion. The degeneration and removal of debris is far more rapid than in adults, yielding a markedly enlarged extracellular space with a relative absence of glia at the vacated postsynaptic "thickenings". Denervated postsynaptic "thickenings" become occupied by neuronal and nonneuronal profiles and profiles of uncertain origin, singly or in various combinations, or the sites may remain partially vacant. One or more axons with synaptic vesicles often aggregated at the site are commonly involved. Certain terminals form contacts on progressively greater lengths of the "thickening" until typical synaptic contacts predominate by 14 days survival. The results suggest a competitive reinnervation process and provide a fine structural explanation for the events leading to alterations in this pathway following neonatal deafferentation.

Loss and reacquisition of hippocampal synapses after selective destruction of CA3-CA4 afferents with kainic acid

Intraventricular injections of kainic acid were used to destroy the hippocampal CA3-CA4 cells bilaterally in rats, thus denervating the inner third of the molecular layer of the fascia dentata and stratum radiatum of area CA1. Electron microscopic studies showed that this lesion reduced the synaptic density of the CA1 stratum radiatum by an average of 86%. The synaptic density of the inner third of the dorsal dentate molecular layer declined by two-thirds and the corresponding zone of the ventral dentate molecular layer by about half. Within 6-8 weeks the synaptic density of these laminae had been restored to the control value or nearly so. In the CA1 stratum radiatum about 72% of the synaptic contacts destroyed by the lesion were replaced, the inner third of the ventral dentate molecular layer recovered 75% of its lost synapses and the inner third of the dorsal dentate molecular layer apparently recovered virtually all of them. The newly formed synapses did not differ noticeably from those normally present. A kainic acid lesion reduced the synaptic density of the outer two-thirds of the dentate molecular layer by 30% within 3-5 days, despite a virtual absence of presynaptic degeneration in that zone. This result implies a substantial disconnection of perforant path synapses. It did not appear to depend on the extent of denervation of the inner zone. The loss of perforant path synapses was completely reversible. We suggest that the dentate granule cells shed a portion of their synapses in response to a substantial loss of neurons to which they project and regained them when their axons had formed new synaptic connections.

Factors specifying the development of synapse number in the rat dentate gyrus: effects of partial target loss

The development of the dentate gyrus has been studied under conditions of partial reduction of granule cell number. Neonatal rats were subjected to X-irradiation, a procedure which reduces the number of granule cells to 20% of control values. In X-irradiated rats, quantitative analyses were performed on cells in the entorhinal cortex which give rise to the perforant path projection to the dentate granule cells, and on the remaining, undamaged dentate granule cells. These residual cells were examined morphologically for possible hyperdevelopment in comparison to granule cells from control animals. Granule cells in X-irradiated animals were similar to granule cells in control animals with respect to dendritic structure and synaptic density. The number of neurons in both the medial and lateral entorhinal cortices in X-irradiated animals appeared normal until day 12, at which time a selective reduction in cell numbers became apparent. By day 30, 25-55% of the cells of origin of the perforant path were absent in X-irradiated animals. It is hypothesized that these cells are subject to retrograde transynaptic degeneration as a result of target removal. Further, it appears that granule cells play an important role in determining the density of their innervation.

Acetylcholine sensitivity of developing ectopic nerve-muscle junctions in adult rat soleus muscles

1. The development of junctional ACh sensitivity has been studied during the formation of ectopic nerve-muscle junctions (n.m.j.s) between the superficial fibular nerve and the denervated soleus muscle of adult rats. 2. When the soleus nerve was cut 2 weeks or more after implanting the fibular nerve, spontaneous m.e.p.p.s and evoked e.p.p.s were first detected in the vicinity of the fibular nerve sprouts 2.5-3 days later. At this time, peaks of local ACh sensitivity greater than the high level of extrajunctional sensitivity induced by denervation were found near the sprouts of the fibular nerve. 3. During the first week of foreign innervation, the extrajunctional sensitivity of the newly innervated muscle fibres fell, but the peaks of sensitivity in the region of the fibular nerve sprouts persisted. Many of these peaks occurred at sites of transmitter release from the fibular nerve terminals. Each innervated fibre had from 1-8 such peaks. 4. When the fibular nerve was cut 2 days or more after cutting the soleus nerve peaks of ACh sensitivity persisted in the region of the degenerated foreign nerve terminals even if the extrajunctional sensitivity was abolished by direct electrical stimulation of the muscle starting soon after cutting the fibular nerve. 5. When the fibular nerve was left intact, more than half of the peaks of sensitivity formed initially in the region of the foreign nerve sprouts had disappeared 2-3 weeks after cutting the soleus nerve. 6. We conclude that during the formation of ectopic n.m.j.s in adult rat muscle the foreign nerve terminals bring about two types of long-lasting change in the distribution and stability of the underlying ACh sensitivity in the muscle fibre membrane; an increase and stabilization of sensitivity at sites of transmitter release which occurs by the time functional transmission at the newly formed n.m.j.s can be detected, and a loss of sensitivity at some of the sites which takes place about 1-2 weeks later.

Preferential neurotoxicity of colchicine for granule cells of the dentate gyrus of the adult rat

Injections of 5-7 microgram (6-9 nmol) of colchicine into the dentate gyrus of the hippocampus of mature rats result in widespread destruction of dentate granule cells with little, if any, damage to other cell populations, including hippocampal pyramidal cells. Selective destruction of dentate granule cells is also observed after intraventricular injections. The destructive effects of colchicine appear as soon as 12 hr after the injection and lead to the disappearance of the granule cells over a period of days. Whereas the effects on nongranule cell populations in the hippocampus appear to be reversed by approximately 11 days after injection, the granule cells are almost completely absent at long intervals after injection. At the long postinjection survival intervals the disappearance of the granule cells is accompanied by elimination of their terminal projections, the mossy fibers, as revealed by Timm staining for heavy metals. Because the preferential neurotoxic effects of colchicine do not result in morbidity or obvious behavioral debilitation, the toxicity may prove useful for studying the functional consequences of removing specific cell populations in the central nervous system.

Failure to detect collateral sprouting of mesolimbic dopaminergic neurons during early postnatal development

Using biochemical parameters the present study sought to assess the normal developmental pattern of the dopaminergic innervation of the olfactory tubercle (OT) and how it is affected by olfactory bulbectomy. In rats, the adult pattern of cellular organization is achieved in the OT gradually over the first 7 days after birth. On the other hand, tyrosine hydroxylase (TH) and [3H]dopamine ([3H]DA) uptake, while present at low levels, start to increase rapidly only after the first 7 days reaching adult levels by 40 and 20 days after birth, respectively. TH in dopamine (DA) cell bodies of A10 was already high, 40% of adult value, at birth, reached 150% by day 14 and decreased back to adult values by day 21 after birth. In 10-day-old rats, bulbectomy resulted, 30 days later, in an increase to 123% of control in TH activity and 137% in [3H]DA uptake within the OT. Comparable changes were found following bulbectomy in adults. However, bulbectomy in 1-day-old rats did not produce any significant changes 40 days later. The findings suggest that during postnatal growth TH activity is increased in DA cell bodies, preceding the changes in DA terminals of the OT, resembling the changes occurring during collateral sprouting in adults. In addition, changes indicative of collateral sprouting do not occur in response to deafferentation of the OT in 1-day-olds but do in 10-day-olds or older animals, a phenomenon probably related to a critical development period of the OT.

Do neurotrophic interactions control synapse formation in the adult rat brain?

The role of axonal transport in the regulation of synaptic contacts was studied in the adult rat dentate gyrus. Colchicine was applied to the fimbria, which includes fibers of the hippocampal commisural system, and axonal transport was measured. Axonal transport in these and other fibers of the fimbria was markedly reduced. The region of commissural termination in the molecular layer of the dentate gyrus was monitored by electron microscopy for changes in the number of synapses per unit area following cholchicine treatment, sodium chloride treatment, or fimbrial transection. Four days after colchicine treatment there was no change in the number of synapses. However, at 11 and 60-70 days after colchicine treatment the number of synapses per unit area significantly increased. This increase occurred throughout the commissural terminal zone, but it did not occur in terminal zones of other afferents in the dentate gyrus. The increased synaptic density appeared to arise from the commissural system itself because removal of commissural fibers eliminates the increase (in addition to the normal commissural input). These findings suggest a role for axonally transported trophic substances in the specific regulation of synaptogenesis in the dentate gyrus of the adult rat.