An electron microscopic study of lesion-induced synaptogenesis in the dentate gyrus of the adult rat. I. Magnitude and time course of degeneration

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.

Quantitative ultrastructural changes in rat cortical synapses during early-, mid- and late-adulthood

Quantitative ultrastructural analysis of rat parietal cortex was undertaken to determine the nature of the synaptic changes occurring in the molecular layer over a series of ages in early- (3 months), mid- (6 and 10 months) and late- (17 months) adulthood. The total number of synapses remained constant until 10 months of age, but decreased significantly by 17 months. Asymmetrical synapses on dendritic shafts were lost earlier (by 6 months) than asymmetrical synapses on dendritic spines (by 17 months). Symmetrical axodendritic synapses remained constant throughout adulthood. Analysis of synaptic terminal parameters revealed the following. Both individual and total presynaptic terminal areas decreased over the age range studied. Individual and total postsynaptic terminal areas, however, remained constant over the 3--17-month period. Positive correlations were obtained for the relationships between presynaptic terminal area and both age and synaptic vesicle number. The presynaptic terminal area was largest and contained the greatest number of vesicles at 3 months of age. This age was, in addition, characterized by the least numbers of mitochondria in the presynaptic terminal and spine apparatus in the postsynaptic terminal. The vacuolar and tubular cisternae of the presynaptic terminal were considerably reduced at 17 months. These data suggest that in the molecular layer of the cerebral cortex the period of adulthood is characterized by a diversity of synaptic changes. The 3-month age may reflect the end of the developmental phase and may be marked by changes in synaptic functional activity. The asymmetrical axodendritic synapses may constitute an intermediate form of synapse, capable of being transformed into axospinous synapses as dendritic spines continue to be formed in the adult.

Effect of anisomycin on stimulation-induced changes in dendritic spines of the dentate granule cells

Tetanic stimulation of the entorhinal area induces significant enlargement of the average dendritic spine area and perimeter in the middle and distal thirds of the dentate molecular layer 4 and 90 min following stimulation. Four minutes after stimulation, the differences between the stimulated and control animals were 20% for the dendritic spine area and 9% for the perimeter in the middle third, and in the distal third 32 and 14%, respectively. Ninety minutes after stimulation the differences were 28 and 11% for the area and perimeter in the middle third, and 33 and 18% in the distal third, respectively. Anisomycin at a dose of 25 mg/kg had no significant effect on the average spine area or perimeter in the various thirds of the dentate molecular layer in the 19 and 105 min post-application intervals. This dose of anisomycin given 15 min prior to the stimulation suppresses the stimulation-induced spine changes in the 4 min interval. In the 90 min interval when the effect of anisomycin on protein synthesis is largely terminated, spine enlargement reappears, being 21% higher than the controls in the middle and distal thirds. The differential effect of anisomycin on dendritic spines in the two post-stimulation intervals is discussed in relation to the effect of anisomycin on protein synthesis. The present experiments thus demonstrate that the stimulation-induced spine enlargement in the dentate fascia can be suppressed by a protein synthesis blocking drug.

Lesion-induced synaptogenesis in the dentate gyrus of aged rats: II. Demonstration of an impaired degeneration clearing response

Previously we reported that a delayed onset in the reinnervation of the outer two-thirds of the dentate molecular layer occurred in aged rats after an entorhinal lesion. Several factors associated with formation of new synaptic contacts and removal of degenerative debris may affect the reinnervation process. In this study the appearance and removal of degeneration was analyzed and evaluated with respect to the delayed reinnervation process in aged rats. After a complete lesion of the entorhinal cortex, 85-90% of the input to the outer two-thirds of the ipsilateral molecular layer is lost. Electron-dense and electron-lucent degeneration are present throughout the outer two-thirds of the denervated molecular layer. In both aged and young adult rats, the electron-lucent degeneration disappears by 10 days postlesion. The predominant electron dense degeneration, however, is removed at a different rate by young adult and aged rats. Young adults demonstrate a biphasic degeneration removal process, with almost half of this degeneration rapidly lost by 10 days postlesion, and nearly all by 60 days postlesion. Aged animals in contrast, have lost only 16% of the dense degeneration at 10 days postlesion, with about 30% of the degeneration remaining at 60 days postlesion. The impaired removal of the degeneration from the denervated zone appears to be reciprocally related to the reinnervation response in both age groups and may be related to the normal astrocyte hypertrophy and elevated corticosteroid levels in aged rats.

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 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.

Reduced ATP concentration as a basis for synaptic transmission failure during hypoxia in the in vitro guinea-pig hippocampus

1. Experiments were performed to determine whether a decrease in tissue ATP contributes to the rapid failure of cerebral synaptic transmission during hypoxia. Transmission between the perforant path and the dentate granule cells in the in vitro hippocampus was studied.2. Hippocampal slice ATP is decreased by approximately 15% at the time that the evoked response begins to diminish in standard Krebs bicarbonate buffer. This is about 2 min after the onset of hypoxia.3. When transmission failure is accelerated by increasing extracellular K(+) from 4.4 to 13.4 mM, the evoked response begins to decay about 30 sec after exposure to hypoxia. There is no decrease in hippocampal slice ATP at this time.4. However, ATP in the molecular layer (the synaptic region of the tissue) is decreased by approximately 15% at the time the evoked response begins to decay in the slices exposed to elevated K(+) concentration.5. Exposing the hippocampal slice to 25 mM-creatine for 3 hr elevates molecular layer phosphocreatine fourfold. Synaptic transmission during hypoxia survives three times as long as it does in the absence of creatine.6. In the creatine fortified medium, molecular layer ATP no longer declines within 30 sec of hypoxia. However the molecular layer ATP does decline within 90 sec of hypoxia, the time at which the evoked response begins to decay in this creatine-fortified buffer.7. The results establish that ATP in the region of the active synapses is lowered when the first signs of electrophysiological failure appear during hypoxia. They also show that maintaining ATP for longer than normal during hypoxia is associated with a prolonged maintenance of the evoked response. They thus suggest that a decline in ATP is one factor causing hypoxic block of synaptic transmission.8. It is further suggested that the very rapid failure of the electroencephalogram during anoxia may also result from a decline in ATP.

Benzodiazepine receptor increases after repeated seizures: evidence for localization to dentate granule cells

Repeated seizures, whether induced by kindling or electroshock, result in increased numbers of benzodiazepine receptors in hippocampal formation membranes. We sought to determine the cellular constituents containing the receptor increases. Binding studies of microdissected samples localized the receptor increases to fascia dentata. [3H]Flunitrazepam autoradiographic studies showed increases of silver grain density over the granule cell and molecular layers of fascia dentata but not in other regions of hippocampal formation. Destruction of granule cells by colchicine or neonatal x-irradiation was associated with marked decrease of benzodiazepine receptor binding. Together, these results provide evidence for localization of the receptor increases to the somata and dendritic tree of the granule cells. We suggest that this cellular localization may provide a clue to the network of altered neural circuitry underlying amygdala kindling.

Desensitization-like changes in GABA receptor binding of rat fascia dentata after entorhinal lesion

Destruction of the hippocampal perforant path fibers reduced the binding of [3H]GABA to membranes prepared from the rat fascia dentata. This result could be detected 1--4 months after surgery, but not in 10 days or less. Such a delayed response appears most compatible with a transsynaptic effect on GABA receptors. Values of low affinity KD and Bmax decreased by about 65%, but no changes were detected in high affinity binding, Hill slope or pharmacological specificity. These findings are consistent with a desensitization of postsynaptic low affinity GABA receptors, possible caused by excessive release of GABA.

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.

Postsynaptic density antigens: preparation and characterization of an antiserum against postsynaptic densities

Long-term immunization of rabbits with postsynaptic densities (PSD) from bovine brain produced an antiserum specific for PSD as judged by binding to subcellular fractions and immunohistochemical location at the light and electron microscope levels. (a) The major antigens of bovine PSD preparations were three polypeptides of molecular weight 95,000 (PSD-95), 82,000 (PSD-82), and 72,000 (PSD-72), respectively. Antigen PSD-95, also present in mouse and rat PSDs was virtually absent from cytoplasm, myelin, mitochondria, and microsomes from rodent or bovine brain. Antigens PSD-82 and PSD-72 were present in all subcellular fractions from bovine brain, especially in mitochondria, but were almost absent from rodent brain. The antiserum also contained low-affinity antibodies against tubulin. (b)Immunohistochemical studies were performed in mouse and rat brain, where antigen PSD-95 accounted for 90 percent of the antiserum binding after adsorption with purified brain tubulin. At the light microscope level, antibody binding was observed only in those regions of the brain where synapses are known to be present. No reaction was observed in myelinated tracts, in the neuronal cytoplasm, or in nonneuronal cells. Strong reactivity was observed in the molecular layer of the dentate gyrus, stratum oriens and stratum radiatum of the hippocampus, and the molecular layer of the cerebellum. Experimental lesions, such as ablation of the rat entorhinal cortex or intraventricular injection of kainic acid, which led to a major loss of PSD in well- defined areas of the hippocampal formation, caused a correlative decrease in immunoreactivity in these areas. Abnormal patterns of immunohistochemical staining correlated with abnormal synaptic patterns in the cerebella of reeler and staggerer mouse mutants. (c) At the electron microscopic level, immunoreactivity was detectable only in PSD. The antibody did not bind to myelin, mitochondria or plasma membranes. (d) The results indicate that antigen PSD-95 is located predominantly or exclusively in PSD and can be used as a marker during subcellular fractionation. Other potential uses include the study of synaptogenesis, and the detection of changes in synapse number after experimental perturbations of the nervous system.

Lesion-induced sprouting of hippocampal mossy fiber collaterals to the fascia dentata in developing and adult rats

A lesion-induced formation of an abnormal projection of hippocampal mossy fiber collaterals to the molecular layer of the fascia dentata was studied in rats. Both immature (1--30 days old) and adult rats were subjected to hippocampal and entorhinal lesions which alone or in combination removed one or more of the major afferents to the dentate molecular layer (commissural, associational, and perforant path). Some lesions in addition transected the main part of the mossy fibers en route from the dentate granule cells to the hippocampal pyramidal cells in regio inferior (CA3). The formation of aberrant mossy fiber terminals in fascia dentata (supragranular mossy fibers) was monitored by the histochemical Timm sulphide silver method, but the presence of aberrant terminals was also observed in the electron microscope. Abnormal amounts of supragranular mossy fiber terminals were found following entorhinal lesions of both immature and adult rats, but not following commissural lesions. Even larger amounts of aberrant terminals were, however, found in immature and adult rats subjected to lesions which removed most of the associational hippocampodentate projection by isolating columns of fascia dentata from major parts of the hilus (CA4). Pure transections of the fascia dentata perpendicular to its longitudinal septotemporal axis did not in itself cause aberrant supragranular terminals, although such lesions partially damaged the associational afferents. When the transections were combined with commissural lesions or entorhinal lesions or both, large amounts of supragranular terminals did, however, form at the denervated levels septal to the transection. After comparison of the amounts and distributions of the aberrant terminals found after the different lesions and in transplants of dentate tissue with different amounts of afferent input, we conclude that it is deafferentation of the dentate molecular layer, and not axotomy of the mossy fibers in the hilus of CA3 (pruning), that causes the aberrant growth of mossy fiber collaterals. Moreover, simultaneous removal of more than one afferent system seems to have a potentiating rather than a simple additive effect on the formation of supragranular mossy fibers.

Interconverting mu and delta forms of the opiate receptor in rat striatal patches

The binding of a radiolabeled "mu receptor" prototype opiate, dihydromorphine (H2morphine), and the binding of a "delta receptor" prototype, [D-Ala2,D-Leu5]enkephalin (D-Enk), to slide-mounted rat caudate slices were simultaneously compared quantitatively and visualized by autoradiography. Generally, D-Enk bound to opiate receptors distributed evenly throughout the entire striatum (type 2 pattern), whereas H2morphine labeled discrete islands or patches of receptors (type 1 pattern). In the presence of Mn2+ (3 mM) or other divalent cations, however, Na+ and GTP at 25 degrees C caused an increase in D-Enk binding at the expense of H2morphine binding at striatal opiate receptor patches. Thus, these conditions shifted D-Enk binding from an even pattern to one that included both an even and patchy distribution. These incubation conditions not only promoted D-Enk binding to striatal patches but also enabled the opiate receptor to regulate adenylate cyclase with the same (P less than 0.01) ligand selectivity pattern as that obtained by the displacement of D-Enk binding. The relative affinity of opiate receptors in striatal patches for opiate peptides, naloxone, and morphine appears to be a function of incubation conditions and coupling to adenylate cyclase and is not indicative of distinctly different opiate receptors. We postulate a three-state allosteric model consisting of mu agonist-, mu antagonists-, and adenylate cyclase-coupled delta-agonist-preferring states, whose equilibrium may be regulated by a sulfhydryl group mechanism.

Fate of the hippocampal mossy fiber projection after destruction of its postsynaptic targets with intraventricular kainic acid

Intraventricular injections of kainic acid were used to create a model of selective cell death in order to study the fate of afferent projections that are deprived of their postsynaptic targets. This treatment rapidly destroyed hippocampal CA3 pyramidal cells, but not those neurons that give rise to their mossy fiber and entorhinal afferents. Light microscopic studies with the Timm's sulfide silver stain indicated that half or more of the mossy fiber boutons in area CA3b were lost within the first 1-3 days after kainic acid administration. This finding was confirmed by electron microscopy. Electron-dense, usually vacuolated mossy fiber boutons accounted for about 10-20% of the total population present at a 4-hour survival time, but were not encountered in control rats nor at survival times longer than 1 day. Other mossy fiber boutons remained electron lucent, but enlarged, became more rounded in shape, and suffered an apparent loss of synaptic vesicles. It is suggested that degeneration of some mossy fiber boutons and resorption of others into the axon may have accounted for the precipitous decline in their number. The dendritic excrescences contacted by these boutons were nearly all undergoing electron-dense degeneration 4 hours after kainic acid administration. In rats that survived 6-8 weeks mossy fiber boutons remained somewhat scarce, individual boutons appeared relatively small, and only one-third the normal percentage were observed to be engaged in more than one synaptic contact within a single cross section. A qualitative electron microscopic study of the entorhinal projection to area CA3 suggested a response to kainic acid treatment similar to that of the mossy fiber projection, except that no entorhinal boutons were seen to become electron dense. These findings suggest that presynaptic fibers in the mature hippocampus adjust the size of their terminal arborizations and number of synaptic contacts to accommodate a reduction in the target cell population. The rapid loss of mossy fiber boutons may be attributable to an unusual fragility of these structures when they are deprived of the mechanical support normally provided by the pyramidal cell. Finally, the ability of kainic acid administration to alter the number and distribution of presynaptic elements must be taken into account whenever this toxin is used to make selective lesions of postsynaptic cells.

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 comparison of distal and proximal dendritic synapses on CAi pyramids in guinea-pig hippocampal slices in vitro

1. In vitro slices of guinea-pig hippocampus have been employed to compare excitatory synapses located distally and proximally on the dendritic tree of CA1 pyramidal cells.2. The main orientation of unmyelinated afferent fibres was found to be parallel to each other and perpendicular to the dendritic axis.3. The density of boutons ending on dendritic spines was roughly similar throughout the greater part of the dendritic tree with an average of 42 +/- 7.2 synapses per 100 mum(2). Their number did, however, decrease in the distal fifth of the apical and in the distal third of the basal dendritic region in parallel with an increase of boutons on the dendritic shafts.4. Negative synaptic field potentials (extracellular field e.p.s.p.s) had their maximum in the region where activated afferent fibres terminated and showed reversal when recorded from sufficiently displaced positions along the dendritic axis. The field e.p.s.p. was preceded by a diphasic presynaptic fibre volley. By cutting all but a narrow bundle of afferent fibres selective activation of a small group of dendritic synapses was possible. Stimulation of fibres crossing tissue bridges (35-100 mum wide) evoked field e.p.s.p.s comparable in amplitude to those seen in slices without lesions. The size of the field e.p.s.p.s evoked via distal and proximal bridges was remarkably similar and linearly related to the size of the appropriate stimulus current and presynaptic volley.5. Selective activation of a small group of afferent fibres gave rise to large amplitude population spikes. Proximal and distal bridges were largely equipotent when they were equally wide. Above the threshold amplitude, the evoked population spikes were linearly related to both the presynaptic volley and the stimulus current. Constant current stimulation of fibres at all apical dendritic levels was equally effective in evoking population spikes, with the exception of the outer fifth of the tree where stimulation was unsuccessful. Input across distal or proximal bridges (400 or 50 mum from the soma) also gave the same high probability of discharge of single units (1.0 for thirty-five of thirty-six cells).6. An input across a narrow and distal bridge (35 mum), representing less than 5% of the fibres synapsing on the apical dendrite, was sufficient to give a firing probability of 1.0 for all cells tested (fifteen).7. For seventeen cells pairs of equally wide distal and proximal apical dendritic bridges were compared. Both inputs gave a mean probability of firing above 0.95 with stimulation strengths less than 2.5 times the spike threshold.8. Intracellular e.p.s.p.s had similar shapes following activation across distal and proximal dendritic bridges. The amplitude of neither type was significantly affected by hyperpolarization of the soma up to 25 mV. The half-width was prolonged to the same moderate degree for both inputs.9. The firing level for the action potential was similar for proximal and distal dendritic inputs and for spikes excited by depolarizing current pulses across the soma membrane.10. The apparent equipotentiality of synchronously activated distal and proximal dendritic synapses is discussed in the light of the known histology of the CA1 pyramidal cells.

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.

Degeneration of hippocampal CA3 pyramidal cells induced by intraventricular kainic acid

Degeneration of hippocampal CA3 pyramidal cells was investigated by light and electron microscopy after intraventricular injection of the potent convulsant, kainic acid. Electron microscopy revealed evidence of pyramidal cell degeneration within one hour. The earliest degenerative changes were confined to the cell body and proximal dendritic shafts. These included an increased incidence of lysosomal structures, deformation of the perikaryal and nuclear outlines, some increase in background electron density, and dilation of the cisternae of the endoplasmic reticulum accompanied by detachment of polyribosomes. Within the next few hours the pyramidal cells atrophied and became electron dense. Then these cells became electron lucent once more as ribosomes disappeared and their membranes and organelles broke up and disintegrated. Light microscopic changes correlated with these ultrastructural observations. The dendritic spines and the initial portion of the dendritic shaft became electron dense within four hours and degenerated rapidly, whereas the intermediate segment of the dendrites swelled moderately and became more electron lucent. No degenerative changes were evident in pyramidal cell axons and boutons until one day after kainic acid treatment. Less than one hour after kainic acid administration, astrocytes in the CA3 area swelled, initially in the vicinity of the cell body and mossy fiber layers. It is suggested that the paroxysmal discharges initiated in CA3 pyramidal cells by kainic acid served as the stimulus for this response. Phagocytosis commenced between one and three days after kainic acid administration, but remained incomplete at survival times of 6-8 weeks. Astrocytes, microglia, and probably oligodendroglia phagocytized the degenerating material. These results point to the pyramidal cell body and possibly also the dendritic spines as primary targets of kainic acid neurotoxicity. In conjunction with other data, they support the view that lesions made by intraventricular kainic acid can serve as models of epileptic brain damage.

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.

Neurogenesis and neuron regeneration in the olfactory system of mammals. III. Deafferentation and reinnervation of the olfactory bulb following section of the fila olfactoria in rat

Axotomy at the level of the lamina cribrosa in rat induces rapid degeneration of the olfactory sensory axons in the bulb. The phenomenon, which is limited to the layer of olfactory fibres and to the glomeruli of the bulb, can be observed as early as 15-24 h after surgery, and peaks at 3-4 days. The glomeruli located in the rostro-ventral portion of the bulb are affected first, and the process extends to the dorso-caudal portion with a delay of 12-24 h. Reactive hypertrophy of the glia coincides with removal of the degenerating terminals, and is maximal 48 h after axotomy. Axotomy does not preclude reinnervation of the bulb by axons originating from new, reconstituted neurons in the olfactory neuroepithelium. These new axons begin to reach the periphery of the bulk approximately at the 20th day post-operative and then reinnervate the glomeruli. The rostro-ventral portion of the bulb is the first to be reinvaded by the new axons. The glomeruli reacquire a morphological pattern similar to controls between 20 to 30 days.

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.

The effect of collateral sprouting on the density of innervation of normal target sites: implications for theories on the regulation of the size of developing synaptic domains

The 'commissural' innervation of the dentate gyrus molecular layer has been analyzed in normal adult rats and in those in which the ipsilateral entorhinal cortex had been removed by aspiration at 14 days post-natal. This ablation severely deafferents the distal two-thirds of the molecular layer and induces 'sprouting' by the commissural afferents which are normally restricted to the more proximal dendritic zone. It was the objective of the present study to employ quantitative electron microscopy to determine (1) the extent of synaptic recovery in the deafferented field; (2) the magnitude of the contribution by the commissural fibers to the reinnervation of the deafferented field; and (3) if sprouting by the commissural projections causes a reduction in the density of the terminal field they generate in their normal target region. The synaptic density of the neonatally deafferented middle molecular layer was found to have returned to near control levels by adulthood. Degeneration studies performed in the adult revealed that commissural endings were located in equivalent numbers in the inner and middle molecular layers of rats in which the entorhinal cortex had been removed at 14 days post-natal; in normal rats (i.e. no neonatal surgery) the commissural terminals were found only in the inner molecular layer. Furthermore, and most importantly, the density of commissural terminals in the inner molecular layer was virtually identical in the 'sprouted' and control rats. Thus the tremendous areal expansion of the commissural terminal field which occurs after early deafferentation of the distal parts of the granule cell dendrites was not accompanied by any loss of input to the normal target of this afferent. Therefore, sprouting in this system represents an exaggeration of normal growth rather than a redistribution of a fixed population of endings. The relevance of these findings to theories concerned with the regulation of axonal growth and terminal proliferation during development is discussed.

Terminal proliferation in the partially deafferented dentate gyrus: time courses for the appearance and removal of degeneration and the replacement of lost terminals

The time courses for the appearance and removal of degenerating terminals and the loss and reappearance of intact terminals were investigated in the partially denervated inner molecular layer of the dentate gyrus of the adult rat. Dense degeneration was evident in the neuropil within 26 hours following contralateral hippocampectomy. These profiles increased rapidly in number until the maximal degree was reached at two to three days postlesion, after which the degenerating terminals were quickly removed from the neuropil. A more rapid rate of removal occurred during the 3-to 5-day survival period than from 6 to 50 days postlesion. The intact terminal population dropped 35% within two days of the lesion and remained at this level until six to eight days postlesion when the number began to steadily increase. The time course for this reappearance can be divided into two phases: a period of rapid terminal addition from 6 to 15 days followed by a phase of slower acquisition. This recovery continued until the normal synaptic density was regained by 50 to 65 days postlesion. These results indicate that a substantial proportion of degenerating endings are removed well in advance of the time at which terminal proliferation begins, suggesting that certain changes other than merely the removal of competitive inputs must take place prior to growth of new terminals. Possible explanation suggested by the present results for the delay in the onset of sprouting include: (1) an absence of appropriate postsynaptic targets during the 2-to 5-day postlesion period and (2) inhibition of axonal growth by the glial cells which are phagocytizing the degenerating products. Beyond the sixth postlesion day the rate at which new terminals appear does correlate with the rate at which degeneration is removed. This suggests that once underway the time course for sprouting may be determined by the avaiabliity of postsynaptic sites.

The specificity of reactive synaptogenesis: a comparative study in the adult rat hippocampal formation

The CA1 region of the hippocampus in the mature rat is shown to possess the capacity to form new synapses following a lesion of either the commissural afferents, which removes 41% of the synaptic input to stratum radiatum, or commissural and associational afferents, which destroys 74% of the synaptic input. With both types of lesion, extensive reinnervation occurs without obvious changes in lamination of afferent fibers and without accompanying changes in the acetylcholinesterase-(AChE) staining pattern. This is in contrast to what is known to occur in the hippocampal dentate gyrus following an ipsilateral entorhinal lesion where afferent lamination is reordered and where AChE-staining intensifies. A comparison between the disparate patterns of reinnervation in these closely related structures affords the opportunity to examine some of the specific factors that may regulate synaptic readjustments in brain.

Protein and glycoprotein composition of synaptic junctions prepared from discrete synaptic regions and different species

Synaptic junction (SJ) were isolated by subcellular fractionation from different areas of the steer brain and from the brains of different species (steer, rat, chicken and human) for the purpose of comparing their protein and glycoprotein composition. The synaptic junction fractions from different brain regions and species were of comparable morphological purity. Analysis of the polypeptide composition of isolated synaptic junction fractions via SDS polyacrylamide gel electrophoresis showed that the major polypeptides were represented in all junctional fractions independent of their source. Tubulin, actin, the major 52,000 Mr postsynaptic density protein and a group of proteins with a molecular weight of 200-250,000 Mr were all similarly represented. Most other components were also similar but quantitative differences were found for a few polypeptides. Interspecies differences were more prevalent than those between different brain areas of the same species. The protein compositions of different brain areas were similar even when an area consisting of primarily one neuronal type was compared to areas containing a mixture of neuronal types. However, two-dimensional gel electrophoresis revealed distinct (but usually minor) polypeptides in enriched quantities in one or more brain areas. Tryptic peptide maps were carried out on the major postsynaptic density protein of different species. These maps showed a high degree of conservation in this protein's primary structure among all species studied. The glycoproteins of isolated synaptic junctions which bind the plant lectin concanavalin A (Con A) were also examined. To identify individual Con A binding components, SJ fractions were solubilized and constituent glycoproteins were separated by SDS gel electrophoresis. Gels were then incubated in 125I-Con A. The glycoproteins which bound Con A in gels were few in number and were not the major Coomassie blue staining bands. The great majority of the Con A binding glycoproteins were similar between species and among brain areas of the same species.

Development of high affinity choline uptake and associated acetylcholine synthesis in the rat fascia dentata

The ontogenic development of hemicholinium-sensitive, high affinity choline uptake and the synthesis of acetylcholine from exogenous choline have been studied in particulate preparations of the rat fascia dentata. Between 6 days of age and adulthood the rate of high affinity choline uptake increases 3-fold, when expressed with respect to protein, and 125-fold, when expressed independently of protein. This process develops most rapidly during the period around 16-17 days of age, similar to the ontogenesis of choline acetyltransferase activity. This observation supports the idea that cholinergic septohippocampal boutons develop mainly at this time. Unlike choline acetyltransferase activity, the velocity of high affinity choline uptake increases to as much as 161% of the adult value at about 30 days of age. It is suggested that at 25-31 days of age a relatively high endogenous septohippocampal firing rate increases the rate of choline uptake. At 6 days of age we detected no synthesis of acetylcholine from the accumulated choline. Uptake-synthesis coupling develops mainly between 6 and 13 days of age, earlier than any other presynaptic cholinergic property. Acetylcholine synthesis from exogenous choline develops in paralled with high affinity choline uptake, but developmental increases in uptake velocity result in comparable increases in synthesis rate only after a delay of several days. Some limiting factor other than choline acetyltransferase activity appears to link the accumulation of exogenous choline to acetylcholine synthesis during development.

Terminal proliferation and synaptogenesis following partial deafferentation: the reinnervation of the inner molecular layer of the dentate gyrus following removal of its commissural afferents

The inner one-third of the dendritic region of the dentate gyrus granule cells in adult rats receives projections primarily from the commissural fibers of the contralateral hippocampus and the associational fibers of the ipsilateral hippocampus. At two to four days following the complete removal of the contralateral hippocampus, approximately 25% of the terminals in the inner molecular layer are observed degenerating. This provides an excellent model system to investigate possible terminal proliferation induced by deafferentation since (1) the experimental lesion is easily reproducible, (2) no retrograde reactions occur in the granule cells as a direct result of the lesion, (3) no shrinkage is detected in this region following commissural deafferentation, (4) the same dendritic region can be relocated precisely in each animal, and (5) the synaptic counts are highly consistent between animals. Results from this study and from previous investigations demonstrate that the commissural projection is contained within a 0-80 mu zone directly above the granule cell layer; Complete photomontages of this zone were taken, but only the 40-80 mu zone was quantified for neuronal and glial changes in three normal, five 2- to 4-day, and five 50- to 75-day postlesion animals. The average synaptic count dropped to 64% of control values by 2 to 4 days, returned to 97% by 50- to 75 days postlesion, The number of terminals showing multiple synaptic contacts increased slightly in the long-term animals. Measurements of average terminal area showed no change between the short- and long-term survival groups. These results indicate that this dendritic region is reinnervated following partial deafferentation and that the reinnervation is due primarily to the formation of new terminals rather than the expansion of pre-existing terminals.

Laminar differentiation of the hippocampus, fascia dentata and subiculum in developing rats, observed with the Timm sulphide silver method

The laminar staining of the rat hippocampal region with the Timm sulphide silver method is from studies on adult rats known to depend on the various fibersystems terminating in these laminae. In order to illustrate the development of these fibersystems the laminar differentiation of the Timm staining of fascia dentata, hippocampus and subiculum is presented for rats between 1 and 31 days old. Corresponding changes in cytoarchitectonics as revealed by thionin staining are briefly demonstrated. Even on the first postnatal day there are indications of the mature, laminar staining pattern, and between three and nine days all the laminae corresponding to the terminal fields of the major afferent and intrinsic systems appear. After 12 days only minor additions to the laminar pattern develop, but there are adjustments of absolute and relative dimensions of layers and fields, and also the staining densities of individual laminae change. These observations are in good correlation with the available information on both hippocampal neurogenesis and cytodifferentiation, and the few fiber tracing studies performed on the developing hippocampal region. Compared to the latter, which ideally marks only one system or one lamina per animal, the Timm method provides an excellent means for getting an overview of the general developmental situation at the different ages. Thus developmental gradients along septotemporal, medio-lateral and basal-apical axes are found; which should be heeded in future studies on hippocampal synaptogenesis. The observations are discussed in relation to current models for neuronal growth and formation of nervous connections.

Identification of the cells of origin of a central pathway which sprouts following lesions in mature rats

Following unilateral destruction of the entorhinal cortical region of the adult rat, the denervated granule cells of the dentate gyrus are reinnervated as a result of the proliferation of a pathway from the surviving contralateral entorhinal area. The present study investigates the cells of origin of this lesion-induced pathway. Following HRP injections into the reinnervated dentate gyrus, heavily labeled cells were evident in layers II and III of the contralateral entorhinal area, in marked contrast to the pattern of labeling in normal animals, where labeled cells are restricted almost entirely to layer III. The atypically labeled cells in the operated animals were found predominantly in the dorsal half of the entorhinal area, and were concentrated in the medial most portion of layer II. These atypically labeled cells in layer II of the operated animals were an average of 16% larger than their unlabeled neighbors in the same lamina. This was not related to the loading with HRP, however, since in normal animals, cells in layer II which are labeled with HRP were no different in size than unlabeled cells. The atypically labeled cells in layer II of operated animals could also be identified at the electron microscopic level, and could be distinguished from the cells in layer III which normally project to regio superior of the contralateral hippocampal formation. While labeled cells were evident in layers II and III following injections into the reinnervated dentate gyrus, no labeled cells were found in the presubiculum or parasubiculum. In combination, these results suggest (1) the pathway which reinnervates the dentate gyrus from the contralateral entorhinal area originates predominantly, if not exclusively, from the cells in layer II, (2) these cells in layer II have the same preferential distribution within the entorhinal area as the rare lightly labeled cells which can be found contralateral to an injection in normal animals and (3) cells which participate in the reinnervation are larger than their unlabeled neighbors which presumably do not give rise to fibers which reinnervate the contralateral dentate gyrus. Since the cells in layer II which sprout following lesions can be identified at both the light and elctron microscopic level, a potentially valuable model system is available in which to analyze cellular changes during sprouting.

Regenerating afferents establish synapses with a target neuron that lacks its cell body

When the axons of crayfish tail-fan mechanoreceptor neurons are severed, the axons regenerate into the central nervous system and after 2 to 6 weeks reestablish functional contacts with their standard interneuronal target cells. Removal of the cell body and hence the genes of the largest of these interneurons does not interfere with the successful reestablishment of synapses between it and its afferents.

Progressive brain damage accelerates axon sprouting in the adult rat

An entorhinal cortical lesion causes undamaged fibers in the deafferented hippocampus to sprout and form new connections within 4 to 7 days after the lesion was made. When a partial lesion of the entorhinal cortex precedes a second, more complete entorhinal lesion by a few days, the rate of axon sprouting is accelerated so that the response to the second lesion occurs within only 2 days. This priming effect is present within 4 days, lasts for a few weeks, and eventually subsides. This acceleration may explain, in part, the faster recovery and reduced deficits seen in behavioral studies that have followed serial lesion paradigms.