Ultrastructural evidence for bouton proliferation in the partially deafferented dentate gyrus of the adult rat

A quantitative morphological study of the changes in the dentate gyrus molecular layer in response to the removal of perforant path afferents was made utilizing electron microscopic techniques. Alterations in 1. the population of remaining afferents, 2. glial cells, and 3. granule cell dendrites are reported. The major observation was an increase in intact bouton density in the region of denervation which began at 5 days post-lesion and continued through 11 days post-lesion, the longest post-lesion survival time studied.

Identification of a protein related to tubulin in the postsynaptic density

The postsynaptic density is a unique subcellular organelle associated with the synaptic complex and appears as an electron-dense area immediately subjacent to the postsynaptic plasma membrane. The postsynaptic density was isolated from the synaptosomal fraction and the protein constituents were analyzed by polyacrylamide gel electrophoresis. Polypeptides closely related to tubulin were identified as a major component of the postsynaptic density on the basis of molecular weight, subunit structure, and peptide map criteria.

Histochemical evidence of altered development of cholinergic fibers in the rat dentate gyrus following lesions. II. Effects of partial entorhinal and simultaneous multiple lesions

It has been concluded previously that the septohippocampal fibers which project to the rat dentate gyrus extend or branch in the denervated area of the molecular layer following a complete ipsilateral entorhinal lesion. The septohippocampal fibers thus appear to replace some of the perforant fibers which degenerate as a result of the lesion. The reactive fibers eventually become localized to a much smaller and more superficial area after lesions of immature rats than after lesions made in adulthood. To determine whether this difference in the response results from a selective reaction to loss of the lateral perforant path in the immature rat, various portions of the entorhinal cortex were removed at the age of 11 days, and the cholinergic septohippocampal fibers were visualized by acetylcholinesterase histochemistry. An alternative possibility, that the difference between immature and adult rats is attributable to an interaction with other reactive afferents, was tested by removing other sources of input (the contralateral entorhinal cortex, contralateral hippocampal formation or both) along with the ipsilateral entorhinal cortex at the age of 11 days and then demonstrating the septohippocampal fibers histochemically. Lesions of the lateral part of the ipsilateral entorhinal cortex (source of the lateral perforant path) at 11 days of age evoked a septohippocampal reaction along the outer edge of the molecular layer, where the lateral perforant path fibers normally terminate. This result matched that produced by a complete entorhinal lesion. Lesions of the medial entorhinal cortex evoked no obvious reaction. In contrast, the septohippocampal fibers in adult rats proliferated in the denervated area of the molecular layer after lesions of either part of the entorhinal cortex. Combining lesions of other sources of innervation to the dentate gyrus with an ipsilateral entorhinal lesion at 11 days of age did not alter the response of septohippocampal fibers, as determined histochemically. Neither did the septohippocampal fibers react to removal of commissural afferents alone. The response at any age was unaffected by prior or subsequent removal of the contralateral entorhinal cortex. These results indicate that in immature rats the septohippocampal fibers respond only to loss of the lateral perforant path, but these same fibers can later react to loss of any part of the perforant path. They are regarded as support for the hypothesis that the reactive septohippocampal fibers preferentially interact with dendritic growth cones. Our results do not support explanations based on a hypothetical attraction between septohippocampal and crossed perforant path fibers (which react in the same area) or on competition with commissural fibers (which reinnervate an adjacent area). We suggest further that proximity to the degenerating elements does not in itself determine the pattern of reinnervation after lesions of the central nervous system.

Histochemical evidence of altered development of cholinergic fibers in the rat dentate gyrus following lesions. I. Time course after complete unilateral entorhinal lesion at various ages

The entorhinal cortex of rats was removed at various times during development, and the reaction of the cholinergic septohippocampal input to the dentate gyrus was examined by use of acetylcholinesterase histochemistry. When the ipsilateral entorhinal cortex is completely removed, the outer 70-75% of the molecular layer of the dentate gyrus is almost completely denervated. After such a lesion at 5 to 33 days of age, the acetylcholinesterase staining initially intensified throughout the denervated area, indicating that the septohippocampal fibers branched or elongated. This reaction could be detected within one day after a lesion at 11 days of age and within three or five days after lesions at earlier or later times. Whereas the initial response of the septohippocampal fibers was independent of the age at which the lesion was made, their final localization depended on the developmental state of the animal. After lesions at the age of 5 or 11 days, the reactive septohippocampal fibers became restricted to the outer one-sixth to one-third of the molecular layer within two days after appearance of their initial reaction. A similar concentration of reactive fibers was demonstrable after lesions at 16, 18 or 21 days of age, but some reaction persisted in the middle third of the molecular layer. Finally, after lesions at 26 or 33 days of age the proliferating cholinergic fibers ultimately were uniformly distributed throughout the outer 60% of the molecular layer. These results suggest that septohippocampal fibers initially extend or sprout throughout the denervated area to replace the lost perforant path fibers. However, the reactive fiber population becomes restricted to the outer edge of the molecular layer if the entorhinal lesion is made before the period of cholinergic synaptogenesis and concentrates in this same zone if it is made while cholinergic synapses are forming. We suggest that either the proliferative reaction continues in the outer part of the molecular layer and subsides in other parts of the denervated area or septohippocampal fibers move outward through the molecular layer to assume a more superficial location. After entorhinal lesions at 16 days of age or later the pale-staining zone (containing fibers that originate in hippocampus regio inferior) immediately deep to the denervated area widened. If the lesion was made earlier, this zone never developed at most septotemporal levels of the dentate gyrus. These results are probably related to the extension of regio inferior fibers into the denervated area.

Development of afferent lamination in the fascia dentata of the rat

In the present study we examine the development of afferent lamination in the fascia dentata of the postnatal rat, as a first step in determining possible mechanisms controlling synaptic specificity in this system. This analysis is based on degeneration-induced argyrophilia as well as autoradiographic labeling of the entorhinal and commissural/associational afferents. Both methods show that in spite of the immaturity of the neonatal fascia dentata, these afferent systems have already established territorial relationships by 4 days of age which persist into adulthood. At 4 days, the entorhinal projection is restricted approximately to the outer 45 mum of the 80 mum wide molecular layer. The commissural/associational projection occupies appoximately the inner 35 mum of the molecular layer. At older ages the commissural/associational zone increases in width very slowly relative to the entorhinal zone. We also discuss these results in relation to potential mechanisms of afferent development and dendritic differentiation.

Identification of glycoproteins and proteins at synapses in the central nervous system

Synaptic plasma membranes, synaptic junctions, and postsynaptic densities have been isolated from rat brain, the proteins resolved by polyacrylamide gel electrophoresis, and the glycoproteins identified. The synaptic junction is composed of a spectrum of polypeptides which range in Mr from 13,000 to 250,000. The overall pattern is similar to synaptic plasma memranes; however, the relative proportions of the polypeptides are distinctive. The postsynaptic density fraction consists primarily of one band with an Mr of 52,000. Polypeptides with an Mr of 55,000, and another five of higher Mr, make up the remaining protein. The polypeptides of the postsynaptic density fraction must be reduced with mercaptoethanol in order to permeate the polyacrylamide gel. Therefore, postsynaptic density proteins are cross-linked by disulfide bonds into supramacromolecular aggregates. Glycoproteins which bind concanavalin A were identified in synaptic junctions by studying the binding of 123I-concanavalin A directly to the polypeptides resolved on the polyacrylamide gels. Only four bands, each with an Mr greater than 95,000, bind concanavalin A. In contrast, the pattern of concanavalin A-binding polypeptides in synaptic plasma membranes is distinctive and more complex. In the postsynaptic density fraction, most of the concanavalin A binding occurs to a glyco-component which migrates at the dye front. These data, together with previous cytochemical data using concanavalin A-ferritin conjugates, indicate a limited and select group of high Mr concanavalin A glycoproteins resides within the synaptic cleft of asymmetric type synapses. Whereas a select group of polypeptides bind concanavalin A, all polypeptides resolved in the synaptic junction fraction are glycoproteins and contain galactosyl or galactosyl-like residues, since they label with tritiated borohydride following galactose oxidase treatment. This suggests that the carbohydrate composition of individual glycoproteins is different.

On the functional coupling of neurotransmitter uptake and release in brain

1 Isolated synaptosomal fractions from mouse forebrains were incubated [14C]-gamma-aminobutyric acid ([14C]-GABA). Release of the accumulated label in high potassium solution was measured. 2 The fractional release dependent upon calcium was decreased by raising the concentration of [14C]-GABA during labeling but was not affected by altering the time allowed for labelling or the time between labelling and stimulation. 3 These data suggest that extracellular GABA gains rapid access to available intraterminal pools. The relative distribution of the accumulated GABA in differerent pools can be influenced by the concentration of GABA in the incubation medium but, once (stored", there is no net redistribution of accumulated GABA in the absence of stimulation.

Potentiation of excitatory synaptic transmission in the normal and in the reinnervated dentate gyrus of the rat

Following destruction of the ipsilateral temporo-ammonic tract, which originates in the entorhinal cortex, and terminates on the granule cells of the dentate gyrus, fibers from the surviving contralateral entorhinal area proliferate forming extensive new connections with the denervated dentate granule cells. Utlizing extracellular recording techniques, we have compared the characteristics of synaptic transmission in the lesion induced afferents with the characteristics of the normal ipsilateral afferents by analyzing the responses of dentate granule cells to paired pulse activation of temporo-dentate circuitry. In the dentate gyrus of the normal rat, and extracellularly recorded EPSP evoked by stimulation of the ipsilateral entorhinal cortex is enhanced by as much as 100% by a "conditioning" pulse to the same afferent system. This is called paired pulse potentiation. In the reinnervated dentate gyrus, the extracellular EPSP evoked by a test stimulus delivered to the contralateral entorhinal cortex is also potentiated by a conditioning pulse. Thepaired pulse potentiation in the reinnervated dentate gyrus has a time course which is comparable to that of the normal ipsilateral afferent system, but the magnitude of the potentiation is somewhat less, averaging approximately 140% of control...

Distribution and mobility of lectin receptors on synaptic membranes of identified neurons in the central nervous system

The distribution and mobility of concanavalin A (Con A) and Ricinus communis agglutinin (RCA) receptors (binding sites) on the external surfaces of Purkinje, hippocampal pyramidal, and granule cells and their attached boutons were studied using ferritin-lectin conjugates. Dendritic fields of these cells were isolated by microdissection and gently homogenized. Cell fragments and pre- and postsynaptic membranes were labeled with the ferritin-lectin conjugates at a variety of temperatures, and the distribution of lectin receptors was determined by electron microscopy. Both classes of these lectin receptors were concentrated at nearly all open and partially open postsynaptic junctional membranes of asymmetric-type synapses on all three neuron types. Con A receptors were most concentrated at the junctional membrane region, indicating that the mature neuron has a specialized nonrandom organization of carbohydrates on its outer surface. Lectin receptors located on postsynaptic junctional membranes appeared to be restricted in their mobility compared to similar classes of receptors on extrajunctional membrane regions. Labeling with ferritin-RCA and -Con A at 37 degrees C produced clustering of lectin receptors on nonjunctional surfaces; however, Con A and RCA receptors retained their nonrandom topographic distribution on the postsynaptic junctional surface. The restricted mobility of lectin receptors was an inherent property of the postsynaptic membrane since the presynaptic membrane was absent. It is proposed that structures in the postsynaptic density may be transmembrane-linked to postsynaptic receptors and thereby determine topographic distribution and limit diffusion of specialized synaptic molecules. Speicalized receptor displays may play an important role in the formation and maintenance of specific synaptic contacts.

Stimulation-dependent depression of readily releasable neurotransmitter pools in brain

Previously accumulated GABA was released from isolated forebrain synaptosomes with repeated calcium stimulation in elevated-potassium medium. Fractional release (calcium-dependent) in response to a second calcium pulse (90-120 sec later) was depressed to approximately 60% of initial release. Neither initial GABA release nor the subsequent depression of release was affected by variations in the labelling duration. Stimulation-dependent depression of labelled GABA and norepinephrine release was demonstrated from both cerebral cortex and cerebellum synaptosomal preparations. In addition, depression resulted from prior stimulation in the presence of veratridine, A23187 or elevated-potassium. Although release of previously accumulated GABA was depressed by calcium stimulation, the release of GABA accumulated between stimulations was not. Release of this recently accumulated GABA was indistinguishable from the initial release of previously accumulated GABA and larger than the subsequently depressed release from the previously accumulated pools. These data imply (1) that the depressed release resulted from a decrease of available transmitter in pools that support secretion processes, (2) that depressed release did not result from a depression of stimulus-secretion coupling processes, and (3) that transmitter accumulated subsequent to release events is released preferentially to transmitter accumulated prior to the intervening stimulation.

Electrophysiological analysis of the projection from the contralateral entorhinal cortex to the dentate gyrus in normal rats

The projection from the contralateral entorhinal cortex to the dentate gyrus is shown to exert a monosynaptic excitatory action. Stimulation of the contralateral entorhinal cortex evokes unitary granule cell discharges in the dentate gyrus. This evoked activity is followed by a period of inhibition lasting about 50 msec. Laminar analyses of field potentials generated by contralateral medial entorhinal stimulation localize the synaptic activity to the middle portion of the granule cell dendrites. This localization is consistent with the termination site of the contralateral entorhinal projection as previously shown anatomically. Earlier studies have indicated that the entorhinal cortex excites only the ipsilateral dentate gyrus. These findings now demonstrate that the cortical input to the dentate gyrus is bilateral in normal rats. However, the contralateral projection appears to be much less efficacious than the ipsilateral input.

GDEE antagonism of iontophoretic amino acid excitations in the intact hippocampus and in the hippocampal slice preparation

Glutamic acid diethylester (GDEE) reversibly antagonized excitations produced by glutamate and aspartate but not those produced by acetylcholine when applied iontophoretically to rat CA1 hippocampal neurons in penthrane (methoxyfluorane) anesthetized rats and to CA1 neurons in in vitro slice preparations. GDEE did not appear to differentiate between the excitations produced by glutamate aspartate and appeared to be a more potent antagonist than has previously been reported. CA1 cells were remarkably sensitive to acetylcholine; 5-50 nA being sufficient to produce marked amino acid-like excitations, which were unrelated to the pH of the acetylcholine. The nature of the responses to applied substances was virtually identical between the intact animal and the in vitro slice preparation. A description of the in vitro technique is given as an Appendix.

Intracellular responses from granule cell layer in slices of rat hippocampus: perforant path synapse

Intracellular responses were recorded in vitro from the denate granule cell layer of hippocampal slices prepared from adult rats. Spontaneous activity of granule cells in vitro consisted of action potentials and small, graded depolarizations, presumably of synaptic origin. Granule cells could be activated by injection of depolarizing current or release of hyperpolarizing current. Individual granule cells spatially summed input from the perforant path and fired multiple action potentials in vitro following strong presynaptic volleys. Depolarization decreased and hyperpolarization increased the EPSP amplitude, which is consistent with a conductance-increase mechanism. Although we could demonstrate postexcitatory inhibition in some cells, granule cells in vitro appeared to receive less inhibitory feedback than in vivo, EPSP amplitude and spike output of granule cells showed frequency potentiation and posttetanic potentiation to perforant path stimulation. These intracellular responses in vitro complement some of the findings from field-potential analyses of the dentate gyrus in intact animals.

Hypertrophy and redistribution of astrocytes in the deafferented dentate gyrus

The response of the astroglial population of the dentate gyrus molecular layer to removal of that region's primary afferent was investigated using Cajal's gold sublimate method. Deafferentation caused the astrocytes to hypertrophy, an effect which was detectable at 24 hr and maximal at 72-96 hr post-lesion. Following this, the astroglia entered a lengthy period of gradual atrophy. Counts of the astrocytes in the various sublayers of the molecular layer led to the conclusion that these cells migrate into denervated dendritic areas from neighboring, nondeafferented zones.

Lesion-induced synaptogenesis in brain: a study of dynamic changes in neuronal membrane specializations

When incoming fibers to a given brain region are damaged and degenerate, the remaining undamaged fibers can, in some cases, form new synapses, and restore physiologically functional circuitry. Synaptic membrane events underlie this reconstruction: the connection between membranes is broken and reformed.

Stimulus-secretion coupling processes in brain: analysis of noradrenaline and gamma-aminobutyric acid release

1. Brain synaptosomal fractions released both endogenous and exogenously loaded noradrenaline and gamma-aminobutyric acid (GABA) in response to calcium. Elevation of magnesium concentrations in the release media decreased the calcium-dependent release. 2. The release of noradrenaline and GABA occurred within 250 msec following the application of calcium. Following the initial response to calcium, release progressively decreased with continued application of calcium. GABA release declined more rapidly than noradrenaline release, consistent with a noradrenaline distribution having greater accessibility to the release process. 3. Sodium was required for the loading of noradrenaline and GABA into pools released by calcium. On the other hand, the presence of sodoium was not required for release from previously loaded pools. 4. Microsomal fractions did not exhibit calcium-dependent release of noradrenaline or GABA. Furthermore, exogenously loaded lysine was not released from synaptosomal fractions in response to calcium. 5. Barium and strontium, but not magnesium, stimulated noradrenaline and GABA release in the absence of calcium. The ordering of alkaline earth efficacies was barium greater than strontium greater than calcium. 6. Manganese inhibited calcium-dependent release of noradrenaline and GABA to a greater extent than magnesium. 7. Release, in response to 1 mM calcium, increased linearly with the log. [K+]0, suggesting that a voltage-dependent calcium inophore limits release. The slope of release vs. log. [K+]0 was greater for noradrenaline than for GABA. 8. For a given [K+]0 less than 55 mM, increases in external calcium concentration above 1 mM increased noradrenaline release but decreased GABA release. These data suggest that calcium can decrease its own permeation and that differences in the release process may exist for different neurotransmitters. 9. In the presence of the artificial calcium ionophore, A23187, both noradrenaline and GABA release increased linearly with the log. [Ca2+]0. The slope for noradrenaline release was greater than that for GABA release. 10. Stimulus-secretion coupling in brain is suggested to be regulated at the level of a voltage dependent calcium permeation mechanism. However, basic differences in the interaction of calcium with the release process may exist between the noradrenaline and GABA systems.

Time-dependent changes in commissural field potentials in the dentate gyrus following lesions of the entorhinal cortex in adult rats

Previous neuroanatomical work has shown that lesions of the entorhinal cortex in adult rats cause the commissural projections to spread from their normally restricted locus in the inner molecular layer approximately 40-50 mum into the outer molecular layer (that is, into the zone deafferented by the lesion). In the present study we measured the effects of the entorhinal lesion on the distribution of short-latency potentials elicited by commissural stimulation in the molecular layer. Studies with animals tested at various times after the lesion and with a preparation that permitted recording from the same rat at several post-lesion intervals both indicated that the commissural response spread 100-150 mum towards the deafferented outer molecular layer, while the maximum response spread 50-100 mum. These effects were first detectable by 9 days after the lesion and were fully developed by 15 days post-lesion. These findings suggest that the growth of the commissural system seen after entorhinal lesions results in the rapid formation of functional terminals and are discussed in relationship to the behavioral consequences of brain lesions.

A dual marking technique for microelectrode tracks and localization recording sites

This paper describes techniques for marking both microelectrode tracks and exact recording loci using a combination of fast green dye and horseradish peroxidase (HRP). The procedure involves coating the exterior of HRP filled microelectrodes with fast green dye in order to identify electrode tracks, and ejecting HRP from the electrode to mark recording loci. Rapid, multiple marks can be made with this technique without harming the recording capabilities of the micropipette.

Changes in protein levels in perfusates of freely moving cats: relation to behavioral state

Perfusates from the brains of freely moving cats, obtained by means of a push-pull cannula, contain high concentrations of proteins. The levels vary in a cyclic fashion and are higher during rapid eye movement sleep than during the waking state. The proteins represent a distinctive class of tissue protein and their changing levels appear to reflect an alteration in the protein content of the extracellular space of brain related to behavioral state.

A neurophysiological analysis of commissural projections to dentate gyrus of the rat

The electrophysiological properties of the commissural projections to the dentate gyrus of the rat were investigated using extracellular field-potential and unit-recording techniques. The following conclusions with respect to those investigations were obtained: 1) The CA3c/CA4 region of the contralateral hippocampus proved to be the most effective site for eliciting the commissural field potentials in the dentate gyrus dorsal and ventral leaves. 2) The location of the short-latency negative field potential in the molecular layer of the dentate gyrus was restricted to a region 50-100 mum distal to the granule cell layers corresponding to the inner one-third of the granule cell dendrites. 3) The negative field potential proved to satisfy a number of criteria for the extracellular representation of the summed EPSPs of synchronously activated granule cells. 4) The excitatory nature of the commissural projections to the dentate was confirmed by the short-latency driving of units recorded from the granule cell layers. 5) A comparison of both commissural and entorhinal cortical stimulation procedures showed the field potentials elicited by the different convergent anatomical systems to be localized within different regions of the dentate molecular layer. 6) The distribution of commissural potentials along the septotemporal axis of the dentate gyrus indicated that stimulation sites homotopic to the recording electrode in the contralateral CA3c/CA4 region were the most effective in eliciting these potentials. 7) These findings were discussed with reference to the mode of activation of the dentate granule cells by the commissural system with specific comparison to the larger and apparently more powerful projections from the entorhinal cortex.

Experimental tests of hypotheses about neurochemical mechanisms underlying behavioral tolerance to the anticholinesterase, diisopropyl fluorophosphate

Neurochemical and psychopharmacological studies of rats were designed to examine four hypotheses which have been proposed to account for the development of behavioral tolerance to the anticholinesterase, diisopropyl fluorophosphate (DFP). The fact that the activity of the enzymes, adenosine triphosphatase, alkaline phosphatase and cytochrome oxidase, did not change concomitantly with behavioral measures during chronic treatment with DFP suggests that nonspecific metabolic changes are unlikely mechanisms of behavioral tolerance. Similarly, a lack of change in choline acetylase activity coupled with constantly high acetylcholine levels (140%) and low cholinesterase activity (28.5%) tends to eliminate end-product inhibition of acetylcholine synthesis as a primary mechanism of tolerance to DFP. Alpha-Methyl-p-tyrosine in doses to 150 mg/kg affected the behaviors of control and DFP-treated rats to a comparable degree, offering no support for the hypothesis that a redundant adrenergic system may replace the cholinergic system during the development of tolerance to DFP. In contrast to these various negative findings, pilocarpine was less effective in suppressing the responding of rats tolerant to DFP than that of control subjects. This confirms other evidence indicating that a decreased sensitivity of cholinergic (muscarinic) receptors is one mechanism underlying the development of tolerance to DFP.

Isolation of postsynaptic densities from rat brain

Most synapses in the central nervous system exhibit a prominent electron-opaque specialization of the postsynaptic plasma membrane called the postsynaptic density (PSD). We have developed a procedure for the isolation of PSDs which is based on their buoyant density and their insolubility in N-lauroyl sarcosinate. Treatment of synaptic membranes with this detergent solubilizes most plasma membranes and detaches PSDs from the plasma membrane so that they can be purified on a density gradient. Isolated PSDs appear structurally intact and exhibit those properties which characterize them in tissue. The isolated PSDs are of the size, shape, and electron opacity of those seen in tissue; they stain with both ethanolic phosphotungstic acid and bismuth iodide-uranyl lead and the fraction contains cyclic 3',5'-phosphodiesterase activity. Quantitative electron microscope analysis of the PSD fraction gives an estimated purity of better than 85%. Inasmuch as the PSD is associated primarily with dendritic excitatory synapses, our PSD fraction represents the distinctive plasma membrane specialization of this specific synaptic type in isolation.

Proteins of the postsynaptic density

An analysis was made of the protein composition of a fraction of postsynaptic densities (PSDs) prepared from rat brain. Protein makes up 90% of the material in the PSD fraction. Two major polypeptide fractions are present, based on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The major polypeptide fraction has a molecular weight of 53,000, makes up about 45% of the PSD protein, and comigrates on gels with a major polypeptide of the synaptic plasma membrane. The other polypeptide band has a molecular weight of 97,000, accounts for 17% of the PSD protein, and is not a prominent constituent of other fractions. Six other polypeptides of higher molecular weight (100,000-180,000) are consistently present in small amounts (3-9% each). The PSD fraction contains slightly greater amounts of polar amino acids and proline than the synaptic plasma membrane fraction, but no amino acid is usually prominent. The PSD apparently consists of a structural matrix formed primarily by a single polypeptide or class of polypeptides of 53,000 molecular weight. Small amounts of other specialized proteins are contained within this matrix.