A quantitative electron microscopic study of synaptogenesis in the dentate gyrus of the rat

Development of place navigation in rats from weaning to puberty

Young hooded rats were trained to escape onto a hidden platform after swimming in a pool of opaque water. Subjects 21, 28, 35, 42, and 64 days of age on the first training day were given 28 trials on 5 consecutive days. Half of the rats were required to localize the platform in relation to external room cues only ("place only" condition) and the other half were helped by the presence of a visible cue on the platform ("cue + place" condition). A deficiency in place navigation was observed in the 21- and 28-day groups; they showed slow escape and took circuitous routes more often than older rats. This deficiency was related to a poor spatial bias toward the training position when the subjects were allowed to swim for 30 s in the absence of the platform, at the end of the 28-trial training period (probe trial). The 35-day group showed adult-like learning ability in both training conditions, but failed to show searching behavior during the probe trial after having been trained in the presence of the proximal cue. Only rats older than 40 days showed typical adult behavior such as swimming directly toward the platform from any starting position and localized searching around the absent platform's position during the probe trial, no matter what the training conditions were. These results suggest that central nervous system structures responsible for place learning in the rat are functional from around 32 days of age, but fail to trigger searching behavior following cued training before the sixth week.

Polyribosomes under developing spine synapses: growth specializations of dendrites at sites of synaptogenesis

We have previously reported that there is a dramatic increase in polyribosomes associated with dendritic spines during periods of synapse growth induced by denervating lesions. We suggested that polyribosomes at the postsynaptic site may somehow be involved in the growth of synapses. To evaluate this hypothesis further, the present study determines whether synapses which are growing in the developmental period also have accumulations of polyribosomes. We examined the dentate gyrus of the developing rat electron microscopically at 7, 10, 15, 20, and 28 days of age, which spans the major period of synaptogenesis in this structure. Qualitative observations revealed dramatic accumulations of polyribosomes under spine synapses in the youngest animals (7 and 10 days of age). With synapse development, the accumulations of polyribosomes became less dramatic, so that by 28 days of age, the neuropil of the dentate gyrus appeared qualitatively mature. To determine the relationship between polyribosomes under spine synapses and synapse development, quantitative electron microscopic methods were use to evaluate synapse density (number of synapses/100 micron 2), and the incidence of polyribosome-containing spines (proportion of spine synapses with underlying polyribosomes) in the neuropil of the dorsal blade of the dentate gyrus at each age. An inverse relationship was found between synapse density and the proportion of spines with polyribosomes. Synapse density increased in an almost linear fashion between 7 and 28 days of age to levels which were actually somewhat higher than in mature rats, whereas the incidence of polyribosome-containing spines was highest at the youngest ages and decreased with development. Thus, polyribosomes were most prominent under spine synapses during the period of maximal synapse growth. These results, together with our previous observations of increased numbers of polyribosomes under spines during lesion-induced growth, suggest that the polyribosomes represent a structural specialization of dendrites at sites of synapse construction. We propose that they produce protein(s) that are involved in synapse growth.

Maturation of kainic acid seizure-brain damage syndrome in the rat. II. Histopathological sequelae

The histopathological sequelae of parenteral administration of kainic acid were investigated in immature rats (3-35 days of age). The brains were fixed 1-14 days after the administration of kainate and the damage evaluated by means of argyrophylic (Fink-Heimer, Gallyas or Nauta-Gygax) and Nissl stains. In animals of less than 18 days of age there was no sign of damage even after 1-2 h of severe tonico-clonic convulsions. Between 18 and 35 days after birth, there was a progressive increase in the severity of the damage, the adult pattern being reached at the latter age. As in adult animals, brain damage was most severe in structures which are part of the limbic system, i.e. the hippocampal formation, lateral septum, amygdaloid complex, claustrum, piriform cortex, etc. In addition to neuronal abnormalities, the following reactions were observed: hypertrophy and swelling of satellite oligodendroglia, proliferation of hypertrophic microglia, proliferation of astroglia and hypertrophy of endothelial cells in the capillary wall. The latter type of change, together with local coagulative necrosis, was almost exclusively restricted to the granular and molecular layers of the fascia dentata. In the hippocampal formation we found a temporal gradient of vulnerability. The earliest and most consistent neuronal alterations were largely restricted to interneurons of the hilar region and to a lesser extent to non-pyramidal neurons of strata oriens and radiatum. The severe necrotic destruction of the pyramidal layer of CA3 is conspicuous at a later age (postnatal day 30-35) and with longer survival times. Our results suggest that: (1) the neurotoxin only induces brain damage once it also causes limbic motor seizures and its associated metabolic activations, notably in the amygdala; (2) the earliest pathological sequelae occur in interneurons of the hilar region and (3) sclerosis of the vulnerable region of the Ammon's horn--the CA3 region--is only obtained once the dentate granules and their mossy fibres are fully operational, thereby reflecting the crucial role of this axonal connection in eliciting hippocampal damage.

Age-related fine structural changes in axons and synapses during deafferentation of the rat pyriform cortex: a possible basis for plasticity

The purpose of this study was to compare the sequence of axonal and synaptic alterations following deafferentating lesions at selected postnatal ages and relate these changes to synaptic organization in the olfactory cortex. Rats received unilateral olfactory bulb ablation at 2 1/2, 6, 9 and 13 days of age and were studied at survivals of 12 h to 30 days. At least three clearly different forms of acute degeneration were seen; flocculent, granular and dense with the granular form an intermediate form. The proportion of granular and especially dense degeneration increases after six days of age as does the presence of glia. The denser the type of degeneration, the greater the retention of remnants of this form of synaptic degeneration at deafferented postsynaptic sites. This as well as the increased presence of glia after six days may be important factors in the limitation of plastic reorganization or reinnervation in more mature individuals. The youngest operated animals show rapid vacating of the receptor site, relative absence of glia and striking evidence of competitive reoccupation of deafferented sites.

A comparison of the postnatal development of post-activation potentiation in the neocortex and dentate gyrus of the rat

The postnatal development of short-term potentiation (STP) and long-term potentiation (LTP) was examined in the neocortex and dentate gyrus of rats aged 7 days to adult. STP and LTP of the transcallosal response in the neocortex could not be demonstrated until the third postnatal week. STP and LTP of the perforant path-dentate response could not be demonstrated until the second postnatal week. In both cases, STP appeared several days before LTP. Structural and neurochemical correlates of STP/LTP development, and their implications for possible STP/LTP mechanisms, are discussed.

The indusium griseum in the mouse: architecture, Timm's histochemistry and some afferent connections

The Indusium griseum (IG) is an enigmatic cortical field classically felt to be a part of the hippocampus (HC). In the mouse, IG lies just dorsal to the corpus callosum at the base of the anterior half of cingulate cortex. In coronal sections the field is small but constitutes a fairly long rostro-caudal strip. The connections of the IG are poorly understood. The Timm's staining pattern of the IG is reminiscent of a mini-dentate gyrus (DG) comprising a layer of granule cells with two bands of staining in the molecular layer. In the DG there are three bands which correspond to inputs from the lateral and medial entorhinal area (LEA and MEA) and the ipsi- and contralateral association systems. Using anterograde transport of HRP we have found that the LEA and MEA also terminate in the molecular layer of the IG. This suggests that the IG is a displaced portion of the DG. The olfactory system is known to have a strong indirect influence on the HC via primary and secondary bulbar projections to the LEA. Wheat germ agglutinin-HRP injections confined to the main olfactory bulb (MOB) show a direct projection from the MOB to IG. Both the olfactory bulb itself and retrobulbar structures such as the piriform cortex (PC) convey olfactory information to the LEA; the LEA supplies a major input to the DG. Our results suggest that there is a more direct pathway whereby olfactory information may influence a cortical region, IG, whose histochemistry and direct afferents from the entorhinal cortex suggest that it is part of or closely related to the DG. Thus, IG may represent a phylogenetically old olfacto-recipient outpost of the hippocampus.

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.

Development of benzodiazepine binding subtypes in three regions of rat brain

An examination of the development of benzodiazepine binding of the high and low affinity triazolopyridizine (TPZ) type was undertaken in rat cortex, hippocampus and cerebellum. Various concentrations of a typical triazolopyridizine, CL-218-872, were used to displace [3H]flunitrazepam from synaptosomal fractions from rats of postnatal ages day 0, 7, 14, 21, 35 and 70. In contrast to the cortex and cerebellum, hippocampus displays high affinity TPZ binding at birth, prior to the periods of dendritic elaboration and synaptogenesis, suggesting that adult proportions of high and low affinity sites are maintained throughout postnatal development. Delayed postnatal development of high affinity TPZ sites is observed in the cerebellum, similar to the postnatal development previously observed in the cortex.

Differentiation of granule cell dendrites in the dentate gyrus of the rhesus monkey: a quantitative Golgi study

The differentiation of granule cell dendrites in the dentate gyrus of the hippocampal region was studied in a series of developing fetal and postnatal rhesus monkeys whose brains were processed by the rapid Golgi method. The total combined lengths of dendrites, the total number of dendritic spines, and their density on the proximal, middle, and distal thirds of the dendritic shafts were determined at embryonic days 58, 95, 120, 153, term (165), postnatal days 3, 20, 60, 150, 365, and adults. At all ages examined, granule cells exhibited various levels of maturation with the more differentiated cells being situated in the superficial strata of the granular layer and the less mature cells lying in progressively deeper positions, thus conforming to the outside-to-inside spatiotemporal gradient of their genesis. Quantitative analysis shows that, in this primate, hippocampal granule cells differentiate mainly in the second half of gestation with all measured parameters attaining mature values by the time of birth. However, the analysis also reveals a transient phase of exuberant postnatal development which involves excessive dendritic branching, regional changes in dendritic length, overproduction of dendritic spines, and redistribution of spines within the molecular layer. After reaching peak values around the middle of the first year of life, these parameters decrease and in adult monkeys fall back to the neonatal level.

Protein phosphorylation in isolated synaptic junctional structures: changes during development

Endogenous phosphorylation in subcellular fractions enriched in synaptic plasma membranes (SPM) and purified synaptic junctions (SJ) has been examined in vitro at various stages of postnatal development. Protein kinase activity was measured using both endogenous and exogenous (histones) proteins as substrates. Protein phosphorylation that is regulated by cyclic AMP also was investigated. SPM and SJ fractions displayed large increments in kinase activities and cyclic AMP-stimulated protein phosphorylation between postnatal days 5 and 15. SJ fractions exhibited a dramatic age-dependent change in the cyclic AMP-stimulated phosphorylation of endogenous proteins of molecular weights 73,000 (1a), 68,000 (1b), 65,000 (1c) and 55,000. Experiments in which isolated SJs were phosphorylated with soluble cyclic AMP-dependent kinase showed that the phosphorylation of proteins 1a, 1b and 1c resulted from their de novo appearance in newborn SJ fractions and not from a maturation-dependent coupling of kinases and substrates that were already present in newborn SJ fractions. The levels of regulatory (R) subunits of cyclic AMP-dependent kinases in synaptic fractions were measured by [3H]cyclic AMP binding and photoaffinity labeling with [32P]8-N3-cyclic AMP and were observed to change little during postnatal development. These findings show that there is a strong correlation between the in situ appearance of synapses and the enzymatic maturation of endogenous, cyclic AMP-stimulated protein phosphorylation in SJ fractions isolated at different stages of development.

Development changes and regional variation in the ganglioside composition of the rat hippocampus

Individual gangliosides were extracted and quantified from intact and subregionally dissected hippocampal formations during the first 3 weeks of postnatal development in the rat. The 4 major gangliosides were all found to be rapidly increasing relative to brain weight prior to the onset of significant synaptogenesis, but quantitative changes in the final proportion of each ganglioside resulted from different rates of accumulation of gangliosides D1a, D1b, and T1b, beginning about the time of synaptic maturation and dendritic arborization but prior to the onset of myelination and neurophysiological activity. A ganglioside pattern difference characterized by lower levels of T1b and D1b in the area dentata of middle and dorsal aspects of the hippocampal formation coincides with developmental gradients previously observed by morphogenetic criteria for these subregions. These ganglioside differences may thus be developmentally regulated, but the persistence into adulthood of higher levels of D1a in the dentate area suggests that some ganglioside pattern differences may be intrinsic to the unique neuronal cell populations present in different areas of the hippocampus.

Mechanisms of septal lamination in the developing hippocampus revealed by outgrowth of fibers from septal implants. III. Competitive interactions

Embryonic septal tissues grafted alongside the hippocampal formation in neonatal rat hosts were used to test for the presence of competitive interactions between developing native axons and those from the septal grafts. When native septal afferents to the target hippocampal formation are removed at the time of implantation, axons from septal grafts visualized with acetylcholinesterase (AChE) histochemistry enter the appropriate laminar zones in the host hippocampus. The pattern of septal implant growth differs markedly when native septohippocampal fibers are left intact. Under these conditions, implant-associated AChE staining is intense in the cortical area surrounding the graft but little or no reaction product is present in the host dentate gyrus and hippocampus proper. A similar result is obtained when implant viability is enhanced by introducing it 3 days after the cavity is made. These data illustrate an apparent hierarchy in competitive interactions for developing septal fibers. Septal afferents can inhibit the growth of other septal fibers and commissural/associational (c/a) axons can exclude septal fibers from their terminal field. Septal fibers and entorhinal afferents will develop concurrently.

Identification and quantification of n-acetylserotonin (NAS) in the developing hippocampus of the rat

1. A specific anti-NAS antibody and fluorescein-labelled second antibody were employed to investigate the presence of NAS in the dentate gyrus of the hippocampus as a function of age. 2. Immunoreactive NAS (INAS) was present in the granular cell layer of the dentate gyrus as early as 20 days postconception. 3. INAS appears to be present primarily in cell bodies. 4. Cross-reactivity and inhibition experiments confirm the positive identification of INAS. 5. INAS is age-dependent and increases with age reaching adult levels by day 30 post-conception/17 days after birth. 6. The appearance and subsequent increase in INAS correlates with the development of dentate granular cells and their subsequent synapse development suggesting a role for NAS in the normal functioning of these cells.

Identification of cell types in rat hippocampal slices maintained in organotypic cultures

We have cultured transverse slices of the hippocampal formation from neonatal rats and have identified the cell types which appear in the outgrowth with cell type specific markers. Tetanus toxin and anti-tetanus toxoid, as well as antisera to neurofilaments and 14-3-2 protein, were used to identify neurons. Astrocytes were identified with antisera to glial fibrillary acidic protein and were the predominant non-neural cell type. Fibroblastic cells were labeled with antisera to fibronectin and to myosin and oligodendroglia were identified with antisera to galactocerebroside. The hippocampal neurons could be classified as 1 or the 3 types present in vivo (pyramidal cells, granule cells, or GABAergic interneurons) on the basis of their size, shape, location, or reaction with antisera to glutamic acid decarboxylase. Outgrowth of glial cells and neurites occurred within hours of explantation. Within a few days granule cell neurons migrated onto the glial cell layer from the explant. Their movement is probably related to their migration during in vivo development of the granule cell layer. Synapse formation was observed by electron microscopic analysis beginning about 3-5 days in vitro and areas of neuropil containing many synapses were observed after 3-4 weeks. This culture system should be useful for further studies on the cellular processes which occur during hippocampal development and plasticity.

Postnatal ontogenesis of hippocampal CA1 area in rats. I. Development of dendritic arborisation in pyramidal neurons

Hippocampal pyramidal neurons (area CA1) in rats, 5, 10, 15, 24, 48 and 90 days old were studied by means of the Golgi-Cox impregnation. Using a video-computer microscope the development of dendritic arborisation was analysed and descriptive as well as quantitative data were obtained. During the suckling period (5 and 10 days) the basal dendrites, the main shaft and within the stratum moleculare branching terminal fibers of the apical dendrite were found to develop. At the weaning period (15 and 24 days) the development progressed with proliferation and ramification of the apical dendrite's lateral branches in the stratum radiatum and of its preterminal fibers branching in the stratum lacunosum. The lateral branches of the apical dendrite developed in a consequent order from the neuronal soma towards fissura hippocampi. Adult numbers of dendrites were set in first, followed by elongation and ramification of the fibers. The final number of segments was in the whole dendrite system, except of the main shaft, established before the maximum of the total dendritic length was reached. The afferent fibers termination upon the pyramidal neurons are known to be distinctly divided within the hippocampal layers. Step by step development of the individual groups of dendrites therefore probably eventuates the linkage of afferent fibers in a successive order.

Developmental changes in morphology and molecular composition of isolated synaptic junctional structures

Synaptic junctional fractions which display subcellular purity that compares favorably to similar fractions prepared from adult have been isolated from immature rat brains. Electron microscopic analysis of immature fractions has revealed age-dependent changes in the morphology of isolated synaptic structures. The recovery of total synaptic junctional protein increased in a linear fashion and was temporally correlated with the appearance of asymmetric synapses in brain. Systematic age-dependent changes were observed in the protein and glycoprotein composition of synaptic membrane and synaptic junction fractions during postnatal development. In isolated synaptic junctions, the major postsynaptic density protein increased approximately 20-fold during postnatal development. Immature synaptic junction fractions contained tubulin and actin in larger relative quantities than are present in synaptic junction fractions isolated from adult brain tissues. Immature synaptic junctions also contained appreciable amounts of postsynaptic membrane glycoproteins that bind concanavalin A (con A).

Development of glutamate binding sites and their regulation by calcium in rat hippocampus

The postnasal development of the Na-independent [3H]glutamate binding sites, which exhibit some characteristics of postsynaptic glutamate receptors, has been studied in rat hippocampal membranes. The amount of binding sites (expressed in pmol/hippocampus) represents 4% of the adult level at postnatal day (PND) 4, increases very rapidly until PND 9, and then increases at a slower rate reaching 80% of the adult value at PND 23. In contrast, the density of binding sites (expressed in pmol/mg protein) exhibits a maximum at PND 9 and slowly decreases to reach the adult value at PND 23. These changes seen to be only quantitative since the affinity (about 450nM) and Hill coefficient (about 1.0) of these binding sites remain constant throughout development. Calcium ions have been shown to markedly stimulate [3H]glutamate binding in adult hippocampal membranes. This effect appears on PND 9--10 and increases rapidly until PND 16 when it is similar to that seen in the adult rat. We also determined the minimum age at which long-term potentiation (LTP) of synaptic transmission could be detected in the CA1 field of hippocampal slice preparations following repetitive electrical stimulation of the Schaffer-commissural pathways. LTP was only rarely detected at PND 8 whereas it could be reliably obtained after PND 9. These results indicate that the postnatal development of Na-independent glutamate binding sites closely parallels synapse formation in the hippocampus, further supporting the idea that the binding sites are associated with a physiological receptor. They also show that the appearance of the stimulatory effect of calcium on glutamate binding occurs at a time when several forms of synaptic plasticity appear in the hippocampus. In particular the correlation of the development of LTP with the calcium-stimulation of glutamate binding suggests that these phenomena have similar cellular mechanisms.

Development of the mossy fibers of the dentate gyrus: I. A light and electron microscopic study of the mossy fibers and their expansions

The postnatal development of the axons of the dentate granule cells--the so-called mossy fibers--was studied at the light microscopic level in Timm and Golgi preparations and also by transmission electron microscopy. In the Timm-stained material, there was a distinctive coloration in the hilus and incipient stratum lucidum, indicating the presence of mossy fibers, on the first postnatal day. Over the next two weeks, the stained areas became more extensive, the size and density of the stained particles increased, and the particles became more intensely stained. These signs of progressive development of the mossy fibers appeared to reflect, temporally and topographically, the developmental gradients followed by their parent granule cells. The Golgi material confirmed the presence of mossy fibers in the hilus on the first postnatal day. Fasciculi of mossy fibers were observed in the stratum lucidum of the 3-day-old hippocampus, and although these immature axons were devoid of large synaptic expansions, they did have prominent growth cones at their termini. Small expansions along the lengths of the axons first appeared on day 7 and these grew to approximately an adult size and complexity by about day 14. The postsynaptic component of the mossy fiber synapse, the "thorny excrescence," did not begin to emerge from the proximal portion of the pyramidal cell dendrites until sometime after day 9. At the electron microscopic level we observed, on the first postnatal day, small, immature mossy fiber expansions which made both symmetric and asymmetric contacts directly with dendritic shafts. These profiles, which were only one tenth the size of mature expansions, grew rapidly between postnatal days 1 and 9 and increased their mean area by a factor of five. On or about day 9, as the "thorny excrescences" emerged, the asymmetric synapses came to be associated with these spinous processes. Taken together, the Golgi and electron microscopic analyses support the suggestion that mossy fibers establish synaptic contact with pyramidal cell dendrites early in the postnatal period, several days before there is any indication of spine development. Furthermore, the "thorny excrescences" develop after the more typical, pedicellate spines have appeared on the distal pyramidal cell dendrites. Finally, while it is clear that the mossy fibers in our 21-day-old material are, for the most part, fully matured, a more subtle and protracted development of the system, long into adulthood, is indicated by the increased area and density of stained particles in the Timm preparations from adult animals.

Interaction of age and sex in sympathetic axon ingrowth into the hippocampus following septal afferent damage

Damage to the medial septal nucleus or to the fimbria/fornix in the adult rat elicits sprouting of vascular sympathetic fibers into the deafferented regions of the hippocampal formation. The following study examines the effects of developmental stage and sex on this sprouting phenomenon using both fluorescence histochemistry and high affinity uptake of noradrenaline. We find that (1) the sprouting, which is reduced in adult and juvenile males relative to females, is equivalent in the two sexes after transections at postnatal day 3, and (2) the period of maximum ingrowth is sexually regulated, occurring near postnatal day 3 in the male and postnatal day 13 in the female.

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.

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.

A comparison of the effect of mid-thoracic spinal hemisection in the neonatal or weanling rat on the distribution and density of dorsal root axons in the lumbosacral spinal cord of the adult

Transecting the thoracic spinal cord of the rat has markedly different effects on behavioral responses of the hindlimbs if the lesion is made at the neonatal or weanling stage of development. The present investigation tested the possibility that the behavioral differences were related to a difference in the distribution or density of dorsal root connections in the lumbosacral spinal cord. In order to use each animal as its own control the distribution and density of dorsal root axons was compared on the two sides of the L5-S1 segments of the lumbosacral spinal cord in adult rats given a mid-thoracic spinal hemisection at the neonatal or weanling stage of development. Comparing the experimental (initially hemisected side) and control sides of the cord, we found no evidence for a change in the distribution of dorsal root axons. The distribution of Fink-Heimer stained degeneration 4--6 days after bilateral spinal root section was virtually identical on the two sides of the cord from animals hemisected at either stage. However, in rats spinally hemisected at the neonatal stage (n = 8), a significantly greater density of dorsal root degeneration was found within the intermediate nucleus of Cajal (INC) on the experimental side using coded material and a blind analysis. No difference in the density of dorsal root degeneration was detected in the group of rats spinally hemisected at the weanling stage (n = 6). Controls indicated that the increased density of degeneration was not due to compression resulting from shrinkage of the INC or to degeneration remaining from the initial hemisection. We conclude that the increased amount of argyrophilia within the INC of neonatally hemisected rats is due to an increased density of dorsal root axons in this zone. This result supports the hypothesis that the behavioral differences found when comparing animals transected at the neonatal or weanling stages of development are related to an increased number of dorsal root connections within the lumbosacral spinal cord.

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.

A regional study of electrophoretically separated proteins from the crude synaptosomal fractions of the developing rat brain

Albino rats of 1, 5, 9, 16, and 25 days of age were sacrificed, and six brain regions were dissected free. Crude synaptosomal preparations from each brain region and age were obtained by differential centrifugation. Protein content in these fractions from the six brain regions increased almost threefold between 1 and 25 days of age. The preparations were delipidated, solubilized, and subjected to polyacrylamide gel electrophoresis. The densitometric patterns of five samples from each age and brain region were analyzed for changes in the relative amount of stained protein bands. Of the polyacrylamide gel-separated protein bands, 16 regional differences at specific ages and 20 developmental changes in specific brain regions were found to be statistically significant. The hippocampal region showed the greatest number of statistical changes in the protein pattern as a function of age, whereas the cerebellum showed none. The results suggest the importance of considering regional differences in studies of biochemical developmental changes.

Development of potentiation in the dentate gyrus of rat: physiology and anatomy

The physiological development of potentiating processes in the rat dentate gyrus were compared to morphological development. Rapid Golgi techniques were coupled with in vitro studies of dentate granule cell frequency potentiation, post-tetanic potentiation and long-term potentiation. Frequency potentiation and long-term potentiation exhibited a developmental progression between 7 and 210 days postnatal. Posttetanic potentiation remained constant across this period. The relation of these findings to synaptogenesis and dendritic spine formation are discussed.

Development of habituation in the dentate gyrus of rat: physiology and anatomy

The physiological development of monosynaptic response habituation in the rat dentate gyrus was compared to morphological development. Rapid Golgi techniques were coupled with in vitro studies of dentate granule cell habituation to several frequencies and intensities of monosynaptic excitation. Except for the youngest group, the degree of habituation increased as a function of age, paralleling the morphological development.

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.

Synapse development within the spinal trigeminal nucleus

Pars interpolaris of the spinal trigeminal nucleus of kittens has been studied with the electron microscope at birth and at several subsequent ages during the first month of life. Attention has been given to ultrastructural maturational changes that occur in this neuropil, especially events in synaptogenesis. The results of this investigation include the following observations: (1) the neuropil, even at the earliest ages studied (three-hour-old kittens), is strikingly mature, necessitating a quantitative assessment in order to determine subtle developmental changes in synaptic patterns; (2) the number of axoaxonic contacts at birth are few, and their emergence is essentially a postnatal phenomenon; (3) it appears that the immature Gray type II or symmetrical synapse possesses distinct cleft material and dense, parallel membrane specializations. Synaptic vesicle accumulation at this contact appears to occur after the membrane specializations have formed. A previous study by Kerr26 has shown a reduced potential for primary afferent reorganization with the spinal trigeminal nucleus when kittens are subjected to trigeminal rhizotomy after three days of age. Our observations on the development of axoaxonic synaptic arrangements in the neonatal period may provide an explanation for these earlier results.

Comparison of electron microscopy and silver staining for the detection of the first entorhinal synapses to develop in the dentate gyrus

The development of the projection from the entorhinal cortex to the dentate gyrus (perforant path) has been studied with the electron microscope. The projection was lesioned in baby rats 5--13 days old and the dentate gyrus examined after 6--72 hr. Degenerating synapses first appeared in small numbers in the dentate neuropile at 7 days and in greater numbers in progressively older animals. There was a sixteen-fold increase in the number of synapses undergoing degeneration between 7 and 13 days. This investigation provides a calibration for the reduced silver method which has been used to trace developing axons (Singh, 1977). By this method the first signs of Wallerian degeneration, after cutting axons in the perforant path, were seen in the dentate neuropile at 9 days.

An autoradiographic study of the development of the entorhinal and commissural afferents to the dentate gyrus of the rat

The development of the entorhinal, ipsilateral associational, and commissural afferents to the dentate gyrus have been studied autoradiographically, following the injection of small amounts of tritiated proline into the medial and lateral parts of the entorhinal cortex, and into fields CA3c and CA4 of the hippocampus, in a series of rats, on the third, sixth, and twelfth postnatal days. Clear labeling of the entorhinal afferents were found at the third postnatal day, and from the earliest stage studied the afferents from the two parts of the entorhinal cortex appear to be spatially segregated within the stratum moleculare of the dentate gyrus: the fibers from the lateral entorhinal area occupying the outermost one-third, or so, of this stratum, while those from the medial entorhinal cortex occupy its middle zone. The ipsilateral hippocampo-dentate associational pathway is present at the third postnatal day, but the commissural projection (which shares with it the inner part of the stratum moleculare) could not be labeled until the sixth postnatal day. By the twelfth day the characteristic adult pattern of distribution of the terminals of the two hippocampo-dentate pathways is established. Although this pattern is best accounted for on the basis of a temporal competition for the available synaptic sites on the proximal parts of the dendrites of the granule cells, the spatial segregation of these two fiber systems from those arising in the entorhinal cortex, is probably due to the selective fasciculation of fibers in each group of afferents and to their early cytochemical specificity.