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

The normal organization of the lateral posterior nucleus of the golden hamster

As a first step in analyzing the influence of various afferent projections on the development of the hamster lateral posterior nucleus, its normal organization was studied using both light and electron microscopic techniques. Rostrolateral, rostromedial, and caudal subdivisions were identified. The rostrolateral subdivision receives dense projections from the ipsilateral superior colliculus and posterior neocortex, as well as sparser, more restricted projections from the contralateral colliculus and retina. The ipsilateral colliculus is by far the major source of medium-sized (M)terminals with round vesicles. These terminals synapse around the shafts of large central dendrites to form distinctive synaptic clusters. The contralateral colliculus and retina contribute a few M-terminals to the clusters. In contrast, axons from the posterior neocortex form very large (RL-)terminals with round vesicles from the posterior neocortex form very large (RL)terminals with round vesicles which synapse onto numerous appendages of single proximal dendrite, are surrounded by glial lamellae, and rarely participate in the clusters. Axons from all four sources also form small (RS)terminals with round vesicles which synapse on the shafts of small dendrites. Finally, F-terminals with flat or pleomorphic vesicles form symmetric synaptic contacts both within and outside the clusters. The only identified projection to the rostromedial subdivision is from the ipsilateral posterior neocortex, which contributes RL- and RS-terminals. F-terminals are also found, but neither M-terminals nor synaptic clusters are present. The caudal subdivision also receives RL- and RS-terminals from the ipsilateral posterior neocortex. Small inputs from the ipsilateral and contralateral colliculi are present, but their axons form only RS-terminals. No M-terminals or synaptic clusters are found. These results indicate that a large neonatal superior colliculus lesion would eliminate the vast majority of the M-terminals in the synaptic clusters of the ipsilateral lateral posterior nucleus. In subsequent studies (Crain and Hall, '80 a,b,c), we will examine how the remaining inputs from the retina, contralateral superior colliculus, and posterior neocortex contribute to the synaptic organization when it develops after such a lesion.

The organization of afferents to the lateral posterior nucleus in the golden hamster after different combinations of neonatal lesions

After a neonatal lesion of the ipsilateral superior colliculus, the projections to the lateral posterior nucleus from the contralateral superior colliculus and retina expand their terminal fields until they share a common border. In the first experiment described in this paper, we removed both superior colliculi at birth and used the Fink-Heimer method to show that the optic tract projection could expand even further and enter the region which would have been occupied by the terminals of the crossed colliculus projection. Similarly, in the second experiment, we showed that the crossed collicular projection could be increased even more if the contralateral eye as well as the ipsilateral colliculus was removed at birth. Another result of a neonatal superior colliculus lesion is that the projection from the optic tract shares a border with the posterior neocortical projection. In the third experiment, we removed both the ipsilateral superior colliculus and the posterior neocortex at birth and demonstrated that the optic tract projection expanded more than after an ipsilateral colliculus lesion alone. Our results support the hypotheses that the projections from the ipsilateral and contralateral superior colliculi and the retina compete for synaptic space in the lateral posterior nucleus, and that a similar competition between the retinal and cortical projections may also occur.

Spatial correlates of hippocampal unit activity are altered by lesions of the fornix and endorhinal cortex

Behavioral and electrophysiological evidence supports the role of the hippocampus in the processing of spatial information. In the present study, neuronal activity recorded from chronically implanted hippocampal microelectrodes was correlated with a rat's spatial orientation while traversing a radial maze for food reward. Place units were found in all fields of the dorsal hippocampus and dentate gyrus. Rotation of the maze relative to extramaze cues failed to disrupt the intact animal's spatial task performance of the spatial correlates of the unit activity. Lesions of the fornix or entorhinal cortex disrupted performance of the task. Unit activity correlated to the animal's spatial orientation was also disrupted by either lesion. There was no correlation between the disruption of the unit activity and location of the unit within hippocampal fields. Unit activity from lesioned animals showed correlation to the physical properties of the maze rather than to the orientation of the maze in space. These results further support the role of the hippocampus in the processing of spatial information.

Sexual dimorphism in extent of axonal sprouting in rat hippocampus

Sympathetic axons, normally innervating the extracerebral vasculature, sprout into denervated regions of the hippocampal formation after lesions of the medial septal nucleus or fimbria in adult female rats. Similar lesions in adult males also elicit the sympathetic ingrowth; however, the number of anomalous axons is greatly reduced and their distribution is altered. In adult males the sympathetic axons do not send out collaterals within the stratum oriens of region CA3 or the molecular layer or deep hilar regions of the area dentata, as they do in adult females. Lesions in juveniles of both sexes result in more vigorous sprouting than in their adult counterparts. In the young males the anomalous axons are distributed more extensively into the dentate molecular layer; in the young females the axons merely send out more collaterals within the same regions as in the adults. This sexually dimorphic response to central nervous system damage suggests either that the sprouting is affected by the hormonal environment of the mature hippocampal system or that this brain region, like the hypothalamus, may express permanent morphological or physiological differences as a result of exposure to sex steroids during development.

Functional effects of lesion-induced plasticity: long term potentiation in formal and lesion-induced temporodentate connections

The crossed temporodentate pathway from the entorhinal cortex of one hemisphere which proliferates in response to a contralateral entorhinal lesion in adult rats was analyzed for its ability to exhibit long term potentiation of synaptic efficacy similar to that which occurs in the normal ipsilateral temporodentate pathway. It was found that while the small synaptic response evoked by contralateral entorhinal cortical stimulation in normal rats does not undergo long term potentiation, after unilateral entorhinal lesions and proliferation of the crossed temporodentate pathway, the crossed pathway acquires a capacity for potentiation of synaptic action which qualitatively resembles that of the normal ipsilateral temporodentate circuit. However, despite the potentiation of synaptic drive, no long term enhancement of cell discharge was observed in the re-innervated dentate gyrus even through potentiation of this parameter was very prominent in the ipsilateral pathway. Mechanisms are discussed by which a previously non-potentiating pathway may acquire, as a consequence of lesion-induced sprouting, an ability to undergo long term potentiation of synaptic efficacy in a fasion similar to the ablated pathway. Reasons for the failure to observe potentiation of cell firing are also considered.

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.

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.

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.

Impulse blockade in frog cardiac ganglion does not resemble partial denervation in changing synaptic organization

Partial denervation of parasympathetic neurons in the frog heart by surgical section of one vagus nerve results in a marked reorganization of functional synaptic connections made by the remaining vagus nerve. These changes are not simply due to a lack of impulse activity per se in the sectioned nerve because blockage of impulses in one vagus with tetrodotoxin-impregnated cuffs did not cause similar changes in the innervation pattern of the ganglion. Furthermore, tetrodotoxin-blocked vagal fibers retain their ability to sprout and can form new synapses on denervated neurons.

Terminal arbors of axons that have formed abnormal connections

Ther terminal arbors of individual retinogeniculate axons that have been induced to grow into an inappropriate geniculate layer have been revealed for light and electron microscopic study by being filled with horseradish peroxidase. After a unilateral ocular enucleation in kittens, single axons from the surviving eye show terminal arbors not only within their own geniculate layers but also in the denervated layers. The new, abnormal arbors arise from the terminal segments of arbors that lie within the nondenervated layer and make patterns of synaptic contacts that appear normal.

Abnormal recrossing retinotectal projections after early lesions in Syrian hamsters: age-related effects

If the superficial layers of the right superior colliculus (SC) of a newborn hamster are removed, fibers from the left eye not only terminate in the surviving deeper layers of the right SC, but also cross the tectal midline and terminate in the medial third of the left SC. If the right eye is also removed at birth, the abnormally recrossing fibers from the left eye will spread over the entire surface of the left SC31. In this series of experiments, we ablated the right SC of hamsters at birth, but enucleated the right eye on different days postnatally in order to examine the spreading of the axon terminal pattern as a function of age. When the animals were young adults, the pattern of retinofugal projection of the left eye was traced using the Fink-Heimer technique or with autoradiography. It was found that the projections from the left eye continue to spread over the entire left SC when the right eye was removed up until day 10, though their distribution was more sparse when the eye was removed on days 7-10 than when it was removed on days 0-6. When the removal of the right eye was delayed until day 12, the lateral spreading of recrossing axons was markedly reduced. When the right eye was removed on day 14, the distribution of the projections from the left eye was restricted to the medial third of the left SC, just as if no early eye removal had been performed. It appears that after a critical age is reached, even when terminal space is available the axons and axon terminals will not move, at least not over any appreciable distance.

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.

Histochemical evidence for a post-lesion reorganization of cholinergic afferents in the hippocampal formation of the mature cat

We have utilized acetylcholinesterase (AChE) histochemistry to analyze possible post-lesion changes in the distribution of AChE containing afferents to the hippocampal formation of the cat following unilateral destruction of the entorhinal cortex. In the cat, the entorhinal area gives rise to a massive projection to the ipsilateral fascia dentata, and to regio inferior and regio superior of the hippocampus proper. Sixty days following unilateral entorhinal lesions, histochemical preparations for AChE indicate a dramatic increase in the density of the reaction product in the zones normally occupied by entorhinal afferents in the fascia dentata and regio inferior of the hippocampus proper, whereas little if any increase in the density of the reaction product was observed in the entorhinal terminal zone in regio superior. In addition to these increases in the density of the AChE reaction product, there was also evidence for a widening of an AChE free zone in the inner stratum moleculare of the fascia dentata denervated by the lesion. The time course of these changes in the pattern of AChE staining was analyzed by sacrificing animals 7, 10, 13, 14, 16, 17,, 19, and 20 days following entorhinal cortical lesions. The increase in the density of the AChE reaction product in the denervated zones was not apparent at seven days post-lesion, while at ten days post-lesion, a slight increase in the density of the AChE reaction product could be observed. By 13 days post-lesion, the differences between the denervated and normally innervated (contralateral) hippocampal formation were prominent, and by 16 days post-lesion, the pattern of staining appeared comparable to that which was observed at longer post-lesion intervals. The present experiments indicate that following entorhinal cortical lesions in mature cats the final post-lesion pattern of altered AChE staining is quite comparable to that which is observed following similar lesions in rats. In the rat, such changes in AChE staining have been interpreted as a reflection of a proliferation of cholinergic septal afferents within the denervated zones. If this interpretation is correct, the present results suggest a similar proliferation of cholinergic afferents following entorhinal lesions in cats. The time course of this apparent proliferation is considerably slower in the cat then in the rat, however, since the earliest changes are observed at approximately five days post-lesion in the rat, and ten days post-lesion in the cat.

Expansion of the half retinal projection to the tectum in goldfish: an electrophysiological and anatomical study

The topographical retino-tectal projection of goldfish was electrophysiologically mapped at various intervals after surgical removal of the nasal half of the retina and pigment epithelium. The remaining projection was initially restricted to the appropriate rostral half of the tectum, even if the nerve was crushed and allowed to regenerate. But later, after 137 days or more, it showed a progressive expansion onto the foreign caudal half of the tectum. The magnification factor, the number of micrometers of tectum per degree in the visual field, doubled in the rostro-caudal but not in the medio-lateral direction. Analysis of the sequence of the expansion showed that a few fibers originally projecting nearest the denervated area were the first to spread over it. Then, progressively more fibers moved caudally until a nearly uniform representation of the half retina was established on the tectum. Radioautography also demonstrated that retinal fiber terminals had invaded the caudal tectum. The retinae of these fish were also examined histologically. The density of ganglion cells had not increased, but they consistently showed the axonal reaction. This was not found to be associated with any initial surgical trauma, but rather with the movement of their fiber terminals within the tectum. Frozen sections through half retinal and normal eyes, were cut and photographed for comparison of ocular geometry. Operated eyes were normal except for a slight but consistent loss of ocular volume. Analysis of the optical geometry showed that recording with fish in air produced two effects: Myopia (10 degrees blur circle, or less) and enlargement of the visual field by 15 percent to 20 percent.

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.

Acetylcholinesterase staining in subdivisions of the cat's inferior olive

The present paper describes a unique distribution of true AChE activity in the IO. In the dorsal accessory olive three areas with high AChE activity can be distinguished. The medial accessory olive can be subdivided into a caudal part which shows rostro-caudally directed bands with different enzymatic activity, and a rostral part which shows a more uniform, medium activity. In the nucleus beta and the dorso-medial cell column, AChE activity is low. The ventral and dorsal lamellae of the principal olive contain areas with high, medium, and low activity. The dorsal cap is strongly positive, while the ventrolateral outgrowth is negative for AChE. Enzyme distribution cannot be fully explained on the base of the known afferent and efferent connections with the IO. However, histochemical results provide evidence that generally supports a subdivision of the IO that mirrors these connections (Brodal, '40; Armstrong et al., '74' Boesten and Voogd, '75; Groenewegen et al., '75).

Quantitative autoradiographic analysis of the time course of proliferation of contralateral entorhinal efferents in the dentate gyrus denervated by ipsilateral entorhinal lesions

The time course of the post-lesion proliferation of contralateral entorhinal afferents which occurs in response to ipsilateral entorhinal lesions was quanititatively analyzed with autoradiographic techniques. The extent of the crossed projection to the dentate granule cells was quantified on the basis of a contralateral/ipsilateral (C/I) ratio of grain density in the entorhinal terminal zones at 6, 8, 10, 12, 14, and over 60 days post-lesion. C/I ratios of grain density indicate little if any change in the crossed projection at 6 days post-lesion. Between 8 and 12 days post-lesion, the extent of the crossed projection increases dramatically, on the basis of the C/I ratio of grain density. C/I ratios do not increase further between 12 and 14 days post-lesion, but are higher at 60 days post-lesion. These results suggest that the crossed pathway proliferates extensively within the denervated zones between 8 and 12 days post-lesion, and may continue to proliferate at a much slower rate after 12 days post-lesion.

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.

Behavioral correlates of denervation and reinnervation of the hippocampal formation of the rat: recovery of alternation performance following unilateral entorhinal cortex lesions

Following unilateral lesions of the entorhinal cortex (E.C.) of the rat, cells in the dentate gyrus which have been deprived of their normal ipsilateral input are reinnervated in part by axons from the contralateral E.C. The proliferation of this crossed projection occurs largely between 8 and 12 days postlesion. The present experiments analyze changes in alternation behavior which occur during this period of afferent proliferation. Rats were trained to alternate responses (L-R) in a T-maze for food reward. Bilateral E.C. lesions resulted in a persistent deficit in alternation performance which did not recover after over 50 days of postoperative testing. Unilateral E.C. lesions, however, resulted in a transient deficit in alternation which recovered over time to preoperative levels. For example, animals permitted a 10-day recovery before the initiation of postlesion testing exhibited no more of a performance deficit than following a 10-day no-training period alone. However, animals permitted only a 3-day postoperative recovery were impaired in the alternation task until 10-12 days postlesion, despite daily training. Thus, recovery of performance following unilateral lesions was dependent on postlesion time rather than the amount of testing/retraining. Since bilateral lesions resulted in a persistent performance deficit while unilateral lesions resulted in a deficit with recovery, we hypothesize that behavioral recovery might be related to the reinnervation of the dentate gyrus by the contralateral E.C. To test this hypothesis, secondary lesions were placed in operated-recovered animals. Secondary lesions of the surviving E.C. resulted in a deficit in alternation performance similar to that following one stage bilateral lesions. In addition, secondary lesions of the dorsal psalterium (the fiber tract which carries the corssed E.C.-dentate projections) also disrupted performance in operated-recovered animals. Primary lesions of the dorsal psalterium alone had only slight and transient effects on alternation performance, however. Thus, the time course of the recovery, the results following bilateral lesions, and the results of secondary lesions are all consistent with the hypothesis that recovery of alternation performance following unilateral E.C. lesions may depend upon the reinnervation of the dentate gyrus by the contralateral E.C.

Behavioral correlates of denervation and reinnervation of the hippocampal formation of the rat: open field activity and cue utilization following bilateral entorhinal cortex lesions

Bilateral lesions of the entorhinal cortex (E.C.) of the rat result in persistent deficits in both spontaneous and reinforced alternation. The present study analyzes the nature of this impairment. To determine if changes in exploratory activity accompanied the deficits in alternation, open field activity was measured daily from 2-22 days following bilateral E.C. lesions. Such lesions resulted in a pronounced transient increase in open field activity which peaked between 5 and 7 days postlesion, but subsequently decreased to near preoperative levels at approximately 11 days postlesion. Alternation performance was also analyzed, to determine which cues are utilized to make the alternation, and whether cue utilization is affected by bilateral E.C. lesions. Utilizing a plus (+) maze, animals readily learned to alternate goal arms, but even with extensive training, failed to learn to alternate turns (left and right). However, the ability to identify the two goal arms in a nonalternation situation (which does not require short term recall of the preceding trial) was not permanently impaired by bilateral E.C. lesions. Since bilateral E.C. lesions do not result in persistent deficits in the ability to identify the two goal arms, but do disrupt alternation performance, we hypothesize that the deficit in alternation might reflect an inability to recall which arm was chosen on preceding trials. The implications of these results for an understanding of the behavioral consequences of postlesion reorganization of neuronal circuitry are discussed.

Reinnervation of dentate gyrus by homologous afferents following entorhinal cortical lesions in adult rats

Granule cells of the rat dentate gyrus which are denervated by unilateral destruction of the entorhinal cortex are reinnervated in part by proliferation of surviving pathways from the contralateral entorhinal cortex. The cells of origin of these lesion-induced projections were identified by retrograde labeling with horseradish peroxidase and were the same cell type which normally project to the ipsilateral dentate gyrus

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

Intact synapses in the denervated area of the rat dentate gyrus are reduced to 14% of those normally present 2-4 days following a unilateral entorhinal lesion. By 160-240 days after lesion, the former entorhinal terminal zone is repopulated with new synapses. In all, there is more than a 5-fold increase in the density of intact synapses in the denervated zone between 2 and 240 days post-lesion, and the denervated zone of the molecular layer is restored to 80% of control values. The synapses are Gray type I and are formed on simple and complex spines which closely resemble those normally present. A few boutons have an abnormally large number of synaptic junctions. Reinnervation seems to progress at differential rates. Synapses are rapidly regained up to 30 days after operation, but thereafter the reacquisition of synaptic connections is much slower. Reinnervation is more rapid in the portion of the denervated zone nearest the granule cells, where the maximal densities are attained within 30 days. The time course of reinnervation differed from that of degeneration. A portion of the new synapses in the reinnervated molecular layer appear to arise by the assembly of new synaptic junctions. Over time, the number of post-synaptic contact sites along a given length of dendritic surface recovers, suggesting the formation of new synaptic sites. Our data indicate that granule cells retain a capacity even into adulthood to manufacture, position and assemble postsynaptic components of a synapse and, in concert with reactive afferents, form normal-appearing synapses.

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

Synapses in the rat dentate gyrus are rapidly lost after removal of the primary input from the entorhinal cortex. In this paper we describe the extent and time course of degeneration and in the subsequent paper the nature of the reinnervation processes. They synapses of entorhinal afferents are remarkably concentrated in their zone of termination. Unilateral removal of the rat entorhinal cortex results in the loss of about 86% of all synapses in the outer three-fourths of the molecular layer of the epsilateral dentate gyrus. Entorhinal synapses are all asymmetric (Gray type I) and terminate on dendritic spines. Analysis of the degeneration reaction provides a means to examine the characteristics of the loss of a relatively homogeneous afferent on a single cell type. The morphological characteristics of the the degenerating terminals showed some heterogeneity; both the electron lucent and electron dense types of degenerating terminals were identified. The electron lucent type was observed only at short survival times. The time course of the loss of degenerating terminals was resolvable into two components, each of which followed first order decay kinetics. Thus degenerating entorhinal terminals behaved as a population which disappeared randomly at a rate dependent on the fraction of terminals present at any time. The loss of degenerating terminals was accompanied by the loss of postsynaptic sites. At short survival times the majority of postsynaptic sites (defined by the presence of a postsynaptic density) had disappeared. There was also a loss of complex spines and some shrinkage of the molecular layer.

A quantitative autoradiographic and electrophysiological study of the reinnervation of the dentate gyrus by the contralateral entorhinal cortex following ipsilateral entorhinal lesions

The post-lesion proliferation of contralateral enthorhinal afferents which occurs in response to ipsilateral entorhinal lesions was quantitatively analyzed with autoradiographic and electrophysiological techniques. In both cases, the extent of the crossed projection to the dentate granule cells was quantified on the basis of a contralateral/ipsilateral (C/I) ratio. Autoradiographic measures of grain density in the entorhinal terminal field indicates that the very sparse crossed entorhinal projection in intact animals proliferates approximately 6-fold following unilateral entorhinal lesions (on the basis of an increased C/I ratio of grain density in animals with long standing unilater entorhinal lesions). Furthermore, the total number of grains in the entorhinal terminal zone (obtained by subtracting background from non-terminal regions) also increases approximately 6-fold, indicating that compression of the neuropil cannot be the factor responsible for the increased grain density. These increases in the anatomical extent of the crossed projection as a consequence of unilateral entorhinal lesions are also reflected electrophysiologically. In operated animals, the C/I ratio of the extracellular population EPSP (a measure of the synaptic current generated by the crossed projections) also increase 5-8 fold. In addition, while in normal animals, no population spikes are observed following stimulation of the contralateral entorhinal area (indicating an absence of synchronous grnaule cell discharge in response to contralateral entorhinal input), such population spikes are quite prominent in the reinnervated dentate gyrus, indicating a large increase in the effective synaptic drive of the proliferated crossed projections.

The ventral lateral geniculate nucleus, pars lateralis of the rat. Synaptic organization and conditions for axonal sprouting

The synaptic organization of the pars lateralis portion of the ventral lateral geniculate nucleus is similar to that of other thalamic nuclei. There are four types of synaptic knobs (RL, RS, F1, F2). RL knobs are large and irregularly shaped, contain round synaptic vesicles and make multiple asymmetrical junctions. They are found primarily in "synaptic islands" making contact with gemmules, spines, small dendrites, and other synaptic profiles containing pleiomorphic synaptic vesicles (F2). Smaller RS knobs contain round vesicles and make asymmetrical junctions with the same type of elements as RL knobs, with the exception of the F2 profiles, but are seldom found in synaptic islands. F1 knobs contain flattened synaptic vesicles and form symmetrical junctions with F2 knobs, gemmules, spines, and small-medium dendrites in synaptic islands, throughout the neuropil, and on the proximal dendrites and soma of the largest type of neuron. F2 knobs are irregularly shaped, contain pleiomorphic synaptic vesicles and make symmetrical junctions primarily with gemmules and spines in synaptic islands. They are postsynaptic to RL and F1 knobs. Occipital decortication indicates that cortical terminals are of the RS type. Bilateral enucleation indicates that retinal terminals are of both the FL and RS type. The large amount of geographic overlap of retinal and cortical terminals on gemmules, spines, and small dendrites found in the neuropil outside of synaptic islands logically would maximize axonal sprouting between these two sources.