Sympathetic sprouting and recovery of a spatial behavior

After lesions of the medial septum, peripheral sympathetic fibers from the superior cervical ganglion appear in the hippocampal formation. To assess the functional significance of this neuronal rearrangement, we analyzed behavior on a spatial/memory task sensitive to hippocampal dysfunction, the radial eight-arm maze. The procedure allowed evaluation of both working and reference memory. All rats were able to master the task. Half of the rats then underwent either medial septal lesions and ganglionectomy or sham neurosurgery and ganglionectomy, and the other half underwent medial septal lesions or sham neurosurgery followed by ganglionectomy after further behavioral testing. Medial septal lesions in both groups disrupted taks performance with recovery of performance occurring with time. However, the rate of recovery was significantly enhanced in rats which had septal lesions and ganglionectomies simultaneously. Removal of the ganglion after recovery produced no effects on maze performance. Our results suggest that sympathetic ingrowth retards recovery processes.     

Long-term effects of nerve growth factor and neural transplants on behavior of rats with medial septal lesions

The present experiment investigated the interaction between exogenous nerve growth factor (NGF) and intrahippocampal septal grafts on the behavior of rats after a medial septum lesion. Young female rats received a bilateral injection of a fetal septal cell suspension into the dorsal hippocampus either immediately (immediate grafts) or 8 days after the lesion (delayed grafts). For delayed grafts, a higher concentration of endogenous neurotrophic factors can be assumed to be present in the deafferentated host tissue at the time of transplantation. One group of rats with lesions received NGF with the immediate grafts, another group received NGF alone. A sham-operated group and 3 groups with lesions (and given either immediate or delayed intrahippocampal saline injections, or no other treatment) constituted controls. The animals were tested for spontaneous alternation and for performance in a radial 8-arm maze, 1, 5 and 9 months postoperatively. Medial septal lesions reduced spontaneous alternation but, 9 months after surgery, recovery was observed in both lesion-control rats and in rats with delayed grafts (but not with immediate grafts). In the radial maze task, lesions produced a persistent impairment, although both immediate and delayed grafts reduced this deficit several months after surgery (more markedly and rapidly in the case of delayed grafts). NGF, however, increased the maze learning deficit especially 5 months postoperatively. These latter results are in contrast to findings of earlier studies showing transient beneficial effects of NGF administration. It is suggested that the effects of NGF in the present study might be due to an enhanced sprouting of sympathetic fibers into the hippocampal formation.     

Pineal and habenula innervation altered by septal lesions

Septal lesions result in a measurable increase in sympathetic innervation of the hippocampus. The cell bodies of origin of these sympathetic fibers reside in the superior cervical ganglion (SCG), which also projects to the pineal gland and medial habenula. If the SCG neurons which project to the pineal are the same neurons which sprout into the hippocampus after septal lesions, a common cell body reaction might mediate changes in innervation of the pineal. If cholinergic denervation is the principal trigger of hippocampal sprouting, a similar response might be observed in habenula which is also innervated cholinergically from septal area neurons. Pineal and medial habenula innervation were quantified using a new microfluorometric method at 20 post-lesion intervals following septal lesions. Changes in intensity and density of innervation were correlated with time in days after the lesions. Pineal intensity, pineal density, and habenula intensity exhibited statistically significant correlations with time in days after septal lesions. These changes occurred during the growth of sympathohippocampal fibers. The data suggest that a common cell body reaction mediated the changes in pineal and the sympathetic sprouting in the hippocampus.     

Evidence for sprouting specificity following medial septal lesions in the rat

Damage to the rat septohippocampal pathway results in the growth of sympathetic axons from nearby blood vessels into the denervated hippocampal formation. Sympathohippocampal sprouting exhibits lesion specificity--that is, only injury to the septohippocampal projection elicits the sprouting response. Whether other perivascular fibers sprout in response to septohippocampal injury (response specificity) has been addressed in the present study. Using cathecholamine histofluorescence and acetylcholinesterase histochemical techniques, we determined the distribution and incidence of perivascular sympathetic and nonsympathetic fibers associated with parahippocampal blood vessels in normal rats and in rats sustaining medial septal lesions. We found that sympathetic fibers are more numerous than acetylcholinesterase-positive fibers at all septotemporal levels of the hippocampal formation and that both types are very rare at dorsal hippocampal levels in normal rats. Following medial septal lesions, however, there is a tremendous increase in the number of perivascular sympathetic fibers at dorsal hippocampal levels but no change in the number of acetylcholinesterase-positive fibers. Electron microscopic observations indicate that the increase in perivascular fibers is due to increases in the number of sympathetic axonal fascicles as well as the number of axons per fascicle. Furthermore, both light and electron microscopic data suggest that parahippocampal veins are normally not accompanied by perivascular fibers but are associated with sympathetic fibers following medial septal lesions. These results indicate that sympathetic sprouting in response to septohippocampal denervation exhibits specificity not only in terms of the lesion which elicits such sprouting but also in terms of the types of fibers that respond to the lesion.     

Functional Consequences of a Single Nerve Growth Factor Administration Following Septal Damage in Rats

This study examined how possible nerve growth factor (NGF)-induced behaviour changes after septal damage might be modulated by the lesion extent, the dose of NGF administered and the delay between surgery and the onset of testing. In a first experiment, young rats which received electrolytic septal lesions of high or low intensity (inducing respectively large and mild lesions) were treated with 10 or 30 μg NGF administered intrahippocampally in a single injection. They were tested 4 months postoperatively for open field ambulation, spontaneous alternation and radial maze performance. It was observed that irrespective of the severity of the lesions rats were impaired in the spontaneous alternation and radial maze tests; however, no obvious changes appeared in the open field test. While an NGF injection did not affect behavioural performances in rats with large lesions, it was capable of ameliorating behavioural deficits in the spontaneous alternation and radial maze tests of rats with mild lesions in both NGF dosage groups. It was also seen that lesions produced a general decrease in hippocampal choline acetyltransferase (ChAT) activity, which was not significantly affected by an NGF administration. There was no significant correlation between ChAT activity and behavioural performance of NGF-treated rats. In a second experiment, young rats received mild septal lesions and were treated with 10 μg NGF. These rats were tested 2 weeks postoperatively for radial maze performance. NGF rats exhibited similar behaviour to controls with regard to all of the variables measured. The present results suggest that a single NGF administration spares some abilities to use spatial information efficiently providing lesions are partial.     

Nerve growth factor and septal grafts: a study of behavioral recovery following partial damage to the septum in rats.

Previous studies have produced conflicting results about the effects of intracerebral injection of NGF after septal damage in rats: in one experiment, behavioral deficits in maze tasks were exacerbated by NGF administration whereas they were alleviated in another one. The present investigation aimed to clarify the effects of NGF and to identify factors liable to induce different behavioral outcomes. Behavioral effects were assessed following a postsurgical delay of five months using various parameters: food consumption in a novel environment, spontaneous activity, locomotion in an open-field, immobility in a tail suspension test, spontaneous alternation in a T-maze and performance in a radial eight-arm maze. Possible influence of intrahippocampal sympathetic fiber ingrowth occurring after septal lesions was ruled out, as the comparison of rats subjected to superior cervical ganglia removal with their lesion-control counterparts showed few behavioral differences, even after NGF administration. All lesioned rats showed reduced adaptability in most of these tests. Grafts partially reversed the lesion-induced deficit in spontaneous alternation. A single intracerebral NGF injection was found to ameliorate radial maze performance, whether rats were grafted or not. However, it appeared that the number of strategies available to NGF-rats in the radial maze task was as limited as for lesion-control rats. These findings suggest that NGF-rats do not recover spatial abilities lost after septal lesions, but are able to make more efficient use of remaining capacities to master the maze task.     

Sympathetic noradrenergic sprouting in response to central cholinergic denervation: A histochemical study of neuronal sprouting in the rat hippocampal formation

An unusual example of neuronal sprouting occurs in the rat brain. Several weeks after fimbrial transection or septal lesions, peripheral sympathetic fibers appear in the dentate and hippocampal gyri. We compared the distribution of normal cholinergic septohippocampal fibers and nerve terminals with the distribution of noradrenergic sympathetic (sympathohippocampal) fibers after septal lesions using anterograde transport of horseradish peroxidase and fluorescence histochemistry. In addition, we destroyed other afferents to the hippocampal formation and examined the effect of subtotal septal lesions on acetylcholinesterase staining and the distribution of sympathohippocampal fibers. The combined results of these experiments suggest that peripheral noradrenergic fibers sprout specifically in response to destruction of central cholinergic fibers after septal lesions. This appears to be the first model of neuronal sprouting in the central nervous system where one identified transmitter system (noradrenergic) sprouts only in response to, and perhaps to replace, another specific transmitter system (cholinergic).     

Sympathetic sprouting reverses decreases in membrane-associated activity of protein kinase C following septohippocampal denervation of the rat hippocampus

Hippocampal sympathetic ingrowth (HSI), a form of neuronal plasticity, is induced by medial septal lesions and consists of the sprouting of peripheral sympathetic fibers, arising from the superior cervical ganglion, into the dentate gyrus and CA₃ region of the hippocampus. HSI has been previously shown to alter learned and spontaneous behaviors, phosphatidyl inositide hydrolysis, and the antagonist binding kinetics of both muscarinic cholinergic receptors and phorbol ester receptors. We now report that sympathetic sprouting reverses decreases in membrane-associated activity of protein kinase C (PKC) following septohippocampal denervation of the rat hippocampus. Further, no changes were found in α, β or γ PKC isoenzymes among experimental groups, suggesting that the group A PKC isoforms do not mediate the observed changes in activity and phorbol ester binding.     

Target regulation of sympathetic sprouting in the rat hippocampal formation

Sympathetic fibers appear in the rat hippocampal formation after lesions of the medial septal nucleus. The distribution of sympathetic axons correlates most closely with the distribution of dentate granule cells and their axons. To test the hypothesis that granule cells are the targets of sympathetic fibers, neurotoxins were injected to destroy either the granule cells (colchicine) or the pyramidal cells (kainic acid) in the hippocampal formation 1 month prior to placing septal lesions. Sympathetic fibers appeared in animals with kainic acid lesions but not in regions lacking granule cells in animals treated with colchicine. These results support the hypothesis that the granule cells of the dentate gyrus are the targets of sprouting sympathetic fibers.     

Anterior thalamic lesions reduce spine density in both hippocampal CA1 and retrosplenial cortex,but enrichment rescues CA1 spines only

Injury to the anterior thalamic nuclei (ATN) may affect both hippocampus and retrosplenial cortex thus explaining some parallels between diencephalic and medial temporal lobe amnesias. We found that standard‐housed rats with ATN lesions, compared with standard‐housed controls, showed reduced spine density in hippocampal CA1 neurons (basal dendrites, ?11.2%; apical dendrites, ?9.6%) and in retrospenial granular b cortex (Rgb) neurons (apical dendrites, ?20.1%) together with spatial memory deficits on cross maze and radial‐arm maze tasks. Additional rats with ATN lesions were also shown to display a severe deficit on spatial working memory in the cross‐maze, but subsequent enriched housing ameliorated their performance on both this task and the radial‐arm maze. These enriched rats with ATN lesions also showed recovery of both basal and apical CA1 spine density to levels comparable to that of the standard‐housed controls, but no recovery of Rgb spine density. Inspection of spine types in the CA1 neurons showed that ATN lesions reduced the density of thin spines and mushroom spines, but not stubby spines; while enrichment promoted recovery of thin spines. Comparison with enriched rats that received pseudo‐training, which provided comparable task‐related experience, but no explicit spatial memory training, suggested that basal CA1 spine density in particular was associated with spatial learning and memory performance. Distal pathology in terms of reduced integrity of hippocampal and retrosplenial microstructure provides clear support for the influence of the ATN lesions on the extended hippocampal system. The reversal by postoperative enrichment of this deficit in the hippocampus but not the retrosplenial cortex may indicate region‐specific mechanisms of recovery after ATN injury. © 2014 Wiley Periodicals, Inc.     

Sympathohippocampal sprouting is directed by a target tropic factor

In order to determine what aspect of septohippocampal denervation elicits sympathetic sprouting, entorhinal lesions were made to first elicit intensification of acetylcholinesterase (AChE) staining in the dentate molecular layer. Transport of horseradish peroxidase (HRP) from the septum confirmed that the dense band of AChE staining reflects the presence of septohippocampal fibers. Animals receiving entorhinal lesions, followed 30 days later by septal lesions, demonstrated the same pattern of sympathetic sprouting as seen in animals without entorhinal lesions. These results suggest that sympathohippocampal sprouting is directed by a tropic factor released by the cholinergic-denervated hippocampal formation.     

Hormonal regulation of axonal sprouting in the hippocampus

Removal of septal fibers to the rat hippocampal formation is followed by an ingrowth of sympathetic axons into the deafferented regions. We have shown previously that the pattern of sprouting is more restricted in males than in females when the lesions are made in mature animals, but is almost the same if lesions are made during the early postnatal period. In the present study, the relationship of circulating sex hormones to the sprouting response was investigated by comparing the extent of sympathetic axon in-growth following fimbrial lesions in intact or gonadectomized adult male and female rats. The effects of manipulating sex steroids during development was examined by comparing sympathetic axonal sprouting after fimbrial lesions in rats which were castrated (male) or treated with testosterone (females and castrated males) on postnatal day 2.We find that (1) gonadectomy of either adult female or male rats does not affect the sprouting response, but (2) neonatal castration of male rats permits sprouting in a pattern similar to normal females, and neonatal testosterone treatment of females or castrated males results in the more limited sprouting response characteristic of normal males. These results indicate that the sex-related differential response to fimbrial lesions may be determined by developmental differences in endogenous steroid levels.     

Cholinergic axons from delayed septal implants and sympathetic fibers co-exist in the denervated dentate gyrus

Adult female rats received a unilateral fornix/fimbria lesion 6 weeks prior to obtaining an implant of embryonic septal tissue. The ingrowth of Cholinergic axons from the delayed implants into the denervated dentate gyrus was visualized by acetylcholinesterase (AChE) histochemistry. Since sprouting of anomalous sympathetic fibers also occurs following fornix/fimbria transection, the organization of these fibers was identified with the monoamine histofluorescence technique. Lesion control specimens with fornix/fimbria lesions demonstrate that sympathetic fibers are present along the septo-temporal axis of the dentate gyrus by 6 weeks postlesion. In specimens with delayed septal implants there is ingrowth of AChE fibers along the septo-temporal axis of the dentate gyrus with the densest distribution of fibers located at the septal pole of the dentate. The sympathetic fibers which are present in the dentate prior to the implantation of the septal tissue still persist in regions which contain a moderate density of AChE fibers but appear absent or diminished in regions with a dense Cholinergic ingrowth. The data suggest that the postsynaptic signals necessary for the selective reinnervation of the dentate gyrus by the septal Cholinergic axons are not abolished by synaptic reorganization in the neuropil which occurs prior to the implantation of the septal tissue. Moreover, there may be some competition between the Cholinergic and sympathetic fibers for this postsynaptic signal and for space within the dentate neuropil.     

Ascending efferent projections of the gustatory parabrachial nuclei in the rabbit

Peripheral noradrenergic fibers from the superior cervical ganglion (SCG) appear in the hippocampal formation of the adult rat central nervous system following damage to the medial septal nucleus or its afferent fibers. The appearance of these fibers coincides with a significant and substantial increase in the concentration of norepinephrine in the dentate gyrus and in the ability of synaptosomes made from the dentate to take up [3H]norepinephrine (NE) in vitro. Up to 7 weeks following a medial septal lesion, dentate norepinephrine levels are significantly lower if the sympathetic preganglionic trunk has been sectioned at the time septal lesions are made. By contrast, the uptake of [3H]NE into dentate synaptosomes is not affected by the preganglionic section. Furthermore, if the sectioned preganglionic trunk is allowed to reinnervate the SCG, the dentate NE concentration rapidly returns to levels equivalent to dentates with intact sympathetic preganglionic trunks. In addition to the ingrowth of the sympathetic fibers, central noradrenergic fibers display a sprouting or 'pruning' response to medial septal lesions. These data show that afferent input regulates the neurotransmitter concentration during sympathetic ingrowth while retrograde influences from the target appear to regulate the density or extent of the growing fibers. The experimental findings are discussed in light of the hypothesis that the hippocampal formation is capable of producing a sympathetic growth factor following septal denervation.     

Food restriction attenuates ischemia-induced spatial learning and memory deficits despite extensive CA1 ischemic injury

The purpose of the present study was to examine whether short-term food restriction (40% less food over a 3-month period) can attenuate ischemia-induced CA1 neuronal degeneration, and whether this attenuation translated into improved recovery of functional impairments following global ischemia. There was a significant loss of pyramidal CA1 neurons in ischemic compared to sham-operated rats but no difference between the ad lib and food-restricted ischemic animals. Although the diet did not influence neuronal damage in ischemic animals, the performance of food-restricted ischemic rats in spatial task such as the radial arm maze was significantly better than that of ad lib fed ischemic rats. Food-restricted ischemic rats made equivalent numbers of working memory errors as sham-operated animals and took the same time to complete a standard 8-arm radial arm maze task. They also displayed higher activity level in the open field compared to ad libitum fed ischemic rats, and spent considerably more time in the open arms of the elevated plus maze compared to the other groups, suggesting decreased anxiety in these ischemic rats. The relative sparing of spatial memory performance in food-restricted ischemic animals suggests that food restriction facilitates functional recovery.     

Failure of chronic physostigmine to ameliorate working memory deficits after medial septal lesions

To assess the potential usefulness of chronic acetylcholinesterase inhibition in the treatment of learning/memory disorders arising from central cholinergic deficient states, physostigmine was administered chronically to rats with medial septal lesions and the retention of a spatial/working memory task investigated. Three dose levels of physostigmine (0.025, 0.05, 0.075 mg/kg) were administered three times per day following medial septal lesions. Retention of a standard radial 8-arm maze task was assessed. Although the lesions transiently disrupted task performance, physostigmine therapy did not improve either daily performance or total recovery time. Our results suggest that chronic acetylcholinesterase inhibition is not effective in ameliorating the working memory deficits that occur after medial septal lesions.     

Behavioural, histological and immunocytochemical consequences following 192 IgG-saporin immunolesions of the basal forebrain cholinergic system

Use of the selective immunotoxin; 192 IgG-saporin, is helping to elucidate the role of the cholinergic system in cognition by overcoming the problems of interpretation associated with the use of non-specific lesioning agents. In separate studies, we have compared the long- and short-term effects of single site and combined saporin lesions of the nucleus basalis magnocellularis and medial septal area, on spatial learning and memory in radial arm and water maze tasks. At 11 months, only rats with combined lesions showed deficits in both radial and water maze tasks, although terminal cholinergic deafferentation was substantial and extensive tissue loss was seen at the injection sites in both single and combined lesions. However, the extensive tissue loss with long-term lesions suggested that behavioural deficits were not solely attributable to cholinergic deafferentation. In contrast, when rats with combined lesions were tested 5 months after lesioning, no deficits were apparent, although there was almost complete loss of choline acetyltransferase- and nerve growth factor receptor-immunoreactivity in the basal forebrain with no tissue damage at the injection sites. This study supports existing literature that selective loss of cholinergic neurons in the basal forebrain does not produce behavioural impairments in standard tasks of learning and memory, but deficits are apparent when damage is non-selective as occurs late after lesioning, confounding interpretation of behavioural data. It further highlights potential problems with this immunotoxin in long-term studies.     

Effect of topiramate on cognitive function and activity level following neonatal seizures

Topiramate, an antiepileptic drug with a number of mechanisms of action including blockade of AMPA/KA receptor subtypes, was assessed as a neuroprotective agent following seizures. We administered topiramate or saline chronically during and following a series of 25 neonatal seizures. After completion of the topiramate treatment, animals were tested in the water maze for spatial learning and the open field for activity level. Brains were then examined for cell loss and sprouting of mossy fibers. Rats treated with topiramate performed significantly better in the water maze than rats treated with saline. Topiramate treatment also reduced the amount of seizure-induced sprouting in the supragranular region. There were no differences between topiramate- and saline-treated rats in activity level in the open field, swimming speed, or weight gain. These findings show that long-term treatment with topiramate after neonatal seizures changes the long-term consequences of seizures and improves cognitive function.     

Transplantation of neurospheres after granule cell lesions in rats: cognitive improvements despite no long-term immunodetection of grafted cells

EGF-responsive C17 murine-derived neural stem cells (neurospheres) were grafted into the dentate gyrus of adult male rats after dentate granule cells lesions produced by colchicine injections. Behavioural performance was evaluated over two post-grafting periods, using tests sensitive to hippocampal dysfunctions. The first period began 1 month after grafting and testing conducted in the water maze and the radial maze distinguished working- and reference-memory performance. The second period began 9 months after grafting and learning performance was also evaluated in a Hebb-Williams maze, in addition to both other tests. The lesions induced lasting deficits in all tests. During the first period, the grafts had no effect in either test. Conversely, during the second period, grafted rats showed a weak improvement in the water maze and a significant increase of reference memory performance in the radial maze. In the Hebb-Williams maze, performance of grafted rats was close to normal.Strengthening the idea that dentate gyrus granule cells play an important role in the acquisition of new (perhaps more configural than only spatial) information, our results, moreover, suggest that neurosphere grafts may foster recovery after damage to point-to-point connection systems in the adult brain.     

Excitotoxic lesions of the rostral thalamic reticular nucleus do not affect the performance of spatial learning and memory tasks in the rat

Rats with cytotoxic lesions of the rostral pole of the thalamic reticular nucleus were compared with surgical control animals on a series of spatial learning and memory tests. While evidence was found for an initial, transient impairment on forced-choice alternation in a T-maze, this rapidly disappeared, and overall performance was unaffected. Subsequent experiments found no evidence that lesions of the rostral reticular nucleus affected the acquisition or performance of tests in the radial arm maze and the Morris water maze. Thus, it appears that the rostral pole of the thalamic reticular nucleus often does not play a necessary role in the performance of tests of spatial learning and memory, in spite of its interconnections with other regions that are required for normal spatial memory.