Neural plasticity and recovery of function after brain injury

Botulinum Neurotoxin-A Injected Intrastriatally into Hemiparkinsonian Rats Improves the Initiation Time for Left and Right Forelimbs in Both Forehand and Backhand Directions

Forelimb stepping is a widely used test for the assessment of forelimb akinesia in hemiparkinsonian (hemi-PD) rats. The initiation time (IT) is considered the most sensitive parameter in the stepping test procedure. Here we propose a novel, reliable, and simple method for the measurement of IT of both forelimbs in both forehand and backhand directions in rats. Evaluating the same videos taken for quantifying adjusting steps, IT measurements were done without additional experiments. This is in contrast to the classical approach introduced by Olsson et al. (1995), in which separate experiments are necessary. We successfully applied our approach to hemi-PD rats intrastriatally treated with botulinum neurotoxin-A (BoNT-A). In naïve rats, an IT of about 0.62 s was found, and in right-sided hemi-PD rats the IT of the left forepaw increased to about 3.62 s. These hemi-PD rats showed, however, reduced ITs of the impaired left forepaws 1 month and the second time 7 months after induction of hemi-PD via the injection of 1 ng BoNT-A into the ipsilateral striatum, depending on post BoNT-A survival time. The method described offers the possibility of a precise and animal-friendly evaluation of IT in rats, including the beneficial effect of BoNT-A treatment in hemi-PD rats.

Music and the brain: the impact of music on an oboist's fight for recovery

A 20-year-old female, alias Sara, was an aspiring professional oboist who studied music performance at college level. While wading across a river she lost her balance, was swept down river, and suffered profound injuries including severe hypothermia, cardiac arrest, and hypoxic brain injury. While recovering, her family and friends surrounded Sara with music. Her oboe teacher placed Sara's oboe in her hand and played tapes of Sara's past performances. Her mother played recordings of her favourite music in hopes that the music would remind her of her life's passion and, thus, stimulate her mind and soul while she recovered. Two years post-injury, Sara continues to strive to improve her quality of life. In this case study, Sara's musical and medical recovery is detailed using quantitative and qualitative data. This data gathered allows one to provide an analysis of the powerful role of music in Sara's fight to recover.

The Oct-2 POU domain gene in the neuroendocrine brain: a transcriptional regulator of mammalian puberty

POU homeodomain genes are transcriptional regulators that control development of the mammalian forebrain. Although they are mostly active during embryonic life, some of them remain expressed in the postnatal hypothalamus, suggesting their involvement in regulating differentiated functions of the neuroendocrine brain. We show here that Oct-2, a POU domain gene originally described in cells of the immune system, is one of the controlling components of the cell-cell signaling process underlying the hypothalamic regulation of female puberty. Lesions of the anterior hypothalamus cause sexual precocity and recapitulate some of the events leading to the normal initiation of puberty. Prominent among these events is an increased astrocytic expression of the gene encoding transforming growth factor-alpha (TGF alpha), a tropic polypeptide involved in the stimulatory control of LHRH secretion. The present study shows that such lesions result in the rapid and selective increase in Oct-2 transcripts in TGF alpha-containing astrocytes surrounding the lesion site. In both lesion-induced and normal puberty, there is a preferential increase in hypothalamic expression of the Oct-2a and Oct-2c alternatively spliced messenger RNA forms of the Oct-2 gene, with an increase in 2a messenger RNA levels preceding that in 2c and antedating the peripubertal activation of gonadal steroid secretion. Both Oct-2a and 2c trans-activate the TGF alpha gene via recognition motifs contained in the TGF alpha gene promoter. Inhibition of Oct-2 synthesis reduces TGF alpha expression in astroglial cells and delays the initiation of puberty. These results suggest that the Oct-2 gene is one of the upstream components of the glia to neuron signaling process that controls the onset of female puberty in mammals.

Expression of platelet-derived growth factor B-chain and beta-receptor in hypoxic/ischemic encephalopathy of neonatal rats

Expression of platelet-derived growth factor B-chain and of its specific receptor (beta-receptor) was investigated in immature brains with hypoxic/ischemic injury. After the left common carotid arteries of seven-day-old rats were ligated and pups were placed in a hypoxic chamber, the protein and messenger RNA of both B-chain and beta-receptor were assessed using immunocytochemistry and northern analysis, respectively. Transcripts for B-chain were localized by in situ hybridization. Faint but definite expression of B-chain and beta-receptor was seen in the brains of untreated neonatal controls. Three to 48 h after hypoxia B-chain protein was generally increased above control levels, but focally decreased expression was seen in infarcted areas. Enhanced induction of messenger RNA of B-chain was seen in the both sides of cerebral cortices and hippocampi at 3 h. Strongly increased positivity for B-chain protein and mRNA occurred in the neurons surrounding the infarct. In situ hybridization still showed this up-regulation seven days after hypoxia. Beta-receptor protein expression was enhanced in some neurons immediately surrounding the infarct at 3 h of hypoxia, and marked up-regulation was seen at 16 h. Beta-receptor messenger RNA remained at control levels. Immunocytochemistry showed strong immunoreactivity for the beta-receptor on the neurons surrounding the infarct at 72 h. These results indicate that a neonatal hypoxic/ischemic insult induces neuronal up-regulation of the platelet-derived growth factor B-chain as well as beta-receptor immediately after hypoxia. While this up-regulation is relatively transient in most neurons, sublethal damage to neurons immediately surrounding an infarct induces sustained up-regulation. Through autocrine and paracrine mechanisms, platelet-derived growth factor B-chain molecules may act as a neuroprotective factor in immature brain experiencing with hypoxic/ischemic injury.

Differential alterations in basal and D-amphetamine-induced behavioural pattern following 6-OHDA or ibotenic acid lesions into the dorsal striatum

It is well known that the corpus striatum is related to the sterotyped activation induced by several psychostimulants. In this study we analyzed the effects of 6-OHDA, in comparison with those of ibotenic acid lesions, into the dorsal striatum, on the behavioural pattern induced by saline or D-amphetamine treatment. A computerized technique for recording the animal motor activity was developed in order to define in a detailed way the behavioural profile in lesioned and sham-operated rats induced by the saline or D-amphetamine treatment. A 6-OHDA lesion into the dorsal striatum modified the basal behavioural pattern which was mainly characterized by reduced motor activation while ibotenic acid lesion affected the structure of the basal behavioral pattern. D-Amphetamine administration in 6-OHDA lesioned rats induced a behavioural stimulation, but a decreased motor and stereotyped activation was observed compared to the sham-operated animals treated with D-amphetamine. In contrast, D-amphetamine administration in the ibotenic acid-lesioned rats induced a motor and stereotyped activity which was not reduced compared to that seen after D-amphetamine treatment in sham-operated rats. These results suggest that these two types of lesion induced differential effects on the behavioural pattern either after saline or after D-amphetamine administration. Dopaminergic neurotransmission in the dorsal striatum plays a permissive role on the emergence of the behavioural responses, while the dorsal striatum circuitry plays a crucial role on the organization of the behavioural pattern. In addition, dopaminergic activity in this structure serves a primary control in the D-amphetamine-elicited motor activation or stereotypy, while the striatal structure is involved in the shaping of the D-amphetamine behavioural pattern.

Behavioural recovery after unilateral lesion of the dopaminergic mesotelencephalic pathway: effect of repeated testing

Functional recovery following a complete unilateral lesion of the nigrostriatal pathway in adult rats was studied. We examined the effect of training on the spontaneous or induced postural bias following the lesion. Two tasks measuring lateralization were used to assess the lesion-induced postural bias: spontaneous asymmetry was evaluated in the Y-maze, whereas induced body bias was measured by hanging the rat by its tail. Recovery was assessed at three different times following the lesion. The effects of lesion in adult rats in the short, medium and long term were evaluated and compared with the effects of dopaminergic transplants. In adult lesioned rats, destruction of dopaminergic innervation of the neostriatum induced initially an ipsilateral bias as measured in the "tail hang test" and the Y-maze. Recovery of function was observed in the tail hang test as ipsilateral bias declined on repeated testing. Apart from this effect, there was a post-lesion interval effect, since the postural bias disappeared more rapidly on repeated testing in the long-term lesioned rats. This spontaneous recovery was impaired by intrastriatal dopaminergic grafts. Furthermore, no spontaneous recovery was observed in the Y-maze test. These observations show that repeated testing can influence the long-term effects of damage to the nigrostriatal dopamine system.

Recovery of cognitive functions following nonprogressive brain injury

It has recently become clear that the adult human brain is capable of more plasticity than previously thought. Investigations into the natural history of change following brain injury demonstrate that partial recovery of function can and does occur. Furthermore, there is increasing evidence that intervention through re-training or provision of compensatory memory aids can result in improved cognitive functioning.

Cerebral diaschisis following cerebellar hemorrhage

In 1914, Von Monakow described diaschisis, the recovery of lost cortical function in regions positionally distant from, but linked by neuronal tracts to, the primary site of cortical damage. Cerebellar diaschisis after cortical insult is detailed in the literature; however, cortical diaschisis after cerebellar insult remains a rarely reported occurrence. We describe a 36-year-old woman with rupture of a right-sided cerebellar arteriovenous malformation who developed such expected cerebellar signs as ataxia, dysmetria, and nystagmus. Days later, the patient developed profound impulsivity, disinhibition, and psychomotor agitation. Single photon emission computed tomography (SPECT) showed decreased perfusion of the bilateral frontal and temporal lobes, consistent with regional loss of neural activity. Eventual clinical improvement corresponded with reperfusion of those regions, identified on follow-up SPECT. This case documents cortical diaschisis following cerebellar insult and shows that diaschisis must be considered in patients with cerebral injury manifesting cortical deficits remote from the site of primary pathology.

The fimbria-fornix/cingular bundle pathways: a review of neurochemical and behavioural approaches using lesions and transplantation techniques

Extensive lesions of the fimbria-fornix pathways and the cingular bundle deprive the hippocampus of a substantial part of its cholinergic, noradrenergic and serotonergic afferents and, among several other behavioural alterations, induce lasting impairment of spatial learning and memory capabilities. After a brief presentation of the neuroanatomical organization of the hippocampus and the connections relevant to the topic of this article, studies which have contributed to characterize the neurochemical and behavioural aspects of the fimbria-fornix lesion "syndrome" with lesion techniques differing by the extent, the location or the specificity of the damage produced, are reviewed. Furthermore, several compensatory changes that may occur as a reaction to hippocampal denervation (sprouting changes in receptor sensitivity and modifications of neurotransmitter turnover in spared fibres) are described and discussed in relation with their capacity (or incapacity) to foster recovery from the lesion-induced deficits. According to this background, experiments using intrahippocampal or "parahippocampal" grafts to substitute for missing cholinergic, noradrenergic or serotonergic afferents are considered according to whether the reported findings concern neurochemical and/or behavioural effects. Taken together, these experiments suggest that appropriately chosen fetal neurons (or other cells such as for instance, genetically-modified fibroblasts) implanted into or close to the denervated hippocampus may substitute, at least partially, for missing hippocampal afferents with a neurochemical specificity that closely depends on the neurochemical identity of the grafted neurons. Thereby, such grafts are able not only to restore some functions as they can be detected locally, namely within the hippocampus, but also to attenuate some of the behavioural (and other types of) disturbances resulting from the lesions. In some respects, also these graft-induced behavioural effects might be considered as occurring with a neurochemically-defined specificity. Nevertheless, if a graft-induced recovery of neurochemical markers in the hippocampus seems to be a prerequisite for also behavioural recovery to be observed, this neurochemical recovery is neither the one and only condition for behavioural effects to be expressed, nor is it the one and only mechanism to account for the latter effects.

Traumatic brain injury of the forelimb and hindlimb sensorimotor areas in the rat: physiological, histological and behavioral correlates

This study characterizes physiological, histological and behavioral effects of traumatic brain injury (TBI) produced by a controlled pneumatic impactor striking the entire right sensorimotor cortex of the anesthetized rat. Damage to both the fore- and hindlimb sensorimotor areas resulted in a hemiparetic animal which allowed us to use four sensitive behavioral/neurological tests to track the recovery sequelae after injury. Initial experiments measured cardiovascular and respiratory effects after cortical impact which depressed the dura to varying depths. Both 0.5 mm and 1 mm cortical depressions produced a momentary decrease (P < 0.05) in mean arterial blood pressure (MABP) while cortical impacts to depths of 2 mm or 3 mm produced a momentary increase (P < 0.05) in MABP. Normotension was re-established within 30 s after the initial response at all injury levels. Respiratory rate was affected only following 3 mm cortical depressions. A 1 mm cortical depression appeared ideal in terms of minimal cardiorespiratory effects, low mortality and lasting behavioral effects. For behavioral and histologic studies, therefore, additional rats were injured by a 1 mm cortical impact and tested for 8 weeks after TBI using four behavioral tests. Injured rats displayed both fore- and hindlimb deficits up to 56 days while traversing a narrow beam (P < 0.001) and up to 28 days when crossing a pegged beam (P < 0.05). Forelimb deficits evaluated on a wire grid platform were evident for 28 days (P < 0.05). Forepaw preference measured in a non-test setting indicated a bias to use the unaffected forepaw for 35 days (P < 0.05). A biphasic pattern of functional recovery was seen on all tests. A period of rapid functional recovery lasting 7 to 10 days was followed by a slower period of functional recovery lasting many weeks. Possible meanings of this biphasic recovery are discussed as issues of behavioral compensation/adaptation versus true neural recovery. Eight weeks after TBI histological analyses indicated that axonal degeneration was present in the areas adjacent to the ipsilateral cortical injury site. Degenerating fibers also extended across the corpus callosum into the homologous area in the contralateral cortex and were seen in the ipsilateral striatum, somatosensory and motor thalamic nuclei and substantia nigra. Significant axonal degeneration occurred bilaterally around the deep cerebellar nuclei. Degenerating fibers extended into the folia and terminated in the cerebellar granule cell layer. Thus the entire sensorimotor control system appeared to have been affected by a cortical injury.

Neuronal regeneration and estrous cycle restoration after locus coeruleus-periventricular gray substance section

The locus coeruleus (LC) was anatomically separated from the periventricular gray substance (PVG) by means of knife cuts in the adult female rat presenting regular estrous cycling. This resulted in a transient suppression of the estrous cycling that lasted 10-13 days after surgery. After this period, irregular or regular cycling activity was observed. The regular cycling was restored 30-45 days after the knife cuts. Golgi impregnation of some of the brains of these rats revealed regenerative elements in the knife-cut-insulted area. Thus, blood vessels, macrophagic-like elements, and glial-like elements were observed in close relation with the knife-cut pathway. Additionally, well-defined stained neurons typical of the LC and PVG were observed in close proximity to the knife-cut pathway. Dendritic and axon projections towards the insulted area were observed. Well defined axons were seen across the knife-cut pathway. These data support, first, that the LC-PVG communication is part of a circuitry for the modulation of gonadotropic activity, and second, that in the restoration of the estrous cyclicity after the knife cut, regenerative processes leading to a LC-PVG functional reconnection occurred after the knife cut.

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 micrograms 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 micrograms 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.

The role of 5-hydroxytryptamine (5-HT) receptor subtypes and plasticity in the 5-HT systems in the regulation of nociceptive sensitivity

This review shows that the role of 5-hydroxytryptamine (5-HT) in the regulation of nociception depends on the 5-HT receptor subtypes involved and on long-term functional changes in the 5-HT receptors. Stimulation of the 5-HT1 receptors, as well as of the 5-HT2 and 5-HT3 receptors, may reduce nociceptive sensitivity. In addition, activation of 5-HT2 and 5-HT3 receptors may also enhance nociceptive sensitivity. Up- or down-regulation of the 5-HT receptors may result in long-lasting changes, plasticity, in the 5-HT systems. Lesioning of 5-HT neurons induces denervation supersensitivity to 5-HT, and prolonged stimulation of 5-HT receptors may produce subsensitivity to 5-HT. In the spinal cord denervation supersensitivity to 5-HT may depend on reduced release of substance P (SP). An increase in the release of SP, on the other hand, may reduce the effects of 5-HT receptor activation. Long-term treatment with antidepressants which are used in clinical pain therapy appears to up-regulate the 5-HT1 receptors and to down-regulate the 5-HT2 receptors.

Unilateral dopamine lesions in neonatal, weanling and adult rats: comparison of rotation and reaching deficits

The aim of this study was to compare the functional effects of neonatal, weanling and adult lesions of the dopaminergic (DA) mesencephalic neurones on paw-reaching behaviour. The mesotelencephalic DA pathway was destroyed unilaterally in neonatal (3 and 7 day), weanling (21 day) and adult (2 months) rats by local injection of 6-hydroxydopamine into the medial forebrain bundle at the level of the lateral hypothalamus, followed by behavioural studies conducted 2 months later. Amphetamine and apomorphine induced similar rates of rotation irrespective of the age of the lesion. By contrast skilled reaching with the contralateral paw was profoundly disrupted by lesions made in adult or weanling rats, but a much reduced deficit was observed in neonatally lesioned rats. Tyrosine hydroxylase immunohistochemistry indicated a similar degree of dopamine cell loss from the substantia nigra in all groups. These observations suggest that the host brain undergoes developmental changes 1-3 weeks postnatally that influence the long-term effects of lesions in the nigrostriatal dopamine system.

A light and electron microscopical study of enkephalin-immunoreactive structures in the rat neostriatum after removal of the nigrostriatal dopaminergic pathway

The pattern of enkephalin immunoreactivity was examined in the adult rat neostriatum, at various times after unilateral removal of the nigrostriatal dopamine input by 6-hydroxydopamine injection into the medial forebrain bundle. Animals were examined 12 days, 26 days or 13 months after the lesion. Enkephalin-immunoreactive synaptic boutons (n = 1018) in the control and the dopamine-depleted neostriatum were analysed in the electron microscope. The area of enkephalin-immunoreactive synaptic bouton profiles was significantly larger in the dopamine-depleted neostriatum and this increase was maximal in rats in which the lesion had been made 26 days or 13 months previously (50% increase). The synaptic specializations of these enkephalin-immunoreactive boutons were significantly longer in the neostriatum from the injected side. Dendritic shafts were the principal postsynaptic target of these boutons (67%) but dendritic spines (18%), perikarya (6.5%) and unidentifiable small dendrites or spines (8.5%) were also contacted. The proportions of enkephalin-immunoreactive boutons on the different postsynaptic targets were not altered by the 6-hydroxydopamine lesion. The increase in enkephalin immunoreactivity observed in the dopamine-depleted neostriatum in previous studies may be explained by the increase in the size of enkephalin-immunoreactive synaptic boutons found in the present ultrastructural investigation. The observations do not rule out the possibility that there is also an increase in the number of immunoreactive synaptic boutons, due to, for example, sprouting of the existing enkephalin-containing fibres.

The recovery of locomotion following partial extirpation of the motor cortex and transplantation of cortical tissue in the white rat

The changes in the parameters of locomotion in three groups of white rats at ages from 5 to 16 weeks were investigated in this study: 1) following unilateral extirpation of an area of cerebral cortex in the zone of the motor representation of the hind limb at the age of 5 weeks; 2) following homotypical transplantation of cortical tissue from newborn rats to the zone of decortication at the same age; 3) intact animals of the same litters (control). An age-related increase in the step length took place in the rats of all groups. By contrast with the intact rats, in which the increase in the values of the step length and width took place smoothly over the course of the entire period, a lag in the locomotion parameters investigated was observed in the operated animals. After two to five weeks the features of recovery were already observed in the grafted rats, while a tendency toward an alteration in the character of the gait was detected after 9 to 11 weeks: their step length exceeded that of both the operated and intact rats. Histological control confirmed the taking of all grafts without signs of rejection or necrotic changes. The possible mechanisms of participation of endogenous and exogenous factors in the elimination of the hemiparetic sequelae are discussed.

Physiological changes in the somatosensory forepaw cerebral cortex of adult raccoons following lesions of a single cortical digit representation

The purpose of this study was to determine whether restricted lesions within primary somatosensory (SmI) cortex cause changes in the functional organization of cortical areas bordering on the site of injury. Focal ablations of cortical tissue were made in the representational area for digit 3 within the SmI forepaw cortex of adult raccoons. Electrophysiological mapping experiments done 15-17 weeks later showed that significant alterations had occurred in the response properties of clusters of neurons within those representational zones adjoining the lesion--the zones for digit 2, digit 4, and the palmar pads. These three cortical areas were modified by the appearance of new, usually weaker secondary inputs and changes in some properties of the normal primary inputs from the forepaw. (i) Many neurons responded to stimulation of previously ineffective skin regions; the new inputs often originated from digit 3 but frequently involved other digits or the pads as well. (ii) Neuronal receptive fields (RFs), mapped at a standard suprathreshold stimulus intensity, were larger than normal. (iii) Skin type and submodality sensitivity typically were less specific than normal; more neurons had RFs that included both glabrous and hairy skin or claws and displayed mixtures of responsiveness to skin touch, hair deflection, or claw touch. (iv) The representation of RF location, skin type, and submodality sensitivity was more variable as a function of horizontal and vertical distance through the cortex. In general, the physiological changes were found to degrade the somatotopic order and response specificity of the intact cortical areas adjoining the lesion.(ABSTRACT TRUNCATED AT 250 WORDS)

Recovery of olfactory behavior following removal of accessory olfactory bulb in adult rat

It has been shown that male rats demonstrate a preference for the odor of the female rat. This preference has been suggested to be a function of the accessory olfactory pathway. Thus, changes in preference of the male rat for the odor of the female were examined following accessory olfactory bulb (AOB) removal. Preference decreased to about 30% of that seen in the intact rat during the 10 days following AOB removal. Thereafter, it increased gradually over one month survival time up to approximately 70% of the level demonstrated by the intact rat. The rats whose main olfactory bulb and AOB were removed did not show any recovery at any survival time. The time course of the behavioral recovery after AOB removal was compared with the time course of the recovery of synaptic density which was observed in the medial amygdaloid nucleus after AOB removal. The relationship between functional recovery and reorganization of synaptic connections is discussed.

Increased basic fibroblast growth factor (bFGF) immunoreactivity at the site of focal brain wounds

Basic fibroblast growth factor (bFGF) is a polypeptide found within the CNS with potent effects on the survival and proliferation of CNS glia and endothelial cells, and on the survival and outgrowth of CNS neurons. Immunohistochemical methods were used to examine relative changes in the levels and distribution of bFGF following focal brain injury. Two monospecific antisera to bFGF were used to immunostain intact mature rat brain, and brain in which a focal mechanical lesion had been made in the dorsolateral cerebral cortex one week previously. In the intact brain, staining was localized primarily in neuronal cell bodies, especially in limbic structures. In injured brain, a marked increase of bFGF immunoreactivity was found at the borders of lesions, localized to the dense accumulation of cells, many of which resembled 'reactive' astroglia. Such increases in local bFGF concentrations may contribute to the cascade of cellular changes--including glial and capillary proliferation, and neural sprouting--that follows focal brain injury.

Plasticity of intra-amygdaloid connections following the denervation of fibers from accessory olfactory bulb to medial amygdaloid nucleus in adult rat: immunohistochemical study of anterogradely transported lectin (Phaseolus vulgaris leucoagglutinin)

The rearrangement of the terminations of intra-amygdaloid fibers was examined in the medial amygdaloid nucleus (MAN) following denervation of the fibers from the accessory olfactory bulb (AOB) of adult rat using immunohistochemistry of anterogradely transported lectin (Phaseolus vulgaris leucoagglutinin, PHA-L). The AOB was removed unilaterally by suction at two months before the PHA-L injection. The PHA-L was injected bilaterally into the posteromedial region of the amygdala (posteromedial amygdaloid nucleus and amygdala-hippocampal transitional area). After the tissue was processed by the routine immunohistochemical method, the immunologically labeled axons and axon terminals in the MAN were observed with the light microscope. On the control side, labeled axons were observed in the cellular part of the MAN, but they were not found in the molecular layer in which the fibers from the accessory olfactory bulb terminate. On the side of the AOB removal, the labeled axons were found not only in the cellular part but also in the molecular layer. Electron microscopic observations showed that the labeled axon terminals made synaptic contacts in the molecular layer. These results indicate that terminals of intra-amygdaloid fibers expand from the cellular part to the molecular layer in the MAN and make synaptic contacts in this layer following the denervation of the AOB fibers.

Synaptic reorganization in the medial amygdaloid nucleus after lesion of the accessory olfactory bulb of adult rat. I. Quantitative and electron microscopic study of the recovery of synaptic density

The time course of the appearance of degenerating synapses and the loss and reappearance of intact synapses was investigated in the medial amygdaloid nucleus (MAN) following lesions of the accessory olfactory bulb (AOB). Degenerating synapses were concentrated in the molecular layer of MAN. A quantitative electron microscopic analysis of the molecular layer was performed at 2, 4, 8, 16, 32 and 64 days after the placement of kainic acid injections within AOB. At each survival time, counts of synapses were made from photomicrographic montages. The degenerating synapses were observed at 2 days after lesion. Their density reached a peak at 2 or 4 days after the lesion and decreased thereafter. No degenerating synapses were found at 64 days following the lesion. Shrinkage of the thickness of molecular layer was observed following the AOB lesion. This was most pronounced at 4 or 8 days after the lesion at which time the thickness was 70-75% of the contralateral molecular layer. The shrinkage receded moderately (85% of contralateral layer at 64 days after lesion). The compensated synaptic density of intact synapses was reduced to less than half of the control density at 4 or 8 days after the lesion. Thereafter synaptic density increased gradually and reached more than 80% of control density at 64 days after lesion. The recovery in number of intact synapses following the lesion suggests the possibility of synaptic reorganization by remaining afferent fibers.

Noradrenergic, serotonergic, and cholinergic sprouting in the hippocampus that follows partial or complete transection of the septohippocampal pathway: contributions of spared inputs

The aminergic and cholinergic fibers innervating the hippocampus reach their target regions via the dorsal (dorsal fornix, fimbria, and cingulum) and ventral routes. The plasticity of this innervation after lesions of the dorsal pathway was investigated in the septal, medial, and temporal regions of the adult rat hippocampus. The extent and time course of sprouting was assessed by determining high-affinity uptake of [3H]noradrenaline and [3H]serotonin, and the activities of cholinergic enzymes 1 week to 6 months after partial or complete transection of the dorsal pathway. The initial decline in these terminal indices resulting from the transection of the medial or lateral half of the dorsal pathway reflected the distribution of their respective terminal sites in the three hippocampal regions. Longer survival after such lesions led to a substantial recovery of cholinergic and noradrenergic markers indicating reinnervation of the hippocampus by spared dorsal and/or by undamaged ventral afferent fibers. Whether the recovery was complete or not, the sprouting of spared dorsal fibers was maximum within a relatively shorter postlesion survival whereas that of the ventral afferents continued for a longer time. In contrast to the extent of noradrenergic and cholinergic restitution, very poor serotonergic recovery was seen in the same animal in spite of the availability of spared serotonergic afferent fibers and regardless of the duration of postlesion survival. The apparently finite capacity of spared axons for postlesion reinnervation was not increased by longer survival. Amplification of this property of central neurons by other interventions may supplement the approach of transplantation for recovery of function following injury.

Homologous cholinergic efferents spared by partial fimbrial lesions contribute to the recovery of hippocampal cholinergic enzymes in adult rats

Cholinergic fibers spared by partial lesions of the fimbrial bundle contribute to the recovery of enzyme markers for hippocampal cholinergic terminals. This recovery, most likely representing structural restoration of cholinergic terminals lost to lesion-induced degeneration, is amplified by a 'conditioning' lesion. This model of functionally relevant postlesion reconstruction, by and of structures within the CNS, is suitable to search for means to enhance homotypic collateral sprouting.