Restoration of function to the denervated spinal cord after implantation of embryonic 5HT- and substance P-containing raphe neurones

Information processing streams in rodent barrel cortex: the differential functions of barrel and septal circuits

Rodent somatosensory cortex contains an isomorphic map of the mystacial whiskers in which each whisker is represented by neuronal populations, or barrels, that are separated from each other by intervening septa. Separate afferent pathways convey somatosensory information to the barrels and septa that represent the input stages for 2 partially segregated circuits that extend throughout the other layers of barrel cortex. Whereas the barrel-related circuits process spatiotemporal information generated by whisker contact with external objects, the septa-related circuits encode the frequency and other kinetic features of active whisker movements. The projection patterns from barrel cortex indicate that information processed by the septa-related circuits is used both separately and in combination with information from the barrel-related circuits to mediate specific functions. According to this theory, outputs from the septal processing stream modulate the brain regions that regulate whisking behavior, whereas both processing streams cooperate with each other to identify external stimuli encountered by passive or active whisker movements. This theoretical view prompts several testable hypotheses about the coordination of neuronal activity during whisking behavior. Foremost among these, motor brain regions that control whisker movements are more strongly coordinated with the septa-related circuits than with the barrel-related circuits.

Monosynaptic connections between pairs of L5A pyramidal neurons in columns of juvenile rat somatosensory cortex

Layer 5 (L5) of somatosensory cortex is a major gateway for projections to intra- and subcortical brain regions. This layer is further divided into 5A and 5B characterized by relatively separate afferent and efferent connections. Little is known about the organization of connections within L5A of neocortical columns. We therefore used paired recordings to probe the anatomy and physiology of monosynaptic connections between L5A pyramidal neurons within the barrel columns of somatosensory cortex in acute slices of approximately 3-week-old rats. Post hoc reconstruction and calculation of the axodendritic overlap of pre- and postsynaptic neurons, together with identification of putative synaptic contacts (3.5 per connection), indicated a preferred innervation domain in the proximal dendritic region. Synaptic transmission was reliable (failure rate <2%) and had a low variability (coefficient of variation of 0.3). Unitary excitatory postsynaptic potential (EPSP) amplitudes varied 30-fold with a mean of 1.2 mV and displayed depression over a wide range of frequencies (2-100 Hz) during bursts of presynaptic firing. A single L5A pyramidal neuron was estimated to target approximately 270 other pyramidal neurons within the same layer of its home barrel column, suggesting a mechanism of feed-forward excitation by which synchronized single action potentials are efficiently transmitted within L5A of juvenile cortex.

Serotonin Reinnervation of the Suprachiasmatic Nucleus by Intrahypothalamic Fetal Raphe Transplants, with Special Reference to Possible Influences of the Target

Survival and development of fetal serotonin (5-HT) neurons grafted to various brain areas in adult mammals have been suggested to be under host influences. The aim of this study was to determine whether the suprachiasmatic nucleus of the hypothalamus (SCN), a region receiving a 5-HT input which is one of the densest and the most heavily synaptic in the brain, can actually support the development of transplanted 5-HT neurons. The time course and extent of 5-HT reinnervation were therefore investigated with 5-HT immunocytochemistry in adult rats subjected to intraventricular injection of 5,7-dihydroxytryptamine and subsequent grafting of fetal cell suspension of mesencephalic raphe neurons. The ultrastructural features of the newly formed 5-HT terminal plexa were also examined. Serotonin reinnervation of the SCN remained partial up to 4 months post-transplantation, with no apparent predilection of the reinnervating fibres for any particular portion of the nucleus, thus differing from the normal 5-HT innervation of the SCN both quantitatively and qualitatively. This was in sharp contrast to the 5-HT hyperinnervation observed in neighbouring areas such as the supraoptic nucleus, a structure normally provided with only few 5-HT fibres, and the ventral wall of the third ventricle. The graft-derived 5-HT axons, however, displayed ultrastructural features that did not appear different from those of their normal counterparts; in particular they re-established defined synaptic contacts with the host population. These results may indicate that the mature SCN specifically lacks a trophic factor necessary for the ingrowth of graft-derived 5-HT fibres, or that it represents an inhibitory environment for such an ingrowth. The limited ability of regrowing 5-HT axons to restore a normal density of 5-HT innervation could also be related to the fact that these neurons normally establish a relatively high number of synaptic contacts in the target region.

Differences in somatosensory processing in S1 barrel cortex between normal and monoamine oxidase A knockout (Tg8) adult mice

Spatio-temporal processing of whisker information was analysed in vivo for single neurons in D2 barrel columns of S1 cortex in Tg8 mutant mice, which lack barrels. Findings were compared with normal C3H mice of the same genetic background. The topographical organization of functional columns was similar in Tg8 and normal mice. Response magnitudes (RMs) to D2 principal whisker deflections in D2 columns for Tg8 were similar to normals for layers I-III and layer IV cells but short latency responses (>10 ms post-stimulus) were twice the magnitude of normal mice. The surrounding whiskers D1 and D3 yielded smaller RMs in layer IV of mutants than normal mice whereas RMs in layers I-III were equipotent (P>0.5). Modal latencies were shorter in Tg8 mice in all layers. Latency distributions for whisker D2 responses in both laminae were bimodal in normal mice, peaking at 6-8 and 12 ms post-stimulus, but unimodal in Tg8 mice in both laminae, peaking at 6-8 ms. Hence, despite an absence of barrels, segregation of columns is enhanced in layer IV and sensory processing is faster in layers I-IV compared with normal mice. This contrasts with adenylyl cyclase knockout mice where both an absence of barrels and enhanced surrounding whisker responses have been observed. These findings suggest that factors other than barrels and clustering of thalamo-cortical terminals define receptive field geometry.

Ipsilateral and contralateral transfer of tactile learning

We examined the spatial organization of perceptual learning in a cortex-dependent task. Rats learned a tactile task using four whiskers on one side of the snout, all others being clipped. These trained whiskers were then clipped and prosthetic whiskers were attached. Subsequent performance was found to be determined by the location of the prosthetic whiskers. There was partial transfer of learning to neighbouring whisker positions. In addition, there was partial transfer of learning to whisker positions on the other side of the snout, but only if the prosthetic whiskers were symmetrically opposite the trained whiskers. These findings suggest that neural changes underlying perceptual learning are distributed according to the topographic organization of the sensory cortical map.

Human spinal cord retains substantial structural mass in chronic stages after injury

In chronic stages of human spinal cord injury, atrophy of the cord has been reported in regions both at and distant to the injury site. Local cord atrophy results from the direct effects of bony impact and ischemia, whereas distant atrophy results from anterograde (Wallerian) and retrograde axonal degeneration. However, the actual extent of degenerative changes in the chronically injured human spinal cord both at and remote from the injury site has rarely been reported, and has not been rigorously quantified to date. In the present study, we quantified the extent of spinal cord atrophy in 12 humans with chronic injury (2-34 years posttrauma) utilizing quantitative stereological assessment of spinal cord magnetic resonance images, and compared the results to uninjured human spinal cords. Focal cystic atrophy of the cord, characterized by signal attenuation on T1-weighted images, was regularly present at the actual site of impact injury and replaced a mean longitudinal area equaling less than one spinal cord segment in length (2.01 +/- 0.60 cm2, or a loss of 89.3 +/- 17.4% of the longitudinal area of one spinal cord segment). Spinal cord segments immediately rostral to the zone of cystic degeneration showed atrophy of only 19.4 +/- 7.5% of normal cord longitudinal area, and spinal cord segments immediately caudal to the zone of cystic degeneration showed atrophy of 16.5 +/- 4.1% of normal cord longitudinal area. Extensive spinal cord atrophy extending beyond the region of injury occurred in two of twelve cases (16.7%), and both were caused by late syrinx formation. Thus, spinal cord atrophy after trauma remains primarily restricted to the original site of injury. Experimental neural repair strategies should take into account the importance of "bridging" relatively short zones of cystic atrophy, then promoting axonal regeneration through potentially long segments of remaining cord parenchyma.

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.

Human fetal tissue research: practice, prospects, and policy

Tissue from human fetal cadavers has long been used for medical research, experimental therapies, and various other purposes. Research within the last two decades has led to substantial progress in many of these areas, particularly in the application of fetal tissue transplantation to the treatment of human disease. As a result, clinical trials have now been initiated at centers around the world to evaluate the use of human fetal tissue transplantation for the therapy of Parkinson's disease, insulin-dependent diabetes mellitus, and a number of blood, immunological and, metabolic disorders. Laboratory studies suggest a much wider range of disorders may in the future be treatable by transplantation of various types of human fetal tissue. A combination of characteristics renders fetal tissue uniquely valuable for such transplantation, as well as for basic research, the development of vaccines, and a range of other applications. Although substitutes for human fetal tissue are being actively sought, for many of these applications there are at present no satisfactory alternatives. Important issues remain unresolved concerning the procurement, distribution, and use of human fetal cadaver tissue as well as the effects of such use on abortion procedures and incidence. These issues can be addressed by the introduction of appropriate guidelines or legislation, and need not be an impediment to legitimate research and therapeutic use of fetal tissue.

Transplants of embryonic motoneurones to adult spinal cord: survival and innervation abilities

One goal of transplantation experiments involving damaged spinal cords is to reconstruct a functional innervation to muscles in the periphery. Embryonic spinal cord grafts have been shown to survive transplantation into adult spinal cord lacking motoneurones. Motoneurones from the graft appear to be able to innervate muscle tissue by being encouraged to grow across a bridge of peripheral nerve. Integration of grafted motoneurones appears to involve their migration from the graft into the host ventral horn, thus replacing depleted host neurones. These results suggest possible strategies of research that might lead to treatments of spinal cord injuries and disorders in which motoneurone loss occurs, such as amyotrophic lateral sclerosis, spinal muscular atrophies and poliomyelitis.

Immunoelectron microscopic analysis of the synaptic connectivity of serotoninergic neurons grafted to the 5,7-dihydroxytryptamine-lesioned rat spinal cord

The connections between the host and 5-hydroxytryptamine-containing neurons grafted to the spinal cord have been analysed using electron microscopic immunohistochemistry. Adult rats with 5,7-dihydroxytryptamine lesions of the brain and spinal cord received implants of embryonic medullary raphé neurons at three sites in the spinal cord. Eight to 10 months after grafting, the transplanted 5-hydroxytryptamine-positive neurons had formed extensive and complex contacts with spines, dendrites, perikarya and vesicle-containing structures in both the dorsal and ventral horns. Reinnervation of laminae IV-VI was less rich. In the graft itself, connections were also made between non-immunoreactive varicosities and 5-hydroxytryptamine-containing dendrites, and somata, but the exact origin of the afferents was not determined. Outside the implant site, no obvious synaptic junctions onto grafted 5-hydroxytryptamine-immunoreactive boutons were obvious, although labelled and unlabelled varicosities were often in close apposition. Synaptic junctions in the dorsal horn were predominantly symmetric, with the presynaptic varicosity containing mostly small agranular vesicles. By contrast, in the ventral horn most junctions were asymmetric, while the presynaptic element contained both small agranular and large dense-core vesicles. The results demonstrate that the types of synaptic contacts formed between the grafted 5-hydroxytryptamine neurons and the host spinal cord are remarkably similar to those found in intact spinal cord. In addition, the division of morphological differences that exists between 5-hydroxytryptamine-containing boutons in the normal dorsal vs ventral horns is also apparent in the transplanted animals. Finally, there appear to be present several anatomical substrates for the regulation by the host of 5-hydroxytryptamine output from the grafted neurons.