Is there capacity for anatomical and functional repair in the adult somatosensory thalamus?

Pattern of long-distance projections from fetal hippocampal field CA3 and CA1 cell grafts in lesioned CA3 of adult hippocampus follows intrinsic character of respective donor cells

Fetal hippocampal grafts transplanted to the lesioned CA3 of adult hippocampus can extend axonal projections to many regions of the host brain. However, the identity of grafted cells that project to specific host regions is unknown. We hypothesize that the pattern of long-distance axonal projections from distinct fetal hippocampal cells grafted to lesioned CA3 is specified by the intrinsic nature of respective donor cells rather than characteristics of the host graft region. We grafted fetal hippocampal CA3 or CA1 cells into kainic acid lesioned CA3 of adult hippocampus at four days post-lesion. Neurons projecting to either the contralateral hippocampus or the ipsilateral septum were then measured in these grafts at four months post-grafting using Fluoro-Gold and DiI tract tracing. CA3 grafts located close to the degenerated CA3 cell layer showed a high propensity for establishing projections into the contralateral hippocampus (commissural projections) compared to similarly located CA1 grafts, which exhibited negligible commissural projections. Similar distinction was observed between the two graft types even when they were located only partially in the lesioned CA3. Among CA3 grafts, those placed near the degenerated CA3 cell layer established significantly greater commissural projections than those placed only partially in the CA3 region. Septal projections, in contrast, were robust from both CA3 and CA1 grafts. This differential projection pattern between CA3 and CA1 grafts resembles projections of CA3 and CA1 cells in intact hippocampus.These results demonstrate that the intrinsic character of grafted fetal cells determines the type of efferent projections from fetal grafts into different targets in the lesioned adult host brain. However, the extent of efferent projections from specific grafts is also influenced by the location of grafted cells within the host region. Thus, graft-mediated appropriate reconstruction of damaged circuitry in the lesioned brain may require grafting of homotopic donor cells. Further, the robust and specific projections observed from CA3 grafts is likely beneficial for functional recovery of hippocampus following CA3 injury and hence of significance towards developing a graft-mediated therapy for human temporal lobe epilepsy.

Development of long-distance efferent projections from fetal hippocampal grafts depends upon pathway specificity and graft location in kainate-lesioned adult hippocampus

Fetal hippocampal cells grafted into the excitotoxically lesioned hippocampus of adult rats are capable of extending axonal projections into the host brain. We hypothesize that the axonal growth of grafted fetal cells into specific host targets, and the establishment of robust long-distance efferent graft projections, require placement of fetal cells in close proximity to appropriate axon guidance pathways. Intracerebroventricular administration of kainic acid in adult rats leads to a specific loss of hippocampal CA3 pyramidal neurons. We grafted 5'-bromodeoxyuridine-labeled embryonic day 19 hippocampal cells into adult hippocampus at four days post-kainic acid lesion, and quantitatively measured the projection of grafted cells into the contralateral hippocampus and the septum after three to four months survival using Fluoro-Gold and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (Dil) tracing. Grafts located in or near the degenerated CA3 cell layer exhibited numerous neurons which established commissural projections with the contralateral hippocampus. However, such projection did not occur in intrahippocampal grafts located away from the CA3 cell layer. In contrast, neurons in all grafts established robust projections into the septum regardless of location within hippocampus although grafts located near the degenerated CA3 cell layer displayed more neurons with such projections. Location of grafted cells clearly influences the development of efferent graft projections into distant targets in the adult host brain, particularly access to axon guidance pathways to facilitate the formation of projections. The establishment of robust long-distance commissural projections of fetal hippocampal grafts is clearly dependent on their placement in or near the degenerated CA3 cell layer, suggesting that appropriate axon guidance pathways for commissural pathways are tightly focussed near this cell layer. However, the establishment of septal projections of these grafts was not dependent on specific location within the CA3 cell layer, suggesting that axonal guidance mechanisms to the septum are more diffuse and not limited to the CA3 dendritic layers. The results underscore that fetal hippocampal grafts are capable of partly restoring lesioned hippocampal circuitry in adult animals when appropriately placed in the host hippocampus.

Fetal hippocampal cells grafted to kainate-lesioned CA3 region of adult hippocampus suppress aberrant supragranular sprouting of host mossy fibers

Selective lesion of the rat hippocampus using an intracerebroventricular administration of kainic acid (KA) represents an animal model for studying both lesion recovery and temporal lobe epilepsy. This KA lesion leads initially to loss of CA3 hippocampal neurons, the postsynaptic target of mossy fibers, and later results in aberrant mossy fiber sprouting into the dentate supragranular layer (DSGL). Because of the close association of this aberrant mossy fiber sprouting with an increase in the seizure susceptibility of the dentate gyrus, delayed therapeutic strategies capable of suppressing the sprouting of mossy fibers into the DSGL are of significant importance. We hypothesize that neural grafting can restore the disrupted hippocampal mossy fiber circuitry in this model through the establishment of appropriate mossy fiber projections onto grafted pyramidal neurons and that these appropriate projections will lead to reduced inappropriate sprouting into the DSGL. Large grafts of Embryonic Day 19 hippocampal cells were transplanted into adult hippocampus at 4 days post-KA lesion. Aberrant mossy fiber sprouting was quantified after 3-4 months survival using three different measures of Timm's staining density. Grafts located near the degenerated CA3 cell layer showed dense ingrowth of host mossy fibers compared to grafts elsewhere in the hippocampus. Aberrant mossy fiber sprouting throughout the dentate gyrus was dramatically and specifically reduced in animals with grafts near the degenerated CA3 cell layer compared to "lesion only" animals and those with ectopic grafts away from the CA3 region. These results reveal the capability of appropriately placed fetal hippocampal grafts to restore disrupted hippocampal mossy fiber circuitry by attracting sufficient host mossy fibers to suppress the development of aberrant circuitry in hippocampus. Thus, providing an appropriate postsynaptic target at early postlesion periods significantly facilitates lesion recovery. The graft-induced long-term suppression of aberrant sprouting shown here may provide a new avenue for amelioration of hyperexcitability that occurs following hippocampal lesions.

Postnatal development of mouse "whisker" thalamus: ventroposterior medial nucleus (VPM), barreloids, and their thalamocortical relay neurons

We followed developmental changes in "barreloid" thalamocortical relay cell (TCR) dendritic arbors between postnatal day 5 (P5; birth = P0) and adulthood. Single neurons in 150- to 250-microns coronal or oblique slices through the somatosensory thalamus in mice of different postnatal ages were injected with lucifer yellow (LY) under direct visualization. Filled cells in the ventroposterior medial nucleus (VPM) were imaged with a confocal microscope, and rendered and analyzed on a computer workstation with special-purpose software. The whisker representation in the thalamus, as revealed by the pattern of barreloids, was demonstrated by oblique illumination of the slices and/or later cytochrome oxidase (CO) staining. VPM cross-sectional area trebles from P5 to adulthood. Barreloids (single-whisker representations) are well delineated in unstained sections until P10-P11; thereafter, barreloids can only be recognized with difficulty with the CO stain. Thalamocortical relay cell (TCR) somal volumes increase rapidly in the first 2 weeks. The number of primary dendrites does not change, nor does the length of the primary dendritic segments, from P5 to adulthood; however, distal dendritic segments elongate and increase in number. Dendritic arbors are confined on P5 to single barreloids; in adults they extend to adjacent barreloids. The postnatal transformation of dendritic arbors by process growth to adjacent barreloids is mainly completed by P18. A change in the developmental role of these cells, from instructing whisker pattern formation to integrating sensory information from more than one whisker, thus occurs after the whisker pattern in the barrel cortex is established. It coincides with the age at which animals are known to begin exploratory whisking behaviors. The mechanism appears to be by growth and remodeling of distal dendrites rather than by oriented growth and regression, as has been reported for stellate cells in cortical whisker barrels.

Dendritic architecture of rat somatosensory thalamocortical projection neurons

This study examines dendrites from physiologically characterized and intracellularly labelled thalamocortical projection (TCP) neurons from the rat ventrobasal complex (VB) and posterior nucleus (POm). The goals were to provide quantitative descriptions of TCP neuron dendrites, examine underlying design principles of dendritic morphology, and determine correlations between dendritic size parameters. Forty-four dendrites from seven VB neurons and 21 dendrites from three POm TCP neurons that responded to low-threshold mechanical stimuli were reconstructed and quantitatively analyzed at the light microscopic level. The dendritic architecture of the neurons was remarkably similar in most parameters studied, including the percentage of dichotomous branching, contribution of terminal branches to total dendritic length, and branching symmetry. There was a positive correlation between stem dendrite diameter and the length of the entire dendrite arbor, making it possible to estimate the total length of a dendritic arbor by measuring the stem dendrite diameter. The correlations of the VB and POm dendrites had different slopes. The path distance (the distance from the soma to a dendritic end point) of individual dendrites showed only a small variation with large differences in the total dendritic length of an arbor. The constant diameter of distal dendrites shows that dendrite diameter is a poor predictor of synaptic location on the dendritic tree. Although the morphology of neurons and their individual dendrites varied considerably in overall size and qualitative appearance, when examined qualitatively, many aspects of dendritic structure were similar within and between groups. We suggest that the rat somatosensory TCP neurons have a stereotyped dendritic architecture and present data which provide a base for future comparative, developmental, and plasticity studies.

Actions of sciatic nerve ligature on sexual behavior of sexually experienced and inexperienced male rats: effects of frontal pole decortication

The action of the hyperalgesia produced by the loose ligature of the sciatic nerve on the sexual behavior of two groups of sexually experienced and inexperienced male rats was studied. The putative changes in the behavior induced by this manipulation were attempted to be counteracted by the removal of the anterior frontal cortex. Independently of the sexual experience, the ligature of the sciatic nerve did not modify any parameter of the sexual behavior, except for a slight prolongation of the intromission latency. Surprisingly, the removal of the cortical frontal pole resulted in drastic changes in the sexual behavior of sexually inexperienced male rats such as an increase in the intromission and ejaculation latencies and the postejaculatory interval. These changes were not observed in the sexually experienced group. Interestingly, the sciatic nerve ligature prevented the changes in the copulatory behavior produced by the frontal cortex removal. These results suggest that the hyperalgesia induced by the sciatic nerve ligature was unable to alter the sexual behavior. By contrast, the frontal cortex removal produced important changes in the sexual behavior that depend upon the previous sexual experience. All results are discussed on the bases of the neural control underlying the possible relationships between algesia and male sexual behavior.