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

Changes in the distribution of the dentate gyrus associational system following unilateral or bilateral entorhinal lesions in the adult rat

The distribution of the dentate gyrus associational system was analyzed in naive adult rats and in those with either unilateral or bilateral lesions of the entorhinal cortex. Horseradish peroxidase histochemistry was used to trace the origin and course of this intrinsic fiber system. The fibers originated in the CA3-4 pyramidal cell field, apparently medial to the origin of the Schaffer collateral system, and followed a trajectory which was essentially identical to that described for this system by Zimmer36. The associational terminal field occupied the inner 26% of the dentate gyrus molecular layer in normal rats and 35-38% of the normal width of that layer following either ipsilateral or bilateral entorhinal lesion. These measurements are quite similar to those previously obtained on the commissural system terminal field in the normal and partially deafferented dentate gyrus. These results are interpreted to reflect axon sprouting by the associational fibers into the adjacent deafferented dendritic field.

Dopamine acetylcholine imbalance in Parkinson's disease. Possible regenerative overgrowth of cholinergic axon terminals

Parkinson's disease is characterised by an imbalance between acetylcholine and dopamine which probably results from the degeneration of a dopaminergic nigrostriatal pathway. A new hypothesis is proposed to explain the development of this imbalance. Applying the concept that degeneration of nerve-fibres in the central nervous system can lead to collateral sprouting of uninjured fibres, it is suggested that the death of dopaminergic nigrostriatal neurons results in sprouting of axons of cholinergic interneurons in the caudate nucleus. This overgrowth could result in the cholinergic innervation of neuronal membranes vacated by degenerated dopaminergic terminals. Thus, the apparent changes in the activity of dopaminergic and cholinergic systems can be accounted for by faulty regeneration in the central nervous system.

Regeneration of central cholinergic neurones in the adult rat brain

The regrowth of lesioned central acetylcholinesterase (AChE)-positive axons in the adult rat was studied in irides implanted to two different brain sites: in the caudal diencephalon and hippocampus, and in the hippocampal fimbria. At both implantation sites the cholinergic septo-hippocampal pathways were transected. At 2-4 weeks after lesion, newly formed, probably sprouting fibres could be followed in abundance from the lesioned proximal axon stumps into the iris transplant. Growth of newly formed AChE-positive fibres into the transplant was also observed from lesioned axons in the anterior thalamus, and to a minor extent also from the dorsal and ventral tegmental AChE-positive pathways and the habenulo-interpeduncular tract. The regrowth process of the sprouting AChE-positive, presumed cholinergic fibres into the iris target was studied in further detail in whole-mount preparations of the transplants. For this purpose the irides were removed from the brain, unfolded, spread out on microscope slides, and then stained for AChE. During the first 2-4 weeks after transplantation the sprouting central fibres grew out over large areas of the iris. The new fibres branched profusely into a terminal plexus that covered maximally about half of the iris surface, and in some areas the patterning of the regenerated central fibres mimicked closely that of the normal autonomic cholinergic innervation of the iris. In one series of experiments the AChE-staining was combined with fluorescence histochemical visualization of regenerated adrenergic fibres in the same specimens. In many areas there was a striking congruence in the distributional patterns of the regenerated central cholinergic and adrenergic fibres in the transplant. This indicates that - as in the normal iris - the sprouting cholinergic axons (primarily originating in the lesioned septo-hippocampal pathways) and adrenergic axons (primarily originating in the lesioned axons of the locus neurones) regenerate together along the deneravated Schwann cell sheaths. From a comparison between the central reinnervation process and the process of reinnervation of the iris by peripheral cholinergic axons after transplantation to the anterior eye chamber, it is concluded that the regenerative capacity of central cholinergic neurones (above all the septo-hippocampal system) is not much inferior to that of their peripheral counterparts when given similar growth conditions. Moreover, central cholinergic neurones seem partly able to replace the peripheral ones in the reinnervation of a denervated peripheral target.

Lesion-induced synaptogenesis in brain: a study of dynamic changes in neuronal membrane specializations

When incoming fibers to a given brain region are damaged and degenerate, the remaining undamaged fibers can, in some cases, form new synapses, and restore physiologically functional circuitry. Synaptic membrane events underlie this reconstruction: the connection between membranes is broken and reformed.

Time-dependent changes in commissural field potentials in the dentate gyrus following lesions of the entorhinal cortex in adult rats

Previous neuroanatomical work has shown that lesions of the entorhinal cortex in adult rats cause the commissural projections to spread from their normally restricted locus in the inner molecular layer approximately 40-50 mum into the outer molecular layer (that is, into the zone deafferented by the lesion). In the present study we measured the effects of the entorhinal lesion on the distribution of short-latency potentials elicited by commissural stimulation in the molecular layer. Studies with animals tested at various times after the lesion and with a preparation that permitted recording from the same rat at several post-lesion intervals both indicated that the commissural response spread 100-150 mum towards the deafferented outer molecular layer, while the maximum response spread 50-100 mum. These effects were first detectable by 9 days after the lesion and were fully developed by 15 days post-lesion. These findings suggest that the growth of the commissural system seen after entorhinal lesions results in the rapid formation of functional terminals and are discussed in relationship to the behavioral consequences of brain lesions.

Neuroplasticity of primary afferents in the neo-natal cat and some results of early deafferentation of the trigeminal spinal nucleus

The plasticity of cervical primary afferents has been investigated in young adult cats in which the trigeminal root, together with most of the ganglion, had been excised via a new approach to these structures during the first week after birth. Once degeneration debris had disappeared, bilateral ganglionectomies of the upper three cervical dorsal roots were done in five animals and the IXth and Xth roots transected on the side of the chronic trigeminal denervation in one instance; the degeneration pattern on the chronically denervated side was compared to that on the normal side. There was mild evidence of increased degeneration of the cervical afferents in the C1 segment and in the medulla (subnucleus interpolaris of V) on the chronically denervated side. The proliferation was more apparent in the kitten operated at three days of age and was only vestigial in animals operated at six or seven days; it had no appreciable tendency to extend into the large contiguous pool of denervated trigeminal neurons. There was no evidence of sprouting of the IXth and Xth nerve afferents. These observations indicate that primary afferents of the upper cervical roots and of the glossopharyngeal and vagus nerves have little neuroplastic potential even at early stages. The literature on neuroplasticity is controversial and a brief review of this together with suggestions regarding the reasons for some of these conflicts is presented. Some structural and functional effects of chronic trigeminal denervation are briefly described.

Hippocampal conditioned responses in the absence of entorhinal input

Hippocampal unit responses to conditioned stimuli were analyzed in intact rats and in rats deprived of the input from the entorhinal cortex to the hippocampus. Cells in normal rats responded more intensely to a conditioned stimulus when it was applied within 1 min from a previous one. The rats with lesions had smaller conditioned hippocampal responses and, in addition, there were no differences between responses to stimuli applied after short or long intertrial intervals. No effects on gross conditioned behavior were noticed.

Crossed pathways from the entorhinal area to the fascia dentata. II. Provokable in rats

In the rat thhe perforant pathways from the entorhinal area normally innervate the fascia dentata only ipsilaterally. However, unilateral ablation of the entorhinal area (deentorhination) induces the formation of an anomalous crossed projection from the intact contralateral entorhinal area to the septal portion of the deafferented fascia dentata. After deentorhination of rats aged 1-30 days the organization of this projection was analyzed (a) by producing secondary lesions in the intact entorhinal area of perforant paths and observing the results anterograde degeneration with Fink-Heimer silver impregnation techniques, and (b) by staining with Timm's sulfide silver method whichmakes the terminal fields of afferent systems stand out in different tones of colors. Both methods showed the crossed entorhino-dentate projection to consist of two separable components. They were named the crossed medial perforant path and the crossed lateral perforant path, corresponding to their similarity in origin, dendritic localization of termination and Timm stainability to the ordinary, uncrossed medial and the lateral perforant pathways (MPP and LPP) which arise in the medial and lateral parts of the entorhinal cortex, respectively. Similarly induced crossed projections were demonstrated to the subcallosal continuation of fascia dentata, the fasciola cinerea. The heaviest terminal field of the crossed entorhino-dentate projection which was found in the most rostral and medial parts of the deafferented fascia dentata correlated with a lack of expected aberrant extension into theMPP and LPP terminal zones of commissural and ipsilateral hippocampodentate fibers. In Fink-Heimer preparations there was little variation in the distribution of the aberrant crossed sustems over the range of ages studied although the chronic operations performed earliest postnatally (5 days) tended to produce the heaviest representation. This latter observation appeared consistent with changes in the Timm staining pattern of the deafferented fascia dentata, since with an increase in age at the primary lesion from 5 to 14 days there was no increase in the spread into the fascia dentata of Timm stainable axon ter minals from CA3, interpreted as a sign of fewer crossed entorhinal afferents succeeding in a presumable competition with the CA3-derived system for available terminal space.

An autoradiographic study of the time of origin and the pattern of granule cell migration in the dentate gyrus of the rat

The dentate gyrus of the rat contains about 600,000 granule cells. These small neurons are generated over a prolonged period from the 14th day of gestation until some time after the second postnatal week. The majority of the cells pass through their last phase of DNA synthesis in the postnatal period, and during the peak period of cell generation, between the fifth and seventh days after birth, up to 50,000 granule cells are formed each day. Contrary to earlier reports, most of the cells pass through their last mitotic division either within the stratum granulosum itself, or within the hilar region of the developing gyrus. The precursor population of cells in the hilar region must therefore constitute a pool of true neuroblasts. The origin of this pool of cells has not been definitely established but it seems probable that its cells are derived from the neuroepithelium lining the lateral ventricle adjacent to the region from which the hippocampal pyramidal cells are generated. Examination of the final location of granule cells labeled at different stages reveals three distinct morphogenetic gradients in the gyrus. The cells in the dorsal blade tend to be formed earlier than those in the ventral blade; cells in the more caudal (or temporal) portions of the gyrus are generated earlier than those in more rostral (or septal) regions; and in all regions the more superficial neurons in the stratum granulosum are formed earlier than the deeper granule cells. The bearing of some of these findings on the development and organization of the connections of the dentate gyrus is discussed.

Neuronal sprouting after hippocampal lesions

It appears that aberrant but functional circuitry can rapidly develop after removal of particular afferent projections. The parameters describing this process appear to vary according to (1) the age at which the experimental manipulation was performed, (2) the specific afferent being studied, and (3) the particular input which is eliminated or possibly the brain region which is deafferented. It must also be emphasized that abnormal growth is not always found after lesions even in situations which a priori appear appropriate. Hopefully, these anatomical and physiological results obtained in a relatively "simple" brain system will be of help in evaluating the role played by abnormal wiring in the development of behavioral deficits.

Heritable aspects of anomalous myelinated fibre tracts in the forebrain of the laboratory mouse

Serial coronal or sagittal sections were stained for myelin and examined in 6 inbred, 4 hybrid, and 2 outbred mouse strains. Absent corpus callosum was seen only in BALB/cJ as reported by Wimer, but a wide range of the size of corpus callosum was also noted. The action of a major gene was not evident in backcross or F2 generations; polygenic and perhaps epistatic inheritance was indicated. In A/J, and to a lesser extent A/HeJ and BALB/cJ, the columns of fornix frequently collided with the anterior commissure and either passed around it to make a normal termination or deflected dorsally to make an abnormal termination in lateral septum. In some BALB/cJ brains the anterior commissure instead was displaced and passed behind or through the columns of fornix. Backcross and F2 data suggested inheritance was polygenic and that genetic variation affected the spatio-temporal coordination of ontogeny of the two tracts. Finally, unusual longitudinal bundles were detected in the septal region of BALB/cJ. Results of crosses were consistent with the hypothesis that a single, incompletely dominant gene was acting, but further study of both the anatomy and heredity of the defect was deemed necessary.

Synaptic rearrangement in the dentate gyrus: histochemical evidence of adjustments after lesions in immature and adult rats

In immature animals, ablation of the entorhinal cortex elicited a rapid intensification of acetylcholinesterase (EC 3.1.1.7) staining in the outer one-quarter of the molecular layer of the dentate gyrus. Subsequent lesions of the septum eliminated this acetylcholinesterase intensification. Electron-microscopic histochemical analysis demonstrated a 30-fold increase in the number of acetylcholinesterase-positive synaptic endings in the intensification zone. The acetylcholinesterase augmentation thus appears attributable, in part at least, to an increase in the number of acetylcholinesterase-rich synaptic endings established by septo-hippocampal fibers. Observations in a comparative study of immature and adult rats point to the animal's developmental state as a major determinant of differences in these lesion-induced neuronal adjustments.