Observations on the development of certain ascending inputs to the thalamus in rats. I. Postnatal development

A Systematic Review of Direct Outputs from the Cerebellum to the Brainstem and Diencephalon in Mammals

The cerebellum is involved in many motor, autonomic and cognitive functions, and new tasks that have a cerebellar contribution are discovered on a regular basis. Simultaneously, our insight into the functional compartmentalization of the cerebellum has markedly improved. Additionally, studies on cerebellar output pathways have seen a renaissance due to the development of viral tracing techniques. To create an overview of the current state of our understanding of cerebellar efferents, we undertook a systematic review of all studies on monosynaptic projections from the cerebellum to the brainstem and the diencephalon in mammals. This revealed that important projections from the cerebellum, to the motor nuclei, cerebral cortex, and basal ganglia, are predominantly di- or polysynaptic, rather than monosynaptic. Strikingly, most target areas receive cerebellar input from all three cerebellar nuclei, showing a convergence of cerebellar information at the output level. Overall, there appeared to be a large level of agreement between studies on different species as well as on the use of different types of neural tracers, making the emerging picture of the cerebellar output areas a solid one. Finally, we discuss how this cerebellar output network is affected by a range of diseases and syndromes, with also non-cerebellar diseases having impact on cerebellar output areas.

Serotonin immunoreactivity in auditory brainstem neurons of the postnatal monoamine oxidase-A knockout mouse

Altered levels of extracellular serotonin (5-HT) during development cause structural abnormalities in the neural projections of sensory systems. To better understand the potential role of 5-HT in the development of auditory system projections, we examined 5-HT immunoreactivity (IR) in auditory brainstem nuclei of postnatal mice. We previously observed 5-HT-IR in the lateral superior olive (LSO) of wild type mice. In the current study, we used a genetic model (monoamine oxidase-A knockout mouse) in which brain 5-HT levels are abnormally high to improve detection of 5-HT. In the cochlear nucleus of this knockout, 5-HT-IR cell bodies were observed in the dorsal cochlear nucleus (DCN), a primary relay to the inferior colliculus (IC). In the superior olivary complex, 5-HT-IR somata were observed in the LSO, another relay to the IC. Labeled somata were also observed within the IC itself. The 5-HT immunostaining in all 3 regions was transient and was not observed beyond postnatal day 8. These results suggest that 5-HT may play a role in the branching and refinement of DCN and LSO axon collaterals within the IC, as well as IC axon collaterals within the medial geniculate body. The pattern of expression indicates that 5-HT has a developmental role in select populations of neurons of the ascending auditory pathway prior to any influences of sound-evoked activity.

Changes of 2-deoxyglucose uptake in the rat auditory pathway after bilateral ablation of the cochlea

It has been reported that the area of decreased glucose metabolism in the FDG-PET of prelingually deaf children correlates significantly with speech performance after cochlear implantation. In this study, we undertook to confirm changes of glucose metabolism in the cerebral cortex using an animal model with age-matching groups to completely exclude the influence of age differences between the deaf and normal-hearing groups. The cochlea was ablated bilaterally at a postnatal 10-14 days in the deaf groups; 3-4 deaf and normal rats were included at each time point at 1, 2, 4 and 8 weeks and 7 months after ablation. After injecting 2-deoxyglucose intraperitoneally, digitalized autoradiographic images were obtained, and analyzed by using two different methods; 3-dimensional voxel-wise statistical analysis and conventional 2-dimensional densitometry. The hypometabolic area analyzed using 3-dimensional analysis and the differences of optical density between normal and deaf as determined by densitometry were widest and most prominent between 4 and 8 weeks after ablation. Differences were not significant before 2 weeks or after 7 months after ablation. This result shows that the hypometabolic area becomes prominent after a critical period and it decreases as the duration of deafness increases. We believe that cross-modal plasticity may be the mechanism of changes in glucose metabolism and that this result reinforced the usefulness of evaluating hypometabolic area using FDG-PET in deaf children.

Cross-modal neuroplasticity in neonatally enucleated hamsters: structure, electrophysiology and behaviour

Potential auditory compensation in neonatally bilaterally enucleated Syrian hamsters was explored anatomically, electrophysiologically and behaviourally. Gross morphology of the visual cortex appeared normal and no obvious cytoarchitectural malformation was discerned. However, enucleation induced a significant increase in the spontaneous firing rate of visual cortex cells. Further, auditory stimuli elicited field potentials and single unit responses in the visual cortex of enucleated, but not normal, animals. About 63% of the cells isolated in the visual cortex of 16 enucleated hamsters responded to at least one type of auditory stimulus. Most of the responses were less vigorous and less time-locked than those of auditory cortex cells, and thresholds were typically higher. Projection tracing with WGA-HRP disclosed reciprocal connections between the visual cortex and the dorsal lateral geniculate nucleus in both intact and enucleated animals. However, in the enucleated animals retrogradely labelled cells were also found in the inferior colliculus, the major midbrain auditory nucleus. Behaviourally determined auditory sensitivity across the hearing range did not differ between enucleated and intact hamsters. Minimum audible angle, as determined by a conditioned suppression task, ranged from around 17 to 22 degrees, with no significant difference between normal and enucleated animals. The two groups also did not differ with regard to the direction of their unconditioned head orientating response to intermittent noise. However, the enucleated animals showed a more vigorous response and were slower to habituate to the noise. These results show that bilateral enucleation of newborn hamsters results in auditory activation of visual targets, in addition to the typical activation of the intact auditory pathway. Behaviourally it appears that enucleated hamsters, compared with their normal littermates, are slower to habituate in their response to an unexpected source of sound.

Deafferentation-induced apoptosis of neurons in thalamic somatosensory nuclei of the newborn rat: critical period and rescue from cell death by peripherally applied neurotrophins

This study shows that unilateral transection of the infraorbital nerve (ION) in newborn (P0) rats induces apoptosis in the contralateral ventrobasal thalamic (VB) complex, as evidenced by terminal transferase-mediated deoxyuridine triphosphate-biotin nick end labelling (TUNEL) and electron miscroscopy. Double-labelling experiments using retrograde transport of labelled microspheres injected into the barrel cortex, followed by TUNEL staining, show that TUNEL-positive cells are thalamocortical neurons. The number of TUNEL-positive cells had begun to increase by 24 h postlesion, increased further 48 h after nerve section, and decreased to control levels after 120 h. Lesion-induced apoptosis in the VB complex is less pronounced if ION section is performed at P4, and disappears if the lesion is performed at P7. This time course closely matches the critical period of lesion-induced plasticity in the barrel cortex. Nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF), applied on the ION stump alone or in combination, are able to partially rescue thalamic neurons from apoptosis. Total cell counts in the VB complex of P7 animals that underwent ION section at P0 confirm the rescuing effect of BDNF and NGF. Blockade of axonal transport in the ION mimics the effect of ION section. These data suggest that survival-promoting signals from the periphery, maybe neurotrophins, are required for the survival of higher-order neurons in the somatosensory system during the period of fine-tuning of neuronal connections. We also propose that anterograde transneuronal degeneration in the neonatal rat trigeminal system may represent a new animal model for studying the pathways of programmed cell death in vivo.

Developmentally regulated expression of Thy-1 in structures of the mouse sensory-motor system

Thy-1 is a cell-surface molecule of the immunoglobulin superfamily which is expressed at high levels in the mature nervous system. Thy-1 has been implicated in regulating axonal outgrowth and synaptic function, but little is known regarding its cellular localization and expression in the central nervous system (CNS) during development or in adulthood. In this study, Thy-1 gene expression and protein localization were examined in sensory-motor and related areas of the adult and postnatally developing mouse CNS. Thy-1 mRNA expression was restricted to neurons; immunoreactivity was densely distributed throughout the neuropil of all regions examined, often delineated the neuronal plasmalemma, and labeled axons in white matter tracts of the brain and spinal cord. In adulthood, immunolabeling was regionally widespread and was present relatively homogeneously throughout all cell-dense layers of sensory-motor cortex, throughout most thalamic nuclei, globus pallidus, and spinal cord. Developmentally, however, Thy-1 expression and localization exhibited a spatially and temporally staggered sequence leading to the adult pattern. In sensory-motor cortex, Thy-1 expression in layer V preceded expression in other layers; in the barrel field, labeling of barrel septa preceeded a gradually increasing intensity of immunolabeling of barrel centers; in the thalamus, Thy-1 exhibited a differential onset and temporal pattern of expression across different nuclei associated with motor, sensory, or limbic systems; in the caudate nucleus, Thy-1 expression was greatest during the first postnatal week of life before declining during subsequent development. Taken together, the adult distribution and developmental patterns leading to it form a unique profile in comparison with other structurally related glycosyl-phosphatidylinositol (GPI)-anchored neural cell adhesion molecules. The pattern and timing of Thy-1 expression across layers and nuclei during early postnatal development are more complex than previously recognized, thus perhaps reflecting varied roles for Thy-1 in aspects of structural or functional maturation which proceed independently of the timing of neurogenesis, migration, and dendritic and axonal growth.

Cross-modal plasticity of the corticothalamic circuits in rats enucleated on the first postnatal day

Reorganization of the reciprocal corticothalamic connections was studied as a possible anatomical substrate of the cross-modal compensation of the missing visual input of the visual cortex by somatosensory-evoked activities in neonatally enucleated rats. The use of quantitative retrograde tract-tracing techniques revealed that the contribution of the lateral posterior thalamic nucleus (LP) is significantly increased following enucleation, while that of the dorsolateral geniculate and the lateral dorsal nuclei is decreased in the thalamocortical afferentation of a region in visual cortical area 17. In contrast with the control rats, a dense terminal arborization of afferents was labelled in the LP after the injection of anterograde tracer into the barrel cortex of the enucleated rats. The injection of anterograde tracer into the visual cortex also demonstrated a massive afferentation into the LP of the enucleated rats. Visual and somatosensory corticothalamic afferents exhibited similar ultrastructural features in the LP after enucleation, but their synaptic organizations differed as regards the diameter of the postsynaptic dendrites. Taken together with the previous observations, these results suggest a central role for the LP in the transmission of the somatosensory-evoked activities to the visual cortex after early blindness.

Morphological development of afferent segregation and onset of synaptic transmission in the trigeminothalamic pathway of the wallaby (Macropus eugenii)

A light and electron microscopic study has been made of the time of formation of whisker-related patterns in trigeminothalamic afferents and the onset of synapse formation between afferents and cells in the ventroposteromedial nucleus (VPM) of the marsupial mammal, the wallaby, by labelling afferents with a carbocyanine dye. A parallel in vitro study was made of the functional development of the trigeminothalamic pathway to the VPM. Evoked synaptic responses could be recorded in the VPM from the time that afferents first reached the VPM at postnatal day 15 (P15). At all stages, the excitatory response comprised both N-methyl-D-aspartate- and non-N-methyl-D-aspartate-mediated components. At P40, the response decreased markedly in duration, coinciding with the onset of synaptogenesis. This implies that transmission is occurring prior to synapse formation, probably through transmitter release from growth cones. At P50, synaptic responses became dominated by a fast, non-N-methyl-D-aspartate potential, and this coincided with the first appearance of whisker-related patterns in the VPM. A gamma-aminobutyric acid (subtype A)-mediated, inhibitory component was also present from the time of afferent arrival. These findings support the idea that functional interactions between afferents and their targets may play a role in pattern formation in the somatosensory thalamus.

Postnatal cytoarchitecture of the rat medial geniculate body

The medial geniculate body (MGB) is a thalamic structure that provides vital information flow to the forebrain for complex acoustic processing. The development of cytoarchitectural features of the MGB was examined in rat to identify age-related patterns of growth in major geniculate compartments that have been described previously (Clerici and Coleman [1990] J. Comp. Neurol. 297:14-31; Clerici et al. [1990] J. Comp. Neurol. 297:32-54): the ventral (MGv), dorsal (MGd), and medial (MGm) divisions. Results show that, on the day of parturition, all major nuclei of each division are characterized, including the ovoid (OV) and ventral (LV) nuclei of MGv; the dorsal, deep dorsal (DD), caudodorsal, limitans, and suprageniculate nuclei of MGd; and the MGm. The MGv and MGd, which display comparable areas at birth, show rapid growth to postnatal day 7 (PND7), which then slows until PND11, around the time of ear canal opening; subsequently, MGv accelerates growth to reach larger adult size. From PND11 to PND16, thionin facilitates parcellation by extensive staining of dendritic processes of MGd, MGm, and lateral posterior nucleus neurons but not neurons of the MGv or the dorsal lateral geniculate nucleus. Golgi stains after birth reveal restricted dendritic arborizations in MGv cells and dichotomous branching patterns of MGd neurons. Somal size in MGB increases dramatically subsequent to afferent innervation and again following onset of auditory function. Somal growth occurs between all postnatal age groups tested for OV, LV, and DD nuclei, although LV segments related to high and low frequencies do not differ. Cell packing density predicts the expanse of major MGB divisions better than somal size. These results demonstrate the integrity and growth patterns of MGB nuclei and divisions from nascence and provide a substrate for subsequent study of anatomical and physiological development of the MGB.

Afferent arrival and onset of functional activity in the trigeminothalamic pathway of the rat

In this study, a novel in vitro slice preparation has been used to study the anatomical and physiological development of the trigeminothalamic pathway in the prenatal and neonatal rat. Anterograde tracing studies showed that the most rostral trigeminal fibres had reached the cephalic flexure by embryonic day (E)15, and entered the diencephalon by E16. By E17 the first few fibres had reached the ventroposteromedial thalamic nucleus (VPM) where they terminated in growth cones. The projection was more substantial and fibres had begun branching by E18, and arbors were more elaborate by E19. The fibres densely filled the nucleus by the day of birth (PO). The physiological studies showed that postsynaptic responses to stimulation of the trigeminal nerve or principal sensory nucleus (Pr5) could first be recorded at E17. Reliable responses to stimulation of either the nerve or Pr5 were recorded from E18 on. Stimulation of Pr5 enabled both axonal and synaptic signals to recorded in VPM. A GABAergic influence was acting to decrease the overall level of excitability in the thalamus from E18. In prenatal animals, the excitatory response was primarily mediated by NMDA receptors, and by P1 a non-NMDA mediated component was beginning to appear. These results demonstrate that the capacity for axonal conduction in the trigeminothalamic fibres and synaptic transmission in the thalamus are present from the time that anatomical connections are first established.

Postnatal development of GABA-immunoreactive terminals in the reticular and ventrobasal nuclei of the rat thalamus: a light and electron microscopic study

The postnatal development of inhibitory GABAergic circuits in the thalamic reticular and ventrobasal nuclei was studied in rats ranging from the day of birth to the end of the third postnatal week by means of a postembedding immunogold staining procedure to visualize GABA. In the reticular nucleus, GABA labeling was present from birth in cell bodies, dendrites, growth cones and a few synaptic terminals, whereas in the ventrobasal nucleus it was exclusively in axonal processes identifiable as growth cones, vesicle-rich profiles and synaptic terminals. In both nuclei, GABA-labeled synaptic terminals were, however, very scarce and immature in neonatal animals and they became numerous and morphologically mature only after the end of the second postnatal week. These findings suggest that inhibitory synaptic responses in the somatosensory thalamus are not yet fully mature throughout the first two postnatal weeks and support the hypothesis that GABA may initially play trophic roles. The relatively late maturation of the thalamic GABAergic system may have important functional consequences, as the reticulothalamic circuits are responsible for the generation of spindle wave oscillations whose cellular mechanisms are also involved in the generation of spike-and-wave (absence) seizures in humans and in animal models.

Branching pattern of corticothalamic projections from the somatosensory cortex during postnatal development in the rat

In adult animals corticothalamic (CT) axons pass through the thalamic reticular nucleus (Rt) where they give off collateral branches innervating the Rt neurons. The postnatal development of CT projections from the somatosensory cortex, with particular reference to the branching pattern within Rt, ventrobasal (VB) and posterior (PO) nuclei, was investigated in the rat with anterograde tracing. Biotinylated dextran-amine (BDA) was iontophoretically injected into the somatosensory cortex of rats ranging from postnatal day (P) 0 to P30. At P1 most of the cortical axons traversed unbranched Rt and terminates in VB and PO, whereas at P3 they formed rudimentary branches in these nuclei. From P6 to P9 a progressive increase in the amount of dense clusters of terminal arborizations was evident in Rt, and by the second postnatal week more complex arborizations with a clear topographic arrangement were observed in Rt, VB and PO. Our findings indicate that CT fibers show a quantitative increase both in R1 and in somatosensory thalamic nuclei during the first postnatal week, although their terminal arborizations are however still incomplete. The pattern of collateralization of CT projections achieves an adult configuration at the end of the second postnatal week.

Expression of neuronal acetylcholine nicotinic receptor alpha 4 and beta 2 subunits during postnatal development of the rat brain

The expression of the alpha 4 and beta 2 subunits of neuronal nicotinic acetylcholine receptors (nAChRs) was studied in developing rat brain using in situ hybridization. The levels of both transcripts were already high at birth in cerebral cortex, medial habenula, CA1/CA3 regions of the hippocampus and several thalamic nuclei. In general, the beta 2 subunit showed a higher density of hybrids than the alpha 4. Beta 2 expression did not change with age in the medial habenula, medial geniculate nucleus or in the hippocampus whereas it decreased in the cortex. The developmental pattern of the hybridization signal for alpha 4 was different according to the brain area considered. The expression of the two transcripts showed a biphasic pattern in some thalamic nuclei: the lowest levels occurring during the first and second postnatal weeks respectively, and the highest levels during the second and fourth postnatal weeks. The ontogenetic profile of the expression of the alpha 4 subunit in the thalamic nuclei coincided with that of [3H]-L-nicotine binding sites. These findings suggest that the two subunits of nAChRs are independently regulated in most of the brain areas examined, and that in some regions, such as the thalamus, the ontogenetic variations reported for the alpha 4 subunit correlate with those observed for the [3H]-L-nicotine binding sites.

Postnatal ontogeny of GABAB binding in rat brain

The postnatal development of GABAB binding sites in rat brain was studied by quantitative receptor autoradiography using [3H]GABA under selective conditions. Binding levels peak at regionally specific times during the first three weeks of life and then decline to adult levels. GABAB binding peaked in the globus pallidus, vestibular and spinal trigeminal nuclei, and the CA3 region of the hippocampus at postnatal day 3; in the striatum, nucleus accumbens, inferior olive, septum, dentate gyrus and CA1 region of the hippocampus at postnatal day 7; in the neocortex and thalamus at postnatal day 14; and in the medial geniculate at postnatal day 21. Following these regionally specific peaks, binding decreased to postnatal day 28 levels. Further significant decreases in binding were observed in all regions examined between postnatal day 28 and adulthood. Comparisons of binding site pharmacology reveal equipotent displacement of GABAB binding by several competitive agonists and antagonists in postnatal day 7 and adult rat brain, indicating that immature and adult binding sites have similar pharmacological properties with regard to these compounds. The GABAB receptor antagonist CGP 54626A, however, inhibited binding more potently in the postnatal day 7 thalamus and neocortex than in these areas in the adult brain. The guanyl nucleotide analogue guanosine 5'-O-(3-thiotriphasphate) inhibited GABAB binding extensively in both postnatal day 7 and adult brain. The non-competitive antagonist zinc also inhibited GABAB binding at both ages and was more potent in postnatal day 7 brain than in adult brain. Saturation analyses reveal two binding sites with similar affinities in both immature and adult rat brain, indicating that postnatal modulation of GABAB binding reflects changes in binding site density rather than modulation of binding site affinity. While immature GABAB binding sites share most pharmacological characteristics with adult binding sites and appear to be coupled to G-proteins at an early age, their interactions with zinc and CGP 54626A suggest that GABAB binding sites in immature brain may have a distinct pharmacological profile. Our data suggest significant regional and pharmacological changes in GABAB binding during development. The implications of these findings are discussed with regards to a possible role of GABAB receptors in the development of the central nervous system.

Transient increase in expression of GAD65 and GAD67 mRNAs during postnatal development of rat spinal cord

Gamma-aminobutyric acid (GABA) is thought to be one of the classic neurotransmitters acting as a developmental signal. To understand the role for GABA in development, we investigated the expression of transcripts encoding two forms of the GABA-synthesizing enzyme glutamate decarboxylase (GAD65 and GAD67) in the cervical enlargement of the rat spinal cord at successive postnatal days--P0, P7, P14, P21, and P90 (adult)--by using in situ hybridization histochemistry. Cells hybridized with two oligonucleotide probes designed to detect GAD65 and GAD67 mRNAs were widely distributed in all laminae, except in motoneurons of the spinal cord. The integrated densities of hybridization signals were measured across all layers of the gray matter. The relative number of GAD mRNA-labeled cells was determined within each of four regions: laminae I-III, laminae IV-VI, laminae VII and VIII, and lamina X. There was a transient increase in both the integrated density and the relative number of hybridized cells between P7 and P14, after which there was a marked decline to adult levels (lowest). An overall decrease in the number of GAD mRNA-labeled cells was evident in all layers, but a dramatic drop occurred in a subpopulation of cells within ventral portions of the spinal cord. The distribution patterns and postnatal changes in expression of the mRNAs encoding GAD65 and GAD67 were similar and closely paralleled reported changes in the abundance of GAD65 and GAD67 proteins and their product, GABA. Transient increases in GAD mRNA expression during the early postnatal period coincide with, and may be linked to, synapse formation and synapse elimination of the developing spinal cord.

Localization of phosphatase inhibitor-1 mRNA in the developing and adult rat brain in comparison with that of protein phosphatase-1 mRNAs

The localization and ontogenic changes in the gene expression for phosphatase inhibitor-1 (I-1) were analyzed by in situ hybridization histochemistry, and they were compared with those for three catalytic subunits of protein phosphatase type 1 (PP-1). At the adult stage, intense expression signals for I-1 were detected in the hippocampal formation, piriform cortex, claustrum, dorsal endopiriform cortex, suprachiasmatic nucleus, choroid plexus, arachnoid membrane, and pineal body. Moderate expression signals for I-1 were observed in the olfactory neuronal layers, caudate putamen, layers II-IV, and VI of the neocortex, and cerebellar granule cells, whereas the expression levels were low in the thalamus, cerebellar Purkinje cells, and brain stem nuclei. Although the expression levels for the three PP-1 mRNAs varied notably in various brain regions, a relatively high and parallel expression of I-1 and PP-1 mRNAs was found in most regions of the forebrain. However, the dissociation in the expression levels between I-1 and PP-1 mRNAs was found in several loci: the laminar expression of I-1 mRNA versus the homogeneous expression of PP-1 mRNAs in the cerebral cortex; low levels of expression of I-1 mRNA versus relatively high expression of PP-1 mRNAs in the brain stem nuclei; high expression of I-1 mRNA in the arachnoid membrane versus low expression of PP-1 mRNAs in it. The unparallel expression was also seen in embryonic brain: No significant expression of I-1 mRNA versus substantial expression of PP-1 mRNAs in the ventricular zone and cerebellar external granular layer; transiently high expression of I-1 mRNA in developing thalamus versus constantly moderate to low expression of PP-1 mRNAs there. These findings suggest that I-1 may play some discrete roles independent of the regulation of PP-1 in certain regions and developing stages of the brain.

Early development of dorsal column-medial lemniscal projections in the clawed toad, Xenopus laevis

In Xenopus laevis fluorescent dextran amines were applied to study the development of the dorsal column-medial lemniscal projection: rhodamine dextran amine was applied at the mesodiencephalic border to retrogradely label the cells of origin of the medial lemniscus in the dorsal column nucleus (DCN); fluorescein dextran amine to the spinal cord to anterogradely label the primary afferent projections to the DCN. The first mesodiencephalic projections were found at stage 51, i.e. almost immediately after spinal afferent fibers had reached the DCN.

Development of the rat septohippocampal projection: tracing with DiI and electron microscopy of identified growth cones

The factors determining the development of specific fiber tracts in the central nervous system as well as the interactions of growth cones with the surrounding micromilieu are largely unknown. Here we investigated the ontogenetic development of the septohippocampal projection in the rat with the lipophilic carbocyanine dye DiI which is transported anterogradely and retrogradely in neurons and can be applied to fixed embryonic tissue. Photoconversion of anterogradely labeled fibers allowed us to study individual growth cones by electron microscopy. The first axons originating from the septal complex were found in the hippocampus as early as on embryonic day (ED) 19, reaching the fimbrial pole of the hippocampus on ED 18. However, on ED 17 we consistently found retrogradely labeled cells in the hippocampus, indicating that the development of the hippocamposeptal projection precedes that of the septohippocampal projection. On ED 19, the majority of the axons directed toward the hippocampal formation passed the hippocampus and grew further into the subicular complex and entorhinal cortex. These axons gave off collaterals that invaded the hippocampus proper. A fairly adult pattern of the septohippocampal projection was reached on postnatal day 10, although may growth cones were still found. A comparative analysis of individual growth cones found in the fimbria and the hippocampus proper revealed no striking differences in their morphology. Electron microscopic analysis showed that growth cones in the fimbria were mainly contacted by other axons, whereas growth cones in the hippocampus had contact with all available elements. This may indicate that growing septohippocampal fibers are guided by axons of the earlier formed hippocamposeptal projection. In the hippocampus proper, other cues, probably derived from the target itself, may guide the septohippocampal axons to their appropriate target cells.

Ontogenesis of the cerebellofugal projection in the rat

Ontogenesis of the cerebellofugal projection was studied in the rat by the tract-tracing method with WGA-HRP. The projection, forming a uniform front of compact fibre bundle tipped by growth cones, began entering the brainstem on embryonic day 17 (E17), grew rapidly and orderly with no random extension of fibers, and arrived at the most rostral part of the thalamus already by E18, distributing dense terminals to various brainstem and thalamic nuclei. The course and termination of this projection in prenatal animals was largely similar to normal adult projection although differences were found. Some projections increased postnatally, whereas some projections which were existent in embryos regressed with age and finally disappeared completely. The adult pattern of the projection was attained by 3 weeks of age. It is worth noting that the projections which appeared transiently are similar to those reported as aberrantly regenerated projections in kittens which are born in more mature state than rats and have no such projections at birth.

Changes in excitatory amino acid modulation of phosphoinositide metabolism during development

Two different developmental patterns of stimulation of phosphoinositide (PI) metabolism by excitatory amino acid (EAA) receptors were observed during the postnatal maturation of various brain regions. A 'burst' in PI metabolism was seen at postnatal day 6 (PND6) in olfactory bulb and cerebellum and at PND9 in hippocampus. In cortex and thalamus/hypothalamus high levels of PI metabolism were observed initially, and then began to decline at PND15 and PND18, respectively. NMDA inhibition of PI metabolism was generally found to parallel the EAA activation but the persistence of inhibition varied in the different brain regions.

Induction of somatic sensory inputs to the lateral geniculate nucleus in congenitally blind mice and in phenotypically normal mice

The incidence of aberrant innervation of the lateral geniculate nucleus by ascending somatic sensory axons was examined following injections of wheat germ agglutinin conjugated with horseradish peroxidase into the dorsal column nuclei of adult mice which were: (1) normal; (2) normal, but bilaterally enucleated on the day of birth; (3) normal, but received a large unilateral lesion of the rostral cortex on the day of birth; (4) normal, bilaterally enucleated, as well as unilaterally lesioned in the rostral cortex on the day of birth; (5) homozygous for an ocular retardation mutation (orj/orj); or (6) homozygous for the orj mutation and received a large unilateral lesion of the rostral cerebral cortex on the day of birth. In the phenotypically normal animals which were untreated, no somatic sensory inputs enter into the dorsal lateral geniculate nucleus. A few labeled axons enter into and arborize within the dorsal lateral geniculate nucleus in normal animals which received bilateral enucleations or unilateral rostral cortical lesions on the day of birth. However, in congenitally blind animals and in phenotypically normal animals which received bilateral enucleations as well as unilateral rostral cortical lesions on the day of birth, a significant number of labeled axons enter into and arborize within the dorsal lateral geniculate nucleus. Among all these experimental groups, the densest innervation of the lateral geniculate nucleus occurred in congenitally blind animals which received rostral cortical lesions on the day of birth. In these, robust arborizations of labeled somatic sensory axons occupy a substantial extent of the lateral geniculate nucleus. These results not only demonstrate that ascending somatic sensory axons can be rerouted to the lateral geniculate nucleus, but also indicate that the ability of a thalamic afferent pathway to undergo extensive reorganization and to innervate inappropriate thalamic targets following early perturbations is not unique to the retinal projection (in which this has previously been demonstrated), and may be a more general characteristic of the major thalamic afferent systems.

Auditory pathway and auditory activation of primary visual targets in the blind mole rat (Spalax ehrenbergi): I. 2-deoxyglucose study of subcortical centers

The blind mole rat Spalax ehrenbergi is a subterranean rodent that shows striking behavioral, structural, and physiological adaptations to fossorial life including highly degenerated eyes and optic nerves and a behavioral audiogram that indicates high specialization for low-frequency hearing. A 2-deoxyglucose functional mapping of acoustically activated structures, in conjunction with Nissl/Klüver-Barrera-stained material, revealed a typical mammalian auditory pathway with some indications for specialized low-frequency hearing such as a poorly differentiated lateral nucleus and a well-developed medial nucleus in the superior olive complex. The most striking finding was a marked 2-deoxyglucose labeling of the dorsal lateral geniculate body and of cortical regions that correspond to visual areas in sighted rodents. The results render the blind mole rat a good model system for studying natural neural plasticity and intermodal compensation. In this report, we confine ourselves to the subcortical levels. The cortical level will be dealt comprehensively in a following paper.