Effects of neonatal monocular and binocular enucleation on transient acetylcholinesterase activity in developing rat visual cortex

Evolution in Neuromodulation-The Differential Roles of Acetylcholine in Higher Order Association vs. Primary Visual Cortices

This review contrasts the neuromodulatory influences of acetylcholine (ACh) on the relatively conserved primary visual cortex (V1), compared to the newly evolved dorsolateral prefrontal association cortex (dlPFC). ACh is critical both for proper circuit development and organization, and for optimal functioning of mature systems in both cortical regions. ACh acts through both nicotinic and muscarinic receptors, which show very different expression profiles in V1 vs. dlPFC, and differing effects on neuronal firing. Cholinergic effects mediate attentional influences in V1, enhancing representation of incoming sensory stimuli. In dlPFC ACh plays a permissive role for network communication. ACh receptor expression and ACh actions in higher visual areas have an intermediate profile between V1 and dlPFC. This changing role of ACh modulation across association cortices may help to illuminate the particular susceptibility of PFC in cognitive disorders, and provide therapeutic targets to strengthen cognition.

Bilateral enucleation alters gene expression and intraneocortical connections in the mouse

Background

Anatomically and functionally distinct sensory and motor neocortical areas form during mammalian development through a process called arealization. This process is believed to be reliant on both activity-dependent and activity-independent mechanisms. Although both mechanisms are thought to function concurrently during arealization, the nature of their interaction is not understood. To examine the potential interplay of extrinsic activity-dependent mechanisms, such as sensory input, and intrinsic activity-independent mechanisms, including gene expression in mouse neocortical development, we performed bilateral enucleations in newborn mice and conducted anatomical and molecular analyses 10 days later. In this study, by surgically removing the eyes of the newborn mouse, we examined whether early enucleation would impact normal gene expression and the development of basic anatomical features such as intraneocortical connections and cortical area boundaries in the first 10 days of life, before natural eye opening. We examined the acute effects of bilateral enucleation on the lateral geniculate nucleus of the thalamus and the neocortical somatosensory-visual area boundary through detailed analyses of intraneocortical connections and gene expression of six developmentally regulated genes at postnatal day 10.

Results

Our results demonstrate short-term plasticity on postnatal day 10 resulting from the removal of the eyes at birth, with changes in nuclear size and gene expression within the lateral geniculate nucleus as well as a shift in intraneocortical connections and ephrin A5 expression at the somatosensory-visual boundary. In this report, we highlight the correlation between positional shifts in ephrin A5 expression and improper refinement of intraneocortical connections observed at the somatosensory-visual boundary in enucleates on postnatal day 10.

Conclusions

Bilateral enucleation induces a positional shift of both ephrin A5 expression and intraneocortical projections at the somatosensory-visual border in only 10 days. These changes occur prior to natural eye opening, suggesting a possible role of spontaneous retinal activity in area border formation within the neocortex. Through these analyses, we gain a deeper understanding of how extrinsic activity-dependent mechanisms, particularly input from sensory organs, are integrated with intrinsic activity-independent mechanisms to regulate neocortical arealization and plasticity.

Optic nerve transection affects development and use-dependent plasticity in neocortex of the rat: Quantitative acetylcholinesterase imaging

We investigated the effects of neonatal optic nerve transection on cortical acetylcholinesterase (AChE) activity in hooded rats during postnatal development and following behavioral manipulation after weaning. AChE reaction product was quantified on digitized images of histochemically stained sections in layer IV of primary somatic sensory, primary visual and visual association cortex. Rats with optic nerve transection were compared to sham-operated littermates. In all cortical regions of both types of animal, AChE reaction product was increased to peak 2 weeks after birth and decreased thereafter, reaching adult levels at the end of the third postnatal week. During postnatal development, reaction product in primary visual cortex was lower in rats deprived of retinal input than in sham-operated littermates and the area delineated by reaction product was smaller. However, optic nerve transection did not modify the time course of postnatal development or statistically significantly diminish adult levels of AChE activity. Behavioral manipulations after weaning statistically significantly increased enzyme activity in sham-operated rats in all cortical areas examined. Compared with cage rearing, training in a discrimination task with food reward had a greater impact than environmental enrichment. By contrast, in the rats with optic nerve transection enrichment and training resulted in statistically significantly increased AChE activity only in lateral visual association cortex. Our findings provide evidence for intra- and supramodal influences of the neonatal removal of retinal input on neural activity- and use-dependent modifications of cortical AChE activity. The laminar distribution of the AChE reaction product suggests that the observed changes in AChE activity were mainly related to cholinergic basal forebrain afferents. These afferents may facilitate the stabilization of transient connections between the somatic sensory and the visual pathway.

Retinal influences specify cortico-cortical maps by postnatal day six in rats and mice

Studies of callosal projections in striate cortex show that the retina is involved in the development of topographical connections. In normal animals callosal fibers connect retinotopically corresponding, nonmirror-symmetric cortical loci, whereas in animals bilaterally enucleated at birth, callosal fibers connect topographically mismatched, mirror-symmetric loci. Moreover, in rodents the overall pattern of visual callosal connections is adult-like by postnatal day 12 (P12). In this study we delayed the onset of retinal deafferentation in rats and mice in order to determine the period when retinal influences are critically needed for the development of retinotopically matched callosal linkages. Callosal maps were revealed by placing small injections of retrogradely and anterogradely transported tracers into different loci of lateral striate cortex. We found that the patterns of callosal linkages in rats enucleated at P12, P8, and P6 were nonmirror-symmetric, as in normally reared rats. In contrast, the patterns of linkages in rats enucleated at P4 closely resembled the mirror-symmetric pattern seen in rats enucleated at birth (P0). A similar reversal in topography (from symmetric to nonsymmetric) occurred in mice when enucleation was delayed from P4 to P6. These findings indicate that retinal input prior to P6, but not prior to P4, is sufficient for specifying normal callosal topography. Moreover, they suggest that development of retinotopically matched callosal linkages depends critically on retinal influences during a brief period between P4 and P6, when callosal connections are still very immature.

Neonatal electrolytic lesions of the basal forebrain stunt plasticity in mouse barrel field cortex

Previous studies have shown that neonatal electrolytic lesions of basal forebrain cholinergic projections in mice lead to a transient cholinergic depletion of neocortex and to permanent alterations in cortical cytoarchitecture and in cognitive performance. The present study examines whether neonatal electrolytic lesions of the basal forebrain modify neocortical plasticity. Using cytochrome oxidase histochemistry, we compared cross-sectional areas of individual barrels in the barrel field of four groups of postnatal day 8 (P8) old mice that on P1 received either (1) right electrolytic lesions of the basal forebrain, (2) left C row 1-4 whisker follicle ablations, (3) combined lesion treatments or (4) ice anesthesia only. The size of barrels in basal forebrain lesioned animals was not significantly different from controls. However, the plastic response to whisker removal was compromised in basal forebrain lesioned animals. An index of plasticity, the ratio of row D/row C areas, was reduced significantly in the combined nBM lesioned/follicle ablation group. Compared to whisker-lesioned mice, the expansion in rows B and D and the shrinkage in the lesioned row C area were diminished in the combined treatment group. The present findings correspond to those from a study of rats injected with a cholinergic immunotoxin [Cereb. Cortex 8 (1998) 63]. These results suggest that cholinergic inputs play a role in regulating plasticity as well as in the morphogenesis of mouse sensory-motor cortex.

Massive cross-modal cortical plasticity and the emergence of a new cortical area in developmentally blind mammals

In the current investigation, the neurophysiological organization of the neocortex was examined in adult animals that were bilaterally enucleated very early in life, before the retino-geniculo-cortical pathway was established. Our results indicate that some aspects of development of cortical fields are not mediated by specific sensory inputs. However, the current study also demonstrates that peripheral innervation plays a large role in the organization of the neocortex, as cortical territories normally involved in visual processing are completely captured by the auditory and somatosensory system. Thus, a large degree of phenotypic variability in cortical organization can be accomplished solely by removing or modifying sensory inputs.

Reduction of transiently expressed acetylcholinesterase activity in developing thalamocortical projections does not affect the mature pattern of basal forebrain projections to visual cortex

Experiments tested the hypothesis that acetylcholinesterase (AChE) activity, expressed transiently in developing thalamocortical projections, serves to limit the growth of basal forebrain cholinergic projections into thalamocortical recipient zones. Newborn rats were subjected to enucleation, a procedure that eliminates transient AChE activity in developing visual cortex. After 3-8 weeks survival, AChE histochemical techniques revealed no alteration in the pattern of AChE positive basal forebrain axons in visual cortex. These data indicate that transient AChE activity in developing sensory cortex does not limit ingrowth of basal forebrain cholinergic axons.

Neonatal infraorbital nerve transection and blockade of axoplasmic transport reduce expression of acetylcholinesterase by thalamocortical axons

Acetylcholinesterase (AChE) is transiently expressed by ventrobasal thalamic neurons and their axons in the primary somatosensory cortex and this enzyme has been used as a marker for these axons in studies concerned with cortical development and plasticity. The present experiment evaluated the effects of both peripheral nerve transection and blockade of axoplasmic transport upon the expression of AChE in thalamic axon terminals in the primary somatosensory cortex. Both manipulations resulted in marked reductions in the density of AChE in thalamocortical axons and less dramatic decreases in the density of this enzyme in thalamic neurons. These results indicate that AChE may not invariably provide unequivocal information about the distribution of somatosensory thalamocortical axon terminals in developing rodents.

Neonatal monocular enucleation-induced cross-modal effects observed in the cortex of adult rat

The cortices of neonatally enucleated rats were explored for somatosensory responses with special reference to an extension into the occipital cortex. Monocular enucleation was performed on rats at birth. The animals were raised and from the age of three months the activity evoked by either electric stimulation of the vibrissa pad or bending of the vibrissae was tested in the contralateral cortex by electric recording and autoradiography. It was found that early enucleation caused an expansion of the somatosensory responses, among others into the visual area. Neurons responsive to visual and somatosensory stimuli were demonstrated in the anterior part of the primary and secondary visual areas, contralateral to the enucleation. Electrophysiological and autoradiographic studies unambiguously proved that early enucleation exerted a significant cross-modal effect on the somatosensory responsive area.

Development of geniculocortical projections to visual cortex in rat: evidence early ingrowth and synaptogenesis

Anterograde movement of DiI and transneuronal transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were used to study the temporal and laminar patterns of ingrowth of the geniculocortical projection to visual cortex in fetal and postnatal rats. The development of this projection was compared to patterns of migration and settling of [3H]-thymidine-labeled neurons destined for cortical layer IV, and to geniculocortical synapse formation. DiI-labeled geniculocortical axons were found in the intermediate zone beneath the lateral cerebral mantle at embryonic day (E)17 and in the subplate layer underlying visual cortex by E18. On E19 they appeared to accumulate and grow radially into an expanding subplate layer and into the deep part of developing cortical layer VI. By postnatal day (P)0, DiI or WGA-HRP-labeled geniculocortical axons were found in developing cortical layers VI and V. By P1, they invaded the deep portion of the cell-dense cortical plate, where they were in position to make initial contact with neurons that would later form layer IV. A few axons traversed the cortical plate to reach the marginal zone. Layer IV became an identifiable layer on P2, and a clear projection to layer IV was evident by P3. These results suggest that geniculocortical afferents grow continuously from the intermediate zone, initially into an expanding subplate layer and then sequentially into each of the developing cortical layers without evidence of "waiting." Electron microscopic data suggest that geniculocortical axons begin to form immature synapses with dendrites and neuronal perikarya as they first encounter cortical neurons, first in the subplate layer and then in developing layers VI, V and marginal zone, in addition to the primary target layer IV. The precise targeting and overall temporal and laminar patterns of ingrowth and synaptogenesis suggest that geniculocortical axons are directed to the visual cortex by guidance cues within the internal capsule and subplate. Further, they reach the occipital pole early enough to influence the specification and histogenesis of cortical area 17, perhaps by exerting an influence on the deep-to-superficial "wave" of neuronal differentiation in sequentially developing subplate and cortical layers VI, V and IV.

Transition from developing to mature patterns of acetylcholinesterase activity in rat visual cortex: implications for the time-course of geniculocortical development

Patterns of acetylcholinesterase (AChE) histochemical staining in cortical area 17 differ in infant and mature rats. In infants, intense AChE activity is seen as a band corresponding to layer IV and deep layer III of the visual cortex, and this staining is associated with terminal fields of geniculocortical neurons. In adult animals, AChE activity is densest in deep layer IV and layer V and is associated with projections originating in the basal forebrain. The present study investigated the transition from developing to mature patterns of AChE staining in visual cortex. Unilateral lesions were placed in either the lateral geniculate body or the basal forebrain of rats postnatal days 8 (P8) to adulthood; the effects of these lesions on patterns of AChE activity in visual cortex were studied with histochemical techniques and optical densitometry. Lesions involving the lateral geniculate body markedly reduce AChE activity in visual cortex of P12 rats, had moderate effects in P20 rats, and had no apparent effect on AChE activity of visual cortex of rats aged P40 and older. Lesions of basal forebrain had little effect on AChE activity in visual cortex of P12 animals, increasing effect in P15-35 rats, and eliminated much of AChE staining in visual cortex of adults. The period of transition from developing to mature patterns of AChE activity in visual cortex of animals bilaterally enucleated at birth was not different from the period of transition in normally sighted animals. These data indicate that mature patterns of AChE activity in visual cortex are not achieved until well into the second month of life. If transient AChE expression is characteristic of geniculocortical neurons during the period of time in which axons are proliferating within visual cortex, then these data indicate that geniculocortical connections may be forming well into the second month of life in the rat.

Transient patterns of serotonergic innervation in the rat visual cortex: normal development and effects of neonatal enucleation

The transient aggregation of serotonin (5-HT)-containing fibers in the early development of rat visual cortex was examined immunohistochemically. The aggregation of 5-HT immunoreactive (IR) fibers consisted of three stages which were classified according to the course of time and degree of space occupied. The primary aggregation appeared in the subplate and moved upward along the development of the cortex. The aggregation proceeded to the secondary stage in presumptive layer IV. The fibers extended in a column-like structure following the secondary aggregation and formed the tertiary aggregation. The upper edge of the tertiary aggregation formed a lattice-like pattern in layer I and its structure was recognized to be similar to the structure of a 'blob' which characterizes the primary visual cortex in monkey. This transient aggregation of 5-HT-IR fibers began in the subplate of the anterior visual cortex on postnatal day 2 (PND 2) and progressed towards the posterior. On PND 11, the secondary and tertiary aggregations were completed in the entire region. No further aggregation of 5-HT-IR fibers was observed on PND 15. The anterior-to-posterior axis in the aggregation process corresponds to the direction of differentiation in the layer structure of cortex. In order to investigate the relationship between the transient aggregation of 5-HT-IR fibers and the development of the visual pathway, the secondary and tertiary aggregation on PND 11 were observed after postnatal monocular or binocular enucleation. Enucleation of eye balls did not affect either the area occupied by the 5-HT-IR fibers in the secondary aggregation or the number of column structures in the tertiary aggregation. However, the contralateral and ipsilateral cortices of monocularly enucleated cases were irregularly shaped in the secondary aggregation. The distribution of 5-HT-IR fiber terminals in the binocular area (Oc1B) increased in density on the contralateral side in the monocular enucleation, while that of both sides in the binocular enucleation was of non-homogeneous density and were shaped irregularly. The above results suggest that the transient aggregation of 5-HT-IR fibers observed in the early stage of development of visual cortex is regulated primarily by the intrinsic factors, and that extrinsic factors, such as visual pathway input, affect the aggregation within the boundary of such intrinsic factors. That is, the visual pathway input and the input balance from both eyes affect the distribution density of 5-HT-IR fibers and the shape of the visual cortex, respectively.

Transneuronal degeneration of thalamic neurons following deafferentation: quantitative studies using [3H]thymidine autoradiography

Transneuronal degeneration of thalamic neurons following partial deafferentation was studied using [3H]thymidine autoradiography. Timed-pregnant female Sprague-Dawley rats received systemic injections of [3H]thymidine on embryonic day (E) 13, 14 and/or 15. On the day of birth, pups were anesthetized by hypothermia and subjected to unilateral enucleation, unilateral removal of the inferior colliculus or sham lesion. Animals were sacrificed on postnatal day 10 or 30 and the brains processed for autoradiography. Material from sham-lesioned animals demonstrates that neurons destined for the dorsal lateral geniculate nucleus (LGd) undergo final mitoses on E13, 14 and 15. Neurons in the ventral medial geniculate nucleus (MGv) undergo final mitoses on E13 and 14. Thirty days following neonatal unilateral eye removal, the contralateral LGd displays a loss of approximately 30-35% of [3H]thymidine labeled neurons. Neonatal unilateral removal of the inferior colliculus results in a loss of approximately 30-40% of labeled neurons in MGv. For both LGd and MGv, shorter survival times reveal less severe cell loss. Late generated (E15) LGd neurons show less severe loss following enucleation than do earlier generated neurons. These results document the degree of cell loss in sensory thalamic nuclei following deafferentation and demonstrate that [3H]thymidine autoradiography provides a useful quantitative method for assessing anterograde transneuronal cell loss in targeted populations of neurons in the developing central nervous system.

Relationships between patterns of acetylcholinesterase activity and geniculocortical terminal fields in developing and mature rat visual cortex

Intraocular injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) result in anterograde transneuronal labeling of geniculocortical axon terminals in cortical area 17. In area 17 of developing rat pups, transported WGA-HRP occurs primarily in layer I and in a band that includes layer IV and deep layer III; this pattern is virtually identical to the laminar pattern of endogenous acetylcholinesterase (AChE) activity. In adult animals, transported WGA-HRP again is localized in layer I and in deep layer III and layer IV, but the endogenous AChE activity is found most prominently in deep layer IV and layer V. These results indicate that geniculocortical terminal fields are co-extensive with transient patterns of AChE activity in the developing rat, but not with the mature pattern of AChE in the adult.

Neural systems contributing to acetylcholinesterase histochemical staining in primary visual cortex of the adult rat

Histochemical studies demonstrate that cortical area 17 (primary visual cortex) of the adult rat displays a characteristic laminar pattern of acetylcholinesterase (AChE) activity. While AChE-positive axons are found throughout the cortical layers, most intense staining occurs in a band that corresponds to layer V and the deep portion of layer IV. The present studies were directed toward determining the neural systems containing this AChE activity. Unilateral electrolytic or excitatory amino acid induced lesions of the basal forebrain result in reductions of AChE staining in ipsilateral visual cortex, particularly in layers IV and V. Electrolytic or scalpel lesions, placed in white matter underlying dorsal and lateral neocortex to interrupt basal forebrain projections to visual cortex, also reduce AChE staining in visual cortex. Lesions in the cingulate bundle and supracallosal stria reduced AChE staining retrosplenial cortex but did not affect staining visual cortex. Placement of electrolytic lesions in the hypothalamus produced no detectable change in the pattern of AChE in visual cortex. Electrolytic lesions in the midbrain tegmentum, placed to interrupt ascending axons from brainstem monoamine neurons, produced no detectable change in the pattern of AChE in visual cortex. Placement of lesions in the dorsal thalamus that include all of the dorsal lateral geniculate nucleus did not alter AChE staining in visual cortex. The results indicate that AChE activity in adult visual cortex is found primarily within afferent axons from the basal forebrain system. These data demonstrate further that the AChE staining characteristic of adult visual cortex is associated with neural systems that are distinctly different from those associated with AChE staining in visual cortex of the infant rat.

Patterns of transiently expressed acetylcholinesterase activity in cerebral cortex and dorsal thalamus of developing rats with cytotoxin-induced microencephaly

Previous studies have demonstrated that acetylcholinesterase (AChE) activity is expressed transiently by thalamocortical neurons of primary sensory systems in developing rat pups. In the present study, prenatal treatment with methylazoxymethanol acetate (MAM) on embryonic day 15, 16, or 17 resulted in rat pups with cerebral cortices markedly reduced in thickness and areal extent. Histochemical studies demonstrated that AChE staining occurs in fiber-like plexuses in primary visual, auditory, and somatosensory regions of developing cerebral cortex of MAM-treated animals, just as in normal developing rats, but that the transient patterns of AChE are found more superficially than normal and they occur in an abnormal patchy distribution. Neuronal somata in thalamic lateral geniculate, medial geniculate and ventral basal nuclei of MAM-treated animals show transient AChE staining indistinguishable from that seen in normal animals. These data indicate: (1) AChE is expressed transiently by thalamocortical neurons in MAM-treated animals, (2) intensity of the transiently expressed AChE is not affected by MAM-induced loss of cortical neurons, and (3) the abnormal AChE patterns in cortex likely reflect the abnormal distributions of thalamocortical terminal fields that are characteristic of MAM-treated animals.

Neonatal enucleations reduce number, size, and acetylcholinesterase histochemical staining of neurons in the dorsal lateral geniculate nucleus of developing rats

Previous studies have demonstrated that transient patterns of acetylcholinesterase (AChE) activity are characteristic of geniculo-recipient regions of rat cortical area 17 during the second and third postnatal weeks of life. Neonatal enucleation results in a marked reduction of this transiently expressed cortical AChE. Parallel studies have demonstrated that the dorsal lateral geniculate nucleus (dLGN) also expresses AChE transiently during development. The present study examines neuronal number and size as well as AChE histochemical staining in the dLGN of normal and neonatally enucleated rat pups to determine whether changes in dLGN neurons could account for the decreased visual cortical AChE staining that results from neonatal enucleation. Changes in 4 parameters in dLGN were noted after neonatal enucleation. First, a 26-37% shrinkage in the volume of dLGN occurred contralateral to enucleation. Second, enucleation resulted in a loss of 16-30% of AChE-stained neuronal somata. Third, remaining AChE-positive neuronal somata appeared shrunken by approximately 40%. Fourth, intensity of AChE histochemical staining of individual dLGN neurons was reduced by approximately 24% following neonatal enucleation. These data suggest that loss of transient AChE activity in cortical area 17 consequent to neonatal enucleation is secondary to enucleation-induced alterations in the dLGN; these alterations include loss of neurons, shrinkage of neurons, and an apparent decrease in the ability of neurons to synthesize AChE. These data support the hypothesis that geniculocortical projection neurons express AChE transiently during development of geniculocortical connectivity and indicate that normal afferent connections and/or activity are important for the transient expression of AChE by these neurons.

Intraocular injections of tetrodotoxin reduce transiently expressed acetylcholinesterase activity in developing rat visual cortex

Geniculo-recipient layers of primary visual cortex in the rat display a transient pattern of acetylcholinesterase (AChE) activity during the second postnatal week of life. Previous work has demonstrated that neonatal enucleations markedly reduce the transient AChE activity in visual cortex. The present studies were undertaken to determine the effects of reduced afferent neural activity on expression of the transient pattern of AChE activity. Rat pups received intraocular injections of tetrodotoxin (TTX) on postnatal days (PND) 3, 5, 7, 9 and 11 and were sacrificed on PND 12. Some animals were enucleated on PND 3. Brain sections were processed for AChE histochemistry and analyzed by optical densitometry. These experiments show that uniocular injections result in a markedly decreased level of AChE activity in layer IV of the medial part of cortical area 17 contralateral to the injected eye. The degree of reduction of AChE activity from repeated TTX injections was similar to the degree of reduction following enucleation on PND 3. Binocular injections of TTX result in a reduction of AChE activity in layer IV throughout cortical area 17, similar to the effects of binocular enucleation on PND 3. Experiments combining injection of horseradish peroxidase along with TTX on PND 11 demonstrate that retinal ganglion cells of TTX injected eyes are still capable of anterograde axonal transport. These data demonstrate that normal innervation and afferent activity are necessary for the transient expression of AChE activity by geniculocortical neurons.

Neurogenesis of the basal forebrain in euploid and trisomy 16 mice: an animal model for developmental disorders in Down syndrome

The neurogenesis and early histochemical differentiation of the basal forebrain in trisomy 16 fetal mice and their euploid littermates were examined by combining [3H]thymidine autoradiography with acetylcholinesterase histochemistry. Neurons of the basal forebrain were being born between embryonic day 11 and 15 in both chromosomally normal (euploid) and aneuploid mice. In euploid littermate controls, neurogenesis proceeded along a caudal to rostral gradient with the peak on embryonic day 11 for caudal portions and embryonic day 13 for rostral portions of the basal forebrain. In contrast, in trisomy 16 mice, rostral sections exhibited a peak of neurogenesis on embryonic day 11, 2 days earlier than in their euploid littermate controls. Hypocellularity of the basal forebrain region was noted in trisomy 16 mice; particularly dramatic was the reduction of the population of cells that expressed acetylcholinesterase. This reduction in cell number in the trisomics was not accompanied by a reduction in cell size or by a dramatic change in the distribution of residual neurons when compared to that of euploid littermate controls. Since trisomy 16 mice do not survive the perinatal period, we examined the pattern of acetylcholinesterase expression in normal C57B1/6J mice from embryonic day 16 to postnatal day 5 to determine the postnatal disposition of these neurons. Already at embryonic day 16, fibers staining for acetylcholinesterase penetrated the striatal anlage, in their course towards targets in the cerebral cortices. By postnatal day 5, the previously expansive distribution of basal forebrain neurons had become consolidated in a more ventral and rostral position by the extensive outgrowth of the striatal neurons, a pattern resembling that seen in adult animals.

Postnatal development of lamina III/IV nonpyramidal neurons in rabbit auditory cortex: quantitative and spatial analyses of Golgi-impregnated material

We have studied the postnatal development of lamina III/IV spine-free nonpyramidal neurons in the auditory cortex of the New Zealand white rabbit. The morphology and dendritic branching pattern of single cells impregnated with a Golgi-Cox variant were analyzed with the aid of camera lucida drawings and three-dimensional reconstructions obtained with a computer microscope. Sample sizes of 20 neurons were obtained at birth (day 0), postnatal day (PD) 3, 6, 9, 12, 15, 21, and 30 days of age. Normative data were also available from PD-60 and young adult rabbits studied previously. At birth, lamina II-IV have not yet emerged from the cortical plate; immature nonpyramidal neurons at the bottom of the cortical plate (presumptive layer IV) are characterized by short, vertically oriented dendrites. Growth-cone-like structures are present along the shafts and at the tips of the dendrites. At birth, soma area and total dendritic length are, respectively, 34 and 10% of adult values. The cortical plate acquires a trilaminar appearance at PD-3. The six-layered cortex is present by PD-6. During the first postnatal week dendritic length increases fourfold and is accompanied by a significant increase in both terminal and preterminal dendritic growth cones. At the onset of hearing at PD-6, there is a significant proliferation of dendrites and branches to 144 and 200% of adult levels, respectively. These supernumerary dendrites are rapidly lost during the second postnatal week, at which time the somata and dendrites become covered with spines. The loss of higher-order dendrites occurs more gradually; the number of dendritic branches is still 116% of adult values at PD-30. Spine density peaks between days PD-12 and PD-15, and then gradually diminishes until the cells are sparsely spined or spine free by PD-30. Total dendritic length increases in a linear fashion up to PD-15, at which time it is 80% of adult values. An analysis of terminal and intermediate branches demonstrated that the increase in total dendritic length after PD-6 is due entirely to the growth of terminal dendrites. Total dendritic length attains adult levels by PD-30. Spatial analyses revealed that a vertical orientation of dendrites is present at birth. Associated with the onset of hearing at PD-6, there is an explosive elaboration of dendrites toward the pial surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolic activity in striate and extrastriate cortex in the hooded rat: contralateral and ipsilateral eye input

The extent of changes in glucose metabolism resulting from ipsilateral and contralateral eye activity in the posterior cortex of the hooded rat was demonstrated by means of the C-14 2-deoxyglucose autoradiographic technique. By stimulating one eye with square wave gratings and eliminating efferent activation from the other by means of enucleation or intraocular TTX injection, differences between ipsilaterally and contralaterally based visual activity in the two hemispheres were maximized. Carbon-14 levels in layer IV of autoradiographs of coronal sections were measured and combined across sections to form right and left matrices of posterior cortex metabolic activity. A difference matrix, formed by subtracting the metabolic activity matrix of cortex contralateral to the stimulated eye from the ipsilateral "depressed" matrix, emphasized those parts of the visual cortex that received monocular visual input. The demarcation of striate cortex by means of cholinesterase stain and the examination of autoradiographs from sections cut tangential to the cortical surface aided in the interpretation of the difference matrices. In striate cortex, differences were maximal in the medial monocular portion, and the lateral or binocular portion was shown to be divided metabolically into a far lateral contralaterally dominant strip along the cortical representation of the vertical meridian, and a more medial region of patches of more or less contralaterally dominant binocular input. Lateral peristriate differences were less than those of striate cortex, and regions of greater and lesser monocular input could be distinguished. We did not detect differences between the two hemispheres in either anterior or medial peristriate areas, thus indicating either completely binocular input (which seems unlikely given the retinotopic organization of these regions), or a greater dependence than in the lateral peristriate on inputs that were not affected by the visual manipulations.

Neonatal lesions of the basal forebrain cholinergic neurons result in abnormal cortical development

The effect of electrolytic lesions of the neonatal forebrain on the morphogenesis of the mouse neocortex has been examined. Balb/C mice were lesioned unilaterally within 24 h of birth. The development of cortical cytoarchitecture was assessed in Nissl-stained sections, and the levels of presynaptic markers for cholinergic, noradrenergic and serotonergic afferents were measured in the fronto-parietal cortex ipsilateral and contralateral to the lesion at various postnatal ages and in adulthood. The basal forebrain (nBM) lesion resulted in a transient but severe reduction of cortical cholinergic markers and in abnormal cortical cytoarchitecture. Cytoarchitectural abnormalities were expressed as delay in the emergence of differentiated cell populations and affected sequentially more superficial layers with maturation following lesion. Furthermore, the location and extent of these morphologic abnormalities appeared to correlate with the degree of cholinergic denervation. Cortical monoamines were also temporarily reduced as a result of the lesion; however, pharmacologic lesions of the monoaminergic projections alone did not result in the abnormal cortical cytoarchitecture. Thus, the basal forebrain cholinergic projection appears to serve a role in regulating cortical differentiation.

Investigations of the origins of transient acetylcholinesterase activity in developing rat visual cortex

Transient acetylcholinesterase (AChE) activity is characteristic of cortical area 17 of the developing laboratory rat during the second and third postnatal weeks of life. This AChE activity is most intense in a band that corresponds to cortical layer IV and the deep part of layer III, but also is found in the outer half of cortical layer I and in layer VI. The morphology of the pattern of the histochemical reaction product indicates that the transient AChE is characteristic of an axonal terminal field. The present report describes results of 3 sets of experiments aimed at determining the source of transient AChE in cortical area 17. First, placement of lesions in portions of the basal forebrain or in the cingulate bundle results in a decrease in the general pattern of AChE throughout occipital cortex and especially in layer I, but the transient bands of AChE in layers III-IV of cortical area 17 are not eliminated. Second, kainic acid or cobalt chloride injections in cortical area 17 result in the loss of many AChE-positive neuronal somata but do not eliminate the transient pattern of AChE in thalamo-recipient layers of cortical area 17. Similarly, treatment of fetuses with mitotic inhibitors that eliminate many of the neurons destined for granular and supragranular layers does not eliminate transient patterns of AChE. Third, lesions that include the lateral geniculate nucleus of the thalamus or geniculocortical projections result in a marked loss of the pattern of AChE in thalamo-recipient layers of cortical area 17, without significant loss in other layers of area 17 or in other regions of occipital cortex. These data support the hypothesis that the transient AChE found in thalamo-recipient layers of cortical area 17 is contained within geniculocortical axon terminals.

Development of 'non-specific' cholinesterase-containing neurons in the dorsal thalamus of the rat

In adult rats, neurons displaying histochemical staining for 'non-specific' cholinesterase (ChE) are found 3 distinct regions of the dorsal thalamus: the thalamic reuniens nucleus (Re), the anterior dorsal nucleus (AD), and a region that includes the lateral part of the central lateral nucleus (CL) and the ventral portion of the lateral dorsal nucleus (LD). Normal development of ChE-positive neurons was studied with cholinesterase histochemical techniques in postnatal infant rats. Although ChE staining of capillary endothelium is detectable shortly after birth, ChE staining of neurons first occurs at about postnatal day 5 (PND 5) with light staining of AD and CL-LD. At PND 7, staining in AD and CL-LD has increased in intensity and staining also is present in neurons of the anterior ventral (AV) and ventral anterior (VA) nuclei. ChE staining of neurons in Re first appears at PND 10. The number of neurons staining for ChE in each of these nuclei, and also the intensity of staining in individual neurons, appear to increase during the next several days until about PND 14. After PND 14, ChE staining intensity in neurons of AD, Re, and CL-LD appears to plateau and the pattern of staining continues into adulthood. In contrast, ChE staining of neurons in VA declines markedly and only a very few neurons in the dorsal part of VA remain ChE-positive after PND 21. ChE staining of neuropil in AV increases markedly, obscuring somatal staining in this nucleus. These results are discussed in regard to transient and continued expression of ChE activity in the dorsal thalamus and possible functional roles of ChE.

Neonatal enucleations reduce specific activity of acetylcholinesterase but not choline acetyltransferase in developing rat visual cortex

Infant rats were subjected to unilateral or bilateral enucleation or sham lesions on the day of birth. On postnatal day 13 or 14, tissue blocks from cortex were assayed for acetylcholinesterase (AChE) and choline acetyltransferase (ChAT). Bilateral enucleation resulted in a decrease in specific activity of AChE in occipital cortex, as compared with control animals. Unilateral enucleation resulted in a decrease of AChE activity in the hemisphere contralateral to the enucleated orbit. However, neither type of enucleation resulted in significant changes of ChAT specific activity. These data suggest that reductions in AChE activity resulting from neonatal enucleations are not due to a direct involvement of cholinergic afferents to visual cortex.

Transient concordant distributions of nicotinic receptors and acetylcholinesterase activity in infant rat visual cortex

For a short period during the second week after birth, layer IV of rat visual cortex displays intense acetylcholinesterase (AChE) activity which demarcates area 17. We now report that this transient AChE pattern is paralleled by a marked increase in [3H]nicotine binding sites in layer IV of area 17. Muscarinic receptors show a different pattern. Neonatal bilateral enucleation results in a reduction of AChE reaction product and [3H]nicotine labelling in area 17, while leaving the pattern of muscarinic labelling virtually unaffected. The close association of [3H]nicotine labelling and AChE activity suggests that presynaptic nicotinic cholinergic receptors may modulate transmitter release while functional geniculocortical synapses are being formed.