Jen Yu

POSTNATAL MIGRATION OF NEURONS AND FORMATION OF LAMINAE IN RAT CEREBRAL CORTEX

Migration of neurons and formation of laminae in the developing neocortex were studied by means of thymidine autoradiography. Timed pregnant rats received a single pulse injection of [3H]thymidine in the morning of embryonic day (E)13, 14, 15, 16, 17, 18 or 19. Pups were killed on postnatal day (P)0, 1, 2, 3, 4, 6, 10, 30, or 60 and brains were processed for autoradiography. Neurons in posterior (visual) cortical areas labeled by [3H]thymidine administration on E13 or E14 were found predominantly in the cortical subplate; cells labeled on El5 in layer VI; cells labeled on E16 in layers VI and V, cells labeled on El7 in layers V and IV; E18 in layers IV and III; and E19 in layers III and II. By the day of birth (PO), neurons labeled from E13-16 injections were already in their mature laminae in cortex. Many of the cells labeled on E17 were still situated within the cell-dense cortical plate (CP) at PO, and within layer V by P1. Cells labeled on E18 were found in the most superficial part of the CP on PO, in the deep part of the CP on P1, and formed layer IV on P2 and P3. At PO, many E19 labeled cells appeared to be in migration to the cortex and were found in the CP on P1, in layer III by P4, and in layer II by P6. Cells in the auditory cortex labeled by [3H]thymidine injections on a particular day were situated more superficially than comparable labeled cells in the visual cortex, indicating a lateral to medial gradient in which the auditory cortex is formed earlier than the visual cortex. Distributions of labeled cells in the somatosensory cortex were similar to those in the visual cortex. These data provide a detailed and comprehensive description of the position of varied populations of cortical neurons during the early postnatal period, as well as a description of the formation of cortical laminae at times when major systems of afferents are growing into the cortex and making synaptic connections with their target cells.

Brain mechanisms and intelligence. Psychometric g and executive function.

Abstract Sternberg [Sternberg, R.J. (1985). Beyond IQ: a triarchic theory of human intelligence. New York: Cambridge Univ. Press.] has proposed that the general intelligence, or the g factor, obtained when batteries of mental tests are factor analyzed, is a reflection of the fact that executive functions (EF) are common to all cognitive tests. Three lines of evidence that fail to support Sternbergs formulation are presented. First, in animal problem solving studies, there is only a modest degree of overlap between brain structures that are critical for g, and brain structures that have been identified as the rodent EF system. Second, children with attention deficit-hyperactivity disorder (ADHD), characterized by EF dysfunction, do not have IQ scores that are lower, on average, than children in the test standardization populations. Third, human frontal lobe patients often have clear EF deficits, but IQ (a next-best estimate of g) may be preserved. These findings cast serious doubt on the plausibility of Sternbergs formulation. Clarifying the distinction between psychometric g and EF can be important for understanding the differences between practical and psychometric intelligence.

AFGF promotes axonal growth in rat spinal cord organotypic slice co-cultures.

This study developed a slice culture model system to study axonal regeneration after spinal cord injury. This model was tested in studies of the roles of acidic fibroblast growth factor (aFGF) and peripheral nerve segments in axonal growth between pieces of spinal cord. Transverse sections of P15-P18 Sprague-Dawley rat spinal cord were collected for organotypic slice cultures. Group I consisted of two slices of spinal cord in contact with each other during the culture period. Group II consisted of two slices that were separated by 3 mm and connected by two segments of intercostal nerves. Group III consisted of single slices for studies of neuron survival. Some cultures from each group included aFGF in the culture medium. Bromodeoxyuridine (BrdU) was included in the medium for some cultures. The results showed three principal findings. First, counts of neurofilament-positive cells demonstrated that treatment with aFGF significantly increased the number of surviving neurons in culture. Second, neurofilament immunostaining and DiI tracing demonstrated axons crossing the junction between the two pieces of spinal cord or growing through the intercostal nerve segments, and these axons were seen only in cultures with aFGF treatment. Third, few cells were double stained for neurofilament and BrdU, and these were found only with aFGF treatment. These results demonstrate that (1) organotypic slice cultures present a useful model to study regeneration from spinal cord injury, (2) aFGF rescues neurons and promotes axonal growth in these cultures, and (3) segments of intercostal nerves promote axon growth between slices of spinal cord.

Absence of selectivity in the loss of neurons from the developing cortical subplate of the rat

Neurons of the cortical subplate display evidence of cell death, although a significant population survives to the mature brain. The present study examined different populations of neurons to determine if the loss of cells was specific for a particular cell type. Immunocytochemical procedures for neurons expressing GluR2/3, GAD, or NPY, were used on tissue sections taken from animals at gestational day 18 to postnatal day 21. The rate of loss of labeled cells was similar for all groups of neurons. Thus, these data reveal no evidence that the loss of subplate neurons is specific to any major cell type.

A l-[3H]glutamate binding site on glia: an autoradiographic study on implanted astrocytes

In the present study cultured astrocytes were implanted into the inferior colliculus of rats to create an astrocyte-enriched field that could be examined autoradiographically. The presence of the astrocytes was confirmed with anti-glial fibrillary acidic protein (GFA) immunocytochemistry. We report the presence of a chloride-dependent glutamate binding site on the implanted astrocytes. In the presence of chloride, the specific glutamate binding detected in the implant area was 5-fold greater than that found in a corresponding contralateral region. When the chloride was replaced with acetate, glutamate binding to the astrocytes decreased by more than 80%. The chloride-dependent binding to the astrocytes was insensitive to inhibition by kainic acid (KA) and N-methyl-D-aspartate (NMDA) and sensitive to quisqualate, L-aspartate, L-2-amino-4-phosphonobutyrate, and L-alpha-aminoadipate. The pharmacology of the binding was very similar to that of the in vitro glutamate binding to membranes from cultured astrocytes and to that of a chloride-dependent transport system identified in a glioma cell line. We conclude that the interaction of glutamate with astrocytes is an important component of the total glutamate binding observed in brain slices.

Arrival of afferents and the differentiation of target neurons: studies of developing cholinergic projections to the dentate gyrus.

This study examined the relationship between the development of cholinergic axons originating from the septum and a group of their target cells, the granule cells of the dentate gyrus of the rat. Acetylcholinesterase histochemistry was used to identify septal cholinergic afferents to the dentate gyrus; parallel studies used anterograde movement of a carbocyanine dye to label the septal projections. Septal cholinergic axons are present in the molecular layer of the internal blade of the dentate gyrus shortly after birth, but these axons do not reach the external blade until several days later. Results demonstrate that acetylcholinesterase positive septal axons grow into the external blade of the dentate gyrus only after the recently generated granule cells have coalesced to form a clearly defined layer. Results from studies using in situ hybridization techniques demonstrate that dentate gyrus granule cells express messenger RNAs for brain derived neurotrophic factor and for neurotrophic factor 3 shortly after formation of the granule cell layer. Ingrowth of septal cholinergic axons follows two days after the formation of the external blade of the dentate gyrus and the expression of neurotrophin messenger RNAs by the dentate granule cells. These data support the hypothesis that target cell development is a prerequisite for attracting the ingrowth of septal afferent axons.

Static magnetic field influence on rat tail nerve function

Motor nerve conduction and excitability were measured on the tail nerve of anesthetized rats before and after the nerve was exposed perpendicularly to a static electromagnetic field of various intensities and durations. There was no significant change in either the distal latencies or the amplitudes of the compound muscle action potential (CMAP) measured from stimulating the tail nerve after it was exposed to the electromagnetic field with a density up to 1.2 Tesla (T) for a duration of 60 seconds. However, the nerve excitability expressed as changes of the amplitudes of the submaximally evoked CMAP increased significantly when the tail nerve was exposed to a magnetic field with a density higher than 0.5T for more than 30 seconds. The finding that an electromagnetic field increases motor nerve excitability suggests a possible mechanism of its therapeutic effects.

Detour problem-solving behavior in rats with neocortical and hippocampal lesions: a study of response flexibility

Young rats previously subjected to discrete neocortical ablations or hippocampal electrolytic lesions received five trials on each of three “climbing” detour problems. Performance on Trial 1 (a measure of response flexibility) of each problem was significantly impaired in those groups with frontal, parietal, or hippocampal lesions; the group with occipital lesions was impaired on only one problem. Performance on Trials 2–5 (a measure of detour habit formation) of each problem was significantly impaired only in the hippocampal group; the neocortical groups showed either no impairment or a mild impairment on these trials. These results suggest that the solution of detour problems on the first trial may require a cognitive process that is qualitatively different from that required on subsequent trials.

Brain mechanisms in problem solving and intelligence: A replication and extension

Abstract Anderson (1993, 1995) recently reported the extraction of a general intelligence (g) factor from a battery of tests administered to laboratory rats, conflicting with historical evidence and our more recent study of rat problem-solving behavior ( Thompson, Crinella, & Yu, 1990) . Andersons findings are most likely due to use of an outbred strain, whereas we and others used inbred rats. However, it has been suggested that these unique findings may be due to Andersons use of a more g-loaded test battery. We tested this hypothesis by analyzing the performance of 120 adult male Sprague-Dawley rats on seven laboratory tests, selected for diversity and g saturation. At 21 to 23 days of age, 96 of the animals were bilaterally lesioned in one of 48 brain sites, and 24 remained unlesioned. Testing began at 42 to 45 days of age. Analysis of data from the 24 unlesioned animals showed no evidence of a g factor. When data from unlesioned and lesioned animals were combined, uniformly positive correlations were obtained, and an unrotated first principal component accounted for 34% of the variance. This component had psychometric properties analogous to g factors extracted from human test batteries. The results support our contention that the disparity between Andersons findings and ours are due to strain differences, and not psychometric properties of the respective test batteries. The results also show that the g loading of a test is directly related to the number of brain structures required to mediate its performance—biological support for the view that tests with higher g loadings sample a proportionately greater number of elementary processes than tests with lower g loadings. Because the psychometric g factor is sensitive to the widest array of brain lesions, it may be the best measure for detecting neuropsychological deficit irrespective of lesion location.

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.