Dendritic anomalies in a freezing model of microgyria: a parametric study

Effects of nerve growth factor in experimental model of focal microgyria

AIM

The effects on neural repair of intraparenchymal nerve growth factor (NGF) administration were evaluated in neonate Wistar rats with experimentally induced focal microgyria.

METHODS

A freezing focal polymicrogyric lesion was induced on the frontal cortex in 35 newborn Wistar rats on postnatal day 1. NGF was administered in 15 cases, with 20 pups as controls. Animals were sacrificed at 72 h and 7 days after NGF administration. Real-time PCR was used for the quantification of the expression of TrkA, p75, and doublecortin (DCX) at the level of the cortical lesion in seven different groups of animals: control 72 h (n = 5), control 7 days (n = 5), microgyria 72 h (n = 5), microgyria 7 days (n = 5), microgyria + NGF 72 h (n = 5), microgyria + NGF 7 days (n = 5), and control + NGF (n = 5).

RESULTS

A significant increase in TrkA expression was found in the microgyria + NGF 7 days group compared to the others. TrkA upregulation was already visible 72 h after NGF administration. Unlike TrkA, p75 expression increased in animals subjected to the experimental focal microgyria and decreased markedly after NGF administration. DCX expression in injured animals was observed to increase strongly 7 days after NGF administration compared with other groups.

CONCLUSIONS

NGF administration interferes with neural repair mechanisms at the polymicrogyric lesion site by means of TrkA and DCX upregulation which possibly counteracts the process of apoptosis caused by the brain injury.

Enhanced infragranular and supragranular synaptic input onto layer 5 pyramidal neurons in a rat model of cortical dysplasia

Cortical dysplasias frequently underlie neurodevelopmental disorders and epilepsy. Rats with a neonatally induced cortical microgyrus [freeze-lesion (FL)], a model of human polymicrogyria, display epileptiform discharges in vitro. We probed excitatory and inhibitory connectivity onto neocortical pyramidal neurons in layers 2/3 and 5 of postnatal day 16-22 rats, approximately 1-2 mm lateral of the lesion, using laser scanning photostimulation (LSPS)/glutamate uncaging. Excitatory input from deep and supragranular layers to layer 5 pyramidal cells was greater in FL cortex, while no significant differences were seen in layer 2/3 cells. The increased input was due to a greater number of LSPS-evoked excitatory postsynaptic currents (EPSCs), without differences in amplitude or kinetics. Inhibitory input was increased in a region-specific manner in pyramidal cells in FL cortex, due to an increased inhibitory postsynaptic current (IPSC) amplitude. Connectivity within layer 5, parts of which are destroyed during lesioning, was more severely affected than connectivity in layer 2/3. Thus, we observed 2 distinct mechanisms of altered synaptic input: 1) increased EPSC frequency suggesting an increased number of excitatory synapses and 2) higher IPSC amplitude, suggesting an increased strength of inhibitory synapses. These increases in both excitatory and inhibitory connectivity may limit the extent of circuit hyperexcitability.

Organization and morphology of thalamocortical neurons of mouse ventral lateral thalamus

The ventral lateral nucleus of the thalamus (VL) serves as a central integrative center for motor control, receiving inputs from the cerebellum, striatum, and cortex and projecting to the primary motor cortex. We aimed to determine the somatotopy and morphological features of the thalamocortical neurons within mouse VL. Retrograde tracing studies revealed that whisker-related VL neurons were found relatively anterior and medial to those labeled following injection of retrograde tracer into hindpaw motor areas. Simultaneous injections of fluorescent microspheres in both cortical regions did not result in double-labeled neurons in VL. Quantitative analysis of dendritic and somatic morphologies did not reveal any differences between hindpaw and whisker thalamocortical neurons within VL. The morphology of the thalamocortical neurons within mouse VL is similar to those in other mammals and suggests that mouse can be used as a model system for studying thalamocortical transformations within the motor system as well as plasticity following sensory deprivation or enrichment.

Developmental timeframes for induction of microgyria and rapid auditory processing deficits in the rat

Induction of a focal freeze lesion to the skullcap of a 1-day-old rat pup leads to the formation of microgyria similar to those identified postmortem in human dyslexics. Rats with microgyria exhibit rapid auditory processing deficits similar to those seen in language-impaired (LI) children, and infants at risk for LI and these effects are particularly marked in juvenile as compared to adult subjects. In the current study, a startle response paradigm was used to investigate gap detection in juvenile and adult rats that received bilateral freezing lesions or sham surgery on postnatal day (P) 1, 3 or 5. Microgyria were confirmed in P1 and 3 lesion rats, but not in the P5 lesion group. We found a significant reduction in brain weight and neocortical volume in P1 and 3 lesioned brains relative to shams. Juvenile (P27-39) behavioral data indicated significant rapid auditory processing deficits in all three lesion groups as compared to sham subjects, while adult (P60+) data revealed a persistent disparity only between P1-lesioned rats and shams. Combined results suggest that generalized pathology affecting neocortical development is responsible for the presence of rapid auditory processing deficits, rather than factors specific to the formation of microgyria per se. Finally, results show that the window for the induction of rapid auditory processing deficits through disruption of neurodevelopment appears to extend beyond the endpoint for cortical neuronal migration, although, the persistent deficits exhibited by P1 lesion subjects suggest a secondary neurodevelopmental window at the time of cortical neuromigration representing a peak period of vulnerability.

Impairment of functional inhibition in the contralateral cortex following perinatally acquired malformations in rats

Neonatal freeze lesions in newborn rats induce focal malformations of the cerebral cortex mimicking human polymicrogyria which is a common cause of epilepsy and neuropsychological deficits in children and adults. Experimental and clinical studies demonstrated hyperexcitability in the malformation itself and peridysplastic cortex associated with a widespread imbalance of excitatory and inhibitory function and extensive alterations in cortical connectivity. We investigated the integrity of functional cortical inhibition using a paired pulse paradigm in brain slice preparations of adult freeze-lesioned rats. In contrast to previous electrophysiological studies focusing on the dysplastic cortex and the ipsilateral hemisphere, we here mapped both hemispheres. Extracellular field potentials were evoked by application of double pulses at the border of layer VI/white matter and recorded in layer II/III. Evaluation of the ratio of the field potential amplitudes at different recording positions allowed an assessment of regional functional inhibition. Using this approach, we observed a significant reduction of functional inhibition in the somatosensory cortex of the contralateral hemisphere, whereas only slight alterations were detected in the ipsilateral lesion surround. Our results provide evidence that focal cortical malformations not only impair cortical excitability in the ipsilateral hemisphere but also induce a disinhibition of the contralateral cortex.

Light microscopic study of GluR1 and calbindin expression in interneurons of neocortical microgyral malformations

Rat neocortex that has been injured on the first or second postnatal day (P0-1) develops an epileptogenic, aberrantly layered malformation called a microgyrus. To investigate the effects of this developmental plasticity on inhibitory interneurons, we studied a sub-population of GABAergic cells that co-express the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor GluR1 subunit and the calcium-binding protein, calbindin (CB). Both malformed and control cortex of adult (P40-60) animals contained numerous interneurons double-stained for CB and GluR1. Immunoreactivity (IR) for CB was up-regulated in perikarya of interneurons within supragranular layers of control cortex between P12 and P40. However, in malformed adult (P40) cortex, CB-IR levels were significantly lower than in adult controls, and fell midway between levels in immature and adult control animals. Between P12 and P40, GluR1-IR was down-regulated in perikarya of interneurons in control cortex. Somatic GluR1-IR levels in malformed adult (P40) cortex were not different from adult controls. These neurons formed a dense plexus of highly GluR1-positive spiny dendrites within layer II. The dendritic plexus in the malformation was more intensely GluR1-immunoreactive than that in layer II of control cortex. This was due to apparent changes in thickness and length of dendrites, rather than to significant changes in the number of interneuronal perikarya in the microgyral cortex. Results indicate that the population of GluR1/CB-containing interneurons is spared in malformed microgyral cortex, but that these cells sustain lasting decreases in their somatic expression of calbindin and alterations of dendritic structure. Potential functional implications of these findings are discussed.

Cellular physiology of the neonatal rat cerebral cortex

The early development of the cerebral cortex is characterized by neurogenesis, neuronal migration, cellular differentiation and programmed cell death. Cajal-Retzius cells, developing cortical plate neurons and subplate cells form a transient synaptic circuit which may serve as a template for the formation of cortical layers and columns. These three neuronal cell types show distinct electrophysiological properties and synaptic inputs. Endogenous or exogenous harmful disturbances during this developmental period may lead to the preservation of early cortical circuits, which may act as trigger zones for the initiation of pathophysiological activity.