Factors influencing functional connectivity during hippocampal development

Synaptic reorganization in subiculum and CA3 after early-life status epilepticus in the kainic acid rat model

PURPOSE

The immature rat brain is highly susceptible to seizures, but has a resistance to pathological changes induced by seizures as compared to adult rats. However, prolonged seizures during early-life enhance cellular injury and hyperexcitability induced by convulsive insults later in adulthood. The mechanisms underlying these phenomena are not understood. In adult models, the CA1 axons reorganize their projections to subiculum. Seizure induced plasticity in this pathway has not been investigated in immature seizure models, and may contribute to the vulnerability to later seizures.

METHODS

On postnatal day 15, rats experienced convulsive status epilepticus with kainic acid (KA). Seizure induced plasticity was examined with Timm histochemistry and iontophoretic injections of sodium selenite, a retrograde tracer. Cellular injury was evaluated with Fluoro-Jade B histochemistry.

RESULTS

Retrograde tracing experiments determined a 67% larger dorsoventral extent of retrograde labeling in the CA1 pyramidal region after tracer injections in subiculum. The synaptic reorganization of the CA1 projection to subiculum was noted in the absence of overt neuronal injury in subiculum or CA1. In contrast, mossy fiber sprouting was detected into the stratum oriens of CA3 with limited neuronal injury to CA3 pyramidal neurons. No mossy fiber sprouting into the inner molecular layer of the dentate gyrus, or CA1 sprouting into the stratum moleculare of CA1 were noted.

CONCLUSIONS

The results indicate that the developing brain has distinct mechanisms of seizure induced reorganization as compared to the adult brain. Our experiments show that the concept of "resistance of the immature brain to excitotoxicity" is considerably more complicated than generally believed. Morphological plasticity in the immature brain appears more extensive in distal, but not proximal, projections of hippocampal pathways, and across hippocampal lamellae. The abnormal connectivity between hippocampal lamellae might play a role in the increased susceptibility to injury and hyperexcitability associated with later convulsive insults.

The dynamics of dendritic structure in developing hippocampal slices

Time-lapse fluorescence confocal microscopy was used to directly visualize the formation and dynamics of postsynaptic target structures (i.e., dendritic branches and spines) on pyramidal neurons within developing tissue slices. Within a 2 week period of time, pyramidal neurons in cultured slices derived from early postnatal rat (postnatal days 2-7) developed complex dendritic arbors bearing numerous postsynaptic spines. At early stages (1-2 d in vitro), many fine filopodial protrusions on dendrite shafts rapidly extended (maximum rate approximately 2.5 microM/minute) and retracted (median filopodial lifetime, 10 min), but some filopodia transformed into growth cones and nascent dendrite branches. As dendritic arbors matured, the population of fleeting lateral filopodia was replaced by spine-like structures having a low rate of turnover. This developmental progression involved a transitional stage in which dendrites were dominated by persistent (up to 22 hr) but dynamic spiny protrusions (i.e., protospines) that showed substantial changes in length and shape on a timescale of minutes. These observations reveal a highly dynamic state of postsynaptic target structures that may actively contribute to the formation and plasticity of synaptic connections during CNS development.

Effect of neonatal degranulation on the morphological development of rat CA3 pyramidal neurons: inductive role of mossy fibers on the formation of thorny excrescences

Intracellular peroxidase injections were made on CA3 pyramidal neurons from slices obtained from 3-60 day old control and experimental rats. Experimental rats had been irradiated at birth with gamma-rays to destroy the granule cells of the fascia dentata. Neonatal gamma-ray irradiation induced a clear cut reduction of the total number of granule cells (80% decrease at P60) and mossy fibers as revealed by Nissl and Timm stains. The number of branching points and segments, total dendritic length, and density of dendritic spines in the stratum radiatum and stratum oriens on CA3 pyramidal cells were not significantly different between control and irradiated rats. To quantify the development of thorny excrescences we measured the number and total area of the thorny excrescences per neuron at different stages of development. gamma-Ray irradiation induced a reduction in the number and total area of thorny excrescences. This effect was readily apparent by postnatal day (P) 12, and a reduction of 70% in the total area was observed at P30. In conclusion, gamma-ray irradiation destroys the majority of granule cells and induces a reduction in the development of thorny excrescences. Our data strongly suggest that this effect is directly caused by the lack of mossy fibers rather than by the irradiation itself since other parameters (i.e., the number of segments and branching points, density of non-mossy spines, and total dendritic length) were not affected. Therefore, we suggest that mossy fibers play an inductive role in the formation of thorny excrescences.

Neurotransmitters in the regulation of neuronal cytoarchitecture

Recent experiments in isolated neurons in cell culture have demonstrated that neurotransmitters and associated electrical activity can directly affect neurite outgrowth. The results indicate that neurotransmitters have considerable potential to control the development of the neuronal circuits in which they participate in information coding in the adult. Cellular mechanisms regulating growth cone motility have been found to be similar to those regulating neurotransmitter release at the synapse and involve electrical activity, calcium and other second messengers. These similarities suggest that the morphological changes in connections observed in adult plasticity may involve the transition of synaptic terminals back to a growth mode. Excitatory and inhibitory neurotransmitters can interact to yield a net effect on neuronal morphology. In the intact nervous system a balance between these neurotransmitter inputs is probably important in maintaining circuits. Studies of neurotransmitter involvement in learning and memory processes indicate that brain function can alter brain structure and that neurotransmitters may control these structural changes. The hippocampus is one brain region in which we are beginning to define roles for neurotransmitters as sculptors of neuronal cytoarchitecture. The neurotransmitter glutamate was found to specifically affect the cytoarchitecture of hippocampal pyramidal neuron dendrites in a graded manner which suggests that glutamate may be involved in: establishing hippocampal circuitry during brain development; maintaining and modifying circuitry in the adult; and inducing neurodegeneration in several disorders including epilepsy, Alzheimer's disease, and stroke. Therapeutic approaches to disorders which affect brain cytoarchitecture may now be devised based upon knowledge of the neurotransmitters and their cellular mechanisms in the pertinent brain region.

Mossy fibres form synapses with identified pyramidal basket cells in the CA3 region of the guinea-pig hippocampus: a combined Golgi-electron microscope study

Mossy fibres, i.e. the axons of dentate granule cells, terminate with characteristic giant boutons on large spines or excrescences of the pyramidal cells in regio inferior of the hippocampus. In addition to pyramidal cells there are several types of non-pyramidal neuron which extend their dendrites into the termination zone of mossy fibres. By using the combined Golgi-electron microscope technique mossy fibre terminals were found, which established asymmetric synaptic contacts with both spines of pyramidal cells and smooth dendrites of identified (Golgi-stained) pyramidal basket cells in the CA3 region of the guinea-pig hippocampus. The observed synaptic connection with pyramidal basket cells suggests an involvement of the mossy fibre system in feed-forward inhibition of the hippocampal pyramidal neurons.