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Structure and function of aberrant perforant path synapses in the hippocampus of neonatally X-irradiated rats
Insights
X-irradiation at birth alters synaptic connections in the rat dentate gyrus. Perforant path fibers form functional synapses with dendritic spines, suggesting aberrant connections with pyramidal cells.
Area of Science:
- Neuroscience
- Neuroanatomy
- Synaptic Plasticity
Background:
- The entorhinal cortex (EC) projects to the dentate gyrus (DG) via the perforant path (PP), a critical pathway for memory formation.
- Granule cell proliferation in the DG is crucial for normal hippocampal development and function.
- X-irradiation at birth disrupts granule cell neurogenesis, offering a model to study compensatory mechanisms and aberrant circuitry.
Purpose of the Study:
- To investigate the functional and morphological characteristics of synapses formed by PP fibers in the medial DG of rats irradiated at birth.
- To determine if these synapses are functional and to identify their postsynaptic targets.
- To explore the implications of altered neurogenesis on synaptic connectivity and potential aberrant connections.
Main Methods:
- Adult rats were X-irradiated at birth to inhibit DG granule cell proliferation.
- Ultrastructural analysis was used to examine the morphology of PP synapses in the medial DG.
- Electrophysiological recordings were performed to assess the functional responses of these synapses upon PP stimulation.
Main Results:
- Ultrastructural evidence confirmed that PP fibers form synaptic contacts with dendritic spines in the medial DG of irradiated rats.
- Stimulation of the PP elicited functional synaptic responses, indicating the presence of active synapses.
- Findings suggest that PP fibers establish aberrant functional connections with the elongated basal dendrites of pyramidal cells.
Conclusions:
- Despite the absence of granule cell proliferation, functional synapses are formed by PP fibers in the medial DG.
- These synapses target dendritic spines, but aberrant connections with pyramidal cell basal dendrites are suggested.
- This study highlights the brain's capacity for forming synaptic connections even under conditions of disrupted neurogenesis, albeit with potential functional consequences.
Abstract:
We have examined the functional and morphological characteristics of synapses made by perforant path fibres projecting from the entorhinal cortex to the medial zone of the dentate area in adult rats which had been X-irradiated at birth, a procedure which prevents the proliferation of granule cells in that zone. We provide ultrastructural evidence that perforant path fibres in this region make synaptic contact with dendritic spines, and demonstrate that these synapses generate functional responses when the perforant path is stimulated. The evidence suggests that aberrant functional connections are made with the elongated basal dendrites of pyramidal cells.