Hippocampal associational and commissural pathways: Anatomical and electrophysiological studies in the rat

Abstract

Hippocampal associational and commissural pathways of the dentate gyrus include inhibitory and excitatory elements, but which inhibitory and excitatory neurons contribute to this pathway remain unclear. In addition, it is not known whether hilar mossy cells establish a recurrent excitatory network with dentate granule cells or generate lateral inhibition by activating inhibitory neurons. Improved methods for detecting neuronal markers and the retrograde tracer Fluoro-Gold (FG) were used to identify associationally and commissurally projecting neurons of the rat hippocampus and describe their three-dimensional organization. FG-positive, commissurally projecting interneurons of the dentate granule cell layer and hilus were detected in numbers greater than previously reported. FG labeling of interneurons was as high as 96% of hilar somatostatin-positive interneurons, 84% of parvalbumin-positive cells of the granule cell layer and hilus, and 33% of hilar calretinin positive cells. Whereas hilar mossy cells and CA3 pyramidal cells were FG-labeled throughout the contralateral longitudinal axis, FG-positive interneurons exhibited a relatively homotopic distribution contralaterally. In addition, retrograde transport revealed that few inhibitory interneurons were among the many retrogradely labeled hilar neurons 2.5-4.5 mm longitudinal and ipsilateral to the FG injection site. Conversely, GluR2-positive hilar mossy cells were the only with significant longitudinal associational projections. During perforant pathway stimulation in urethane-anesthetized rats, diffusion of the GABA(A) receptor antagonist bicuculline methiodide from the tip of a glass recording electrode evoked granule cell discharges and c-Fos expression in granule cells, mossy cells, and inhibitory interneurons within a ∼400 μm radius of the tip. This focally-evoked activity powerfully suppressed distant granule cell responses recorded ∼2.5-4.5 mm longitudinally. Three days after kainic acid-induced status epilepticus or prolonged perforant pathway stimulation, longitudinal inhibition was intact in rats with <40% hilar neuron loss, but was consistently abolished after extensive (>85%) hilar cell loss. These data suggest that hilar mossy cells establish surround inhibition by activating inhibitory neurons and that mossy cell loss, which is observed in the disease process of temporal lobe epilepsy, may represent a network-level mechanism underlying post-injury hippocampal dysfunction and epileptic network hyperexcitability.