Dentate hilar mossy cells and somatostatin-containing neurons are immunoreactive for the α8 integrin subunit: characterization in normal and kainic acid-treated rats

Abstract

Integrins are heterodimeric cell surface receptors composed of different α and β subunits that mediate cell–cell and cell–extracellular matrix interactions. They have been implicated in the regulation of neuronal migration, differentiation, process outgrowth, and plasticity. The α8 integrin subunit associates exclusively with the β1 subunit to form a receptor (α8β1) for fibronectin, vitronectin, tenascin, and osteopontin. In a previous study, we demonstrated that hippocampal dentate hilar neurons are immunoreactive for α8. The present study identifies the major types of α8-immunoreactive hilar neurons and characterizes the effects of kainic acid-induced seizures on α8-immunoreactivity in these cells. Examination of the hilus in normal rats revealed α8-immunoreactivity in the somatodendritic compartments of large hilar neurons identified as mossy cells, including a subset of dendritic thorny excrescences that were contacted by large mossy fiber terminals. α8-immunoreactivity also was found in approximately 71% of somatostatin-containing hilar cells. Kainic acid-induced seizures dramatically and rapidly altered the levels and distribution of α8-immunoreactivity in hilar neurons. After 1.5 h of seizures, α8-immunoreactivity in their dendrites was reduced greatly. One day after kainic acid treatment, labeling was diminished throughout the somatodendritic compartments of most hilar cells. This decrease appeared to be transient, since α8 labeling returned to normal levels in surviving hilar neurons within 2 weeks of treatment. In addition, many α8-immunoreactive hilar neurons, particularly in caudal dentate regions, were lost 3–5 weeks after kainic acid treatment. Our findings suggest that α8β1 may mediate adhesive interactions of the dendritic processes of mossy cells and somatostatin-containing hilar neurons with other cellular elements or with extracellular matrix components. They also suggest that α8 may be susceptible to activity-dependent proteolysis that could modulate its function in the somatodendritic compartment of these cells.