HEAT-SHOCK PROTEIN EXPRESSION IN VULNERABLE CELLS OF THE RAT HIPPOCAMPUS AS AN INDICATOR OF EXCITATION-INDUCED NEURONAL STRESS

The inducible 72 kDa heat shock protein (HSP72) has been shown to be protective in non-neuronal cells and neurons in culture, but its function and the control of its expression in the CNS are poorly understood. Although HSP72 is induced in neurons in vivo by neurotoxic compounds that produce seizures and neuronal damage, it is unknown if its expression is a specific response to excitation per se or to "stressful" or potentially injurious excitation, or if it is a marker or mediator of irreversible injury. We have attempted to identify the nature of the stimulus for HSP72 expression by utilizing focal electrical stimulation that can either excite or destroy postsynaptic cells, depending on the duration of afferent stimulation. Previous studies have demonstrated that intermittent stimulation of the main hippocampal afferent pathway for 24 hr evokes synchronous discharges in dentate granule cells but does not injure them. However, the same stimulation irreversibly destroys three of the four cell populations innervated by the granule cells. The three vulnerable populations are the dentate hilar mossy cells, the somatostatin/neuropeptide Y (NPY)-immunoreactive hilar neurons, and the CA3c pyramidal cells. The fourth and relatively resistant population is the GABA-immunoreactive dentate basket cells. In this study, we have localized HSP72 expression immunocytochemically in the hippocampal dentate gyrus in response to nontoxic durations of potentially neurotoxic afferent stimulation. One hour of perforant path stimulation, which evoked granule cell spikes and epileptiform discharges and aberrant NPY expression in virtually all granule cells 1 d later, evoked no detectable HSP72 immunoreactivity in any hippocampal neurons. However, 1 d after stimulation for 2-6 hr, HSP72 immunoreactivity was observed in the seizure-susceptible dentate hilar cells and CA3c neurons that would have died had stimulation continued. Despite being directly and repetitively activated by the afferent stimulation, the relatively seizure-resistant granule cells and dentate pyramidal-shaped basket cells did not express detectable HSP72 immunoreactivity. These results indicate that HSP72 expression is a sensitive and cell-specific response to potentially injurious neuronal stress caused by repetitive excitation, and is a marker neither of irreversible cell injury nor of nonstressful excitation.