A paradox after systemic kainate injection in rats: lesser damage of hippocampal CA1 neurons after higher doses.

The pyramidal neuron loss in dorsal rat hippocampus was determined 4 days after i.p. administration of 10 or 20 mg/kg kainic acid (KA). Histological examination revealed that subtotal-to-total loss of the pyramidal neurons in both the CA3 and CA1 regions of hippocampus was produced after 10 mg/kg KA. At the higher dose, severe damage was evident in the CA3 region while no or only sporadic damage was observed in the CA1 region. These findings suggest that the high KA dose damaged the CA3 pyramidal neurons before excitatory input through the Schaffer collaterals produced irreversible damage to the CA1 pyramidal neurons.