Ultrastructure of parvalbumin-immunoreactive neurons in the CA1 area of the rat hippocampus following a kainic acid injection.

Non-pyramidal neurons which contain the calcium-binding protein parvalbumin (PV) were examined, at the ultrastructural level, in the CA1 area of the normal hippocampus and following a unilateral intracerebroventricular kainic acid (KA) injection. Many degenerating PV-immunoreactive (IR) neurons were identified in the ipsilateral strata oriens and pyramidale at 3 days post-lesion and some were seen in stratum oriens of the contralateral CA1 area. A few PV-IR neurons remained resistant to the effects of KA. A chronic, almost total loss of PV-IR terminals was detected around the soma of the ipsilateral CA1 pyramidal neurons. However, the PV-IR terminals around the axon initial segments of the CA1 pyramidial neurons remained intact at all post-lesion survival times in both ipsilateral and contralateral tissue. The examination of serial ultrathin sections established the origin of the PV-IR terminals around the axon initial segments to be the KA-resistant PV-IR neurons in stratum pyramidale. This data provides evidence for the loss of non-pyramidal neurons following a KA lesion together with evidence for a surviving inhibitory circuit that could, if functional, provide a very strong inhibitory control of pyramidal neurons.