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 parval bumin (PV) were examined, at the ultrastructural level, in the CA1 are a of the normal hippocampus and following a unilateral intracerebroven tricular kainic acid (KA) injection. Many degenerating PV-immunoreacti ve (IR) neurons were identified in the ipsilateral strata oriens and p yramidale at 3 days post-lesion and some were seen in stratum oriens o f the contralateral CA1 area. A few PV-IR neurons remained resistant t o the effects of KA. A chronic, almost total loss of PV-IR terminals w as detected around the soma of the ipsilateral CA1 pyramidal neurons. However, the PV-IR terminals around the axon initial segments of the C A1 pyramidial neurons remained intact at all post-lesion survival time s in both ipsilateral and contralateral tissue. The examination of ser ial ultrathin sections established the origin of the PV-IR terminals a round the axon initial segments to be the KA-resistant PV-IR neurons i n stratum pyramidale. This data provides evidence for the loss of nonp yramidal neurons following a KA lesion together with evidence for a su rviving inhibitory circuit that could, if functional, provide a very s trong inhibitory control of pyramidal neurons.