AFFERENTS TO THE SEIZURE-SENSITIVE NEURONS IN LAYER-III OF THE MEDIALENTORHINAL AREA - A TRACING STUDY IN THE RAT

Neurons in layer III of the medial entorhinal area (MEA) in the rat ar e extremely vulnerable to local injections of amino-oxyacetic acid and to exprimentally induced limbic seizures. A comparable specific patho logy has been noted in surgical specimens from patients with temporal lobe epilepsy. Efforts to understand this preferential neuronal vulner ability led us to study the neural input to this layer in the rat. Ion tophoretic injection of the retrograde tracer fast blue, aimed at laye r III of the MEA, resulted in retrogradely labeled neurons in the pres ubiculum in all the injected hemispheres. The nucleus reuniens thalami , the anteromedial thalamic nucleus, the ventral portion of the claust rum (endopiriform nucleus), the dorsomedial parts of the anteroventral thalamic nucleus, and the septum-diagonal band complex were labeled l ess frequently. In only one experiment, retrogradely labeled neurons w ere observed in the ventrolateral hypothalamus and in the brainstem nu cleus raphe dorsalis. Since projections from claustrum to the entorhin al cortex has not been studied in the rat with modern sensitive antero grade tracing techniques, iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were placed into the ventral portion of the claustrum. Anterogradely labeled fibers in the entorhi nal area proved not to be confined to the MEA, since a prominent proje ction distributed to the lateral entorhinal area as well. In both area s, the densest terminal labeling was present in layers IV-VI, whereas layer III appeared to be only sparsely labeled. The present data indic ate that of all potential afferents only those from the presubiculum d istribute preferentially to layer III of the MEA. This, in turn, sugge sts a potentially important role of the presubiculum in the seizure-re lated degeneration of neurons in layer III of the MEA.