PARVALBUMIN-IMMUNOREACTIVE NEURONS IN THE ENTORHINAL CORTEX OF THE RAT - LOCALIZATION, MORPHOLOGY, CONNECTIVITY AND ULTRASTRUCTURE

We studied the distribution, morphology, ultrastructure and connectivi ty of parvalbumin-immunoreactive neurons in the entorhinal cortex of t he rat. Immunoreactive cell bodies were found in all layers of the ent orhinal cortex except layer I. The highest numbers were observed in la yers II and III of the dorsal division of the lateral entorhinal area whereas the lowest numbers occurred in the ventral division of the lat eral entorhinal area. Most such neurons displayed multipolar configura tions with smooth dendrites. We distinguished a type with long dendrit es and a type with short dendrites. We also observed pyramidal immunor eactive neurons. A dense plexus of immunoreactive dendrites and axons was prominent in layers II and III of the dorsal division of the later al entorhinal area and the medial entorhinal area. None of the parvalb umin-immunoreactive cells became retrogradely labelled after injection of horseradish peroxidase into the hippocampal formation. By electron microscopy, immunoreactivity was observed in cell bodies, dendrites, myelinated and unmyelinated axons and axon terminals. Immunoreactive d endrites and axons occurred in all cortical layers. We noted many myel inated immunoreactive axons. Immunoreactive axon terminals were medium sized, contained pleomorphic synaptic vesicles, and established symme trical synapses. Both horseradish peroxidase labelled and unlabelled i mmunonegative cell bodies often received synapses from immunopositive axon terminals arranged in baskets. Synapses between immunoreactive ax on terminals and unlabelled dendritic shafts and spines were abundant. Synapses with initial axon segments occurred less frequently. In addi tion, synaptic contacts were present between immunopositive axon termi nals and cell bodies and dendrites. Thus, the several types of parvalb umin-containing neuron in the entorhinal cortex are interneurons, conn ected to one another and to immunonegative neurons through a network o f synaptic contacts. Immunonegative cells projecting to the hippocampa l formation receive axe-somatic basket synapses from immunopositive te rminals. This connectivity may form the morphological substrate underl ying the reported strong inhibition of cells in layers II and III of t he entorhinal cortex projecting to the hippocampal formation.