GABAergic neurons in the rat dentate gyrus are innervated by subcortical calretinin-containing afferents.

Abstract

Fibers of supramammillary origin establish putatively excitatory asymmetric synaptic connections with dentate granule cells. The present study was designed to determine whether hippocampal gamma-aminobutyric acid (GABA)-ergic nonprincipal cells are also targets of these calretinin (CR)-containing subcortical afferents. Light and electron microscopic double immunostaining for CR and parvalbumin (PA) or calbindin (CB) were performed in the rat dentate gyrus ipsilateral and contralateral to a unilateral fimbria-fornix transection. GABA-postembedding immunostaining was performed on ultrathin sections of this double-labeled material. Contralateral to the transection, CR-immunoreactive fibers formed multiple large boutons in the inner molecular layer. These fibers also impinged on PA-containing basket cells located adjacent to the granular layer and on CB-immunoreactive hilar neurons. Ipsilateral to the transection, CR-containing fibers in the inner molecular layer and boutons impinging on PA-containing or CB-immunoreactive neurons were absent. Parent cell bodies of extrinsic CR-containing afferents were traced using wheat germ agglutinin-conjugated horseradish peroxidase. Additional CR immunostaining of the subcortical region unveiled retrogradely labeled neurons that were also immunostained for CR only in the supramammillary area and the nucleus reuniens. The latter projection, however, terminates in CA1 and not in the dentate gyrus. Subcortical afferents impinging on dentate nonprincipal cells formed exclusively asymmetric synapses. Postembedding immunostaining demonstrated that CB-containing cells contain GABA, whereas CR-positive axon terminals forming asymmetric synapses are devoid of this labeling. These data indicate that dentate inhibitory neurons receive a putative excitatory input originating from the supramammillary nucleus. Thus, the supramamillo-hippocampal pathway may exert a powerful feed-forward inhibitory control of the signal flow in the rat dentate gyrus.