PARVALBUMIN-CONTAINING CELLS OF THE ANGULAR PORTION OF THE VERTICAL LIMB TERMINATE ON CALBINDIN IMMUNOREACTIVE NEURONS LOCATED AT THE BORDER BETWEEN THE LATERAL AND MEDIAL SEPTUM OF THE RAT

In the septal complex, both parvalbumin and calbindin neurons cocontai n GABA. In the same area, a large number of GABA-GABA synaptic connect ions can be observed. In order to further characterize their neurochem ical nature, as well as the extrinsic and/or intrinsic origin of these GABA terminals, the following experiments were performed: (1) correla ted light- and electron-microscopic double immunostaining for calbindi n and parvalbumin on septal sections of control rats; (2) light micros copic parvalbumin immunostaining of septal sections after surgical iso lation (5 days) of the septum from its telencephalic or (3) hypothalam ic afferents; and (4) parvalbumin immunostaining of sections prepared from the entire brain 2 days following horseradish peroxidase injectio n into the border between the lateral and medial septum. The results d emonstrated that: (1) in a well-circumscribed, vertically longitudinal area located between the lateral and medial septum, 0.1-0.6 mm anteri or to the bregma, a group of calbindin-containing, nonsomatospiny neur ons are surrounded by parvalbumin-immunoreactive baskets; (2) these ba sket-forming axon terminals establish symmetric synaptic contacts with their targets; and (3) their cells of origin are not in the medial se ptum, but in the angular portion of the vertical limb. These observati ons indicate that a portion of the septal complex GABA-GABA synaptic c onnections represent functional interaction between two different type s of GABAergic neurons. The presynaptic GABAergic neurons contain parv albumin, and the postsynaptic GABAergic cells are immunoreactive for c albindin. Furthermore, a population of the medial septum/diagonal band parvalbumin neurons project only to the hippocampus, while others, wh ich may also send axons to the hippocampus, terminate on lateral septu m calbindin cells as well.