DEVELOPMENT OF CHOLINERGIC AND GABAERGIC NEURONS IN THE RAT MEDIAL SEPTUM - EFFECT OF TARGET REMOVAL IN EARLY POSTNATAL-DEVELOPMENT

During normal development of the nervous system, the target fields inf luence the survival and differentiation of projection neurons, but the factors regulating this interaction remain obscure. In the present st udy, we have raised the question whether the target region is essentia l for the postnatal development and maintenance of two different types of central projection neurons, cholinergic and GABAergic septohippoca mpal cells. In early postnatal rats (P5, P10), the hippocampus was eli minated by unilateral intrahippocampal injections of the excitotoxin N -methyl-D-aspartate. After a long survival time (at P70), we have immu nostained serial sections of the septal region with antibodies against choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzy me, or the calcium-binding protein parvalbumin (PARV) which is known t o be contained in GABAergic septohippocampal neurons. In the medial se ptum ipsilateral to the lesioned side, about 60% of ChAT-immunoreactiv e neurons and 62% of PARV-immunoreactive neurons were found in adultho od even after complete elimination of the hippocampus. Some immunoreac tive cells appeared heavily shrunken, but electron microscopic analysi s revealed ultrastructural characteristics typical for medial septal n eurons obtained from controls. Our results indicate that target elimin ation during development affected both types of projection cells, alth ough only the cholinergic cells are known to be responsive to target-d erived factors. (C) 1997 Wiley-Liss, Inc.