CULTURED BASAL FOREBRAIN CHOLINERGIC NEURONS IN CONTACT WITH CORTICAL-CELLS DISPLAY SYNAPSES, ENHANCED MORPHOLOGICAL FEATURES, AND DECREASED DEPENDENCE ON NERVE GROWTH-FACTOR

Prior studies examining the dependence of basal forebrain cholinergic neurons (BFCNs) on nerve growth factor (NGF) for survival have reached differing conclusions depending on the experimental paradigm employed , suggesting the importance of environmental and developmental variabl es. The present study examined the NGF dependence of BFCNs and modulat ory effects of target (cortical) neurons under the controlled conditio ns of dissociated cell cultures. Initial experiments found BFCNs (iden tified by using choline acetyltransferase immunocytochemistry) in pure basal forebrain (BF) cultures to be dependent on NGF between the 2nd and 4th week in vitro. During that developmental period, NGF deprivati on for 3 days, induced by application of anti-NGF antibody, resulted i n degeneration of over 80% of BFCNs, whereas at earlier or later times , BFCNs were largely resistant to NGF deprivation. When BF neurons wer e plated together with cortical neurons (as dissociated co-cultures), the BFCNs grew neuritic processes (labeled with acetylcholinesterase h istochemistry) that-appeared to specifically target cortical neurons; electron microscopy revealed that synapses formed between these cells. BFCNs in co-cultures were more resistant to NGF deprivation, were lar ger, and had much more extensive neuritic growth than BFCNs in pure BF cultures. The resistance of BFCNs to NGF deprivation provided by cort ical neurons could be largely reproduced by addition of other trophic factors (brain-derived neurotrophic factor, BDNF; neurotrophin 3, NT3; neurotrophin 4/5, NT4/5; or glial-derived neurotrophic factor, GDNF) during NGF deprivation in pure BF cultures. These results suggest that developing BFCNs undergo a critical period requiring trophic influenc es that may be provided by NGF or other trophic factors, as well as un known factors derived from cortical neurons. (C) 1996 Wiley-Liss, Inc.