TARGET-DEPENDENT SYNAPTOGENESIS - INDUCTIVE EFFECT OF PITUITARY MELANOTROPHS ON THEIR CENTRAL INNERVATION

The glandular activity of the vertebrate pituitary intermediate lobe ( IL) is regulated by direct cellular innervation, in contrast with the purely humoral regulation of adjacent pituitary anterior lobe (AL). Th us in the rat IL, melanotrophs receive a dopaminergic and GABAergic in nervation from the basal hypothalamus, which tonically inhibit their g landular activity. We studied this model of neuron-target interactions in cocultures in defined medium of fetal hypothalamic neurons with ne onate pituitary glandular cells. In the cocultures with IL cells, neur oglandular contacts occurred after 4 days in vitro (DIV) but required another 8 DIV to exhibit ultrastructural and immunocytochemical featur es of fully differentiated functional synapses; by contrast, neuroneur onal synapses developed much faster and could already be detected afte r 4 DIV. In the cocultures with AL cells, neuroglandular contacts neve r mature in differentiated synapses. Confocal microscope observation r evealed that dopaminergic neurons, which represented less than 1% of t otal neurons in the cocultures, established 50% of the synapses detect ed on the melanotrophs. These cells are thus able, contrary to the AL cells, to promote the establishment of functional synapses and, to som e extent, to select their specific innervation. (C) Wiley-Liss, Inc.