EMERGENCE OF A SYNAPTIC NEURONAL NETWORK WITHIN PRIMARY STRIATAL CULTURES SEEDED IN SERUM-FREE MEDIUM

In order to investigate the basic cellular mechanisms involved in neur onal interactions within the striatum, we prepared a primary striatal cell culture from rat fetal brain in chemically defined medium. Using morphological and whole-cell recording methods, we observed that an in tensive neuritic elongation with a progressive build up of a sodium-de pendent electrogenesis occurred during the first week of culture. Morp hologically mature synapses began to develop after 10 days in vitro. B y this time, most of the neurons (82 +/- 9%) received spontaneously sy naptic potentials, which led them to fire (71 +/- 11%). The spontaneou s firing was prevented by cadmium (200 mu M) and tetrodotoxin (5 mu M) , which suggested that a Ca2+-dependent release of neurotransmitters w as involved in the synaptic activation. We further obtained evidence t hat GABA, and to a lesser extent acetylcholine, contributed to these s pontaneous synaptic potentials. At 15 days in vitro, it was possible t o observe up to four synaptic contacts on a given dendrite. By this ti me, whole-cell recordings performed on pairs of neurons showed that th e mature neurons were interconnected by excitatory synapses. As the nu mber of synapses increased, the striatal neurons gradually formed a la rge network in which spontaneous activity developed, which tended to b e organized into synchronized bursting patterns.