Enhancement of quantal transmitter release and mediatophore expression by cyclic AMP in fibroblasts loaded with...

Neuronal properties such as neurotransmitter uptake and release can be expressed in non-neuronal cells. We show here that fibroblasts-mouse cell line L-M(TK-)-are able to take up acetylcholine from the external medium and to release it in response to a calcium influx. Release was assessed biochemically by a luminescence method, but it was also elicited from individual fibroblasts and recorded in real-time using a Xenopus myocyte as an acetylcholine detector. After treatment for three to six days with dibutyryl-cyclic AMP, the cells changed their shape and acetylcholine release was greatly enhanced. Surprisingly, in differentiated fibroblasts the time-course transmitter release exhibited a high degree of variability even for the successive responses evoked from the same cell; many currents recorded in myocytes on electrical stimulation of fibroblasts had an extremely long duration (up to 1 s or more). This suggested that the release sites were kept open for a very long time. Cyclic AMP treatment also caused a marked increase in the expression of mediatophore 16,000 mol. wt proteolipid in fibroblast membranes. Mediatophore is an acetylcholine-translocating protein which is abundant in cholinergic presynaptic plasma membranes. It is concluded that cyclic AMP differentiation of fibroblasts prolongs the duration of acetylcholine release at individual sites and enhances the expression of the 16,000 mol. wt proteolipid-forming mediatophore.