GANGLION CELL DEPOLARIZATION AND TRANSMISSION BLOCK BY ACh: INDEPENDENT EVENTS

¹ Woman's Medical College of Pennsylvania, Philadelphia, Pennsylvania

Acetylcholine (ACh) was injected (10 µ of 2, 5.5 or 16.5 mM) during intracellular recording of evoked orthodromic responses in frog sympathetic ganglia. Transmission block and ganglion cell depolarization by ACh were dissociated in the following ways. 1) Transmission block frequently occurred at levels of ganglion cell depolarization lower than the maximum attained, and persisted longer than ganglion cell depolarization. 2) Passage of inward current, during transmission block and ganglion cell depolarization, restored membrane potential to the resting level but did not restore transmission. 3) A number of cells were not depolarized at all by low doses of ACh (2 mM), yet transmission block still occurred. 4) Orthodromic responses were not abolished during ganglion cell depolarization (10-15 mV) produced by outward current, but equivalent ganglion cell depolarization produced by ACh caused transmission block in approximately 80% of cells. 5) The ACh doses tested do not abolish antidromic and direct spikes. These results suggest that transmission block and ganglion cell depolarization by ACh are not mechanistically linked. They also suggest, with the lack of evidence for specific interaction between ACh and a synaptic region of the cell (Ginsborg and Guerrero, 1964; Riker. 1967a), that there is no reason to assign ACh transmission block to a postsynaptic site. It is proposed that ACh transmission block occurs at the unmyelinated terminal portions of the presynaptic nerve.