THE PRODUCTION OF STIMULUS-BOUND REPETITION BY NEOSTIGMINE IN SYMPATHETIC GANGLION CELLS

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

Intracellular recordings were made in isolated sympathetic ganglia of bullfrogs. With orthodromic stimulation (0.1 impulse/sec) in drug-free Ringer's solution, stimulus-bound repetition (SBR) occurred in about 10% of the ganglion cells, appearing as two spike responses to a single stimulus. In these cells the second, or repetitive, spike arises from the relatively persistent and large synaptic potential. After orthodromic tetanic conditioning (50 stimuli/sec, 40 sec), approximately 18% of the cells exhibited SBR. In the presence of neostigmine (0.06-6 µ M), 62% of the cells displayed SBR without tetanic conditioning, whereas after conditioning nearly every cell fired repetitively, with the number of stimulus-bound repetitive spikes ranging between two and nine. Underlying the increased incidence and degree of SBR was an increased duration and amplitude of the synaptic potential. SBR was particularly dependent on orthodromic stimulus frequency, with repetitive spikes being abolished progressively as frequency was raised above 0.1 impulse/sec. All cells in the presence of neostigmine displayed spontaneous miniature synaptic potentials which were sometimes large enough to initiate spikes. Antidromic stimuli always evoked single spike responses, and antidromic tetanization (50 stimuh/sec, 40 sec) never elicited SBR. Repetitive spiking did not occur at higher concentrations of neostigmine (0.6 mM). The results suggest that SBR reflects a prejunctional action of neostigmine, the mechanism of which remains to be determined.