Effects of denervation and lack of calcium on the action of [formula omitted] venom on rat sympathetic ganglion

Abstract

Black-widow spider or Latrodectus mactans venom (L.V.) is known to block transmission of impulses in the isolated cord of the cockroach (1, 2) as well as the activity of the crayfish stretch receptor (3). It appears that, at least in the crayfish, this block is due to a direct depolarizing action of the venom on the somadendritic membrane (Obara and Mauro, personal communication). In contrast, L.V. blocks neuromuscular transmission in the frog, where, by acting presynaptically, it causes a large increase in the frequency of miniature end-plate potentials (MEPP), eventually leading to their exhaustion (4). A similar effect of L.V. on the neuromuscular junction was observed in the cat by Okamoto et al. (5) with no significant change in the muscular response evoked by direct electrical stimulation or by intra-arterial injection of acetylcholine (ACh.) When the venom is applied to isolated sympathetic ganglion of the rat it causes action potentials in the postganglionic cells and blocks synaptic transmission (6). It appears that this blocking action of the venom is again due to release and “exhaustion” of transmitter, since a) a previous curarization of the ganglion prevents the ganglion cell discharges and b) the venom causes a 50 per cent decrease in the labelled ACh content of the resting unstimulated ganglion which has been previously incubated with H 3-choline for 2 hr while undergoing repetitive stimulation (6). The present study is a further attempt to elucidate the mode of action of L.V. First we have sought further evidence for the presynaptic action by studying its effect on previously denervated ganglia. A second approach has considered the effect on ganglia incubated in a Ca-free medium. This condition is known to prevent the release of ACh normally caused by presynaptic depolarization due to a nerve impulse or to a high-K medium (7). In fact, Longenecker et al. (4) reported that Ca 2+ ions are not necessary in obtaining the effect of L.V. at the frog neuromuscular junction (viz. increased frequeency of MEPP).