Worm Breeder's Gazette 14(1): 41 (October 1, 1995)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Effects of Conus venoms on the pharynx

Leon Avery

(leon@eatworms.swmed.edu) Department of Biochemistry, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, TX 75235-9038

Cone snails are predatory snails. They harpoon their prey, which may be small fish, marine worms, or other snails, then inject a paralyzing venom through the hollow harpoon (Olivera et al, Science 249: 257). The venom is a complex mixture of tiny peptide toxins, different in each species. Several of these toxins have been shown to act on cell-surface molecules important for nervous system function such as the N-type Ca++ channel or the voltage-gated Na+ channel (Gray et al, Ann Rev Biochem 57: 665).

I have been testing Conus venoms (donated by Baldomero Olivera) on the pharynx. Pharynxes are dissected into 200 Ál saline solution and an EPG recorded. After 1 min crude venom is added to the solution. Most of the venoms were tested on wild-type pharynxes in saline, wild-type pharynxes in the presence of 1 ÁM serotonin, and eat-18 mutant pharynxes in the presence of 1 ÁM serotonin. eat-18 is necessary for the action of the putative pacemaker neuron MC on pharyngeal muscle. Serotonin speeds up pumping by activating MC and pharyngeal muscle. So far I have tested venoms from six species: C caracteristicus, C geographus, C imperialis, C magus, C marmoreus, and C textile. All venoms cause pumping to slow down or stop, usually temporarily. Four venoms produce specific effects in addition to this general inhibition.

The effects of C caracteristicus venom are consistent with a block of all pharyngeal nervous system activity. Unfortunately, very high concentrations (2.4 mg/ml nominal) are required for this effect, so I have not investigated it carefully.

C magus venom consistently makes muscle contractions feeble (0.45 mg/ml nominal), and at high concentration (4.5 mg/ml nominal) paralyzes muscle motion. In addition, the muscle action potentials often become long-lasting. C geographus has similar but weaker effects. Joe Dent has shown that both these effects can be produced by lowering the Ca++ concentration or adding Ca++ channel blockers (WBG 13(1): 44), and C magus and C geographus venoms are known to contain omega-conotoxins, which block voltage-gated Ca++ channels (Gray et al, Ann Rev Biochem 57: 665). Therefore I suspect these effects are caused by a muscle Ca++ channel block. There is no striking decrease in MC or M3 transmission, so the effect may be specific to the muscle Ca++ channel. In vertebrates, in contrast, omega-conotoxins specifically block neuronal Ca++ channels.

C imperialis venom has the most interesting effect: it stimulates rapid pumping. It probably acts by (directly or indirectly) stimulating MC, since it has little or no effect on eat-18 mutant pharynxes, and the rapid pumping has the electrophysiological signatures of MC activity. The figure is a recording from a wild-type pharynx without serotonin. In the minute before addition of venom there were three brief bursts of pumping. The addition of 0.45 mg/ml C imperialis venom at 1 min can be seen as a burst of noise. After a delay (perhaps because of the general inhibitory effect all the venoms have), the pharynx began to pump at a rate of 4-5 Hz. In other pharynxes this stimulation has been seen to last at least 9 min.