Neurophysiological Effects of Naturally Occurring Defensive Compounds on the Freshwater Snail Planorbis corneus: Comparison with Effects in Insects

Abstract

The essential oil constituents citral, geraniol, and eugenol have toxic or repellent properties that are utilized by a variety of organisms to deter natural enemies. Their mechanism of action is unknown, but some essential oils such as eugenol are claimed to act on insects by specific binding to octopamine receptors. We studied their effects on the isolated buccal ganglia of Planorbis corneus, having demonstrated that they caused cessation of feeding and death when added to the aquarium water (approximately 5 × 10⁻⁴ mol l⁻¹). They abolished spike activity at 2 × 10⁻³ mol l⁻¹ but at lower doses (threshold 5 × 10⁻⁵ mol l⁻¹) they resembled octopamine in eliciting burst firing indicative of the fictive feeding sequence. However, the octopamine antagonists phentolamine (10⁻⁵ mol l⁻¹), yohimbine (10⁻⁴ mol l⁻¹; which blocked hyperpolarizing octopamine responses), and metoclopramide (10⁻⁴ mol l⁻¹; which blocked depolarization) had no effect on any of the responses to the oils. The oils produced incomplete block of excitatory and inhibitory responses to octopamine even at high doses (10⁻³ mol l⁻¹), and they had similar effects on responses to dopamine and acetylcholine. The oils (10⁻⁵–2 × 10⁻³ mol l⁻¹) all increased the frequency of contractions of the isolated esophagus and progressively reduced their amplitude. The effects were similar to those of octopamine but were not blocked by phentolamine, metoclopramide, or prolonged exposure of the esophagus to octopamine. The results suggest a variety of actions that could contribute to toxic effects in molluscs, but there was no evidence for specific actions on octopamine receptors, either as agonists or antagonists. Evidence is presented that nonspecific depolarization produces their octopamine-like actions.