Caulerpenyne, a toxin from the seaweed Caulerpa taxifolia, depresses afterhyperpolarization in invertebrate neurons.

The massive invasion of the Mediterranean Sea by the tropical seaweed Caulerpa taxifolia (Vahl) C. Agardh has stimulated several investigations in order to test the environmental risk from an ecotoxicological point of view. The studies carried out on various experimental models have shown that caulerpenyne, the major metabolite synthesized by the seaweed, affects several cellular and molecular targets. In addition, neurological disorders have been reported in patients who accidentally ate C. taxifolia, but no evidence about the potential effects of the seaweed and of its metabolites on nerve cells were up to now available. Herein we describe that caulerpenyne modifies the electrical properties of touch mechanosensory cells of the leech Hirudo medicinalis. The physiological firing of these cells causes an afterhyperpolarization that is mainly due to the activity of the Na+/K+-ATPase and to a lesser extent to a calcium-dependent potassium current. Caulerpenyne depressed this afterhyperpolarization; the effect was dose-dependent and partially reversible. Experiments have been carried out in order to understand the mechanism through which caulerpenyne reduced the afterhyperpolarization. The action of the biotoxin has been tested in the presence of pharmacological blockers of calcium-dependent potassium channels such as cadmium and apamin. In these experimental conditions, caulerpenyne still reduced the residual afterhyperpolarization, suggesting a direct effect of the toxin on the Na+/K+-ATPase. In order to test this hypothesis, we have performed experiments where the Na+/K+-ATPase was activated by the intracellular injection of sodium and where also its basal activity was modified as well. From the data collected we suggest that caulerpenyne inhibits both the basal and the sodium-induced activity of the Na+/K+-ATPase in leech touch neurons.

Neurotoxic effects of caulerpenyne.

1. In this paper the authors tested the effect of caulerpenyne (CYN), a sesquiterpene synthesized by the green alga Caulerpa taxifolia onto the central nervous system of the leech Hirudo medicinalis. Investigations have been performed with three different approaches: neuroethological, electrophysiological and neurochemical techniques. 2. CYN application mimics the effect of a nociceptive stimulation (brushing), eliciting a clear-cut potentiation of the animal swim response to the test stimulus (non associative learning process such as sensitization). This effect is similar to that one induced by the endogenous neurotransmitter serotonin (5HT). 3. CYN strongly reduces the after-hyperpolarization (AHP) recorded from T sensory neurons. This effect overlaps that one produced by 5HT, but it is not affected by the serotonergic antagonist methysergide. 4. The decrease of AHP amplitude due to CYN application is observed also in presence of apamin, a blocking agent of Ca++-dependent K+ channels, suggesting that CYN is acting through the inhibition of the Na+/K+ electrogenic pump. 5. The depression of the AHP driven by CYN is not prevented by application of MDL 12330A, an adenylate cyclase inhibitor. On the other hand MDL 12330A counteracts the reduction of AHP due to 5HT application. 6. Incubation of the leech central nervous system with CYN induces the phosphorylation of proteins of 29, 50, 66 and 100 kDa. This pattern of phosphorylation is similar to that one elicited by 5HT treatment. 7. The data demonstrate that CYN exerts remarkable effects on leech neurons by acting onto specific molecular targets such as the Na+/K+ ATPase. This effect may influence important neural integrative functions and may explain the sensitizing action produced by the toxin on swim induction. Finally, caulerpenyne does not act through the pathways involved in the 5HT action, and its effect is not mediated by the second messenger cyclic AMP. The mechanism of action of CYN are still under investigations.

Neurobiological principles of learning and memory.

An increasing flow of evidences collected on elementary forms of learning processes in selected animal models evidentiates some mechanisms which can represent the basic cellular principles underlying plastic changes: 1. 5HT and second messengers of nucleotide type (like cAMP) have a pivotal role in the learning process. 2. In almost all short-term learning processes the modifications are subserved by a mechanism of protein phosphorylation. 3. In various animal models the modulation of K+ and Ca2+ channels is the molecular mechanism for learning. Experiments performed in sensory T neuron of the leech indicate that the modulation of Na+/K+ electrogenic pump is one of the fundamental mechanism for learning. 4. In long-term plastic changes, the most important finding is that newly synthesized proteins are formed. 5. In addition to what has been observed in the Aplysia model, where changes in synaptic efficacy represent the basic principles of memory storage, in the leech it has been demonstrated that a molecular machinery present in a single neuron can adapt the activity of the cell to environmental stimuli.