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We reared 120 Rana sphenocephala tadpoles in 1.5 m long chambers (10 tadpoles per chamber) made from vinyl rain gutters. The chambers contained predatory late-instar dragonfly naiads (Odonata: Libellulidae) confined at one end in clear tubes drilled with aeration holes. The number of confined predators varied between control (n = 0), low (n = 1), and high density conditions (n = 5). Tadpoles were free to swim throughout each chamber, although food was isolated at one end adjacent to the predator tubes. In an additional high density predator condition, food was isolated at the opposite end of the chamber away from the predator tubes. There were a total of three replicates for each of the four conditions. Tadpoles from each condition were individually weighed (g) and their total length (mm) was recorded at regular intervals throughout the trial. Tadpoles raised in the high density condition with multiple predators surrounding their only food source were significantly shorter in length and weighed significantly less that those reared in all other conditions. When multiple predators and larval food were isolated at opposite ends of the chamber, tadpole growth did not statistically differ from that of control animals. Tadpoles raised with multiple predators surrounding their food also possessed slightly deeper tails, although these results were not statistically significant. Our results suggest that the inhibition of tadpole growth observed in this study may be linked to behavioral changes when perceived predation risks are high.
The M-current is a voltage-activated outward K current that plays important roles in the regulation of neuronal resting membrane potential and action potential firing patterns. Retigabine is an investigational anticonvulsant drug that acts to enhance M-current in neurons or cells expressing M-channel subunits. The present study examines the actions of retigabine on the M-current of bullfrog sympathetic neurons, the cells in which the M-current was originally described. Using whole cell patch-clamp recordings from isolated bullfrog neurons, we found that retigabine (10 µM) produced a -12 mV hyperpolarization of the resting membrane potential. After controlling for this hyperpolarization, neurons were stimulated with depolarizing current pulses to induce action potential firing. Retigabine produced a 92% decrease in action potential production. Voltage clamp analysis revealed that retigabine produced a 71% increase in outward current amplitude at -30 mV, an increase that was accompanied by a large enhancement in M-current amplitude. The increased level of M-current appeared to be due to a retigabine-induced 19 mV hyperpolarizing shift in the activation threshold voltage for the M-current. All responses observed in the presence of retigabine were readily reversed after wash-out of the drug. Our findings support the conclusion that retigabine dampens neuronal excitability by acting as an M-current agonist. This drug, which is currently in clinical trials, may thus prove to be an effective anticonvulsant agent.