Recordings in an integrating central neuron reveal the mode of action of isoeugenol

Abstract

Although isoeugenol is one of the most widely used anesthetics in fish, its actual mode of action and thus its applicability for particular interventions is poorly understood. Here we determined effects of isoeugenol on various aspects of sensory and neural function, taking advantage of intracellular in vivo recordings in a uniquely suited identified neuron, the Mauthner neuron in the brain of goldfish. We show that isoeugenol strongly affects hearing and vision, but sensitivity and time course of action differed largely in these two senses. The action potential, chemical and electric synaptic transmission at the central neuron were not affected at low but efficient anesthesia. Effects seen at high concentration thereby do not support current views of how isoeugenol might act on central neurons. We show that isoeugenol is highly useful to anesthetize fish for handling, but that in more severe treatment its application needs to be carefully adapted to task.

The effect of eugenol anesthesia on the electric organ discharge of the weakly electric fish Apteronotus leptorhynchus

“…These observations are consistent with the results of experiments carried out on rat neocortical and hippocampal slices that demonstrated reversible and dose-dependent suppression by eugenol of epileptiform field potentials and spreading depression, presumably via inhibition of synaptic plasticity (Müller et al 2006 ). On the other hand, it is important to keep in mind that direct comparison of our study and that of Machnik et al ( 2023 ) is not possible because not only were different fish species ( A. leptorhynchus vs. C. auratus ) and different neural systems (pacemaker nucleus vs. Mauthner neuron) examined but also different, though closely related, agents (eugenol vs. isoeugenol) were employed.…”

“…Despite the wide use of eugenol and related compounds as anesthetics in fish, little is known about their effects on CNS function. A first attempt to fill this critical gap in knowledge was undertaken recently by employing an antidromic-stimulation regime of the axon of the Mauthner cell to study the effect of different concentrations of isoeugenol on the generation and propagation of action potentials in goldfish ( Carassius auratus ) (Machnik et al 2023 ). No effect was observed at 10 mg/L and 20 mg/L, even after exposure of the fish to the anesthetic for more than 1 h. At 40 mg/L isoeugenol, a slight decrease in the slope of action potentials was evident after 10–30 min.…”

“…Clove oil and each of its three active ingredients have been widely used as fish anesthetics. Yet, only very recently possible effects of eugenol on physiological functions in the central nervous system (CNS) have attracted attention by investigators (Machnik et al 2023 ). The results of our investigation indicate a pronounced, dose-dependent effect of eugenol on the frequency of the neural oscillations of the Pn, commencing within less than a minute after the fish’s exposure to the anesthetic and lasting for up to 30 min after the fish’s removal from the immersion solution.…”