Recording from an Identified Neuron Efficiently Reveals Hazard for Brain Function in Risk Assessment

Abstract

Modern societies use a continuously growing number of chemicals. Because these are released into the environment and are taken up by humans, rigorous (but practicable) risk assessment must precede the approval of new substances for commerce. A number of tests is applicable, but it has been very difficult to efficiently assay the effect of chemicals on communication and information processing in vivo in the adult vertebrate brain. Here, we suggest a straightforward way to rapidly and accurately detect effects of chemical exposure on action potential generation, synaptic transmission, central information processing, and even processing in sensory systems in vivo by recording from a single neuron. The approach is possible in an identified neuron in the hindbrain of fish that integrates various sources of information and whose properties are ideal for rapid analysis of the various effects chemicals can have on the nervous system. The analysis uses fish but, as we discuss here, key neuronal functions are conserved and differences can only be due to differences in metabolism or passage into the brain, factors that can easily be determined. Speed and efficiency of the method, therefore, make it suitable to provide information in risk assessment, as we illustrate here with the effects of bisphenols on adult brain function.

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

“…The recording electrode was moved in the brain using a motorized micromanipulator (MP-285; Sutter Instrument, USA). We used established techniques to localize and to identify one of the two MNs for intracellular in vivo recording 21,30,37 . Recordings were filtered (Hum Bug Noise Eliminator; Quest Scientific, Canada) and digitized (A/D converter Micro1401; Cambridge Electronic Design Limited, UK) at 50 kHz.…”

“…This creates a challenging situation: In principle, several anesthetics are available for use in fish and some of them are used extensively, but often with different traditions in aquaculture and in research 15,18,19 . In both, however, evidence is largely missing about their mode of action, the required concentrations, the required time of exposure and time till full recovery and it is often not known how anesthetic agents affect the functions that should be studied in the anesthetized animal [20][21][22] . In contrast to mammalsfor which many more studies are availablesuch information is presently mostly lacking for fish and is urgently needed 18,22 .…”

“…Specifically, it is still unclear whether isoeugenol acts as a local anesthetic 18,23,24 i.e., prevents sensory information from reaching the brain 20 or inhibits sodium channels of neurons in the brain 25 and thus acts as a systemic anesthetic 20,22 . Here we use an approach that we introduced recently 21,26,27 to examine the mode of action of isoeugenol and to provide evidence critical for using it in research labs (Fig. 1).…”

“…To study the anesthetic effects a substance can have in fish and to narrow down its mode of action, the Mauthner neuron (MN) thereby is an ideal substrate. The MN is one of the very few neurons in the vertebrate brain that can be identified individually from one animal to the next and that is readily accessible to intracellular in vivo recording 21,26,30 . It has therefore been a major source of insight into fundamental mechanisms of synaptic communication in the vertebrate CNS 31 .…”

“…By probing the MN, it is thus possible to obtain information on diverse aspects of brain and sensory function. Applying different sensory stimuli in vivo allows to work out the effect that the anesthetic has on each given sensory channel, from sensory transduction, the transmission of this information to the CNS and central processing 21,27 . By stimulating the axon antidromically, the effect of an anesthetic on action potential generation and spreading in the CNS can be studied,which allows to test hypotheses of whether an anesthetic acts generally on axons in the CNS.…”