Use of non-mammalian alternative models for neurotoxicological study

Abstract

The field of neurotoxicology needs to satisfy two opposing demands: the testing of a growing list of chemicals, and resource limitations and ethical concerns associated with testing using traditional mammalian species. National and international government agencies have defined a need to reduce, refine or replace mammalian species in toxicological testing with alternative testing methods and non-mammalian models. Toxicological assays using alternative animal models may relieve some of this pressure by allowing testing of more compounds while reducing expense and using fewer mammals. Recent advances in genetic technologies and the strong conservation between human and non-mammalian genomes allow for the dissection of the molecular pathways involved in neurotoxicological responses and neurological diseases using genetically tractable organisms. In this review, applications of four non-mammalian species, zebrafish, cockroach, Drosophila, and Caenorhabditis elegans, in the investigation of neurotoxicology and neurological diseases are presented.

Figure 1

Transgenic zebrafish expressing a blue fluorescent protein from the cardiac myosin light chain 2 promotor. Image courtesy of Peter Schlueter.

Figure 2

Development of C. elegans from larvae to adult over 72 h. Extinction (optical density) versus time of flight (length) for nematodes incubated at 20°C for 0, 24, 48, or 72 h. Each point corresponds to a single nematode.

Figure 3

Neurotoxic effects of chlorpyrifos on adult C. elegans feeding. Fitted concentration-response curve of a representative experiment based on observed mean size-adjusted fluorescence measurements as a percent of the control for chlorpyrifos. Each point represents approximately 120 nematodes on average, with counts ranging from 104 to 141 (For details, see Boyd et al., 2007).

Figure 4

Effect of cadmium-exposure on mtl-1::GFP transgenic C. elegans. Transgenic nematode expressing GFP under the control of the C. elegans metallothionein promoter (mtl-1) were grown in the absence (upper panel) or presence (lower panel) of 100 μM cadmium for 24 h. Constitutive mtl-1 transcription is observed in the pharynx of the nematodes, whole metal-inducible transcription occurs in the nematode intestine.

Figure 5

A schematic diagram of the sodium channel protein, indicating four homologous domains (I-IV), each having six transmembrane segments (1 to 6). The locations of the sequences corresponding to the two mutually exclusive exons G1/G2 in the cockroach BgNa_v gene are indicated with solid blocks. The corresponding exons l and k in the Drosophila para gene are indicated in parenthesis. An alignment of amino acid sequences encoded by exons G1 and G2 are presented below the topology diagram. The residues in G2 that are identical to those in G1 are marked with dots.

Figure 6

Glutamate-activated currents in cockroach thoracic ganglion neurons recorded by the whole-cell patch clamp method. Three types of currents were evoked following the U-tube applications of 100 μM glutamate for 8 sec at a holding potential of −60 mV, with the symmetrical chloride concentrations between internal and external solutions (Zhao et al., 2004b).

Figure 7

Dose-response relationships of fipronil block of slow- desensitizing and fast-desensitizing GluCls of cockroach neurons. The currents were evoked by 20-sec U-tube and bath application of 100 μM glutamate and various concentrations of fipronil at a holding potential of −60 mV. The maximum peak current was measured for the fast-desensitizing current and the steady-state current was measured for the slow-desensitizing current (Zhao et al., 2004a).