Citation:

Jung, G., E. Pitzer, T. Beasley, K. McDaniel, R. Sills, R. Herbert, C. Ganta, R. Fisher, H. Jensen, J. Harry, AND D. Herr. Neurophysiological assessment of peripheral nerve, somatosensory, and brainstem auditory function after perinatal exposure to emamectin in Long Evan rats. Society of Toxicology, Salt Lake City, UT, March 10 - 14, 2024.

Impact/Purpose:

This work provides neurophysiological evaluation of animals perinatally exposed to emamectin. The data will be used to support AOP development for developmental neurotoxicity.

Description:

Background: We have previously observed altered ontogeny of locomotor activity and uncoordinated hindlimb gait and decreased startle response in adult animals following perinatal exposure to emamectin benzoate (EB), which acts as a positive allosteric modulator for GABA-gated chloride channels.  This work examined peripheral nerves and somatosensory evoked responses as potential neuroanatomical locations related to the altered hindlimb gait and motor activity. Similarly, auditory evoked potentials were examined as potential neuronal sites related to the changed startle response. Methods: Pregnant Long Evans rats were gavaged with deionized water (vehicle), 2.29, or 3.78 mg/kg EB (5 mL/kg dosage volume) from gestational day 6 to postnatal day (PND) 21. On approximately PND 76 the male offspring (n=18-21/treatment) were surgically implanted with epidural screw electrodes (1 mm2 area) over the somatosensory cortex and cerebellum, along with ground and reference electrodes. About one week later, compound nerve action potentials (CNAP) and nerve conduction velocity (NCV; tail nerves) were assessed, along with somatosensory function by recording evoked potentials (SEPs) over the somatosensory cortex and cerebellum. Brainstem auditory evoked responses were recorded using click, 4, 16 and 64 kHz tone pips as stimuli. A subset of the animals was perfused with buffered saline and paraformaldehyde, fixed in situ, sectioned, and stained with H&E or immunochemical staining for parvalbumin. A mixed model analysis of variance with appropriate distributions and covariance structures was used to analyze the data. Results: The evoked responses were altered by the various stimulus manipulations (intensity, frequency), demonstrating that the preparations were under stimulus control and responding as expected. Perinatal treatment with EB did not alter the amplitudes, latencies, or NCV of compound action potentials in the tail nerve. The evoked responses recorded over the cerebellum or somatosensory cortex were not statistically changed by treatment with EB. However, in the cerebellar response, there was a ~25-30% increase in the amplitude of peak P55 in the 3.78 mg/kg treatment group using a 2 or 3 mA stimulus. In the somatosensory cortex response, there was a 54% increase in the amplitude of peak N53 using a 3 mA stimulus. The effect sizes of the changes in the responses from the cerebellum and cortex are in the medium – large range. The amplitudes of peaks of the brainstem auditory responses were not statistically altered by EB treatment. However, there was a reduction in the amplitude of the response in region of peaks P4–P5 using an 80 dB click or 16 kHz stimulus. The decrease in peak P4 amplitude ranged from 24-15%, and is considered a medium effect size. The change in auditory potential shape resulted in changes in the latencies of peaks in the P5-P6 region of the response. Preliminary analysis showed no significant difference in H&E and parvalbumin staining in the neuronal cell bodies. Conclusions: Perinatal treatment with EB did not cause changes in the neurophysiological measures of peripheral nerves. Possible changes in somatosensory potentials involved increased amplitudes of later portions of the response, indicating changes in cortical or cerebellar processing. Possible decreases in amplitudes of auditory responses suggest altered neurotransmission in the region of the lateral lemniscus (peak P4). The data suggest possible alterations in the balance of excitatory/inhibitory neurons, rather than loss of groups of nerve cells. These hypotheses should be investigated using intra-cortical recordings and additional histopathological assessment. This is an abstract of a proposed presentation and does not necessarily reflect US EPA policy.

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