Histopathological Evaluation of the Nervous System in National Toxicology Program Rodent Studies

Abstract

This article outlines the changes and underlying rationale for modifications to the histopathological evaluation of the nervous system during toxicology and carcinogenesis studies conducted by the National Toxicology Program (NTP). In the past, routine evaluation of the nervous system was mostly limited to three sections of brain, and occasionally the spinal cord and peripheral nerves. Factors such as the increasing occurrence of human neurological diseases and associated economical cost burden, the role of unidentified environmental stressors in neurodegenerative disorders, multiple therapeutic drug-induced neuropathies noted in human clinical trials, and the exponential use of environmental chemicals with unknown neurotoxic potential necessitate a more extensive evaluation of the nervous system. The NTP has modified its protocol to include examination of key anatomic subsites related to neurodegenerative diseases such as Parkinson's disease. Modifications include four additional sections of the brain. Increasing the number of brain sections permits examination of a greater number of specific anatomic subsites with unique vulnerability. In addition, the spinal cord, peripheral nerves, trigeminal ganglion, and intestinal autonomic ganglia will be evaluated as needed. It is expected that this modified approach will increase the sensitivity of detecting neurotoxicants and neurocarcinogens important in human neurologic and neurodegenerative disorders.Keywords: neuropathology; histopathology; brain; nervous system; NTP; nervous system; screening.Since its inception in 1978, one of the major goals of the National Toxicology Program (NTP) has been to identify carcinogens (based on carcinogenicity for laboratory animals under conditions of study as adopted by NTP since 1983) based on 2-year rodent bioassays. The standard three sections of the brain examined in such routine studies were taken at the levels of (1) the basal ganglia, including the cerebrum at the frontal cortical level; (2) the thalamus at the mid-infundibular level; and (3) a mid-cerebellar section that included the cerebellum and medulla. For studies in which clinical neurological signs were noted, or studies in which agents were suspected to be neurotoxic, sections of the spinal cord and the sciatic nerve were also examined. In addition to the identification of carcinogens, the NTP has recently started a number of initiatives to assess adverse, nonneoplastic toxicological effects. Since environmental factors that affect nonneoplastic conditions such as immune-mediated diseases (Virgolini et al. 2005) and reproductive disorders (Davey et al. 2008) are increasingly common, initiatives within the NTP have included enhanced immunopathology as well as revised criteria for histopathological evaluation in reproductive and developmental toxicity studies. Similarly, increasing attention to the role of environmental factors in neurological disorders such as Parkinson's disease ) and autism (Cavagnaro 2007) also emphasizes the need for improved toxicity e...

Subsite Awareness in Neuropathology Evaluation of National Toxicology Program (NTP) Studies

“… Note: Although not identical, the trimming planes are comparable to the planes depicted in Rao et al, 2011 and Bolon et al, 2013. It is important to note that modest variations in trimming planes are expected variations in brain sampling among animals within a single study, across multiple studies, and among institutions.…”

“…Specifically, this review is designed to complement the original manuscript (Rao et al, 2011) by providing a navigational guide to the neuroanatomic localization and functional significance of selected subsites within the NTP-7. It is not intended to replace existing neuroanatomic texts by any means, but is designed primarily for toxicologic pathologists involved in routine evaluation of H&E stained sections in general toxicity and safety evaluation studies.…”

“…The following review is limited to the identification and brief functional relevance (based on susceptibility to toxicity or association with neurological disorders) of approximately 50 neuroanatomic subsites (Table 1 and Figure 2) noted in seven transverse sections of brain adopted in the recently revised NTP protocol (Rao et al 2011). The manuscript is organized to begin with each of the seven transverse sections of the brain (labeled as NTP-7, Level 1 through NTP-7, Level 7 taken progressively from the rostral to the caudal brain).…”

Juvenile Toxicology

“…Nonclinical juvenile animal toxicity studies should be scientifically justified and well designed with appropriate endpoints (Bolon et al 2013; R. M. Parker 2014b; Rao et al 2011). …”

STP Position Paper

“…In particular, brain and spinal cord sampling schemes suitable for executing these recommendations exist already in the published literature. For instance, the revised sampling protocol for the rodent brain put forth by the U.S. National Toxicology Program (NTP) provides an instructive example of these nervous system sampling proposals as they might be applied to general toxicity studies in rodents (Rao et al 2011). Two contract research organizations subsequently have tested this updated NTP approach on rodent general toxicity studies in their own facilities.…”