Clinical and anatomic pathology effects of serial blood sampling in rat toxicology studies, using conventional or microsampling methods.

UNLABELLED: As a general practice in rodent toxicology studies, satellite animals are used for toxicokinetic determinations, because of the potential impact of serial blood sampling on toxicological endpoints. Besides toxicological and toxicokinetic determinations, blood samples obtained longitudinally from a same animal may be used for the assessment of additional parameters (e.g., metabolism, pharmacodynamics, safety biomarkers) to maximize information that can be deduced from rodents. We investigated whether removal of up to 6 × 200 µL of blood over 24h can be applied in GLP rat toxicology studies without affecting the scientific outcome. METHODS: 8 week-old female rats (200-300 g) were dosed for up to 1 month with a standard vehicle and subjected or not (controls) to serial blood sampling for sham toxicokinetic/ancillary determinations, using miniaturized methods allowing collection of 6 × 50, 100 or 200 µL over 24h. In-life endpoints, clinical pathology parameters and histopathology of organs sensitive to blood volume reduction were evaluated at several time points after completion of sampling. RESULTS: In sampled rats, minimal and reversible changes in red blood cell mass (maximally 15%) and subtle variations in liver enzymes, fibrinogen and neutrophils were not associated with any organ/tissue macroscopic or microscopic correlate. CONCLUSION: Serial blood sampling (up to 6 × 200 µL over 24h) is compatible with the assessment of standard toxicity endpoints in adult rats.

Negative impact of melatonin ingestion on the photopic electroretinogram of dogs.

Melatonin follows a circadian rhythm entrained by the light/dark cycle and plays a role in promoting light sensitivity at night. It has been suggested that melatonin and dopamine reciprocal inhibition may contribute to the switch between day and night vision. The purpose of this study was to investigate the impact of a high dose of melatonin administration on the photopic and scotopic electroretinogram (ERG) of dogs in the daytime, when it is not thought to be present. Photopic and scotopic ERG luminance response functions were obtained from 7 anaesthetized beagle dogs (3 males and 4 females), once without melatonin (control) and once after oral administration of melatonin (90 mg/dog). Vmax (maximal b-wave amplitude achieved) and logK (retinal sensitivity) were calculated from the derived luminance response function. Photopic flicker ERG was also recorded. In photopic condition, a-wave amplitude (control: -126.90 µV; with melatonin: -49.64 µV; p<0.001) and Vmax (control: 252.50 µV; with melatonin: 115.40 µV; p<0.001) were decreased. A significant reduction of the photopic flicker ERG amplitude was observed after melatonin ingestion. In scotopic condition, an overall difference was reported before and after melatonin ingestion for the a- and b-wave amplitude, but no change was significant for Vmax. Melatonin ingestion at a high dose during the day decreases the photopic amplitude of a- and b-wave, but has no impact on implicit time. This negative impact of melatonin on photopic system may serve to promote night vision.

The ERG of the beagle dog: evidence associating a post b-wave negativity with the Tapetum lucidum.

As previously reported in the literature, the electroretinogram (ERG) of the Beagle dog includes a large post b-wave negativity, the origin of which is not yet established. In the course of our investigations on the electroretinogram in dogs, we examined two Beagle dogs (2 years apart) who had one eye devoid of a Tapetum Lucidum (TL). Photopic (cone-mediated) and scotopic (rod-mediated) ERGs were obtained according to the guidelines for clinical electroretinography in dog. In both dogs the short-latency ERG components (i.e. a- and b-waves) were found to be within the normal range in amplitude, peak time and morphology O.U. However, the large negative component that, in Beagle dogs, normally follow the b-wave was absent from the photopic as well as the scotopic signals obtained from the TL-free eye. Our results thus suggest a possible contribution of the TL to the ERG of Beagle dogs.

Comparing the photopic ERG i-wave in different species.

The i-wave, a post b-wave component of the human photopic electroretinogram (ERG), is claimed to originate at the level of the retinal ganglion cells (RGC) or more distally. We investigated whether this wave is a feature common to all species. Photopic ERGs were obtained from the following species: Beagle dog, European cat, New Zealand white rabbit, Göttingen minipig, Cynomolgus monkey, Sprague-Dawley and brown Norway rats, Hartley guinea pig, and CD1 and C57BL6 mice. Results were compared with those obtained from normal human subjects. Except for rats and mice, all species yielded a well-demarcated i-wave, easily identifiable and separated from the a-b-wave complex by approximately 20 ms. Our sample suggests that the i-wave is a feature common to the photopic ERG of most species including humans. In view of its suggested origin, the i-wave would offer a unique opportunity to test, with the flash ERG, the functional integrity of the retinal ganglion cells in animals where use of a pattern stimulus is not always easily obtained.