Critical analysis of potential body temperature confounders on neurochemical endpoints caused by direct dosing and maternal separation in neonatal mice: a study of bioallethrin and deltamethrin interactions with temperature on brain muscarinic receptors.

Abstract

The present investigation was conducted to understand better possible confounding factors caused by direct dosing of neonatal mice during the pre-weaning developmental period. By direct dosing, pups might encounter thermal challenges when temporarily removed from their 'natural habitat'. Typically, this leads to a cold environment and food deprivation (impaired lactation) and modulation of the toxic potency of the substance administered. Growth retardation as a consequence of such behavioural changes in pups makes it increasingly difficult to differentiate specific from non-specific mechanisms. Neonatal NMRI mice were dosed daily by gavage (0.7 mg kg(-1) body wt.) from postnatal day (PND) 10-16 with S-bioallethrin, deltamethrin or the vehicle. Then the pups, including their non-treated foster dams, were subjected temporarily for 6 h day to a hypo-, normo- or hyperthermic environment, which was followed by normal housing. The measured temperatures in the environmental chambers were ca. 21, 25 and 30 degrees C, respectively. Thus, temperatures in the hypo- and normothermic groups are comparable to the temperatures commonly present in testing laboratories, whereas the hyperthermic condition is that temperature typically present in the 'natural habitat' of pups. A deviation from the normal behaviour of both pups and dams was observed in the hypo- and normothermic groups. In these groups the rectal temperatures of pups were markedly decreased, especially in the early phase of the study (PND 10-12). Neonates that received either test substance displayed changes in body weights and brain weights at terminal sacrifice (PND 17) when subjected temporarily to a non-physiological environment. An enormous influence of environmental temperature on the density of muscarinic receptors in the crude synaptosomal fraction of the cerebral cortex was ascertained. In summary, these results demonstrate that the direct dosing of thermolabile neonatal mice by gavage is subject to significant artefacts that render the interpretation of findings from such studies difficult. It appears that if direct dosing of neonatal pups is mandated, and inhalation is a relevant route of exposure, the combined inhalation exposure of dams with their litters is an alternative procedure that does not cause disruption of the 'natural habitat' of pups. However, owing to their higher ventilation, under such conditions the pups may receive dosages at least double those of the dams.