Neonatal maternal separation and sex‐specific plasticity of the hypoxic ventilatory response in awake rat

Abstract

We tested the hypothesis that neonatal maternal separation (NMS), a form of stress that affects hypothalamo-pituitary-adrenal axis (HPA) function in adult rats, alters development of the respiratory control system. Pups subjected to NMS were placed in a temperature and humidity controlled incubator 3 h per day for 10 consecutive days (P3 to P12). Control pups were undisturbed. Once they reached adulthood (8-10 weeks old), rats were placed in a plethysmography chamber for measurement of ventilatory and cardiovascular parameters under normoxic and hypoxic conditions. Measurement of c-fos mRNA expression in the paraventricular nucleus of the hypothalamus (PVH) combined with plasma ACTH and corticosterone levels confirmed that NMS effectively disrupted HPA axis function in males. In males, baseline minute ventilation was not affected by NMS. In contrast, NMS females show a greater resting minute ventilation due to a larger tidal volume. The hypoxic ventilatory response of male NMS rats was 25% greater than controls, owing mainly to an increase in tidal volume response. This augmentation of the hypoxic ventilatory response was sex-specific also because NMS females show an attenuated minute ventilation increase. Baseline mean arterial blood pressure of male NMS rats was 20% higher than controls. NMS-related hypertension was not significant in females. The mechanisms underlying sex-specific disruption of cardio-respiratory control in NMS rats are unknown but may be a consequence of the neuroendocrine disruption associated with NMS. These data indicate that exposure to a non-respiratory stress during early life elicits significant plasticity of these homeostatic functions which persists until adulthood.

Influence of juvenile housing conditions on the ventilatory, thermoregulatory, and endocrine responses to hypoxia of adult male rats

“…This housing condition is standard in our facilities. Since data obtained from these animals were nearly identical to those reported previously in our laboratory (14,20), data from 83 males housed in pairs (see details below) from these earlier studies were added to the current data set to reduce and optimize use of laboratory animals. The remaining 60 animals were housed in triads.…”

“…Results obtained from these animals were compared with those subjected to neonatal stress (see below). To constitute proper controls (20,52), pups were undisturbed and continuously remained with the dam from postnatal days 3-12. The litter was placed in a larger cage where animal care technicians' attention could be dismissed for 10 consecutive days so as to leave animals completely free from human interactions.…”

“…This posthypoxic sample was handled the same way as baseline (normoxic) blood and stored in the Ϫ80°C freezer until total CORT, testosterone, progesterone, and estradiol assays were performed. Analysis of CORT was performed in our laboratory, as we have done previously (14,20), using the Correlate EIA ELISA kits (Assay Design, Ann Arbour, MI) and a microplate spectrophotometer ( -Quant, Bio-Tek Instruments, Winooski, VT). CORT concentrations were calculated from the parameters of the standard curve linearized by a log-log transformation.…”

“…Consequently, experiments were performed on both "control" rats, which received no experimental treatment before weaning, and rats subjected to neonatal maternal separation (NMS; 3 h/day from postnatal days [3][4][5][6][7][8][9][10][11][12], a form of neonatal stress that enhances responsiveness to environmental conditions. NMS disrupts hypothalamo-pituitary-adrenal (HPA) axis function; at adulthood, baseline levels of endocrine (ACTH, CORT) and neuronal [paraventricular nucleus of the hypothalamus (PVN), fos, mRNA] indicators of HPA axis activation of NMS rats are greater than those of controls (20). In NMS rats, postweaning isolation (single-cage housing) promotes food intake and body weight (W b ) gain (45).…”

“…In the present context, the NMS group, therefore, represents an excellent "tool" to detect changes in animal physiology induced by housing. With respect to respiratory regulation, the HVR of adult male rats previously subjected to NMS is augmented by ϳ25% (20), owing in part to elevated CORT levels (14,29). Because augmented O 2 chemoreflex predis-poses to respiratory instability during sleep (12,30,53), understanding the impact of external conditions such as housing on this physiological function may provide valuable insight into the environmental determinants of health.…”