Neonatal maternal separation and enhancement of the inspiratory (phrenic) response to hypoxia in adult rats: disruption of GABAergic neurotransmission in the nucleus tractus solitarius.

Neonatal maternal separation (NMS) alters respiratory control development. Adult male rats previously subjected to NMS show a hypoxic ventilatory response 25% greater than controls. During hypoxia, gamma-aminobutyric acid (GABA) release within the nucleus tractus solitarius (NTS) modulates the magnitude of the ventilatory response. Because development of GABAergic receptors is sensitive to NMS, we tested the hypothesis that in adults, a change in responsiveness to GABA within the NTS contributes to NMS-related enhancement of the inspiratory (phrenic) response to hypoxia. Pups subjected to NMS were placed in an incubator for 3 h/day for 10 consecutive days [postnatal days 3 to 12]. Controls were undisturbed. Adult (8-10 weeks old) rats were anaesthetized (urethane; 1.6 g/kg), paralysed and artificially ventilated to record phrenic activity. Rats either received a 50-nL microinjection of GABA (5 microm) or phosphate-buffered saline (sham) within the caudal NTS, or no injection prior to being exposed to hypoxia (FiO(2) = 0.12; 5 min). NMS enhanced both the frequency and amplitude components of the phrenic response to hypoxia vs controls. GABA microinjection attenuated the phrenic responses in NMS rats only. This result is supported by ligand binding autoradiography results showing that the number of GABA(A) receptors within the NTS was 69% greater in NMS vs controls. Despite this increase, the phrenic response to hypoxia of NMS rats is larger than controls, suggesting that the higher responsiveness to GABA microinjection within the NTS is part of a mechanism that aims to compensate for: (i) a deficient GABAergic modulation; (ii) enhancement of excitatory inputs converging onto this structure; or (iii) both.

Neonatal maternal separation and enhancement of the hypoxic ventilatory response in rat: the role of GABAergic modulation within the paraventricular nucleus of the hypothalamus.

Neonatal maternal separation (NMS) affects respiratory control development as adult male (but not female) rats previously subjected to NMS show a hypoxic ventilatory response 25% greater than controls. The paraventricular nucleus of the hypothalamus (PVN) is an important modulator of respiratory activity. In the present study, we hypothesized that in awake rats, altered GABAergic inhibition within the PVN contributes to the enhancement of hypoxic ventilatory response observed in rats previously subjected to NMS. During normoxia, the increase in minute ventilation following microinjection of bicuculline (1 mm) within the PVN is greater in NMS versus control rats. These data show that regulation of ventilatory activity related to tonic inhibition of the PVN is more important in NMS than control rats. Microinjection of GABA or muscimol (1 mM) attenuated the ventilatory response to hypoxia (12% O2) in NMS rats only. The higher efficiency of microinjections in NMS rats is supported by results from GABAA receptor autoradiography which revealed a 22% increase in GABAA receptor binding sites within the PVN of NMS rats versus controls. Despite this increase, however, NMS rats still show a larger hypoxic ventilatory response than controls, suggesting that within the PVN the larger number of GABAA receptors either compensate for (1) a deficient GABAergic modulation, (2) an increase in the efficacy of excitatory inputs converging onto this structure, or (3) both. Together, these results show that the life-long consequences of NMS are far reaching as they can compromise the development of vital homeostatic function in a way that may predispose to respiratory disorders.

Neonatal maternal separation induces sex-specific augmentation of the hypercapnic ventilatory response in awake rat.

Neonatal maternal separation (NMS) is a form of stress that exerts persistent, sex-specific effects on the hypoxic ventilatory response. Adult male rats previously subjected to NMS show a 25% increase in the response, whereas NMS females show a response 30% lower than controls (8). To assess the extent to which NMS affects ventilatory control development, we tested the hypothesis that NMS alters the ventilatory response to hypercapnia in awake, unrestrained rats. Pups subjected to NMS were placed in a temperature- and humidity-controlled incubator 3 h/day for 10 consecutive days (P3 to P12). Control pups were undisturbed. At adulthood (8 to 10 wk old), rats were placed in a plethysmography chamber for measurement of ventilatory parameters under baseline and hypercapnic conditions (inspired CO(2) fraction = 0.05). After 20 min of hypercapnia, the minute ventilation response measured in NMS males was 47% less than controls, owing to a lower tidal volume response (22%). Conversely, females previously subjected to NMS showed minute ventilation and tidal volume responses 63 and 18% larger than controls respectively. Although a lower baseline minute ventilation contributes to this effect, the higher minute ventilation/CO(2) production response observed in NMS females suggests a greater responsiveness to CO(2)/H(+) in this group. We conclude that NMS exerts sex-specific effects on the hypercapnic ventilatory response and that the neural mechanisms affected by NMS likely differ from those involved in the hypoxic chemoreflex.

Neonatal maternal separation and early life programming of the hypoxic ventilatory response in rats.

The neonatal period is critical for central nervous system (CNS) development. Recent studies have shown that this basic neurobiological principle also applies to the neural circuits regulating respiratory activity as exposure to excessive or insufficient chemosensory stimuli during early life can have long-lasting consequences on the performance of this vital system. Although the tactile, olfactory, and auditory stimuli that the mother provides to her offspring during the neonatal period are not directly relevant to respiratory homeostasis, they likely contribute to respiratory control development. This review outlines the rationale for the link between maternal stimuli and programming of the hypoxic ventilatory response during early life, and presents recent results obtained in rats indicating that experimental disruption of mother-pup interaction during this critical period elicits significant phenotypic plasticity of the hypoxic ventilatory response.

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

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.