Neonatal maternal separation opposes the facilitatory effect of castration on the respiratory response to hypercapnia of the adult male rat: Evidence for the involvement of the medial amygdala

Abstract

Respiratory manifestations of panic disorder (PD) include a greater respiratory instability and enhanced responsiveness to CO compared to normal individuals. Although the prevalence of PD is approximately three times greater in women compared to men, the origins of this sexual dimorphism remain poorly understood. Similar to PD patients, adult female rats previously subjected to neonatal maternal separation (NMS) show an increase in their ventilatory response to CO . Because this effect of NMS is not observed in males, we hypothesised that testosterone prevents NMS-induced hyper-responsiveness to CO . Pups subjected to NMS were placed in an incubator for 3 h d from postnatal days 3-12. Control pups remained undisturbed. At adulthood (8-10 weeks of age), rats were then subjected either to sham surgery or castration. Fourteen days later, breathing was measured at rest (room air) and during acute exposure to hypercapnia (5 and 10% CO for 10 minutes each) using plethysmography. To gain insight into the mechanisms involved, c-fos expression was used as an indicator of neuronal activation. Brains were collected following air or CO exposure for quantification of c-fos positive cells by immunohistochemistry in selected regions, including the paraventricular nucleus of the hypothalamus, the dorsomedial hypothalamus and the amygdalar complex. Castration produced a 100% increase of hyperventilatory response to 10% CO in control rats. Unexpectedly, castration had no effect on the hyperventilatory response of NMS rats. The intensity of the hypercapnic response was inversely correlated with c-fos expression in the medial amygdala. We conclude that testosterone prevents the hyper-responsiveness to CO , whereas NMS attenuates sensitivity to hormone withdrawal. We propose that an inhibitory influence from the medial amygdala contributes to this effect.

Understanding rat emotional responses to CO2

“…In rats exposed to 10% CO 2 , the medial and central amygdala show increased c-Fos expression. This increase is associated with augmented minute ventilation, breathing frequency, and tidal volume 63 . Johnson et al 64 found that rats exposed to the 20% CO 2 challenge tended to increase c-Fos expression in the central amygdala, and this increase was related to increased fecal boli production (indicative of fear and anxiety) and thigmotaxis in the open field tests, but no effect of hypercapnia on c-Fos expression was detected in the medial and basolateral amygdala.…”

“…The dorsomedial region of hypothalamus (DMH) is involved in the execution of behavior responses to aversive stimuli 73,74 . One study reported that rats exposed to 10% CO 2 did not differ from controls in the number of labeled cells on the DMH 63 . However, other studies have found that the DMH shows high c-Fos expression in rats exposed to 5% CO 2 and the 20% CO 2 challenge 30,64,70 , particularly in orexin neurons 34 found only in the DMH and perifornical nucleus of the hypothalamus 75 .…”

“…One other study reports no significant increase in c-Fos expression in the PVN of rats exposed to 10% CO 2 (ref. 63 ).…”

Effects of Early-Life Stress on the Brain and Behaviors: Implications of Early Maternal Separation in Rodents

“…To address this issue, one of the most convincing evaluation methods is an analysis of c-Fos expression, a product of the immediate-early gene c-fos , which is expressed in an activity-dependent manner [ 50 ]. Via this approach, many pieces of evidence have shown that MS stimulates various brain regions in different ways depending on the timing and duration of MS [ 51 , 52 ]. The current authors previously examined c-Fos expression after MS, mainly focusing on the hypothalamus and limbic brain regions, and demonstrated that repeated MS and a single incidence of MS during different developmental stages and periods showed different patterns of c-Fos expression [ 46 , 53 ].…”

Disruption of estradiol regulation of orexin neurons: a novel mechanism in excessive ventilatory response to CO2 inhalation in a female rat model of panic disorder

“…The NMS protocol was identical to the one used in our previous studies 29,30 . Briefly, virgin females were mated and 3 days after delivery, each litter was separated daily from their mother 3 h/day (09:00-12:00) from postnatal day 3-12.…”