Maternal Influences on the Stress Response

The human body is partially regulated by hormones. Hormonal signals are activated by the Hypothalamic Pituitary Adrenal Axis. Regulation of hormones is controlled by negative and positive feedback loops (Betts et al. 2022, 17.2). A negative feedback loop involves a signal being sent to a target (Betts et al., 2022, 1.5). The target then produces a response, which feeds back to halt the signal (Betts et al., 2022, 1.5). This helps the body maintain homeostasis. When a signal continues to amplify a response, this represents a positive feedback loop (Betts et al. 2022, 1.5). The response does not feed back, and continues to be amplified until the signal is removed (Betts et al, 2022, 1.5). Negative feedback loops are the primary form of hormonal regulation (Betts et al., 2022, 17.2). There are positive feedback loops associated with the reproductive system, including lactation. With lactation, a mechanical stimulus such as suckling, activates the HPA axis (Unit 9 Lecture slide 142). This prompts the hypothalamus to release prolactin releasing factors (Unit 9 Lecture slide 142). This causes the anterior pituitary gland to release prolactin ((Unit 9 Lecture slide  142). The release of prolactin leads to milk production from mammary glands ((Unit 9 Lecture slide 142). 

A negative feedback loop that humans experience in their everyday life is that of the stress response. The stress response is an adaptive mechanism that helps prepare humans to respond to threats in their environment (Beery & Francis, 2011). Stimuli that trigger the stress response are referred to as stressors (Lovallo, 2016). Stressors can be both physical and psychological (Lovallo, 2016). Once a stimulus is appraised as a stressor, this activates the HPA axis (Lovallo, 2016). The hypothalamus then releases corticotropin-releasing factor (Herman et al., 2016). This then causes the pituitary gland to release adrenocorticotropin hormone (Herman et al., 2016). This causes the adrenal cortex to release cortisol into major organ systems of the body (Herman et al., 2016). Cortisol is a glucocorticoid (Herman et al., 2016). It then feeds back to inhibit further release from the hypothalamus (Herman et al., 2016). 

There are epigenetic that can influence the stress response. Epigenetics involve the interaction between genes and the environment (Berry & Francis, 2011). This means that genetic changes can occur as a result of environmental or social interactions (Beery & Francis, 2011). Genetic changes have been observed coincidingly with variations in maternal stress, early life experiences, and maternal care (Cao-Lei et al., 2016; Berry & Francis, 2011). For example, maternal cortisol levels negatively impact the development of the HPA axis in the fetus (Cao-Lei et al., 2016). Glucocorticoids released by the mother in a time of stress can reach the fetus through the placenta and lead to dysregulation of the HPA axis (Cao-Lei et al., 2016). Cortisol from a mother can also increase cortisol levels for the fetus (Cao-Lei et al., 2016). Rats studies have also demonstrated that maternal grooming directly affects offspring’s vulnerability to stress. Whether or not they were born to a high licking mother, rats who are reared by a low-licking mother, had higher stress reactivity (Beery & Francis, 2011). The quality of maternal care influenced the development and programming of the HPA axis (Berry & Francis, 2011). Maternal depressed mood in the third trimester has been found to be associated with increased methylation of glucocorticoid receptor gene, which predicted an increase in the infants’ HPA stress reactivity (Cao-Lei et al., 2016). 

DNA methylation is an epigenetic mechanism that can modify DNA without changing the DNA sequence (Cao-Lei et al., 2016). It makes genes less likely to be expressed (Cao-Lei et al., 2016). The gene for the glucocorticoid receptor plays a key role in regulating HPA axis functioning (Beery & Francis, 2011; Cao-Lei et al., 2016). Glucocorticoid receptors in the hippocampus sense the feedback of glucocorticoids released by the adrenal cortex (Beery et al., 2011). Methylation of the glucocorticoid receptor gene makes it less likely to be expressed. This means that it is less likely to sense the glucocorticoids being fed back. This makes HPA axis activation more likely to continue instead of cease like it is supposed to. This makes a person more sensitive to stress in their lives.