1.0 Introduction

Stress initiates an immediate response of multiple neural and endocrine systems (Bains et al, 2015). Both humans and animals respond to environmental anxiety with a stress response that allows for an adaption to the stressor to maintain homeostasis (Sheng et al, 2020). The plasticity of the hypothalamus is its ability to change and adapt to new information and stressors. Recent studies suggest that stressful experiences leave indelible marks on the paraventricular nucleus (PVN) of the hypothalamus and alter the ability of their synapses to undergo plasticity (Bains et al, 2015). Dysregulation of the hypothalamicpituitary-adrenal (HPA) axis has been related to a range of affective and stress related disorders (Levy and Tasker, 2012).

2.0 The Hypothalamus and its plasticity

The hypothalamus is located within the diencephalon of the brain along with the thalamus and consists of several subnuclei, having a major role in regulating the autonomic nervous system (ANS), subdivided into the parasympathetic (PNS) and sympathetic nervous system (SNS) (Klein, 2013). The hypothalamus secretes hormones and sends numerous neural impulses signalling the hypophysis, commonly known as the pituitary gland. The hypophysis has a major role in maintaining homeostasis of many biological processes, for example controlling the cardiovascular system, fluid distribution and thermoregulation (Klein, 2013).

Neural and hormonal signals are delivered to target tissue by the SNS and PNS division. Nerve impulses are usually rapid and have a short duration of action, in contrast the endocrine system relies on chemical messengers that will produce a slower end result but with often long-lasting effects (Aspinall, 2003).

The HPA axis (see figure 1) is one such neuroendocrine mechanisms that mediates the effects of stressors by regulating many physiological processes such as immune responses, metabolism and the ANS. The HPA axis conveys a cascade of endocrine pathways that respond to communication with the hypothalamus, pituitary- and adrenal glands via a negative feedback loop (Sheng et al, 2020).