Stress, while an essential response in the short term, can be very damaging to health when experienced chronically. Stress has been linked to the development or exacerbation of many disorders in humans, including coronary heart disease, depression, type n diabetes, even colds and 'flu. The conventional approach to investigating the problem of an overactive stress response and potential treatments is to examine pathways that drive responses to stress. An alternative way of addressing this problem, however, is to develop treatments that enhance the brain's own inhibitory stress-protective mechanisms.
The medial prefrontal cortex (mPFC) has been strongly implicated in control of a variety of behavioural, autonomic and endocrine responses to stress. In particular, the hypothalamic-pituitary-adrenal (HPA) axis, a key element of the endocrine stress response, is suppressed by activation of the mPFC. Importantly, anatomical studies have failed to demonstrate direct projections from the mPFC to the medial parvocellular region of the hypothalamic paraventricular nucleus (mpPVN) at the apex of the HPA axis. Accordingly, the mechanism by which the mPFC modulates HPA axis stress responses is likely to involve a relay or relays through other brain regions. Additionally, there has yet been no direct attempt to determine the role of inputs to the mPFC in activation of the region in response to stress. Of the neurotransmitter systems associated with mPFC function, the dopamine system appears to be particularly sensitive to stress. However, there is much evidence to suggest that noradrenaline also interacts with dopamine in the mPFC to modulate subcortical activation. Thus questions remain as to the role of these neurotransmitter systems in the mPFC in the stress response. The primary aim of this thesis was therefore to investigate how the mPFC modulates the HPA axis and associated brain regions in response to stress.
Initially it was necessary to properly characterise the animals' response to the stressor used in these investigations, air puff (Chapter Two). Brain patterns of expression of the protein product, Fos, of the immediate-early gene, c-fos, were examined, as were changes in plasma adrenocorticotropic hormone and corticosterone, to determine the typical response to this stressor.
The experiments described in Chapter Three aimed to examine how the mPFC affects the HPA axis response to air puff and to investigate other stress-sensitive brain regions that could act as a relay of information between the mPFC and the mpPVN. Such candidate relay populations include the bed nucleus of the stria terminalis (BNST), amygdala, paraventricular nucleus of the thalamus (PVT), and brainstem catecholamine cell groups. These populations both receive inputs from the mPFC and have been implicated in the control of mpPVN function during stress. Excitotoxic lesions of the mPFC resulted in an increase in neuronal activation in both the mpPVN and the ventral (v) BNST in response to air puff compared with sham-lesioned animals, thus demonstrating the importance of the mPFC and highlighting a role for the BNST in mPFC regulation of the mpPVN. Furthermore, retrograde tracing from the PVN (Chapter Three) demonstrates that there is a population of neurons, activated by air puff that projects from the vBNST to the mpPVN. Further experiments involving lesions of the BNST (Chapter Six) reinforce that the BNST does have an important role in mpPVN activation in response to stress, in addition to influencing many other brain regions. Retrograde tracing from the BNST (Chapter Six), however, shows that there is no evidence for a direct mPFC-BNST projection activated in response to air puff, thus implying an indirect pathway for any mPFC modulation of the vBNST.
Another brain region potentially involved in mPFC modulation of the neuronal responses to stress, the PVT, was also examined (Chapter Seven). While a lesion of the PVT revealed a role for this region in regulation of the central amygdala response to stress, no effect was seen on the HPA axis response to air puff.
Retrodialysis of dopamine antagonists into the mPFC of animals exposed to either air puff or to an immune challenge, injection of the proinflammatory cytokine interleukin-1β, revealed that the response to different types of stressor can be processed differently in the mPFC (Chapter Five). While retiodialysis of both D1 and D2 dopamine antagonists caused an attenuation of neuronal activation in response to interleukin-1β in all brain regions examined, only the D2 antagonist caused an attenuation of the response to air puff, and then only in the mpPVN and BNST. As well as indicating that the response to different stressors may be processed differently by the mPFC, these results further highlight an important role for the BNST, at least in the mPFC-modulated response to air puff.
In addition, catecholamine terminal specific lesions of the mPFC demonstrate a role for the catecholaminergic inputs to the mPFC in modulation of the response to stress (Chapter Four). Again these experiments particularly highlight the importance of the BNST, this region displaying a significantly altered response to air puff with a catecholamine terminal specific lesion of the mPFC. These experiments also indicate that while the dopamine input to the mPFC may be important, especially in local mPFC activation, there is also a fundamental influence of the noradrenergic input to the mPFC on the responses to stress.
The investigations presented in this thesis demonstrate an important role for the mPFC in regulation of the HPA axis response to air puff. Furthermore, it is suggested that the BNST in particular may play a pivotal role in this regulation. Thus a pathway is proposed whereby the mPFC indirectly influences the vBNST in response to air puff. It is suggested that this influence leads, in tum, to a down-regulation of vBNST activation, decreasing the direct excitatory output of the vBNST to the mpPVN. It is clear that regulation of the inputs to the mPFC also play an important role in the stress response and it is also unlikely that the BNST is the only brain region by which the mPFC communicates with the mpPVN. However, the suggestion presented in this thesis of an important role for the BNST in mPFC-regulated HPA axis responses to stress identifies a potential target for future drug treatments of overactive stress responses.