Shuaike Ma

Endogenous opioids and attenuated hypothalamic-pituitary-adrenal axis responses to immune challenge in pregnant rats

In late pregnant rats, the hypothalamic-pituitary-adrenal (HPA) axis is hyporesponsive to psychogenic stressors. Here, we investigated attenuated HPA responses to an immune challenge and a role for endogenous opioids. ACTH and corticosterone were assayed in blood samples from virgin and 21 d pregnant rats before and after endotoxin [lipopolysaccharide (LPS); 1 μg/kg, i.v.], interleukin-1β (IL-1β; 500 ng/kg, i.v.), or vehicle. In virgins, plasma ACTH concentrations increased 1 h after LPS and 15 min after IL-1β, as did corticosterone, with no responses in pregnant rats. In situ hybridization revealed increased corticotrophin releasing hormone (CRH) mRNA expression in the dorsomedial parvocellular paraventricular nucleus (pPVN) and increased anterior pituitary pro-opiomelanocortin mRNA expression 4 h after IL-1β in virgins; these responses were absent in pregnant rats. In contrast, immunocytochemistry showed that Fos expression was similarly increased in the nucleus tractus solitarius (NTS) A2 region in virgin and pregnant rats 90 min and 4 h after IL-1β. Naloxone pretreatment (5 mg/kg, i.v.) restored ACTH and pPVN CRH mRNA responses after IL-1β in pregnant rats but reduced the CRH mRNA response in virgins without affecting ACTH. Proenkephalin-A and μ-opioid receptor mRNA expression in the NTS was significantly increased in the pregnant rats, indicating upregulated brainstem opioid mechanisms. IL-1β increased noradrenaline release in the PVN of virgin, but not pregnant, rats. However, naloxone infused directly into the PVN increased noradrenaline release after IL-1β in pregnant rats. Thus, the HPA axis responses to immune signals are suppressed in pregnancy at the level of pPVN CRH neurons through an opioid mechanism, possibly acting by preterminal autoinhibition of NTS projections to the pPVN.

Norepinephrine Release in Medial Amygdala Facilitates Activation of the Hypothalamic-Pituitary-Adrenal Axis in Response to Acute Immobilisation Stress

Activation of the brain noradrenergic system during stress plays an important integrative function in coping and stress adaptation by facilitating transmission in many brain regions involved in regulating behavioural and physiological components of the stress response. The medial amygdala (MeA) has been implicated in modulation of stress‐induced activation of the hypothalamic‐pituitary‐adrenal (HPA) axis, and MeA is a target of innervation from brainstem noradrenergic neurones. However, it is not known whether, and to what extent, activation of the ascending noradrenergic innervation of MeA might modulate stress‐induced adrenocorticotropic hormone (ACTH) secretion. In the first experiment in this study, we measured extracellular norepinephrine (NE) levels in MeA using in vivo microdialysis. The concentration of NE in dialysate samples collected in MeA was elevated by more than three‐fold over baseline in response to acute immobilisation stress, providing evidence of a possible modulatory role for NE in the MeA during stress. This potential role was then assessed in the second experiment by measuring changes in the elevation of plasma ACTH concentration induced by acute immobilisation stress immediately following bilateral microinjections of α1‐ or β‐adrenergic receptor antagonists directly into MeA. Compared to vehicle‐injected controls, the α1‐receptor antagonist benoxathian dose‐dependently and significantly attenuated the ACTH response to acute stress, whereas combined β1/β2‐receptor blockade in MeA had only a modest effect. These results indicate that MeA does play a role in the stress response, and support the hypothesis that stress‐induced activation of NE release in MeA, acting primarily through α1 receptors, facilitates activation of the HPA axis in response to acute stress.

Chronic cold stress sensitizes brain noradrenergic reactivity and noradrenergic facilitation of the HPA stress response in Wistar Kyoto rats

Many psychiatric disorders, including depression, post-traumatic stress disorder and other anxiety disorders, result from an interaction between genetic factors and exposure to a sufficiently sensitizing environmental stressor. The inbred Wistar Kyoto (WKY) rat strain has been proposed as a model of stress vulnerability, exhibiting an exaggerated hypothalamic-pituitary-adrenal (HPA) response to stress and susceptibility to gastric ulceration. Previously, we showed that stress-activation of the brain noradrenergic system was deficient in WKY rats, and they lacked noradrenergic facilitation of the HPA response in the lateral bed nucleus of the stria terminalis (BSTL), compared to outbred Sprague-Dawley (SD) controls. Deficient modulatory function of the noradrenergic system may contribute to the stress susceptibility of WKY rats. Thus, we investigated the influence of a sensitizing stimulus, chronic intermittent cold exposure, on neuroendocrine and noradrenergic stress reactivity, and on noradrenergic facilitation of the HPA response in these two strains. Chronic cold exposure (7 days, 4 h/day, 4 degrees C) potentiated activation of the HPA axis by acute immobilization stress, assessed by measuring plasma adrenocorticotropic hormone (ACTH), in both strains, although to a greater extent in WKY rats, and enhanced stress-induced norepinephrine (NE) release in BSTL of WKY but not SD rats. We then compared the influence of chronic cold exposure on noradrenergic modulation of the HPA stress response in BSTL, by measuring changes in acute stress-induced elevation of plasma ACTH after microinjecting the alpha(1)-adrenoreceptor antagonist benoxathian into the BSTL. As shown previously, benoxathian attenuated stress-induced ACTH secretion in control SD but not control WKY rats. After chronic cold, the ACTH response to acute stress was attenuated by benoxathian administration into BSTL of both strains, such that the WKY response was not different from that of SD rats. Thus, chronic cold not only sensitized the release of NE in BSTL of WKY rats, but also restored noradrenergic facilitation of their already-elevated HPA response. Such functional sensitization of a previously-deficient facilitatory system may be one mechanism whereby exposure to repeated or severe stress may induce pathologic dysregulation of the stress response in susceptible individuals, resulting in psychiatric illness.

Induction of Fos expression by acute immobilization stress is reduced in locus coeruleus and medial amygdala of Wistar-Kyoto rats compared to Sprague-Dawley rats

Activation of the brain noradrenergic system during acute stress is thought to play an important integrative function in coping and stress adaptation by facilitating transmission in many brain regions involved in regulating behavioral and physiologic components of the stress response. Compared with outbred control Sprague-Dawley (SD) rats, inbred Wistar-Kyoto (WKY) rats exhibit an exaggerated hypothalamic-pituitary-adrenal (HPA) response as well as increased susceptibility to certain forms of stress-related pathology. However, we have also shown previously that WKY rats exhibit reduced anxiety-like behavioral reactivity to acute stress, associated with reduced activation of the brain noradrenergic system. Thus, to understand better the possible neurobiological mechanisms underlying dysregulation of the stress response in WKY rats, we investigated potential strain differences in stress-induced neuronal activation in brain regions that are both involved in regulating behavioral and neuroendocrine stress responses, and are related to the noradrenergic system, either as targets of noradrenergic modulation or as sources of afferent innervation of noradrenergic neurons. This was accomplished by visualizing stress-induced expression of Fos immunoreactivity in the paraventricular nucleus of the hypothalamus, lateral bed nucleus of the stria terminalis, central nucleus of the amygdala, and medial nucleus of the amygdala (MeA), as well as the noradrenergic nucleus locus coeruleus (LC). Stress-induced Fos expression was found to be decreased in the LC and MeA of WKY rats compared with similarly stressed SD rats, whereas no strain differences were observed in any of the other brain regions. This suggests that strain-related differences in activation of the MeA may be involved in the abnormal neuroendocrine and behavioral stress responses exhibited by WKY rats. Moreover, as the MeA is both an afferent as well as an efferent target of the brainstem noradrenergic system, reduced MeA activation may either be a source of reduced noradrenergic reactivity seen in WKY rats, or possibly a consequence. Nonetheless, understanding the mechanisms underlying altered stress reactivity in models such as the WKY rat may contribute to a better understanding of stress-related psychopathologies such as depression, post-traumatic stress disorder or other anxiety disorders.

Autoreceptor‐mediated inhibition of norepinephrine release in rat medial prefrontal cortex is maintained after chronic desipramine treatment

Alterations in noradrenergic neurotransmission are important in the mechanism of action of many antidepressant drugs, including selective norepinephrine (NA) reuptake inhibitors such as desipramine (DMI). It has been suggested that chronic NA reuptake blockade induces a desensitization of inhibitory α2‐adrenergic autoreceptors. This hypothesis was tested in experiment 1 using in vivo microdialysis to examine the degree of α2‐autoreceptor‐mediated inhibition of NA release in rat medial prefrontal cortex exerted by endogenous NA following chronic treatment with vehicle or DMI. This was accomplished by measuring the elevation of extracellular NA levels induced by acute administration of the α2‐receptor antagonist yohimbine. An 8‐fold increase in basal NA levels was observed after 21 days of DMI treatment. Further, acute yohimbine administration induced a robust elevation in NA levels which was not attenuated, and in fact at lower doses was greater in DMI‐treated rats compared with vehicle‐treated controls. In experiment 2, we addressed directly the functional status of terminal α2‐autoreceptors in frontal cortex in vitro, in the absence of potentially confounding competition from elevated levels of endogenous NA, after chronic reuptake blockade. We observed no difference in the degree to which the α2‐receptor agonist clonidine inhibited potassium‐evoked [3H]‐NA release from cortical slices taken from DMI‐ or vehicle‐treated rats. Together, these data suggest that endogenous activation of α2‐autoreceptors persists in restraining NA neurotransmission in the face of tonically elevated basal NA levels following chronic reuptake blockade.