Richard Windle

Oxytocin attenuates stress-induced c-fos mRNA expression in specific forebrain regions associated with modulation of hypothalamo-pituitary-adrenal activity.

We reported previously that the neuropeptide oxytocin attenuates stress-induced hypothalamo–pituitary–adrenal (HPA) activity and anxiety behavior. This study sought to identify forebrain target sites through which oxytocin may mediate its anti-stress effects. Ovariectomized, estradiol-treated rats received intracerebroventricular infusions of oxytocin (1 or 10 ng/hr) or vasopressin (10 ng/hr), and the patterns of neuronal activation after restraint stress were determined by semiquantitative mapping of c-fos mRNA expression. Oxytocin administration significantly attenuated the release of ACTH and corticosterone and the increase in corticotropin-releasing factor mRNA expression in the hypothalamic paraventricular nucleus (PVN) in response to 30 min restraint. Restraint also induced the expression of c-fos mRNA in selective regions of the forebrain, including the PVN, paraventricular thalamic nucleus, habenula, medial amygdala, ventrolateral septum (LSV), most subfields of the dorsal and ventral hippocampus, and piriform and endopiriform cortices. In most cases, this level of gene expression was unaffected by concomitant administration of oxytocin. However, in the PVN, LSV, and throughout all subfields of the dorsal hippocampus, restraint evoked no detectable increase in c-fos mRNA in animals treated with either dose of oxytocin. Vasopressin had no effects on either HPA axis responses or neuronal activation in response to restraint, indicating that the effects were highly peptide selective. These data show that central oxytocin attenuates both the stress-induced neuroendocrine and molecular responses of the HPA axis and that the dorsal hippocampus, LSV, and PVN constitute an oxytocin-sensitive forebrain stress circuit.

Endocrine and Behavioural Responses to Noise Stress:Comparison of Virgin and Lactating Female Ratsduring Non‐Disrupted Maternal Activity

The behavioural and endocrine responses to a 10 min white noise stress have been characterized in female virgin and undisturbed lactating Sprague‐Dawley rats. Animals were continuously video‐taped and frequent blood samples were collected using an automated sampling system. Noise stress caused hypothalamo‐pituitary‐adrenal (HPA) activation, as indicated by a rapid increase in plasma corticosterone and ACTH in the virgins: corticosterone concentrations peaked 20 min after initiation of the stress before declining rapidly back to basal concentrations. In contrast, noise stress had no significant effect on either plasma corticosterone or ACTH concentrations in the lactating animals. However, 72 h after weaning the corticosterone response of the ex‐lactating rats was of comparable magnitude, but longer duration to that seen in the virgins. Plasma prolactin concentrations were significantly higher in the lactating animals and declined in response to the noise whereas, a transient but reproducible increase was seen in the virgin group. In situ hybridization revealed a significantly lower basal expression of CRF mRNA in the paraventricular nucleus of lactating rats as compared to the virgins, but noise stress had no further effect. Virgin animals showed behavioural responses to the stress, including an increase in the total activity, exploratory behaviours (rearing) and displacement behaviours (grooming). Lactating animals also showed behavioural responses to the noise, but their activities were principally directed towards the pups. These data show that although lactating rats showed normal behavioural reactivity to a psychological stress they showed no statisitically significant activation of the HPA axis, suggesting a dissociation of behavioural and neuroendocrine responses to this mild stress.

Chapter 8 Peripartum plasticity within the hypothalamo-pituitary-adrenal axis

The hypothalamo-pituitary-adrenal (HPA) axis plays important roles in the adaptive changes in physiology that occur during pregnancy and lactation. Although the axis still exhibits a pulsatile pattern of secretion, the normal diurnal rhythm of pulse amplitude is lost during lactation, such that mean basal levels remain constant throughout the day. In addition, the peripartum period is associated with a remarkable plasticity in stress-induced HPA activity, in that the increase of HPA activity normally seen in response to either physical or psychological stresses in the non-reproductive state become severely attenuated or absent in the lactating animal. This stabilization of both basal and stress-induced HPA activity may be important for maintaining a constant endocrine environment, thereby preventing any programming effects on the developing offspring. Attenuation of the stress response is initiated in late pregnancy and is temporally associated with luteolysis, indicating possible steroid hormone involvement. Indeed, mimicking the luteolytic changes in oestrogen and progesterone levels in non-pregnant animals induces a similar attenuation of the stress response. Furthermore down-regulation of the stress response is, at least in part, centrally mediated since in the period following luteolysis rats will show a decreased level of stress-induced neuronal activation of the PVN, as measured by the expression of either c-fos or CRH mRNAs. Persistence of this adapted state is dependent upon the continued suckling stimulus, as removal of the offspring litter rapidly leads to resumption of HPA responses to and the appearance of an exaggerated diurnal rhythm. The underlying mechanisms responsible for this stress hyporesponsiveness may include plasticity of noradrenergic and oxytocin pathways. In view of its role in other reproductive behaviors, a stress-inhibiting effect of oxytocin may reflect a more widespread co-ordinating role in the peripartum animal.

Increased Corticosterone Pulse Frequency During Adjuvant-Induced Arthritis and its Relationship to Alterations in Stress Responsiveness

Frequent blood sampling from males rats was used to study hypothalamic‐pituitary‐adrenal (HPA) axis activation during arthritis and its association with diminished responses to acute psychological stress. In control rats, corticosterone release occurred in a series of 13 ± 1 pulses per 24 h. Induction of arthritis by Mycobacterium‐adjuvant injection initially increased the rate of hormone release within each pulse and, by day 14 postinjection, when hind‐paw inflammation was established, caused a marked increase in pulse frequency to 22 ± 1 per 24 h leading directly to elevated circulating corticosterone levels. In both control and adjuvant‐treated rats, there was a marked response to a 10‐min noise stress when the stimulus coincided with a rising or interpulse phase of the endogenous corticosterone rhythm. However, when the noise stress coincided with a falling phase of this rhythm, the response was greatly diminished. Since corticosterone pulse frequency was markedly increased and hence interpulse interval decreased by day 14, there was an increased probability of the noise stress occurring during the nonstress responsive falling phase of the corticosterone secretory cycle. As a result, the group mean response to noise stress was significantly smaller in the arthritic than the controls (70.2 ± 9.2 versus 107.8 ± 13.0 ng/ml, respectively). In contrast to the differential response to noise stress, all rats showed similar responses to the acute immunological challenge with i.v. lipopolysaccharide. Thus, altered basal pulse frequency is a major factor influencing HPA activation during acute psychological stress.

The hypothalamic-pituitary-adrenal axis response to endotoxin is attenuated during lactation.

Pregnancy and lactation are times of prolonged physiological changes affecting the neuroendocrine and immunological systems. One well‐characterized change is the neuroendocrine hyporesponsiveness to acute stressful stimuli. We have now designed studies to see whether there is an alteration in the response of the hypothalamic‐pituitary‐adrenal (HPA) axis to an immunological inflammatory challenge and to ascertain whether lactating animals show altered neural and endocrine responses to inflammatory stimuli. Lactating (day 9–12 postpartum) or virgin control Sprague‐Dawley female rats were injected with either 200 μg of endotoxin (lipopolysaccharide, LPS ) or sterile saline given i.p. Trunk blood or jugular blood was collected from the animals at 2 h or hourly over 6 h after injection. Both plasma adrenocorticotropic hormone (ACTH) and corticosterone concentrations were significantly higher in saline treated lactating animals compared with the virgin group. LPS significantly elevated circulating levels of plasma ACTH and corticosterone in both virgin and lactating animals compared with saline controls, however, hormone responses to LPS were significantly reduced in lactating animals relative to virgin controls. Corticosterone‐binding globulin concentrations were lower in lactating animals compared to virgin animals and LPS decreased concentrations in virgin, but not lactating rats. Analysis of cfos mRNA in the paraventricular nucleus (PVN) of the hypothalamus revealed that 2 h following injection there was a increase in cfos expression only in the virgin animals treated with LPS, compared to all other treatment conditions. Corticotropin‐releasing hormone (CRH) mRNA expression was overall greater in virgin animals, but was increased to similar extent in both virgin and lactating animals treated with LPS. Primary arginine vasopressin (AVP) mRNA transcripts were increased 2 h following LPS injection, but a greater increase in expression was seen in virgin animals. These data demonstrate that there is a lower level of free circulating glucocorticoid in response to inflammatory stimuli and suggests that communication between the immune and endocrine systems may be altered during lactation.

Daily patterns of secretion of neurohypophysial hormones in man: effect of age

The neurohypophysial hormone vasopressin contributes to control of urine output and, while urine flow shows a clear daily rhythm, there has been debate as to whether this is true of neurohypophysial hormones. A study was performed on fifteen adult males, with a mean age of 25 years, over a 24 h period, nine blood samples being taken at regular intervals for the determination of neurohypophysial hormones and indices of fluid balance. Samples were taken via an indwelling cannula so that sleep was undisturbed. A daily variation in the plasma concentrations of oxytocin and vasopressin was demonstrated with concentrations reaching a nadir in the late afternoon. Concentrations of both hormones peaked at 02.00 h. Vasopressin concentrations were inversely correlated with packed cell volume, indicating that the altered hormone release was affecting fluid retention. Consistent with this was the observation that the relationship of plasma osmolality to vasopressin depended on the time of day. To determine the effect of ageing, a similar study was performed on nine healthy elderly subjects with a mean age of 70 years. The nocturnal peak of vasopressin was markedly attenuated, while oxytocin release was similar to that in the younger group. These observations confirm the existence of a daily rhythm in the plasma concentrations of neurohypophysial hormones and indicate that the amplitude of the vasopressin change decreases with age.

Hypothalamic-Pituitary-Adrenal Function

Basal hypothalamic-pituitary-adrenal (HPA) function is characterised by pulses of corticosterone secretion followed by a transient refractory period when the axis appears to be inhibited. In females pulses of corticosterone secretion occur approximately once per hour with variation in pulse amplitude underlying a diurnal rhythm. Males show smaller pulses of secretion which become widely spaced during the early light phase nadir. Pulsatility is altered by genetic programming, early life experiences and reproductive status. Activation of the HPA axis during adjuvant induced arthritis results in an increase in the pulse frequency. This is associated with a marked change in hypothalamic gene expression with a diminution of CRH mRNA and a marked increase of AVP mRNA which becomes the predominant HPA secretagogue.

Reduced Response of the Hypothalamo-Pituitary-Adrenal Axis toα 1-Agonist Stimulation during Lactation1

To determine whether altered noradrenergic activation of the hypothalamo-pituitary-adrenal (HPA) axis contributes to the attenuated neuroendocrine response to stress observed during lactation, the effect of intracerebroventricular injection of the α1-agonist methoxamine (100 μg) was compared between virgin and lactating rats. Virgin rats showed significant increases in plasma corticosterone after methoxamine, reaching 317 ± 44 ng/ml at 10 min and remaining significantly elevated for more than 120 min, but lactating rats showed no significant increase in corticosterone levels. Furthermore, methoxamine induced an increase in paraventricular nucleus (PVN) CRF messenger RNA expression in virgin, but not lactating, animals. Both groups of rats exhibited comparable elevations in plasma PRL after methoxamine treatment. Arginine vasopressin messenger RNA expression within the parvocellular PVN was greater in the lactating animals than in the virgin controls, but methoxamine injection was without further effect. S...

Region-specific immediate-early gene expression following the administration of corticotropin-releasing hormone in virgin and lactating rats

Central administration of corticotropin-releasing hormone (CRH) induces immediate-early gene (IEG) expression (c-fos and NGFI-B) in forebrain structures in a pattern similar to that observed following restraint stress. Lactating rats display modified neuroendocrine and behavioural responses to stress which have been hypothesized to be at least partially mediated through changes within the circuitry converging on the PVN, including CRH activated pathways. Quantitative measures of regional expression of c-fos and NGFI-B mRNA representative of two classical intracellular pathways, were used to define modification of the circuitry involved in the altered response to central CRH in the lactating female. Compared to saline controls, virgin female rats injected with 5 micrograms CRH i.c.v. displayed significantly increased immediate-early gene expression in the hypothalamic paraventricular nucleus (PVN), arcuate nucleus, lateral septum, bed nucleus of the stria terminalis, central, medial and cortical nuclei of the amygdala, and all subfields of the hippocampal formation. In lactating rats treated with CRH there was a significant increase in c-fos gene expression in the CeA and in the hippocampal subfields CA1, CA4 and dentate gyrus but not in the other areas examined. The i.c.v. administration of CRH significantly increased NGFI-B expression in the PVN, arcuate nucleus, medial amygdala and all hippocampal subfields of virgin rats. Lactating rats treated with CRH failed to show a significant increase in NGFI-B expression in the PVN, median eminence, arcuate nucleus, medial amygdala, CA2 and CA3 subfields of the hippocampus. These results further suggest that changes in specific neural circuits might at least partially underlie the modified responses to CRH and perhaps to stress in the lactating female.

Differential Effects of Psychological and Immunological Challenge on the Hypothalamo-Pituitary-Adrenal Axis Function in Adjuvant-induced Arthritisa

ABSTRACT: Inability to mount a suitable glucocorticoid response to a stressor can be life‐threatening. Rats with hind‐paw inflammation, associated with the development of adjuvant‐induced arthritis (AA), are unable to mount a hypothalamo‐pituitary‐adrenal (HPA) axis response to acute stress. In the present study we have compared the effects of acute psychological stress (noise) and acute immunological challenge (lipopolysaccharide [LPS] injection), on the activation of the HPA axis in rats with the chronic inflammatory stress of AA. We conclude that the increase in HPA axis activity in AA is principally due to an increase in corticosterone pulse frequency and not to any alteration in pulse magnitude. The lack of response to acute stress can be accounted for by the increase in pulse frequency and the associated refractory period following each pulse, producing dramatic but specific changes in basal HPA function. These changes may account for the loss of responsiveness to acute stress, but not to acute immunological challenge, because the HPA axis is able to respond to LPS in male rats with AA. However, there appears to be an impaired adrenal responsiveness in female rats with AA that is not inherent, but occurs as a consequence of the development of inflammation.