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.

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.

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.

Lack of effect of protein deprivation-induced intrauterine growth retardation on behavior and corticosterone and growth hormone secretion in adult male rats: a long-term follow-up study.

To further define the neuroendocrine consequences of intrauterine growth retardation (IUGR), we have used a rat model of maternal protein restriction throughout pregnancy to examine the pattern of corticosterone and GH secretion under basal conditions and in response to psychological stress in male offspring at 4, 9, and 18 months of age. The findings were correlated with studies of behavioral activity. Despite a consistent reduction in birth weight and failure of catch-up growth, there were no significant differences in GH secretory profiles between IUGR and control rats at any age. We were unable to demonstrate a difference in the number, amplitude, length, or area of corticosterone secretory pulses between control and IUGR animals; although again, there was a significant decrease with age. The mean peak plasma concentration of corticosterone in response to a noise stress also declined with age but was unaffected by IUGR. There were no consistent, statistically significant differences in behavioral responses between normal control and IUGR animals or between groups of animals at different ages. These results do not, therefore, support the presence of major functional abnormalities in either GH or corticosterone secretory responses in adult male rats subjected to IUGR.

Chronic iodine deprivation attenuates stress-induced and diurnal variation in corticosterone secretion in female Wistar rats.

Many millions of people throughout the world are at risk of developing iodine deficiency-associated disorders. The underlying effects of iodine deficiency on neuroendocrine function are poorly defined. We have studied stress-induced and diurnal variation in corticosterone secretion in female rats rendered chronically hypothyroid by feeding them an iodine-free diet for 6 months. Corticosterone secretory responses in iodine deficient animals were compared to those seen in animals rendered hypothyroid with propylthiouracil and untreated controls. By using a well-validated, automated blood sampling system to collect small samples of blood over the complete daily cycle in unrestrained animals, we have demonstrated for the first time that the normal diurnal rhythm of corticosterone secretion is lost in chronic iodine deficiency and that the corticosterone secretory response to the psychological stress of 10 min exposure to white noise is attenuated. Despite restoration of circulating triiodothyronine and thyrotropin releasing hormone- and thyroid stimulating hormone beta-transcript prevalence in the hypothalamus and pituitary, respectively, 1 month after restoration of normal iodine-containing diet both the diurnal variation in corticosterone levels and the corticosterone secretory response to the noise stress remained reduced in amplitude compared to control animals. Thus, chronic hypothyroidism induced by iodine deficiency significantly attenuates hypothalamo-pituitary-adrenal axis activity, an effect that persists after functional recovery of the thyroid axis.

Early-life exposure to endotoxin alters hypothalamic-pituitary-adrenal function and predisposition to inflammation.

We have investigated whether exposure to Gram-negative bacterial endotoxin in early neonatal life can alter neuroendocrine and immune regulation in adult animals. Exposure of neonatal rats to a low dose of endotoxin resulted in long-term changes in hypothalamic-pituitary-adrenal (HPA) axis activity, with elevated mean plasma corticosterone concentrations that resulted from increased corticosterone pulse frequency and pulse amplitude. In addition to this marked effect on the development of the HPA axis, neonatal endotoxin exposure had long-lasting effects on immune regulation, including increased sensitivity of lymphocytes to stress-induced suppression of proliferation and a remarkable protection from adjuvant-induced arthritis. These findings demonstrate a potent and long-term effect of neonatal exposure to inflammatory stimuli that can program major changes in the development of both neuroendocrine and immunological regulatory mechanisms.

Significance of pulsatility in the HPA axis.

A stress-free automated blood sampling system has been employed to demonstrate pulsatile hypothalamo-pituitary-adrenal (HPA) activity in the rat. In females, pulses of corticosterone secretion occur approximately once/hour throughout the 24 h cycle, 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. Ageing does not affect the occurrence of pulses but the diurnal variation is lost. Analysis of the relationship between the HPA response to an acute noise stress and its coincidence with the various phases of the pulse, suggests that pulsatile activity arises from alternating periods of activation and suppression. Responses to i.v. corticotropin-releasing factor are not affected by pulse phase, indicating that this relationship is not generated at the pituitary-adrenal level. This phase relationship holds for all strains of rat except the hyperresponsive Fischer-344 in which an exaggerated stress response arises from a lack of phase-dependent suppression. Patterns of pulsatile activity are also modulated by neonatal programming or chronic HPA activation arising from adjuvant-induced arthritis, with consequent impact upon the response to acute stimuli. Thus, variations in the patterns of pulsatile activity are important determinants of both basal secretion and acute responses of the HPA axis.

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 microg 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.

Differential effects of psychological and immunological challenge on the hypothalamo-pituitary-adrenal axis function in adjuvant-induced arthritis.

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.

The pulsatile characteristics of hypothalamo-pituitary-adrenal activity in female Lewis and Fischer 344 rats and its relationship to differential stress responses.

The dynamic patterns of basal and stimulated hypothalamo-pituitary-adrenal (HPA) activity of freely moving female Lewis and Fischer 344 rats were compared using an automated blood-sampling system. Both strains showed pulsatile corticosterone release throughout the 24 h cycle. Lewis rats showed clear circadian variation in both pulse frequency (8.4 +/- 0.4 pulses between 1700-2300 h vs. 5.3 +/- 0.8 pulses between 0500-1100 h; P < 0.05) and height (198 +/- 27 ng/ml between 1700-2300 h vs. 107 +/- 14 ng/ml between 0500-1100 h; P < 0.05). Fischer rats exhibited pulses of similar frequency and height to those in Lewis rats during the evening, but showed no circadian variation, resulting in higher mean daily corticosterone concentrations. Although both strains showed behavioral and HPA responses to white noise stress (10 min; 114 dB), Fischer rats showed much greater increases in total activity, grooming, and rearings, and two important differences in the corticosterone responses were observed. First, in Lewis rats a clear relationship existed between basal and stimulated HPA activities, in that a significant response was seen only when the stress coincided with the rising (secretory active) phase of a basal pulse. Noise stress coinciding with a falling (nonsecretory) phase elicited no significant response. In contrast, Fischer rats showed similar responses regardless of the underlying pulse phase. Second, after the peak response at 20 min (Lewis, 237 +/- 67 ng/ml; Fischer, 390 +/- 57 ng/ml), corticosterone levels fell rapidly in Lewis rats, but remained maximally elevated for 20 min in Fischer rats, resulting in a significantly greater integrated response. The corticosterone response to i.v. CRF was unaffected by pulse phase in both strains, suggesting that a suprapituitary mechanism mediates the phase-dependent response to stress in the Lewis strain. CRF-induced corticosterone levels rose more rapidly in Fischer rats, peaking at 10 min (473 +/- 95 ng/ml) compared with 30 min (390 +/- 75 ng/ml) in Lewis rats, suggesting greater pituitary sensitivity in this strain. Thus, differences in both central and pituitary control of the HPA axis contribute to the strain difference in stress responsiveness between female Lewis and Fischer rats.

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.

Ultradian rhythm of basal corticosterone release in the female rat: dynamic interaction with the response to acute stress.

The present study investigated the dynamic regulation of the hypothalamo-pituitary-adrenal axis and its significance to acute stress responsiveness in the female rat. An automated, frequent blood-sampling technique allowed the circadian rhythm of corticosterone to be resolved into a series of pulses. These were equally distributed (mean interval, 50.9 +/- 3.7 min) throughout the 24-h cycle, but their magnitude varied significantly, being higher between 1800-2200 h (137 +/- 9 ng/ml) than between 0600-1000 h (75 +/- 17 ng/ml). This pattern of release indicates continuous, but variable, activity of the axis throughout the day. The pulsatile ultradian rhythm suggested alternate periods of secretion and inhibition, which were found to have a profound effect on the corticosterone responses to acute stress. Noise stress (10 min, 114 decibels) evoked a transient increase in corticosterone, which reached a maximum (377 +/- 87 ng/ml) 20 min after onset. However, within this group (n = 26) the response varied depending on the underlying basal activity. When stress coincided with a rising (secretory) phase of a pulse, corticosterone concentrations rose to 602 +/- 150% of mean basal concentrations (P < 0.001). In contrast, when stress coincided with a falling (nonsecretory) phase of a pulse, a significantly smaller response, no greater than a basal pulse, was evoked. Thus, the alternate periods of secretion and inhibition generating basal hypothalamo-pituitary-adrenal activity are an important determinant of responses to acute stress.

Reduced response of the hypothalamo-pituitary-adrenal axis to alpha1-agonist stimulation during lactation.

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 alpha1-agonist methoxamine (100 microg) 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. Studies performed on ovariectomized virgin rats and ovariectomized rats receiving estradiol or progesterone replacement failed to reproduce the attenuated HPA responses seen after methoxamine treatment, although methoxamine-induced PRL levels were greatly increased by estradiol, probably arising from an effect on hormone synthesis. In vitro electrophysiological recordings of PVN neurons in hypothalamic slices from proestrous virgin and lactating rats showed that 45-52% of neurons in both groups exhibited excitatory responses to 10(-4) M methoxamine, but there was a differential response to 10(-5) M methoxamine, with PVN neurons from lactating animals failing to show a response. These data show a selective down-regulation of alpha1-mediated activation of the HPA axis in lactating animals. This may contribute to the attenuated stress-induced activation of the HPA axis during lactation.

Renal responses to oxytocin during the 4 days of the oestrous cycle in the rat.

Oxytocin was administered to virgin female rats at doses of 25-200 pmol/min during 0.077 mol NaCl/l infusion at 150 microliters/min on each day of the oestrous cycle. The resultant rates of urine flow, glomerular filtration (GFR) and electrolyte excretion were determined. Oxytocin caused significant increases in urine flow (P < 0.001) and sodium excretion (P < 0.001); both responses being dose-dependent (P < 0.02 and P < 0.01 respectively). Significant variations in the renal responsiveness to the hormone occurred over the 4 days of the oestrous cycle. On oestrus the lowest dose of 25 pmol oxytocin/min produced a significant increase in urine flow (from 139.5 +/- 4.3 to 165.6 +/- 7.1 microliters/min, P < 0.005) and a dose of 50 pmol/min produced a significant increase in sodium excretion (from 10.6 +/- 0.1 to 14.5 +/- 0.7 mumol/min, P < 0.005). Significant increases in urine flow and sodium excretion were seen on pro-oestrus with hormone administration rates of 50 and 100 pmol/min respectively and on dioestrus day 2 with a rate of 100 pmol/min. On dioestrus day 1 no increase in urine flow or sodium excretion was seen over the dose range of oxytocin administration. A dose of 100 pmol oxytocin/min significantly increased GFR on pro-oestrus and dioestrus day 2, but not on the other 2 days of the cycle. The circulating hormone concentrations produced by oxytocin infusion were similar on each day of the cycle and so could not account for the differences seen. Therefore, these results suggest varying renal responsiveness to oxytocin during the reproductive cycle of the female rat.

Central oxytocin administration reduces stress-induced corticosterone release and anxiety behavior in rats.

Endocrine responses to noise stress and anxiety-related behaviors were measured in groups of ovariectomized, estradiol-treated female rats given central infusions of oxytocin. Control animals receiving isotonic saline showed a large increase in plasma corticosterone concentrations in response to 10 min of white noise. This response to noise stress was significantly and dose dependently decreased by oxytocin administered intracerebroventricularly at 10 or 100 ng/h for 5 days. Oxytocin also significantly decreased rearing behavior during this stress. When a second noise stress was given 3 days after cessation of oxytocin infusion, corticosterone responses did not differ between the control and previously oxytocin-infused animals. Administration of vasopressin had no significant effect on either the corticosterone or behavioral responses to noise stress. Anxiety-related behaviors were measured on the elevated plus-maze. No significant differences were seen in maze exploration between saline- and oxytocin-treated animals when housed and tested in the same environment. However, when animals were mildly stressed by testing in an unfamiliar environment, oxytocin-treated animals showed a higher proportion of open arm entries and spent significantly more time in the open arms of the maze. Thus, oxytocin exerts a central anxiolytic-like effect on both endocrine and behavioral systems and could play a role in moderating behavioral and physiological responses to stress.

Endocrine and behavioural responses to noise stress: comparison of virgin and lactating female rats during 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 statistically significant activation of the HPA axis, suggesting a dissociation of behavioural and neuroendocrine responses to this mild stress.

Effect of oxytocin receptor antagonists on the renal actions of oxytocin and vasopressin in the rat.

The effect of three oxytocin receptor antagonists on the renal actions of oxytocin and vasopressin was investigated in conscious male rats infused with hypotonic saline. Infusion of oxytocin at 100 pg/min produced plasma concentrations of 12.7 +/- 3.3 pmol/l and led to significant increases in sodium excretion, urine flow and glomerular filtration rate (GFR). The increase in sodium excretion of 42 +/- 9% during oxytocin infusion was significantly decreased by all three antagonists to 15 +/- 5% (10 ng [mercapto-proprionic acid1, D-Tyr(Et)2,Thr4,Orn8]-oxytocin/min), 13 +/- 5% (5 ng desGly9[D-Trp2,Thr4,Orn8]-dC6oxytocin/min) and 4 +/- 5% (1 ng d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr(NH2)9]-vasotocin/min). Similarly, the increase in urine production of 22 +/- 5% associated with oxytocin infusion was significantly decreased to 4 +/- 3% (5 ng desGly9[D-Trp2,D-Thr4,Orn8]-dC6 oxytocin/min) and 1 +/- 4% (1 ng d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr(NH2)9]-vasotocin/ min). All three antagonists blocked the oxytocin-induced increase in GFR when infused at 10 ng/min. Infusion of vasopressin at 160 pg/min produced plasma concentrations of 10.1 +/- 2.1 pmol/l and this led to a significant increase in sodium excretion and a significant decrease in urine flow rate. None of the antagonists had any effect on the natriuretic or antidiuretic actions of vasopressin suggesting that different receptors are involved in these renal actions of the two peptides.

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.

The effect of pinealectomy on osmotically stimulated vasopressin and oxytocin release and Fos protein production within the hypothalamus of the rat.

Osmotically stimulated vasopressin and oxytocin release were measured in pinealectomized and sham operated male rats infused with hypertonic sodium chloride. Neuronal activation in the hypothalamic regions associated with oxytocin and vasopressin release was investigated by quantitative assessment of Fos protein production. The osmotically stimulated release of both vasopressin and oxytocin was significantly lower in pinealectomized animals as compared to sham operated controls. The slope of regression lines between plasma osmolality and hormone concentrations in the sham animals showed a 1.0 +/- 0.1 pmol per mosm/kg rise in vasopressin and 2.0 +/- 0.4 pmol per mosm/kg rise in oxytocin whilst in the pinealectomized animals these values were significantly lower at 0.4 +/- 0.1 pmol vasopressin per mosm/kg and 0.8 +/- 0.2pmol oxytocin per mosm/kg. The osmotic thresholds for hormone release were unaffected by pinealectomy. Fos production was also significantly lower in the supraoptic nucleus and organ vasculosum of the lamina terminalis in the pinealectomized rat at 62 +/- 20 and 59 +/- 9 Fos immunoreactive cells/section as compared to corresponding values of 202 +/- 31 and 123 +/- 20 Fos immunoreactive cells/section in the shams. These observations suggest that reduced hormone release in the pinealectomized animal is due to lowered responsiveness of central osmoregulatory mechanisms and that melatonin may therefore influence the activation of the magnocellular system.