Protective effects of endotoxin in a rat model of chronic inflammation are accompanied by suppressed secretion of pro-inflammatory cytokines and biphasic alteration in hypothalamo-pituitary-adrenal axis activity.

We have previously demonstrated that Gram-negative bacterial endotoxin can exert long-term protective effects against the chronic inflammatory disease adjuvant arthritis in rats. The present study was designed to investigate the mechanisms and time-course of hypothalamo-pituitary-adrenocortical (HPA) axis activity and cytokine secretion underlying this phenomenon. Rats were injected with endotoxin (lipopolysaccharide) and blood was collected either 7 or 21 days later. Priming with endotoxin induced a biphasic alteration in secretion of adrenocorticotrophic hormone and corticosterone in response to a second injection of endotoxin, with decreased secretion observed after 7 days whereas robust secretion was observed at 21 days. Seven days following priming with endotoxin, plasma concentrations of pro-inflammatory cytokines interleukin (IL)-6 and interferon (IFN)-gamma were reduced by 90%, and tumour necrosis factor (TNF)-alpha by 70%, compared to saline-treated rats, whereas robust secretion of the anti-inflammatory cytokine IL-10 was maintained in both groups. A similar net change favouring an anti-inflammatory cytokine secretory milieu was also observed 21 days following priming with endotoxin. This study provides evidence that the long-term protective effects of endotoxin on inflammation are associated with a sustained reduction in secretion of pro-inflammatory cytokines. HPA axis hypoactivity at 7 days suggests that corticosterone is not involved in suppressing IL-6, IFN-gamma and TNF-alpha at this time point. Conversely, hypersecretion of corticosterone at 21 days may underlie synchronous suppression of IL-6 and IFN-gamma. These data provide novel insight into interactions between HPA axis activity and cytokine secretion following endotoxin priming prior to induction of inflammatory disease.

Organizational role for testosterone and estrogen on adult hypothalamic-pituitary-adrenal axis activity in the male rat.

Organizational effects of testosterone during a critical period of neonatal life have major irreversible effects on adult sexual behavior. We have investigated whether perinatal androgen changes also affect another major sexually differentiated system, the hypothalamo-pituitary-adrenal axis. This was assessed in male rats who had been exposed to perinatal flutamide or 1,4,6-androstatriene-3,17-dione (ATD). Once the animals reached adulthood, an automated sampling system was used to collect blood from freely moving animals at 10-min intervals over 24 h, followed by a noise stress and then the administration of lipopolysaccharide (LPS). Perinatal flutamide- and ATD-treated rats not only had higher mean corticosterone levels and increased frequency and amplitude of corticosterone pulses over the 24 h compared with vehicle-injected controls, but they also showed markedly increased corticosterone responses to both noise and LPS. All parameters of increased hypothalamo-pituitary-adrenal activity resembled the normal physiological state of the intact adult female rather than that of the intact adult male rat. Furthermore, 3 h after LPS administration, both flutamide- and ATD-treated animals had markedly higher levels of corticotropin-releasing factor mRNA in the parvocellular paraventricular nucleus (PVN) and proopiomelanocortin mRNA in the adenohypophysis. Flutamide-treated rats also had a greater level of PVN arginine vasopressin mRNA. PVN glucocorticoid receptor mRNA levels were significantly lower in both the flutamide- and the ATD-treated male rats. These data highlight the importance of perinatal exposure to both testosterone and estrogen(s) on the development of a masculinized circadian corticosterone profile and stress-induced hypothalamo-pituitary-adrenal axis activity in the adult male rat.

Postnatal masculinization alters the HPA axis phenotype in the adult female rat.

The ability of postnatal testosterone propionate (TP) to masculinize both behaviour and gonadal cyclicity in the female rat is well documented. We have investigated whether postnatal androgen also has an organizational effect on another sexually dimorphic neuroendocrine system--the hypothalamo-pituitary-adrenal (HPA) axis. Female rats were exposed to a single injection of testosterone propionate (TP) or oil within 24 h of birth. As adults, rats were either ovariectomized and given 17beta-oestradiol replacement (OVXE2) or sham ovariectomized with cholesterol implants (SHOVX). An automated sampling system collected blood from unanaesthetized adult female rats every 10 min over a 24-h period, during a mild psychological stress (noise) and following an immunological lipopolysaccharide stress (LPS). Neonatal TP-treated SHOVX rats had a significant reduction in the number, height, frequency and amplitude of corticosterone pulses over the basal 24-h period, compared to both the neonatal oil-treated and TP-treated OVXE2 animals. The corticosterone response to both noise and LPS was also significantly decreased for the TP-treated SHOVX females. Three hours post-LPS administration, TP females had significantly lower values of paraventricular nucleus (PVN) corticotrophin releasing hormone (CRH), arginine vasopressin (AVP) and anterior pituitary proopiomelanocortin (POMC) mRNAs and greater PVN glucocorticoid receptor (GR) mRNA expression compared to the oil-treated controls. E2 replacement in adult TP rats normalized all the mRNA levels, except for PVN GR mRNA which did fall towards the levels of the oil-control animals. A single injection of TP within 24 h of birth disrupts the development of the characteristic female pattern of corticosterone secretion and the normal female HPA response to stress, resulting in a pattern similar to that seen in males. These effects can be reversed by E2 treatment in the adult TP female rat.

Immunohistochemical staining of endomorphin 1 and 2 in the immune cells of the spleen.

Endomorphin 1 (EM-1) and EM-2 have been widely reported in the cells of the central nervous system (CNS) but limited research has been done regarding their distribution in the peripheral system. The occurrence of EM-1 and -2 in the spleen as measured by RIA and their ability to mediate immune function imply a role for EMs in this area. The current study examines the localization of EM-1 and -2 in the immune cells of the spleen of male and female rats via an immunohistochemical procedure. In both genders, EM-1 and -2 immunoreactive staining was predominantly present in macrophages and B cells with minimal EM immunoreactive staining in T cells. This is the first evidence of a differential distribution of EM-1 and -2 in cells of the immune system.

Gonadal steroid replacement reverses gonadectomy-induced changes in the corticosterone pulse profile and stress-induced hypothalamic-pituitary-adrenal axis activity of male and female rats.

We investigated the effects of gonadal hormone replacement on the pulsatile parameters underlying basal circadian corticosterone secretion in castrated male and ovariectomized female rats using an automated sampling system. Blood was collected from freely moving, unanaesthetized rats every 10 min over a 24-h period and sampling was continued during a noise stress and after lipopolysaccharide (LPS) administration. Castrated male rats had markedly higher corticosterone levels than intact controls. This was reflected by increased number and frequency of pulses in addition to an increase in the pulse height and amplitude under both basal circadian and stress conditions. Hormone replacement with either testosterone or dihydrotestosterone returned these corticosterone levels and circadian profile to those found in intact males, confirming an androgen-mediated effect. Ovariectomized females had significantly lower basal and stress-induced corticosterone levels with lower frequency and amplitude of corticosterone pulses than intact females. 17beta-oestradiol replacement returned basal levels, pulsatile measurements and stress-induced corticosterone levels to those found in intact females. Three hours post-LPS administration, castrated males demonstrated significantly higher values of parvocellular paraventricular nucleus (PVN) arginine vasopressin and corticotrophin-releasing factor and anterior pituitary pro-opiomelanocortin mRNA while ovariectomized females showed significantly lower levels of all three transcripts compared to intact controls. PVN glucocorticoid receptor mRNA levels 3 h post-LPS administration were significantly decreased in castrated males and significantly increased in ovariectomized female rats. Replacement of gonadal steroids resulted in a return to the levels found in intact controls after LPS. Gonadal steroid replacement is sufficient to reverse changes in the pulsatile characteristics of corticosterone release after gonadectomy. In addition, gonadal steroid replacement reverses stress-induced alterations in hypothalamic-pituitary-adrenal (HPA) activity. These data demonstrate a major contribution of gonadal steroids to the regulation of HPA axis activity and to the pulsatile characteristics of corticosterone release.

A single lipopolysaccharide administration is sufficient to induce a long-term desensitization of the hypothalamic-pituitary-adrenal axis.

We have previously shown that a single exposure of adult rats to a severe emotional stressor such as immobilization is able to exert a long-term desensitization of the response of the hypothalamic-pituitary-adrenal (HPA) axis to the same stimulus when applied days to weeks later. Surprisingly, the intensity of the effect increased with time elapsed between the two exposures, suggesting that we are dealing with a new type of stress-associated phenomenon. Taking into account the clinical importance of tolerance to endotoxin, in the present study we assessed whether a single exposure to an immunological stressor such as lipopolysaccharide can induce effects similar to those of immobilization. Rats injected with lipopolysaccharide (1 mg/kg) showed a reduction of the response of the corticotropin-releasing factor mRNA in the paraventricular nucleus of the hypothalamus after a new lipopolysaccharide injection 4, but not 2 weeks later. In an additional experiment using a different blood sampling procedure, adrenocorticotropin hormone, corticosterone and tumor necrosis factor-alpha responses were reduced approximately to the same extent by previous experience with lipopolysaccharide either 1 or 4 weeks before. Our data suggest that a previous single exposure to lipopolysaccharide induces a long-lasting tolerance of the HPA axis that likely involves some kind of learning-like brain plasticity.

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.

A pilot study into the therapeutic effects of music therapy at a cancer help center.

CONTEXT: Since the mid-1980s, music therapy has been a regular feature of the residential program at the internationally renowned Bristol Cancer Help Centre, United Kingdom. Music therapy complements other therapeutic interventions available to residents at the center. OBJECTIVE: To compare the therapeutic effects of listening to music in a relaxed state with the active involvement of music improvisation (the playing of tuned and untuned percussion instruments) in a music therapy group setting and to investigate the potential influence of music therapy on positive emotions and the immune system of cancer patients. DESIGN: A quantitative pre-posttest, psychological/physiological measures, and qualitative focus group design. SETTING: A cancer help center that offers a fully integrated range of complementary therapies, psychological support, spiritual healing, and nutritional and self-help techniques addressing the physical, mental, emotional, and spiritual needs of cancer patients and their supporters. PARTICIPANTS: Twenty-nine cancer patients, aged 21 to 68 years. INTERVENTION: Group music therapy interventions of listening to recorded/live music in a relaxed state and improvisation. MAIN OUTCOME MEASURES: Increased well-being and relaxation and less tension during the listening experience. Increased well-being and energy and less tension during improvisation. Increased levels of salivary immunoglobulin A and decreased levels of cortisol in both experiences. RESULTS: Psychological data showed increased well-being and relaxation as well as altered energy levels in both interventions. Physiological data showed increased salivary immunoglobulin A in the listening experience and a decrease in cortisol levels in both interventions over a 2-day period. Preliminary evidence of a link between positive emotions and the immune system of cancer patients was found. CONCLUSIONS: These findings, which link listening to music in a relaxed state and improvisation to alterations in psychological and physiological parameters, may provide a better understanding of the effectiveness of music therapy for cancer patients.

Acetaldehyde, a metabolite of ethanol, activates the hypothalamic-pituitary-adrenal axis in the rat.

Cyanamide is a potent inhibitor of aldehyde dehydrogenase (ALDH: EC 1.2.1.3) used in the treatment of alcoholics. In the presence of ethanol, cyanamide causes an accumulation of acetaldehyde, a highly toxic metabolite of ethanol, with unpleasant side-effects. A similar accumulation is seen in some Oriental people with low ALDH activity. We have investigated the effects of ethanol and cyanamide administration on the activation of the hypothalamic-pituitary-adrenal (HPA) axis using in situ hybridization histochemistry and radioimmunoassay. Ethanol plus cyanamide resulted in a significant increase in corticotrophin-releasing factor and arginine vasopressin mRNA in the paraventricular nucleus, and pro-opiomelanocortin mRNA in the anterior pituitary. Plasma corticosterone concentrations were also significantly elevated following ethanol plus cyanamide administration. The blood concentration of acetaldehyde in the ethanol plus cyanamide group increased significantly. These results suggest that acetaldehyde, induced by blocking ethanol metabolism, is able to activate the HPA axis operating through a central mechanism.

The hypothalamo-pituitary-adrenal axis response to experimental traumatic brain injury.

Alterations in the hypothalamo-pituitary-adrenal (HPA) axis following traumatic brain injury have not been documented in detail. We used fluid percussion injury (FPI) to evaluate the early changes in components of the HPA axis following experimental traumatic brain injury. Wistar rats were sacrificed at 2 or 4 h following sham or FPI surgery. In situ hybridization histochemistry was used to determine the expression of mRNAs of corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP) in the hypothalamus and pro-opiomelanocortin (POMC) in the pituitary. A group of animals undergoing no surgery were used as control. Repeated blood sampling from an indwelling catheter demonstrated that plasma corticosterone (CORT) levels peaked 30 min following surgery in sham and FPI animals but there was no significant difference in CORT concentration between these groups at any time. Pituitary POMC expression was increased following sham and FPI surgery (compared with control non-operated animals) but with no significant difference between the two groups undergoing surgery. Hypothalamic CRH mRNA expression was significantly higher in animals undergoing FPI compared with sham surgery. Hypothalamic AVP mRNA expression was not significantly increased when compared with control nonoperated animals. These data indicate that the anaesthesia and/or surgery associated with FPI or sham surgery induces a generalised activation of the HPA axis. The selective increase in CRH mRNA in animals undergoing FPI may be due to specific effects of traumatic brain injury rather than a general stress response and may suggest an additional neurotransmitter role for CRH following head injury. The absence of an AVP response suggests that the effects of FPI may be mediated through the CRH-alone-containing subpopulation of neurons.

Substance P is involved in terminating the hypothalamo- pituitary-adrenal axis response to acute stress through centrally located neurokinin-1 receptors.

The neurokinin substance P (SP) has been previously shown to inhibit basal hypothalamo-pituitary-adrenal (HPA) axis activity. This study was designed to investigate the effects of central injection of the specific neurokinin-1 receptor antagonist RP67580 on the HPA axis response to acute restraint stress. In non-restrained rats injected with RP67580, plasma ACTH and corticosterone levels were elevated at 30 and 60 min compared to rats injected with vehicle, but there were no differences between vehicle and RP67580 groups at 4h. In restrained rats injected with vehicle, plasma ACTH and corticosterone levels were significantly elevated at 30 min and 60 min following initiation of the stress but had returned to basal levels at 4h. In restrained rats injected icv with RP67580, plasma corticosterone and ACTH levels were significantly elevated at 30 min and 60 min, with no significant differences compared to the restraint stressed vehicle-injected group. However, in the RP67580-injected group, corticosterone and ACTH levels remained significantly elevated at 4h following onset of restraint compared to those in the restraint stressed vehicle-injected group. Corticotrophin-releasing factor mRNA levels in the parvocellular subdivision of the paraventricular nucleus of the hypothalamus and POMC mRNA levels in the anterior pituitary were significantly increased in the stressed group 4h following injection with RP67580 compared to the stressed group injected with vehicle alone. These data show that endogenous SP does not inhibit the initial magnitude of the HPA axis response to restraint stress, but does act through neurokinin-1 receptors at a central level to reduce the duration of the response to stress. This suggests that SP may be an important central agent controlling the transition between acute and chronic stress.

Onset and severity of inflammation in rats exposed to the learned helplessness paradigm.

OBJECTIVE: To test the hypothesis that there is an association between susceptibility to inflammation and a hyporesponsive hypothalamo-pituitary-adrenal (HPA) axis. METHODS: Animals were separated on the basis of behaviour in the learned helplessness (LH) paradigm into groups of LH(+) (i.e. animals which did not escape footshock) and LH(-) animals. Adjuvant-induced arthritis (AA) was subsequently induced in the LH(+) and LH(-) animals. RESULTS: Plasma corticosterone was significantly increased in response to the LH test in the LH(-) compared with the LH(+) rats. We observed an earlier onset and increased inflammation in the LH(-) rats in spite of the greater corticosterone response to the acute stress. We noted lower levels of plasma testosterone in the LH(-) animals suggesting a possible influence for this protective factor in AA. CONCLUSION: These data suggest that increased onset and severity of inflammation in AA is not a simple consequence of an attenuated HPA axis response to stress as proposed in the Lewis rat. Indeed we have observed the converse. Together these data suggest that the balance of pro- and anti-inflammatory factors released in response to stress may influence the progress of AA.

Central single and chronic administration of morphine stimulates corticosterone and interleukin (IL)-6 in adjuvant-induced arthritis.

Adjuvant-induced arthritis (AA) in the rat is a chronic inflammatory stress in which circulating corticosterone and interleukin (IL)-6 levels are elevated. In addition, there are profound neuroendocrine changes associated with the development of hind-paw inflammation which have major implications for the ability of the rat to respond to stress. Central injection of morphine is able to increase plasma corticosterone and circulating IL-6 concentration in control animals. In present study we have determined the effects of a single and repeated injection of morphine into the lateral ventricle of control and AA animals on plasma corticosterone, circulating IL-6 levels and course of hind-paw inflammation in AA rats. In the present study we found a sustained increase in plasma corticosterone both after single and repeated injection of morphine in control and AA rats and an increase of the level of circulating IL-6 in AA rats after repeated injection of morphine. These data suggest that alternative systems distinct from Athose activated in response to acute stress are activated by morphine in the AA animals. The similarity with the sustained increase in corticosterone following LPS injection suggest that central opiates may be involved in mediating HPA axis and cytokines response to inflammatory stress.

Central LPS-induced c-fos expression in the PVN and the A1/A2 brainstem noradrenergic cell groups is altered by adrenalectomy.

The A1 and A2 brainstem noradrenergic cell groups project to the hypothalamic paraventricular nucleus (PVN), which is involved in integrating the stress response. Bi-directional communication between the brain and immune system is well established, with both neuroendocrine and immune responses being activated by lipopolysaccharide (LPS). The mechanisms underlying such activation and differences between alternative routes of administration remain unclear. We examined activation of the PVN and A1/A2 cell groups, by assessing c-fos mRNA, or counting Fos-positive neurons in either the PVN or in brainstem A1/A2 cell groups 3 h after intracerebroventricular (i.c.v.) LPS, in control and adrenalectomized (ADX) rats. We also measured corticotropin-releasing hormone (CRH) mRNA in the PVN, and plasma corticosterone (CORT) levels. A group of ADX/CORT-replaced animals received i.c.v. LPS, and CRH mRNA and Fos peptide in the PVN were analysed. ADX increased CRH mRNA in the PVN, as did LPS, but no enhancement of this response was seen in LPS/ADX animals. C-fos mRNA also increased in both the PVN and the A2 cell group following LPS, but this response was potentiated by ADX. Fos peptide-containing cells increased in the PVN and A2 following LPS, and this change was amplified by ADX. Only 11.25% of Fos was found in DBH-positive (putative noradrenergic) neurons, suggesting activation of neurons containing other transmitters. ADX/LPS/CORT animals showed numbers of Fos neurons in the brainstem, and CRH mRNA levels in the PVN which were comparable to intact/LPS animals. Central LPS activates the hypothalamo-pituitary-adrenal axis, a process mediated partly by brainstem noradrenergic neurons, suggesting the involvement of afferent/efferent pathways within the brain. Peripheral administration of LPS involves activation of vagal inputs leading to the nucleus tractus solitarius. We suggest that centrally administered LPS activates the A2 cell group by a mechanism independent of the vagus. In the absence of CORT, despite the lack of a CRH mRNA response, an exaggerated c-fos and peptide response to LPS is observed, which is reversed following CORT pretreatment.

Dissociation between c-fos mRNA in the paraventricular nucleus and corticosterone secretion in rats with adjuvant-induced arthritis.

Increased c-fos mRNA or fos immunoreactivity within the central nervous system has been used as a marker of neuronal activation. Acute stress and acute immune challenge result in an increase in c-fos mRNA in corticotrophin-releasing factor (CRF)-containing neurons in the paraventricular nucleus (PVN). It has often been implied that an increase in fos in the PVN can be equated to an increase in the activity of CRF itself, although there is some evidence to suggest these events are not linked. In the present study we have used the rat model of adjuvant-induced arthritis (AA), in which, despite the activation of the pituitary-adrenal system associated with inflammation, there is a paradoxical decrease in CRF mRNA and CRF peptide release. AA rats are unable to mount a hypothalamo-pituitary-adrenal (HPA) axis response to acute stress. They are, however, able to mount a response to acute immune stimulation, e.g. lipopolysaccharide injection. Despite the lack of HPA axis response to stress, there is an increase in c-fos mRNA to these challenges in AA. This suggests that the increase in c-fos mRNA in response to acute stress is not related to a subsequent increase in CRF mRNA in this model. We can conclude that under these conditions, c-fos mRNA is not a good marker of HPA axis activation and independent estimation of the involvement of CRF in the stimulation of the HPA axis should always be obtained. The AA model may prove useful for the comparison of the relationship between immediate early genes and heteronuclear RNAs in response to acute stress and immune stimuli with which to tease apart the molecular mechanisms underlying the control of releasing factor activation at the level of the PVN.

Central injection of morphine stimulates plasma corticosterone and interleukin (IL)-6 and IL-6 R mRNAs in the pituitary and adrenals in adjuvant-induced arthritis.

Adjuvant-induced arthritis (AA) in the rat is a T-cell mediated, chronic inflammatory stress in which circulating interleukin (IL)-6 levels are elevated. In addition, there are profound neuroendocrine changes associated with the development of hind-paw inflammation which have major implications for the ability of the rat to respond the to stress. Central injection of morphine is also able to increase circulating IL-6 concentration in control animals. In the present study we have determined the effects of a single injection of morphine into the lateral ventricle of control and AA animals on plasma corticosterone levels, on changes in plasma corticosterone and on IL-6 and IL-6 receptor mRNAs in the pituitary and adrenal gland. IL-6 and IL-6 receptor mRNAs were increased in the anterior pituitary of AA rats given moprhine compared with saline-treated AA rats. In the adrenal cortex, IL-6 mRNA was unaltered and IL-6 receptor mRNA was significantly decreased under these same conditions. AA rats were unable to mount corticosterone response to acute stress but were able to respond to acute stimulation with e.g. LPS. In the present study we found a sustained increase in plasma corticosterone in control animals which was still significantly elevated 2 hours following morphine injection, with a further significant increase in AA rats. These data suggest that alternative systems distinct from those activated in response to acute stress are activated by morphine in the AA animals. The similarity with the sustained increase in corticosterone following LPS injection suggest that either similar pathways are involved, or that central opiates may be involved in mediating HPA axis response to stress.

PACAP gene expression in neurons of the rat hypothalamo-pituitary-adrenocortical axis is induced by endotoxin and interleukin-1beta.

Inflammatory stress due to infection by various micro-organisms is known to activate the hypothalamo-pituitary-adrenocortical (HPA) axis through inflammatory mediators. Recently, pituitary-adenylate-cyclase-activating polypeptide (PACAP) was shown to be located in corticotropin-releasing factor containing neurons of the medial parvocellular part of the hypothalamic paraventricular nucleus (mpPVN). In the present study, we demonstrate that PACAP gene expression is induced in neurons of the mpPVN after intraperitoneal administration of bacterial lipopolysaccharide (LPS) which was accompanied by a marked increase in PACAP immunoreactivity in the external zone of the median eminence. As determined by quantitative in situ hybridization, PACAP gene expression was rapidly induced after 4 h and was elevated for 48 h, declining to normal levels after 72 h. A significant increase in PACAP mRNA was also observed following intraperitoneal injection of interleukin-1beta. PACAP gene expression was not induced by LPS in vagotomized animals, suggesting that the increase in PACAP mRNA following immune activation by LPS is mediated via the vagus nerve. The findings suggest that PACAP may function as a hypothalamo-pituitary-releasing factor during acute inflammation.

Acute and long-term treatments with the selective serotonin reuptake inhibitor citalopram modulate the HPA axis activity at different levels in male rats.

It is well established that the maximal therapeutic effect of selective serotonin reuptake inhibitors (SSRI) are achieved in depressive patients after several weeks of treatment, but the adaptive processes leading to the therapeutic effects are unclear. It has been shown that hyperactivity in the hypothalamic-pituitary-adrenal (HPA) axis in depressive patients is affected by long-term antidepressant treatment. These changes occur in association with the mood normalising effect, suggesting that antidepressants affect the HPA axis and this effect is associated with the therapeutic effect. Male Wistar rats were treated with the SSRI, citalopram, to investigate time-related changes in components that may be involved in the desensitization of the HPA axis. A single injection of citalopram (10 mg/kg, s.c.), increased the plasma levels of ACTH and corticosterone in a dose-dependent manner and increased the number of c-Fos containing cells in the hypothalamic paraventricular nucleus. A daily treatment with the same compound (10 mg/kg, s.c.) for 14 days decreased the expression of POMC mRNA ( approximately 40%). In addition, a blunted response to citalopram was observed in animals long-term treated with citalopram. Also CRF-stimulated cAMP accumulation in the pituitary was altered. In conclusion, acute citalopram activated the HPA-axis at the hypothalamic level and long-term citalopram treatment desensitized the HPA-axis at the pituitary level. These results support the hypothesis that the therapeutic effects of long-term antidepressant treatments reduce HPA axis responsiveness.

Activation of the hypothalamic-pituitary axis in adrenalectomised rats: potentiation by chronic stress.

The influence of chronic stress on the status of the hypothalamo-pituitary-adrenal (HPA) axis of sham-operated and adrenalectomised rats was assessed. Animals underwent bilateral adrenalectomy (ADX) and 3 days later they were either left undisturbed or subjected daily to immobilization for 2 h each morning for 14 days (chronic IMO). In situ hybridization histochemistry revealed that ADX increased corticotropin-releasing factor (CRF) mRNA levels in the paraventricular nucleus of the hypothalamus (PVN) and proopiomelanocortin (POMC) mRNA levels in the anterior pituitary, in both control and chronically stressed rats as measured on the day following the last exposure to stress. Chronic IMO increased CRF mRNA levels in the PVN and POMC mRNA levels in the anterior pituitary of sham-operated rats, as measured on the day following the last exposure to stress. Chronic IMO potentiated the increase in CRF mRNA in the PVN following ADX and resulted in further increases in CRF mRNA above levels seen in adrenal-intact animals. Finally, chronic stress, while not altering basal ACTH levels of ADX rats, reduced the ACTH response of these animals to a novel stressor (tail-shock for 30 min). These results suggest that chronic stress exerts a stimulatory influence at the hypothalamic level that is partially restrained by daily stress-induced glucocorticoid release. Despite the potentiation by chronic stress of CRF mRNA content in the PVN of ADX rats, a blunted circulating ACTH response to an acute short-term stressor was apparent in ADX-chronically stressed rats, suggesting that chronic stress might also alter POMC processing and/or ACTH secretory patterns in the anterior pituitary in ADX animals.