Oxytocin in Brattleboro rats: increased synthesis is contrasted by blunted intrahypothalamic release from supraoptic nucleus neurones

Adult male Brattleboro rats were used to investigate the impact of the congenital absence of vasopressin on the release pattern of oxytocin (OXT) within the hypothalamic supraoptic nucleus (SON) in response to a 10-min forced swimming session and osmotic stimulation. Both immunohistochemical and in situ hybridisation data suggest that vasopressin-deficient animals have more oxytocin-synthesising neurones in the SON than homozygous wild-type controls. Unexpectedly, both forced swimming and peripheral osmotic stimulation resulted in a blunted release profile of oxytocin within the SON of vasopressin-deficient rats compared to controls. A similar intranuclear OXT response to direct osmotic stimulation of the SON by retrodialysis with hypertonic Ringer's solution in both genotypes confirmed the capability of SON neurones to locally release oxytocin in vasopressin-deficient rats, indicating an altered processing of information originating from multisynaptic inputs rather than a deficit in release capacity. Taken together with data obtained in previous studies, the present findings provide evidence suggesting that autocrine and paracrine signalling of magnocellular neurones differs within the paraventricular nucleus and the SON. Thus, significant alterations in intra-SON oxytocin mRNA levels cannot easily be extrapolated to intranuclear release profiles and the local signal intensity of this neuropeptide after physiological stimulation.

Congenital absence of vasopressin and age-dependent changes in ACTH and corticosterone stress responses in rats

The hypothalamic components of the hypothalamo-pituitary-adrenal axis (HPA) are corticotropin-releasing hormone (CRH) and vasopressin. To test the hypothesis that HPA regulation changes with age, we compared ether and bacterial lipopolysaccharide (LPS) injection induced stress reactions in adult and 10-day-old Brattleboro rats, which naturally lack vasopressin owing to mutation of the gene (di/di). The LPS stimulus was used also with V(1b) receptor antagonist pretreatment (SSR149415). In adult di/di or V(1b) pretreated rats, we observed normal pituitary and adrenocortical secretory responses, while in all 10-day-old rats stress-induced serum corticosterone increases were marked, but adrenocorticotropin (ACTH) increases were significantly smaller. Compared to control pups the adenohypophysis of the 10-day-old di/di rats responded normally to CRH, but their adrenal glands were hyper-responsive to ACTH, while in adults there was greater secretion at both levels with no difference between the genotypes. The serum transcortin level was higher in adults than pups, with the di/di pups having higher transcortin levels than controls. Hence, using the same stressors in adults and pups with both a genetic model and pharmacological pretreatment, we have shown that the role of vasopressin in ACTH regulation is more important during the neonatal period than in adulthood. Blunted hypophysial sensitivity to CRH and similar adrenal gland sensitivity to ACTH in the pups compared to adults suggest that hypothalamic factors could be responsible for the neonatal stress hyporesponsive period.

The relative importance of hypothalamic neurons containing corticotropin-releasing factor or vasopressin in the regulation of adrenocorticotropic hormone secretion

Corticotropin-releasing factor (CRF-41) and arginine vasopressin (AVP) are the two major factors that regulate adrenocorticotropic hormone (ACTH) secretion. The two neurohormones are co-localized in the parvocellular neurons of the paraventricular nuclei (PVN) of the hypothalamus and are capable of potentiating each others' action on freshly excised anterior pituitary fragments or cells in vitro. Transection of all axons entering the medial basal hypothalamus from anterior and lateral directions blocks ACTH release induced by either adrenalectomy or ether-surgery stress. Adrenalectomy-induced ACTH release is almost completely suppressed by a long-term lesion of the PVN. Stress-induced ACTH release is blocked for only a few days after PVN lesion and the pituitary-adrenal response to ether-surgery stress returns to a large extent by a few weeks after PVN lesioning. This remarkable plasticity can be observed also in the homozygous Brattleboro rat, therefore it is not dependent on mediation by AVP. When parvocellular CRF-41- and AVP-containing cells are present, and the anterior lobe ACTH cells are desensitized to the stimulating effects of AVP, the ACTH response to haemorrhage and immobilization is markedly decreased. This indicates that AVP may partially mediate ACTH release under normal conditions. The hypothalamic control of the pituitary-adrenocortical system has a remarkable degree of redundancy which may compensate, at least under stressful conditions, for disruption of the function of CRF-41-containing cells of the paraventricular nucleus, the major source of CRF-41 in the stalk-median eminence.

Signs of attenuated depression-like behavior in vasopressin deficient Brattleboro rats

Vasopressin, a peptide hormone functioning also as a neurotransmitter, neuromodulator and regulator of the stress response is considered to be one of the factors related to the development and course of depression. In the present study, we have tested the hypothesis that congenital deficit of vasopressin in Brattleboro rats leads to attenuated depression-like behavior in tests modeling different symptoms of depression. In addition, hypothalamic-pituitary-adrenocortical axis activity was investigated. Vasopressin deficient rats showed signs of attenuated depression-like behavior in forced swimming and sucrose preference tests, while their behavior on elevated plus maze was unchanged. Vasopressin deficiency had no influence on basal levels of ACTH and corticosterone and had only mild impact on hormonal activation in response to forced swimming and plus-maze exposure. However, vasopressin deficient animals showed higher level of dexamethasone induced suppression of corticosterone response to restraint stress and higher basal levels of corticotropin-releasing hormone mRNA in the hypothalamic paraventricular nucleus. In conclusion, present data obtained in vasopressin deficient rats show that vasopressin is involved in the development of depression-like behavior, in particular of the coping style and anhedonia. Moreover, behavioral and endocrine responses were found to be dissociated. We suggest that brain vasopressinergic circuits distinct from those regulating the HPA axis are involved in generating depression-like behavior.

Maternal genotype influences stress reactivity of vasopressin-deficient brattleboro rats

The role of vasopressin, cosecreted with corticotropin-releasing hormone (CRH), in stress is debated, because both normal as well as reduced adrenocorticotropin hormone (ACTH) rise to an acute challenge has been reported in Brattleboro rats genetically lacking vasopressin (di/di). Because di/di pups could be born either from di/+ (heterozygous) or from di/di mothers, and maternal influence is known to modify adult responsiveness, we investigated whether the influence of maternal genotype could explain the variability. Adult rats from mothers with different genotypes were stressed with 60 min restraint and trunk blood was collected for measuring hormone content by radioimmunoassay at the end of stress. All offspring of di/+ mothers had similar ACTH responses to restraint, while the di/di rats born to, and raised by di/di mothers showed reduced ACTH reactivity to restraint. The di/di rats showed elevated water turnover and required a daily cage cleaning every day, which meant frequent handling. To offset the role of handling, all rats had daily cage cleaning in the next series, but the results were the same as in the first series. To investigate whether lactation, the behaviour of the mother or some other factor during the pregnancy is responsible for the differences, pups from di/+ dams were raised by di/di foster mothers and vice versa. We found that the genotype of parental mother is more important than that of the foster mother. The corticosterone and prolactin elevation normally seen after acute stress was unchanged by family history, maternal or personal genotype. Furthermore, in studies with mutant animals, the rearing conditions should be controlled by the experimenter. In experiments with Brattleboro rats, the use of homozygous and heterozygous rats from the same litters of di/+ dams and di/di males is recommended. Our results suggest that vasopressin is not indispensable for ACTH release, and that the di/di genotype of the parental mother can decrease the stress reactivity of the di/di Brattleboro rats.

Role of hypothalamic inputs in maintaining pituitary-adrenal responsiveness in repeated restraint

The role of hypothalamic structures in the regulation of chronic stress responses was studied by lesioning the mediobasal hypothalamus or the paraventricular nucleus of hypothalamus (PVH). Rats were acutely (60 min) and/or repeatedly (for 7 days) restrained. In controls, a single restraint elevated the plasma adrenocorticotropin (ACTH), corticosterone, and prolactin levels. Repeated restraint produced all signs of chronic stress, including decreased body and thymus weights, increased adrenal weight, basal corticosterone levels, and proopiomelanocortin (POMC) mRNA expression in the anterior pituitary. Some adaptation to repeated restraint of the ACTH response, but not of other hormonal responses, was seen. Lesioning of the mediobasal hypothalamus abolished the hormonal response and POMC mRNA activation to acute and/or repeated restraint, suggesting that the hypothalamo-pituitary-adrenal axis activation during repeated restraint is centrally driven. PVH lesion inhibited the ACTH and corticosterone rise to the first restraint by approximately 50%. In repeatedly restrained rats with PVH lesion, the ACTH response to the last restraint was reduced almost to basal control levels, and the elevation of POMC mRNA level was prevented. PVH seems to be important for the repeated restraint-induced ACTH and POMC mRNA stimulation, but it appears to partially mediate other restraint-induced hormonal changes.

Non-genomic effects of glucocorticoids in the neural system. Evidence, mechanisms and implications

Complementing the classical concept of genomic steroid actions, here we (i) review evidence showing that important neural effects of glucocorticoids are exerted by non-genomic mechanisms; (ii) describe known mechanisms that may underlie such effects; (iii) summarize the functions and implications of non-genomic mechanisms and (iv) outline future directions of research. The role of non-genomic mechanisms is to shape the response of the organism to challenges that require a substantial reorganization of neural and somatic functions and involve massive behavioral shifts. Non-genomic effects may (i) prepare the cell for subsequent glucocorticoid-induced genomic changes, (ii) bridge the gap between the early need of change and the delay in the expression of genomic effects and (iii) may induce specific changes that in some instances are opposite to those induced by genomic mechanisms. The latter can be explained by the fact that challenging situations require different responses in early (acute) and later (chronic) phases. Data show that non-genomic mechanisms of glucocorticoid action play a role in both pathological phenomena and the expression of ameliorative pharmacological effects. Non-genomic mechanisms that underlie many glucocorticoid-induced neural changes constitute a for long overlooked controlling factor. Despite the multitude and the variety of accumulated data, important questions remain to be answered.

The effect of glucocorticoids on the anxiolytic efficacy of buspirone

RATIONALE

The serotonergic system and the hypothalamus-hypophysis-adrenocortical axis reciprocally influence each other. Therefore, the interaction between stress and serotonergic anxiolytics should be of major concern for both laboratory investigations and clinical treatment.

OBJECTIVES

We have studied the effects of the serotonergic anxiolytic buspirone in rats in which basal levels of glucocorticoids were low and stable, while acute stress reactions were inhibited or exogenously induced.

METHODS

Rats were adrenalectomised. Subcutaneous corticosterone pellets maintained basal glucocorticoid concentrations while acute changes were mimicked by corticosterone injections. Anxiety was assessed by the social interaction test. Temporal changes were evaluated by submitting rats to the same manipulations three times at two-day intervals.

RESULTS

Buspirone applied to animals with stable and low plasma glucocorticoid concentrations induced a dramatic increase in social interactions. A slight locomotor suppressive effect was also noticed. The effects of buspirone proved to be stable over time in these animals. Acute treatment with corticosterone doubled the locomotor suppressive effects of buspirone and reversed its anxiolytic effects: the buspirone-corticosterone combination was anxiogenic after the first application. During the second and third treatment, the impact of corticosterone on buspirone efficacy gradually decreased, but the combined treatment remained about half as effective in reducing anxiety as buspirone alone.

The effect of hypothalamic lesions on hypothalamo-pituitary-adrenal axis activity and inflammation in adjuvant-induced arthritis

Adjuvant-induced arthritis (AA) was induced in control and in hypothalamic lesioned Piebald-Viral-Glaxo (PVG) rats. Following discrete paraventricular nucleus (PVN) lesions plasma corticosterone was increased 14 days after adjuvant injection as in controls, when hind paw inflammation was apparent. PVN lesion did not affect the severity of inflammation.In contrast, following medial basal hypothalamus (MBH) lesions adjuvant did not increase corticosterone levels and the increase in paw volume at day 14 was potentiated. Basal proopiomelanocortin(POMC) mRNA expression in the anterior lobe was unchanged by PVN lesions and decreased by MBH lesions. AA increased POMC mRNA in controls and in both PVN and MBH lesioned rats. After complete MBH lesion, surviving anterior pituitary tissue maintained morning levels of corticosterone.Thus, AA may activate the hypothalamo-pituitary-adrenal axis without the mediation of PVN neurones projecting to the median eminence. However, the loss of the corticosterone response to AA and the increase in severity of inflammation in the MBH lesioned rats suggests a central (non-PVN) component mediates effects of inflammation. Furthermore, the increase in POMC mRNA in the MBH lesioned AA rats suggests that part of this process is not mediated by releasing factors in the hypothalamo-hypophysial portal system, and that extrahypothalamic(peripheral) mediators act on the pituitary during chronic inflammation.

Vasopressin pressor receptor-mediated activation of HPA axis by acute ethanol stress in rats

The plasma arginine vasopressin (AVP), ACTH, and corticosterone levels and the hypothalamic corticotropin-releasing hormone (CRH) content were measured after oral administration of 1 ml of 75% ethanol to rats, a model known to induce acute gastric erosions and stress. Elevated plasma AVP, ACTH, and corticosterone levels were detected 1 h after ethanol administration. Treatment with the vasopressin pressor (V(1)) receptor antagonist [d(CH(2))(5)Tyr(Me)-AVP] before ethanol administration significantly reduced the ACTH and corticosterone level increases. A higher hypothalamic CRH content was measured at 30 or 60 min after ethanol administration. V(1) receptor antagonist injection, 5 min before ethanol administration, inhibited the rise in hypothalamic CRH content. The protein synthesis blocker cycloheximide prevented the hypothalamic CRH content elevation after stress. The AVP-, CRH-, and AVP + CRH-induced in vitro ACTH release in normal anterior pituitary tissue cultures was also prevented by pretreatment with the V(1) receptor antagonist. The results support the hypothesis that stress-induced AVP may not only act directly on the ACTH producing anterior pituitary cells but also indirectly at the hypothalamic level via the synthesis and release of CRH.

Ultradian corticosterone rhythm and the propensity to behave aggressively in male rats

Ultradian fluctuations in plasma glucocorticoids have been demonstrated in a variety of species including humans. The significance of such rhythms is poorly known, although disorganized ultradian glucocorticoid rhythms have been associated with behavioural disorders. Here we report that ultradian glucocorticoid rhythms may establish the propensity to behave aggressively in male rats. Male rats were significantly more aggressive in the increasing phase of their corticosterone fluctuation when confronting a male intruder than counterparts in the decreasing phase of their corticosterone fluctuations facing such opponents. Corticosterone fluctuations were mimicked by a combination of treatments with the corticosterone synthesis inhibitor metyrapone and corticosterone. Again, males with increased plasma corticosterone levels were more aggressive than counterparts with a decreased plasma corticosterone concentration. These data suggest that the behavioural response to an aggressive challenge may vary in the same animal across the day due to the pulsating nature of corticosterone secretion. Aggressive behaviour is also episodic in humans; moreover, intermittent explosive behaviour is recognized as a psychological disorder. It can be hypothesized that a temporal coincidence between the occurrence of a challenge and a surge in plasma corticosterone concentration may be one of the factors that promote episodic aggressive outbursts.

Housing conditions and the anxiolytic efficacy of buspirone: the relationship between main and side effects

Serotonergic anxiolytics yield contradictory results both in the laboratory and clinically. In an attempt to investigate the cause of discrepancies, the anxiolytic effect of buspirone (0, 3 or 10 mg/kg, single treatment) was tested 1 h and 4 h after injection in rats in different housing conditions. At 1 h after drug administration, buspirone increased corticosterone production and decreased locomotor behaviour in both the elevated plus-maze and the social interaction tests. No anxiolytic-like effect was produced in either test. At 4 h after drug injection, no corticosterone or locomotor effects of buspirone were observed. In contrast, anxiolytic effects emerged in this phase. Open arm exploration and social investigation were increased in the plus-maze and social interaction test, respectively. In the plus-maze, the anxiolytic effect was significant in isolated animals only. In the social interaction test, the anxiolytic effect was stronger in isolated than in group-housed animals. When corticosterone secretion was inhibited by adrenalectomy, a full anxiolytic effect of buspirone was observed 1 h after drug administration. It appears that the side effects of buspirone have a shorter duration than the main anxiolytic effect. The buspirone-induced increase in corticosterone may have abolished the anxiolytic effects of the drug shortly after injection. Individual housing enhanced the anxiolytic efficacy of buspirone 4 h after administration.

Radiodetoxified lipopolysaccharide fails to activate the hypophyseal- pituitary-adrenal axis in the rat

Lipopolysaccharide (LPS) is known to raise the concentration of the circulating stress hormones such as ACTH, corticosterone and beta-endorphin. This effect of endotoxin is mediated by different immune system-released hormone-like factors (e.g. interleukins, tumor necrosis factor etc.). Gamma-ray irradiation of LPS alters its biological properties and results in a radiodetoxified LPS preparation with numerous beneficial effects and decreased toxicity. In this study we compared the neuroendocrine effects of a commercial LPS and our native and radiodetoxified LPS preparations in rats. Plasma ACTH, corticosterone and beta-endorphin levels were measured by specific radioimmunoassays 120 min after intraperitoneal LPS administration. Control animals were injected with saline. Results show a dramatic increase in all hormones after administration of commercial and our native LPS preparation. Hormone levels in saline-injected controls and radiodetoxified LPS-treated rats did not rise significantly. These results suggest that radio-detoxification disintegrated that part of the LPS molecule complex which is responsible for toxicity including an enhanced production of cytokines, which trigger the hypothalamo-pituitary-adrenal axis.

Defeat is a major stressor in males while social instability is stressful mainly in females: towards the development of a social stress model in female rats

Social stress models appear useful in elucidating the interrelationship between stress, mood disorders, and drug efficacy. However, reliable social stress models for females are virtually lacking. The aim of this study was to determine stress-related consequences of (a) defeat in aggressive encounters and (b) social instability, in male and female rats. Defeat in male and female subjects was induced by aggressive male residents and female residents made aggressive by surgery (mediobasal hypothalamic lesion [MBHL]), respectively. Aggressiveness of resident males and resident MBHL females was remarkably similar. Alternating isolation and mixed-sex crowding phases with membership rotation were used to induce social instability. Aggression was kept low in the latter paradigm by manipulating crowding group composition. Defeat stress reduced weight gain, and increased both adrenals and plasma corticosterone in males. Only adrenal weight was affected in females. Social instability reduced weight gain, and induced thymus involution, adrenal hypertrophy and elevated plasma corticosterone levels in females. Only weight gain and thymus weights were affected in males. It is concluded that defeat stresses males more than females, while social instability is more stressful for females than for males, if aggressive contacts are low. It is suggested that the social instability model is a good model of social stress in females.

Effect of cholinergic drugs on the concentration of intracellular free calcium of rat pituitary intermediate lobe cells

We tested the effect of cholinergic drugs on the concentration of intracellular free calcium in rat melanotropes. Acetylcholine, muscarine, carbachol, and nicotine resulted in a significant rise in this parameter. Effect of acetylcholine was reduced by atropine (non-selective muscarinic antagonist), pirenzepine (M1 muscarinic antagonist), and 4-DAMP (M3 > M1 muscarinic antagonist), but exposure to the M1 muscarinic agonist McN-A 343 resulted in a significantly smaller calcium-response than that seen in response to acetylcholine or to muscarine. This suggests the involvement of both M1 and M3 muscarinic receptors in the acetylcholine-induced calcium-rise. On the other hand, in the presence of atropine the acetylcholine-induced calcium-rise was not eliminated: this fact indicates that nicotinic receptors are also involved in the acetylcholine-induced intracellular calcium-rise. The acetylcholine-, and nicotine-induced calcium-rise was significantly reduced in presence of the neuronal-type nicotinic antagonist, mecamylamine. This suggests the involvement of a neuronal-type nicotinic receptor in the acetylcholine-induced intracellular calcium-response. Moreover, because in a further experiment almost 80% of the cells investigated responded to muscarine as well as nicotine, we conclude that both functionally active muscarinic and nicotinic receptors are present on the same cell.

Simultaneous blockade of two glutamate receptor subtypes (NMDA and AMPA) results in stressor-specific inhibition of prolactin and corticotropin release

Many neurons express simultaneously two or more isotypes of glutamate receptors, so that pharmacological modulation of more than one receptor may be necessary to reveal the role of glutamate in mediating physiological processes. The present studies were aimed at evaluating involvement of endogenous glutamate in triggering plasma prolactin (PRL) and adrenocorticotropic hormone (ACTH) levels in response to three different stress stimuli (footshock, immobilization and ether stress). Blockade of glutamate receptor subtypes was achieved by the administration of the NMDA antagonist dizocilpine (MK-801, 0.2 mg/kg) and the selective AMPA antagonist GYKI 52466 (10 mg/kg). Rats were pretreated for 4-5 days and then exposed to stressful stimulation. Basal hormone levels were not affected by the antagonists. In male rats, combined, but not separate blockade of NMDA and AMPA/kainate subtypes of glutamate receptors prevented the rise in plasma PRL in response to footshock stress. In female rats, footshock-induced PRL release was inhibited even by separate blockade of NMDA receptors by dizocilpine, suggesting that the PRL system of females is more sensitive to the effect of NMDA antagonists than that of males. None of the treatments affected PRL release during immobilization or ether stress. Simultaneous blockade of NMDA and AMPA receptor subtypes resulted in a mild inhibition of immobilization-induced ACTH release without any effect on ACTH response to footshock or ether stress. The data suggest that involvement of glutamatergic pathways in neuroendocrine response during stress is selective for discrete stress stimuli and stress hormones. In addition a concerted action of glutamate on both NMDA and non-NMDA receptor subtypes is involved in the control of PRL release during footshock stress.

Alpha 2-adrenoreceptor subtypes regulate ACTH and beta-endorphin secretions during stress in the rat

The effect of different alpha 2-adrenoreceptor subtype agonists and antagonists on adrenocorticotrop hormone (ACTH) and beta-endorphin release induced by ether stress was examined. Ether inhalation-induced ACTH and beta-endorphin increase was inhibited by i.c.v. administration of 30 micrograms but not 1 and 10 micrograms clonidine (alpha 2-adrenoreceptor agonist). I.c.v. oxymetazoline (alpha 2A-adrenoreceptor agonist; 1-10-30 micrograms) or the alpha 1-agonist methoxamine (100 micrograms/rat) failed to inhibit the stress-induced rise. Pretreatment with the alpha 1/alpha 2B.C-antagonist prazosin (0.5 mg/kg, i.p.) prevented the effect of clonidine on the ether stress, while the alpha 1/alpha 2A-antagonist WB-4101 (0.5 mg/kg, i.p.) was unable to counteract the inhibitory effect of clonidine. Prazosin alone had no effect on the ether-induced plasma ACTH and beta-endorphin elevation. These results suggest that noradrenaline in the central nervous system may inhibit the stress-induced hypothalamo-pituitary-axis and pituitary beta-endorphin activation via alpha 2B.C-adrenoceptor subtypes and prazosin may antagonize its effect on these receptors.

Muscarinic M1 and M3 receptors are present and increase intracellular calcium in adult rat anterior pituitary gland

Physiological and biochemical evidence indicates the existence of functional muscarinic cholinergic receptors in the anterior pituitary. The selectivity of these receptors has been characterised by studying the binding of [3H]quinuclidinyl benzilate ([3H]QNB) and [3H]diphenyl-acetoxy-N-methyl-piperidine ([3H]4-DAMP) in membrane preparation of male rat anterior pituitary at 25 degrees C. Competition experiments with receptor selective muscarinic antagonists were used to characterise specific selective muscarinic receptor binding. Both [3H]QNB and [3H]4-DAMP bound to anterior pituitary membranes at low concentrations, binding was saturable and was potently displaced by 4-DAMP (M1, M3 subtypes selective antagonist) > atropine (general) > pirenzepine (M1). Methoctramine (M2) didn't antagonise the [3H]QNB binding efficiently. Acetylcholine and carbachol increased the intracellular Ca2+ level in 62% and 65% of cultured rat anterior pituitary cells in a dose-dependent manner, and this effect was prevented by pirenzepine. Based on these results we suggest that both M1 and M3 muscarinic receptors are present and active in the majority of cells in the rat anterior pituitary gland, but their physiological role in the adult rat remains to be examined.

Local regulation of vasopressin and oxytocin secretion by extracellular ATP in the isolated posterior lobe of the rat hypophysis

It is now widely accepted that ATP functions as a signalling substance in the nervous system. The presence of P2 receptors mediating the action of extracellular ATP in brain regions involved in hormonal regulation raises the possibility that a similar role for ATP might also exist in the neuroendocrine system. In this study, the release from the rat isolated neurohypophysis preparation of endogenous ATP, oxytocin and vasopressin (AVP) were measured simultaneously using luciferin-luciferase and RIA techniques. After 70 min preperfusion, electrical field stimulation caused a rapid increase in the amount of ATP in the effluent and the release of AVP and oxytocin also increased stimulation-dependently. Inhibition of voltage-dependent Na+ channels by tetrodotoxin (1 microM) reduced the stimulation-evoked release of AVP and oxytocin; however, the evoked release of ATP remained unaffected. The effect of endogenous ATP on the hormone secretion was tested by suramin (300 microM), the P2 receptor antagonist. Suramin significantly increased the release of AVP, and the release of oxytocin was also enhanced. ATP, when applied to the superfusing medium, decreased the release of AVP, but not that of oxytocin, and its effect was prevented by suramin. ATP (60 nmol), added to the tissues, was readily decomposed to ADP, AMP and adenosine measured by HPLC combined with ultraviolet light detection, and the kinetic parameters of the enzymes responsible for inactivation of ATP (ectoATPase and ecto5'-nucleotidase) were also determined (Km=264+/-2.7 and 334+/-165 microM and vmax=6.7+/-1.1 and 2.54+/-0.24 nmol/min per preparation (n=3) for ectoATPase and ecto5'-nucleotidase respectively). Taken together, our data demonstrate the stimulation-dependent release, P2 receptor-mediated action and extracellular metabolism of endogenous ATP in the posterior lobe of the hypophysis and indicate its role, as a paracrine regulator, in the local control of hormone secretion.

Social stress of variable intensity: physiological and behavioral consequences

Stress effects in humans depend on stress type, intensity, and duration. Animal models of social stress serve as good ways to mimic stress experienced in humans. However, the available stress paradigms pay little attention to the relationship between the intensity and the type of social stressors. The aim of the present work is to study behavioral and endocrinological consequences of social stress by varying the intensity and type of agonistic social contacts. Subjects were exposed to the attacks of an experienced fighter resident rat once a day for 4 consecutive days. Mild versus strong effects were studied by varying the length of daily confrontations (30 min vs. 4 h). The type of social confrontations was varied by ceasing or maintaining sensory contacts among contestants between encounters. Endocrinological variables were measured on the 5th day. Anxiety was assessed by means of the elevated plus-maze. The stress state depended on the length of daily encounters: 30-min encounters did not, whereas 4-h encounters did result in weight loss and chronic elevation of plasma corticosterone. The type of contacts between subjects and dominants also affected the resulting stress state: adrenal hypertrophy was obtained only when contacts between contestants were maintained between encounters. Although the mildest stress procedure (30-min encounters on 4 consecutive days) did not affect endocrinological variables, it resulted in subtle behavioral modifications that changed the anxiety-related effects of additional acute stressors. Thus, anxiety-related behavioral changes resulting from repeated mild stressors may be hidden factors that can have long-term consequences on the development of anxiety-like behavioral deficits. Results outline the necessity of studying the effects of social stressors of different intensities and different types.

Acute effects of glucocorticoids: behavioral and pharmacological perspectives

There has been evidence since the early eighties that glucocorticoids, apart from their well known chronic effects, may have acute, short-term effects. However, a lack of understanding of the molecular mechanisms of action has hampered appreciation of these observations. Mounting evidence over the years has continued to confirm the early observations on a fast corticosterone control of acute behavioral responses. We summarize experimental data obtained mainly in rats but also in other species which show: (1) that glucocorticoid production is sufficiently quick to affect ongoing behavior; (2) that there exist molecular mechanisms that could conceivably explain the fast neuronal effects of glucocorticoids (although these are still insufficiently understood); (3) that glucocorticoids are able to stimulate a wide variety of behaviors within minutes; and (4) that acute glucocorticoid production (at least in the case of aggressive behavior) is linked to the achievement of the behavioral goal (winning). The achievement of the behavioral goal reduces glucocorticoid production. It is argued that glucocorticoids are regulatory factors having a well-defined behavioral role. Both the acute (stimulatory) effects and the chronic (inhibitory) effects are adaptive in nature. The acute control of behavior by corticosterone is a rather unknown process that deserves further investigation. The pharmacologic importance of the acute glucocorticoid response is that it may readily affect the action of pharmacologic agents. An interaction between acute glucocorticoid increases and noradrenergic treatments has been shown in the case of offensive and defensive agonistic behavior. Non-behavioral data demonstrate that acute increases in glucocorticoids may interfere with other neurotransmitter systems (e.g., with the 5HT system) as well. These observations show the importance of taking into account endocrine background and endocrine responsiveness in behavior pharmacological experiments.

Aggressive experience affects the sensitivity of neurons towards pharmacological treatment in the hypothalamic attack area

Early investigators of brain stimulation-evoked complex behaviours (attack, escape, feeding, self-grooming, sexual behaviour) reported that experience may affect the behavioural outcome of brain stimulation. This intriguing example of functional neuronal plasticity was later totally neglected. The present experiment investigated the behavioural outcome of in vivo microdialysis perfusion of the glutamate agonist kainate and/or the GABAA antagonist bicuculline into the hypothalamic attack area (HAA) of (1) animals naive to dyadic encounters; (2) animals with a recent aggressive experience (the probe being implanted 6-24 h after the last of a series of dyadic encounters); and (3) animals with an earlier aggressive experience (probe being implanted 2 weeks after the last aggressive experience). On the experimental day, rats received two 5-min infusions during a dyadic encounter lasting 35 min with an unknown opponent. Flow rate was 1.5-2 microliters/min, drug concentrations were 1.8 x 10(-5) and 1.5 x 10(-5) M for kainate and bicuculline, respectively. Behaviour was analysed before, during and after perfusions. Only the combined kainate + bicuculline treatment had significant effects on behaviour at the doses studied. A significant increase in aggressive behaviour was elicited only in animals with a recent aggressive experience, while naive animals and with an earlier experience responded to the treatments by grooming. These results appear to support early observations indicating that one important aspect of brain stimulation effects is previous experience.

Alterations in corticotropin-releasing hormone gene expression of central amygdaloid neurons following long-term paraventricular lesions and adrenalectomy

Corticotropin-releasing hormone messenger RNA expression in the amygdala of rats after adrenalectomy and bilateral lesions of the hypothalamic paraventricular nucleus was examined by in situ hybridization histochemistry. Corticotropin-releasing hormone messenger RNA-containing cells are abundant in the intermediate subdivision of the central amygdaloid nucleus. Some corticotropin-releasing hormone-labeled cells are scattered in other subdivisions of the central nucleus and throughout the anterior amygdaloid area. Five days after bilateral adrenalectomy, the number of corticotropin-releasing hormone messenger RNA-containing cells was reduced both in the central nucleus and the anterior area of the amygdala. This reduction was prevented by corticosterone replacement and contrasts sharply with the known rise of corticotropin-releasing hormone messenger RNA in the paraventricular nucleus after adrenalectomy. Corticotropin-releasing hormone messenger RNA expression in the amygdala was up-regulated in rats with six-week bilateral lesions of the paraventricular nucleus. This elevation in corticotropin-releasing hormone messenger RNA was not influenced by adrenalectomy or corticosterone, and it did not correlate with plasma levels of adrenocorticotrophic hormone or corticosterone. The possible direct innervation of the amygdala by the paraventricular nucleus is supported by the demonstration of labeled axons from the paraventricular nucleus to the amygdala after injection of an anterograde tracer, Phaseolus vulgaris leucoagglutinin, into the paraventricular nucleus. Labeled fibers take two courses: through the lateral hypothalamus ventral amygdalofugal path and through the stria terminalis. Data presented here suggest that the paraventricular nucleus-amygdala connection is likely to be inhibitory to corticotropin-releasing hormone neurons in the central amygdala. These neurons may participate in behavioral responses to stress effected through brainstem autonomic centers rather than directly through the hypothalamo-pituitary adrenal axis.

Monosodium glutamate lesions inhibit the N-methyl-D-aspartate-induced growth hormone but not prolactin release in rats

Large doses of glutamate administered to newborn rats damage permanently the neurones in the hypothalamic arcuate nucleus containing the growth hormone releasing hormone and the prolactin inhibiting dopamine neuron cell bodies. Since adult animals that underwent neonatal glutamate treatment still have a relatively well functioning growth hormone and prolactin system, we tested whether in the adults the excitatory amino acid sensibility is changed. After i.v. injection of different doses (10 or 30 mg/kg) of N-methyl-D-aspartate (excitatory amino acid receptor subtype agonist) growth hormone levels were significantly increased in the control groups but there was no rise in neonatally glutamate treated male and female rats. The level of prolactin was increased by N-methyl-D-aspartate, too, but the glutamate treatment had no effect on the rise. Our study suggests that systemic administration of N-methyl-D-aspartate increases plasma growth hormone level by activating the growth hormone releasing cells in the arcuate nucleus, but the intact tuberoinfundibular dopaminergic pathway is not essential for its prolactin stimulatory effect.

Corticosterone increase inhibits stress-induced gastric erosions in rats

The role of glucocorticoids released in response to stress in the pathogenesis of stress-induced gastric erosions has been reevaluated. Gastric erosions elicited in male rats by 3-h cold-restraint or water-restraint stresses were studied after acute reduction of corticosterone release or occupation of glucocorticoid receptors by the antagonist RU-38486 during stress. Stress-induced corticosterone production was reduced by creating a lesion on the hypothalamic paraventricular nucleus (PVN) 4 days before stress as well as by pretreatment with a rabbit antiserum to adrenocorticotropin (ACTH) 30 min before stress. RU-38486 (10 mg/kg po) was administered 20 min before and 60 min after the onset of stress. Corticosterone for replacement was injected 15 min before the onset of stress to mimic stress-induced corticosterone response. Plasma corticosterone levels were measured by fluorometry or RIA. Gastric erosions were quantitated by measuring the area of damage. Four days after PVN lesion, stress-induced corticosterone release was decreased and gastric erosions were increased. Injecting corticosterone significantly attenuated the effect of PVN lesion on gastric erosions. The ACTH antiserum inhibited corticosteroid secretion in response to stress and markedly increased gastric erosions. The administration of the glucocorticoid/progesterone antagonist RU-38486 significantly potentiated the formation of stress-induced gastric erosions. These observations support the suggestion that glucocorticoids released during stress have a gastroprotective action rather than an ulcerogenic effect as was generally accepted.

Lesioning of the hypothalamic paraventricular nucleus inhibits ether-induced ACTH but not prolactin release

Stress mediators, CRF-41 and vasopressin known to be synthesized in, and released from the parvocellular neurosecretory neurons of the hypothalamic paraventricular nucleus (PVN) are essential to release adrenocorticotropin (ACTH) in response to stress. In addition, suckling-induced prolactin (PRL) release also depends on the integrity of the PVN. In the present study, ether stress-induced adrenocorticotrop hormone (ACTH) and prolactin (PRL) release was studied 2, 5 and 42 days after placing lesions in the hypothalamic paraventricular nucleus (PVN) of male rats. Ether-induced ACTH secretion was strongly inhibited 2 and 5 days after lesions whereas 6 weeks later the lesion induced inhibition was fading. In contrast, PVN lesion failed to inhibit ether-induced PRL release at any time studied. The results suggest that contrary to previous suggestions the peptidergic neurons essential for stress-induced PRL release are outside the PVN.

Catecholaminergic involvement in the control of aggression: hormones, the peripheral sympathetic, and central noradrenergic systems

Noradrenaline is involved in many different functions, which all are known to affect behaviour profoundly. In the present review we argue that noradrenaline affects aggression on three different levels: the hormonal level, the sympathetic autonomous nervous system, and the central nervous system (CNS), in different, but functionally synergistic ways. Part of these effects may arise in indirect ways that are by no means specific to aggressive behaviour, however, they are functionally relevant to it. Other effects may affect brain mechanisms specifically involved in aggression. Hormonal catecholamines (adrenaline and noradrenaline) appear to be involved in metabolic preparations for the prospective fight; the sympathetic system ensures appropriate cardiovascular reaction, while the CNS noradrenergic system prepares the animal for the prospective fight. Indirect CNS effects include: the shift of attention towards socially relevant stimuli; the enhancement of olfaction (a major source of information in rodents); the decrease in pain sensitivity; and the enhancement of memory (an aggressive encounter is very relevant for the future of the animal). Concerning more aggression-specific effects one may notice that a slight activation of the central noradrenergic system stimulates aggression, while a strong activation decreases fight readiness. This biphasic effect may allow the animal to engage or to avoid the conflict, depending on the strength of social challenge. A hypothesis is presented regarding the relevance of different adrenoceptors in controlling aggression. It appears that neurons bearing postsynaptic alpha2-adrenoceptors are responsible for the start and maintenance of aggression, while a situation-dependent fine-tuning is realised through neurons equipped with beta-adrenoceptors. The latter phenomenon may be dependent on a noradrenaline-induced corticosterone secretion. It appears that by activating very different mechanisms the systems working with adrenaline and/or noradrenaline prepare the animal in a very complex way to answer the demands imposed by, and to endure the effects caused by, fights. It is a challenge for future research to elucidate how precisely these mechanisms interact to contribute to functionally relevant and adaptive aggressive behaviour.

Catecholaminergic control of intracellular free calcium and beta-endorphin secretion of rat pituitary intermediate lobe cells

Individual melanotropes and intermediate lobes were tested to elucidate the role of alpha- and beta-adrenergic and D-2 dopamine receptors in the regulation of concentration of intracellular free calcium ([Ca2+]i) and release of beta-endorphin. Hormone secretion was studied in a superfusion system, while [Ca2+]i was measured microspectrofluorimetrically. Noradrenaline (1 microM) resulted in a slight decrease, then a marked increase in [Ca2+]i and secretion of beta-endorphin. The nonselective beta-adrenergic agonist isoproterenol (1 microM) increased [Ca2+]i and secretion of beta-endorphin; this effect was blocked by the beta-antagonist propranolol (10 microM). The alpha-adrenergic agonist phenylephrine (1 microM) increased [Ca2+]i and beta-endorphin secretion, but this effect was not blocked by terazosin or prazosin (alpha1-adrenergic antagonists, 1 microM). Administration of the alpha2-adrenergic agonist xylazine (1 microM) increased [Ca2+]i but did not affect secretion of the hormone. Biphasic effect of noradrenaline was tested in presence of adrenergic and dopaminergic antagonists. The noradrenaline-induced rise in [Ca2+]i and beta-endorphin secretion was decreased by propranolol, but this drug did not modify the inhibition. In the presence of 1 microM sulpiride (selective D-2 dopaminergic antagonist), the inhibitory phase of the curve was abolished, and the subsequent increase was reduced. This suggests that activation of dopamine D-2 receptors is involved not only in the inhibition, but also in the subsequent increase, which may originate from a rebound after the termination of the activation of these inhibitory receptors. Our data suggest the presence of several distinct types of catecholamine receptors in the rat intermediate lobe, and the dominant involvement of D-2 and beta-adrenergic receptors in the noradrenaline-induced regulation of melanotropes.

Age-dependent muscarinic stimulation of beta-endorphin secretion from rat neurointermediate lobe in vitro

The effect of acetylcholine on the neurointermediate lobe beta-endorphin secretion was studied in the neonatal and in the adult rat in vitro. Acetylcholine stimulated beta-endorphin secretion from the 2-day- and 5-day-old neurointermediate lobe, the effect was dose dependent and more pronounced in the presence of the cholinesterase inhibitor eserine. The 10-day-, the 21-day-old and the adult rat neurointermediate lobes did not respond to acetylcholine, even in the presence of eserine. Basal beta-endorphin secretion was elevated by the D2 receptor antagonist sulpiride, but acetylcholine was without effect in the 10-day-old and in the adult neurointermediate lobe even after dopamine receptor blockade. The beta-endorphin stimulatory response to acetylcholine was diminished by the M1 muscarinic receptor antagonist pirenzepine and blocked by the M3 > M1 antagonist 4-diamino-phenyl-piperidine (4-DAMP). The selective M2 antagonist methoctramine and nicotine had no effect. These data indicate that the neurointermediate lobe beta-endorphin secretion is under special muscarinic cholinergic regulation for a relatively short time after birth. The disappearance of this stimulatory cholinergic effect in later life might be due to changes in the intracellular secretory machinery in the IL and/or to the uncoupling of the cholinergic receptors from the intracellular signal transduction system(s) responsible for the stimulated secretion in the rat melanotrope cells.

The mechanism of action of alpha 2 adrenoceptor blockers as revealed by effects on open field locomotion and escape reactions in the shuttle-box

The precise mechanism of action of alpha 2 adrenoceptor blockers in not known, although in principle they have two main effects: (i) they stimulate the norepinephrinergic system by inhibiting the negative feed back of norepinephrine release (presynaptic effect) and (ii) they inhibit the effects of norepinephrine on postsynaptic alpha 2 adrenoceptors. We postulate that if the presynaptic actions of the antagonists prevail, the enhanced norepinephrine release leads to an activation of postsynaptic alpha 1 or beta adrenoceptors. In this case the effects of alpha 2 adrenoceptor blockers can be reversed by antagonists acting on the latter two adrenoceptors, since postsynaptic alpha 2 adrenoceptors are also blocked. If the postsynaptic blockade of alpha 2 adrenoceptors is the main cause of effects, than the blockade of alpha 1 or beta adrenoceptors should not reverse the action of alpha 2 blockers. The alpha 2 blocker idazoxan (dose 0.5-5 mg/kg) increased locomotor activity in an open field, an effect that was abolished by both alpha 1 and beta receptor blockers (prazosin and propranolol, respectively). Escape responses in a shuttle box were strongly suppressed by idazoxan (0.5-2 mg/kg). However, this effect was not changed by concomitant alpha 1 or beta receptor blockade. These results suggest that the mechanism of action of alpha 2 adrenoceptor blockers depends on which effects are studied. Exploration seems to be affected by a presynaptic mechanism as neurons bearing postsynaptic alpha 1 or beta adrenoceptors are involved in the control of this behavior, while escape reactions appear to be affected by the postsynaptic blockade of alpha 2 adrenoceptors (i.e. neurons bearing postsynaptic alpha 2 adrenoceptors are involved in its control). Thus, there is no generalized mechanism of action for alpha 2 adrenoceptor blockers; their precise mode of action should be investigated in each particular case.

Domperidone stimulates prolactin secretion in rats with complete destruction of the mediobasal hypothalamus

The main objective of this study was to further elucidate the functional relationship between endogenous dopamine and the prolactin (PRL)-releasing effect of the dopamine antagonists domperidone and haloperidol. We studied the effect of the above dopamine antagonists on the PRL secretion in control and mediobasal hypothalamus (MBH)-lesioned rats. Significant increase in basal plasma PRL levels was detected 7 days after complete surgical destruction of the MBH. Haloperidol injection (0.5 mg/kg, i.v.) was followed by an increased plasma PRL concentration in the sham-operated animals; however, in the MBH-lesioned rats where the basal PRL levels were high haloperidol failed to produce additional PRL release. In contrast to haloperidol, domperidone (0.1 mg/kg, i.v.) was able to further elevate the MBH-lesion induced high plasma PRL concentration. Moreover, the change in plasma PRL levels of the MBH-lesioned rats was parallel with that in the sham-lesioned animals after domperidone injections. When haloperidol was given prior to the domperidone injection it did not influence the PRL releasing effect of domperidone in MBH-lesioned animals. The PRL stimulatory effect of domperidone (0.3 mg/kg, i.v.) in MBH-lesioned rats was antagonized by dopamine (20 micrograms/kg, i.v.) and bromocryptine (20 micrograms/kg, i.v.). The above results suggest that the stimulatory effect of domperidone on the pituitary PRL secretion is mediated--at least in part--through the pituitary D2 dopamine receptors, but not by the displacement of endogenous dopamine originating from the MBH and reaching the pituitary via portal vessels.

Acute behavioural effects of corticosterone lack specificity but show marked context-dependency

Rats were injected intraperitoneally with corticosterone (1 mg/kg). Ten min later they were submitted to dyadic encounters for 15 min. Rats naive to aggressive encounters responded to the corticosterone treatment by a reduction in resting times and an increase in exploration directed towards the novel environment. Resident rats acquainted with aggressive encounters responded to the treatment by a reduction of resting and an increase in threat behaviours. In resident rats acquainted with aggressive encounters and reared previously with a female an increase in attack frequency was noticed. It is concluded that an acute increase in blood corticosterone results in a stimulation of behaviour during social challenge, however, the specific effect is highly context dependent. These data confirm sporadic reports that noticed a behavioural stimulatory effect of acute corticosterone treatments. However, as far as a social challenge situation is concerned, the effect of corticosterone seems to be rather context then behaviour specific. Since corticosterone is known rather to change neuronal excitability than to activate neurons by itself, one can hypothesize that the context dependency of the effect is determined by the different configuration of activated centres in different situations. Attack may be stimulated by corticosterone only when centres involved in attack are already activated, that is, when a high frequency of attack is already present.

The effects of the alpha 2 adrenoceptor blocker idazoxan on defeat-induced immobility and plasma corticosterone in rats is antagonized by administration of adrenocorticotrophin-antiserum

Adult male Wistar rats were introduced into the home cage of residents accustomed to aggressive encounters. The attacks of the residents induced lasting immobility in the subjects. The alpha-2 adrenoceptor blocker idazoxan (which elicits an increase in brain norepinephrine release probably by the blockade of presynaptic autoreceptors) induced an enhancement of defeat-induced corticosterone secretion as well as a reduction in immobility. Pre-treatment with adrenocorticotrophin (ACTH) antiserum abolished both the increased adrenomedullar stress reaction and the reduction in immobility. It is concluded that norepinephrine-induced corticosterone secretion is involved in the immobility-lowering effects of alpha-2 adrenoceptor blockers. In previous experiments we have shown that the norepinephrine-induced increase in the aggressiveness of the residents depends on the ability to secrete corticosterone. Therefore it appears that the effects of norepinephrine on social conflict are at least partly mediated by corticosterone. This implies the involvement of acute corticosterone changes in fight/flight behaviours.

Corticotropin-releasing hormone expression in supraoptic neurons after bilateral lesioning of the paraventricular nucleus in rats

In situ hybridization histochemistry was used to demonstrate corticotropin releasing hormone (CRH) mRNA expression in the supraoptic nucleus of rats. Labeled cells with a range of grain densities were located mainly in the dorsal area of the nucleus. Long-term (6-weeks) lesioning of the hypothalamic paraventricular nucleus, which eliminates the major CRH pool from the hypothalamo-hypophyseal system resulted in an increased CRH mRNA density within supraoptic neurons compared to within sham-operated rats. Adrenalectomy failed to effect CRH mRNA content either in sham-operated or paraventricular-lesioned animals. CRH gene expression in supraoptic neurons of long-term paraventricular lesioned rats may exhibit a compensatory mechanism in the hypothalamus by which supraoptic neurons can take over some of the functions of the lesioned paraventricular CRH cells.

Effects of anterolateral and posterolateral cuts around the medial hypothalamus on the immunoreactive ACTH and beta-endorphin levels in selected brain regions of the rat

To evaluate the relative weight of the ACTH-ergic and beta-endorphin-ergic pathway(s) leaving the medial hypothalamus (MH) in anterior or posterior directions immunoreactive ACTH and beta-endorphin (ir-ACTH and ir-betaE) were quantified in selected brain regions of the rat 7-8 days after placing anterolateral (ALC) or posterolateral (PLC) cut around the MH. Retrograde accumulation of both peptides was observed in the MH after ALC, but not after PLC. ALC resulted in dramatic decrease in ir-ACTH/ir-betaE concentrations in all extra-MH brain regions tested (extra-MH hypothalamus, septum, thalamus, hippocampus, amygdala, and medulla oblongata). In contrast, ir-ACTH and ir-betaE levels decreased only in the thalamus and in the medulla oblongata after PLC. The present data indicate (a) ACTH- and betaE-like substances synthesized in the arcuate region of the hypothalamus are axonally transported to extrahypothalamic brain regions by neuronal pathways leaving the MH primarily anterolateral, anterodorsal, or anteromedial direction (even the fibers of certain posteromedial or posterolateral projections leave the MH some anterior directions); (b) the posterior ACTH-/betaE-ergic projections seem to be of minor importance except for the thalamus and the medulla oblongata where it contributes to about one-third of the peptide content. Our biochemical study provide quantitative complementary data to the detailed immunohistochemical picture of the ACTH/betaE-ergic projections in the rat brain described by Khachaturian et al.

Effect of posterior pituitary denervation (PPD) on prolactin (PRL) and alpha-melanocyte-stimulating hormone (alpha-MSH) secretion of lactating rats

Previous data have clearly suggested that the posterior pituitary (PP), consisting of neural lobe (NL) and intermediate lobe (IL), has a role in the control of anterior pituitary PRL secretion. However, basic aspects of this regulatory mechanism like (1), the role of an intact hypothalamic innervation of the PP as well as (2) the site of production of previously found PRL releasing substance(s) have not yet been characterized. Denervation of the PP (PPD) is an effective method for having a selective lesion of the innervation of PP, indeed, PPD results in a disappearance of neurosecretory materials from NL and tyrosine hydroxylase (TH) immunoreactivity from IL, leaving blood supply of all three lobes intact. Blood samples were taken from freely moving sham an PP-denervated lactating rats before and after 4-h separation from their pups and during the suckling stimulus. PPD blocks separation-induced depletion but only attenuates suckling induced release of PRL. Furthermore, it doubles plasma level of alpha-MSH during the entire sampling period, which has been used as a marker for in vivo secretory activity of IL cells. Lack of the separation-induced depression in plasma PRL of PPD animals can be partially restored by normalizing the diabetes insipidus with treatment of a vasopressin analogue, 1-desamino-8-D-arginine-vasopressin (dDAVP). In contrast, dDAVP, neither alone nor in combination with oxytocin (OXY), can change PPD-induced elevation of plasma alpha-MSH as well as attenuation of PRL response induced by suckling. It is concluded that: (1) contribution of the THDA system parallel to the confirmed role in the regulation of alpha-MSH seems to be crucial for the depletion of plasma PRL induced by separation but not for the elevation due to suckling stimulus, (2) intact hypothalamic innervations of both NL and IL, regulating water intake and the secretion of alpha-MSH, respectively, are necessary for normal secretory responses of AL during lactation, (3) as well as for the presence of PRF activity in PP, (4) which does not solely responsible for suckling-induced PRL release. Therefore, an interplay between several substances produced by NIL of the pituitary gland must have been responsible for the intact regulation of PRL secretion during lactation.

Acetylcholine stimulates growth hormone secretion in the neonatal rat pituitary

The direct effect of acetylcholine on pituitary growth hormone secretion during the postnatal period of the rat was studied using a superfusion system. Acetylcholine elicited a dose related stimulatory effect on growth hormone (GH) secretion in the pituitaries from 2-day old rats. M1 muscarinic agonist McN A343 mimicked the GH releasing effect of acetylcholine, nicotine was ineffective. The GH release elicited by acetylcholine diminished with postnatal development, it was small by the end of the third postnatal week and was not demonstrable in the adult pituitaries. The effect of acetylcholine was potently antagonized by pirenzepine (M1 antagonist) and 4-DAMP (M3 and M1 antagonist) but not by methoctramine (M2 antagonist). It is concluded that unlike in the adult, in the newborn rat the cholinergic regulation of pituitary GH secretion plays a prominent role directly at pituitary level most likely via M1 receptors.

The physiology of social conflict in rats: what is particularly stressful?

The involvement of the opponent's behavior in the aggression-induced stress response was tested. Subjects (male Wistar rats) faced stimulus rats previously submitted either to mediobasal hypothalamic lesion (MBHL) or to sham operation. MBHL intruders were more aggressive but were also more ready to submit compared with sham-operated rats; as residents they were more efficient in inducing submissive behavior in the opponent. The subject's stress response at 15 min was neither dependent on the intensity of aggressive behavior nor on residence status but on dominant-submissive relationships. The submission of the stimulus rat reduced corticosterone plasma levels in the subjects, while the subjects' own submissive behavior elicited a very significant increase in its plasma corticosterone. In contrast, plasma glucose seemed to depend on residence status, whereas plasma free fatty acids reacted to any novel event (including cage switch).

The effect of alpha 2 adrenoceptor blockers on aggressive behavior in mice: implications for the actions of adrenoceptor agents

The effects of three alpha 2 adrenoceptor blockers (idazoxan, yohimbine and CH-38083) on isolation-induced aggressive behavior was studied in male mice. The three drugs produced different behavioral profiles. Idazoxan reduced aggressiveness dose-dependently by decreasing the duration of offensive/aggressive interactions and increasing the duration of defensive behaviors. The other two drugs produced only parts of the dual action of idazoxan: yohimbine affected mainly defensive behaviors, while CH-38083 affected only the time spent with fighting. Saline injections per se also influenced behavior and, in contrast to alpha 2 adrenoceptor blockers, induced an increase in aggressiveness. These results are different from those previously obtained in rats, which show bell-shaped dose-response curves in response to alpha 2 adrenoceptor blockers (small doses increased, while large doses decreased aggression). It is postulated that the strong behavioral reaction of mice to the injection per se may mask the aggression-heightening effects of small doses of alpha 2 adrenoceptor blockers in this species. A theory is also presented regarding the complexity of adrenoceptor interactions when both pre-, and postsynaptic alpha 2 adrenoceptors are blocked.

Compression of the pituitary stalk elicits chronic increases in CSF vasopressin, oxytocin as well as in social investigation and aggressiveness

The neurochemical and behavioural effects of a novel stereotaxic surgical method developed for interrupting the nerve fibres running through the rat pituitary stalk to the posterior pituitary gland was studied. The cerebrospinal fluid (CSF) vasopressin (AVP) and oxytocin (OT) content as well as changes in aggressiveness were measured in rats one week and one month after the surgical intervention. The main results are as follows: (1) the compression of the pituitary stalk elicits a chronic increase in water consumption, as well as in CSF vasopressin and oxytocin content; (2) the surgical intervention increased the frequency of clinch fighting after one week. The increase in aggressiveness accentuated after one month and, in addition, operated animals showed reduced scores of resting while exploratory and social behaviours increased; (3) there was a strong positive correlation between water consumption, vasopressin, and aggressiveness; (4) oxytocin changes showed a positive correlation with variation in social behaviour. The surgical intervention may serve as a model for lesions of the pituitary stalk and formation of ectopic neurohypophyses in humans.

The physiology of social conflict in rats: what is particularly stressful?

The involvement of the opponent's behavior in the aggression-induced stress response was tested. Subjects (male Wistar rats) faced stimulus rats previously submitted either to mediobasal hypothalamic lesion (MBHL) or to sham operation. MBHL intruders were more aggressive but were also more ready to submit compared with sham-operated rats; as residents they were more efficient in inducing submissive behavior in the opponent. The subject's stress response at 15 min was neither dependent on the intensity of aggressive behavior nor on residence status but on dominant-submissive relationships. The submission of the stimulus rat reduced corticosterone plasma levels in the subjects, while the subjects' own submissive behavior elicited a very significant increase in its plasma corticosterone. In contrast, plasma glucose seemed to depend on residence status, whereas plasma free fatty acids reacted to any novel event (including cage switch).

Hypothalamic-pituitary corticotroph function after shunting of magnocellular vasopressin and oxytocin to the hypophyseal portal circulation

Surgical pituitary stalk compression (PSC) causes neural lobe denervation and development of ectopic magnocellular terminals in the external zone of the median eminence, resulting in shunting of magnocellular vasopressin (VP) and oxytocin (OT) to the pituitary portal circulation. To determine the effects of PSC on hypothalamic and pituitary function, VP and CRH receptors and POMC messenger RNA (mRNA) in the pituitary, and CRH, VP, and OT mRNA levels in the PVN were examined in 7-day PSC and sham-operated rats. Immunohistochemical staining revealed marked accumulation of immunoreactive VP in the swollen pituitary stalk and the external zone of the distal median eminence of PSC rats. Plasma ACTH and corticosterone levels were significantly increased by PSC, an effect that was attenuated by minipump infusion of desamino-[D-Arg8]-vasopressin (DDAVP) given to correct the diabetes insipidus. [3H]VP binding to anterior pituitary membranes from PSC rats was reduced by 50% compared to that in sham-operated controls, but VP V1b receptor mRNA levels measured by in situ hybridization were unchanged. Pituitary CRH receptors measured by binding autoradiography and CRH receptor mRNA levels did not change after PSC. POMC mRNA was unchanged in the anterior pituitary lobe, but markedly increased in the intermediate lobe of PSC rats, with or without DDAVP infusion. In the hypothalamic supraoptic and paraventricular nuclei, PSC resulted in reduction of VP mRNA (-83%) and OT mRNA (-38%) levels, probably reflecting retrograde degeneration of magnocellular neurons. This decrease was more pronounced for VP mRNA than for OT mRNA CRH mRNA levels in parvicellular cells of the paraventricular nuclei of PSC rats were reduced by 55%. Correction of the diabetes insipidus by DDAVP treatment further decreased hypothalamic VP mRNA levels, but restored CRH mRNA to control levels. The data show that continuous exposure of the pituitary to high VP levels from ectopic magnocellular fibers in the median eminence results in VP, but not CRH, receptor loss. Pituitary VP receptor down-regulation and decreased hypothalamic CRH expression are likely to contribute to the reduced corticotroph responsiveness to VP reported in this experimental model.

Hypothalamic alpha 2A-adrenoceptors stimulate growth hormone release in the rat

Oxymetazoline, the relatively selective alpha 2A-adrenoceptor agonist (with more than 60-fold selectivity over the alpha 2B-adrenoceptor subtype), was administered into the lateral ventricle (i.c.v.) of rats and plasma growth hormone (GH) levels were measured. Oxymetazoline was more potent to release GH after i.c.v. administration than was clonidine; 0.01 microgram i.c.v. oxymetazoline already caused a significant release of GH, while at least 0.1 microgram clonidine had to be administered to cause a similar response. The dose-response curve was of an inverted U shape since with 10 micrograms of oxymetazoline the plasma GH did not rise. When oxymetazoline was injected i.c.v. to rats with somatostatin fibres to the median eminence transected by an anterolateral cut in the hypothalamus there was a significant rise in plasma GH, suggesting that oxymetazoline stimulated GHRH rather than inhibited somatostatin release. Pretreatment with CH-38083 (7,8-(methylenedioxy)-14-alpha-hydroxy-alloberban HCl, selective for alpha 2-adrenoceptors but not differentiating between alpha 2A and alpha 2B subtypes), prevented the plasma GH rise normally elicited by 1 microgram i.c.v. oxymetazoline. The alpha 2A- and alpha 1-selective adrenoceptor antagonist, WB-4101 (2-(2,6-dimethoxyphenoxyethyl)-aminomethyl-1,4-benzodioxane hydrochloride), prevented the GH rise normally induced by oxymetazoline while prazosin, the alpha 2B- and alpha 1-selective adrenoceptor antagonist, prolonged the elevation occurring in the control rats between 30 and 60 min after oxymetazoline injection. Since both prazosin and WB-4101 are alpha 1-adrenoceptor antagonists but differ in their action on alpha 2A and alpha 2B subtypes as well as in their action on oxymetazoline-induced GH secretion, the antagonist studies suggest that oxymetazoline stimulates GH release through activation of alpha 2A-adrenoceptors stimulatory to GHRH release, and not by an action through alpha 2B- or alpha 2C- or alpha 1-adrenoceptors. Since WB-4101 also antagonized clonidine action on GH release we also suggest that the major component may be the stimulation of the alpha 2A-adrenoceptors in the clonidine action on GH release.

Do alpha-2 adrenoceptors modify coping strategies in rats?

Previous research has shown that resident rats treated with alpha 2 adrenoceptor blockers display a modified aggressive response towards intruding animals. In the present study we report data on the behavioral changes induced by alpha 2 adrenoceptor blockers in intruder animals. In experiments 1 and 2 intruders smaller in body weight than the residents were treated with 0.0, 0.5, and 1.0 mg/kg CH-38083 and idazoxan, respectively; in experiment 3 weight matched intruders were injected with 1 mg/kg CH-38083 or idazoxan. The treatment of smaller intruders did not change the behavior of residents. In contrast, weight-matched intruders injected with alpha 2 adrenoceptor blockers elicited increased aggression in residents. Social behaviors, exploration and offensive aggression showed insignificant variation in intruders. Defensive behaviors, in contrast, showed major changes: in experiments 1 and 2 a dose-dependent decrease in immobility and a dose-dependent increase in defensive upright was noticed. In experiment 3, high scores of defensive upright were apparent, precluding detection of drug-induced changes. However, when the last 5 min of the encounter were analysed separately, results similar to the first two experiments were observed. Significant negative correlations were found between immobility and defensive upright scores. The results suggest that alpha 2 adrenoceptor blockers induce a shift from a passive (immobility) towards a more active (defensive upright) coping style. These and previous data show that alpha 2 adrenoceptor blockers, other than yohimbine, seem to exert a behavior-activating effect in rats.

Marked changes of arginine vasopressin, oxytocin, and corticotropin-releasing hormone in hypophysial portal plasma after pituitary stalk damage in the rat

Mechanical compression of the pituitary stalk with the help of a blunt stereotaxic knife results in posterior pituitary denervation (PPD) and sprouting proximal to the injury, leading to formation of an ectopic neurohypophysis in the stalk. This provides an experimental model for those cases in which traumatic damage severs the nerve fibers to the neural lobe but does not obliterate the hypophysial-portal circulation. The effect of PPD on the hypophysial-portal concentration profile of putative ACTH secretagogues as well as basal and stimulated ACTH secretion in vitro were investigated at varying times after PPD. The contents of arginine vasopressin (AVP) and oxytocin (OT) in extracts of the stalk median eminence 1 week after PPD were markedly elevated, whereas corticotropin-releasing hormone (CRH) content was unaffected. Levels of these three neuropeptides in hypophysial-portal blood collected under anesthesia from the proximal stump of the transected stalk (or the ectopic neural lobe) were measured at weekly intervals in groups of rats after sham or PPD surgery. Hypophysial-portal AVP levels showed a monotonic increase with time after PPD from a 1.8-fold elevation at 1 week post-PPD to a maximum concentration 6-fold greater than that in sham groups at 4 weeks post-PPD. Portal plasma OT levels also exhibited extreme elevation. In contrast, portal plasma CRH levels showed an initial 72% decline 1 week post-PPD. We suggest that mechanical damage to the pituitary stalk and the subsequent sprouting redirected secretion of AVP and OT from the neural lobe to the pituitary stalk. This caused sustained elevations of portal plasma concentrations of AVP and OT. The resulting tonic exposure to AVP and/or OT may down-regulate anterior pituitary receptors to these neurohypophyseal peptides and indirectly decrease CRH release into the portal circulation.

Paraventricular nucleus controls 5-HT2C receptor-mediated corticosterone and prolactin but not oxytocin and penile erection responses

There is evidence that administration of the 5-HT2C receptor agonist, m-chlorophenylpiperazine (m-CPP), increases plasma concentrations of oxytocin, prolactin, corticosterone, induces penile erections and excessive grooming in male rats. To test the hypothesis that the hypothalamic paraventricular nucleus mediates these neuroendocrine and behavioural responses, we measured the effects of m-CPP (0.6 mg/kg i.v.) on these parameters in chronically cannulated, freely moving male rats after surgical lesion of the paraventricular nucleus or sham operation. Paraventricular nucleus lesion markedly attenuated prolactin, corticosterone and excessive grooming, but not oxytocin and penile erection responses to m-CPP. In contrast, both oxytocin and corticosterone responses to the dopamine receptor agonist, apomorphine (0.05 mg/kg i.v.), were attenuated in lesioned rats. The present studies suggest that the paraventricular nucleus mediates m-CPP-induced prolactin, corticosterone and probably also excessive grooming responses, and the mechanisms of apomorphine and m-CPP action on oxytocin secretion and penile erection responses differ.

Adrenocorticotropin, prolactin and beta-endorphin stimulatory actions of alpha-2-adrenoceptor antagonists

We studied the effect of glucocorticoid pretreatment, mediobasal hypothalamus lesion (MBHL) and the interaction between clonidine and yohimbine in male Wistar rats to elucidate the sites and/or mechanisms of endocrine actions of alpha 2-antagonists. The pretreatment of 1 mg/kg s.c. dexamethasone for 4 days effectively prevented the stimulatory effect of alpha 2-antagonists yohimbine (5 mg/kg i.p.) and CH-38083 (1 mg/kg i.p.) on adrenocorticotropin (ACTH) secretion, while the action of these antagonists on prolactin (PRL) and beta-endorphin (beta E) remained unchanged. The central (i.c.v.) pretreatment of 5 micrograms/rat clonidine failed to antagonize the prolactin (PRL) and beta E releasing effect of yohimbine. However, it inhibited the yohimbine-induced ACTH secretion. MBHL resulted in a significant enhancement in basal plasma PRL and beta-endorphin (beta E) levels. But basal plasma ACTH levels have not been changed. Yohimbine failed to stimulate ACTH secretion in MBH-lesions rats, while PRL and beta E response to the yohimbine was maintained in these animals. This study confirms that the alpha 2-antagonists stimulate ACTH secretion by a corticosteroid-sensitive mechanism which is located centrally. In contrast, alpha 2-antagonists affect PRL and beta E secretion via a corticosteroid-insensitive mechanism located at the periphery, possible within the pituitary gland.

Diurnal alteration in opiate effects on the hypothalamo-pituitary-adrenal axis: changes in the mechanism of action

Opioid control of the hypothalamo-pituitary-adrenocortical axis has a characteristic circadian rhythm (Kiem, Kanyicska, Stark and Fekete, 1987). To elucidate the mechanisms leading to circadian alterations of opioid control of the hypothalamo-pituitary-adrenocortical axis, and to look for the receptor type at which the p.m. inhibitory action of opioids occurs, we examined the effect of morphine at different doses and the interaction between naloxone and morphine at different times of day in intact male Wistar rats. In the morning: morphine (10 and 30 mg/kg s.c.) significantly increased corticosterone secretion, while 3 mg/kg s.c. had no effect. Naloxone in a dose of 2.5 mg/kg i.p. significantly antagonized the corticosterone-releasing effect of morphine, suggesting that the secretion of corticosterone induced by morphine is mediated via mu-opioid receptors. In the afternoon: basal plasma corticosterone levels were higher than those in the morning, and morphine caused a significant corticosterone increase only at the dose of 30 mg/kg, and had no effect in the dose of 10 mg/kg. Morphine significantly decreased corticosterone levels in the dose of 3 mg/kg. This inhibitory action lasted approximately 3 h after morphine injection and was able to inhibit the circadian evening rise of corticosterone. The effect of morphine and the interaction between naloxone and morphine on prolactin secretion remained unchanged from a.m. to p.m.; naloxone (2.5 mg/kg i.p.) which inhibited the 30 mg/kg morphine-induced corticosterone rise in the morning failed to antagonize the 3 mg/kg morphine-induced decrease of corticosterone secretion in the afternoon. A high dose of naloxone (10 mg/kg i.p.) effectively prevented the 3 mg/kg morphine-induced p.m. inhibition of corticosterone secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone secretion and activation of cyclic AMP by growth hormone releasing hormone and gamma-aminobutyric acid in the neonatal rat pituitary

The effect of factors influencing pituitary growth hormone secretion may be mediated by a combination of several intracellular mechanisms. The involvement of cyclic AMP (cAMP) in the GH stimulatory effect of tau-aminobutyric acid (GABA) and of growth hormone releasing hormone (GHRH) was studied in neonatal rat pituitaries. In the pituitaries of the newborn rats GH secretion was stimulated by forskolin and by isobutylmethylxantine (IBMX). GHRH but not GABA elevated pituitary cAMP concentration, whereas both drugs increased GH secretion from 2-day old pituitaries. IBMX did not augment the cAMP stimulating effect of GHRH in 2-day old, but potentiated it in older (7, 14 and 21-day old) pituitaries. The results indicate the presence of a functioning, but relatively immature intracellular signal transmission system in the 2-day old rat pituitary.