Paraventricular nucleus mediates prolactin secretory responses to restraint stress, ether stress, and 5-hydroxy-L-tryptophan injection in the rat

Restraint stress increases prolactin-mediated phosphorylation of signal transducer and activator of transcription 5 in the hypothalamus and adrenal cortex in the male mouse

Prolactin is a pleiotropic peptide hormone produced by the lactotrophs in the anterior pituitary. Its rate of secretion is primarily regulated by a negative-feedback mechanism where prolactin stimulates the activity of the tuberoinfundibular dopaminergic (TIDA) neurones, increasing their release of dopamine, which accesses the pituitary via the median eminence to suppress further prolactin secretion. In addition to its well established role in lactation, circulating prolactin is secreted in response to stress, although the mechanism by which this is achieved or its cellular targets remains unknown. In the present study, we show that 15 minutes of restraint stress causes an approximately seven-fold increase in circulating prolactin concentration in male mice. Monitoring prolactin receptor activation, using immunohistochemistry to determine the level and distribution of tyrosine phosphorylated signal transducer and activator of transcription 5 (pSTAT5), we show that this stress-induced increase in prolactin interacts with both central and peripheral targets. Restraint stress for 15 minutes significantly increased pSTAT5 staining in the arcuate nucleus, median eminence and the zona fasciculata of the adrenal cortex. In each case, this response was prevented by pretreating the animals with bromocriptine to block prolactin secretion from the pituitary. Interestingly, in contrast to many cells in the arcuate nucleus, stress reduced pSTAT5 staining of the TIDA neurones (identified by dual-labelling for tyrosine hydroxylase). This suggests that there is reduced prolactin signalling in these cells and thus potentially a decline in their inhibitory influence on prolactin secretion. These results provide evidence that prolactin secreted in response to acute stress is sufficient to activate prolactin receptors in selected target tissues known to be involved in the physiological adaptation to stress.

Effects of anesthetics and terminal procedures on biochemical and hormonal measurements in polychlorinated biphenyl treated rats

This investigation reports the effects of various terminal procedures, and how they modified the responses to a toxicant (polychlorinated biphenyls [A1254], 130 mg/kg/day × 5 days) administered by gavage to Sprague-Dawley male rats. Terminal procedures included exsanguination via the abdominal aorta under anesthesia (isoflurane inhalation or Equithesin injection), decapitation with or without anesthesia, or narcosis induced by carbon dioxide inhalation. Effects of repeated anesthesia were also tested. Terminal procedures induced confounding stress responses, particularly when Equithesin was used. The terminal procedures modified the conclusions about effects of A1254 on the concentrations of corticosterone, insulin, glucagon, glucose, alkaline phosphatase, lactate dehydrogenase, uric acid, and blood urea nitrogen, from nonstatistically significant to significant changes, and in the case of luteinizing hormone from a statistically significant increase to a significant decrease. Investigations of effects of toxicants should be designed and interpreted considering potential changes induced by the selection of a terminal procedure.

Prolactin induces phosphorylation of the STAT5 in adrenal glands of Hatano rats during stress

AIMS

To investigate the signaling of prolactin (PRL) in the adrenal gland during stress in Hatano high- (HAA) and low-avoidance (LAA) rats.

MAIN METHODS

Adrenal glands of both strains were collected at 0, 15 and 30 min after stress. The protein levels of phosphorylated STAT5 and the mRNA levels of melanocortin receptor 2 (MC2R) and PRL receptor (PRLR) were analyzed. Furthermore, the effects of bromocriptine-induced hypoprolactinemia on adrenocortical responses to stress were investigated.

KEY FINDINGS

Adrenocorticotropic hormone (ACTH) concentrations in HAA were greater than LAA, while the difference in PRL concentrations were found only at 120 min after stress induction. No strain differences were observed in corticosterone or progesterone in response to stress. The stress-induced increase in MC2R mRNA expression was higher in HAA, but there was a lowered PRLR mRNA expression. STAT5 become highly phosphorylated in response to stress in both strains, but bromocriptine led to a reduction the STAT5 phosphorylation. Exposure to bromocriptine was associated with a reduction in plasma PRL in response to stress in both strains, while the ACTH levels were not altered. However, the decrease in corticosterone and progesterone in response to stress was observed only in bromocriptine-treated LAA rats.

SIGNIFICANCE

These data show that PRL plays an important role in the regulation of corticosterone and progesterone release in LAA but not in HAA during stress. These results suggest that PRL increase in response to stress, and it acts on the adrenal cortex and thereby plays an important physiologic role in protecting against acute stress.

Neonatal maternal separation affects endocrine and metabolic stress responses to ether exposure but not to restraint exposure in adult rats

We investigated prolactin secretion and metabolic changes in stress response in adult male rats submitted to periodic maternal separation (MS; 180 min/day) at 2 weeks of life. Restraint and ether exposure were randomly performed when the animals were 10-12 weeks of age. Restraint exposure: the animals were placed into plastic tubes (21 cm long, 4.5 cm diameter) for 20 min. Ether exposure: the rats were exposed to ether for 10 min. Atrial cannulation for blood sampling was performed through the jugular vein 5 days before the experiments. In both protocols, blood samples were taken immediately before (0), and 5, 15 and 20 min after the beginning of stress exposure. Ours results showed attenuated endocrine and metabolic responses to ether exposure in the maternal separation (MS) group compared to the control group. The measured metabolic parameters, plasma glucose, prolactin, lactate, and insulin secretion, were 32%, 55%, 41%, 73% lower (P < 0.01), respectively, in MS than in control animals. On the other hand, the endocrine and metabolic stress responses to restraint exposure were not affected by maternal separation. There was no difference between the MS and the control groups in any of the parameters studied. Our data demonstrated that early life experiences affect the hormonal systems beyond the hypothalamic-pituitary-adrenal axis, such as the central neuronal pathways, and their activities related to hormonal and metabolic responses to stress in adulthood. More importantly, these modifications were specific, but dependent on stress situation affecting mainly the circuitry related to the stress response to ether exposure.

GABAergic mediation of stress-induced secretion of corticosterone and oxytocin, but not prolactin, by the hypothalamic paraventricular nucleus

The hypothalamus-pituitary-adrenal axis (HPA) participates in mediating the response to stressful stimuli. Within the HPA, neurons in the medial parvocellular region of paraventricular nucleus (PVN) of the hypothalamus integrate excitatory and inhibitory signals triggering secretion of corticotropin-releasing hormone (CRH), the main secretagogue of adrenocorticotropic hormone (ACTH). Stressful situations alter CRH secretion as well as other hormones, including prolactin and oxytocin. Most inputs to the PVN are of local origin, half of which are GABAergic neurons, and both GABA-A and GABA-B receptors are present in the PVN. The objective of the present study was to investigate the role of GABA-A and GABA-B receptors in the PVN's control of stress-induced corticosterone, oxytocin and prolactin secretion. Rats were microinjected with saline or different doses (0.5, 5 and 50 pmol) of GABA-A (bicuculine) or GABA-B (phaclofen) antagonists in the PVN. Ten minutes later, they were subjected to a stressor (ether inhalation) and blood samples were collected 30 min before and 10, 30, 60, 90 and 120 min after the stressful stimulus to measure hormone levels by radioimmunoassay. Our results indicate that GABA acts in the PVN to inhibit stress-induced corticosterone secretion via both its receptor subtypes, especially GABA-B. In contrast, GABA in the PVN stimulates oxytocin secretion through GABA-B receptors and does not alter prolactin secretion.

Specific and time-dependent effects of glucocorticoid receptor agonist RU28362 on stress-induced pro-opiomelanocortin hnRNA, c-fos mRNA and zif268 mRNA in the pituitary

This study examined the effects of the glucocorticoid receptor (GR) agonist RU28362 on stress-induced gene expression in the pituitary of rats to investigate mechanisms of glucocorticoid negative feedback in vivo. In an initial experiment, acute restraint stress produced rapid (within 15 min) induction of c-fos mRNA, zif268 mRNA and pro-opiomelanocortin (POMC) hnRNA within the anterior and intermediate/posterior pituitary as determined by quantitative real-time polymerase chain reaction. Treatment with RU28362 (150 microg/kg, i.p.) 60 min before restraint inhibited adrenocorticotrophic hormone (ACTH) and corticosterone secretion and selectively suppressed the stress-induced increase in POMC hnRNA in the anterior pituitary gland. The failure of RU28362 to surpress the stress-induced rise in c-fos and expression of zif268 mRNA suggests that the central release of ACTH secretagogues was not affected at this time point by treatment with the GR agonist. Rather, the inhibition of ACTH release appeared to be due to a direct effect of RU28362 within the pituitary. A follow-up time-course study varied the interval (10, 60 or 180 min) between RU28362 pretreatment and the onset of restraint. The stress-induced increase in POMC hnRNA was completely blunted by RU28362 treatment within 10 min of treatment, although the stress induced hormone secretion, c-fos mRNA and zif268 mRNA were unaffected. The rapid inhibition of the stress-induced rise in POMC hnRNA in the anterior pituitary appears to reflect direct, GR-mediated suppression of POMC gene expression. RU28362 pretreatment 180 min before restraint onset was sufficient to suppress the stress-induced expression in the anterior pituitary gland of all three genes examined. Thus, the delayed negative feedback effects on hypothalamic-pituitary-adrenal axis activity that emerged after 180 min after glucocorticoid treatment were not evident at 60 min. Taken together, the data suggest that the inhibition of the stress-induced release of ACTH apparent within the first hour of glucocorticoid exposure is effected at the level of the pituitary gland. The delayed glucocorticoid effects evident 180 min after RU28362 treatment may include glucocorticoid actions in the brain and additional actions within the pituitary.

Growth hormone, cortisol and prolactin responses to physical exercise: higher prolactin response in depressed patients

This study was designed to compare growth hormone, cortisol and prolactin responses to physical exercise in depressed patients and healthy comparison subjects. Patients fulfilled the DSM-IV diagnostic criteria for current major depressive disorder; subjective depressive symptoms were rated with Montgomery-Asberg Depression Rating Scale (MADRS) immediately before the experiment. Growth hormone, cortisol and prolactin were measured before and immediately after physiologically stressful bicycle cardiopulmonary exercise test. After exercise, there were three additional hormone measurements, with 30-min intervals. No significant difference was found in baseline growth hormone, cortisol or prolactin levels between patients and the control group. Plasma growth hormone and cortisol levels increased significantly during physical exercise in both patients and controls and returned to baseline in 90 min. There was no significant difference in growth hormone or cortisol responses to physical exercise between the two groups. However, prolactin levels increased only in the depressed patients group during the exercise. We hypothesize that acute exercise may have a stronger effect on serotonin (5-HT) release in depressed patients, which is reflected in increased plasma prolactin concentration.

[18F]altanserin binding to human 5HT2A receptors is unaltered after citalopram and pindolol challenge

The aim of the present study was to develop an experimental paradigm for the study of serotonergic neurotransmission in humans using positron emission tomography and the 5-HT2A selective radioligand [18F]altanserin. [18F]altanserin studies were conducted in seven subjects using the bolus/infusion approach designed for attaining steady state in blood and brain 2 hours after the initial [18F]altanserin administration. Three hours after commencement of radiotracer administration, 0.25 mg/kg of the selective serotonin reuptake inhibitor, citalopram (Lundbeck, Valby, Denmark), was administered to all subjects as a constant infusion for 20 minutes. To reduce 5-HT1A-mediated autoinhibition of cortical 5-HT release, four of the seven subjects were pretreated with the partial 5-HT1A agonist pindolol for 3 days at an increasing oral dose (25 mg on the day of scanning). In each subject, the baseline condition (120 to 180 minutes) was compared with the stimulated condition (195 to 300 minutes). Despite a pronounced increase in plasma prolactin and two subjects reporting hot flushes compatible with an 5-HT-induced adverse effect, cortical [18F]altanserin binding was insensitive to the citalopram challenge, even after pindolol pretreatment. The biochemical and cellular events possibly affecting the unsuccessful translation of the citalopram/pindolol challenge into a change in 5-HT2A receptor binding of [18F]altanserin are discussed.

Pivotal Role of an Endogenous Tetrahydroisoquinoline, Salsolinol, in Stress- and Suckling-Induced Release of Prolactin

In mammals, the role of a prolactin-releasing factor (PRF) in the acute changes of prolactin (PRL) secretion that usually occur after challenges (e.g., suckling stimulus or stress) of homeostasis has been suspected for a long time. We have recently observed that 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, salsolinol (SAL), produced by the hypothalamus and the neuro-intermediate lobe (NIL) of the pituitary gland, can selectively release PRL from the anterior lobe (AL). Moreover, binding sites for SAL have been detected in areas like median eminence, NIL, and AL. It has been proposed that SAL is a putative endogenous PRF. We have also found that a structural analogue of SAL, 1-methyl-3,4-dihydroisoquinoline (1MeDIQ), is able to block dose-dependently SAL-, suckling-, and immobilization (IMO) stress-induced release of PRL without having any influence on alpha-methyl-p-tyrosine (alphaMpT)-induced PRL responses. Neither SAL nor 1MeDIQ has any effect on alpha-melanocyte-stimulating hormone (alphaMSH), adrenocorticotrophic hormone (ACTH), beta-endorphin (beta-END) and arginine-vasopressin (AVP) secretion. Moreover, SAL-induced PRL response was attenuated in male rats pretreated with dexamethasone (DEX). These results strongly suggest that SAL has an important role in the regulation of PRL release induced by physiologic and environmental stimuli; therefore, it can be considered as the strongest candidate for being the PRF in the hypothalamo-hypophysial system. Our findings also indicate that the adrenal steroids may play an inhibitory feedback role in SAL-mediated PRL response.

Blunted prolactin response to fentanyl in depression. Normalizing effect of partial sleep deprivation

There is some evidence that sleep deprivation (SD) might exert its antidepressant properties by involving endogenous opioid mechanisms. The authors investigated the effects of mu-receptor agonist administration on prolactin release in depressed patients before and after partial SD. Medication-free female depressed inpatients (N=18) were participating in two fentanyl challenge tests after partial SD and undisturbed sleep, 3 days apart in random order. Healthy volunteer women (N=10) were enrolled after full night sleep as comparison subjects. Five of them had placebo trials. Participants were given an intravenous injection of 0.1 mg/70 kg fentanyl at 9:00 AM. The prolactin secretory response to the opiate agonist was investigated for 1 h with serial blood sampling. After a night of undisturbed sleep, fentanyl administration prompted increases in plasma prolactin concentrations with blunted responses found in the depressed group. Following partial SD, the stimulated prolactin secretion of depressed patients increased significantly and was comparable to the response of comparison subjects. These findings suggest that SD acts via an opioid/dopamine-related mechanism. An alternative explanation, based on serotonin involvement is addressed in the discussion.

Identification of a novel first exon of prolactin receptor gene expressed in the rat brain

A novel first exon, E1(4), whose sequence was distinct from those of the three known first exons, E1(1), E1(2), and E1(3), of the rat PRL receptor (PRL-R) gene was identified by cDNA cloning for the 5'-end region of PRL-R mRNA expressed in the rat brain. Sequence analysis revealed the presence of two different length E1(4) cDNAs. The longer cDNA contained the 243-bp E1(4) sequence, and the shorter cDNA lacked the 139-bp sequence at the 5'-end of the longer one. Neither E1(4) cDNA has a second exon sequence, indicating that the E1(4) first exon is extensively spliced to the third exon. E1(4)-containing PRL-R mRNAs were detected only in the brain by RT-PCR and ribonuclease protection assay. The longer E1(4) mRNA was expressed as the major PRL-R mRNA species in the brain and was greatly increased in pregnant (d 18) and lactating (d 5) rats. A genomic clone containing the E1(4) first exon together with its 5'- and 3'-flanking regions was isolated from a rat kidney genomic library. Ribonuclease protection assay revealed that the position corresponding to the 5'-end of the shorter E1(4) cDNA is the major transcription start point for the E1(4) exon. The 5'-flanking region of E1(4) contained a TATA box-like element 23 bp upstream of the major transcription start point. Other putative transcription factor-binding sites, such as CCAAT, Sp1, and glucocorticoid-responsive elements, were observed at further upstream regions. These results suggest that PRL-R gene expression in rat brain is controlled by the promoter for the E1(4) first exon.

Prolactin release during exercise in normal and adrenodemedullated untrained rats submitted to central cholinergic blockade with atropine

To study the role of the central cholinergic system in pituitary prolactin (PRL) release during exercise we injected atropine (5 x 10(-7) mol) into the lateral cerebral ventricle of intact or adrenodemedullated (ADM) untrained rats, at rest or submitted to exercise on a treadmill (18 m x min(-1), 5% grade) until exhaustion. The rats were implanted with chronic jugular catheters for blood sampling and with unilateral intracerebroventricular (icv) cannulas placed in the right lateral ventricle. Blood prolactin concentrations were measured before and every 10 min after the start of exercise for a period of 60 min. After the animals started running, plasma prolactin levels rose rapidly in both normal and ADM rats, reaching near maximum at 10 min. Close to exhaustion (19.8 +/- 2.9 min for intact rats and 23.5 +/- 4.1 min for ADM) they were still high, remained increased until 30 min, and returned to preexercise levels at 40 min. Icv injections of atropine decreased the time to exhaustion by 67% in intact rats and by 96.2% in ADM and also reduced the exercise-induced PRL release in both intact (50%) and ADM rats (90%). The results showed that prolactin release induced by exercise was dependent on the exercise workload and could be observed as early as after 10 min of running, remaining increased until 30 min. These data indicate that adrenodemedullation does not affect prolactin secretion induced by exercise, although adrenodemedullated rats proved to be more sensitive to the reducing effect of central cholinergic blockade on their maximal capacity for exercise.

Bromocriptine-induced dissociation of hyperglycemia and prolactin response to restraint

The present study investigated the effects of immobilization (restraint stress) on rat chronically treated with a D(2) receptor agonist (bromocriptine, 0.4 mg/100 g body weight, injected daily intraperitoneally (ip) for 2 weeks) on plasma glucose, prolactin, and insulin levels. During restraint, the plasma prolactin of vehicle-treated (VEH) rats increased rapidly, reaching a peak at 10 min (57.9 +/- 8.1 ng/ml, P < .01). In contrast, restraint failed to induce any significant change in the plasma prolactin levels of bromocriptine-treated (BR) rats. The hyperglycemic response to immobilization was 97% higher (P < .05) in BR rats than in VEH rats. Our data demonstrate that prolactin secretion and hyperglycemia in response to restraint can be dissociated by chronic treatment with BR, which also increased the hyperglycemic response to immobilization probably due to central D(2) dopaminergic activity.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Prolactin and corticosterone secretion in response to acute stress after paraventricular nucleus lesion by ibotenic acid

The cellular organization of the paraventricular nucleus (PVN) is complex and eight distinct regions have been identified by Nissl staining. Three consist of magnocellular neurons and five of parvocellular neurons. Ibotenic acid, a glutamate analogue, is a toxin with neuroexcitatory properties which acts on N-methyl-D-aspartate and metabotropic receptors. Depending on the dose used, ibotenic acid causes extensive damage of parvocellular neurons of the paraventricular nucleus but preserves magnocellular neurons and passage fibers, in contrast to electrolytic lesions, which causes diffuse and nonspecific destruction. We studied the prolactin (PRL) and corticosterone secretion in response to acute stress induced by exposure to the ether, 3 weeks after selective neurotoxic lesion of parvocellular neurons of the paraventricular nucleus by microinjection of ibotenic acid. There was no significant difference in the basal levels of PRL and corticosterone between control and lesioned animals. The plasma PRL increased in the sham and lesioned groups after stress of similar manner. However, the increase in plasma corticosterone in response to stress was significantly higher in lesioned animals. In conclusion, the selective lesion of parvocellular neurons of the PVN did not change basal or stress induced PRL secretion but it caused hypersensitivity of the hypothalamus-pituitary-adrenal axis 3 weeks later, probably by corticotropin releasing hormone (CRH) from hypothalamic areas others than parvocellular neurons of the PVN; hypersensitivity of corticotropes to the secretagogues others than CRH; or hyperresponsiveness of AVP receptors in the adenohypophysis. Furthermore, we cannot rule out a putative inhibitory factor of the hypothalamus-pituitary axis produced by parvocellular neurons of the PVN. This factor modulator of corticotropin secretion could be absent after recuperation of the response of the hypothalamus-pituitary axis to the stress.

The citalopram challenge test in patients with major depression and in healthy controls

Neuroendocrine challenge tests in depressed patients have revealed a blunted hormonal reaction to serotonergic stimuli. In the present study, citalopram was chosen as the serotonergic agent for neuroendocrine stimulation. Compared to earlier challenge agents, citalopram has the advantage of serotonergic selectivity, its application is well tolerated and the possibility of intravenous application reduces pharmacokinetic interference. Sixteen patients suffering from an acute episode of major depression and 16 healthy controls underwent the stimulation procedure with 20 mg of citalopram and placebo. Whereas significant differences in the secretion of prolactin and cortisol between citalopram and placebo challenge were observed in the control group, no differences were found in the group of depressed patients. Comparison of depressed patients and controls showed a significantly blunted prolactin secretion in patients. Differences in cortisol secretion following serotonergic stimulation with citalopram did not become significant. The stimulation procedure was well tolerated in all subjects, although a higher number of side effects was observed in the control group. The amount of side effects did not correlate with the hormone responses. These results are in line with the hypothesis of serotonergic hypofunction in depressed patients. In conclusion, the 20-mg citalopram challenge test is thought to be a promising tool for further investigation of serotonergic function in psychiatric illness.

Forebrain pathways mediating stress-induced hormone secretion

Exposure to hostile conditions initiates the secretion of several hormones, including corticosterone/cortisol, catecholamines, prolactin, oxytocin, and renin, as part of the survival mechanism. Such conditions are often referred to as "stressors" and can be divided into three categories: external conditions resulting in pain or discomfort, internal homeostatic disturbances, and learned or associative responses to the perception of impending endangerment, pain, or discomfort ("psychological stress"). The hormones released in response to stressors often are referred to as "stress hormones" and their secretion is regulated by neural circuits impinging on hypothalamic neurons that are the final output toward the pituitary gland and the kidneys. This review discusses the forebrain circuits that mediate the neuroendocrine responses to stressors and emphasizes those neuroendocrine systems that have previously received little attention as stress-sensitive hormones: renin, oxytocin, and prolactin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABAA, histamine, and serotonin receptors alter the neuroendocrine stress response. The effects of these drugs are discussed in relation to their effects on forebrain neural circuits that regulate stress hormone secretion. For psychological stressors such as conditioned fear, the neural circuits mediating neuroendocrine responses involve cortical activation of the basolateral amygdala, which in turn activates the central nucleus of the amygdala. The central amygdala then activates hypothalamic neurons directly, indirectly through the bed nucleus of the stria terminalis, and/or possibly via circuits involving brainstem serotonergic and catecholaminergic neurons. The renin response to psychological stress, in contrast to those of ACTH and prolactin, is not mediated by the bed nucleus of the stria terminalis and is not suppressed by benzodiazepine anxiolytics. Stressors that challenge cardiovascular homeostasis, such as hemorrhage, trigger a pattern of neuroendocrine responses that is similar to that observed in response to psychological stressors. These neuroendocrine responses are initiated by afferent signals from cardiovascular receptors which synapse in the medulla oblongata and are relayed either directly or indirectly to hypothalamic neurons controlling ACTH, prolactin, and oxytocin release. In contrast, forebrain pathways may not be essential for the renin response to hemorrhage. Thus current evidence indicates that although a diverse group of stressors initiate similar increases in ACTH, renin, prolactin, and oxytocin, the specific neural circuits and neurotransmitter systems involved in these responses differ for each neuroendocrine system and stressor category.

Anterior pituitary response to stress: time-related changes and adaptation

A wide array of physical and psychological stressors alter the secretion of anterior pituitary hormones. However, both the qualitative and the quantitative features of the stressors as well as its duration markedly influence the final endocrine response. In addition, among all anterior pituitary hormones, only ACTH and prolactin levels appear to reflect the intensity of the stress experienced by the animals. Although physical stressors show a somewhat specific neuroendocrine profile, the response of the pituitary-adrenal (PA) and sympathomedulloadrenal axes are common to almost all stressors. After an initial stimulatory effect of stress, an inhibition of all anterior pituitary hormones, except ACTH, can be found provided the stressor is intense enough. The mechanisms responsible for this biphasic response to stress are likely to be located at sites above the pituitary. When the animals are repeatedly exposed to the same stressor, some behavioural and physiological consequences of stress exposure are reduced, suggesting that the animals become adapted to the stimulus. This process has been also termed habituation. Among all the pituitary hormones, only ACTH and prolactin levels are reduced as a consequence of repeated exposure to the same (homotypic) stressor, although some negative results have been reported. However, it has been recently reported that subtle changes in the characteristics of the stressors or in their regularity can greatly influence adaptation, and these factors might explain failure to find adaptation of ACTH and prolactin in some works. Habituation of ACTH and prolactin, when observed, appears to be specific for the chronically applied stressor so that the potentiality of the PA axis and prolactin to respond to a novel (heterotypic) stressor can be preserved. In the case of the PA axis, an intact or potentiated response to a novel stressor is observed in spite of presumably negative feedback exerted by daily stress-induced glucocorticoid release and the high resting levels of glucocorticoids. This phenomenon has been termed as facilitation and can be unmasked alternating stress. Although with the exception of the PA axis, developmental aspects of anterior pituitary response to stress have been poorly studied, available data suggest that dramatic changes occur in some hormones during weaning, with some, but less profound, change thereafter. Responsiveness to stressors appears to mature with age, but developmental patterns differ among the various anterior pituitary hormones.

p-Chlorophenylalanine and fluoxetine inhibit D-fenfluramine-induced Fos expression in the paraventricular nucleus, cingulate cortex and frontal cortex but not in other forebrain and brainstem regions

D-Fenfluramine, a putative serotonin releaser and reuptake inhibitor, is commonly prescribed for the treatment of obesity. Brain sites activated by D-fenfluramine have been mapped via the expression of the immediate early gene Fos. However, it is not clear that serotonin release in the brain mediates the effects of D-fenfluramine on Fos expression. The present study determined whether D-fenfluramine induces the expression of Fos in the brain through the release of serotonin. Rats were pretreated either with the serotonin depleting drug p-chlorophenylalanine (PCPA) or with the serotonin reuptake inhibitor fluoxetine. Both drugs inhibited D-fenfluramine-induced Fos expression in the cingulate cortex, frontal cortex, and the parvocellular subdivision of the paraventricular nucleus of the hypothalamus. Neither drug reduced D-fenfluramine-induced Fos responses in several other brain areas, including the caudate-putamen, amygdala, and brainstem regions such as the lateral parabrachial nucleus and nucleus of the solitary tract. These results indicate regional specificity of mechanisms mediating D-fenfluramine-induced Fos expression. It is likely that D-fenfluramine-induced Fos expression at various sites in the brain is mediated via a combination of serotonin release and other, as yet unidentified, neurotransmitters.

Effect of sleep deprivation on neuroendocrine response to a serotonergic probe in healthy male subjects

Neuroendocrine responses to stimulation with a selective serotonin reuptake inhibitor (citalopram) were measured to investigate the effects of all-night sleep deprivation on serotonergic function in healthy male subjects (n = 7). We studied citalopram-stimulated prolactin and cortisol plasma concentrations in a placebo-controlled cross-over protocol following sleep and sleep deprivation. Citalopram infusion (20 mg i.v. at 14:20-14:50 h) after a night of undisturbed sleep prompted robust increases in both plasma prolactin and cortisol concentrations. Following a night of sleep deprivation, by contrast, the citalopram-induced prolactin response was blunted, but the cortisol response was not significantly altered. This differential response pattern relates to the distinct pathways through which serotonin may activate the corticotrophic and the lactotrophic systems. While an unchanged cortisol response does not indicate (but also does not refute the possibility of) an altered serotonergic responsivity following sleep deprivation, the suppressed prolactin response could reflect a downregulation of 5-HT1A or 2 receptors. An alternative, not mutually exclusive, explanation points to the possibility that sleep deprivation activates the tubuloinfundibular dopaminergic system, the final inhibitory pathway of prolactin regulation.

Morphine withdrawal alters anterior pituitary hormone secretion, brain endopeptidase activity and brain monoamine metabolism in the rat

Rats were made tolerant to morphine by a 5-day regimen with increasing doses. The time course of changes in serum anterior pituitary hormone levels, brain endo- and exopeptidase activity, levels of brain biogenic amines and body weight were studied during abrupt morphine withdrawal. Cold stimulated secretion of thyrotropin and the secretion of growth hormone were both decreased whereas that of prolactin was increased. In the hypothalamus both prolyl endopeptidase and dipeptidyl peptidase IV activities were concomitantly increased. The hypothalamic 5 hydroxyindole acetic acid levels were also increased. Changes in hormone secretion, peptidase activity and monoamine turnover had returned to baseline levels by 92 hr. Our results indicate that morphine withdrawal and the associated stress produce alterations in anterior pituitary thyrotropin and growth hormone secretion. Concomitant increases in hypothalamic prolyl endopeptidase and dipeptidyl peptidase activities may contribute to these changes.

Serotonergic regulation of renin and prolactin secretion

Drugs that, directly or indirectly produce activation of serotonin (5-HT) receptors increase plasma concentrations of both prolactin and renin. The serotonergic regulation of prolactin and renin secretion share several common characteristics. Serotonergic neurons originating in the dorsal raphe and terminating in the hypothalamus stimulate the secretion of both prolactin and renin. Destruction of cells in the hypothalamic paraventricular nucleus (PVN) inhibits both the prolactin and renin responses to 5-HT agonists and 5-HT-releasing drugs. Activation of 5-HT2 receptors increases the secretion of both prolactin and renin, while activation of other 5-HT receptor subtypes has differential effects on these hormones. However, there are also differences between the serotonergic mechanisms that regulate the secretion of prolactin and renin. Activation of 5-HT1A receptors increases the secretion of prolactin but not of renin. In addition, activation of peripheral 5-HT2 receptors stimulates the secretion of renin, while activation of peripheral 5-HT3 receptors increases plasma levels of prolactin but not renin. In humans, the effect of 5-HT-releasing drugs and 5-HT agonists on plasma prolactin concentrations has been studied to a greater extent than effects on most other hormones. In contrast, the renin response to 5-HT agonists and 5-HT releasers has not been well characterized in humans. Because of the important role of the renin-angiotensin system in cardiovascular regulation, studies on the serotonergic regulation of renin release in humans could increase our understanding of cardiovascular disorders associated with altered serotonergic function. Examples include anxiety and consequences of cocaine abuse. In conclusion, comparing the serotonergic regulation of prolactin and renin secretion indicates similarities that might shed light on common brain mechanisms that regulate neuroendocrine function.

Serum prolactin levels and behavior in infants

This study examined the relationship between serum prolactin levels and behavior in infants and toddlers who experienced two potentially stressful experiences (developmental testing and venipuncture). Serum prolactin levels showed considerable consistency over a 3-month period (r = 0.64 between study entry and three months, p < 0.001, n = 50). There was also stability in having either a normal or a high value (> or = 25 ng/ml). Among children who had a normal value on initial testing, 97% also has a normal value after 3 months; 55% of those with initial high values continued to have high values (chi 2 = 19.26, p < 0.001). Children with high serum prolactin levels were more likely to be rated as unusually hesitant and unhappy during developmental testing. Overall, 53% of the children with serum prolactin levels > or = 25 ng/ml were considered abnormal in affect, compared to 20% of those with lower serum prolactin values (total n = 138, chi 2 = 13.56, p < 0.001). These results suggest that, even in early life, serum prolactin levels may reflect characteristic individual behavioral and neuroendocrine responses to stress.

Hypothalamic paraventricular, but not supraoptic neurons, mediate the serotonergic stimulation of oxytocin secretion

The purpose of the present studies was to determine whether cells in the hypothalamic paraventricular (PVN) or supraoptic (SON) nuclei mediate the serotonergic stimulation of oxytocin secretion. The serotonergic stimulus consisted of injection of the 5-HT-releasing drug p-chloroamphetamine (8 mg/kg, IP). The validity of this approach was verified by comparing this drug with another 5-HT releaser, d-fenfluramine (5 mg/kg, IP). Both 5-HT releasers increased plasma oxytocin concentration. Furthermore, the 5-HT uptake blocker fluoxetine (10 mg/kg, IP) blocked the effects of both p-chloroamphetamine and d-fenfluramine on plasma oxytocin concentrations, suggesting that both 5-HT releasers must be taken up through the 5-HT transporter into 5-HT nerve terminals to increase oxytocin secretion. In the lesion experiments, cells in the hypothalamic PVN or SON were destroyed by injection of the cell-selective neurotoxin ibotenic acid. The PVN lesions reduced basal levels and inhibited the effect of p-chloroamphetamine (8 mg/kg, IP) on plasma oxytocin concentration. In contrast, SON lesions did not alter basal oxytocin levels and did not reduce the oxytocin response to p-chloroamphetamine, suggesting that the SON is not involved in the serotonergic stimulation of oxytocin secretion. Site specificity of the PVN lesions was confirmed when injections of ibotenic acid into the hypothalamic dorsomedial nucleus (DMN), immediately caudal to the PVN, potentiated the oxytocin response to p-chloroamphetamine, suggesting that the DMN exerts an inhibitory influence on the secretion of oxytocin. Taken together, the data suggest that the serotonergic stimulation of oxytocin secretion involves PVN, but not SON, oxytocin neurons.

5-Hydroxytryptophan-stimulated prolactin levels in cafeteria diet fed rats: an in vivo evaluation of the central serotonergic tonus

Hyperphagia in rats fed a cafeteria diet might be related to the palatability of the diet or to diet-induced changes in central neurotransmitters regulating the feeding behavior. In this study the central serotonergic tonus in adult male Wistar rats was evaluated in vivo after 6 weeks of feeding a cafeteria diet by the prolactin response to the administration of 5-hydroxytryptophan (5HTP), the immediate serotonin precursor. Blood was taken just before, 30, 60 and 90 min after the ip injection of 50 mg/kg 5HTP for the determination of prolactin concentrations were comparable between cafeteria fed rats and control rats, fed normal laboratory chow (12.7 +/- 5.4 vs 7.7 +/- 4.5 ng/ml). The 5HTP-stimulated prolactin secretion in the cafeteria diet fed rats, determined by the peak value (95.8 +/- 17.2 vs 119.1 +/- 27.0 ng/ml) as well as by the integrated area under the curve (5478 +/- 774 vs 5916 +/- 2275 ng/ml. 90 min) was not significantly lower than in the control rats. In conclusion, our results did not show a significantly decreased 5HTP-induced prolactin release in cafeteria-fed rats, suggesting that a low hypothalamic serotonergic tonus is probably not involved in the overeating of this dietary-induced obesity model.

Effects of hyperprolactinemia on plasma glucose and prolactin in rats exposed to ether stress

The present study was designed to characterize the effect of chronic hyperprolactinemia on plasma glucose and prolactin (PRL) during ether stress in male and female rats. Wistar rats of both sexes were divided into a hyperprolactinemic group (bearing pituitary grafts) and a control group (sham operated). They were exposed to ether during 10 min and had blood samples taken immediately before, and 5, 15, 40, and 70 min after ether exposure and assayed for glucose and PRL concentrations. Ether stress induced significant increase in plasma PRL of control rats at 5 min (male 171%, female 161%; P < 0.01), but only a mild PRL increase occurred in grafted rats (male 66%, female 62%; P < 0.05). Control male rats sustained elevated plasma PRL longer than females. There was a marked elevation of glucose levels at 5 min (P < 0.01) which peaked at 15 min in all groups. Grafted rats had glucose levels significantly above control at baseline (female 15%, P < 0.05) and at 40 min (male 25%, female 29%; P < 0.05). It is concluded that ether-induced PRL release produced a rapid and transient response, more intense in males than in females and which was impaired by previous hyperprolactinemia (pituitary grafts). There was an acute hyperglycemic response, plasma glucose being increased in grafted rats, supporting the hypothesis of a hyperglycemic effect of PRL.

Clomipramine-induced sleep disturbance does not impair its prolactin-releasing action

The present study was undertaken to examine the role of sleep disturbance, induced by clomipramine administration, on the secretory rate of prolactin (PRL) in addition to the direct drug effect. Two groups of supine subjects were studied under placebo-controlled conditions, one during the night, when sleeping (n = 7) and the other at daytime, when awake (n = 6). Each subject received a single 50 mg dose of clomipramine given orally 2 hours before blood collection. Plasma PRL concentrations were analysed at 10 min intervals and underlying secretory rates calculated by a deconvolution procedure. For both experiments the drug intake led to significant increases in PRL secretion, acting preferentially on tonic secretion as pulse amplitude and frequency did not differ significantly from corresponding control values. During the night clomipramine ingestion altered the complete sleep architecture in that it suppressed REM sleep and the sleep cycles and induced increased wakefulness. As the relative increase in PRL secretion expressed as a percentage of the mean did not significantly differ between the night and day time studies (46 +/- 19% vs 34 +/- 10%), it can be concluded that the observed sleep disturbance did not interfere with the drug action per se. The presence of REM sleep was shown not to be a determining factor either for secretory pulse amplitude and frequency, as, for both, mean nocturnal values were similar with and without prior clomipramine ingestion.

Osmotic inhibition of prolactin secretion in rats

Chronic hyponatremia is known to cause inhibition of pituitary vasopressin (AVP) and oxytocin (OT) secretion in response to most physiological stimuli, as well as a marked inhibition of synthesis of these peptides. Because many studies have implicated neurohypophyseal peptides in the regulation of pituitary prolactin (PRL) secretion, we investigated the effects of chronic hyponatremia on basal and stimulus-induced PRL secretion in rats. Hyponatremia was induced by subcutaneous infusion of 1-deamino-[8-D-arginine]-vasopressin (dDAVP) (5 ng/h) to rats fed a nutritionally balanced liquid diet, and plasma [Na+] was maintained < or = 115 mmol/l for 10-12 days. After this period, hyponatremic rats and normonatremic controls fed the same diet without dDAVP were subjected to one of the following stimuli known to stimulate PRL release in rats: 3 min exposure to ether, hemorrhage (20 ml/kg), intravenous injection of 5-hydroxytryptophane (5-HTP, 10 mg/kg), or intravenous injection of estradiol (5 micrograms/kg). A baseline blood sample was collected before each stimulus, and 3-6 additional blood samples were collected at selected intervals after the stimulus. Baseline levels of plasma PRL were not different between normonatremic and hyponatremic rats. However, PRL responses induced by either or estradiol, but not those induced by hemorrhage or 5-HTP, were very significantly blunted in the chronically hyponatremic rats. Plasma AVP and OT responses were measured as an index of magnocellular secretion, but did not correlate with the PRL responses for any of the stimuli tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Repeated cocaine exposure inhibits the adrenocorticotropic hormone response to the serotonin releaser d-fenfluramine and the 5-HT1A agonist, 8-OH-DPAT

The influence of cocaine exposure on serotonergic neurons and postsynaptic 5-HT1A receptor-mediated responses was evaluated by measuring neuroendocrine responses to a serotonin (5-HT) releaser or a 5-HT1A agonist. Male rats received cocaine (15 mg/kg, i.p.) or saline twice daily for 7 days. Forty-two hr after the final cocaine injection, the 5-HT releaser d-fenfluramine (0, 0.2, 0.6, 2, or 5 mg/kg, i.p.) or the 5-HT1A agonist, 8-OH-DPAT (0, 10, 50, 200 or 500 micrograms/kg, s.c.) were administered. Blood samples were then collected for analysis of plasma ACTH, prolactin, and renin concentrations. The ACTH responses to d-fenfluramine and 8-OH-DPAT were inhibited in cocaine pretreated rats. However, the prolactin responses to d-fenfluramine and 8-OH-DPAT were not significantly modified by cocaine exposure. Additionally, the renin response to d-fenfluramine was unaltered by repeated cocaine administration, while 8-OH-DPAT did not alter renin secretion in either pretreatment group. In contrast to published reports which show that cocaine exposure produces supersensitive 5-HT2A and/or 5-HT2C receptor-mediated responses, the present data suggest that repeated cocaine exposure produces subsensitivity to at least some postsynaptic 5-HT1A receptors. Cocaine-induced deficits in the ACTH response to 5-HT releasers may reflect 5-HT1A receptor subsensitivity, but presynaptic deficits cannot be excluded. Examination of the ACTH response to 5-HT1A agonists may represent a valuable approach to determine deficits in 5-HT function in human cocaine abusers.

c-Fos proto-oncogene activity induced by mating in the preoptic area, hypothalamus and amygdala in the female rat: role of afferent input via the pelvic nerve

In order to identify brain areas which receive afferent genitosensory input important for mating-induced prolactin release, we compared numbers of Fos-immunoreactive (Fos-IR) cells in brains of intact estrous females 1 h after differential mating stimulation. Numbers of Fos-IR cells were approximately 3-fold higher in the preoptic area (POA), medial amygdala (mAMYG) and bed nucleus of the stria terminalis (BNST) when females received intromissions (I) from males than when they received mounts-without-intromission (M) or were taken directly from their home cage. In the ventrolateral portion of the ventromedial nucleus (VL-VMN), the paraventricular nucleus (PVN) of the hypothalamus and the midbrain central tegmental field (CTF) numbers of Fos-IR cells were significantly higher than home cage levels in groups of females exposed to males regardless of type of mating stimulation received. Bilateral transection of the pelvic nerve eliminated the increases in Fos-IR in POA and mAMYG which occurred in sham-transected females in these areas after intromissions from males. These data demonstrate that afferent input via the pelvic nerve activates cell groups within the POA, mAMYG and BNST and suggests that these areas may be involved in initiation of mating-induced prolactin surges.

Median eminence-afferent vasoactive intestinal peptide (VIP) neurons in the hypothalamus: localization by simultaneous tract tracing and immunocytochemistry

Retrograde tract tracing and immunocytochemistry were used to investigate the CNS source of the VIP that is present in high concentrations in the hypophysial portal blood and has been shown to have a stimulatory effect on pituitary prolactin secretion. Fluoro-gold (FG), which enters the CNS through areas devoid of the blood-brain barrier, such as median eminence, was injected peripherally. Brain sections from FG-treated animals were immunostained for VIP. A small population of VIP-containing cell bodies in the parvocellular and periventricular parts of the paraventricular nucleus (PVN) was also labeled with FG. Vasoactive intestinal peptide-immunoreactive perikarya not labeled with FG were also observed in the PVN, as well as FG-labeled cells that did not contain VIP. The results suggest that some VIP-producing neurons in the PVN project to the median eminence and are, therefore, functionally related to pituitary regulation; the function of other VIP neurons in the PVN is unknown.

Hypothalamic involvement in stress-induced hypocalcemia in rats

Although hormonal regulation of blood calcium homeostasis has been intensively investigated in the peripheral organs, the involvement of the central nervous system in calcium regulation is still poorly understood. In the present study, we found that (1) bilateral lesions of the ventromedial nucleus of the hypothalamus (VMH), but not those of the paraventricular hypothalamic nucleus or the lateral hypothalamic area, eliminated immobilization (IMB)-induced hypocalcemia, and (2) electrical stimulation of the VMH decreased the blood calcium level. The results suggest that the VMH has a hypocalcemic function and plays a role in IMB-induced hypocalcemia.

Excitotoxin paraventricular nucleus lesions: stress and endocrine reactivity and oxytocin mRNA levels

Electrolytic lesion of the paraventricular nucleus (PVN) of the hypothalamus blocks the tachycardia response to stress. The current study examined the effects of chemical lesion of PVN parvocellular neurons on the cardiovascular and endocrine responses to stress and on the content of hypothalamic oxytocin (OT) mRNA levels. Acute footshock stress increased heart rate in both ibotenic acid lesion and control groups of animals; however, the tachycardia was significantly lower in animals with a PVN lesion than the controls. Lesion of the PVN also attenuated the increase in plasma OT induced by stress, 4-fold in the lesion group versus 20-fold for the controls. There was not a generalized decrease in hormonal responsiveness since the OT response to an osmotic challenge was exaggerated in the lesion group. There was no difference between the groups in the arterial pressure and vasopressin responses to acute stress. Neurotoxin lesions of the PVN also resulted in significant depletions of VP and OT in all levels of the spinal cord and decreased OT levels in the dorsal brainstem. Ibotenic acid lesions of the PVN resulted in no significant changes in OT mRNA in the PVN, SON and PP. In addition, the 48-h dehydration resulted in a significant increase in plasma OT and OT mRNA in the PVN. These data indicate that the parvocellular neurons of the PVN play a role in integration of cardiovascular and endocrine responses to both stressful and osmotic stimuli and provide further evidence that parvocellular OT and VP neurons project to the brainstem and spinal cord.

Decreased 5-HT-mediated prolactin release in major depression

The prolactin response to intravenous clomipramine, a 5-HT uptake inhibitor, was significantly attenuated in 12 patients with major depression. In contrast, in a further 12 depressed patients, the PRL responses to thyrotropin-releasing hormone, which acts directly on the pituitary to release PRL, were not reduced. These findings suggest that the reduction in 5-HT-mediated PRL release seen in depressed patients is due to an impairment of brain 5-HT function rather than a pituitary abnormality.

Serotonergic function during lithium augmentation of refractory depression

Serotonergic mechanisms have been implicated in the pathophysiology of depression and in the neuropharmacology of antidepressant treatment. One measure of central serotonergic function is the prolactin (PRL) response to IV L-tryptophan (L-TRP). We used the L-TRP test to assess the role of serotonin in the mechanism of action of lithium augmentation in refractory major depression. Twenty-six patients with antidepressant-refractory major depression each received three L-TRP tests (after 2 weeks of placebo, after 4 weeks of active primary antidepressant, and after 1 week of lithium augmentation). Primary antidepressant treatment did not increase the PRL response, but lithium augmentation resulted in a statistically significant increase in PRL response as compared to both placebo pretreatment (P less than 0.04) and antidepressant treatment alone (P less than 0.025). This study supports a role for serotonergic mechanisms in the action of lithium augmentation.

The integrity of the ventral noradrenergic bundle (VNAB) is not necessary for a normal neuroendocrine stress response

The paraventricular nucleus of the hypothalamus (PVN) receives a dense noradrenergic innervation originating in the caudal brainstem and conveyed by the ventral noradrenergic bundle (VNAB). To evaluate the importance of this pathway, rats were bilaterally injected with 6-hydroxydopamine (6-OHDA) into the VNAB, posterior to the locus coeruleus to avoid the lesion of the dorsal noradrenergic system. These lesions reduced noradrenaline (NA) levels in the PVN by 60% without any significant change of NA levels in the cortex or of dopamine or serotonin in any part of the brain, indicating the specificity of the lesion. After one or three weeks, the neuroendocrine responses to stress were monitored. The secretion of adrenocorticotropic hormone (ACTH), corticosterone and prolactin were studied under basal conditions and after exposure to a novel environment. The activity of the sympathetic nervous system (SNS) was studied in catheterized rats. Plasma catecholamines were measured in basal conditions, and in response to gentle handling or exposure to footshocks. Apart from a transient increase of the adrenocortical axis activity which disappeared 3 weeks after surgery, the lesion did not change either basal levels of the hormones measured or their response to stress, indicating that the noradrenergic input to the PVN conveyed by the VNAB is not necessary for a normal neuroendocrine stress response to occur.

The physiology and mechanisms of the stress-induced changes in prolactin secretion in the rat

It is well known that stress in a number of forms induces the secretion of prolactin (PRL) in a number of species. What is not well known is that under certain conditions stress will also induce a decrease in PRL secretion. The conditions whereby stress decreases PRL are those where PRL secretion is elevated such as during the proestrous afternoon surge and during the nocturnal surge of pseudopregnancy. The physiologic significance of the stress-induced increase of PRL is suggested to be important in maintaining the competence of the immune system. The significance of the stress-induced decrease of PRL does not appear to have a major consequence on the physiology of reproduction in the rat and it is suggested that future studies be directed towards its significance in the immune system. The literature is reviewed dealing with the regulation of PRL secretion with emphasis on the factors that generate PRL surges in the rat. In addition the mechanism(s) of the stress-induced increase and decrease is (are) also examined. A hypothesis is presented suggesting an interaction between tuberoinfundibular dopamine secretion and a hypothalamic prolactin releasing factor in the generation of PRL surges and the differential effects of stress on PRL secretion.

Clinical studies of 5-HT function using i.v. L-tryptophan

1. Preclinical studies reveal that long-term treatment with antidepressant drugs induces significant changes in serotonergic (5-HT) receptor sensitivity. Similarly, clinical studies suggest that brain 5-HT function is abnormal in depression. Of the available methodologies for conducting such clinical studies, the pharmacological challenge strategy has proven particularly useful. 2. I.v. L-TRP has emerged as the most frequently used challenge agent in diagnostic and neuropsychopharmacological studies of 5-HT function. I.v. L-TRP increases serum prolactin (PRL) in humans, probably via 5-HT mechanisms. Under carefully standardized conditions, this PRL response to L-TRP appears to be a reasonably sensitive and valid measure of net 5-HT function. 3. The PRL response to L-TRP is blunted in depressed patients compared with healthy controls. Blunting has not been observed in panic disorder, obsessive compulsive disorder, or schizophrenia, although preliminary findings suggest it may occur in bulimia. 4. The PRL response to L-TRP is enhanced by certain classes of thymoleptic drugs (TCAs, MAOIs, 5-HT reuptake inhibitors, lithium) in a differentially time-dependent fashion. So-called "atypical" antidepressants (trazodone, mianserin) and benzodiazepines have no effect. Such findings are generally consistent with preclinical electrophysiological findings. 5. These clinical studies of the PRL response to L-TRP, in conjunction with emerging evidence that experimentally reduced plasma TRP can reverse the therapeutic effects of some antidepressants, suggest that antidepressant drug action may be more accurately conceptualized as 5-HT dependent rather than 5-HT enhancing. The availability of more selective 5-HT-active drugs promises to further clarify 5-HT mechanisms of neuropsychiatric disease and drug action at the clinical level.

Neuroendocrine aspects of the serotonergic hypothesis of depression

This review examines the role of serotonin (5-HT) in depression. Dysfunction of serotonergic neurons has been implicated as one of the causes of endogenous depression. Since serotonergic neurons innervate the hypothalamus and these neurons send collaterals to several other brain areas, it is possible that hypothalamic sites which control hormone secretion receive the same serotonergic afferents that innervate other limbic areas in the brain. Several investigators have devised neuroendocrine challenge tests measuring the effect of 5-HT agonists on plasma cortisol and prolactin in depressed patients. These tests help to identify dysfunctional 5-HT neurons, and are a "window into the brain." The secretion of cortisol and prolactin is increased predominantly by 5-HT1 receptors. However, changes in 5-HT2 receptors have also been implicated in depression. Results from our laboratory and by others suggest that brain serotonergic neurons stimulate renin and vasopressin secretion by activation of 5-HT2 receptors. Therefore, the renin and vasopressin response to 5-HT agonists should be included in neuroendocrine tests of serotonergic function in affective disorders. Since antidepressants produce a decrease in the density of 5-HT2 receptors, renin and vasopressin could be used to evaluate the antidepressant potential of new drugs.

Growth hormone secretion and ultradian rhythms in growth-retarded rats with dorsomedial hypothalamic lesions

Rats with lesions of the dorsomedial nucleus of the hypothalamus (DMN-L) are hypophagic and have reduced linear growth and body weight, but normal body composition. Male Sprague-Dawley rats (170-190 g) housed individually under a 12:12 L:D schedule with lights out at 1430 hr received jugular cannulas, and on return to precannulation body weight (4.2 +/- 0.6 days), they received bilateral electrolytic DMN-L or sham-operations (SHAM). Rats with DMN-L (n = 8) were hypophagic postsurgery and weight less (p less than 0.05) than SHAM at six days postlesion surgery. The difference in body weight between the two groups continued to expand over the next four weeks. Six days postsurgery, the rats were bled (RBC's returned in 10% BSA-saline) every 15 minutes between 0600-1215 hr and growth hormone (GH) subsequently assayed. The total GH secretion, as computed from the area under each rat's ultradian pattern, was similar in both groups (DMN-L versus SHAM, 2952.2 +/- 346.5 versus 2950.4 +/- 337.5). Using the PULSAR computer program, the baseline secretion (12.2 +/- 4.0 versus 11.8 +/- 2.7 ng/ml), total number of peaks (2.4 +/- 0.4 versus 2.4 +/- 0.2), and interpeak interval (2.8 +/- 0.5 versus 2.7 +/- 0.4, hr) were not significantly different between the DMN-L and SHAM rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Chromaffin cell proliferation in the adult rat adrenal medulla

Epinephrine and norepinephrine-containing chromaffin cells proliferate in the adrenal glands of normal adult rats throughout life. Moreover, their rate of proliferation is markedly increased by short-term administration of reserpine, one of many agents which in long-term experiments are associated with the development of adrenal medullary tumors. Current data suggest that chromaffin cell proliferation in the adult rat adrenal is mediated by the interaction of neurogenic and hormonal signals. Reserpine is known to directly deplete catecholamine stores, and to reflexively increase the activity of the splanchnic nerve endings innervating the adrenal medulla to stimulate both secretion and synthesis of catecholamines and other secretory granule constituents. Its effect on chromaffin cell proliferation suggests that the same signals may regulate chromaffin cell number to meet physiological needs. The reserpine model might shed light on signal transduction mechanisms which normally promote or prevent proliferation of chromaffin cells and of other neuroendocrine cells during development or in adult life, and on ways in which such mechanisms are altered in the course of the development and progression of tumors. It also suggests the possibility that chromaffin cells might be propagated in vitro for use in basic biological studies or in transplants for the treatment of Parkinson's disease.

Cardiovascular reactivity and plasma prolactin response to mental stress in normals and hypertensives

Central dopaminergic activity (CDA) may be involved in blood pressure control as a negative modulator of sympathetic outflow. In this study the plasma PRL changes produced by mental stress (a colour-word conflict test, CWT) were investigated in normals (NT, n.15) and stable hypertensives (HT, n.16) and the PRL response, as a possible index of CDA was correlated to the cardiovascular and the plasma renin activity (PRA) responses as indexes of peripheral sympathetic outflow. A significant (p less than 0.05) slight decrease in mean values of PRL was observed in normals after the CWT but no change was found in hypertensives. No correlation was found between the PRL responses to CWT and the maximal mean arterial pressure changes or the PRA changes whether the groups were considered separately or together. These findings indicates that PRL does not appear to be a reliable index of the CDA involved in blood pressure control.

Dynamism of chemoarchitecture in the hypothalamic paraventricular nucleus

The hypothalamic paraventricular nucleus (PVN) has been implicated in a remarkable number of functions including control of pituitary-adrenocortical activity in response to stress, body fluid homeostasis, milk ejection reflex, prolactin secretion, thyroid hormone secretion, analgesia, food intake, gastrointestinal functions, cardiovascular functions, and control of pineal melatonin synthesis. Paraventricular neurons produce hormones of key importance in neuroendocrine regulation such as vasopressin (VP), oxytocin (OX), 41-residue corticotropin releasing factor (CRF), thyrotropin releasing hormone (TRH), somatostatin (SOM) and the putative prolactin releasing factor vasoactive intestinal polypeptide (VIP). Three recent advances pertinent to the organization of the PVN include: (1) the evidence that the structure of the PVN is compartmental in nature, topographically segregated cellular units seem to carry out different functions; (2) the discovery that paraventricular neurons are capable of expressing a multitude of neuromediators simultaneously, thus cellular units can be best specified by a certain combination of neuromediators; (3) evidence that the composition of the neuromediator "cocktail" in individual neurons is variable and depends on the physiological status of the animal. Hence, the PVN may be best considered as a dynamic mosaic of chemically specified subgroups of neurons. The flexibility of neurotransmitter status in paraventricular neurons may play a central role of a functional plasticity of fixed anatomical circuits.

Neuroendocrine significance of vasoactive intestinal polypeptide

The new data reported here, and available in the literature, are interpreted to indicate that acute release of PRL in stress is probably mediated by secretion of VIP and PHI arising from a subpopulation of paraventricular cells in the tuberoinfundibular system, and that this secretion is under serotonergic control, presumably by way of the raphe nuclear projection to the hypothalamus. The acute PRL response to suckling response is only partially under VIP/PHI control, and may be regulated by an as yet unidentified neural lobe hormone. In addition to the hypothalamic component of PRL regulation, there is a well-defined population of VIP cells within the pituitary, representing the only known example of VIP expression outside of nerve cells. This population of VIP cells is exquisitely responsive to thyroid status, and in common with the thyrotrope cell, is activated by hypothyroidism. Since VIP secretion is enhanced in the hypothyroid pituitary, and VIP release is stimulated by TRH, it is reasonable to postulate that paracrine VIP secretion may play a role in the reasonable to postulate that paracrine VIP secretion may play a role in the hyperprolactinemia prolactinemia that occurs in the hypothyroid human, although this is clearly not the case for the rat in whom hyperprolactinemia was not demonstrable. The role of VIP/PHI in human pituitary disease is unknown. We have been unable to identify any tumors that contain immunoreactive material using tissues prepared by standard methods. It may be that the demonstration of VIP/PHI is more demanding and will require better techniques for staining. The role of tuberoinfundibular VIP hypersecretion remains to be established but the evidence for stress-induced PRL hypersecretion in man encourages us to believe that at least some cases may be due to excessive hypothalamic activity. Additional potential neuroendocrine actions of VIP are in the secretomotor control of the ovary and thyroid, and in the regulation of somatostatin secretion and synthesis. In dispersed cell cultures (but not in whole hypothalamic slices from adult animals), VIP stimulates somatostatin secretion and independently stimulates the formation of somatostatin mRNA, an effect that can be duplicated in mixed cultures by treatment with forskolin, a postreceptor cAMP stimulator. In work carried out by Montminy and colleagues, the cAMP action has shown to be mediated by formation of a soluble protein that appears to activate the somatostatin gene promotor through interaction with a specific gene sequence.

The effect of bombesin on basal, alpha-methyl-p-tyrosine, haloperidol, morphine, bremazocine and stress-induced prolactin secretion

Intravenously administered bombesin lowered basal PRL levels in conscious male rats and prevented the morphine, bremazocine and stress-induced PRL secretion. The same dose of bombesin had no effect on PRL levels in alpha-methyl-p-tyrosine pretreated rats and did not affect haloperidol-stimulated PRL release. These results show that bombesin given intravenously acts as an inhibitor of PRL secretion and suggests that it does not act on the lactotrope itself but rather by an increase of the inhibitory dopaminergic tone.

Modulation of T-suppressor cell activity by central nervous system catecholamine depletion

This study extends our previous findings, which indicate that depletion of CNS catecholamines has a marked inhibitory effect on humoral immune responsiveness. These data show that depletion of CNS catecholamines by injection of 6-hydroxydopamine (6-OHDA) into the cisterna magna in conjunction with immunization enhances the activity of a population of splenic T-suppressor cells as evidenced by the transfer of these cells to normal recipients. Increased suppressor cell activity does not result solely from 6-OHDA treatment, but rather requires concomitant immunization. Further characterization shows that these suppressor cells are not antigen specific. Hypophysectomy abrogates the effects of 6-OHDA injection suggesting that catecholamine depletion modulates immune function via the release of pituitary hormones. Thus, depletion of CNS catecholamines impairs immune responsiveness by inducing enhanced T-suppressor cell activity, providing additional evidence of the involvement of the CNS in regulation of immune responsiveness.