Catecholamine turnover alterations in discrete areas of the median eminence of the 4- and 5-day cyclic rat

Hypothalamus as an endocrine organ

The endocrine hypothalamus constitutes those cells which project to the median eminence and secrete neurohormones into the hypophysial portal blood to act on cells of the anterior pituitary gland. The entire endocrine system is controlled by these peptides. In turn, the hypothalamic neuroendocrine cells are regulated by feedback signals from the endocrine glands and other circulating factors. The neuroendocrine cells are found in specific regions of the hypothalamus and are regulated by afferents from higher brain centers. Integrated function is clearly complex and the networks between and amongst the neuroendocrine cells allows fine control to achieve homeostasis. The entry of hormones and other factors into the brain, either via the cerebrospinal fluid or through fenestrated capillaries (in the basal hypothalamus) is important because it influences the extent to which feedback regulation may be imposed. Recent evidence of the passage of factors from the pars tuberalis and the median eminence casts a new layer in our understanding of neuroendocrine regulation. The function of neuroendocrine cells and the means by which pulsatile secretion is achieved is best understood for the close relationship between gonadotropin releasing hormone and luteinizing hormone, which is reviewed in detail. The secretion of other neurohormones is less rigid, so the relationship between hypothalamic secretion and the relevant pituitary hormones is more complex.

Direct actions of estradiol on the anterior pituitary gland are required for hypothalamus-dependent lactotrope proliferation and secretory surges of luteinizing hormone but not of prolactin in female rats

Estradiol induces surges of prolactin (PRL) and luteinizing hormone (LH) secretion as well as lactotrope proliferation in female rats. We examined whether these hypothalamus-dependent events require the direct action of estradiol on the anterior pituitary gland by selective blockade of its peripheral actions, using ICI182,780 (ICI), an antiestrogen that cannot cross the blood-brain barrier. Injection of ICI into ovariectomized rats, at a dose of 250 microg/day for 4 days, almost completely inhibited estradiol-induced growth of the uterus, proliferation of lactotropes as determined by bromodeoxyuridine incorporation, and afternoon surges of LH secretion. However, ICI only partially inhibited estradiol-induced surges of PRL secretion and had no effect on estradiol-induced tonic inhibition of LH secretion even at the highest dose of 1,000 microg/day. The inhibitory effects of ICI found at 250 microg/day were attributable to its selective peripheral, but not central actions since ICI did not alter hypothalamic expression of progesterone receptors, an estradiol-dependent brain process. Estradiol-induced increases in the number of progesterone receptor-immunoreactive cells in the hypothalamic ventromedial nucleus and the medial preoptic area were not inhibited by this dose of ICI but were inhibited by 500 microg/day tamoxifen, an antiestrogen that can cross the blood-brain barrier. Treatment of cycling female rats with 250 microg/day ICI beginning from diestrus day 2 was also effective in blocking estrous lactotrope proliferation and preovulatory surges of LH secretion but not PRL secretion. Finally, in ovariectomized estradiol-treated pup-deprived lactating rats, ICI did not affect suckling-induced PRL secretion but completely blocked lactotrope proliferation. These results suggest that a direct estradiol action on the anterior pituitary gland is required for lactotrope proliferation and the positive feedback action on LH secretion but not for the secretory surges of PRL or for negative feedback.

Cyclic changes in the release of norepinephrine and dopamine in the medial basal hypothalamus: effects of aging

Push-pull perfusion and HPLC were used to measure the release of norepinephrine (NE) and dopamine (DA) in the medial basal hypothalamus of young (4-5 months old), middle-aged (8-10 months old), and old (22-24 months old) rats. In the young animals, the afternoon of proestrus was characterized by a gradual increase in NE release and a simultaneous gradual decrease in DA release. The peak in NE release and the nadir in DA release occurred at about the time when the proestrous surges in serum LH and PRL are known to occur. No changes in NE and DA releases occurred in the afternoon of diestrus when serum LH and PRL are known to remain stable. In the middle-aged proestrous animals, the patterns of NE and DA releases were similar to those in the young proestrous animals, but the peak in NE release was attenuated and did not reach statistical significance. This corresponded with the reported attenuation in the LH surge in middle age. In the old persistently diestrous animals, NE and DA were released at constant rates, which correlated with the well-documented constant levels of serum LH and PRL in old age. These data provide an explanation for the simultaneous proestrous surges of LH and PRL and lead us to conclude that NE plays a facilitatory role in the LH surge, while DA, through its inhibitory action, regulates the PRL surge.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic and age-related changes in norepinephrine concentrations in the medial preoptic area and arcuate nucleus

High-performance liquid chromatography with electrochemical detection (HPLC-EC) and Palkovits' microdissection technique were used to measure norepinephrine (NE) concentrations in the medial preoptic area (MPA) and arcuate nucleus (AN) during various stages of the estrous cycle. NE was measured seven times at 2-h intervals between 1000 h and 2200 h on the days of proestrus and diestrus in young (4-month-old) rats and four times at 2-h intervals between 1400 h and 2000 h in old (20-22-month-old) persistently diestrous rats. On the day of proestrus in young animals, NE increased progressively from low levels at 1000 h to peak levels at 2000 h, followed by a sharp decline at 2200 h. In contrast, no changes in NE occurred on the day of diestrus. Unlike the young proestrous rats, but similar to the young diestrous rats, no changes in NE concentrations either in the MPA or in the AN occurred in the old persistently diestrous rats. These data demonstrate that NE concentrations in the MPA and AN change during the estrous cycle. We believe the increase in NE on the afternoon of proestrus is related to the surge in serum luteinizing hormone (LH) that occurs simultaneously in this stage of the estrous cycle. The lack of change in NE concentrations in the young diestrous and persistently diestrous old animals is consistent with the well-established absence of changes in serum LH in these animals.

Rat brain hypothalamic and hippocampal monoamine and hippocampal beta-adrenergic receptor changes during pregnancy

The concentration of noradrenaline (NA), dopamine (DA), serotonin (5-HT), and their metabolites was measured in hypothalamic and hippocampal brain tissue obtained from non-pregnant, 15- or 20-day pregnant and 4-day postpartum rats. At 20 days of pregnancy, hypothalamic NA and DA concentrations were significantly decreased and their turnover increased relative to postpartum and estrous values, respectively. Hippocampal 3-methoxy-4-hydroxy-phenylglycol (MHPG) levels were significantly decreased at 15 days of pregnancy and 4 days postpartum compared to estrous and 20-day pregnant levels and the MHPG/NA ratio was significantly reduced at 4 days postpartum relative to the estrous value. Hippocampal 5-HT and 5-hydroxyindole-3-acetic acid (5-HIAA) levels were significantly decreased at 15 days of pregnancy while 5-HIAA levels and the 5-HIAA/5-HT ratio were significantly decreased at 20 days of pregnancy. Hippocampal beta-adrenergic receptor density was significantly lower at 4 days postpartum than at 15 days of pregnancy. A positive correlation was observed between plasma progesterone and hippocampal beta-adrenoceptor Kd values, suggesting a possible causal relationship between these two variables. The monoamine and beta-adrenoceptor changes which occur during pregnancy may be an important contributing factor in determining the mood changes which occur during pregnancy and postpartum.

Estrogen effects on the tuberoinfundibular dopaminergic system in the female rat brain

Estrogen effects on tyrosine hydroxylase (TH), monoamine oxidase types A and B (MAO), and dopamine (DA) in microdissected regions of the hypothalamus, preoptic area and substantia nigra (SNR) of the female rat brain were investigated. Ovariectomized (OVX) young adult female rats were implanted with single silastic capsules containing 100% estradiol valerate (EV). Control rats received empty silastic capsules. Two weeks following capsule insertion, EV decreased TH activity and DA concentration in the arcuate nucleus (AN) while no significant changes in TH activity or DA concentration were observed in the SNR, ventromedial nucleus (VMN), suprachiasmatic nucleus, paraventricular nucleus, medial preoptic nucleus, or the periventricular preoptic nucleus. Although estrogen suppressed TH and DA in the AN, 2 weeks following removal of the estrogen containing capsules, TH activity and DA concentration were restored to control (OVX) levels. Suppression of MAO activity occurred in both the AN and the VMN of rats implanted with EV capsules and returned to OVX levels following the removal of the estradiol load. These results revealed that estrogen effects on TH and MAO activities and DA concentration in the midbrain are region specific and reversible; and that among the dopaminergic systems studied, estrogen effects on TH and DA are confined to the tuberoinfundibular dopaminergic system (TIDAS). Furthermore, these results support our hypothesis that estrogen is a key regulator of DA function in the TIDAS via effects on TH. The importance of these findings to the control of gonadotropin secretion and reproductive cyclicity is discussed.

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.

Metoclopramide effect on serum prolactin LH and FSH in patients with polycystic ovary syndrome

In order to investigate the DA activity in polycystic ovary syndrome (PCOS) we studied the response of LH, FSH and PRL to a dopamine receptor antagonist metoclopramide (MCP-10 mg iv) in 12 PCO subjects (7 with normal and 5 with elevated levels of prolactin). The prolactin and LH responses to metoclopramide were compared to those obtained in 6 normal cycling women. Although a significant increase in PRL levels was documented after MCP administration in all PCO patients and normal cycling women (p less than 0.01), the highest increment in PRL levels was observed in normoprolactinemic PCO subjects. In contrast a blunted PRL response was observed in hyperprolactinemic PCO patients. There was a negative correlation between basal PRL levels and the maximum net increase in PRL after MCP. In both groups of PCO subjects MCP administration caused initial decrease in LH levels followed by an increase after 4 h. In hyperprolactinemic PCO patients this observed MCP effect on LH was more pronounced and significantly different in comparison with normoprolactinemic PCO patients (p less than 0.01). MCP administration did not cause significant acute alterations in LH levels in normal cycling women and no significant FSH changes in either PCO or control subjects. A relative dopamine deficiency might cause hypersecretion of PRL and LH in patients with PCOS and hyperprolactinemia.

Regional patterning of hormones in the female rat anterior pituitary: disproportionate changes over the estrous cycle

The present study addressed the possibility that regional differences exist in the typical patterning of anterior pituitary hormones seen over the estrous cycle. The results show that LH in the rostral area of the pituitary, significantly higher than in other regions on diestrus, rises markedly on proestrus before falling late on that afternoon to concentrations comparable to other areas. Smaller cyclic changes were seen for lateral and caudo-central areas. The pattern of regional FSH concentrations on diestrus, while similar to LH, does not show the proestrus fall in the rostral region. Prolactin declines from a high on diestrus I to a low on late proestrus, but no regional disparities are present. TSH shows distinct regional differences and a fall in concentrations that coincides with the reported rise in serum estradiol. These findings indicate that the preovulatory surge of LH may represent a regionally disproportionate event, possibly involving inequalities in the relative amounts of gonadotropin-releasing hormone reaching various areas of the pituitary or differences in the ability of this releasing hormone to trigger LH release.

Acute effects of L-dopa and bromocriptine on serum PRL, LH and FSH levels in patients with hyperprolactinemic and normoprolactinemic polycystic ovary syndrome

We have investigated the importance of the dopaminergic control of gonadotropin secretion by studying LH, FSH and PRL responses to L-dopa and bromocriptine in patients with polycystic ovary syndrome (PCOS). Both L-dopa and bromocriptine administration were followed by a statistically significant decrease in LH in the hyperprolactinemic PCO patients (compared to the normoprolactinemic subgroup - p less than 0.01 and control group - p less than 0.05); the decline was proportional to the basal level of LH. A significant positive correlation between basal LH levels and maximum net decrease of LH was observed after administration of both agents (p less than 0.01). Although both subgroups of PCO patients showed a similar decrease in PRL levels it was statistically significant only in the normoprolactinemic patients (p less than 0.01). Prolactin sensitivity to the inhibitory effect of bromocriptine and L-dopa showed a significant correlation with the basal PRL level (p less than 0.01). The response of serum FSH was variable and not significant. These results suggest that a reduction of an inhibitory influence of hypothalamic dopamine might be a cause of inappropriately elevated LH and PRL levels found in patients with polycystic ovary syndrome and hyperprolactinemia.

Hypothalamic monoamines and their catabolites in relation to the estradiol-induced luteinizing hormone surge

Monoamines and non-conjugated catabolites (serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA), 3,4-dihydroxyphenyl-acetic acid (DOPAC), homovanillic acid (HVA), 4-hydroxy-3-methoxyphenylethyleneglycol (MHPG), norepinephrine (NE), and dopamine (DA] were measured in the medial basal hypothalamus (MBH) and preoptic area (POA) of ovariectomized (OVX) and OVX estradiol (E2)-treated rats using high-performance liquid chromatography with electrochemical detection. These E2 treatments were sufficient to induce an LH surge. The use of MHPG/NE ratios as estimates of NE release was validated in the rat hypothalamus by the major decreases of MHPG after injection of the alpha 2-adrenergic agonist, clonidine, and by MHPG increases after the alpha 2-antagonist, yohimbine. The ratio, MHPG/NE, decreased between morning and afternoon in the MBH but not in the POA; there were no differences between OVX and E2-treated rats. Previous studies using a variety of methods indicate that NE turnover increases during LH surges. The present data suggest that unconjugated MHPG is not a sensitive measure of NE release in the rat hypothalamus, but can detect the large changes produced by stimulating or inhibiting the alpha 2-adrenergic autoreceptor. The ratios of DOPAC/DA and 5-HIAA/5-HT in the MBH decreased consistently between morning and afternoon in OVX rats, with or without E2 treatment. This suggests that the release of DA and 5-HT decreases during the day regardless of steroidal milieu.

Effects of different dopamine agonists and antagonists on post-menopausal hot flushes

The dopaminergic system seems to be involved in both pulsatile luteinizing hormone (LH) secretion and hot flushes in post-menopausal women. With the aim of further clarifying its role, the effectiveness of dopaminergic and antidopaminergic drugs in the treatment of hot flushes was studied. Self-assessed scores for vasomotor symptoms were evaluated in 5 groups of 15 patients treated for 20 days with one of the following agents: placebo; the dopamine receptor agonist, bromocriptine; the indirect dopaminergic agent, Liposom; the antidopaminergic drug, veralipride or the peripheral antidopaminergic agent, domperidone. All of these treatment regimens were effective in alleviating hot flushes, but the pharmacological agents proved to be more effective than the placebo. A direct dopaminergic action is hypothesized in the case of bromocriptine and Liposom, while the antidopaminergic drugs might act through different indirect mechanisms such as the short-loop feedback exerted by hyperprolactinaemia on tuberoinfundibular dopamine (TIDA) neurons with a secondary dopamine-like activity, or stimulation of the opioid system.

The influence of gonadal steroids on the dopamine inhibitory effect on gonadotropin release in men

The aim of this study was to ascertain whether an interrelationship exists between gonadal steroids and the inhibition of gonadotropin secretion by dopamine. The effect of dopamine infusion (4 micrograms/kg/minute intravenously) on gonadotropin plasma levels in four castrated men (18 to 23 years of age) and in four age-matched normal men was studied. Normal subjects were studied before and after treatment with a specific nonsteroidal estrogen receptor antagonist, CC. LH plasma levels in normal subjects receiving CC had a maximum decrease percentage and a net decrease significantly greater (P less than 0.005 and P less than 0.012, respectively) than those before CC treatment. In castrated subjects the maximum decrease percentage was significantly greater (P less than 0.005) than in control subjects, but it did not show any difference from that of normal subjects receiving CC. In none of the group were significant changes in FSH concentration observed. The findings suggest that whenever there is a gonadal steroid deficiency, dopamine infusion causes an increased sensitivity to LH inhibition. This may be due to a lower endogenous dopaminergic influence on LH secretion.

Estradiol increases dopamine turnover in intermediate and posterior pituitary lobes of ovariectomized rats

Dopamine levels were specifically increased in the intermediate lobe of the pituitary gland of ovariectomized rats following acute 17-beta-estradiol administration. Estradiol treatment increased the dopamine turnover rate 10-fold in the intermediate lobe and 2-fold in the posterior lobe of ovariectomized rats. In contrast, estrogen treatment had no effect on the endogenous levels or the turnover rate of norepinephrine in these tissues. Our results suggest that estrogens can selectively modulate dopamine metabolism in the posterior and intermediate lobes of the rat pituitary gland.

Dopamine-stimulated adenylate cyclase in hypothalamus: influence of estrous cycle in female and castration in male rats

The activity of dopamine- and norepinephrine-stimulated adenylate cyclase in hypothalamus and amygdala was studied during the estrous cycle of the female and following castration of the male rat. In the medial hypothalamus but not in the anterior hypothalamus or amygdala, stimulation of adenylate cyclase by dopamine was enhanced during proestrus and following castration. There were no changes in norepinephrine-stimulated adenylate cyclase in medial hypothalamus. Thus, an interaction may exist between gonadal hormones and dopamine receptors linked to adenylate cyclase in medial hypothalamus. The rapidity of the changes in receptor sensitivity suggests that this interaction plays an important role in physiological regulation.

Effects of manipulating catecholamines on the incidence of the preovulatory surge of of luteinizing hormone and ovulation in the rat: evidence for a necessary involvement of hypothalamic adrenaline in the normal or 'midnight' surge

The preovulatory surge of luteinizing hormone reaches a maximum at 18.00 h on the day of pro-oestrus in female rats maintained with regular lighting from 06.00 to 20.00 h. This surge is initiated by a discharge of luteinizing hormone-releasing hormone into hypophysial portal blood. In this study, drugs which affect catecholamine-mediated neurotransmission were administered on the day of pro-oestrus and the effects on serum concentrations of luteinizing hormone and on subsequent ovulation were observed. alpha-Methyl-p-tyrosine, diethyldithiocarbamate and SKF 64139 inhibit catecholamine synthesis at the level of tyrosine hydroxylase, dopamine beta-hydroxylase and phenylethanolamine N-methyltransferase, respectively. Although alpha-methyl-p-tyrosine suppressed ovulation, it had a negligible effect on the incidence of the preovulatory surge. In contrast, the various treatments with diethyldithiocarbamate and SKF 64139 resulted in a minimal occurrence of the 18.00 h surge; at relatively low doses, however, these drugs frequently elicited a surge at 22.00 or 24.00 h which invariably resulted in ovulation. The failure of the surge after diethyldithiocarbamate or SKF 64139 was not associated with a loss of pituitary sensitivity to luteinizing hormone-releasing hormone. In terms of the hypothalamic concentration of dopamine, noradrenaline, adrenaline and 5-hydroxytryptamine at 18.00 h on pro-oestrus, the only common effect of diethyldithiocarbamate and SKF 64139, given in a dose which blocks the surge, was a severe depletion of adrenaline; alpha-methyl-p-tyrosine failed to produce this effect despite inducing a marked depression of dopamine and a moderate loss of noradrenaline. Neither the increase in hypothalamic dopamine after diethyldithiocarbamate, nor the alpha 2 receptor blocking properties of SKF 64139 appear to be relevant in this context since injections of L-dopa or piperoxane, an alpha 2 receptor antagonist, were without effect on the surge or ovulation. The failure of the surge after prazosin, an alpha 1 receptor antagonist, indicates that the function of adrenaline may be mediated postsynaptically by alpha 1 receptors. Clonidine, an alpha 2 receptor agonist which reduces the turnover rate of hypothalamic adrenaline, had effects of the surge and ovulation which were comparable to those of diethyldithiocarbamate and SKF 64139, the relatively low doses causing some of the surges to occur at 24.00 instead of 18.00 h and higher doses suppressing the surge at both times and thus preventing ovulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of manipulating central catecholamines on puberty and the surge of luteinizing hormone and gonadotropin releasing hormone induced by pregnant mare serum gonadotropin in female rats

We have investigated the effect of manipulating central catecholamines on the timing of puberty (as assessed by vaginal opening) in female rats and the surge of luteinizing hormone (LH) and gonadotropin releasing hormone (GnRH) induced by pregnant mare serum gonadotropin (PMSG) in immature female rats. Manipulation of the catecholamines was carried out with either 6-hydroxydopamine (6-OHDA) administered with or without either desipramine (DMI) or pargyline, or alpha-methyl-p-tyrosine (alpha-MPT). The neonatal administration of 6-OHDA delayed puberty, an effect which was potentiated by pretreatment with DMI and was associated with a reduction in the rate of body growth. Catecholamine fluorescence in animals aged 60--65 days that had been treated with DMI followed by 6-OHDA was diminished only in the caudatus--putamen; treatment with 6-OHDA alone resulted in diminished fluorescence in the hypothalamus and in the intermediate but not the external layer of the median eminence. The neonatal administration of alpha-MPT had no significant effect on either the growth rate or the timing of puberty. Regular oestrous cycle occurred after puberty in animals treated with either 6-OHDA or alpha-MPT. The PMSG-induced LH surge was significantly enhanced by 6-OHDA (administered i.v.) plus DMI, and reduced by 6-OHDA injected in to the lateral ventricle (v). The inhibitory effect of 6-OHDA (v) was reduced by DMI, but in animals given 6-OHDA (i.v.) after pargyline there was a marked reduction in the height of the LH surge. There was a good correlation between the changes in the concentrations of LH in peripheral plasma and the concentrations of GnRH in pituitary stalk plasma in that the PMSG-induced surge of GnRH was significantly increased by 6-OHDA (i.v.) plus DMI and reduced by 6-OHDA (v). In animals treated with 6-OHDA (i.v. plus DMI catecholamine fluorescence was reduced only in the external layer of the median eminence, while after 6-OHDA (v) plus DMI degeneration was seen in the medial forebrain bundle. These results demonstrate a marked difference between the long-term and acute effects of 6-OHDA on the gonadotropin control system. Neonatal treatment with 6-OHDA plus DMI significantly delays puberty and the rate of body growth, but does not affect cyclical gonadotropin release and has no persistent effect on the hypothalamic catecholaminergic systems. The acute administration of 6-OHDA, depending upon the route of administration and whether it is given after DMI, can either potentiate or inhibit the PMSG-induced surge of GnRH and consequently LH by mechanisms which involve destruction, respectively, of either dopaminergic terminals in the median eminence or catecholaminergic fibres in the dorsal hypothalamus.

Tanycytes and their relation to the hypophyseal gonadotrophic function

It is shown by the enzyme-histochemical method that after prenatal androgenization (the administration of 25 mg testosterone propionate to the mother on the 19th day of pregnancy) female rats lose the sex differences in the age dynamics of the activity of some dehydrogenases in the beta 1-tanycytes, which were detected earlier in the 'critical period'. The data of the present study are regarded as further evidence in favor of the earlier advanced assumption of beta 1-tanycyte hypersensitivity to the sex hormones. The authors develop an original hypothesis on the possible role of beta 1-tanycytes in the feedback control of gonadoliberin release from the nerve terminals of the median eminence.

Inhibition of LH and prolactin release in the cycling rat following inhibition of dopamine-beta-hydroxylase

The effect of fusaric acid, an inhibitor of dopamine-beta-hydroxylase (DBH), on luteinizing hormone (LH) and prolactin levels during the estrous cycle was determined. Fusaric acid was found to cause a selective dose- and time-dependent inhibition of DBH activity in the medial basal hypothalamus without altering tyrosine hydroxylase activity. When DBH was inhibited during the afternoon of diestrus, the proestrous surges of both LH and, to a lesser extent, prolactin were inhibited. These results suggest that noradrenergic neuronal activity in the mediobasal hypothalamus is required during the afternoon preceding proestrus in order for the LH and prolactin surges to occur. It is possible that the rise in serum estrogen during late diestrus 2 is blocked by fusaric acid treatment. This estrogen increase is necessary for LH and prolactin surges to occur during proestrus. When DBH was inhibited during the afternoon of proestrus, the LH and prolactin surges were completely eliminated. This indicates that noradrenergic neuronal activity in the mediobasal hypothalamus during the afternoon of proestrus is important for both the LH and prolactin surges to occur.

Catecholamine synthesizing enzymes in the hypothalamus during the estrous cycle

The activities of tyrosine hydroxylase and dopamine-beta-hydroxylase were measured in the medial basal hypothalamus and remaining hyothalamic tissue of female rats at various times during diestrus 2, proestrus and estrus. Tyrosine hydroxylase activity in the medial basal hypothalamus was significantly lower at 12.00 h compared with other times on proestrus. This decrease preceded the elevation of serum prolactin and LH during the afternoon of proestrus. Tyrosine hydroxylase activity did not change significantly during diestrus 2 or estrus nor was it altered at any time in the remainder of the hypothalamus. Dopamine-beta-hydroxylase activity in the basal medial hypothalamus was significantly elevated at 12.00 h on proestrus and at 14.00 h on diestrus. The results provide further evidence for a decrease in dopaminergic neuron activity in the medial basal hypothalamus which may precipitate the series of events leading to the LH surge during proestrus. The increase in dopamine-beta-hydroxylase activity suggests that an increase in noradrenergic neuron activity may also be involved in triggering the release of LH.

Changes in the distribution of non-neuronal elements in rat median eminence and in anterior pituitary hormone secretion after activation of tuberoinfundibular dopamine neurones by brain stimulation or nicotine

In order to investigate the posssibility of acute functional changes in non-neuronal elements (mainly tanycytes) of the median eminence, the proportion of portal capillary surface covered by such elements was measured by quantitative electron microscopy in ovariectomized, oestrogen-progesterone-pretreated rats. In some of these animals, the functional state of the tuberoinfundibular dopamine (DA) neurones was assessed by histochemical microfluorimetry. Serum concentrations of luteinizing hormone (LH), growth hormone (GH) and prolactin were determined by radioimmunoassay. Two different types of treatment, i.e. systemic administration of nicotine (1 mg/kg, s.c.) or electrical stimulation in the medial amygdaloid nucleus, markedly reduced the percentage of capillary surface covered by non-neuronal profiles within 20 and 15 min, respectively. At the same time, the tuberoinfundibular DA system responded by an increase in cellular fluorescence intensity, reflecting neuronal activation. Medial preoptic stimulation had basically the same effect but with more variability in the change in capillary coverage by tanycytes. The action of nicotine was prevented by pretreatment with the DA receptor blocking agent, pimozide (5 mg/kg), which indicates (1) that a dopaminergic mechanism was involved in the nicotine effect and (2) that the tanycyte response was elicited by DA released from nerve terminals acting at some receptor site. Nicotine also lowered serum levels of GH and prolactin. Pimozide antagonized only the effect on prolactin. While the reaction of DA neurones and capillary coverage by tanycytes were correlated with each other in individual rats, no statistically significant correlation was observed between tanycyte response and hormone levels, so that no conclusions can as yet be drawn as to the neuroendocrine significance of the tanycyte reaction. These results indicate that rapid changes in the proportion or portal capillary surface covered by non-neuronal profiles can be elicited by stimulation of extrahypothalamic brain areas or by activation of cholinergic mechanisms. The tanycyte response appears to be mediated at least in part by the tuberoinfundibular DA neurones.

Effects of estrogen and progesterone on plasma gonadotropins and on catecholamine levels and turnover in discrete brain regions of ovariectomized rats

Estradiol benzoate (EB) was administered, either alone or followed 48 h later by progesterone to ovariectomized rats. Plasma gonadotropins (FSH and LH) and steady state levels of norepinephrine (NE) and dopamine (DA) in 17 individual brain nuclei were assayed. In addition, catecholamines were measured after administration of the synthesis inhibitor alpha-methyltyrosine (alpha-MT) in order to assess hormonal influences on turnover. Treatment with EB, which lowered plasma FSH and LH, reduced the depletion of NE produced by alpha-MT in the lateral septum, interstitial nucleus of the stria terminalis, and central gray catecholamine area, and reduced the depletion of DA in the nucleus of the tractus diagonalis. EB enhanced NE depletion in the periventricular and anterior hypothalamic nuclei, and raised steady state levels of NE in the medial amygdaloid nucleus. These effects were reversed by subsequent treatment with progesterone, which stimulated FSH and LH release. EB plus progesterone enhanced the alpha-MT-induced depletion of NE over that observed with EB alone in the arcuate nucleus, and similarly enhanced DA depletion in the interstitial nucleus of the stria terminalis. EB plus progesterone prevented the depletion of NE by alpha-MT in the paraventricular and ventromedial nuclei, and also lowered resting NE levels in the paraventricular nucleus. The results suggest that catecholamine neurons in several discrete brain regions participate in the stimulatory and inhibitory feedback effects of ovarian hormones on gonadotropin secretion, and perhaps also on the hormonal induction of sexual receptivity.

Changes in catecholamine content in discrete brain nuclei during the estrous cycle of the rat

Norepinephrine (NE) and dopomine (DA) concentrations were measured in discrete brain nuclei in female rats at different stages of the 4-day estrous cycle. In the medial preoptic and paraventricular nuclei, NE levels were reduced on the days of proestrus and estrus as compared to metestrus-diestrus. NE levels sharply increased between estrus and metestrus in the lateral septal nucleus but did not vary significantly at other times. In the mesencephalic central gray catecholamine area, NE levels were elevated on metestrus as compared to all other days of the cycle. DA concentrations also varied in certain nuclei during the estrous cycle. In the lateral septum, DA levels were highest during diestrus, and in the nucleus of the tractus diagonalis, DA levels were highest on metestrus. The DA concentration of the caudate nucleus decreased between proestrus and estrus and remained low through metestrus. DA in the medial preoptic nucleus showed a similar pattern of change as did NE. In the median eminence DA levels increased significantly from proestrus afternoon to estrus, but did not vary on the other days. It is concluded that cyclic activity of catecholamines in a few discrete areas of the brain may be related to the fluctuations of the gonadotropins and ovarian hormones to influence ovulation and mating behavior.

The effect of castration, thyroidectomy and haloperidol upon the turnover rates of dopamine and norepinephrine and the kinetic properties of tyrosine hydroxylase in discrete hypothalamic nuclei of the male rat

Adult male rats were either castrated, thyroidectomized, or treated with haloperidol and the rates of turnover of dopamine (DA) and norepinephrine (NE) in the median eminence (ME), the arcuate and dorsomedial nuclei of the hypothalamus were estimated from the rate of decay of DA and NE concentrations as determined by radioenzymatic assay following blockade of catecholamine synthesis by alpha-methyl-p-tyrosine. The ME of animals similarly prepared was also examined for changes in the total activity and kinetic properties of tyrosine hydroxylase (TH). Four days following the administration of haloperidol (400 microgram/kg) or 10 days after castration, there was a significant increase in the rate of turnover of DA but not NE in the ME accompanied by an increase in the Vmax but not Km for the substrate or cofactor of TH. Furthermore, the administration of haloperidol to hypophysectomized rats also significantly increased the TH activity in the ME, indicating that such changes may occur independently of any changes in serum prolactin levels. Ten days after thyroidectomy, or three weeks after treatment with prophylthiouracil, there was a significant increase in the turnover rate of DA in both the ME and dorsomedial nucleus but not in the arcuate nucleus. No changes in the turnover rates of NE in any of the three areas were observed following thyroidectomy. In the ME, the increase in turnover of DA was accompanied by an increase in the total TH activity (Vmax) as welll as a decrease in Km for tetrahydrobiopterin but not tyrosine. From these results 4 conclusions were drawn: (1) following halperidol, castration, and thyroidectomy there are increases in the activity of dopaminergic terminals within the ME; (2) castration, haloperidol and thyroidectomy may influence the activity of dopaminergic terminals within the ME by different mechanisms; (3) changes in tyrosine hydroxylase and turnover of catecholamines within the ME may occur independently of changes in prolactin levels; and (4) local recurrent afferent circuits may exist in the arcuate nucleus region of the hypothalamus.

Catecholamines in discrete areas of the hypothalamus of obese and castrated male rats

Levels of norepinephrine (NE) and dopamine (DA) were measured in eight discrete regions of the hypothalamus in three groups of male rats; genetically obese (fafa), non-obese (FaFa) and castrated non-obese (FaFa). DA levels showed no significant differences among the groups in any of the regions. NE levels in the paraventricular nucleus (PVN) were significantly lower in the obese and castrated animals than in the normal animals. In the median eminence (ME), NE levels were significantly decreased for the castrated group. None of the other regions sampled showed significant differences in NE levels.

Hypothalamic-pituitary-ovarian interactions during reproductive senescence in the rat

The neuroendocrine status of Long-Evans female rats was evaluated at several key stages of reproductive senescence. Young (4-8 mo), middle-aged (10-14 mo) and old (24-30 mo) animals were studied according to reproductive state. The reproductive states studied were (1) regularly cycling, (2) constant estrus and (3) pseudopregnant, as determined by vaginal smear cytology. Neuroendocrine parameters at the levels of the hypothalamus, pituitary and steroid-producing organs were compared between each group. DA3, E and NE concentrations in the median eminence of the hypothalamus were determined by a highly sensitive radioenzymatic assay. LRF content in the median eminence was measured by radioimmunoassay. Circulating levels of LH, FSH, PRL and six steroids were determined. Changes in hormone and neurotransmitter concentrations were deomonstrated in association with the various stages of reproductive senescence and with age advancement. These changes involved the hypothalamic, pitiutary and steroid systems. NE content in the median eminence, FSH in serum and circulating androstenedione were all significantly increased in middle-aged, cyclic rats prior to the onset of senescent anovulation. DA concentration in 24 mo. old constant estrous rats (30.7 +/- 7.7 pg/microgram, N = 6) and in 30 mo. old pseudopregnant rats (27.5 +/- 7.1 pg/microgram, N = 6) was significantly reduced compared to young (6 mo. old), cyclic controls on proestrous (55.0 +/- 4.7 pg/microgram, N = 12). This DA reduction was associated with a 3-fold increase in circulating prolactin. The results are discussed in terms of a regulatory cascade model of female reproductive senescence (Finch, 1976).