Estrogen increases galanin immunoreactivity in hyperplastic prolactin-secreting cells in Fisher 344 rats.

Galanin is a widely distributed regulatory peptide which modulates the pituitary secretion of PRL and GH. Estrogen administration strongly stimulates galanin gene expression in the rat anterior pituitary. In adult female Fischer 344 rats, estrogen also induces hyperplasia of lactotropes. We used immunocytochemical analysis to assess the effects of estrogen on galanin-like immunoreactivity (Gal-IR) in the rat pituitary and hypothalamus during sc diethylstilbestrol (DES) implantation and after its removal at 30 days. In the anterior pituitary, DES implantation increased the portion of Gal-IR-containing cells from less than 2% in the control rats to 18.3% after 3 days of DES and 36% after 30 days. These changes paralleled the lactotrope hyperplasia exhibited in response to DES exposure. Ten and 30 days after removal of the DES capsules, the percentage of Gal-IR-containing cells in the anterior pituitary decreased to 6.3% and 1.5%, respectively. Colocalization studies revealed that Gal-IR-containing cells were predominantly lactotropes. Immunoelectron microscopy demonstrated that Gal-IR was concentrated in the Golgi region of these hyperplastic lactotropes and suggests that little of the synthesized galanin is secreted. The distribution of Gal-IR in the hypothalamus, median eminence, and neurohypophysis was unaffected by DES treatment. These data demonstrate that galanin is synthesized by hyperplastic pituitary lactotropes of Fischer 344 rats and that peptide accumulation is dependent on the presence of circulating estrogens. In contrast, neuronal galanin synthesis in the hypothalamus does not appear to be regulated by estrogen.

Immunohistochemical identification of galanin and growth hormone-releasing factor-containing neurons projecting to the median eminence of the rat.

Through the combined demonstration of the retrograde transport of True blue and the immunohistochemical staining of galanin (GAL), the GAL neurons that project to the median eminence were identified. Moreover, the distribution of GAL and growth hormone-releasing factor (GRF) was analyzed in the arcuate nucleus (ARC) with an elution-restaining procedure. Following the injection of True blue into the median eminence, GAL-positive cells labeled with True blue were found mainly in the ARC. In addition, a few GAL neurons in the periventricular, the paraventricular and the supraoptic nuclei were labeled with True blue. The elution-restaining results revealed that many retrogradely labeled GAL neurons in the ARC contained GRF. These findings suggest that a subpopulation of GAL neurons in the ARC sending axons to the median eminence produces, stores and releases a GRF-like peptide.

Sexual differentiation of growth hormone feedback effects on hypothalamic growth hormone-releasing hormone and somatostatin.

To investigate possible sex differences in the feedback regulation of growth hormone (GH) secretion, concentrations of immunoreactive GH-releasing hormone (GRF) and somatostatin (SS) were measured in the median eminence (ME) and the hypothalamus of male and female rats bearing the MtTW15 tumor, which secretes high amounts of GH and prolactin (PRL). Four weeks after tumor implantation in male rats, the GRF concentration in the whole hypothalamus, including the ME, was decreased by 37% (0.29 +/- 0.02 vs. 0.46 +/- 0.02 ng/mg protein in intact male controls; p less than 0.001) and the concentration of SS was increased by 40% (11.5 +/- 0.7 vs. 8.1 +/- 0.3 ng/mg protein in male controls; p less than 0.01). In female rats, the presence of tumor for 4 weeks caused a smaller (18%) reduction in GRF concentrations (0.27 +/- 0.02 vs. 0.33 +/- 0.03 ng/mg protein in intact female controls; p less than 0.05) and no significant change in SS concentrations (10.2 +/- 0.08 vs. 9.7 +/- 0.8 ng/mg protein in female controls). Tumor-related changes in GRF and SS concentrations were also more pronounced in male rats than in females, when determined separately in the microdissected ME and in the remaining hypothalamus. These differences occurred despite similar increases in serum GH, PRL and insulin-like growth factor I concentrations in male and female tumor-bearing rats. To assess which hormone (GH or PRL) was responsible for these changes, intact male rats were treated for 10 days with 2 daily s.c. injections of rat GH (rGH; 100 and 250 micrograms/day), rat PRL (100 and 250 micrograms/day) or vehicle.(ABSTRACT TRUNCATED AT 250 WORDS)

Galanin-like immunoreactivity is influenced by estrogen in peripubertal and adult rats.

Galanin gene expression in the anterior pituitary is potently stimulated by estrogen in adult rats. To evaluate the influence of estrogen on galanin during the peripubertal period 30- to 32-day-old female rats were treated with pregnant mare serum gonadotropin (PMSG, 10 IU s.c., 10.00 h). Galanin-like immunoreactivity (galanin-LI) in hypothalamic and pituitary tissues was evaluated 1, 2 or 3 days after PMSG treatment between 17.00 and 19.00 h. The PMSG treatment stimulated 17 beta-estradiol secretion, which induced a midafternoon LH surge 2 days after the PMSG treatment. Concentrations of galanin-LI at the time of this LH surge were elevated 82% in the anterior pituitary and 58% in the hypothalamus (without the median eminence) when compared to saline-treated female rats. On the 3rd day after the PMSG injection, galanin-LI was increased 236% in the anterior pituitary, 88% in the neurointermediate lobe and 39% in the median eminence compared to saline-treated female rats. These changes in galanin-LI were not observed in similarly aged male rats or ovariectomized rats treated with PMSG. In adult male rats, daily injections with 17 beta-estradiol valerate (10 micrograms/daily s.c.) for 1 week increased galanin-LI in the median eminence and neurointermediate lobe to an extent similar to that seen in juvenile female rats following PMSG treatment. In contrast, the high serum levels of 17 beta-estradiol achieved after 17 beta-estradiol valerate treatment increased galanin-LI in the anterior pituitary 65-fold. These studies indicate that galanin-LI is influenced by estrogen in peripubertal and adult rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Stress-induced suppression of the prolactin afternoon surge in ovariectomized, estrogen-treated rats and the nocturnal surge in pseudopregnant rats are accompanied by an increase in median eminence dihydroxyphenylacetic acid concentrations.

Experiments were performed to determine whether the suppression of prolactin (PRL) surges during restraint was accompanied by changes in the activity of tuberoinfundibular dopamine (TIDA) neurons in the median eminence. Animals were either ovariectomized and estrogen-treated (OVX-PEP) or cervically stimulated to induce pseudopregnancy (PSP). Restraint stress was administered by tying the hind legs together with plastic-coated bell wire. Animals were decapitated following 15 or 30 min of restraint stress or immediately after removal from the animal room (control) when PRL levels were basal (10.00 h), at the peak of the afternoon PRL surge in OVX-PEP animals (17.00 h) or the nocturnal PRL surge in PSP animals (05.00 h). Median eminence dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) were significantly decreased in control rats at 17.00 h when compared to control rats at 10.00 h (103.1 +/- 3.7 vs. 85.8 +/- 3.3 and 11.4 vs. 7.1 +/- 0.4 pg/micrograms protein, respectively) and plasma PRL was markedly elevated. Restraint stress at 10.00 h resulted in a significant increase in serum PRL, but this increase was not accompanied by a change in DA or DOPAC when compared to control animals (103.1 +/- 3.7 vs. 107.9 +/- 4.8 and 11.4 +/- 0.4 vs. 10.4 +/- 0.6 pg/micrograms protein, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II antiserum decreases luteinizing hormone-releasing hormone in the median eminence and preoptic aea of the rat

Ovariectomized female rats were sacrificed 3 h after intracerebroventricular microijnjection of normal rabbit serum (NRS), specific antiserum against angiotensin II (AB-AII) or atrial natriuretic factor (AB-ANF). AB-AII decreased plasma LH by 50% and LH-RH content by 70% in the median eminence and medial preoptic area, respectively, but did not change plasma FSH when compared to animals which receivede NRS. There was no difference in these parameters when the animals were injected with AB-ANF or NRS.These results indicate that endogenous AII plays a physiological role in LH release acting directly or indirectly through LH-RH neurons of the median eminence and medial peroptic area

Tyrosine hydroxylase in the stalk-median eminence and posterior pituitary is inactivated only during the plateau phase of the preovulatory prolactin surge.

This study examined changes in the activity of tyrosine hydroxylase (TH) in the stalk-median eminence (SME) and posterior pituitary (PP) during the preovulatory PRL surge. Immature female rats were injected with PMSG on day 28. Blood PRL levels were low on the morning of day 30, rose to a peak from 1400-1600 h, remained at a lower plateau from 1800-2400 h, and declined to basal levels on the morning of day 31. SME, PP, and striatum were removed from PMSG-treated rats at selected times during the periovulatory period and from age-matched control rats. TH activity was determined in tissue homogenates by a coupled hydroxylation-decarboxylation assay. Apparent Km and maximum velocity values with respect to 6-methyl tetrahydropterine were estimated from substrate saturation curves. The kinetic parameters for TH in either the SME or the PP of control rats were similar at 1100 and 1800 h on day 30. However, the apparent Km in both tissues was significantly lower than that in the striatum. The affinity of TH in the SME and PP was unchanged before and during the peak phase of the PRL surge, reduced significantly during the late plateau, and returned to presurge levels in the morning of day 31. TH activity in the striatum was similar at all times examined. To determine the state of activation of the enzyme, tissue homogenates were preincubated with cAMP, ATP, and magnesium. TH activity in the SME during the peak phase was unchanged by cAMP, and that in the PP was modestly increased. The relatively inactive enzyme in both tissues during the plateau phase was markedly activated by a cAMP-dependent mechanism. The low affinity of striatal TH was greatly increased by cAMP at both times. These data suggest that TH in the SME and PP exists in an activated state most of the time and is transiently inactivated during the plateau phase of the PRL surge. In contrast, TH in the striatum is relatively inactive in the basal state and is not affected by hormonal changes induced by PMSG. We conclude that the peak PRL surge occurs in spite of active dopamine (DA) neurons, suggesting that it is generated by a nondopaminergic mechanism. Decreased TH activity in DA neurons in the SME and PP may prolong the PRL surge during the plateau phase, whereas increased DA activity coincides with the termination of the surge.

Topographic relations between tyrosine hydroxylase- and luteinizing hormone-releasing hormone-immunoreactive fibers in the median eminence of adult rats.

Employing electron microscopic double immunolabeling, we determined a close apposition of tyrosine hydroxylase (TH) and luteinizing hormone-releasing hormone (LHRH) nerve fibers in the rat median eminence (ME). These axo-axonic contacts occurred frequently in the internal and palisade zones, i.e. at the level of the fiber preterminals. In the superficial area of the ME, major TH fibers abutted on the basal lamina and some were projected into the pericapillary space of the portal vessels. Conversely, LHRH fibers were arrested by the endfeet of tanycytes in reaching the basal lamina.

Influence of age on the control of thyrotropin secretion by thyrotropin-releasing hormone in the male rat.

Alterations with age in the control of thyrotropin (TSH) secretion by thyrotropin-releasing hormone (TRH) were evaluated at the hypothalamic and pituitary levels in young (3-5 months) and old (22-24 months) male rats. In the hypothalamus, TRH was quantified in the median eminence and in the mediobasal hypothalamus; in the adenohypophysis the membrane receptors for TRH were evaluated as well as the accumulation of TRH in the gland. As for TSH, its concentration was determined in the anterior pituitary gland and in plasma. In the hypothalamus, the concentration of TRH did not differ between young and old rats in the whole mediobasal hypothalamus, but it was significantly less in the old rats at the level of the median eminence (29.9 +/- 2.8 vs. 52.2 +/- 4.3 ng/mg protein). In the adenohypophysis, the density of receptors for TRH was greater in the old than in the young rats (23.2 +/- 3.2 vs. 13.7 +/- 1.1 fmol MeTRH/mg gland)--with no change in the affinity constant--, and the amount of TRH detected was larger (10.8 +/- 1.8 vs. 2.8 +/- 0.6 pg/mg gland), illustrative of an age-related increase in TRH accumulation in the pituitary gland. The latter results are contrasting with the findings of unchanged pituitary and plasma concentrations of TSH as well as unmodified TSH response to TRH in old rats. The present data concerning TRH and the analogy with previous observations regarding dopamine in old rats are indicative of reduced neuronal activities with age at the hypothalamic level associated with impairments in the processing of the hypothalamic hormones at the pituitary level.

Potential involvement of galanin in the regulation of fluid homeostasis in the rat.

A physiological role for galanin, a 29-amino acid neuropeptide, has not been established. However, anatomical studies have demonstrated the presence of galanin in brain regions associated with the control of water balance in the rat, most notably in the paraventricular nucleus (PVN) of the hypothalamus and the neurointermediate lobe of the pituitary gland (NIL). In the PVN, galanin coexists with arginine vasopressin (AVP) in magnocellular neurons. The present study demonstrates that homozygous Brattleboro rats, which lack AVP, produce galanin. Galanin concentrations in the median eminence (ME) of the homozygous Brattleboro rat do not differ from the galanin concentrations in the ME of either heterozygous Brattleboro or Sprague-Dawley rats. However, galanin concentrations in the NIL of the homozygous Brattleboro rat were reduced by 75%. Similarly, dehydration induced by salt-loading reduced galanin concentrations in the NIL and produced transient changes in the ME. These data demonstrate that galanin concentrations are influenced by changes in fluid homeostasis and suggest that galanin may be an important component in the regulation of neurohypophyseal function and AVP secretion.

Anterior pituitary type II thyroxine 5'-deiodinase activity is not affected by lesions of the hypothalamic paraventricular nucleus which profoundly depress pituitary thyrotropin secretion.

Bilateral destruction of the hypothalamic paraventricular nuclei (PVN) produced a profound depression of plasma TSH and the median eminence TRH concentration in hypothyroid rats. Anterior pituitary type II iodothyronine 5'-deiodinase (5'-D) activity was consistently lower but not significantly different in sham- and PVN-lesioned rats. Treatment with suboptimal replacement doses of 0.15 and 0.75 micrograms T4/100 g BW.day produced a graded depression of plasma TSH in the PVN (P less than 0.02), but not in the sham (P greater than 0.8) groups. Adenohypophyseal 5'-D was depressed in both sham and PVN groups by the highest T4 dose. Plasma T4 was much lower in PVN than in sham rats given comparable doses of T4 (P less than 0.001), but plasma T3 was not significantly different. This suggests that an increase in peripheral T4 metabolism was produced by PVN lesions. Our data indicate that changes in adenohypophyseal 5'-D activity are not responsible for the decrease in plasma TSH in PVN-lesioned rats and that neither the PVN nor endogenous TRH plays a significant role in the regulation of anterior pituitary 5'-D activity.

Immunoreactive growth hormone-releasing hormone in rat placenta.

It has been shown that immunoreactive and biologically active GH-releasing hormone (GHRH)-like material is present in rat placenta. To investigate the role of placental GHRH, we measured it in human and rat placenta of different gestational stages, using specific RIA systems. GHRH and somatostatin contents in median eminence, pituitary GH contents, and plasma GHRH levels were also quantified in rats. Immunoreactive GHRH was detectable in rat placenta [13 days of gestation, 1.4 +/- 0.4 (+/- SD); 16 days, 1.4 +/- 0.2; 20 days, 1.7 +/- 0.4 ng/g] but not in human placenta (less than 0.06 ng/g in both full-term and mid-term placenta). GHRH concentrations in rat placenta did not change significantly during pregnancy, but total contents increased progressively in relation to placental growth. GHRH and somatostatin contents in median eminence of pregnant rats were not different from those of control female rats. In contrast, rat pituitary GH contents in pregnant rats were significantly lower than those of control female rats. Immunoreactive GHRH was not detectable in plasma of either pregnant rats or nonpregnant rats. Molecular sieve chromatography revealed two peaks of immunoreactive GHRH in rat placental extracts: a major peak eluted in the position of synthetic rat GHRH and a minor peak in the higher molecular weight region. In contrast, a single peak in the position of rat GHRH was observed in rat median eminence extracts. Detection of immunoreactive GHRH in rat placenta but not in human may suggest that the mechanism of GHRH gene expression in placenta is species specific. Failure of detection of immunoreactive GHRH in rat maternal circulation suggests that placental GHRH may not affect the maternal hypothalamic pituitary axis. Presence of high molecular weight materials of immunoreactive GHRH in rat placenta but not in median eminence suggests that posttranslational processing of the GHRH precursor molecule may be different in the two organs. Placental GHRH may have a paracrine function or may be secreted into fetal circulation and contribute to fetal growth.

Galanin in the hypothalamo-hypophyseal system.

In the rat median eminence immunoreactive galanin nerve fibers and terminals are present in high numbers in the external layer, and fibers in moderate numbers are seen in the internal layer. The possible sources of these galanin-containing fibers were studied by means of radioimmunoassay and immunohistochemistry in rats with different types of hypothalamic lesions. Galanin-like neurons were found both (1) in the magnocellular hypothalamo-neurohypophyseal system and (2) in the parvocellular hypothalamo-median eminence-anterior pituitary system. Cell bodies containing galanin-like immunoreactivity were localized in the supraoptic, magnocellular paraventricular and accessory magnocellular neurons with axons traversing the internal layer and terminating in the posterior pituitary. Surgical isolation of these neurons from the median eminence resulted in a marked depletion of immunoreactive galanin from the internal layer of the median eminence and the posterior pituitary. Due to the retrograde accumulation of axonally transported substances in cells proximal to the lesions, immunoreactive galanin-like cells became visible in the supraoptic and paraventricular nuclei ipsilateral to the knife cuts, and levels of galanin-like immunoreactivity increased in these nuclei 7 days after bilateral transections of the hypothalamo-hypophyseal tract. Immunoreactive galanin fibers in the external layer of the median eminence around the portal capillaries were found to be of paraventricular and arcuate nucleus origin. Bilateral paraventricular lesions caused marked (70%) reduction in levels of galanin-like immunoreactivity in the median eminence. The remaining 30% of the galanin immunoreactivity in the external layer may arise from the arcuate nucleus, which contains a great number of galanin-containing cell bodies.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of luteinizing hormone-releasing hormone (LHRH) neurons that project to the median eminence.

The neuropeptide, luteinizing hormone-releasing hormone (LHRH), is released from nerve terminals in the median eminence and carried via the hypophysial portal system to the anterior pituitary, where it stimulates the release of gonadotropins. LHRH-containing neurons are located in many different regions of the rodent brain, including olfactory, septal, preoptic, and hypothalamic structures. Since those LHRH neurons that project to the median eminence form the final common pathway for the regulation of the pituitary/gonadal axis, we wished to determine which of these cell groups are afferent to this structure. A retrograde tracer, the lectin wheat germ agglutinin (WGA), was placed directly on the exposed surface of median eminence. Following survival times of 8-13 hr, brains were prepared for the dual immunocytochemical detection of WGA and LHRH. Approximately 50% of the LHRH neurons from the level of the septal nuclei caudalward were found to contain WGA immunoreactivity and therefore to project to the median eminence. The remaining single-labeled LHRH neurons were intermingled with the double-labeled cells. The 2 populations were not distinguishable from each other on either cytological or cytoarchitectonic criteria. Those LHRH neurons that were not retrogradely labeled following an injection of tracer into the median eminence are presumed to project to other regions of the central nervous system. We conclude that the LHRH neurons that are directly involved in the regulation of reproductive function are very heterogeneous, widely scattered in telencephalic and diencephalic regions.

[Effect of 5,6-hydroxytryptamine and various light schedules on the hypothalamo-hypophyseo-gonadal system of the female rat]. / Vliianie 5,6-oksitriptamina i raznykh svetovykh rezhimov na gipotalamo-gipofizarno-gonadnuiu sistemu samok krys.

Hypothalamic luliberin and plasma lutropin concentration were studied under normal light-darkness cycle or under constant illumination in rats. Some of them were injected with 5,6-dihydroxytryptamine (5,6-HTA) into the lateral brain ventricle. Constant illumination led to a persistent oestrus with a significant drop in plasma lutropin and a decrease in hypothalamic luliberin. Destruction of serotoninergic terminals with 5,6-HTA resulted in an opposite effect and prevented to some extent inhibiting influence of constant illumination. Luliberin synthesis and secretion seem to be suppressed by serotoninergic nerve terminals on the luliberin producing cells as well as by serotonin reaching the hypothalamus from the pineal gland.

Histamine-immunostaining in the rat median eminence: an unexpected form of cross-reactivity with LH-RH.

By use of two antisera (alpha HA-1, alpha HA-2) raised to histamine (HA) a similar distribution of HA-positive cell bodies and fibres in the rat brain has been demonstrated, except for a dense plexus in the median eminence which was only found with alpha HA-1. The HA-immunostaining in the median eminence was similar in distribution to that of luteinizing hormone-releasing hormone (LH-RH). In gelatin models containing LH-RH a concentration-dependent increase in immunofluorescence intensity (range: 0.01-10 microM) was found with alpha HA-1 but not with alpha HA-2. To evaluate the significance of the cross-reaction of alpha HA-1 with LH-RH, antero- or postero-lateral deafferentations of the rat mediobasal hypothalamus were made to discriminate between LH-RH and HA projections to the median eminence. The LH-RH- and HA-immunostaining in the median eminence was not affected by posterolateral deafferentations but was abolished in rats with an anterolateral lesion. We conclude that the HA-immunostaining in the rat median eminence is due to a cross-reaction of alpha HA-1 with LH-RH.

Purification of FSH-releasing factor: its dissimilarity from LHRH of mammalian, avian, and piscian origin.

Sheep stalk median eminence fragments were lyophylized, extracted and filtered through a column of Sephadex G-25. The fractions were then assayed for the presence of LHRH by radioimmunoassay (RIA) and bioassayed for FSH and LH-releasing activity following their IV injection into ovariectomized, estrogen progesterone-blocked rats. The radioimmunoassayable LHRH emerged from the column at the same position from which it emerged many years before when LH was measured by bioassay. This same region also contained the LH-releasing activity as measured by bioassay. FSH-releasing activity was present in two tubes just preceding the emergence of the bio- and immunoassayable LHRH. The activity was highly significant and there was no LH-releasing activity in the fractions. They contained much less LHRH as determined by RIA than is sufficient to evoke LH release in this assay. The FSH-releasing activity was recovered in the same fractions in which it was found many years ago with this same assay but with measurement of plasma FSH by bio-rather than immunoassay as employed here. A dose-related release of LH was obtained by injection of LHRH in this assay but there was no significant FSH release even with a dose of 27 ng of LHRH per rat. To determine if one of the LHRHs of lower forms might be FSH-RF, Chicken I and II LHRH and Salmon LHRH were also assayed for FSH- and LH-releasing activity. Each of these peptide possessed LH-releasing activity, albeit much less than that of the mammalian peptide but had no FSH-releasing activity whatsoever.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and modulation of corticotropin-releasing factor in the neurointermediate pituitary gland.

The present study attempts to determine whether part of the corticotropin-releasing factor (CRF)-like materials present in the 'posterior pituitary' is composed of authentic CRF and examines whether the concentration of that peptide may be modulated by circulating glucocorticoids. Analysis of crude extracts of neurointermediate lobes (NIL) of rat pituitaries by reverse-phase HPLC, coupled with a specific radioimmunoassay (RIA), revealed the presence of a major component eluting with the same retention time as rat CRF (rCRF) and, importantly, which was indistinguishable by RIA from the synthetic peptide. Also, two minor forms eluted earlier than rCRF upon HPLC; one of these forms matched the elution position of r[Met(O)21,38]CRF. All three species did show biological activity and stimulated ACTH release from pituitary cells. Essentially the same elution profile was generated by median eminence (ME) extracts. Immunoreactive CRF (CRFi) content of the NIL was about 3% of that of the ME and was found to undergo a significant increase as a result of long-term adrenalectomy while, in contrast, CRFi content of the ME was decreased. This effect of adrenalectomy was completely antagonized by dexamethasone treatment. This study thus provides strong evidence for the presence of authentic CRF within the NIL of the rat pituitary and also shows that tissue concentration of that peptide was modulated by glucocorticoids.

cAMP-dependent ACTH secretagogues facilitate corticotropin releasing activity of angiotensin II on rat anterior pituitary cells in vitro.

Both arginine vasopressin (AVP) and angiotensin II (AII) potentiate the corticotropin-releasing activity of CRF41 via a potentiation of CRF41-induced cAMP production. In perfused rat anterior pituitary cells, AII (10(-8) mol) showed a transitory 2-fold increase in its ACTH-releasing activity, when tested after application extract of rat stalk median eminence. In order to determine whether this facilitating effect on AII corticotropin-releasing activity occurred through cAMP-dependent mechanisms, the ACTH-releasing activity of AII was tested after stimulation with CRF41, AVP or forskolin, three secretagogues with known effects on cAMP production. When given 16 min after CRF41, 10 micrograms/l, AII (10(-8) mol) showed a significant increase (210%) in its ACTH-releasing activity, which returned to the normal level when AII-stimulation was repeated at 32 min (121%) and 48 min (100%). Similarly, forskolin, 3 X 10(-6) mol, produced a significant transitory increase (208%) in the subsequent AII-induced ACTH release whereas AVP, 10 micrograms/l and 100 micrograms/l, had no effect on the following ACTH response to AII. These results suggest that the AII-induced ACTH secretion--which is cAMP independent--nevertheless may be modulated by previously stimulation of the cAMP pathway.

The influence of suckling on the hypothalamic and pituitary secretion of immunoreactive alpha-melanocyte stimulating hormone.

The effect of suckling on the secretion of alpha-melanocyte stimulating hormone (alpha-MSH) from the hypothalamus and pituitary was determined by a specific radioimmunoassay. There was an increase in the neurointermediate lobe (NIL) content of alpha-MSH 1 h after the onset of suckling. The values were restored to control levels within 3 h. The anterior lobe content of alpha-MSH was not affected by suckling. Plasma alpha-MSH levels were also unaffected by suckling, indicating that suckling probably affects the synthesis of NIL alpha-MSH, and not its release. Suckling lowered the alpha-MSH content in the mediobasal hypothalamus of lactating rats, and had no effect on the median eminence content of this peptide. In vitro, hypothalami from lactating rats released more alpha-MSH than hypothalami of random cycling females under basal, and stimulated (56 mM potassium) conditions. These results suggest that hypothalamic alpha-MSH may play a role in mediating some of the hormonal changes occurring during lactation.