Blockade of ERK1/2 activation with U0126 or PEP7 reduces sodium appetite and angiotensin II-induced pressor responses in spontaneously hypertensive rats.

Spontaneously hypertensive rats (SHRs) have increased daily or induced sodium intake compared to normotensive rats. In normotensive rats, angiotensin II (ANG II)-induced sodium intake is blocked by the inactivation of p42/44 mitogen-activated protein kinase, also known as extracellular signal-regulated protein kinase1/2 (ERK1/2). Here we investigated if inhibition of ERK1/2 pathway centrally would change sodium appetite and intracerebroventricular (icv) ANG II-induced pressor response in SHRs. SHRs (280-330 g, n = 07-14/group) with stainless steel cannulas implanted in the lateral ventricle (LV) were used. Water and 0.3 M NaCl intake was induced by the treatment with the diuretic furosemide + captopril (angiotensin converting enzyme blocker) subcutaneously or 24 h of water deprivation (WD) followed by 2 h of partial rehydration with only water (PR). The blockade of ERK1/2 activation with icv injections of U0126 (MEK1/2 inhibitor, 2 mM; 2 µl) reduced 0.3 M NaCl intake induced by furosemide + captopril (5.0 ± 1.0, vs. vehicle: 7.3 ± 0.7 mL/120 min) or WD-PR (4.6 ± 1.3, vs. vehicle: 10.3 ± 1.4 mL/120 min). PEP7 (selective inhibitor of AT1 receptor-mediated ERK1/2 activation, 2 nmol/2 µL) icv also reduced WD-PR-induced 0.3 M NaCl (2.8 ± 0.7, vs. vehicle: 6.8 ± 1.4 mL/120 min). WD-PR-induced water intake was also reduced by U0126 or PEP7. In addition, U0126 or PEP7 icv reduced the pressor response to icv ANG II. Therefore, the present results suggest that central AT1 receptor-mediated ERK1/2 activation is part of the mechanisms involved in sodium appetite and ANG II-induced pressor response in SHRs.

Arginine vasopressin as a thyrotropin-releasing hormone.

Although hypothyroidism (with concomitant increased levels of thyroid-stimulating hormone) has been associated with elevated plasma vasopressin, the role that vasopressin plays in controlling thyroid-stimulating hormone secretion from the adenohypophysis is not understood. In two in vitro pituitary cell systems, vasopressin caused a specific and dose-related release of thyroid-stimulating hormone from cells that was equal in potency to that elicited by thyrotropin-releasing hormone, the primary acknowledged regulator of thyroid-stimulating hormone release. When injected into the hypothalamus, however, vasopressin specifically inhibited the release of thyroid-stimulating hormone. Thus, vasopressin may exert differential regulatory effects on thyroid-stimulating hormone secretion in the hypothalamus and pituitary gland.

Interaction of central angiotensin II and aldosterone on sodium intake and blood pressure.

Recent studies demonstrated an important natriorexigenic mechanism activated by aldosterone acting in the hindbrain. Studies have also shown that aldosterone effects are intensified by angiotensin II (ANG II) and vice-versa. Thus, the aim of the present work was to test if angiotensinergic mechanisms in the forebrain are involved on sodium appetite to aldosterone infused into the 4th V and also if aldosterone into the 4th V might facilitate ingestive and cardiovascular responses to central ANG II. Male Holtzman rats with stainless steel cannulas implanted into the 4th ventricle (4th V) and lateral ventricle (LV) had access to 1.8% NaCl during 2 h/day. Chronic infusion of aldosterone (100 ng/h) into the 4th V for 7 days strongly increased 1.8% NaCl intake (16.1 ±â€¯2.2 ml/2h/day). Losartan (AT1 receptor antagonist, 50 µg/1 µl) acutely injected into the LV reduced 1.8% NaCl intake induced by aldosterone infusion into the 4th V (8.8 ±â€¯2.3 ml/2h/day). The pressor response to ANG II (50 ng/1 µl) into the LV increased in rats treated with aldosterone into the 4th V (45 ±â€¯5 mmHg, vs. vehicle infusion: 26 ±â€¯4 mmHg). Similarly, fluid intake (water + 1.8% NaCl) also increased when rats receiving aldosterone infusion were treated with ANG II acutely into the LV. These results suggest that forebrain angiotensinergic mechanisms are important for sodium intake produced by aldosterone acting in the hindbrain. In addition, aldosterone in the hindbrain produces sensitization of the central pressor mechanisms activated by ANG II acting in the forebrain.

Cardiovascular actions of orexin-A in the rat subfornical organ.

Orexin-A is a neuropeptide, primarily produced in the lateral hypothalamic/perifornical hypothalamus. Orexin receptors and immunoreactive neuronal fibres are widely distributed throughout the brain, suggesting integrative neurotransmitter roles in a variety of physiological systems. Intracerebroventricular injections of orexin-A increase blood pressure and stimulate drinking, and the subfornical organ (SFO), a circumventricular structure implicated in autonomic control, is a potential site at which orexin may act to exert these effects. We have therefore used microinjection techniques to examine the effects of orexin-A administered directly into the SFO on blood pressure and heart rate in urethane anaesthetised male Sprague-Dawley rats. Orexin-A microinjection (50 fmol) into the SFO caused site-specific decreases in blood pressure (SFO: mean area under curve (AUC) = -681.7 +/- 46.8 mmHg*s, n = 22 versus non-SFO: 63.68 +/- 54.69 mmHg*s, n = 15, P < 0.001), and heart rate (SFO: mean AUC = -26.7 +/- 2.8 beats, n = 22, versus non-SFO: mean AUC = 1.62 +/- 2.1 beats, n = 15, P < 0.001). Vagotomy did not alter the hypotensive or bradycardic responses elicited by orexin-A microinjection. Prior alpha-adrenoceptor blockade with phenoxybenzamine (1 mg/kg, i.v.) masked the orexin-A induced blood pressure (mean AUC = -122.6 +/- 17.6 mmHg*s, n = 4, P < 0.01 paired t-test) and heart rate (mean AUC = -6.7 +/- 1.7 beats, n = 4, P < 0.05, paired test) response. The orexin-A induced heart rate response was attenuated when beta-adrenoceptors were blocked with propranolol (1 mg/kg, i.v.; mean AUC = 0.6 +/- 2.8 beats, n = 5, P < 0.01 paired t-test). These studies demonstrate that microinjection of orexin-A into the SFO causes site specific decreases in blood pressure and heart rate which is mediated by a reduction in sympathetic tone.

Glucagon-like polypeptides in canine brain.

Glucagon-like material has been detected by radioimmunoassay in several areas of the canine brain. High concentrations of glucagon-like immunoreactivity (GLI), measured with antibodies directed against the N-terminal region of glucagon, have been found in the hypothalamus, amygdala, and mesencephalon, but a high concentration of immunoreactive glucagon (IRG), measured with antibodies directed against the C-terminal region of glucagon, has been found only in the hypothalamus. The predominant molecular forms of GLI isolated from brain extracts by affinity chromatography are the same as those isolated from gut extracts. The predominant form of IRG in brain extracts is of the same (approximate) molecular weight as pancreatic glucagon.

Natriuretic peptides A family of hormones.

The natriuretic peptides are a family of structurally related peptides, products of at least three genes, which share a common, 17-membered, internal disulfide ring. Three major subtypes (A-, B-, and C-type natriuretic peptides) exist, and each is present in various tissues in multiple N- and C-terminally extended or shortened forms. The recent description of the C-type peptide CNP, the major brain-derived peptide, has suggested that original studies examining the endocrine actions of the natriuretic peptides using A-type (ANP) or B-type (BNP) isoforms might have underestimated some of the biopotency of the natriuretic peptides or even failed to detect the full spectrum of the family's bioactivity. The identification of multiple, often species-specific, subtypes of the natriuretic peptides, together with the characterization of at least two classes of receptors for these hormones, indicates a variety of potential avenues for the development of therapeutic strategies for the use of these factors. Importantly, the description of protease inhibitors that prolong and enhance the actions of the natriuretic peptides in humans suggests potential for the recruitment of the natriuretic peptides in the treatment of endocrine as well as cardiovascular disease.

The hypocretin/orexin story.

Newly described peptides, produced in neurons in the lateral hypothalamic area, have been shown to stimulate appetite and stereotypic behaviors associated with feeding. Discovered independently by two groups, the hypocretins/orexins stimulate autonomic function and have been shown to be physiological regulators of the arousal state. Neuroendocrine and metabolic effects of these peptides, some related to sleep-wakefulness and arousal state, are just now being discovered.

Adrenomedullin: A newly discovered hormone controlling fluid and electrolyte homeostasis.

Neural and humoral mechanisms controlling fluid and electrolyte homeostasis employ a diverse array of physiologic mechanisms that often, when aberrant, are the underlying cause of disease. Behavioral, hormonal, renal, and vascular responses to volume and osmotic challenges must be coordinated to achieve the goal of homeostasis. In recent years, it has become apparent that there exist a number of hormonal factors produced throughout the body that can coordinate these multiple regulatory mechanisms by complementary effects in several tissues. Thus, in addition to their vasoactive properties, recently characterized hormones such as the natriuretic peptides and the endothelins, as well as the better established renin-angiotensin system, exert central nervous, renal, cardiac, and pituitary effects that regulate normal fluid and electrolyte balance. Now a new player, adrenomedullin, has been added to the cast, and the interplay of multiple hormonal factors involved in the physiology and pathophysiology of volume and osmotic status continues to be elucidated.

Dehydration-induced alterations in rat brain vasopressin and atrial natriuretic factor immunoreactivity.

Potent natriuretic and spasmolytic peptides present in cardiac extracts recently have been identified. These atrial natriuretic factors (ANF) exert vascular and renal actions quite contrary to those of vasopressin (AVP). The ability of ANF to inhibit AVP secretion suggested a role for the peptides in the control of AVP release. The present studies report the measurement of ANF-like immunoreactivity within brain regions associated with the hypothalamo-neurohypophyseal tract and demonstrate significant water deprivation-induced reductions in ANF content of several structures (neural lobe, organum vasculosum lamina terminalis, suprachiasmatic and supraoptic nuclei) but not in others (median eminence, paraventricular nucleus, cortex and pituitary). The data suggest the production of ANF-like peptides within the brain and, further, the involvement of central ANF in extracellular fluid volume regulation.

The novel vasoactive hormone, adrenomedullin, inhibits water drinking in the rat.

The novel hormone, adrenomedullin (AdM), which exerts potent hypotensive effects in the periphery and natriuretic actions in the kidney, was found to be antidipsogenic. Cerebroventricular injection of AdM (22, 44, and 88 pmol) resulted in a dose-related diminution of water drinking in response to subsequent central administration of 100 pmol angiotensin II. Additionally, 88 pmol AdM significantly inhibited the drinking response to overnight water deprivation and hyperosmotic challenge. No significant effects of AdM in the doses tested were observed on blood pressure, heart rate, or motor activity. These results suggest that this novel hormone can act within the nervous system to complement its peripheral actions on fluid and electrolyte homeostasis, independent of a central action on cardiovascular function or locomotion.

Interactions of dopaminergic and peptidergic factors in the control of prolactin release.

Oxytocin (OT) has been shown to play a role in the control of physiological PRL release and has been demonstrated to have a direct effect on the pituitary to stimulate PRL secretion. Administration of OT into the third ventricle, however, lowers PRL levels. This reduction could be mediated by either an inhibition of the release of endogenous OT into the hypohysial portal circulation or via an alteration in the release of some other PRL releasing (PRF) or PRL release-inhibiting (PIF) factor. In order to determine if centrally administered OT lowers PRL levels by increasing secretion of dopamine (DA) into the portal circulation, endogenous dopaminergic tone was blocked by injection of the DA antagonist domperidone (DOM). Subcutaneous administration of DOM resulted in elevated PRL levels which could be further augmented by iv infusion of OT (at 0.01 or 0.1 microgram OT/kg.min) or partially, but significantly, reduced by pretreatment with anti-OT antiserum (0.75 ml) indicating that under conditions of DA blockade, OT (which has little PRF activity during conditions of normal dopaminergic tone) can stimulate PRL secretion by a direct pituitary action. Treatment with DOM did not prevent, however, the reduction in PRL levels produced by central administration of OT (2 micrograms). This suggests that the effect of OT to alter PRL secretion when administered into the third ventricle was not mediated via an increase in DA release into the portal circulation. Furthermore, central administration of the OT antagonist CAV-259 (1-deamino-2-D-Trp-4-Val-8-Orn-OT) after DOM treatment resulted in a significant increase in PRL secretion indicating that endogenous levels of OT within the hypothalamus inhibit PRL secretion through a nondopaminergic mechanism. This stimulatory effect of the OT antagonist was not blocked by pretreatment with anti-OT antiserum (iv) which had been demonstrated previously to reduce the PRL surges in lactating mothers and steroid-primed ovariectomized rats, as well as to block the increase in PRL secretion seen after central administration of vasoactive intestinal peptide (VIP). Thus the central effect of OT to alter PRL secretion was probably not due to a change in the release of OT into the portal circulation. Intravenous administration of a VIP antagonist (D-4-Cl-6-Phe-17-Leu-VIP, previously demonstrated to be capable of reducing the PRL surge seen in lactating mothers) into DOM-treated rats does not alter PRL levels but blocks the ability of central administration of the OT antagonist CAV-259 to increase PRL levels under these conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenomedullin inhibits salt appetite.

Adrenomedullin (AM) exerts profound natriuretic and vasodilatory effects in conscious animals. This newly discovered hormone also acts in the central nervous system to inhibit water drinking and in the pituitary gland to reduce basal and stimulated ACTH release. We investigated whether the natriuretic action of AM in kidney was matched by a central nervous system action to decrease salt intake. Isotonic hypovolemia induced in male rats by pretreatment with polyethylene glycol potently stimulates both water and salt water (0.3 mol/liter NaCl) drinking. Saline drinking was significantly inhibited when AM was administered into the lateral cerebroventricle before the drinking interval. The effect was dose related (dose range, 44-88 pmol), long lasting (> 5 h), and reversible (resolved at 24 h). When hypovolemic rats were administered antiserum to AM (intracerebroventricular administration) before the drinking interval, a significant 2-fold augmentation of saline drinking was observed. These data suggest that in addition to peripheral actions on cardiovascular and renal function and pituitary actions to inhibit ACTH release, AM may act within the central nervous system to determine fluid and electrolyte balance and, ultimately, blood pressure.

Comparison of the pituitary effects of the mammalian endothelins: vasoactive intestinal contractor (endothelin-beta, rat endothelin-2) is a potent inhibitor of prolactin secretion.

Direct pituitary effects of vasoactive intestinal contractor (VIC), which has been described recently to be the rat form of endothelin-2 (ET-2), were compared to those previously reported for rat ET-1, rat ET-3, and human ET-2. In static incubations of cultured dispersed anterior pituitary cells, the minimum effective dose of VIC necessary to inhibit PRL release after 1-h incubation was 1 pM, and the maximum effective dose was 1 nM. Similar inhibition was observed with human ET-2. The minimum effective inhibitory dose of ET-1 was also 1 pM; however, that of ET-3 was 0.1 nM. PRL release inhibition by VIC was not mediated via the D2-dopamine receptor and was not prevented by calcium channel blockade with 100 nM nifedipine. The inhibitory effect of VIC was not present in cells treated with 100 nM staurosporine, a dose that inhibits protein kinase-C activity. Time-course studies revealed a transient stimulation of PRL release with higher doses of VIC (10 and 100 nM), which occurred within the first 15 min of incubation and was unaffected by calcium channel blockade or inhibition of protein kinase-C activity. No stimulation of PRL release was observed with doses of VIC lower than 10 nM. Instead, we observed the maintenance of the inhibitory effect for 4 h of incubation. GH release was not significantly affected by doses of VIC ranging from 10(-13)-10(-7) M; however, the release of LH was slightly, yet significantly, stimulated by 10 and 100 nM VIC. This release was prevented by pretreatment with nifedipine, but unaffected by protein kinase-C inactivation. A physiological role for VIC (rat ET-2) in the control of lactotroph function is suggested by its effectiveness at picomolar doses and its long-lasting action.

A novel vasoactive peptide, adrenomedullin, inhibits pituitary adrenocorticotropin release.

The potent hypotensive peptide, adrenomedullin (AdM), originally isolated from a human pheochromocytoma is present in a variety of rat and human tissues. We examined its potential effects in anterior pituitary gland, reasoning that it may be a feedback regulator of adrenocorticotropin (ACTH) secretion. Rat AdM11-50 inhibited basal ACTH secretion from dispersed, rat anterior pituitary cells in a significant, dose-related fashion (maximum inhibition at 10(-9) M). Rat AdM11-50 also inhibited, in a dose-related fashion, corticotropin releasing hormone (CRH)-stimulated ACTH secretion, but did not block the ability of CRH to stimulate cAMP accumulation in these cells. These findings suggest that in addition to peripheral actions in the vasculature and kidney, adrenomedullin may act within the anterior pituitary gland to control fluid and electrolyte homeostasis.

Atrial natriuretic factor inhibits luteinizing hormone secretion in the rat: evidence for a hypothalamic site of action.

The presence of atrial natriuretic factor (ANF) immunoreactivity and receptors for ANF in the median eminence, hypothalamus, and anterior pituitary gland suggests a role for the peptide in the hypothalamic control of anterior pituitary function. In conscious ovariectomized female rats, transient elevation of plasma levels of ANF by volume loading, a stimulus known to release endogenous ANF from the heart, or by bolus iv injection of 0.1, 1.0, or 10 micrograms synthetic ANF failed to result in altered circulating levels of LH or GH. Constant iv infusion of ANF for 30 min, such that 2- to 3-fold elevations in plasma ANF were detected by RIA resulted, however, in significant inhibition of LH release in ovariectomized female rats (0.05 and 0.1 micrograms ANF/kg.min) and orchidectomized male rats (0.1 microgram ANF/kg.min). It was unlikely that this effect was exerted at the level of the anterior pituitary, since ANF failed to alter basal or LHRH-stimulated LH release from cultured anterior pituitary cells in vitro and since iv infusion of 0.1 microgram ANF/kg.min failed to alter pituitary responsiveness in vivo to a 10-ng bolus injection of LHRH. Significant inhibition of LH secretion was also observed after third cerebroventricular injection of 1.0 or 2.0 nmol ANF. As with iv infusion, central administration of ANF failed to significantly alter GH secretion. LHRH release from median eminence explants incubated in vitro in the presence of dopamine (60 or 120 microM) was inhibited by 10(-7) M ANF, suggesting a median eminence site of action of the peptide. Finally, an opiate involvement in the mechanism of ANF's action was suggested, since naloxone (0.5 mg, iv, followed by a 60-min infusion of an additional 1 mg) completely blocked the ability of ANF (0.1 or 0.5 microgram/kg.min, infused over the last 30 min of naloxone administration) to inhibit LH release. These data suggest that ANF can act centrally to alter the hypothalamic control of gonadotropin secretion, possibly by interacting with central dopaminergic and peptidergic systems. They further suggest actions of ANF within the brain unrelated to its previously described effects on fluid and electrolyte homeostasis.

A possible role of prostacyclin to stimulate prolactin and growth hormone release by hypothalamic and pituitary actions, respectively.

Prostacyclin (PGI2) (1-5 micrograms in 3 microliters 0.05 M Tris/HCl buffer, pH 7.5) and its stable metabolite, 6-oxo-PGF1 alpha, were microinjected into the third ventricle of ovariectomized rats, and plasma FSH, GH, PRL, and TSH levels were measured by RIA. Control animals received 3 microliters buffer. Injection of 5 micrograms PGI2 dramatically elevated plasma PRL values (4- to 5-fold) at 5 and 15 min, whereas the same dose of 6-oxo-PGF1 alpha produced a significant but smaller (2-fold) stimulatory effect. A delayed increase (1.5-fold) in plasma GH occurred after intraventricular PGI2 at 30 and 60 min. 6-Oxo-PGF1 alpha failed to alter GH levels. There were no alterations in plasma FSH and TSH after intraventricular injection of PGI2. Dispersed, overnight cultured cells from anterior pituitaries of ovariectomized rats were tested with 10(-4)-10(-7) M PGI2 and its metabolite. After 15 min of incubation, 3 X 10(-5) PGI2 produced a highly significant elevation in GH release (P less than 0.001), whereas there was no alteration in PRL levels. Only pharmacological doses of 6-oxo-PGF1 alpha (10(-4) M) stimulated GH release. There was no alteration in PRL release by the cultured cells even in the presence of 10(-4) PGI2. These results suggest that PGI2 stimulates PRL release by a hypothalamic action either to increase the release of PRL-releasing factor, or to decrease release of PRL-inhibiting factor, or by both mechanisms. The delayed stimulatory effect of PGI2 on the release of GH may be exerted via an effect on the anterior lobe itself, since PGI2 was effective in stimulating GH release by the incubated pituitary cells.

A nonoxytocinergic prolactin releasing factor and a nondopaminergic prolactin inhibiting factor in bovine neurointermediate lobe extracts: in vitro and in vivo studies.

Several peptidergic PRL-releasing factors (PRFs) have been described; however, none have been proven to be of primary physiological importance in the control of hormone release. Similarly dopamine withdrawal alone cannot completely explain the profiles of PRL secretion observed under a variety of conditions. We describe here the isolation in semipurified form of both a PRF and a PRL-inhibiting factor (PIF) from bovine neurointermediate lobe (NIL) extracts. Acid extracts of bovine NILs stimulated, in a dose-related manner, PRL release from cultured anterior pituitary cells, even after immunoabsorption of endogenous oxytocin from the extract. PIF and PRF activities were semipurified from NIL extracts by Sephadex chromatography and detected by in vitro and in vitro bioassays. The PRF material could be separated from oxytocin by gel sieving and was active in the presence of dopamine in vitro unlike synthetic oxytocin and in cell preparations in which the oxytocin-responsive lactotrophs had been removed by selective cytotoxin cell targeting using an oxytocin-ricin A chain cytotoxic conjugate. The PRF material stimulated PRL secretion in a dose-dependent fashion in conscious male rats after iv injection. The PIF material comigrated on sizing gel chromatography with immunoreactive oxytocin and was active in vitro during dopamine blockade with domperidone and in vivo in the presence of endogenous dopaminergic tone. These data suggest that novel factors present in the NIL might exert physiologically relevant control over lactotroph function and add to the growing literature on the presence of a PRF in the NIL.

Possible neuroendocrine actions of endothelin-3.

The presence of endothelin (ET) immunoreactivity and binding sites in hypothalamus and pituitary gland suggests potential neuroendocrine actions of this family of vasoactive peptides. ET-3, the predominant member of the ET family in brain, exerted significant dose-related (1, 10, and 100 nM) inhibitory effects on PRL release from dispersed anterior pituitary cells in static incubations. The effect was not dependent on voltage-sensitive calcium channels, since the dihydropyridine calcium channel antagonist nifedipine failed to block this action. Nifedipine did, however, significantly reduce the transient acute stimulatory effect of ET-3 on PRL release in cultured cells incubated in dynamic perifusion. The longer lasting inhibitory effect on PRL release that followed the brief stimulatory action was not affected by nifedipine. ET-3 also stimulated a transient but significant release of LH from cells harvested from random cycle female rats, an effect that was not antagonized by a LHRH antagonist, but was blocked by nifedipine, suggesting the mobilization of extracellular calcium as a mechanism of action of ET-3. Nifedipine also reversed the acute stimulatory effect of ET-3 on GH secretion from these cells. Cerebroventricular injections of ET-3 (6 or 60 ng) failed to significantly alter PRL or LH secretion in conscious rats, suggesting that brain-derived ET does not act within the hypothalamus to alter the release of these two hormones. Similarly, iv infusion of even pressor doses of ET-3 (10, 30, or 300 ng) failed to significantly alter PRL, LH, or GH release; thus, it is unlikely that ET of peripheral origin acts within the gland. Our results suggest that locally produced ET may act as a neuroendocrine or paracrine factor controlling pituitary function in the rat.

Hypothalamic and pituitary sites of action of oxytocin to alter prolactin secretion in the rat.

To determine whether oxytocin (OT) could alter the release of PRL and other hormones from the anterior pituitary gland, the effects of OT were examined in two in vitro and two in vivo test systems. Cells dispersed from anterior pituitary glands of intact adult male rats were incubated in medium containing OT at doses of 10(-8), 10(-7), 10(-6), and 10(-5) M in two trials. OT stimulated PRL release 1.5-fold (P less than 0.01) and 2- to 3-fold (P less than 0.001) above control levels at 10(-8) and 10(-7) M doses, respectively, thus indicating a dose-dependent relationship. Higher doses did not produce a further elevation above that obtained with 10(-7) M OT. Arginine vasopressin (AVP) caused a slight decrease in PRL release from dispersed cells while TRH produced a small (25%), significant, but nondose-related increase in PRL release. Hemipituitary glands from adult male rats, incubated with 10(-6) and 10(-5) M OT, released twice as much PRL (P less than 0.01) into the medium as paired controls, but 10(-7) M OT was ineffective. The iv injection of 1 or 10 micrograms OT into conscious male rats elevated plasma PRL by 50% (P less than 0.05) or 500% (P less than 0.001), respectively, above basal values at 5 min only. Vehicle or 0.1 microgram OT were without effect. When 0.1 microgram OT was microinjected into the third ventricle (3V) of conscious male rats, it paradoxically reduced plasma PRL by 40% at 30 min (P less than 0.05), whereas 1 microgram OT significantly lowered PRL at 5-60 min, with the maximum suppression (60%, P less than 0.001) occurring at 30 min. These latter findings may indicate that an ultrashort loop feedback mechanism exists whereby exogenous OT decreases hypothalamic OT secretion, thereby reducing the OT stimulus for PRL release. The specificity of the OT effect on PRL was attested to by the failure of OT to alter significantly FSH, LH, and TSH in each system. GH was unchanged except that 3V-injected OT (1 microgram only) elevated (P less than 0.001) plasma GH at 15-30 min. These results support the view that OT acts directly on the cells of the anterior pituitary gland at low to high doses to release PRL specifically and in a dose-related fashion. In contrast, 3V injection of OT reduces PRL secretion, thereby suggesting that OT may decrease its own neurosecretion by ultrashort loop feedback and thus reduce an OT stimulus for PRL release.

C-type natriuretic peptide mediates the hypothalamic actions of the natriuretic peptides to inhibit luteinizing hormone secretion.

Both A- and C-type natriuretic peptides (ANP and CNP, respectively) significantly reduce LH secretion when injected into the third cerebral ventricle of conscious rats. To establish which natriuretic peptide receptor subtype transduces these inhibitory messages, we have employed novel cytotoxin cell targeting techniques to selectively destroy cells in the hypothalamus that respond to ANP or CNP. Rats pretreated with ANP conjugated to the toxic A-chain of the plant cytotoxin ricin failed 1 week later to respond to central injection of ANP with the normal inhibition of LH secretion. These rats did, however, respond with significant inhibition of LH secretion to central injection of CNP. In fact, the LH inhibition observed after CNP injection was significantly greater than that expressed after similar injection of CNP in rats pretreated with unconjugated ricin A-chain (toxin control). Those control rats displayed significant reduction of LH levels in response to ANP injection as well. Plasma LH levels were not significantly affected by central administration of either ANP or CNP in rats pretreated with ricin A-chain conjugated to CNP. These results further demonstrate the power of this novel technology and provide positive evidence supporting our hypothesis that ANP exerts its LH-inhibiting effect by displacing endogenous CNP from clearance receptors within the brain. This endogenous CNP, then, like exogenously applied CNP, activates the guanyl cyclase-B receptors on cells, which are part of the network controlling the release of LHRH.