Chronic hyponatremia reduces survival of magnocellular vasopressin and oxytocin neurons after axonal injury.

Axonal injury to hypothalamic magnocellular vasopressin (AVP) and oxytocin (OT) neurons causes degeneration of a substantial subpopulation of these neurons. In this study, we investigated the influence of osmolality on this injury-induced cell death. Normonatremic, chronically hypernatremic, and chronically hyponatremic rats received pituitary stalk compression (SC), which causes degeneration of AVP and OT terminals in the neurohypophysis. Twenty-one days after SC, rats were perfused and hypothalami were serially sectioned and alternately stained for AVP-neurophysin and OT-neurophysin immunoreactivities. Normonatremic and hypernatremic rats exhibited a triphasic pattern of water intake after SC, with peak intakes 3 times higher than those exhibited by sham-operated normonatremic rats. In contrast, hyponatremic SC rats exhibited peak water intakes of 600 ml/24 hr, approximately 9-10 times the water intakes of sham-operated normonatremic rats. In normonatremic rats, SC caused degeneration of 65% of the AVP neuron population in the SON and 73% in the PVN, but only 31% of the OT neuron population in the SON and 35% in the PVN. Similar results were found in hypernatremic rats after SC. However, in hyponatremic rats SC caused degeneration of 97% of the AVP neuron population in the SON and 93% in the PVN, and 90% of the OT neuron population in the SON and 84% in the PVN. Our results, therefore, demonstrate that injury-induced degeneration of magnocellular AVP and OT neurons is markedly exacerbated by chronic hypo-osmolar conditions, but neuronal survival is not enhanced by chronic hyperosmolar conditions.

The hypothalamic ventromedial nucleus sends a met-enkephalin projection to the preoptic area's periventricular zone in the female rat.

The female but not the male rat possesses a dense network of methionine enkephalin (m-Enk) fibers in the periventricular zone of the preoptic area (pePOA). The potential source of these fibers was determined by injection of the tracer fluorogold, FAu, into the preoptic area of adult female rats. Twenty-four hours before they were killed, the rats were administered colchicine (intraventricularly) to enable immunocytochemical visualization of m-Enk cells. Upon examination of the brains with fluorescence microscopy, double-labeled cells showing fluorogold and immunofluorescence for m-Enk were consistently observed in the preoptic area, the ventrolateral division of the ventromedial nucleus of the hypothalamus (VMHvl) and nearby medial tuberal area (MTA), the arcuate nucleus, periventricular area of the hypothalamus, perifornical area, and dorsomedial nucleus of the hypothalamus. A series of lesion and knife cut experiments using glass, Halasz, and wire knives determined that the pePOA m-Enk fibers arose from the hypothalamus, near or within the VMH. Ibotenic acid lesions further determined that the source of the m-Enk projection was the VMHvl with a possible additional contribution from the MTA.

Cholecystokinin activates catecholaminergic neurons in the caudal medulla that innervate the paraventricular nucleus of the hypothalamus in rats.

Stimulation of gastric vagal afferents by systemic administration of cholecystokinin octapeptide (CCK) inhibits gastric motility, reduces food intake, and stimulates pituitary secretion of oxytocin and adrenocorticotropic hormone in rats. To characterize further the central neural circuits responsible for these effects, the present study used triple-labeling immunocytochemical methods to determine whether or not exogenous CCK activates cFos expression in catecholaminergic neurons in the caudal medulla that project to the paraventricular nucleus of the hypothalamus (PVN). To identify these neurons, the retrograde tracer fluorogold (FG) was iontophoresed into the PVN of anesthetized rats under stereotaxic guidance. After 2 weeks, rats were injected with CCK (100 micrograms/kg, i.p.) and then anesthetized and killed 1 hour later by perfusion fixation. Medullary sections were processed for triple immunocytochemical localization of cFos, retrogradely transported FG, and tyrosine hydroxylase (TH). In rats with FG injections centered in the PVN (n = 10), approximately 70% of the FG-labeled neurons in the caudal nucleus of the solitary tract (NST) and ventrolateral medulla (VLM) expressed cFos. Of these activated PVN-projecting neurons, approximately 78% in the NST and 89% in the VLM were catecholaminergic (TH positive). These results indicate that PVN-projecting catecholaminergic neurons within the caudal medulla are activated by peripheral administration of CCK, further implicating these ascending catecholaminergic pathways in the neuroendocrine, physiological, and behavioral effects produced by gastric vagal stimulation.

Effect of chronic hyponatremia on central and peripheral oxytocin and vasopressin secretion in rats.

Previous studies have shown that many treatments that stimulate the peripheral secretion of oxytocin (OT) and vasopressin (AVP) from the pituitary simultaneously increase the levels of these peptides in the cerebrospinal fluid (CSF). Since osmotically and nonosmotically stimulated pituitary secretion of OT and AVP is markedly blunted in hyponatremic rats, the present studies evaluated whether central OT and AVP secretion into the CSF is similarly inhibited during sustained hyponatremia. Adult male rats with indwelling cisterna magna cannulae were rendered hyponatremic (plasma [Na+] < 110 mmol/l) by s.c. infusion of desmopressin (dDAVP; 10 ng/h) in combination with ingestion of a liquid diet for 3 days, then subjected to osmotic (i.v. or i.p. injection of 2 M NaCl; HS) or nonosmotic (6 mmol/kg of 0.15 M LiCl i.p.) stimulation. In normonatremic rats both i.v. and i.p. HS caused marked increases in plasma OT and AVP levels 30 min after treatment. Significant elevations of OT, but not AVP, were also present in CSF. Despite similar increases in plasma Na+ concentrations, plasma OT responses in the hyponatremic rats were absent after HS i.v. and were significantly blunted after HS i.p., but neither group had increased plasma AVP. In parallel with the plasma results, CSF OT responses were absent in hyponatremic rats given HS i.v. and significantly blunted in hyponatremic rats given HS i.p., but neither group had increased CSF AVP. Nonosmotic stimulation with isotonic LiCl increased OT levels both in plasma and CSF in normonatremic rats 20 min after treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Naloxone disinhibits magnocellular responses to osmotic and volemic stimuli in chronically hypoosmolar rats.

Normonatremic and chronically hyponatremic rats were pretreated with naloxone (5 mg/kg) or isotonic (150 mM) NaCl, then were given i.v. injections of 2 M NaCl (2 ml) or were hemorrhaged (20 ml/kg). Baseline and post-stimulus blood samples were withdrawn through indwelling jugular venous catheters. Baseline levels of plasma vasopressin (AVP) and oxytocin (OT) were similar in both normonatremic and hyponatremic rats and did not change after naloxone pretreatment. Increases in plasma AVP and OT levels in response to both hypertonic saline and hemorrhage were markedly blunted in the hyponatremic rats compared to the normonatremic rats. Naloxone pretreatment caused augmented AVP and OT secretion in response to hypertonic saline stimulation and hemorrhage in both the normonatremic and hyponatremic rats; the magnitude of the naloxone augmentations in the hyponatremic rats were sufficient to normalize the OT response to hypertonic saline and both the OT and AVP responses to hemorrhage. Our results therefore suggest that endogenous opioids are likely involved in the inhibition of stimulus-induced AVP and OT release that accompanies chronic hypoosmolality.

Effects of footshock stress upon spleen and peripheral blood lymphocyte mitogenic responses in rats with lesions of the paraventricular nuclei.

To assess the role of the hypothalamic paraventricular nucleus (PVN) in mediating stressor-induced immune alterations, male Lewis rats were subjected to a 1-h session of intermittent footshock stress or home cage conditions 6 days after receiving bilateral or sham PVN lesions. Splenic and peripheral blood lymphocyte proliferative responses to the non-specific mitogens, concanavalin A (ConA) and phytohemagglutinin (PHA), were subsequently measured as were plasma corticosterone levels. In sham-operated rats, footshock markedly elevated plasma corticosterone levels and concurrently suppressed the proliferative responses of peripheral blood and splenic lymphocytes. In PVN-lesioned rats, however, the shock-induced suppression of lymphocyte proliferation in the peripheral blood and the elevation of plasma corticosterone were significantly attenuated, while lymphocyte proliferation in the spleen was suppressed below the level of the sham-treated animals. Thus, by utilizing ablation studies, we have determined that the PVN may play a direct role in the alteration of lymphocyte function during stress, and an intact PVN buffers the effect of stress on the responsiveness of spleen lymphocytes to non-specific mitogens.

Osmotic inhibition of prolactin secretion in rats.

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

Hyponatremia inhibits sprouting of oxytocinergic axons following bilateral lesions of the paraventricular nucleus in rats.

The external zone of the median eminence in rats contains very few oxytocinergic (OT) fibers. However, 6 weeks after lesioning the paraventricular nucleus (PVN) many OT axons can be found in the external zone, possibly as part of a mechanism compensating for the loss of secretion originating from the PVN. In this study, we investigated whether such structural reorganization of the median eminence occurs in chronically hyponatremic rats, which are known to have significantly inhibited OT and vasopressin (AVP) neurosecretory responses to most physiological stimuli. Normonatremic and chronically hyponatremic rats received bilateral PVN lesions using a rotating knife. One week after PVN lesions both vasopressin-neurophysin (AVP-NP) and oxytocin-neurophysin (OT-NP) staining intensities decreased in the internal zone of the median eminence, and AVP-NP immunoreactivity almost entirely disappeared from the external zone. Six weeks after PVN lesions OT-NP but not AVP-NP immunostaining appeared in the external zone of the median eminence in the normonatremic rats. In contrast, no increase in OT-NP staining occurred in the external zone of the median eminence in the hyponatremic rats by six weeks after PVN lesions. These results indicate that chronic hyponatremia inhibits not only synthesis and secretion of AVP and OT, but also impairs cellular processes involved in the sprouting response normally seen following bilateral PVN lesions in rats.

Gastric motility and food intake in rats after lesions of hypothalamic paraventricular nucleus.

Systemic administration of cholecystokinin (CCK) or LiCl inhibits gastric motility and food intake in rats. Brain stem-projecting oxytocin (OT) neurons in the hypothalamic paraventricular nucleus (PVN) have been proposed to mediate the inhibitory effects of CCK and LiCl on gastric motility and food intake. In the present studies, we found that basal gastric motility was elevated in rats 12-20 h after knife-cut lesions of the PVN; however, this effect disappeared 3 days later. Furthermore, CCK and LiCl inhibited gastric motility at 12-20 h, 3 days, and 3 wk after PVN lesions, although their effects were blunted. Injection of the local anesthetic lidocaine into the PVN had effects similar to acute PVN lesions. In rats with PVN lesions, the inhibitory effects of CCK and LiCl on food intake were indistinguishable from those in sham-lesioned rats. We conclude that the PVN tonically inhibits gastric motility and that it participates in, but is not essential for, the inhibitory effects of CCK and LiCl on gastric motility and food intake in rats.

Functional neurolobectomy induced by controlled compression of the pituitary stalk.

Degeneration of magnocellular nerve terminals in the neurohypophysis was induced by compressing the pituitary stalk of anesthetized rats for 30 s using a triangle-shaped wire. Immediately after stalk compression (SC), rats exhibited markedly increased water intake characteristic of diabetes insipidus, followed by a triphasic pattern of fluid intake. In SC rats, arginine vasopressin (AVP) and oxytocin (OT) contents of the neurointermediate lobe (NIL) of the pituitary gland were significantly reduced to approximately 2.5% and approximately 10% of sham-operated controls, respectively. In contrast, OT, but not AVP, content of the stalk-median eminence (SME) of SC rats was significantly increased. Histological examination of the pituitaries showed substantial degeneration of the neural lobe with very scarce AVP-neurophysin and OT-neurophysin immunoreactivity, while both the anterior and the intermediate lobes appeared to be intact. Plasma AVP and OT responses to infusion of hypertonic NaCl were significantly blunted in SC rats compared to sham-operated controls. However, two days after surgery the secretory patterns of LH in SC rats were similar to those in the controls. These results indicate that controlled compression of the pituitary stalk results in selective degeneration of the neural lobe without causing permanent ischemic damage to the anterior pituitary, and produces marked sustained functional deficits in pituitary AVP and OT secretion. Consequently, SC provides an alternative means to achieve selective neurolobectomy in rats.

Vasopressin and oxytocin secretion in chronically hyposmolar rats.

Neurohypophysial secretion of vasopressin (AVP) and oxytocin (OT) was studied in rats maintained under hyposmolar conditions for 10-24 days. Graded intravenous infusions of hypertonic saline solutions had no consistent effect on plasma AVP and OT levels until plasma sodium concentration ([Na+]) exceeded 130 mM, after which levels of both hormones increased as an exponential function of plasma [Na+]. Detectable increases in plasma AVP and OT began at significantly lower plasma [Na+] in hyposmolar rats than in normosmolar control rats (10.8 mM lower for AVP and 18.4 mM lower for OT). AVP and OT secretion in hyposmolar rats was also markedly blunted in response to nonosmotic stimuli, including acute and chronic hypovolemia and systemic administration of cholecystokinin. Cessation of 1-desamino-8-D-arginine vasopressin-induced antidiuresis resulted in an appropriately rapid correction of plasma [Na+] to normal levels within 24 h. Consequently, although chronic hyposmolarity caused a moderate downward resetting of the osmotic thresholds for AVP and OT secretion, it did not cause sustained deficits in osmoregulation. These results suggest that osmoreceptor activity is regulated to maintain extracellular fluid and plasma osmolality within narrow absolute ranges rather than responding to relative changes in osmolality.

Hyponatremia-induced inhibition of magnocellular neurons causes stressor-selective impairment of stimulated adrenocorticotropin secretion in rats.

Chronically hyponatremic rats were subjected to various stressors in order to evaluate the possible contribution of magnocellular neurons to the regulation of ACTH secretion, since such rats have markedly inhibited secretion and synthesis of magnocellular arginine vasopressin (AVP) and oxytocin (OT). Stress caused by a novel environment or by insulin-induced hypoglycemia resulted in moderate increases in plasma ACTH, which were of similar magnitude in both hyponatremic and normonatremic rats, and these stressors caused no increase in plasma AVP and OT levels in either group of rats. However, when exposed to ether, hyponatremic rats exhibited a significantly blunted ACTH response compared to normonatremic controls (331 +/- 49 vs. 740 +/- 124 pg/ml; P less than 0.01, respectively), and plasma AVP levels were markedly increased in the normonatremic, but not in the hyponatremic, rats. Intravenous infusion of 2 M NaCl also caused an ACTH release in hyponatremic rats that was significantly smaller than that in their normonatremic counterparts (228 +/- 52 vs. 479 +/- 85 pg/ml; P less than 0.05, respectively), and in this case both plasma AVP and OT levels were markedly increased in the normonatremic, but not in the hyponatremic, rats. However, hyponatremic rats exhibited greatly increased plasma ACTH levels 2 and 96 h after adrenalectomy (ADX), which were statistically equivalent to the increases in ACTH levels in normonatremic rats after ADX. Seven days after ADX parvocellular neurons of the paraventricular nucleus showed strongly increased CRF-41 and AVP-neurophysin, but not OT-neurophysin, immunoreactivities in both normonatremic and hyponatremic rats. These results show that parvocellular CRF-41/AVP-producing neurons in the paraventricular nucleus are not inhibited by chronic hyponatremia, in contrast to magnocellular neurons, and suggest that ACTH secretion induced by ether or hypertonic saline, but not by novel environment or insulin-induced hypoglycemia, is partially mediated by magnocellular AVP and/or OT.

Oxytocinergic neurons in rat hypothalamus. Dexamethasone-reversible increase in their corticotropin-releasing factor-41-like immunoreactivity in response to osmotic stimulation.

Recent results have demonstrated altered corticotropin-releasing factor (CRF)-41 content of the neurointermediate lobe (NIL) of the pituitary gland in response to various manipulations including osmotic stimulation. This study was undertaken to determine whether changes in CRF-41 content of the NIL are accompanied by changes in intensity of CRF-41-like immunoreactivity (CRF-41-LI) of neurosecretory neurones of the hypothalamus in response to osmotic stimulation. Wistar rats of both sexes given either tap water ad libitum, 2% NaCl solution, or access to tap water was limited to 20 min daily, for 7 days. Subsets of rats from each group were adrenalectomized (ADX) or treated with dexamethasone (DEX). Thirty-six hour before perfusion with fixative consisting of buffered formaldehyde and picric acid, animals received 75 micrograms colchicine i.c.v. Forty micrometer thick vibratome sections were stained for CRF-LI, arginine vasopressin (AVP-LI) and oxytocin (OXY-LI) using the avidin-biotin-peroxidase complex method. In response to both types of osmotic stimulation magnocellular neurones of the paraventricular (PVN) and supraoptic nuclei (SON) showed increased CRF-LI, AVP-LI and OXY-LI, while CRF-LI of parvocellular perikarya of the PVN decreased. The enhanced CRF-LI seemed to appear in a subset of magnocellular neurones with OXY-LI but not AVP-LI. Increased staining intensities were also observed in magnocellular neurones in ADX rats challenged osmotically. In contrast, systemic DEX administration, as well as implantation of DEX in the area on the SON, sharply attenuated CRF-LI but not AVP-LI or OXY-LI of magnocellular neurones in osmotically stimulated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Osmotic stimulation affects neurohypophysial corticotropin releasing factor-41 content: effect of dexamethasone.

In this study we examined the effects of 2% saline loading (S), partial restriction of water consumption (R) or a combination of S or R with dexamethasone (DEX) treatment for 14 days on corticotropin releasing factor (CRF)-41 content of the neurointermediate lobe (NIL) and supraoptic nuclei (SON) of male Wistar rats. Arginine vasopressin (AVP) and oxytocin (OXY) contents of the NIL and SON were also assayed as well as plasma corticosterone, ACTH, [Na+] and [Cl-] concentrations. S or R for 14 days resulted in an increase in CRF-41 content and a concomitant drop in AVP and OXY contents of the NIL. Dexamethasone treatment enhanced the effect of S but not of R on NIL CRF-41 content. Dexamethasone treatment abolished the decrease in the AVP content and partially reversed the decrease in the OXY content of the NIL in response to S but not in response to R. No changes were observed in CRF-41, AVP and OXY content of the SON. Unstressed plasma corticosterone concentrations were not changed in S rats but were elevated in R rats; DEX did not prevent this elevation. Plasma ACTH concentrations were low in all groups examined. Plasma [Na+] and [Cl-] increased in response to both S and R. Increases in [Na+] and [Cl-] evoked by S but not R were prevented by DEX treatment. The results show that in the NIL, osmotic stimulation decreases AVP and OXY content, while it increases the CRF-41 content.

Long-term salt loading impairs pituitary responsiveness to ACTH secretagogues and stress in rats.

Male Wistar rats were allowed to drink tap water ad lib (W), 2% saline (S) or 2% saline containing dexamethasone (S + D, 1 mg/l) for 7 days. On the 8th day rats were subjected to a 3-min ether stress. Plasma ACTH, corticosterone and prolactin concentrations were determined before and after ether exposure. Prestress concentrations of plasma ACTH were low and did not vary among the three groups. In response to ether stress W rats exhibited twice as high plasma ACTH concentrations as did S rats. Rats of the S + D group exhibited a small but statistically significant ACTH response. Plasma corticosterone concentration in S rats was increased while in S + D rats was significantly decreased under resting conditions compared to that in W rats. Ether stress caused large increases in plasma corticosterone concentrations in W and S rats while a small but statistically significant increase was observed in S + D rats. Prolactin responses to ether were smaller in groups S and S + D than in group W. To test whether the decreased ACTH response to ether exposure was a result of a decreased sensitivity of corticotrope cells to corticotropin releasing factor (CRF)-41 or arginine vasopressin (AVP), adenohypophysial fragments from W, S and S + D rats were incubated in the presence of different doses of CRF-41 or AVP. Pituitary fragments obtained from W rats secreted larger amounts of ACTH than did pituitaries from S rats in response to either CRF-41 or AVP.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of restoration of ACTH release in rats with long-term lesions of the paraventricular nuclei.

The effects of lesions in the paraventricular nucleus (PVN) on the adrenocortical response to ether stress were investigated in neurohypophysectomized and intact rats. During the first 4 days after placement of lesions in the PVN, the corticosterone response to ether stress was almost completely inhibited. It then gradually increased and, within 4-6 weeks of surgery, was restored to about 60% of that in sham-operated rats. Basal plasma concentrations of corticosterone were low in rats after placement of lesions in the PVN and/or after neurointermediate lobectomy (NILX). Corticosterone responses to ether stress were similar in groups submitted to PVN lesions and/or NILX, and lower than those in the appropriate sham-operated groups. In all lesioned groups, plasma ACTH concentrations after a combination of stressors (ether plus laparotomy) were also lower than those in the sham-operated groups. Six weeks after lesioning of the PVN, immunoreactive rat corticotrophin-releasing factor-41 (rCRF-41) concentrations in stalk-median eminence (SME) extract fell to about 5% of that in sham-operated rats, while immunoreactive arginine vasopressin (AVP) concentrations did not change. Immunohistochemistry revealed a substantial decrease in rCRF-41 immunostaining of the median eminence 6 weeks after lesioning of the PVN, though randomly located clusters of stained terminals were still seen in the whole rostro-caudal extent of the median eminence. A mixture containing synthetic rCRF-41 and AVP, in proportions similar to those in SME extracts from sham-operated rats, caused significantly less release of ACTH from anterior pituitary cell cultures than did SME extracts from sham-operated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Maturation of the circadian rhythm of the adrenocortical functions in human neonates and infants.

The cortisol levels in the peripheral blood were measured radioimmunologically at 08.00, 12.00,. 16.00, 20.00, 24.00 and 04.00 h in human neonates and infants with ages of 1, 2 and 3 days, 1, 2 and 4 weeks, 2, 3 and 6 months and 1 and 3 years. During the first 2 days of extrauterine life the neonates had high plasma cortisol levels without a circadian rhythm. During the first 2 months there were decreased cortisol levels in the infants, but the diurnal rhythms were still absent, and free-running fluctuations were observed. The typical circadian rhythms of the plasma cortisol levels were present in infants aged 3 months; these rhythms were identical with the diurnal changes of 1- and 3-year-old infants. In the groups of patients studied the data suggest that the circadian rhythm of the adrenocortical function may develop to the adult-type pattern already during early infancy.

Effect of prostaglandins on hypothalamic-pituitary-testicular function in vitro.

The effect of prostaglandins (PG) A1, E1, E2 and F2 alpha in the concentration range of 10(-7)--10(-4) M were studied in vitro on a rat hypothalamic tissue, collagenase-digested isolated anterior pituitary cell and Leydig cell suspension system by measuring the testosterone production of incubated Leydig cells. PGs did not change the testosterone production and the hCG sensitivity of the Leydig cells, nor the LH secretion and the LHRH sensitivity of the anterior pituitary cells. PGE2 at concentrations of 10(-6), 10(-5) and 10(-4) M significantly increased the hypothalamic tissue-induced pituitary-testicular activation, and this stimulatory effect of PGE2 was dose dependent. PGA1, PGE1 and PGF2 alpha did not alter hypothalamic LHRH release measured in vitro. The results suggest that PGE2 has a direct stimulatory effect on hypothalamic LHRH release.