Involvement of calmodulin in depolarization-induced release of corticotrophin-releasing hormone-41 from the rat hypothalamus in vitro

We have employed an acute explant system of the rat hypothalamus in vitro, as previously described, to examine the role of calcium and calmodulin in the release of corticotrophin-releasing hormone-41 (CRH-41). Release of CRH-41, as determined by radioimmunoassay, was stimulated in a dose-dependent manner by the membrane-depolarizing agents KCl and veratridine. Stimulation was also observed with the calcium ionophore A23187. The calcium channel blocker verapamil (1-100 mumol/l) inhibited both KCl- and veratridine-induced release in a dose-dependent manner (maximum inhibition of 75% and 60% respectively), thus providing further evidence that calcium entry is required for secretion of CRH-41 following membrane depolarization. Trifluoperazine (1-100 mumol/l), an inhibitor of calmodulin-calcium interaction, decreased both KCl- and veratridine-evoked CRH-41 secretion in a dose-dependent fashion (maximum inhibition of 50% and 30% respectively). Similarly, phenytoin, a calmodulin-dependent kinase inhibitor, in the concentration range of 1-100 mumol/l, also decreased depolarization-induced CRH-41 release in a dose-dependent manner. The basal release of CRH-41 was unaffected by either treatment. Finally, both calmodulin inhibitors (10 mumol/l) decreased CRH-41 release induced by the calcium ionophore A23187 (10 mumol/l). These data provide evidence for the role of calcium in membrane depolarization-induced stimulus-secretion coupling of rat hypothalamic CRH-41. Furthermore, inhibition of the stimulatory responses by two separate classes of calmodulin inhibitors suggests a role for calmodulin, at least in part, in this process.

Lack of modulation of the adrenocortical response to ACTH by an opioid peptide

Eight normal male subjects received 1 mg dexamethasone at 23.00 h and 0.5 mg on wakening followed by a physiological intravenous dose of synthetic ACTH1-24 250 ng, with and without the administration of a stable met-enkephalin analogue (guanyl-DAMME, 100 micrograms) 10 minutes prior to the ACTH. The opioid analogue caused no change in the peak, incremental, or incremental area under the curve responses of plasma cortisol to the ACTH. This study does not support a role for opioid peptides in the acute modulation of the adreno-cortical response to ACTH.

Densitometric analysis of Southern blot autoradiographs and its application to monitoring patients with chronic myeloid leukemia

The characteristic genetic exchange in chronic myeloid leukemia (CML) is the fusion of the ABL proto-oncogene and a specific part of the BCR or phl gene. Detection of this exchange by cytogenetic or Southern blot analysis is highly diagnostic for CML. The latter approach has not previously been used to quantify the relative proportions of leukemic and non-leukemic cells. We have assessed the feasibility of estimating the relative proportion of leukemic cells present in a sample by densitometric analysis of autoradiographs of Southern blots. In dilution experiments of CML cells with normal cells, a linear relationship could be demonstrated between the relative intensity of the autoradiograph band corresponding bcr rearrangement and the proportion of leukemic cells present. This relationship was found to be largely independent of autoradiograph exposure time. Six patients receiving various therapies have been evaluated for as long as 4.5 years by repeated densitometric and cytogenetic analysis. In general, a declining proportion of Philadelphia (Ph) chromosome positive cells was paralleled by decreasing intensity of the autoradiograph band representing bcr rearrangement. Densitometric changes were often seen prior to the detection of Ph negative cells. This analysis appears to provide a sensitive method for monitoring patients with CML.

The use of venous catheterization in the diagnosis and localization of bilateral phaeochromocytomas

OBJECTIVE

The purpose was to assess the value of venous catheter sampling as a method for the location of phaeochromocytomas, particularly when imaging techniques have been equivocal or ambiguous.

DESIGN

Venous catheter sampling was carried out in cases of suspected phaeochromocytoma, and compared with samples obtained from other patients without phaeochromocytomas undergoing adrenal venous catheterization.

PATIENTS

Three patients had phaeochromocytomas (subsequently confirmed by histology); five patients had no clinical or biochemical evidence of phaeochromocytoma but were being investigated for other conditions.

MEASUREMENTS

Catecholamine and cortisol assays were performed on plasma samples from the adrenal veins and elsewhere, and the noradrenaline to adrenaline (NA:AD) ratio was calculated. RESULTS In patients without phaeochromocytomas the NA:AD ratio was less than 1 in nine adrenal vein samples; in four adrenal vein samples with NA:AD ratio of greater than 1, the presence of a phaeochromocytoma was subsequently confirmed. An extra-adrenal tumour was also located by comparison of central and peripheral venous catecholamine concentrations.

CONCLUSIONS

Venous catheterization with measurements of catecholamines, and determination of NA:AD ratios, allows for the rapid and confident diagnosis and localization of unilateral, bilateral, and extra-adrenal phaeochromocytomas.

Megavoltage pituitary irradiation in the management of prolactinomas: long-term follow-up

OBJECTIVE To determine the long-term effects of external beam megavoltage radiotherapy (RT: 4500 cGy via three portals at 180 cGy or less total daily dose) on endocrine function in prolactinomas. DESIGN Longitudinal study following radiotherapy with periodic re-assessment at regular intervals, at least 2 months off dopamine agonist therapy. PATIENTS Thirty-six female patients, age range 19-50 years, with either macroprolactinomas (12 patients) or microprolactinomas (24 patients), but without significant suprasellar extensions. MEASUREMENTS Clinical appraisal, and anterior and posterior pituitary assessment: basal levels at yearly intervals or less, with dynamic screening with TRH, LHRH and hypoglycaemic stimulation every 2-3 years. RESULTS Before RT, serum prolactin (PRL) levels ranged from 1150 to 34,000 mU/l; after RT (mean 8.5 years, range 3-14), serum PRL fell to normal (i.e. less than 360 mU/l) in 18 of the 36 patients (50%), and to just above the normal range (378-780 mU/l) in a further 10 (28%). Two patients had PRL levels at their last follow-up higher than those at presentation, with one patient showing evidence of tumour recurrence on CT scan. A total of eight of the 36 patients (23%) developed post-RT gonadal deficiency by the end of follow-up at 8 +/- 3.1 years (+/- SD, range 3-11), but six were aged over 40 years at that time. GH deficiency was frequent, occurring in 94% of patients, usually from 2 to 3 years post-RT, while TSH deficiency and reduced ACTH reserve was uncommon (each 14%), and occurred later. In the subgroup of 12 patients with macroprolactinomas, results were broadly comparable. CONCLUSIONS Megavoltage RT produces a progressive fall in serum prolactin in the great majority of patients with prolactinomas, with a relatively low incidence of TSH or ACTH deficiency. As it is now clear that dopamine agonist therapy alone provides sufficient management for microprolactinomas, RT may be used for the long-term control of macroprolactinomas, together with interim dopamine agonist therapy. It allows pregnancy to be safely undertaken but, in view of the delayed onset of gonadal deficiency, its administration should be timed with respect to the desired onset of conception in women.

Isolation and characterization of a sulfur-regulated gene encoding a periplasmically localized protein with sequence similarity to rhodanese

During sulfur-limited growth, the cyanobacterium Synechococcus sp. strain PCC 7942 loses most of its photosynthetic pigments and develops an increased capacity to acquire sulfate. Sulfur deprivation also triggers the synthesis of several soluble polypeptides. We have isolated a prominent polypeptide of 33 kDa that accumulates specifically under sulfur-limiting conditions. This polypeptide was localized to the periplasmic space. The gene for this protein (designated rhdA) was isolated and discovered to lie within a region of the Synechococcus sp. strain PCC 7942 genome that encodes components of the sulfate permease system. The mRNA for the 33-kDa protein accumulates to high levels within an hour after the cells are deprived of sulfur and drops rapidly when sulfur is added back to the cultures. The amino acid sequence of the protein has similarity to bovine liver rhodanese, an enzyme that transfers the thiol group of thiosulfate to a thiophilic acceptor molecule, and a rhodaneselike protein of Saccharopolyspora erythraea. A strain in which rhdA was interrupted by a drug resistance marker exhibited marginally lower levels of rhodanese activity but was still capable of efficiently utilizing a variety of inorganic sulfur sources. The possible role of this protein in the transport of specific sulfur compounds is discussed.

Marked differences in immunocytological localization of [3H]estradiol-binding protein in rat pancreatic acinar tumor cells compared to normal acinar cells

[3H]Estradiol can bind to a specific protein in normal rat pancreatic acinar cells. Electron microscopic immunocytochemical analysis has shown this protein to be localized primarily in the rough endoplasmic reticulum and mitochondria. Rat exocrine pancreatic tumor cell lines, whether grown in tissue culture (AR42J) or as a tumor mass after sc injection into rats (DSL-2), lacked detectable amounts of this [3H]estradiol-binding protein (EBP), as determined by the dextran-coated charcoal assay. Furthermore, primary exocrine pancreatic neoplasms induced with the carcinogen azaserine contained little or no detectable [3H]estradiol-binding activity. However, electron immunocytochemical studies of transformed cells indicated the presence of material that cross-reacted with antibodies prepared against the [3H]EBP. The immunopositive reaction in transformed cells was localized almost exclusively in lipid granules. Such lipid organelles in normal acinar cells, although present less frequently than in transformed cells, have never been observed to contain EBP-like immunopositive material. Presumably, the aberrant localization of EBP in these acinar tumor cells results in loss of function of this protein, which in normal pancreatic acinar cells appears to exert a modulating influence on zymogen granule formation and the process of secretion.

Isolated ACTH deficiency with absent response to corticotrophin-releasing factor--41. Evidence for a primary pituitary defect

An 8-year-old girl presenting in hypoglycaemic coma was shown to have isolated deficiency of adrenocorticotrophic hormone (ACTH) secretion. Failure to secrete ACTH in response to intravenous administration of synthetic ovine corticotrophin-releasing factor (CRF-41) suggests that this disorder was due to a primary pituitary defect, rather than of hypothalamic origin.

High-dose naloxone modifies cardiovascular and neuroendocrine function in ambulant subjects

To determine the role of the opioids in the control of the cardiovascular system in awake ambulatory subjects, eight healthy men were infused with a high dose of naloxone (10 mg bolus plus 7 mg/hr), or saline placebo, for 12 hr. Ambulatory monitoring of blood pressure and heart rate every 10 min indicated no differences between trials for blood pressure (p greater than 0.80), but a significant difference for the maximal heart rate response during stair climbing or 1 km walks (p less than 0.02). Plasma cortisol concentration were increased during the naloxone trials (p less than 0.05), as was total urinary epinephrine and dopamine output (p = 0.005 and less than 0.03, respectively). Plasma FSH and LH concentrations were elevated during naloxone infusion (FSH: p less than 0.02, LH: p less than 0.01), but neither exercise or mental tasks significantly altered their levels (p greater than 0.20). The cardiovascular responses during moderate mental tasks were not affected by naloxone (p greater than 0.05). These results indicate that in the normal ambulatory state the opioid system has a minor role in cardiovascular regulation, as demonstrated by the urinary catecholamines. Its role becomes more evident when considerable stress is imposed.

Adrenergic stimulation of the human pituitary-adrenal axis is attenuated by an analog of met-enkephalin

It has previously been suggested that endogenous opioid peptides may suppress the pituitary-adrenal axis in man by inhibiting an excitatory alpha 1-adrenoceptor input to neural mechanisms liberating corticotrophin-releasing factor or factors (CRFs). This hypothesis has been tested here by investigating the effect of the met-enkephalin analog, DAMME (FK-33,824), on the elevation in serum cortisol induced by the catecholamine-releasing agent d-amphetamine (10 and 25 mg p.o.) and the direct alpha 1-adrenoceptor agonist methoxamine (6 micrograms/kg/min i.v.) in two groups of 6 normal male subjects. In both studies, the rise in serum cortisol was significantly attenuated by the analog of met-enkephalin. These data suggest that exogenous opioids act as a site downstream to the alpha 1-adrenoceptor input to CRF release; it appears that opioids modulate adrenocorticotrophic hormone release in man at a minimum of two distinct and separate sites.

Similar high molecular weight forms of growth hormone-releasing hormone are found in rat brain and testis

We have utilized a new radioimmunoassay for rat growth hormone-releasing hormone (GHRH) to investigate the presence of GHRH in different organ systems of adult rat, and specifically the rat central nervous system (CNS). The highest concentration of GHRH was found, as expected, in the hypothalamus, but significant amounts were also located in the brain cortex, predominantly the frontal cortex, as well as in the testis. Smaller amounts were identified in the cerebellum and brain stem. Sephadex and reversed phase high performance liquid chromatography demonstrated that while hypothalamic GHRH exclusively eluted at the position of rat GHRH (1-43), in testis and brain the major form was predominantly (testis) or wholly (brain) of a higher molecular weight. While this molecular species has yet to be further characterized, the data suggest the similar GHRH-like species exist in the CNS as well as the testis.

Interleukin 4 expressed in situ selectively alters thymocyte development

Using a transgenic mouse model we show that increased intrathymic expression of interleukin 4 (IL-4) significantly perturbs the development of thymocytes. Transgenic double-positive (CD4+CD8+) thymocytes, which are present in dramatically reduced numbers, exhibit increased T cell receptor (TCR) expression and increased mobilization of calcium mediated by these receptors. In contrast, transgenic single-positive (CD4+CD8- and CD4-CD8+) thymocytes and peripheral T cells exhibit decreased TCR-mediated calcium mobilization. The development of CD4-CD8+ thymocytes is significantly perturbed by IL-4 expressed in vivo; only peripheral CD4+ T cells are found in significant numbers in transgenic mice, while CD4-CD8+ thymocytes are present in increased numbers, apparently because of their failure to emigrate to the periphery. In contrast to these selective effects on T cell development, no significant differences in the numbers of B cells or mast cells, or in the plasma levels of IgE and IgG1 are observed between transgenic and control mice. These observations suggest that IL-4 in vivo exerts its major effects locally rather than systemically, even when its expression is constitutively increased.

Light-harvesting proteins of diatoms: their relationship to the chlorophyll a/b binding proteins of higher plants and their mode of transport into plastids

We have cloned and characterized members of a gene family encoding polypeptide constituents of the fucoxanthin, chlorophyll a/c protein complex, a light-harvesting complex associated with photosystem II of diatoms and brown algae. Three cDNA clones encoding proteins associated with this complex in the diatom Phaeodactylum tricornutum have been isolated. As deduced from the nucleotide sequences, these light-harvesting proteins show homology to the chlorophyll a/b binding polypeptides of higher plants. Specifically, the N-terminal regions of the fucoxanthin, chlorophyll a/c-binding proteins are homologous to the chlorophyll a/b binding proteins in both the third membrane-spanning domain and the stroma-exposed region between membrane-spanning domains 2 and 3. Like the chlorophyll a/b-binding proteins, the mature fucoxanthin, chlorophyll a/c polypeptides have three hydrophobic alpha-helical domains which could span the membrane bilayer. The similarities between the two light-harvesting proteins might reflect the fact that both bind chlorophyll molecules and/or might be important for maintaining certain structural features of the complex. There is little similarity between the N-terminal sequences of the primary translation products of the fucoxanthin, chlorophyll a/c proteins and any transit sequences that have been characterized. Instead, the N-terminal sequences have features resembling those of signal sequences. Thus either transit peptides used in P. tricornutum show little resemblance to those of higher plants and green algae or the nuclear-encoded plastid proteins enter the organelle via a mechanism different from that used in higher plants.

Subcutaneous growth hormone-releasing hormone augments pulsatile nocturnal GH release in GH-insufficient children, but may also raise basal GH secretion

Growth hormone-releasing hormone (GHRH) when given s.c. to GH-insufficient children either as pulses, or once or twice daily, promotes linear growth. These treatment regimens, however, are not ideal as they require frequent drug administration and a significant proportion of patients do not show improved growth. We have now investigated the GH response to a nocturnal s.c. infusion of GHRH (1-29)NH2, at two dosages, 5 and 10 micrograms/kg/h, in a group of five GH-insufficient children. The s.c. infusion of GHRH between 2100 h and 0600 h augmented nocturnal pulsatile GH release in all five children. There was a dose-dependent response for the GH area under the curve (AUC), and mean total GH concentration. The AUC for GH was significantly greater after the 10 than 5 micrograms/kg/h GHRH which in turn was greater than that after placebo; mean (SD) AUC: 14816 (3978), 8125 (1931), 3032 (1582) mU min/l respectively (P less than 0.01 and P less than 0.05). There was no significant change in the number of GH pulses during the 9-h infusions when the subjects were infused with GHRH 10 or 5 micrograms/kg/h compared to placebo, and they occurred at similar times although the number of pulses tended to be greater after GHRH; the mean (SD) numbers of GH pulses were 5.0 (0.7), 3.8 (0.8), 3.2 (0.8), respectively. There was however a significant rise in the mean baseline GH concentration in all patients during the infusion of GHRH 10 micrograms/kg/h compared to placebo, but not with 5 micrograms/kg/h. Thus, GHRH(1-29)NH2 given s.c. augmented nocturnal pulsatile GH release in GH-insufficient children but it also increased baseline GH secretion. These results suggest that a sustained release preparation of GHRH could be a potential treatment for GH-insufficient children, and that a dose of 5 micrograms/kg/h would promote pulsatile GH release, but that at higher dosage it may also raise basal GH secretion.

Atrial natriuretic peptides inhibit the release of corticotrophin-releasing factor-41 from the rat hypothalamus in vitro

Atrial natriuretic peptide, ANP(99-126), is derived from cardiac atrial tissue and has potent effects on salt and water homeostasis, including the inhibition of aldosterone and vasopressin release. Recent studies have also suggested that it may suppress the pituitary-adrenal axis. In addition, N-truncated forms of ANP, such as ANP(103-126), have been identified within the central nervous system, with a prominent hypothalamic localization in the paraventricular nucleus. We have therefore investigated whether ANP(99-126) and ANP(103-126) are able to modulate the release of the principal ACTH-releasing factor, corticotrophin-releasing factor-41 (CRF-41), from the rat hypothalamus in vitro. The static incubation system has been previously described in detail. Male Wistar rats were decapitated between 09.00 and 09.30 h, their hypothalami rapidly removed, and four half-hypothalami incubated for 20-min intervals following a period of stabilization. The effect of the ANP peptides on the basal (B) and KCl (28 mmol/l)-stimulated (S) release of immunoreactive CRF-41 was studied by means of successive incubations in the absence (B1, S1) and presence (B2, S2) of the peptides. The ratios B2:B1 and S2:S1 were compared with parallel control incubations by ANOVA. Neither form of ANP had any effect on the basal release of CRF-41. ANP(99-126) caused a dose-dependent inhibition of CRF-41 release in the concentration range 1-100 nmol (P less than 0.01). ANP(103-126) also suppressed the release of CRF-41 in the concentration range 100 pmol/l-100 nmol/l (P less than 0.01), with a minimum S2:S1 ratio at 10 nmol/l, and a decrease in effect at 100 nmol/l. Finally, the stimulation of CRF-41 release induced by noradrenaline (10 nmol/l and 1 mumol/l) was non-competitively antagonized by 100 nmol ANP(99-126)/l and 10 nmol ANP(103-126)/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Highly complex genetic rearrangement involving at least seven breakpoints in a case of chronic myeloid leukemia

Cytogenetic analysis for an atypical case of chronic myeloid leukemia (CML) showed a complex karyotype with four chromosome breakpoints (5q12, 12q21, 12q24, and 22q11) and translocation products that included a typical Philadelphia chromosome but apparently normal chromosomes 9. Molecular genetic analyses using four breakpoint cluster region (bcr) probes indicated that three breaks were probably present on chromosome 22. Two apparently independent breaks appeared to exist within the bcr, one of which was probably associated with a deletion of some bcr sequences. By combining the molecular and cytogenetic data, we could infer a total of seven breaks. This case illustrates the extensive and complex types of genetic alteration that may be associated with a c-abl and bcr fusion.

Pyridostigmine, an Acetylcholinesterase Inhibitor, Stimulates Growth Hormone Release, but has no Effect on Basal Thyrotrophin or Adrenocorticotrophin Levels, or the Thyrotrophin Response to Thyrotrophin-Releasing Hormone

Abstract Pyridostigmine, an acetylcholinesterase inhibitor, stimulates growth hormone (GH) release and is thought to act by inhibiting hypothalamic somatostatin release. There are few data concerning the effect of pyridostigmine on other pituitary hormones apart from GH. We have studied the effect of pyridostigmine on basal GH, thyrotrophin (TSH), prolactin, adrenocorticotrophin and cortisol release, and thyrotrophin-releasing hormone (TRH)-stimulated TSH and prolactin release, in two studies involving nine healthy male subjects. Pyridostigmine stimulated GH release in all subjects but had no effect on adrenocortocotrophin or cortisol levels, or basal or TRH-stimulated TSH and prolactin levels. There are some data to suggest that somatostatin inhibits TRH-stimulated TSH release. Our findings, however, suggest that either endogenous somatostatin tone has little effect on the TSH response to TRH compared to its effects on GH or pyridostigmine acts through a mechanism other than altering somatostatin tone. Pyridostigmine did not alter adrenocorticotrophin or cortisol levels in the presence of a clear action on GH release, providing further evidence that the previously reported effects of cholinergic drugs on cortisol release are stress-related.

Transplantation studies in mice with congenital hemolytic anemia

Sphha/sphha anemic mice have an abnormality in the erythroid membrane protein, alpha spectrin, and exhibit multiple related clinical abnormalities, including spherocytosis, shortened red cell survival, chronic hemolysis, hemosiderosis, and extramedullary hematopoiesis. In addition, these mutant mice exhibit a granulocytosis and lymphocytosis, lymph node hyperplasia, elevated serum immunoglobulins, membranoproliferative glomerulonephritis, and decreased lifespan--abnormalities that are less clearly attributable to a spectrin defect. In order to further elucidate the mechanisms of disease in these animals, we undertook a series of bone marrow transplantation experiments. Transplantation of anemic marrow into lethally irradiated congenic +/+ mice resulted in chronic spherocytosis, hemolytic anemia, peripheral leukocytosis, and extramedullary hematopoiesis. Additionally, transplant recipients of anemic marrow which had received a higher radiation dose (12 Gy) had increased numbers of peripheral blood CD4+ and CD8+ lymphocytes, a hypocellcular thymus, and a severe pneumonitis characterized by nodular areas of consolidation and edema. Mice receiving congenic +/+ marrow and irradiated with the same radiation dose exhibited minimal pulmonary abnormalities. Anemic mice transplanted with congenic +/+ marrow usually died, but the survivors exhibited reversal of some clinicopathological changes. These results would suggest that the clinical abnormalities of sphha/sphha mice are in part attributable to abnormalities of hematopoietic stem cells but may also involve defects in other cell types. The pathogenesis of the accompanying lymphoid abnormalities observed in this mutant anemic mouse and any correlation with the erythroid spectrin defect are presently unknown. The pulmonary disease that develops in the transplant recipients of anemic marrow needs to be characterized further but may represent a unique model of lung injury.

Opiate receptor subtype regulation of CRF-41 release from rat hypothalamus in vitro

We have previously shown that the release of corticotrophin-releasing factor 41 (CRF-41) induced by a variety of neurotransmitters and depolarizing agents from the rat hypothalamus in vitro is inhibited by morphine. In order to further characterize the opiate receptors mediating this inhibitory action, we have now investigated the effects of a variety of opioid compounds with relatively high selectivity for mu-, kappa- and delta-opiate receptors on K(+)-stimulated CRF-41 release. The selective mu-opioid receptor agonist 202-250 inhibited K(+)-evoked CRF-41 release in a dose-dependent manner with a maximum inhibition of approximately 60% at 10(-5) M (p less than 0.01), as did the kappa-selective agonists PD-117,302 and U-50,488, with a similar plateau in response of approximately 40% inhibition at 10(-6) M (p less than 0.05). The effects of these agonists were specifically reversed by the mu- and kappa-receptor antagonists naloxone and MR2266, respectively, while the specific delta-receptor antagonist ICI 154,129 was ineffective. Both naloxone and MR2266 slightly but significantly increased the basal release of CRF-41. The delta-agonist D-Pen2,5-enkephalin was without significant effect in the same dose range. These data suggest that both mu- and kappa-receptors, but not delta-receptors, mediate the inhibitory effect of opiates on stimulated CRF-41 release from the rat hypothalamus.

Gamma-Aminobutyric Acid Modulation of Corticotrophin-Releasing Factor-41 Secretion from the Rat Hypothalamus in vitro

Abstract There is increasing evidence for a centrally mediated inhibitory effect of the amino-acid neurotransmitter y-aminobutyric acid (GABA) on the hypothalamo-pituitary-adrenal axis. In the present study, the direct effect of GABA in modulating the release of the 41-residue corticotrophin-releasing factor (CRF-41), the major CRF identified so far, was investigated in acute hypothalamic explants by utilizing previously validated incubation and assay techniques. While GABA (10(-7)'to 10(-5) M) had no effect on basal CRF-41 release (P > 0.05), it significantly suppressed K (-) (28 mM)-stimulated release in a dose-dependent manner (P < 0.01). A similar inhibitory effect was observed with the GABA agonist muscimol (10(-7) to 10(-5) M). Noradrenaline (10(-6) M) -induced CRF-41 release was also significantly inhibited by GABA 10(-6) M. The inhibitory effect of GABA on K(+)-stimulated CRF-41 secretion was completely. reversed by the GABA antagonists bicuculline and picrotoxin (10(-6) to 10(-5) M) in a dose-dependent fashion. Both bicuculline and picrotoxin stimulated basal and K(+) (28 mM)-stimulated CRF-41 release, indicating the presence of tonic inhibition by endogenous GABA in the basal state. Finally, GABA 10(-5) M was able to significantly inhibit the stimulated release of CRF-41 from the isolated median eminence. In summary, the present data provide strong evidence that GABA-induced inhibition of the hypothalamo-pituitary-adrenal axis is mediated, at least in part, through an inhibitory action on CRF-41 secretion. It is likely that these GABA receptors are located directly on CRF-41 neurons, probably on nerve terminals in the median eminence.