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Regulation of hypothalamic somatostatin by glucocorticoids
J. Steroid Biochem. Molec. Biol. Vol. 53, No. 1-6, pp. 277-282, 1995 Copyright ~: 1995 Elsevier Science Ltd 0960-0760(95)00065-8 Printed in Great Britain. All rights reserved 0960-0760/95 $9.50 + 0.00
Pergamon
Regulation of Hypothalamic Somatostatin by Glucocorticoids Jest~s D e v e s a , 1. M 6 n i c a G a r c i a B a r r o s , 1 M i g u e l G o n d a r , 1 J. A. F. T r e s g u e r r e s 2 a n d V i c t o r A r c e 1 1Department of Physiology, School of Medicine, University of Santiago de Compostela and 2Department of Physiology, School of Medicine, University of Madrid, Spain G l u c o c o r t i c o i d s (GCs) p l a y a key r ol e in t he p h y s i o l o g y o f t he h y p o t h a l a m i c - s o m a t o t r o p h axis, since t h e se s t e r o i d s e n h a n c e g r o w t h h o r m o n e (GH) gene t r a n s c r i p t i o n a n d i n c r e a s e G H R H r e c e p t o r synthesis. H o w e v e r , GC excess i n h i b i t s n o r m a l g r o w t h in all species st udi ed. T hi s is m a i n l y d u e to t h e i m p a i r e d G H s e c r e t i o n o b s e r v e d d u r i n g h y p e r c o r t i s o l i s m , a s i t u a t i o n in w h i c h G H r e s p o n s e s to a n u m b e r o f s t i m u l i , i n c l u d i n g G H R H , a r e b l u n t e d . T h e i n h i b i t o r y effect of GCs on G H s e c r e t i o n s e e m s to be d e p e n d e n t on e n h a n c e d h y p o t h a l a m i c SS s e c r e t i o n . Since SS r e l e a s e is s t i m u l a t e d by i g - a d r e n e r g i c a g o n i s m we t e s t e d t he pos s i bi l i t y t h a t GC i n h i b i t i o n o f G H s e c r e t i o n w o u l d d e p e n d on i n c r e a s e d f l - a d r e n o c e p t o r a c t i v i t y in S S - p r o d u c i n g n e u r o n s . T h e e x p e r i m e n t a l design c o n s i s t e d in e v a l u a t i n g t h e G H r e s p o n s e to G H R H in n o r m a l subj ect s a f t e r h a v i n g i n d u c e d h y p e r c o r t i s o l i s m , w i t h DEX, a n d bl ocki ng j g - a d r e n o c e p t o r s w i t h p r o p r a n o l o l ( P R O ) . M o r e o v e r , to i n v e s t i g a t e the specificity o f this m e c h a n i s m , G H R H - i n d u c e d G H r e l e a s e was t e s t e d a f t e r i n d u c i n g h y p e r c o r t i s o l i s m a n d e n h a n c i n g ~2-adrenergic or m u s c a r i n i c c h o l i n e r g i c tone, by giving c l o n i d i n e (CLO) or p y r i d o s t i g m i n e (PD), r e s p e c t i v e l y . As e x p e c t e d , n o c t u r n a l D E X a d m i n i s t r a t i o n i n h i b i t e d t h e G H r e s p o n s e to G H R H . In this s i t u a t i o n o f h y p e r c o r t i s o l i s m , b o t h P R O a n d CLO, b u t n o t PD , w e r e able to r e v e r s e t h e i n h i b i t o r y effect o f D E X on G H R H - e l i c i t e d release. H o w e v e r , t he p o t e n t i a t i n g effect o f th e se d r u g s on th e G H R H - i n d u c e d G H s e c r e t i o n was onl y o b s e r v e d f o r P R O . T h e s e d a t a c o n f i r m t h a t GC excess i n h i b i t s G H r e l e a s e by i n c r e a s i n g h y p o t h a l a m i c SS s e c r e t i o n , a n d t h a t t he m e c h a n i s m is m e d i a t e d by G C - i n d u c e d e n h a n c e d f l - a d r e n e r g i c r e s p o n s i v e n e s s . T h e r e f o r e , t he d e f e c t i v e G H R H s e c r e t i o n o b s e r v e d in c h r o n i c h y p e r c o r t i s o l i s m m u s t be a c o n s e q u e n c e of t h e c o n t i n u o u s b l o c k a d e t h a t SS excess e x e r t s on G H R H - p r o d u c i n g n e u r o n s . O u r p o s t u l a t e agrees with o t h e r d a t a in th e l i t e r a t u r e showing t h a t GCs m o d u l a t e t he s e c r e t i o n o f s o m e h y p o t h a l a m i c p e p t i d e s by c h a n g i n g th e r e s p o n s i v e n e s s o f t he p r o d u c i n g n e u r o n s f r o m ~ 2 - a d r e n o c e p t o r s to t h a t o f j J - a d r e n o c e p t o r s .
J. Steroid Biochem. Molec. Biol., Vol. 53, No. 1-6, pp. 277-282, 1995
INTRODUCTION
secretion, by blocking both G H R H release and the secretagogue effect of the peptide, interrupts pituitary G H secretion. T he rhythmic lack of SS release, most likely governed by suprahypothalamic structures, will allow G H R H to be secreted and therefore induce G H release to the blood. These concepts are shown schematically in Fig. 1. On these bases, the factors involved in G H control would mainly act by affecting hypothalamic SS secretion. Therefore, in order to ascertain how G H is regulated, the putative effect of each factor studied on hypothalamic SS release must first be investigated [1]. Such an investigation would take into account that adrenergic inputs to SS-producing neurons play the main role in SS control, it now being clear that while ct2-adrenergic stimulation inhibits SS release in all
Hypothalamic SS plays the main role in growth hormone (GH) control [1]. In fact, although hypothalamic G H releasing hormone ( G H R H ) must reach the pituitary somatotropes in order for G H secretion to be elicited, the release of this peptide seems to be modulated by SS [2], which otherwise counteracts the GH-releasing effect of G H R H at the somatotroph level. According to this hypothesis, the hypothalamicsomatotroph rhythm (HSR) [3] which establishes the episodic pattern of physiological G H secretion must have its main oscillator in SS-producing neurons. SS Proceedings of the I X International Congress on Hormonal Steroids, Dallas, Texas, U.S.A., 24-29 September 1994. *Correspondence to J. Devesa. 277
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Fig. 1. H y p o t h a l a m i c g r o w t h h o r m o n e control. SS i n h i b i t s G H s e c r e t i o n b y c o u n t e r a c t i n g the effect of G H R H at t h e s o m a t o t r o p e s level a n d d i r e c t l y i n h i b i t i n g t h e h y p o t h a l a m i c r e l e a s e of the peptide. I n t u r n , SS s e c r e t i o n is m a i n l y d e p e n dent on the balance between a2-adrenergic (inhibitory) and ~ - a d r e n e r g i c ( s t i m u l a t o r y ) a c t i v i t i e s at t h e S S - p r o d u c i n g n e u r o n s . G H d i r e c t l y s t i m u l a t e s SS release. >s t i m u l a t i o n . - - - --, i n h i b i t i o n .
species in which it has been studied [4], fl-adrenergic agonism stimulates the secretion of this inhibitory peptide. GLUCOCORTICOIDS AND GROWTH HORMONE SECRETION
Glucocorticoids (GCs) are a key component for the physiological functioning of the hypothalamicsomatotroph axis [1]. These steroids enhance G H gene transcription [5] and increase G H R H receptor synthesis [6]; consequently, it seems to be logical that a synthetic G C such as dexamethasone (DEX) stimulates both G H R H - i n d u c e d G H release [7] and spontaneous secretion of the hormone [8]. In line with this, an adequate G C replacement is needed to restore a normal G H secretion in patients with idiopathic A C T H deficiency [9]. However, it is well known that G C excess inhibits normal growth in all species studied [10]. Although this results from the conjunction of a n u m b e r of peripheral alterations, chronic hypercortisolism is associated with impaired G H secretion [11]. Either the spontaneous secretion of the hormone or its release in response to a n u m b e r of stimuli, including G H R H , are blunted in situations of G C excess. T h e r e f o r e paradoxically GCs seem to play two opposite roles in G H control: facilitatory, exerted at the pituitary level; and inhibitory, most likely exerted on the hypothalamic structures. T h e former, a physiological effect, must be associated with the permissive role that these steroids play in the organism [1]; however, when trying to provide a physiological explanation for the inhibitory action that GCs exert on G H secretion, a n u m b e r of questions arise. T h a t is, why does this effect occur? Where and
et al.
how does the inhibition take place? What is the physiological meaning of this effect? In order to provide an adequate answer to these questions, we must first know how G H secretion occurs (for review see Ref. [1]). G H is released into peripheral blood episodically. This episodic pattern is necessary for the optimal induction of physiologic effects at the peripheral level. T h e rhythm, most likely established by suprahypothalamic structures, is basically modulated by the S S - G H relationships. G H release stimulates SS secretion [12], which in turn interrupts the secretory activity in somatotropes. Interestingly, the lack of G H does not appear to affect the rhythmic secretion of SS [4], while in the absence of this peptide a tonic G H secretion can be observed. According to this, as pointed out in the Introduction, SS plays the main role in G H control. T h e rhythmic interruption of SS release allows G H R H to be secreted; then G H R H induces a rapid, dose-dependent, release of G H somatotropes, but also the transcription of the G H gene. In turn, increased G H would represent a positive signal to SS-producing neurons (Fig. 1). T h e i r response would optimize, by blocking G H R H secretion and action, the amount of G H released and consequently the pituitary stores in the hormone. A number of reports suggested a primary G C induced G H R H blockade as the mechanism directly responsible for the lack of G H seen in hypercortisolism [13, 14]. However, we hypothesized that the inhibitory effect of GCs on G H secretion had to be exerted by stimulating SS secretion [1, 15]. In our hypothesis, SS excess being the primary event, SS-dependent G H R H blockade would therefore take place [2]. In fact, in rats, the administration of pharmacological doses of D E X for 3 days increases the hypothalamic content of SS [16], whereas the administration of SS antiserum enhances the blunted G H response to G H R H observed in animals given D E X for 4 days [17]. T h e possibility that hypercortisolism led to increased SS secretion was tested according to our recent model indicative of how the G H responses to physiological or pharmacological challenges should be interpreted [1]. Moreover, since SS control is mainly exerted by inhibitory ~2-adrenergic pathways and stimulatory fl-adrenergic pathways, our experimental model was based on the possibility that an increased SS tone would be dependent on enhanced fl-adrenoceptor responsiveness (Fig. 1). T h e support for such a hypothesis proceeds from several findings demonstrating that, at the peripheral level, GCs increase fl-agoniststimulated adenyl cyclase activity and fl-adrenoceptor number [18]. In fact, the sequence homologous to glucocorticoid-responsive elements has been identified in the 3' flanking region of the fl2-adrenoceptor gene [19, 20]. Our study [15], in normal humans, basically consisted of the evaluation of the G H response to G H R H after inducing hypercortisolism with DEX, and then
Glucocorticoids and Somatostatin DEX.
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Fig. 2. This d i a g r a m s c h e m a t i c a l l y shows the e x p e r i m e n t a l p r o t o c o l followed. In a c o n t r o l study, the GH r e s p o n s e t o G H R H was a n a l y z e d a f t e r giving d e x a m e t h a s o n e (DEX) or placebo (P) 10h before. The GH r e s p o n s e t o G H R H w a s t h e n t e s t e d u n d e r ~ - a d r e n e r g i c blockade with p r o p r a n o l o l (PRO), o r ~ t 2 - a d r e n e r g i c a g o n i s m w i t h e l o n i d i n e ( C L O ) , o r c h o l i n e r g i c a g o n i s m with p y r i d o s t i g m i n e (PD), These studies were r e p e a t e d u n d e r h y p e r c o r t i s o l i s m elicited by giving DEX 10 h b e f o r e t h e G H R H challenge.
blocking /3-adrenoceptors with propranolol (PRO) (Fig. 2). With this experimental design we attempted to ascertain whether /?-adrenoceptor blockade would be able to counteract the inhibitory effect of GCs on the G H response to G H R H . In such a case, a logical conclusion would be that the mechanism postulated for GCs at the peripheral level [18] would also be operative on the SS-producing neurons. In addition, and to better elucidate the specificity of this putative mechanism of action, we also tested the G H R H - i n d u c e d G H release in the same subjects after inducing hypercortisolism and enhancing e2-adrenergic or muscarinic cholinergic tone, by giving clonidine (CLO) or pyridostigmine (PD), respectively (Fig. 2). T h e rationale for this was based on (1) the direct SS-release inhibiting effect found for C L O by our group [21]; and (2) the fact that while cholinergic pathways play an important role in SS control [22], their effects seem to be mediated by catecholamines [23], as it occurs in the periphery. On these bases, it would be expected that C L O , acting at
the post-synaptic level in the SS-producing neurons, would be able to counteract the inhibitory effect of D E X on the G H response to G H R H , while this would not occur with PD, acting proximal to adrenergic neurons. Results from this study [15] confirmed the experimental hypothesis. Nocturnal D E X administration induced the existence of a refractory phase [3] in all the subjects at the time of G H R H tests, 9 h later. This indicated a high amount of SS inputs reaching the somatotropes. In this situation, and as previously described in other studies, G H R H challenge was not able to evoke a significant G H release. Both PRO and C L O , but not PD, were able to reverse the inhibitory effect of D E X on the G H R H - e l i c i t e d G H response. However, while the PRO-enhanced G H R H - i n d u c e d G H secretion remained unaffected by hypercortisol±sin, this was no longer observed with CLO. From these data, summarized in Table 1, we concluded that the inhibitory effect of G C excess on G H release
Table 1. Plasma G H peaks (pg/l; mean +' S E M ) elicited by G H R H in the different experimental situations. In the control study the G H response to G H R H was significantly (P < 0.01) enhanced after pretreatment with propranolol (PRO), clonidine (CLO) or pyridostigmine (PD). Nocturnal dexamethasone administration significantly (P < 0.05) reduced the G H response to G H R H . This blunted G H response was totally or partially restored after [I-adrenergic blockade, with PRO, or o~2-adrenergic agonism, with CLO, respectively, but unaffected by cholinergic agonism with PD
Control Hypercortisolism
P or DEX
PRO
20.3 ± 5.5* 10.7 ± 3.9
43 ± 4.6** 39 ± 5.5**
CLO 55.6 ± 5.6** 25.9 ± 3.9*
PD 51.2_+ 7** 12.9 ± 3.1
*P <0.05 vs control study under hypercortisolism (DEX). **P < 0.01 vs placebo study (P, control).
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PITUITARY SOMATOTROPES Fig. 3. G C s i n h i b i t G H s e c r e t i o n b y e n h a n c i n g S S r e l e a s e . T h i s effect s e e m s to be d u e to i n c r e a s e d ~8-adrenergic a c t i v i t y in S S n e u r o n s (1). t h e d e f e c t i v e G H R H s e c r e t i o n o b s e r v e d in h y p e r c o r t i s o l i s m m u s t o c c u r as a consequence o f t h e t o n i c i n h i b i t i o n t h a t SS e x e r t s on G H R H n e u r o n s (2). G H s e c r e t i o n is t h e n b l u n t e d (2). / / - a d r e n e r g i c b l o c k a d e w i t h P R O i m p e d e s SS to be s e c r e t e d . C L O a c t i n g d i r e c t l y o n ~ 2 - a d r e n o c e p t o r s w o u l d a l l o w a certain degree o f SS i n h i b i t i o n , therefore leading to a G H r e s p o n s e to G H R H c h a l l e n g e . s t i m u l a t i o n . - - - --, i n h i b i t i o n .
is due to increased hypothalamic SS secretion which appears to be dependent on D E X - i n d u c e d enhanced fl-adrenergic responsiveness [15] (Fig. 3). Moreover, the study allowed us to confirm our postulate regarding the major role that central adrenergic pathways play in SS, and therefore in G H , control. In effect, the lack of action of the enhancement of cholingeric tone, with PD, under hypercortisolism, compared to that of C L O in the same experimental situation, would be related to the different site of action of these drugs [23]. P D acting proximal to adrenergic neurons would be unable to antagonize fl-adrenergicinduced SS release, whereas C L O directly stimulating ~2-adrenoceptors in SS neurons would partially counteract the D E X - i n d u c e d increased fl-adrenergic activity (Fig. 3). Since C L O is a rather selective ~t2-adrenergic agonist, it is likely that the opposing effects of endogenously enhanced fl-adrenergic responsiveness and exogenously induced ~2-adrenergic stimulation could account for the G H responses to G H R H seen after giving D E X + CLO. T h a t is, those subjects in which the effect of D E X on fl-adrenoceptors had been low; or those in which this effect had partially disappeared at the time of C L O administration, exhibited the normal C L O - e n h a n c e d G H R H - i n d u c e d G H release [21]. This is consistent with the fact that the cellular responses of many tissues to N E depend on the relative availability of its receptor subtypes
(~1, ~2, #). Our postulate that glucocorticoids increase SS release by enhancing fl-adrenoceptor responsiveness is also supported by a recent study by Huang et a l . [24]. These authors described that GCs modulate irANP secretion in hypothalamic neurons by changing the
responsiveness of the cells from ~2-adrenoceptors to that of fl-adrenoceptors. T h e possibility of enhanced SS tone being responsible for the inhibitory effect of G C on G H release seems to dispute data indicating that it is the lack of G H R H which accounts for the blunted G H secretion seen in chronic hypercortisolism. This argument is based on the finding that pyridostigmine administration is unable to restore G H release in Cushing's patients, whereas priming with G H R H partially reinstated the G H response to cholinergic agonism in these patients [25]. However, G H R H administration does not elicit G H release in the presence of elevated SS inputs to the pituitary, and P D is not able to block /%adrenergic dependent SS-enhanced release [15]. Therefore, although it is likely that a G H R H defective secretion exists during chronic hypercortisolism, it must be due to the tonic inhibitory effect that SS hypertone exerts on G H R H neurons (Fig. 3). A G H R H challenge after a few days fl-adrenergic blockade in Cushing's patients would clarify this aspect. Assuming that GCs stimulate hypothalamic SS secretion, the observation that an acute effect of these steroids is to consistently induce a clear G H release [26] appears to be quite surprising. T h u s , the administration of D E X (4 mg, i.v.) 3 h prior to a G H R H challenge increases basal plasma G H values and potentiates the G H response to G H R H . How can we conciliate this finding with the SS-mediated inhibitory effect described above? Interestingly, the GH-releasing effect of G C is consistently observed 3 h after the steroid administration; it is not potentiated by SS-inhibiting drugs [26, 27], but its apparition is delayed by the induction of SS
Glucocorticoids and Somatostatin release w i t h atropine. T h e s e data i n d i c a t e that the secretion of G H i n d u c e d b y D E X is m e d i a t e d by i n h i b i t e d S S secretion. M o r e o v e r , a f u n c t i o n a l l y intact h y p o t h a l a m i c - s o m a t o t r o p h axis is n e e d e d i n o r d e r to o b s e r v e the D E X - i n d u c e d G H secretion [28]. T h e r e fore, a direct effect of G C at the s o m a t o t r o p i c level as the factor r e s p o n s i b l e for the acute G H r e s p o n s e m a y be discarded. C o n s i d e r i n g the above, a n d b e i n g aware of the existence of a H S R m o d u l a t e d b y G H - S S r e l a t i o n ships, these data lead us to speculate that D E X - i n d u c e d G H release is also a c o n s e q u e n c e of the D E X - e l i c i t e d SS secretion. T h a t is, the d i s r u p t i o n of the H S R b y D E X a d m i n i s t r a t i o n w o u l d reset the r h y t h m i c i t y of the system. T h u s , in a first step D E X w o u l d increase SS release, therefore b l o c k i n g G H secretion. T h e lack of G H w o u l d in t u r n i n t e r r u p t SS secretion t h u s allowing G H to be secreted. T h e r e are n o e x p e r i m e n t a l data to s u p p o r t the h y p o t h e s i s that such a m e c h a n i s m w o u l d be r e s p o n s i b l e for the acute G H - r e l e a s i n g effect of G C s ; h o w e v e r , the r e s y n c h r o n i z a t i o n of the H S R f o l l o w i n g its r u p t u r e b y c l o n i d i n e has b e e n r e p o r t e d b y o u r g r o u p [29]. T h e r e f o r e , the fact that D E X a d m i n i s t r a t i o n acutely s y n c h r o n i z e s the r h y t h m of G H secretion exactly 3 h afterwards [26] m i g h t be e x p l a i n e d as d e s c r i b e d above. T e s t i n g the G H r e s p o n s e to G H R H at short t i m e intervals after g i v i n g D E X a n d h a v i n g i n d u c e d or n o t f l - a d r e n e r g i c blockade w o u l d d e m o n s t r a t e w h e t h e r or n o t this h y p o t h e s i s is correct. I n s u m m a r y , the i n h i b i t o r y effect that G C s play o n G H secretion is clearly m e d i a t e d b y f l - a d r e n e r g i c i n d u c e d SS release. T h e lack of G H R H o b s e r v e d in c h r o n i c h y p e r c o r t i s o l i s m is m o s t likely d e p e n d e n t o n the c o n t i n u o u s blockade that SS h y p e r t o n e exerts o n G H R H - p r o d u c i n g n e u r o n s . T h e acute, facilitatory role that G C s play in G H c o n t r o l is m o s t likely m e d i a t e d b y the s t i m u l a t i n g effect of these steroids o n SS release as well, r e s u l t i n g in a r e s y n c h r o n i z a t i o n of the H S R . T h i s is apart f r o m the well k n o w n effects of these steroids o n the n u m b e r of G H R H receptors [6] a n d o n G H gene e x p r e s s i o n [5]. T h e s e postulates agree w i t h o u r e x p e r i m e n t a l data i n d i c a t i n g that SS plays the m o s t i m p o r t a n t role in G H c o n t r o l a n d that SS secretion is m a i n l y r e g u l a t e d by the b a l a n c e b e t w e e n c~2- a n d f l - a d r e n e r g i c activity in S S p r o d u c i n g n e u r o n s [1]. T h e c o n t r o l of h y p o t h a l a m i c SS secretion by G C w o u l d be related to the r e g u l a t o r y role that these steroids play in the o r g a n i s m . Acknowledgement--This study was supported by DGICYT (Madrid,
Spain), grant PM88-0208 and grant PM90-0024.
REFERENCES
1. Devesa J., Lima L. and Tresguerres J. A. F.: Neuroendocrine control of growth hormone secretion in humans. Trends Endocr. Metab. 3 (1992) 173-181.
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2. Lima L., Arce V., Tresguerres J. A. F. and Devesa J.: Clonidine potentiates the growth hormone (GH) response to GH-releasing hormone in norepinephrine synthesis-inhibited rats: evidence for an ~2-adrenergic control of hypothalamic release of somatostatin. Neuroendocrinology 57 (1993) 1155-1160. 3. Devesa J., Lima L., Lois N. et al.: Reasons for the variability in growth hormone (GH) responses to GHRH challenge: the endogenous hypothalamic-somatotroph rhythm. Clin. Endocr. (Oxf). 30 (1989) 367-377. 4. Arce V., Garcia Barros M., Vara E., Lima L., Tresguerres J. A. F. and Devesa J.: Clonidine potentiates the GH response to a GHRH challenge in hypothalamic GHRH-deficient rats. Neuroendocrinology 61 (1995) 552-558. 5. Karin M., Castrillo J. L. and Theill L. E.: Growth hormone gene regulation: a paradigm for cell-type-specific gene activation. Trends Genet. 6 (1990) 92-96. 6. Seifert H., Perrin M., Rivier J. and Vale W.: Growth hormonereleasing factor binding sites in rat anterior pituitary membrane homogenates: modulation by glucocorticoids. Endocrinology 117 (1985) 424-426. 7. Casanueva F. F., Burguera B., Tom6 M. A. et al.: Depending on the time of administration, dexamethasone potentiates or blocks growth hormone-releasing hormone-induced growth hormone release in man. Neuroendocrinology 47 (1988) 46-49. 8. Casanueva F. F., Burguera B., Muruais C. and Dieguez C.: Acute administration of corticoids: a new and peculiar stimulus of growth hormone secretion in man. ,7- Clin. Endocr. Metab. 70 (1990) 234-237. 9. Giustina A., Romanelli G., Candrina R. and Giustina G.: Growth hormone deficiency in patients with idiopathic adrenocorticotrophin deficiency resolves during glucocorticoid replacement. J. Clin. Endocr. Metab. 68 (1989) 120-124. 10. Blodgett F. M., Burgin L , Iezzoni D., Gribetz D. and Talbot D. N.: Effects of prolonged corticosterone therapy on statural growth, skeletal maturation and metabolic status of children. New Engl. J. Med. 254 (1956) 636--641. 11. Krieger D. T. and Glick S. M.: Growth hormone and cortisone responsiveness in Cushing's syndrome: relation to a possible central nervous system etiology. Am. J. Med. 52 (1972) 25-40. 12. Kelijman M. and Frohman L. A.: The role of the cholinergic pathway in growth hormone feedback. J. Clin. Endocr. Metab. 72 (1991) 1081-1087. 13. Kaufman K., Jones K. L., Wehrenberg W. B. and Culler F. L.: Inhibition by prednisone of growth hormone (GH) response to GH releasing hormone in normal men. J. Clin. Endocr. Metab. 67 (1988) 1258-1261. 14. Leal-Cerro A., Pumar A., Villamil F., Astorga R., Dieguez C. and Casanueva F. F.: Growth hormone releasing hormone (GHRH) priming increases growth hormone secretion in patients with Cushing's syndrome. Clin. Endocr. (Oxf). 38 (19933 399-403. 15. Lima L., Arce V., Diaz M. J., Tresguerres J. A. F. and Devesa J.: Glucocorticoids may inhibit growth hormone release by enhancing fl-adrenergic responsiveness in hypothalamic somatostatin neurons. J. Clin. Endocr. Metab. 76 (1993) 439-444. 16. Nakagawa K., Ishizuka T., Obara T., Matsubara M. and Akikawa K.: Dichotomic action of glucocorticoids on growth hormone secretion. Acta Endocr. (Copenh). 116 (1987) 165-171. 17. Wehrenberg W. B., Janowski B., Piering A. W., Culler F. and Jones K. L.: Glucocorticoids: potent inhibitors and stimulators of growth hormone secretion. Endocrinology 126 (1990) 3200-3203. 18. Collins S., Caron M. G. and Lefkowitz R. J.: Regulation of adrenergic receptor responsiveness through modulation of receptor gene expression. A. Rev. Physiol. 53 (1991) 497-508. 19. Emorine L. J., Marullo S., Delavier-Klutchko C., Kaveri S. V. and Durieu-Trautmann O.: Structure of the gene for human fl2-adrenergic receptors: expression and promoter characterization (glucocorticoid induction/transcriptionfactor Spl binding sites). Proc. Nam. Acad. Sci. U.S.A. 84 (1987) 6995-6999. 20. Nakada M. T., Haskell K. M., Ecker D. J., Stadel J. M. and Crooke S. T.: Genetic regulation of fl2-adrenergic receptors in 3T3-L1 fibroblasts. Biochem. J. 260 (1989) 53-59. 21. Devesa J., Arce V., Lois N., Tresguerres J. A. F. and Lima L.: ~2-Adrenergic agonism enhances the growth hormone response to GH-releasing hormone through an inhibition of hypothalamic
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somatostatin release in normal men. J. Clin. Endocr. Metab. 71 (1990) 1581-1588. 22. Casanueva F, F., Villanueva L., Cabranes J. A., Cabezas-Cerrato J. and Fern~indez-Cruz A.: Cholinergic mediation of GH secretion elicited by arginine, clonidine and physical exercise in man. J. Clin. Endocr. Metab. 59 (1984) 526--530. 23. Devesa J., Diaz M. J., Tresguerres J. A. F., Arce V, and Lima L.: Evidence that ~2-Adrenergic pathways play a major role in growth hormone (GH) neuroregulation: ~2-adrenergic agonism counteracts the inhibitory effect of muscarinic cholinergic blockade on the GH response to GH-releasing hormone, while ~2-adrenergic blockade diminishes the potentiating effect of increased cholinergic tone on such stimulation in normal men. J. Clin. Endocr. Metab. 73 (1991) 251-256. 24. Huang W., Lee D., Yang Z., Copolov D. L. and Lira A. T.: Plasticity of adrenoceptor responsiveness on ir ANP secretion and pro-ANP mRNA expression in hypothalamic neuron cultures: modulation by dexamethasone. Endocrinology 131 (1992) 1562-1564.
25. Difiguez C., Mallo F., Alvarez C. et al.: Role of glucocorticoids in the neuroregulation of GH secretion. In Regulation ~ Growth Hormone and Somatic Growth (Edited by L. D. De la Cruz). Elsevier, Amsterdam (1992) pp. 219-225. 26. Casanueva F. F., Burguera B., Muruais C. and Di6guez C.: Acute administration of corticoids: a new and peculiar stimulus of growth hormone secretion in man. J. Clin. Endocr. Metab. 70 (1990) 234-237. 27. Burguera B., Muruais C., Pefialva A. et al.: Dual and selective action of glucocorticoids upon basal and stimulated growth hormone release in man. Neuroendocrinology 51 (1990) 51-58. 28. Popovic V., Damjanovic S., Micic D. et al.: Modulation by glucocorticoids of growth hormone secretion in patients with different pituitary tumors. Neuroendocrinology 58 (1993) 465--472. 29. Lima L., Arce V., Diaz M. J., Tresguerres J. A. F. and Devesa J.: Clonidine pretreatment modifies the growth hormone secretory pattern induced by short term continuous GRF infusion in normal man. Ch'n. Endocr. (Oxf). 35 (1991) 129-135.
Pergamon
Regulation of Hypothalamic Somatostatin by Glucocorticoids Jest~s D e v e s a , 1. M 6 n i c a G a r c i a B a r r o s , 1 M i g u e l G o n d a r , 1 J. A. F. T r e s g u e r r e s 2 a n d V i c t o r A r c e 1 1Department of Physiology, School of Medicine, University of Santiago de Compostela and 2Department of Physiology, School of Medicine, University of Madrid, Spain G l u c o c o r t i c o i d s (GCs) p l a y a key r ol e in t he p h y s i o l o g y o f t he h y p o t h a l a m i c - s o m a t o t r o p h axis, since t h e se s t e r o i d s e n h a n c e g r o w t h h o r m o n e (GH) gene t r a n s c r i p t i o n a n d i n c r e a s e G H R H r e c e p t o r synthesis. H o w e v e r , GC excess i n h i b i t s n o r m a l g r o w t h in all species st udi ed. T hi s is m a i n l y d u e to t h e i m p a i r e d G H s e c r e t i o n o b s e r v e d d u r i n g h y p e r c o r t i s o l i s m , a s i t u a t i o n in w h i c h G H r e s p o n s e s to a n u m b e r o f s t i m u l i , i n c l u d i n g G H R H , a r e b l u n t e d . T h e i n h i b i t o r y effect of GCs on G H s e c r e t i o n s e e m s to be d e p e n d e n t on e n h a n c e d h y p o t h a l a m i c SS s e c r e t i o n . Since SS r e l e a s e is s t i m u l a t e d by i g - a d r e n e r g i c a g o n i s m we t e s t e d t he pos s i bi l i t y t h a t GC i n h i b i t i o n o f G H s e c r e t i o n w o u l d d e p e n d on i n c r e a s e d f l - a d r e n o c e p t o r a c t i v i t y in S S - p r o d u c i n g n e u r o n s . T h e e x p e r i m e n t a l design c o n s i s t e d in e v a l u a t i n g t h e G H r e s p o n s e to G H R H in n o r m a l subj ect s a f t e r h a v i n g i n d u c e d h y p e r c o r t i s o l i s m , w i t h DEX, a n d bl ocki ng j g - a d r e n o c e p t o r s w i t h p r o p r a n o l o l ( P R O ) . M o r e o v e r , to i n v e s t i g a t e the specificity o f this m e c h a n i s m , G H R H - i n d u c e d G H r e l e a s e was t e s t e d a f t e r i n d u c i n g h y p e r c o r t i s o l i s m a n d e n h a n c i n g ~2-adrenergic or m u s c a r i n i c c h o l i n e r g i c tone, by giving c l o n i d i n e (CLO) or p y r i d o s t i g m i n e (PD), r e s p e c t i v e l y . As e x p e c t e d , n o c t u r n a l D E X a d m i n i s t r a t i o n i n h i b i t e d t h e G H r e s p o n s e to G H R H . In this s i t u a t i o n o f h y p e r c o r t i s o l i s m , b o t h P R O a n d CLO, b u t n o t PD , w e r e able to r e v e r s e t h e i n h i b i t o r y effect o f D E X on G H R H - e l i c i t e d release. H o w e v e r , t he p o t e n t i a t i n g effect o f th e se d r u g s on th e G H R H - i n d u c e d G H s e c r e t i o n was onl y o b s e r v e d f o r P R O . T h e s e d a t a c o n f i r m t h a t GC excess i n h i b i t s G H r e l e a s e by i n c r e a s i n g h y p o t h a l a m i c SS s e c r e t i o n , a n d t h a t t he m e c h a n i s m is m e d i a t e d by G C - i n d u c e d e n h a n c e d f l - a d r e n e r g i c r e s p o n s i v e n e s s . T h e r e f o r e , t he d e f e c t i v e G H R H s e c r e t i o n o b s e r v e d in c h r o n i c h y p e r c o r t i s o l i s m m u s t be a c o n s e q u e n c e of t h e c o n t i n u o u s b l o c k a d e t h a t SS excess e x e r t s on G H R H - p r o d u c i n g n e u r o n s . O u r p o s t u l a t e agrees with o t h e r d a t a in th e l i t e r a t u r e showing t h a t GCs m o d u l a t e t he s e c r e t i o n o f s o m e h y p o t h a l a m i c p e p t i d e s by c h a n g i n g th e r e s p o n s i v e n e s s o f t he p r o d u c i n g n e u r o n s f r o m ~ 2 - a d r e n o c e p t o r s to t h a t o f j J - a d r e n o c e p t o r s .
J. Steroid Biochem. Molec. Biol., Vol. 53, No. 1-6, pp. 277-282, 1995
INTRODUCTION
secretion, by blocking both G H R H release and the secretagogue effect of the peptide, interrupts pituitary G H secretion. T he rhythmic lack of SS release, most likely governed by suprahypothalamic structures, will allow G H R H to be secreted and therefore induce G H release to the blood. These concepts are shown schematically in Fig. 1. On these bases, the factors involved in G H control would mainly act by affecting hypothalamic SS secretion. Therefore, in order to ascertain how G H is regulated, the putative effect of each factor studied on hypothalamic SS release must first be investigated [1]. Such an investigation would take into account that adrenergic inputs to SS-producing neurons play the main role in SS control, it now being clear that while ct2-adrenergic stimulation inhibits SS release in all
Hypothalamic SS plays the main role in growth hormone (GH) control [1]. In fact, although hypothalamic G H releasing hormone ( G H R H ) must reach the pituitary somatotropes in order for G H secretion to be elicited, the release of this peptide seems to be modulated by SS [2], which otherwise counteracts the GH-releasing effect of G H R H at the somatotroph level. According to this hypothesis, the hypothalamicsomatotroph rhythm (HSR) [3] which establishes the episodic pattern of physiological G H secretion must have its main oscillator in SS-producing neurons. SS Proceedings of the I X International Congress on Hormonal Steroids, Dallas, Texas, U.S.A., 24-29 September 1994. *Correspondence to J. Devesa. 277
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Fig. 1. H y p o t h a l a m i c g r o w t h h o r m o n e control. SS i n h i b i t s G H s e c r e t i o n b y c o u n t e r a c t i n g the effect of G H R H at t h e s o m a t o t r o p e s level a n d d i r e c t l y i n h i b i t i n g t h e h y p o t h a l a m i c r e l e a s e of the peptide. I n t u r n , SS s e c r e t i o n is m a i n l y d e p e n dent on the balance between a2-adrenergic (inhibitory) and ~ - a d r e n e r g i c ( s t i m u l a t o r y ) a c t i v i t i e s at t h e S S - p r o d u c i n g n e u r o n s . G H d i r e c t l y s t i m u l a t e s SS release. >s t i m u l a t i o n . - - - --, i n h i b i t i o n .
species in which it has been studied [4], fl-adrenergic agonism stimulates the secretion of this inhibitory peptide. GLUCOCORTICOIDS AND GROWTH HORMONE SECRETION
Glucocorticoids (GCs) are a key component for the physiological functioning of the hypothalamicsomatotroph axis [1]. These steroids enhance G H gene transcription [5] and increase G H R H receptor synthesis [6]; consequently, it seems to be logical that a synthetic G C such as dexamethasone (DEX) stimulates both G H R H - i n d u c e d G H release [7] and spontaneous secretion of the hormone [8]. In line with this, an adequate G C replacement is needed to restore a normal G H secretion in patients with idiopathic A C T H deficiency [9]. However, it is well known that G C excess inhibits normal growth in all species studied [10]. Although this results from the conjunction of a n u m b e r of peripheral alterations, chronic hypercortisolism is associated with impaired G H secretion [11]. Either the spontaneous secretion of the hormone or its release in response to a n u m b e r of stimuli, including G H R H , are blunted in situations of G C excess. T h e r e f o r e paradoxically GCs seem to play two opposite roles in G H control: facilitatory, exerted at the pituitary level; and inhibitory, most likely exerted on the hypothalamic structures. T h e former, a physiological effect, must be associated with the permissive role that these steroids play in the organism [1]; however, when trying to provide a physiological explanation for the inhibitory action that GCs exert on G H secretion, a n u m b e r of questions arise. T h a t is, why does this effect occur? Where and
et al.
how does the inhibition take place? What is the physiological meaning of this effect? In order to provide an adequate answer to these questions, we must first know how G H secretion occurs (for review see Ref. [1]). G H is released into peripheral blood episodically. This episodic pattern is necessary for the optimal induction of physiologic effects at the peripheral level. T h e rhythm, most likely established by suprahypothalamic structures, is basically modulated by the S S - G H relationships. G H release stimulates SS secretion [12], which in turn interrupts the secretory activity in somatotropes. Interestingly, the lack of G H does not appear to affect the rhythmic secretion of SS [4], while in the absence of this peptide a tonic G H secretion can be observed. According to this, as pointed out in the Introduction, SS plays the main role in G H control. T h e rhythmic interruption of SS release allows G H R H to be secreted; then G H R H induces a rapid, dose-dependent, release of G H somatotropes, but also the transcription of the G H gene. In turn, increased G H would represent a positive signal to SS-producing neurons (Fig. 1). T h e i r response would optimize, by blocking G H R H secretion and action, the amount of G H released and consequently the pituitary stores in the hormone. A number of reports suggested a primary G C induced G H R H blockade as the mechanism directly responsible for the lack of G H seen in hypercortisolism [13, 14]. However, we hypothesized that the inhibitory effect of GCs on G H secretion had to be exerted by stimulating SS secretion [1, 15]. In our hypothesis, SS excess being the primary event, SS-dependent G H R H blockade would therefore take place [2]. In fact, in rats, the administration of pharmacological doses of D E X for 3 days increases the hypothalamic content of SS [16], whereas the administration of SS antiserum enhances the blunted G H response to G H R H observed in animals given D E X for 4 days [17]. T h e possibility that hypercortisolism led to increased SS secretion was tested according to our recent model indicative of how the G H responses to physiological or pharmacological challenges should be interpreted [1]. Moreover, since SS control is mainly exerted by inhibitory ~2-adrenergic pathways and stimulatory fl-adrenergic pathways, our experimental model was based on the possibility that an increased SS tone would be dependent on enhanced fl-adrenoceptor responsiveness (Fig. 1). T h e support for such a hypothesis proceeds from several findings demonstrating that, at the peripheral level, GCs increase fl-agoniststimulated adenyl cyclase activity and fl-adrenoceptor number [18]. In fact, the sequence homologous to glucocorticoid-responsive elements has been identified in the 3' flanking region of the fl2-adrenoceptor gene [19, 20]. Our study [15], in normal humans, basically consisted of the evaluation of the G H response to G H R H after inducing hypercortisolism with DEX, and then
Glucocorticoids and Somatostatin DEX.
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HYPERCORTISOLISM I t Blood samples for GH analysis 2300
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Fig. 2. This d i a g r a m s c h e m a t i c a l l y shows the e x p e r i m e n t a l p r o t o c o l followed. In a c o n t r o l study, the GH r e s p o n s e t o G H R H was a n a l y z e d a f t e r giving d e x a m e t h a s o n e (DEX) or placebo (P) 10h before. The GH r e s p o n s e t o G H R H w a s t h e n t e s t e d u n d e r ~ - a d r e n e r g i c blockade with p r o p r a n o l o l (PRO), o r ~ t 2 - a d r e n e r g i c a g o n i s m w i t h e l o n i d i n e ( C L O ) , o r c h o l i n e r g i c a g o n i s m with p y r i d o s t i g m i n e (PD), These studies were r e p e a t e d u n d e r h y p e r c o r t i s o l i s m elicited by giving DEX 10 h b e f o r e t h e G H R H challenge.
blocking /3-adrenoceptors with propranolol (PRO) (Fig. 2). With this experimental design we attempted to ascertain whether /?-adrenoceptor blockade would be able to counteract the inhibitory effect of GCs on the G H response to G H R H . In such a case, a logical conclusion would be that the mechanism postulated for GCs at the peripheral level [18] would also be operative on the SS-producing neurons. In addition, and to better elucidate the specificity of this putative mechanism of action, we also tested the G H R H - i n d u c e d G H release in the same subjects after inducing hypercortisolism and enhancing e2-adrenergic or muscarinic cholinergic tone, by giving clonidine (CLO) or pyridostigmine (PD), respectively (Fig. 2). T h e rationale for this was based on (1) the direct SS-release inhibiting effect found for C L O by our group [21]; and (2) the fact that while cholinergic pathways play an important role in SS control [22], their effects seem to be mediated by catecholamines [23], as it occurs in the periphery. On these bases, it would be expected that C L O , acting at
the post-synaptic level in the SS-producing neurons, would be able to counteract the inhibitory effect of D E X on the G H response to G H R H , while this would not occur with PD, acting proximal to adrenergic neurons. Results from this study [15] confirmed the experimental hypothesis. Nocturnal D E X administration induced the existence of a refractory phase [3] in all the subjects at the time of G H R H tests, 9 h later. This indicated a high amount of SS inputs reaching the somatotropes. In this situation, and as previously described in other studies, G H R H challenge was not able to evoke a significant G H release. Both PRO and C L O , but not PD, were able to reverse the inhibitory effect of D E X on the G H R H - e l i c i t e d G H response. However, while the PRO-enhanced G H R H - i n d u c e d G H secretion remained unaffected by hypercortisol±sin, this was no longer observed with CLO. From these data, summarized in Table 1, we concluded that the inhibitory effect of G C excess on G H release
Table 1. Plasma G H peaks (pg/l; mean +' S E M ) elicited by G H R H in the different experimental situations. In the control study the G H response to G H R H was significantly (P < 0.01) enhanced after pretreatment with propranolol (PRO), clonidine (CLO) or pyridostigmine (PD). Nocturnal dexamethasone administration significantly (P < 0.05) reduced the G H response to G H R H . This blunted G H response was totally or partially restored after [I-adrenergic blockade, with PRO, or o~2-adrenergic agonism, with CLO, respectively, but unaffected by cholinergic agonism with PD
Control Hypercortisolism
P or DEX
PRO
20.3 ± 5.5* 10.7 ± 3.9
43 ± 4.6** 39 ± 5.5**
CLO 55.6 ± 5.6** 25.9 ± 3.9*
PD 51.2_+ 7** 12.9 ± 3.1
*P <0.05 vs control study under hypercortisolism (DEX). **P < 0.01 vs placebo study (P, control).
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1.-GC c,o _ PRO IIml
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PITUITARY SOMATOTROPES Fig. 3. G C s i n h i b i t G H s e c r e t i o n b y e n h a n c i n g S S r e l e a s e . T h i s effect s e e m s to be d u e to i n c r e a s e d ~8-adrenergic a c t i v i t y in S S n e u r o n s (1). t h e d e f e c t i v e G H R H s e c r e t i o n o b s e r v e d in h y p e r c o r t i s o l i s m m u s t o c c u r as a consequence o f t h e t o n i c i n h i b i t i o n t h a t SS e x e r t s on G H R H n e u r o n s (2). G H s e c r e t i o n is t h e n b l u n t e d (2). / / - a d r e n e r g i c b l o c k a d e w i t h P R O i m p e d e s SS to be s e c r e t e d . C L O a c t i n g d i r e c t l y o n ~ 2 - a d r e n o c e p t o r s w o u l d a l l o w a certain degree o f SS i n h i b i t i o n , therefore leading to a G H r e s p o n s e to G H R H c h a l l e n g e . s t i m u l a t i o n . - - - --, i n h i b i t i o n .
is due to increased hypothalamic SS secretion which appears to be dependent on D E X - i n d u c e d enhanced fl-adrenergic responsiveness [15] (Fig. 3). Moreover, the study allowed us to confirm our postulate regarding the major role that central adrenergic pathways play in SS, and therefore in G H , control. In effect, the lack of action of the enhancement of cholingeric tone, with PD, under hypercortisolism, compared to that of C L O in the same experimental situation, would be related to the different site of action of these drugs [23]. P D acting proximal to adrenergic neurons would be unable to antagonize fl-adrenergicinduced SS release, whereas C L O directly stimulating ~2-adrenoceptors in SS neurons would partially counteract the D E X - i n d u c e d increased fl-adrenergic activity (Fig. 3). Since C L O is a rather selective ~t2-adrenergic agonist, it is likely that the opposing effects of endogenously enhanced fl-adrenergic responsiveness and exogenously induced ~2-adrenergic stimulation could account for the G H responses to G H R H seen after giving D E X + CLO. T h a t is, those subjects in which the effect of D E X on fl-adrenoceptors had been low; or those in which this effect had partially disappeared at the time of C L O administration, exhibited the normal C L O - e n h a n c e d G H R H - i n d u c e d G H release [21]. This is consistent with the fact that the cellular responses of many tissues to N E depend on the relative availability of its receptor subtypes
(~1, ~2, #). Our postulate that glucocorticoids increase SS release by enhancing fl-adrenoceptor responsiveness is also supported by a recent study by Huang et a l . [24]. These authors described that GCs modulate irANP secretion in hypothalamic neurons by changing the
responsiveness of the cells from ~2-adrenoceptors to that of fl-adrenoceptors. T h e possibility of enhanced SS tone being responsible for the inhibitory effect of G C on G H release seems to dispute data indicating that it is the lack of G H R H which accounts for the blunted G H secretion seen in chronic hypercortisolism. This argument is based on the finding that pyridostigmine administration is unable to restore G H release in Cushing's patients, whereas priming with G H R H partially reinstated the G H response to cholinergic agonism in these patients [25]. However, G H R H administration does not elicit G H release in the presence of elevated SS inputs to the pituitary, and P D is not able to block /%adrenergic dependent SS-enhanced release [15]. Therefore, although it is likely that a G H R H defective secretion exists during chronic hypercortisolism, it must be due to the tonic inhibitory effect that SS hypertone exerts on G H R H neurons (Fig. 3). A G H R H challenge after a few days fl-adrenergic blockade in Cushing's patients would clarify this aspect. Assuming that GCs stimulate hypothalamic SS secretion, the observation that an acute effect of these steroids is to consistently induce a clear G H release [26] appears to be quite surprising. T h u s , the administration of D E X (4 mg, i.v.) 3 h prior to a G H R H challenge increases basal plasma G H values and potentiates the G H response to G H R H . How can we conciliate this finding with the SS-mediated inhibitory effect described above? Interestingly, the GH-releasing effect of G C is consistently observed 3 h after the steroid administration; it is not potentiated by SS-inhibiting drugs [26, 27], but its apparition is delayed by the induction of SS
Glucocorticoids and Somatostatin release w i t h atropine. T h e s e data i n d i c a t e that the secretion of G H i n d u c e d b y D E X is m e d i a t e d by i n h i b i t e d S S secretion. M o r e o v e r , a f u n c t i o n a l l y intact h y p o t h a l a m i c - s o m a t o t r o p h axis is n e e d e d i n o r d e r to o b s e r v e the D E X - i n d u c e d G H secretion [28]. T h e r e fore, a direct effect of G C at the s o m a t o t r o p i c level as the factor r e s p o n s i b l e for the acute G H r e s p o n s e m a y be discarded. C o n s i d e r i n g the above, a n d b e i n g aware of the existence of a H S R m o d u l a t e d b y G H - S S r e l a t i o n ships, these data lead us to speculate that D E X - i n d u c e d G H release is also a c o n s e q u e n c e of the D E X - e l i c i t e d SS secretion. T h a t is, the d i s r u p t i o n of the H S R b y D E X a d m i n i s t r a t i o n w o u l d reset the r h y t h m i c i t y of the system. T h u s , in a first step D E X w o u l d increase SS release, therefore b l o c k i n g G H secretion. T h e lack of G H w o u l d in t u r n i n t e r r u p t SS secretion t h u s allowing G H to be secreted. T h e r e are n o e x p e r i m e n t a l data to s u p p o r t the h y p o t h e s i s that such a m e c h a n i s m w o u l d be r e s p o n s i b l e for the acute G H - r e l e a s i n g effect of G C s ; h o w e v e r , the r e s y n c h r o n i z a t i o n of the H S R f o l l o w i n g its r u p t u r e b y c l o n i d i n e has b e e n r e p o r t e d b y o u r g r o u p [29]. T h e r e f o r e , the fact that D E X a d m i n i s t r a t i o n acutely s y n c h r o n i z e s the r h y t h m of G H secretion exactly 3 h afterwards [26] m i g h t be e x p l a i n e d as d e s c r i b e d above. T e s t i n g the G H r e s p o n s e to G H R H at short t i m e intervals after g i v i n g D E X a n d h a v i n g i n d u c e d or n o t f l - a d r e n e r g i c blockade w o u l d d e m o n s t r a t e w h e t h e r or n o t this h y p o t h e s i s is correct. I n s u m m a r y , the i n h i b i t o r y effect that G C s play o n G H secretion is clearly m e d i a t e d b y f l - a d r e n e r g i c i n d u c e d SS release. T h e lack of G H R H o b s e r v e d in c h r o n i c h y p e r c o r t i s o l i s m is m o s t likely d e p e n d e n t o n the c o n t i n u o u s blockade that SS h y p e r t o n e exerts o n G H R H - p r o d u c i n g n e u r o n s . T h e acute, facilitatory role that G C s play in G H c o n t r o l is m o s t likely m e d i a t e d b y the s t i m u l a t i n g effect of these steroids o n SS release as well, r e s u l t i n g in a r e s y n c h r o n i z a t i o n of the H S R . T h i s is apart f r o m the well k n o w n effects of these steroids o n the n u m b e r of G H R H receptors [6] a n d o n G H gene e x p r e s s i o n [5]. T h e s e postulates agree w i t h o u r e x p e r i m e n t a l data i n d i c a t i n g that SS plays the m o s t i m p o r t a n t role in G H c o n t r o l a n d that SS secretion is m a i n l y r e g u l a t e d by the b a l a n c e b e t w e e n c~2- a n d f l - a d r e n e r g i c activity in S S p r o d u c i n g n e u r o n s [1]. T h e c o n t r o l of h y p o t h a l a m i c SS secretion by G C w o u l d be related to the r e g u l a t o r y role that these steroids play in the o r g a n i s m . Acknowledgement--This study was supported by DGICYT (Madrid,
Spain), grant PM88-0208 and grant PM90-0024.
REFERENCES
1. Devesa J., Lima L. and Tresguerres J. A. F.: Neuroendocrine control of growth hormone secretion in humans. Trends Endocr. Metab. 3 (1992) 173-181.
281
2. Lima L., Arce V., Tresguerres J. A. F. and Devesa J.: Clonidine potentiates the growth hormone (GH) response to GH-releasing hormone in norepinephrine synthesis-inhibited rats: evidence for an ~2-adrenergic control of hypothalamic release of somatostatin. Neuroendocrinology 57 (1993) 1155-1160. 3. Devesa J., Lima L., Lois N. et al.: Reasons for the variability in growth hormone (GH) responses to GHRH challenge: the endogenous hypothalamic-somatotroph rhythm. Clin. Endocr. (Oxf). 30 (1989) 367-377. 4. Arce V., Garcia Barros M., Vara E., Lima L., Tresguerres J. A. F. and Devesa J.: Clonidine potentiates the GH response to a GHRH challenge in hypothalamic GHRH-deficient rats. Neuroendocrinology 61 (1995) 552-558. 5. Karin M., Castrillo J. L. and Theill L. E.: Growth hormone gene regulation: a paradigm for cell-type-specific gene activation. Trends Genet. 6 (1990) 92-96. 6. Seifert H., Perrin M., Rivier J. and Vale W.: Growth hormonereleasing factor binding sites in rat anterior pituitary membrane homogenates: modulation by glucocorticoids. Endocrinology 117 (1985) 424-426. 7. Casanueva F. F., Burguera B., Tom6 M. A. et al.: Depending on the time of administration, dexamethasone potentiates or blocks growth hormone-releasing hormone-induced growth hormone release in man. Neuroendocrinology 47 (1988) 46-49. 8. Casanueva F. F., Burguera B., Muruais C. and Dieguez C.: Acute administration of corticoids: a new and peculiar stimulus of growth hormone secretion in man. ,7- Clin. Endocr. Metab. 70 (1990) 234-237. 9. Giustina A., Romanelli G., Candrina R. and Giustina G.: Growth hormone deficiency in patients with idiopathic adrenocorticotrophin deficiency resolves during glucocorticoid replacement. J. Clin. Endocr. Metab. 68 (1989) 120-124. 10. Blodgett F. M., Burgin L , Iezzoni D., Gribetz D. and Talbot D. N.: Effects of prolonged corticosterone therapy on statural growth, skeletal maturation and metabolic status of children. New Engl. J. Med. 254 (1956) 636--641. 11. Krieger D. T. and Glick S. M.: Growth hormone and cortisone responsiveness in Cushing's syndrome: relation to a possible central nervous system etiology. Am. J. Med. 52 (1972) 25-40. 12. Kelijman M. and Frohman L. A.: The role of the cholinergic pathway in growth hormone feedback. J. Clin. Endocr. Metab. 72 (1991) 1081-1087. 13. Kaufman K., Jones K. L., Wehrenberg W. B. and Culler F. L.: Inhibition by prednisone of growth hormone (GH) response to GH releasing hormone in normal men. J. Clin. Endocr. Metab. 67 (1988) 1258-1261. 14. Leal-Cerro A., Pumar A., Villamil F., Astorga R., Dieguez C. and Casanueva F. F.: Growth hormone releasing hormone (GHRH) priming increases growth hormone secretion in patients with Cushing's syndrome. Clin. Endocr. (Oxf). 38 (19933 399-403. 15. Lima L., Arce V., Diaz M. J., Tresguerres J. A. F. and Devesa J.: Glucocorticoids may inhibit growth hormone release by enhancing fl-adrenergic responsiveness in hypothalamic somatostatin neurons. J. Clin. Endocr. Metab. 76 (1993) 439-444. 16. Nakagawa K., Ishizuka T., Obara T., Matsubara M. and Akikawa K.: Dichotomic action of glucocorticoids on growth hormone secretion. Acta Endocr. (Copenh). 116 (1987) 165-171. 17. Wehrenberg W. B., Janowski B., Piering A. W., Culler F. and Jones K. L.: Glucocorticoids: potent inhibitors and stimulators of growth hormone secretion. Endocrinology 126 (1990) 3200-3203. 18. Collins S., Caron M. G. and Lefkowitz R. J.: Regulation of adrenergic receptor responsiveness through modulation of receptor gene expression. A. Rev. Physiol. 53 (1991) 497-508. 19. Emorine L. J., Marullo S., Delavier-Klutchko C., Kaveri S. V. and Durieu-Trautmann O.: Structure of the gene for human fl2-adrenergic receptors: expression and promoter characterization (glucocorticoid induction/transcriptionfactor Spl binding sites). Proc. Nam. Acad. Sci. U.S.A. 84 (1987) 6995-6999. 20. Nakada M. T., Haskell K. M., Ecker D. J., Stadel J. M. and Crooke S. T.: Genetic regulation of fl2-adrenergic receptors in 3T3-L1 fibroblasts. Biochem. J. 260 (1989) 53-59. 21. Devesa J., Arce V., Lois N., Tresguerres J. A. F. and Lima L.: ~2-Adrenergic agonism enhances the growth hormone response to GH-releasing hormone through an inhibition of hypothalamic
282
Jest~s Devesa et al.
somatostatin release in normal men. J. Clin. Endocr. Metab. 71 (1990) 1581-1588. 22. Casanueva F, F., Villanueva L., Cabranes J. A., Cabezas-Cerrato J. and Fern~indez-Cruz A.: Cholinergic mediation of GH secretion elicited by arginine, clonidine and physical exercise in man. J. Clin. Endocr. Metab. 59 (1984) 526--530. 23. Devesa J., Diaz M. J., Tresguerres J. A. F., Arce V, and Lima L.: Evidence that ~2-Adrenergic pathways play a major role in growth hormone (GH) neuroregulation: ~2-adrenergic agonism counteracts the inhibitory effect of muscarinic cholinergic blockade on the GH response to GH-releasing hormone, while ~2-adrenergic blockade diminishes the potentiating effect of increased cholinergic tone on such stimulation in normal men. J. Clin. Endocr. Metab. 73 (1991) 251-256. 24. Huang W., Lee D., Yang Z., Copolov D. L. and Lira A. T.: Plasticity of adrenoceptor responsiveness on ir ANP secretion and pro-ANP mRNA expression in hypothalamic neuron cultures: modulation by dexamethasone. Endocrinology 131 (1992) 1562-1564.
25. Difiguez C., Mallo F., Alvarez C. et al.: Role of glucocorticoids in the neuroregulation of GH secretion. In Regulation ~ Growth Hormone and Somatic Growth (Edited by L. D. De la Cruz). Elsevier, Amsterdam (1992) pp. 219-225. 26. Casanueva F. F., Burguera B., Muruais C. and Di6guez C.: Acute administration of corticoids: a new and peculiar stimulus of growth hormone secretion in man. J. Clin. Endocr. Metab. 70 (1990) 234-237. 27. Burguera B., Muruais C., Pefialva A. et al.: Dual and selective action of glucocorticoids upon basal and stimulated growth hormone release in man. Neuroendocrinology 51 (1990) 51-58. 28. Popovic V., Damjanovic S., Micic D. et al.: Modulation by glucocorticoids of growth hormone secretion in patients with different pituitary tumors. Neuroendocrinology 58 (1993) 465--472. 29. Lima L., Arce V., Diaz M. J., Tresguerres J. A. F. and Devesa J.: Clonidine pretreatment modifies the growth hormone secretory pattern induced by short term continuous GRF infusion in normal man. Ch'n. Endocr. (Oxf). 35 (1991) 129-135.