Effects of gonadectomy and testosterone replacement on growth hormone response to α2 adrenergic stimulation in the male rat

Psychoneuroendocrinology, Vol. 7, No.

2/3, pp. 245 - 248, 1982.

0306 - 4530/82/020245 - 04 $03.00/0 © 1982 Pergamon Press Ltd.

Printed in Great Britain.

SHORT C O M M U N I C A T I O N EFFECTS OF G O N A D E C T O M Y A N D T E S T O S T E R O N E R E P L A C E M E N T ON G R O W T H H O R M O N E R E S P O N S E TO a2 A D R E N E R G I C S T I M U L A T I O N IN THE M A L E RAT J. O. JANSSON, E. ERIKSSON, S. EDI~N a n d K. MODIGH Departments of Physiology and Pharmacology, University of G0teborg, G0teborg, Sweden (Received 18 August 1981; in final form 29 April 1982) SUMMARY The growth hormone (GH) response to clonidine in reserpine-pretreated rats is a putative in vivo model to reflect activation of central postsynaptic ct2 receptors. In the present study the influence of testosterone on the responsiveness of central a2 receptors was investigated using this method. One week after operation the GH response to clonidine was drastically reduced in gonadectomized adult male rats compared to sham-operated controls. Testosterone replacement completely antagonized the effect. The results suggest an influence of testosterone on central postsynaptic a2 receptors or on structures connected to these receptors.

INTRODUCTION TESTOSTERONE has been a t t r i b u t e d i m p o r t a n t influences on m e n t a l f u n c t i o n s such as

m o o d , l i b i d o a n d a g g r e s s i o n ( H e r r m a n & Beach, 1976; Rose, 1978). A l t h o u g h r e c e p t o r b i n d i n g sites for the h o r m o n e have been d e m o n s t r a t e d in the b r a i n (Barley et aL, 1975), little is k n o w n a b o u t how a c t i v a t i o n o f these r e c e p t o r s m o d u l a t e s various n e u r o t r a n s m i t t e r systems. In m a l e rats the t u r n o v e r o f c a t e c h o l a m i n e s in the central n e r v o u s system is increased after c a s t r a t i o n ( A n t o n - T a y & W u r t m a n , 1968; D o n o s o et al., 1969; Engel et al., 1979), a n d this effect is p r e v e n t e d by t e s t o s t e r o n e r e p l a c e m e n t (Engel et al., 1979). O t h e r steroid h o r m o n e s , i.e. estrogens a n d g l u c o c o r t i c o i d s , have been r e p o r t e d to m o d u l a t e the sensitivity o f central c a t e c h o l a m i n e r e c e p t o r s ( H r u s k a & Silbergeld, 1980; M o b l e y & Sulser, 1980). In the present study we have investigated w h e t h e r t e s t o s t e r o n e exerts a n influence on n o r a d r e n a l i n e ( N A ) n e u r o t r a n s m i s s i o n at o r b e y o n d the r e c e p t o r level. N A m e c h a n i s m s are involved in the r e g u l a t i o n o f g r o w t h h o r m o n e ( G H ) secretion ( D u r a n d et al., 1977; Ed6n & M o d i g h , 1977). A d m i n i s t r a t i o n o f reserpine in a d o s e that effectively depletes b r a i n N A causes c o m p l e t e s u p p r e s s i o n o f the n o r m a l pulsatile G H secretion in the rat (Ed6n & M o d i g h , 1977). A d m i n i s t r a t i o n o f clonidine, an a g o n i s t for b o t h ct, a n d ct2 receptors, to r e s e r p i n e - p r e t r e a t e d a n i m a l s causes increased p l a s m a levels Correspondence to: Elias Eriksson, Department of Pharmacology, University of GOteborg, Box 33031, S-400 33 GOteborg, Sweden. 245

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J. 0 . J~NSS()N el al.

of G H (Ed6n & Modigh, 1977). This effect can be blocked by the selective c~2 receptor antagonist yohirnbine but not by the ct, receptor blocking agent phenoxybenzamine (Eriksson et al., 1982) and hence should be mediated by postsynaptic c~2 receptors. Recent in vitro radioligand binding studies have revealed that ~2 receptors are indeed located not only on the NA neurons, as so-called autoreceptors, but also postsynaptically (U'Prichard et al., 1979). These receptors are widely distributed in the brain and may play important roles in mental functions. However, their physiological significance and regulation are not well elucidated due to lack of valid in vivo models reflecting their activation. The G H response to clonidine, being fairly constant in magnitude, provides a possibility to study these receptors, e.g. with respect to changes in sensitivity (Eriksson et al., 1980; Eriksson et al., 1982). Indeed, chronic treatment with a receptor antagonist, which should cause denervation supersensitivity of central NA receptors, also causes enhanced G H responses to clonidine, while chronic agonist treatment results in decreased responses (Eriksson et al., 1982). In the present study, the clonidine/GH model was applied in castrated male rats with or without testosterone replacement in order to clarify whether this hormone influences the responsiveness of central a2 receptors. MATERIALS AND METHODS Male Sprague - Dawley rats (Anticimex, Stockholm), weaned at 20 days of age, were housed under controlled conditions with food and water freely available. At 50 days of age the rats were gonadectomized or shamoperated under ketamine anaesthesia (Ketalar R, 150 mg/kg). At the same time, intra-aortic cannulae were implanted, allowing frequent blood sampling from undisturbed animals (Ed6n, 1978). One group of gonadectomized animals received daily subcutaneous injections of testosterone propionate (Testoviron R, Schering) (5 mg/kg) for six days, starting on the day of surgery. The hormone was dissolved in sesame oil and injected in vols. of 0.2 ml. The other group of gonadectomized rats and sham-operated controls received only sesame oil for the same time period. The last injections were given 24 hr before the experiment. On the day of the experiment all rats received reserpine (Serpasil R, CIBA) (10 mg/kg) and 5 hr later clonidine HCI (Boebringer Ingelheim) (0.5 mg/kg). Reserpine was dissolved in a few drops of glacial acetic acid, and the final vol. was made up with 5.5% glucose solution. Clonidine was dissolved in 0.9% saline with moderate heating. Both drugs were administered in vols. of 10 ml/kg. Blood samples (0.3 ml) were withdrawn at 15-rain intervals starting 4.5 hr after the adminstration of reserpine. Plasma GH was.measured by radioimmunoassay using a double antibody technique (Ed6n, 1978). Values are given as mean _+ S.E.M. Significances between groups were calculated on logarithmically transformed data by analysis of variance followed by Tukey's test for multiple comparisons (Box et al., 1978). Values for p less than 0.05 were considered significant. RESULTS

The effect of clonidine on plasma levels of G H in sham-operated (SHAM), gonadectomized (GX) and testosterone treated GX (GX plus TEST) rats is shown in Fig. i. Calculations of the area under the curve for each individual rat (SHAM: 98.9 420.6 n g / m l / h r , GX: 28.8 _+ 4.9 n g / m l / h r , GX plus TEST: 180.7 _+ 30.1 n g / m l / h r ) revealed a significantly decreased secretory response in the GX rats compared to the sham-operated controls (p < 0.01). This effect of gonadectomy was effectively prevented by testosterone replacement (GX vs GX plus TEST, p < 0.01). DISCUSSION

One week after operation the G H response to clonidine was drastically reduced in the gonadectomized male rats compared to the sham-operated controls. Since this effect could be prevented by testosterone replacement, it in all probability was due to androgen

247

TESTOSTERONE AND C E N T R A L ~ 2 - R E ( ' E P T O R S

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SHRH GX GX + TEST

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FnG. 1. Effect of clonidine on plasma levels of GH in sham-operated (SHAM), gonadectomized (GX) and testosterone treated gonadectomized (GX + TEST) male rats. All animals were pretreated with reserpine 5 hr before the injection of clonidine. There were 7 - 10 animals in each group. * p < 0.05 vs gonadectomized rats; • * p < 0.01 vs gonadectomized rats; t P < 0.05 vs sham-operated rats.

deficiency. A high r e p l a c e m e n t dose o f testosterone was used, leading to s u p r a n o r m a l p l a s m a testosterone levels ( J a n s s o n , u n p u b l i s h e d data), a n d indeed the G H response to c l o n i d i n e a p p e a r e d e n h a n c e d in the testosterone treated a n i m a l s in c o m p a r i s o n to the s h a m - o p e r a t e d controls. T h e observed effects of c a s t r a t i o n a n d testosterone r e p l a c e m e n t o n the G H response to clonidine m a y reflect changes in the responsiveness of central p o s t s y n a p t i c ct~ receptors. Alternatively, one m a y speculate u p o n a n influence of testosterone o n the G H p r o d u c i n g cells in the pituitary or o n i n t e r m e d i a t e steps, e.g. G H releasing factor (Eden et al., 1981). However, c a s t r a t i o n of male rats does n o t cause a n y significant decrease in pituitary levels of G H (Birge et al., 1967). Moreover, s p o n t a n e o u s G H secretion is n o t reduced in adult g o n a d e c t o m i z e d male rats (Mode et al., 1982). Therefore, changes i n p i t u i t a r y G H c o n t e n t could hardly explain the p r o n o u n c e d differences in G H response to clonidine. Interestingly, a decreased G H response to c l o n i d i n e is a c o m m o n n e u r o e n d o c r i n e a b n o r m a l i t y in m e n t a l depression in m a n (Matussek et al., 1980). In view o f the present results, one m a y speculate that a testosterone deficiency, leading inter alia to subsensitivity o f central p o s t s y n a p t i c ct, receptors, m a y c o n t r i b u t e in some cases to the p a t h o p h y s i o l o g y o f m e n t a l depression. Reduced testosterone levels, as well as a beneficial effect of a n d r o g e n t r e a t m e n t , in e n d o g e n o u s depression have been reported (Klaiber et al., 1974; H e r r m a n n & Beach, 1976).

The authors are grateful to Dr. A. E. Wilhelmi for supplying the rat GH reference preparation and to the NIAMDD Rat Pituitary Hormone Distribution Program and Dr. A. F. Parlow for generously providing antiserum to rat GH (NIAMDD-Anti -rat GH-S3). The study was supported by grants from the Swedish Medical Research Council° (project No. 04752), the Faculty of Medicine, University of G0teborg, Svenska L~ikares~llskapet, Ake Wibergs stiftelse and Anna Ahrenbergs fond. Excellent technical assistance was given by Miss Lena Andersson.

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J . O . JANSSON et al. REFERENCES

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