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Decreased serotonin turnover in the dorsal hippocampus of rat brain shortly after adrenalectomy: selective normalization after corticosterone substitution
Brain Research, 239 (1982) 659-663 Elsevier Biomedical Press
659
Decreased serotonin turnover in the dorsal hippocampus of rat brain shortly after adrenalectomy: selective normalization after corticosterone substitution
E. R. DE KLOET, G. L. KOV/~CS, G. SZABO, G. TELEGDY, B. BOHUS and D. H. G. VERSTEEG Rudolf Magnus lnstitute ]br Pharmacology, University of Utrecht, Medical Faculty, Vondellaan 6, 3521 G D Utrecht (The Netherlands) and ( G.L.K., G.S. and G. T. ) Department of Pathophysiology, University Medical School, Szeged (Hungary)
(Accepted January 28th, 1982) Key words: corticosterone - - hippocampus - - serotonin
Pargyline-induced accumulation of serotonin (5-HT) was used as an index of 5-HT turnover rate in the dorsal hippocampus. One hour after bilateral removal of the adrenals, 5-HT turnover was significantly reduced when compared to that of the sham-operated controls. A low dose of corticosterone given immediately after adrenalectomy restored the 5-HT response, while the same dose of dexamethasone was ineffective. Pretreatment with dexamethasone blocked the 5-HT response to corticosterone in the acutely adrenalectomized rat. The specificity of the 5-HT response in the hippocampus corresponds to the properties of the glucocorticoid receptor system in rat hippocampa[ neurons. H i p p o c a m p a l neurons contain adrenal steroid receptor sites with a stringent specificity for corticosterone, the principal glucocorticoid o f the rat 11. W h e n corticosterone is administered to adrenalectomized rats, a large a m o u n t of the glucocorticoid receptor complex is retained selectively in cell nuclei of hippocampal neurons 5. In contrast, the potent synthetic glucocorticoid dexamethasone has a lower affinity for these glucocorticoid receptor sites and is only poorly retained by the hippocampa[ neuron cell nuclei3,7,14. The function o f the hippocampal glucocorticoid receptor system was obscure until it was recently found that such receptors may be implicated in the modulation by corticosterone of certain adaptive behavioral responses2,t2, a3. Studies in this laboratory have demonstrated a behavioral deficit in rats at 1 h after bilateral removal of the adrenalsL The deficient behavior could be restored by giving a low dose o f corticosterone immediately after adrenalectomy (ADX). Dexamethasone, as well as the mineralocorticoids deoxycorticosterone and aldosterone, were ineffective. Pretreatment with dexamethasone even prevented the normalizing effect of corticosterone on behavior o f A D X rats 2. A close correlation between brain 5-HT turnover8, 9 or t r y p t o p h a n hydroxylase activity with the corticosterone level has been reported t. Behavioral alterations with or without corticostereid treatment 2 m a y well be related to changes in serotonergic neurotransmission in the brain. Therefore, 5-HT turnover in 0006-8993/82/000(~0000/$02.75 © Elsevier Biomedical Press
660 the brains of acutely adrenalectomized rats with or without steroid replacement was studied using the same time and dose scheme as in the behavioral studies. In the first experiment rats were subjected to bilateral A D X or to sham-adrenalectomy (S-ADX) under ether anesthesia and received 30 min later either pargyline (150 mg/kg i.p.) or vehicle. The rats were decapitated 60 min after surgery. The brains were cut into sections 2 m m thick and the dorsal hippocampus (about 24 mg wet tissue weight) dissected at 0 °C. Serotonin was assayed according to the spectrofluorimetric method of Snyder et al. t6. Pargyline was used according to Tozer 18. Pargyline, a monoamine oxidase inhibitor, produces an accumulation of brain serotonin (5-HT) which is approximately linear for at least 30 min after its administration. The difference in 5-HT level between pargyline- and vehicle-treated animals was taken as an index of the synthesis or of the turnover rate of 5-HT 8,18. The steady-state 5-HT level in the dorsal hippocampus was not significantly different for A D X and S-ADX (Table I). Pargyline induced a significant increase in 5-HT concentration in both groups. The pargyline-induced accumulation of 5-HT was significantly decreased after A D X , when compared to S-ADX. This index was 55 %1 lower after A D X than after SADX. In the second experiment the A D X rats received as replacement low doses (30 #g/100 g body weight s.c.) of either corticosterone or dexamethasone (Table l I) immediately after surgery. The replacement with corticosterone resulted in a 2-fold increase in 5-HT accumulation in the dorsal hippocampus, and consequently this index for 5-HT turnover rate was restored to that of the sham-operated control animals. Dexamethasone treatment inhibited the pargyline effect: no pargylineinduced 5-HT accumulation was observed. The difference was significant in comparison with corticosterone-substituted A D X animals. The steady state 5-HT levels were not significantly different for any of the groups. In a third experiment the animals received dexamethasone (30/~g/100 g body weight s.c.) 60 rain prior to A D X and corticosterone replacement at ADX. Dexame-
TABLE 1 Effect of adrenalectomy ( ADX) and sham-adrenalectomy ( S-ADX) on the pargyline-induced accumulation of serotonin (5-H7) in the dorsal hippocampus
Data are means from 12-14 animals ± S.E.M. The difference in 5-HT level between pargyline- and vehicle-treated animals was taken as index for the synthesis or turnover rate of 5-HT. Treatment at ~0
rain
S-ADX ADX
--30 rain
SAL PARG SAL PARG
5-HT concentration (#g/g tissue)
Diff
0.85 ± 0.06 1.57 ± 0.11" 0.94 i 0.13 1.26 v~ 0.08".**
÷ 0.72 -t 0.32
* Significantly different (P < 0.05) from the respective SAL group. SAL = saline. ** Significantly different (P < 0.05) from S-ADX-PARG. PARG = pargyline.
661 TABLE 1I Effect o f corticosterone and dexamethasone on the pargyline-induced accumulation o f serotonin (5-HT) in the dorsal hippocampus o f the acutely (one hour) adrenalectomized (ADX)
Data are mean of 12-15 (1) or (~8 (1I) animals 3: S.E.M. The data were analyzed by one way ANOVA and subsequently by Student's t-test (two-tailed). Treatment at --120 mitt
--60 rain
--30 min
SAL
ADX/SAL
SAL
ADX/DEX
SAL
ADX/CORT
DEX
ADX/CORT
SAL PARG. SAL PARG. SAL PARG. SAL PARG.
5- H T concentration (l~g/g tissue)
0.88 +~ 0.06 1 1.12 ± 0.05* 0.92 ~ 0.1l II 0.79 ± 0.05"*,*** 0.94 _k 0.06 I 1.42 ± 0.07*,** 0.97 ± 0.07 1I 1.15 -c 0.08***
Diff"
t 0.24 --0.13 F 0.48 ~ 0.18
* Significantly different (P < 0.05) from the respective SAL group. Significantly different (P < 0.05) from SAL-ADX/SAL-PARG. *** Significantly different (P < 0.05) from SAL-ADX/CORT-PARG. **
thasone pretreatment did not affect the 5-HT steady-state level, but prevented the enhancement in pargyline-induced 5-HT accumulation after corticosterone replacement of the A D X rats (Table ]l). The validity of using pargyline for estimation of the 5-HT turnover rate was shown by recent work of van Loon et al. 9. Essentially the same response of the 5-HT system was obtained in a comparison of 4 different methods: accumulation of 5-HT and decline of 5-hydroxyindoleacetic acid (5-HIAA) following pargyline administration, m-hydroxybenzylhydrazine-induced accumulation of 5-hydroxytryptophan (5HTP) and accumulation of 5-HIAA following probenecid. These authors showed that the 4 indices of 5-HT turnover rate were reduced in hippocampus, hypothalamus and brainstem 2 h after adrenalectomy. Subsequent studies using one index, showed that corticosterone replacement and dexamethasone administration restored 5-HT turnover in the hypothalamus in the direction of that in sham-operated controls. Hypothalamic 5-HT has been implicated in the neuroendocrine control of pituitary A C T H release 20. Corticosterone and dexamethasone suppress stress-induced pituitary A C T H release, and since this suppression is exerted in part at the hypothalamic level • it would explain why both steroids are effective in the restoration of the 5-HT response in the hypothalamus of A D X rats. The present study shows an effect on 5-HT turnover rate in the hippocampus, that is specific for corticosterone. The 5-HT response to corticosterone was shown by means of a classical endocrine approach: removal of the adrenals decreased 5-HT turnover; corticosterone replacement restored 5-HT turnover to the level of that in sham-operated controls. Dexamethasone, however, failed to restore the 5-HT response. Accordingly, the specificity of the corticosterone effect on 5-HT corresponds
662 to the stringent specificity of the corticosterone receptors in hippocampal neurons. The 1 h time interval after ADX allowed considerable depletion of the hippocampal corticosterone receptor system leaving only about 40% of the sites occupied with endogenous hormoneL The replacement dose of 30 #g corticosterone was sufficient to maintain receptor occupancy up to the level of S-ADX (70-80 ~ occupation) and at the same time normalized the 5-HT response'-'. 5-HT projections arising from the midbrain raphe nuclei innervate the hippocampus 4. The index for 5-HT turnover rate was derived from the pargyline-induced 5H T accumulation, and probably represents a presynaptic event. The activation of 5H T turnover may depend on occupation by corticosterone of receptors in the cell bodies of hippocampal neurons, that are located postsynaptically. If this corticosterone receptor interaction is involved in the modulation of hippocampal 5-HT turnover, such an effect seems to take place in a transsynaptic fashion. Dexamethasone did not mimic the corticosterone effect. However, since the synthetic glucocorticoid inhibits the pargyline-induced 5-HT accumulation, it affects 5H T metabolism. Pretreatment with dexamethasone prevented the 5-HT response to corticosterone replacement of ADX rats. Since dexamethasone is poorly retained by neuronal cell nuclei of rat hippocampus v, it is unlikely that the antagonism of the synthetic steroid was exerted via interaction with the neuronal corticosterone receptor s2cstem. The action of dexamethasone on 5-HT may, therefore, occur via another mechanism. For instance, there are reports of a direct influence of glucocorticoids on the uptake of the amino acid precursor tryptophan by nerve endings t,x7, and this would affect the catalytic efficiency of tryptophan hydroxylase. The role of the hippocampus in the control of pituitary ACTH release is not well defined. Hippocampal function seems to be more implicated in the control of adaptive behavior 3. Certain behaviors are disturbed after removal of the adrenals and selectively restored with corticosterone'~,12,13. The present study shows that the hippocampal 5-HT response to corticosterone is subject to the same stringent steroid specificity. The specificity of the behavioral and serotonin responses correspond to the properties of the corticosterone rezeptor system in hippocampal neurons. The skilfut technical assistance of Mrs Katalin Kov~ics, Agnes Sz6ke, Marjanne van Ittersum, Harma Scholtens, Henny de Vos Burchart-Lodewijks, Dorothy Voorhuis and Mr. Henk Spierenburg i.s gratefully acknowledged. These studies were supported in part by a twinning grant of the European Training Program in Brain and Behaviour Research (ETPBBR).
1 Azmitia, E. C. and McEwen, B. S., Adrenalcortical influenceon rat brain tryptophan hydroxylase activity, Brain Research, 78 (1974) 291-302. 2 Bohus, B. and De Kloet, E. R., Adrenal steroids and extinction behaviour: antagonism by progesterone, deoxycorticosterone and dexamethasone of a specificeffect of corticosterone, Life Sci., 28 (1981) 433-440. 3 Bohus, B. De Kloet, E. R. and Veldhuis, H. D., Adrenal steroids and behavioural adaptation: relationships to brain receptors. In D. W. Pfaff and D: Ganten (Eds.), Current Topics in Neuroendocrinology, Vol. 2: Adrenal action on the brain, Springer, Berlin, in press.
663 4 Fuxe, K., H6kfelt, T. and Ungerstedt, U., Localization of indole amines in CNS. In S. Garattini and P. A. Shore (Eds.), Advances in Pharmacology, Vol. 6, Part A, Academic Press, New York, 1968, pp. 235-251. 5 Gerlach, J. L. and McEwen, B. S., Rat brain binds adrenal steroid hormone: radioautography of hippocampus with corticosterone, Science, 175 (1972) 1133-1136. 6 Jones, M. T., Hillhouse, E. W. and Burden, J. L., Structure-activity relationship of corticosteroid feedback at the hypothalamic level, J. Endocr., 74 (1977) 415424. 7 De Kloet, E. R., Wallach, G. and McEwen, B. S., Differences in corticosterone and dexamethasone binding to rat brain and pituitary, Endocrinology, 96 (1975) 598-609. 8 Kov~ics, G. L., Kishonti, J., Lissak, K. and Telegdy, G., Dose-dependent dual effect of corticosterone on cerebral 5-HT metabolism, Neurochem. Res., 2 (1977) 311-322. 9 van Loon, G. R., Shum, A. and Sole, M. J., Decreased brain serotonin turnover after short-term (two-hour) adrenalectomy in rats: a comparison of four turnover methods, Endocrinology, 108 (1981) 1392-1402. 10 Martini, L., Focchi, M., Gavazzi, G. and Pecile, A., Inhibitory action of steroids on the release of corticotrophin, Arch. int. Pharmacodyn., 140 (1962) 156-163. 11 McEwen, B. S., Weiss, J. M. and Schwartz, L. S., Retention of corticosterone by cell nuclei from brain region of adrenalectomized rats, Brain Research, 17 (1970) 471-482. 12 Micco, D. J., Jr. and McEwen, B. S., Glucocorticoids, the hippocampus and behaviour: interactive relation between task activation and steroid hormone binding specificity, J. comp. physiol. Psychol., 94 (1980) 624-633. 13 Micbeau. J., Destrade, C. and Soumireu-Mourat, B., Intraventricular corticosterone injection facilitates memory of an appetitive discriminative task in mice, Neurosci. Lett., Suppl., 7 (1981) S 242. 14 Rees, H. D., Stumpf, W. E. and Sar, M., Autoradiographic studies with 3H-dexamethasone in the rat brain and pituitary. In W. E. Stumpf and L. D. Grant (Eds.), Anatomical Neuroendocrinology, S. Karger, Basel, 1975, pp. 262-269. 15 Rousseau, G. G., Baxter, J. D. and Tomkins, G. M., Glucocorticoid receptors: relation between steroid binding and biologic effects, J. molec. Biol., 67 (1972) 99-107. 16 Snyder, S. H., Axelrod, J. and Zweig, M., A sensitive and specific fluorescence assay for tissue serotonin, Biochem. Pharmacol., 14 (1965) 831-835. 17 Sze, P. Y., Neckers, L. and Towle, A. C., Glucocorticoids as a regulatory factor for brain tryptophan hydroxylase, J. Neurochem., 26 (1976) 169-173. 18 Tozer, T. N., Neff, N. H. and Brodie, B. B., Application of steady-state kinetics to the synthesis rate and turnover time of serotonin in the brain of normal and reserpine treated rats, J. Pharmacol. exp. Ther., 153 (1966) 177-182. 19 Vermes, I., Smelik, P. G. and Mulder, A. H., Effects of hypophysectomy, adrenalectomy and corticosterone treatment on uptake and release of putative central neurotransmitters by rat hypothalamic tissue in vitro, Life Sci., 19 (1976) 1719-1726. 20 Weiner, R. I. and Ganong, W. F., Role of brain monoamines and histamine in regulation of anterior pituitary secretion, Physiol. Rev., 58 (1978) 905-975.
659
Decreased serotonin turnover in the dorsal hippocampus of rat brain shortly after adrenalectomy: selective normalization after corticosterone substitution
E. R. DE KLOET, G. L. KOV/~CS, G. SZABO, G. TELEGDY, B. BOHUS and D. H. G. VERSTEEG Rudolf Magnus lnstitute ]br Pharmacology, University of Utrecht, Medical Faculty, Vondellaan 6, 3521 G D Utrecht (The Netherlands) and ( G.L.K., G.S. and G. T. ) Department of Pathophysiology, University Medical School, Szeged (Hungary)
(Accepted January 28th, 1982) Key words: corticosterone - - hippocampus - - serotonin
Pargyline-induced accumulation of serotonin (5-HT) was used as an index of 5-HT turnover rate in the dorsal hippocampus. One hour after bilateral removal of the adrenals, 5-HT turnover was significantly reduced when compared to that of the sham-operated controls. A low dose of corticosterone given immediately after adrenalectomy restored the 5-HT response, while the same dose of dexamethasone was ineffective. Pretreatment with dexamethasone blocked the 5-HT response to corticosterone in the acutely adrenalectomized rat. The specificity of the 5-HT response in the hippocampus corresponds to the properties of the glucocorticoid receptor system in rat hippocampa[ neurons. H i p p o c a m p a l neurons contain adrenal steroid receptor sites with a stringent specificity for corticosterone, the principal glucocorticoid o f the rat 11. W h e n corticosterone is administered to adrenalectomized rats, a large a m o u n t of the glucocorticoid receptor complex is retained selectively in cell nuclei of hippocampal neurons 5. In contrast, the potent synthetic glucocorticoid dexamethasone has a lower affinity for these glucocorticoid receptor sites and is only poorly retained by the hippocampa[ neuron cell nuclei3,7,14. The function o f the hippocampal glucocorticoid receptor system was obscure until it was recently found that such receptors may be implicated in the modulation by corticosterone of certain adaptive behavioral responses2,t2, a3. Studies in this laboratory have demonstrated a behavioral deficit in rats at 1 h after bilateral removal of the adrenalsL The deficient behavior could be restored by giving a low dose o f corticosterone immediately after adrenalectomy (ADX). Dexamethasone, as well as the mineralocorticoids deoxycorticosterone and aldosterone, were ineffective. Pretreatment with dexamethasone even prevented the normalizing effect of corticosterone on behavior o f A D X rats 2. A close correlation between brain 5-HT turnover8, 9 or t r y p t o p h a n hydroxylase activity with the corticosterone level has been reported t. Behavioral alterations with or without corticostereid treatment 2 m a y well be related to changes in serotonergic neurotransmission in the brain. Therefore, 5-HT turnover in 0006-8993/82/000(~0000/$02.75 © Elsevier Biomedical Press
660 the brains of acutely adrenalectomized rats with or without steroid replacement was studied using the same time and dose scheme as in the behavioral studies. In the first experiment rats were subjected to bilateral A D X or to sham-adrenalectomy (S-ADX) under ether anesthesia and received 30 min later either pargyline (150 mg/kg i.p.) or vehicle. The rats were decapitated 60 min after surgery. The brains were cut into sections 2 m m thick and the dorsal hippocampus (about 24 mg wet tissue weight) dissected at 0 °C. Serotonin was assayed according to the spectrofluorimetric method of Snyder et al. t6. Pargyline was used according to Tozer 18. Pargyline, a monoamine oxidase inhibitor, produces an accumulation of brain serotonin (5-HT) which is approximately linear for at least 30 min after its administration. The difference in 5-HT level between pargyline- and vehicle-treated animals was taken as an index of the synthesis or of the turnover rate of 5-HT 8,18. The steady-state 5-HT level in the dorsal hippocampus was not significantly different for A D X and S-ADX (Table I). Pargyline induced a significant increase in 5-HT concentration in both groups. The pargyline-induced accumulation of 5-HT was significantly decreased after A D X , when compared to S-ADX. This index was 55 %1 lower after A D X than after SADX. In the second experiment the A D X rats received as replacement low doses (30 #g/100 g body weight s.c.) of either corticosterone or dexamethasone (Table l I) immediately after surgery. The replacement with corticosterone resulted in a 2-fold increase in 5-HT accumulation in the dorsal hippocampus, and consequently this index for 5-HT turnover rate was restored to that of the sham-operated control animals. Dexamethasone treatment inhibited the pargyline effect: no pargylineinduced 5-HT accumulation was observed. The difference was significant in comparison with corticosterone-substituted A D X animals. The steady state 5-HT levels were not significantly different for any of the groups. In a third experiment the animals received dexamethasone (30/~g/100 g body weight s.c.) 60 rain prior to A D X and corticosterone replacement at ADX. Dexame-
TABLE 1 Effect of adrenalectomy ( ADX) and sham-adrenalectomy ( S-ADX) on the pargyline-induced accumulation of serotonin (5-H7) in the dorsal hippocampus
Data are means from 12-14 animals ± S.E.M. The difference in 5-HT level between pargyline- and vehicle-treated animals was taken as index for the synthesis or turnover rate of 5-HT. Treatment at ~0
rain
S-ADX ADX
--30 rain
SAL PARG SAL PARG
5-HT concentration (#g/g tissue)
Diff
0.85 ± 0.06 1.57 ± 0.11" 0.94 i 0.13 1.26 v~ 0.08".**
÷ 0.72 -t 0.32
* Significantly different (P < 0.05) from the respective SAL group. SAL = saline. ** Significantly different (P < 0.05) from S-ADX-PARG. PARG = pargyline.
661 TABLE 1I Effect o f corticosterone and dexamethasone on the pargyline-induced accumulation o f serotonin (5-HT) in the dorsal hippocampus o f the acutely (one hour) adrenalectomized (ADX)
Data are mean of 12-15 (1) or (~8 (1I) animals 3: S.E.M. The data were analyzed by one way ANOVA and subsequently by Student's t-test (two-tailed). Treatment at --120 mitt
--60 rain
--30 min
SAL
ADX/SAL
SAL
ADX/DEX
SAL
ADX/CORT
DEX
ADX/CORT
SAL PARG. SAL PARG. SAL PARG. SAL PARG.
5- H T concentration (l~g/g tissue)
0.88 +~ 0.06 1 1.12 ± 0.05* 0.92 ~ 0.1l II 0.79 ± 0.05"*,*** 0.94 _k 0.06 I 1.42 ± 0.07*,** 0.97 ± 0.07 1I 1.15 -c 0.08***
Diff"
t 0.24 --0.13 F 0.48 ~ 0.18
* Significantly different (P < 0.05) from the respective SAL group. Significantly different (P < 0.05) from SAL-ADX/SAL-PARG. *** Significantly different (P < 0.05) from SAL-ADX/CORT-PARG. **
thasone pretreatment did not affect the 5-HT steady-state level, but prevented the enhancement in pargyline-induced 5-HT accumulation after corticosterone replacement of the A D X rats (Table ]l). The validity of using pargyline for estimation of the 5-HT turnover rate was shown by recent work of van Loon et al. 9. Essentially the same response of the 5-HT system was obtained in a comparison of 4 different methods: accumulation of 5-HT and decline of 5-hydroxyindoleacetic acid (5-HIAA) following pargyline administration, m-hydroxybenzylhydrazine-induced accumulation of 5-hydroxytryptophan (5HTP) and accumulation of 5-HIAA following probenecid. These authors showed that the 4 indices of 5-HT turnover rate were reduced in hippocampus, hypothalamus and brainstem 2 h after adrenalectomy. Subsequent studies using one index, showed that corticosterone replacement and dexamethasone administration restored 5-HT turnover in the hypothalamus in the direction of that in sham-operated controls. Hypothalamic 5-HT has been implicated in the neuroendocrine control of pituitary A C T H release 20. Corticosterone and dexamethasone suppress stress-induced pituitary A C T H release, and since this suppression is exerted in part at the hypothalamic level • it would explain why both steroids are effective in the restoration of the 5-HT response in the hypothalamus of A D X rats. The present study shows an effect on 5-HT turnover rate in the hippocampus, that is specific for corticosterone. The 5-HT response to corticosterone was shown by means of a classical endocrine approach: removal of the adrenals decreased 5-HT turnover; corticosterone replacement restored 5-HT turnover to the level of that in sham-operated controls. Dexamethasone, however, failed to restore the 5-HT response. Accordingly, the specificity of the corticosterone effect on 5-HT corresponds
662 to the stringent specificity of the corticosterone receptors in hippocampal neurons. The 1 h time interval after ADX allowed considerable depletion of the hippocampal corticosterone receptor system leaving only about 40% of the sites occupied with endogenous hormoneL The replacement dose of 30 #g corticosterone was sufficient to maintain receptor occupancy up to the level of S-ADX (70-80 ~ occupation) and at the same time normalized the 5-HT response'-'. 5-HT projections arising from the midbrain raphe nuclei innervate the hippocampus 4. The index for 5-HT turnover rate was derived from the pargyline-induced 5H T accumulation, and probably represents a presynaptic event. The activation of 5H T turnover may depend on occupation by corticosterone of receptors in the cell bodies of hippocampal neurons, that are located postsynaptically. If this corticosterone receptor interaction is involved in the modulation of hippocampal 5-HT turnover, such an effect seems to take place in a transsynaptic fashion. Dexamethasone did not mimic the corticosterone effect. However, since the synthetic glucocorticoid inhibits the pargyline-induced 5-HT accumulation, it affects 5H T metabolism. Pretreatment with dexamethasone prevented the 5-HT response to corticosterone replacement of ADX rats. Since dexamethasone is poorly retained by neuronal cell nuclei of rat hippocampus v, it is unlikely that the antagonism of the synthetic steroid was exerted via interaction with the neuronal corticosterone receptor s2cstem. The action of dexamethasone on 5-HT may, therefore, occur via another mechanism. For instance, there are reports of a direct influence of glucocorticoids on the uptake of the amino acid precursor tryptophan by nerve endings t,x7, and this would affect the catalytic efficiency of tryptophan hydroxylase. The role of the hippocampus in the control of pituitary ACTH release is not well defined. Hippocampal function seems to be more implicated in the control of adaptive behavior 3. Certain behaviors are disturbed after removal of the adrenals and selectively restored with corticosterone'~,12,13. The present study shows that the hippocampal 5-HT response to corticosterone is subject to the same stringent steroid specificity. The specificity of the behavioral and serotonin responses correspond to the properties of the corticosterone rezeptor system in hippocampal neurons. The skilfut technical assistance of Mrs Katalin Kov~ics, Agnes Sz6ke, Marjanne van Ittersum, Harma Scholtens, Henny de Vos Burchart-Lodewijks, Dorothy Voorhuis and Mr. Henk Spierenburg i.s gratefully acknowledged. These studies were supported in part by a twinning grant of the European Training Program in Brain and Behaviour Research (ETPBBR).
1 Azmitia, E. C. and McEwen, B. S., Adrenalcortical influenceon rat brain tryptophan hydroxylase activity, Brain Research, 78 (1974) 291-302. 2 Bohus, B. and De Kloet, E. R., Adrenal steroids and extinction behaviour: antagonism by progesterone, deoxycorticosterone and dexamethasone of a specificeffect of corticosterone, Life Sci., 28 (1981) 433-440. 3 Bohus, B. De Kloet, E. R. and Veldhuis, H. D., Adrenal steroids and behavioural adaptation: relationships to brain receptors. In D. W. Pfaff and D: Ganten (Eds.), Current Topics in Neuroendocrinology, Vol. 2: Adrenal action on the brain, Springer, Berlin, in press.
663 4 Fuxe, K., H6kfelt, T. and Ungerstedt, U., Localization of indole amines in CNS. In S. Garattini and P. A. Shore (Eds.), Advances in Pharmacology, Vol. 6, Part A, Academic Press, New York, 1968, pp. 235-251. 5 Gerlach, J. L. and McEwen, B. S., Rat brain binds adrenal steroid hormone: radioautography of hippocampus with corticosterone, Science, 175 (1972) 1133-1136. 6 Jones, M. T., Hillhouse, E. W. and Burden, J. L., Structure-activity relationship of corticosteroid feedback at the hypothalamic level, J. Endocr., 74 (1977) 415424. 7 De Kloet, E. R., Wallach, G. and McEwen, B. S., Differences in corticosterone and dexamethasone binding to rat brain and pituitary, Endocrinology, 96 (1975) 598-609. 8 Kov~ics, G. L., Kishonti, J., Lissak, K. and Telegdy, G., Dose-dependent dual effect of corticosterone on cerebral 5-HT metabolism, Neurochem. Res., 2 (1977) 311-322. 9 van Loon, G. R., Shum, A. and Sole, M. J., Decreased brain serotonin turnover after short-term (two-hour) adrenalectomy in rats: a comparison of four turnover methods, Endocrinology, 108 (1981) 1392-1402. 10 Martini, L., Focchi, M., Gavazzi, G. and Pecile, A., Inhibitory action of steroids on the release of corticotrophin, Arch. int. Pharmacodyn., 140 (1962) 156-163. 11 McEwen, B. S., Weiss, J. M. and Schwartz, L. S., Retention of corticosterone by cell nuclei from brain region of adrenalectomized rats, Brain Research, 17 (1970) 471-482. 12 Micco, D. J., Jr. and McEwen, B. S., Glucocorticoids, the hippocampus and behaviour: interactive relation between task activation and steroid hormone binding specificity, J. comp. physiol. Psychol., 94 (1980) 624-633. 13 Micbeau. J., Destrade, C. and Soumireu-Mourat, B., Intraventricular corticosterone injection facilitates memory of an appetitive discriminative task in mice, Neurosci. Lett., Suppl., 7 (1981) S 242. 14 Rees, H. D., Stumpf, W. E. and Sar, M., Autoradiographic studies with 3H-dexamethasone in the rat brain and pituitary. In W. E. Stumpf and L. D. Grant (Eds.), Anatomical Neuroendocrinology, S. Karger, Basel, 1975, pp. 262-269. 15 Rousseau, G. G., Baxter, J. D. and Tomkins, G. M., Glucocorticoid receptors: relation between steroid binding and biologic effects, J. molec. Biol., 67 (1972) 99-107. 16 Snyder, S. H., Axelrod, J. and Zweig, M., A sensitive and specific fluorescence assay for tissue serotonin, Biochem. Pharmacol., 14 (1965) 831-835. 17 Sze, P. Y., Neckers, L. and Towle, A. C., Glucocorticoids as a regulatory factor for brain tryptophan hydroxylase, J. Neurochem., 26 (1976) 169-173. 18 Tozer, T. N., Neff, N. H. and Brodie, B. B., Application of steady-state kinetics to the synthesis rate and turnover time of serotonin in the brain of normal and reserpine treated rats, J. Pharmacol. exp. Ther., 153 (1966) 177-182. 19 Vermes, I., Smelik, P. G. and Mulder, A. H., Effects of hypophysectomy, adrenalectomy and corticosterone treatment on uptake and release of putative central neurotransmitters by rat hypothalamic tissue in vitro, Life Sci., 19 (1976) 1719-1726. 20 Weiner, R. I. and Ganong, W. F., Role of brain monoamines and histamine in regulation of anterior pituitary secretion, Physiol. Rev., 58 (1978) 905-975.