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Effects of estradiol and dexamethasone on choline acetyltransferase activity in various rat brain regions
Brain Research, 453 (1988) 389-392
389
Elsevier BRE22962
Effects of estradiol and dexamethasone on choline acetyltransferase activity in various rat brain regions Herman Kaufman 1, Csaba Vadasz 2 and Abel Lajtha 2 Divisions of 1Molecular Biology and Neural Regeneration and 2Neurochemistry, The Nathan S. Kline Institute for Psychiatric Research, Orangeburg, NY10962 (U.S.A.)
(Accepted 22 March 1988) Key words: Choline acetyltransferase; Estradiol; Dexamethasone; Brain region; Mechanism of action
Estradiol, administered to ovariectomized rats, increased choline acetyltransferase (CHAT) activity in the caudate nucleus, cortex, hippocampus, and hypothalamus, suggesting possibly widespread central cholinergic involvement in estrus-related behavior. Dexamethasone also, except in hypothalamus, increased ChAT activity, notably (50%) in hippocampus. ChAT activity changes did not correlate with reported regional hormone receptor density. Estradiol's effect in the caudate suggests that hormone receptor and affected enzyme may not necessarily coexist intraneuronaUy.
Steroid hormones, released into the circulation by the gonads and adrenals, evoke or modulate discrete behavioral patterns in mature animals 2°. Estradiol, for example, elicits characteristic female mating behavior when injected systemically2 or into the ventromedial hypothalamus of ovariectomized rats 26. As might be reasonably supposed, the selective activation of neuronal pathways in the brain that mediate the translation of hormonal signal to behavioral response involves changes in the regional activities of enzymes concerned with neurotransmitter metabolism. The activity of several such enzymes, including choline acetyltransferase (CHAT), has been found to be altered after estradiol administration 5'9't4'15"17. Such studies were in part guided by extension to the brain of a 'genomic model' which was based on (a) observations that in uterine cells, estradiol formed complexes with specific intracellular receptors, which then interacted with the genome to induce specific protein synthesis 3, and (b) by later findings showing the presence, in regionally differing densities, of specific intracellular receptors for various steroid hormones in both brain and pituitary 2°.
CHAT, which catalyzes acetylcholine synthesis, is a marker of cholinergic neurons. Changes in its activity in diverse brain areas in response to systemic steroid hormone treatment may, then, be indicative of the range and relative regional participation of the brain's cholinergic pathways in the hormonally evoked behavioral patterns. Further, relating any such induced regional changes in ChAT activity to the corresponding regional densities of the hormone receptors may also offer some insight regarding mechanism(s) of action. We have examined the effect of estradiol administration to ovariectomized rats on ChAT activity in the caudate nucleus, frontoparietal cortex, hippocampus and hypothalamus. The first two brain regions are particularly poor in estradiol receptors, whereas the hypothalamus is relatively rich in such receptors 24. We have also carried out a similar study using dexamethasone, a highly potent adrenocorticotropic hormone (ACTH)-suppressing synthetic glucocorticosteroid, which has its own characteristic regional binding distribution. Sprague-Dawley rats (200-220 g), ovariecto-
Correspondence: H. Kaufman, Division of Molecular Biology and Neural Regeneration, The Nathan S. Kline Institute for Psychiat-
ric Research, Orangeburg, NY 10962, U.S.A. 0006-8993/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)
390 mized more than 3 weeks earlier, were group-housed on a 10 h light, 14 h dark cycle. They were injected s.c. with 17fi-estradiol or dexamethasone in 0.2 ml sesame oil or oil alone (controls) in two equal doses 36 h apart. There were 4-5 animals per treated group. One group of estradioi-treated rats received 10/~g hormone and the other 100/~g per day. Dexamethasone-treated rats received 80/~g on each of the 2 days. Two days after the first injection, each animal was decapitated, and its brain dissected over ice. Each caudate nucleus, hippocampus, cortex, and hypothalamus was frozen by dry ice and stored at -88 °C. They were taken for homogenization and enzyme assay during a confirmed period of ChAT stability. Homogenization was at 0 °C in 1 mM EDTA, 0.1% Triton X-100 solution, pH 7.0. Homogenates, stored at -88 °C, were usually assayed for ChAT activity the following day by the radiochemical method of Fonnum 7, somewhat modified. Protein was determined by a modified procedure of Lowry et al. 13. Estradiol elicited significant increases in ChAT activity in all 4 brain regions examined (Table I). The effect bore no apparent relation to the density of estradiol receptors reported for those regions 24. Dexamethasone also elevated ChAT activity in all the regions tested except for the hypothalamus. The high dose of estradiol (100ktg/animal/day for two days) led to an increase of 31% in ChAT activity of the caudate nucleus, and somewhat lower increases in the frontoparietal cortex and hippocampus. [Luine et al. l~ using a different treatment schedule, also found a rise in hippocampal ChAT activity (12%, versus 17% here).] ChAT activity rose 32% in the hypothalamus. Lowering the dose of estradiol to 10/~g did not uniformly alter the effect on activity.
Although dexamethasone also increased ChAT activity in 3 of the regions, the activity profile differed somewhat from that caused by a roughly similar dosage of estradiol. Particularly noteworthy was a much larger increase in activity in the hippocampus following dexamethasone treatment than was elicited by a comparable dosage of estradiol (50% vs 17%). A close numerical similarity in effects by dexamethasone and low dosage estradiol was noted in the caudate nucleus, frontoparietal cortex, and hypothalamus. The diversity of brain regions in which ChAT activity changed after estradiol administration suggests a possibly widespread central cholinergic involvement in estrus-related behaviors. Our results also confirm and extend findings of estradiol-induced regional changes in ChAT activity 14'17. The present observations, particularly regarding estradiol's effect on the caudate, also bear on the possible mechanism(s) by which the hormonal effects are generated. The principal model invoked for relating steroid hormone action to resultant 'long-term' changes in regional enzymatic activities has had two components: (a) that interaction of hormone with intracellular receptor leads to a quantitative alteration of specific gene transcription, and (b) that a regional correspondence between hormone receptor density and hormonally induced changes in enzymatic activity is likely. Consistent with the foregoing were findings that an estradiol-elicited rise in ChAT activity in the preoptic area of rat brain was accompanied by a commensurate increase of ChAT protein 16. However, our results showed no necessary correspondence between a region's reported density of receptors and changes in its ChAT activity. Estradiol, for example,
TABLE I Effect of estradiol and of dexamethasone treatment of ovariectomized rats on the specific activity of choline acetyltransferase in various brain regions
Data are expressed as ratios of specific activities of hormone-treated to control animals. Specificactivities are expressed in pmol product/ min/mg protein and are mean values + S.E.M. for 4-5 animals per group. Statistical significancewas evaluated by Student's t-test. Caudate nucleus
Hippocampus
Cortex (frontoparietal)
Hypothalamus
fl-estradiol
(100pg) (10~ug)
1.31 (P < 0.01) 1.18 (P < 0.02)
1.17 (P < 0.05) 1.18 (P < 0.05)
1.09 (P < 0.01) 1.20 (P < 0.001)
1.32 (P < 0.001) 0.90 (P < 0.10)
Dexamethasone
(80/~g)
1.18 (P < 0.001)
1.50 (P < 0.001)
1.20 (P < 0.01)
1.02
1.00 (1963 + 41.7)
1.00 (453 _+27.0)
1.00 (378 _+5.5)
(348 _+2.5)
Control (spec. act.)
1.00
391 substantially increased ChAT activity in the caudate nucleus and the frontoparietal cortex, which have very few estradiol receptors 24. In principle, of course, a regional correspondence between receptor population and affected enzyme would not be necessary. The receptors could be in the perikarya in one region and the enzyme in the termini of the same; neurons in another. It is difficult to see, though, how the results for the caudate can be understood in this way. This region has particularly few estradiol receptors 24. Further, its very rich complement of cholinergic neurons is primarily intrinsic and none of the 3 major pathways to the caudate are cholinergic I1. Yet estradiol did induce a large increase in ChAT activity in this region (31%). This indicates that at least in some brain regions the hormone-triggered change in activity of a given enzyme may not devolve from the coexistence in the same neuron of receptor and affected enzyme. How might the caudate result be explained? One speculative possibility is that the observed effect is a consequence of estradiol-induced events in neurons that are afferent to those in the caudate. Lesioning of the nigrostriatal pathway 22 or the medial septal area l° has been reported to increase ChAT activity in the caudate-putamen body or striatum. The heavy innervation by dopaminergic neurons from the nigrostriatal pathway of cholinergic neurons in the caudate nucleus is usually considered to be mainly inhibitory of the latter. Estrogen, cited by Euvrard et al. 6 as antidopaminergic in the striatum, might thereby disinhibit the caudate's cholinergic neurons. Increased impulse traffic in cholinergic neurons was noted by Ekstr6m 4 to increase ChAT activity. Such a sequence might then account for estradiol's observed effect on ChAT in the caudate. Moreover, estradiolcaused changes in secretions of pituitary hormones might play a role. This possibility is supported by examination of data from Luine et a1.14, showing that hypophysectomy reduced basal ChAT activity and also lessened an estradiol-induced increase in ChAT activity in the preoptic area. Dexamethasone also increased ChAT activity in the caudate nucleus, frontoparietal cortex, and hippocampus. The binding of this glucocorticoid is known to be rather uniformly distributed among various brain regions of the rat 19. The large increase in ChAT activity (ca 50%) in the hippocampus by dexa-
methasone contrasts strikingly with the observation by Meyer et al. 21 that corticosterone replacement in adrenalectomized male rats produced no change in activity of ChAT (or 15 other enzymes) in this region t h o u g h the density of putative receptors for dexamethasone in the hippocampus is considerably less than that for the native glucocorticoid, corticosterone 19. Again, it would appear that regional receptor density and induced changes in enzymatic activity by steroid hormones are not necessarily coterminous. Conceivably, as suggested previously for estradiol, dexamethasone's effect may at least in part be mediated by indirect means, particularly taking into account its powerful depressive effect on the anterior pituitary's secretion of hormones. Another possibility might be via its effects on membrane properties. (Dexamethasone has been reported to increase membrane fluidity and thereby the activities of some plasma membrane-bound enzymes 1, and considerable ChAT in intrinsic membrane-bound form is present in brain 23.) With regard to some of the notably similar effects on ChAT activity by low dosage estradiol and higher dosage dexamethasone in 3 regions, it may be worth noting that although the present data cannot assign cause, there are some precedents for the possibility that these hormones might participate in binding to the same target sites or receptors. Stumpf and Gar 27 have suggested that a neuron may be addressed by different steroid hormones and the work of MacLusky et al.t8 suggests that estrogen and progestin receptors may coexist in certain cells of some regions. Receptors for dexamethasone and estrogen have been found to cohabit cells of some non-neural tissues s'25 and in chick oviduct gland cells these hormones synergistically induce ovalbumin 8. Interestingly, Lisk and Reuter 12 found estradiol retention in the pituitary decreased after dexamethasone treatment, noted that any hypothesized dexamethasone-receptot interaction must be much weaker than the estradiol-receptor interaction, and observed that dexamethasone can apparently synergize with estrogen in facilitating sexual receptivity. In summary, the activity of CHAT, an enzyme characteristic of cholinergic neurons, has been found to be increased in a number of widely diverse brain regions as a consequence of the intervention of both fl-estradiol and dexamethasone. Such changes in en-
392 z y m e activity are not necessarily c o i n c i d e n t with the
c a m p u s by d e x a m e t h a s o n e ( a b o u t t h r e e - f o l d higher
regional density of r e c e p t o r s for such steroid hor-
than that i n d u c e d by estradiol). In A l z h e i m e r ' s dis-
m o n e s . T h e initiating h o r m o n a l effects m a y h a v e a
ease an especially large loss of C h A T activity in hip-
different origin, a possibility n o t e d by M c E w e n et
p o c a m p u s has b e e n o b s e r v e d (see r e v i e w by T e r r y
al. 2° and u n d e r l i n e d by the p r e s e n t results. It is, of
and Davies" ), and it has b e e n s u g g e s t e d that agents
course, quite c o n c e i v a b l e that the i n d u c e d e n z y m a t i c
that increase C h A T activity in this a r e a m i g h t a m e -
changes are e f f e c t e d directly by the ' g e n o m i c m o d e l '
liorate the m e m o r y loss.
in certain brain regions, and indirectly in others. N o t e is also m a d e of the possible clinical r e l e v a n c e
W e are grateful to Dr. B r u c e S. M c E w e n for his
of s o m e of t h e s e r e s u l t s , p a r t i c u l a r l y of the large in-
thoughtful a t t e n t i o n and r e v i e w of the original m a n u -
crease in C h A T activity ( 5 0 % ) i n d u c e d in the h i p p o -
script and a t t e n d a n t stimulating discussions•
1 Brasitus, T.A., Dudeja, P.K., Dahiya, R. and Halline, A., Dexamethasone-induced alterations in lipid composition and fluidity of rat proximal-small-intestinal brush-border membranes, Biochem. J., 248 (1987) 455-461. 2 Davidson, J.M., Rodgers, C.H., Smith, E.R. and Bloch, G.J., Stimulation of female sex behavior in adrenalectomized rats with estrogen alone, Endocrinology, 82 (1968) 193-195• 3 DeAngelo, A.B. and Gorski, J., Role of RNA synthesis in the estrogen induction of a specific uterine protein, Proc. Natl. Acad. Sci. U.S.A., 66 (1970) 693-700. 4 Ekstr6m, J., Acetylcholine synthesis and its dependence on nervous activity, Experientia, 34 (1978) 1247-1253. 5 Eleftheriou, B.C. and Dobson, C.L., Effects of neonatal and adult treatments with gonadal hormones on choline acetylase activity in brain regions of male mice after fighting, Psychopharmacologia, 27 (1972) 45-52. 6 Euvrard, C., Oberlander, C. and Boissier, J.R., Antidopaminergic effect of estrogens at the striatal level, J. Pharmacol. Exp. Ther., 214 (1980) 179-185. 7 Fonnum, F., A rapid radiochemical method for the determination of choline acetyltransferase, J. Neurochem., 24 (1975) 407-409• 8 Hager, L.J., McKnight, G.S. and Palmiter, R.D., Glucocorticoid induction of egg white mRNAs in chick oviduct, J. Biol. Chem., 255 (1980) 7796-7800. 9 Iramain, C.A., Owasoyo, J.O. and Egbunike, G.N., Influence of estradiol on acetylcholinesterase activity in several female rat brain areas and adenohypophysis, Neurosci. Lett., 16 (1980) 81-84. 10 Kuhar, M.J., Sethy, V.H., Roth, R.H. and Aghajanian, G.K., Choline: selective accumulation by central cholinergic neurons, J. Neurochem., 20 (1973) 581-593• 11 Ladinsky, H. and Consolo, S., The effect of altered function of dopaminergic neurons on the cholinergic system in the striatum, Prog. Brain Res., 49 (1979) 411-419• 12 Lisk, R.D. and Reuter, L.A., Dexamethasone: increased weights and decreased (3H)-estradiol retention of uterus, vagina and pituitary in the ovariectomized rat, Endocrinology, 99 (1976) 1063-1070. 13 Lowry, O.H., Rosebrough, N.J., Farr, A.L. and Randall. R.J., Protein measurement with the Folin-phenol reagent, J. Biol. Chem., 193 (1951)265-275. 14 Luine, V.N., Khylchevskaya, R.I. and McEwen, B.S., Effect of gonadal steroids on activities of monoamine oxidase and choline acetylase in rat brain, Brain Research, 86 (1975) 293-306. 15 Luine, V.N., McEwen, B.S. and Black, I.B., Effect of 17flestradiol on hypothalamic tyrosine hydroxylase activity,
Brain Research, 120 (1977) 188-192. 16 Luine, V.N., Park, D., Joh, T., Reis, D. and McEwen, B.S, Immunochemical demonstration of increased choline acetyltransferase concentration in rat preoptic area after estradiol administration, Brain Research, 191 (1980) 273-277. 17 Luine, V.N., Estradiol increases choline acetyltransferase activity in specific basal forebrain nuclei and projection areas of female rats, Exp. Neurol., 89 (1985) 484-490. 18 MacLusky, N.J., Krey, L., Lieberburg, I. and McEwen, B.S., Estrogen modulation of progestin receptors in the bonnet monkey (M. radiata) and the rat, Endocrinol. Soc. Annu. Meet., Miami, t978. 19 McEwen, B.S., deKloet, R. and Wallach, G., Interactions in vivo and in vitro of corticoids and progesterone with cell nuclei and soluble macromolecules from rat brain regions and pituitary, Brain Research, 105 (1976) 129-136. 20 McEwen, B.S., Davis, P.G., Parsons, B. and Pfaff, D.W., The brain as a target for steroid hormone action, Annu. Rev. Neurosci., 2 (1979) 65-112. 21 Meyer, J.S., Luine, V.N., Khylchevskaya, R.I. and McEwen, B.S., Glucocorticoids and hippocampal enzyme activity, Brain Research, 166 (1979) 172-175. 22 Nagy, J.I., Vincent, S.R. and FiNger, H.C., Altered neurotransmitter synthetic enzyme activity in some extrapyramidal nuclei after lesions of the nigro-striatal dopamine projection, LiJe Sci., 22 (1978) 1777-1782. 23 Peng, J.H., McGeer, P.L. and McGeer, E.G., Membranebound choline acetyltransferase from human brain: purification and properties, Neurochem. Res., 11 (1986) 959-971. 24 Pfaff, D. and Keiner, M., Atlas of estradiol-concentrating cells in the central nervous system of the female rat, J. Comp. Neurol., 151 (1973) 121-158. 25 Renkawitz, R., Schtitz, G.. yon der Ahe, D. and Beato. M., Sequences in the promoter region of the chicken lysozyme gene required for steroid regulation and receptor bindihg, Cell, 37 (1984) 503-510. 26 Rubin, B.S. and Barfield, R.J., Priming of estrous responsiveness by implants of 17fl-estradiol in the ventromedial hypothalamic nucleus of female rats, Endocrinology, 106 (1980) 504-509. 27 Stumpf, W.E. and Gar, M., Steroid hormone target sites in the brain: the differential distribution of estrogen, progestin, androgen and glucocorticosteroid, J. Steroid Biochem., 7 (1976) 1163-1170. 28 Terry, R.D. and Davies, P., Dementia of the Alzheimer type, Annu. Rev. Neurosci., 3 (1980) 77-95.
389
Elsevier BRE22962
Effects of estradiol and dexamethasone on choline acetyltransferase activity in various rat brain regions Herman Kaufman 1, Csaba Vadasz 2 and Abel Lajtha 2 Divisions of 1Molecular Biology and Neural Regeneration and 2Neurochemistry, The Nathan S. Kline Institute for Psychiatric Research, Orangeburg, NY10962 (U.S.A.)
(Accepted 22 March 1988) Key words: Choline acetyltransferase; Estradiol; Dexamethasone; Brain region; Mechanism of action
Estradiol, administered to ovariectomized rats, increased choline acetyltransferase (CHAT) activity in the caudate nucleus, cortex, hippocampus, and hypothalamus, suggesting possibly widespread central cholinergic involvement in estrus-related behavior. Dexamethasone also, except in hypothalamus, increased ChAT activity, notably (50%) in hippocampus. ChAT activity changes did not correlate with reported regional hormone receptor density. Estradiol's effect in the caudate suggests that hormone receptor and affected enzyme may not necessarily coexist intraneuronaUy.
Steroid hormones, released into the circulation by the gonads and adrenals, evoke or modulate discrete behavioral patterns in mature animals 2°. Estradiol, for example, elicits characteristic female mating behavior when injected systemically2 or into the ventromedial hypothalamus of ovariectomized rats 26. As might be reasonably supposed, the selective activation of neuronal pathways in the brain that mediate the translation of hormonal signal to behavioral response involves changes in the regional activities of enzymes concerned with neurotransmitter metabolism. The activity of several such enzymes, including choline acetyltransferase (CHAT), has been found to be altered after estradiol administration 5'9't4'15"17. Such studies were in part guided by extension to the brain of a 'genomic model' which was based on (a) observations that in uterine cells, estradiol formed complexes with specific intracellular receptors, which then interacted with the genome to induce specific protein synthesis 3, and (b) by later findings showing the presence, in regionally differing densities, of specific intracellular receptors for various steroid hormones in both brain and pituitary 2°.
CHAT, which catalyzes acetylcholine synthesis, is a marker of cholinergic neurons. Changes in its activity in diverse brain areas in response to systemic steroid hormone treatment may, then, be indicative of the range and relative regional participation of the brain's cholinergic pathways in the hormonally evoked behavioral patterns. Further, relating any such induced regional changes in ChAT activity to the corresponding regional densities of the hormone receptors may also offer some insight regarding mechanism(s) of action. We have examined the effect of estradiol administration to ovariectomized rats on ChAT activity in the caudate nucleus, frontoparietal cortex, hippocampus and hypothalamus. The first two brain regions are particularly poor in estradiol receptors, whereas the hypothalamus is relatively rich in such receptors 24. We have also carried out a similar study using dexamethasone, a highly potent adrenocorticotropic hormone (ACTH)-suppressing synthetic glucocorticosteroid, which has its own characteristic regional binding distribution. Sprague-Dawley rats (200-220 g), ovariecto-
Correspondence: H. Kaufman, Division of Molecular Biology and Neural Regeneration, The Nathan S. Kline Institute for Psychiat-
ric Research, Orangeburg, NY 10962, U.S.A. 0006-8993/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)
390 mized more than 3 weeks earlier, were group-housed on a 10 h light, 14 h dark cycle. They were injected s.c. with 17fi-estradiol or dexamethasone in 0.2 ml sesame oil or oil alone (controls) in two equal doses 36 h apart. There were 4-5 animals per treated group. One group of estradioi-treated rats received 10/~g hormone and the other 100/~g per day. Dexamethasone-treated rats received 80/~g on each of the 2 days. Two days after the first injection, each animal was decapitated, and its brain dissected over ice. Each caudate nucleus, hippocampus, cortex, and hypothalamus was frozen by dry ice and stored at -88 °C. They were taken for homogenization and enzyme assay during a confirmed period of ChAT stability. Homogenization was at 0 °C in 1 mM EDTA, 0.1% Triton X-100 solution, pH 7.0. Homogenates, stored at -88 °C, were usually assayed for ChAT activity the following day by the radiochemical method of Fonnum 7, somewhat modified. Protein was determined by a modified procedure of Lowry et al. 13. Estradiol elicited significant increases in ChAT activity in all 4 brain regions examined (Table I). The effect bore no apparent relation to the density of estradiol receptors reported for those regions 24. Dexamethasone also elevated ChAT activity in all the regions tested except for the hypothalamus. The high dose of estradiol (100ktg/animal/day for two days) led to an increase of 31% in ChAT activity of the caudate nucleus, and somewhat lower increases in the frontoparietal cortex and hippocampus. [Luine et al. l~ using a different treatment schedule, also found a rise in hippocampal ChAT activity (12%, versus 17% here).] ChAT activity rose 32% in the hypothalamus. Lowering the dose of estradiol to 10/~g did not uniformly alter the effect on activity.
Although dexamethasone also increased ChAT activity in 3 of the regions, the activity profile differed somewhat from that caused by a roughly similar dosage of estradiol. Particularly noteworthy was a much larger increase in activity in the hippocampus following dexamethasone treatment than was elicited by a comparable dosage of estradiol (50% vs 17%). A close numerical similarity in effects by dexamethasone and low dosage estradiol was noted in the caudate nucleus, frontoparietal cortex, and hypothalamus. The diversity of brain regions in which ChAT activity changed after estradiol administration suggests a possibly widespread central cholinergic involvement in estrus-related behaviors. Our results also confirm and extend findings of estradiol-induced regional changes in ChAT activity 14'17. The present observations, particularly regarding estradiol's effect on the caudate, also bear on the possible mechanism(s) by which the hormonal effects are generated. The principal model invoked for relating steroid hormone action to resultant 'long-term' changes in regional enzymatic activities has had two components: (a) that interaction of hormone with intracellular receptor leads to a quantitative alteration of specific gene transcription, and (b) that a regional correspondence between hormone receptor density and hormonally induced changes in enzymatic activity is likely. Consistent with the foregoing were findings that an estradiol-elicited rise in ChAT activity in the preoptic area of rat brain was accompanied by a commensurate increase of ChAT protein 16. However, our results showed no necessary correspondence between a region's reported density of receptors and changes in its ChAT activity. Estradiol, for example,
TABLE I Effect of estradiol and of dexamethasone treatment of ovariectomized rats on the specific activity of choline acetyltransferase in various brain regions
Data are expressed as ratios of specific activities of hormone-treated to control animals. Specificactivities are expressed in pmol product/ min/mg protein and are mean values + S.E.M. for 4-5 animals per group. Statistical significancewas evaluated by Student's t-test. Caudate nucleus
Hippocampus
Cortex (frontoparietal)
Hypothalamus
fl-estradiol
(100pg) (10~ug)
1.31 (P < 0.01) 1.18 (P < 0.02)
1.17 (P < 0.05) 1.18 (P < 0.05)
1.09 (P < 0.01) 1.20 (P < 0.001)
1.32 (P < 0.001) 0.90 (P < 0.10)
Dexamethasone
(80/~g)
1.18 (P < 0.001)
1.50 (P < 0.001)
1.20 (P < 0.01)
1.02
1.00 (1963 + 41.7)
1.00 (453 _+27.0)
1.00 (378 _+5.5)
(348 _+2.5)
Control (spec. act.)
1.00
391 substantially increased ChAT activity in the caudate nucleus and the frontoparietal cortex, which have very few estradiol receptors 24. In principle, of course, a regional correspondence between receptor population and affected enzyme would not be necessary. The receptors could be in the perikarya in one region and the enzyme in the termini of the same; neurons in another. It is difficult to see, though, how the results for the caudate can be understood in this way. This region has particularly few estradiol receptors 24. Further, its very rich complement of cholinergic neurons is primarily intrinsic and none of the 3 major pathways to the caudate are cholinergic I1. Yet estradiol did induce a large increase in ChAT activity in this region (31%). This indicates that at least in some brain regions the hormone-triggered change in activity of a given enzyme may not devolve from the coexistence in the same neuron of receptor and affected enzyme. How might the caudate result be explained? One speculative possibility is that the observed effect is a consequence of estradiol-induced events in neurons that are afferent to those in the caudate. Lesioning of the nigrostriatal pathway 22 or the medial septal area l° has been reported to increase ChAT activity in the caudate-putamen body or striatum. The heavy innervation by dopaminergic neurons from the nigrostriatal pathway of cholinergic neurons in the caudate nucleus is usually considered to be mainly inhibitory of the latter. Estrogen, cited by Euvrard et al. 6 as antidopaminergic in the striatum, might thereby disinhibit the caudate's cholinergic neurons. Increased impulse traffic in cholinergic neurons was noted by Ekstr6m 4 to increase ChAT activity. Such a sequence might then account for estradiol's observed effect on ChAT in the caudate. Moreover, estradiolcaused changes in secretions of pituitary hormones might play a role. This possibility is supported by examination of data from Luine et a1.14, showing that hypophysectomy reduced basal ChAT activity and also lessened an estradiol-induced increase in ChAT activity in the preoptic area. Dexamethasone also increased ChAT activity in the caudate nucleus, frontoparietal cortex, and hippocampus. The binding of this glucocorticoid is known to be rather uniformly distributed among various brain regions of the rat 19. The large increase in ChAT activity (ca 50%) in the hippocampus by dexa-
methasone contrasts strikingly with the observation by Meyer et al. 21 that corticosterone replacement in adrenalectomized male rats produced no change in activity of ChAT (or 15 other enzymes) in this region t h o u g h the density of putative receptors for dexamethasone in the hippocampus is considerably less than that for the native glucocorticoid, corticosterone 19. Again, it would appear that regional receptor density and induced changes in enzymatic activity by steroid hormones are not necessarily coterminous. Conceivably, as suggested previously for estradiol, dexamethasone's effect may at least in part be mediated by indirect means, particularly taking into account its powerful depressive effect on the anterior pituitary's secretion of hormones. Another possibility might be via its effects on membrane properties. (Dexamethasone has been reported to increase membrane fluidity and thereby the activities of some plasma membrane-bound enzymes 1, and considerable ChAT in intrinsic membrane-bound form is present in brain 23.) With regard to some of the notably similar effects on ChAT activity by low dosage estradiol and higher dosage dexamethasone in 3 regions, it may be worth noting that although the present data cannot assign cause, there are some precedents for the possibility that these hormones might participate in binding to the same target sites or receptors. Stumpf and Gar 27 have suggested that a neuron may be addressed by different steroid hormones and the work of MacLusky et al.t8 suggests that estrogen and progestin receptors may coexist in certain cells of some regions. Receptors for dexamethasone and estrogen have been found to cohabit cells of some non-neural tissues s'25 and in chick oviduct gland cells these hormones synergistically induce ovalbumin 8. Interestingly, Lisk and Reuter 12 found estradiol retention in the pituitary decreased after dexamethasone treatment, noted that any hypothesized dexamethasone-receptot interaction must be much weaker than the estradiol-receptor interaction, and observed that dexamethasone can apparently synergize with estrogen in facilitating sexual receptivity. In summary, the activity of CHAT, an enzyme characteristic of cholinergic neurons, has been found to be increased in a number of widely diverse brain regions as a consequence of the intervention of both fl-estradiol and dexamethasone. Such changes in en-
392 z y m e activity are not necessarily c o i n c i d e n t with the
c a m p u s by d e x a m e t h a s o n e ( a b o u t t h r e e - f o l d higher
regional density of r e c e p t o r s for such steroid hor-
than that i n d u c e d by estradiol). In A l z h e i m e r ' s dis-
m o n e s . T h e initiating h o r m o n a l effects m a y h a v e a
ease an especially large loss of C h A T activity in hip-
different origin, a possibility n o t e d by M c E w e n et
p o c a m p u s has b e e n o b s e r v e d (see r e v i e w by T e r r y
al. 2° and u n d e r l i n e d by the p r e s e n t results. It is, of
and Davies" ), and it has b e e n s u g g e s t e d that agents
course, quite c o n c e i v a b l e that the i n d u c e d e n z y m a t i c
that increase C h A T activity in this a r e a m i g h t a m e -
changes are e f f e c t e d directly by the ' g e n o m i c m o d e l '
liorate the m e m o r y loss.
in certain brain regions, and indirectly in others. N o t e is also m a d e of the possible clinical r e l e v a n c e
W e are grateful to Dr. B r u c e S. M c E w e n for his
of s o m e of t h e s e r e s u l t s , p a r t i c u l a r l y of the large in-
thoughtful a t t e n t i o n and r e v i e w of the original m a n u -
crease in C h A T activity ( 5 0 % ) i n d u c e d in the h i p p o -
script and a t t e n d a n t stimulating discussions•
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