3,5 cyclic adenosine monophosphate mediates the salmon calcitonin-induced increase in hypothalamic tyrosine hydroxylase activity.

This study examined the effect of salmon calcitonin (sCT) on hypothalamic tyrosine hydroxylase (TH) activity and evaluated the cellular signaling mechanisms involved in the response. Fetal hypothalamic cells were cultured in a defined medium and treated with sCT and/or specific protein kinase inhibitors on day 14 in vitro. sCT (0.1-10 nM) increased both TH activity and cellular cAMP content in a concentration-dependent manner. sCT (10 nM) increased TH activity to 150-175% of control values and resulted in a 10-fold increase in cellular cAMP content. Both the C1a and C1b CT receptor isoforms were present in the cultures, as assessed by RT-PCR. Rp-adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS), a cAMP antagonist, and H-8, a cyclic nucleotide kinase inhibitor, blocked the sCT-induced increase in TH activity, with complete abolition of the response observed at concentrations of 1 mM and 5 microM, respectively. sCT (10 nM) increased radiolabeled phosphate incorporation into TH protein to 169% of control values and 1 mM Rp-cAMPS completely blocked this effect. In contrast, neither Calphostin C, a protein kinase C inhibitor, nor U-73122, a phospholipase C inhibitor, significantly altered the ability of sCT to increase TH activity. Likewise, the sCT-induced increase in TH activity was observed after pretreating the cells with either BAPTA/AM, an intracellular calcium chelator, or thapsigargin, an inhibitor of the endoplasmic reticulum calcium pump. These data indicate that sCT has a profound stimulatory effect on TH activity in fetal hypothalamic cells and that enhanced phosphorylation of TH coincides with the sCT-induced increase in enzyme activity. Moreover, CT receptors, which are linked to cAMP production, are expressed in the hypothalamic cells and a cAMP-dependent mechanism mediates the sCT-induced activation and phosphorylation of TH.     

Effects of stress on catecholamines and tyrosine hydroxylase activity of individual hypothalamic nuclei.

The effects of acute stress on norepinephrine (NE) and dopamine (DA) concentrations and of repeated stress on tyrosine hydroxylase (TH) activity were measured in individual hypothalamic nuclei and other rat brain regions. A microdissection technique and radioisotopic enzymatic assays enables these studies to be performed. NE and DA concentrations were reduced and TH activity increased selectively in the arcuate nucleus. These results suggest that the arcuate nucleus may be selectively involved in the stress response and support the hypothesis that catecholamines in the medial basal hypothalamus mediate certain of the neuroendocrine changes observed in stress.     

Sex-related alterations in hypothalamic tyrosine hydroxylase after neonatal monosodium glutamate treatment

This study compared tyrosine hydroxylase (TH) mRNA signal levels, relative quantity of TH protein, and the catalytic activity of TH in the tuberoinfundibular dopaminergic neurons (TIDA) of male and ovariectomized (OVEX) female rats. In addition, the effects of monosodium glutamate (MSG) neurotoxicity on these parameters of TH regulation were evaluated. Neonatal rats were injected with MSG (4 mg/g body weight) or 10% sodium chloride (controls) on alternate days for the first 10 days of life. Females were ovariectomized on day 45 of age, and all rats were used between 60 and 80 days of age. The TH mRNA signal levels, as assessed by an in situ hybridization technique, were 2-fold higher in control females than in control males, whereas the number of TH mRNA-containing cells was similar between sexes. The TH immunostainings of the TIDA perikarya in the arcuate nucleus and of the nerve terminals in the median eminence were qualitatively more intense in females than males. The catalytic activity of TH, as determined by in vitro DOPA accumulation in the stalk-median eminence, was 3-fold greater in females than males. Neonatal MSG-treatment resulted in a marked reduction in the number of TH mRNA-containing cells and TH-immunopositive cells in the arcuate nucleus of both sexes, as well as a decrease in the intensity of TH immunostaining in the median eminence. The cellular mRNA signal levels for TH were markedly reduced in females after MSG treatment, but were unchanged in males. MSG treatment reduced TH activity to 20% of control levels in females, but did not alter enzyme activity in males.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serotonin-induced decrease in hypothalamic tyrosine hydroxylase activity and corresponding increase in prolactin release are abolished at midpregnancy and by transplants of rat choriocarcinoma cells.

We investigated the effect of central serotonin (5-hydroxytryptamine, 5-HT) administration on hypothalamic tuberoinfundibular dopamine neurons and related changes in neuronal activity to circulating PRL levels in two physiological models: 1) pregnant rats expressing (day 8) or not expressing (days 11 and 16) PRL surges, and 2) ovariectomized rats transplanted with rat choriocarcinoma cells, which secrete functional placental lactogen-I. Over a 4-min period between 0900 and 1400 h, rats were administered either vehicle or 5-HT (20 micrograms/6 microliters) through lateral ventricular cannulae. Plasma PRL levels were determined by RIA. NSD 1015 (25 mg/kg intraarterial), a dihydroxyphenylalanine (DOPA) decarboxylase inhibitor, was injected 20 min after initiation of ventricular infusion. Ten min later, the stalk-median eminence (SME) was dissected. The rate of DOPA accumulation, determined by measuring DOPA levels in the SME by HPLC, was used as an index of tyrosine hydroxylase catalytic activity, indicating tubero infundibular dopamine neuronal activity. In day-8 pregnant rats 5-HT reduced DOPA accumulation to 57% of vehicle-injected controls and increased circulating PRL levels 13-fold. In contrast, on days 11 and 16 of pregnancy 5-HT did not alter DOPA accumulation in the SME or plasma PRL levels. In nonpregnant rats ovariectomized for 24 h, 5-HT decreased DOPA accumulation in the SME to 43% of vehicle-infused controls and increased PRL levels approximately 26-fold. However, in nonpregnant rats with rat choriocarcinoma cells, 5-HT produced no changes in either DOPA accumulation in the SME or in circulating PRL levels. The inability of 5-HT to reduce tyrosine hydroxylase activity after mid-pregnancy may account for the lack of a PRL response. Placental lactogens secreted at midpregnancy, particularly placental lactogen-1, may induce this loss of 5-HT effect.     

Calmodulin and a cyclic nucleotide-dependent protein kinase facilitate the prolactin-induced increase in tyrosine hydroxylase activity in tuberoinfundibular dopaminergic neurons

Many aspects of tuberoinfundibular dopaminergic neuronal function are increased by elevated prolactin (PRL) levels, including the activity of tyrosine hydroxylase, the rate-limiting enzyme in the biosynthesis of dopamine. This study evaluated the roles of calmodulin, cyclic nucleotide-dependent protein kinase, and calcium/calmodulin-dependent protein kinase II in the PRL-induced increase in tyrosine hydroxylase activity. Ovariectomizerd rats were treated with haloperidol or ovine PRL (oPRL) for 20–30 h before the experiment, respectively. Treatment with haloperidol increased circulating PRL levels 8-fold and tyrosine hydroxylase activity in the stalk-median eminence 1.8-fold. Treatment with oPRL increased tyrosine hydroxylase activity 1.9-fold. W-7, a calmodulin antagonist, reversed both the haloperidol- and oPRL-induced increase in tyrosine hydroxylase activity to control levels. H-8, a cyclic nucleotide-dependent protein kinase inhibitor, also reversed the haloperidol induced increase in tyrosine hydroxylase activity. KN62, a selective calcium/calmodulin-dependent protein kinase II inhibitor, attenuated the haloperidol-induced increase in tyrosine hydroxylase activity, but KNO4, a structurally related control compound, had no effect. By contrast, the oPRL- and haloperidol-induced increases in tyrosine hydroxylase activity were not altered by KN93, a selective calcium/calmodulin-dependent protein kinase II inhibitor. These data indicate that calmodulin and a cyclic nucleotide-dependent protein kinase contribute to the PRL-induced increase in tyrosine hydroxylase activity, but the role of calcium/calmodulin-dependent protein kinase II is still unclear.     

Immunocytochemical colocalization of hypothalamic progestin receptors and tyrosine hydroxylase in steroid-treated monkeys.

Progesterone (P)-induced PRL secretion in estradiol (E)-primed monkeys is not due to direct pituitary stimulation, because lactotropes do not express progestin receptors (PR). However, the hypothalamus, particularly the tuberoinfundibular dopaminergic system (TIDA), plays a major role in the regulation of PRL secretion. To determine whether hypothalamic dopamine neurons are progestin target cells, the colocalization of PR and tyrosine hydroxylase (TH), a phenotypic marker of dopaminergic neurons, was examined with double immunocytochemistry. Two methods for visualizing the antigens were applied; the first was a dual peroxidase method, and the second was a peroxidase-alkaline phosphatase method. In addition, the question of whether E induces PR in dopamine neurons was explored. Spayed female monkeys were treated with empty Silastic capsules, E-filled capsules for a period of 28 days, or E capsules supplemented with P capsules for the last 14 days of E treatment. Only the E- plus P-treated monkeys exhibited an increase in serum PRL during the P treatment period. Frontal sections at the level of the optic chiasm and arcuate nucleus were examined for the colocalization of TH and PR. After E treatment, hypothalamic PR-positive cells increased in both intensity and number. Neurons expressing both TH and PR were detected in the rostral hypothalamus, lateral to the third ventricle (A11-rostral) and in a discrete subventricular population (A11-subvent). The lateral population continued caudally (A11-caudal). The A11-subvent population exhibited little steroid regulation. Of the remaining A11 TH neurons, approximately 20% exhibited PR in the spayed and E-treated groups. Addition of P doubled the percentage of PR-containing TH neurons in this group. Although very few TH-positive neurons in the ventral arcuate nucleus contained PR (A12-ventral), many double labeled neurons were observed in the dorsal arcuate region (A12-dorsal). Ventral arcuate TIDA neurons were not regulated by steroids, but E plus P increased PR expression in A12-dorsal. Double labeled cells were rarely seen in the zona incerta (A13) or the emerging ventral tegmental area (A10). In summary, P probably does not act directly on ventral arcuate TIDA neurons to stimulate PRL secretion. However, the frequency of PR-positive dopamine neurons in the A11-rostral, A11-caudal, and A12-dorsal groups increased with E and P treatment. Therefore, the contribution of the PR-positive periventricular dopamine neurons to progestin-stimulated PRL secretion may be important.     

Adrenal and vascular tyrosine hydroxylase activity in Goldblatt hypertension

To examine the role of the sympathetic nervous system in hypertension, the in vitro activity of tyrosine hydroxylase was examined in one-kidney, one clip (1K1C) and two-kidney, one clip (2K1C) hypertensive rabbits and their respective controls 2 weeks after surgical procedures. The in vitro activity of tyrosine hydroxylase provides a measure of catecholamine synthesis and serves as a biochemical index of activity of noradrenergic neurons and the adrenal medulla. Mean atrial pressure rose from 91.5 +/- 1.0 to 128.5 +/- 5.6 mm Hg (p less than 0.01) in the 1K1C group and from 91.8 +/- 1.3 to 106.5 +/- 5.0 mm Hg (p less than 0.02) in the 2K1C group, whereas no change in blood pressure was found in their respective controls. Adrenal tyrosine hydroxylase activity was increased 85% in the 1K1C group, as compared with values in one-kidney controls (from 11.8 +/- 1.5 to 21.8 +/- 1.1 pmol CO2/min/mg; p less than 0.0002), and was increased 49% in the 2K1C group, as compared with values in two-kidney controls (from 8.01 +/- 1.2 to 11.9 +/- 1.1 pmol CO2/min/mg; p less than 0.02). In the 1K1C group, proximal mesenteric tyrosine hydroxylase activity was decreased 46% compared with values in one-kidney controls (from 23.5 +/- 5.0 to 12.8 +/- 2.5 pmol CO2/min/mg; p less than 0.03) and distal mesenteric tyrosine hydroxylase activity was decreased 42% (from 7.73 +/- 1.2 to 4.46 +/- 0.8 pmol CO2/min/mg; p less than 0.03). In the 2K1C group, neither proximal nor distal mesenteric tyrosine hydroxylase activity was altered. Tyrosine hydroxylase activity was not detectable in the femoral arteries, or in the thoracic and abdominal aorta.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prolactin-regulated tyrosine hydroxylase activity and messenger ribonucleic acid expression in mediobasal hypothalamic cultures: the differential role of specific protein kinases

Prolactin secretion from the anterior pituitary is tightly regulated by feedback onto the hypothalamic neuroendocrine dopaminergic (NEDA) neurons. Prolactin stimulates these neurons to synthesize and secrete dopamine, which acts via the pituitary portal vasculature to inhibit prolactin secretion from the pituitary lactotrophs. Despite the physiological importance of this feedback, relatively little is known about the signaling mechanisms responsible for prolactin activation of NEDA neurons. This issue has been examined here using a cell culture preparation of the fetal rat mediobasal hypothalamus. Prolactin stimulated a time- and concentration-dependent increase in catecholamine synthesis, which was maximal after 60-120 min (1 microg/ml prolactin) and inhibited by the prolactin antagonist Delta1-9-G129R-hPRL. This prolactin response was accompanied by a rise in the site-specific (ser-19, -31, and -40) phosphorylation of tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis. Consistent with this observation, the prolactin-induced increase in catecholamine synthesis was abolished by inhibitors of protein kinase A and protein kinase C (PKC). Prolactin incubation also resulted in a PKC-dependent activation of the MAPK pathway, although this was not required for the stimulation of catecholamine synthesis. In addition to increasing TH phosphorylation and catecholamine synthesis, prolactin also increased TH mRNA expression. In contrast to catecholamine synthesis, this latter response was not suppressed by inhibition of protein kinase A or PKC. These results indicate that although prolactin controls catecholamine synthesis in NEDA neurons by regulating both TH activity and TH mRNA expression, it employs distinct, nonoverlapping, signaling pathways to achieve these ends.     

In situ activity and phosphorylation of tyrosine hydroxylase in the median eminence

Intracellular activation and phosphorylation of tyrosine hydroxylase (TH; E.C. 1.14.16.2) in the median eminence of the rat brain were investigated. The in situ activity of TH was assayed by the accumulation of L-dihydroxyphenylalanine (DOPA) in the median eminence of hypothalamic fragments incubated in the presence of NSD 1015. When hypothalamic fragments were incubated with veratridine (0-10(-3) M), maximal stimulation of TH activity was observed at 10(-4) M. The mean concentration of DOPA in the median eminence of hypothalamic fragments incubated with 10(-4) M veratridine was 3 times that seen in its absence. Phosphorylation of TH in the median eminence was evaluated by autoradiographic quantification of [32P]TH in 32P-labelled median eminence tissue. The amount of [32P]TH in 32P-labelled median eminence incubated with 10(-4) M veratridine was 2 times that seen in the absence of veratridine. These data are consistent with the view that in the median eminence phosphorylation and activation of TH are linked events and that phosphorylation may be a means of regulating the biosynthesis of dopamine in this region of the brain.     

Trans-synaptic induction of adrenomedullary tyrosine hydroxylase activity by choline: evidence that choline administration can increase cholinergic transmission.

Twenty-four hours after rats receive choline chloride (20 mmol/kg, by stomach tube) the activity of tyrosine hydroxylase [tyrosine 3-monooxygenase; L-tyrosine, tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2] increases by 31% within adrenomedullary chromaffin cells. This treatment also causes major elevations in the levels of choline and acetylcholine within the adrenal gland; however, acetylcholine levels return to normal by 16 hr after the choline is given. The daily administration of 10 or 20 mmol/kg of choline for 4 days elevates adrenal tyrosine hydroxylase activity by 29% or 51%, respectively. Such increases in tyrosine hydroxylase activity are not observed in animals given ammonium chloride, another basic chloride-containing compound, by stomach tube or in animals treated with cycloheximide, an inhibitor of adrenal protein synthesis. They are also absent in denervated adrenals. These observations demonstrate that the increase in presynaptic acetylcholine levels produced by giving animals the neurotransmitter's precursor (choline) can be associated with parallel changes in the transmission of signals across cholinergic synapses, probably because more of the transmitter is released per nerve impulse.     

Estradiol and the inhibition of hypothalamic gonadotropin-releasing hormone pulse generator activity in the rhesus monkey.

In mammals, gonadal function is controlled by a hypothalamic signal generator that directs the pulsatile release of gonadotropin-releasing hormone (GnRH) and the consequent pulsatile secretion of luteinizing hormone. In female rhesus monkeys, the electrophysiological correlates of GnRH pulse generator activity are abrupt, rhythmic increases in hypothalamic multiunit activity (MUA volleys), which represent the simultaneous increase in firing rate of individual neurons. MUA volleys are arrested by estradiol, either spontaneously at midcycle or after the administration of the steroid. Multiunit recordings, however, provide only a measure of total neuronal activity, leaving the behavior of the individual cells obscure. This study was conducted to determine the mode of action of estradiol at the level of single neurons associated with the GnRH pulse generator. Twenty-three such single units were identified by cluster analysis of multiunit recordings obtained from a total of six electrodes implanted in the mediobasal hypothalamus of three ovariectomized rhesus monkeys, and their activity was monitored before and after estradiol administration. The bursting of all 23 units was arrested within 4 h of estradiol administration although their baseline activity was maintained. The bursts of most units reappeared at the same time as the MUA volleys, the recovery of some was delayed, and one remained inhibited for the duration of the study (43 days). The results indicate that estradiol does not desynchronize the bursting of single units associated with the GnRH pulse generator but that it inhibits this phenomenon. The site and mechanism of action of estradiol in this regard remain to be determined.     

Sex hormones and tyrosine hydroxylase activity in vascular and adrenal tissue.

Vascular tyrosine hydroxylase (TH) activity did not appear to be affected by the sex hormones. There were no differences in enzyme activity in the mesenteric artery or vein taken from male and female normotensive or spontaneously hypertensive rats. Castration of either male or female rats did not alter mesenteric artery or vein TH activity, and the administration of estradiol, progesterone, or testosterone also had no effect on vascular TH activity. However, the sex hormones did alter activity in other tissues. Estradiol and progesterone administration to intact female rats increased adrenal TH activity, whereas castration of the male rat decreased it. Although the sex hormones were not important regulators of TH in blood vessels, vascular TH activity did appear to be under some hormonal regulation since hypophysectomy decreased mesenteric artery enzyme activity. Hypophysectomy studies also indicated that adrenal TH activity was under some hormonal regulation.     

Estradiol stimulates GDNF expression in developing hypothalamic neurons

Estrogens stimulate the differentiation of neurons and neural networks in the CNS. The concordance of the cellular responses of estrogens and growth factors suggests that both factors may interact on the cellular level to ensure their developmental role. We have put forward this hypothesis and analyzed the effect of estrogens on the expression of glial cell line-derived neutrotrophic factor (GDNF) in developing hypothalamic cells. Using Western blotting and competitive RTPCR, we have demonstrated that 17beta-estradiol (E2) increases the expression of GDNF in hypothalamic cell cultures. E2-induced GDNF expression was seen in neurons but not astrocytes. GDNF induction by E2 appeared to be transmitted through nonclassical estrogen action, since the application of the nuclear estrogen receptor antagonists ICI 182, 780 did not abolish this effect. Only inhibitors of intracellular Ca(2+) and cAMP/protein kinase A signaling were effective in preventing E2 effects. We conclude that E2 is capable of influencing GDNF expression in the developing hypothalamus. Thus, it is conceivable that developmental E2 effects in the hypothalamus are partially mediated through the regulation of other important developmental signals such as growth factors.     

Immunostaining reveals accumulation of serotonin and coexistence with tyrosine hydroxylase in hypothalamic neurons of acutely stalk-sectioned baboons

The distribution of serotonin (5-HT) and tyrosine hydroxylase (TH) was examined in the hypothalamus of juvenile baboons, 24 h after infundibular stalk section. Simultaneous immunostaining for 5-HT with peroxidase-antiperoxidase (PAP) and TH with 15 nm colloidal gold (IGS) was performed on Vibratome sections from 3 operated and 1 control female. Light microscopy revealed fine 5-HT immunopositive (5-HT+) fibers, presumably axons, in the suprachiasmatic nuclei and ventromedial hypothalamus (VMH) after stalk section. In addition, focal accumulations of swollen and heavily stained 5-HT+ fibers occurred on the side of the surgical approach. Enlarged fibers were densest in the medial preoptic area, lateral and VMH areas, and the median eminence. TH immunoreactivity (TH+) in VMH cell bodies and axons was only slightly increased over that in controls. Electron microscopy of areas of 5-HT+ and TH+ overlap (medial VMH and adjacent periventricular zone) showed that 5-HT+ profiles were mostly unmyelinated axons and irregular varicosities. A few myelinated 5-HT+ axons were also observed. TH+ perikarya, dendrites, axons and terminals showed gold labeling characteristic for this enzyme. However, colocalization of 5-HT (PAP) and TH (IGS) was present in a number of fiber varicosities in experimental animals only. Both single- and double-labeled profiles occurred in individual thin sections, thus arguing against antibody cross-reactivity. These results indicate that: hypothalamic 5-HT+ fibers project to the median eminence in primates; 5-HT fibers become more obvious after stalk section due to accumulation of transmitter; focal 5-HT+ immunoreactivity in the hypothalamus can increase dramatically after distant and mild surgical trauma, and coexistence of 5-HT and TH in single neurons can appear after acute stalk section and/or trauma in experimental animals. These findings might represent uptake of exogenous 5-HT or amplified expression of endogenous neurotransmitter, suggesting that plasticity of transmitter phenotype might follow acute surgical and/or endocrine intervention in mature primate brain. Neuroendocrine studies employing the stalk-sectioned primate might thus be radically affected.     

Influence of ACTH on tyrosine hydroxylase activity in the locus coeruleus of mouse brain

Tyrosine hydroxylase activity in noradrenergic neurons of the locus coeruleus was found to rise in bilaterally adrenalectomized adult mice, with maximum increase of the enzyme activity (52% above control) occurring 7 days after the surgery. No such increase of tyrosine hydroxylase activity was found in dopaminergic neurons of the substantia nigra. The increase of enzyme activity in the locus coeruleus following adrenalectomy was totally prevented by corticosterone replacement. Moreover, the adrenalectomy effect was abolished by hypophysectomy, indicating the involvement of the pituitary rather than the adrenocortical function. Chronic administration of ACTH (20 IU/kg, i.p., daily) resulted in an increase of tyrosine hydroxylase activity in the locus coeruleus, with a time course and magnitude similar to those found after adrenalectomy. Additionally, the effects of four ACTH analogs were determined. ACTH 1-24 and ACTH 4-10 were as effective as the whole ACTH molecule, whereas ACTH 4-10,7-D-Phe and ACTH 11-24 were ineffective. The effect of ACTH 4-10, a peptide fragment with no adrenocorticotrophic activity, further indicates that glucocorticoids are not involved. From these data, it appears that tyrosine hydroxylase in the locus coeruleus neurons is under the regulatory influence of pituitary ACTH. It remains to be determined whether the hormone can be transported from its pituitary origin to the locus coeruleus and exerts a direct action on the noradrenergic neurons. Regardless of the mechanism, the response of the noradrenergic neurons to pituitary activity may be an important component in physiological adaptation of the central nervous system to chronic stress.     

Estradiol attenuates angiotensin-induced aldosterone secretion in ovariectomized rats

Estrogen replacement therapy significantly reduces the risk of cardiovascular disease in postmenopausal women. Previous studies indicate that estradiol (E2) decreases angiotensin II (AT) receptor density in the adrenal and pituitary in NaCl-loaded rats. We used an in vivo model that eliminates the potentially confounding influence of ACTH to determine whether the E2-induced decrease in adrenal AT receptor expression affects aldosterone responses to angiotensin II (Ang II). Female rats were ovariectomized, treated with oil (OVX) or E2 (OVX+E2; 10 microg, s.c.) for 14 days, and fed a NaCl-deficient diet for the last 7 days to maximize adrenal AT receptor expression and responsiveness. On days 12-14 rats were treated with dexamethasone (DEX; 25 microg, i.p., every 12 h) to suppress plasma ACTH. On day 14 aldosterone secretion was measured after a 30-min infusion of Ang II (330 ng/min). Ang II infusion increased the peak plasma aldosterone levels to a lesser degree in the OVX+E2 than in the OVX rats (OVX, 1870 +/- 290 pg/ml; OVX+E2, 1010 +/- 86 pg/ml; P < 0.05). Ang II-induced ACTH and aldosterone secretion was also studied in rats that were not treated with DEX. In the absence of DEX, the peak plasma aldosterone response was also significantly decreased (OVX, 5360 +/- 1200 pg/ml; OVX+E2, 2960 +/- 570 pg/ml; P < 0.05). However, E2 also reduced the plasma ACTH response to Ang II (P < 0.05; OVX, 220 +/- 29 pg/ml; OVX+E2, 160 +/- 20 pg/ml), suggesting that reduced pituitary ACTH responsiveness to Ang II contributes to the effect of E2 on Ang II-induced aldosterone secretion. Adrenal AT1 binding studies confirmed that E2 significantly reduces adrenal AT1 receptor expression in both the presence and absence of DEX in NaCl-deprived rats. These results indicate that E2-induced decreases in pituitary and adrenal AT1 receptor expression are associated with attenuated pituitary ACTH and adrenal aldosterone responses to Ang II and suggest that estrogen replacement therapy may modulate Ang II-stimulated aldosterone secretion as part of its well known cardioprotective actions.     

Electrical stimulation of preganglionic nerve increases tyrosine hydroxylase activity in sympathetic ganglia.

The effect of synaptic stimulation on tyrosine hydroxylase [tyrosine 3-monooxygenase: L-tyrosine, tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2] activity in the rat superior cervical ganglion was studied. The preganglionic cervical sympathetic trunk was stimulated unilaterally at 10 Hz for 30 min. Forty-eight hours later tyrosine hydroxylase activity was 33% higher on the stimulated than on the control side. The enzyme activity restimulated than on the control side. The enzyme activity remained elevated in the stimulated ganglia for 2 days. No change was observed in total ganglion protein. Comparable increases in tyrosine hydroxylase activity were observed in anesthetized and conscious animals.