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

Salsolinol induces a decrease in cyclic AMP at the median eminence and an increase at the adenohypophysis in lactating rats

Investigating the cellular events in the pituitary gland, the intracellular cyclic AMP (cAMP) of the median eminence (ME), neuro-intermediate lobe (NIL) and the anterior lobe (AL) have been measured following 15-min of intravenous injection of salsolinol (SAL). Parallel to the elevation of plasma prolactin (PRL), SAL induced a significant decrease of cAMP concentration in the ME. In contrast, SAL injection resulted in a significant increase of cAMP at the level of the AL. Changes in cAMP of the NIL as well as in the plasma level of vasopressin (VP) could not be detected. The observed changes in the level of cAMP following the acute treatment of SAL in the ME and the AL seems to be related to interacting neuroendocrine signals delivered from the ME to the AL through the long portal vessels to release PRL.

A study of the cellular mechanism by which lead affects catecholamine secretion

Our purpose was to determine the role of protein kinases in the mediation of the stimulatory effects of lead on catecholamine secretion. Pheochromocytoma cells were incubated for 90 minutes with W-7 (calmodulin antagonist), calphostin C (protein kinase C inhibitor), Sp-cAMPS (cAMP agonist), Rp-cAMPS (cAMP antagonist), forskolin (activator of adenylyl cyclase), or lead nitrate. Catecholamines were measured by liquid chromatography. Lead had a stimulatory effect on catecholamine secretion, whereas W-7 was inhibitory. In the presence of both lead and W-7, the response was markedly decreased compared to that seen with lead alone. Calphostin C suppressed the secretion of catecholamines; however, in the presence of lead and calphostin C, the secretion was similar to that seen with lead alone. Compared to control, Sp-cAMPS was stimulatory. Co-incubation of Sp-cAMPS and lead had a slight synergistic effect. Rp-cAMPS decreased catecholamine secretion, but co-incubation of Rp-cAMPS and lead resulted in a slight reduction compared to lead alone. Forskolin markedly increased the secretion of catecholamines, and co-incubation of lead and forskolin resulted in a synergistic increase. In the absence of calcium, lead had no effect. We conclude that lead stimulates catecholamine secretion by acting through the calcium/calmodulin-dependent protein kinase II system and not through the protein kinase C or protein kinase A system, and requires the presence of calcium for its action.

Tuberoinfundibular dopaminergic neurons and lactotropes in young and old female rats

Aging in female rats is accompanied by several endocrine dysfunctions, such as reproductive decline associated with characteristic hyperprolactinemia, lactotrope hyperplasia, and functional impairment of hypothalamic tuberoinfundibular dopaminergic (TIDA) neurons. The aim of this morphometrical, immunocytochemical, and densitometrical study was to gain a better anatomical knowledge of TIDA neurons and axons as well as of lactotropes in old female rats with (A) or without (NA) pituitary adenomas, compared with young animals. At the hypothalamic level, we found that tyrosine hydroxylase (TH)-labeled neurons in the arcuate nucleus were comparable in young and old NA yet their size and TH-content were increased in A animals. Also the TH-labeled median eminence axons did not differ significantly between young and old NA but were more numerous in the old A rats. Independently from adenomas, both number of prolactin (PRL)-labeled structures and content of immunoreactive PRL were increased in pituitaries of old rats, the plasma PRL levels, however, were high only in A. Our findings support the documented lactotrope hypertrophy and hyperplasia in old female rats and suggest that TIDA-neuron changes only occur in hyperprolactinemic animals carrier of adenomas.

Right-left asymmetry of tyrosine hydroxylase in rat median eminence: influence of arterial baroreflex nerves

Arterial baroreceptor nerves consist of 2 pairs of nerves, carotid sinus and aortic. The latter nerves differ bilaterally as to size and origin suggesting that central baroreceptor activity could be asymmetrical. This was tested by studying activity of tyrosine hydroxylase (TH), the rate limiting enzyme for catecholamine synthesis, on right and left sides of the brain in median eminence (ME), arcuate nucleus (ARCN), paraventricular nucleus (PVN) and supraoptic nucleus (SON). Seven days after right or left sinoaortic denervation (SAD), bilateral SAD or sham-operation (SO), in situ TH activity was determined by the rate of dihydroxyphenylalanine (DOPA) accumulation following administration of a dopa decarboxylase inhibitor. Unexpectedly, the intact, SO rats had significantly lower DOPA concentrations on the left than right sides of ME in two separate studies; right-left differences averaged 24% and 38%. Evidence supporting asymmetry of TH activity was the lower concentrations of catecholamines on left than right sides of the brain in intact SO rats, namely, dopamine in ME and ARCN and norepinephrine and epinephrine in ARCN. Unilateral and bilateral SAD further suppressed TH activity on the left side of the ME; the maximum right-left difference was 58% in rats with right SAD. Their ARCN also showed asymmetry of TH activity. Asymmetry of TH activity was attributable to asymmetry of TH protein which was significantly lower on the left sides of ME and ARCN. This is the first demonstration of bilateral asymmetry of TH in mediobasal hypothalamus and its enhancement by interruption of left or right afferents from the arterial vascular system.

Quantitative study of tyrosine hydroxylase mRNA in catecholaminergic neurons and adrenals during development and aging

Using a solution hybridization-S1 nuclease protection assay, we quantitatively studied tyrosine hydroxylase mRNA (mRNATH) in catecholaminergic cells of the substantia nigra, hypothalamus, superior cervical ganglion, and adrenal of male rats from early neonatal life to old age. Throughout this time, the lowest level of mRNATH in any tissue was found in the youngest animals, and their development was associated with an increase in the quantity of mRNATH. However, the extent of the increase as well as the pattern of change was dependent on the tissue. The amount of mRNATH in the substantia nigra of 1-day-old pups was 162 +/- 7 attomoles (mean and S.E.M.), increasing to 877 +/- 39 amol at 14 days of age. Then, the amount fell to 480 +/- 25 amol at 6 weeks of age, but changed little between 6 weeks and 23 months of age. In the hypothalamus of 1-day-old pups, the quantity of mRNATH was 24 +/- 3 amol, increasing to 60 +/- 6 amol at 2 weeks and changed little thereafter. mRNATH in the superior cervical ganglion increased gradually until 10 months of age; at which time the amount was 3 times that of neonatal animals. In the adrenal, mRNATH increased continuously throughout the period of observation. The amount of mRNATH in the adrenal of 23-month-old animals was 25 times that in the adrenal of 4-day-old pups. These data suggest that tyrosine hydroxylase gene expression does not diminish in aged rats.

Dopamine autoreceptors modulate the phosphorylation of tyrosine hydroxylase in rat striatal slices

The hypothesis that dopamine (DA) autoreceptors modulate the phosphorylation of tyrosine hydroxylase (TH; EC 1.14.16.2) was investigated in rat striatal slices. Tissue was prelabeled with 32P inorganic phosphate, and TH recovered by immunoprecipitation with anti-TH rabbit serum. The TH monomer was resolved on sodium dodecyl sulfate polyacrylamide gels, and the extent of phosphorylation was determined by scanning densitometry of autoradiographs. Depolarization of striatal slices with 55 mM K+ markedly increased the incorporation of 32P into several proteins, including the TH monomer (Mr = 60,000). A similar increase in TH phosphorylation occurred in response to the adenylate cyclase activator forskolin and the cyclic AMP analog dibutyryl cyclic AMP. An increase in TH phosphorylation was not observed in response to the D1-selective agonist SKF 38393. The D2-selective DA autoreceptor agonist pergolide decreased the phosphorylation of TH below basal levels and blocked the increase in phosphorylation elicited by 55 mM K+. The inhibitory effect of pergolide was antagonized by the D2-selective antagonist eticlopride. Changes observed in the phosphorylation of TH were mirrored by changes in tyrosine hydroxylation in situ. These observations support the hypothesis that a reduction in TH phosphorylation is the mechanism by which DA autoreceptors modulate tyrosine hydroxylation in nigrostriatal nerve terminals.

Quantification of the mass of tyrosine monooxygenase in the median eminence and superior cervical ganglion

A procedure is described that enables one to quantify the mass of TH in a fraction of the ME and SCG of rats. This procedure is specific and sensitive. It should be possible to study the biosynthetic activity of catecholaminergic neurons as a function of the mass of TH in such cells.

Saline ingestion stimulates the in situ molar activity of tyrosine hydroxylase in the median eminence and superior cervical ganglion

The effects of drinking saline for 7 days on the mass and in situ activity of tyrosine hydroxylase (TH) in the median eminence (ME) and superior cervical ganglion (SCG) of rats were investigated. TH mass was quantified by immunoblot assay. In situ TH activity was calculated from the rate of intracellular accumulation of L-dihydroxyphenylalanine (DOPA). In rats that drank 10 mM, 30 mM, and 100 mM NaCl for 7 days, TH activity in the ME was 34 +/- 4, 36 +/- 5, and 45 +/- 3 (mean and S.E.M.) mol of DOPA.h-1.mol of TH-1, respectively, compared to 30 +/- 2 for rats that drank water. The activity of TH in the SCG of animals that drank 10 mM, 30 mM, and 100 mM NaCl was 143 +/- 24, 167 +/- 12, and 272 +/- 13 mol DOPA.h-1.mol TH-1, respectively, compared to 119 +/- 10 for animals that drank water. The mass of TH in the ME and SCG decreased as a function of the concentration of NaCl in the drinking water. In animals that drank water, 10 mM, 30 mM, and 100 mM NaCl, the amounts (pmol) of TH were, respectively, 0.28 +/- 0.03, 0.31 +/- 0.04, 0.23 +/- 0.02, and 0.21 +/- 0.01 per ME and 0.67 +/- 0.06, 0.72 +/- 0.11, 0.37 +/- 0.01, and 0.34 +/- 0.02 per SCG. TH activity in the ME or SCG was unaffected by treatment for 7 days with arginine vasopressin.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonal modulation of the quantity and in situ activity of tyrosine hydroxylase in neurites of the median eminence

The role of ovarian hormones in the control of the quantity and activity of tyrosine hydroxylase (TyrOHase) in neurites of the median eminence of the rat was investigated. TyrOHase was quantified by an immunoblot assay using purified rat TyrOHase as the standard. Treatment of ovariectomized animals with progesterone, but not estradiol, resulted in a significant reduction in the amount of TyrOHase in the median eminence. The in situ activity of the enzyme was assayed by measuring the rate of synthesis of L-3,4-dihydroxyphenylalanine (dopa), and the results were expressed as mol of dopa per hr per mol of TyrOHase. In animals treated with both estradiol and progesterone for 3 days, the in situ activity of TyrOHase in the median eminence was 114 +/- 13.5 (mean +/- SEM) compared to 26 +/- 4.7 for the controls. Estradiol or progesterone alone was much less effective than was the combination of estradiol and progesterone. To ascertain whether the effect of estradiol and progesterone on TyrOHase activity was reflected in the secretion of dopamine into hypophyseal portal blood, ovariectomized rats were treated for 3 days with both estradiol and progesterone or with the solvent vehicle. The concentration of dopamine in portal plasma of the hormone-treated animals was 1.93 +/- 0.533 ng/ml compared to 0.34 +/- 0.094 ng/ml in vehicle-treated animals. We conclude that the quantity and in situ molar activity of TyrOHase in neurites of the median eminence as well as the secretion of dopamine from these neurites are modulated by the combined action of estradiol and progesterone.