Role of testosterone in the regulation of tuberoinfundibular dopaminergic neurons in the male rat

The effects of testosterone on the tuberoinfundibular dopamine (DA) neuronal activity was examined by determining the rate of DA synthesis-accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of a decarboxylase inhibitor and the concentration of a DA metabolite,--3,4-dihydroxyphenylacetic acid (DOPAC)--in the median eminence in the male rat. Within 1 week after orchidectomy, there was an increase in the accumulation of DOPA and the concentration of DOPAC in the median eminence, but there was no change in the concentration of DA. Conversely, 1 day after testosterone administration to orchidectomized rats, the elevated DOPAC concentrations in the median eminence returned to levels comparable to those in gonadally intact rats. Neither orchidectomy nor testosterone replacement had any effect on plasma prolactin concentrations, but inhibition of prolactin secretion following administration of the DA agonist bromocriptine blocked the increase in DOPA accumulation in the median eminence of orchidectomized rats; this latter effect was reversed by intracerebroventricular administration of prolactin. On the other hand, intracerebroventricular injection of prolactin caused a similar increase in the accumulation of DOPA in the median eminence of gonadally intact, orchidectomized, and testosterone-treated orchidectomized rats. Immobilization stress decreased the accumulation of DOPA and the concentration of DOPAC in the median eminence of orchidectomized rats, but had no effect in intact or testosterone-treated orchidectomized rats. These results indicate that testosterone inhibits the basal activity of tuberoinfundibular DA neurons and blocks the inhibitory effects of physical restraint on these neurons, but does not alter the ability of these neurons to respond to delayed activation by prolactin.     

Evidence that prolactin mediates the stimulatory effects of estrogen on tuberoinfundibular dopamine neurons in female rats.

The effects of ovariectomy and estrogen on prolactin secretion and/or the activity of tuberoinfundibular dopamine (TIDA) neurons were examined by either concurrently measuring concentrations of prolactin in plasma and 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence of female rats or by determining the rate of DA synthesis (accumulation of 3,4-dihydroxyphenylalanine (DOPA) after the administration of a decarboxylase inhibitor) in the median eminence. For comparison, concentrations of alpha-melanocyte-stimulating hormone (alpha MSH) in plasma and DOPAC in the intermediate lobe of the pituitary (an index of the activity of tuberohypophysial DA neurons) were also determined. Ovariectomy produced a time-dependent decrease in the accumulation of DOPA and the concentrations of DOPAC in the median eminence and prolactin in plasma with maximal effects occurring by 7 days. Estrogen administration to ovariectomized rats increased plasma prolactin and median eminence DOPAC concentrations to levels comparable to those in diestrous controls. In contrast, neither ovariectomy nor estrogen replacement altered the concentrations of alpha MSH in plasma or DOPAC in the intermediate lobe. Administration of the DA agonist bromocriptine blocked the ability of estrogen to increase plasma prolactin and median eminence DOPAC concentrations. Also, administration of antiserum to rat prolactin blocked the stimulatory action of estrogen on median eminence DOPAC concentrations. Taken together, these results indicate that the stimulatory effect of estrogen on the activity of TIDA neurons is mediated by prolactin.     

Stress-induced secretion of alpha-melanocyte-stimulating hormone is accompanied by a decrease in the activity of tuberohypophysial dopaminergic neurons.

The purpose of the present study was to examine the acute effects of stress on the secretion of alpha-melanocyte-stimulating hormone (alpha MSH) and the activity of tuberohypophysial dopamine (DA) neurons in female and male rats. The activity of tuberohypophysial DA neurons was estimated by measuring the accumulation of 3,4-dihydroxyphenylalanine (DOPA) following administration of the decarboxylase inhibitor NSD 1015, and the concentrations of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the intermediate and neural lobes of the posterior pituitary. The combination of brief (2 min) ether exposure followed by 30 min of supine restraint (immobilization in the supine position) decreased the rate of DOPA accumulation in the intermediate, but not in the neural lobe of both female and male rats. Similarly, brief ether exposure followed by 10, 20 or 30 min of supine restraint increased plasma alpha MSH concentrations and decreased DOPAC concentrations in the intermediate lobe of female and male rats. In the absence of ether, tube restraint (confinement in a cylindrical acrylic tube) increased alpha MSH secretion and decreased intermediate lobe DOPAC concentrations, whereas ether in the absence of physical restraint had no effect. These results suggest that the stress-induced activation of alpha MSH secretion in both female and male rats may be due, in part, to a decrease in the activity of tuberohypophysial DA neurons in the intermediate lobe of the posterior pituitary.     

Direct pituitary inhibition of prolactin secretion by dopamine and noradrenaline in sheep

The effects of dopamine, noradrenaline and 3,4-dihydroxyphenylacetic acid (DOPAC) on the release of prolactin were examined in ovariectomized ewes. Infusion of dopamine (0.5 or 1 microgram/kg per min for 2 h i.v.) reduced plasma prolactin concentrations in a dose-dependent manner, whereas DOPAC (5 or 10 micrograms/kg per min for 2 h i.v.) had no effect. In a further series of experiments, ovariectomized hypothalamopituitary disconnected ewes were given dopamine or noradrenaline (each at 0.5 or 1 microgram/kg per min for 2 h i.v.), and both amines reduced mean plasma concentrations of prolactin with similar potency in a dose-dependent manner. These effects were blocked by treatment with pimozide and prazosin respectively. During the infusion of dopamine, the peripheral plasma concentrations of DOPAC and 3,4-dihydroxyphenylethyleneglycol (DHPG) were increased (DOPAC, 22 +/- 7 (S.E.M.) to 131 +/- 11 nmol/l; DHPG, 2.9 +/- 0.3 to 6.4 +/- 0.2 nmol/l), but plasma concentrations of dopamine and noradrenaline did not change. Finally, administration of domperidone, a specific dopamine receptor antagonist that does not cross the blood-brain barrier, resulted in a sustained increase in plasma prolactin concentrations in ovariectomized ewes. We conclude that the secretion of prolactin from the pituitary gland is under dual inhibitory regulation by both dopamine and noradrenaline in the sheep.     

Neurotensin activates tuberoinfundibular dopamine neurons and increases serum corticosterone concentrations in the rat

The activity of tuberoinfundibular dopamine neurons, as estimated from the amount of dihydroxyphenylalamine (DOPA) formed in the median eminence after the inhibition of DOPA decarboxylase and the concentration of dihydroxyphenylacetic acid (DOPAC) in this brain region, was significantly increased 1-8 h following the intracerebroventricular (i.c.v.) administration of neurotensin (20 micrograms). Neurotensin (5 and 20 micrograms i.c.v.) also significantly increased DOPAC concentrations in the n. accumbens but had no effect in the striatum. Serum concentrations of corticosterone in rats treated with neurotensin (1-20 micrograms i.c.v.) were 5-7 times those in vehicle-treated animals. [D-Trp11]-neurotensin (0.5 micrograms i.v.c.) also significantly increased DOPAC concentrations in the median eminence and serum corticosterone concentrations. It is concluded that neurotensin acutely increases the activity of tuberoinfundibular and mesolimbic dopamine neurons and the secretion of ACTH.     

Kappa-opioid-receptor-mediated regulation of alpha-melanocyte-stimulating hormone secretion and tuberohypophyseal dopaminergic neuronal activity

The effects of the kappa-opioid receptor agonist trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzene- acetamide methanesulfonate hydrate (U-50488) were examined on alpha-melanocyte-stimulating hormone (alpha-MSH) secretion and the activity of tuberohypophysial dopamine (DA) neurons in the male rat. Tuberohypophysial DA neuronal activity was estimated by measuring: (1) the rate of DA synthesis [accumulation of 3,4-dihydroxyphenylalanine (DOPA) following inhibition of aromatic L-amino acid decarboxylase], and (2) DA metabolism [concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC)] in the intermediate lobe of the pituitary. U-50488 produced a dose- and time-dependent increase in plasma concentrations of alpha-MSH which was accompanied by a decrease in the accumulation of DOPA and in the intermediate lobe. The effects of U-50488 were blocked by pretreatment with the DA agonist apomorphine but not by the beta-adrenergic antagonist propranolol. The effects of U-50488 on plasma alpha-MSH concentrations and intermediate-lobe DOPA accumulation were blocked by pretreatment with the selective kappa-opioid receptor antagonist nor-binaltorphimine. These results indicate that U-50488, by acting on kappa-opioid receptors, inhibits the activity of intermediate-lobe tuberohypophysial DA neurons, and through this action increases the secretion of alpha-MSH from melanotrophs.     

Concentrations of dopamine and noradrenaline in hypophysial portal blood in the sheep and the rat

The concentrations of dopamine, noradrenaline and their respective primary neuronal metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylethyleneglycol (DHPG) were measured in the hypophysial portal and peripheral plasma of sheep and rats by combined gas chromatography-mass spectrometry. Hypophysial portal and jugular blood samples were taken at 5- to 10-min intervals for 3-7 h from six conscious ovariectomized ewes. Blood was also collected for 30 min under urethane anaesthesia from the cut pituitary stalk from 16 pro-oestrous female and five intact male rats. In ovariectomized ewes, noradrenaline concentrations were higher in hypophysial portal plasma than in peripheral plasma (6.6 +/- 0.8 vs 2.2 +/- 0.4 nmol/l). In contrast, dopamine was undetectable (less than 1 nmol/l) in the portal and peripheral plasma of all ewes. Plasma levels of DOPAC and DHPG in portal and jugular samples were similar. In all pro-oestrous female rats, plasma concentrations of dopamine were higher in portal blood than in jugular blood (8.0 +/- 1.4 vs 4.8 +/- 0.6 nmol/l). Detectable concentrations of dopamine were measured in the portal plasma of two out of five male rats. Noradrenaline concentrations were higher in portal plasma than in peripheral plasma of both female (8.3 +/- 1.7 vs 3.7 +/- 0.6 nmol/l) and male (14.8 +/- 2.7 vs 6.1 +/- 1.2 nmol/l) rats. These data show that noradrenaline, but not dopamine, is secreted into the long portal vessels in sheep. The results suggest that there are species differences in the secretion of hypothalamic dopamine into hypophysial portal blood.     

Effect of electrical stimulation of the ventromedial nucleus and the dorsomedial nucleus on the activity of tuberoinfundibular dopaminergic neurons

Perikarya and terminals of tuberoinfundibular dopaminergic (TIDA) neurons are located in the arcuate nucleus (ARN) and in the median eminence (ME), respectively. Dopamine (DA) released from TIDA terminals in the ME inhibits prolactin secretion from the anterior pituitary. Anatomical studies have described the sources of afferents to ARN and ME, but not to TIDA neurons per se. The ventromedial nucleus (VMN) and the dorsomedial nucleus (DMN) of the hypothalamus project to ARN and ME and have a role in prolactin regulation. In the present study, VMN and DMN were investigated as possible sources of TIDA afferents. Alterations in the activity of TIDA neurons were estimated by measuring plasma concentrations of prolactin and the rates of DA synthesis (3,4-dihydroxyphenylalanine - DOPA - accumulation after administration of the decarboxylase inhibitor NSD 1015) and metabolism (concentrations of the DA metabolite 3,4-dihydroxyphenylacetic acid - DOPAC) in the ME following electrical stimulation of ARN, VMN, and DMN in ovariectomized female rats. Thirty minutes of bilateral stimulation of ARN or DMN increased DOPA accumulation in the ME; stimulation of the VMN had no effect. 5-Hydroxytryptamine synthesis in the ME was unaffected by stimulation of any region. Plasma prolactin levels declined during DMN stimulation, varying with the frequency and duration of the electrical stimulus. DA metabolism within TIDA neurons increased with DMN stimulation, as evidenced by increased DOPAC concentrations in the ME. In females whose basal TIDA activity has been increased by haloperidol treatment or decreased by bromocriptine treatment, DMN stimulation was still able to increase DOPA accumulation in the ME. The present data suggest the presence of stimulatory TIDA afferents originating from or passing through the DMN.     

Protective Effect of Riluzole on MPTP-Induced Depletion of Dopamine and Its Metabolite Content in Mice

The neuroprotective effects of riluzole (2-amino-6-trifluoromethoxy benzothiazole), a Na⁺ channel blocker with antiglutamatergic activity, MK-801, a blocker of N-methyl-D-aspartate (NMDA) receptors and monoamine oxidase (MAO) inhibitor pargyline were compared in the model of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced depletion of dopamine and its metabolite 3, 4-dihydroxyphenylacetic acid (DOPAC) levels in mice. The mice received four intraperitoneal injections of MPTP (10 mg/kg) at 1-hr intervals and then the brains were analyzed at 1, 3 and 7 days after the treatments. Dopamine and DOPAC levels were significantly decreased in the striatum from 1 day after MPTP treatments. A severe depletion in dopamine and DOPAC levels was found in the striatum 3 and 7 days after MPTP treatments. Riluzole dose-dependently antagonized the MPTP-induced decrease in dopamine and DOPAC levels in the striatum. Pargyline also protected against MPTP-induced decrease in dopamine levels in the striatum. However, this drug showed no significant change in the striatal DOPAC levels. On the other hand, MK-801 failed to protect against MPTP-induced decrease in dopamine levels in the striatum. However, MK-801 reversed the MPTP-induced decrease in DOPAC levels. These results suggest that riluzole can protect against MPTP-induced striatal dopamine and DOPAC depletion in mice. This protective effect may be caused by inactivation of voltage-dependent Na⁺ channels by riluzole. Furthermore, the present study suggests that the activation of NMDA receptors does not mainly contribute to MPTP-induced neurodegeneration, whereas MAO, especially MAO type B(MAO-B) plays a crucial role in MPTP-induced degeneration of the nigrostriatal dopaminergic neuronal pathway.     

Comparison of the effects of ether and restraint stress on the activity of tuberoinfundibular dopaminergic neurons in female and male rats

The purpose of the present study was to characterize the acute inhibitory effects of restraint stress on the activity of tuberoinfundibular dopamine (DA) neurons as estimated by measuring concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence. The time course of the effects of two types of physical restraint (immobilization in the supine position or confinement in an acrylic cylindrical tube) was determined in unanesthetized and diethylether (ether)-exposed female and male rats. The combination of brief (2 min) exposure to ether followed by 10 and 20 min of supine restraint increased concentrations of prolactin in plasma and decreased DOPAC concentrations in median eminence of both female and male rats. Thirty minutes of supine restraint decreased DOPAC concentrations in the median eminence of female rats that were not exposed to ether, and brief exposure to ether enhanced this effect. By contrast, 30 min of supine restraint failed to alter DOPAC concentrations in the median eminence in either unanesthetized or ether-exposed male rats. Tube restraint in the absence of ether failed to alter DOPAC concentrations in the median eminence of either female or male rats; but in female rats preexposed to ether, 30 min of tube restraint decreased DOPAC concentrations in the median eminence. On the other hand, in the absence of physical restraint, 2 min ether exposure caused a transient increase in prolactin secretion and a concurrent decrease of DOPAC concentrations in median eminence of both female and male rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Activation of tuberoinfundibular but not tuberohypophysial dopaminergic neurons following intracerebroventricular administration of alpha-melanocyte-stimulating hormone

The effect of alpha-melanocyte-stimulating hormone (alpha MSH) on the activity of different central dopaminergic neurons in the male rat was determined by measuring the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) and the accumulation of 3,4-dihydroxyphenylalanine (DOPA) following the administration of a decarboxylase inhibitor in brain regions that contain terminals of nigrostriatal (striatum), mesolimbic (nucleus accumbens), tuberoinfundibular (median eminence) and tuberohypophysial (neural and intermediate lobe of the pituitary) dopaminergic neurons. Intracerebroventricular (i.c.v.) administration of alpha MSH caused a prompt (within 30 min) increase in the concentration of DOPAC and the accumulation of DOPA in the median eminence, but was without effect in the other brain regions. The alpha MSH-induced increase in tuberoinfundibular dopaminergic neuronal activity was temporally related to a decrease in circulating concentrations of prolactin. Twelve hours after the i.c.v. administration of prolactin DOPA accumulation increased in the median eminence but not in the neural or intermediate lobes of the pituitary. DOPA accumulation was not altered in any brain region 12 h after the i.c.v. administration of alpha MSH. These results suggest that alpha MSH acts acutely to selectively activate tuberoinfundibular dopaminergic neurons and thereby cause the secretion of prolactin from the anterior pituitary to decrease.     

Dissociation of prolactin secretion from tuberoinfundibular dopamine activity in late pregnant rats

This study investigated whether the PRL surge that precedes parturition is accompanied by a decrease in activity of hypothalamic tuberoinfundibular dopamine (TIDA) neurons, as occurs during the PRL surges of early pregnancy. Serial blood samples were collected at regular intervals during early and late pregnancy via chronic indwelling jugular cannulae, and concentrations of plasma PRL were determined by RIA. In addition, pregnant rats were killed at either 1200 and 0300 h on different days throughout pregnancy. Levels of TIDA neuronal activity were estimated using concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence as an index of dopamine metabolism. During early pregnancy, plasma PRL concentrations showed characteristic diurnal and nocturnal surges peaking at 1700 and 0300 h, respectively, whereas during late pregnancy, there was a broad nocturnal surge throughout the night preceding parturition. During early pregnancy, DOPAC was elevated at 1200 h, associated with suppressed plasma PRL, whereas at 0300 h, during the nocturnal PRL surge, DOPAC was significantly reduced (P < 0.05). On the last day of pregnancy DOPAC levels were significantly reduced at both 1200 and 0300 h compared with those at 1200 h in early pregnancy regardless of the PRL concentration. This experiment was repeated with additional groups to further characterize the timing of the fall in TIDA activity during late pregnancy. DOPAC concentrations were elevated throughout the second half of pregnancy, then fell significantly between 0300-1200 h on day 21, approximately 36 h before parturition. As in the previous experiment, the timing of changes in DOPAC concentrations in the median eminence was dissociated from the antepartum PRL surge. These data indicate that the regulation of PRL secretion during late pregnancy is different from that of early pregnancy. Despite the prolonged reduction in activity of TIDA neurons during late pregnancy, PRL secretion still occurs as a nocturnal surge, suggesting that dopamine is not the only regulator of PRL secretion at this time.     

Melatonin synergizes with low doses of L‐DOPA to improve dendritic spine density in the mouse striatum in experimental Parkinsonism

The dopamine precursor, L‐3,4‐dihydroxyphenylalanine (L‐DOPA), is the preferred drug for Parkinson's disease, but long‐term treatment results in the drug‐induced dyskinesias and other side effects. This study was undertaken to examine whether melatonin could potentiate low dose L‐DOPA effects in 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐induced experimental parkinsonism. Mice were treated with the parkinsonian neurotoxin, MPTP, and different doses of melatonin and low doses of L‐DOPA. Behavior, striatal histology, and dopamine metabolism were evaluated on the 7th day. MPTP‐induced striatal dopamine loss was not modified by melatonin administration (10–30 mg/kg; i.p. at 10‐hr intervals, 6 times; or at 2‐hr intervals, by day). However, low doses of L‐DOPA (5 mg/kg, by oral gavage) administered alone or along with melatonin (10 mg/kg, i.p.) twice everyday for 2 days, 10 hr apart, after two doses of MPTP significantly attenuated striatal dopamine loss and provided improvements in both catalepsy and akinesia. Additionally, Golgi‐impregnated striatal sections showed preservation of the medium spiny neurons, which have been damaged in MPTP‐treated mouse. The results demonstrated that melatonin, but not L‐DOPA, restored spine density and spine morphology of medium spiny neurons in the striatum and suggest that melatonin could be an ideal adjuvant to L‐DOPA therapy in Parkinson's disease, and by the use of this neurohormone, it is possible to bring down the therapeutic doses of L‐DOPA.     

Evidence for a physiological reduction in brain dopamine but not norepinephrine metabolism during the preovulatory phase in normal women

To investigate the role of brain catecholamine (CA) activity in the mechanisms related to physiological ovulatory function, we used high-performance liquid chromatography with electrochemical detector to measure the levels of urinary dopamine (DA), norepinephrine (NE), epinephrine (E), vanillylmandelic acid (VMA), homovanillic acid (HVA), 3,4-dihydroxyphenylacetic acid (DOPAC), and total 3-methoxy-4-hydroxy-phenylglycol (MHPG) in a group of 12 normal women during both the early follicular and pre-ovulatory phases of the menstrual cycle. The mean (+/- SEM) concentrations of HVA and DOPAC were significantly lower (P less than 0.001) during the pre-ovulatory phase than during the early follicular phase, whereas those of DA, NE, E, VMA and total MHPG were unaltered. A significant negative correlation between urinary HVA and plasma LH (r = -0.70, P less than 0.01) was also found during the pre-ovulatory period, whereas no significant negative correlations were found between urinary HVA and plasma PRL, progesterone and oestradiol. These data show: 1) reduced brain DA activity and 2) unchanged brain NE activity at the time of the midcycle surge in normal women, suggesting a physiological variation of the central DA metabolism in ovulatory function.     

Effects of cyclic hydrostatic pressure on the brain biogenic amines concentrations in the flounder, Platichthys flesus

The present study evaluated the effects of cyclic variations of hydrostatic pressure (HP) on neurotransmitters in the whole brain of flounder. The concentrations of the biogenic amines L-3,4-dihydroxyphenylalanine (L-DOPA), dopamine (DA), norepinephrine (NE), epinephrine (E), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 3-methoxytyramine (3-MT), 5-hydroxytryptamine (5-HT) and 5-hydroxyindolacetic acid (5-HIAA) were measured. Fish were subjected to HP cyclic variations which mimic naturally occurring conditions for a period of 14 days. DA, NE and 5-HT concentrations were significantly smaller by 21, 24 and 36%, respectively, compared to control fish. The concentrations of monoamine metabolites HVA, 3-MT and 5-HIAA were also smaller than those in control fish. These results suggest that central monoaminergic systems were influenced during long exposure to cyclic HP. The decreases of central neurotransmitters content might be involved in the physiological and behavioral responses to intermittent HP in fish.     

Median eminence dopamine and serotonin neuronal activity. Temporal relationship to preovulatory prolactin and luteinizing hormone surges

Using a high-performance liquid chromatography (HPLC) system coupled with an electrochemical detector, the concentrations of dopamine (DA) and 5-hydroxytryptamine (5-HT) and their major specific metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindole-3-acetic acid (5-HIAA), respectively, were measured in the median eminence (ME) throughout the rat estrous cycle. The ME DA content remained fairly constant throughout the estrous cycle except on estrus when 17.00 h values were significantly lower than 10.00 h values (40% decrease, p less than 0.05). The ME 5-HT content determined at 10.00 h was higher on proestrus than on any other day of the cycle. The ME DOPAC concentrations did not differ between 10.00 and 17.00 h on diestrus I, diestrus II or estrus. On the contrary, there was an almost linear decline between 10.00 and 17.00 h on proestrus (36% decrease, p less than 0.05). The ME 5-HIAA content did not differ between 10.00 and 17.00 h on any day of the estrous cycle. Significant changes were recorded for the DOPAC/DA and 5-HIAA/5-HT ratio in the ME on proestrus. There was a progressive decrease, starting from 10.00 h in the DOPAC/DA ratio with minimal values (42% decrease, p less than 0.05) at 16.00 h followed by an increase from 16.00 to 19.00 h. On the other hand, the 5-HIAA/5-HT ratio increased between 10.00 and 17.00 h (97% increase, p less than 0.05) and subsequently declined until 19.00 h (67% decrease vs. 17.00 h, p less than 0.05).2+hese data show that a concomitant     

Dopaminergic input to the ventromedial hypothalamus facilitates the oestrogen-induced luteinizing hormone surge in ewes

In this study we examined the release of dopamine and noradrenaline in the ventromedial hypothalamus (VMH) of ovariectomized ewes during the oestrogen-induced luteinizing hormone (LH) surge by measuring their respective metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and methoxyhydroxyphenylglycol (MHPG) using microdialysis. Further we investigated whether inhibition of catecholamine synthesis in the VMH by bilateral reverse dialysis of alpha-methyl-p-tyrosine (alpha-MPT) would block the oestrogen-induced LH and/or prolactin surges. Oestradiol treatment (50 microg oestradiol benzoate) of ovariectomized ewes resulted in a biphasic LH response, significantly (p < 0.05) decreasing LH concentrations from 2.5 to 10.5 h after injection, followed by an LH surge beginning at 16 h. Prolactin concentrations were also significantly (p < 0.05) increased in oestradiol-treated ewes from 13 h. VMH DOPAC concentrations in oil-vehicle-treated animals were at the level of detection (0.02 ng/ml) in most samples over the 24-hour sampling period. In oestradiol-treated ewes, VMH DOPAC levels were initially low before and up to 8 h after oestradiol injection but then increased significantly (p < 0.05) at 10-12 h and remained elevated up to 20 h after injection. In contrast, oestradiol injection had no effect on MHPG concentrations in the VMH. Bilateral reverse microdialysis of alpha-MPT into the VMH significantly (p < 0.05) delayed the time from oestradiol injection to the onset of the LH surge, the time to peak LH concentration and attenuated the LH surge compared with reverse dialysis of Ringer solution alone. In contrast, alpha-MPT treatment had no effect upon the oestradiol-induced increase in prolactin concentrations. This study provides evidence that the VMH is an important hypothalamic site in the neuro-endocrine control of the LH surge in ewes. The results suggest that dopaminergic neurons with terminals in the VMH are part of a neuronal pathway mediating the positive feedback effects of oestradiol on gonadotropin-releasing hormone secretion and the LH surge.     

Effects of tyrosine and tryptophan ingestion on plasma catecholamine and 3,4-dihydroxyphenylacetic acid concentrations

The effect of oral tyrosine or tryptophan ingestion on plasma concentrations of norepinephrine (NE), dopamine (DA), epinephrine (EPI), and 3,4-dihydroxyphenylacetic acid (DOPAC) was investigated in a double blind, placebo-controlled study in fasted men. Tyrosine ingestion induced within 45 min a significant but short-lasting (approximately 30 min) increase in plasma concentrations of NE, EPI, and DA and a coincident decrease in plasma DOPAC levels. Ingestion of tryptophan or lactose placebo did not after plasma DA, EPI, NE, or DOPAC levels. Since plasma catecholamines derive from peripheral sources, while circulating DOPAC may reflect both brain and peripheral DA turnover, these results suggest that the oral ingestion of tyrosine can exert acute effects on catecholamine systems within and outside the brain.     

Caste differences in dopamine-related substances and dopamine supply in the brains of honeybees (Apis mellifera L.)

We determined the mechanisms underlying caste differences in the brain levels of dopamine-related substances in adult honeybees (Apis mellifera L.). Brain levels of dopamine, DOPA (a dopamine precursor), and N-acetyldopamine (a dopamine metabolite, NADA) were significantly higher in three-day-old virgin queens than same-aged workers. Caste differences in dopamine and NADA levels were also found in the hemolymph. The in vitro enzymatic activities of DOPA decarboxylase (DDC) during dopamine synthesis in brains were not significantly different between castes. The DDC activity in adult queens was mainly found in the brain, but with lower levels of activity detected in the mandibular glands, salivary glands and ovaries. Oral application of DOPA to workers led to DOPA uptake in the brain and significantly higher dopamine and NADA levels in the brain, suggesting that dopamine synthesis could be controlled by the amount of DDC substrate. Royal jelly samples taken from queen cells had a >25-fold higher concentration of dopamine compared with honey samples collected from honey cells. However, oral application of the same concentration of dopamine did not significantly enhance the brain levels of dopamine and NADA. These results suggest that the higher levels of brain dopamine in queens compared with workers can be explained by the higher level of DDC substrate, rather than DDC activity in the brain and other tissues of queens as well as exogenous dopamine in the royal jelly.     

Enkephalin inhibits dopamine synthesis in vitro in the median eminence portion of rat hypothalamic slices

The effect of enkephalin on dopamine synthesis in vitro in tuberoinfundibular dopaminergic (TIDA) neurons was investigated in rat hypothalamic slices. Dopamine synthesis in vitro in TIDA neurons was estimated by 3,4-dihydroxyphenylalanine (DOPA) accumulation in the median eminence after incubation of slices with a DOPA decarboxylase inhibitor. The enkephalin agonist [D-Ala2]Met-enkephalinamide (ENKamide) decreased the rate of basal DOPA accumulation in the median eminence portion of hypothalamic slices from ovariectomized rats at concentrations over 2 microM. The inhibitory action of ENKamide was more pronounced in hypothalamic slices from haloperidol-treated rats in which basal DOPA accumulation in the median eminence was stimulated by increased PRL secretion. In contrast, ENKamide decreased neither the rate of depolarization- induced CA2+-dependent DOPA accumulation nor the rate of (Bu)2cAMP- or forskolin-induced DOPA accumulation in the median eminence of normal or haloperidol-treated rats. The rank order of the potencies of enkephalins and their analogs for inhibition of DOPA accumulation in the median eminence was similar to that of their binding capacities for opioid receptors. ENKamide inhibited basal DOPA accumulation even when hypothalamic slices were incubated in Ca2+-free medium to which tetrodotoxin was added or when the median eminence was incubated alone without the remainder of the hypothalamic slice. These results suggest that enkephalin, by acting directly on axon terminals of TIDA neurons in the median eminence, inhibits basal dopamine synthesis.