Release of endogenous dopamine from tuberoinfundibular neurons

Characterisation of the DA-ergic system in the mediobasal hypothalamus: a new approach to simultaneously monitor the release of DA from the TIDA neurons and the PRL secretion from the adenohypophysis in awake rats

The TIDA neurons, which constitute part of the arcuate nucleus-ME complex, play an important inhibitory role in the regulation of the PRL secretion from the adenohypophysis. To simultaneously study the release of DA from the TIDA neurons and the PRL secretion from the adenohypophysis in awake rats, a microdialysis probe was implanted into the MBH together with a permanent heartcannula in male rats. The extracellular levels of DA in the MBH as measured by microdialysis decreased to 25% of basal values after local infusion of TTX (1 mumol/l), indicating that the released DA was directly derived from neuronal activity. DOPAC levels were not affected. This local infusion of TTX into the MBH induced parallel to the immediate decrease in DA levels, a profound increase in PRL concentration in the blood (from 10 to 55 ng/PRL-RP-2/ml) directly after infusion. Thus, the area in which the dialysis probe was inserted indeed included the DA-ergic neurons that regulate the PRL secretion. Evidence for a functional re-uptake system in the MBH was obtained by local infusion of the re-uptake inhibitor nomifensine (5 mumol/l) which induced an increase in DA release to 350% of basal values, without affecting the DOPAC levels. In spite of this increase in DA levels, the PRL concentration in the blood was not affected. In pseudopregnant female rats, relatively high levels of extracellular DA in the MBH were obtained during the interphase during which the PRL levels are low, while lower DA levels were apparent during the phase the spontaneous nocturnal PRL surge normally appears. Taken together, the approach presented in this study, i.e. the simultaneous measurements of DA in the MBH and PRL in the blood, establishes an advanced method enabling studies on the DA-PRL interactions in awake animals.

Effects of thyrotropin-releasing hormone and phorbol ester on dopamine release from dispersed rat tuberoinfundibular dopaminergic neurons

We have investigated the intracellular mechanisms underlying thyrotropin-releasing hormone (TRH)-mediated [3H]dopamine ([3H]DA) release from dispersed rat tuberoinfundibular dopaminergic (TIDA) neurons. The specific binding of [3H]Me-TRH to these cells is characterized by a single, high-affinity binding site (Kd = 1.2 nM) with a Bmax value of 178 fmol/mg protein. Thyrotropin-releasing hormone markedly increased [3H]DA release and intracellular free calcium concentration ([Ca2+]i) in TIDA neurons, and its effect was abolished by treatment with EGTA (5 mM) or chlordiazepoxide, a specific TRH receptor antagonist (10 microM). Furthermore, to examine the involvement of protein kinase C on [3H]DA release, we investigated the effect of phorbol myristate acetate (PMA), which is known to activate protein kinase C directly. Phorbol myristate acetate induced a significant increase in [3H]DA release in a concentration-dependent manner. Treatment with TRH (1 microM) plus PMA (100 nM) resulted in an additive increase in [3H]DA release. Thyrotropin-releasing hormone (1 microM) still increased [3H]DA release even after preincubation with PMA (500 nM) for 24 h, but PMA (100 nM) did not under the same conditions. These results suggest that TRH may induce DA release in dispersed rat TIDA cells by increasing calcium influx and activating the protein kinase C system.

Effects of reuptake inhibitors on dopamine release from the stalk-median eminence and posterior pituitary in vitro

Similar to other dopaminergic systems, the dopaminergic neurons innervating the stalk-median eminence (SME) and posterior pituitary (PP) possess an uptake mechanism for dopamine (DA). However, the extent of DA reuptake in these tissues and its physiological significance are debated since much of the released DA is removed by the hypophysial portal vasculature before recapture. The objectives of this study were: (1) to establish in vitro conditions for examining the effects of reuptake inhibitors on DA release from the PP and SME; (2) to compare the effects of nomifensine, diclofensine and amphetamine on DA release from the SME and PP; and (3) to distinguish between reuptake and releasing properties of these drugs. Individual SME and PP were dissected from ovariectomized rats and incubated in either a static or perifusion system. Media DA was extracted with alumina and quantitated by high performance liquid chromatography with electrochemical detection. The reuptake inhibitors, nomifensine, dichlofensine and amphetamine, in the presence of pargyline, a monoamine oxidase inhibitor, stimulated both basal and K(+)-evoked release of DA from the SME and PP under static incubation conditions. The drugs elicited a 2-3-fold higher increase in basal DA release from the SME as compared to the PP. Only amphetamine stimulated DA release in the perifusion system whereas nomifensine and diclofensine were without effects. We concluded that: (1) a mechanism for the reuptake of DA is operable in both the SME and PP; (2) the reuptake of DA appears to be more active in the SME than the PP; and (3) unlike amphetamine, nomifensine and diclofensine are pure reuptake inhibitors devoid of direct DA releasing activities.

Possible involvement of Ca++ ions, protein kinase C and Na(+)-H+ antiporter in insulin-induced endogenous dopamine release from tuberoinfundibular neurons

Insulin (63 microM) stimulated endogenous dopamine (DA) release from tuberoinfundibular neurons. This effect was independent on the presence of extracellular glucose and did not involve the outward transport of DA, mediated by its membrane carrier. By contrast this effect was completely prevented by the removal of extracellular Ca++ ions in presence of the Ca(++)-chelator ethyleneglycol-2-(2-aminoethyl)-tetracetic acid (EGTA). Furthermore 1-(5-isoquinolinyl-sulfonyl)-2-methyl-piperazine (H7), a compound which behaves as a putative inhibitor of protein kinase C (PK-C) (10 microM), completely counteracted the stimulation of endogenous DA release induced by insulin. Amiloride (300 microM) and its 5-amino nitrogen atom-substituted derivative, 5-(N-methyl-N-(guanidinocarbonylmethyl) amiloride (MGCMA) (10 microM), a highly selective inhibitor of the Na(+)-H+ membrane antiporter, were both able to prevent the stimulatory action exerted by insulin on endogenous DA release. Collectively, these results suggest that the transductional events by which insulin stimulated endogenous DA release from TIDA neurons may involve the activation of PK-C, the enhancement of Ca++ influx and the stimulation of the Na(+)-H+ exchange system.

Pure uptake blockers of dopamine can reduce prolactin secretion: studies with diclofensine

The effects of diclofensine, a pure dopamine (DA) uptake inhibitor on 1) 3H-DA uptake in rat arcuate-periventricular nucleus-median eminence synaptosomes, 2) basal and K+-evoked endogenous DA release from tuberoinfundibular dopaminergic (TIDA) neurons and 3) in vivo prolactin (PRL) secretion were studied. Diclofensine, in concentrations of 0.01, 0.1 and 1 microM caused a marked decrease of 3H-DA uptake. In addition, it was unable to stimulate basal endogenous DA release which, on the contrary, was elicited by d-amphetamine in the same concentration (50 microM). On the other hand, diclofensine (50 microM) caused a 3 fold enhancement of K+-evoked DA release. Finally, the compound, when administered in vivo to male rats, significantly reduced basal serum PRL levels. The results of the present study seem to indicate that the pharmacological blockade of DA uptake in TIDA neurons is a condition sufficient to cause a reduction of PRL release.

The metabolism of dopamine in the median eminence reflects the activity of tuberoinfundibular neurons

The purpose of the present study was to characterize the metabolism of dopamine (DA) in tuberoinfundibular (TI) neurons terminating in the median eminence and to examine the effects of procedures that alter the synthesis and turnover of DA in these neurons on the concentrations of dihydroxyphenylacetic acid (DOPAC) in the median eminence. The DA uptake inhibitor nomifensine (25 mg/kg, i.p.; 30 min) failed to alter median eminence DOPAC concentrations indicating that very little released DA is recaptured and metabolized by TIDA neurons. Within 5 min following the administration of the monoamine oxidase inhibitor pargyline (50 mg/kg, i.v.) median eminence DOPAC concentrations declined to 15% of control demonstrating that this metabolite has a high turnover rate and is rapidly removed from the median eminence. Median eminence DOPAC concentrations in diestrous female rats, whose TIDA neuronal activity is higher than in the male, were two-fold greater than in male rats. Prolactin (10 micrograms/rat, i.c.v.; 12 h), which increases TIDA neuronal activity, produced a corresponding increase in median eminence DOPAC concentrations in male rats. Restraint stress (30 min), which decreases TIDA neuronal activity, produced a corresponding decrease in median eminence DOPAC concentrations in diestrous female rats. The results from the present study suggest that DOPAC concentrations in the median eminence can be used as an index of TIDA neuronal activity.

Maitotoxin and Bay-K-8644: two putative calcium channel activators with different effects on endogenous dopamine release from tuberoinfundibular neurons

In the present study we report on the effects of two putative calcium channel activators, maitotoxin and the dihydropyridine BAY-K-8644, on endogenous dopamine release from tuberoinfundibular dopaminergic neurons. Maitotoxin stimulated basal dopamine release and this effect was calcium-dependent. By contrast BAY-K-8644 failed to produce any modification of basal or high potassium-induced dopamine release. These results suggest that maitotoxin, unlike BAY-K-8644, represents a suitable tool to investigate the functional role of calcium channels in central dopaminergic neurons.

Endogenous dopamine release from tuberoinfundibular neurons: does calmodulin play any role?

The possible involvement of calmodulin in the process of endogenous dopamine (DA) release from arcuate-periventricular nuclei-median eminence fragments, containing tuberoinfundibular dopaminergic (TIDA) neurons, has been investigated in an in vitro incubation system. For this purpose the basal and K+-stimulated DA release was examined in the presence and in the absence of the different putative calmodulin antagonists, pimozide, trifluoperazine, penfluridol and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7). Trifluoperazine and pimozide in concentrations up to 100 microM were both uneffective in blocking K+-evoked DA release. Penfluridol in doses of 5 and 10 microM, did not prevent 35 mM K+-induced endogenous DA release. It was able to reduce K+-stimulated DA release only at the very large concentration of 100 microM. W-7 added in vitro to the hypothalamic fragments, prevented endogenous DA release evoked by 35 mM K+ in a dose-dependent manner. W-5, a chlorine deficient analogue of W-7, that interacts only weakly with calmodulin, failed to modify K+-stimulated endogenous DA release in doses up to 200 microM. All the putative calmodulin antagonists used in the present study did not induce any change of basal DA release. In conclusion the fact that most of the agents, except W-7, known to antagonize calmodulin-dependent processes in many biological systems failed to interfere with the release of endogenous DA from TIDA neurons seems to suggest that calmodulin does not play a crucial role in the process of DA release and that the inhibitory effect of W-7 on endogenous DA release may be better attributed to other mechanisms different from its anticalmodulin action.

Effect of different organic and inorganic blockers of calcium entry on the release of endogenous dopamine from tuberoinfundibular neurones

In the present study the effect of different blockers of calcium entry belonging to different chemical classes on basal and K+-elicited release of endogenous dopamine (DA) from tuberoinfundibular dopaminergic neurones was studied in vitro. For this purpose fragments of hypothalamus containing arcuate-periventricular nuclei and median eminence were incubated in vitro and endogenous DA released into the medium was assayed by radioenzymatic assay. The organic blockers of calcium entry, nitrendipine, nimodipine, nifedipine, diltiazem and flunarizine did not modify basal or K+-evoked release of endogenous DA, unless very large concentrations (100 microM) of nifedipine or diltiazem were used. The phenylalkylamine methoxyverapamil (D-600) consistently inhibited K+-stimulated release of endogenous DA in concentrations of 50 and 100 microM. Cobalt and lanthanum, two ions with an ionic radius similar to that of calcium and which are known to inhibit calcium fluxes through nerve membranes, significantly blocked release of endogenous DA elicited by 35 mM K+. In summary, the results of the present study showed that calcium channels in the tuberoinfundibular dopaminergic system displayed a different sensitivity to various classes of blockers of calcium entry. Inorganic blockers of calcium entry, like lanthanum and cobalt, appeared to be the most effective in blocking Ca2+-dependent release of endogenous DA, whereas, among the organic calcium antagonists, phenylalkylamines seemed to possess a certain degree of effectiveness.

Reduced tuberoinfundibular dopaminergic neuronal function in rats after long-term withdrawal of estrogen treatment

Hypothalamic fragments from female rats treated repeatedly with estradiol valerate (EV) and bearing prolactin (PRL)-secreting tumors contained, seven months after the last EV injection, lower concentrations of dopamine (DA) than age-matched controls. Depolarizing concentrations of K+ (35 mM) and amphetamine (50 microM) evoked in PRL-secreting tumor bearing rats an endogenous DA release significantly lower than in controls.

Effect of aging on in vitro dopamine biosynthesis in the median eminence of rat hypothalamic slices

The rates of basal and cyclic AMP-dependent DOPA accumulation in the median eminence of hypothalamic slices were not different between ovariectomized young and aged rats. However, the rate of Ca2+-dependent, depolarization-induced DOPA accumulation was smaller in aged rats, suggesting that the Ca2+ system in the regulation of dopamine biosynthesis in tuberoinfundibular dopaminergic neurons is altered by aging.

In vitro dopamine biosynthesis in the median eminence of rat hypothalamic slices: involvement of voltage-dependent Ca2+ channels

It was investigated whether Ca2+ is involved in the regulation of basal depolarization-induced dopamine biosynthesis in tuberoinfundibular dopaminergic neurons. The rate of dopamine biosynthesis was estimated by in vitro dihydroxyphenylalanine (DOPA) synthesis in the median eminence following incubation of rat hypothalamic slices with a DOPA decarboxylase inhibitor. Depolarizing agents such as K+ and veratridine increased the synthesis rate of DOPA in the median eminence in a dose-dependent manner with a maximal synthesis rate obtained at concentrations of 50 mM and 50 microM, respectively. Removal of Ca2+ and addition of EGTA (1 mM) into the medium did not influence basal DOPA synthesis in the median eminence but blocked the K+- and veratridine-induced DOPA synthesis. The Ca2+ channel blockers verapamil (100 microM) and Co2+ (4 mM) were effective in reducing the depolarization-induced DOPA synthesis. A23187 (10 microM), a Ca2+ ionophore, stimulated basal DOPA synthesis in the median eminence. On the other hand, tetrodotoxin (2 microM), a Na+ channel blocker, did not change the basal and K+-induced DOPA synthesis in the median eminence whereas it completely inhibited the veratridine-induced DOPA synthesis. These results suggest that depolarization-induced synthesis of dopamine in tuberoinfundibular neurons requires Ca2+ influx through voltage-dependent Ca2+ channels.

Phencyclidine-induced inhibition of rat prolactin secretion: increased portal blood dopamine

Intraperitoneal administration of phencyclidine (PCP, 2.5-20 mg/kg) produced a dose-related inhibition of the increase in serum PRL concentrations produced by alpha-methylparatyrosine (AMPT) or reserpine, but not morphine. Phencyclidine was more potent in antagonizing the PRL-releasing effect of reserpine than that of AMPT, suggesting a greater effect of PCP on the cytoplasmic than the storage dopamine (DA) pool. Phencyclidine had no effect on PRL release from rat pituitary glands in vitro. Intravenous administration of PCP (10 mg/kg) to anesthetized male rats produced a two-fold increase in pituitary stalk (DA) concentrations, suggesting that PCP inhibits rat serum PRL by increasing the release of DA from the tuberoinfundibular neurons, and possibly by blocking its reuptake as well.