Letter: Catecholamines in the median eminence: new evidence for a major noradrenergic input

Central administration of serotonin decreases tyrosine hydroxylase catalytic activity and messenger ribonucleic Acid signal levels in the hypothalamus of female rats

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 prolactin (PRL) levels. Ovariectomized rats were treated with either vehicle or 5-HT through a lateral ventricular cannula in one of two dose paradigms: 1) a bolus of 20 μg, with tissues taken at 30 min, or 2) the same bolus immediately followed by 20 μg/30 min via a syringe pump for 120 min, and tissues taken at 120 min. Blood samples were taken throughout experiments and plasma PRL determined by radioimmunoassay. Under both paradigms, NSD 1015, a dihydroxyphenylalanine (DOPA) decarboxylase inhibitor (25 mg/kg intraarterially) was injected 10 min before decapitation and brain excision followed by stalk-median eminence dissection. The rate of DOPA accumulation, determined by measuring DOPA levels in the stalk-median eminence by high-performance liquid chromatography with electrochemical detection was used as a measure of tyrosine hydroxylase (TH) catalytic activity. Stalk-median eminence DOPA accumulation in control rats was 29.9 ± 4.2 and 28.8 ± 4,4 ng/mg protein (30 and 120 min experiments, respectively). DOPA accumulation in 5-HT-treated rats was significantly reduced (P<0.05) after 30 min to 17.8 ± 1.2 ng/mg protein, but it was similar (21.7 ± 3.9) to controls after 120 min of 5-HT infusion. 5-HT levels in the stalk-median eminence of rats treated with 5-HT were 13- to 17-fold greater than controls (16.9 to 18.5 ng/mg protein). Plasma PRL levels in both groups increased 10-fold after 5-HT treatment with a peak at 5 min, returning to baseline by 120 min. TH mRNA levels were determined by in situ hybridization in a second group of rats which were treated with the 20μg bolus and subsequent 120 min infusion of 5-HT. TH mRNA signal levels in the arcuate nucleus of control rats averaged 144 ± 21 grains/cell. After treatment with 5-HT, TH mRNA levels in the arcuate nucleus were significantly lower (P<0.0001) with 69±14 grains/cell. In a third group of rats, the effects of the 30 min 5-HT treatment on TH catalytic activity and circulating PRL levels was challenged with two 5-HT(2) receptor antagonists, LY53857 (5 mg/kg intraperitoneally) or ketanserin (10 mg/kg intraperitoneally). Neither the 5-HT-induced decrease in TH catalytic activity nor the increase in PRL was altered by pretreatment (120 min) with 5-HT(2) antagonists. These data suggest that central 5-HT is capable of decreasing TH activity and TH mRNA levels in the tuberoinfundibular dopamine neurons and that the decrease in dopaminergic neuronal activity may contribute to the 5-HT-induced PRL rise. The changes in TH catalytic activity and PRL after intracerebroventricular administration of 5-HT do not appear to be mediated by 5-HT(2) receptors.

Neurokinin A in the anterior pituitary of female rats: effects of ovariectomy and estradiol

The effect of acute and chronic ovariectomy and the substitutive treatment with 17-beta estradiol and/or progesterone on anterior pituitary levels of neurokinin A (NKA) was studied in female rats. Acute ovariectomy did not result in significant changes of NKA in the anterior pituitary gland as compared with the levels in diestrous intact rats, but a single injection of 5 micrograms of estradiol in ovariectomized rats significantly decreased NKA levels in the anterior pituitary gland. Progesterone was without effect and did not modify the decrease of NKA in the anterior pituitary gland induced by estradiol. In rats examined 11 to 17 days after ovariectomy, NKA in the anterior pituitary gland was significantly higher than in diestrous intact rats. In the hypothalamus, ovariectomy resulted in decreased levels of NKA in the median eminence-arcuate nucleus. Estradiol significantly reduced NKA stores in the anterior pituitary gland but increased them in the whole hypothalamus and in the median eminence-arcuate nucleus. Thus, estradiol seems to be a powerful regulator of NKA stores in the adenohypophysis and also in the hypothalamus.

Effects of immobilization stress on tuberoinfundibular dopaminergic (TIDA) neuronal activity and prolactin levels in lactating and non-lactating female rats

The effects of immobilization stress on the prolactin secretory response and on the activity of the tuberoinfundibular dopaminergic (TIDA) neurons were determined in intact, virgin female rats on the morning of diestrus or proestrus and in post-partum, lactating female rats. The virgin females exhibited a significant increase in circulating levels of prolactin which was evident by 1 minute and persisted during the immobilization (5 minutes). In contrast, the prolactin secretory response in lactating females was significantly attenuated compared to non-lactating animals. The activity of the TIDA neurons was not altered by the 5 minutes of stress. Even after 30 minutes of immobilization, TIDA neuronal activity was not affected in either the lactating or cycling females. These data suggest that the cycling female rat is capable of a prolactin secretory response to the stressor without inhibition of TIDA neuronal activity. It seems likely that prolactin releasing factors mediate this response. In contrast, stress did not produce a similar prolactin increase during lactation. It seems likely that, during lactation, the pituitary is not sensitive to releasing factors unless the TIDA neurons are inhibited. There appear to be differences in the sensitivity of the pituitary depending on the physiological state of the model employed.

Short-term treatment with 17-beta estradiol enhances spontaneous [3H] dopamine release from cultured rat tuberoinfundibular neurons

The effects of 17-beta estradiol (E2) on spontaneous [3H] dopamine ([3H]DA) release was investigated using primary cultured cells from the tuberoinfundibular region of rat hypothalamus, which includes DA neurons. [3H] DA uptake by the neurons in the presence of E2 at 10(-8) mol/l was similar to that by control cells. Pretreatment with E2 at 10(-9) mol/l or more resulted in dose-dependent increase in spontaneous [3H] DA release from the cultured hypothalamic cells. The spontaneous [3H] DA release reached almost a plateau on pretreatment with E2 at 10(-9) mol/l for 6 hours. Pretreatment with 1 nM E2 also enhanced DA release induced by 10 microM ionophore A23187. These results indicate that estrogen stimulates tuberoinfundibular DA neuronal activity.

Neurokinin A levels in the hypothalamus of rats and mice: effects of castration, gonadal steroids and expression of heterologous growth hormone genes

Neurokinin A is a decapeptide with pharmacological activities and localizations similar to those of substance P. In this report we describe the effects of castration and administration of testosterone, dihydrotestosterone and estradiol, on neurokinin A levels in the hypothalamus of male rats. The effects of estradiol and testosterone on hypothalamic neurokinin A were also examined in normal mice and in transgenic mice carrying the genes for human or bovine growth hormone (hGH, bGH, respectively). Either acute or prolonged castration was followed by a decrease of neurokinin A concentrations in the hypothalamus of male rats. The substitutive administration of testosterone propionate or estradiol benzoate for 14 days resulted in an increase of hypothalamic neurokinin A levels above the values found in intact animals. A lower dose of testosterone propionate or dihydrotestosterone also increased hypothalamic neurokinin A levels in the hypothalamus of castrated rats. In normal intact male mice a single injection of estradiol benzoate significantly increased hypothalamic neurokinin A levels. A similar effect was observed in transgenic mice carrying the bGH gene with phosphoenolpyruvate carboxykinase (PEPCK) promoter, while mice carrying the hGH gene failed to show any response to estradiol. In castrated male mice, either normal or transgenic, carrying the bGH gene with metallothionein promoter, a single injection of testosterone propionate significantly increased neurokinin A levels in the hypothalamus. It is concluded that sex steroids may regulate the levels of neurokinin A in the hypothalamus of rats and mice.

Substance P affects the GABAergic system in the hypothalamo-pituitary axis

In the present work we examined the effect of the neutralization of endogenous substance P by the administration of an anti-substance P serum (ASPS) on GABA concentration in the anterior pituitary in hyperprolactinemic conditions induced by 5-hydroxytryptophan or by grafting anterior pituitaries. ASPS reduced the increase in the anterior pituitary GABA concentration induced by hyperprolactinemia. In vitro experiments showed that substance P inhibited K(+)-evoked GABA efflux from hypothalamic fragments and decreased GABA concentration in the anterior pituitary but ASPS increased it. Our results demonstrate that substance P modifies hypothalamic GABA release and anterior pituitary GABA concentration and suggest that an interaction exists between substance P and GABA.

Vasoactive intestinal peptide affects the GABAergic system in the hypothalamic-pituitary axis

The effect of a specific antiserum against vasoactive intestinal peptide (VIP) on GABA in the hypothalamic-pituitary axis was studied. The administration of anti-VIP serum (A-VIP) increased anterior pituitary GABA concentration in control rats, but decreased this neurotransmitter in rats with hyperprolactinemia induced by acute or chronic treatments with estrogens, or by the implanting of anterior pituitary glands under the kidney capsule. Besides, the injection of the A-VIP serum in the morning in proestrous rats causes a decrease in anterior pituitary GABA concentration, measured in the afternoon of the same day. The in vitro effect of A-VIP and VIP on endogenous GABA release from hypothalamic fragments and on anterior pituitary GABA concentration was studied. A-VIP increased both basal and high K(+)-evoked GABA effluxes whereas VIP produced a decrease in evoked GABA efflux from hypothalamic fragments. Furthermore, A-VIP inhibited the normal degradation of GABA that occurs in the isolated gland whereas VIP increased it. These results suggest that VIP modifies hypothalamic GABA release and anterior pituitary GABA concentration.

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.

Cobalt-sensitive and dihydropyridine-insensitive stimulation of dopamine release from tuberoinfundibular neurons by high extracellular concentrations of barium ions

Recently, it has been demonstrated that Ca2+ entrance into the neuronal cytoplasm can occur upon the activation of 3 different types of specific voltage-dependent channels which can be characterized according to the following criteria: (1) voltage threshold for activation; (2) tendency to inactivation; (3) bivalent cation permeability; and (4) drug sensitivity. In this study we investigated, in tuberoinfundibular dopaminergic (TIDA) hypothalamic neurons, the biochemical and pharmacological properties of Ca2+ channels, by comparing the effects of high extracellular concentrations of Ba2+ and Ca2+ ions on [3H]dopamine (DA) release from TIDA neurons. The results obtained show that extracellular Ba2+ ion concentrations dose-dependently (10-20 mM) stimulated [3H]DA release from superfused TIDA neurons and that this effect was prevented by Co2+ ions (2 mM). In addition, superfusion of TIDA neurons with a concentration of Ca2+ ions equimolar to that of Ba2+ ions (20 mM) failed to modify [3H]DA release. The fact that tetraethylammonium (10 mM), a blocker of K+ currents in excitable cells, did not mimick the stimulatory action of Ba2+ ions on [3H]DA release, seems to exclude that the effect of Ba2+ ions was dependent on the inhibition of K+ channels in TIDA neurons. The omission of Ca2+ ions from the extracellular medium did not prevent the stimulatory effect on [3H]DA release elicited by elevated concentrations of Ba2+ ions, but rather reinforced this effect. Finally, nitrendipine (50 microM) did not modify the stimulatory effect of high extracellular Ba2+ ions on [3H]DA release from TIDA neurons.

Effect of ethanol on GABA uptake and release from hypothalamic fragments

A study was performed on the effect of ethanol on the basal and K+-evoked efflux of endogenous GABA from rat hypothalamic fragments. The amount of GABA present in the medium and in the tissue was measured by radioreceptor assay. In vitro addition of ethanol (50 and 100 mM) enhanced the K+-evoked efflux of GABA in a Ca++-dependent manner, and increased tissue GABA content. Since K+-evoked outflow induced by ethanol was not affected by the presence of nipecotic acid, ethanol appears to alter the uptake of endogenous GABA. An inhibitory effect of ethanol on 3H-GABA uptake was observed under K+ depolarization. On the other hand, acute ethanol administration produced a decrease in basal and K+-evoked efflux from hypothalamic fragments and in tissue GABA concentration. Changes in GABA efflux may lie behind some of the neuropharmacological effects of ethanol.

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.

Differential effects of haloperidol, clozapine, and fluperlapine on tuberoinfundibular dopamine neurons and prolactin secretion in the rat

Two atypical neuroleptic agents, clozapine and fluperlapine, produced rapid elevations in plasma PRL concentrations that were similar in magnitude to those produced by haloperidol. However, the PRL response to clozapine or fluperlapine was of much shorter duration than that elicited by haloperidol. Clozapine, but neither fluperlapine nor haloperidol, produced a rapid increase in the activity of tuberoinfundibular dopamine (TIDA) neurons, as evidenced by an enhanced accumulation of dihydroxyphenylalanine (DOPA) in the median eminence after the inhibition of DOPA decarboxylase. The clozapine-induced increase in DOPA accumulation was evident within 30 minutes after its administration and persisted for at least 4 hours. The clozapine-induced increase in the activity of TIDA neurons may account, in part, for the abbreviated PRL response to this neuroleptic. In addition, ability to produce a short-lived increase in PRL secretion in the rat appears to be common to the atypical neuroleptic drugs.

Feed-back effect of growth hormone on hypothalamic opioid and somatocrinin producing neurons

Reportedly, most acromegalics are refractory to the growth hormone (GH)-releasing effect of central nervous system-acting stimuli. For instance, the synthetic analogue of met-enkephalin (Enk) viz. FK 33-824 fails to alter the high circulating GH levels of acromegalics. The most likely interpretation of such finding is that circulating GH disrupts, for a negative feedback effect, hypothalamic opioid function and/or GH-releasing hormone (GHRH) producing neurons, through which opioids exert their action. To address this issue, we have evaluated in intact and hypophysectomized male rats the effect of a high-dose GH regimen on the hypothalamic stores of endogenous opioid peptides, beta-endorphin (beta-EP) and met-enkephalin (met-enk). Moreover we have evaluated in intact male rats the effect of exogenous GH on median eminence (ME) GHRH stores and the ability of FK 33-824 to stimulate GH and prolactin (PRL) secretion and of exogenous GHRH to induce GH secretion. Human GH (25 and 250 micrograms bid for 4 days) administered to hypophysectomized rats strikingly reduced beta-EP and met-enk-like immunoreactivity (LI) in the medial basal hypothalamus, the effect being already maximal with the lower hGH dose. The higher dose of hGH diminished, though to a lower extent, hypothalamic beta EP-LI content also in intact rats, and reduced GHRH-LI content in the ME. Despite these profound biochemical alterations, the GH responsiveness to GHRH and FK 33-824 administration was preserved, while the latter drug induced a lower PRL rise in GH-treated than in control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of evidence for an impairment of tuberoinfundibolar dopaminergic neurons in aged male rats of the Sprague-Dawley strain

Circulating prolactin (PRL) levels, dopamine (DA) content, in vitro basal and stimulus-evoked endogenous DA release from arcuate-periventricular nuclei median-eminence fragments were studied in young (4 months) and old (24-25 months) male rats of Sprague-Dawley strain. Serum PRL levels did not differ in young and aged animals. In addition DA tissue content, basal and K+- or d-amphetamine evoked endogenous DA release did not show age-related differences. These results suggest that in male rats of the Sprague-Dawley strain the activity of tuberoinfundibular dopaminergic (TIDA) neurons does not change during senescence, unlike what happens in other strains of rats.

Rapid release of [3H]dopamine from median eminence and striatal synaptosomes

Release of preaccumulated, tritium-labeled dopamine ([3H]DA) from preparations of isolated nerve terminals (synaptosomes) of rat median eminence (ME) and corpus striatum (CS) was examined over short time intervals (1-20 s). In both preparations, basal efflux of [3H]DA was linear with time. Depolarization with high K+ resulted in an initial rapid release of [3H]DA which stabilized by 20 s, whereas veratridine elicited an increased rate of release over basal levels that was linear over the first 20 s. The calculated rate constants of release for both the initial phase of K+- and the veratridine-stimulated release were approximately threefold greater in CS than in ME synaptosomes. The major component of the high K+-induced release of [3H]DA from both synaptosome preparations increased as a graded function of [Ca2+]o. However, a smaller component, independent of external Ca2+, existed in both ME and CS synaptosomes. Increasing the [Mg2+] in the external solution resulted in a right shift of both the [K+]o and the [Ca2+]o dose-response curves, consistent with actions of Mg2+ on screening surface membrane charges and blocking voltage-dependent Ca2+ channels. In all studies, steady-state uptake of the [3H]DA was about twofold greater into CS than into ME synaptosomes. Moreover, the fraction of incorporated [3H]DA released by stimulation from the CS was much greater than that released from ME synaptosomes. These data are consistent with differences between these two types of dopaminergic terminals with respect to packaging and/or distribution of the accumulated neurotransmitter in intraneuronal pools, as well as marked differences in the apparent kinetics of DA release.

Effect of hypophysectomy and growth hormone replacement on hypothalamic GHRH

Long-term (7 and 14 days) hypophysectomy resulted in a striking decrease in growth hormone releasing hormone-like immunoreactivity (GHRH-LI) in the median eminence (ME) of adult male rats, evaluated by both radioimmunoassay and immunohistochemistry. Treatment with human GH (125 micrograms/rat, twice daily IP for 14 days) prevented, though partially, depletion of GHRH-LI from the ME, as assessed by both methods. These results demonstrate that circulating GH levels regulate the function of GHRH-producing structures, via a feedback mechanism.

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.

Prolactin release and tuberoinfundibular dopaminergic neuronal activity following single and double injections of morphine

It is well established that opiate agonists alter tuberoinfundibular dopaminergic activity and consequently prolactin release. The purpose of this study was to characterize the effects of morphine on prolactin secretion and tuberoinfundibular dopaminergic neuronal activity with respect to time after administration. Additionally, the effect of an initial morphine injection on the response produced by a second injection of morphine was determined. The rate of depletion of median eminence dopamine content following synthesis inhibition by alpha-methyl-p-tyrosine was used as an index of dopaminergic neuronal activity. Male rats given a single injection of morphine sulfate (15 mg/kg, s.c.) showed a significant increase in circulating prolactin levels and had a lower rate of median eminence dopamine turnover 1 h after injection. Four hours after injection, circulating prolactin levels were similar to those in vehicle treated rats, while dopamine turnover was significantly higher than controls. When two injections of morphine sulfate (15 mg/kg, s.c.) were given 4 h apart, the stimulation of prolactin release produced by the second injection was significantly attenuated. Although this second injection caused a significant decrease in dopamine turnover, the turnover rate following this injection was significantly greater than that following the initial injection. The combination of fluoxetine and 5-hydroxytryptophan (FLX/5-HTP) caused an initial increase in prolactin secretion with plasma values returning to basal levels by 4 h. When rats were pretreated with FLX/5-HTP instead of morphine, the prolactin response to an injection of morphine 4 h later was not attenuated. Similarly a FLX/5-HTP pretreatment had no influence on a second injection of FLX/5-HTP administered 4 h later.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Histamine-induced prolactin release and activity of tuberoinfundibular dopaminergic neurons in male rats

The role of tuberoinfundibular dopamine (DA) in the histamine-induced rise of plasma prolactin was studied in male rats. Right lateral ventricle (icv) histamine injection (30 micrograms/rat) caused a significant rise of plasma prolactin at 15 and 30 min; at 60 min values returned to basal levels. Histamine does not modified steady-state DA concentrations. For turnover evaluation, histamine was icv injected immediately or 30 min after the tyrosine hydroxylase inhibitor alpha-methyltyrosine (250 mg/kg i.p.) and DA concentration in the median eminence was measured at 0, 1 and 2 hours after the inhibitor injection. Rate constants of DA decline and DA synthesis rates were found similar in both controls (cerebrospinal fluid) and histamine-injected rats. These results indicate that the stimulatory action of histamine on prolactin release might not be associated to DA. It would be due mainly to its action on prolactin-releasing factors and to a minor extent, to changes in the dopaminergic system.

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 single or repeated estrogen administration on tuberoinfundibular GABA neurons and anterior pituitary GABA receptors: biochemical and functional studies

The effect of single or protracted administration of estradiol valerate on the hypothalamo-pituitary gamma-aminobutyric acid (GABA)ergic system and on plasma prolactin levels has been evaluated in female rats 2 months after the last (chronic treatment) or the single dose of the steroid. In the group of animals receiving one dose of estrogen, no modifications were detected in the activity of the tuberoinfundibular GABAergic neurons as implied by unchanged GABA accumulation either in the median eminence or the anterior pituitary after blockade of GABA catabolism with ethanolamine-O-sulphate. However, a complete disappearance of the low affinity population of GABA receptors in the anterior pituitary was observed. In this experimental condition, where baseline prolactin levels were 3-fold higher than in control rats, muscimol, a potent GABA agonist, was effective in significantly lowering plasma prolactin concentrations. Chronic estradiol valerate administration reduced GABA accumulation in the median eminence and the anterior pituitary at 4, but not at 2 h, after intracerebroventricular injection of ethanolamine-O-sulphate. Moreover, in this instance, a complete disappearance of the high affinity population of GABA receptors in the anterior pituitary was detected. Long-term estrogen administration induced also a 55-fold increase of plasma prolactin titers and muscimol was ineffective in reducing prolactin concentrations in plasma. The ability of muscimol to inhibit prolactin release only in single-estrogen-treated animals strongly suggests that the high affinity population of anterior pituitary GABA receptors is that involved in the mechanisms whereby GABA inhibits prolactin release from anterior pituitary.

Hypothalamic neurotransmitter function in experimentally induced hyperprolactinemia

It is known that animals or patients bearing a prolactin (PRL)-secreting tumor (PST) do not suppress PRL levels after administration of indirectly acting dopamine agonists, namely nomifensine (Nom), and are not responsive to the PRL releasing effect of antidopaminergic drugs and opioid peptides. Since the action of these drugs is mediated through the tuberoinfundibular dopaminergic (TIDA) system, these findings have been taken to indicate that animals and humans bearing prolactinomas have a defective TIDA function. Alternatively, PRL unresponsiveness to these drugs could be due to hyperfunction of TIDA system for the feedback action of high PRL levels. To clarify whether hypo- or hyperfunction of the TIDA system was responsible for such behaviour, we tested the effect of a synthetic opioid peptide (FK 33-824), a DA receptor antagonist, domperidone (Dom), and of Nom on PRL secretion in two experimental models of non-tumoral hyperprolactinemia, i.e. rats bearing ectopic pituitaries since 3 days (TP rats), or treated with ovine PRL (oPRL 250 micrograms, twice daily for 3 days), in which existence of an increased TIDA function has been demonstrated. FK 33-824 (0.5 mg/kg i.p.) increased significantly plasma PRL levels in control rats but failed to do so in TP rats and it elicited a significantly lower PRL response than in controls in rats treated with oPRL. In both experimental models, a PRL secretagogue, e.g. 5-hydroxytryptophan (50 mg/kg i.p.), elicited the same response as in controls, indicating that the pituitary PRL pool was preserved.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional changes in monoamine metabolism in the aging constant estrous rat

Studies were undertaken to determine levels of monoamines and their metabolites in brain regions in young (3-4 months) normally cycling and old (25-26 month) constant estrous female rats. Dopamine (DA) concentrations were reduced in old rats in the median eminence (ME), medial basal hypothalamus (MBH), preoptic area-anterior hypothalamus (POA-AH) and the striatum. Similarly, concentrations of dihydroxyphenylacetic acid (DOPAC), the major acid metabolite of DA, were reduced significantly in all 4 regions. In the ME, a strong positive correlation was observed between DA and DOPAC concentrations in both young and old rats. Concentrations of norepinephrine (NE) were reduced in old rats in the MBH and POA-AH but not in the ME or striatum. Concentrations of serotonin (5HT) and its major metabolite, 5-hydroxyindoleacetic acid (5HIAA) were generally unchanged with age in all of the regions examined. These studies indicate the age-related regional alterations in DA and 5HT metabolism can be monitored by methods which quantitate monoamines and their metabolites.

Regulation of prolactin secretion during suckling: involvement of the hypothalamo-pituitary GABAergic system

Previous results demonstrated that GABA exerts a dual control on PRL secretion, one excitatory mediated in part by the impairment of the tubero-infundibular dopaminergic (TIDA) system function, the other inhibitory occurring at the level of the anterior pituitary (AP), where 3H-GABA and 3H-Muscimol (3H-M) recognition sites have been described. This report provides evidence for a physiological role of the tubero-infundibular GABAergic system (TI-GA-BA) on PRL secretion in the rat. In lactating rats separated for 4 h from their pups reinstitution of suckling for different periods resulted in an increase either in glutamic acid decarboxylase (GAD) activity in the mediobasal hypothalamus (MBH) or in AP-GABA content. Dynamic changes of the GABAergic function in the MBH-AP system seemed to have a certain degree of specificity because suckling did not affect GAD activity in the caudate nucleus. In lactating rats 2, 4, 8 and 24 h after removal of the offsprings AP-GABA concentrations and plasma PRL titers significantly decreased with respect to values present in rats never separated from their pups. Since it has been demonstrated that the PRL lowering effect of GABA is a receptor-mediated event, we have investigated the plasticity of AP-GABA receptors during suckling. The inhibitory action of GABA seems to be mediated mainly by the activation of the high affinity binding sites. This proposition is supported by the fact that in lactating rats, where only the high affinity receptor population is present, M was still able to decrease significantly plasma PRL concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual effect of verapamil on K+-evoked release of endogenous dopamine from arcuate nucleus-median eminence complex

The effect of verapamil, a calcium-entrance blocker, on K+-evoked release of endogenous dopamine from tuberoinfundibular neurons incubated in vitro was studied. This compound, added to the incubation medium, at the dose of 10(-6) M, significantly reinforced K+-induced dopamine release, whereas, at higher doses (10(-5), 5 X 10(-5) and 10(-4) M), it completely prevented the stimulated dopamine release. The results obtained with the higher doses showed the calcium dependence of K+-evoked release of endogenous dopamine from central neurons. The opposite effect, seen with the lower dose of verapamil, could be due to different pharmacological properties of the drug.

Release of endogenous dopamine from tuberoinfundibular neurons

Release of endogenous dopamine(DA) from arcuate-periventricular nucleus-median eminence fragments has been analyzed in an in vitro static incubation system. Exposure of these hypothalamic fragments to increasing concentrations of K+ ions produced a dose-dependent release of endogenous DA. The highest rate of K+-stimulated DA efflux occurred in the first 10 minutes, thereafter it progressively declined reaching prestimulated levels at 30 minutes. If two consecutive depolarizing stimuli of 40 mM KCl were applied to the same hypothalamic fragment, after a 40 minutes rest period, an equivalent release of endogenous DA occurred. Removal of Ca++ ions from the incubation medium containing the Ca++ chelator EGTA caused a decrease of basal DA efflux and completely prevented the K+-induced release of DA. Furthermore when verapamil, a blocker of Ca++ entrance, was added to the incubation medium in a concentration of 50 microM, the K+-induced DA efflux was completely counteracted, whereas spontaneous release was unmodified. Finally nomifensine, a potent blocker of DA uptake, added in vitro in a final concentration of 10 microM, significantly reinforced K+-induced release of endogenous DA. Since nomifensine did not modify basal DA release, this study confirmed its prevalent uptake blocking property rather than its releasing action on DA.

Dopaminergic neurons in the mediobasal hypothalamus of old rats: evidence for decreased affinity of tyrosine hydroxylase for substrate and cofactor

The effect of aging on the activity of tyrosine hydroxylase (TH) and on the number of TH-positive perikarya in the hypothalamus was studied in old and young female rats. The activity of TH in the mediobasal hypothalamus (MBH) of old rats was significantly (P less than 0.025) less than that in young rats. In old rats, the Km of TH for tyrosine as well as cofactor, 6-methyl-5,6,7,8-tetrahydropterine (6MPH4), was markedly greater than the Km in young rats. The maximal velocity was only slightly reduced in old animals. Contiguous coronal sections of the brain of an old and a young female rat were immunocytochemically stained for TH, and the TH-positive perikarya in the hypothalamus were counted. In the circumventricular region, 6793 TH-positive perikarya were present in the young brain and 6632 in the old brain. In the arcuate region, 2868 and 2760 TH-positive perikarya were counted in the young and old brain, respectively. It is concluded that the reduced TH activity in the MBH of old rats is not a consequence of a reduction in the number of TH-positive perikarya in the arcuate or circumventricular regions of the hypothalamus but is due to a reduction in the affinity of TH for its substrate and cofactor.

Mechanisms involved in the prolactin-releasing effect of benserazide

The mechanism(s) underlying the prolactin (PRL)-releasing effect of benserazide (Bz), a peripheral inhibitor of L-aromatic amino-acid decarboxylase, was investigated in the rat. In intact male and female rats, Bz was ineffective to increase significantly plasma PRL at 0.8 mg/kg i.p. but elicited an already maximal effect at 1.6 mg/kg. Bz added to in vitro incubated anterior pituitaries (APs) did not alter PRL secretion at the dose of 3.8 X 10(-6)M but increased PRL release at 10(-4)M. Bz, even at very high doses (up to 10(-3) M), did not displace [3H]spiroperidol binding from AP membrane preparations. In rats having had mechanical ablation of the medio basal hypothalamus (MBH), Bz (15 mg/kg i.p.) induced no rise in plasma PRL and did not counteract the striking inhibitory effect of a dopamine (DA) infusion (5 micrograms/kg per min per 120 min). Administration of Bz (15 mg/kg i.p.) into intact male rats decreased significantly the DA concentrations in the median eminence (ME) but not in the residual hypothalamus and the AP. In the same rats 1-dopa (50 mg/kg i.p.) increased significantly the DA concentrations not only in the ME but also in the hypothalamus and the AP. Bz given concurrently with 1-dopa markedly reduced the rise in DA concentrations induced by 1-dopa in the ME, and greatly potentiated the increase in DA concentrations in the hypothalamus. These data indicate that the mechanism whereby a single administration of Bz increases PRL secretion in the rat is not consistent with the postulated DA receptor antagonist action of the drug, but instead implies inhibition of the decarboxylation of 1-dopa at dopaminergic nerve terminals of the ME.

Uptake and release of [3H]dopamine by the median eminence: evidence for presynaptic dopaminergic receptors and for dopaminergic feedback inhibition

The accumulation and release of [3H]dopamine by the median eminence in vitro was studied after treatments with different pharmacological agents, to determine whether such a procedure would be useful for measuring neuronal activity in the tuberoinfundibular dopaminergic system. The accumulation of [3H]dopamine was temperature, time, and sodium dependent, and reduced by unlabelled dopamine and by a potent dopamine uptake blocker, nomifensine. The outflow of tritium was studied after blocking the oxidative deamination of dopamine by nialamide. The outflow of tritium was elicited consistently by biphasic square wave electrical pulses and by high molarity potassium ions. The response to electrical stimulation was dependent largely on calcium and partially on sodium. The response to high molarity potassium ions was reduced in the absence of calcium ions. The response to electrical stimulation was increased by nomifensine and by a dopaminergic antagonist, haloperidol, and was reduced by dopamine and by a dopaminergic agonist, piribedil. The inhibitory action of dopamine was antagonized by haloperidol. These results indicate the existence of uptake and release mechanisms in the tuberoinfundibular dopamine neurons, and suggest that dopamine may inhibit its own release via dopaminergic receptors. This in vitro method may be useful for measuring dopamine uptake and release by tuberoinfundibular dopaminergic neurons.

A comparison of domperidone and haloperidol effects on different dopaminergic neurons in the rat brain

Domperidone, a dopamine (DA) receptor antagonist with reportedly preferential actions outside of the blood-brain barrier, and haloperidol, a centrally active DA antagonist, were compared with respect to their abilities to increase the activity of dopaminergic neurons in the rat brain. The activity of nigrostriatal, mesolimbic, tuberohypophyseal and tuberoinfundibular dopamine nerves was estimated by measuring the in vivo rate of DA synthesis (dihydroxyphenylalanine accumulation following administration of an inhibitor of aromatic L-amino acid decarboxylase) in the striatum, olfactory tubercle, posterior pituitary and median eminence, respectively. In an initial study, the rates of DA synthesis in striatum, olfactory tubercle, and posterior pituitary were determined at 2, 8, and 16 h after subcutaneous administration of 0.25, 2.5, or 25 mg/kg domperidone. At the lowest dose of domperidone, DA synthesis was increased only in the posterior pituitary at 8 and 16 h; at the intermediate dose, DA synthesis increased in the posterior pituitary at 8 and 16 h and in the olfactory tubercle at 8 h. Only at 8 h after the highest dose of domperidone was DA synthesis increased in the striatum. When 2.5 mg/kg of domperidone or haloperidol were administered, DA synthesis in posterior pituitary and median eminence was increased in a similar fashion (in the latter region only at 16 h). In contrast, domperidone promoted only modest and delayed increases in DA synthesis in the olfactory tubercle and had no effect in the striatum. These results indicate that systemically administered domperidone preferentially increases DA synthesis in neurons terminating outside the blood-brain barrier, but after a pronounced delay, high doses of the drug can also activate DA neurons which project to the forebrain.

Measurement of 5-hydroxytryptamine synthesis and metabolism in selected discrete regions of the rat brain using high performance liquid chromatography and electrochemical detection: pharmacological manipulations

High performance liquid chromatography coupled with electrochemical detection (LCEC) was employed to measure 5-hydroxytryptamine (5-HT), 5-hydroxyindole-3-acetic acid (5-HIAA) and 5-hydroxytryptophan (5-HTP) in the suprachiasmatic (SCN), medial preoptic (MPO) and arcuate (AN) nuclei as well as the median eminence (ME) and striatum (ST) of individual rat brains. Biochemical estimations of changes in 5-HT neuronal activity were made by measuring: (1) concentrations of 5-HT and 5-HIAA and (2) the rate of 5-HT synthesis (5-HTP accumulation following the administration of NSD 1015, an inhibitor of aromatic L-amino acid decarboxylase) after the administration of pharmacological agents known to influence these neurons. Pargyline increased the concentration of 5-HT and decreased the concentration of 5-HIAA while probenecid increased the concentration of 5-HIAA in all 5 brain regions. At both 2 and 24 hours after reserpine the concentration of 5-HT decreased, 5-HIAA increased or did not change, and the rate of 5-HT synthesis increased. In most of the brain regions blockers of 5-HT neuronal uptake (fluoxetine, chlorimipramine) did not influence 5-HT or 5-HIAA concentrations dramatically, but increased the rate of 5-HT synthesis. L-tryptophan generally increased the concentrations of 5-HT and 5-HIAA as well as the rate of accumulation of 5-HTP in all regions except the ME where 5-HIAA and 5-HTP concentrations both were unaffected. These results reveal that the method using LCEC is sensitive enough to detect pharmacologically-induced changes in 5-HT metabolism and synthesis in discrete regions of rat brain. The drugs examined in the present study generally caused similar changes in 5-HT dynamics in all 5 brain regions examined.

The effect of morphine on 5-hydroxytryptamine synthesis and metabolism in the striatum, and several discrete hypothalamic regions of the rat brain

The effects of morphine on 5-hydroxytryptamine (5-HT) synthesis (accumulation of 5-hydroxytryptophan following inhibition of aromatic L-amino acid decarboxylase) and metabolism (concentration of 5-HT and its primary metabolite, 5-hydroxyindole-3-acetic acid [5-HIAA]) were determined in discrete nuclei of the rat brain using high performance liquid chromatography coupled with electrochemical detection (LCEC). Morphine (10 mg/kg, s.c.) increased 5-HT synthesis in the medial preoptic (MPO), suprachiasmatic (SCN) and arcuate (AN) nuclei as well as the striatum (ST) 1 hour following its administration. 5-HT synthesis in the median eminence (ME) was not affected at any time examined. A lower dose of morphine (5 mg/kg) also stimulated 5-HT synthesis in the AN. Although steady state concentrations of 5-HT were not greatly affected by morphine administration, the concentration of 5-HIAA in the AN, MPO, and ST increased following morphine (10 mg/kg, s.c., 1 hour). The increase in 5-HT synthesis observed in the MPO, SCN, AN, and ST 1 hour following morphine involved the activation of opiate receptors as administration of an opiate receptor antagonist, naloxone, blocked this effect. These results, indicate that morphine causes an increase in 5-HT synthesis and metabolism via an opiate receptor-mediated mechanism in the AN, MPO, SCN, and ST but not in the ME.

In situ voltammetric microelectrodes: application to the measurement of median eminence catecholamine release during simulated suckling

Catechol-sensitive microelectrodes (10-30 microns) were developed and then used to study the dynamic regulatory role of the prolactin inhibiting factor, dopamine, under conditions of simulated suckling. Current flow resulting from the electrochemical oxidation of catecholamines at the microelectrode surface was linearly related to the concentration of catecholamines present in solution over the range of 5-100 microM. Endogenous catecholamine levels in the rat median eminence were readily detectable and the electrochemical signal corresponding to dopamine release responded in an appropriate manner to various pharmacologic manipulations. We then implanted carbon microelectrodes into the medial median eminence region among capillaries of the primary portal plexus of urethane anesthetized lactating rats. Catecholamine release into the extracellular fluid was electrochemically measured once each minute before, during and after electrical stimulation (15 Hz, 5-30 V, 15 min) of a surgically isolated mammary nerve trunk. This simulated suckling paradigm reliably evoked prolactin secretory episodes qualitatively similar to those observed during suckling of the nipples by the young. During the period of nerve stimulation, a transient (3-5 min) 65% decline in electrochemically detectable catecholamine release was observed. Following cessation of nerve stimulation an oscillatory pattern of catecholamine release was observed with an overall trend toward an increased level of release. This latter observation corresponds with previous reports of increased hypothalamic dopamine turnover during or following suckling and with the increased levels of dopamine measured in hypophysial portal blood following mammary nerve stimulation. The transient nature of the decline of catecholamine release during the nerve stimulation period may explain why a similar observation has not been forth-coming from experiments utilizing the stalk blood collection technique (unless the standard collection periods are considerably shortened). These observations lead us to reject the hypothesis of a mirror image relationship between stalk blood dopamine and peripheral prolactin levels. Instead, we suggest that a transient decline in dopamine secretion coincident with the onset of suckling acts to prepare the pituitary lactotrophs to respond to a prolactin releasing factor which then facilitates prolactin secretion.

Dopaminergic neuronal function, anterior pituitary dopamine content, and serum concentrations of prolactin, luteinizing hormone and progesterone in the aged female rat

The serum concentrations of prolactin (PRL), progesterone and luteinizing hormone (LH), and the content and rate of synthesis of dopamine (DA) in selected brain regions were determined in young (3-6 months), intermediate (13-15 months) and aged (24-25 months) female Long-Evans rats. Young rats were examined on the days of diestrus 2 and estrus. Intermediate rats were divided into 2 groups, a group which was cycling regularly (examined on the day of estrus) and a group which exhibited constant estrus. Aged rats were divided into 3 groups one which cycled regularly (examined on day of estrus), one which exhibited constant estrus, and one which exhibited repetitive pseudopregnancies. Serum PRL was increased in all intermediate and aged rats when compared to values in young animals. Serum LH was increased and progesterone decreased in those intermediate and aged rats which exhibited constant estrous reproductive patterns. The DA content was generally decreased in the median eminence, posterior pituitary, striatum, nucleus accumbens and olfactory tubercle of all aged rats, while the rate of DA synthesis was decreased only in the median eminence of aged, non-cycling rats. This suggests that all DA neuronal systems except those in the tuberoinfundibular system are able to compensate for the age-related loss. Despite the apparent reduction of tuberoinfundibular DA neuronal function the concentration of DA in the anterior pituitary, which is believed to represent amine released from the neurons, is dramatically increased in intermediate age rats in constant estrus, and in all groups of aged rats. The maintenance of high PRL secretion despite the elevated content of DNA in the anterior pituitary suggests an age-related defect in the dynamics of DA in this gland; this defect may contribute to the loss of reproductive function in the aging rat.

Characterization of high affinity dopamine uptake into the dopamine neurons of the hypothalamus

The study of hypothalamic dopamine (DA) neurons is complicated by the difficulty in distinguishing DA neurons from norepinephrine (NE) neurons and by the fact that they comprise only a small proportion of the catecholamine neuron population of the hypothalamus. We have studied DA uptake into nerve terminals of hypothalamic DA neurons using a synaptosomal preparation. Desmethylimipramine (DMI) was used to prevent uptake into synaptosomes from NE neurons, thus pharmacologically isolating dopaminergic from noradrenergic nerve terminals. This DMI-insensitive DA uptake in hypothalamus had all the properties of a high affinity uptake process; it was saturable, dependent on incubation time and incubation temperature, increased linearly with increasing amounts of tissue and was completely abolished by excess unlabeled DA. Also, it was completely abolished by benztropine, an inhibitor of amine uptake into DA neurons. We believe that DMI-insensitive DA uptake into hypothalamic synaptosomes represents uptake into DA nerve terminals. The DMI-insensitive DA accumulation in discrete areas of hypothalamus correlated well with the known prevalence of DA neurons relative to NE neurons in these areas: median eminence greater than median eminence-arcuate nucleus greater than mediobasal hypothalamus greater than whole hypothalamus. Comparison of the affinity constants for DA uptake into synaptosomes incubated without DMI revealed a 2-fold higher affinity constant for DA uptake in median eminence compared with striatum, but affinity constants in all the other hypothalamic regions examined (mediobasal hypothalamus, hypothalamus minus median eminence, whole hypothalamus) were identical to that of striatum. In contrast, comparison on the affinity constants for DA uptake in the presence of DMI revealed a 2-3-fold higher affinity constant for DA uptake in all these hypothalamic regions compared with striatum. It appears the tuberoinfundibular DA neurons and the other DA neurons of the hypothalamus have a high affinity uptake system for DA, although affinity for DA in all of these hypothalamic DA neurons appears to be 2-3-fold lower than that in striatal DA neurons. The data also suggest that the much larger mediobasal hypothalamus may serve as a model for studies of DA uptake into tuberoinfundibular DA neurons of the median eminence.

Secretion of hypothalamic dopamine into pituitary stalk blood of aged female rats

Dopamine secreted into the pituitary stalk blood of old constant estrous rats (20-24 months of age) was significantly less than that of young estrous rats (3-4 months of age). Reduced concentrations of dopamine were also observed in the median eminence and in the neurointermediate lobe of the pituitary gland of old female rats compared to those of young female rats. The low rate of secretion of dopamine into pituitary stalk blood of old female rats was associated with high secretion of prolactin into arterial blood. The impaired hypothalamic secretion of dopamine observed in old rats was not affected by increased availability of L-tyrosine. However, when L-DOPA was given to old rats, very high concentrations of dopamine were measured in pituitary stalk blood, whereas the concentrations of dopamine in the arterial blood were very low. On the basis of these data, it is concluded that the neurosecretory activity of the dopaminergic neurons of the hypothalamus is impaired in old constant estrous rats. This impaired activity can be overcome by increasing the availability of L-DOPA but not L-tyrosine.

Effect of inhibitors of catecholamine synthesis on the pituitary response to LH-RH

Cathecolamines have an important role in the regulation of LH secretion. The effect of two selective blocking drugs of cathecolamines synthesis upon pituitary response to LH-RH was studies. Basal levels of serum LH in rats treated with alpha-methyl-p-tyrosine and diethyldithiocarbamate were significantly lower than those of control rats. Diethyldithiocarbamate induced a greater decrease of LH levels than alpha-methyl-p-tyrosine. The rats treated with diethyldithiocarbamate showed an LH increase after LH-RH stimulation that was significantly higher as compared with control group as well as in alpha-methyl-p-tyrosine treated group.

Morphine differentially alters synthesis and turnover of dopamine in central neuronal systems

On the basis of biochemical indices of dopamine (DA) nerve activity (decline of DA after inhibition of tyrosine hydroxylase, accumulation of DOPA after inhibition of DOPA decarboxylase) it was revealed that morphine increases the activity of nigrostriatal and mesolimbic DA nerves which terminate in the striatum, nucleus accumbens and olfactory tubercle, but reduce the activity of tuberoinfundibular DA nerves which terminate in the median eminence. Morphine had no effect on tuberohypophyseal DA nerves which project to the posterior pituitary. Naloxone was without effect per se, but blocked the effects of morphine on DOPA accumulation. Thus, morphine differentially alters the diverse DA neuronal systems in the rat brain.