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

A New Method for the Visualization of Living Dopaminergic Neurons and Prospects for Using It to Develop Targeted Drug Delivery to These Cells

This is the first study aiming to develop a method for the long-term visualization of living nigrostriatal dopaminergic neurons using 1-(2-(bis(4-fluorophenyl)methoxy)ethyl)-4-(3-phenylpropyl)piperazine-BODIPY (GBR-BP), the original fluorescent substance, which is a derivative of GBR-12909, a dopamine uptake inhibitor. This method is based on the authors’ hypothesis about the possibility of specifically internalizing into dopaminergic neurons substances with a high affinity for the dopamine transporter (DAT). Using a culture of mouse embryonic mesencephalic and LUHMES cells (human embryonic mesencephalic cells), as well as slices of the substantia nigra of adult mice, we have obtained evidence that GBR-BP is internalized specifically into dopaminergic neurons in association with DAT via a clathrin-dependent mechanism. Moreover, GBR-BP has been proven to be nontoxic. As we have shown in a primary culture of mouse metencephalon, GBR-BP is also specifically internalized into some noradrenergic and serotonergic neurons, but is not delivered to nonmonoaminergic neurons. Our data hold great promise for visualization of dopaminergic neurons in a mixed cell population to study their functioning, and can also be considered a new approach for the development of targeted drug delivery to dopaminergic neurons in pathology, including Parkinson’s disease.

Determination of release and uptake parameters from electrically evoked dopamine dynamics measured by real-time voltammetry

Quantifying mechanisms underlying extracellular signaling by the neurotransmitter dopamine (DA) is a difficult task, particularly in the complex extracellular microenvironment of the intact brain. In this study, two methods for evaluating release and uptake from DA dynamics monitored by real-time voltammetry are described. Both are based on a neurochemical model characterizing electrically evoked levels of DA as a balance between these opposing mechanisms. The theoretical basis of what is called here nonlinear regression and single curve analyses is given. Fitting simulated data tests the reliability of the methods. The two analyses are also compared with an experimental data set describing the effects of pharmacologically inhibiting the DA transporter in the caudate-putamen (CP) and nucleus accumbens (NAc). The results indicate that nonlinear regression and single curve analyses are suitable for quantifying release and uptake mechanisms underlying DA neurotransmission. Additionally, the most important experimental finding of this technical study was the independent confirmation of high affinity (approximately 0.2 microM) DA uptake in the intact striatum.

Cocaine's effects on neuroendocrine systems: clinical and preclinical studies

This review examines the effects of cocaine on the neuroendocrine system and summarizes findings from clinical studies of cocaine abusers and preclinical studies in rodents and rhesus monkeys. The effects of acute and chronic cocaine administration on anterior pituitary, gonadal, and adrenal hormones are described, and the functional consequences of chronic cocaine exposure are discussed. Many of cocaine's acute effects on the endocrine system are consistent with its actions as a monoamine reuptake inhibitor. Acute cocaine administration stimulates release of gonadotropins, ACTH, and cortisol or corticosterone and suppresses prolactin levels. It has been difficult to detect changes in basal levels of most hormones or alterations in hormone responsiveness to a challenge dose of cocaine or other agents after chronic cocaine treatment. Interpretation of clinical data is often complicated by polydrug abuse involving opiates and alcohol as well as cocaine. However, preclinical studies of the effects of chronic cocaine exposure on integrated neuroendocrine function have revealed disruptions of the estrous cycle in rats and the menstrual cycle in rhesus monkeys. Furthermore, the menstrual cycle disorders observed in rhesus monkeys parallel those reported in women who abuse cocaine. Much remains to be learned about cocaine's interactions with the endocrine system and the consequences of cocaine abuse for reproductive function.

Lack of evidence for dopamine autoreceptors in the mediobasal hypothalamus: a microdialysis study in awake rats

To determine the functional presence of dopamine (DA) autoreceptors on tuberoinfundibular dopamine (TIDA)-ergic neurons in awake rats, a microdialysis probe was implanted into the mediobasal hypothalamus (MBH). In the presence of the re-uptake inhibitor nomifensine, which increased DA levels to 350% of basal values, systemic administration of the non-selective D1/D2 antagonist haloperiol induced an immediate increase in DA and DOPAC levels to 145% of pretreatment values. However, neither local infusion of the selective D2 antagonist sulpiride nor of the D1 antagonist SCH 23390 affected the nomifensine-elevated extracellular DA or DOPAC levels in the MBH. Systemic administration of the D2 antagonist raclopride equally did not affect the nomifensine-elevated DA release in the MBH. Upon basal extracellular DA levels (without nomifensine), local infusion of the D2 agonist (-)N-0437 equally did not affect the DA or DOPAC levels in the MBH. Furthermore, the increase in DA levels induced by haloperidol could not be antagonized by the D1 agonist CY 208-243. Therefore, the present study does not provide support for the concept of functional autoreceptors located on TIDA neurons regulating the release of DA. Possibly, the effect of haloperidol was non-DA-ergic in character.

Detection of neutral endopeptidase (NEP, enkephalinase, E.C.3.4.24.11) in relation to dopaminergic and gonadoliberinergic nerve endings in the median eminence of the male rat: a double labeling ultrastructural study

The existence of neutral endopeptidase (Enkephalinase, NEP, E.C.3.4.24.11) in membranes of nerve endings in the rat median eminence suggests that some neuropeptides have paracrine and/or autocrine actions in this region. In vitro, neutral endopeptidase is capable of hydrolysing a variety of regulatory peptides but in vivo, many works indicate that in the central nervous system this enzyme is highly implicated in the biological inactivation of enkephalins and tachykinins. In addition there is evidence that NEP is also involved in the inactivation of neurotensin in vivo. The modulation of the release of gonadotrophin releasing hormone (GnRH) is one of the documented actions of enkephalins within the median eminence. However, it is at present unclear whether enkephalins act on dopamine endings, on GnRH endings or on both. As the technical parameters and particularly the tissue fixation used to detect neutral endopeptidase are compatible with immunocytochemical detection of GnRH and tyrosine-hydroxylase (the rate limiting enzyme in the synthesis of catecholamines), two double immunolabelings were realised at the ultrastructural level to determine if GnRH and dopamine nerve endings have the enzyme inserted within their plasma membrane. Our study shows the presence of neutral endopeptidase on tyrosine-hydroxylase-immunoreactive nerve endings while presence of the enzyme on GnRH-immunoreactive nerve endings is not demonstrated. Consequently, our results provide morphological arguments for possibilities of paracrine and/or autocrine actions by neuropeptides inactivated by neutral endopeptidase on tuberoinfundibular dopaminergic nerve endings. Conversely, action of the same peptides on GnRH boutons seems more unlikely.

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 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.

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.

Systematic presence of GABA-immunoreactivity in the tubero-infundibular and tubero-hypophyseal dopaminergic axonal systems: an ultrastructural immunogold study on several mammals

Immunoreactivities for tyrosine hydroxylase (TH), gamma-aminobutyric acid (GABA) and, in some cases, glutamic acid decarboxylase (GAD) were detected by light and electron microscopy in axons projecting into the median eminence and pituitary gland of various mammals (rats, mice, guinea pigs, cats, rabbits and hares). Light microscope immunoperoxidase reactions were performed on adjacent semithin sections of plastic-embedded samples. In the median eminence external zone, the distributions of the TH- and GAD- or GABA-immunoreactive endings were very similar in the anterior and lateral areas, while medially the GABA-labelled endings predominated. Comparable distribution patterns were found in the various species examined. In the pituitary gland, the distributions of GABA- and TH-immunoreactivities were superimposable in the intermediate lobes of all species examined, except in the rabbit and hare in which both types of innervation were lacking. For electron microscopy, the immunogold procedure was applied to sections of lowicryl-embedded samples; simultaneous detection of GABA- and TH-immunoreactivities was enabled by recto-verso double labelling with gold particles of distinct diameters. In the median eminence, GABA-immunoreactivity occurred systematically in the TH-positive endings, while distinct GABA-positive/TH-negative axons were also detected. In the intermediate lobe, the colocalization of TH- and GABA-immunoreactivities was a constant feature of the axons innervating the melanotrophic cells in all the species examined, except in the Leporidae. The functional significance of this colocalization remains to be determined.

D-2 dopamine autoreceptor selective drugs: do they really exist?

The catecholamine dopamine plays an important role as a neurotransmitter or neurohormone in the brain and pituitary gland. Dopamine exerts its effects through activation of two types of receptors called D-1 and D-2. These receptors are distinguished by their different pharmacological characteristics and signal transduction mechanism(s). Release of dopamine inhibits the activity of dopaminergic neurons through activation of so-called dopamine autoreceptors which are of the D-2 type. In general, these receptors occur both in the soma-dendritic region of the dopaminergic neuron, where they are involved in the inhibition of the firing rate and on the dopaminergic terminals where they mediate the inhibition of dopamine synthesis and release. D-2 receptors occur also on the target cells of dopaminergic neurons both in the brain (postsynaptic D-2 receptors) and pituitary gland. On the basis of data gathered from in vivo (behavioral- as well as electrophysiological) studies it has been concluded that D-2 agonists are much more potent at dopamine autoreceptors as compared to postsynaptic D-2 receptors, indicating the possibility of a pharmacological distinction between these differentially located D-2 receptors. This concept led to the introduction of a whole group of drugs allegedly displaying a selective agonist profile at the dopamine autoreceptor. In contrast, biochemical (in vitro) studies with brain tissue as well as the pituitary gland, did not reveal any significant difference between the pharmacological profiles of autoreceptors and postsynaptic D-2 receptors. In the present minireview a balanced discussion is presented of these in vivo and in vitro findings and it is concluded that both autoreceptors as well as postsynaptic D-2 receptors are similar if not identical entities.

Sodium and calcium dependence of purine release from rat cerebral cortical slices

Electrically evoked purine release from rat cerebral cortical slices was evaluated, using a HPLC analysis combined with radioactivity measurement of the identified fractions. Two different pools of released purines have been identified: one probably related to cell metabolism and the other strictly linked to the nervous transmission. Since a linear increase, due to the stimulation frequencies, was found for the purines released from this second pool, a possible dependence on sodium and calcium transmembrane fluxes was evaluated. Pretreatment of the slices with TTX (5 x 10(-7) M) caused only a partial inhibitory effect on purine release (50%). This effect was probably related to the drug activity on the neuronal component of slices, since TTX induces an almost complete inhibition of purine release from isolated neurons "in cultures" and does not affect it from glial cells. Verapamil (1 x 10(-4) M), a calcium-channel blocker at glial and neuronal level, and TEA (3 x 10(-2) M), a specific inhibitor of calcium-mediated potassium efflux from glial cells, administered to the slices alone or in combination, showed a partial calcium-dependence of purine release. These results suggest a glial role in modulation of electrically-evoked purine release. These cells could exert a "buffering action" that regulates the calcium-mediated potassium availability, by which neuronal activity might be influenced.

Peptide- and transmitter-containing neurons in the mediobasal hypothalamus and their relation to GABAergic systems: possible roles in control of prolactin and growth hormone secretion

Indirect immunofluorescence histochemistry was used to study the relation among GABAergic, catecholaminergic, cholinergic, and peptidergic neurons in the rat mediobasal hypothalamus. By employing a direct double-labelling procedure using sheep antiserum against glutamic acid decarboxylase (GAD), mouse monoclonal and rabbit antibodies to neurotensin (NT) and rabbit antisera to tyrosine hydroxylase (TH), choline acetyltransferase (ChAT), galanin (GAL), growth hormone-releasing factor (GRF), or somatostatin (SOM), it was demonstrated that GAD-positive fibers and terminals in the external part of the median eminence co-contained immunoreactivity for TH, NT, GAL or GRF, but not for SOM. In the internal part of the median eminence-infundibular stalk, GAD-positive/NT-, GAL-, and GRF-negative and GAD-positive/TH-positive fiber plexa were shown. When a recently developed direct triple-labelling procedure with biotin-conjugated mouse secondary antibodies in conjunction with diethylaminocoumarin (DAMC)-conjugated avidin was employed, presence of GAD/GAL/NT- as well as GAD/GRF/NT-containing varicosities could be demonstrated close to hypophysial portal vessels. In colchicine-pretreated animals, GAD was shown to coexist with TH, NT, or GAL in cell bodies in both the dorsomedial and ventrolateral domains of the arcuate nucleus, but with GRF only in the ventrolateral division. ChAT-positive neurons in the ventrolateral region were also TH-positive. In the ventrolateral arcuate nucleus, triple-labelling followed by elution-restaining showed GAD/NT/GAL/TH-immunoreactivities in the same cells. Similarly, double-labelling with two following elution-restaining steps showed several NT/GAL/GRF/TH-containing cell bodies in this part of the arcuate nucleus. GAD-positive cells in the anterior hypothalamic periventricular area and fibers in the pituitary neurointermediate lobe were also TH-positive. The results demonstrate complex patterns of storage of chemical messengers in neurons of the arcuate nucleus-median eminence complex. Possible neuroendocrine interactions of these systems in the control of prolactin and growth hormone secretion are discussed.

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.

D2-dopamine receptors regulate the release of [3H]dopamine in rat basal hypothalamus and neurointermediate lobe of the pituitary gland

Homogenates of the rat basal hypothalamus and the neurointermediate lobe of the pituitary gland contained relatively high levels of dopamine as was estimated by high-performance liquid chromatography (HPLC). The release of [3H]dopamine was studied in these regions and desipramine was used to prevent uptake of [3H]dopamine in noradrenergic nerve terminals. The release of radioactivity could be stimulated electrically and was calcium-dependent. It appeared that the release of radioactivity could be inhibited by drugs stimulating D2-dopamine receptors in both regions. The radioactivity released during electrical stimulation was analysed by cation exchange chromatography and appeared to consist predominantly of [3H]dopamine. It is our conclusion that D2-receptors mediate the inhibition of the release of [3H]dopamine from dopaminergic nerve terminals in the basal hypothalamus and in the neurointermediate lobe.

Selective effects of ECT on hypothalamic-pituitary activity

The hypothesis that ECT produces selective effects on hypothalamic-pituitary activity was investigated by determining the effect of ECT on pituitary hormone release in nine depressed patients. After ECT there were massive and rapid increases in the plasma concentrations of nicotine- and oestrogen-stimulated neurophysin (NSN and ESN), prolactin (PRL) and adrenocorticotropin (ACTH), smaller increases in plasma luteinizing hormone (LH) and cortisol, a significant decrease in plasma growth hormone (GH) concentration but no change in plasma thyrotropin (TSH). There was significant attenuation of PRL responses with repeated ECT. The hormonal responses to ECT cannot simply be attributed to stress, since a similar pattern of increases in plasma hormone concentrations did not occur in psychologically normal patients in whom plasma hormone concentrations were measured during induction of anaesthesia and abdominal incision for cholecystectomy. Analysis of these hormonal responses in terms of the knowledge available on the neurotransmitter control of pituitary hormone release suggests that some of these hormonal responses to ECT may be mediated by the activation of serotonergic neurones, while others are probably due to direct stimulation of the neuroendocrine neurones themselves.

Biochemical characterization of the uptake and release of [3H]dopamine by dopaminergic hypothalamic neurons: a developmental study using serum-free medium cultures

The development of the biochemical properties of mouse hypothalamic dopaminergic neurons has been analyzed in vivo and in cultures of cell taken on the 16th day of gestation and grown in serum-free medium for up to 3 weeks. In the course of in vivo development, the dopamine (DA) content remains low during fetal life (10% of the adult value), beginning to increase on the 19th fetal day. In contrast, the specific accumulation of [3H]DA increased markedly during the last days of gestation from 20% of the adult value on the 16th fetal day to 70-80% of the adult value on Postnatal Day 3. Hypothalamic DA neurons in culture accumulate endogenous DA although at a lower level than in vivo. They take up [3H]DA by an active transport system which is specific for DA, and which shows time, temperature, and sodium dependency (Km = 1 microM). HPLC analysis showed that the newly taken up [3H]DA was not metabolized in the short run under the conditions used. It was stored in a form that could be released when neurons were depolarized in a high K+ (60 mM) medium. The K+-evoked [3H]DA release was found to be strictly dependent on extracellular Ca2+. Moreover the release of [3H]DA was also stimulated by veratridine in a Ca2+-dependent manner. Similar data have been obtained with the release of endogenous dopamine. No specific uptake and no K+-evoked dopamine release occurred in 2-day-old cultures. The specific [3H]DA uptake and the K+-evoked release appeared in 5-day-old cultures and increased with time in culture at least until Day 15. We examined the effects on [3H]DA release of polyunsaturated fatty acid, triiodothyronine, and corticosterone, all of which have been shown to play an important role in synaptogenesis in culture. These components, either separately or together, did not modify the percentage of the basal or the stimulated [3H]DA release. These results showed that hypothalamic DA neurons grown in serum-free medium progressively acquired the functional properties of adult DA neurons as concerns DA synthesis, DA uptake, and release. From a development point of view, this study suggests that the capacity to specifically take up [3H]DA and to respond to high K+ concentration is not expressed at early stages of neuronal development.(ABSTRACT TRUNCATED AT 400 WORDS)

D-2 dopamine-receptors regulate the release of [3H]dopamine in rat cortical regions showing dopamine immunoreactive fibers

Using an antibody raised against dopamine the occurrence of dopamine-containing fibers was demonstrated in the prefrontal cortex, anterior cingulate cortex, parietal neocortex, piriform cortex and entorhinal cortex. In extracts of these cortical regions significant amounts of dopamine, although approximately a 100-fold less than in the neostriatum or nucleus accumbens, were detected with high performance liquid chromatography. The release of [3H]dopamine from slices of all these cortical regions was studied in vitro in a superfusion system and desipramine was used to prevent the uptake of [3H]dopamine in noradrenergic nerve terminals. It appeared that the electrically evoked release of radioactivity was inhibited by drugs stimulating D-2 dopamine-receptors in all the regions studied. Cation-exchange column chromatography revealed that the radioactivity released consisted predominantly of [3H]dopamine, indicating that D-2 receptors mediate the inhibition of the release of [3H]dopamine from dopaminergic nerve terminals. Likewise, in the neostriatum as well as in the nucleus accumbens D-2 receptor stimulation inhibits the release of [3H]dopamine. Therefore it is our conclusion that D-2 receptors regulate the release of dopamine from dopaminergic neurons originating in the ventral tegmental area as well as in the substantia nigra.

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.

The hypothalamo-hypophyseal rat explant in vitro: endocrinological studies of the pars intermedia dopaminergic neural input

Short-term in vitro incubation of hypothalamo-hypophyseal tissue from young rats was undertaken to discern more clearly the functional relationship between putative dopaminergic neural projections in the pars intermedia and the secretory activity of melanophore stimulating hormone (MSH). This explant consisted of a portion of the mediobasal hypothalamus containing the dopamine neurone cell bodies of interest, with the attached pituitary neuro-intermediate lobe (n.i.l.). The n.i.l. was inserted into the end of a 1 mm diameter tube attached to a perfusion pump which allowed uninterrupted sampling of medium neighbouring the n.i.l. A 'real-time' analysis of hormone secretion was obtained by immediately and continuously bioassaying for MSH. A bipolar stimulating electrode was placed on the ventral floor of the mediobasal hypothalamus either directly on the arcuate nucleus, median eminence or infundibular stalk. Electrical stimulation for 5 min (0.1-20.0 Hz) caused a transient inhibition of basal MSH secretion, while continuous stimulation (0.1-5.0 Hz) led to a much greater, long-term, reversible inhibition. In the latter, the degree of inhibition was generally dependent on stimulation rate up to a maximum at 5 Hz. Application of the dopamine D2 receptor antagonist, 1-sulpiride (0.001-0.1 microM) to the perfusion medium not only completely and reversibly blocked the stimulus-induced inhibition of MSH release but by itself, significantly increased the basal secretion rate. Applied to the isolated n.i.l., 1-sulpiride did not alter release but did prevent the inhibitory response caused by exogenously applied dopamine (0.1 microM). The gamma-aminobutyric acid receptor antagonist, bicuculline (0.01-1.0 microM), had no effect on any of the parameters studied. In explants, cutting the infundibular stalk linking the mediobasal hypothalamus with the n.i.l., mimicked the effects of 1-sulpiride by interrupting impulse flow to the gland. Thus, electrical stimulation of hypothalamic neurones in these explants apparently causes a release of dopamine from nerve terminals in the pars intermedia to inhibit MSH secretion and perhaps other pro-opiomelanocortin-derived peptides as well.

Dopamine and the action of opiates: a reevaluation of the dopamine hypothesis of schizophrenia. With special consideration of the role of endogenous opioids in the pathogenesis of schizophrenia

It is suggested that the antipsychotic efficacy of opioids in patients suffering from schizophrenia may result from an interaction of opioids with the dopaminergic system. The modulatory effect of opioids on dopaminergic functions has already been demonstrated in basic experiments: Anatomical and biochemical data reveal an interaction between opioid receptors and dopamine (DA) actions on dopaminergic nerve terminals, cell bodies, and afferent nerve endings. Endogenous enkephalin levels correlate well with the endogenous dopamine content in various brain areas. Systemic or iontophoretic administration of morphine alters the spontaneous activity of ventral tegmental dopaminergic neurons. Morphine and enkephalin effectively enhance pituitary prolactin release, whereas dopamine inhibits it. Opioid agonists effectively alter DA release, DA reuptake, and DA metabolism in the striatum and substantia nigra. In reverse, chronic neuroleptic treatment enhances the synthesis and release of pituitary beta-endorphin. Opioids affect contralateral rotation elicited by dopamine agonists in animals with unilateral lesions of the nigrostriatal pathway. Phencyclidine, a psychotropic drug that shares certain pharmacological characteristics with the putative sigma-opioid receptor ligand SKF 10,047, indirectly mimics the effects of dopamine agonists on prolactin release, release of acetylcholine, etc. It is suggested that an imbalance of opiate-DA interaction might be involved in the pathogenesis of schizophrenia. Consequently, clinical studies on the effects of opioids on psychotic symptoms should also examine opioid influence on dopaminergic functions in these patients.

Effects of GABA receptor agonists on [3H]dopamine release from median eminence and pituitary neurointermediate lobe

The effects of GABAA and GABAB receptor agonists were examined on stimulus-induced release of [3H]dopamine ([3H]DA) from a synaptosomal preparation of median eminence (ME) and pituitary neurointermediate lobe (NI). Muscimol was found to inhibit [3H]DA release from ME in a bicuculline-sensitive, strychnine-insensitive manner, but had no effect in NI. Homocarnosine also inhibited [3H]DA release from ME. Baclofen had no effect in either area. These results demonstrate an interaction between two putative prolactin release inhibitory factors, DA and GABA, at the site of secretion into hypophysial portal blood.

In vitro dopamine biosynthesis in the median eminence of rat hypothalamic slices: possible involvement of a Na+-Ca2+ exchange mechanism

Equimolar replacement of Na+ in medium with choline chloride or sucrose and experimental manipulations known to increase [Na+]i, such as ouabain addition and K+ deprivation from medium, caused a marked increase in in vitro DOPA synthesis in the median eminence of rat hypothalamic slices in a Ca2+-dependent manner. These results suggest that a Na+-Ca2+ exchange mechanism is closely involved in the regulation of dopamine biosynthesis in tuberoinfundibular neurons.

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.

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.