Lack of autoreceptor mediated regulation of the spontaneous dopamine turnover in the isolated neurointermediate lobe of the rat pituitary gland in vitro

Summary Isolated neurointermediate lobes of the rat pituitary gland were incubated in Krebs-HEPES solution and the spontaneous outflow of endogenous dopamine and its metabolites (DOPAC, HVA and MOPET) was determined by HPLC with electrochemical detection.The spontaneous outflow of dopamine metabolites (about 1500 fmol/10 min) largely exceeded that of dopamine (about 60 fmol/10 min). Apomorphine concentration-dependently (IC₅₀, 205 nmol/l) reduced the spontaneous out-flow of the dopamine metabolites. The effect of apomorphine developed slowly and was progressive over an observation period of 70 min. After 1 h of exposure to a maximall effective concentration of apomorphine (10 gmol/l), the out-flow of metabolites was inhibited by 43%. The effect of apomorphine was not affected by the dopamine D₂ receptor antagonist (–)-sulpiride nor by the dopamine D₁ receptor antagonist SCH 23390. Neither quinpirole nor fenoldopam significantly affected the spontaneous outflow of dopamine metabolites.It was previously shown that the high rate of spontaneous outflow of dopamine metabolites from the dopaminergic nerves in the neurointermediate lobe reflects largely the immediate catabolism of newly synthesized dopamine. This high rate of spontaneous dopamine synthesis in the neurointermediate lobe is not controled by dopamine autoreceptors. Apomorphine appears to inhibit the spontaneous dopamine turnover by an inhibition not mediated by dopamine receptors.Abbreviations DOPAC 3,4-dihydroxyphenylacetic acid - HVA homovanillic acid - MOPET 3-methoxy-4-hydroxyphenylethanol - NIL neurointermediate lobe Send offprint requests to K. Racké at the above address     

The role of cytoplasmic (newly synthesized) dopamine for the spontaneous and electrically evoked release of dopamine and its metabolites from the isolated neurointermediate lobe of the rat pituitary gland in vitro

Summary Isolated rat NILs were incubated in Krebs-HEPES solution. The release of dopamine and its metabolites (DOPAC, HVA and MOPET) was determined by HPLC with electrochemical detection.The spontaneous release of the sum of metabolites was about 40 times that of dopamine. The spontaneous outflow of dopamine metabolites was unaffected after inhibition of dopamine uptake (by GBR 12921) or after pretreatment with reserpine (5 mg/kg, 12 h before the experiments), but it was reduced by 50% after preincubation with the irreversible DOPA decarboxylase inhibitor, (MFMD, 10 M, for 10 min). The combination of pretreatment with reserpine and preincubation with MFMD resulted in an 80% inhibition of the spontaneous outflow of dopamine metabolites. Treatment with reserpine caused a 98% depletion of the dopamine tissue content, whereas 60 min after exposure to MFMD the dopamine tissue content was decreased by 40%.Electrical stimulation of the pituitary stalk (3–15 Hz, in the presence of GBR 12921) caused a frequency-dependent release of dopamine. Stimulation at 7 or 15 Hz caused also a significant release of dopamine metabolites. After pretreatment with reserpine, the release of dopamine evoked by stimulation at 15 Hz was abolished, whereas the evoked release of the metabolites was only reduced by about 55%. After MFMD, the evoked release of dopamine decreased by a percentage similar to that of dopamine tissue content, but the reduction of the evoked release of metabolites was more pronounced.In conclusion, the spontaneous release of dopamine metabolites from the dopaminergic nerve endings in the NIL largely reflects the catabolism of newly synthesized dopamine. Therefore, a permanent dopamine synthesis (also under resting conditions) is required to maintain the vesicular dopamine pool from which the stimulation-evoked transmitter release is supposed to occur.Abbreviations DOPAC Dihydroxyphenylacetic acid - HVA homovanillic acid - MAO monoamine oxidase - MFMD a-monofluoromethyldopa - MOPET 3-methoxy-4-hydroxyphenylethanol - NIL neurointermediate lobe Send offprint requests to K. Racké at the above address     

SCH 23390 blocks D-1 and D-2 dopamine receptors in rat neostriatum in vitro

Summary The agonistic and antagonistic effects of the new compound SCH 23390 were tested in functional model systems for the D-1 dopamine receptor and for the D-2 dopamine receptor in vitro. In superfused rat neostriatal slices the increase in the efflux of cyclic AMP was used as a parameter for D-1 receptor stimulation. D-2 receptor stimulation was measured as the decrease in the K⁺-evoked release of [³H]-acetylcholine. SCH 23390 had no agonistic activity in these two models. SCH 23390 was a potent antagonist of the stimulating effect of dopamine in the D-1 receptor model (apparent pA₂=7.28). SCH 23390 also antagonized the effect of the D-2 receptor agonist LY 141865 in the D-2 receptor model (apparent pA₂=6.34). This D-2 receptor antagonism proved to be of a competitive nature.     

Cyclic AMP facilitates the electrically evoked release of radiolabelled noradrenaline,dopamine and 5-hydroxytryptamine from rat brain slices

Summary The adenylate cyclase activator forskolin as well as 8-bromo-cyclic AMP enhanced the electrically evoked release of³H-noradrenaline and³H-5-hydroxytryptamine from superfused rat neocortical slices and that of³H-dopamine from neostriatal slices with comparable EC₅₀'s of about 0.5 and 50 M, respectively, without affecting spontaneous tritium efflux. The phosphodiesterase inhibitor ZK 62771 (3–100 M) also enhanced³H-noradrenaline and³H-dopamine release but slightly reduced³H-5-hydroxytryptamine release. However, this drug profoundly enhanced spontaneous tritium release in the latter case. The facilitatory effect of forskolin (0.3 M) on the release of the amine neurotransmitters was potentiated in the presence of ZK 62771 (30 M). Therefore, cyclic AMP appears to exert a general facilitatory effect on the release of these biogenic amines from central nerve terminals.     

Presynaptic regulation of electrically evoked dopamine overflow in nucleus accumbens: a pharmacological study using fast cyclic voltammetry in vitro

Summary Fast cyclic voltammetry has been used to measure electrically evoked dopamine overflow from slices of rat nucleus accumbens in vitro. The substance detected was shown voltammetrically and biochemically to be dopamine of neuronal origin. Enough dopamine was released by a single electrical pulse to be easily detectable, and under these conditions there was no auto-inhibition by the endogenous transmitter (as demonstrated by the failure of dopamine antagonists to increase the amount released). There was no significant inhibition, or enhancement, of release by agonists at the following receptor types: dopamine D₁, 5-hydroxytryptamine, cholinoceptors, ₁-, ₂-, -adrenoceptors, cholecystokinin or neurotensin receptors. However, the dopamine D₂ receptor agonist, quinpirole, was capable of totally inhibiting the release; this effect was concentration-dependently antagonized by the D₂ antagonists haloperidol, sulpiride, metoclopramide and clozapine, with potencies which corresponded to their affinities for D₂ receptors in striatal tissue. The results show that the presynaptic receptors on dopaminergic nerve terminals are of the D₂ type and apparently identical to those in the corpus striatum.     

Inhibition of evoked dopamine release by monopropionylcadaverine in vitro

A naturally occurring diamine, cadaverine, and one of its acyl derivatives, monopropionylcadaverine, were tested for their effects on the in vitro release of endogenous dopamine from slices of the rat neostriatum. Dopamine release was allowed to occur spontaneously and was evoked by elevating the potassium concentration in the incubation medium or by electric field stimulation. Monopropionylcadaverine had no effect on spontaneous release of dopamine and little effect on potassium-evoked release of dopamine, but at concentrations as low as 10^?8 M in the medium it significantly depressed the electrically induced dopamine release.     

Ethanol and negative feedback regulation of mesolimbic dopamine release in rats

 The objectives of this study were to examine the relationship between somatodendritic and terminal field dopamine (DA) release following manipulation of DA D₂ receptors in the ventral tegmental area (VTA), systemic administration of ethanol, and inhibition of DA uptake in the nucleus accumbens (ACB). Perfusion of 5, 25 and 100 μM quinpirole (a D₂ agonist), or sulpiride (a D₂ antagonist) through the microdialysis probe in the VTA produced dose-related decreases or increases, respectively, in the extracellular levels of DA in both the VTA and ACB of adult Wistar rats. The IP administration of 2–3 g/kg ethanol produced a sustained increase in the extracellular levels of DA (150–200% of baseline) in the ACB for at least 2 h after injection, whereas only a transient increase was observed in the VTA. Local perfusion of the ACB with 100 μM GBR12909, a DA uptake inhibitor, elevated the extracellular levels of DA in the ACB to approximately 400% of baseline, but decreased the extracellular levels of DA in the VTA to approximately 50% of baseline. Overall, the results suggest that (a) there is an association between somatodendritic and terminal field DA release when D₂ cell body autoreceptors in the VTA are manipulated, (b) elevating synaptic levels of DA in the terminal field activates a long-loop negative feedback system to the VTA, and (c) different mechanisms may be mediating the actions of ethanol on DA neuronal activity and terminal DA release. Received: 18 November 1997 / Final version: 29 January 1998     

Increase in dopamine release from the nucleus accumbens in response to feeding: a model to study interactions between drugs and naturally activated dopaminergic neurons in the rat brain

The aim of the present study was to investigate the interactions between the in vivo release of dopamine and certain drugs, during conditions of increased dopaminergic activity. Dopaminergic neurons in the nucleus accumbens were activated by feeding hungry rats.48–96 h after implantation of a microdialysis probe 30 min food ingestion by hungry rats induced an immediate eating response that was accompanied with a reproducible and long-lasting increase in extracellular dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC).The effect of various drugs (infused into the nucleus accumbens via the microdialysis probe), on the extracellular levels of dopamine and DOPAC were recorded, and the effect of eating was determined.Infusion of 5 mol/l nomifensine and 3.4 mmol/l calcium increased dopamine release respectively 5.4 and 2-fold but did not modify the eating related increase in dopamine and DOPAC release. Infusion (1 mol/l) as well as intraperitoneal administration (20 mg/kg) of sulpiride induced an increase in basal dopamine release to 220 and 195% of controls, respectively. Both routes of sulpiride pretreatment enhanced the eating related increase in extracellular dopamine and DOPAC.The results of the sulpiride experiments indicate that a behaviorally induced stimulation of dopamine release is modified by autoinhibition. Correspondence to: B. H. C. Westerink at the above address     

Post-training minaprine enhances memory storage in mice: involvement of D1 and D2 dopamine receptors

Post-training administration of minaprine (2.5, 5 and 10 mg/kg) dose-dependently improved retention of an inhibitory avoidance response in mice. Animals receiving nine daily injections of 5 mg/kg and administered a challenge dose post-training showed an improvement in memory consolidation similar to that produced by acute injection of 10 mg/kg. The effects on retention performance induced by the drug appear to be due to an effect on memory consolidation. They were observed when drugs were given at short, but not long, periods of time after training, i.e. when the memory trace was susceptible to modulation. Moreover, these effects are not to be ascribed to an aversive or a rewarding or non-specific action of the drugs on retention performance, as the latencies during the retention test of those mice that had not received a footshock during training were not affected by post-training drug administration. The effects of an acutely injected dose (10 mg/kg) of minaprine as well as those of a challenge dose (5 mg/kg) of the drug administered to repeatedly treated animals were reversed by pretreatment with either selective D₁ or D₂ dopamine receptor antagonists SCH 23390 and (-)-sulpiride administered at per se non-effective doses (0.025 and 6 mg/kg, respectively), thus suggesting that D₁ and D₂ receptor types are similarly involved in the effects of minaprine on memory consolidation. These results show that minaprine improves memory consolidation and that repeated drug administration leads to potentiation of this effect. Moreover, the effects of minaprine on memory consolidation are related to its dopaminergic action.     

Lack of evidence for an involvement of nucleus accumbens dopamine D1 receptors in the initiation of heroin self-administration in the rat

The involvement of dopamine D₁ receptor systems in the reinforcing properties of opiate reward was studied by examining the effect of the dopamine D₁ antagonist SCH23390 on the initiation of heroin self-administration in rats. The D₁ antagonist was administered daily systemically or locally in the nucleus accumbens (NAC), after which the animals were allowed to self-administer heroin (IV) in a 3-h session for 5 consecutive days. Systemic treatment with SCH23390 (0.17 and 0.5 mg.kg^–1) significantly decreased heroin intake during initiation of heroin self-administration, while a dose of 0.06 mg.kg^–1 was not effective. Local administration of SCH23390 (0.5 and 2.5 µg/site) in the NAC did not affect heroin intake. Both systemic and intra-accumbal administration of SCH23390 dose dependently decreased motor behavior measured in a small open field. The attenuation of heroin intake during initiation of heroin self-administration by blockade of dopamine D₁ receptor systems may be due to a decrease in the reinforcing effects of heroin or more likely to a reduction in non-reinforcement-related behavior. The dopamine D₁ receptors present in the NAC are probably not involved in opiate reward.     

Involvement of calcium in the release of immunoreactive beta-endorphin-like peptide from dispersed cells of the neurointermediate lobe of the rat pituitary gland

The presence of Ca2+ in the incubation medium was required for stimulation of the release of the immunoreactive beta-endorphin-like peptide (IR-beta-EP) from the dispersed cells of the neurointermediate lobe of rat pituitary gland by adenosine 3',5'-monophosphate (cAMP) analogs, a phosphodiesterase inhibitor, L-isoproterenol, cholera toxin and forskolin. The basal release observed in the absence of the stimulants was also dependent on the addition of Ca2+. A calcium antagonist (verapamil) inhibited the effects of the stimulants. A calcium ionophore (A23187) enhanced the release of IR-beta-EP, but did not stimulate the formation of cAMP. These findings suggest that Ca2+ has the essential role in the release of beta-endorphin from the neurointermediate lobe of rat pituitary gland.     

Neurotensin effects on evoked release of dopamine in slices from striatum,nucleus accumbens and prefrontal cortex in rat

Summary The effects of neurotensin (NT) on the K⁺-evoked release of endogenous and tritiated dopamine in striatum and on ³H-dopamine in slices from nucleus accumbens and prefrontal cortex were investigated. In striatum, NT (1–1000 nM) elicited a dose-dependent increase in endogenous and ³H-dopamine release. The dose-response curves were comparable with the two methods. Concerning the comparison of NT modulation of ³H-dopamine release in the three cerebral structures, the peptide induced a more marked effect in striatum with a maximal effect of 150% increase. In accumbens, NT (1–1000 nM) potentiated the K⁺-evoked 3H-dopamine release, but in contrast with striatum, the plateau corresponded to a 50% increase. In prefrontal cortex, NT (1–1000 nM) induced small but significant effects, with a maximal increase of 50% at 100 nM. Acetyl-NT (8–13) displayed an action similar to the natural peptide while NT (1–8) did not exhibit any effect, suggesting that the action of NT involved a receptor. The presence of tetrodotoxin did not alter the facilitating effects of NT in the three structures, indicating that interneurons were not involved in the action of NT. The comparison of the effects of NT showed that in terms of efficacy, NT induced an increase in dopamine release more marked in striatum than in nucleus accumbens and prefrontal cortex. These results are consistent with differences in NT receptors localization in these three dopaminergic structures. Send offprint request to: A. Boireau     

Effects of selective D1 and D2 dopamine antagonists on cocaine self-administration in the rat

The effect of the selective D1 antagonist, SCH 23390, and the selective D2 antagonist, spiperone, was investigated in rats trained to self-administer intravenous cocaine on a fixed-ratio (FR) 5 schedule of reinforcement. Both SCH 23390 and spiperone pretreatment increased responding up to doses of 10.0 µg/kg, and decreased responding at higher doses. Since rate of responding maintained by a drug can be influenced by factors other than its reinforcing efficacy, behavior maintained by cocaine was also investigated under a progressive-ratio schedule. The breaking point obtained under this schedule is used as a measure of the efficacy of the reinforcer and this value is not exclusively determined by response rate. With the progressive-ratio schedule, both SCH 23390 and spiperone produced dose-dependent decreases in the highest ratio completed in rats self-administering cocaine. The results obtained using the FR 5 and progressive-ratio schedules suggest that both D1 and D2 receptors are involved in mediating the reinforcing effects of cocaine.     

Amphetamine induced release of endogenous dopamine in vitro is not reduced following pretreatment with reserpine

Summary The release of endogenous dopamine evoked by electrical stimulation or by exposure to (+)-amphetamine (10 M) was determined in superfused striatal slices of the rat.The spontaneous and the electrically-evoked release of dopamine were significantly increased in the presence of nomifensine (10 M). After reserpine pretreatment (5 mg/kg, s.c., 24 h), the striatal dopamine content was reduced by about 90%. Exposure to 10 M (+)-amphetamine during 2 min released similar amounts of dopamine from striatal slices of untreated or reserpine pretreated rats. Similar results were obtained when monoamine oxidase activity was inhibited in vivo with pargyline.Pretreatment with reserpine does not modify the (+)-amphetamine-induced release of dopamine, in spite of the marked reduction of the striatal dopamine content. These results provide direct evidence for the view that (+)-amphetamine releases dopamine from a special, reserpine-resistant pool of newly synthetized transmitter.Some of the results described in this publication have been presented at the British Pharmacological Society Meeting (Arbilla et al. 1984a)     

Inhibition by the putative potassium channel opener pinacidil of the electrically-evoked release of endogenous dopamine and noradrenaline in the rat vas deferens

Summary The effect of pinacidil on the release of endogenous noradrenaline and dopamine from the sympathetic innervation of the rat vas deferens was examined. Amine release was evoked by electrical stimulation (1, 2, 5 and 10 Hz) or by depolarization with high potassium (75 mmol/l) in the medium. Dopamine and noradrenaline were measured by means of high pressure liquid chromatography with electrochemical detection.Pinacidil (1, 5, 10 and 50 mol/l) produced a concentration-dependent inhibition of the electrically stimulated (2 Hz) overflow of noradrenaline and dopamine. Only pinacidil 50 mol/l increased the spontaneous loss of dopamine and noradrenaline. The inhibitory effects of pinacidil (5 mol/l) on amine overflow were also observed at other frequencies of stimulation (1, 5 and 10 Hz). The magnitude of the inhibitory effect on noradrenaline release was approximately the same at all frequencies (63% to 56% reduction); for dopamine, the higher the frequency of stimulation, the greater the inhibitory effect of pinacidil (up to 73% reduction). When the preparations were continuously stimulated for 70 min at 2 Hz, pinacidil (5 mol/l) reduced the overflow of dopamine and noradrenaline during the first 40 or 30 min of stimulation only. The addition of phentolamine (1 mol/l) to the perifusion medium slightly reduced the inhibitory effect of pinacidil on amine overflow, but the inhibition by pinacidil remained statistically significant. Tetraethylammonium (10 mmol/l) completely abolished the inhibitory effect of pinacidil (10 mol/l). Pinacidil (5 mol/l) did not reduce the potassium-evoked release of the amines.The results demonstrate that pinacidil impairs transmitter release from the sympathetic innervation of the rat vas deferens, probably as a consequence of the opening of potassium channels. Send offprint request to P. Soares-da-Silva at the above adress     

In vivo release of endogenous dopamine from rat caudate nucleus by phenylethylamine

The in vivo release of endogenous dopamine (DA) from the rat caudate nucleus has been measured in the presence and absence of beta-phenylethylamine. A push-pull cannula was implanted into the brain and the tissue was perfused with artificial cerebrospinal fluid (CSF) containing phenylethylamine in concentrations ranging from 5 X 10(-3) to 5 X 10(-7) M. The DA released into the perfusate was determined radioenzymatically. Dopamine was released at rates significantly greater than its resting rate by concentrations of phenylethylamine of 5 X 10(-3) to 5 X 10(-5)M; 5 X 10(-6)M phenylethylamine caused a slight increase in release, but the difference from the resting rate was not significant. The absence of calcium in the perfusing medium did not significantly alter either the unstimulated release rate of DA or the release rate stimulated by 5 X 10(-5)M phenylethylamine. The concentrations of phenylethylamine required to increase release of DA in vivo are discussed briefly in relation to the doses required to elicit behavioural effects.     

Effects of dopamine D1- or D2-like receptor antagonists on the hypermotive and discriminative stimulus effects of (+)-MDMA

Rationale Both dopamine (DA) and serotonin (5-HT) release are evoked by (+)-MDMA; however, little is known of the contribution of DA D₁- and D₂-like receptors (D₁R and D₂R, respectively) in the behavioral effects of (+)-MDMA.Objectives To test the hypothesis that a D₁R or D₂R antagonist would attenuate the hypermotive or discriminative stimulus effects of (+)-MDMA.Methods Male Sprague-Dawley rats (n=164) were pretreated with the D₁R antagonist SCH 23390 (3.125–50 g/kg, SC) or the D₂R antagonist eticlopride (12.5–50 g/kg, SC) prior to treatment with (+)-MDMA (3 mg/kg, SC) and locomotor activity was recorded using photobeam monitors. Twelve additional rats trained to discriminate (+)-MDMA (1 mg/kg, IP) from saline in a two-lever water-reinforced FR20 task were administered SCH 23390 (6.25 g/kg, IP) or eticlopride (12.5 g/kg, IP) prior to (+)-MDMA (0.375–1.0 mg/kg, IP). Rats were then placed in the drug discrimination chambers and the percent (+)-MDMA appropriate responding and response rate were measured.Results Both SCH 23390 and eticlopride blocked (+)-MDMA-evoked hyperactivity in a dose-related manner; the highest doses of the antagonists also effectively suppressed basal locomotor activity. In rats trained to discriminate (+)-MDMA from saline, SCH 23390 (6.25 g/kg), but not eticlopride (12.5 g/kg), blocked the stimulus effects of (+)-MDMA without altering response rate.Conclusion These data indicate that DA released indirectly by (+)-MDMA administration results in stimulation of D₁R and D₂R to enhance locomotor activity. Furthermore, the D₁R appears to play a more prominent role than the D₂R in the discriminative stimulus properties of (+)-MDMA.     

A paradigm to investigate the self-regulation of cocaine administration in humans

Introduction Current laboratory paradigms of human cocaine administration generally dictate the timing of drug access in ways that may limit assessing aspects of cocaine-taking behavior. Patient-controlled analgesia (PCA) methods, which allow individuals less restricted access to narcotic (i.e., opiate) analgesics, have proven safe and clinically effective for self-regulated treatment of pain. The current study assessed the feasibility, safety, and validity of a model of ad libitum cocaine self-administration, in which participants self-selected the timing of cocaine infusions, using PCA techniques.Methods Eight nontreatment seeking, otherwise medically healthy, experienced cocaine users participated in a double-blind, placebo-controlled, escalating-dose regimen of intravenous cocaine (0, 8, 16, and 32 mg per 70 kg) on 4 test days, during which time participants had 2 h of access to cocaine via manual presses of a corded PCA pump button under a fixed ratio 1: time-out 5-min schedule.Results Procedures were well-tolerated by participants, and no significant adverse events were noted. Measures of cocaine self-administration (e.g., number of responses and interinfusion intervals) indicated a significant main effect of cocaine dose, consistent with predicted dose–response relationships (i.e., decreasing responses and increasing interinfusion intervals with increasing injection dose). Participants appeared to regulate their cocaine intake in a carefully controlled manner, using considerably less cocaine (about half) that permitted by pump loading, PCA parameters, and session duration.Conclusions Data from this study support the validity of our PCA paradigm. Moreover, results suggest the apparent feasibility and safety of allowing experienced users to self-select the timing of cocaine infusions to intervals as short as 5 min. Such procedures may enhance our ability to identify effective pharmacological treatments for cocaine addiction.     

In vitro studies on the mechanism by which (+)-norfenfluramine induces serotonin and dopamine release from the vesicular storage pool

(+)-Norfenfluramine is the main metabolite of the serotoninergic anorectic agent (+)-fenfluramine. Both compounds inhibit 5-HT reuptake and stimulate its release, although they induce release from different pools, with (+)-norfenfluramine acting primarily on the cytoplasmic pool. Moreover, (+)-norfenfluramine was more potent than the parent drug in inducing dopamine release. In order to investigate whether (+)-norfenfluramine induces a Ca²⁺-dependent vesicular release, like some amphetamine derivatives, in the present study we preloaded synaptosomes with the [³H]neurotransmitter ([³H]5-HT or [³H]dopamine), superfused (washed) them for 47 min in the absence of pargyline and then exposed them to the releasing stimulus. With this protocol, the cytoplasmic pool should be absent and the [³H]neurotransmitter should mainly be stored in synaptic vesicles, where (+)-norfenfluramine should act to induce release. This was confirmed by a significant decrease of (+)-norfenfluramine-induced [³H]5-HT and [³H]dopamine release after reserpine pretreatment. The dose-response curves of (+)-norfenfluramine-induced [³H]5-HT release were superimposable in hippocampus and hypothalamus, and also superimposable on the curve for (+)-fenfluramine-induced [³H]5-HT release; the dopamine releasing potency of (+)-norfenfluramine in the striatum was more than ten times lower. The [³H]5-HT release induced by (+)-norfenfluramine was partly (about 50%) but significantly Ca²⁺-dependent, and it was also markedly (68%) inhibited by Cd²⁺, a non-specific blocker of voltage-dependent Ca²⁺ channels, suggesting that the Ca²⁺-dependent release is mediated by entry of Ca²⁺ into the synaptosomes through these channels. The [³H]dopamine release induced by 5 μM (+)-norfenfluramine was completely Ca²⁺-independent whereas at higher concentrations (10 and 20 μM) it was only slightly (20%) Ca²⁺-dependent. We have no clear explanation why (+)-norfenfluramine has these different effects on serotoninergic and dopaminergic synaptosomes. Received: 6 April 1998 / Accepted: 9 June 1998     

Release of dynorphin-like immunoreactivity from rat anterior pituitary gland in vitro

Summary Rat anterior pituitary quarters were incubated in vitro and the release of dynorphin A_1–13-like immunoreactivity (Dyn A_1–13-IR) into the incubation medium was studied. An elevation of the potassium concentration of the incubation medium to 40 mmol/l increased the release of Dyn A_1–13-IR in a calcium-dependent manner. Addition of luteinizing hormone-releasing hormone (LHRH) stimulated the release of Dyn A_1–13-IR in a dose-dependent manner. The LHRH-induced release of Dyn A_1–13-IR was almost completely prevented by omission of calcium ions from the incubation medium or by addition of the LHRH-antagonistd-pGlu¹,d-Phe²,d-Trp^3,6-LHRH. This is the first demonstration that dynorphin-like immunoreactivity can be released from the rat adenohypophysis through calcium-dependent mechanisms by both high potassium concentration and receptor-mediated stimulation.This study was supported by the Deutsche Forschungsgemeinschaft (Kn 220/1-1)