The effect of pimozide on variable-interval performance: A test of the ‘anhedonia’ hypothesis of the mode of action of neuroleptics

A quantitative behavioural test system based on Herrnstein's (1970) equation was used to test a prediction derived from the anhedonia hypothesis of neuroleptic action, that pimozide should increase the value of the behavioural parameter K _H (the reinforcement frequency needed to maintain the half-maximal response rate in variable-interval schedules). On the basis of theoretical considerations, it was shown that the equation implies that a drug which exerts such an effect on K _H must have a more profound suppressant effect on performance maintained by low reinforcement frequencies than on performance maintained by high reinforcement frequencies. Fifteen rats were trained under variable-interval 10-s and variable-interval 100-s schedules, and the effect of pimozide (0.125, 0.25, 0.33, and 0.5 mg/kg) was tested on performance maintained under each schedule. The drug suppressed performance maintained under both schedules in a dose-dependent manner, and there was no tendency for the drug to exert a greater effect on performance maintained under the lower reinforcement frequency. These results do not provide any evidence that the effect of pimozide on variable-interval performance is due to an anti-hedonic effect; rather, they are compatible with the hypothesis that pimozide impairs the capacity to respond.     

Radioimmunoassay of β-endorphin basal and stimulated levels in extracted rat plasma

Summary A sensitive radioimmunoassay for -endorphin is described. Antibodies against human -endorphin which exhibit a high avidity for the C-terminal of the peptide were raised in rabbits following the injection of thyroglobulin-coupled human -endorphin (h-E) as immunogen. Methionineenkephalin, - -endorphin, as well as ACTH peptides did not cause interference in the radioimmunoassay. -Lipotropin, however, showed a 50% cross-reactivity. The sensitivity of the assay is 25 pg/0.5 ml tube volume for -endorphin. -Endorphin was extracted with a high recovery from the rat plasma using silicic acid and -endorphin levels as low as 100 pg/ml could be measured.Basal levels of -endorphin-like immunoreactivity in plasma of rats were about 400 pg/ml. -Endorphin levels in adrenalectomized rats and in animals chronically treated with the cortisol synthesis blocker metyrapone were found to be markedly increased (about 7-fold). Exposure of the rats to electrically induced foot-shocks caused a similar increase of immunoreactive -endorphin in plasma. A significant increase was also seen after insulin injection.     

Absorption Enhancing Effect of Cyclodextrins on Intranasally Administered Insulin in Rats

The absorption enhancing effect of -, -, and -cyclodextrin (CD), dimethyl--cyclodextrin (DMCD), and hydroxypropyl--cyclodextrin (HPCD) on intranasally administered insulin was investigated in rats. Coadministration of 5% (w/v) DMCD to the insulin solution resulted in a high bioavailability, 108.9 ± 36.4% (mean ± SD, n = 6), compared to i.v. administration, and a strong decrease in blood glucose levels, to 25% of their initial values. Coadministration of 5% -CD gave rise to an insulin bioavailability of 27.7 ± 11.5% (mean ± SD, n = 6) and a decrease in blood glucose to 50% of its initial value. The rate of insulin absorption and the concomitant hypoglycemic response were delayed for the -CD-containing solution as compared to the DMCD preparation. The other CDs, HPCD (5%), -CD (1.8%), and -CD (5%), did not have significant effects on nasal insulin absorption. DMCD at a concentration of 5% (w/v) induces ciliostasis as measured on chicken embryo tracheal tissue in vitro, but this effect is reversible. In conclusion, DMCD is a potent enhancer of nasal insulin absorption in rats.     

Intrathecal substance P depresses the tail-flick response — Antagonism by naloxone

Summary Substance P injected into the lumbar subarachnoid space of rats depressed the tail-flick response to radiant heat in a dose-dependent way. The effective doses ranged from 0.1 g to 100 g per rat (ED 50: 1.5 g/rat). The maximum of the effect was reached 20 min after intrathecal injection and the effect lasted for about 30 min. An antinociceptive effect was also observed after intrathecal injection of substance P 1 g to spinal rats. The depression of the tail-flick response produced by intrathecal administration of substance P was abolished by intrathecal (5 g/rat) or i.p. (0.5 mg/kg) injections of naloxone.Supported by the Sonderforschungsbereich 38 Membranforschung     

Can two, four or eight‐hour urine collections after voluntary voiding be used instead of twenty ‐four hour collections for the estimation of creatinine clearance in healthy subjects

The accuracy of creatinine clearance estimations obtained from 4hour (16:0020:00, 20:0024:00, 08:0012:00, 12:0016:00) and 8hour (16:0024:00, 24:0008:00 and 08:0016:00) urine collections and the Cockcroft Gault formula compared with the standard 24hour collection, as well as the cyclical variation in creatinine excretion were studied in a group of 22 healthy subjects (Serum creatinine < 1.5mg/dl, Blood Urea Nitrogen < 50mg/dl) after voluntary voiding. The mean 4hour and 8hour creatinine clearances were not significantly different from the 24hour values. Clearance values from 8hour collections between 24:0008:00 and 16:0024:00 were found to be the most accurate and gave the best correlations. Furthermore only the mean absolute percentage deviations of the 8hour from the 24hour clearance values were significantly less than 20%. Significant cyclical variations in creatinine clearance over 24 hours were not observed. Time intervals between 23:0007:00 and 07:0009:00 were chosen for the comparisons between 8hour, 2hour, Cockcroft Gault creatinine clearance estimations and the 24hour values in 21 healthy subjects. The mean 2hour and 8hour creatinine clearances were not significantly different from the 24hour values. However, once again only the 8hour clearance values differed by less than 20% from the 24hour values and they were more accurate and better correlated than the 2hour values. As expected, in both groups of subjects, the percentage of clearance values that deviated by more than 20% from the 24hour values decreased as the length of the collection times increased. The Cockcroft Gault formula in both groups of volunteers gave less accurate clearance estimations, smaller correlation coefficients (not statistically significant in Group I subjects) and percentage deviations from the 24hour values greater than 20%. Undetected early stage renal insufficiency in three volunteers and the use of actual instead of normalized Scr values may have been the cause of these poor clearance estimations. In healthy subjects (Scr < 1.5mg/dl) 24hour creatinine clearance may be estimated from an 8hour urine collection with voluntary voiding if a 20% deviation from the 24hour value is considered clinically acceptable.     

Enhancement of Solubility and Bioavailability of β-Lapachone Using Cyclodextrin Inclusion Complexes

Purpose. To explore the use of cyclodextrins (CD) to form inclusion complexes with -lapachone (-lap) to overcome solubility and bioavailability problems previously noted with this drug. Methods. Inclusion complexes between -lap and four cyclodextrins (-, -, -, and HP-CD) in aqueous solution were investigated by phase solubility studies, fluorescence, and ¹H-NMR spectroscopy. Biologic activity and bioavailability of -lap inclusion complexes were investigated by in vitro cytotoxicity studies with MCF-7 cells and by in vivo lethality studies with C57Blk/6 mice (18-20 g). Results. Phase solubility studies showed that -lap solubility increased in a linear fashion as a function of -, -, or HP-CD concentrations but not -CD. Maximum solubility of -lap was achieved at 16.0 mg/ml or 66.0 mM with HP-CD. Fluorescence and ¹H-NMR spectroscopy proved the formation of 1:1 inclusion complexes between -CD and HP-CD with -lap. Cytotoxicity assays with MCF-7 cells showed similar biologic activities of -lap in -CD or HP-CD inclusion complexes (TD₅₀ = 2.1 M). Animal studies in mice showed that the LD₅₀ value of -lap in an HP-CD inclusion complex is between 50 and 60 mg/kg. Conclusions. Complexation of -lap with HP-CD offers a major improvement in drug solubility and bioavailability.     

Clockwise hysteresis or proteresis

The authors propose the word proteresis to designate the clockwise hysteresis, i.e., when an effect increases more rapidly than the observed drug concentrations. Such a phenomenon has been recently described for aspirin and nicotine. Indeed hysteresis means which comes after, while proteresis, the greek symmetrical word, means which comes earlier, a more appropriate term for the described situation.     

Sex differences in the medication choice for hypertension in general practice. A study with written case simulations

The objective of this study was to explore explanations for the preference of physicians to prescribe blockers to hypertensive men and diuretics to hypertensive women.A qualitative study among 12 family physicians was conducted with a combination of written case simulations, semistructured interviews and statements on attitudes of physicians towards antihypertensive drug choice.Among the male hypertensive cases the most frequently prescribed drugs were blockers, whereas among the female hypertensive cases diuretics were more often prescribed. Physician characteristics associated with a preferred prescribing of blockers to hypertensive men and diuretics to hypertensive women were: older age (no residency in family medicine), the believe that blockers are more effective in men with regard to lowering blood pressure and that diuretics are more effective in women, a nonevidence based attitude and a sexrelated attitude towards the choice of blockers and diuretics in general, and in particular towards the prescribing of blockers to hypertensive men because men have a higher absolute risk of coronary heart disease than women. An additional explanation for these findings may be the higher prevalence of ankle oedema among women. Patient characteristics associated with more prescribing of blockers to hypertensive men and diuretics to hypertensive women were: current employment and a "highrisk" profile in terms of blood pressure level and additional cardiovascular risk factors.Although, most considerations underlying a preferred prescribing of blockers to hypertensive men and diuretics to hypertensive women were not evidencebased, the actual choice of antihypertensive drug (diuretic or blocker) was evidencebased. These considerations may also play a role in the sex difference in the choice of calcium antagonists and angiotensin converting enzyme inhibitors and require further investigation.     

Cardiac ventricular β2-adrenoceptors in guinea-pigs and rats are localized on the coronary endothelium

Summary In mammalian heart tissue ₂ are known to coexist with ₁. In the present study, evidence that ₂ in guinea-pig and rat ventricles are primarily localized on the coronary endothelium is provided by competition binding studies with the subtype-selective -adrenoceptor antagonists ICI 89.406 (₁) and ICI 118.551 (₂) on four different plasma membrane preparations. (1) Following density gradient centrifugation of cardiac ventricular microsomes from rats or guinea-pigs, endothelial plasma membranes migrated at slightly higher density than the sarcolemmal membranes, as verified by endothelial (angiotensin converting enzyme) and sarcolemmal markers (adenylate cyclase, [³H] ouabain binding). At the activity peak of angiotensin converting enzyme, the relative amount of ₂ in guinea-pigs and rats was 25% and 65%, respectively. (2) On sarcolemmal membranes corresponding to the activity peak of adenylate, cyclase, -adrenoceptors consisted of the ₁ exclusively (guinea-pig), or to at least 90% (rat). (3) Cultures of coronary endothelial cells derived from guinea-pigs revealed only ₂. (4) Isolated guinea-pig cardiomyocytes contained only ₁, a finding recently established in rat myocytes as well.     

Über die Wirkung einiger herzwirksamer Bisguanylhydrazone auf den Veratrineffekt am Frosch-Sartoriusmuskel

Zusammenfassung Die Wirkung einiger Guanylhydrazone sowie von Tyramin auf die Veratrine-Response des Froschsartoriusmuskels wurden untersucht. Von den drei herzglykosidartigen Bisguanylhydrazonen zeigten BG 60 und BG 31 den unspezifischen Veratrinantagonismus des Chinidins. BG 85 hingegen beeinflußte die VR herzglykosidartig. Von den drei tyraminartigen Substanzen BG 81, MG 41 und Tyramin wirkten die letzten zwei chinidinartig, während BG 81 trotz seiner tyraminartigen Wirkung die VR herzglykosidartig beeinflußte.

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Summary A series of guanylhydrazones was synthesized, some of the substances had a cardiac glycoside-like activity on the mammalian heart whereas other ones acted tyramine-like. The effect of some guanylhydrazones of each kind of action on the veratrine response (VR) of the frog's sartorius muscle should be investigated. Following substances were synthesized: 1. with cardiac glycoside-like activity 3,3-dimethyl-4,4-diacetyldiphenylbisguanylhydrazone (BG 60), progesteronebisguanylhydrazone (BG 31), p-aceto-m-methyl-phenyl-acetonyl-ether-bisguanylhydrazone (BG 85) 2. with sympathomimetic activity: p-aceto-phenyl-acetonyl-ether-bisguanylhydrazone (BG 81) and p-hydroxybenzaldehyd-monoguanylhydrazone (MG 41). The characteristic influence of the cardiac glycosides on the VR was only to be seen with BG 85 and BG 81. BG 60, BG 31 and BG 41 showed a quinidine-like effect on the VR. That means that from 3 guanylhydrazones with cardiac glycoside-like activity on the mammalian heart two of them had a quinidine-like effect on the VR and only one of them showed the typical cardiac glycoside-like effect on the VR. On the other hand among the guanylhydrazones with tyramine-like activity on the mammalian heart there is one, BG 81, which influenced the VR like a cardiac glycoside.

     

Differential modulation by muscimol and baclofen on antinociception induced by morphine, β-endorphin,d-Pen2,5-enkephalin and U50,488H administered intracerebroventricularly in the mouse

The present study was designed to investigate the modulatory effects of stimulation of GABA_A and GABA_B receptors at supraspinal sites on antinociception induced by supraspinally administered -, -, -, and -opioid receptor agonists. The effects of the GABA_A and GABA_B receptor agonists, muscimol and baclofen respectively, on the antinociception induced by morphine (a -receptor agonist), -endorphin (an -receptor agonist), D-Pen^2,5-enkephalin (DPDPE, a -receptor agonist) and U50,488H ({trans-3,4-di-chloroN-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeocetamide}; a -receptor agonist) injected intracerebroventricularly (i.c.v.) were studied. The anti-nociception was assayed using the tail-flick and hot-plate tests. Muscimol at doses of 25–200 ng, administered i.c.v. alone did not affect the latencies of tail-flick and hot-plate thresholds, but attenuated dose-dependently the inhibition of the tail-flick and hot-plate responses induced by i.c.v. administered morphine (2 g), -endorphin (1 g), DPDPE (10 g), and U50,488H (60 g). Baclofen (1.25–10 ng) administered i.c.v. alone did not affect the latencies of the tail-flick and hot-plate responses, but attenuated dose-dependently the inhibition of the tail-flick and hot-plate responses induced by -endorphin and U50,488H, without affecting morphine-or DPDPE-induced responses. Our results indicate that activation of GABA_A receptors at the supraspinal sites by i.c.v. injection of muscimol antagonizes antinociception induced by supraspinally administered -, -, -, and -opioid receptor agonists. On the other hand, activation of GABA_B receptors at supraspinal sites by i.c.v. baclofen antagonizes antinociception induced by i.c.v. administered - and -opioid agonists, but not - or -opioid agonists.     

Effects of locally applied dopamine to the nucleus accumbens on the motor activity of normal rats and following α-methyltyrosine or Reserpine

The motor activity of rats was investigated following bilateral application of various doses (0–80 g) of dopamine to the nucleus accumbens. A high dose (80 g) of dopamine increased the motor activity of normal as well as -methyltyrosine- and reserpine-treated rats. It also increased the late motor activity (6–9 min) of normal rats, probably due to stimulation of postsynaptic dopamine receptors. Lower doses (10–40 g) of dopamine suppressed initial (0–3 min) motor activity of normal rats, perhaps due to stimulation of dopamine autoreceptors on the dopamine nerve terminals in the nucleus accumbens with a subsequent inhibition of dopamine neurotransmission. An intermediate dose (40 g) of dopamine was able to restore the motor activity of -methyltyrosine-treated but not of reserpine-treated rats at all time intervals. This difference, indicating a restoration of the normal pattern of habituation by dopamine only in animals pretreated with -methyltyrosine, suggests that normal behaviour is dependent on release of dopamine by nerve impulses.     

Structure-activity relationship studies on mescaline: II. Tolerance and Cross-tolerance between mescaline and its analogues in the rat

Summary Using the CAR shuttle box technique, with male hooded rats as subjects, the development of tolerance to mescaline, 3,4-dimethoxyphenyl-ethylamine (DMPE) and NN-dimethylmescaline (DMM) and possible cross-tolerance between these drugs was studied. It was found that seven successive daily doses of mescaline induced tolerance to its inhibitory effect, but that the excitatory effect was increased. Tolerance developed to the predominantly inhibitory effect of DMPE, and to the excitatory effect of DMM.Partial cross-tolerance existed between DMPE and mescaline, and when the procedure was reversed, between mescaline and DMPE. Cross-tolerance was shown in the same manner between DMM and mescaline, and mescaline and DMM.The problems of quantification and interpretation of these results is discussed. It was concluded, on the basis of subsidiary experiments, that the inhibitory effect of mescaline was probably not caused by ataxia or by a peripheral effect.     

Cardiostimulatory effects of cyclic 3′,5′-adenosine monophosphate and its acylated derivatives

Summary The effects of cyclic 3,5-AMP and of two acylated derivatives, dibutyryl (DBA) and dihexanoyl-3,5-AMP (DHA) were investigated in isolated perfused hearts of guinea pigs, rats and rabbits.In guinea pig hearts, DBA (Ca- and Na-salt) and DHA-Na in high doses (10 moles) produced strong and long lasting increases in the rate and amplitude of contractions, coronary flow, and moderate increases in phosphorylase activity in the majority of experiments. The positive ino- and chronotropic effects occured 3–5 min after injection of the drug, mostly in a fluctuating manner with several maxima. Theophylline augmented the effects of DBA-Na and revealed positive inotropic actions of non substituted 3,5-AMP.In rat hearts, similar, but more pronounced and dose-dependent effects were observed after 1, 5 and 10 moles DBA-Na. Propranolol (50 g) did not block the action of 10 moles DBA-Na. Non substituted 3,5-AMP, 5-AMP and ATP in doses of 10 moles had no significant positive inotropic effects.In rabbit hearts, DBA-Na (50 moles) produced moderate, non fluctuating rises in the amplitude of contraction.The results provide evidence that under certain conditions cyclic 3, 5-AMP itself, like its acylated derivatives DBA and DHA, may produce strong and direct positive inotropic and chronotropic effects in the heart. These findings support the view that cyclic 3,5-AMP is the cellular mediator of the cardiostimulant actions of substances that increase its rate of production in the myocardial cell.The excellent technical help of Mrs. Vera Bauer is gratefully acknowledged by the authors.     

Metabolism,Distribution, and Transdermal Permeation of a Soft Corticosteroid,Loteprednol Etabonate

The soft corticosteroid, loteprednol etabonate (chloromethyl 17-ethoxycarbonyloxy-11-hydroxy-3-oxoandrosta-1,4-diene-17-carboxylate), I, was designed based on the inactive metabolite approach. Accordingly, I should be metabolized by hydrolysis to the corresponding inactive cortienic acid derivative, II. The in vitro and in vivo metabolism of I indeed yielded mainly this inactive metabolite, which is more hydrophilic and thus readily eliminated from the body. Relatively high levels of I were found in tissues after intravenous administration of the drug in rats. The permeability of I through hairless mouse skin was comparable to what has been found for related hard steroids, without significant metabolism taking place in the skin.     

Structural and Functional Differences Between Glycosylated and Non-glycosylated Forms of Human Interferon-β (IFN-β)

Purpose. Two recombinant IFN- products have been approved for the treatment of multiple sclerosis, a glycosylated form with the predicted natural amino acid sequence (IFN--la) and a non-glycosylated form that has a Met-1 deletion and a Cys-17 to Ser mutation (IFN--lb). The structural basis for activity differences between IFN--la and IFN--lb, is determined. Methods. In vitro antiviral, antiproliferative and immunomodulatory assays were used to directly compare the two IFN- products. Size exclusion chromatography (SEC), SDS-PAGE, thermal denaturation, and X-ray crystallography were used to examine structural differences. Results. IFN-- la was 10 times more active than IFN-- Ib with specific activities in a standard antiviral assay of 20 × 10⁷ lU/mg for IFN--la and 2 × 10⁷ lU/mg for IFN--lb. Of the known structural differences between IFN--la and IFN--lb, only glycosylation affected in vitro activity. Deglycosylation of IFN--la produced a decrease in total activity that was primarily caused by the formation of an insoluble disulfide-linked IFN precipitate. Deglycosylation also resulted in an increased sensitivity to thermal denaturation. SEC data for IFN--lb revealed large, soluble aggregates that had reduced antiviral activity (approximated at 0.7 × 10⁷ lU/mg). Crystallographic data for IFN--la revealed that the glycan formed H-bonds with the peptide backbone and shielded an uncharged surface from solvent exposure. Conclusions. Together these results suggest that the greater biological activity of IFN--la is due to a stabilizing effect of the carbohydrate on structure.     

The metabolism of the amyloid precursor protein and its relevance to Alzheimer's disease

Summary The pathology of Alzheimer's disease is primarily characterized by the deposition of -amyloid/A peptide as the major component of senile or neuritic plaques. The A peptide is produced as a result of proteolytic cleavage of the transmembrane protein precursor, APP, during its normal cellular metabolism. The free amino terminus of the A peptide is generated by an endopeptidic cleavage between Met⁶⁷¹-Asp⁶⁷² by a protease termed -secretase. Increased cleavage at this site takes place in a rare, inherited double mutation (Lys⁶⁷⁰-Met⁶⁷¹ to Asn⁶⁷⁰-Leu⁶⁷¹), leading to increased A production and consequent development of Alzheimer's disease on an accelerated time scale in the affected individuals, underscoring the pathological importance of -secretase activity. Cellular studies provide direct evidence that inhibition of -secretase activity would appear to be effective in inhibiting A production as a rational approach to developing therapeutics for the disease.     

The effects of different doses of methylpentynol on escape/avoidance conditioning in two strains of rats selectively bred for high and low “emotionality”

Summary Animals from strains selectively inbred for high and low emotionality have been administered varying doses of methylpentynol and then subjected to a discrete escape/avoidance task (buzzer-shock). Results indicate an initial increase in avoidance scores associated with dose level followed by a decrease. There is also a clear dose and strain effect. No direct difference between the sexes was noted but there is a strong sex X strain interaction. The usual superiority of the non-emotional strain on tasks of this type of conditioning is reversed by the drug. The possible biphasic and dual effect of methylpentynol is discussed.We are indebted to The Nicholas Research Institute Ltd., Slough for the supply of Oblivon used throughout this experiment and for the partial financial support of one of us, B.D.G.     

Behavioural and anticonvulsant effects of Ca2+ channel toxins in DBA/2 mice

This study investigated the behavioural and anticonvulsant effects of voltage-sensitive calcium channel blockers in DBA/2 mice.-Conotoxin MVIIC (0.1, 0.3 g ICV/mouse) and-agatoxin IVA (0.1, 0.3, 1 g ICV), which act predominantly at P- and/or Q-type calcium channels, prevented clonic and tonic sound-induced seizures in this animal model of reflex epilepsy (ED₅₀ values with 95% confidence limits for protection against clonic sound-induced seizures were 0.09 (0.04–0.36) g ICV and 0.09 (0.05–0.15) g ICV, respectively and against tonic seizures 0.07 (0.03–0.16) g ICV and 0.08 (0.04–0.13) g ICV, respectively). The N-type calcium channel antagonists-conotoxin GVIA and-conotoxin MVIIA were also tested in this model.-Conotoxin GVIA was anticonvulsant in DBA/2 mice, but only at high doses (3 g ICV prevented tonic seizures in 60% of the animals; 10 g ICV prevented clonic seizures in 60% and tonic seizures in 90% of the animals), whereas-conotoxin MVIIA did not inhibit sound-induced seizures in doses up to 10 g ICV. Both-conotoxin GVIA and-conotoxin MVIIA induced an intense shaking syndrome in doses as low as 0.1 g ICV, whereas-conotoxin MVIIC and-agatoxin IVA did not produce shaking at any of the doses examined. Finally,-conotoxin GI (0.01–1 g ICV) and-conotoxin SI (0.3–30 g ICV), which both act at acetylcholine nicotinic receptors, were not anticonvulsant and did not induce shaking in DBA/2 mice. These results confirm that blockers of N- and P-/Q-type calcium channels produce different behavioural responses in animals. The anticonvulsant effects of-conotoxin MVIIC and-agatoxin IVA in DBA/2 mice are consistent with reports that P- and/or Q-type calcium channel blockers inhibit the release of excitatory amino acids and are worthy of further exploration.     

The sympathetic axons innervating the sinus node of the rabbit possess presynaptic opioid к- but not μ- or δ-receptors

Summary The postganglionic sympathic nerves of rabbit isolated hearts were stimulated with pulses delivered at 5 Hz and train durations of 1–5 s. Ethylketocyclazone 0.01–1 mol/l and fentanyl 1 and 10 mol/l but not morphine 1 and 10 mol/l, Met-enkephalin 1 and 4 mol/l or d-Ala², d-Leu⁵-enkephalin 0.5 and 5 mol/l diminished the stimulation-evoked increase in heart rate. The effect of ethylketocyclazocine 0.1 mol/l was antagonized by naloxone 1 and 10 mol/l. In contrast, the effect of fentanyl was not changed by naloxone 10 mol/l. Ethylketocyclazocine 0.03 and 1 mol/l did not reduce the tachycardia elicited by exogenous noradrenaline. The results suggest that, under in vitro conditions, only presynaptic opioid - but not - or -receptors inhibit the release of noradrenaline from the sympathetic neurones innervating the sinus node.