Different types of opioid receptors mediating analgesia induced by morphine,DAMGO, DPDPE,DADLE and β-endorphin in mice

Summary The effects of intracerebroventricular (i.c.v.) administration of d-Phe-Cys-Tyr-d-Try-Orn-Thr-Pen-Thr-NH₂ (CTOP), a selective mu-opioid receptor antagonist, (Allyl)2-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174864) and (N,N-Bisallyl-Tyr-Gly-Gly--(CH₂S)-Phe-Leu-OH (ICI 154129), selective delta-opioid receptor antagonists on blocking analgesia induced by -endorphin, morphine, d-Ala²-NMePhe⁴-Gly-ol-enkephalin (DAMGO), d-Ala²-d-Leu⁵-enkephalin (DADLE) and d-Pen²-enkephalin (DPDPE) administered i.c.v. were studied in male ICR mice. The analgesia was assessed by the tail-flick and paw-licking (hot-plate) tests. The potencies of opioid agonists injected i.c.v. for producing analgesia were DAMGO > DADLE > -endorphin > morphine > DPDPE. Intracerebroventricular administration of CTOP (0.05 g) selectively antagonized inhibition of the tail-flick and paw-licking response induced by morphine, DAMGO or DADLE but not -endorphin or DPDPE. ICI 174864 (5 g) and ICI 154129 (5 g) injected i.c.v. selectively antagonized analgesia induced by DPDPE or DADLE but not -endorphin, morphine or DAMGO injected i.c.v. These results indicate that analgesia induced by morphine and DAMGO is mediated by the stimulation of mu-opioid receptors while analgesia induced by DPDPE is mediated by the stimulation of delta-opioid receptors. DADLE-induced analgesia is mediated by the stimulation of both mu- and delta-opioid receptors. Analgesia induced by -endorphin is mediated by neither munor delta-opioid receptors.Abbreviations i.c.v. intracerebroventricular - i.t. intrathecal - CTOP d-Phe-Cys-Tyr-d-Try-Orn-Thr-Pen-Thr-NHZ - DAMGO d-Ala²-NMePhe²-Gly-ol-enkephalin - DADLE d-Ala²-d-Leus-enke-phalin - DPDPE dd-Pen²-dd-Pen⁵-enkephalin - ICI 174864 (Allyl)2Tyr-Aib-Aib-Phe-Leu-OH - ICI 154129 (N,N-Bisallyl-Tyr-Gly-Gly-(CH₂S)-Phe-Leu-OH     

Opiate receptors in the rabbit iris

Summary Evidence pointing to the presence of opiate receptors in the rabbit iris was obtained in an in vivo study of the effects on the pupil of the intraocular injection of morphine (10–100 g) and d-Ala²-met-enkephalinamide (d-Ala-E) (5–50 g). Both opiates induced a significant dose-dependent decrease in pupillary size and an appreciable fluctuation of pupillary size. I.v. administration of naloxone (0.5 mg/kg) before or after injection of opiates prevented the miotic effect and the fluctuation of pupil size. The parallelism of the dose-response curves of opiates in the presence and absence of naloxone indicated competitive reversible antagonism.     

Calcium channel modulation by dihydropyridines modifies sufentanil-induced antinociception in acute and tolerant conditions

Summary The study was aimed at elucidating the possible participation of l-type Ca²⁺ channel in the acute analgesic effect of an opiate and the development of tolerance to this action. Sufentanil, a selective p agonist, and two dihydropyridines, the Ca²⁺ antagonist nimodipine and the Ca²⁺ agonist Bay K 8644, were selected. The tail-flick test was used to assess the nociceptive threshold. In naive rats, nimodipine (200 g/kg) potentiated the analgesic effect of sufentanil reducing the ED₅₀ from 0.26 to 0.08 g/kg. Similar results were observed with its (–)-enantiomer Bay N 5248, while the (+) enantiomer Bay N 5247 was ineffective. Tolerance to the opiate was induced by chronic subcutaneous administration of sufentanil with minipumps (2 g/h, 7 days). In these conditions the dose-response curve to sufentanil was displaced to the right and the ED₅₀ was increased to 1.49 g/kg. In tolerant rats, nimodipine preserved its potentiating ability and prevented the displacement to the right of the sufentanil dose response-curve (ED₅₀ = 0.48 g/kg). When nimodipine was pumped (1 g/h, 7 days) concurrently with sufentanil, the development of tolerance to the opioid was not disturbed. However, the expression of tolerance was abolished and even the effect of acutely administered sufentanil was markedly potentiated (ED₅₀ = 0.03 g/kg). Similar experiments were performed with Bay K 8644. In naive rats, Bay K 8644 at a low dose (20 g/kg) that behaves as a calcium agonist, antagonized the analgesic effect of sufentanil (ED₅₀ = 0.58 g/kg), whereas at a high dose (200 g/kg) it potentiated this action (ED₅₀ = 0.15 g/kg). In tolerant rats, Bay K 8644 (20 g/kg) preserved its antagonizing ability inducing a displacement to the right of the sufentanildose-response curve (ED₅₀ = 4.2 g/kg). When Bay K 8644 was pumped (1 g/h, 7 days) concurrently with sufentanil, it enhanced the expression of tolerance to the opiate (ED₅₀ = 3.8 g/kg). These results suggest that the calcium fluxes through the l-type channel in neurones are functionally linked to the activation of the opiate receptor: the blockade of the channel increased the potency of sufentanil, whereas its activation reduced the potency of the opiate. In chronic experiments, DHPs concurrently administered with sufentanil did not affect the development of tolerance to the opiate. However, nimodipine prevented the expression of this phenomenon. Even more, the animals became hypersensitive to the opiate suggesting that the adaptative mechanisms induced by chronic opiate could be affected by chronic nimodipine.This work was supported by grants from Universidad de Cantabria-Caja Cantabria (1988) and Bayer AG, Wuppertal, FRGPredoctoral Fellow: Fondo de Investigaciones Sanitarias de la Seguridad Social.Send offprint requests to: M. A. Hurlé at the above address     

Selective potentiations in opioid analgesia following scopolamine pretreatment

The effects of the muscarinic receptor antagonist scopolamine upon analgesia induced by d-ala-d-leu-enkephalin (DADL), beta-endorphin (BEND) and morphine were examined. While scopolamine (10 mg/kg, IP) significantly potentiated the analgesic responses following DADL (40 g, ICV) and morphine (5 mg/kg, SC) on the jump test, it failed to alter significantly BEND (1 g, ICV) analgesia.     

Enhancement of δ- but not μ-opiate agonist binding by calcium

Summary Present evidence for distinction of 2 types of opiate receptor sites in rat brain homogenates originates from different relative affinities of morphine-like alkaloids and enkephalins to -or enkephalin and - or morphine-receptor sites. We now report that Ca²⁺ in a physiological dose range (0.5–3 mM) enhances the binding of ³H-enkephalin in hypotonically treated rat brain membranes, whereas specific binding of ³H-morphine-like alkaloids is not affected. Furthermore, the potency of [d-Ala², d-Leu⁵]-enkephalin to inhibit [³H]-diprenorphine and [³H]-ethylketazocine binding increased in the presence of Ca²⁺, whereas an increase in potency of [d-Ala², d-Leu⁵]-enkephalin to inhibit binding of -receptor ligands was not observed. Kinetic analysis revealed that Ca²⁺ decreased the rate of dissociation of [d-Ala², d-Leu⁵]-enkephalin without affecting the rate of association, thereby increasing the affinity. However, in saturation binding studies, performed in diencephalic membranes, in which [d-Ala², d-Leu⁵]-enkephalin binds predominantly to -receptors, Ca²⁺ also increased the binding affinity of [³H]-[d-Ala², d-Leu⁵]-enkephalin. Double reciprocal analysis suggested a mixed competitive-noncompetitive type of inhibition of [d-Ala², d-Leu⁵]-enkephalin binding by dihydromorphine. Thus, the interactions of - and -opiate ligands with -receptors may involve topographically different, but closely related binding sites, located on a single receptor molecule.Abbreviations DADL [d-Ala², d-Leu⁵]-enkephalin - DHM dihydromorphine - met-enkephalin methionine-enkephalin - leu-enkephalin leucine-enkephaline - FK 33-824 [d-Ala², MePhe⁴, Met(O)-ol]-enkephalin - EGTA ethyleneglycol-bis-(-aminoethylether) N, N'-tetraacetic acid - TRIS Tris (hydroxymethyl)-aminomethan     

The binding spectrum of narcotic analgesic drugs with different agonist and antagonist properties

Summary Four groups of narcotic analgesic drugs have been assessed for their opiate activities by using three binding assays and three pharmacological bioassays. In the binding assays, their inhibition constants (K _I, nM) were determined against the binding of the -ligand, [³H]-[d-Ala ² ,MePhe ⁴ , Gly-ol⁵]enkephalin, of the -ligand, [³H]-[d-Ala ² ,d-Leu ⁵]enkephalin and of the -ligand, [³H]-(±)-ethylketazocine after suppression of - and -binding by 100 nM of the unlabelled -ligand and 100 nM of the unlabelled -ligand. The pharmacological agonist or antagonist activities were assayed on the guinea-pig ileum, mouse vas deferens and rat vas deferens.The first group of compounds were pure agonists in all three pharmacological bioassays. The majority of the compounds showed preference to -binding but phenazocine and particularly etorphine had also high affinities to the - and -binding sites.The second group consisted of N-allyl and N-cyclopropylmethyl homologues of the morphine, 3-hydroxymorphinan and normetazocine series which had agonist and antagonist activities in the guinea-pig ileum and mouse vas deferens but were pure antagonists in the rat vas deferens. In the binding assays, -binding and -binding were prominent.The third group was made up by the ketazocine-like compounds which in the guinea-pig ileum and mouse vas deferens were pure agonists and in the rat vas deferens pure antagonists. The binding spectrum showed particularly high binding to the -binding site.The fourth group was the antagonists which were devoid of agonist activity with the exception of diprenorphine and Mr 2266 which had retained some agonism. The binding spectrum showed considerable variation, naloxone in low concentration being a selective -antagonist, Mr 2266 having high affinities to the - and -binding sites and diprenorphine having considerable affinities to the -, - and -binding sites.Since each of the four groups of compounds, whether pure agonists, agonist-antagonists, ketazocine-like drugs or pure antagonists, shows independent varittions in the affinities to the - and -binding sites, their different pharmacological behaviour cannot be solely due to difference in the binding spectra.     

Behavioral activities of opioid peptides and morphine sulfate in golden hamsters and rats

The behavioral effects of -endorphin, [D-Ala², D-Leu⁵]-enkephalin and morphine were investigated in golden hamsters and in rats. In golden hamsters, -endorphin and [D-Ala², D-Leu⁵]-enkephalin induced loss of righting reflex, whereas morphine caused no such effect. Both opiate peptides and morphine caused the inhibition of tail-flick response and catalepsy in rats. -Endorphin was the most potent, followed by [D-Ala², D-Leu⁵]-enkephalin and then by morphine. The catalepsy induced in rats by [D-Ala², D-Leu⁵]-enkephalin was different from that of -endorphin and morphine in that it produced catalepsy without muscular rigidity. -Endorphin and [D-Ala², D-Leu⁵]-enkephalin caused hypothermia in golden hamsters; morphine was less active in altering the body temperature. -Endorphin caused hypothermia at high doses and hyperthermia at low doses in rats. These heterogenous behavioral responses indicate that multiple types of receptors mediate the effects of opiates in the central nervous system.     

Sodium dependent 3H-noradrenaline release from rat neocortical slices in the absence of extracellular calcium presynaptic modulation by μ-opioid receptor and adenylate cyclase activation

Summary In Ca²⁺-free EGTA (1 mmol/l)-containing medium veratrine (3 mol/l) and ouabain (100 mol/l) strongly enhanced the efflux of ³H-noradrenaline from superfused rat brain neocortical slices prelabelled with the radioactive amine. In both cases ³H-noradrenaline release was prevented by tetrodotoxin (1 mol/l). These effects of veratrine and ouabain were virtually additive and independent of whether the noradrenaline uptake carrier was blocked with 1 mol/l desipramine or not. The adenylate cyclase activator forskolin (10 nmol/l–10 mol/l) strongly enhanced veratrine- and ouabain-induced ³H-noradrenaline release, without affecting spontaneous tritium efflux. The release induced by both stimuli was profoundly inhibited by the selective -opioid receptor agonist [d-Ala, MePhe⁴, Gly-ol⁵]enkaphalin (DAGO, 3 nmol/l–1 mol/l) in a concentration-dependent manner. The inhibitory effects of 1 mol/l DAGO were abolished by 1 mol/l naloxone. On the other hand, preincubation of the slices for 1 h with the -opioid receptor-selective irreversible ligand fentanyl isothiocyanate (1 pmol/l) did not change the inhibitory effects of DAGO.These data show that veratrine- and ouabain-induced ³H-noradrenaline release from central noradrenergic nerve terminals is facilitated by increasing intracellular cyclic AMP levels and reduced by activation of presynaptic -opioid receptors, indicating the involvement of exocytotic neurotransmitter release. The results provide further evidence for the hypothesis that under these conditions neurotransmitter release from central noradrenergic neurons is triggerred by a Na⁺-induced efflux of Ca²⁺ ions from intracellular stores.Abbreviations DAGO [d-Ala², McPhe⁴, Gly-ol⁵]enkephalin Send offprint requests to A. N. M. Schoffelmeer at the above address     

Opioid activities of human β-casomorphins

Summary Opioid activities of human -casomorphin-4,-5,-7 and -8 and, for comparison, of the corresponding bovine -casomorphins were studied in the guinea-pig ileum preparation. Binding parameters, i.e. K _d -values and binding site concentrations, for the interaction of human and bovine -casomorphins with opioid receptors in rat brain homogenates were determined in inhibition experiments, using [³H]-(d-Ala², MePhe⁴, Gly-ol⁵)enkephalin, [³H]-(d-Ala², d-Leu⁵)enkephalin and [³H]ethylketazocin as -, - and -opioid receptor ligands. Analysis of binding data was performed using a non-linear curve fitting program. All -casomorphins examined displayed opioid activity. The affinity was highest for -receptors, less so for -receptors and lowest for -receptors. It is suggested that human -casomorphins might play a role as food hormones.     

β-endorphin-sensitive opioid receptors in the rat tail artery

Summary Isolated tail arteries of rats were perfused and field-stimulated every 2 min with 2 pulses at 1 Hz. Different opioid peptides depressed the contractile responses to stimulation; their concentration-response curves showed a maximum at about 40% inhibition. The rank order of potency of the peptides was -endorphin (IC₅₀ = 97 nmol/1) BAM-22P > FK-33824 > DAGO > [d-Ala²,d-Leu⁵]-enkephalin metorphamide > dynorphin A-(1-13) [Met⁵]enkephalin. All these substances have in common a certain activity at opioid -receptors, although the enkephalins are preferential -, and the dynorphins preferential -agonists. However, the selective -agonist [d-Pen²,l-Pen⁵]enkephalin was ineffective at up to 10 mol/l, and the -agonists ethylketocyclazocine and U-50488 acted only at concentrations higher than 3 mol/l. Whereas the effects of -endorphin, DAGO and [d-Ala²,d-Leu⁵]enkephalin could be reduced by the -preferential antagonist naloxone, the effects of ethylketocyclazocine and U-50488 were not changed. The -selective antagonist ICI 174864 did not influence the action of [d-Ala²,d-Leu⁵]enkephalin. Naloxone in a concentration (1 mol/l) which nearly abolished the effect of DAGO 3 mol/l, slightly enhanced responses to stimulation. Neither -endorphin nor DAGO influenced vasoconstriction evoked by the application of noradrenaline or adenosine triphosphate; U-50488 reduced it. In arteries preincubated with [³H]noradrenaline DAGO depressed, whereas naloxone enhanced the tritium overflow and vasoconstriction evoked by field stimulation (0.4 Hz, 24 pulses every 14 min). In addition, naloxone antagonized the effect of DAGO. We suggest that the axon terminals of postganglionic sympathetic neurones in the rat tail artery possess -endorphin-sensitive opioid receptors of the -type. The activation of these receptors by exogenous or endogenous opioids inhibits the release of the neuroeffector transmitter.This work was supported by the Deutsche Forschungsgemeinschaft (SFB 325) Send offprint requests to P. Illes at the above address     

Morphine-like discriminative stimulus effects of opioid peptides: possible modulatory role ofd-Ala2-d-Leu5-enkephalin (DADL) and dynorphin A(1-13)

The role of the different opioid receptors was studied in rats trained to discriminate SC injections of 3.0 mg/kg morphine from saline by tests for generalization to graded doses of morphine and receptor-selective peptides administered into the lateral cerebral ventricle. Dose-dependent morphine-like stimulus effects were produced over a wide range of doses (0.001–30 g), depending upon ligand and animal, by morphine, by themu-selective peptides DAGO[d-Ala²-NMePhe⁴-Gly(ol)-enkephalin] and FK33824[d-Ala²,NMePhe⁴-Met(O)⁵-(ol)-enkephalin], and by thedelta-selective peptide, DADL[d-Ala²,d-Leu⁵enkephalin]. The order of relative potency of these substances was: FK33824>DAGO>morphine>DADL. In contrast, DPLPE[d-Pen²,l-Pen⁵)enkephalin], which has much greaterdelta receptor selectivity than does DADL, and dynorphin A(1-13) (0.1–10 g), akappa-receptor agonist, engendered choice responding appropriate for saline. When 1.0 g DADL, a dose lacking morphine-like discriminative effects, was administered concurrently with SC morphine, the stimulus effects of morphine were potentiated. Concurrent administration of 10 g dynorphin A(1-13) and morphine attenuated the stimulus effects of morphine inconsistently. These results support previous findings thatmu-opioid receptors are of primary importance in mediating the morphine-like discriminative effects of opioid peptides. They also suggest that morphine-like discriminative effects can be modulated by other types of opioid receptors.     

Peripheral effects of opioid drugs on capsaicin-sensitive neurones of the guinea-pig bronchus and rabbit ear

Summary The effect of a potent opioid agonist, [d-Met², Pro⁵]-enkephalinamide was investigated on two responses involving capsaicin-sensitive afferent neurones, namely, atropine-resistant contractions of the guinea-pig bronchus evoked by electrical field stimulation and the nociceptor stimulation to intraarterial injections of acetylcholine or capsaicin into the vascularly isolated rabbit ear. The hypotheses to be tested were whether (a) opioid receptor activation may inhibit mediator release from primary afferent neurones and (b) the opioid could exert an analgesic effect at a peripheral site of action. Non-cholinergic contractions of the guinea-pig isolated main bronchi due to electrical stimulation were concentration-dependently inhibited by [d-Met², Pro⁵]-enkephalinamide (10 nM–1 M). This effect was abolished by naloxone (1 M). Naloxone alone induced no change in the stimulation-evoked contractions of the bronchus, indicating that no endogenous opioid control was present. Substance P and neurokinin A induced bronchial contractions that were not influenced by [d-Met², Pro⁵]-enkephalinamide. This indicates that [d-Met², Pro⁵]-enkephalinamide inhibits electrically-evoked bronchial contractions by reduced mediator release from capsaicin-sensitive sensory nerve endings, since these contractions are most probably brought about by tachykinins, released from afferent neurones. Capsaicin-induced bronchial contractions were in contrast to electrical stimulation not influenced by [d-Met², Pro⁵]-enkephalinamide which suggests a different site of action. The activation of sensory neurones in the rabbit ear by i. a. injection of acetylcholine and capsaicin was not reduced under infusion of [d-Met², Pro⁵]-enkephalinamide (1 and 10 M) or lofentanil (1 and 10 M). The enhancement of the effect of acetylcholine by infusion of prostaglandin E₂ (0.15 M) also remained unchanged under infusion of 10 M [d-Met², Pro⁵]-enkephalinamide. A peripheral analgesic action of the two opioid agonists studied is therefore not indicated. Send offprint requests to F. Lembeck     

EEG spectral analysis for the evaluation of the central effects of δ6-tetrahydrocannabinol in rabbits

The effects of ⁶-THC on the frequency spectrum of the hippocampal EEG were examined with the assistance of an electronic integrator. Six rabbits bearing chronically implanted cortical and hippocampal electrodes were dosed intravenously with ⁶-THC (2.5% solution in polyethylene glycol 300; doses of 10, 50, 100, 250 and 1000 g/kg) once a week. A comparison of epochs of arousal in the pre-drug control and post-drug periods demonstrated that a dose as low as 50 g/kg produced a consistent, statistically significant diminution in power of the 7, 8, 9 Hz frequencies of the hippocampal EEG, with 8 Hz the most sensitive. Doses above 250 g/kg produced an overall decrease in the power spectrum from 7 to 27 Hz.Spectral analysis of the EEG offers a method which is sufficiently sensitive to reveal a central effect of ⁶-THC in rabbits at doses equivalent to the human threshold for psychic effects.Presented, in part, at the VIIIth International Congress of Electroencephalography and Clinical Neurophysiology, Marseilles, France, September 1–7, 1973.M. D. W. was supported by the Medical Research Council of Canada during the tenure of this work.The technical assistance of Mr. E. Deodati is gratefully acknowledged.     

Subclassification of presynaptic α2-adrenoceptors: α2D-autoreceptors and α2D-adrenoceptors modulating release of acetylcholine in guinea-pig ileum

The study was designed to classify in terms of _2A, _2B, _2C and _2D the presynaptic ₂-autoreceptors, as well as the ₂-receptors modulating the release of acetylcholine, in the myenteric plexus-longitudinal muscle (MPLM) preparation of the guinea-pig ileum. A set of antagonists was chosen that was able to discriminate between the four subtypes. Small pieces of the MPLM preparation were preincubated with ³H-noradrenaline or ³H-choline and then superfused and stimulated electrically.The stimulation periods used (3H-noradrenaline: 3 trains of 20 pulses, 50 Hz, train interval 60 s; ³H-choline: single trains of 30 pulses, 0.2 Hz) did not lead to ₂-autoinhibition or inhibition of ³H-acetylcholine release by endogenous noradrenaline. The ₂-selective agonist 5-bromo6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) reduced the evoked overflow of tritium in both ³H-noradrenaline and ³H-choline experiments. Most (³H-noradrenaline) or all (³H-choline) of the 10 antagonists shifted the concentration-inhibition curves of UK 14,304 to the right. pK_d values of the antagonists were calculated from the shifts. pK_d values from ³H-noradrenaline experiments correlated with pK_d values from ³H-choline experiments (r = 0.981).It is concluded that ₂-autoreceptors and ₂-heteroreceptors modulating the release of acetylcholine in the MPLM preparation are of the same subtype. Comparison with antagonist affinities for prototypic native ₂ binding sites, binding sites in cells transfected with ₂ subtype genes, and previously classified presynaptic ₂-adrenoceptors — all taken from the literature — indicates that both are _2D. The results are consonant with the hypothesis that at least the majority of ₂-autoreceptors belong to the _2A/D branch of the ₂-adrenoceptor tree, across mammalian or at least across rodent and lagomorph species. The same may hold true for ₂-adrenoceptors on non-noradrenergic neurones.     

Tryptophan antidepressant ‘physiological sedative’: Fact or fancy?

Some recent publications relating to the allegedly antidepressive and sedative effects of L-tryptophan the precursor of 5-HT have been reviewed.The evidence to date suggests that the amino acid is as effective as standard tricyclic drugs in alleviating the symptoms of depression, especially those cases presenting with mainly psychomotor retardation, and is synergistic with MAOIs. L-Tryptophan would also appear to be a physiological sedative. This action, however, appears to be related to the time of administration and at present has only been demonstrated at night, when endogenous levels of 5-HT are at their peak.In terms of practical therapeutics L-tryptophan would appear to have greater potential as an antidepressant than as a sedative.     

Behavioural effects of selective μ-, κ-, and δ-opioid agonists in neonatal rats

The behavioural effects of selective -, - and -opioid agonists in 5-, 10- and 20-day-old rats were investigated by observational analysis. The predominant response to -agonists was behavioural depression. High doses (10 mg/kg IP) of morphine and DAGO (d-Ala², NMe-Phe⁴, Glyol⁵-enkephalin) produced overt sedation in all the age groups and also induced catalepsy which was particularly apparent in the 5- and 10-day-old animals. These compounds did not produce any signs of behavioural activation in the neonatal rats. In contrast, rat pups treated with the -agonists U50,488H and PD 117,302 (1,10 mg/kg IP) exhibited marked hyperactivity with increases in wall-climbing and locomotion. Sedative effects of the highest dose of the -agonists began to emerge, however, as the animals grew older, resulting in significant decreases in behaviours such as gnawing and grooming at 20 days of age. The -agonist (+)-tifluadom (0.1–10 mg/kg), but not its corresponding (-)-isomer, produced an increase in activity in 5-day-old rats, thus extending the observations made with U50,488H and PD 117,302 and establishing the stereoselective nature of the response. The involvement of -receptors in opioid-induced hyperactivity was further substantiated by using a variety of opioid antagonists. In this context, the increase in activity induced by U50,488H (10 mg/kg) in 5-day-old neonates was attenuated by naltrexone (1 mg/kg IP) but not by larger doses (10 mg/kg) of either M8008 (which has low affinity for -receptors) or the selective -receptor antagonist ICI 174,864. Finally, DPDPE (d-Pen², d-Pen⁵-enkephalin) which acts selectively at -opioid receptors, did not exert any behavioural effects in either the 5-, 10- or 20-day-old rat pups at doses of up to 10 mg/kg. These results demonstrate behavioural effects of - and -but not -agonists in neonatal rats. There is a clear differentiation between - and -receptor effects and both - and -mediated behaviours show dissimilarities from the adult profile.     

Effects of Δ8-THC,and Δ9-THC on the acquisition of a discriminative positional habit in rats

Using a reversal learning paradigm the dissociative effects of two tetrahydrocannabinols (THC) on the acquisition and reversal of a discriminative positional habit in rats were studied. A T-shaped water maze was used. From these experiments it is concluded that learning under the influence of ⁸-THC (10 and 20 mg/kg), and ⁹-THC (5 mg/kg) is state-dependent (StD) in the rat.Numbering system according to IUPAC rules.Parts of the results were presented at the Symposium on the chemistry and biological activity of cannabis, Stockholm, October 26–28, 1971 and at the Symposium on medical plants of Brazil, Sao Paulo, April 17–20, 1972.     

α-Adrenoceptor activity of arylalkylimidazoles is improved byα-methylation and impaired byα-hydroxylation

Summary To investigate the effects of hydroxyl and methyl substitution of the alkyl bridge bond on the-adrenoceptor activity of arylalkylimidazole derivatives, the cardiovascular effects of the molecules were studied in anaesthetized and pithed rats. The compounds studied were 4(5)-substituted imidazole derivatives with a methano, ethano or etheno bridge between the imidazole and the 2-, 2,3- or 2,6-methyl substituted phenyl rings. The hypotensive and bradycardic activities of the molecules in the anaesthetized rat were always reduced by-hydroxylation and usually augmented by-methylation of the bridge between the imidazole and phenyl rings. Hydroxylation was associated with a consistent, marked decrease in vasopressor and sympatho-inhibitory activity in the cardiovascular system of the pithed rat, but a methyl moiety as a bulky substituent in the-position of the alkyl bridge did not decrease but even caused an increase in-adrenoceptor activity in this test system. The detrimental effect of-hydroxylation of the compounds at ₁- and ₂-adrenoceptors supports the notion that the interaction of the imidazoles at-adrenoceptor is different from that of the classical, noradrenaline-like phenethylamines. The results also suggest that the alkyl bridge between the phenyl and imidazole rings of the imidazoles may contribute directly to the binding process.     

[3H]5′-N-ethylcarboxamide adenosine binds to both Ra and Ri adenosine receptors in rat striatum

Summary Adenosine analogs such as 5-N-ethylcarboxamide adenosine and N⁶-cyclohexyladenosine stimulate or inhibit adenosine cyclase activity in preparations of rat striatum depending on the assay conditions. N⁶-cyclohexyladenosine inhibits but does not stimulate adenosine cyclase activity in preparations of hippocampus. These findings suggest that the striatum contains both R _a (stimulatory) and R _i (inhibitory) adenosine receptors while the hippocampus contains only R _i receptors. We have previously shown that [³H]N⁶-cyclohexyladenosine binds to R _i receptors in rat hippocampus (Yeung and Green 1983). Comparisons of the characteristics of [³H]5-N-ethylcarboxamide adenosine and [³H]N⁶-cyclohexyladenosine binding to hippocampus show that [³H]5-N-ethylcarboxamide adenosine also binds to R _i receptors with high affinity. [³H]5-N-ethylcarboxamide adenosine binds to R _i receptors in the striatum and to a second site that is present in striatum but not hippocampus. High affinity binding of both ligands to R _i receptors can be blocked by treatments with N-ethylmaleimide that do not markedly affect [³H]5-N-ethylcarboxamide adenosine binding to the second site in the striatum. The pharmacological characteristics of the second site indicate that it is the R _a adenosine receptor.The abbreviations used are NEM N-ethylmaleimide - Gpp(NH)p 5-guanylylimidodiphosphate - NECA 5-N-ethylcarboxamide adenosine - l-PIA N⁶-(l-phenylisopropyl)adenosine - d-PIA N⁶-(d-phenylisopropyl) adenosine - DPX 1,3-diethyl-8-phenylxanthine     

Glucose both inhibits and stimulates insulin secretion from isolated pancreatic islets exposed to maximally effective concentrations of sulfonylureas

Summary Isolated pancreatic islets from mice were perifused with media containing maximally effective concentrations of glibenclamide (0.1–10 mol/l) or glipizide (1 mol/l). In these islets an increase of the glucose concentration from 10 mmol/l to 40 mmol/l or addition of d-glyc-eraldehyde (20 mmol/1) caused a temporary decrease in insulin release which was followed by a sustained enhancement of release. -Ketoisocaproate (3 or 20 mmol/1) did not inhibit insulin release; at high concentration it was an even stronger secretagogue than d-glucose or d-glyceraldehyde. It is concluded that high energy phosphates couple B-cell fuel metabolism and insulin release by acting both on the ATP-dependent K⁺ channel and on other targets not yet identified.Some of the results described here are part of the medical thesis of A. WallaschSend offprint requests to U. Panten at the above address