Kappa-receptor Agonist Research Articles

Intracellular calcium and hormone release from nerve endings of the neurohypophysis in the presence of opioid agonists and antagonists.

Rat neural lobes and isolated nerve terminals from the neurohypophysis were stimulated in the presence of different opioid agonists and antagonists. The secretion of arginine vasopressin and oxytocin and rise in cytoplasmic calcium induced by depolarization were analyzed by radioimmunoassay and the fluorescent probe fura-2, respectively. The kappa-agonists dynorphin A(1-13) and dynorphin A(1-8) did not affect electrically evoked release of vasopressin, although oxytocin release was slightly reduced. U-50 488, a relatively specific kappa-receptor agonist, had no effect on the amount of vasopressin or oxytocin secreted, although it significantly reduced K(+)-evoked changes in [Ca2+]i in isolated nerve endings. Two kappa-receptor antagonists, MR 2266 and diprenorphin, alone had no effect on vasopressin and oxytocin secretion from isolated nerve endings depolarized with potassium. Opioid agonists less selective for the kappa receptors, etorphin and ethylketocyclazocin, were found to inhibit the release of both vasopressin and oxytocin significantly. Naloxone, a nonselective opiate receptor antagonist, alone had no effect on vasopressin release but potentiated the electrically evoked release of oxytocin. Naloxone also could overcome the inhibitory effect of etorphin on oxytocin and vasopressin release observed after electrical stimulation of the neural lobe. A number of inconsistencies therefore exist between the effects of opioid agonists and antagonists on neuropeptide release and on the evoked changes in [Ca2+]i. In view of these inconsistencies and the high concentrations of opioid agonists and antagonists necessary to modify release, we conclude that it is doubtful that opioid molecules have a physiological role in controlling neurohypophysial secretion.

Cardiovascular actions of the kappa-agonist, U-50,488H, in the absence and presence of opioid receptor blockade.

1. The cardiovascular actions of U-50,488H, a kappa-receptor agonist, were studied in rat isolated perfused hearts, and in anaesthetized rats, over concentrations or doses generally above those required to produce kappa-receptor-mediated effects. 2. U-50,488H dose-dependently decreased left-ventricular peak systolic pressure and beating rate in vitro and reduced blood pressure and heart rate in vivo. 3. Over the concentration range of 1-30 microM in vitro, and the dose-range of 0.5-32 mumol kg-1 in vivo, U-50,488H prolonged the P-R, QRS and Q-T intervals of the ECG. 4. The effects of U-50,488H were not antagonized by an opioid receptor antagonist, naloxone (1 microM or 8 mumol kg-1). Similarly, the opioid receptor antagonist, MR 2266, at 8 mumol kg-1 did not significantly reduce the cardiovascular actions of U-50,488H in vivo. 5. The actions of U-50,488H on responses to electrical stimulation were also studied. Over the dose range of 0.5-32 mumol kg-1, U-50,488H altered thresholds and effective refractory period. It had a biphasic action on thresholds for induction of ventricular fibrillation. Thresholds were decreased at lower doses (0.5-4 mumol kg-1) but increased at higher doses (8-32 mumol kg-1). The effects of lower doses were blocked by naloxone. Effective refractory period and threshold pulse width only increased with dose. 6. In conclusion, U-50,488H at high concentration, had direct depressant actions on cardiac contractility, electrical excitability and the ECG. These depressant effects were not antagonized by the opioid receptor antagonists, naloxone and MR 2266, and probably do not involve opioid receptors. Furthermore, some of the observed effects were those expected to result from sodium channel blockade.

Differential inhibitory effects of opioids on cigarette smoke, capsaicin and electrically-induced goblet cell secretion in guinea-pig trachea.

1. Goblet cell secretion in guinea-pig airways is under neural control. Opioids have previously been shown to inhibit neurogenic plasma exudation and bronchoconstriction in guinea-pig airways. We have now examined the effects of morphine and opioid peptides on tracheal goblet cell secretion induced by either electrical stimulation of the cervical vagus nerves, exogenous capsaicin, or acute inhalation of cigarette smoke. The degree of goblet cell secretion was determined by a morphometric method and expressed as a mucus score which is inversely related to mucus discharge. 2. Morphine, 1 mg kg-1, completely blocked goblet cell secretion induced by electrical stimulation of the vagus nerves. Morphine also inhibited the response to cigarette smoke given either at a low dose (10 breaths of 1:10 diluted in air), which principally activates cholinergic nerves, or at a high dose (20 breaths of undiluted), which activates capsaicin-sensitive sensory nerves, by 100% and 73% respectively. In contrast, morphine had no significant inhibitory effect on capsaicin-induced goblet cell secretion. The inhibitory effect of morphine was reversed by naloxone. 3. Selective mu- or delta-opioid receptor agonists, [D-Ala2, NMePhe4, Glyol5]enkephalin (DAMGO) or [D-Pen2, D-Pen5]enkephalin (DPDPE) respectively, caused a dose-related inhibition of low dose cigarette smoke-induced goblet cell discharge, with DPDPE more potent than DAMGO. A kappa-receptor agonist, trans-3,4-dichloro-N-methyl-N-(2-(1-pyrollidinyl)cyclohexyl) benzeneacetamine (U-50,488H), had no inhibitory effect. DPDPE had no inhibitory effect on goblet cell secretion induced by exogenous methacholine. 4. DAMGO dose-dependently blocked the response to high dose cigarette smoke with a maximal inhibition of 95% at 2 x 10(-7) mol kg-1. Neither DPDPE nor U-50,488H had any significant inhibitory effect. The increase in goblet cell secretion induced by exogenous substance P was not affected by DAMGO.5. We conclude that opioids inhibit neurally-mediated goblet cell secretion via actions at prejunctional delta and mu-receptors on cholinergic nerves and at mu-receptors on sensory nerve endings, and that capsaicin activation of sensory nerves is via a different mechanism from that of electrical or cigarette smoke activation.

Effects of acute and chronic ketocyclazocine and its modulation by oxytocin or vasopressin on food intake in rats

The effects of acute and chronic ketocyclazocine (KCZ, a kappa receptor agonist) and its interactions with oxytocin (OXY) or vasopressin (AVP) were investigated on food intake in free-fed rats. Acute treatment with KCZ (1 mg/kg) produced a generalized hyperphagia during the light phase (0–6 h) without influencing dark phase (6–24 h) food intake. On chronic administration, tolerance developed to hyperphagic effect during light phase, whereas an enhancement in the food intake was seen during dark phase. OXY or AVP (both at 10 μg/kg) per se, did not affect the food intake response during either the light or the dark phase, after acute as well as chronic treatment. In the interaction studies, acute AVP or OXY attenuated the hyperphagia of KCZ during the light phase. On chronic treatment, both AVP and OXY blocked (a) the tolerance, and (b) the “reverse tolerance” to the food intake response to KCZ during light and dark phases, respectively. These results are discussed in light of complex opioid-OXy/AVP interactiona during food intake in rats.

Synthesis, antinociceptive activity, and opioid receptor profiles of substituted trans-3-(decahydro- and octahydro-4a-isoquinolinyl)phenols.

A series of trans-3-(6- and 7-substituted-decahydro-4a-isoquinolinyl)phenols and trans-3-(octahydro-4a-isoquinolinyl)phenols have been synthesized as potential opioid analgesics. Using a combination of in vitro and in vivo test systems, the receptor profiles of selected compounds have been assessed and in some instances distinguish between mu- and kappa-receptor agonists. In general, introduction of a 6-exocyclic methylene group into the trans-3-(decahydro-4a-isoquinolinyl)phenol system enhanced both antinociceptive activity and kappa-opioid receptor selectivity. For each series, analogues bearing an N-cyclopropylmethyl substituent exhibited greater kappa-receptor selectivity while N-methyl derivatives showed greater mu-receptor selectivity. The 7-substituted compounds (3b) were significantly less potent antinociceptive agents than their 6-substituted counterparts (3a), the octahydroisoquinoline analogues exhibiting intermediate activity. The axial 8-methyl-6-exocyclic methylene isoquinoline (20) is the most potent compound in the mouse abdominal constriction assay (ED50 = 0.05 mg/kg sc), whereas the equatorial 8-methyl isomer (16) was significantly less potent (ED50 = 3.3 mg/kg sc).

The role of mu- and kappa-opioid receptors in cocaine-induced conditioned place preference.

Effects of buprenorphine, U-50,488H, naltrexone and lithium chloride on cocaine conditioned place preference were examined. Buprenorphine, a mixed opioid agonist-antagonist, blocked the cocaine-induced place preference. Furthermore, the kappa-receptor agonist U-50,488H and the mu-receptor antagonist naltrexone both antagonized the cocaine preference. U-50,488H or naltrexone alone induced a place aversion in a dose-dependent manner. However, the cocaine-induced conditioned place preference was not blocked by lithium chloride, although the latter induced a conditioned place aversion, indicating that the antagonism of cocaine-induced place preference by U-50,488H or naltrexone does not result from a functional antagonism. These results suggest that mu- and kappa-opioid receptors may be involved in cocaine-induced conditioned place preference.

The Role of Mu- and Kappa-Opioid Receptors in Cocaine-Induced Conditioned Place Preference

Effects of buprenorphine, U-50,488H, naltrexone and lithium chloride on cocaine conditioned place preference were examined. Buprenorphine, a mixed opioid agonist-antagonist, blocked the cocaine-induced place preference. Furthermore, the kappa-receptor agonist U-50,488H and the mu-receptor antagonist naltrexone both antagonized the cocaine preference. U-50,488H or naltrexone alone induced a place aversion in a dose-dependent manner. However, the cocaine-induced conditioned place preference was not blocked by lithium chloride, although the latter induced a conditioned place aversion, indicating that the antagonism of cocaine-induced place preference by U-50,488H or naltrexone does not result from a functional antagonism. These results suggest that mu- and kappa-opioid receptors may be involved in cocaine-induced conditioned place preference.

Analgesic and discriminative stimulus properties of U-62,066E, the selective kappa-opioid receptor agonist, in the rat.

The analgesic and discriminative stimulus properties of U-62,066E, a selective kappa-opioid receptor agonist, were investigated in the rat and compared with those of morphine. In the hot-plate test, U-62,066E produced a potent analgesic effect almost comparable to that of morphine. U-62,066E-induced analgesia was far less sensitive to antagonism by naloxone than was morphine-induced analgesia, but was potently reversed by MR-2266, a kappa-receptor antagonist. Although tolerance occurred to both U-62,066E and morphine analgesia, there was no cross-tolerance between these drugs. U-62,066E did show cross-tolerance to U-50,488H, another selective kappa-receptor agonist. Rats were trained to discriminate either 1.0 mg/kg U-62,066E or 3.2 mg/kg morphine from saline in a two-level food-reinforced procedure. The stimulus effect of U-62,066E was substituted for by U-50,488H and E-2078 a stable dynorphin derivative, but not by morphine. None of the kappa-agonists substituted for the morphine stimulus. Although U-62,066E stimulus by itself was not antagonized by MR-2266 or naloxone up to as high a dose as 10 mg/kg, the U-62,066E-like stimulus effect of U-50,488H was markedly blocked by MR-2266. The dopamine antagonists haloperidol and sulpiride substituted for the U-62,066E stimulus cue that was, however, not attenuated by the dopamine agonist lisuride. Lisuride reversed the U-62,066E-like stimulus induced by U-50,488H.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intravenous mu and kappa opioid receptor agonists on sensory responses of convergent neurones in the dorsal horn of spinalized rats.

1. Electrophysiological experiments have been performed to assess the effects of intravenously administered mu and kappa opioid agonists on the responses to noxious thermal and mechanical and non-noxious tactile stimuli of single convergent neurones in laminae III-VI of the dorsal horn of spinalized rats anaesthetized with alpha-chloralose. 2. The mu receptor agonists tested were fentanyl (1-16 micrograms kg-1) and morphine (0.5-16 mg kg-1) and the kappa-receptor agonists U-50,488 (1-16 mg kg-1) and tifluadom (0.1-1.6 mg kg-1). Multiple drug tests were made on each cell so that compounds could be compared under closely comparable conditions. 3. In one protocol, thermal and mechanical nociceptive responses of matched amplitudes were elicited alternately. Both mu and kappa agonists dose-dependently reduce the neuronal responses. Thermal nociceptive responses were as sensitive to the kappa agents as were the mechanical nociceptive responses; the mu agonists similarly reduced both types of response in parallel. 4. In another protocol, nociceptive and non-nociceptive responses were elicited alternately to permit the degree of selective antinociception to be assessed. The mu agonists were scarcely selective, fentanyl reducing nociceptive only slightly (but significantly at 4-16 micrograms kg-1) more than non-nociceptive responses. The kappa-opioid agonist U50,488 reduced tactile responses somewhat more than nociceptive responses. 5. The spontaneous discharge of these cells with ongoing activity was reduced to a significantly greater degree than the evoked responses; this is likely to have contributed to the non-selectivity of the reduction of the evoked responses. 6. The results are discussed with respect firstly to previous reports that K opioids are ineffective in tests of thermal nociception, and secondly to the likely spinal mechanisms by which opioid receptor agonists mediate antinociception.

Effect of opioid peptides on circular muscle of canine duodenum.

1. The effects of opioid peptides on inhibitory transmission in the circular muscle layer of canine duodenum were investigated in vitro using simultaneous mechanical and intracellular electrical recording techniques. 2. Exogenously added [Met5]enkephalin, [Leu5]enkephalin and dynorphin (1-13) decreased the amplitude of non-adrenergic, non-cholinergic inhibitory junction potentials (IJPs) evoked by transmural nerve stimulation. 3. A selective delta-receptor agonist, DPDPE ([D-Pen2, D-Pen5]enkephalin), and a selective mu-receptor agonist, PL017 (Try-Pro-NMePhe-D-Pro-NH2), decreased the amplitude of IJPs whereas a selective kappa-receptor agonist, U-50,488H ([trans-3,4-dichloro-N-methyl-N-(2-91-pyrolidinyl)-cyclohexyl]- benzeneacetamide methanesulphonate), in large doses (1 microM) produced only a small reduction. 4. A selective delta-receptor antagonist, ICI-174,864, blocked the effect of DPDPE but not that of PL017 suggesting the presence of distinct delta- and mu-opioid receptors on inhibitory motor nerves. 5. Exogenously added dynorphin (1-13) decreased the amplitude of IJPs. delta-Opioid receptors appeared to be involved because ICI-174,864, a selective delta-antagonist, blocked the inhibitory effect of exogenously added dynorphin (1-13). 6. The inhibitory effect of the opioid peptides was still observed in preparations of circular muscle devoid of myenteric and submucosal plexuses, indicating that the site of action was on inhibitory motor nerve fibres located within the circular muscle layer and not on neuronal cell bodies in the enteric plexuses. 7. It was concluded that in the canine small intestine, opioid peptides could modulate release of inhibitory transmitter(s) at or near nerve terminals of inhibitory motor nerves innervating circular muscle cells.

P-163 - Effect of spiradoline, a kappa receptor agonist, on Ca2+-induced contraction and intracellular Ca concentration in porcine coronary artery.

P-163 - Effect of spiradoline, a kappa receptor agonist, on Ca2+-induced contraction and intracellular Ca concentration in porcine coronary artery.

Identification of opioid receptors on gastric muscle cells by selective receptor protection.

Opioid receptors on isolated gastric smooth muscle cells were characterized pharmacologically by a technique in which synthetic selective opioid agonists and antagonists were used to protect and thus enrich a specific receptor type while all other receptors were inactivated by N-ethylamaleimide. Treatment of the cells with the selective mu-receptor agonist DAGO or antagonist CTAP preserved only the response to DAGO; treatment with the selective delta-receptor agonist DPDPE or antagonist naltrindole preserved only the response to DPPE; and treatment with the selective kappa-receptor agonist U50,488H or antagonist nor-binaltorphimine preserved only the response to U50,488H. The results established the presence of distinct kappa-, delta-, and mu-opioid receptors capable of mediating contraction of isolated gastric muscle cells. The pattern of interaction of endogenous opioid peptides with protected receptors implied that dynorphin-(1-13) and Met-enkephalin were selective agonists for kappa- and delta-opioid receptors, respectively, and Leu-enkephalin a preferential agonist of mu-opioid receptors. The results were confirmed by a reverse approach in which opioid receptors were inactivated by site-directed irreversible antagonists. beta-Funaltrexamine, a mu-selective antagonist, abolished the response to mu-receptor agonists, whereas beta-chlornaltrexamine, a mu- and kappa-selective antagonist, abolished the response to mu-receptor agonists and partially inhibited the response to kappa-receptor agonists.

Epidural Morphine With Butorphanol for Postoperative Analgesia After Cesarean Delivery

Epidural morphine has been used more and more to provide long-lasting postoperative analgesia after cesarean delivery. However, the incidence of pruritus (20%-93%) and nausea (17%-60%) detract from the usefulness of epidural morphine. The purpose of this study was to evaluate, in 30 patients having epidural anesthesia for cesarean delivery, the analgesic efficacy and side effects when a combination of epidural morphine, a mu-receptor agonist, and butorphanol, a mu-receptor antagonist and kappa-receptor agonist, was administered. After clamping of the umbilical cord, patients received 4 mg epidural morphine with 3 mL of normal saline (group 1), 4 mg epidural morphine with 1 mg butorphanol and 2 mL of normal saline (group 2), or 4 mg epidural morphine with 3 mg butorphanol (group 3). Patients were monitored for 24 h after administration of the study medications. There were no significant differences between the groups in visual analogue pain scores, time to first analgesic request, respiratory rate, or Trieger dot test performance in the 24 h immediately after these epidural injections. There were three patients in group 1 and one patient in group 2 who experienced oxygen saturations less than 90%. (No patients in group 3 developed an oxygen saturation less than 92%.) The patients in group 3 did not require treatment for pruritus or nausea, a response significantly different (P less than 0.001 and P less than 0.05, respectively) from group 1 or group 2.(ABSTRACT TRUNCATED AT 250 WORDS)

? and ? Opiate receptors in primary astroglial cultures from rat cerebral cortex

The effects of mu, delta, and kappa receptor-agonists on forskolin stimulated cyclic adenosine-3',5'-monophosphate (cAMP) formation were examined in astroglial enriched primary cultures from the cerebral cortex of newborn rats. Intracellular cAMP accumulation was quantified by radioimmunoassay. Morphine was used as a mu-receptor agonist, D-Ala-D-Leu-Enkephalin (DADLE) as a delta-receptor agonist and dynorphin 1-13 (Dyn) as a kappa-receptor agonist. Basal cAMP levels were unaffected by either the opiate agonists or the antagonists used. In the presence of the cAMP stimulator forskolin, morphine had no significant effect on the cytoplasmic cAMP levels. DADLE caused a dose related inhibition of the forskolin stimulated cAMP accumulation. The effects of this delta receptor stimulation was blocked with the selective antagonist ICI 174.864. In the presence of Dyn, the forskolin stimulated cAMP accumulation was inhibited in a dose related manner. This kappa receptor stimulation was blocked with the selective antagonist MR 2266. Co-administration of DADLE and Dyn resulted in a non additive inhibition of the forskolin stimulated accumulation of cAMP. These findings indicate that astroglial enriched cultures from the cerebral cortex of rats express delta and kappa-receptors co-localized on the same population of cells, and that these receptors are inhibitory coupled to adenylate cyclase.

Different profile of electrocortical power spectrum changes after micro‐infusion into the locus coeruleus of selective agonists at various opioid receptor subtypes in rats

1. The effects of various opioid receptor agonists given directly by means of a chronically implanted cannula into the locus coeruleus (LC) on behaviour and ECoG activity, continuously analysed, and quantified as total power spectrum (0-16 Hz) and in preselected frequency bands (0-3; 3-6; 6-9; 9-12 and 12-16 Hz), were studied in rats. 2. Dermorphin (0.05, 0.5, 1, 2 and 5 pmol) and Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (DAMGO; 1, 10, 30, 100 pmol and 1 nmol), two typical mu-receptor agonists, applied unilaterally or bilaterally directly into the LC, produced a typical dose-dependent ECoG synchronization with a significant increase in total power spectrum as well as in the lower frequency bands. Dermorphin was found to be approximately 30 times more powerful than DAMGO in producing similar quantitative ECoG changes. 3. D-Ala-D-Leu-Thr-Gly-Gly-Phe-Leu (DADLE; 1, 10, 50 and 100 pmol), a selective delta-receptor agonist, micro-infused into the LC produced dose-dependent behavioural soporific effects and ECoG increase in total power spectrum as well as in 3-6, 6-9, 9-12 Hz frequency bands. In comparison to dermorphin, the ECoG power spectrum effects of DADLE were 10 fold less potent, whereas in comparison to DAMGO it was approximately 3 times more potent. A lower dose (0.1 pmol) was ineffective in changing behaviour and ECoG power spectrum. 4. The microinfusion into the LC of U 50, 488H, a selective Kappa-opioid receptor agonist, (0.25, 1, 2.5, 5 and lOpmol) produced a typical pattern characterized by a first short-lasting (3-25 min) phase of behavioural arousal and ECoG desynchronization, followed by a longer lasting (20-130min according to the dose) phase of behavioural sleep and ECoG synchronization. A lower dose (0.1 pmol) was ineffective in changing behaviour and ECoG power spectrum. 5. Dextromethorphan and ketamine, two selective agonists at sigma-receptors given into the LC (1, 5 and 1Opmol) induce behavioural arousal, increase in locomotor activity and an intense pattern of stereotypedm movements. However, by increasing the dose of ketamine (50 and lOOpmol), marked sedation, postural changes and an increase in low frequency ECoG bands, sometimes associated with high amplitude fast frequency potentials, were observed. 6. Naloxone applied directly into the LC (1 and 2 pmol 15min before) was able to prevent the behavioural and ECoG effects induced by dermorphin, DAMGO and DADLE. Higher doses of naloxone (1Opmol into the LC) were however, required to antagonize the behavioural and ECoG soporific effects induced by the Kappa-receptor agonist U 50,488H. In contrast, naloxone (1Opmol into the LC) was unable to prevent or reduce the behavioural and ECoG effects induced by subsequent administration into the same site of dextromethorphan and ketamine. 7. In conclusion, the present experiments confirm that behavioural and ECoG effects elicited following stimulation of mu-, delta-, Kappa- and sigma-opioid receptors located in the LC are quite different. Activation of ,mu-, band Kappa-receptors induced sedative effects whereas dextromethorphan and ketamine, two sigma-receptor agonists, induced behavioural arousal and ECoG desynchronization. In addition, the present results strongly support the crucial role played by opioid mechanisms, in the locus coeruleus, in the mediation of the soporific effects of drugs acting as agonists at opioid receptors.

Evidence that the agonist action of dynorphin A(1–8) in the guinea‐pig myenteric‐plexus may be mediated partly through conversion to [Leu5]enkephalin

1. The agonist action of the opioid peptide dynorphin A(1-8) on the myenteric plexus-longitudinal muscle of the guinea-pig ileum has been characterized. 2. The endogenous opioid peptide dynorphin A(1-8) was rapidly degraded by slices of myenteric plexus-longitudinal muscle of the guinea-pig ileum. 3. A product of the degradation was the delta-receptor preferring [Leu5]enkephalin. Levels of [Leu5]enkephalin were markedly increased in the presence of the peptidase inhibitors bestatin, thiorphan and captopril. 4. In the myenteric plexus dynorphin A(1-8) acted as a kappa-receptor agonist. In the presence of bestatin, thiorphan and captopril a mu-receptor agonist effect was observed. This mu-agonist action was lost in the presence of N-[1-(RS)-carboxy-2-phenylethyl]Ala-Ala-Phe-p-aminobenzoate, an inhibitor of the endopeptidase enzyme EC 3.4.24.15. 5. The results suggest that formation of [Leu5]enkephalin from dynorphin A(1-8) may be an important conversion process. The enzyme responsible may be the Zn2(+)-metalloendopeptidase, EC 3.4.24.15.

Effects of RP 60180, a kappa-receptor agonist, on dopamine metabolism and utilization in rat and guinea-pig brain

Effects of RP 60180, a kappa-receptor agonist, on dopamine metabolism and utilization in rat and guinea-pig brain

Kappa‐Selective Opioid Receptor Agonists Leumorphin and Dynorphin Inhibit Supraoptic Neurons in Rat Hypothalamic Slice Preparations

Abstract To clarify the effects of opioid peptides, and in particular the effects of kappa-receptor agonists on the activity of supraoptic neurons, extracellular recordings were made from 71 spontaneously firing neurons in the rat hypothalamic slice preparation. Of 71 neurons, 28 showed a phasic firing pattern (phasic neurons: putative vasopressin neurons). The mean firing rate of phasic neurons was 2.6 spikes/s (intraburst firing rate 5.4 +/- 2.2 spikes/s). The mean firing rate of neurons classified as non-phasic neurons (putative oxytocin neurons) was 4.5 spikes/s. Following bath application of leumorphin (LM) at 10(-7) M, which has potent opioid activity at kappa-receptors, 17 (61%) of 28 phasic neurons were inhibited and 22 (51%) of 43 non-phasic neurons were inhibited. Excitation was observed in only one non-phasic neuron. The dose-dependence of the response to LM was tested in five supraoptic neurons. There was an inverse relationship between LM concentration and percent change in firing rate. The threshold concentration of LM was approximately 10(-8) M. The relatively selective kappa-receptor antagonist, MR-2266, completely blocked the LM-induced responses. Its effects were long-lasting and only partial recovery was observed 2 h after the application of MR-2266. Dynorphin had similar inhibitory effects on supraoptic neurons to those obtained with LM when tested on the same neurons. In another series of experiments the mu-receptor agonist morphine and the delta-receptor agonist [D-Ala, D-leu]-enkephalin (DADLE) were applied to 28 supraoptic neurons (12 phasic and 16 non-phasic neurons) at 10(-7) M and their actions compared directly with that of LM. Only two of 12 phasic neurons tested were inhibited by DADLE and none of five phasic neurons tested was inhibited by morphine, while eight of the 12 neurons tested were inhibited by LM. By contrast the non-phasic neurons tested were inhibited by application of each of the peptides; seven of 16 neurons tested were not only inhibited by LM, but also five of 11 neurons by DADLE and seven of 15 by morphine. The magnitude of the responses varied from cell to cell. These results suggest that LM acts at the same receptors as dynorphin, and that opioids acting preferentially at kappa-receptors inhibit both vasopressin and oxytocin neurons while delta- and mu-receptor agonists inhibit primarily oxytocin neurons.

Modulation of cholinergic neurotransmission in guinea‐pig airways by opioids

1. Opioid receptors have been localised on sensory fibres in the vagus nerve and opioids have previously been shown to inhibit non-adrenergic, non-cholinergic (NANC) neurotransmission in guinea-pig bronchi in vitro and in vivo. We have now investigated whether an inhibitory effect could be demonstrated on cholinergic neurotransmission. 2. Electrical field stimulation (EFS) (8 Hz, 0.5 ms, 40 V for 20 s) produced only a rapid, cholinergic response in the upper trachea but in the lower trachea and main bronchi a cholinergic response which was atropine-sensitive and a longer lasting NANC contraction that was atropine-insensitive was demonstrated. This slow contraction could be blocked by tetrodotoxin and capsaicin pretreatment. 3. [D-Ala2, NMePhe4, Gly-ol5]enkephalin (DAMGO), a selective mu-opioid receptor agonist, inhibited the cholinergic response to EFS at 8 Hz in a dose-dependent manner in main bronchi (IC50 = 113 nM with a maximal inhibition of 35.7 +/- 5.6% 10 microM, n = 5). In the lower trachea, DAMGO inhibited the cholinergic response to a similar extent (inhibition of 35.8 +/- 3.5% at 10 microM, n = 5). However, DAMGO had no effect on the contractile response to exogenously applied acetylcholine in the main bronchi. By contrast, opioids had no inhibitory effect on cholinergic neurotransmission in the upper trachea. DAMGO (1 microM) inhibited the cholinergic response to EFS in a frequency-dependent manner in the main bronchi with greater inhibition at lower frequencies of stimulation. 4. The delta-opioid receptor agonist [D-Pen2, D-Pen5]enkephalin (DPDPE) significantly inhibited the cholinergic component of the constrictor response to EFS at 8 Hz in the bronchi but at the highest dose used (10 microM). U-50,488H, a Kappa-receptor agonist, had no inhibitory effect on the cholinergic constrictor component in the main bronchi (10microM). 5. DAMGO also inhibited the NANC responses to EFS in the main bronchi in a dose-dependent manner (with an IC50 = 36 nm and a maximal inhibition of 63.4 + 8.3%, at 1 microM, n = 5). DAMGO had no effect on contractile responses to exogenously applied substance P (SP). DPDPE (10 microM) was less effective in inhibition of the NANC bronchoconstriction with a maximal inhibition of 29.2 + 4.2% (n = 7), and U-50,488H (1O microM) had no inhibitory effect. 6. After capsaicin pretreatment, which depleted sensory nerves of neuropeptides, the inhibitory effect of DAMGO (1 microM) on cholinergic constriction in main bronchi at 8 Hz was only 13.4 + 1.9% (n = 13) compared with 32.9 + 4.0% (n = 9) inhibition in vehicle-treated controls (P < 0.001). 7. Opioids may reduce the cholinergic neural responses in airways partly via an inhibitory action on excitatory NANC nerves and partly by a direct effect on cholinergic neurotransmission. The opioid receptor involved is of the mu-opioid receptor subtype.

The effect of transcutaneous electrical nerve stimulation (TENS) on catecholamine metabolism during pacing-induced angina pectoris and the influence of naloxone

Two invasive studies (invasive study I and invasive study II) showed positive effects of transcutaneous electrical nerve stimulation (TENS) in pacing-induced angina pectoris in terms of increased tolerance to pacing, improved lactate metabolism and less anginal pain. Invasive study I demonstrated a decrease in left ventricular afterload by TENS treatment as reflected by a fall in systolic blood pressure and this fact was thought to be explained by reduced sympathetic activity since arterial levels of epinephrine and norepinephrine dropped during TENS in TENS responders. In invasive study II, the influence of naloxone on the effects of TENS in pacing-induced angina pectoris was studied in 11 patients with severe coronary artery disease. The patients were catheterized and treated with TENS on 2 occasions; one with a single intravenous (i.v.) dose of saline as placebo and one with a single i.v. dose of 50 mg naloxone, double-blind, in random order. Treatment with TENS increased tolerance to pacing ( P < 0.01 with placebo and P < 0.01 with naloxone, respectively) and improved lactate metabolism ( P < 0.05 with placebo and P < 0.01 with naloxone, respectively). The positive effects of TENS were thus reproducible and not reversed by single i.v. doses of naloxone. The results of this study indicate that the effects of TENS on the heart are not mediated by beta-endorphin but do not exclude activation of more short-acting opioids like delta or kappa receptor agonists (met-enkephalin and/or dynorphin) since naloxone has a low affinity for these receptors. It is also possible that non-opioid mechanisms are of importance. However, this study did not exhibit any data that confirmed the hypothesis that the effects of TENS in angina pectoris can be explained by a reduced sympathetic activity, since the arterial levels of epinephrine and norepinephrine did not change and the systolic blood pressure did not decrease during TENS treatment.