Species differences in mu- and delta-opioid receptors.

The mu opioid receptor ligand [D-Ala2, NMePhe4, Gly-ol5]enkephalin (DAGO) and delta opioid receptor ligand [D-Pen2,D-Pen5]enkephalin (DPDPE) show similar specificity in competition binding studies in whole brain homogenate in rat and mouse. However, in saturation studies, the density and affinity of DPDPE binding sites were substantially greater in the mouse. There was no difference between the mouse and rat in the density and affinity of DAGO sites. Results from dose-response studies for analgesia using the same ligands administered i.c.v. in both species paralleled the binding studies. DAGO was approximately 2 times more potent in the mouse compared to the rat; while DPDPE was more than 15 times more potent in the mouse. Thus, binding capacity and affinity differences appear to be related to the functional potency of the mu and delta ligands in the two species. These results suggest that the difference in potency of DPDPE between rat and mouse is related to the differences in brain delta opioid receptors.     

Relative potencies of metal ions on transmitter release at mouse motor nerve terminals.

1. The effects of a range of metal ions were systematically studied at the mouse neuromuscular junction in order to investigate the type of calcium channel present at the nerve terminal. 2. Endplate potentials and miniature endplate potentials were recorded from the phrenic nerve diaphragm muscle preparation with glass microelectrodes. 3. Endplate potential amplitudes and quantal contents were reduced by manganese (IC50 220 microM), cadmium (IC50 11 microM), cobalt (IC50 350 microM), and nickel (IC50 420 microM). Miniature endplate potentials were not affected by these ions at concentrations equal to the IC50s. Gadolinium did not reduce endplate potentials up to 100 microM. 4. Comparisons made with known channel types in neuroblastoma cell lines suggest that the calcium channels at the motor nerve terminal are different from those types studied in the cell lines, although most similarity is shown to the high-voltage activated calcium channel types.     

Diversity among mouse motor nerve terminals with respect to release transmitter quanta.

1. The aim of this work was to reexamine whether a positive correlation exists between the frequency (F, sec-1) of miniature endplate potentials (m.e.p.ps) and the quantal content (m) of endplate potentials (e.p.ps) or between quantal content, frequency and twin-pulse facilitation of transmitter release at a large number of neuromuscular junctions in the mouse. 2. The values of F and m were both measured intracellularly at endplates of mouse diaphragm in a high Mg2+/low Ca2+ bathing solution. 3. Values of both F and m varied from junction to junction. Smaller values of F were correlated with smaller values of m, and vice versa, resulting in a linear relationships. Histograms of F and m were skewed towards smaller values. 4. E.p.ps evoked by twin pulses gave the quantal contents of the first (m1) and second (m2) responses. 5. The ratio of m2 to m1 varied from junction to junction. A histogram of this ratio was skewed towards smaller values. 6. The ratio of m2 to m1 showed larger fluctuations at junctions with smaller values of F or m1 but was focused around 1 at junctions with larger values of F or m1. 7. The skewed parts of the histograms of F, m and m2/m1 accounted for the major population of junctions. 8. These results support the hypothesis that an intrinsic ability to release transmitter plays a role in regulation of the evoked output of transmitter at neuromuscular junctions in the mouse. 9. Such an ability is not correlated with the twin-pulse facilitation of transmitter release.     

Contribution of supraspinal mu- and delta-opioid receptors to antinociception in the rat.

This study evaluated the contribution of supraspinal opioid receptors to the production of antinociception, in the rat. I.c.v. administration of a selective mu- (DAMGO) and a selective delta- (DPDPE), but not a selective kappa- (U50,488H) opioid receptor agonist, produced significant dose-dependent increase in mechanical nociceptive thresholds. ICI 174,864, a delta-opioid receptor antagonist, completely blocked the antinociceptive effects produced by DPDPE ([D-Pen2,D-Pen5]enkephalin) at a dose that had no effect on the increases in nociceptive thresholds produced by DAMGO ([D-Ala2,N-MePhe4,Gly5-ol]enkephalin). The simultaneous i.c.v. administration of a low-antinociceptive dose of DAMGO or DPDPE given in combination with sequentially increasing doses of the other opioid agonist, produced synergy (i.e., a more than additive antinociceptive effect), at the lower doses tested. The results of these experiments provide evidence to support the suggestion that both supraspinal mu- and delta-opioid receptors contribute to the production of antinociception, in the rat.     

Differential binding properties of oripavines at cloned mu- and delta-opioid receptors.

This study examines the possibility that oripavine opioid receptor agonists bind equally to both high and low affinity states of the mu-opioid receptor. Studies were performed in C6 cells expressing mu- or delta-opioid receptors; high and low agonist affinity states of the receptors were defined by the absence and presence, respectively of Na+ ions and the GTP analog Gpp(NH)p. At the mu-opioid receptor dihydroetorphine and etorphine were full agonists, buprenorphine had moderate efficacy while diprenorphine was an antagonist. At the delta-opioid receptor, dihydroetorphine, etorphine, and diprenorphine had moderate efficacy while buprenorphine was an antagonist. The binding affinities of the oripavines at the mu-opioid receptor decreased only one to 2-fold in the presence of NaCl and Gpp(NH)p. In contrast, decreases in oripavine affinity at the delta-opioid receptor correlated with delta-opioid receptor efficacy. The ability of oripavine agonists to bind with high affinity to the low agonist affinity state of the nu-opioid receptor may explain the high potencies of these compounds in vivo.     

Enhancement by an antagonist of transmitter release from frog motor nerve terminals.

1 The effect of Ba2+ on the synchronous release of acetylcholine from frog motor nerve terminals was studied by conventional electrophysiological techniques. 2 When Ca2+ and Ba2+ were the only divalent cations in the bathing fluid, Ba2+ caused a presynaptic reduction in the amplitude of the endplate potential (e.p.p.). This effect was surmountable by increasing the Ca2+ concentration. 3 The affinity constant (KA) for Ba2+, calculated on the assumption that Ba2+ is a competitive inhibitor of the agonist, Ca2+, was 1.1 +/- 0.4 mM-1 (mean +/- s.e. mean, n = 8). 4 When e.p.ps were depressed by the addition of 1 mM Mg2+, addition of Ba2+ (1 to 3 mM) caused either a further presynaptic depression of moderate magnitude or had no additional effect. 5 When e.p.p.s were depressed with [Mg2+] greater than or equal to 2 mM, addition of Ba2+ greater than or equal to 0.9 mM enhanced the e.p.p. amplitude by a presynaptic mechanism. 6 The interaction of the divalent cation antagonists Mg2+ and Ba2+ with the agonist, Ca2+ is discussed. It is demonstrated that a model which considers the nonequilibrium, kinetic properties of binding can be used to describe interactions between divalent cations at the external surface of the motor nerve ending.     

The in vivo effect of lipopolysaccharide on the spontaneous release of transmitter from motor nerve terminals.

1. The in vivo effect of E. coli lipopolysaccharide (LPS) on the spontaneous release of transmitter was studied in the isolated phrenic nerve-diaphragm preparation of the mouse. 2. The resting membrane potential was decreased and frequency of miniature endplate potentials (m.e.p.ps) was increased by treatment with LPS. 3. Pretreatment of diaphragms with ouabain markedly increased the frequency of m.e.p.ps in control group but not in the LPS group. 4. When mice were treated with polymyxin B (a LPS neutralizer), pentoxifylline (an inhibitor of tumor necrosis factor-alpha formation) and NG-nitro-L-arginine (an inhibitor of nitric oxide (NO) synthase) the effects of LPS were reversed. 5. These results suggest that LPS increases the spontaneous transmitter release through, at least in part, the pathways of tumour necrosis factor-alpha and NO followed by an inhibition of the Na(+)-pump activity in the endplate area.     

RGSZ1 and GAIP regulate mu- but not delta-opioid receptors in mouse CNS: role in tachyphylaxis and acute tolerance.

In the CNS, the regulators of G-protein signaling (RGS) proteins belonging to the Rz subfamily, RGS19 (G(alpha) interacting protein (GAIP)) and RGS20 (Z1), control the activity of opioid agonists at mu but not at delta receptors. Rz proteins show high selectivity in deactivating G(alpha)z-GTP subunits. After reducing the expression of RGSZ1 with antisense oligodeoxynucleotides (ODN), the supraspinal antinociception produced by morphine, heroin, DAMGO ([D-Ala2, N-MePhe4,Gly-ol5]-enkephalin), and endomorphin-1 was notably increased. No change was observed in the effect of endomorphin-2. This agrees with the proposed existence of different mu receptors for the endomorphins. The activities of DPDPE ([D-Pen2,5]-enkephalin) and [D-Ala2] deltorphin II, agonists at delta receptors, were also unchanged. Knockdown of GAIP and of the GAIP interacting protein C-terminus (GIPC) led to changes in agonist effects at mu but not at delta receptors. The impairment of RGSZ1 extended the duration of morphine analgesia by at least 1 h beyond that observed in control animals. CTOP (Cys2, Tyr3, Orn5, Pen7-amide) antagonized morphine analgesia when given during the period in which the effect of morphine was enhanced by RGSZ1 knockdown. Thus, in naive mice, morphine tachyphylaxis originated in the presence of the opioid agonist and during the analgesia time course. The knockdown of RGSZ1 facilitated the development of tolerance to a single dose of morphine and accelerated tolerance to continuous delivery of the opioid. These results indicate that mu but not delta receptors are linked to Rz regulation. The mu receptor-mediated activation of Gz proteins is effective at recruiting the adaptive mechanisms leading to the development of opioid desensitization.     

Pharmacological profile of various kappa-agonists at kappa-, mu- and delta-opioid receptors mediating presynaptic inhibition of neurotransmitter release in the rat brain.

1. The potency, relative efficacy and selectivity of a series of kappa-opioid receptor agonists at the mu-, delta- and kappa-opioid receptors mediating inhibition of electrically-induced (radiolabelled) neurotransmitter release from superfused rat brain slices was determined. 2. With regard to their potencies at kappa-receptors mediating inhibition of striatal [3H]-dopamine release, the highest pD2 value (8.7) was found for bremazocine and the lowest (7.1) for U50488; the pD2 values for ethylketocyclazocine (EKC), tifluadom, U69593 and PD117302 were between 8.0 and 8.3. There were no marked differences between the relative efficacies of the kappa-agonists (maximum inhibition being 60-70%). In contrast to the other kappa-agonists, at a concentration of 1 microM, PD117302 caused a significant (25-40%) increase of the spontaneous efflux of tritium. 3. None of the kappa-agonists significantly affected striatal [14C]-acetylcholine (ACh) release, with the exception of a slight inhibitory effect of EKC. The delta-receptor-mediated inhibitory effect of [D-Ala2, D-Leu5]enkephalin (DADLE) on [14C]-ACh release was antagonized in a concentration-dependent manner by bremazocine (0.1 and 1.0 microM) and also partially by EKC (1 microM), but not by the other kappa-agonists. The pA2 value for bremazocine as an antagonist at the delta-receptors involved was 8.0, compared to 7.6 for naloxone. 4. None of the kappa-agonists significantly affected cortical [3H]-noradrenaline (NA) release, with the notable exception of tifluadom, which strongly inhibited release by activating mu-receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of mu- and delta-opioid receptors in the nucleus accumbens in turning behaviour of rats

The role of mu-, delta1- and delta2-opioid receptors in the nucleus accumbens in pivoting was investigated in freely moving rats. Unilateral injections of the mu-opioid receptor agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO, 1 and 2 microg) and the delta2-opioid receptor agonist, deltorphin II (1 and 2 microg), but not the delta1-opioid receptor agonist, [D-Pen(2,5)]-enkephalin (DPDPE, 1-4 microg), into the shell or the core of the nucleus accumbens significantly induced contraversive pivoting. The pivoting induced by DAMGO (2 microg) and deltorphin II (2 microg) was inhibited significantly by the mu-opioid receptor antagonist, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH2 (CTOP, 0.1 and 1 microg), and the delta2-opioid receptor antagonist, naltriben (NTB, 0.1 and 1 mg/kg, i.p.), respectively. The DAMGO (2 microg)- or deltorphin II (2 microg)-induced pivoting was also inhibited significantly by co-administration of the dopamine D1/D2 receptor antagonist, cis(Z)-flupentixol (1 and 10 microg). The pivoting induced by unilateral injections of a mixture of dopamine D1 (SKF 38393, 5 microg) and D2 (quinpirole, 10 microg) receptor agonists into the shell was significantly inhibited by cis(Z)-flupentixol (1 and 10 microg) or NTB (1 and 3 mg/kg, i.p.), but not CTOP (1 microg) or delta1-opioid receptor antagonist, (E)-7-benzylidenenaltrexone (1 mg/kg, i.p.). The contraversive pivoting elicited by the cholinergic agonist, carbachol (5 microg), into the core was inhibited by co-administration of the muscarinic M1 antagonist, pirenzepine (1 microg), but not cis(Z)-flupentixol (1 microg). The results suggest that unilateral activation of mu- or delta2-opioid, but not delta1-opioid, receptors in the core and/or shell of the nucleus accumbens elicits contraversive pivoting that requires intact dopamine D1/D2 receptors in the shell, but not intact muscarinic M1 mechanism in the core. The study also shows that delta2-opioid, but not mu- and delta1-opioid, receptors in the core and/or shell modulate the shell-specific, dopamine D1/D2 receptor mechanisms involved in the production of pivoting.     

Regulation of transmitter synthesis and release in mesolimbic dopaminergic nerve terminals: Effect of ethanol

Slices from rat olfactory tubercle were incubated in freshly oxygenated Krebs-Ringer phosphate (KRP) and in the presence of l-tyrosine [¹⁴C-U] as dopamine (DA) precursor. Thereafter, the newly synthesized [¹⁴C]DA and the [¹⁴C]DA released into the incubation media were isolated by Alumina column, and ion-exchange, chromatography. The presence of K⁺ depolarizing concentrations (25–70 mM) in the incubation media markedly increased the formation of [¹⁴C]DA from [¹⁴C]tyrosine, following a rather complex and biphasic pattern. Dibutyryl cyclic AMP (dB-cAMP) and theophylline also increased the formation of newly synthesized [¹⁴C]DA. Ethanol (0.2 to 0.4%. w/v significantly blocked the stimulation of [¹⁴C]DA biosynthesis that was induced by low K⁺ depolarizing concentrations (25 mM) and by dB-cAMP (5 × 10^?4M) or theophylline (1 × 10³M). In contrast, only higher ethanol concentrations (0.8 to 1.1%, w/v) blocked the increase in DA formation induced by high K⁺ depolarizing concentrations (40 and 55 mM). Potassium depolarization (40 mM) markedly evoked the release of newly synthesized [³H]DA or [³H]DA previously taken up by the slices. The release was shown to be dependent upon the presence of Ca²⁺ and inhibited by an excess of Mg²⁺ (12mM). Ethanol (0.8 to 1.1%, w/v) produced no effect on K⁺-induced release of [³H]DA. The model described in this paper can be used as a simple experimental tool to study neurotransmitter synthesis and release from nerve terminals belonging to the mesolimbic dopaminergic system. The results reported suggest the existence of at least two mechanisms by which neuronal depolarization increases transmitter formation in mesolimbic dopaminergic terminals. Ethanol, at relatively low concentrations, seems to produce a specific inhibitory effect upon the mechanism that predominates under low depolarizing conditions. The possibility is raised that the effects described for ethanol may play a role in the neuropharmacological responses induced by this agent in vivo.     

Differential effects of various secretagogues on quantal transmitter release from mouse motor nerve terminals treated with botulinum A and tetanus toxin

Electrophysiological and electron microscopic techniques were used to investigate the actions of potassium depolarization, black widow venom (BWSV), Ca2+-ionophore A 23187 and hyperosmotic solution on mouse hemidiaphragms poisoned in vitro with botulinum A toxin (BoTx) and tetanus toxin (TeTx). These neurotoxins reduced the frequency of miniature endplate potentials (m.e.p.ps) from 5/s of the control to 2/min and 21/min, respectively. High potassium (25 mmol/l) increased the m.e.p.p.-frequency at BoTx- and TeTx-poisoned endplates to 30/min and 50/s, respectively. The ultrastructure of endplates showed no obvious changes. BWSV (0.04 glands/ml) was just as effective in promoting transmitter release from BoTx-treated endplates as in control preparations. Electron micrographs revealed depletion of vesicles as well as swollen and disrupted mitochondria. When preparations were pretreated with TeTx, BWSV only moderately increased transmitter release and no alterations of the ultrastructure could be observed. At TeTx- or BoTx-poisoned endplates Ca2+-ionophore A 23187 usually produced an extreme reduction of m.e.p.p.-frequency (0.005/s), sometimes preceded by a short burst-like release. The ultrastructure of these endplates was not obviously affected. Application of hyperosmotic solution to BoTx- or TeTx-poisoned preparations further reduced the already low m.e.p.p.-frequency. These results further demonstrate that TeTx and BoTx act at different sites in the transmitter releasing process.     

Modification of transmitter release from periarterial nerve terminals by dipyridamole in canine isolated splenic artery

1. The aim of the present study was to determine the modulatory effects of dipyridamole on purinergic and adrenergic transmission in the canine isolated, perfused splenic artery. 2. Periarterial nerve electrical stimulation readily induced a double-peaked vasoconstriction consisting of an initial transient, predominantly P2X receptor-mediated constriction followed by a prolonged, mainly alpha1-adrenoceptor-mediated response. 3. Exposure of tissues to dipyridamole (0.1-1 micro mol/L) dose-dependently inhibited both the first and second peaks of the vasoconstrictor response at a low frequency of stimulation (1 Hz), whereas at an intermediate frequency of stimulation (4 Hz), the first peak of the response was depressed without any significant effect being observed on the second peak of constriction. 4. At a higher dose (1 micro mol/L) dipyridamole potentiated vasoconstrictor responses to noradrenaline (0.03-1 nmol). At any doses used, dipyridamole had no effect on the vasoconstrictor responses to ATP (0.03-1 micro mol). 5. Tyramine (0.01-0.3 micro mol) induced vasoconstriction in a dose-dependent manner. The dose-response curves for tyramine were shifted to the right following treatment with dipyridamole (0.1-1 micro mol/L). 6. The present results indicate that dipyridamole may inhibit purinergic and adrenergic transmission presynaptically, whereas postsynaptically dipyridamole may potentiate the adrenergic vascular constriction by inhibition of transmitter uptake.     

Effect of opiates on transmitter release from visualized hypogastric boutons innervating the rat pelvic ganglia.

1. The effect of opiates on neurotransmission between visualized hypogastric nerve boutons and postganglionic cell bodies has been examined using extracellular recording of nerve bouton impulses (NBIs) and excitatory postsynaptic currents (e.p.s.cs). 2. Morphine (10 to 40 microM) did not affect neurotransmission in the ganglia. Dynorphin-A (4 microM) and U50488H (1 microM) decreased quantal transmitter release and naloxone (10 microM) reversed these effects. 3. Morphine (10 microM), dynorphin-A (4 microM) and U50488H (1 microM) did not affect either the time course or consistency with which the NBI was recorded. 4. Dynorphin-A (1 to 4 microM) and U50488H (1 microM) decreased the average amplitude of e.p.s.cs by increasing the number of failures to release quanta from single or small groups of 2 to 4 boutons during continuous nerve stimulation at 0.1 Hz. 5. The decrease in quantal release induced by dynorphin-A and U50488H in 0.2 to 0.5 mM [Ca2+]zero was readily reversed by increasing the extracellular calcium ion concentration to 1 mM. 6. It was concluded that kappa-opioid receptors are located on the boutons of the hypogastric nerve and when activated by kappa-opioid receptor agonists reduce quantal release without affecting the NBI.     

Binding site of loperamide: automated docking of loperamide in human mu- and delta-opioid receptors

Loperamide is a piperidine analogue, acting as agonist on peripheral opioid receptors, exhibiting affinity and selectivity for the cloned mu human opioid receptor compared with the delta human opioid receptor. Automatic docking studies of loperamide, using AutoDock, on human mu- and delta-opioid receptors is described. Whilst no meaningful difference was detected concerning the docking of the arylpiperidine moiety, mu/delta selectivity could be explained as a different accommodation of the two phenyl groups in two lipophylic pockets of receptors.     

Presynaptic nicotine receptors mediating a positive feed-back on transmitter release from the rat phrenic nerve

Summary The effects of 1,1-dimethyl-4-phenylpiperazinium (DMPP) and of nicotine receptor antagonists on [³H]acetylcholine release from the rat phrenic nerve preincubated with [³H]choline were investigated in the absence and presence of cholinesterase inhibitors (presynaptic effects). Additionally, the effects of hexamethonium and tubocurarine on the muscle contraction of the indirectly stimulated diaphragm were examined (postsynaptic effects).DMPP (1–30 M) increased (76–92%), whereas hexamethonium (0.001–1 mM) and tubocurarine (1–10 M) decreased (52–60%) the release of [³H]acetylcholine following a train of 100 pulses at 5 Hz. The release caused by a longer train (750 pulses at 5 Hz) was only slightly affected by DMPP and tubocurarine. In the presence of neostigmine (10 M) neither tubocurarine nor DMPP significantly modulated the evoked [³H]acetylcholine release. High DMPP concentrations (10 and 30 M) enhanced the evoked release only when the pretreatment interval was reduced from 15 min to 20 s.Tubocurarine and hexamethonium concentration-dependently inhibited the end-organ response. Hexamethonium was 250-fold more potent on presynaptic than on postsynaptic nicotine receptors.It is concluded that the motor nerve terminals are endowed with presynaptic nicotine receptors. These autoreceptors mediate a positive feed-back mechanism that can be triggered by previously released endogenous acetylcholine. Receptor desensitization can be produced by high agonist concentrations (endogenous or exogenous agonists) and is probably one mechanism to limit the autofacilitatory process. The presynaptic receptors appear to differ in their pharmacological properties from the post-synaptic receptors.     

Interaction of co-expressed mu- and delta-opioid receptors in transfected rat pituitary GH(3) cells

mu- and delta-Opioid agonists interact in a synergistic manner to produce analgesia in several animal models. Additionally, receptor binding studies using membranes derived from brain tissue indicate that interactions between mu- and delta-opioid receptors might be responsible for the observation of multiple opioid receptor subtypes. To examine potential interactions between mu- and delta-opioid receptors, we examined receptor binding and functional characteristics of mu-, delta-, or both mu- and delta-opioid receptors stably transfected in rat pituitary GH(3) cells (GH(3)MOR, GH(3)DOR, and GH(3)MORDOR, respectively). Saturation and competition binding experiments revealed that coexpression of mu- and delta-opioid receptors resulted in the appearance of multiple affinity states for mu- but not delta-opioid receptors. Additionally, coadministration of selective mu- and delta-opioid agonists in GH(3)MORDOR cells resulted in a synergistic competition with [(3)H][D-Pen(2,5)]enkephalin (DPDPE) for delta-opioid receptors. Finally, when equally effective concentrations of [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO) and two different delta-opioid agonists (DPDPE or 2-methyl-4a alpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a alpha-octahydroquinolino-[2,3,3-g]-isoquinoline; TAN67) were coadministered in GH(3)MORDOR cells, a synergistic inhibition of adenylyl cyclase activity was observed. These results strongly suggest that cotransfection of mu- and delta-opioid receptors alters the binding and functional characteristics of the receptors. Therefore, we propose that the simultaneous exposure of GH(3)MORDOR cells to selective mu- and delta-opioid agonists produces an interaction between receptors resulting in enhanced receptor binding. This effect is translated into an augmented ability of these agonists to inhibit adenylyl cyclase activity. Similar interactions occurring in neurons that express both mu- and delta-opioid receptors could explain observations of multiple opioid receptor subtypes in receptor binding studies and the synergistic interaction of mu- and delta-opioids in analgesic assays.     

Effect of morphine on the nerve terminal impulse and transmitter release from sympathetic varicosities innervating the mouse vas deferens.

1 The effect of morphine on both the propagation of the nerve terminal impulse along the sympathetic varicose axons as well as the evoked and spontaneous transmitter release has been evaluated. 2 Morphine (1 microM) did not significantly change the shape or the regularity by which the nerve terminal impulse was recorded while evoked transmitter release was greatly reduced. 3 Morphine induced a uniform decrease in evoked transmitter release irrespective of the release probability of individual varicosities of their position along terminal branches. 4 Procedures which are thought to increase intracellular calcium concentration such as increasing the extracellular calcium concentration, stimulation of the nerve with trains of impulses and increasing the duration of the action potential with 4-aminopyridine reduced the ability of morphine to decrease evoked transmitter release. 5 Morphine had to act directly on the varicosities to induce a decrease in evoked transmitter release. 6 The decrease in evoked quantal release does not involve an affect on the nerve terminal impulse or the vesicle release process and morphine may affect the dependence of the secretory process on calcium.     

Inhibition of enkephalin metabolism and activation of mu- or delta-opioid receptors elicit opposite effects on reward and motility in the ventral mesencephalon.

The coexistence of endogenous opioid systems and dopaminergic neurones in the midbrain tegmental area suggests functional interactions between dopamine and enkephalins. Nevertheless, the identification of the specific opioid receptors associated with modulation of tegmental dopamine activity and its behavioural concomitants on motility and reward is far from clear, considering the mixed nature of the ligands usually employed. In this way, kelatorphan, a potent inhibitor of enkephalinases and selective agonists for mu- and delta-opioid receptor subtypes (DAGO and DSTBULET, respectively) were infused directly into the ventral tegmental area of the rat to study the role of endogenous enkephalins and opioid receptors in regulating spontaneous motor activity and intracranial self-stimulation behaviour. A greater increase in the rate of intracranial self-stimulation behaviour was found after activation of mu-opioid receptors in the ventral tegmental area, as compared to activation of delta-opioid receptors, whereas enhancement of endogenous enkephalins by inhibiting their metabolism through kelatorphan, reduced the rate of intracranial self-stimulation behaviour. On the contrary, spontaneous motor activity was reduced by the delta-opioid receptor agonist, whereas kelatorphan increased the movements of the animal. Taken together, these results show that inhibition of the metabolism of enkephalins in the ventral tegmental area decreased positive reinforcement from the lateral hypothalamic medial forebrain bundle and increased spontaneous movements. On the contrary, activation of both mu- or delta-opioid receptors in the ventral tegmental area significantly increased self-stimulation and decreased spontaneous motor activity, supporting the view that different mechanisms underlie the behavioural effects, resulting from enhancement of endogenous enkephalins and from activation of specific opioid receptors in the ventral mesencephalon.