Mutual interaction between presynaptic alpha 2-adrenoceptors and opioid kappa-receptors at the noradrenergic axons of rabbit brain cortex

Locus coeruleus kappa-opioid receptors modulate reinstatement of cocaine place preference through a noradrenergic mechanism

Activation of kappa-opioid receptors (KORs) in monoamine circuits results in dysphoria-like behaviors and stress-induced reinstatement of drug seeking in both conditioned place preference (CPP) and self-administration models. Noradrenergic (NA) receptor systems have also been implicated in similar behaviors. Dynorphinergic projections terminate within the locus coeruleus (LC), a primary source of norepinephrine in the forebrain, suggesting a possible link between the NA and dynorphin/kappa opioid systems, yet the implications of these putative interactions have not been investigated. We isolated the necessity of KORs in the LC in kappa opioid agonist (U50,488)-induced reinstatement of cocaine CPP by blocking KORs in the LC with NorBNI (KOR antagonist). KOR-induced reinstatement was significantly attenuated in mice injected with NorBNI in the LC. To determine the sufficiency of KORs in the LC on U50,488-induced reinstatement of cocaine CPP, we virally re-expressed KORs in the LC of KOR knockout mice. We found that KORs expression in the LC alone was sufficient to partially rescue KOR-induced reinstatement. Next we assessed the role of NA signaling in KOR-induced reinstatement of cocaine CPP in the presence and absence of a α2-agonist (clonidine), β-adrenergic receptor antagonist (propranolol), and β(1)- and β(2)-antagonist (betaxolol and ICI-118,551 HCl). Both the blockade of postsynaptic β(1)-adrenergic receptors and the activation of presynaptic inhibitory adrenergic autoreceptors selectively potentiated the magnitude of KOR-induced reinstatement of cocaine CPP but not cocaine-primed CPP reinstatement. Finally, viral restoration of KORs in the LC together with β-adrenergic receptor blockade did not potentiate KOR-induced reinstatement to cocaine CPP, suggesting that adrenergic receptor interactions occur at KOR-expressing regions external to the LC. These results identify a previously unknown interaction between KORs and NA systems and suggest a NA regulation of KOR-dependent reinstatement of cocaine CPP.

Involvement of Ca2+-dependent PKCs in the adaptive changes of mu-opioid pathways to sympathetic denervation in the guinea pig colon

In the guinea pig colon, chronic sympathetic denervation entails supersensitivity to inhibitory mu-opioid agents modulating cholinergic neurons. The mechanism underlying such adaptive change has not yet been unravelled, although protein kinase C (PKC) may be involved. A previous study indirectly demonstrated that activation of mu-opioid receptors on myenteric neurons facilitates PKC activity. Such coupling may counteract the inhibitory action of mu-opioid agents on acetylcholine overflow, since PKC, per se, increases this parameter. After chronic sympathetic denervation such restraint abates, representing a possible mechanism for development of supersensitivity to mu-opioid agents. In the present study, this hypothesis was further investigated. After chronic sympathetic denervation, Ca(2+)-dependent PKC activity was reduced in colonic myenteric plexus synaptosomes. The mu-opioid agent, DAMGO, increased Ca(2+)-dependent PKC activity in synaptosomes obtained from normal, but not from denervated animals. In myenteric synaptosomes obtained from this experimental group, protein levels of Ca(2+)-dependent PKC isoforms betaI, betaII and gamma decreased, whereas alpha levels increased. In whole-mount preparations, the four Ca(2+)-dependent PKC isoforms co-localized with mu-opioid receptors on subpopulations of colonic myenteric neurons. The percentage of neurons staining for PKCbetaII, as well as the number of mu-opioid receptor-positive neurons staining for PKCbetaII, decreased in denervated preparations. The same parameters related to PKCalpha, betaI or gamma remained unchanged. Overall, the present data strengthen the concept that mu-opioid receptors located on myenteric neurons are coupled to Ca(2+)-dependent PKCs. After chronic sympathetic denervation, a reduced efficiency of this coupling may predominantly involve PKCbetaII, although also PKCbetaI and gamma, but not PKCalpha, may be implicated.

Presynaptic opioid receptors on noradrenergic and serotonergic neurons in the human as compared to the rat neocortex

1. Electrically evoked release of [3H]-noradrenaline ([3H]-NA) or [3H]-5-hydroxytryptamine ([3H]-5-HT) in slices of human and the rat neocortex was used to characterize presynaptic opioid receptors. 2. Release of [3H]-NA in rat neocortical slices was reduced only by the mu-receptor agonist DAMGO (pIC50: 7.27, CI95: [7.22, 7.32]; Imax: 77.6+/-1.6%; antagonized by naloxone: pA2: 8.88, CI95: [8.78, 8.98]). 3. Release of [3H]-NA in human neocortical slices was unaffected by DAMGO, but inhibited by the delta-receptor agonist DPDPE (Imax: 25.7+/-2.2%) and the kappa-receptor agonist U-50,488H (19.7+/-2.7% inhibition at 1 microM). Both effects were antagonized by naltrindole (1 microM). 4. Release of [3H]-5-HT in rat neocortical slices, was inhibited by DAMGO (10 microM) and U-50,488H (1 and 10 microM) only in the presence of the 5-HT receptor antagonist methiotepin (1 microM). 5. Release of [3H]-5-HT in human neocortical slices was unaffected by DPDPE, but U-50,488H (Imax: 40.8+/-8.3%; antagonized by 0.1 microM norbinaltorphimine) and DAMGO (16.4+/-3.9% inhibition at 1 microM; antagonized by 0.1 microM naloxone) acted inhibitory. 6. Release of [3H]-5-HT in human neocortical slices was reduced by nociceptin/orphanin (0.1 and 1 microM). These effects were antagonized by the ORL1 antagonist J-113397 (1-[(3R,4R)-1-cyclo-octylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one; 0.1 microM). 7. This study provides evidence for significant species differences in opioid receptor-mediated modulation of NA and 5-HT-release in human vs rat neocortex. In rats, mu-opioid receptors modulate NA release, but 5-HT release is only weakly affected by mu- and kappa-opioids. In contrast, NA release in human neocortex is modulated via delta-opioid receptors, but 5-HT release mainly via kappa-opioid receptors. In addition also the ORL1 receptor seems to be involved in 5-HT release modulation.

Kappa opioid agonist-induced changes in IOP: correlation with 3H-NE release and cAMP accumulation

Opioid receptors have been demonstrated to modulate various functions in the eye. This research project was designed to determine and compare the effects of kappa opioid agonists on selected parameters that influence ocular hydrodynamics. Experiments determined the effects of two relatively selective kappa opioid receptor agonists, ICI 204 448 (ICI), which has limited ability to penetrate the blood-brain barrier, and spiradoline mesylate on: (1) in vivo parameters, intraocular pressure (IOP) and pupil diameter (PD); and (2) in vitro parameters, neurotransmitter release and cAMP accumulation, in the ciliary body. Dark-adapted, reverse light cycle New Zealand white (NZW) male rabbits were used in all experiments. In in vivo experiments, intraocular pressures and pupil diameters were measured by a pneumatonometer and an optistick, respectively, before and after drug administration. Baseline readings were taken at 0.5 and 0 hr prior to agonist administration. Postdrug IOP and PD measurements were made at 0.5, 1, 2, 3, 4 and 5 hr after agonist application. In some experiments, the relatively selective kappa antagonist, norbinaltorphimine was applied 30 min prior to agonist application. In in vitro experiments, the release of tritiated norepinephrine (3H-NE) was measured from perfused electrically stimulated iris ciliary bodies and expressed as the percent change of the control. Basal and isoproterenol-stimulated cyclic AMP concentrations in iris ciliary bodies were quantified by radioimmunoassay techniques in the presence and absence of ICI and spiradoline. ICI and spiradoline decreased IOP in a dose-dependent manner in normal rabbits, but only spiradoline produced significant changes in PD. The kappa opioid receptor antagonist, norbinaltorphimine, antagonized the hypotensive effects of spiradoline and ICI in IOP experiments. Both kappa agonists inhibited the release of norepinephrine from perfused iris ciliary bodies. Isoproterenol- stimulated cAMP levels in iris ciliary bodies were suppressed by both kappa receptor agonists. The antagonism by norbinaltorphimine suggests that ICI and spiradoline lower IOP by activating kappa opioid receptors in the eye. The bilateral effects of unilaterally applied spiradoline on PD indicate that this kappa agonist activates receptors in the iris and/or the brain. The inhibition of norepinephrine release and cAMP accumulation in the iris ciliary body by ICI and spiradoline suggests that there are both pre- and postjunctional sites of action for kappa agonists.

Lack of interaction between alpha(2)-autoreceptors and prejunctional receptors mediating a facilitatory effect on noradrenaline release

The present study was undertaken to investigate the effect of alpha(2)-autoreceptor blockade on the facilitatory influence exerted by activation of beta -, A(2A)-adenosine- and angiotensin II receptors. Segments of a rat-tail artery, previously incubated with(3)H-noradrenaline, were subjected to electrical stimulation. The influence of isoprenaline, the compound CGS21680 and angiotensin II on the overflow of tritium evoked by electrical stimulation was checked before and after alpha(2)-adrenoceptor blockade. All the agonists used caused concentration-dependent increases of tritium overflow, the maximal effect representing increases of 44.2, 27.4 and 41.2% for isoprenaline, CGS21680 and angiotensin II, respectively. In the presence of alpha(2)-adrenoceptor blockade by phenoxybenzamine ( 1 microm) or yohimbine (33 or 100 nm), the facilitatory influence of isoprenaline, CGS21680 and angiotensin II was not significantly changed. Since this facilitatory influence, which involves the activation of G(s)- or G(q)-proteins, was not enhanced by alpha(2)-adrenoceptor blockade, it is concluded that the enhancement of the negative modulation resulting from activation of A(1)-adenosine-, muscarine- and kappa -receptors, as previously shown, should be due to the fact that the involved systems share signal transduction mechanisms, or at least G-proteins.

Interaction of the alpha-2 adrenergic- and opioid receptor with the cGMP system in the mouse cerebellum

The alpha-2 adrenergic agonist dexmedetomidine (Dex), 3-300 microg/kg, i.p., decreased cerebellar cGMP in a dose-dependent manner. Fentanyl (F), an opioid agonist, increased cerebellar cGMP at 0.3 mg/kg, s.c., and decreased it at doses >/=1 mg/kg. The inhibitory effect was receptor specific, that of Dex being blocked by the alpha-2 adrenergic antagonist yohimbine, 5 mg/kg, i.p.; that of F by the opioid antagonist naloxone, 5 mg/kg, i.p. In contrast the stimulatory effect of F was blocked by both naloxone and yohimbine. Yohimbine also enhanced the inhibitory effect of F. In mice pretreated with pertussis toxin, 2 microgram/mouse, given i.c.v. 72 h before the agonists, the decrease in cGMP induced by Dex or F was not affected, while the stimulatory effect of F was reversed to an inhibitory effect. When inhibiting doses of F and Dex were administered together, the cGMP response was smaller than the sum of the individual responses. Dex attenuated in a dose-dependent manner the decrease in cGMP induced by F, and unmasked or enhanced the stimulatory effect of F. These results show that the alpha-2 adrenergic- and opioid-receptors are coupled to the cGMP effector system and suggest that the two pathways converge at a common post-receptor site in the cascade of events transducing the receptor signal to cGMP regulation.

Interactions between the presynaptic alpha2-autoreceptor and presynaptic inhibitory heteroreceptors on noradrenergic neurones

The noradrenergic neurones of the autonomic nervous system (postganglionic sympathetic neurones) and of the central nervous system are endowed with presynaptic receptors by which noradrenaline release is inhibited by noradrenaline itself (via the alpha2-autoreceptor) and by other transmitters and mediators (via heteroreceptors). Frequently, but not consistently, inhibitory interactions exist between auto- and heteroreceptors. This holds true for the following heteroreceptors: adenosine A1, cannabinoid CB1, dopamine D2/D3, histamine H3, 5-hydroxytryptamine (serotonin) 5-HT(1B), imidazoline, muscarine M2, delta opioid, kappa opioid, mu opioid, orphan opioid (ORL1), prostaglandin EP3, and somatostatin SRIF1. Such interactions (which have also been obtained in human tissue) may, if not considered, prevent the identification of a putative heteroreceptor or the quantitative estimation of the effect mediated by this receptor, and they may explain drug interactions in humans in vivo; many ligands at the alpha2-autoreceptor and at the heteroreceptors may be simultaneously used for therapeutic reasons (e.g., carbachol, clonidine, dopamine, sumatriptan, mianserin, and misoprostol) or abused (e.g., heroin, LSD, and delta9-tetrahydrocannabinol in hashish or marijuana).

Modulation of neurotransmitter release by opioid mu- and kappa-receptors from adrenergic terminals in the myenteric plexus of the guinea-pig colon: effect of alpha 2-autoreceptor blockade

We have studied the effect of [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO, opioid mu-receptor agonist) and ICI-204,448 (kappa-receptor agonist) on endogenous noradrenaline release in the guinea-pig isolated distal colon. DAMGO enhances noradrenaline over-flow and this effect is antagonized by naloxone (pIC50 = 10.27) and nor-binaltorphimine (pIC50 = 7.97), and concentration-dependently turned into inhibition by yohimbine. ICI-204,448 inhibits noradrenaline overflow and is antagonized by naloxone (pIC50 = 9.38) and nor-binaltorphimine (pIC50 = 10.48), but is not affected by yohimbine. Evidence is thus given that mu- and kappa-opioid receptors modulate noradrenaline release in the guinea-pig colon. Modifications by yohimbine of the effect of DAMGO indicate the existence of a functional relationship between mu-receptors and alpha(2)-autoreceptors in this model.

Hypertension due to chronic blockade of P1-purinoceptors

1. Long-term treatment of rats with 1,3-dipropyl-8-sulphophenylxanthine (DPSPX), a non-selective antagonist of adenosine receptors, causes a hypertensive state. 2. In DPSPX-hypertensive rats, prejunctional alpha 2-adrenoceptors become supersensitive to the selective alpha 2-adrenoceptor agonist UK-14,304, while postjunctional adrenoceptor-mediated responses are not changed; furthermore, prejunctional beta-adrenoceptor-mediated facilitation of noradrenaline release is also enhanced. 3. In DPSPX-hypertensive rats, there are important morphological alterations of the small arteries, their lumina appearing strongly reduced and occasionally occluded by proliferation of the intimal cells. 4. In DPSPX-hypertensive rats, there is an increase in plasma renin, and captopril prevents not only the development of the hypertension but also the morphological changes in the arteries. 5. Other important changes occur in DPSPX-hypertensive rats: an alteration of the adrenergic regulation of the cardiac functions and an enhancement of perivascular neurotransmission. 6. These results suggest that adenosine may play an important role in the development of some kinds of human hypertension.

Tonic modulation of neurotransmitter release in the guinea-pig myenteric plexus: effect of mu and kappa opioid receptor blockade and of chronic sympathetic denervation

We have studied the effects of mu- and kappa-opioid receptor blockade on endogenous acetylcholine and noradrenaline overflow from the myenteric plexus of the guinea-pig isolated colon. Cyprodime (putative mu-selective antagonist) and nor-binaltorphimine (kappa-selective antagonist) had a concentration-dependent facilitatory effect on both acetylcholine and noradrenaline overflow. Moreover, in colonic specimens obtained from sympathetically denervated animals, the effect of opioid antagonists on acetylcholine overflow was significantly higher with respect to normal preparations. Evidence is thus given in favour of an involvement of mu- and kappa-opioid receptor pathways in the tonic modulation of neurotransmitter release at the colonic level. Enhanced sensitivity to the effect of mu and kappa antagonists after chronic sympathetic denervation is strongly suggestive for the existence of a functional link between opioid and adrenergic pathways in this model.

A subtype of opioid kappa-receptor is coupled to inhibition of Gi1-mediated phospholipase C activity in the guinea pig cerebellum

PLC activity was stimulated either by 1-100 microM of GTP or by 100-3,000 microM Ca2+ in lysed synaptosomal membranes of the guinea pig cerebellum. The kappa-opioid receptor agonist selectively inhibited the PLC activity stimulated by 100 microM GTP, but not by 100-3,000 microM Ca2+. Pretreatment of membranes with PTX abolished such a kappa-agonist-induced inhibition of PLC activity. The reconstitution of Gi1, but not of Go purified from porcine brains with PTX-treated membranes showed a complete recovery of the kappa-agonist-inhibition of PLC activity. These findings suggest that a novel subtype kappa-receptor mediates inhibition of PLC through inhibiting the intrinsic activity of PTX-substrate G-proteins.

Prejunctional interaction of alpha 2-adrenoceptors and D2 dopamine receptors on perivascular sympathetic nerves

1. Potential interaction between release-inhibiting prejunctional alpha 2-adrenoceptors and D2 dopamine receptors was investigated by measuring D2 receptor-mediated inhibition of stimulation-evoked tritium overflow from perfused rat tail arteries incubated with [3H]-noradrenaline. 2. In the presence of cocaine (10(-5)M), which enhanced stimulation-evoked tritium overflow, the D2 dopamine agonist N-0923 [(S)-(-) 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin] (10(-8)M) was less effective at inhibiting stimulation-evoked tritium overflow compared to inhibition in the absence of cocaine. 3. In the presence of cocaine, yohimbine (10(-6)M) enhanced stimulation-evoked [3H]-noradrenaline release. Under these conditions, inhibition produced by N-0923 was enhanced compared to tissues without yohimbine. 4. In the presence of cocaine, the alpha 2-adrenoceptor agonist UK-14,304 (10(-7)M) reduced stimulation-evoked tritium overflow. However, in this case, N-0923-mediated inhibition was not significantly altered. 5. In most circumstances increasing or reducing activation of prejunctional alpha 2-adrenoceptors resulted in attenuation or enhancement, respectively, of D2 receptor activation. However, in the case of UK-14,304 this relationship did not hold. Thus, most but not all the evidence supports an interaction between prejunctional alpha 2-adrenoceptors and D2 dopamine receptors.

Long-term administration of 1,3-dipropyl-8-sulphophenylxanthine (DPSPX) alters alpha 2-adrenoceptor-mediated effects at the pre- but not at the postjunctional level

The present investigation was undertaken to see whether a long-term inhibition of adenosine receptors--leading to hypertension--interferes with alpha 2-adrenoceptor-mediated modulation of noradrenaline release. Rat tail arteries were removed from normal and from hypertensive animals obtained by chronic treatment with intraperitoneally infused DPSPX (1,3-dipropyl-8-sulphophenylxanthine) or orally administered L-NAME (NG-Nitro-L-arginine methyl ester). To study prejunctional effects, the influence of UK-14,304 (5-bromo-6(imidazoline-2-ylamino)-quinoxaline) and yohimbine on the overflow of tritium evoked by electrical stimulation (100 V; 1 Hz; 2 ms; 5 min) from tissues preloaded with 3H-noradrenaline was analysed. To study postjunctional effects, concentration-response curves to UK-14,304 were determined. In DPSPX-treated rats there was an enhancement of the prejunctional effects of UK-14,304: its Ec30% was reduced from 381 (250; 579) to 85 (73; 99) nmol.l-1 (n = 5; P < 0.05) and its maximal effect--expressed as percent reduction of tritium overflow-increased from 45 +/- 5% to 61 +/- 5% (n = 6; P < 0.05). In L-NAME-treated rats there was no change in either of these two parameters. At the postjunctional level, there was no change in the sensitivity to UK-14,304 in tissues from either DPSPX- or L-NAME-treated rats. Yohimbine (10-1000 nmol.l-1) caused a concentration-dependent increase of tritium overflow evoked by electrical stimulation in both control and hypertensive animals (either DPSPX- or L-NAME-treated).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of opioid receptor subtype-selective agonists on purinergic and adrenergic components of neurogenic contractions of mouse vas deferens

1. Effects of opioid agonists on the purinergic and adrenergic components of neurogenic contractions and in some experiments on transmitter overflow were studied in the mouse isolated vas deferens. 2. When the vas deferens was stimulated every 2 min by pairs of pulses 2 s apart in the presence of prazosin 0.3 microM (to isolate the purinergic component) or alpha,beta-methylene-ATP 3 microM (to isolate the adrenergic component), each pulse elicited a separate twitch. The opioid agonists [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO, mu-receptor-selective), [D-Pen2,D-Pen5]enkephalin (DPDPE, delta-selective) and trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide (U-50488, kappa-selective) concentration-dependently reduced both purinergic and adrenergic contractions. For each agonist, maximal effects and concentrations causing half-maximal effects were very similar for inhibition of the purinergic component on the one hand and for inhibition of the adrenergic component on the other hand, although the adrenergic component was inhibited with a slight preference. Moreover, effects on contractions elicited by the first and the second pulse of the pairs were very similar. 3. When vasa deferentia preincubated with [3H]-noradrenaline were stimulated with trains of 100 pulses delivered at 20 Hz, morphine 10 microM reduced significantly both evoked tritium overflow and evoked contractions. Its effect was antagonized by naloxone. 4. It is concluded that, in contrast to drugs acting at some other presynaptic receptors, opioid mu-, delta- and kappa-agonists inhibit purinergic and adrenergic neurogenic contractions of the mouse vas deferens in a similar manner. In contrast to a previous report, no enhancement by morphine of the release of noradrenaline elicited by high frequency pulse trains was observed.

Mutual interaction of histamine H3-receptors and ?2-adrenoceptors on noradrenergic terminals in mouse and rat rain cortex

Brain cortex slices were preincubated with 3H-noradrenaline and superfused with physiological salt solution containing desipramine. We studied the inhibition of the electrically evoked tritium overflow caused by histamine in the presence of alpha-adrenoceptor ligands (mouse and rat brain cortex), and the inhibition caused by talipexole (the former B-HT 920) in the presence of H3-receptor ligands (mouse brain cortex). In mouse brain cortex slices, the inhibitory effect of histamine on the tritium overflow evoked by 36 pulses, 0.3 Hz was not changed by the alpha 1-adrenoceptor antagonist prazosin, but increased by the alpha 2-adrenoceptor antagonist rauwolscine. When the current strength or the duration of electrical pulses was reduced to compensate for the increase in evoked tritium overflow produced by rauwolscine, the latter still enhanced the effect of histamine. The histamine-induced inhibition of tritium overflow evoked by 360 pulses, 3 Hz was not affected by the alpha 1-adrenoceptor agonist phenylephrine but attenuated by the alpha 2-adrenoceptor agonist talipexole. Finally, the inhibition by histamine of the tritium overflow evoked by 3 pulses, 100 Hz was attenuated by talipexole but not affected by rauwolscine. Conversely, the inhibitory effect of talipexole on tritium overflow elicited by 360 pulses, 3 Hz was slightly attenuated by the H3-receptor agonist R-(-)-alpha-methylhistamine but not affected by the H3-receptor antagonist thioperamide. In rat brain cortex slices, histamine only tended to inhibit tritium overflow evoked by 360 pulses, 3 Hz, both in the absence of alpha-adrenoceptor antagonists and in the presence of prazosin. However, histamine markedly inhibited the evoked overflow in the presence of rauwolscine.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the interaction between presynaptic alpha 2-adrenoceptors and adenosine A1 receptors located on noradrenergic nerve terminals

The aim of the present study was to obtain a more detailed understanding of the interaction between presynaptic alpha 2-adrenoceptors and A1 adenosine receptors mediating inhibition of noradrenaline release in the central nervous system. Slices of rabbit hippocampus, prelabelled with [3H]noradrenaline, were superfused in the presence of the re-uptake inhibitor (+)-oxaprotiline and electrically stimulated during superfusion. During stimulation with 36 pulses at 3 Hz the alpha 2-adrenoceptor antagonist yohimbine induced a five-fold increase of noradrenaline release indicating a pronounced autoinhibition of approximately 80%. In these experiments the inhibition of release caused by R-PIA, a preferential A1 agonist, as well as its facilitation caused by DPCPX, a selective A1 antagonist, were smaller in comparison to the effects of these compounds on release virtually free of autoinhibition (i.e. by stimulating the tissue with 4 pulses at 100 Hz (POP-stimulation) or with 36 pulses at 3 Hz in presence of yohimbine). Clonidine, an alpha 2-adrenoceptor agonist, was used to impose a distinct alpha 2-adrenoceptor-mediated inhibition of release elicited by POP-stimulation. Only, however, in the presence of 30 nmol/l clonidine, causing maximum inhibition of approximately 80% of 3H-overflow, but not in the presence of 6 nmol/l clonidine, causing approximately 50% inhibition, a significant diminution of the inhibitory effect of R-PIA was seen. Similarly, the alpha 2-adrenoceptor mechanism was affected only by 10 mumols/l R-PIA causing maximum inhibition of approximately 80%, but remained unchanged in the presence of 30 nmol/l R-PIA diminishing release by 50%.(ABSTRACT TRUNCATED AT 250 WORDS)

Antidiuretic effect of bremazocine and U-50,488 in rats after alpha 2-adrenoceptor blockade

The role of alpha 2-adrenoceptors and kappa-opioid receptors in urination was studied in rats. In water-loaded rats (40 mL kg-1 p.o.) the kappa-opioid agonist bremazocine (0.05 0.2 mg kg-1 i.p.) induced a dose-related diuretic response in the second hour after administration, but had no effect in the first hour. When rats were pretreated with the alpha 2-adrenoceptor antagonist idazoxan (1 mg kg-1 s.c.), bremazocine induced a dose-related antidiuretic response in the first hour; thereafter the rats showed an increase of urination similar to that with bremazocine alone. The antidiuretic effect of bremazocine was dependent on the dose of idazoxan with maximal response after 1-3 mg kg-1. Similar results were obtained with bremazocine in the presence of yohimbine (1 mg kg-1 s.c.). The antidiuretic profile of bremazocine after idazoxan was shared by U-50,488 (2.5-10 mg kg-1 i.p.), although this compound alone at the high dose reduces urine output in the first hour. The antidiuresis induced by bremazocine in the presence of idazoxan in water-loaded rats was completely antagonized by 10 but not 2 mg kg-1 i.p. of the opioid antagonist naloxone. Thus, kappa-opioid agonists, in addition to their diuretic effect, also produce an antidiuretic response which may be mediated by alpha 2-adrenoceptors.

Inhibitory prejunctional muscarinic receptors at sympathetic nerves do not operate through a cyclic AMP dependent pathway

In mouse atria previously incubated with [3H]-noradrenaline, carbachol (1.0 mumo1/l) significantly inhibited the fractional stimulation-induced (S-I) outflow of radioactivity. The inhibitory effect of carbachol was greater in the presence of the alpha-adrenoceptor antagonist phentolamine (1.0 mumol/l), which by itself significantly increased the S-I outflow of radioactivity. In both cases the inhibitory effect of carbachol was blocked by atropine (0.3 mumol/l), suggesting that the effect was mediated through muscarinic receptors. 8-Bromo cyclic AMP (270 mumol/l) in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX, 100 mumol/l), was used to maximally enhance the S-I outflow of radioactivity through the cyclic AMP mechanism. The inhibitory effect of carbachol either in the presence or in the absence of phentolamine, was not reduced in the presence of 8-bromo cyclic AMP and IBMX. Similar results with carbachol in the presence of 8-bromo cyclic AMP and IBMX were also found in rat right atrial strips which had been incubated with [3H]-noradrenaline. These results suggest that the effects through inhibitory prejunctional muscarinic receptors are not mediated by cyclic AMP. The protein kinase inhibitor, staurosporine (0.1 mumol/l), significantly blocked the enhancing effects of 8-bromo cyclic AMP (270 mumol/l) plus IBMX (100 mumol/l) on the S-I outflow of radioactivity from rat atrial strips. The inhibitory effect of carbachol (1.0 mumol/l) however, was not reduced in the presence of staurosporine, suggesting that protein kinases affected by staurosporine (protein kinase A, protein kinase C) are not involved in the post-receptor mechanism for inhibitory prejunctional muscarinic receptors. This finding further rules out the involvement of cyclic AMP in muscarinic inhibition. The inhibitory effect of carbachol either by itself or in the presence of phentolamine, was not reduced in atria from mice that had been pretreated with pertussis toxin (1.5 or 3.0 micrograms). Furthermore, in rat atrial strips, the inhibitory effect of carbachol either in the presence or in the absence of phentolamine, was also not altered by pretreating the rats with pertussis toxin (8.4 micrograms). The results suggest that in both tissues the major mechanism for inhibition of noradrenaline release through muscarinic receptors does not involve a pertussis toxin sensitive G protein.

Stimulation intensity as critical determinant of presynaptic receptor effectiveness

A number of reports in the literature suggest that there is an inverse correlation between the intensity of nerve activation and the effectiveness of presynaptic receptors that inhibit transmitter release. This common feature of many presynaptic inhibitory receptors may provide important insights into the mechanism by which transmitter release is controlled. Sue Duckles and Dénes Budai discuss the implications such a relationship between the pattern of nerve activation and effectiveness of presynaptic receptors has for understanding the physiological role of neuromodulators in general.

Blockade of alpha 2-adrenoceptors increases opioid mu-receptor-mediated inhibition of the firing rate of rat locus coeruleus neurones

In pontine slices of the rat brain, the frequency of spontaneous action potentials of locus coeruleus (LC) neurones was recorded extracellularly. Noradrenaline 0.1-100 mumol/l, UK 14,304 0.01-100 nmol/l, [Met5]-enkephalin 1-10,000 nmol/l and [D-Ala2,D-Leu5]enkephalin 0.1-1,000 nmol/l, all depressed the firing rate. Rauwolscine 1 mumol/l antagonized the effects of both noradrenaline and UK 14,304, but potentiated the effects of [Met5]enkephalin and [D-Ala2, D-Leu5]enkephalin. Idazoxan 1 mumol/l acted in a similar manner. Prazosin 1 mumol/l did not change the effects of either noradrenaline or [Met5]enkephalin. Naloxone 0.1 mumol/l antagonized both [Met5]enkephalin and [D-Ala2, D-Leu5]enkephalin, but failed to alter the effects of either noradrenaline or UK 14,304. Rauwolscine, idazoxan and prazosin, all 1 mumol/l, as well as naloxone 0.1 mumol/l, did not influence the firing rate when given alone. Desipramine 1 mumol/l inhibited the discharge of action potentials in a rauwolscine-antagonizable manner. Noradrenaline 10 mumol/l produced the same depression of firing, both in the presence of noradrenaline 1 mumol/l and [Met5]enkephalin 0.03 mumol/l. Likewise, the effect of [Met5]enkephalin 0.3 mumol/l was the same, irrespective of whether it was added to a medium containing [Met5]enkephalin 0.03 mumol/l or noradrenaline 1 mumol/l. The spontaneous activity of LC neurones is inhibited by somatic alpha 2-adrenoceptors and opioid mu-receptors. We suggest that the two receptors interact with each other at a site located between themselves and not in the subsequent common signal transduction system.

Serotonin uptake blockers influence serotonin autoreceptors by increasing the biophase concentration of serotonin and not through a "molecular link"

The mechanism of the attenuation, by serotonin uptake blockers, of the release-inhibiting effect of exogenous serotonin autoreceptor agonists was studied in rabbit brain cortex and rat hypothalamus slices. The slices were preincubated with 3H-serotonin and then superfused and stimulated electrically. In rabbit brain slices stimulated by trains of 4 pulses at 100 Hz, 5-carboxamidotryptamine and 5-methoxytryptamine reduced the evoked overflow of tritium, and their concentration-response curves were not changed by any of three serotonin uptake inhibitors, namely citalopram, fluvoxamine and 6-nitroquipazine. In contrast, when the slices were stimulated by trains of 10 pulses at 0.033 Hz, fluvoxamine shifted the concentration-response curve of 5-methoxytryptamine to the right. Experiments with the autoreceptor antagonist metitepine indicated that little, if any, endogenous autoinhibitory tone developed in the course of trains of 4 pulses/100 Hz, irrespective of the absence or presence of uptake inhibitors, as well as during trains of 10 pulses/0.033 Hz in the absence of uptake inhibitors, whereas marked autoinhibition developed when 10 pulses/0.033 Hz were applied in the presence of fluvoxamine. In rat hypothalamic slices stimulated by trains of 4 pulses at 100 Hz, citalopram also failed to change the concentration-response curve of 5-methoxytryptamine. These results indicate that serotonin uptake blockers attenuate the effect of exogenous autoreceptor agonists by an increase in the biophase concentration of released serotonin and, hence, in endogenous autoinhibitory tone, and not by some direct "molecular link" unrelated to the biophase concentration of released serotonin.

Studies of receptor-mediated inhibition of 45Ca accumulation into synaptosomes

1. The effects of alpha 2-adrenoceptor and kappa-opiate receptor activation on 45Ca accumulation into rat cortical synaptosomes were examined. 2. Clonidine (1 microM) and U50488H (1 microM) significantly reduced 45Ca accumulation under both resting (5 mM K+) and depolarizing (15-30 mM K+) conditions. 3. The inhibitory effects of the agonists on 45Ca accumulation into synaptosomes were enhanced in the presence of the Na(+)-Ca2+ exchange inhibitor sodium orthovanadate (vanadate, 2 mM), and were not present in mitochondrial preparations. 4. When the agonists were used together, their inhibitory effects were not additive but were, in fact, attenuated. 5. In the presence of the alpha 2-adrenoceptor antagonist idazoxan (1 microM), the inhibitory effect of U50488H on 45Ca accumulation was enhanced. A similar increase in the inhibitory effectiveness of clonidine was observed in the presence of naloxone (20 microM). 6. When synaptosomes were pretreated with 3-isobutyl-1-methylxanthine (IBMX, 0.5 mM), dibutyrylcyclic AMP (db-cyclic AMP, 10 microM) or 8-bromo-cyclic AMP (8Br-cyclic AMP, 10 microM), the inhibitory effects of clonidine and U50488H were abolished, suggesting that a decrease in cyclic AMP production is part of the receptor-effector coupling mechanism of both receptor systems. 7. The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA, 0.05 microM) increased 45Ca accumulation but did not alter the inhibitory effects of clonidine or U50488H, thus showing that the effects of the agonists are not mediated by protein kinase C. 8. We conclude that alpha 2-adrenoceptor and Kappa-opiate receptor activation dramatically reduce 45Ca influx through Ca21 channels (e.g., by 50%), that there is a functional antagonism between the two receptor systems and that in both cases, the receptor effector mechanism involves a decrease in cyclic AMP production.

Mutual interaction between presynaptic alpha 2-adrenoceptors and 5-HT1B receptors on the sympathetic nerve terminals of the rat inferior vena cava

Segments of the rat vena cava preincubated with [3H]noradrenaline and superfused with [3H]noradrenaline-free solution containing desipramine and corticosterone were stimulated electrically (standard parameters: 150 mA, 0.3 ms, 0.66 Hz; duration 6 min). In some experiments the stimulation parameters were modified in order to obtain similar absolute release values despite the presence of an alpha-adrenoceptor agonist or antagonist or of 5-hydroxytryptamine (5-HT). In a first set of experiments, the vascular segments were first exposed to an alpha-adrenoceptor ligand, which was kept present throughout the remainder of superfusion, and then to 5-HT. The release-inhibiting effect of 5-HT was attenuated by the alpha 2-adrenoceptor agonists clonidine and B-HT 920 whereas it was enhanced by the alpha-adrenoceptor antagonists phentolamine and idazoxan. The alpha 1-adrenoceptor antagonist prazosin did not change the 5-HT-induced inhibition of noradrenaline (NA) release. In a second set of experiments, 5-HT was administered first and kept present in the superfusion fluid for the remainder of the experiment. In the presence of 5-HT, the overflow-inhibiting effects of B-HT 920 and clonidine and the overflow-enhancing effect of idazoxan were reduced. The results demonstrate that activation of one kind of receptor decreased the inhibition of noradrenaline release produced by activation of the other. These effects were not the consequence of the change of release per se induced by the interacting drugs, since they also occurred when the release was adjusted to similar levels by modification of the stimulation parameters.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrically evoked noradrenaline release in the rat hypothalamic paraventricular nucleus studied by in vivo electrochemistry: autoregulation by alpha-2 receptors

Evoked noradrenaline release was monitored every 1 s in vivo from the hypothalamic paraventricular nucleus by differential pulse amperometry at +105 mV combined with carbon fiber electrodes. Noradrenaline release was evoked by electrical stimulations of the ventrolateral medulla for 20s every 10 min at a physiological frequency (3-20 Hz) (see accompanying paper). The evoked noradrenaline release was dose-dependently attenuated by clonidine (10-100 micrograms/kg, i.v.) and strongly enhanced by alpha-2 antagonists: yohimbine (2 mg/kg), piperoxane (2 mg/kg) and idazoxan (0.05-1 mg/kg). Moreover, the effect of clonidine (50 micrograms/kg) was prevented by yohimbine (5 mg/kg) or idazoxan (1 mg/kg). Haloperidol (50 micrograms/kg) or propranolol (10 mg/kg) did not affect evoked noradrenaline release while prazosin (0.05-1 mg/kg) induced a moderate increase. However, prazosin did not prevent the effect of clonidine (50 micrograms/kg). Reserpine (5 mg/kg) pretreatment for 1 h induced a pronounced decrease in the evoked noradrenaline release and abolished the effect of yohimbine (2 mg/kg). Pretreatment by desipramine 30 min before injection abolished the effect of clonidine (50 micrograms/kg) but not of yohimbine (2 mg/kg). The amplitude of the yohimbine (2 mg/kg) effect depended on the frequency of the stimulation: it was maximal between 3 and 7 Hz and gradually declined from 10 to 20 Hz. These results show that noradrenaline release is presynaptically controlled by an alpha-2 adrenoreceptor and suggest that, in physiological conditions, endogenous extracellular noradrenaline inhibits its own phasic release. In conclusion, noradrenergic terminals act as a high pass filter which converts impulse flow into noradrenaline release and the features of this filter are modulated by extracellular noradrenaline via an alpha-2 adrenoreceptor.

Electrically evoked noradrenaline release in the rat hypothalamic paraventricular nucleus studied by in vivo electrochemistry: characterization and facilitation by increasing the stimulation frequency

The hypothalamic paraventricular nucleus is densely innervated by noradrenergic terminals mainly originating in the A1 group within the ventrolateral medulla. An oxidation signal corresponding to extracellular catechols was recorded from the paraventricular nucleus of urethane anaesthetized rats every 1 s by differential pulse amperometry at + 105 mV combined with carbon fiber electrodes. In basal conditions, both extracellular noradrenaline and DOPAC, which are synthesized by noradrenergic terminals, contributed to this oxidation signal. Electrical stimulations of the rostral part of the A1 group were applied for 10 or 20 s every 10 min at physiological frequency (3-20 Hz). They induced an immediate increase in the oxidation signal which lasted as long as the stimulation. This increase was due to the evoked noradrenaline release since it was enhanced by pargyline, desipramine and amphetamine and it was attenuated by alpha-methyl-p-tyrosine and reserpine. The amplitude of the evoked noradrenaline release depended non-linearly on the frequency of the stimulation (from 3 to 20 Hz). When expressed per pulse, noradrenaline release was facilitated four-fold as the frequency increased from 3 to 20 Hz. Central noradrenergic neurons exhibit a tonic activity in a single spike discharge pattern with a mean frequency below 5 Hz but they respond to physiological stimuli by short bursts of action potentials at 20 Hz. Therefore, the present data show that noradrenergic terminals convert physiological impulse flow into noradrenaline release as a high pass filter which enhances the signal-to-noise ratio in response to phasic stimuli.

Presynaptic alpha 2-adrenoceptor and kappa-opiate receptor occupancy promotes closure of neuronal (N-type) calcium channels

Synaptosomes prepared from rat cerebral cortex by homogenization in isotonic sucrose and centrifugation on four-step discontinuous percoll density gradients were loaded with the fluorescent indicator fura-2 to allow measurement of intrasynaptosomal free calcium concentrations [( Ca2+]i). Incubation of fura-2 loaded synaptosomes with either the kappa-opiate agonist U-50,488H (0.1-100 microM) or the alpha 2-adrenoceptor agonist clonidine (0.1-100 microM), resulted in a dose-dependent reduction in [Ca2+]i and these changes were completely antagonised by prior inclusion of naloxone (20 microM) or idazoxan (RX781094) (2 microM) respectively. When the 1,4-dihydropyridine Ca2+-channel blocker nifedipine (1 microM) was incubated with synaptosomes for 1 min, there was a 17.0% decrease in [Ca2+]i and when it was combined with either U-50,488H (1 microM) or clonidine (1 microM) there was a reduction in [Ca2+]i of 35.0 and 48.1% respectively i.e. the effects were additive. The increases in the depression of [Ca2+]i produced by these drug combinations were antagonised by the inclusion of naloxone (20 microM) or idazoxan (2 microM) which resulted in decreases in free [Ca2+]i of 26.5 and 14.1% respectively. These data indicate that the effects of clonidine and U-50,488H are not mediated by L-type Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Antagonistic action of naloxone on central histamine receptors-stimulated corticosterone secretion in rats under stress

In rats under a mild stress of restraint the interaction between central opioid receptors and histaminergic stimulation of the pituitary-adrenocortical activity was investigated indirectly through corticosterone secretion. In order to avoid a possible direct action on adrenal glands, all the tested drugs were administered intracerebroventricularly (icv). Naloxone an opioid antagonist and cimetidine, a H2- and mepyramine a H1-receptor antagonists were given 15 min before histamine and histamine agonists. One hour after histaminergic drug injection the rats were restrained for 10 min and decapitated. Histamine, 2-pyridylethylamine (PEA), a histamine H1-receptor agonist, and 4-methylhistamine (MeHA) and dimaprit, H2-receptor agonists, significantly intensified the stress-induced increase in serum corticosterone levels. Naloxone, given alone icv or ip, did not substantially alter the stress-induced corticosterone response. Like mepyramine naloxone abolished the corticosterone response to PEA in stressed rats. Naloxone also decreased significantly, though not totally, the corticosterone response to MeHA, dimaprit and histamine, its efficiency being similar to that of cimetidine, a H2-receptor antagonist. These results suggest that in stressed rats central opioid receptors are considerably involved in the histamine H1-receptor - and, to a lesser degree, in the H2-receptor stimulation of the hypothalamo-pituitary-adrenocortical axis.

Alpha 2-adrenergic, kappa-opiate, and P1-purinergic autoreceptors have mutually antagonistic effects: a new regulatory mechanism?

Rat cortical synaptosomes prepared on four-step discontinuous Percoll density gradients were loaded with the fluorescent Ca2+-indicator fura-2 to allow measurement of the intrasynaptosomal free calcium concentration ([Ca2+]i). When P1-purinergic, alpha 2-adrenergic, or kappa-opiate agonists were incubated with these synaptosomes for 1 min, there was a highly significant, dose-dependent reduction in [Ca2+]i. The effects of these agonists were blocked by inclusion of appropriate specific antagonists. When alpha 2-adrenergic and P1-purinergic agonists were coincubated, a mutual antagonism of their effects was observed, and, in fact, an increase rather than a decrease in [Ca2+]i was apparent. This mutual antagonism was reversed by addition of either a P1-purinergic or a alpha 2-adrenergic antagonist. Parallel studies in which kappa-opiate and P1-purinergic agonists were coincubated also demonstrated a mutual antagonism between the individual effects that was reversed by prior inclusion of either a kappa-opiate or P1-purinergic antagonist. As these mutually antagonistic effects have been observed between alpha 2-adrenergic, kappa-opiate, and P1-purinergic receptor-mediated events, we suggest that this may be a general phenomenon and may be a regulatory mechanism at nerve endings.

Interaction between the cardiovascular effects of clonidine and the kappa-opioid agonist U-50,488H in the anterior hypothalamic area of the rat brain

In thiobutabarbitone-anaesthetized rats, microinjection of clonidine (1-40 nmol) into the anterior hypothalamic area (AHy) produced dose-dependent reductions in mean arterial blood pressure and heart rate. Microinjection of the kappa-opioid agonist U-50,488H (3 and 10 nmol) did not modify these parameters. Simultaneous co-administration of clonidine (4 nmol) and U-50,488H (10 nmol) into the AHy resulted in significant potentiation of the clonidine-induced hypotension and marked attenuation of the bradycardia. A lower dose of U-50,488H (3 nmol) co-administered with clonidine (4 nmol) did not influence the cardiovascular responses to clonidine. These findings suggest that AHy neurons involved in the cardiovascular responses to clonidine may be modulated by kappa-opioid receptor stimulation.

Comparison of the pre-junctional effect of opioids in two blood vessels of the rabbit

Inhibition of contractile responses to adrenergic nerve stimulation by opioid peptides was compared using in vitro ring segments of rabbit ear and saphenous arteries transmurally stimulated with short (8 Hz, 1 s) and long (2 Hz, 25 s) stimulation trains. Both delta- ([Leu5]enkephalin or [Met5]enkephalin) and kappa-selective agonists (dynorphin-(1-13) or ethylketocyclazocine) inhibited nerve stimulation-evoked contraction; however, with delta-selective agonists, maximum inhibitory responses were significantly less in the saphenous than in the ear artery. If has been hypothesized that activation of pre-junctional alpha 2-adrenoceptors inhibits effects of opioid receptor activation. Thus the actions of opioid agonists were compared with and without yohimbine (10(-7) M). With yohimbine, inhibitory responses to opioids were not increased at any frequency tested (3-8 Hz) in either vessel. Thus delta and kappa opioid receptors inhibit adrenergic transmitter release in both blood vessels, with delta receptor effects being more prominent in the ear artery. Secondly, studies with alpha 2-adrenoceptor blockade suggest that there is no interaction between pre-junctional opioid and alpha 2-adrenoceptors.

Transmitter release patterns of noradrenergic, dopaminergic and cholinergic axons in rabbit brain slices during short pulse trains, and the operation of presynaptic autoreceptors

Slices of rabbit brain were field-stimulated either by single electrical pulses or by trains of 4 or 8 pulses at 1 or 100 Hz in order to study transmitter release patterns and the autoinhibition of transmitter release. The slices were preincubated with 3H-noradrenaline (cortex), 3H-dopamine (caudate nucleus) or 3H-choline (caudate nucleus). Slices preincubated with 3H-noradrenaline were superfused with medium containing desipramine 1 mumol/l. The overflow of tritium elicited by single pulses amounted to 0.19% of the tritium content of the tissue. The overflow elicited by 4 pulses/1 Hz was similar, whereas that elicited by 4 pulses/100 Hz was 5.1-fold higher. Yohimbine 10-1000 nmol/l increased up to 2.5-fold the overflow evoked by 4 pulses/1 Hz but did not change the overflow evoked by single pulses or 4 pulses/100 Hz. - Slices preincubated with 3H-dopamine were superfused with medium containing nomifensine 1 mumol/l. The overflow of tritium elicited by single pulses was 0.39% of the tritium content of the tissue. The overflow elicited by 4 pulses/1 Hz was 1.3-fold and the overflow elicited by 4 pulses/100 Hz 1.4-fold higher. Domperidone 1-100 nmol/l and sulpiride 10-1000 nmol/l increased up to 2.4-fold the overflow evoked by 4 pulses/1 Hz but increased only slightly the overflow evoked by single pulses or 4 pulses/100 Hz. - Slices preincubated with 3H-choline were superfused either with physostigmine-free medium or with medium containing physostigmine 1 mumol/l. In physostigmine-free medium, atropine did not increase the evoked overflow of tritium at any stimulation condition. In physostigmine-containing medium, the overflow elicited by single pulses was 0.18% of the tritium content of the tissue. The overflow elicited by 8 pulses/1 Hz was 2.0-fold and the overflow elicited by 8 pulses/100 Hz 2.2-fold higher. Atropine 2-200 nmol/l increased up to 2.4-fold the overflow evoked by 8 pulses/1 Hz but increased only slightly the overflow evoked by single pulses or 8 pulses/100 Hz. In physostigmine-free medium, sulpiride 10-1000 nmol/l did not change the single-pulse-evoked overflow of tritium in the absence but increased it in the presence of nomifensine 1 mumol/l. Single pulses elicit a large release of 3H-noradrenaline, 3H-dopamine and 3H-acetylcholine under the conditions of these experiments. Release elicited by single pulses is not subject to autoinhibition except for a small inhibition by spontaneously released transmitter in the case of dopaminergic and cholinergic axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of 5-HT and alpha 1 adrenoceptor antagonists on kappa opioid-induced sedation

The kappa opioid agonists PD-117302 and U-50488 were found to produce dose-dependent reductions in spontaneous locomotor activity in mice. The magnitude of the response to a given dose of each kappa agonist was found to be clearly potentiated by pretreating the animals with either ketanserin (1 mg/kg) or prazosin (0.5 mg/kg). Pretreatment with the selective 5-HT2 receptor antagonist ritanserin given at a high dose (1 mg/kg), the nonselective 5-HT antagonist methysergide or the 5-HT synthesis inhibitor parachlorophenylalanine did not alter the magnitude of the response to the kappa agonist. These results suggest that 5-HT systems are not involved in the sedative effects of kappa opioid agonists and that the potentiating effect seen in animals pretreated with ketanserin is due to the alpha 1 blocking properties of this compound since the effect was mimicked by the alpha 1 antagonist prazosin.

Uptake, release, and modulation of release of noradrenaline in rabbit superior colliculus

The noradrenaline content, the uptake of [3H]noradrenaline, and the release of previously incorporated [3H]noradrenaline were studied in slices of rabbit superior colliculus. The concentration of endogenous noradrenaline was higher in superficial than in deep layers of the superior colliculus. Upon incubation with [3H]noradrenaline, tritium was accumulated by a mechanism that was strongly inhibited by oxaprotiline but little inhibited by 6-nitroquipazine. Electrical stimulation at 0.2 or 3 Hz increased the outflow of tritium from slices preincubated with [3H]noradrenaline; the increase was almost abolished by tetrodotoxin or a low calcium medium. Clonidine reduced the evoked overflow of tritium, whereas yohimbine increased it and antagonized clonidine. The evoked overflow was also reduced by the dopamine D2-receptor-selective agonists apomorphine and quinpirole, an effect antagonized by sulpiride. The preferential opioid kappa-receptor agonist ethylketocyclazocine produced an inhibition that was counteracted by naloxone. Nicotine accelerated the basal outflow of tritium; part of the acceleration was blocked by hexamethonium. The muscarinic agonist oxotremorine slightly diminished the electrically evoked overflow, and its effect was abolished by atropine. The oxaprotiline-sensitive uptake of [3H]noradrenaline as well as the tetrodotoxin-sensitive and calcium-dependent overflow of tritium upon electrical stimulation (presumably reflecting the release of [3H]noradrenaline) indicate that noradrenaline is a neurotransmitter in the superior colliculus. The release of [3H]noradrenaline is modulated through alpha 2-adrenoceptors as well as dopamine D2-receptors, opioid kappa-receptors and nicotine and muscarine receptors. No clear evidence was found for modulation through beta-adrenoceptors, D1-receptors, serotonin receptors, opioid mu- or delta-receptors or receptors for GABA or glutamate. Only the alpha 2-adrenoceptors receive an endogenous agonist input, at least under the conditions of these experiments. The pattern of presynaptic modulation resembles that found for noradrenaline release in other rabbit brain regions, suggesting that all noradrenergic axons arising in the locus coeruleus possess similar presynaptic receptor systems.

Presynaptic alpha 2-adrenoceptor, opioid kappa-receptor and adenosine A1-receptor interactions on noradrenaline release in rabbit brain cortex

The interaction of presynaptic, release-inhibiting alpha 2-adrenoceptors, opioid kappa-receptors and adenosine A1-receptors was studied in slices of the occipito-parietal cortex of the rabbit. The slices were preincubated with 3H-noradrenaline and then superfused and stimulated electrically twice for 2 min each (S1, S2). The stimulation-evoked overflow of tritium was taken to reflect action potential-evoked release of noradrenaline. One of two release-modulating compounds to be examined for interaction was kept in the medium throughout superfusion, the other one was added before S2. In many experiments, the stimulation parameters were adjusted (frequency 0.5-7 Hz; voltage drop 2-5 V/cm) in order to obtain similar reference release (S1) values despite the presence of the first release-modulating compound. The selective kappa-receptor agonist ethylketocyclazocine (EK) attenuated markedly the release-inhibiting effects of the alpha 2-adrenoceptor-selective agonists clonidine and alpha-methylnoradrenaline as well as the release-facilitating effect of the alpha 2-adrenoceptor-selective antagonist yohimbine. The attenuation occurred both when the parameters of electrical stimulation were kept constant and when they were adjusted to obtain similar S1 release values. The selective A1-receptor agonist R-N6-phenylisopropyladenosine (PIA) also attenuated the effects of clonidine and yohimbine. Conversely, clonidine attenuated and yohimbine enhanced the release-inhibiting effect of PIA. Yohimbine also enhanced the release-facilitating effect of the adenosine receptor antagonist 8-phenyltheophylline. Again, these changes occurred both at constant stimulation parameters and when stimulation parameters were adjusted.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutual antagonism of kappa-opiate and alpha 2-adrenoceptor agonist effects on intrasynaptosomal free [Ca2+]i

Synaptosomes prepared from rat cerebral cortices on Percoll discontinuous density gradients were loaded with the fluorescent EGTA analogue Quin 2 to allow measurement of intracellular free [Ca2+]i. When either kappa-opiate or alpha 2-adrenoceptor agonists were incubated with the synaptosomes, there was a highly significant (p less than 0.004, p less than 2.7 X 10(-6), respectively) reduction in intrasynaptosomal free [Ca2+]i relative to controls. As these synaptosomes are not depolarised, the data suggest that both alpha 2-adrenoceptor agonists and kappa-opiate agonists inhibit neurotransmitter release, decreasing the availability of intraneuronal [Ca2+]i rather than altering Ca2+ entry. However, when these two agonists were coincubated, there was a complete abolition of the effects of either agonist; in fact, there was an apparent increase in the intrasynaptosomal free [Ca2+]i. Neither morphine nor [D-Ala2-D-Leu5]enkephalin, mu and delta opiate agonists respectively, had any significant effect on intrasynaptosomal free [Ca2+]i. These results show that the individual effects of clonidine and dynorphin A1-13 are in keeping with the role of these substances at autoreceptors controlling neurotransmitter release. The mutual antagonism of their effects on [Ca2+]i is more difficult to explain but it may be a mechanism that prevents the occurrence of excessive inhibition of neuronal systems.

Protection of presynaptic alpha-adrenoceptors against irreversible blockade by phenoxybenzamine: preservation of the modulatory effects of exogenous noradrenaline and yohimbine

1. Receptor protection experiments were carried out in order to study the site of action of alpha-adrenoceptor agonists and antagonists on the release of noradrenaline. Cerebrocortical slices from rabbits were preincubated with 3H-noradrenaline. They were then superfused with medium containing cocaine 30 mumol/l and stimulated electrically (3 Hz) three times, after 60, 250 and 295 min of superfusion (S1, S2, S3). Phenoxybenzamine 10 mumol/l, when used, was added between S1 and S2 for 30 min; putative protecting drugs (clonidine 100 mumol/l or yohimbine 10 mumol/l) were present 5 min before and during the exposure to phenoxybenzamine and then washed out together with the latter. Either the voltage drop between the electrodes at S2 and S3 or the Ca2+ -concentration of the superfusion medium at S2 and S3 was diminished, if necessary, in order to bring the overflow evoked by S2 close to the overflow at S1. Blockade by phenoxybenzamine, or protection against the blockade, was examined by addition of the test compounds noradrenaline 0.1 mumol/l or yohimbine 1 mumol/l before S3. 2. In slices not exposed previously to alpha-adrenoceptor ligands, noradrenaline 0.1 mumol/l greatly reduced, whereas yohimbine 1 mumol/l greatly increased the evoked overflow of tritium. Both effects were abolished in slices treated with phenoxybenzamine 10 mumol/l alone between S1 and S2. 3. In contrast to phenoxybenzamine alone, exposure to phenoxybenzamine 10 mumol/l in the presence of either clonidine 100 mumol/l or yohimbine 10 mumol/l failed to abolish the effects of the test compounds noradrenaline 0.1 mumol/l and yohimbine 1 mumol/l, although the effects were reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic opioid receptors in the portal vein of the rabbit

The effects of opioids on sympathetic neuroeffector transmission were studied in superfused strips of the portal vein of the rabbit. In the first series of experiments, the tissue was stimulated electrically every 10 min with 10 pulses at 8 Hz and a current strength of 200 mA. The contractions evoked were concentration dependently inhibited by the slightly kappa-selective agonist ethylketocyclazocine and by the delta-selective agonists [D-Ala2,D-Leu5]enkephalin (DADLE) and [D-Pen2,D-Pen5]enkephalin (DPDPLE). The mu-selective agonist [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO) had no effect. The estimated EC50 values for ethylketocyclazocine and DADLE were 5.5 X 10(-8) and 2.9 X 10(-8) mol/l, respectively. Naloxone shifted the concentration-response curves of ethylketocyclazocine and DADLE to the right; the estimated Kb values were (1.7 +/- 0.4) X 10(-8) mol/l and (2.9 +/- 0.8) X 10(-8) mol/l, respectively. The delta-selective antagonist ICI 174864 antagonized only the inhibitory effect of DADLE but not that of ethylketocyclazocine. In the second series of experiments, the tissue was preincubated with [3H]noradrenaline then superfused and stimulated electrically three times for 3 min at 1 Hz and a current strength of 100 mA. Ethylketocyclazocine and DADLE inhibited the evoked overflow of tritium. The inhibition produced by ethylketocyclazocine was antagonized by naloxone. Our results show for the first time that opioid agonists inhibit noradrenaline release and hence, neurogenic vasoconstriction in a vein. As in other cardiovascular tissues of the rabbit, the presynaptic opioid receptors are of the kappa- and delta- but not the mu-type.