Role of histamine receptor in mesencephalic nucleus dorsalis raphe in cardiovascular regulation

Summary The effects of microinjection of histamine and its antagonists into mesencephalic nucleus dorsalis raphe, were investigated on mean arterial pressure and heart rate in cats to elucidate the nature and role of histaminergic receptors in cardiovascular regulation. Microinjection of histamine (5 and 10 g) into nucleus dorsalis raphe elicited both inhibitory and excitatory cardiovascular responses respectively. On the other hand, microinjection of H₂-receptor blocker, cimetidine (10 g) resulted in hypertension and tachycardia while H₁-receptor antagonist, mepyramine (10 g) microinjection evoked hypotension and bradycardia. Furthermore, local pretreatment with cimetidine and mepyramine blocked the inhibitory and excitatory cardiovascular responses of graded doses of histamine microinjection. These H₁ and H₂ receptors are localized in nucleus dorsalis raphe since microinjection of histamine into adjoining neural structures did not evoke any cardiovascular change. Furthermore, both the inhibitory and excitatory cardiovascular responses to histamine microinjection could not be observed in animals with spinal cord transection and in animals pretreated with p-chlorophenylalanine while they could be observed in bilateral cervical vagotomized animals. Thus, it appears that these cardiovascular responses to microinjection of histamine into nucleus dorsalis raphe, are due to modulation of serotonergic bulbospinal influence on sympathetic preganglionic neurones in the spinal cord. Moreover, the excitatory cardiovascular responses of high dose of histamine (10 g) seem to result from a local release of noradrenaline since they were blocked by prior microinjection of guanethidine and piperoxan into nucleus dorsalis raphe. A release of noradrenaline in turn, modulates the activity of the neurones of the nucleus by acting on adrenoceptors and thereby alters the activity of sympathetic preganglionic neurones. These adrenoceptors appear to be of ₁ type (Saxena et al. 1985, 1987) since phenylephrine microinjection evoked excitatory cardiovascular responses could be blocked by piperoxan. Send offprint requests to K. K. Tangri at the above address     

Effects of intrathecal or intracerebroventricular pretreatment with pertussis toxin on antinociception induced by β-endorphin or morphine administered intracerebroventricularly in mice

We have previously demonstrated that -endorphin and morphine, when administered supraspinally, produce antinociception by activating different descending pain inhibitory systems in both rats and mice. However, the signal transduction mechanisms involved in the descending pain-inhibitory systems that are activated by -endorphin and morphine administered intracerebroventricularly (i.c.v.) have not been characterized. Therefore, in the present study, the effects of intrathecal (i.t.) and i.c.v. pretreatments with pertussis toxin (PTX) on antinociception induced by -endorphin or by morphine administered i.c.v. were studied in ICR mice. Antinociception was assessed by the tail-flick assay and by the hotplate assay. Intrathecal pretreatment with PTX (0.5 g) for 6 days effectively reduced the inhibition of the tail-flick response induced by -endorphin (1 g) or by morphine (1 g) administered i.c.v. However, i.t. pretreatment with PTX was not effective in reducing the inhibition of the hot-plate response induced by -endorphin or by morphine administered i.c.v. Intracerebroventricular pretreatment with PTX (0.5 g) for 6 days effectively reduced the inhibition of the tail-flick and hot-plate responses induced by morphine (1 g), but not that induced by -endorphin (1 g), administered i.cv. Our results suggest that there are PTX-sensitive G proteins coupled to the spinal descending pain inhibitory systems that are activated by -endorphin and morphine administered i.c.v. At a supraspinal level, i.cv. morphine- but not -endorphin-induced antinociception is mediated by PTX-sensitive G proteins. Correspondence to: Hong W. Suh at the above address     

Inhibition of dopamine-sensitive adenylate cyclase by opioids: possible involvement of physically associated µ- and δ-opioid receptors

Summary D-1 dopamine receptor-stimulated cyclic AMP efflux from rat neostriatal slices (induced by 30 M dopamine + 10 M (–)sulpiride) was concentration-dependently reduced by morphine, [abetd-Ala-abetd-Leu]-enkaphalin (DADLE), [d-Pen-abetd-Pen]enkephalin (DPDPE) and bremazocine. Naloxone (0.1 M) selectively antagonized the inhibitory effect of (a submaximally effective concentration of) morphine, whereas ICI 174864 (0.75 M) completely blocked the inhibitory effects of DADLE, DPDPE and bremazocine without affecting that of morphine, indicating a role of µ- as well as -opioid receptors. Upon simultaneous activation of D-1 dopamine receptors and b-opioid receptors the (-receptor-mediated) inhibitory effect of morphine was abolished, while it was not changed following simultaneous activation of D-1 and (inhibitory) D-2 dopamine receptors. Cyclic AMP efflux induced by isoprenaline or adenosine was not affected by the opioids and that induced by vasoactive intestinal peptide (VIP) was inhibited by morphine and DADLE only. In the latter case naloxone, but not ICI 174864, antagonized the inhibitory effects.These data show that D-1 dopamine receptor-stimulated adenylate cyclase activity in rat neostriatum, but not that stimulated through other receptors, is inhibited by two pharmacologically distinct opioid receptor subtypes. It is speculated that these - and -opioid receptors share a common inhibitory guanine nucleotide binding protein and may represent closely associated recognition sites of a functional opioid receptor complex. Send offprint requests to A. N. M. Schoffelmeer at the above address     

Anti-nociceptive effect of morphine,opioid analgesics and haloperidol injected into the caudate nucleus of the rat

Summary The effect of intracaudate (i.c.) microinjections of morphine, opioid analgesics and haloperidol was determined on the tail-flick response evoked by radiant heat in rats. Bilateral injections (0.2 l on each side) into the caudate nuclei of morphine 5 g, pethidine 50 g, levorphanol 4 g, dextrorphan 10 g and haloperidol 5 g significantly increased the reaction time of the tail-flick response. The antinociceptive effect of an i.c. injection of morphine or levorphanol was abolished by an intraperitoneal (i.p.) injection of naloxone 0.2 mg/kg or apomorphine 2 mg/kg. The anti-nociceptive effect of pethidine, dextrorphan and haloperidol was reduced but not abolished by an i.p. injection of naloxone 0.2 mg/kg. An i.p. injection of apomorphine 2 mg/kg abolished the effect of an i.c. injection of haloperidol. A bilateral i.c. injection of naloxone 5 g or apomorphine 10 g reduced the anti-nociceptive effect of an i.p. injection of morphine 2 mg/kg or haloperidol 2 mg/kg, but did not abolish it. It is concluded that (1) an anti-nociceptive effect can be achieved by an action on the caudate nucleus of the drugs tested; (2) the anti-nociceptive effect exerted by morphine and levorphanol in the caudate nucleus is due to a specific action mediated by opiate receptors, whilst that produced by pethidine and dextrorphan is due to a specific and/or unspecific action; (3) the anti-nociceptive effect of haloperidol in the caudate nucleus is due to an impairment of dopaminergic impulse transmission, which is also involved in the effect of morphine and levorphanol.Supported by the Sonderforschungsbereich 38 Membranforschung     

Effects of morphine on release of acetylcholine in the rat striatum: an in vivo microdialysis study

Summary We examined the effect of morphine on the release of acetylcholine (ACh) in the striatum of freely moving rats using the in vivo microdialysis method. The basal level of ACh was 3.01 ± 0.51 pmol/30 l/15 min in the presence of neostigmine (10 M). Tetrodotoxin (1 M), a selective blocker of voltage-dependent Na⁺ channels, rapidly decreased the release of ACh in the striatal perfusates. Morphine at a dose of 10 mg/kg (i.p.) caused a reduction of ACh release in the striatum at 90–150 min. However, a lower dose of morphine (5 mg/kg, i.p.) did not affect ACh release in the striatum. The reduction following intraperitoneal administration of morphine was abolished by naloxone (1.0 mg/kg).After microinjection of the neurotoxin 6-hydroxydopamine (6 g/3 l, 7 days before) in the substantia nigra, the morphine (10 mg/kg)-induced decrease of ACh was attenuated, and a similar result occurred following reserpine (2 mg/kg, i.p.) 24 h before combined with -methyl-p-tyrosine (300 mg/kg, i. p.) 2.5 h before.These findings indicate that morphine exerts an inhibitory influence on striatal ACh release in freely moving rats and that this inhibitory effect is mediated by the nigro-striatal dopaminergic system.Correspondence to K. Taguchi at the above address     

The involvement of central cholinergic system in the pressor effect of intracerebroventricularly injected U-46619, a thromboxane A2 analog, in conscious normotensive rats

The aim of this study was to determine the involvement of the central cholinergic system in the rise in blood pressure evoked by the thromboxane A2 (TxA2) analog, U-46619, given centrally. Intracerebroventricular (i.c.v.) injections of U-46619 (0.5, 1.0 and 2.0 g) caused dose- and time-related increases in blood pressure and decreased heart rate in awake rats. U-46619 (1 g; i.c.v.) also produced an approximately 65% increase in posterior hypothalamic extracellular acetylcholine and choline levels. Pretreatment with SQ-29548 (8 g; i.c.v.), selective TxA2 receptor antagonist, completely inhibited both the cardiovascular responses and the increase in acetylcholine and choline levels to subsequent injection of U-46619 (1 g; i.c.v.). Atropine (10 g; i.c.v.), nonselective muscarinic receptor antagonist, pretreatment did not affect the cardiovascular responses observed after U-46619 (1 g; i.c.v.). Pretreatment with the nonselective nicotinic receptor antagonist, mecamylamine (50 g; i.c.v.) attenuated the pressor effect of U-46619 (1 g; i.c.v.). Higher doses of mecamylamine (75 and 100 g; i.c.v.) pretreatments did not change the magnitude of the blockade of pressor response to U-46619; however, they abolished the bradycardic effect of U-46619 dose-dependently. Interestingly, pretreatment of rats with methyllycaconitine (10 g; i.c.v.) or -bungarotoxin (10 g; i.c.v.), selective antagonists of 7 subtype of nicotinic acetylcholine receptors (7nAChRs), partially abolished the pressor response to i.c.v. injection of U-46619 (1 g). Similar to the mecamylamine data, the use of higher doses of methyllycaconitine (25 and 50 g; i.c.v.) produced the same magnitude of blockade that was observed after the 10 g methyllycaconitine pretreatment, but it completely abolished the bradycardic effect of U-46619 (1 g; i.c.v.) at the dose of 25 g. The present results show that central administration of U-46619 produces pressor and bradycardic effect and increase in hypothalamic acetylcholine and choline levels by activating central TxA2 receptors. The activation of central nicotinic receptors, predominantly 7nAChRs, partially mediates the cardiovascular responses to i.c.v. injection of U-46619.     

Opioid dependence in myenteric neurons innervating the circular muscle of guinea-pig ileum

Summary Guinea-pigs were treated with morphine for 6–8 days by subcutaneous implantation of pellets, each containing a mixture of morphine base (120 mg) and morphine hydrochloride (35 mg). Each guinea-pig received a single pellet. Mechanical activity of the circular muscle was recorded in vitro in preparations comprising the circular muscle and myenteric plexus. Exposure to morphine was maintained by addition of 1 M morphine to the organ baths. After 90 min, morphine was withdrawn, either by repeatedly washing tissues in morphine-free Krebs' solution , or by addition of naloxone to reduce the occupancy of the opioid receptors by morphine. Withdrawal of morphine resulted in markedly enhanced contractile activity compared with that in circular muscle-myenteric plexus preparations from untreated control guinea-pigs. The withdrawal contractions were abolished by tetrodotoxin (600 nM) and greatly reduced by hyoscine (1 M), indicating that they resulted from action potential discharge in myenteric neurons that release acetylcholine onto the circular muscle. Activation of the cholinergic excitatory motor neurons was not secondary to synaptic activation by cholinergic interneurons, because hexamethonium (100 M) did not affect withdrawal contractions. The withdrawal response may therefore arise in the cholinergic excitatory motor neurons themselves, or in neurons that activate them via noncholinergic mechanisms. Send offprint requests to S. Johnson at the above address     

Cardiovascular responses elicited by microinjection of monoamines into mesencephalic nucleus dorsalis raphe in cats

Summary The effect of monoaminergic agonists and antagonists microinjected into mesencephalic nucleus dorsalis raphe has been studied on blood pressure and heart rate to elucidate the nature and role of these monoaminergic receptors in cardiovascular regulation. Microinjection of monoamines, noradrenaline, phenylephrine and 5-hydroxytryptamine (5-HT) into nucleus dorsalis raphe elicited hypertension and tachycardia which could be blocked by local pretreatment with piperoxan (an -adrenoceptor blocker) and methysergide (a 5-HT receptor blocker) respectively. However, isoprenaline microinjections failed to evoke any response. Bilateral vagotomy did not prevent these cardiovascular responses evoked by monoamines microinjection, while cervical spinal cord (C₁) transection with bilateral vagotomy prevented these responses. These monoaminergic receptors seem to be localized in nucleus dorsalis raphe since microinjection of monoamines into neural structures adjoining nucleus dorsalis raphe, failed to induce any cardiovascular response. Monoaminergic receptors are present in nucleus dorsalis raphe which modulate cardiovascular activity by influencing sympathetic preganglionic neurons in the intermediolateral columns of the spinal cord.     

Role of muscarinic cholinergic mechanisms in the substantia nigra pars reticulata in mediating muscular rigidity in rats

Summary Bethanechol chloride (5–25 g), when injected into the substantia nigra pars reticulata (SNR) of rats, produced muscular rigidity in a dose-dependent way, and in addition, catalepsy and ipsilateral posture. The effects of bethanechol in the dose of 25 g were prevented by coadministration of 10 g scopolamine hydrochloride. Injections of 25 g betanechol or 10 g scopolamine into the reticulata only slightly affected the muscular rigiditiy produced by 15 mg/kg i.p. morphine hydrochloride. The results suggest that muscarinic cholinergic mechanisms in the substantia nigra pars reticulata, although effective by themselves, affect by expression of at least one striatal functional alteration, the muscular rigidity, in a less effective way than GABAergic or endogenous opioid mechanisms do.     

Human isolated ileum: motor responses of the circular muscle to electrical field stimulation and exogenous neuropeptides

Summary (1) Circularly-oriented muscle strips from the human ileum responded to electrical field stimulation (1–50 Hz) with frequency-related primary relaxation at low frequency and primary contractions at high frequencies of stimulation. Both responses were abolished or markedly reduced by tetrodotoxin (1 M). (2) Atropine (3 M) or omega conotoxin (0.1 M) reduced but dit not abolish contraction to electrical field stimulation and enhanced the relaxation. Omega conotoxin (0.1 M) did not affect carbachol-induced contraction nor isoprenaline-induced relaxation. (3) Neurokinin A and substance P (1 nM-1 M) produced a concentration-dependent contraction. The NK-1 receptor selective agonist, [Pro⁹]SP sulfone and the NK-2 receptor selective agonist [Ala⁸]NKA(4-10) prodneed a contraction superimposable to that of substance P and neurokinin A, respectively. On the other hand, [MePhe⁷]-neurokinin B, an NK-3 receptor selective agonist was ineffective up to 1 [M. The response to substance P or neurokinin A was unaffected by atropine (3 M). (4) Galanin, up to 0.1 M produced a weak and inconsistent contraction. (5) Vasoactive intestinal polypeptide (10 nM - 1 M) produced a concentration-dependent relaxation while human alpha calcitonin gene-related peptide exerted a weak and inconsistent relaxant effect. (6) These findings indicate that both cholinergic excitatory and non-cholinergic inhibitory nerves affect the motility of the circular muscle of the human small intestine. Transmitter release from excitatory nerves seems largely mediated by activation of omega conotoxin-sensitive (N-type) calcium channels. Tachykinins exert a potent contractile effect independently of cholinergic nerves via NK-1 and NK-2 receptors. Send offprint requests to C. A. Maggi at the above address     

An ATP-sensitive potassium channel blocker abolishes the potentiating effect of morphine on the bicuculline-induced convulsion in mice

ICV bicuculline, a selective GABA_A antagonist, dose-dependently induced clonic-tonic convulsions in mice. Coadministration of ICV morphine ( opioid agonist) significantly potentiated ICV bicuculline-induced convulsions, and this effect of morphine was completely blocked by pretreatment with-funaltrexamine (-FNA), a antagonist. ICV glibenclamide, a selective ATP-sensitive potassium (K_ATP) channel blocker, at a dose which alone did not affect the convulsive threshold of bicuculline, was capable of blocking the exacerbation of ICV bicuculline-induced convulsions by morphine. The present data further suggest that K_ATP channels may play a tonic regulatory role in the potentiative effect of morphine on ICV bicuculline-induced convulsions.     

On the mechanism of the decrease in cerebellar cyclic GMP content elicited by opiate receptor agonists

Summary Morphine, dextromoramide (4 mol/kg i.p.) and vimonol R₂ (17 mol/kg i.p.) in analgesic doses (28 to 112 mol/kg i.p.) decreased 3,5-cyclic guanosine monophosphate (cGMP) in rat cerebellar cortex; morphine also decreased the cGMP content in deep cerebellar nuclei. Intrastriatal but not intracerebellar injections of morphine (20 g) decreased cerebellar cGMP content. Naltrexone, an opiate receptor antagonist, but not apomorphine, a dopaminergic receptor agonist, blocked the effect of morphine on cerebellar cGMP. Pretreatment with 3-acetylpyridine (3-AP) which destroys the climbing fibers, failed to antagonize the effect of morphine on cerebellar cGMP. These results suggest that activation of opiate receptors in striatum decreases cerebellar cGMP content presumably by reducing activity in the mossy fiber excitatory input to cerebellum.     

Suppression of deprivation-induced water intake in the rat by opioid antagonists: central sites of action

The effects of naltrexone methobromide, a quaternary derivative of the opioid antagonist naltrexone, were investigated on deprivation (24 h)-induced water intake in the unilaterally cannulated rats. Naltrexone methobromide reduced post-deprivational water intake with an ED₅₀ of 7.3 g when tested at 30 min (peak effect) after intracere-broventricular administration. It also dose-dependently (0.3–10 g) depressed water intake, with peak effects at 15 min, after microinjection into the paraventricular hypothalamic nucleus and into the supraoptic hypothalamic nucleus. The drug did not produce any other effects on behaviors. The ED₅₀s were 1.4 g when given into the paraventricular nucleus, and 3.3 g when given into the supraoptic nucleus, respectively. Although injections of higher doses (1.0, 3.0 and/or 10 g) of the drug into the preoptic area, zona incerta, and corpus callosum significantly suppressed water intake, other behavioral manifestations, such as rotational behaviors, convulsions, body shakes, head swaying, and/or backward locomotion were manifested simultaneously with the reduction in drinking. When injected into the lateral hypothalamic area, water intake was not significantly affected by the drug. These findings suggest that the paraventricular and supraoptic hypothalamic nuclei are important sites of action in the naltrexone-induced suppression of water intake.Supported by Grant DA00541 and by K05DA00008, both from the National Institute on Drug Abuse     

Electrolytic lesions and pharmacological inhibition of the dorsal raphe nucleus prevent stressor potentiation of morphine conditioned place preference in rats

Rationale Exposure to a single session of uncontrollable inescapable shock (IS), but not to identical controllable escapable shock, produces a potentiation of morphine's rewarding properties that is unusual in that the stressor can be given a number of days before the drug administration in an environment quite different from the drug context. Many other behavioral outcomes of stressors that depend on the uncontrollability of the stressor are mediated by alterations in serotonergic (5-HT) neurons within the dorsal raphe nucleus (DRN).Objectives The present experiments examined the role of the DRN and 5-HT in mediating the effect of IS on the rewarding properties of morphine as assessed by conditioned place preference (CPP).Methods In experiment 1, subjects received small electrolytic lesions of the DRN and were tested for morphine (3.0 mg/kg, SC) CPP after IS or control treatment. In experiment 2, subjects received an intra-DRN microinjection of the 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 1.0 g/0.5 l) either before IS or before morphine (3.0 mg/kg, SC) injections during CPP testing.Results IS potentiated morphine CPP in controls, but both DRN lesion and intra-DRN 8-OH-DPAT, either before IS or before morphine administration, completely blocked this effect.Conclusions These data implicate alterations in DRN 5-HT neurons in the potentiation of morphine reward produced by uncontrollable stress.     

The effect of chronic treatment with naltrindole,a selective δ-opioid antagonist,on μ-opioid receptor-mediated antinociception in diabetic mice

The effects of chronic treatment with naltrindole (NTI), a selective -opioid receptor antagonist, on the antinociceptive effects of -opioid agonists, such as morphine and [D-Ala²,N-MePhe⁴, Gly-ol⁵]enkephalin (DAMGO) were examined in diabetic mice. Antinociception induced by morphine (10 g, ICV) and DAMGO (0.5 g, ICV) was significantly lower in diabetic mice than in non-diabetic mice. The low sensitivities to the antinociceptive potencies of ICV morphine (10 g) and DAMGO (0.5 g) in diabetic mice were reversed compared with those in saline-treated non-diabetic mice when diabetic mice had been pretreated with NTI (2 mg/kg per day, SC) for 14 days. Naive mice which had been injected with spleen mononuclear cells from saline-treated diabetic mice were less sensitive to DAMGO-induced antinociception. However, adoptive transfer of spleen mononuclear cells from NTI-treated diabetic mice to naive mice had no effect on the recipients' antinociceptive sensitivity to DAMGO. These results suggest that the effect of NTI on the sensitivity to -opioid agonists in diabetic mice may be due to the immunosuppressive effects of NTI.     

Opioid receptors in the caudate nucleus can mediate EMG-recorded rigidity in rats

Summary Systemic administrations of opioids are known to induce catatonia or lead pipe rigidity in rats. The relevance of the caudate nucleus in inducing rigidity was tested. For this purpose, several opioids (or saline) were injected into the head of the caudate nucleus (intrastriatal injection) through an implanted cannula, and the electromyographical activity (EMG) was recorded in the gastrocnemius-soleus muscle (GS). Morphine (7.5–30 g), injected unilaterally, induced a continuous EMG activity in the ipsilateral GS muscle. This effect could be antagonized by systemic administration of naloxone (1 or 2 mg/kg i.p.). D-ala²-met⁵-enkephalinamide (3 g) and levorphanol (22.5 g) induced an EMG activity, too, whereas an equimolar dose of dextrorphan was ineffective, indicating that this effect was stereospecific and mediated via opioid receptors in the caudate nucleus. The EMG activity observed after systemic morphine administration (15 mg/kg i.p.) was antagonized by intrastriatal injection of naloxone (5 g). From our results, it can be concluded that the striatum — at least the head of the caudate nucleus — plays an important role in mediating the rigidity observed after systemic administration of morphine and other opioids.with technical assistance of Ch. Bode and H. Kügler     

The cardiovascular effects of intraventricularly administered histamine in the anaesthetised rat

Summary In urethane-anaesthetised rats intraventricular (i.c.v.) injections of histamine (0.1–10.0 g) elicited dose-related rises in both the resting blood pressure and heart rate. These cardiovascular effects of histamine were antagonised in a dose-dependent manner by i.c.v. pretreatments with the histamine H₁-receptor antagonists mepyramine (10, 50 and 100 g) and diphenylpyraline (100 and 200 g). Pretreatment with the histamine H₂-receptor antagonist metiamide (100 and 200 g i.c.v.) failed to modify either of the responses. A dose-related antagonism of the hypertensive response to histamine i.c.v. was elicited by phentolamine (100 and 200 g i.c.v.) but the positive chronotropic effect was not modified by this pretreatment. The cardiovascular responses to histamine i.c.v. were abolished by mecamylamine (5.0 mg/kg i.v.) and greatly reduced by 6-hydroxydopamine (3×250 g i.c.v.), but only the tachycardia was significantly modified by atropine (100 g i.c.v.) and propranolol (1 mg/kg i.v.). Propranolol (100 g i.c.v.), bilateral vagotomy, or acute bilateral adrenal demedullation failed to modify the cardiovascular responses to histamine i.c.v. The results suggest that histamine is able to modify the resting blood pressure and heart rate by independent central modes of action, which involve central adrenergic and cholinergic mechanisms.Preliminary findings of this study were presented at the Autumn meeting of the British Pharmacological Society (Finch and Hicks, 1975).     

A method to estimate the potency of the μ-component of an opioid having mixedμ-, andκ- and/orδ-agonist activities

The SC administration of either typical-agonists such as morphine, pethidine, fentanyl and levorphanol or a mixed- and-agonist like [d-Ala²,d-Leu⁵]-enkephalin to 10-day-old rats produced loss of righting reflex. Additionally, the loss of righting reflex induced by these opioid agonists was antagonized by naloxone, an opioid antagonist having a preference for-receptors, but by neither nor-binaltorphimine nor naltrindole, a specific- or-antagonist, respectively, indicating that the loss of righting reflex was produced by the interaction of an opioid with-receptors. Moreover, the potency of each opioid agonist relative to that of morphine estimated by the present in vivo method was similar to that determined by the traditional in vitro isolated preparation. In contrast to-agonists, neither typical-agonists such as U-50, 488H, ketocyclazocine, pentazocine and butorphanol, nor a selective-agonist like [d-Pen²,d-Pen⁵]-enkephalin affected the righting reflex of 10-day-old rats, indicating that-agonists, but neither- nor-agonists, produced the naloxone-reversible loss of righting reflex in infant rats. By employing the present in vivo method to estimate the-agonist activity of an opioid with mixed agonist activities, it was indicated that the-agonist activity of ethylketocyclazocine, which had been employed as a representative-agonist, was essentially the same as that of morphine, a representative-agonist.     

Influence of N-allyl-normetazocine on acetylcholine release from brain slices: involvement of muscarinic receptors

Summary The effects of (±)N-allyl-normetazocine on the release of acetylcholine from different areas of guinea-pig and rat brain were investigated. 1. The drug did not modify the electrically (2 Hz) evoked tritium efflux from guinea-pig cerebral cortex, thalamus and caudate nucleus slices, preloaded with ³H-choline 0.1 mol/l and superfused with Krebs solution containing hemicholinium-3 10 mol/l. 2. (±)N-allyl-normetazocine 10 mol/l. enhanced the evoked ³H efflux from guinea-pig brain slices superfused with Krebs solution containing physostigmine 30 mol/l or oxotremorine 0.3 -1 gmol/l; the effect was naloxone-insensitive and was abolished by atropine 0.15 mol/l, but not by pirenzepine 1 mol/l. 3. (±)N-allyl-normetazocine 5 mol/l enhanced the electrically evoked release of endogenous acetylcholine as well, in a naloxone-insensitive way. 4. Both (±) and (+)N-allyl-normetazocine were without effect on ³H efflux from rat caudate nucleus slices electrically stimulated at 0.2 Hz frequency, after preloading with ³H-choline and during superfusion with hemicholinium-3. 5. The results are discussed in view of the antimuscarinic properties of the drug. Send offprint requests to A. Siniscalchi     

In vivo apparent pA2 analysis in rats treated with either clocinnamox or morphine

Experiments tested the hypothesis that loss of agonist potency or effectiveness following irreversible antagonist or chronic agonist treatment may result from affinity changes at opioid receptors. Apparent affinity of naltrexone or nalbuphine for opioid receptors was measured in vivo in rats treated with either a single dose of the irreversible antagonist clocinnamox or repeated doses of morphine. Apparent affinity of each antagonist was estimated from its potency as an antagonist of discriminative stimulus or rate-decreasing effects of morphine in rats trained to discriminate 3.2 mg/kg morphine and saline. In control rats, apparent pA₂ values for naltrexone and nalbuphine were 7.5–7.6 and 5.3, respectively. In clocinnamox-treated rats, apparent pA₂ values for naltrexone were 7.2–7.7, suggesting that clocinnamox treatment did not alter affinity of naltrexone for sites through which morphine exerts behavioral effects. In rats treated repeatedly with morphine, apparent pA₂ values for nalbuphine were 5.1–5.3, suggesting that repeated morphine treatment did not alter affinity of nalbuphine for these sites. The observation that neither clocinnamox nor repeated morphine treatment altered in vivo affinity estimates for naltrexone or nalbuphine, respectively, suggests that the reductions in agonist potency produced by these treatments do not result from changes in affinity at opioid receptors.This work was supported by US Public Health Service Grant DA03796 and NIDA Research Scientist Development Award K02 DA00132 to A.M. Young. A portion of this work is presented in abstract form [Walker EA, Richardson TM, Young AM (1995) Apparent affinity estimates for opioid antagonists in rats treated with clocinnamox or chronic morphine. In: Harris LS (ed) NIDA Res Monogr 153: 450]