Blockade by morphine of acetylcholine release from the caudate nucleus in the mid-pontine pretrigeminal cat

Fentanyl produces cholinergically-mediated analeptic and EEG arousal effects in rats

Fentanyl (20 micrograms/kg i.p.), administered to naltrexone-pretreated, pentobarbital-anesthetized rats, produced a shortening of the duration of narcosis. This analeptic effect was blocked by atropine, but not by methylatropine, indicating that a central cholinergic mechanism was involved. Fentanyl also increased sodium-dependent high affinity uptake of choline activity in the hippocampus and cortex that had been depressed by the barbiturate. Injection of 0.8 ng of fentanyl into the pontis oralis in the pontine reticular formation also produced analepsis in naltrexone-pretreated, pentobarbitalized rats. Hippocampal EEG recordings also showed the appearance of cholinergically-mediated theta activity, which was indicative of arousal activity in the hippocampus. These results suggest that fentanyl, in addition to possessing potent opiate activity, also activates a nonopioid-mediated central cholinergic arousal system.

Enhancement of opioid cataleptic response by cortical frontal deafferentation or intrastriatal injection of NMDA-receptor antagonists

The cataleptic activity of morphine and methadone was markedly potentiated in frontally decorticated rats with no apparent changes in the onset or duration of action. Enhancement of the opioid cataleptic response was not due to changes in the availability of the drugs in the brain. The potentiation of methadone-induced catalepsy in decorticated rats was mimicked in naive rats by intrastriatal (i.s.) application of 2-amino-7-phosphonoheptanoic acid (AP7), a potent and selective antagonist of N-methyl-D-aspartate (NMDA) receptors. Therefore, degeneration of glutamatergic synapses following decortication could be responsible for the changes in behavioral effects caused by opioids. The failure of AP7 to elicit an effect after injection into the n. accumbens fits with the possibility of a selective involvement of the striatum in this phenomenon. That the striatum plays a critical role in the expression of opioid-induced catalepsy was substantiated by the findings that: (1) naloxone, an opioid antagonist, injected i.s., prevents the potentiation of catalepsy induced by methadone and morphine in decorticated animals, (2) oxotremorine, a muscarinic agonist, injected i.s., reverses the enhancement of opioid-catalepsy in decorticated rats, (3) earlier studies by others showed that ablation of the striatum had a facilitatory action on opioid-induced catalepsy. In conclusion, evidence is given that the corticostriatal pathway exerts an inhibitory effect upon narcotic-induced cataleptic behavior. The possibility that this effect is mediated through striatonigral GABAergic output is discussed. The data further suggest that the neuronal mechanisms through which the corticostriatal pathway mediates narcotic catalepsy is operative through activation of NMDA receptors within the striatum.

Codeine produces a cholinergically mediated analeptic effect in rats and rabbits

The intravenous administration of codeine to diazepam-narcotized rabbits resulted in a shortened duration of loss of righting reflex. Coadministration of naltrexone plus codeine enhanced this analeptic effect and was also effective in shortening the duration of pentobarbital narcosis. The analeptic effect was blocked by atropine, but not by methylatropine, indicating involvement of a central cholinergic mechanism. In rats the analeptic activity correlated with the reversal of the diazepam-induced fall in sodium dependent high affinity choline uptake in hippocampal and cortical synaptosomes. These findings may represent the pharmacological basis of the recently reported antinarcoleptic action of codeine in man.

Bradykinin antagonizes morphine-induced analgesia in rats

The intraventricular injection of 5 micrograms of bradykinin or Met-Lys-bradykinin in rats antagonized the increase in nociception threshold induced by morphine (5 micrograms i.c.v. or 10 mg/kg i.p.) in the tail flick test. The effect of the two kinins was prevented by atropine (30 mg/kg i.p.). The results suggest that bradykinin counteracts opiate activity in the central nervous system by enhancing acetylcholine release.

Acetylcholine release in the cat caudate nucleus measured with the choline oxidase method

A chemiluminescent assay for the estimation of acetylcholine (ACh) was used to measure ACh release in caudate nuclei (CN) of halothane-anaesthetized cats implanted with push-pull cannulae. The validity of the entire experimental approach used was shown by the fact that ACh release was calcium-dependent and was increased by depolarizing agents (potassium ions, veratridine) as well as by atropine. The effects of GABA (10(-5) M, 30 min) unilateral application into the ventralis medialis and ventralis lateralis thalamic nuclei on ACh release in both CN were then examined. This treatment, known to increase DA release bilaterally, decreased ACh release in both CN. These data further reveal the role of thalamic nuclei in the bilateral regulation of the activity of neurons identified within the basal ganglia and are discussed in the light of the well-known inhibitory influence of nigrostriatal DA neurons on striatal cholinergic neurons.

Effect of neuroactive peptides on labeled 5-hydroxytryptamine release from rat spinal slices in vitro

Effects of neuroactive peptides on the release of labeled 5-hydroxytryptamine (5-HT) from preloaded rat spinal cord slices were investigated. The 5-HT release was significantly stimulated by somatostatin (10-50 microM) and substance P (10-50 microM), but not by neurotensin (50 microM), beta-endorphin (30 microM) and methionine-enkephalin (met-enk) (100 microM). Somatostatin-stimulated 5-HT release was markedly inhibited by gamma-aminobutyric acid (GABA) (30 microM), but not by baclofen (30 microM) and met-enk (100 microM). Substance P-stimulated 5-HT release was strongly inhibited by GABA (30 microM) and baclofen (30 microM), but not by met-enk (100 microM). High concentrations (20 mM) of potassium also stimulated 5-HT release. The high potassium-stimulated 5-HT release was not affected by GABA (30 microM) and met-enk (100 microM). These results suggested further evidence on the important role of somatostatin and substance P as modulators of serotonergic neurones.

The cholinergic system in rat striatum during morphine tolerance and dependence

Male Sprague-Dawley rats were used in the present study to assess the effects of chronic treatment of morphine on the striatal cholinergic system. The results demonstrate that neither short nor long-term morphine treatment had an effect on choline acetyltransferase (ChAT) activity or 3H-quinuclidinylbenzilate (3HQNB) binding in discrete striatal regions of the rat brain.

Acetylcholinesterase changes in the central nervous system of mice during the development of morphine tolerance addiction and withdrawal

The distribution of acetylcholinesterase in the brain is studied during the development of morphine tolerance and through a period of withdrawal to elucidate the possible role of this enzyme in producing physical dependence in mice. Tolerance and physical dependence are produced in male albino mice by giving morphine sulphate subcutaneously at eight hourly intervals, in an increasing dose of 10 mg/kg body weight every 24 hours, for 15 days. The animals are considered addicted, when they received an otherwise lethal dose, 150 mg/kg three times a day. The enzyme shows a marked elevation in the overall distribution during the development of physical dependence. The habenular complex, nuclei anterioventralis and medialis thalami, nucleus caudatus putamen, amygdaloideus lateralis, septal nuclei, nucleus nervi hypoglossi, nucleus reticularis lateralis, tuberculum olfactorium, nucleus tractus diagonalis brocae, stratum pyramidale hippocampi, nucleus paraventricularis thalami, nucleus dorsalis nervi vagi, nucleus tractus spinalis nervi trigemini and nucleus reticularis thalami show an increase in the enzyme activity. This enhancement is not linear with the increase in dosage. Withdrawal is characterised by a sudden fall in the activity of acetylcholinesterase in the above mentioned areas of brain.

Interactions of morphine with putative neurotransmitters in the mesencephalic reticular formation

Acetylcholine (ACh) and norepinephrine (NE) have been identified previously as putative nociceptive neurotransmitters in the mesencephalic reticular formation (MRF) of the rat because they frequently mimic the change in neuronal firing (usually an increase) evoked by a noxious stimulus (NS). The purpose of this study was to determine if 1.) morphine (M) acts to prevent the increase in firing evoked by a NS by blocking the effects of either of these two neurotransmitters and 2.) if this effect is a specific narcotic effect. Using the technique of microiontophoresis in conjunction with extracellular recording, we located single units in the MRF in which 1.) neuronal firing was accelerated by a NS: 2.) M blocked this response; and 3.) either ACh or NE mimicked the effect of the NS. Neurons meeting these three criteria were studied further to determine if morphine would also block the response to either of the neurotransmitters and if this was a specific narcotic effect. We found that morphine blocked the increase in neuronal firing evoked by the NS and ACh or the NS and NE in over 50% of the cells meeting the above criteria. Some neurons were found in which both ACh and NE mimicked the NS and M blocked all three responses. This blockade of these neurotransmitters was a specific narcotic effect because it could be reversed by the systemic administration of naloxone. These data lead to the tentative hypothesis that M, acting via an opiate receptor, blocks the increase in neuronal firing evoked by a NS by blocking the postsynaptic effects of either ACh or NE. This may be one of the mechanisms by which morphine acts to produce analgesia.

Changes in acetylcholine in caudate nucleus tissue isolated in situ detected by gas chromatography mass spectrometry selected ion monitoring

Acetylcholine neurons in the striatum are believed to be primarily intrinsic. In cats a cylinder of caudate tissue was isolated unilaterally from all afferent and efferent connections while preserving the blood supply through the base of the cylinder. Cats were decapitated two and four weeks after operation. Dorsal, intermediate and ventral portions of the viable isolated cylinder, and corresponding tissue from the contralateral hemisphere serving as control, were removed, weighed and frozen. Deuterated internal standards at concentrations approximating those of acetylcholine and choline in the tissue were added and acetylcholine and choline extracted, reacted with propionyl chloride and demethylated with sodium benzenethiolate. The derivatives were separated by gas chromatography, and the acetylcholine and choline concentrations in the tissue evaluated using selected ion monitoring of m/z 58, 60 and 64. The observed decrease in the acetylcholine concentration in the dorsal portion could be due either to the interruption of the cholinergic thalamic input or to the more severe ischemia in this portion. The increase in the acetylcholine concentration in the ventral portion, where blood circulation is normal, may reflect accumulation of acetylcholine in inactive intrinsic neurons after severance of their input, or indicate sprouting of intrinsic acetylcholine neurons.

Candidate mechanisms for inhibition of neurotransmitter release by narcotic analgesics and endorphins

Some neurones are endowed with receptors for endorphins and narcotic analgesics. Activation of these receptors results in a depression of the release of transmitter per impulse. It is currently believed that narcotic analgesics and endorphins depress the stimulus-induced influx of calcium (Ca2+) into the terminal and thereby modify the amount of the ion which triggers the release of the transmitter from intracellular stores. The influx of Ca2+ is largely governed by the Ca2+ "channel", which opens during depolarization of the neuronal membrane either after an action potential (electrical stimulus) or in the presence of high extracellular potassium (K+) or nicotinic stimulants (chemical stimulus). The evoked influx of Ca2+ can be affected by a direct action on the Ca2+ "channel" or by primary actions on other membrane properties that subsequently regulate the Ca2+ "channel". In many tissues narcotic analgesics and endorphins fail to inhibit transmitter release. This may be accounted for by the possibility that either such neurones lack presynaptic opiate receptors or that the function of existing receptors remains latent under the experimental conditions employed. Currently, there is insufficient evidence for endorphins physiologically modulating transmitter release.

Effect of beta-endorphin on calcium uptake in the brain

The uptake of 45Ca2+ by nerve-ending fractions from brains of mice was inhibited in vitro by 10(-9)M concentrations of beta-endorphin and in mice injected intraventricularly with 7 picomoles of beta-endorphin. That the effect was a specific opiate agonist response of beta-endorphin was demonstrated by use of the opiate antagonist, naloxone, which reversed the action. A role for beta-endorphin in the regulation of calcium flux and neurotransmitter release should be considered.

Effect of muscarinic cholinergic drugs on morphine-induced catalepsy, antinociception and changes in brain dopamine metabolism

The effects of drugs acting on muscarinic cholinergic receptors on the catalepsy, antinociception and changes in rectal temperature and in brain dopamine metabolism induced by morphine were studied in Wistar rats. Scopolamine (0.3 - 30 mg/kg) was about three times as potent as atropine (1 - 30 mg/kg) in potentiating the cataleptic effect of morphine. Methylscopolamine and methylatropine did not alter the cataleptic effect of morphine. Pilocarpine (100 mg/kg) and arecoline (10 mg/kg) slightly but significantly and RS86 (20 - 40 mg/kg) clearly antagonized the morphine-catalepsy. RS86 antagonized the atropine-induced potentiation of morphine catalepsy. The antinociceptive effect of pilocarpine was additive and that of RS86 less than additive with morphine. The antimuscarinic compounds did not alter the antinociceptive effect of morphine. Antimuscarinic compounds enhanced the hypothermic effect of morphine, but none of the compounds studied altered the hyperthermic effect of morphine. The antimuscarinic drugs reduced the concentration of striatal homovanillic acid (HVA) in about same proportion in control and morphine-treated rats. Both the muscarinic compounds and morphine increased the concentration of striatal HVA, but when combined their effects were not significantly different from those of morphine alone. Scopolamine antagonized and pilocarpine accelerated the morphine-induced increase in the rate of depletion of cerebral dopamine content. The present results show that the effects of muscarinic aand antimuscarinic cholinergic drugs on the cataleptic effect of morphine were opposite to their effects on the catalepsy induced by neuroleptic compounds.

Effect of cholingeric drugs on methadone-induced catalepsy and stereotypies in rats treated chronically with methadone

The effects of antimuscarinic (atropine, scopolamine, methylscopolamine), muscarinic (RS86, pilocarpine), antinicotinic (mecamylamine, hexamethonium) and nicotinic (nicotine) cholinergic drugs on the catalepsy and stereotypies induced by acute methadone in rats treated chronically with methadone were studied. The antimuscarinic drugs potentiated and the muscarinic drugs antagonized the cataleptic effect of methadone, whereas the antimuscarinic drugs tended to antagonize and the muscarinic drugs potentiated the methadone-induced stereotypies. Nicotine initially slightly potentiated, and mecamylamine antagonized the cataleptic effect of methadone. The actions of the cholinergic drugs on the extrapyramidal motor effects of methadone were most probably central, because methylscopolamine and hexamethonium had only very weak actions. These results show that the effects of antimuscarinic and muscarinic drugs on the catalepsy and stereotypies induced by methadone are opposite to their effects on the catalepsy and stereotypies produced by drugs which are thought to act on the postsynaptic dopaminergic receptors.

Morphine-naloxone interaction in the central cholinergic system: the influence of subcortical lesioning and electrical stimulation

1 The opiate antagonist naloxone, injected or topically applied to the cerebral cortex, had no significant effect on the spontaneous output of cortical acetylcholine (ACh) in rats. 2 Morphine (2.5 mg/kg) administered intravenously inhibited the release of cortical ACh. A subsequent injection of naloxone rapidly reversed morphine-induced inhibition, and produced a sustained increase in the release of ACh. Topical application of naloxone solutions, after morphine, produced a slow and weak reversal of its inhibitory action. 3 Destruction of the medial thalamus abolished both the inhibitory effects of morphine on the cortical ACh release, and its antagonism by naloxone administered after the agonist. 4 Injection of naloxone in a low dose (0.1 mg/kg) increased the release of cortical ACh provoked by electrical stimulation of either the medial thalamus or the reticular formation in normal rats. In the morphine-dependent rat, naloxone also facilitated the evoked release and its action was greater than in control animals. The facilitatory effect of naloxone on the cortical release evoked by stimulation of the medial thalamus was greater than its effect on the release evoked by stimulation of the reticular formation in both normal and morphine-dependent rats. 5 Naltrexone, a narcotic antagonist, also facilitated the electrically stimulated release of cortical ACh. 6 It is suggested that (a) morphine and naloxone act at a subcortical site, probably the medial thalamus, to modify the cortical ACh release and that (b) naloxone may facilitate the electrically-induced release of ACh in the CNS by antagonizing the effect of the endogenous morphine-like factor, enkephalin.