Effects of morphine, nalorphine and naloxone on neocortical release of acetylcholine in the rat

Physiologic and neuroendocrine responses to intravenous naloxone in subjects with Alzheimer's disease and age-matched controls

BACKGROUND

Prior work showed that administration of naloxone HCl had different behavioral effects in patients with Alzheimer's disease (AD) than controls. The aim of the present study was to contrast the physiologic and neuroendocrine responses to administration of a wide range of doses of intravenous naloxone of patients with probable Alzheimer's disease to aged-matched controls.

METHODS

This was a double-blind, placebo-controlled, study of 12 patients with probable Alzheimer's disease and 8 age-matched normal controls who each received intravenous infusions of naloxone HCl on 3 different days in doses of 0.1 mg/kg and 2.0 mg/kg preceded by test doses of 0.5 mcg/kg. Order of treatment condition was randomized. Vital signs and plasma cortisol and prolactin were obtained at regular intervals.

RESULTS

Both groups showed increased cortisol after naloxone 0.1 mg/kg and 2.0 mg/kg (p < .0001), but the increase was significantly greater and longer lived in controls than in patients. Patients, but not controls, also experienced a significant hypothermic response after naloxone 2.0 mg/kg (p < .05). Prolactin, heart rate, and blood pressure did not change following naloxone and did not differ between groups.

CONCLUSIONS

These findings support a growing body evidence that HPA axis activity is increased in AD, and further suggest that at least part of this may be due to decreased opiatergic tonic inhibition.

Lesioning of the nucleus basalis of Meynert has differential effects on mu, delta and kappa opioid receptor binding in rat brain: a quantitative autoradiographic study

Opioid receptor binding was investigated in rat brain following lesioning of the nucleus basalis of Meynert (nbM). The nbM, which provides cholinergic input to the cortex, was lesioned unilaterally using ibotenic acid. The efficacy of lesioning was confirmed by the observation of a significant decrease in choline acetyltransferase (ChAT) activity in the ipsilateral prefrontal cortex. The specific laminar and regional distribution of mu, delta and kappa opioid receptor binding was quantitated in various cortical and limbic structures in the rat using autoradiography. Distinct medial to lateral gradients of mu and kappa opioid binding were observed in regions of the cerebral cortex. In the lesioned hemisphere the levels of mu, delta and kappa opioid binding were altered in localized areas of the cerebral cortex and the hippocampus. The direction of these binding changes varied with the opioid receptor type being assessed. Delta opioid binding was increased in the lateral portions of the frontal, occipital, perirhinal and retrosplenial granular cortices. Kappa opioid binding was increased in the lateral portion of the occipital cortex and in the CA3 region of the hippocampus. In contrast, mu opioid binding was decreased in the lateral portions of the frontal, entorhinal and forelimb cortices. These opioid receptor changes are discussed with respect to the interactions between the cholinergic and opioid systems, and relevance of the nbM lesion model to Alzheimer's disease.

Effects of morphine on adrenaline responses of uteri from progesterone or estradiol treated mice

1. The effect of a chronic morphine treatment on the in vitro contractile responses of the mouse uterus to adrenaline was studied. 2. Chronic morphine treatment induced a supersensitivity state in the uteri from both progesterone and estradiol treated mice. 3. The acute administration of morphine to the uteri from morphine tolerant-dependent and progesterone treated mice induced a further increase of the contractile effect of adrenaline. 4. Reserpine administration did not further increase the supersensitivity of the mouse uterus to adrenaline induced by a chronic morphine treatment. 5. Reserpine suppressed the acute effects of morphine in the uteri from tolerant-dependent mice.

Morphine inhibition of calcium fluxes, neurotransmitter release and protein and lipid phosphorylation in brain slices and synaptosomes

Morphine (1-100 microM) was found to inhibit several concomitant events in brain slices and synaptosomes which are augmented by depolarizing agents. Thus, 45Ca2+ uptake, amino acid neurotransmitter release, increases in 3',5' cyclic AMP levels and 32Pi incorporation to proteins and lipids induced by veratrine (25 microM) and by potassium (56 mM), were each inhibited in a dose related manner. These inhibitory actions of morphine were all prevented by naloxone (1 microM). Evidence is presented that morphine binding to a receptor on the synaptic membrane affects intracellular mechanisms involved in neurotransmitter release possibly via a second messenger system. An enhancing action of GTP on the inhibitory influences of morphine suggests that its actions are mediated at least in part, via a coupling of the receptor to adenyl cyclase in the outer membrane. This is supported by its inhibitory action on the capacity of depolarizing agents to increase cyclic AMP levels.

Naltrexone and Alzheimer's disease

Naltrexone, an oral opiate antagonist, was administered to nine patients with a diagnosis of Alzheimer's-type dementia (ATD) in a two-phase design: an open dose-ranging phase and a double-blind placebo-controlled trial for patients who showed improvement during the open phase. After a three day placebo (baseline) period, patients received increasing doses of naltrexone over two weeks up to a maximum daily dose of 100 mg. Assessments were made at baseline and at daily dose of 5 mg, 50 mg and 100 mg. Testing was done 2 to 4 hours after medication was administered. Any patient who showed cognitive/behavioral improvement on a given dose of naltrexone was then treated with this dosage in a double-blind crossover comparison to placebo. Criteria for inclusion in the double-blind phase consisted of improvement on three behavioral scales and at least one cognitive test on a given dose of naltrexone. Each double-blind phase followed a one-week washout and was two weeks long. Two of the nine patients demonstrated apparent cognitive enhancement on 100 mg daily of naltrexone and were then tested in the double-blind crossover period. Only one of these patients improved during active naltrexone administration. We conclude that the opiate antagonist naltrexone in a dosage range of 5-100 mg daily is not efficacious in ATD.

Cholinergic mechanisms of analgesia produced by physostigmine, morphine and cold water swimming

This study concerns the cholinergic involvement in three experimental procedures which produce analgesia. Rats were given one of seven treatments: saline (1.0 ml/kg, i.p.); morphine sulfate (3.5, 6.0 or 9.0 mg/kg, i.p.); physostigmine salicylate (0.65 mg/kg, i.p.); warm water swim (3.5 min at 28 degrees C); and cold water swim (3.5 min at 2 degrees C). Each rat was tested on a hot plate (59.1 degrees C) once prior to and 30 min after treatment. Immediately after the last test the rats were killed with focussed microwave radiation. Levels of acetylcholine (ACh) and choline (Ch) in six brain areas (brain stem, cerebral cortex, hippocampus, midbrain, cerebellum and striatum) were analyzed by gas chromatograph-mass spectrometer. Morphine (9.0 mg/kg), physostigmine and cold water swimming caused significant analgesia. Morphine elevated the levels of ACh in the cerebellum and striatum, cold water swimming--in the cerebellum, striatum and cortex, and physostigmine--in the striatum and hippocampus. Levels of choline were elevated by morphine in the cerebellum, cortex and hippocampus, while cold water swimming elevated levels of choline in the cerebellum, cortex, striatum and hippocampus. Physostigmine did not change levels of choline in any of the brain areas studied. These data suggest that the analgetic effects of morphine or cold water swimming may be mediated by components of the cholinergic system that differ from those involved in the analgetic effects of physostigmine.

Morphine suppression of neurotransmitter release evoked by sensory stimulation in vivo

The effects of morphine and naloxone on the release of acetylcholine and amino acid neurotransmitters from sensorimotor cortex were studied employing an in vivo superfusion cannula. Morphine (20 mg/kg) reduced the spontaneous release of acetylcholine but had no detectable effect on the spontaneous release of amino acids. It also suppressed the release of acetylcholine and amino acid neurotransmitters evoked by sensory stimulation of the contralateral sensorimotor cortex via the brachial plexus. Naloxone (5 mg/kg) prevented all of these inhibitory actions of morphine. Naloxone also caused a significant increase in spontaneous acetylcholine release.

Opiate and muscarinic ligand binding in five limbic areas after bilateral olfactory bulbectomy

Bilateral olfactory bulbectomy (BBX) in mice leads to a variety of neutrochemical changes in 5 limbic areas associated with the bulbs. Within one week after BBX, opiate ligand binding declined by 73% in the amygdala, rose by 82% in the hypothalamus and then returned to sham levels by 4 weeks in both areas. Opiate binding also declined by 47% in the olfactory tubercle and to essentially zero in the piriform cortex and the olfactory peduncle after 16 weeks. Muscarinic cholinergic binding declined in the amygdala and hypothalamus at 16 weeks after BBX, but reductions in muscarinic binding were observed in the piriform cortex and the olfactory peduncle by two weeks postsurgery. Muscarinic binding in the olfactory tubercle was unaffected by BBX, as was binding of beta-adrenergic and benzodiazepine ligands in the limbic areas. Binding of [3H]spiroperidol rose 61% in the olfactory tubercle two weeks after surgery and then declined to normal levels. Choline acetyltransferase activity rose by 64% within one week after BBX in the piriform cortex and remained elevated throughout the study. Activity of this enzyme also rose in the olfactory peduncle and the olfactory tubercle after surgery. BBX had only moderate effects on glutamic acid decarboxylase in the limbic areas, and enzyme activity increased 25% in the olfactory tubercle and the piriform cortex 4 weeks after BBX. BBX also resulted in a moderate decrease (22%) in DOPA decarboxylase activity in the olfactory tubercle two weeks after BBX. The implications of these neurochemical changes are discussed in terms of what is known about bulb-limbic system connections.

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.

Sensitivity of mouse vas deferens to neurotransmitters: changes after morphine treatment

1. The pharmacological responses of the isolated vas deferens of the mouse were investigated after acute and chronic treatment with morphine. 2. The addition of morphine to the bath did not alter the responses of the vas deferens to exogenous noradrenaline, adrenaline or dopamine. 3. Low doses of morphine depressed the responses to acetylcholine. Very high concentrations of the opioid (8.5 x 10(-4) M) completely abolished, in about 50% of the preparations, the responses to exogenous acetylcholine, while in the other 50% a potentiation of the responses to low concentrations of acetylcholine was observed. 4. The vas deferens of mice chronically treated with morphine showed increased sensitivity to exogenous noradrenaline, but decreased sensitivity to acetylcholine. 5. A fresh amount of morphine added to the bath enhanced the responses of morphine-tolerant preparations to noradrenaline but not to dopamine or acetylcholine. The specificity of this phenomenon was demonstrated by the use of pentobarbitone instead of the opioid. 6. These results are in agreement with the theory that tolerance could result from a form of disuse supersensitivity.

Acetylcholine antirelease effect of morphine and its modification by calcium

The effects of morphine (10 mg/kg) and calcium (10 mg/kg) were studied on the neocortical release of acetylcholine (ACh) in ketamine-anesthetized rats. Morphine decreased ACh release as measured by the dorsal leech muscle as well as by an enzymatic assay. Calcium was ineffective alone but antagonized the action of morphine. This study supports the hypothesis that the ACh antirelease action of narcotics is mediated through an interaction with calcium.

Acute effects of morphine on regional brain levels of acetylcholine in mice and rats

Morphine increased levels of acetylcholine in mouse striatum in a dose-dependent manner, the increase occurring at the lowest dose previously found to produce analgesia and coinciding with the time of peak analgesic effect. Naloxone blocked this increase. After repeated injections of high doses of morphine, no effect was seen. The hippocampus was the only other brain region showing an effect, and this after a high dose. In the rat, morphine (30 and 90 mg/kg) increased striatal acetylcholine levels. At these and lower doses (2.5 mg/kg and 10 mg/kg) the ratios in the striatum of levels of acetylcholine to levels of dopamine were significantly increased. Only at the highest dose did morphine increase the levels of dopamine in the striatum and of acetylcholine in the hippocampus. Morphine did not change the levels of dopamine in the striatum and of acetylcholine in the hippocampus. Morphine did not change the levels of norepinephrine in either the hypothalamus or cortex of the rat.

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.

The role of supersensitivity to acetyl- choline in the production of tolerance to morphine in stimulated guinea-pig ileum

1 Morphine caused a dose-dependent reduction in both the height of contraction and acetylcholine release from coaxially stimulated strips of guinea-pig ileum.2 Exposure of the tissue to morphine for 90 min produced acute tolerance to the effect of subsequent doses of morphine on contraction height.3 There was no change in the ability of morphine to suppress acetylcholine release.4 The responses of morphine-tolerant ileum to exogenous acetylcholine were enhanced 3 to 10-fold.5 If the ileum did not show tolerance to morphine it did not become more sensitive to acetylcholine.6 The results presented suggest that tolerance to morphine could result from a form of disuse supersensitivity.