Effect of beta-endorphin on calcium uptake in the brain

Analytic Reviews : Potential New Therapies for Acute Ischemic Stroke

Evidence from animal models of acute stroke suggests ischemia may be reversible if blood flow is restored in the first few hours. Studies of human stroke using posi tron emission tomography demonstrate areas with re duced blood flow and relatively preserved metabolism, indicating potentially reversible ischemic brain. Resto ration of blood flow during this reversible phase should improve outcome after stroke.

Many therapeutic strategies for treatment of acute ischemic stroke have been proposed, including increas ing collateral flow, removing vascular obstructions, and interfering with the intracellular cascade of events that lead to neuronal cell death. Hypervolemic hemodilution reduces viscosity and increases cerebral blood flow, and this may hopefully raise blood flow above the critical threshold of irreversible ischemia. Naloxone, calcium channel blockers, and glutamate antagonists alter blood flow and influence intracellular events during and after acute ischemia. Thrombolytic therapy restores blood flow by lysis of obstructing clot. These therapies show promise in preliminary studies, but additional ran domized controlled studies are needed.

Immunoneutralization of agmatine sensitizes mice to micro-opioid receptor tolerance

Systemically or centrally administered agmatine (decarboxylated arginine) prevents, moderates, or reverses opioid-induced tolerance and self-administration, inflammatory and neuropathic pain, and sequelae associated with ischemia and spinal cord injury in rodents. These behavioral models invoke the N-methyl-D-aspartate (NMDA) receptor/nitric-oxide synthase cascade. Agmatine (AG) antagonizes the NMDA receptor and inhibits nitric-oxide synthase in vitro and in vivo, which may explain its effect in models of neural plasticity. Agmatine has been detected biochemically and immunohistochemically in the central nervous system. Consequently, it is conceivable that agmatine operates in an anti-glutamatergic manner in vivo; the role of endogenous agmatine in the central nervous system remains minimally defined. The current study used an immunoneutralization strategy to evaluate the effect of sequestration of endogenous agmatine in acute opioid analgesic tolerance in mice. First, intrathecal pretreatment with an anti-AG IgG (but not normal IgG) reversed an established pharmacological effect of intrathecal agmatine: antagonism of NMDA-evoked behavior. This result justified the use of anti-AG IgG to sequester endogenous agmatine in vivo. Second, intrathecal pretreatment with the anti-AG IgG sensitized mice to induction of acute spinal tolerance of two micro-opioid receptor-selective agonists, [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin and endomorphin-2. A lower dose of either agonist that, under normal conditions, produces moderate or no tolerance was tolerance-inducing after intrathecal pretreatment of anti-AG IgG (but not normal IgG). The effect of the anti-AG IgG lasted for at least 24 h in both NMDA-evoked behavior and the acute opioid tolerance. These results suggest that endogenous spinal agmatine may moderate glutamate-dependent neuroplasticity.

Naloxone lowers cerebrospinal fluid levels of excitatory amino acids after thoracoabdominal aortic surgery

OBJECTIVE

Although naloxone has been used to prevent ischemic spinal cord injury (SCI), its effect on excitatory amino acids (EAAs) has not been understood. We investigated the clinical significance of naloxone by measuring EAAs in the cerebrospinal fluid (CSF) in patients undergoing thoracoabdominal aortic surgery.

METHODS AND SUBJECTS

Twenty-seven patients (15 men and 12 women; mean age, 66 +/- 12 years) undergoing prosthetic replacement of the thoracoabdominal aorta (n = 19) or the descending thoracic aorta (n = 8) from April 1997 to June 2003 under distal perfusion and mild hypothermia were enrolled in this cohort study with historical controls. Their etiology was 7 dissections and 20 nondissections. In 16 patients (naloxone group), intravenous infusion of naloxone (1 microg/kg/h) was continued until the patients became alert. In the remaining 11 patients (control group) naloxone was not given. CSF drainage was used in all patients. CSF levels of EAAs, glutamate, aspartate, and glycine were measured at 6 points in time until 72 hours postoperatively, using a high-performance liquid chromatography method.

RESULTS

In 5 patients with SCI (2 patients in control group, 3 in naloxone group), CSF levels of glutamate and glycine continued to increase even at 72 hours postoperatively, and were significantly more elevated than those in patients without SCI ( P < .0001, glutamate; P = .0006, glycine). Postoperative maximum levels of CSF glutamate and glycine were also significantly higher in patients with postoperative SCI than in patients without SCI (glutamate: 215.3% +/- 158.6% vs 32.9% +/- 37.3% increase from baseline, P < .0001; glycine: 309.1% +/- 218.2% vs 89.2% +/- 103.1% increase from baseline, P = .0036). CSF levels of glutamate and aspartate in naloxone group were significantly lower than those in control group ( P = .0161, glutamate; P < .0001, aspartate). Postoperative maximum level of CSF aspartate was also significantly lower in the naloxone group than in the control group (8.3% +/- 75.5% vs 119.7% +/- 120.6% increase from baseline, P = .0077). In multivariate logistic regression analysis, postoperative maximum CSF glutamate >100% from baseline ( P < .001) and postoperative maximum level of CSF glycine ( P = .005)were identified as the independent risk factors for SCI. Both SCI ( P < .001) and postoperative maximum level of CSF glycine ( P = .005) were the independent predictors for postoperative maximum level of CSF glutamate >100% from baseline.

CONCLUSIONS

CSF levels of EAAs are elevated in patients with SCI. CSF glutamate is the strongest independent predictor of SCI. Naloxone is effective in lowering CSF levels of EAAs.

The opioid/anti-opioid balance in shock: a new target for therapy in resuscitation

Teleologically, pain is of paramount importance for survival and induces the organism to cope in an active way with aggressions from a basically hostile environment. While the activation of endogenous analgesic (opioid) systems typically occurs in conditions of surrender (pre-terminal conditions, sustained tortures, etc.), the activation of endogenous anti-analgesic systems triggers mechanisms of active or passive defence (such as camouflage) aimed at survival. The distinctive features of the main anti-analgesic systems (melanocortinergic, cholecystokininergic, thyroliberinergic) and the dramatic results obtained in experimental pre-terminal conditions (hemorrhagic shock, prolonged respiratory arrest) with the administration of their neuropeptide transmitters (ACTH and several ACTH-fragments, including alpha-MSH, CCK peptides and thyrotropin-releasing hormone) are here reviewed. The study of the mechanisms underlying the resuscitating effects of these neuropeptides has led to the discovery of the (often extremely potent) resuscitating effect of other drugs (protoveratrines, nicotine, centrally-acting cholinergic agents, ganglion-stimulating drugs). It is particularly remarkable that in pre-terminal conditions these neuropeptides and drugs have highly impressive effects on cardiocirculatory parameters at doses that are almost or actually inactive under normal conditions, and that their resuscitating effect is obtained without the need for any other supportive treatment and at dose-levels well below toxic ranges. Finally, in hemorrhage-shocked animals, the treatment with anti-analgesic neuropeptides shortly after bleeding considerably extends the time-limit for an effective and definitively curing blood reinfusion. This would be of self-evident importance in clinical practice, because an extremely simple, non-toxic first-aid treatment in the field, shortly after a massive hemorrhage, could resuscitate the patient for a period sufficient to effectively set up the most appropriate in-hospital treatment.

Effects of naloxone on sodium- and potassium-activated and magnesium-dependent adenosine-5'-triphosphatase activity and lipid peroxidation and early ultrastructural findings after experimental spinal cord injury

Endorphins have been implicated in the pathophysiology of spinal cord injury. The effect of naloxone on the sodium- and potassium-activated and magnesium-dependent adenosine-5'-triphosphatase (Na(+)-K+/Mg+2 ATPase, EC.3.6.1.3.) activity, lipid peroxidation, and early ultrastructural findings were studied in rats at the early stage of spinal cord injury, produced with an aneurysm clip on the T2-T7 segments. The rats were divided into four groups. The 10 rats in Group I, which had no injury and received no medication, were used for determining Na(+)-K+/Mg+2 ATPase activity, the extent of lipid peroxidation (by measuring the level of thiobarbituric acid-reactive substances as malondialdehyde), and normal ultrastructural findings. On the 15 rats in Group II, without spinal cord injury, only laminectomy was performed to determine the effect of surgery on the biochemical indices and findings. In the 15 rats in Group III, physiological saline was administered intraperitoneally in an amount equivalent to that of the naloxone administered immediately after spinal cord injury. In the 15 rats in Group IV, 0.5 mg of naloxone was administered intraperitoneally as a single dose immediately after injury and again 60 minutes after injury. The Na(+)-K+/Mg+2 ATPase activity was promptly reduced after spinal cord injury and remained in a lower level than the levels of Groups I and II during 120 minutes after injury. Naloxone treatment, immediately after trauma, attenuated the inactivation of Na(+)-K+/Mg+2 ATPase. On the other hand, there was a significant difference in the malondialdehyde content between animals in Groups I and III. Naloxone treatment reduced the malondialdehyde content in Group IV.(ABSTRACT TRUNCATED AT 250 WORDS)

Opening of brain potassium-channels inhibits the ACTH-induced behavioral syndrome in the male rat

In adult male rats, the intracerebroventricular (i.c.v.) injection of pinacidil, a potassium channel opener, at the doses of 100, 200 or 300 micrograms/rat, dose-dependently reduced the display of the most typical behavioral symptoms (excessive grooming, stretching, yawning, penile erections) induced by the i.c.v. administration of ACTH-(1-24) (4 micrograms/rat). These data indicate that the complex mechanism of the melanocortin-induced behavioral syndrome involves closure of potassium channels in target neurons, and provide further experimental support to the idea that melanocortins are functional antagonists of opioids.

Antagonization of fentanyl-induced muscular rigidity by neurotensin at the locus coeruleus of the rat

We evaluated the interaction between neurotensin (NT) and mu-opioid receptors at the locus coeruleus (LC), using fentanyl-induced muscular rigidity as our experimental index. Adult, male Sprague-Dawley rats anesthetized with ketamine (120 mg/kg, i.p., with 24 mg/kg/h i.v. infusion supplements) were used. Intravenous injection of fentanyl (100 micrograms/kg) consistently promoted a significant increase in the electromyographic activity recorded from the sacrococcygeus dorsalis lateralis muscle. This implied muscular rigidity was appreciably and dose-dependently antagonized by prior intracerebroventricular (i.c.v.) application of NT (15, 30 or 60 nmol/5 microliter). Microinjection of the tridecapeptide (300 or 600 pmol/100 nl) into the bilateral LC produced similar results. This suppressive effect of NT on fentanyl-induced muscular rigidity was antagonized by simultaneously administered NT antiserum (1:80), or partially blocked by its antagonist, (D-Trp11)-NT (300 pmol), but not by normal rabbit serum (1:80). These results suggest that NT may interact with the mu-opioid receptors at the LC, resulting in the suppression of fentanyl-induced muscular rigidity in the rat.

Role of neuronal and vascular Ca(2+)-channels in the ACTH-induced reversal of haemorrhagic shock

1. In a rat model of volume-controlled haemorrhagic shock causing the death of all control (saline-treated) animals within 30 min, the intravenous (i.v.) bolus injection of ACTH-(1-24) at a dose of 160 micrograms kg-1 produced an impressive and sustained restoration of arterial pressure, pulse pressure and respiratory function, with 100% survival at the end of the observation period (2 h). 2. Both intracerebroventricular (i.c.v., 0.015-0.06 microgram kg-1) and i.v. (5 micrograms kg-1) pretreatment with the N-calcium channel blocker, omega-conotoxin GVIA, and i.v. (but not i.c.v.) pretreatment with the L-calcium channel blocker, nicardipine (125-500 micrograms kg-1) dose-dependently prevented the ACTH-induced shock reversal. 3. These results further indicate that the effect of ACTH in haemorrhagic shock may involve a neuronal link and the eventual restoration of vascular tone mediated by N- and L-type calcium channels, respectively.

A double blind randomized pilot trial of naloxone in the treatment of acute ischemic stroke

Attention has focused on naloxone, an opiate receptor antagonist, because of its potential benefit in reversing neurological damage after acute cerebral ischemia. To evaluate the safety and possible efficacy of high-dose naloxone in ischemic stroke patients we planned a double blind pilot study. Between January 1989 and May 1990 24 patients were randomly assigned to the naloxone or placebo group according to age and neurological deficit. Naloxone was given in a loading dose of 5 mg/kg over 10 minutes followed by a 24-hour infusion at the rate of 3.5 mg/kg/h. 10 patients experienced minor side effects but none of them had to discontinue the treatment. 9 patients improved: 6 in the naloxone group and 3 in the placebo group, but no significant difference was found using the non parametric Mann-Whitney test. Our study suggests that naloxone is safe at the dose used, but the results do not support the planning of similar trials on a larger scale.

Interactions between analgesics and calcium channel blockers

1. The findings, derived from different experimental models, examined in this review, provide evidence that the calcium channel blockers and related drugs possess analgesic effects. 2. The antinociceptive action that some analgesic drugs exhibit may be related to calcium channel blockade. 3. Evidence from a variety of biochemical and pharmacological experimental approaches, support the existence of an interelation between the calcium modulators and the opioid drugs. 4. This idea agrees with the novel neuropharmacological hypothesis that a common very high affinity binding site for multiple neurotransmitters could exist, as has been proposed by Pasternak and Wood (1986). 5. This hypothesis could be extended to the neuromodulators or other neuromediators.

The effects of naloxone on cerebral blood flow and cerebral function during relative cerebral ischemia

CBF and somatosensory evoked potentials (SEPs) were measured in a model of moderate cerebral ischemia in anesthetized spontaneously hypertensive rats. The rats were bled to reduce SEP amplitudes to about 50% of prebleeding control. The consequent blood pressure fall reduced CBF to 77% of control as measured by the laser-Doppler technique. Naloxone (5 mg kg-1 i.v. plus 25 mg kg-1 h-1 i.v. for 30 min) caused a significant increase in SEP amplitudes, while CBF did not change significantly. In addition, the latency of the first SEP component decreased toward prebleeding values. Heart rate (HR) decreased, but MABP was held constant by a pressure-regulating reservoir. In unbled rats, naloxone (5 mg kg-1 i.v.) caused a transient small increase in MABP and SEP amplitudes and decrease in HR. These results indicate that sensory input is regulated by opioid systems. Increased opioid activity may inhibit ascending sensory pathways during relative cerebral ischemia and thereby depress SEP responses. Thus, naloxone can release this inhibition and enhances SEP independently of CBF during relative cerebral ischemia. Similar mechanisms might explain the apparently beneficial effects of naloxone in some stroke models.

Hypotensive effect of taurine. Possible involvement of the sympathetic nervous system and endogenous opiates

We studied the role of diminished sympathetic nervous system (SNS) activity and endogenous opiate activation in the hypotensive action of taurine, a sulfur amino acid, in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Supplementation of taurine could prevent the development of DOCA-salt hypertension in rats, but failed to change blood pressure in vehicle-treated control rats. Cardiac NE turnover, which was determined from the rate of decline of tissue NE concentration after the administration of alpha-methyl-p-tyrosine, was markedly accelerated in DOCA-salt rats, but 1% taurine supplement restored it to normal. Moreover, naloxone (2 mg/kg), the specific opiate antagonist, increased blood pressure in taurine-treated DOCA-salt rats, restoring it to levels similar to those in the DOCA-salt rats. In contrast, taurine did not decrease cardiac NE turnover in the control rats, nor did naloxone increase blood pressure in the taurine-treated control rats. Moreover, supplementation of taurine increased both beta-endorphin-like immunoreactive material and taurine contents in the hypothalamus of DOCA-salt rats, whereas it did not increase beta-endorphin in that of control rats despite increased taurine contents. Thus, taurine not only normalized the increased cardiac SNS activity but also elicited an opiate-mediated vasodepressor response only in DOCA-salt rats. It is suggested, therefore, that endogenous opiate activation, which is intimately related to SNS suppression, may contribute to the antihypertensive effect of taurine in sodium chloride hypertension.

Effect of opiate antagonists on middle cerebral artery occlusion infarct in the rat

The authors examined the effect of the opiate antagonists naloxone and thyrotropin-releasing hormone (TRH) on neurological outcome and the size of areas of cerebral infarction in a rat model of focal cerebral ischemia. The middle cerebral artery (MCA) was permanently occluded in 66 adult Sprague-Dawley rats. The rats were randomly divided into three groups. In 20 Group I rats, TRH in normal saline was administered initially as a 2-mg/kg bolus followed by continuous infusion of 2 mg/kg/hr for 4 hours. In 20 Group II rats, naloxone in normal saline was administered initially as a 2-mg/kg bolus followed by continuous infusion of 2-mg/kg/hr for 4 hours. In 26 Group III rats, physiological saline was administered as an initial 0.5-cc bolus followed by continuous infusion of 0.5 cc/hr for 4 hours. All solutions were given in volumes of 0.5 cc for the bolus and 0.5 cc/hr for continuous infusion, and all infusions were begun within 10 minutes of MCA occlusion. Twenty-four hours after treatment, the rats underwent a careful neurological examination and were then sacrificed immediately. The size of areas of cerebral infarction was evaluated using 2,3,5-triphenyltetrazolium chloride staining techniques. The neurological grade of the rats correlated with the size of infarcted areas among all grades, irrespective of treatment (p less than 0.01). Neither naloxone nor TRH improved neurological function or reduced the size of infarction compared to saline-treated control rats. Treatment with TRH caused a significant increase in mean arterial blood pressure during infusion, but naloxone had no effect. These results suggest that neither TRH nor naloxone are effective in the treatment of acute focal cerebral ischemia.

Adrenocorticotropin reversal of experimental hemorrhagic shock is antagonized by morphine

ACTH-(1-24) dose-dependently improved cardiovascular function in rats and dogs subjected to experimental hemorrhagic shock, and intravenous dose of 160 and 100/microgram/kg, respectively, completely restoring arterial blood pressure and pulse amplitude. All saline-treated animals died within 30 min of bleeding, while all ACTH-treated animals were still alive at the end of the observation period (2 hr). The injection of ACTH-(1-24) also dramatically improved the respiratory function. Morphine, i.v. injected into rats at the dose of 2.5 mg/kg, antagonised the effect of ACTH-(1-24) to a greater or lesser degree, depending on the dose of peptide employed: at 160/microgram/kg, antagonism was complete, at 320/microgram/kg antagonism was only partial, while at 480/microgram/kg antagonism was almost completely overcome. These data further support the idea that melanocortins are physiological antagonists of opioids, and suggest that melanocortin peptides may prove to be rational and effective drugs in the treatment of hypovolemic shock.

ACTH-(1-24) and alpha-MSH antagonize feeding behavior stimulated by kappa opiate agonists

ACTH-(1-24) and alpha-MSH, intracerebroventricularly (ICV) injected at the doses of 4 and 10 micrograms/animal, respectively, markedly inhibited spontaneous feeding in adult Sprague-Dawley rats, the effect remaining significant for 6-9 hours. At these same doses, ACTH-(1-24) and alpha-MSH abolished the feeding-stimulatory effect of the kappa opiate receptor agonist pentazocine, intraperitoneally (IP) injected at the dose of 10 mg/kg. The same antagonism was obtained by ICV injection of ACTH-(1-24) into rats IP treated with other kappa opiate agonists, bremazocine and tifluadom, at the doses of 1 and 5 mg/kg, respectively. These data suggest that melanocortin peptides play an inhibitory role in the complex regulation of food intake, and further support and extend the hypothesis of a melanocortin-opioid homeostatic system, its two neuropeptide components usually having opposite, mutually-balancing effects.

Characterization of opioid receptors in the cat carotid body involved in chemosensory depression in vivo

The effects of selective opioid receptor agonists and antagonists on neural discharge recorded from carotid body arterial chemoreceptors in vivo were studied in anaesthetized cats. Mean ID50 values were determined for each agonist and used to assess chemodepressant potency on intracarotid (i.c.) injection in animals artificially ventilated with air. [Met]enkephalin, [Leu]enkephalin, [D-Ala2, D-Leu5]enkephalin and [D-Pen2, D-Pen5]enkephalin were more potent chemodepressants than [D-Ala2, Me-Phe4, Gly-ol5]enkephalin, dynorphin (1-8) or ethylketocyclazocine; morphiceptin (mu-agonist) was inactive. The rank order of potency was compatible with the involvement of delta-opioid receptors in opioid-induced depression of chemosensory discharge. ICI 154129, a delta-opioid receptor antagonist, was used in fairly high doses and caused reversible dose-related antagonism of chemodepression induced by [Met]enkephalin. It also antagonized depression caused by single doses of [Leu]enkephalin, [D-Ala2, D-Leu5]enkephalin, [D-Ala2, Me-Phe4, Gly-ol5]enkephalin or dynorphin (1-8). ICI 174864, a more potent and selective delta-opioid receptor antagonist, also antagonized chemodepression induced by [Met]enkephalin or by the selective delta-receptor agonist [D-Pen2, D-Pen5]enkephalin. Comparison of background or 'spontaneous' chemosensory discharge during the 30 min periods immediately before and after injecting ICI 174864 (0.1-0.2 mg kg-1 i.c.) showed a significant increase in discharge in one experiment, but in four others discharge was either unaffected or decreased after the antagonist, which argues against a toxic depression of chemosensors by endogenous opioids under resting conditions in our preparation. Sensitivity of the carotid chemoreceptors to hypoxia (ventilating with 10% O2) was increased significantly after ICI 174864, which could be taken as evidence that endogenous opioids depress chemosensitivity during hypoxia. In contrast, responsiveness to hypercapnia was reduced after the antagonist, implying that endogenous opioids may potentiate chemoreceptor sensitivity during hypercapnia. The results obtained using 'selective' agonists and antagonists provide evidence that depression of chemosensory discharge caused by injected opioids involves a delta type of opioid receptor within the cat carotid body. Endogenous opioids may modulate arterial chemoreceptor sensitivity to physiological stimuli such as hypoxia and hypercapnia.

Magnetic field inhibition of morphine-induced analgesia and behavioral activity in mice: evidence for involvement of calcium ions

An exposure for 60 min to a 0.5 Hz rotating magnetic field (1.5-90 G) significantly reduced the day-time analgesic (in CF-1 mice) and locomotory (in C-57BL mice) effects of morphine (10 mg/kg). Intracerebroventricular (i.c.v.) injections of a calcium chelator, EGTA, blocked these effects, while administration of the calcium ionophore, A23187, potentiated the inhibitory actions. In a parallel fashion, i.c.v. administration of Ca2+ reduced, in a dose-related manner, the analgesic and locomotory effects of morphine in control CF-1 and C57 mice. These latter inhibitory effects could also be blocked by EGTA and augmented by A23187, indicating that opiate effects on activity and nociception are both sensitive to antagonism by calcium. Taken together these results suggest that exposure to magnetic stimuli may alter morphine-induced responses in mice, in a manner compatible and consistent with effects on Ca2+ and possibly other divalent ions.

Morphine and beta-endorphin antagonize posture and locomotor disorders induced by the injection of ACTH 1-24 in the rat locus coeruleus

The unilateral microinjection of ACTH 1-24 (20 nmol) into the locus coeruleus (LC) produced a long lasting (2-3 hr) posture asymmetry and movement disorder in all rats tested. This response was readily suppressed by the subsequent local microinjection of an equimolar dose of beta-endorphin or morphine or by the intraperitoneal injection of morphine sulphate (50 mg/kg). Microinjection of naloxone (20 nmol) into the LC produced the above syndrome in a lower percentage of animals. The results support the hypothesis that ACTH peptides and opioids play opposite roles in the control of different brain functions.

Magnetic fields inhibit opioid-induced feeding in the slug, Limax maximus

Exposure to rotating and elevated magnetic fields significantly reduced over three hours the ingestive effects of the opiate agonist, morphine (10 mg/kg), in free-feeding slugs, Limax maximus. Magnetic field exposure also inhibited the opioid-mediated increased ingestive responses of slugs that had been food-deprived for 24 hr. These results suggest that magnetic stimuli inhibit opiate-mediated behavioral and physiological functions in invertebrates in a similar manner as observed in vertebrates.

Opioid peptides increase calcium uptake by synaptosomes from brain regions

The fast-phase calcium uptake by depolarized hippocampal synaptosomes was increased significantly following treatment with 10 nM or 10 microM D-Ala2,D-Leu5-enkephalin (DADLE). No significant changes of calcium uptake by depolarized cortical or striatal synaptosomes were observed for 10 nM or 10 microM treatment with this enkephalin analog. Dynorphin 1-17 analog at 10 nM produces a significant increase in calcium uptake by depolarized striatal synaptosomes. beta-Endorphin and the dynorphin 1-13 fragment analog (10 nM) caused no significant change in the calcium uptake by depolarized synaptosomes from any of the three brain regions. However, calcium uptake by non-depolarized striatal and cortical synaptosomes was increased significantly in the presence of 10 nM beta-endorphin and DADLE. Opioid peptide action on neural calcium uptake is complex and appears to vary somewhat from one brain region to another.

Effects of (D-Met2,Pro5)-enkephalinamide and naloxone on pial vessels in cats

To elucidate the fundamental actions of endogenous opioids and naloxone on the cerebral circulation, the effects of (D-Met2,Pro5)-enkephalinamide and naloxone on pial vessels were investigated in cats. Pial arteries (165.7 +/- 24.9 microns) were found to dilate after the intravenous administration of 1 mg/kg of (D-Met2,Pro5)-enkephalinamide, and a definite dilatation of 7.1-7.6% persisted for 15 min. Pial veins (100.6 +/- 20.2 microns) also dilated but to a lesser degree. The MABP (118.7 +/- 10.5 mm Hg) decreased by 20 mm Hg immediately after the injection, but gradually returned to the initial value 15 min later. The observed cerebral vasodilatation may be attributable to sympathetic inhibition mediated either by the presynaptic opiate receptors of the cerebral vessels or by the opiate receptors in the brainstem. After the intravenous administration of 1 mg/kg of naloxone, pial arteries (122.0 +/- 17.2 microns) showed a slight but significant dilatation of 2.3-5.3%. There were no significant changes in pial veins (87.0 +/- 12.4 microns). MABP (130.4 +/- 12.3 mm Hg) was slightly increased after the injection. Although the mechanism involved was unclear, the cerebral vasodilatation occurring after the administration of naloxone may contribute to its ameliorating effect on the neurological symptoms following cerebral ischemia.

Naloxone in cerebral ischemia: preliminary data

The effect of the opiate antagonist naloxone was evaluated in 11 unselected patients with cerebral ischemia. Naloxone reversed neurological deficits in the 4 patients with less evident signs of vascular damage.

Effects of opiates on synaptosomal calmodulin and calcium uptake

Acute opiate administration in vivo increases the level of cytoplasmic calmodulin in isolated rat brain synaptosomes. These synaptosomes do not, however, display decreased K+-stimulated 45Ca uptake in vitro. Opiates affect neither cytoplasmic calmodulin nor Ca uptake after incubation of synaptosomes with the drugs in vitro. In contrast to the interpretation of electrophysiological data, these results suggest that the observed inhibition by opiates of the release of several transmitters may not be mediated by presynaptic opiate receptors that inhibit Ca uptake.

Enkephalin inhibition of inhibitory input to CA1 and CA3 pyramidal neurons in the hippocampus

Enkephalin-induced excitation in the hippocampus has been attributed to the attenuation of inhibitory input as well as to augmentation of excitatory input to pyramidal neurons. We have further examined these possible mechanisms of enkephalin action, as well as the possibility that enkephalins may be affecting intrinsic membrane properties, by recording intracellularly from CA1 and CA3 pyramidal cells in the guinea pig hippocampal brain slice preparation. It was observed that the inhibitory synaptic potential was significantly decreased in the presence of leucine enkephalin and D-alanine, D-leucine-enkephalin (DADL), whereas the excitatory synaptic potential, revealed by block of the inhibitory postsynaptic potential (IPSP) by bicuculline, was unaltered. In addition, the response of pyramidal cells to pressure-applied GABA was unaffected by enkephalin, as were the voltage-dependent membrane conductances. The increase in excitability which was observed in both field potential and intracellular recordings to drop application of DADL must, then, be due to a purely presynaptic block of inhibitory interneurons in both the CA1 and CA3 areas of the hippocampus.

Histamine release from isolated rat mast cells induced by opiates: effect of sterical configuration and calcium

The stereospecificity of the action of opiates on rat mast cells was investigated by means of the l- and d-isomers levorphanol and dextrorphan. The dose-response curves for histamine release induced by the 2 drugs were of a similar shape with a maximum at 2 X 10(-3) M and a pronounced minimum at 5 X 10(-3) M. At concentrations below 5 X 10(-3) M the effect of both drugs resembled that of morphine, i.e. the release occurred rapidly and inhibition was observed with naloxone, codeine, and antimycin A. Levorphanol, dextrorphan, and the antagonist levallorphan at concentrations above 5 X 10(-3) M seemed to be toxic to mast cells. The uptake of 45Ca in connection with histamine release induced by pethidine, levorphanol, and dextrorphan was lower than that of control cells, whereas the uptake induced by morphine did not differ from that of controls. The inhibition of compound 48/80-induced histamine release by morphine was paralleled by a reduced 45Ca uptake. The time course for the inhibitory effect of preincubation with morphine was similar for the histamine released induced by morphine and by compound 48/80. Washing of the cells after preincubation with morphine was without effect on the inhibition of morphine-induced histamine release, whereas the inhibition of compound 48/80 was reduced. The present observation with morphine and compound 48/80 support our previous impression of similarities in their mode of action, but a mechanism implying an interference by morphine with the disposition of calcium could also account for the findings. The observed antagonism between morphine and calcium resembles that of opiate receptors, but histamine release induced by opiates does not involve stereospecific opiate receptors.

Effects of taurine on tolerance to [D-Ala2, Met5]enkephalinamide in rats

Effects of taurine on tolerance to [D-Ala2, Met5]enkephalinamide (DAME) were investigated in rats. Tolerance was produced by five intraventricular administrations of DAME (50 microgram) during 3 consecutive days. The magnitude of developed tolerance to DAME was not uniform for each behavioral parameter; tolerance to analgesia effects developed more intensively and rapidly from the repeated injections of the peptide than that to akinesia effects. Pretreatment with taurine (9.5 X 10(-2) M) which was injected in a volume of 10 microliter intraventricularly 10 min prior to every administration of DAME suppressed the development of tolerance to both analgesia and akinesia effects of this peptide, whereas pretreatment with L-leucine at the same concentration did not. Spontaneous locomotor activity was measured for 1 h after the 90-min behavioral observation period was completed. That activity increased with the number of the peptide injections. Taurine pretreatment inhibited the induction of 'hyper'-locomotor activity. These results support the view that taurine may possess an ability to inhibit development of tolerance to morphine-like peptides in rats.

Opioid peptides decrease calcium-dependent action potential duration of mouse dorsal root ganglion neurons in cell culture

We investigated opioid peptide actions on somatic calcium-dependent action potentials of dorsal root ganglion (DRG) neurons grown in primary dissociated cell culture. We report that leucine-enkephalin decreased the duration and amplitude of DRG somatic calcium-dependent action potentials. The opioid peptide action was dose-dependent over 20 nM to 5 microM and was antagonized by naloxone, consistent with mediation by opiate receptors. Thus, DRG neuron membranes have opiate receptors which act to decrease calcium influx. It is more likely, therefore, that opiate receptors on the somata of DRG neurons in culture are functionally similar to opiate receptors on primary afferent terminals.

Enhancement of delta- but not mu-opiate agonist binding by calcium

Present evidence for distinction of 2 types of opiate receptor sites in rat brain homogenates originates from different relative affinities of morphine-like alkaloids and enkephalins to delta- or enkephalin and mu- or morphine-receptor sites. We now report that Ca2+ in a physiological dose range (0.5-3 mM) enhances the binding of 3H-enkephalin in hypotonically treated rat brain membranes, whereas specific binding of 3H-morphine-like alkaloids is not affected. Furthermore, the potency of [D-Ala2,D-Leu5]-enkephalin to inhibit [3H]-diprenorphine and [3H]-ethylketazocine binding increased in the presence of Ca2+, whereas an increase in potency of [D-Ala2,D-Leu5]-enkephalin to inhibit binding of mu-receptor ligands was not observed. Kinetic analysis revealed that Ca2+ decreased the rate of dissociation of [D-Ala2,D-Leu5]-enkephalin without affecting the rate of association, thereby increasing the affinity. However, in saturation binding studies, performed in diencephalic membranes, in which [D-Ala2,D-Leu5]-enkephalin binds predominantly to mu-receptors, Ca2+ also increased the binding affinity of [3H]- [D-Ala2,D-Leu5]-enkephalin. Double reciprocal analysis suggested a mixed competitive-noncompetitive type of inhibition of [D-Ala2,D-Leu5]-enkephalin binding by dihydromorphine. Thus, the interaction of delta- and mu-opiate ligands with mu-receptors may involve topographically different, but closely related binding sites, located on a single receptor molecule.

Modulation of brain polyphosphoinositide metabolism by ACTH and beta-endorphin: structure-activity studies

This study describes effects of ACTH1-24 and beta-endorphin on brain polyphosphoinositide metabolism in vitro. The interconversion of these polyanionic phospholipids was studied by incubation of a lysed synaptosomal fraction with [gamma-32P]ATP. Of the membrane phospholipids only PA, DPI and TPI became labeled. The reference peptide ACTH1-24 stimulated the formation of TPI and inhibited the production of PA. For effects on TPI formation both the sequences ACTH5-7 and ACTH10-16 were needed. Effects on PA formation required the sequences ACTH7-10 and ACTH10-16. The basic amino acids in ACTH10-16 seemed to be of crucial importance for the peptide effects. A stimulatory effect on DPI was visible when ACTH was shortened from the N-terminus, and the essential information was in ACTH7-10. beta-endorphin inhibited PA formation and this effect was abolished by C-terminal shortening to gamma-endorphin. Other fragments of the C-terminus of beta-LPH, including the enkephalins, were ineffective. It is concluded that the structure-activity relationship on TPI/PA formation correlates with a similar relationship obtained on excessive grooming behavior in vivo. A possible correlation between the effects on polyPI metabolism and opiate-like effects, and effects on extinction of active avoidance behavior in vivo is discussed.

Morphine-theophylline interaction: antagonism or facilitation?

1 Morphine-theophylline interactions were investigated in both acute and narcotic-dependent preparations, in vitro and in vivo, using four different experimental models: LD50 doses of morphine and naloxone in the mouse; naloxone-induced contractions in the electrically-stimulated and opiate-dependent isolated ileum of the guinea-pig; naloxone-induced jumps in the mouse; an calcium uptake in synaptosomal preparations. 2 The LD50 of morphine was significantly increased by theophylline. 3 The lethal effect of theophylline was potentiated by pretreatment of the animals with naloxone. 4 Theophylline displayed protective effects in the inhibitory response to morphine and antagonism to the withdrawal response induced by naloxone in the electrically-stimulated isolated ileum of the guinea-pig. 5 The number of jumps induced by naloxone in morphine-dependent mice was significantly diminished by theophylline. 6 The inhibitory effect of morphine on the synaptosomal uptake of calcium was decreased by theophylline. 7 The effects of both morphine and theophylline on the cyclic nucleotides and the possible role of calcium in these actions are discussed.

Pain and tryptophan

Tryptophan, the precursor for serotonin, was given to five patients with recurrent pain and diminished sensory deficits following rhizotomy and cordotomy. Their sensory deficits for both touch and pinprick reexpanded to the maximum extent initially recorded after their operative procedures. Their pain was again relieved. No clear understanding of the mechanisms responsible for these changes can be proposed at this time. It is speculated that a metabolic modulation of neurotransmitter functions is responsible for the clinical observations.

Inhibitory actions of methionine-enkephalin and morphine on the cat carotid chemoreceptors

1 The effects of intracarotid injections of methionine-enkephalin (Met-enkephalin) and morphine on chemoreceptor activity recorded from the peripheral end of a sectioned carotid sinus nerve have been studied in cats anaesthetized with pentobarbitone. 2 Met-enkephalin caused a rapid, powerful, inhibition of spontaneous chemoreceptor discharge, the intensity and duration of which was dose-dependent. 3 Morphine was a less potent inhibitor of spontaneous chemoreceptor discharge, and the inhibition it evoked was rather variable and tended to be biphasic. Low doses of morphine caused a slight increase in discharge. 4 Naloxone (0.2 mg i.c.) slightly increased spontaneous discharge, greatly reduced the chemo-inhibition caused by morphine, and reduced the inhibitory effect of Met-enkephalin. A higher dose of naloxone (0.8 mg) caused a substantial reduction of the Met-enkephalin effect. 5 Chemo-excitation evoked by intracarotid injections of acetylcholine, CO2-saturated Locke solution, and sodium cyanide were only slightly and somewhat variably reduced following injections of Met-enkephalin, whereas the inhibitory effect of dopamine was potentiated. Following morphine administration, response to acetylcholine and sodium cyanide were reduced slightly, whereas those to CO2 and dopamine were potentiated. 6 Responses to acetylcholine and CO2 were slightly potentiated during infusion of Met-enkephalin (50 micrograms/min, i.c.) and the response to sodium cyanide was slightly reduced. 7 It is concluded that naloxone-sensitive opiate receptors are present in the cat carotid body; when activated they cause inhibition of spontaneous chemoreceptor discharge. The physiological role of these receptors and the identity of any endogenous ligand remains to be established.