Analeptic and antianaleptic effects of naloxone and naltrexone in rabbits.

Naloxone (5 mg/kg), but not naltrexone, shortened the duration of anaesthesia in rabbits pretreated with pentobarbital. This analeptic effect was blocked by atropine, but not by methylatropine; it thus appears that a central cholinergic mechanism is involved. In contrast, smaller doses of both naloxone and naltrexone attenuated the arousal property of thyrotropin releasing hormone (TRH). Naloxone, but not naltrexone, also antagonized the analeptic property of d-amphetamine. In conscious animals naloxone potentiated, whereas naltrexone attenuated, the excitatory effects of TRH and d-amphetamine.     

In vivo and in vitro effect of naloxone on prolactin response to TRH in rat

In adult male Wistar rats submitted to a standardized noise stress, intravenous TRH induced a prolactin (PRL) secretory response. Prior IV naloxone administration not only lowered plasma PRL levels in those stressed rats but abolished also the stimulatory action of TRH. This effect was further studied by superfusion experiments on enriched PRL cell suspensions (70% lactotrophs) from female adult Wistar rats. Naloxone kept unaffected the basal PRL secretion but lowered significantly that induced by TRH. These experiments suggest a dual effect of naloxone on rat PRL secretion, one exerted on central opioid receptors lowering stress-related increased basal PRL levels, the other inhibiting the TRH-dependent PRL secretion exerted at the lactotroph level itself.     

Neuropeptides and thermoregulation: the interactions of bombesin, neurotensin, TRH, somatostatin, naloxone and prostaglandins

In this study we have examined the interactions of bombesin (1 microgram ICV), neurotensin (1 microgram ICV), TRH (10 micrograms ICV), somatostatin (10 micrograms ICV), PGE2 (10 micrograms ICV) and naloxone (10 mg/kg SC) on thermoregulation in the rat at room temperature (20 +/- 1 degree C). Given alone, bombesin, neurotensin, somatostatin and naloxone all produced hypothermia (bombesin greater than neurotensin greater than somatostatin congruent to naloxone). PGE2 was hyperthermic, and TRH had no effect. Bombesin and PGE2 neutralized one another's effects. Neurotensin had no effect on PGE2-induced hyperthermia. Naloxone enhanced the hypothermic effect of bombesin and somatostatin enhanced the rate of onset of hypothermia after bombesin. TRH had no effect on bombesin-induced hypothermia. TRH, somatostatin and naloxone had no effect on neurotensin-induced hypothermia. TRH antagonized the hypothermia due to naloxone and somatostatin.     

Blood pressure responses of conscious rats to intravenous administration of enkephalin derivatives (D-ala methionine and leucine enkephalinamide, and methionine and leucine enkephalinamide)

The present study was designed to determine the blood pressure (BP) responses of conscious rats given intravenous (IV) injections of enkephalin derivatives (D-ala²-methionine enkephalinamide, DAMEA; D-ala²-leucine enkephalinamide, DALEA; methionine enkephalinamide, MEA; leucine enkephalinamide, LEA) and the receptor mechanisms mediating the resultant change in BP. IV injection of 1.6–16.0 nmoles of DAMEA or DALEA caused a transient but potent decrease in mean arterial pressure (MAP) and mean heart rate (MHR). LEA and MEA (16.0 nmoles) given IV produced slight pressor responses, which were not associated with concomitant tachycardia whereas 48 nmoles of MEA elicited a hypotensive effect accompanied by a fall in MHR. Pretreatment studies whereby various receptor antagonists (naloxone, diprenorphine, phentolamine, D-L-propranolol or atropine) were given IV 5 min before subsequent IV administration of DAMEA, DALEA, MEA or LEA (16 nmoles) showed that naloxone, diprenorphine and atropine blocked the depressor and bradycardic effects of DALEA and DAMEA. Naloxone and phentolamine suppressed the pressor reponse of both MEA and LEA (16.0 nmoles) while diprenorphine blocked the rise in MAP to only MEA. The results show that DAMEA and DALEA mediate their depressor actions in conscious rats via a negative chronotropic effect through an interaction of muscarinic cholinergic receptors on the myocardium. It is suggested that the pressor response of MEA and LEA may be produced via an -receptor mediated effect on the peripheral vasculature to cause vasoconstriction.     

Failure of reversal of cardiovascular responses of clonidine by centrally administered naloxone in anaesthetised rats

Central effects of naloxone on the cardiovascular responses of centrally administered clonidine were studied in anaesthetised normotensive, renal DOCA-salt hypertensive and morphine dependent rats. Clonidine (5 micrograms/ICV) produced significant decrease in blood pressure and heart rate in all the groups of rats in a dose dependent manner. Naloxone (2 micrograms/ICV) failed to reverse the responses of clonidine in all the rat groups. In morphine dependent normotensive and morphine dependent renal DOCA-salt hypertensive rats, the responses of clonidine were further enhanced in the presence of naloxone. Our observations clearly indicate that clonidine does not influence endogenous opioid system for producing cardiovascular effects.     

Naloxone exerts a dose-dependent gastric cytoprotective effect

Previous work has shown that naloxone inhibits the ulcerogenic effects of indomethacin and stress, although the site, mechanism or dose are unknown. We investigated whether naloxone possessed gastric cytoprotective properties, generating a dose-response curve existed for both intragastric (IG) and intravenous (IV) administration. One hundred and two rats were subjected to a four hour period of restraint, with the last two hours at 4 degrees C. Naloxone was given hourly during restraint at doses of 0 (Control), 1, 5, 10, 20 mg/kg. After sacrifice, the residual gastric volume, and pH were measured and the number of mucosal lesions scored. The cytoprotection offered by naloxone was different from control (p = 0.0001), with the intravenous route having a greater effect (p = 0.038). While this protective effect did not correlate with changes in gastric acidity, it correlated with the dose of naloxone.     

The effect of opiate receptor ligands on emotional cardiovascular reactions in lower primates

Intravenous naloxone injection (0.1 mg/kg) facilitated blood pressure increase in response to conditioned sound stimulus followed by electrocutaneous shock in conscious chair-restrained baboons (Papio hamadryas). Naloxone at a dose of 1.0 mg/kg had an opposite effect and led to the decrease in blood pressure and heart rate in conditioned fear reflex. Naloxone microinjections (50 microM) into the periventricular hypothalamus led to a significant diminution of blood pressure and heart rate increment in response to electrocutaneous shock; naloxone microinjections into tractus solitarius nuclei suppressed blood pressure and heart rate reactions both to conditioned (sound) and unconditioned (electrocutaneous shock) stimuli. Microinjections of equimolar morphine quantities in these brain regions facilitated such reactions. It is concluded that endogenous opioid system participates in the formation of cardiovascular reactions to emotional stimuli in monkeys, with multiple opioid receptors of periventricular hypothalamus and tractus solitarius nuclei involved in the generation of such reactions.     

Antagonism by N-methyl levallorphan-methane sulphonate (SR 58002 C) of morphine-elicited acute and chronic central and peripheral effects

The peripheral activity of the quaternary narcotic antagonist N-methyl levallorphan-methane sulphonate (SR 58002 C) at opioid sites located in the periphery and in the central nervous system (CNS), was studied by different approaches in rats after subcutaneous injection (s.c.). Pretreatment with SR 58002 C 2,8 or 32 mg/kg s.c. 10, 50 or 110 min before buprenorphine consistently reduced buprenorphine in vivo binding only in the small intestinal longitudinal muscle with attached myenteric plexus (MP), whereas naloxone (1 mg/kg s.c.) 10 min before buprenorphine lowered buprenorphine binding in MP and brain (without cerebellum). Plasma levels were not altered by SR 58002 C or naloxone. The same doses of SR 58002 C injected 10, 50 or 110 min before morphine selectively antagonized the inhibition of transit of a charcoal meal along the small intestine (mainly a peripheral effect) induced by the agonist, but did not antagonize morphine-elicited analgesia in the hot-plate test (central effect). Naloxone (1 mg/kg s.c.) injected 10 min before morphine antagonized both agonist effects simultaneously. In morphine-dependent rats SR 58002 C (0.25, 1, 4 and 32 mg/kg s.c.) induced diarrhea, dose-dependently, in most animals within the first 30 min, while jumping, measured in the same rats, occurred in some animals, not dose-dependently, from 60 min on. Naloxone (1 mg/kg s.c.) induced both effects in most rats. These findings suggest that, although SR 58002 C probably penetrates the blood-brain barrier in some morphine-dependent rats, it discriminates peripheral and CNS opioid effects.     

Effects of naloxone on hemodynamics and sympathetic activity after exercise.

The effects of highdose naloxone (0.4 mg/kg iv) on hemodynamics and muscle sympathetic nerve activity (MSNA) after exercise were studied in nine normotensive young men randomly allocated the opioid antagonist or vehicle 30 min before treadmill exercise at 70% of resting heart rate reserve. Mean arterial pressure (MAP) was lower after exercise; cardiac output was increased. Mean values for MSNA and plasma norepinephrine were similar before and after exercise, but in individual subjects changes in resting MAP 60 min after exercise were inversely related to changes in sympathetic activity, suggesting that arterial baroreflex regulation of MSNA had been shifted to a lower set point. Naloxone did not prevent postexercise hypotension but transformed these inverse correlations into positive relationships. Naloxone attenuated both calf and systemic vasodilation without altering mean values for MSNA, indicating a peripheral effect of opioid antagonism. In normotensive subjects, naloxone alters the regulation of sympathetic outflow and vascular resistance during recovery from exercise but does not prevent the fall in MAP.     

Cerebral and peripheral blood flow effects of TRH in the rat--a role of vagal nerves

The cardiovascular effects of the IV infusion of TRH were studied in the rat. TRH tended to increase the MAP and markedly increased the CBF(tot) in the control group, in vagotomized animals and in methylatropine-pretreated rats. A marked vasodilation was noted in the pancreas, gastric mucosa, duodenum and cardiac muscle. This effect was turned to vasoconstriction, the heart excluded, in vagotomized animals. Muscarinic blockade attenuated the vasodilating effect of TRH in the duodenum and gastric mucosa. The results indicate that TRH elicits cerebral vasodilation and a partly nonmuscarinic parasympathetically mediated vasodilation in several gastrointestinal organs in parallel with a vasoconstriction which is unmasked by vagotomy.     

The effects of naloxone and picrotoxin on the sedative and anticonflict effects of benzodiazepines

Interactions between naloxone and the benzodiazepine, chlordiazepoxide (CDP), were investigated in rats in a Conditioned Suppression of Drinking Test (CSD), which is a model of experimental conflict behavior. Naloxone reversed the anticonflict activity of CDP in this test. Naloxone and picrotoxin were then tested against CDP in rats using the Geller Conflict Test, which is an operant model of conflict behavior. Both naloxone and picrotoxin antagonized the anticonflict effects of CDP. Naloxone and picrotoxin were also tested for their abilities to reverse CDP-induced loss of righting reflexes in mice. Both naloxone and picrotoxin antagonized the loss of righting reflexes induced by CDP. Naloxone had no effect on the loss of righting reflexes induced by barbiturates or meprobamate. These results suggest that naloxone may be useful in the management of benzodiazepine overdoses.     

Naloxone or TRH fails to improve neurologic deficits in gerbil models of “stroke”

The effects of naloxone or thyrotropin releasing hormone (TRH) upon neurologic outcome were evaluated in gerbil models of cerebral ischemia. Following temporary bilateral carotid occlusion, hypotension was transiently reversed by these endorphin antagonists. However, neither drug altered time to awaken, time to death, or the severity of neurologic signs (ptosis, movement, retracted paws, circling, righting reflexes, seizures, or opisthotonus) when evaluated by a blinded rater. Hot plate escape and roto-rod performance were also unaltered by naloxone or TRH; TRH, but not naloxone, increased respiratory rates. Thus, the transient improvement of cardiorespiratory function produced by these drugs is unrelated to the morbidity and mortality associated with temporary cerebral ischemia in the gerbil. Additional studies evaluating the effects of naloxone or TRH upon neurologic outcome following permanent unilateral carotid occlusion also failed to show any therapeutic effects of these drugs. Both morphine and TRH exacerbated the effects of ischemia. Of gerbils which developed neurologic impairment, the deficit was usually ipsilateral to the occluded carotid. Collectively, these results indicate that neither naloxone nor TRH prevents ischemic deficits in the gerbil. Further studies with different cerebral ischemia models in other species are required to clarify the possible therapeutic effects of these drugs in experimental stroke.     

Opioid modulation of ingestive behaviors in the spiny mouse (Acomys cahirinus)

Feeding and drinking behavior were studied in deprived or sated spiny mice (Acomys cahirinus) at various time intervals following peripheral administration of naloxone hydrochloride and butorphanol tartrate. Naloxone attenuated both food and water intake, but not latency to respond, indicating existence of functional opioid-sensitive feeding and drinking systems in this species. Butorphanol tartrate, a mixed opioid agonist/antagonist produced a dose-related enhancement or suppression of feeding, the former naloxone reversible, but had no measureable effect on drinking.     

Systemic methionine-enkephalin evokes cardiostimulatory responses in the human

The cardiovascular effects of bolus doses of methionine-enkephalin (Met5-ENK) (1 to 100 micrograms/kg) were studied in 9 subjects in whom, at cardiac catheterization for evaluation of chest pain, patent coronary arteries were found. Met5-ENK produced a simultaneous increase in blood pressure and heart rate beginning within 20 sec, reaching maximal values between 30 and 40 sec, and then terminating by 60 sec. Heart rate, systolic, diastolic, and mean arterial blood pressures increased significantly (p less than 0.0005); pulse pressure remained unchanged. Positive dose-effect relationships were observed for heart rate (p less than 0.002), systolic, diastolic, and mean arterial blood pressures (p less than 0.05). Naloxone (0.5 mg/kg), given to 4 subjects, prevented the heart rate and blood pressure changes associated with Met5-ENK administration, demonstrating that the cardiovascular changes were mediated by opiate receptors. Subjects also described cutaneous paresthesias which were not prevented by naloxone pretreatment. These data suggest a role for peripheral enkephalins in cardiovascular regulation.     

Opioid modulation of thyrotropin releasing hormone induced prolactin secretion

It is known that opioids stimulate prolactin (PRL) secretion by an action on hypothalamic neurons, but in vitro studies have suggested a direct action on the lactotrophs. The present study was performed on male rats known to have little or no PRL response to TRH. A beta-endorphin (beta EP) injection in the third ventricle stimulated PRL secretion and induced furthermore a PRL secretory reaction to TRH injected intravenously 20 min later. Pretreatment with naloxone 10 min before beta EP injection abolished not only the PRL response to beta EP but also the conjugated effect of beta EP and TRH. Pretreatment with naloxone methyl bromide (Br-naloxone), a quaternary naloxone derivative, which does not cross the blood-brain barrier, had no effect on the PRL response to beta EP but prevented the conjugated effect of beta EP and TRH on PRL secretion. Pretreatment of the animals with -methyl-parathyrosine resulting in a dopamine depletion or with haloperidol, a dopamine antagonist, could not induce lactotroph responsiveness to TRH. These results suggest that beta EP in male rat sensitizes the PRL cell to TRH by a direct effect and not through an inhibition of the dopaminergic tone.     

Neuropeptides in spinal cord injury: Comparative experimental models

The possible role of endogenous opioids in the pathophysiology of spinal cord injury was evaluated utilizing a variety of experimental models and species. In the cat, we have shown that β-endorphin-like immunoreactivity was increased in plasma following traumatic spinal injury; such injury was associated with a decrease in spinal cord blood flow (SCBF) which was reversed by the opiate receptor antagonist naloxone. Naloxone treatment also significantly improved functional neurological recovery after severe injury. Thyrotropin-releasing hormone (TRH), possibly through its “anti-endorphin” actions, was even more effective than naloxone in improving functional recovery in the cat. In a rat model, utilizing a similar trauma method, TRH proved superior to naloxone in improving SCBF after injury. In addition, naloxone at high doses attenuated the hindlimb paralysis produced by temporary aortic occlusion in the rabbit. The high doses of naloxone required to improve neurological function after spinal injury suggest that naloxone's actions, if opiate receptor mediated, may be mediated by non-μ receptors. Dynorphin, an endogenous opioid with a high affinity for the κ receptor, produced hindlimb paralysis following intrathecal administration in rats. Taken together, these findings suggest that endogenous opioids, possibly acting at κ receptors in the spinal cord, may serve as pathophysiological factors in spinal cord injury.     

Cold acclimation increases physiological responsiveness to intraventricular injection of beta-endorphin in pentobarbital anaesthetized rats. 1. Cardiovascular functions

Intraventricular injection of beta-endorphin (3, 7, 10 and 30 nmol/kg) into the third ventricular of pentobarbital-anaesthetized male Sprague-Dawley rats resulted in a dose-dependent increase in mean arterial pressure (MAP) while injection of the same volume of 0.9% NaCl solution did not cause significant changes in MAP. Naloxone, which did not produce any significant change in MAP, antagonized the vasopressor effect of beta-endorphin, indicating that the response is mediated via the naloxone sensitive opiate receptors. Rats acclimated to cold (5 degrees) for 3 weeks showed a potentiated and prolonged increase in MAP following beta-endorphin injection, indicating an increased responsiveness to the peptide. This increased responsiveness in the cardiovascular system is probably of adaptive value in cold acclimation. Naloxone itself did not alter MAP either, but abolished the cardiovascular response to beta-endorphin completely in cold acclimated rats, indicating an increased effectiveness in its antagonistic effect following cold acclimation as well.     

The effect of vagotomy on the satiety effects of neuropeptides and naloxone

As abdominal vagotomy blocks the satiety effect of cholecystokinin-octapeptide, we felt it would be worthwhile to examine whether the satiety effect of any of the other putative satiety neuropeptides was mediated through the vagus. We confirmed that the satiety effect of peripherally administered cholecystokinin (10 micrograms/kg) was mediated through the vagus. In addition, the satiety effect of peripherally administered TRH (8 mg/kg) also was not present in vagotomized animals. Vagotomy had no effect on the satiety effects of peripherally administered bombesin, calcitonin and naloxone. Nor did vagotomy alter the satiety effect produced by central administration of bombesin, TRH, calcitonin nor naloxone.     

The cardiovascular effects of a chimeric opioid peptide based on morphiceptin and PFRTic-NH2

MCRT (YPFPFRTic-NH₂) is a chimeric opioid peptide based on morphiceptin and PFRTic-NH₂. In order to assess the cardiovascular effect of MCRT, it was administered by intravenous (i.v.) injection targeting at the peripheral nervous system and by intracerebroventricular (i.c.v.) injection targeting at the central nervous system. Naloxone and l-NAME were injected before MCRT to investigate possible interactions with MCRT. Results show that administration of MCRT by i.v. or i.c.v. injection could induce bradycardia and decrease in mean arterial pressure (MAP) at a greater degree than that with morphiceptin and PFRTic-NH₂. When MCRT and NPFF were coinjected, we observed a dose-dependent weakening of these cardiovascular effects by MCRT. Because naloxone completely abolished the cardiovascular effects of MCRT, we conclude that opioid receptors are involved in regulating the MAP of MCRT regardless of modes of injection. The effect of MCRT on heart rate is completely dependent on opioid receptors when MCRT was administered by i.c.v. instead of i.v. The central nitric oxide (NO) pathway is involved in regulating blood pressure by MCRT under both modes of injection, but the peripheral NO pathway had no effect on lowering blood pressure mediated by MCRT when it was administered by i.c.v. Based on the results from different modes of injection, the regulation of heart rate by MCRT mainly involves in the central NO pathway. Lastly, we observed that the cardiovascular effects of MCRT such as bradycardia and decrease of blood pressure, were stronger than that of its parent peptides. Opioid receptors and the NO pathway are involved in the cardiovascular regulation by MCRT, and their degree of involvement differs between intravenous and intracerebroventricular injection.     

Activation of central opioid receptors determines the timing of hypotension during acute hemorrhage-induced hypovolemia in conscious sheep

After an initial compensatory phase, hemorrhage reduces blood pressure due to a widespread reduction of sympathetic nerve activity (decompensatory phase). Here, we investigate the influence of intracerebroventricular naloxone (opioid-receptor antagonist) and morphine (opioid-receptor agonist) on the two phases of hemorrhage, central and peripheral hemodynamics, and release of vasopressin and renin in chronically instrumented conscious sheep. Adult ewes were bled (0.7 ml x kg(-1) x min(-1)) from a jugular vein until mean arterial blood pressure (MAP) reached 50 mmHg. Starting 30 min before and continuing until 60 min after hemorrhage, either artificial cerebrospinal fluid (aCSF), naloxone, or morphine was infused intracerebroventricularly. Naloxone (200 microg/min but not 20 or 2.0 microg/min) significantly increased the hemorrhage volume compared with aCSF (19.5 +/- 3.2 vs. 13.9 +/- 1.1 ml/kg). Naloxone also increased heart rate and cardiac index. Morphine (2.0 microg/min) increased femoral blood flow and decreased hemorrhage volume needed to reduce MAP to 50 mmHg (8.9 +/- 1.5 vs. 13.9 +/- 1.1 ml/kg). The effects of morphine were abolished by naloxone at 20 microg/min. It is concluded that the commencement of the decompensatory phase of hemorrhage in conscious sheep involves endogenous activation of central opioid receptors. The effective dose of morphine most likely activated mu-opioid receptors, but they appear not to have been responsible for initiating decompensation as 1) naloxone only inhibited an endogenous mechanism at a dose much higher than the effective dose of morphine, and 2) the effects of morphine were blocked by a dose of naloxone, which, by itself, did not delay the decompensatory phase.