The role of hot plate and general environmental stimuli in morphine analgesic tolerance

Rats given one injection of morphine (5 mg/kg) paired with a hot plate test displayed greater analgesic tolerance than rats given nine injections of morphine paired with a distinct room in which the hot plate apparatus was located. Hot plate stimuli, rather than general environmental stimuli, are prepotent in the acquisition of morphine analgesic tolerance assessed by the hot plate procedure.     

Effects of naloxone,metenkephalin, and morphine on phencyclidine-induced behavior in the rat

The effects of naloxone, metenkephalin, and morphine were tested on phencyclidine(PCP)-induced stereotyped behaviors, ataxia, and hyperactivity in the rat. Naloxone (8 mg/kg) significantly decreased stereotypy, ataxia, and hyperactivity across all PCP doses tested (2.0, 4.0, and 6.0 mg/kg). Metenkephalin (40 g/kg) and morphine (5 and 10 mg/kg) increased ataxia at the 4.0 and 6.0 mg/kg PCP doses. Stereotypy was altered by the opiates in a dose-dependent manner; enhanced by metenkephalin (40 g/kg) at 2.0 mg/kg and inhibited by metenkephalin (40 g/kg) and morphine (10 mg/kg) at 4.0 and 6.0 mg/kg PCP. Locomotor activity was increased by morphine (5 mg/kg) at 2 mg/kg PCP. These results suggest an involvement of central opiate receptor mechanisms in the mediation of PCP-induced behaviors in the rat.     

Effects of morphine, beta-endorphin and naloxone on catecholamine levels and sexual behavior in the male rat

Intraperitoneal administration of the opiate antagonist naloxone hydrochloride (30 mg/kg) to sexually experienced male rats caused a significant reduction in mount and intromission latencies, number of mounts preceding ejaculation and ejaculation latencies. Intraperitoneal adminstration of naloxone (30 mg/kg) also stimulated persistant non-copulators to begin mating and to ejaculate within a twenty minute test period. Conversely, intraperitoneal administration of morphine sulphate (6 mg/kg) as well as intraventricular injection of the endogenous opiate beta-endorphin (6 micrograms) produced a complete loss of copulatory behavior in male rats. The deficit in sexual behavior induced by beta-endorphin was correlated with a significant increase in hypothalamic norepinephrine levels. It is suggested that the endogenous opiates may be involved in the mediation of sexual behavior via an interaction with central catecholaminergic systems.     

Comparison of subcutaneous and spinal subarachnoid injections of morphine and naloxone on analgesic tests in the rat

Small catheters were chronically implanted subdurally in the rat so that repeated microinjections could be made into the subarachnoid space at the lumbar area. Morphine, injected intrathecally (i.th.) produced analgesia as measured by the tail-flick test at doses $\text{1}\text{100}$ of those by the subcutaneous (s.c.) route. Analgesia from i.th. morphine was reversed by either i.th. or s.c. injected naloxone. The dose of naloxone by the i.th. route was about $\text{1}\text{30}$ of that by the s.c. route. However, i.th. injection was no more effective than s.c. injection of naloxone in reversing analgesia produced by s.c. injection of morphine. When [³H]-labelled naloxone was injected s.c. or i.th. in the above experiment of morphine antagonism, there was a more rapid entry of the labelled material in the brain by the i.th. route of administration. The results raise questions on the relative importance of the spinal mechanism of analgesia produced by s.c. injections of morphine. Analgesia was also measured by the tail-shock vocalization test in which morphine producd a dose-dependent elevation of shock threshold at s.c. doses above those prolonging tail-flick latencies. Morphine injected i.th. at doses above those which elevated tail-flick latency produced hypersensitivity, hyper-reflexia, and convulsive seizure of the hindquarters. The spinal analgesic effect of morphine, when administered localy, appears to have a low ceiling of efficacy.     

Effect of intracerebroventricular administration of morphine upon intestinal motility in rat and its antagonism with naloxone

Morphine, intracerebroventricularly (i.c.v.) or intraperitoneally (i.p.) administered to rats, inhibited intestinal propulsion as tested by a charcoal meal. Such an inhibition was shown to be linearly related to the log of administered doses for both routes of administration and the two linear regressions are parallel, so that morphine was calculated to be 206 times more potent when administered i.c.v. than i.p. A dose of morphine fully active by the i.c.v. route was completely inactive when injected by the i.v., i.p., i.m. and s.c. routes. Naloxone, administered i.c.v., blocked the antipropulsive effect of morphine i.c.v. or i.p. The pA2 of naloxone versus morphine, both administered i.c.v. was determined and calculated to be 7.14 (6.76-7.62).     

The stinging response of the honeybee: Effects of morphine,naloxone and some opioid peptides

Changes in responsiveness for the stinging reaction of honeybees fixed in a holder after receiving 3 electrical shocks delivered with 1 min interval, was registered and used as measurement for the effect of 2 μl of different solutions injected. Every shock consisted of a train of pulses of 1 msec each, delivered for 2 sec at a frequency of 100 Hz. Injection of morphine-HCl (50 to 200 n-moles/bee) produced a dose dependent reduction of the honeybee stinging response to the electrical shocks. The morphine dose that produced a 50% inhibition of the response (D₅₀) was 148 n-moles/bee (927 μg/g), i.e., a value far greater than that reported for vertebrates in behavioral test of analgesia. Naloxone 1.1 μg/g produces a significant reduction of morphine D₅₀ effect and at 4–5 μg/g, a full disinhibition. Thus, whereas the D₅₀ of morphine for honeybees is far greater than that for vertebrates, the doses of naloxone that antagonize morphine are similar for bees and vertebrates. Possible explanations of this difference are mentioned. Injections of met-enkephalin, leu-enkephalin, kyotorphin and (D-Ala²) methionine-enkephalinamide, given in doses of 200 n-moles/bee, an amount greater than that of the morphine D₅₀, exhibited no effect on the stinging response.     

Influence of morphine and naloxone on the release of noradrenaline from rat brain cortex slices

Summary In slices of rat brain cortex preincubated with (–)-³H-noradrenaline, the influence of morphine and naloxone on the efflux of tritium was investigated. The spontaneous outflow of tritium was not changed by 10^–7–10^–5 M morphine and by 10^–6–10^–4 M naloxone, but was accelerated by 10^–4 M morphine. Electrical field stimulation augmented tritium outflow. The overflow evoked per ppulse decreased as the frequency of stimulation was increased from 0.3 to 3 Hz, but remained approximately constant when it was further increased to 10 Hz. At frequencies of 0.3, 1, and 3 Hz, but not at 10 Hz, morphine in concentrations of 10^–7–10^–5 M depressed the stimulation-induced overflow of tritium. 10^–4 M morphine did not influence the overflow induced by stimulation at 0.3 and 1 Hz and increased that evoked by stimulation at 10 Hz. Naloxone (10^–6–10^–4 M) did not change the response to stimulation. In the presence of 10^–4 M naloxone, 10^–6 M morphine did not diminish, and 10^–5 M morphine even enhanced the stimulation-induced overflow of tritium. The inhibitory effect of 10^–6 M morphine was not reduced, after tyrosine hydroxylase had been blocked by -methyltyrosine-methylester. It is concluded that morphine through an action on specific opiate receptors inhibits the release of transmitter from cerebrocortical noradrenergic neurones evoked by nerve impulses. By an action unrelated to opiate receptors, morphine at high concentrations increases the stimulation-induced overflow of noradrenaline, presumably by inhibiting its re-uptake into nerve endings.     

Effects of methionine-enkephalin and morphine on spontaneous locomotor activity: Antagonism by naloxone

Methionine-enkephalin (MEK), a postulated endogenous ligand for the opiate receptors, when administered intracerebroventricularly (ICV) in a dose of 1.75 μmoles/kg) of morphine sulfate had no effect on motor activity. Higher doses (7.0 μmoles/kg) of both MEK and morphine produced profound depression in motor activity; the decrease was significant from 6 to 30 min after their administration. Subcutaneous administration of naloxone (1 mg/kg) did not alter the motor activity. However, administration of naloxone (1 mg/kg) 2 min prior to MEK (7.0 μmole//kg) administration completely blocked the effect of the latter for 15 min while antagonizing the effect of 7.0 μmoles/kg of morphine for 10 min. At 30 min after 7.0 μmoles/kg of either MEK or morphine administration, the motor activity in these groups was identical with that of the naloxone treated group. Methionine-enkephalin in doses of 0.2 and 0.4 μmole/kg had no effect on motor activity for 1 hr observation period. A significant increase in motor activity was recorded 45 and 60 min after 0.2 μmole/kg of morphine sulfate, whereas 0.4 μmole/kg dose of morphine showed significant increase in motor activity only at 60 min after its administration. Furthermore, 0.2 μmole/kg of morphine produced a greater increase in motor activity compared with 0.4 μmole/kg dose at 45 and 60 min after administration. It is concluded that morphine and MEK produce differential effects on mouse spontaneous motor activity with respect to time and dose and that naloxone can inhibit the effects of MEK more effectively than that of morphine on motor activity.     

The action of morphine and naloxone on acid secretion by the rat isolated stomach

The action of morphine and naloxone on acid secretion by the rat isolated stomach has been studied. Morphine (10^?7 to 10^?4 M) had no effect upon spontaneous acid secretion. Morphine (10^?6 M) did not modify the acid output in response to sub-maximal stimulation by pentagastrin, histamine, bethanechol or isoprenaline. Naloxone (10^?6 M) was without effect on the response to pentagastrin or histamine. Our results suggest that opiate receptors do not modify acid secretion in this preparation.     

Effect of morphine and naloxone on intestinal transit in mice

Morphine caused a dose-dependent slowing of the rate of intestinal transit in mice. This inhibitory effect of morphine was antagonised by naloxone administration. Pretreatment with a single dose of morphine did not induce any detectable tolerance to the inhibitory effect of a second dose of morphine given 5 h later. However, naloxone was more effective in antagonising this inhibitory effect of morphine-pretreated mice than in saline-pretreated animals. Molecular sieve morphine pellet implantation for 24 h induced detectable tolerance to the inhibitory effect of morphine administered 3 h after removal of the pellet. In addition, the antagonistic effect of naloxone was also augmented when compared with blank pellet-implanted control animals. The present study has shown that the enhanced naloxone potency against the inhibitory effect of morphine on intestinal transit was observable before the development of overt tolerance, and that tolerance to the effect of morphine on the small intestine could be induced by implantation of a molecular sieve morphine pellet for 24 h.     

Behavioral effects of morphine and naloxone following chronic morphine administration

The effects of morphine, naloxone, and combinations of these drugs were examined in squirrel monkeys under shock-postponement schedules. In the absence of a lever press, shocks could be presented every 4s, and each response postponed shock for 20s. Acutely, morphine (0.10–3.00 mg/kg) produced not only overall response-rate decreases, but also increases in the number of shocks, whereas naloxone (0.10–30.00 mg/kg) had little effect on responding. When given in combination with morphine, several doses of naloxone antagonized the rate-reducing and shock-increasing effects of morphine. Daily administration of morphine resulted in a substantial decrease in the number of shocks received and a moderate attenuation of the rate-decreasing effects of morphine (tolerance). Lower doses substituted for the fixed daily dose resulted in a smaller effect on behavior than under acute administration. Naloxone given in combination with the daily morphine dose or substituted for the daily administration of morphine, produced effects similar to those seen prior to chronic drugging. Thus, behavioral effects of naloxone were not altered even though tolerance to morphine was observed. Larger doses of naloxone continued to antagonize the effects of morphine for at least 24h. No signs of physical dependence were noted when naloxone was administered or when administration of morphine ended.     

Influence of morphine and naloxone on the release of noradrenaline from rat cerebellar cortex slices

Summary In slices of rat cerebellar cortex preincubated with (-)-³H-noradrenaline, the influence of morphine and naloxone on the efflux of tritium was investigated. The spontaneous outflow was not changed by 10^–5 M of either morphine or naloxone. On the other hand, morphine caused a concentration-dependent decrease of the overflow, of tritium evoked by electrical field stimulation. Naloxone did not change the stimulation-induced overflow, but prevented its inhibition by morphine. It is concluded that morphine, through an action on opiate receptors located on cerebellar noradrenergic neurones, inhibits the secretion of the transmitter in response to nerve impulses.     

Effects of d-amphetamine,morphine, naloxone,and drug combinations on visual discrimination in rats

The effects of d-amphetamine, morphine, and naloxone on visual discrimination were investigated using a two-choice discrete-trial procedure in which rats were trained to discriminate the position of a lightflash. Morphine (0.3–5.6 mg/kg) but not amphetamine (0.1–1.0 mg/kg) caused a significant dose-dependent disruption in discriminative performance. Both amphetamine and morphine increased response latencies. Naloxone (1.0 mg/kg) prevented the disruption of any aspect of performance by up to 100 mg/kg morphine. Performance after naloxone/amphetamine co-administration was not significantly different from that observed after amphetamine alone. Naloxone alone (0.3–10 mg/kg) had no effect on discrimination, spatial bias or response latencies. These results suggest that morphine and amphetamine affect different components of discrimination performance. Offprint requests to: S.G. Holzman     

Chronic variable stress or chronic morphine facilitates immobility in a forced swim test: reversal by naloxone

The behaviors displayed in a forced swim test were investigated in rats previously exposed to a chronic variable stress treatment or chronic administration of morphine. In addition, to further explore the participation of an endogenous opiate mechanism in these behavioral effects, naloxone was either administered during the chronic treatment (prior to each stress or morphine exposure) or immediately prior to the forced swim test. Animals were submitted daily to a different stressor for 1 week or injected with morphine (10 mg/kg, IP) for 6 days, whereas controls were unmanipulated except for the injection process. On the day following the last stressor, control and stressed animals were administered saline or naloxone (2 mg/kg, IP) 15 min prior to the forced swim test. Morphine treated animals were similarly tested on the third day following the last morphine injection. In a separate group of rats, naloxone (2 mg/kg, IP) was administered daily 10 min prior to each stressor of the chronic stress regime or each daily morphine injection. A significant increase in the time spent in immobility was observed in stressed animals as well as in rats chronically treated with morphine. In both groups, this potentiated immobility was attenuated by naloxone pretreatment prior to the forced swim test or when given before each daily stressor or morphine injection. In addition, the concurrent exposure to stress or morphine along with naloxone administration enhanced struggling in the first 5 min of the forced swim test. Taken together, the results of these experiments support the conclusion that the increase in immobility seen following chronic variable stress or repeated morphine exposure is modulated by the activation of an endogenous opiate mechanism, given that this effect is attenuated by naloxone administration.     

Effects of morphine,naloxone and their interaction in the learned-helplessness paradigm in rats

The aim of this study was to investigate the possible involvement of the mu-opioid system on the learned-helplessness paradigm, an experimental model of depression, in rats. In this test, rats were first exposed to inescapable foot-shocks (IS); 48 h later, they were submitted to a daily shuttle-box session (30 trials) for 3 consecutive days. Avoidance responses, escape failures and animal activity during each intertrial interval were recorded. Twice daily injections of morphine (0.25–8 mg/kg per day, SC), a mu-opioid agonist, reduced the increased escape failures induced by IS, as did tricyclic antidepressants. Significantly higher intertrial activity was observed in rats treated with morphine (2–8 mg/kg per day) compared with their associated control groups. Naloxone (1 and 2 mg/kg, IP), a mu-opioid antagonist, injected 10 min before each shuttle-box session impaired escape behavior in non-stressed rats and worsened the escape deficit induced by IS. Morphine-induced improvement of escape behavior and increase in intertrial activity were clearly reversed by a low inactive dose of naloxone (0.5 mg/kg). These results suggest that mu-opioid receptor mediation is involved in the deleterious effects of uncontrollable stress.     

Effects of acute morphine pretreatment on the rate-decreasing and antagonist activity of naloxone

Rats responded under a schedule in which every 30th lever press (fixed ratio 30) produced a food pellet during sessions divided into six 5-min ratio components separated by 10-min timeout (TO) periods. Cumulative doses of morphine or naloxone were administered at the start of consecutive TO periods. When given alone, morphine decreased response rates in a dose-dependent manner, abolishing responding at 10 or 17.8 mg/kg. Naloxone doses of 0.1 and 1.0 mg/kg restored rates and patterns of behavior suppressed by a dose of 17.8 mg/kg morphine; doses of 0.32 to 10 mg/kg prevented the rate-decreasing effects of cumulative morphine doses. When administered alone, naloxone initially decreased response rates at a cumulative dose of 32 to 100 mg/kg; with repeated testing and intervening morphine exposure, the required cumulative dose was decreased to 10 or 32 mg/kg. An acute 10 mg/kg morphine pretreatment, given 4 h before the session, decreased the cumulative naloxone dose required to suppress rates an additional 10- to 30-fold. This effect was time-dependent and dose-dependent, and the usual naloxone dose-response function could be recaptured 1 week after the pretreatment effect was obtained. In contrast, acute morphine pretreatment did not alter either the cumulative dose of morphine itself required to suppress rates or the naloxone dose required to reverse or prevent morphine's rate-decreasing effects.     

Effects of morphine and naloxone on thresholds of ventral tegmental electrical self-stimulation

Summary The involvement of opioid systems in self-stimulation reward was investigated by studying the effects of the opioid antagonist naloxone (10 mg/kg s.c.) and graded doses of morphine (0.3–5.0 mg/kg s.c.) on intracranial electrical self-stimulation (ICSS) in rats with electrodes in the ventral tegmental area. Lever pressing for ICSS was analyzed using three different procedures: (1) determination of response rate i.e. the number of responses to high and threshold currents, (2) measuring threshold current when response rate was kept low and relatively constant, (3) determination of behavioural threshold using a two-lever procedure in which a response on one lever resulted in a reset of the decreasing current to a high current contingent on a response to the other lever. It was found that low doses of morphine increased the response rate of ICSS behaviour and decreased the threshold whereas the higher doses decreased the response rate but also decreased the threshold current when measured with a rate insensitive procedure. Naloxone raised the threshold for ICSS and caused a corresponding decrease of response rate.In a second series of experiments in which the behaviour of rats which had been tested in one procedure was analysed using one of the other methods, it was observed that naloxone caused smaller changes, while the effects of morphine were at least comparable to those observed in the first series of experiments.The present data suggest that response rate insensitive procedures to analyse ICSS should be preferred to response rate sensitive ones, especially when the interaction of depressant drugs such as morphine with reward mechanisms is investigated. It is concluded that opioid systems are involved in ICSS elicited by electrical stimulation in the ventral tegmental area, and that the synergistic action of electrical stimulation and the pharmacological activation of ICSS reward circuits due to morphine may be related to the addictive properties of this drug.     

Blockade of the discriminative effects of morphine in the rat by naltrexone and naloxone

The capacity of the specific narcotic antagonists naltrexone and naloxone to block the discriminative effects produced by morphine in the rat were evaluated using a two-choice, discrete-trial avoidance task. The antagonists produced a dose-dependent and time-dependent blockade of morphine's effects as measured by responding on the morphine-appropriate choice lever. Naltrexone and naloxone were equipotent when given subcutaneously concomitantly with subcutaneously administered morphine. However, when the antagonists were administered orally at 0, 2, 4 or 8 h prior to s.c. morphine, naltrexone was more potent that naloxone at every time point and had a duration of action at least twice that of oral naloxone. The discriminitive effects of the narcotic analgesics morphine and methadone were also compared after oral and subcutaneous administration. Both drugs produced dose-related discriminative effects and were one-tenth as potent by the oral as by the subcutaneous route of administration. These results suggest that the discriminative effects produced by morphine in the rat can provide an animal model for the quantitative evaluation of the narcotic antagonist properties of drugs that might be considered for use in narcotic antagonist maintenance programs for the treatment of narcotic addiction.Predoctoral trainee of the Public Health Service Graduate Pharmacology Training Grant 2T1 GM179. This project represents partial fulfillment of the degree of Doctor of Philosophy at Emory UniversityRecipient of Research Scientist Development Award K02 DA00008     

Time course of antagonism of morphine antinociception by intracerebroventricularly administered naloxone in the rat

This kinetic profile and half-life of naloxone were studied for possible use in determination of pA₂ and K_B in vivo. Rats were given morphine subcutaneously and after 15 min naloxone or saline, intracerebroventricularly. A further 15 min later, and at 15 min intervals up to 135 min after morphine, the animals were tested for analgesia in the tail flick test. The dose-response curves of the naloxone group were shifted to the right of those for the saline group. Theamount of displacement decreased with time, indicative of the disappearance of naloxone. The graph of log (dose ratio-1) vs. time was linear with negative slope, in agreement with the time-dependent form of the equation for competitive antagonism. From this slope, the half-life of naloxone was calculated to be 13.3 min. These results demonstrate that the time-dependent method is useful in obtaining the kinetics of centrally acting opiate antagonists.