The effects of acute and chronic administration of morphine on the turnover of brain and adrenal catecholamines in rats

Brain and adrenal catecholamine turnover in adult female rats treated with morphine was investigated. A different time course response of brain and adrenal catecholamines to -methyl-p-tyrosine methylester (AMT) administration in normal rats was observed; the catecholamine turnover rate in adrenal glands appeared to be much slower than in the brain. Acute morphine increased the turnover of brain dopamine and noradrenaline as well as of adrenal catecholamines, whereas chronic morphine treatment induced a decrease in the turnover of brain noradrenaline. Withdrawal induced by nalorphine produced an increase in the utilization of brain noradrenaline and adrenal catecholamines; this effect could be related to the withdrawal stress situation induced by the opiate antagonist. Although the mechanism of morphine action may implicate other neurotransmitters besides catecholamines, our results contribute to evidence that brain and adrenal catecholamines could be involved in the mechanism of morphine tolerance and/or dependence.     

Chronic administration of morphine in cats: effects on adrenal and urinary catecholamines.

Adrenaline and noradrenaline levels in the adrenal glands and the excretion of both bioamines in urine of adult cats were investigated after chronic administration of morphine and nalorphine-induced withdrawal. After 7 days of daily consecutive morphine treatment, a significant increase in the adrenal noradrenaline content and a drop in adrenaline content were observed. After 2 weeks of daily injection of morphine, no significant changes were observed in the adrenal catecholamine level. One month of treatment with the opioid caused a significant increase in the adrenal content of both adrenaline and noradrenaline. Urinary excretion of catecholamines was significantly increased during the 4 weeks of treatment. In animals subjected to spontaneous or induced withdrawal with nalorphine, the adrenal content of catecholamines was altered and the ratio adrenaline/noradrenaline in the adrenal gland was shifted towards noradrenaline. A first injection of morphine produced an excitant manic response characterized by hyperexcitement and aggressive behaviour; animals chronically treated with the drug showed a progressively diminished response to this effect of the drug. It is concluded that physical dependence on morphine is reached by cats chronically treated with morphine and that this effect of the drug influences adrenomedulllary function in a different fashion depending on the stage of morphine treatment.     

Intraseptally injected opiate agents: effects on morphine-induced behaviour of cats

Behavioural effects of intraseptally administered opiate agents were analyzed in cats pretreated with an intraperitoneal injection of morphine. In this way, it became possible to investigate (1) the involvement of septal opiate receptors in the behavioural response of cats to systemic administration of morphine, and (2) the pharmacological character of septal opiate receptors. The following results were obtained with intraseptal injections 15-16 min after intraperitoneal morphine: (1) naloxone decreased frequencies of head and limb movements, and (2) morphine was ineffective. The following results were obtained with intraseptal injections 40-41 min after intraperitoneal morphine: (1) beta-endorphin and, to a lesser extent, fentanyl increased frequencies of locomotor patterns, (2) morphine and Met-enkephalin were ineffective, (3) naloxone and naltrexone decreased frequencies of locomotor patterns in a dose-dependent way, (4) naloxone and naltrexone antagonized the effects of beta-endorphin and fentanyl, and (5) morphine did not attenuate the effect of naloxone. The intraseptal injections affected only the frequencies of the systemically evoked behaviour patterns; the nature of the behaviour patterns remained unchanged. It is concluded that (1) systemically administered morphine does not affect behaviour via a direct action on septal opiate receptors, and (2) the receptors mediating the septally evoked effects are most probably epsilon-type opiate receptors. The hypothesis is put forward that systemic administration of morphine results in an increased release of beta-endorphin from hypothalamo-septal neurons and, as a consequence, changes the beta-endorphin activity at the epsilon-type opiate receptors in the septum.     

Brain catecholamines during ethanol administration, effect of naloxone on brain dopamine and norepinephrine responses to withdrawal from ethanol

The variations in brain dopamine (DA) and norepinephrine (NE) levels after ethanol administration have been studied in rats. Acute ethanol administration significantly decreased brain DA and NE levels. After chronic ethanol intake no changes were observed in brain catecholamines. Ethanol withdrawal induced significant decreases in DA and NE concentrations in the brain. The administration of naloxone, antagonist of opiate receptors, blocked the effects of ethanol deprivation on brain catecholamines. These data suggest that endogenous opioids may be involved in the ethanol-withdrawal syndrome.     

Tricyclic antidepressants activate the pituitary-adrenal axis in the rat. Tolerance to repeated drug administration

The effects of acute chronic administration of tricyclic antidepressants on the pituitary-adrenal axis were studied in adult male rats. Acute administration of desipramine, imipramine and chlorimipramine activated the pituitary-adrenal axis. The effect of imipramine was found to be of short duration with a significant increase at 30 and 60 min after the administration of the drug and a return to baseline levels at 180 min. The effect of imipramine was dose-dependent. Twenty hours after the last drug administration rats chronically injected with chlorimipramine showed normal basal and saline-induced levels of corticosterone. However the response of these rats to an acute dose of chlorimipramine was significantly lower than that of drug-naive rats, indicating tolerance to the drug. The mechanisms involved in the activation of the pituitary-adrenal axis caused by antidepressants and the tolerance to repeated drug administration are still unknown.     

Effects of memantine on estrogen-dependent acute tolerance to the morphine analgesia in female rats

Both human and animal studies suggest that there are sex differences in responding to noxious stimulation as well as in effects of opiate analgesic drugs. Development and/or expression of tolerance to opiate analgesia are also affected by the hormonal status of the experimental subjects. The present study aimed to compare acute tolerance to morphine in cycling and ovariectomized female rats and to evaluate the effects of N-methyl-D-aspartate (NMDA) receptor channel blocker memantine as well as 17-beta-estradiol on tolerance development using the tail-flick test. Acute tolerance to morphine analgesia was observed as a substantial reduction in the response to a test dose of morphine (10 mg/kg) given 6 h after the tolerance-inducing dose (10 mg/kg). Significant acute tolerance was observed in proestrous female rats and was prevented by memantine (3 or 10 mg/kg) treatment. Ovariectomized rats did not demonstrate tolerance to morphine analgesic effects but chronic estradiol administration (5 microg/day, 5 days) reinstated induction of tolerance. Both estrogen receptor modulator tamoxifen (5 mg/kg/day, 5 days) and memantine (3 mg/kg/day, 5 days) prevented estradiol-induced tolerance in ovariectomized rats. Thus, estrogens were found to play a key role in induction of acute tolerance to morphine antinociception. Estradiol-induced acute morphine tolerance may have NMDA receptor-dependent mechanisms.     

Naloxone inhibits and morphine potentiates the adrenal steroidogenic response to ACTH

The administration of morphine to hypophysectomized rats potentiated the steroidogenic response of the adrenal cortex to exogenous adrenocorticotrophic hormone (ACTH) in a dose-dependent fashion. Conversely, the opiate antagonist naloxone inhibited the adrenal response to ACTH. Naloxone pretreatment also antagonized the potentiating effect of morphine on ACTH-induced steroidogenesis in a dose-dependent manner. Neither morphine nor naloxone, administered to hypophysectomized rats, had any direct effect on adrenal steroidogenesis. These adrenal actions were stereospecific since neither the (+)-stereoisomer of morphine, nor that or naloxone, had any effect on the adrenal response to ACTH. The administration of human beta-endorphin to hypophysectomized rats had no effect on the adrenal corticosterone concentration nor did it alter the response of the adrenal gland to ACTH. These results indicate that morphine can potentiate the action of ACTH on the adrenal by a direct, stereospecific, dose-dependent mechanism that is prevented by naloxone pretreatment and which may involve competition for ACTH receptors on the corticosterone-secreting cells of the adrenal cortex.     

Previous chronic chlorimipramine treatment did not modify some physiological responses to acute and chronic stress in rats

The effects of previous chronic administration of the tricyclic antidepressant drug chlorimipramine (CMI) on some physiological responses of adult male rats to stress has been studied. CMI significantly reduced food intake and body weight gain, but did not alter either adrenal weight or basal serum corticosterone levels. Corticosterone response to 1 h of immobilization stress was the same in saline and CMI-treated rats. When the rats previously treated with CMI were subjected to chronic immobilization stress, it was found that the drug did not alter the anorexic effects of the stressor, but reduced the rate of adaptation of adrenocorticotropin response to stress. These data indicate that previous chronic CMI administration does not prevent changes in the secretory activity of the pituitary-adrenal system or the reduction of food intake and body weight caused by strong stressors. Offprint requests to: A. Armario     

Effects of morphine and naloxone on stress ulcer formation and gastric acid secretion

The effect of both acute and chronic morphine and naloxone on restraint-stress gastric ulcer formation and on basal gastric acid secretion were examined in the conscious rat. Acute morphine administration produced a dose-related decrease in stress ulceration and basal gastric acid secretion. Acute naloxone treatment resulted in a dose-related increase in stress ulcer formation and markedly augmented gastric acid secretion. Naloxone (4.0 mg/kg) antagonized the ulcer-reducing effects at all doses of morphine tested (4.0-32.0 mg/kg). Morphine-dependent rats, restrained during spontaneous or naloxone-precipitated withdrawal, exhibited the most severe ulceration. However, only those subjected to naloxone-precipitated withdrawal produced a significant increase in gastric acid output. Chronic treatment with naloxone or with chronic naloxone followed by morphine (16.0 mg/kg) resulted in augmented stress ulcer formation relative to all acutely treated groups. Both chronic naloxone-treated groups exhibited markedly enhanced gastric secretion. These data suggest that central and/or peripheral opiate receptors can modulate both basal and stress-perturbed gastric function.     

Involvement of brain biogenic amines in the electroencephalographic and behavioural effects of morphine in post-addict rats

Rats were prepared with chronic cortical and temporalis muscle electrodes for recording of the electroencephalogram (EEG) and electromyogram and with chronic indwelling intravenous cannulae for drug administration. To induce dependence on morphine, progressively increasing intravenous doses were delivered automatically over a period of 6–7 days, after which time the injections were discontinued. Administration of morphine (10mg/kg, i.p.) to post-addict rats abstinent for 7 days was followed by the immediate appearance of EEG desynchronization and marked behavioural hyperarousal. After pretreatment of rats with (±) α-methyl-para-tyrosine (α-MPT), para-chlorophenylalanine (p-CPA), or a combination of α-MPT and p-CPA during the period of morphine addiction and withdrawal, these EEG and behavioural effects of morphine, given on the eighth day of abstinence, were unaltered. Brain levels of the catecholamines norepinephrine (NE) and dopamine (DA) measured in un-treated post-addict rats after a morphine test dose (10mg/kg, i.p.) likewise remained unchanged. Brain levels of serotonin (5-HT), determined 30 min after the morphine challenge, were significantly elevated. Determinations of brain NE and DA levels, made 30 min after injection of morphine to postaddict rats pretreated 4 hr earlier with a-MPT, showed a reduction in brain DA utilization. These results suggest that both dopaminergic and serotonergic brain pathways may be involved in the mediation of EEG and behavioural responses to morphine in post-addict rats.     

Riluzole decreases the abstinence syndrome and physical dependence in morphine-dependent mice.

The effects of the antiglutamatergic agent, riluzole, were examined on the antinociceptive action of morphine, on the induction of physical dependence, and on the expression of the abstinence syndrome to the opiate in mice. Morphine was administered as a single dose (200 mg/kg) of a slow-release preparation. Acute and chronic administration of riluzole decreased the analgesic response to morphine, the intensity of abstinence behavior (administered 30 min before a dose of naloxone), and the development of physical dependence (repeatedly administered during the period of chronic morphine treatment).     

Evidence of place conditioning after chronic intrathecal morphine in rats

The acute administration of either systemic or intrathecal morphine produced an antinociceptive reaction on the tail flick test; only systemic morphine produced a sedative effect on motor activity. Rats receiving chronic intrathecal injections were hyperactive relative to saline injected control subjects. After a series of four injections, a place preference was induced by both routes of administration: rats who had previously received morphine either by subcutaneous or intrathecal injection spent significantly more time in the context in which the drug had been given, relative to rats who were comparably injected with saline. These data suggest that chronic intrathecal morphine in rats may elicit a discriminable cue, either through direct pharmacological stimulation of supraspinal sites, or, indirectly, as a consequence of spinal opiate action.     

The synergistic effect of concurrent spinal and supraspinal opiate agonisms is reduced by both nociceptive and morphine pretreatment

The antinociceptive effect of morphine administered into the periaqueductal gray (PAG), the intrathecal space (ITH) and concurrently, into both sites (in a 1:1 dose ratio), was assessed in 1) nontolerant rats, 2) rats made tolerant to the effect of morphine on the tail-flick (TF) test and 3) rats that were tested on the TF during chronic saline administration. In nontolerant rats, concurrent morphine injections produced a multiplicative antinociceptive effect (ED50 = 0.392 microgram, total dose) relative to that obtained after separate PAG (ED50 = 2.8 micrograms) or ITH (ED50 = 6.7 micrograms) injections. The multiplicative effect of concurrent morphine administration was significantly reduced in rats made tolerant to morphine (one 3 mg/kg SC injection and TF test per day for six days). Opiate synergy was also reduced but to a smaller extent in rats that were repeatedly tested on the TF during chronic saline administration (one SC injection and TF test per day for six days). Neither chronic morphine nor saline pretreatment altered the dose-response function to intrathecal morphine. However, both morphine and saline pretreatment significantly reduced the antinociceptive effect of morphine administered into the PAG. The data indicate that concurrent morphine administration into the PAG and ITH space results in a synergistic antinociceptive action which is reduced by performance of the nociceptive response, even in the absence of opiate administration. We suggest that the decrease in opiate synergism produced by nociceptive assessment (behavioral tolerance) is mediated supraspinally, while the additional decline resulting from morphine administered in conjunction with the nociceptive tests (opiate tolerance) is mediated by a combined action at spinal and supraspinal sites.     

Similar decrease in spontaneous morphine abstinence by methadone and RB 101, an inhibitor of enkephalin catabolism.

1. The dual inhibitor of enkephalin degrading enzymes, RB 101, is able to block endogenous enkephalin metabolism completely, leading to potent antinociceptive responses potentiated by blockade of CCKB receptors. In this study we have investigated the effects induced by RB 101 given alone, or with the CCKB antagonist, PD-134,308, on a model of spontaneous morphine withdrawal and substitutive maintenance in rats. 2. Animals were chronically treated with morphine for 7 days followed, 36 h after the interruption of drug administration, by a maintenance treatment for 5 days with methadone (2 mg kg-1, i.p.), clonidine (0.025 mg kg-1, i.p.), RB 101 (40 mg kg-1, i.p.), PD-134,308 (3 mg kg-1, i.p.) or a combination of RB 101 plus PD-134,308. Several behavioural observations were made during this period in order to evaluate the acute effects as well as the consequence of chronic maintenance induced on spontaneous withdrawal by the different treatments. 3. Methadone was the most effective compound in decreasing the spontaneous withdrawal syndrome after acute administration. Both, methadone and RB 101 had similar effectiveness in reducing opiate abstinence during the period of substitutive treatment. PD-134,308 did not show any effect when administered alone and did not modify the effect of RB 101. 4. Naloxone (1 mg kg-1, s.c.) failed to precipitate any sign of withdrawal when injected at the end of the chronic maintenance treatment suggesting that, under the present conditions, methadone and RB 101 did not induce significant physical opiate-dependence. 5. The mildness of the side effects induced by chronic RB 101, suggests that systemically active inhibitors of enkephalin catabolism could represent a promising treatment in the maintenance of opiate addicts.     

Effects of morphine in the isolated mouse urinary bladder

Acute morphine increased the responses to acetylcholine of the isolated mouse urinary bladder. A chronic morphine treatment did not change the responses of the urinary bladder to acetylcholine or ATP. The acute administration of morphine did not modify the contractile response to ATP in the urinary bladders from untreated or chronically morphine treated mice. Methadone and ketocyclazocine decreased the responses to the electrical stimulation of the urinary bladder. These depressant effects were not modified by naloxone. The results suggest the nonexistence of opiate receptors in the mouse urinary bladder and the lack of direct effects of morphine on the neuroeffector junction.     

Morphine differentially affects ventral tegmental and substantia nigra brain reward thresholds

In order to differentiate the roles of the nigrostriatal and mesolimbic-mesocortical dopamine systems in the action of opiates on dopaminergically mediated intracranial self-stimulation (ICSS), the effects of chronic morphine administration and acute naloxone administration of ICSS were tested in rats with electrode placements in the substantia nigra pars compacta (A-9) and the ventral tegmentum (A-10). Acute morphine (5.0 mg/kg SC) did not affect ICSS thresholds of rats with electrodes in the A-9 nucleus when tested 1, 3, 5, and 23 hours after administration. With repeated daily administration, though, these animals showed increases in thresholds which grew progressively larger with each day of morphine treatment. This threshold elevation was not reversed by naloxone given 0.5 hour after the final morphine treatment on the fifth day. In contrast, acute morphine significantly lowered self-stimulation thresholds in rats with A-10 placements. Tolerance to this facilitatory effect was evident with chronic administration. Naloxone attenuated the lowering of threshold caused by opiate administration in these A-10 animals. The present data suggest a specificity of action of opiates on different brain systems subserving reward and reinforcement. These findings also suggest that the mesolimbic-mesocortical system may play an important role in mediating the rewarding properties of morphine.     

The effects of neuropeptide Y and its fragments upon basal and electrically stimulated ion secretion in rat jejunum mucosa.

1. The effects of acute or chronic morphine treatment on the changes in blood pressure and pulse rate in response to ganglionic stimulation or blockade and to vagal stimulation, and of isolated atria to field stimulation or noradrenaline, were studied. 2. In pithed rats, intravenously injected hexamethonium significantly depressed the blood pressure responses to sympathetic nerve stimulation. The ganglionic blocking effects of hexamethonium were significantly greater in chronically morphine-treated rats, but were not significantly affected by acute morphine administration in naive animals. 3. Intravenous administration of nicotine dose-dependently increased blood pressure and pulse rate. The magnitudes of these changes were not significantly affected by acute or chronic morphine pretreatment. 4. Studies with rat isolated atrial preparations revealed that the changes in atrial contractile rate and force in response to noradrenaline or field stimulation were not influenced by either acute or chronic morphine treatment. 5. Cervical vagal stimulation produced voltage- or frequency-dependent decreases in pulse rate and blood pressure. The responses were not significantly affected by chronic morphine treatment. 6. These findings suggest that the site of the changes in sympathetic function following prolonged exposure to the opiate appears to be on the preganglionic nerve fibres.     

Arterial catecholamine levels in morphine-treated rats subjected to sympathetic nerve stimulation.

1. The effect of acute or chronic morphine treatment on the changes in arterial noradrenaline and adrenaline levels in response to sympathetic nerve stimulation was studied in rats. 2. Rats which had been chronically treated with morphine in their drinking fluid for 21 days were shown to be morphine-tolerant, as revealed by the tail-immersion test for analgesia. 3. It was found that animals given either acute or chronic morphine treatment had similar basal concentrations of arterial catecholamines to their controls. 4. Sympathetic nerve stimulation produced significant increases in arterial noradrenaline and adrenaline levels in both the control and morphine-treated animals. However, the degree of arterial noradrenaline elevation was significantly less in morphine-tolerant animals. 5. This phenomenon was not observed in acutely morphine-treated rats or at 2 weeks following opiate withdrawal in animals which had been treated previously with morphine for 3 weeks. 6. The findings suggest that chronic morphine treatment in rats not only leads to opiate tolerance but also reduces catecholamine release in response to sympathetic nerve stimulation.     

Ventricular noradrenaline concentrations in naïve and morphine-treated rats subjected to acute myocardial ischaemia.

1 Ventricular noradrenaline concentrations in morphine-treated rats subjected to acute left coronary artery ligation were measured by high performance liquid chromatography with electrochemical detection. 2 In naïve rats, acute left coronary artery ligation induced a significant increase in right ventricular noradrenaline concentration at 5 min and significant decreases in left ventricular noradrenaline concentration at 3 and 10 min. 3 Acute morphine treatment did not significantly alter ventricular noradrenaline concentrations in rats subjected to acute coronary artery ligation. 4 Chronic morphine treatment caused significant declines in ventricular noradrenaline concentrations in rats subjected to acute coronary artery ligation. The reductions increased with duration of opiate treatment, and were reversed by opiate withdrawal. 5 These findings indicate that there is an increase in sympathetic activity during acute myocardial ischaemia. It is suggested that chronic morphine treatment may be able to retard this response, and consequently to lessen the occurrence of early ventricular arrhythmias resulting from acute myocardial ischaemia.     

Drug onset cues, conditioned withdrawal, and drug relapse: comment on McDonald and Siegel (2004)

Previous research has shown that under certain conditions environmental cues associated with morphine administration induce drug-opposite conditioned effects that mimic symptoms of opiate withdrawal. R. V. McDonald and S. Siegel (see record 2004-10475-001) extend these observations by demonstrating that acute exposure to a low dose of morphine induces symptoms of opiate withdrawal in rats previously exposed to a high dose of morphine. They hypothesized that early drug onset cues, repeatedly paired with later, larger drug effects, mediate the paradoxical effect of the low drug dose on behavior. They also hypothesized that conditioned withdrawal symptoms induced by the early drug onset cues may mediate the "priming" effect of drugs on relapse and craving. The authors of this comment discuss the degree to which the literature supports this hypothesis.