Acute morphine administration and withdrawal from chronic morphine increase afterdepolarization amplitude in rat supraoptic nucleus neurons in hypothalamic explants

Supraoptic nucleus (SON) neurons secrete either oxytocin or vasopressin into the bloodstream from their axon terminals in the posterior pituitary gland. SON neurons are powerfully inhibited by the classical μ-opioid receptor agonist, morphine. Oxytocin neurons develop morphine dependence when chronically exposed to this opiate, and undergo robust withdrawal excitation when morphine is subsequently acutely antagonized by naloxone. Morphine withdrawal excitation is evident as an increased firing rate and is associated with an increased post-spike excitability that is consistent with the expression of an enhanced post-spike afterdepolarization (ADP) during withdrawal. Here, we used sharp electrode recording from SON neurons in hypothalamic explants from morphine naïve and morphine treated rats to determine the effects of morphine on the ADP, and to test the hypothesis that morphine withdrawal increases ADP amplitude in SON neurons. Acute morphine administration (0.05-5.0 μM) caused a dose-dependent hyperpolarization of SON neurons that was reversed by concomitant administration of 10 μM naloxone, or by washout of morphine; counter-intuitively, acute exposure to 5 μM morphine increased ADP amplitude by 78 ± 11% (mean ± SEM). Naloxone-precipitated morphine withdrawal did not alter baseline membrane potential in SON neurons from morphine treated rats, but increased ADP amplitude by 48 ± 11%; this represents a hyper-activation of ADPs because the basal amplitude of the ADP was similar in SON neurons recorded from explants prepared from morphine naïve and morphine treated rats. Hence, an enhanced ADP might contribute to morphine withdrawal excitation of oxytocin neurons.     

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.     

Prostaglandins in rat placenta following acute and chronic administration of morphine.

The objective of the present study was to obtain information on prostaglandin (PG) biosynthesis in placenta following narcotic administration. PG biosynthetic capacity was determined in whole rat placenta homogenates in the presence of Na arachidonate, by evaluating net production of PGs assayed against PGE1 on rat stomach strips. An enhanced prostaglandin-like activity is shown in homogenates of placenta from rats treated subcutaneously with 10 mg/kg of morphine. This increase was prevented by naloxone pre-treatment. Continual morphine administration during gestation, results in a normalization of placental biosynthetic capacity thus suggesting the development of a tolerance to the narcotic effect.     

Changes in stimulation analgesia during chronic morphine administration

It has been shown in rat experiments that repeated administration of morphine produces tolerance to the effect of stimulation analgesia (SA). In the control group, the same stimulation of the periaqueductal gray matter (PGM) failed to produce tolerance. Naloxone (2 mg/kg i.p.) exerted a partially antagonistic influence on SA in the control group and did not affect the antinociceptive action of PGM stimulation in morphine-treated animals.     

Intermittent morphine administration induces dependence and is a chronic stressor in rats.

Although constant treatment with morphine (implanted pellets) does not activate the hypothalamic-pituitary-adrenal (HPA) axis, intermittent injections of morphine may constitute a chronic stressor in rats. To test this hypothesis, we compared the effects of morphine in escalating doses (10-40 mg/kg, s.c.) or saline injected twice daily for 4 days on energy balance, hormones, HPA responses to novel restraint and central corticotropin-releasing factor (CRF) mRNA 12 h and 8 days after the last morphine injection in adult male Sprague-Dawley rats. Weight gain stopped at the onset of morphine, weight loss was marked 36 h postmorphine; thereafter, body weight gain paralleled saline controls. At 12 h, insulin, leptin, and testosterone concentrations were reduced but normalized by 8 days. Restraint and tail nicks caused facilitated ACTH responses at 12 h, under-responsiveness at 8 days. CRF mRNA, measured only at 12 h, was increased in the paraventricular (PVN) and Barrington's nuclei (BAR), decreased in the bed nuclei of the stria terminalis (BNST) and unchanged in the amygdala (CeA) in morphine-treated rats. After stress, CRF mRNA increased in PVN in both groups, increased in BAR and decreased in BNST in saline but not morphine groups, and was unchanged in CeA in both groups. Results from all variables characterize intermittent morphine injections as a chronic stressor. In contrast to constant treatment, injected morphine probably allows some withdrawal during each 12 h interval, causing repeated stress. Drug addicts treat themselves intermittently, and stress causes relapse after withdrawal. Thus, intermittent morphine, itself, may promote relapse.     

Serum morphine concentrations after buccal and intramuscular morphine administration.

1. This study compared serum concentrations of morphine after administration of a buccal tablet (25mg) with those after intramuscular injection (10mg). 2. Buccal morphine was administered to eleven healthy volunteers and intramuscular morphine was given to five preoperative surgical patients. Serum morphine concentrations were assayed by high performance liquid chromatography (h.p.l.c.) in samples taken up to 8 h after drug administration. 3. Mean maximum morphine concentrations were eight times lower after buccal administration than after intramuscular injection and occurred at a mean of 4 h later. Individual morphine concentration-time profiles showed marked interindividual variability after administration of the buccal tablet, consistent with considerable variation in tablet persistence time on the buccal mucosa.     

Changes in brain tryptophan and tyrosine following acute and chronic morphine administration

Morphine increased brain concentrations of tryptophan and tyrosine 1–2 hr after administration in a dose-dependent manner in male rats. Concentrations of these amino acids in blood serum decreased 30–45 min post-injection and then rose towards control values. The rise in brain amino acids was antagonized by pretreatment with naloxone. In addicted rats there was only a slight increase in brain tryptophan and no increase in tyrosine. Thirty minutes after naloxone-precipitated withdrawal, tryptophan and tyrosine concentrations were elevated in brain, in contrast to the decline in these amino acids seen after naloxone administration in acutely morphinized rats.These results support the hypothesis that the elevated turnover of brain monoamines induced by morphine administration is related to increased availability of precursor amino acids in morphinized animals.     

Changes in liver tyrosine aminotransferase after acute and chronic administration of morphine in the rat.

Acute administration of Morphine (20 mg/kg/s.c.) in the rat results in a rise of liver tyrosine aminotransferase (TAT) expressed as mumoles of p-hydroxyphenylpyruvate/100 mg/h. With chronic administration, a tolerance develops to this enzymatic effect. TAT induction is not evident in pregnant rats, given the narcotic, in which enzyme levels are already initially high. After delivery TAT returns to normal levels and it is possible to show both induction and tolerance developing to morphine. Enzyme activity in fetal livers is much lower than that of adult animals: after maternal administration of morphine only a modest TAT increase is seen which is not, however, statistically significant. TAT activity is fully evident in livers of offspring, with much higher mean levels in newborn rats from morphine-treated animals, as a possible consequence of morphine deprivation. In this latter group of newborn rats narcotic administration causes TAT activity to return to levels as high as those of naive animals. On the other hand, morphine administration to the prole of naive rats results in an induction of liver TAT.     

Effects of acute and chronic morphine administration on glucose tolerance in mice

The effects of acute and chronic morphine treatment on glucose tolerance were investigated in mice. In acute experiments, a single dose of morphine (20 mg/kg i.p.) increased the serum and muscle glucose level. After glucose loading (1.5 g/kg), the rate of increase and the peak of serum glucose concentration were significantly lowered in morphine-treated mice, while the availability and the half-life of glucose were similar to those of controls. In morphine-dependent mice, the fasting serum and muscle glucose levels were similar to those of control but the liver glucose was significantly greater. After glucose loading the rate of increase in serum glucose level was faster and the availability of glucose was 10% greater than that in naive mice. Again, there was no difference in the half-life of serum glucose between naive and morphine-dependent mice.     

Effects of morphine on the turnover of brain catecholamines and serotonin in rats — chronic morphine administration

The turnover of brain monoamine was studied in rats in which different degrees of tolerance to and dependence on morphine were induced by pellet implantation. The degree of tolerance to morphine was assessed by measuring the increase in effective dose for an antinociceptive effect (vocalization test). The rate of depletion of brain dopamine (DA) and serotonin (5-HT) after α-methyl-p-tyrosine (AMT) or α-propyl-dopacetamide (dop-acetamide) was not changed by chronic morphine treatment. In contrast, the accumulation of brain homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) after probenecid was significantly increased, but there was no correlation between the biochemical changes and the degree of tolerance/dependence of the animals; at a very high degree of dependence 5-HIAA accumulation even became normal. In rats in which smaller amounts of morphine were repeatedly injected every 8 hr for 1 week the increased accumulation of HVA and 5-HIAA persisted in spite of complete tolerance to the antinociceptive effect.The rate of depletion of brain noradrenaline (NA) after AMT or dopacetamide was not changed and the accumulation of brain 3-methoxy-4-hydroxy-phenylglycol sulphate (MHPG-SO₄) after probenecid was not affected in most chronic morphine groups. In the group with the highest degree of tolerance/dependence NA depletion after AMT was even retarded.The results suggest that chronic morphine treatment increases the synthesis and the intraneuronal destruction of newly synthesized DA and 5-HT without changing the rate of functional utilization of the monoamines. It is unlikely that the changes in monoamine metabolism are causally related to processes leading to morphine tolerance/dependence.     

Changes in hypothalamic paraventricular nucleus catecholaminergic activity after acute and chronic morphine administration

The participation of hypothalamic noradrenaline in the expression of neuroendocrine signs of morphine withdrawal has been proposed. The present study in rats examined: (1) the relationships between corticosterone secretion and the possible modifications in noradrenaline and dopamine content and turnover in the hypothalamic paraventricular nucleus after acute and chronic morphine administration; (2) the changes in cyclic adenosine monophosphate (cAMP) levels in the paraventricular nucleus after the same treatments. The results showed that acute morphine injection in control rats increased corticosterone release, 3-methoxy-4-hydroxyphenylethylene glycol (MHPG) production, and noradrenaline turnover. Dopamine turnover in the paraventricular nucleus was decreased and the cAMP levels remained unchanged. In chronic morphine-treated rats, there was no elevation in noradrenaline turnover or in corticosterone secretion, indicating that tolerance developed to the acute effects of the opioid. Correspondingly, no alterations in dopamine turnover were observed when chronic morphine-treated rats were compared with control rats acutely injected with morphine. cAMP levels in the paraventricular nucleus were unchanged during the tolerant state. The results raise the possibility that noradrenergic afferents play a significant role in the alterations of paraventricular nucleus function and pituitary-adrenal axis activity in response to acute and chronic morphine and suggest that these modifications are not mediated through adenylate cyclase activation. The present data provide further support for the idea of adaptive changes in noradrenergic neurons projecting to the paraventricular nucleus during chronic morphine exposure.     

Acute and chronic intracerebroventricular morphine infusions affect long-term potentiation differently in the lateral perforant path.

Experiments were performed to investigate the effects of acute and chronic intracerebroventricular (icv) morphine infusions via osmotic minipumps on long-term potentiation (LTP) in the lateral perforant path (LPP)-granule cell synapse of the rat dentate gyrus. Although significant antinociceptive activity was observed when morphine was infused (25 nmol/microl/h) for 30 min or 1 h, the activity was not observed in rats receiving morphine chronically for 72 h, and the tail-flick latency of this group was comparable to that of rats receiving saline. LTP induction was significantly attenuated after acute morphine infusion (1 h) in LPP-granule cell synapses of the dentate gyrus. In contrast, LTP induction was augmented after chronic morphine infusion for 72 h. Bath-perfused morphine augmented the baseline population spike (PS) amplitude in rats treated with saline, whereas it attenuated the LTP induced by chronic morphine infusion. Returning the LTP to the level of saline infusion after in vitro morphine perfusion suggests that enhancement of the LTP is a withdrawal-like phenomenon. These results suggest a difference between the effects of acute and chronic intracerebroventricular morphine infusions on synaptic plasticity in the LPP-granule cell synapses of the dentate gyrus.     

The effect of acute and chronic administration of morphine and morphine withdrawal on intestinal transit time in the rat

The effects of acute and chronic morphine administration and of morphine-withdrawal on intestinal transit time of a liquid meal were investigated using rats. Many experiments have assessed the effects of acute morphine administration on intestinal transit, but the intestinal effects of chronic morphine administration have been neglected. Our results showed no significant differences between morphine-dependent and control animals when assessing the leading edge of the liquid meal infusion, its distribution and geometric centre (G.C.). However, during naloxone-precipitated withdrawal from morphine, the leading edge of the infusion and its G.C. were significantly distal to values obtained from other groups. Acute morphine administration caused delayed intestinal transit of a meal infusion, an effect partly caused by significant retention of the infusion in the stomach and duodenum. The leading edge of the meal infusion and G.C. were significantly proximal to values obtained from other groups of animals. The results show that morphine-dependent rats develop complete tolerance to the delayed intestinal transit of a meal observed after acute morphine administration and that withdrawal from morphine accelerates intestinal transit of a liquid meal.     

The effects of acute and chronic morphine administration on GABA receptor binding

The effect of acute and chronic morphine administration and naloxone-precipitated withdrawal on the binding of [3H]GABA to its receptor sites in rat brain membranes was investigated. Acute morphine (25 mg/kg) administration produced a decrease in the GABA binding in cerebellum, cortex and striatum. This decrease appears to be due to a selective decrease in the number of high-affinity GABA receptor binding sites. In contrast, rats chronically treated with morphine by pellet implantation did not exhibit any changes in GABA receptor binding, except for an increase in pons medulla. However, in rats which were physically dependent, as indicated by naloxone-precipitated withdrawal, GABA binding was decreased significantly in cerebellum and striatum, relative to chronic morphine treatment or placebo pellet controls. This decrease was due to a decrease in the number of low affinity GABA receptor binding sites. Both chronic morphine and naloxone-precipitated withdrawal treatments produced an increase in GABA binding in the pons medulla. These results suggest that morphine may produce some of its effects by modulating GABAergic systems and that high and low affinity GABA receptor sites may play a differential role during various morphine treatments.     

Effect of acute and chronic morphine administration on brain cholinergic muscarinic receptors

The binding of [3H]quinuclidinyl benzilate (QNB) to various brain regions was determined in rats after acute or chronic treatment with morphine. Morphine and naloxone, in vitro, inhibited the binding of [3H]QNB to striatal membranes only at high concentrations. Thirty minutes after a single injection, morphine (5 or 40 mg/kg s.c.) did not alter the Bmax or Kd values for [3H]QNB binding to striatal receptors. The binding of [3H]QNB to membranes of different brain regions was not changed in morphine tolerant-dependent rats or rats undergoing abrupt or naloxone precipitated withdrawal. The results suggest that central cholinergic muscarinic receptors are unaffected by acute or chronic treatment with morphine, or during abstinence.     

Acute and chronic effects of morphine on lipolysis in rat epididymal fat pads

The effects of morphine and adrenaline on lipolysis have been studied. In vivo studies show that both acute and chronic morphinization enhance lipase activity. In vitro addition of morphine produces a “dose-dependent” increase in the rate of fatty acid release, maximal activation being produced by 2.5 mM morphine. Chronically morphinized fat pad is insensitive to morphine added in vitro. However chronic morphinization does not completely abolish the sensitivity of the tissue to adrenaline. When normal fat pad is incubated in the presence of 2.5 mM morphine and submaximal concentrations of adrenaline (1–50 μM), there is an additive effect. Results from this study indicate that morphine may exert its effect via the same mechanism as adrenaline and suggest that the receptor on the plasma membrane has higher affinity for adrenaline than for morphine.     

Serum morphine concentration after oral administration of diamorphine hydrochloride and morphine sulphate.

1 Venous blood was obtained from patients with far-advanced cancer receiving either diamorphine (diacetylmorphine, heroin) hydrochloride (65 samples) or morphine sulphate (24 samples) regularly by mouth in doses from 2.5 mg to 90 mg every 4 h. 2 Samples were obtained within 30 min of the 09.00 h drug round. 3 Serial samples were also obtained over a 4 h period from three patients receiving diamorphine hydrochloride. 4 Assay of serum 'morphine equivalents' was by radioimmunoassay using an antibody that cross reacts almost equally with diamorphine, 6-0 monoacetylmorphine and morphine. 5 The serum concentration of opiates expressed as 'morphine equivalents' ranged from 11 ng/ml to 1440 ng/ml. 6 A highly significant positive linear correlation exists between the dose administered and the serum concentration (P less than 0.001) with respect to both drugs. 7 There was no difference between the two drugs in relation to the serum concentration achieved per 10 mg of opiate administered. 8 Higher oral doses of both diamorphine and morphine are now being used when indicated rather than, as before, resorting to injections when an oral dose in excess of 40 mg is indicated.