The psychopharmacologic and prolactin response after large doses of naloxone in man

Methylnaltrexone: its pharmacological effects alone and effects on morphine in healthy volunteers

RATIONALE

Methylnaltrexone bromide (MTNX) is a peripherally acting mu-opioid receptor antagonist, prescribed for the treatment of opioid-induced constipation in patients with advanced illness who are receiving palliative care. Studies have used this drug to determine if other opioid-induced effects besides constipation are altered by MTNX in humans and have suggested, based on their results, that these other effects are altered by peripheral opioid actions.

OBJECTIVE

The primary objective of this report is to present results that provide indirect evidence that MTNX has centrally mediated effects, albeit slight, and secondarily to describe the effects of MTNX on psychopharmacological effects of morphine.

METHODS

In a crossover, randomized, placebo-controlled, double-blind study, 29 healthy volunteers received 0.45 mg/kg MTNX or saline subcutaneously, followed by saline intravenously. In three other conditions, 0.143 mg/kg of morphine sulfate administered intravenously was preceded by subcutaneous administration of 0, 0.225, or 0.45 mg/kg MTNX. Before and after drug administration, subjective and physiological measures, including pupil diameter, were assessed.

RESULTS

Two separate analyses confirmed that 0.45 mg/kg MTNX alone induced a slight degree of miosis, a centrally mediated opioid agonist effect. This dose had minimal subjective effects. MTNX at either or both the 0.225 and 0.45 mg/kg dose reduced some subjective effects of morphine without altering miosis.

CONCLUSIONS

We present indirect evidence that MTNX crosses the blood-brain barrier in humans. Therefore, whether the reductions in subjective effects of morphine by MTNX that were observed in past studies and in this study can be attributed to peripheral mechanisms is open to question.

Nalmefene induced elevation in serum prolactin in normal human volunteers: partial kappa opioid agonist activity?

In humans, mu- and kappa-opioid receptor agonists lower tuberoinfundibular dopamine, which tonically inhibits prolactin release. Serum prolactin is, therefore, a useful biomarker for tuberoinfundibular dopamine. The current study evaluated the unexpected finding that the relative mu- and kappa-opioid receptor selective antagonist nalmefene increases serum prolactin, indicating possible kappa-opioid receptor agonist activity. In all, 33 healthy human volunteers (14 female) with no history of psychiatric or substance use disorders received placebo, nalmefene 3 mg, and nalmefene 10 mg in a double-blind manner. Drugs were administered between 0900 and 1000 on separate days via 2-min intravenous infusion. Serial blood specimens were analyzed for serum levels of prolactin. Additional in vitro studies of nalmefene binding to cloned human kappa-opioid receptors transfected into Chinese hamster ovary cells were performed. Compared to placebo, both doses of nalmefene caused significant elevations in serum prolactin (p<0.002 for nalmefene 3 mg and p<0.0005 for nalmefene 10 mg). There was no difference in prolactin response between the 3 and 10 mg doses. Binding assays confirmed nalmefene's affinity at kappa-opioid receptors and antagonism of mu-opioid receptors. [(35)S]GTPgammaS binding studies demonstrated that nalmefene is a full antagonist at mu-opioid receptors and has partial agonist properties at kappa-opioid receptors. Elevations in serum prolactin following nalmefene are consistent with this partial agonist effect at kappa-opioid receptors. As kappa-opioid receptor activation can lower dopamine in brain regions important to the persistence of alcohol and cocaine dependence, the partial kappa agonist effect of nalmefene may enhance its therapeutic efficacy in selected addictive diseases.

Buprenorphine: considerations for pain management

New effective analgesics are needed for the treatment of pain. Buprenorphine, a partial mu-opioid agonist which has been in clinical use for over 25 years, has been found to be amenable to new formulation technology based on its physiochemical and pharmacological profile. Buprenorphine is marketed as parenteral, sublingual, and transdermal formulations. Unlike full mu-opioid agonists, at higher doses, buprenorphine's physiological and subjective effects, including euphoria, reach a plateau. This ceiling may limit the abuse potential and may result in a wider safety margin. Buprenorphine has been used for the treatment of acute and chronic pain, as a supplement to anesthesia, and for behavioral and psychiatric disorders including treatment for opioid addiction. Prolonged use of buprenorphine can result in physical dependence. However, withdrawal symptoms appear to be mild to moderate in intensity compared with those of full mu agonists. Overdoses have primarily involved buprenorphine taken in combination with other central nervous system depressants.

Enkephalin release and opioid receptor activation does not mediate the antinociceptive or sedative/hypnotic effects of nitrous oxide

In previous studies using Fos expression as a marker of neuronal activation, we showed that nitrous oxide (N(2)O) activates bulbospinal noradrenergic neurons in rats and that destruction of these neuronal pathways leads to loss of N(2)O antinociceptive action. Based on previous rat studies it has been proposed that these noradrenergic neurons are activated through opioid receptors through the release of endogenous opioid ligands in the periaqueductal gray. Using mice with a disrupted preproenkephalin gene (Penk2 -/-) and the opioid receptor antagonist naltrexone, we investigated the role of enkephalinergic mechanisms and opioid receptor activation in the behavioral and bulbospinal neuron responses to N(2)O in mice. The antinociceptive response to N(2)O was investigated using the tail-flick, hot-plate, and von Frey assays, the sedative/hypnotic response was measured using rotarod and loss of righting reflex, and bulbospinal neuronal activation was assessed with pontine Fos immunostaining. No differences were observed between wild-type and Penk2 -/- mice for the antinociceptive, sedative/hypnotic, and pontine neuronal activation effects of N(2)O. Similarly, naltrexone did not block N(2)O-induced antinociception, sedation, or hypnosis. We conclude that neither enkephalin nor opioid receptors participate in either the antinociceptive or the sedative/hypnotic actions of N(2)O in mice.

Effects of naloxone on nitrous oxide actions in healthy volunteers

A number of studies have examined the effects of naloxone on nitrous oxide-induced analgesia with conflicting results. In the present study the effects of a relatively high dose of naloxone was examined to determine its effects on nitrous oxide-induced analgesia, as well as on the subjective and psychomotor effects of nitrous oxide. Fourteen subjects participated in a four-session crossover trial in which they received intravenous injections of either saline or 30mg/70kg naloxone 10min into a 35min period in which they were inhaling either 100% oxygen or 30% nitrous oxide in oxygen. Ten minutes after the naloxone administration, subjects were tested on the cold pressor test. Mood and psychomotor performance were also assessed before, during and after the inhalation period. Subjects reported higher pain ratings after the naloxone injection than the saline injection, but there was no evidence of naloxone reversing the analgesic effects of nitrous oxide. Similarly while naloxone also affected mood and impaired psychomotor performance, there was no evidence of naloxone reversing the effect of nitrous oxide on these measures. The results of this study call into question the role of the opioidergic system in mediating various effects of nitrous oxide in humans.

High-dose naloxone (1.0 mg/kg): psychological and endocrine effects in normal male subjects pretreated with one milligram of dexamethasone

The possible participation of the endogenous opioid system (EOS) in the negative feedback of the hypothalamic-pituitary-adrenal axis (HPA-a) activated by low doses (1 mg) of dexamethasone (Dex) was investigated. Ten male healthy subjects (mean age 31.5 +/- 1.9 SEM) were studied on 2 separate days, in a double-blind, cross-over and placebo-controlled design. All subjects were pretreated with 1.0 mg Dex orally the night (2300 h) before both test days. On the study days, subjects were admitted at 0700 h for cannula insertion; the administration of an i.v. bolus of either naloxone (Nal) (1.0 mg/kg) or saline solution (Sal) i.v. was started at 0900 h. Before and following each infusion, mood was measured by a Visual Analogue Scales (VAS) and by the Affective Quality Scale (AQS) every 30 min and blood samples were taken at 15-min intervals. Blood pressure and heart rate were also monitored. Before Dex administration, plasma cortisol levels were within the normal range in all subjects (210.4 +/- 13 ng/ml), while after 9 h after Dex cortisol levels showed the expected significant (p < 0.01) decrease (11.5 +/- 1.9 and 15.04 +/- 0.7 ng/ml for Sal and Nal test days respectively). There were no detectable increases in plasma cortisol levels following either Nal nor Sal administration. However, there was a Nal-induced significant increase in LH (p < 0.01) thus indicating that an effective opioid blockade at the level of the hypothalamic-pituitary unit occurred. There were also a mild and selective Dex + Nal-induced dysphoric (mood factors related to subjects perception of their cognition) and bradycardic effects (p < 0.05). These results suggest that the EOS is not directly involved in the negative feedback triggered by low doses of Dex of the HPA-a, and that there might be a possible glucocorticoid-opioid interaction for the modulation of some aspects of mood.

Effects of naloxone on the subjective and psychomotor effects of nitrous oxide in humans

The effects of naloxone on the mood-altering and psychomotor-impairing effects of nitrous oxide were examined in two studies. Each of the double-blind, randomized trials tested effects of three doses of naloxone or saline placebo during inhalation of 30% nitrous oxide in oxygen or 100% oxygen placebo. Experiment 1 tested a range of naloxone doses used clinically to reverse opiate-induced respiratory depression (0, 0.01, 0.1 1.0 mg/70 kg) and Experiment 2 included a dose approximately 25 times higher than that needed to reverse opiate-induced respiratory depression (0, 1.0, 3.0, 10 mg/70 kg). Nitrous oxide increased subject-rated reports of "feel drug effect," "carefree," "drunk," "sedated," and "high", and decreased psychomotor performance in both experiments. Naloxone had no effects by itself in either experiment, and, for the most part, did not significantly interact with nitrous oxide-induced changes in mood or psychomotor performance. Naloxone, in doses of 10 mg or less, does not appear to affect the subjective and psychomotor effects of nitrous oxide.

Morphine and naloxone act similarly on glutamate-caused guinea pig ileum contraction

Both morphine (M) and naloxone (NL) have been reported to have NMDA receptor blocking effects, regarded as the reason of opiate physical dependence development. On the other hand, glutamate (GLU) has been known to induce the contraction of isolated guinea pig ileum via acetylcholine release. Therefore, different concentrations of M or NL were investigated on the 1 mM GLU-induced contraction of isolated guinea pig ileum fixed at a resting tension of 1 g in isolated organ bath. The mean value (359.3 +/- 20 mg) of the GLU-elicited contraction force was significantly reduced (318.4 +/- 19.4) by 25 nM M concentration in the medium. Consequently, 500 and 750 nM M caused further decreases in a rather dose-dependent manner (270.8 +/- 17.4 and 167.8 +/- 16.5 mg, respectively). One micromolar M contraction nearly abolished (8.0 +/- 8.2 mg) the GLU-induced contraction. A similar effect was obtained with the naloxone concentrations of 10, 20, 40, and 50 microM. In addition, NL has been shown to elicit the contraction of the isolated M-dependent guinea pig ileum. In the present study, 20- and 30-microM NL concentrations in the bathing medium caused the contraction of the ileum made M-dependent by preincubation with M (333.0 +/- 32.4 and 309.5 +/- 17.7 mg, respectively). These contraction forces were significantly reduced when the NL concentration was increased to 40 microM. And, 50 microM NL concentration not only failed to induce contraction but caused a relaxation (-10.6 +/- 2.3) as well. The results were considered supporting evidence for the fact that both M and NL are NMDA receptor blockers.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of 12-hour infusion of naloxone on mood and cognition in normal male volunteers

The effects of a 12-hour naloxone infusion on mood, cognition, and plasma cortisol levels were evaluated in eight normal subjects. The dosage used was a 10 mg dose plus 7 mg/hr (total = 94 mg). Naloxone induced a significant rise in serum cortisol and also induced cognitive impairment, as shown by increased choice reaction time, reduced ability to recall the order of letters and numbers, and reduced accuracy of spatial orientation. The rise in cortisol induced by naloxone was significantly correlated with the rise in the Profile of Mood Scale (POMS) score, indicative of dysphoria. Finally, performance on the spatial orientation task was highly negatively correlated with the peak POMS score after naloxone. From these data and previous studies it is concluded that opioid receptors of low sensitivity to naloxone may mediate a common mechanism regulating the pituitary-adrenal axis' mood, and cognitive function during stress. Personality traits may account for the large individual variability reported in previous studies.

Previous chronic blockade of NMDA receptors intensifies morphine dependence in rats

Chronic exposure of receptors to antagonists generally results in upregulation and/or supersensitivity. On the other hand, the noncompetitive NMDA receptor antagonists ketamine (K) and dextromethorphan (DM) suppress opiate abstinence syndrome by blocking NMDA receptors. Therefore, 40 mg/kg ketamine (K), 5 mg/kg dextromethorphan (DM), 5 mg/kg morphine (M) and 2 mg/kg naloxone (NL) alone or in combination with NL were IP administered to the rats five times during the daytime only for five days to see whether they would intensify abstinence syndrome through upregulation and/or supersensitivity of NMDA receptors. Three days following the implantation of three M-containing pellets, abstinence syndrome was brought about by 2 mg/kg NL injection. Jumping, wet dog shake, writhing, teeth chattering, diarrhoea, defecation and ptosis were observed for ten min. All drugs used alone or in combination with NL increased the intensity of abstinence syndrome. Since K and DM are noncompetitive NMDA receptor antagonists, the intensifying effect of NL or M was considered to be related to their interactions with NMDA receptors. Furthermore, on the basis of the results of the previous and present study, NL was claimed to act on NMDA receptors, like other opioids, but with higher affinity for and weaker blocking effect on NMDA receptors.

Discrimination of butorphanol and nalbuphine in opioid-dependent humans

The purpose of the study was to evaluate the agonist and antagonist stimulus properties of the mixed opioid agonist antagonists butorphanol and nalbuphine in opioid-dependent subjects. Opioid-dependent volunteers (methadone 30 mg/day, PO) were trained in a three-choice drug discrimination procedure to discriminate between the effects of saline (2 ml), hydromorphone (10 mg/70 kg) and naloxone (0.15 mg/70 kg) administered IM. Subjects earned monetary reinforcement for correctly identifying the training drugs by letter code. Other subjective, behavioral and physiological measures were also collected. Hydromorphone and naloxone increased drug-appropriate responses and other characteristic subjective effects measures. Butorphanol and nalbuphine produced increases in naloxone-appropriate discrimination responding and in those subjective effect measures increased by naloxone. Butorphanol produced greater than 80% naloxone-appropriate responding at 1.05 mg/70 kg; nalbuphine produced 100% naloxone-appropriate responding at 2.1 mg/70 kg. Neither butorphanol nor nalbuphine showed opioid agonist-like effects in these subjects maintained at moderate levels of physical dependence. In opioid-dependent subjects, the stimulus effects of butorphanol and nalbuphine are antagonist-like.

Multidimensional scaling of subjective judgements of drug similarities among ketocyclazocine, morphine, cyclazocine, naloxone and placebo

Profiles of the subjective and physiologic effects of opioid drugs in man cannot be assigned with precision to specific opioid drug-receptor interactions. We administered a set of training doses of ketocyclazocine, morphine, cyclazocine, naloxone and placebo to 10 drug-using volunteers and obtained similarity judgements between each of 2 test doses of the drugs and a training dose. These data were submitted to multidimensional scaling analysis (INDSCAL) using both neighboring cells estimates and root mean square estimates to estimate missing cells in the data matrices. The results of these analyses are convergent, appear valid and indicate that there are three drug dimensions expressed in this data set: morphine versus placebo and naloxone; cyclazocine and ketocyclazocine versus placebo and naloxone; and ketocyclazocine versus cyclazocine. We interpret this result as supporting evidence that in the set of five drugs studied, three subjective states are induced.

Endogenous opiates: 1988

This paper is the eleventh installment in our annual review of the research during the past year involving the endogenous opiate system. It is concerned with nonanalgesic and behavioral studies of the opiate peptides that were published during 1988. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.