Research report
Brain opioid receptor adaptation and expression after prenatal exposure to buprenorphine

https://doi.org/10.1016/S0165-3806(98)00117-5Get rights and content

Abstract

Previous in vivo studies revealed that buprenorphine can down-regulate μ and up-regulate δ2 and κ1 opioid receptors in adult and neonatal rat brain. To assess gestational effects of buprenorphine on offspring, pregnant rats were also administered this drug and opioid receptor binding parameters (Kd and Bmax values) were measured by homologous binding assays of postnatal day 1 (P1) brain membranes. Buprenorphine concentrations of 2.5 mg/kg injected into dams elicited an up-regulation of κ1 opioid receptors as detected with the κ1-selective agonist 3H-U69593. Parallel studies with the μ-selective agonist [d-ala2,mephe4,gly-ol5] enkephalin revealed a buprenorphine-induced down-regulation in receptor density at 0.3, 0.6 or 2.5 mg/kg drug treatment. A greater down-regulation of μ receptors for P1 males than for their female counterparts was observed. Buprenorphine did not cause a reduction in binding affinity in these experiments. Changes in opioid receptor adaptation induced by buprenorphine were further supported by data from cross-linking of 125I-β-endorphin to brain membrane preparations. RT-PCR analysis of opioid receptor expression was also estimated in P1 brains. However, significant changes in neither μ nor κ receptor message were detected in P1 brains as a result of prenatal buprenorphine treatment under the conditions of these experiments. Since buprenorphine is being evaluated in clinical trials for the treatment of heroin abuse, the in utero actions of the drug have ramifications for its use in the treatment of maternal drug abuse.

Introduction

Buprenorphine, an oripavine alkaloid derivative, is currently marketed as an analgesic agent and is undergoing clinical evaluation for the treatment of heroin addiction in the United States. In countries such as France and Israel, it has been approved for use in opiate addiction rehabilitation. Acute and chronic administration of buprenorphine can attenuate or even block the effects of opioid agonists, including morphine, heroin and hydromorphone in humans 7, 26, 38, 40. This mixed agonist–antagonist displays fewer adverse side effects and less abuse potential than other opioid agonists used in drug rehabilitation [18]. Two promising approaches have been studied recently, the use of a combination of buprenorphine and the opioid antagonist naloxone in opiate-dependent users 21, 27or the substitution of buprenorphine for methadone 23, 32, 35. Nevertheless, little is known about the effects of mixed agonist–antagonists such as buprenorphine on opioid receptor adaptation and expression, especially during early brain development. This information would have implications for the adoption of buprenorphine in the treatment of maternal drug abuse, since opioid binding sites in developing brain are particularly susceptible to opiate exposure 22, 36.

Pathological manifestations of prenatal opioid exposure including physiological and psychological delays have been described [22]. Opioid receptors in developing brain readily undergo adaptation upon opiate exposure. The advantages of buprenorphine over methadone include: a longer lasting action and thus, use on an alternate-day schedule; less physical dependence; a lower risk of overdose and a less intense withdrawal state 25, 41. Hence, the in utero actions of the drug have ramifications for its adoption in rehabilitation of maternal drug abuse and its pre- and postnatal effects should be assessed in a developing animal model system.

Surprisingly, little data are available on the physiological consequences of prenatal buprenorphine exposure. Early biochemical studies suggested that in utero buprenorphine exposure did not alter met- or leu-enkephalin levels in rodents 28, 29, 39. Consistent with the down-regulation of μ receptors of both adult and P7 rats by this drug [5], administration of 0.5 mg/kg buprenorphine to pregnant rats caused μ receptor down-regulation in brain membranes of their P1 offspring [4]. Data on possible behavioral effects associated with prenatal buprenorphine exposure are also limited. Prenatal buprenorphine exposure does not alter activity or developmental milestones 28, 29, although there is one conflicting report [31]. These findings are in contrast with effects of prenatal methadone exposure on activity and developmental milestones 13, 42, 43. Similarly, rest-activity cycles were disrupted by prenatal methadone exposure [17]but not prenatal buprenorphine exposure [16]. Prenatal buprenorphine exposure does appear to alter sexually dimorphic nonreproductive behaviors including saccharin preference and juvenile spontaneous parental behaviors [1]. Because of interest in its adoption for treatment of opiate addiction, further research on in utero effects of buprenorphine on opioid receptor expression and adaptation is warranted. Here, we report on μ and κ opioid binding in brain membrane preparations from P1 rats after gestational buprenorphine treatment of their mothers. To assess prenatal effects of buprenorphine on brain opioid receptors, pregnant rats were administered three doses of buprenorphine (0.3, 0.6 or 2.5 mg/kg) and levels of P1 pup brain μ and κ opioid sites were estimated by binding and cross-linking assays. In addition, we evaluated the gestational effects of buprenorphine on opioid receptor expression by measuring μ and κ mRNA levels in P1 brain extracts by RT-PCR.

Section snippets

Chemicals

[d-Ala2,mephe4,gly-ol5] enkephalin (DAMGE), 3H-DAMGE (37 Ci/mmol), β-endorphin, 125I-β-endorphin (2000 Ci/mmol) were obtained from Multiple Peptide Systems (San Diego, CA); buprenorphine and U69593 were from NIDA Drug Supply (Research Triangle, NC). 3H-U69593 (56 Ci/mmol) was from Amersham (Arlington Heights, IL). The following were obtained from the indicated sources: Ultraspec™ RNA isolation solution (Biotecx, Houston, TX); RNEasy™ total RNA preparation kit (Qiagen, Chatsworth, CA);

Opioid binding parameters of brain membrane preparations from P1 rats prenatally exposed to 2.5 mg/kg buprenorphine

A significant down-regulation of μ-opioid binding in P1 brain membranes was induced by in utero buprenorphine (0.5 mg/kg) treatment [4]. However, changes in κ binding were not detected under the same conditions. In the present studies the concentration of buprenorphine was elevated to 2.5 mg/kg and both 3H-DAMGE (μ) and 3H-U69593 (κ) binding parameters (Bmax and Kd values) were measured in P1 brain membranes. Buprenorphine exposure reduced μ binding density by 78% in brains of P1 pups without

Discussion

In the present study, we show that in utero exposure to buprenorphine results in up-regulation of κ1 receptors as well as down-regulation of μ opioid receptors in P1 pup brains. Up-regulation of κ1 opioid binding required higher doses of buprenorphine than did down-regulation of μ receptors, as seen previously for both adult and P7 rats. Also consistent with previous investigations, binding affinities were unaffected by buprenorphine treatment under conditions in which brains were harvested

Acknowledgements

This work was supported by NIH grants DA05412 (CJC), HL5T32 H207050 (LMB) and NRCIA DA05766 (LMB). We thank Matt Mabery for assistance in densitometric analysis.

References (44)

  • F. Sehba et al.

    In vivo homologous regulation of mu-opioid receptor gene expression in the mouse

    Eur. J. Pharmacol.

    (1997)
  • A. Tempel et al.

    Antagonist-induced opiate receptor upregulation in cultures of fetal mouse spinal cord-ganglion explants

    Dev. Brain Res.

    (1986)
  • G.K.L. Tiong et al.

    Effects of exposure in utero to methadone and buprenorphine on enkephalin levels in the developing rat brain

    Neurosci. Lett.

    (1988)
  • E.A. Warner et al.

    Pharmacotherapy for opioid and cocaine abuse

    Med. Clin. N. Am.

    (1997)
  • I.S. Zagon et al.

    Development of motor activity in young rats following methadone exposure

    Pharmacol. Biochem. Behav.

    (1979)
  • X.-Z. Zhu et al.

    Comparison of the butyrate effects on neurotransmitter receptors in neurohybrids NG108-15 and NCB-20 cells

    Life Sci.

    (1987)
  • M.M. Belcheva et al.

    Buprenorphine differentially alters opioid receptor adaptation in rat brain regions

    J. Pharmacol. Exp. Ther.

    (1996)
  • M.M. Belcheva et al.

    Agonist-induced desensitization and down-regulation of δ opioid receptors alter the levels of their 125I-β-endorphin cross-linked products in subcellular fractions from NG108-15 cells

    Biochemistry

    (1996)
  • M.M. Belcheva et al.

    Differential down- and up-regulation of rat brain opioid receptor types and subtypes by buprenorphine

    Mol. Pharmacol.

    (1993)
  • M. Belcheva et al.

    Up-regulation of δ opioid receptors in neuroblastoma hybrid cells; evidence for differences in the mechanisms of action of sodium butyrate and naltrexone

    J. Pharmacol. Exp. Ther.

    (1991)
  • W.K. Bickel et al.

    A clinical trial of buprenorphine: comparison with methadone in the detoxification of heroin addicts

    Clin. Pharmacol. Ther.

    (1988)
  • L.M. Bohn et al.

    Evidence for κ and μ opioid receptor expression in c6 glioma cells

    J. Neurochem.

    (1998)
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