Protein Kinase A Maintains Cellular Tolerance toMu Opioid Receptor Agonists in Hypothalamic Neurosecretory Cells with Chronic Morphine Treatment: Convergence on a Common Pathway with Estrogen in Modulating Mu Opioid Receptor/Effector Coupling1
Abstract
The present study examined protein kinase A (PKA) and protein kinase C (PKC) involvement in the maintenance of cellular tolerance tomu opioid receptor agonists resulting from chronic opiate exposure in neurosecretory cells of the hypothalamic arcuate nucleus (ARC). The possibility that the diminution of muopioid receptor/effector coupling produced by acute 17β-estradiol or chronic opiate exposures is mediated by a common kinase pathway also was investigated. Intracellular recordings were made in hypothalamic slices prepared from ovariectomized female guinea pigs. Themu opioid receptor agonistd-Ala2, N-Me-Phe4, Gly-ol5-enkephalin (DAMGO) produced dose-dependent hyperpolarizations of ARC neurons. Chronic morphine treatment for 4 days reduced DAMGO potency 2.5-fold with no change in the maximal response. This effect was mimicked by a 20-min bath application of the PKA activator cAMP, Sp-isomer, or the PKC activator phorbol-12,13-dibutyrate. A 30-min bath application of the broad-spectrum protein kinase inhibitor staurosporine completely abolished the reduced DAMGO potency seen in morphine-tolerant neurosecretory cells, including those immunopositive for gonadotropin-releasing hormone. The effect of staurosporine was mimicked by the PKA inhibitor cAMP, Rp-isomer, but not by the PKC inhibitor calphostin C. Finally, a 20-min bath application of 17β-estradiol did not further reduce DAMGO potency in morphine-tolerant ARC neurons. Therefore, increased PKA activity maintains cellular tolerance to mu opioid receptor agonists in ARC neurosecretory cells caused by chronic morphine treatment. Furthermore, acute 17β-estradiol and chronic opiate treatments attenuate mu opioid receptor-mediated responses via a common PKA pathway.
Footnotes
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Send reprint requests to: Edward J. Wagner, Ph.D., Dept. of Physiology & Pharmacology, L334, Oregon Health Sciences University, 3181 S.W. Sam Jackson Park Road, Portland, OR 97201.
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↵1 The experiments described in this study were supported by PHS Grants DA05158 and DA00192 (RSDA to M.J.K.).
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↵2 Supported by PHS training grants 5T32 DA07262 and 5T32 HD07133.
- Abbreviations:
- ANOVA
- analysis of variance
- ARC
- arcuate nucleus
- cAMP
- cyclic adenosine monophosphate
- DAMGO
- d-Ala2, N-Me-Phe4, Gly-ol5-enkephalin
- Δg
- increase in conductance
- ΔVmax
- maximum steady-state hyperpolarization
- E2
- 17β-estradiol
- FITC
- fluorescein isothiocyanate
- GnRH
- gonadotropin-releasing hormone
- HAP
- hyperpolarizing after potential
- LSD
- least significant difference
- PDBu
- phorbol-12,13-dibutyrate
- PKA
- protein kinase A
- PKC
- protein kinase C
- Rp-cAMP
- adenosine 3′,5′-cyclic monophosphothioate, Rp-isomer
- Sp-cAMP
- adenosine 3′,5′-cyclic monophosphothioate, Sp-isomer
- τ
- membrane time constant
- TH
- tyrosine hydroxylase
- TTX
- tetrodotoxin
- V/I
- voltage-current
- Vm
- membrane potential
- HEPES
- N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid
- aCSF
- artificial cerebrospinal fluid
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- Received October 17, 1997.
- Accepted February 13, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
