Abstract
The cellular correlates of desensitization or tolerance are poorly understood. To address this, we studied acute and long-term μ-opioid desensitization, with respect to Ca2+ currents, in cultured rat dorsal root ganglion (DRG) neurons. Exposure of DRG neurons to the μ-agonist [d-Ala2,N-MePhe4,Gly-ol5]-enkephalin (DAMGO; 3 μM) reduced whole-cell currents ∼35%, but with continued agonist application, 52% of the response was lost over 10 to 12 min. In contrast, exposure of DRG neurons to DAMGO for 24 h resulted in a nearly complete loss of Ca2+ channel regulation after washing and re-exposure to DAMGO. Responses to the γ-aminobutyric acidB agonist baclofen were not affected in these neurons. Acute desensitization preferentially affected the voltage-sensitive component of μ-opioid and γ-aminobutyric acidB responses. Facilitation of both the DAMGO- and baclofen-inhibited current by a strong depolarizing prepulse was significantly attenuated in acutely desensitized neurons. Because Gβγ-subunits mediate neurotransmitter-induced changes in channel voltage-dependent properties, these data suggest an altered interaction of the Gβγ-subunit with the Ca2+channel. Block of N-type Ca2+ channels with ω-conotoxin GVIA revealed a component of the opioid response that did not desensitize over 10 min. We conclude that acute and long-term μ-opioid desensitization in DRG neurons occurs by different mechanisms. Acute desensitization is heterologous and functionally compartmentalized: the pathway targeting non-N-type channels is relatively resistant to the early effects of continuous agonist exposure; the pathway targeting N-type channels in a largely voltage-insensitive manner is partially desensitized; and the pathway targeting N-type channels in a largely voltage-sensitive manner is completely desensitized.
Footnotes
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Send reprint requests to: Gary Samoriski, Ph.D., University of Rochester School of Medicine & Dentistry, Center for Aging and Developmental Biology, Box 645, 601 Elmwood Ave., Rochester, NY 14642. E-mail: gary_samoriski{at}urmc.rochester.edu
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↵1 This study was supported by Grants DA07232 (to G.M.S.) and DA10514 (to R.A.G.) from the National Institutes of Health. Preliminary results of this study have been presented at the 27th (1997 Oct 25–30, New Orleans, LA) and 28th (1998 Nov 7–12, Los Angeles, CA) annual meetings of the Society for Neuroscience.
- Abbreviations:
- PKC
- protein kinase C
- DAMGO
- [d-Ala2,N-MePhe4,Gly-ol5]-enkephalin
- DRG
- dorsal root ganglion
- Vh
- holding potential
- CTOP
- d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2
- GABA
- γ-aminobutyric acid
- IR
- inhibition ratio
- Received December 28, 1999.
- Accepted May 3, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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