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 acid_B agonist baclofen were not affected in these neurons. Acute desensitization preferentially affected the voltage-sensitive component of μ-opioid and γ-aminobutyric acid_B responses. Facilitation of both the DAMGO- and baclofen-inhibited current by a strong depolarizing prepulse was significantly attenuated in acutely desensitized neurons. Because G_βγ-subunits mediate neurotransmitter-induced changes in channel voltage-dependent properties, these data suggest an altered interaction of the G_βγ-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.