The 5-HT3B subunit affects high-potency inhibition of 5-HT sub(3) receptors by morphine

BACKGROUND AND PURPOSE Morphine is an antagonist at 5-HT sub(3)A receptors. 5-HT sub(3) and opioid receptors are expressed in many of the same neuronal pathways where they modulate gut motility, pain and reinforcement. There is increasing interest in the 5-HT3B subunit, which confers altered pharmacology to 5-HT sub(3) receptors. We investigated the mechanisms of inhibition by morphine of 5-HT sub(3) receptors and the influence of the 5-HT3B subunit. EXPERIMENTAL APPROACH 5-HT-evoked currents were recorded from voltage-clamped HEK293 cells expressing human 5-HT3A subunits alone or in combination with 5-HT3B subunits. The affinity of morphine for the orthosteric site of 5-HT sub(3)A or 5-HT sub(3)AB receptors was assessed using radioligand binding with the antagonist [ super(3)H]GR65630. KEY RESULTS When pre-applied, morphine potently inhibited 5-HT-evoked currents mediated by 5-HT sub(3)A receptors. The 5-HT3B subunit reduced the potency of morphine fourfold and increased the rates of inhibition and recovery. Inhibition by pre-applied morphine was insurmountable by 5-HT, was voltage-independent and occurred through a site outside the second membrane-spanning domain. When applied simultaneously with 5-HT, morphine caused a lower potency, surmountable inhibition of 5-HT sub(3)A and 5-HT sub(3)AB receptors. Morphine also fully displaced [ super(3)H]GR65630 from 5-HT sub(3)A and 5-HT sub(3)AB receptors with similar potency. CONCLUSIONS AND IMPLICATIONS These findings suggest that morphine has two sites of action, a low-affinity, competitive site and a high-affinity, non-competitive site that is not available when the channel is activated. The affinity of morphine for the latter is reduced by the 5-HT3B subunit. Our results reveal that morphine causes a high-affinity, insurmountable and subunit-dependent inhibition of human 5-HT sub(3) receptors.