Abstract:
:Morphine remains widely used in clinical settings due to its potent analgesic properties. However, one of the gravest risks of all opioids is their ability to induce respiratory depression and subsequent brain hypoxia that can lead to coma and death. Due to these life-threatening effects, our goal was to examine the effects of intravenous morphine at a wide range of doses (0.1-6.4 mg/kg) on changes in brain oxygen levels in freely moving rats. We used oxygen sensors coupled with high-speed amperometry and conducted measurements in the nucleus accumbens (NAc) and subcutaneous (SC) space, the latter serving as a proxy for blood oxygen levels that depend on respiratory activity. We also examined the effects of morphine on NAc, muscle, and skin temperature. Morphine induced dose-dependent decreases in SC oxygen levels, suggesting respiratory depression, but differential effects on NAc oxygen: increases at low and moderate doses (0.1-1.6 mg/kg) and decreases at the highest dose tested (6.4 mg/kg). Morphine also increased brain temperature at low and moderate doses but induced a biphasic, down-up change at high doses. The oxygen increases appear to result from a neurovascular coupling mechanism via local vasodilation and enhanced oxygen entry into brain tissue to compensate for blood oxygen drops caused by modest respiratory depression. At high morphine doses, this adaptive mechanism is unable to compensate for the enhanced respiratory depression, resulting in brain hypoxia. Hence, morphine appears to be safe when used as an analgesic at clinically relevant doses but poses great risks at high doses, likely to be abused by drug users. NEW & NOTEWORTHY With the use of oxygen sensors coupled with amperometry, we show that morphine induces differential effects on brain oxygen levels, slightly increasing them at low doses and strongly decreasing them at high doses. In contrast, morphine dose dependently decreases oxygen levels in the SC space. Therefore, morphine engages opposing mechanisms affecting brain oxygen levels, enhancing them through neurovascular coupling at low, clinically relevant doses and decreasing them due to dramatic respiratory depression at high doses, likely to be abused.
journal_name
J Neurophysioljournal_title
Journal of neurophysiologyauthors
Solis E Jr,Afzal A,Kiyatkin EAdoi
10.1152/jn.00445.2018subject
Has Abstractpub_date
2018-11-01 00:00:00pages
2513-2521issue
5eissn
0022-3077issn
1522-1598journal_volume
120pub_type
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