Date of Award

8-21-2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Psychology

First Advisor

Anne Z. Murphy, Ph.D. - Chair

Second Advisor

Walter Wilczynski, Ph.D.

Third Advisor

Michael M. Morgan, Ph.D.

Fourth Advisor

Kyle J. Frantz, Ph.D.

Fifth Advisor

Paul S. Katz, Ph.D.

Abstract

Morphine is the most widely prescribed opiate for alleviation of persistent pain; however, it is becoming increasingly clear that morphine is less potent in women compared to men. Morphine primarily binds mu opioid receptors, which are densely localized in the midbrain periaqueductal gray (PAG). Anatomical and physiological studies conducted in the 1960s identified the PAG, and its projections to the rostral ventromedial medulla (RVM) and spinal cord dorsal horn, as an essential neural circuit mediating opioid-based analgesia. Remarkably, the majority of studies since then were conducted in males with the implicit assumption that this circuit was the same in females; this is not the case. It is now well established that morphine produces greater analgesia in males compared to females in a wide range of vertebrates, however, the mechanism(s) driving this sex difference is not clear. Our recent studies indicate that two factors appear to be contributing to the sexually dimorphic effects of morphine. First, there are sex differences in the anatomy and physiology of the descending inhibitory pathway on which morphine acts to produce analgesia. Specifically, the projections from the PAG to the RVM are sexually dimorphic and activated to a greater degree by both inflammatory pain and systemic morphine in males. In the absence of pain, the PAG-RVM circuit is activated to a greater degree in males compared to females, while this activation steadily declines during the development of tolerance in males only. We also have evidence of a sexually dimorphic expression of mu opioid receptor within the PAG that appears to contribute to sex differences in morphine potency. Microinjection of morphine directly into the PAG produces significantly greater analgesia in males, indicating that the PAG is sufficient for eliciting this sexually dimorphic behavior. Furthermore, mu opioid receptor-expressing PAG neurons are necessary for eliciting a sexually dimorphic response to morphine as lesioning mu opioid receptor-expressing neurons attenuates analgesia in males only. Together, these data indicate that the PAG-RVM pathway and mu opioid receptor expression in the PAG is sexually dimorphic and provides a primary mechanism for sex differences in morphine potency.

DOI

https://doi.org/10.57709/1062333

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