Elsevier

Behavioural Brain Research

Volume 177, Issue 1, 12 February 2007, Pages 126-133
Behavioural Brain Research

Research report
PAG mu opioid receptor activation underlies sex differences in morphine antinociception

https://doi.org/10.1016/j.bbr.2006.10.028Get rights and content

Abstract

Given the findings that (1) systemic opioid antinociception varies by estrous stage in females and (2) the magnitude of sex differences in opioid antinociception is negatively correlated with opioid agonist efficacy, we hypothesized that sex differences in the function of the descending pain modulatory system are likely influenced by estrous stage in females and by the number of available opioid receptors therein. The present study tested these hypotheses by (1) comparing antinociception produced by morphine microinjection to the ventral periaqueductal gray (vPAG) in females at different stages of the estrous cycle and (2) examining systemic morphine antinociception in males versus females under conditions of reduced vPAG mu opioid receptor availability. When estrous stage of females was not controlled for (Experiment 1), there was no significant sex difference in tail withdrawal antinociception following morphine microinjection (0.3ā€“10Ā Ī¼g), although morphine was more potent in males than females in producing immobility. Experiment 2 showed that intra-vPAG morphine produced less antinociception and immobility in estrus than in diestrus females; that is, only estrus femalesā€™ response to morphine was lower than that of males. Experiment 3 showed that microinjection of the irreversible mu opioid antagonist Ī²-funaltrexamine (Ī²-FNA) into the vPAG shifted the systemic morphine doseā€“effect curve farther to the right in females than in males. That is, a reduction in available vPAG mu opioid receptors had a greater impact on opioid antinociception in females than in males, suggesting that females have fewer vPAG mu opioid receptors than males. Overall, these data suggest that ovarian hormones and PAG mu opioid receptor density contribute to sex differences in antinociception produced by morphine.

Introduction

Sex differences in opioid antinociception have been observed in mice [7], [38], rats [2], [9], [10], [11], monkeys [40] and humans [19], [20], [21]. In animal models, mu opioid agonists have been shown to be more potent or efficacious in males than in females [2], [10], [11], [38], although this is not always the case [e.g., 38]. One possible explanation for sex differences in opioid antinociception is sex differences in the structure or function of the descending pain modulatory system, including the midbrain periaqueductal gray (PAG).

Opioid administration into the PAG in the rat [24], [25], [31], [34], [53] and cat [41], as well as electrical stimulation of the PAG in humans [23] produces antinociception. This antinociception can be attenuated by concurrent administration of an opioid antagonist such as naloxone [1], [17], [26], indicating that antinociception evoked from the PAG is opioid receptor-mediated. Moreover, the PAG appears to be a critical structure for the antinociception produced by systemic opioids because selective inactivation of the PAG attenuates this antinociception [33], [44], [57]. It has been suggested that the caudal ventral PAG (vPAG, including the dorsal raphe nucleus) is the region most sensitive to mu opioid receptor-mediated antinociception [56].

The prominent role of the PAG in opioid antinociception suggests that sex differences in its structure or function contribute to sex differences in antinociception produced by mu agonists. In fact, sex differences in the anatomical and functional organization of the descending pain modulatory pathway have been reported recently. Female rats have more PAG to rostral ventromedial medulla (RVM) output neurons than do males, and persistent pain activates more output neurons in males than in females [35]. Furthermore, mu opioid receptor expression in vPAG is two-fold higher in male than in female rats [55]. Sex differences in antinociception have been observed after opioid administration to the vPAG [31], [53], [55] and RVM [6]; however, whereas several studies have shown that supraspinal administration of morphine or DAMGO produces greater antinociception in males [6], [28], [29], [31], other studies have reported greater antinociception in females [53] or no sex difference [27], [29]. Disagreement among such studies may result from differences in the type/intensity of the nociceptive stimulus, efficacy of the mu agonist, use of awake versus anesthetized animals, estrous phase of females, and genotype (strain/vendor) of the rodent [11], [12], [38]. In regard to estrous phase, previous studies of the antinociceptive effects of systemically administered mu agonists show that female rats in estrus typically are less sensitive than females in other stages [15], [50], [51]. Furthermore, Bodnar and colleagues demonstrated in 1989 that morphine administered into the lateral ventricles produced less antinoception in female rats in estrus than those in proestrus or diestrus [28]; subsequent studies conducted in the same laboratory consistently reported less antinociception in estrus females compared to males when mu agonists were administered to the vPAG [30], [31], [32], suggesting that the estrous phase effect occurs within vPAG.

The objective of the present study was to determine whether sex differences in morphine antinociception mediated by the caudal vPAG depend on estrous stage in females and mu opioid receptor availability. To test the hypothesis that sex differences in morphine antinociception depend on estrous stage in females, we first compared males and females without regard to estrous stage (Experiment 1), and then directly compared the behavioral responses to intra-vPAG morphine in estrus versus diestrus females (Experiment 2). Given that microinjection of morphine into the vPAG produces both antinociception and immobility [39], both of these behaviors were assessed. The lower density of mu opioid receptors in female compared to male rats suggests that reducing mu opioid receptor availability will produce a greater inhibition of opioid antinociception in female than male rats. This hypothesis was tested by measuring systemic morphine antinociception following microinjection of the irreversible mu receptor-selective antagonist Ī²-FNA into the vPAG (Experiment 3).

Section snippets

Subjects

Age-matched 3- to 6-month old male (375ā€“500Ā g) and female (240ā€“300Ā g) Sprague-Dawley rats (bred in-house from Taconic Farms stock, Germantown, NY) were used. Female rats were randomly selected rather than pre-screened for estrous cycle length or regularity. Rats were housed in same-sex pairs, males and females in separate rooms maintained at 21.5Ā Ā±Ā 1.0Ā Ā°C on a 12:12Ā h light:dark cycle (lights on at 07:00Ā h). Except during testing, rats received food and water ad libitum. Testing was conducted between

Morphine's antinociceptive effects

Antinociceptive effects were examined first as a function of vlPAG or DRN (or other) cannula placement. Fig. 1 shows individual placements, mapped according to magnitude of effect. There were no clear differences in antinociception between rats with vlPAG versus DRN cannula placements; thus, data from these two areas were combined for further analysis of sex differences.

Analysis of combined data showed that there were no significant sex differences in control (saline) tail withdrawal latencies:

Discussion

The present study examined sex differences in morphine-induced antinociception and immobility mediated by the vPAG. In Experiment 1, femalesā€™ estrous stage was not selected (and most females were in diestrus-1 or -2 at the time of testing), and there was no significant sex difference in tail withdrawal antinociception after intra-vPAG microinjection of morphine. Morphine was, however, more potent in males than in females in producing immobility. Experiment 2 showed that morphine's behavioral

Acknowledgements

Supported by NIDA grant DA 10284 (R.M.C.) and minority supplement to DA 10284 (S.A.B.).

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