Brain Research 879 2000 17–22 www.elsevier.com locate bres
Research report
A comparison of morphine analgesic tolerance in male and female mice
a,b,c , c
b,c
Benjamin Kest , Christina Palmese , Eileen Hopkins
a
Department of Psychology 4S-223, The College of Staten Island City University of New York, 2800 Victory Blvd., Staten Island, NY 10314, USA
b
CSI IBR Center for Developmental Neuroscience , Staten Island, NY 10314, USA
c
Neuropsychology Doctoral Subprogram , Queens College City University of New York, Flushing, NY 11367, USA
Accepted 5 July 2000
Abstract
Studies comparing morphine tolerance in males and females are rare, and all studies to date have utilized the rat. To generalize from findings with rats morphine tolerance was investigated in male and female mice using the tail-withdrawal test. Three and 7 days of
systemic morphine injections produced significant but unequal rightward shifts in the morphine dose–response curve such that females displayed greater increases in analgesic ED
values when compared to males. In a separate experiment, males and females displayed
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similar reductions in morphine analgesic sensitivity when MPE maximum possible effect and total area under the curve were compared after 3 days of morphine. Differences in initial morphine sensitivity between sexes were not observed in either study. The data
demonstrate that, in contrast to rats, female mice undergo greater reductions in morphine analgesia relative to males following chronic morphine, but this sex difference may depend on the method of assessing analgesia. Furthermore, the duration and or cumulative dose of
morphine treatment does not affect the expression of sex differences in morphine tolerance.
2000 Elsevier Science B.V. All rights
reserved.
Theme : Sensory systems
Topic : Pain modulation: pharmacology
Keywords : Morphine; Tolerance; Sex differences; Analgesia
1. Introduction morphine administration is analgesic tolerance, or loss in
the relative analgesic potency of morphine. Tolerance It is becoming increasingly appreciated that sex may
results from a myriad of complex molecular and biochemi- determine opioid analgesic sensitivity in both human and
cal changes whose relative contributions are as yet not rodent populations see reviews [5,28]. With regard to
clearly understood [10,14,16,34]. The relative participation morphine, males typically display greater analgesic sen-
of any or several of these variables in tolerance may sitivity than females across several nociceptive assays
depend on several interacting methodological factors such following systemic administration in both rats and mice
as dose and dosing schedules, route of administration, and [2,7–9,19,21–24]. A strong case that sex differences in
the nociceptive response under study [3,15,35]. It is morphine analgesia may be mediated by differential central
therefore not surprising that differences in initial sensitivity nervous system CNS mechanisms can be made based on
to morphine in rats and mice are often not predictive of the the observations that males of both species display greater
magnitude in tolerance subsequent to chronic morphine analgesic effect than females following central administra-
administration [17,18,30,32,36,37]. Thus, despite apparent tion [6,25,29,31].
sex differences in morphine analgesia, it is not possible to One of the most noticeable consequences of repeated
predict whether male and female rodents will undergo similar changes in morphine analgesic sensitivity following
chronic administration. Badillo-Martinez [3] reported a
Corresponding author. Tel.: 11-718-982-4070; fax: 11-718-982-
greater attenuation of morphine analgesia on the hot-plate
3794. E-mail address
: kestpostbox.csi.cuny.edu B. Kest.
in male relative to female rats treated daily with morphine
0006-8993 00 – see front matter
2000 Elsevier Science B.V. All rights reserved. P I I : S 0 0 0 6 - 8 9 9 3 0 0 0 2 6 8 5 - 8
18 B
over 14 days. A similar pattern of greater tolerance in Abuse Rockville, MD, USA, and was dissolved in 0.9
males relative to females on the hot-plate test was ob- physiological saline. All morphine and saline control
served after twice daily or once weekly morphine in- injections were delivered via subcutaneous s.c. injection
jections as well [11]. Studies using the tail-flick test in a volume of 10 ml kg.
however present inconsistent findings. Whereas repeated administration of morphine at weekly intervals for 3 weeks
2.2. Tail-withdrawal assay resulted in a greater loss in total morphine analgesia area
under the curve; AUC in males relative to females [11], Mice were assessed for nociceptive sensitivity on the
no significant sex differences in morphine ED estimates
498C tail-withdrawal test. In this assay of acute, thermal
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were found following twice daily morphine injections for 7 nociception, the mouse is gently restrained and the distal
days in either of the two strains of rats tested [20]. These half of the tail is immersed in water maintained at
conflicting findings may be attributable to differences in 49.060.28C by an immersion circulator pump Fisher
methodology between the two studies. Although the identi- Isotemp Model 71. Latency to reflexive withdrawal of the
cal nociceptive assay was employed, they differed in the tail was measured twice by an experimenter to the nearest
frequency weekly vs. twice daily injections and duration 0.1 s, with each determination separated by 20 s. The two
3 weeks vs. 7 days of morphine injections, as well as in determinations were averaged to reflect each animal’s
the method of quantifying analgesia AUC vs. ED mean withdrawal latency. The tail-withdrawal test was
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estimates. chosen because of its stability even after repeated exposure
The present study had three aims. First, we wanted to to this noxious water temperature [12]. A cut-off latency of
assess whether sex differences in morphine tolerance are 15 s was employed to prevent the possibility of tissue
also observed in mice. Although it is always useful to other damage, and was thus used to calculate MPE see
investigators when findings from one species can be below.
generalized to others, the increasing use of inbred and transgenic mouse models in genetic approaches to the
2.3. Tolerance induction problems of chronic opioid use makes our understanding
of sex differences in this species particularly vital. Second, Mice in the morphine treatment groups were injected
we attempted to determine whether sex differences in three times daily tid for either 3 10, 20, and 40 mg kg
tolerance depend on duration of morphine exposure and or on Days 1, 2, and 3, respectively or 7 10, 20, 40, 40, 80,
cumulative dose administered by comparing male and 80, and 100 mg kg, respectively days. Mice in saline
female for analgesia after both 3 and 7 days of injections control groups received an equal number of saline in-
using a single analgesic measure ED values, nocicep-
jections but were not tested on Day 1.
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tive assay tail-withdrawal, and repeated injection three times daily paradigm. Third, we assessed whether sex
2.4. Dose–response studies differences in tolerance to morphine depends on the
method of quantifying analgesia by comparing dose–re- To reduce the number of mice, ED
values were
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sponse data i.e., ED estimates with time–response data
derived from cumulative dose–response curves as previ-
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peak analgesia and AUC in both sexes following identi- ously described [26]. Briefly, mice were injected with a 1.0
cal morphine administration paradigms. mg kg dose of morphine and received increasing doses
|0.25 log units until each became analgesic. Analgesia was operationally defined as a doubling of each subject’s
2. Materials and methods mean baseline withdrawal latency on consecutive determi-
