The effects of diazepam dependence and withdrawal on morphine-induced antinociception and changes in locomotion in male and female rats

Sex and estrous cycle influence diazepam effects on anxiety and memory: Possible role of progesterone

Studies with rodents and humans show the relationship between female sex hormones and cognitive/emotional tasks. However, despite the greater incidence of anxiety disorders in women, the data are still inconclusive regarding the mechanisms related to this phenomenon. We evaluated the effects of a classical anxiolytic/amnestic drug (diazepam; DZP) on female (at different estrous cycle phases) and male rats tested in the plus-maze discriminative avoidance task (PMDAT), that allows the concomitant evaluation of memory and anxiety-like behavior. Further, in order to investigate the role of progesterone and its metabolites in the effects of DZP in the PMDAT, female rats were pre-treated with the progesterone receptor antagonist mifepristone or the 5-alpha-reductase inhibitor finasteride. The main findings were: (1) DZP caused memory impairment and anxiolysis in both sexes, but only the highest dose induced the anxiolytic effect in females; (2) females in proestrus did not present the amnestic and anxiolytic effects of DZP (at 2.0 and 4.0mg/kg, respectively) and (3) the co-administration of mifepristone reestablished both amnestic and anxiolytic effects of DZP, while finasteride reinstated the amnestic effect in proestrus female rats. These results suggest that changes in the endogenous levels of progesterone and its metabolites are important in the modulation of emotional/cognitive behavior in female rats. Based on the influence on different aspects of DZP action, the mechanisms related to this modulation are probably linked to GABAergic transmission, but this point remains to be investigated. Further, the variation in therapeutic and adverse effects of DZP depending on sex and hormonal state is of great relevance considering the higher prevalence of anxiety disorders in women.

In vivo and in vitro comparison of female and male nociceptors

While it is generally accepted that women have lower pain thresholds for diverse forms of noxious stimuli, the mechanistic basis for this sexual dimorphism in nociceptive pain remains to be elucidated. We confirmed, in the rat, that females have lower cutaneous mechanical nociceptive thresholds and established a similar sexual dimorphism in muscle. To determine if a peripheral mechanism underlies this sexual dimorphism in pain threshold, we compared biophysical properties of cultured dorsal root ganglion (DRG) neurons that innervated the gastrocnemius muscle in female and male rats. DRG neurons from female rats, which innervated the gastrocnemius muscle, had a more hyperpolarized resting membrane potential. To determine if this was associated with a higher mechanical nociceptive threshold, in contradiction to our working hypothesis, we compared the function, in vivo, of nociceptive afferents innervating the gastrocnemius muscle in male and female rats. C-fiber nociceptors innervating muscle in female rats had higher mechanical thresholds than those in males. Other response characteristics of these nociceptors were not significantly different. Thus, both in vitro and in vivo electrophysiology experiments support the idea that lower mechanical nociceptive threshold in females may be due to sexual dimorphism in central nervous system mechanisms, a difference large enough to overcome an opposing difference in peripheral pain mechanisms.

PERSPECTIVE

This article unifies in vivo and in vitro electrophysiology with behavioral data examining the differences in mechanical nociceptive threshold between male and female rats. The data provide a novel perspective on the peripheral and behavioral outcomes of noxious mechanical stimulation.

The Effects of Flumazenil on the Antinociceptive Actions of Morphine in Rats

The 8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazol[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester (Flumazenil)-morphine interaction on analgesia (acute pain model, tail-flick test) was tested after intraperitoneal (IP) and intrathecal (IT) routes of administration in female rats. Analgesia was enhanced by the concurrent administration of Flumazenil with morphine (IP), in a dose-related way. Flumazenil alone (IP) did not produce analgesia. In contrast, morphine analgesia was not enhanced by Flumazenil by the IT route. These data demonstrate that Flumazenil enhances morphine-mediated antinociception by mechanisms that are likely to involve benzodiazepine receptors at sites other than the spinal cord.

Sex, gender, and pain: an overview of a complex field

Traditionally, biomedical research in the field of pain has been conducted with male animals and subjects. Over the past 20-30 yr, it has been increasingly recognized that this narrow approach has missed an important variable: sex. An ever-increasing number of studies have established sex differences in response to pain and analgesics. These studies have demonstrated that the differences between the sexes appear to have a biological and psychological basis. We will provide brief review of the epidemiology, rodent, and human experimental findings. The controversies and widespread disagreement in the literature highlight the need for a progressive approach to the questions involving collaborative efforts between those trained in the basic and clinical biomedical sciences and those in the epidemiological and social sciences. In order for patients suffering from acute and/or chronic pain to benefit from this work, the approach has to involve the use or development of clinically relevant models of nociception or pain to answer the basic, but complex, question. The present state of the literature allows no translation of the work to our clinical decision-making.

Morphine sex-dependently induced place conditioning in adult Wistar rats

The present study was conducted to investigate the potential sex-differences in morphine-induced conditioned place preference. A 3-day unbiased conditioning procedure was used to establish conditioned place preference in adult male and female Wistar rats (weighing 200-250 g). The effect of morphine on locomotor activity of subjects was also studied. Naloxone (0.5-2 mg/kg, i.p.), a selective antagonist of mu-opioid receptor or sulpiride (0.5-2 mg/kg, s.c.), a selective antagonist of dopamine D(2) receptor was administered, during conditioning, to indicate the receptor-mediated mechanisms governing upon possible sex-differences to the opioid response. Results show that morphine (0.5-10 mg/kg, s.c.) differently produced a significant place preference in female and male Wistar rats. Although, the opioid maximum response in both sexes was observed at 7.5 mg/kg, but, it was found that female rats acquired conditioned place preference at a lower dose (0.5 mg/kg, s.c.) of morphine compared to male rats. Moreover, the increase in morphine-induced response at higher doses (5-10 mg/kg, s.c.) was more pronounced in females than the males, indicating that female Wistar rats are more sensitive to the place conditioning induced by morphine. Also, the females were more sensitive to locomotor activation induced by morphine at least at one dose (7.5 mg/kg). Animals' body-weight at 10 mg/kg of opioid was increased, the effect that was not dependent to sex. The results also demonstrate that naloxone (1 and 2 mg/kg, i.p.) induced a significant place preference in two sexes with no significant effect on animals' locomotor activity. The antagonist in males but not in females showed a significant effect on animals' body-weight. Naloxone (0.5-2 mg/kg, i.p.) prior-administration to morphine, during conditioning, attenuated the opioid response in two sexes. The attenuation of the morphine response was more pronounced in males than the other sex at the higher dose (2 mg/kg) of the antagonist. In addition, the preadministration of naloxone, during morphine conditioning, both attenuated the drug-induced hyperactivity in females and decreased the animals' body-weight, albeit more effectively in females than the males. Sulpiride injections (1 and 2 mg/kg s.c.), during the conditioning period, induced a significant aversion in males but not in females with no significant effect either on locomotor activity or body-weight in both sexes. When sulpiride (0.5-2 mg/kg, s.c.), during conditioning, was morphine pre-injected, the antagonist at higher doses significantly attenuated the opioid response in males, reflecting the involvement of dopamine D(2) receptor in sex-dependent morphine-conditioned place preference. Prior-injections of sulpiride to morphine produced a significant effect on locomotor activity of females. The effect of the antagonist preinjections on body-weight was also observed in males. Present results indicate sex-differences both in reinforcing and locomotor activity effects of morphine in Wistar rats.

The four-plates test: anxiolytic or analgesic paradigm?

The four-plates test (FPT) is an animal model of anxiety in which the exploration of the novel surroundings is suppressed by the delivery of a mild electric foot shock. The anti-nociceptive system has been reported to be activated by a variety of stressful stimuli such as footshock. The present study was thus designed to compare effects of drugs in the FPT and in the hot-plate test (an animal model of pain), in order to disambiguate the drug-induced anti-punishment effects obtained in the FPT from alterations in pain sensitivity. Various compounds, known to be implicated in anxiety states as well as nociception, have been studied. Although morphine induced a strong anti-nociceptive effect, it did not modify the number of shocks received in the FPT. Alprazolam and diazepam induced an anxiolytic-like effect in the FPT, at doses that did not induce any effect in the hot-plate test. The antidepressants previously reported anxiogenic (desipramine, maprotiline) in the FPT were found to be analgesic at the same doses. Milnacipran, venlafaxine and paroxetine did not modify the pain threshold, whereas they have previously been shown to induce anxiolytic-like effects in the FPT. The dopaminergic antidepressant agent nomifensine was without effect on both tests. Our results suggest that the reported drug-induced anti-punishment effects in the FPT are not related to modifications of pain threshold but to a pure anxiolytic-like effect.

Morphine tolerance in male and female rats

Several studies indicate greater sensitivity to morphine (MOR) analgesia in male compared to female rats under the acute dosing condition. The present study investigated whether the same sex difference in sensitivity persists in MOR-tolerant rats. MOR was administered chronically (7 mg/kg twice daily) until tolerance developed in each rat. Tolerant rats were treated randomly with higher graded doses of MOR (10-25 mg/kg). Analgesia (tail-flick test) and spontaneous motor activity (total locomotion) were measured. The present data confirmed previous studies showing a greater sensitivity to acute MOR in male than in female rats. However, the sex differences seen in MOR sensitivity were abolished in tolerant rats. The rate of acquisition of tolerance was similar in male and female rats. The analgesic response was not affected by motor depression.

Modification of morphine analgesia and tolerance by flumazenil in male and female rats

This study assessed the effect of the central benzodiazepine receptor antagonist, 8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester (flumazenil), on morphine-induced analgesia, locomotor effects, and development of tolerance in rats. The thermally evoked pain (tail flick) response was determined after acute and chronic intraperitoneal (i.p.) administration of morphine and flumazenil, alone and in combination. In acute studies, flumazenil induced weak analgesia unrelated to dose and sex, whereas morphine-induced analgesia was dependent on both dose and sex (male>female). Flumazenil dose-dependently enhanced the analgesic effect of morphine in female but not in male rats. Isobolographic analysis suggested synergism between flumazenil and morphine in female rats, but antagonism in male rats. Flumazenil-induced locomotor changes (alone and with morphine) were related to sex but not dose. Chronic coadministration of flumazenil with morphine enhanced analgesia and attenuated tolerance development in female rats. The findings suggest a possible role for flumazenil as an adjunct with opioids in acute and chronic pain therapy.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).