Chronic opioid exposure differentially modulates oxycodone self-administration in male and female rats

Abstract

Withdrawal from opioid painkillers can produce short-lived physical symptoms and protracted psychological symptoms including anxiety and depressive-like states that often lead to opioid misuse and opioid use disorder (OUD). Studies testing the hypothesis that opioid withdrawal potentiates the reinforcing effects of opioid self-administration (SA) are largely inconclusive and have focused on males. Although some clinical evidence indicates that women are more likely than men to misuse opioids to self-medicate, preclinical studies in both sexes are lacking. Based on clinical reports, we hypothesized that withdrawal from escalating-dose morphine injections that approximates a prescription painkiller regimen would lead to increased oxycodone SA to a greater extent in female compared to male rats. After escalating-dose morphine (5-30 mg/kg or vehicle, twice/day for 12 days), rats underwent a 2-week abstinence period during which withdrawal signs were measured. The impact of this treatment was assessed on oxycodone SA acquisition, maintenance, dose response, and progressive ratio responding, with additional analyses to compare sexes. We found that both sexes expressed somatic withdrawal, whereas only males demonstrated hyperalgesia in the warm water tail flick assay. During SA acquisition, males with prior morphine exposure took significantly more oxycodone than females. Finally, females with prior morphine exposure demonstrated the lowest motivation to SA oxycodone in the progressive ratio test. Contrary to our initial hypothesis, our findings suggest that prior opioid exposure increases vulnerability to initiate misuse more in males and decreases the reinforcing efficacy of oxycodone in females.

Keywords: hyperalgesia; intravenous self-administration; opioid use disorder; progressive ratio; sex differences; somatic withdrawal.

FIGURE 1

Experimental design of the study. A, Timeline of behaviors and jugular catheter surgeries (“catheter”, d22). Days are numbered sequentially starting with the first day of morphine injections and ending on the last day of PR. B, Escalating dose morphine regimen: 2 injections per day (10:00 and 16:00) for 12 days (12d), with doses increasing from 5 to 30 mg/kg/injection, IP. AS, acoustic startle; DR, dose response; inj, injection; Morph, morphine; Oxy, oxycodone; Prep, Preparation; Prog Ratio, progressive ratio; SA, self‐administration; SW, somatic withdrawal; TF, tail flick; Veh, vehicle

FIGURE 2

Body weight in male and female rats over the course of morphine injections, withdrawal, and oxycodone SA. A, Vehicle‐treated, but not morphine‐treated, male rats gain weight during the 12d morphine/vehicle injection regimen. Female rats, regardless of treatment, gain a nominal amount of weight. Inset (dashed box): A daily pattern of weight changes emerges at the end of the 12d morphine injection regimen. Morphine‐treated male and female rats gain a small amount of weight during the day (from AM to PM measurements), whereas vehicle‐treated rats tend to lose a small amount of weight during the day. N = 25–33 per group. Each day shows body weights prior to the two injections: White background is AM injection, grey background is PM injection. B, Body weights remain significantly decreased in morphine‐treated male rats on withdrawal days 3, 4, and 5, although weights tend to be lower in morphine‐withdrawn males throughout the withdrawal period. No change in body weights is observed in female rats during withdrawal. N = 25–33 per group. C, Body weights of male rats previously exposed to morphine remain qualitatively lower throughout the oxycodone SA tests, but oxycodone SA does not in itself alter body weights in either males or females. N = 15–18 per group. *p < 0.05, **p < 0.01 comparing within‐sex body weights on specific days or between two timepoints indicated by brackets; ^{^^} p < 0.01 main effect of sex

FIGURE 3

Spontaneous morphine withdrawal and protracted withdrawal effects on tail flick (TF) latency and acoustic startle (AS) amplitude. A, from left to right, panels represent the observed number of grooming, wet dog shakes, and ptosis bouts observed in the 15‐min recording sessions, with morphine‐withdrawn rats showing more of each withdrawal sign. N = 25–32 per group. B, Latency to withdraw tail from warm water bath (TF assay) measured on withdrawal day 5. Male, but not female, rats previously exposed to chronic morphine show a decrease in TF latency N = 19–21 per group. C, Amplitude of acoustic startle response measured on withdrawal day 7. Regardless of treatment, female rats showed lower startle amplitudes because of their lower body weights. N = 25–30 rats per group. *p < 0.05, **p < 0.01 compared to vehicle treated of the same sex or groups indicated with brackets; ^{^} p < 0.05, ^{^^} p < 0.01 main effect of sex; ^## p < 0.01 main effect of treatment

FIGURE 4

Effects of previous morphine experience and withdrawal on oxycodone self‐administration (SA; 0.06 mg/kg/infusion, FR1 schedule of reinforcement). (A–C) Prior morphine experience and withdrawal potentiate acquisition of oxycodone SA in male, but not female, rats. Total number of oxycodone infusions (A), active lever presses (B), and inactive lever presses (C) in 1‐h daily sessions. (D–F) Prior morphine experience and withdrawal reduce the amount of oxycodone SA in female, but not male, rats during the last 4 days of the 2‐h/days maintenance phase. Total number of oxycodone infusions (D), active lever presses (E), and inactive lever presses (F) in 2‐h daily sessions. *p < 0.05 comparing female morphine to female vehicle; ^### p < 0.001 main effect of time; ^{^} p < 0.05 main effect of treatment. Insets: For each graph (A–F), data are also expressed as the total of infusions or lever presses over either the acquisition or maintenance phases of SA. *p < 0.05. **p < 0.01 comparing groups indicated with brackets; ^## p < 0.01 main effect of sex. Data are expressed as mean ± SEM; N = 20–26/group

FIGURE 5

Effects of previous morphine experience and withdrawal on oxycodone dose–response in male and female rats. A, In preparation for the dose response test, rats went through a small extinction procedure in which saline was substituted for oxycodone for 4 days until the number of infusions (saline d3, 4) was significantly less than those on saline d1 (^{^^} p < 0.01). Subsequently, rats self‐administered either oxycodone (0.01 mg/kg/infusion) or saline on alternate days for a total of 6 days to demonstrate that rats could behaviorally differentiate between saline and oxycodone (^# p < 0.05 comparing infusions on oxycodone days to saline d3, 4). B, In the dose response test, rats self‐administered one dose/day of oxycodone (0.003–0.3 mg/kg/infusions; one dose/trial/day), presented in random order. Data are presented as the total number of oxycodone infusions during each days' 2‐h dose–response test. ^{^^} p < 0.01 main effect of dose. N = 7–14 rats/group

FIGURE 6

Effects of previous morphine experience and withdrawal on motivation to SA oxycodone in the progressive ratio (PR) test. Data are the average (+SEM) of 2d of 2 h/days PR testing. A, Total number of infusions, B, active lever presses, and C, break point, which is the last ratio completed to earn a reinforcer. N = 7–12 rats/group. *p < 0.05 compared to Veh of the same sex. D, Pearson r correlation was conducted on the sum of infusions on maintenance days 8–10 compared to the PR break point. A significant positive correlation was found (r = 0.758) such that the more oxycodone self‐administered at the end of the maintenance phase of SA, the higher the PR break point. Data from females previously exposed to morphine and withdrawal are represented with grey circles, whereas all other groups are represented with dark squares. Linear regression analysis was also conducted: R ² = 0.574, p < 0.001