Inbred rat heredity and sex affect oral oxycodone self-administration and augmented intake in long sessions: correlations with anxiety and novelty-seeking

Abstract

Oxycodone abuse frequently begins with prescription oral oxycodone, yet vulnerability factors (e.g. sex, genetics) determining abuse are largely undefined. We evaluated genetic vulnerability in a rat model of oral oxycodone self-administration (SA): increasing oxycodone concentration/session (0.025-0.1mg/ml; 1-, 4-, and 16-h) followed by extinction and reinstatement. Active licks and oxycodone intake were greater in females than males during 4-h and 16-h sessions (p < 0.001). Both sexes increased intake between 4-h and 16-h sessions (p < 2e-16), but a subset of strains augmented intake at 16-h (p = 0.0005). Heritability (h2) of active licks during 4-h sessions at increasing oxycodone dose ranged from 0.30 to 0.53. Under a progressive ratio (PR) schedule, breakpoints were strain-dependent (p < 2e-16). Cued reinstatement was greater in females (p < 0.001). Naive rats were assessed using elevated plus maze (EPM), open field (OF), and novel object interaction (NOI) tests. We correlated these behaviors with 28 parameters of oxycodone SA. Anxiety-defining EPM traits were most associated with SA in both sexes, whereas OF and NOI traits were more associated with SA in males. Sex and heredity are major determinants of motivation to take and seek oxycodone; intake augments dramatically during extended access in specific strains; and anxiety correlates with multiple SA parameters across strains.

Fig 1. Protocol and schedule of operant oxycodone self-administration.

The first three sessions were conducted daily. Subsequent sessions were conducted on alternate days. The final scheduled FR5 session was the reinstatement of extinguished oxycodone seeking.

Fig 2. Number of licks during oxycodone SA.

Rats showed a strong preference for the active over inactive spout in females (A) and males (B) throughout the HRDP. (C) The number of licks on the active spout was greater in females (n =  157) than in males (n = 136) during both 4-h (p =  0.003) and 16-h sessions (p =  0.03) in the 23 strains where both sexes were studied. (D) The number of licks on the inactive spout was not different between sexes (p > 0.05) during either the 4-h or 16-h sessions.

Fig 3. Sex differences in oxycodone rewards and intake.

The mean oxycodone reward (A) and intake (B) per session (FR5) were significantly greater in females than males in the 23 strains where both sexes were studied (p < 0.001 for both). Intake was also significantly greater in females than in males across different stages (C) of SA (p < 0.001 for all). In both sexes, the greatest intake occurred during 16-h sessions.

Fig 4. Initial 1-h and stable 4-h oxycodone intake within each sex across the 36 HRDP strains.

Mean intake during the first three sessions (top 2 panels) showed large strain differences in each sex. Similarly, mean intake during the last three 4-h sessions (stable intake; lower 2 panels) varied across the HRDP in each sex. (note: the difference in X axis scales).

Fig 5. Oxycodone (0.1 mg/kg) intake in 4-h and 16-h sessions within each sex.

A, B: Extending access from 4-h to 16-h increased total drug intake/session in many strains and both sexes. Within specific strains, increased 16-h intake was sex-specific. C, D: The correlation of oxycodone intake between 4 h and 16 h sessions was significant in both females (r =  0.832, p < 0.0001) and males (r =  0.835, p < 0.0001).

Fig 6. Augmented oxycodone intake in 16-h vs 4-h sessions.

A subset of strains (7 female, 6 male) augmented their oxycodone intake during long access 16-h vs 4-h sessions by at least 3-fold (Females: F_1,12 = 15.03, p = 0.002; Males: F_1,10 =  18.22, p =  0.002, panels B). Inter-sex comparisons were not conducted because only three strains were common to both sexes. At 4-h, non-augmenters and augmenters had similar oxycodone intake within each sex (Females; F_1,12 =  0.05, p =  0.84, panels A; Males: F_1,10 =  0.08, p =  0.79, Panel B). In the 4-h sessions (panels C, D), maximum oxycodone intake occurred in both sexes of augmenters and non-augmenters during the first hour and declined thereafter toward baseline levels by 3-4 hours. In the 16-h sessions (panels E, F), intake in non-augmenters was maximal at 1 hour, declining gradually thereafter in both sexes. In contrast, early intake, from 1-6 hours, was much greater in the augmenters of both sexes, rapidly decreasing from approximately 7-11 hours, and gradually thereafter. Augmenter types: 7 females, FXLE19/Stm, WAG/RijCrl, SR/JrHsd, LEXF2B/Stm, (SHR/OlaIpcv x BN/NHsdMcwi) F1, FXLE15/Stm, BDIX/NemOda; 6 males, F344/Stm, LEXF5/Stm, FXLE15/Stm, BXH2, LE/Stm, M520/NMcwi). Control non-augmenter types: females, SHR/OlaIpcv, BXH2, BN/NHsdMcwi, HXB23, HXB31, F344/Stm, M520/NMcw; males, (SS/JrHsdMcwi x SHR/Olalpcv) F1, BXH6/Cub, F344/DuCrl, BN-Lx/Cub, HXB31, HXB23.

Fig 7. Breakpoints during progressive ratio schedule of increasing active licks per oxycodone dose (0.1 mg/kg).

In each sex, the breakpoints reached were significantly strain-dependent (A. females, F_32,148 =  5.2, p =  1.8e-12; B. males, F_25,103 =  2.6, p =  0.0003) and greater in females than males (F_22,227 =  6.4, p =  2.5e-14). X-axis is in log scale.

Fig 8. Cue-induced reinstatement of extinguished oxycodone seeking.

Reinstatement was conducted after the number of licks on the active spout was less than 100 during two consecutive extinction sessions. There was a strain difference in the number of active licks during reinstatement in females (A. F_31,142 =  2.9, p =  1.3e-5) and males (B. F_25,94 =  3.2, p =  2.0e-5). In the 23 strains where reinstatement was studied in both sexes, female active licks were greater (F_23,221 =  3.8, p =  1.5e-7).

Fig 9. In the 23 common strains, the correlation in male (M) vs. female (F) oxycodone (0.1 mg/kg) intake/session during 4-h and 16-h sessions.

A. During 4-h sessions with stable oxycodone intake, M vs F intake was highly correlated across the HRDP (Pearson r =  0.54, p =  0.008). B. However, during 16-h sessions, intake between sexes was not significantly correlated (r =  0.35, p =  0.104).

Fig 10. Representative correlations between oxycodone SA parameters and behavioral traits.

Panels A-D show significant correlations for EPM vs oxycodone intake in 4-h SA sessions at 0.25 and 0.5 mg/ml in female (panels A, C) and male (B, D) HRDP strains. Significant correlations in male HRDP strains between NOI (distance to center) and PR (breakpoint) or total oxycodone intake are in panels E and F, respectively. Correlations between NOI and PR or total intake were not significant in females (see supplementary S1 Fig). Panels G and H, show significant correlations between two SA parameters, PR and total intake, in males and females, respectively. P values were not adjusted for multiple comparisons.

Fig 11. Behavioral traits that significantly correlate with oxycodone SA parameters.

This bar graph groups all the behavioral traits measured in EPM, OFT, and NOI trials that were associated with each SA parameter; this grouping is based on the significant correlations (p  <  0.05) identified between a single behavioral trait and an SA parameter (supplementary data, S1 and S2 Figs).